5_Sajona Etal_Tertiary and Quaternanry Magmatism in Mindana and Leyte

Journal of Asian Eurih Sciences, Vol. 15. Tim 2-3; pp. 121-153, 1997 Q 199’7 Published by Elsevier Science. All rights reserved Printed in Great Britain 0743-9547197 s17.00 + 0.00 PII:SO743-9547(97)00002-O

Pergamon

ertiary and Quaternary magmatism in Mindana yte (Philippines): geochronology, geochemistr tectonic setting Fernando G. Sajona,*T Hervk Bellon,? Rent: C. Maury,? Manuel Pubellier,$ Ramon D. Quebral,*$ Joseph Cotten,? Francis Edward Bayon, Ericson Pagadog and Percival Pamatian “Mines and Geosciences Bureau, North Avenue, Diliman, 1100 Quezon City, Philippines 1_URA-CNRS 1278 et Univ. de Bretagne Occidentale, 6 Avenue le Gorgeu - B.P.809 - F-292E5 Brest Cedex, France $URA-CNRS 1315 et Univ. Paris et Marie Curie, 4 Place Jussieu, F-75252 Paris Cedex 05, France $Philippine National Oil Company, Merritt Road, Fort Bonifacio, Metro Manila, Philippines Abstract-A set of 230 studied for their major dating). Several volcanic tectonic framework and

igneous rock samples from Leyte and Mindanao (Philippines) has been and trace element chemistry, mineralogy and geochronology (4”K-‘*Ar sectors have been distinguished according to their geographic position, geologic history.

(a) In Leyte, the ophiolitic basement at 50 Ma is MORB-type but characterized by negative Nb anomalies typical of back-arc basalts. Miocene (20 and 11 Ma) and Late Pliocene to Quaternary magmas (2.5-0.4) show typical island arc talc-alkaline characteristics. In Eastern Mindanao, Eocene (47-46 Ma) magmatic rocks are arc-tholeiitic, while those of Miocene (1X-6 Ma) and Early Pliocene (4.5 Ma) age are generally talc-alkaline. Dacites of 12 Ma age have adakitic compositions. The young ( > 3 Ma) volcanics in Surigao and North Davao are mostly adakitic, although talc-alkaline basalts and basaltic andesites dated at 0.9 Ma overlie adakites in Surigao. (b) In Daguma, Oligocene (30 Ma) arc tholeiitic diorites intrude older arc sequences. R4iocene (17.0-7.7 Ma) dikes and flows in both Daguma and Sarangani Peninsula are mostly talc-alkaline andesites, although minor adakites dated back to 18 Ma have been found overlying undated breccias in Daguma. In southeast Daguma, the talc-alkaline volcanic substratum of Pliocene--Recent volcanoes is 8-6 Ma old. Quaternary (1.83 Ma to present) andesitic and dacitic volcanics in Mt t Blit Parker and Mt Matutum (southwest Daguma) are either talc-alkaline or adakitic, while (northwest Daguma) is built-up by 2.5-Ma-old HFSE-enriched basalts, l-Ma-old normal talc-alkaline andesites and O&Ma-old adakites. (c) In Central Mindanao, Miocene andesites yield ages of 20 and 16 Ma. Pliocene (2.5 Ma) volcanism is dominated by talc-alkaline and HFSE-enriched basalts and basaltic andesites. Quaternary lavas (1 Ma to present) range in character from talc-alkaline to shoshonitic and exhibit wide variations in their trace element behavior. (d) Miocene lavas of 19-11 Ma age in Zamboanga are mostly talc-alkaline, though a lone IX-Ma-old adakite was also encountered. Pliocene-Quaternary basalts and basaltic andesites dated back to 2.2-l Ma are HFSE-enriched, such that some display positive Nb anomalies in spidergrams. Adakites of 3.8-0.7 Ma age are located nearer the Sulu trench. The compositional variety of Tertiary-Recent island arc magmatism in Leyte and Mindanao could be linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the Philippine archipelago. The identification of adakites at certain time periods is used to date timings of subduction initiations and. arc polarity reversals. Major and abrupt changes in geochemical compositions are well exhibited by the Pliocene-Pleistocene volcanism, involving the collision-related shift from talc-alkaline to shoshonitic volcanism in Central Mindanao and adakite production in all the volcanic sectors defined. The origin of the HFSE-enrichments among lavas younger than 3 Ma could be attributed to the presence in their mantle source of an OIB component, or, alternatively, to the enrichment of this mantle source through percolation of slab-derived melts. 0 1997 Published by Elsevier Science Ltd

Introduction The Philippine archipelago provides a challenging subject for petrologists, as its widespread magmatic activity results from an intricate history of subduction, subduction reversals and arc-arc or arc-continent collisions. Most of the previous petrological studies were

focused on the Luzon-Taiwan arc, which includes the island of Luzon, th.e largest island of the Philippines and also the richest in ore deposits. Previous studies of magmatism in Mindanao and Leyte (Bellon and Rangin 1991; Sajona et al. 1993, 1994) have shown that the petrologic characteristics of young magmatism vary according to age and geographic location and are 121

122

F. G. Sajona et al.

strongly dependent on the tectonic evolution in these islands. This paper presents a general overview of magmatic evolution of Mindanao through time and its relationship to the geodynamic history of the Philippine archipelago, on the basis of analytical studies (geochemistry by ICP-AES, 40K-40Ar dating) carried out on 230 samples of magmatic rocks collected from Mindanao and the neighboring islands.

Geodynamic framework Leyte and Mindanao islands are located in the southern portion of the Philippine archipelago, which is composed of tectonic terranes that are either autochtonous or allochtonous with respect to the Eurasian margin (EUR) (Fig. 1). In the archipelago’s present tectonic configuration, Leyte and Eastern and Central Mindanao belong to the Philippine Mobile Belt (PMB; Gervasio 1971) and are of Philippine Sea Plate (PSP) affinity (Pubellier et al. 1991a; Rangin and Silver 1991), while Western Mindanao, consisting of the Zamboanga Peninsula and the Daguma-Sarangani region, is considered to have an EUR origin. For the purpose of this study, four main volcanic sectors have been identified (Fig. lb): Leyte and Eastern Mindanao, Central Mindanao, Daguma-Sarangani and Zamboanga. Both islands are bounded to the east by the Philippine trench (Fig. 1). Two other trenches frame Mindanao: to the west, the Sulu trench, which is the locus of subduction of the Miocene Sulu Sea back-arc basin, and, to the southwest, the Cotabato trench linked to the Eocene Celebes Sea basin subduction (Rangin and Silver 1991). Three geographically distinct arc systems are associated with the trenches (Fig. 2). The southern arc-arc collision zone between Sangihe and Halmahera arcs (Silver and Moore 1978) extends northward in Central Mindanao, as evidenced by seismicity studies (Acharya and Aggarwal 1980; McCaffrey et al. 1980; Quebral 1994) pointing to the presence of detached portions of the Molucca Sea slab beneath the island (Fig 2). The still active voluminous volcanic field in this region may be a consequence of this collision process (Pubellier et al. 1991a; Sajona et al. 1994). The sinistral Philippine fault, a major lithospheric structure affecting the entire archipelago, traverses Leyte island and continues down south through Eastern Mindanao (Fig. 1). In Western Mindanao, the left-lateral Cotabato fault running along a NW-SE direction seems to connect the Sulu-Negros trench with the Sangihe thrust zone and could represent the boundary between the PMB and the EUR in Mindanao.

Analytical methods Chemical analyses on 230 rock samples were obtained by ICP-AES (except Rb by atomic absorption spectroscopy), using AC-E, BE-N, JB-2 and MICA-Fe powders. Relative as standards, on agate-grinded standard deviations for major elements were I 2%, except for MnO and P205, and those for trace elements were I 5%. Details of the analytical methods were given by Cotten et al. (1995).

40K-40Ar analysis were performed on 104 whole-rock samples and on separated feldspars for highly porphyritic rocks. For whole-rock analysis, samples were crushed and sieved to 160-500 pm fraction, then cleaned with distilled water before removal of magnetic minerals. Feldpars were separated magnetically from 80-160 pm aliquots. Details of the analytical procedure for Ar analysis has been described by Bellon and Rangin (1991). Age calculations were carried out using the constants recommended by Steiger and Jager (1977) and error (1 o) was calculated according to Mahood and Drake (1982). These isotopic ages are shown in Table 1, together with parameters characteristic of the samples.

Geological and geochronological

results

Leyte and Eastern Mindanao Leyte. Two ophiolitic bodies outcrop in the southwestern and northeastern portions of the island (Fig. 3). The southwestern massif is dated back to Late Cretaceous based on faunas from associated pelagic sediments (Florendo 1987), while the northeastern one, paleontologically undated, gives K-Ar ages of 50.0 ) 3.9 Ma (JICA-MMAJ-MGB 1986) and 47.21 +_2.15 on gabbros and 55.23 +_ 1.20 Ma on a basalt (Table 1). Late Eocene to Early Oligocene volcaniclastics which unconformably overlie the northeastern ophiolite are truncated by Late Oligocene (NP25) limestones and Middle Miocene (NN6) calcareous turbidites (Aurelio 1992). These turbidites are, in turn, unconformably overlain by two other sequences, a Late Miocene to Early Pliocene (NNl l-13) volcaniclastic unit and Pliocene-Pleistocene (NN19-20) carbonates and volcanic sands. A few ages have been reported for Pre-Pliocene to Quaternary magmatism in northern Leyte: 20.9 +_2.3 Ma for a gabbro (JICA-MMAJ-MGB 1986) and 11.1 + 0.4 Ma for an andesite in lahar near Lake Danao (Bogie 1983). A Late Miocene to Pliocene unconformity seems to correspond to the initiation of the important magmatic activity which built up the Pliocene-Quaternary composite stratovolcanoes distributed near or along the Philippine fault (Fig 3). The young activity of this major fault is documented by the splitting of the largest volcanic center in north-central Leyte by almost 8 km (Aurelio 1992). With the exception of an age of 5.50 & 2.25 Ma obtained for microdiorite recovered from a well in the PNOC geothermal field in Tangonan, most of the ages, including results by JICA-MMAJ-MGB (1986) Bruinsma (1983) and those listed in Table 1, range from ca. 3 to 0.25 Ma. No peculiar time and space pattern of Pliocene-Quaternary volcanic activity can be observed from Biliran to Pana-on. Swigao. Late Cretaceous (Santonian) schistosed marbles outcrop near the ophiolitic basement (Fig. 4), which is overlain by Late Eocene serpentine-bearing sandstones associated with altered volcanics (Quebral 1994). Pillow basal& dated at 17.49 + 0.45 (MN0 88-54) and 13.25 + 0.65 (PH 93-88) (Table 1) are intercalated within limestones that overlie the older units. Pliocene andesites which cut and mineralize Middle Miocene turbidites in Placer town give whole rock ages of 3.78 + 0.15 Ma (PH 93-91) and 2.91 + 0.15 Ma (MN0

Magmatism in Mindanao

123

and Leyte (Philippines)

15’

South China Sea 15'

~hilip~j~e

3

Sea

5’

almahera

cl

Eurasian

marginal

IIndex

basin

B: Leyte-Eastern

Terrane Eurasia Active plate

allochtonous subduction

to

Central

or intra-

Mindanao

Mindaflao

Daguma-Sarangani

thrusting

Thrust zone (dashed inactive) Strike-slip

where

r

Zamboanga

fault

Fig. 1. Tectonic map of the Philippines and surrounding areas, modified from Rangin et ai. (1990). Lane A-A’ corresponds to a synthetic section shown in Fig. 2. Abbreviations: B, Bicol peninsula; S: Samar; P, Panay. D, Dinagat; M, Mati; P: Pujuda peninsula; PF, Philippine fault; CF, Cotabato Fault. Index map A shows relative position of the Philippine Mobile Belt (PMB) with respect to the Eurasian (EUR) and Philippine (PSP) sea plates. Index map B shows the division of the Leyte-Mindanao archipelago into several volcanic sectors that are described in the text.

124

F. G. Sajona et al.

Zamboanga

Fig. 2. Schematic

cross-section

representation

Daguma-

Central

Sarangani

Mindanao

Eastern Mindanao

of line A-A’ in Fig. 1, based on geophysical 1994).

data (Cardwell

et al. 1980; Quebral

88-49) and feldspar ages of 7.66 ) 0.38 and 3.64 & 0.28 Ma, respectively. The older feldspar ages could be interpreted as due to excess argon trapped in the early formed feldspar phenocrysts. Age correction (as described by Sajona et al. 1994) leads to ages of 3.37 and 2.79 Ma, respectively. It must be noted that MN0 88-49 has been previously dated at 4.5 Ma (Bellon and Rangin 1991). A re-analysis after removal of secondary minerals gave analytical and corrected ages reported in Tables 1 and 2, respectively. North of Lake Mainit, andesitic to dacitic plugs and domes were emplaced along the Philippine fault and its subsidiary splays trending N 135-140” and N 45” (Fig. 4). A dacite (Q 90-68) from the base of Mt Pace, the biggest volcanic center of the area, gives a whole-rock age of 1.08 + 0.06 Ma and an older feldspar age of 1.78 ) 0.13 Ma. The corrected age is 0.90 Ma (Table 2). One of the andesite domes rimming the breached crater of the volcano is dated at 0.21 _+0.05 Ma (PH 92-28), while a thin basaltic flow on top of one of these domes gives an age of 0.09 + 0.04 Ma (PH 92-30). Andesitic to dacitic dikes cutting the Malimono ridge on the western coast of Surigao peninsula give ages of 2.31 2 0.24 (MN0 88-46) and 2.21 + 0.10 Ma (PH 92-37). Separated feldspars from PH 92-37 give an apparent age of 4.92 _t 0.43 Ma (Table 1); the corresponding corrected whole-rock age is 1.8 Ma (Table 2). ophiolite body in Pujada Davao . An undated peninsula is in fault contact with pillow basalts, and pelagic cherts in Mati (Fig. 5; Quebral 1994). These rocks were not encountered in the study area, where basement correlated with Late Cretaceous units in Pujada is represented by altered volcaniclastics, lava flows and tuffs occurring on the western side of the Hijo fault (a portion of the Philippine fault). East of this fault, lava flows dated at 47.16 k 1.58 Ma (PH 92-82) (Table 1) and associated volcanic breccias are overlain by Late Oligocene to Middle Miocene elastic sediments and limestones. A fresh diorite boulder coming from a pluton intruding the Eocene volcanic sequence on the upper portions of Panuraon river is dated at 46.14 -& 1.58 Ma. The main dioritic body further east intrudes Paleogene sequences and is overlain by Late Oligocene to Early Miocene limestones, indicating a likely Eocene or Early Oligocene age for the pluton. A small diorite body in San Francisco, far north outside the study area (Fig. 5, index map), gives a whole-rock age of 32.27 4 0.78 Ma (PH 93-82), confirmed by the

feldspar age of 31.44 ) 0.82 Ma. In Amacan, North Davao, a diorite from a PNOC drillhole gives whole-rock and feldspar ages of 18.09 if: 0.54 and 17.49 _+0.45 Ma, respectively. An andesite dike (PH 92-15) cutting the diorite is dated at 12.35 + 0.46 Ma. Small andesite dikes that seal gougy faults cutting Paleogene sequences along Hijo river are dated at 12.02 + 0.30 Ma (PH 92-83), while a float from a basic andesite flow (PH 92-77) along Panuraon river yields an 11.10 + 0.65 Ma age. Young andesite to dacitic volcanism is mostly represented by ash and lapilli cones and domes emplaced along the Philippine fault and its tensional splays trending N 140” and N 35”. A dike (PH 92-80) intruding chloritized rocks of the Paleogene volcanics is dated at 6.46 i_ 0.19 Ma. However, separated feldspars from this sample give a 40.74 + 1.30 Ma age, and the corresponding corrected age is 4.52 Ma (Table 2). Most of the samples come from Lake Leonard, an active cinder volcano with a 1.5-km-wide caldera surrounded by andesitic domes and diatremes. A sample (Q 90-21) gives a whole-rock age of 0.3 1 + 0.11 Ma, a feldspar age of 0.59 t_ 0.26 Ma (Table 1) and a corrected age of 0.24 Ma (Table 2). The latest activity of Lake Leonard could be reflected by the 1.8Ka 14C age obtained from a carbonized wood from pyroclastics rimming the crater (Barnett 1983). General Mindanao

This roughly triangular region is bounded by the Cotabato fault and the Cotabato basin to the southwest and the Agusan-Davao basin to the east (Figs 1 and 6). Its western portion which coincides with the northeastern Zamboanga peninsula, has a substratum of lava flows overlain by Early Miocene limestones, Middle Miocene (NN5) elastic sediments and Late Miocene (NNl l-13) sandstones and marls. An andesitic lava flow (P 90-69) underlying Middle Miocene sediments is dated at 17.27 _t 0.69 Ma (Table 1). These old units are mostly buried under two contiguous active Quaternary volcanoes, Malindang and Ampiro, built-up of basaltic and lesser amounts of andesitic flows intercalated with abundant pyroclastics. A flow (PH 93-59) at the base of Mt Malindang is dated at 0.64 + 0.22 Ma, while another (TIP 5) at a 500-m elevation gives an age of 0.40 + 0.07 Ma. Samples from the flanks of Mt Ampiro yield ages ranging from 0.70 to 0.29 Ma.

BLN 29 BLN 10 PH 92-11 PH 92-171 LEY 90-37A PH 92-14 PH 92-167 PH 92-173

83234-7 B3434-7 B3232-5 B3236-9 B3011-8 B3233-6 B3235-8 B3278-8

Pre- Pliocene B3276-6 B3304-8 BJ292-5 B3685-8 B2292- 1 B3259-7 53260-8 83640-4 B3644-8 B3355-7 B3354-6

B3252-9 B3251-8 B3001-7 B3002-8 B2991-4 B3029-9 B3247-4 B3248-5 B2077-2 B3719-7 83669-7 B3662-8 B3669-7 B3423-5 B3427-9

type7

type

FP 88-46 # 88-49 WR FP PH 93-91 # WR FP PH 92-80 # WR FP

Fp PH 93-82 WR FP PH 92-75 PH 92-82

PH 92-77 PH 92-83 PH 92-15 # PH 93-88 MN0 88-54 PH 92-16 WR

MN0 MN0

Dike

CAKA

Pluton

Pluto11 Flow

IATDi

IATDi IABA

Flow Dike Dike Flow Pillow Pluton

Flow

CAKD

CABA CAD CAKA CAB CAKB CADi

Boulder Flow

Boulder

CAKD

CAKD CAA

Boulder

CAKA

?R FP PH 92-37 # WR

Q 90-68 #

Flow Dome Dome

Pillow lava Pluton

Flow Flow Core sample Flow Flow Core sample Boulder Flow

Sample

CAB CAA CAD

BABB BABG

CAKA ‘CAB CABA SBA CAA CAA CAKA CAKA

Rock

Davao

North North

Davao Davao

San Francisco

North Davao North Davao North Davao Placer Surigao North Davao

North

Placer

Surigao Placer

Surigao

Surigao

5 _t _t + f + f f

0.03 0.11 0.16 0.06 0.24 0.13 0.08 0.07

11.10 12.02 12.32 13.25 17.16 18.09 17.49 32.27 31.44 46.14 47.16

0.09 0.21 0.31 0.59 1.08 1.78 2.21 4.92 2.31 2.91 3.64 3.78 7.66 6.46 40.74 & i I i & & + f _t + +

* & + + + f + + + + + + f + * 0.49 0.29 0.44 0.62 0.36 0.51# 0.43 0.17 0.78 1.12 1.58

0.02 0.03 0.075 0.18 0.165 0.13 0.083 0.43 0.24 0.13s 0.20 0.135 0.24 0.175 1.11 0.69 1.51 2.14 0.95 1.32 1.53 1.82 0.56 0.29 0.40 0.16

0.28 2.91 0.33

1.32

1.46 2.06 2.30 1.30 2.75 0.80 2.82 0.75 2.77 1.27 0.54

MINDANAO

0.16 0.15

2.74 1.10 1.28 1.19 1.67 1.69 2.1 2.43

rock samples

LEYTE

47.21 f 1.68 55.23 & 1.54

0.24 0.61 0.85 1.24 1.31 1.39 1.61 2.53

Age (Ma) ( + g.)

for igneous

EASTERN

results

Surigao Surigao North Davao

Tacloban Tacloban

Biliran Bihrdn Tongonan Silago Tongonan Biliran Pana-on Sogod

Provenance

1. 4”K-40Ar dating

PH 92-30 PH 92-28 # Q 90-21 # WR

Ophiolite basement B3414-5 PH 88-02B B3415-6 PH 88-02A

Sample*

Lab. no.

Tabie

2.48 13.45 10.92 4.07 7.34 8.97 10.31 5.93 2.97 6.03 2.46

0.04 0.14 0.23 0.24 0.88 0.46 2.01 1.19 2.07 1.19 0.63 0.16 0.69 6.08 4.38

2.47 2.71

0.21 0.22 0.35 0.47 0.74 0.75 1.10 1.98

4”Ar$$ (1O-'cm’/g)

16.8 5.9 24.6 17.7 73.0 32.6 42.0 61.0 46.9 40.0 21.3

1.8 3.3 6.7 5.3 11.6 11.3 20.1 9.6 19.7 16.8 10.8 22.6 17.6 32.7 29.1

18.8 24.7

4.7 3.4 4.0 12.5 10.6 7.8 15.1 24.8

@ArDJ (%)

from Leyte and Mindanao

3.46 2.07 1.13 6.00 0.91 5.59 4.15 0.93 0.82 2.23 2.22

1.20 1.53 0.52 3.11 2.46

1.71

0.44 0.79 0.65 1.05 1.84 0.79 2.13 2.65 2.84

2.48 1.64

2.65 1.67 1.35

1.69

1.03 1.52 2.47 0.69

(10 ~‘Icm’/g)

___.ZhAr

0.5081 0.5281 1.0034 1.0043 0.4215 0.4096

1.0104

0.5017 0.7031 0.7055 0.8462

1.0071 0.6057 0.8012 0.8018 0.8042 0.8209 0.6226 0.3017 1.0042 1.0050 0.6056 1.0264 0.6009 0.3099 0.3011

0.3104 0.3037

0.8140 0.6237 0.8006 0.8104 0.8251 0.8020 0.8322 0.5104

Weight (6)

-

B 1.10

1.53 1.19 Table

1.19

1.75 2.32 1.92 0.71 3.66 0.94

1.65

1.61

A B l-Continued

A

B C B A C A

B

C

A B

B 1.14

B C

A B A A B A A

A

A A A

0.92 1.26

Freshness7 _--_______

0.51 0.67 0.17

1.85 2.25

0.6 1.10 2.37 0.67 0.72 2.02 0.79 1.43

LOI (wt 1%)

t;:

WR FP

CAB CAA

CAKD

Mt Blit Mt Matutum Mt Matutum Mt Blit Mt Matutum Mt Matutum Mt Blit Mt Parker

CAD CAA CAA CAA CAKA

Boulder Neck Neck Boulder Lava neck

PH 92-116 # 93-85 # 93-86 # PH 92-115 PH 93-84 # MAT 75 # PI-1 92-114 PH 92-109 #

Mt Parker

Boulder

CAA

PH 92-108 #

WR FP

Boulder

CAD CAA

MAT 77 # PH 92-95 #

B3391-9 B3433-6 B3409-9 B3286-8 B3296-9 B3305-9 B3635-8 B3636-9 B3693-7 B3645-9 B3373-9 B3287-9 B3426-8 B3695-9

Flow Flow Flow

1ATA CAKBA CAKA

Pre-Pliocene MN0 89-12 B2468-7 P 90-69 B3007-4 MN0 89-20 B2248-2

Flow Boulder

Flow Flow Flow Flow Flow Flow Flow Flow Flow Flow Flow Flow Flow

Mt Mat&urn Mt Matutum

Malay-balay Manucan Buda Road

Camiguin Is. Mt Kalatungan Mt Balingoan Maramag Mt Kalatungan Camiguin Is. Talisayan Maramag Mt Kalatungan Mt Apo Kinoguitan Mt Apo Quezon

Iligan Kitabud Mts Lana0

Mt Ampiro Mt Ampiro Mt Malindang Mt Ampiro Mt Malindang Mt Ampiro

Provenance

CAA CAD CAA CAB CAB CABA CAKB SB SBA CAKBA CAKB CAKB CABA

type

Eastern portion B3712-9 MO 11 B3689-3 KL 143c # PCBAL 4 B3692-6 B2252-6 MN0 89-16B B371 l-8 KL 188 BOO2 B3683-6 TALISAYAN B3010-7 MN0 89-16A B2251-5 KL 162 B3710-7 B2253-7 MN0 89-37 KINOGUITAN B3008-5 PH 92-120 B3425-7 MN0 89-17 B2254-8

Flow Flow Flow

Flow Flow Flow Flow Flow Flow

Sample

IATB CAKA CABA

type?

Central portion B3730 PH 92-153 B3017-6 P 90-9 B3356-8 PH 92-151

Rock

CAKA CAKBA SB SB CABA CAD

Sample*

Western portion B3020-9 TUDELA B3018-7 ALORAN TIP 5 B3688-2 B3016-5 P 90-75B B3709-6 PH 93-59 B3009-6 P 90-7514 #

Lab. no.

-.

+ * t_ f f *

0.07 0.09 0.06 0.09 0.20 0.43

+ * f + + * + * f + i: f f

0.00 0.00 0.08 0.07 0.04 0.03 0.08 0.05 0.04 0.06 0.11 0.10 0.27

0.00 0.00 0.17 0.47 0.35 0.76 0.68 0.98 1.06 1.47 1.83 2.50 4.37 5.38

* 0.00 * 0.00 + 0.05 2 0.05 + 0.24 f 0.05 f 0.02 5 0.03 & 0.05 ri7_0.04 f 0.04 + 0.10 i 0.835 f 0.41

1.68 1.72 1.14 1.45 1.33 1.26 2.63 3.21 3.93 1.86 2.41 1.48 1.00

0.26 2.48 1.19

2.47 2.35 2.75 2.77 1.29 1.63

0.66 1.93 2.46

1.72 2.10 2.10 1.31 0.35 1.54 1.93 2.10 1.22 2.88 2.42 0.39 1.19 0.24

DAGUMA-SARANGANI

16.32 & 0.85 17.27 + 0.69 19.86 + 0.36

0.00 0.00 0.14 0.25 0.27 0.34 0.36 0.40 0.52 0.62 0.65 0.80 1.15

0.16 + 0.14 0.46 + 0.08 2.31 f 0.11

0.29 0.39 0.40 0.43 0.64 0.70

CENTRAL

MINDANAO

1. (Continued)

Age (Ma) ( f c)

Table

0.00 0.21 0.12 0.20 3.94 0.38 0.53 0.42 0.42 1.37 1.42 0.31 1.68 0.42

3.49 10.8 0.36

0.00 0.00 0.05 0.12 0.12 0.14 0.30 0.41 0.66 0.37 0.50 4.25 0.37

0.01 0.37 0.89

0.23 0.29 0.35 0.38 0.27 0.37

0.0 0.0 3.0 5.3 0.8 8.1 16.1 16.1 11.9 30.4 32.4 12.7 4.1 7.9

35.0 47.0 92.1

0.0 0.0 1.2 5.6 3.9 5.0 6.8 15.9 10.2 19.3 9.8 0.4 8.5

0.6 8.6 18.0

6.1 6.7 4.8 7.9 2.5 3.4

40Ar§f (“/)

0.64 1 .oo 0.58 0.75 1.04 0.94 0.31 0.29 0.70 0.70 0.44 0.38 12.27 1.14

2.17 3.68 0.46

0.42 1.18 1.11 0.43 0.64 0.48 0.97 0.67 1.65 0.47 1.14 1.77 1.19

0.80 0.90 0.49

0.78 0.94 2.02 1.07 3.19 3.07

j6Ar (1 O-’ cm’/g)

0.5116 0.5321 0.5027 0.8012 0.6309 0.7003 0.8054 0.8006 1.0162 1.0020 0.6196 1.0097 0.3089 0.6889

1.2226 0.8092 1.0046

1.0150 1.0035 1.0040 1.0235 1.0362 0.8216 0.8394 1.0003 1.0262 1.0169 0.8257 0.3030 1.0233

1.0032 0.8257 0.3995

0.8007 0.8017 1.0015 0.8123 1.0126 0.8015

Weight (g;i

A A A C A A A A B B B B

0.54 0.66 0.57 0.54 1.07 1.20 0.84 1.03

B C D

A A B B A A A A B A B A A

A A B

B A A A A A

Freshnesslj

0.22 0.59 0.59 3.04

1.30 2.89 2.42

-0.08 0.57 1.55 1.33 0.22 0.16 0.49 0.68 0.99 0.02 1.60 0.26 0.99

0.81 0.92 1.51

1.10 1.03 0.90 0.03 0.38 0.95

LO1 (wt %)

F_

& 2

0

Z? L.

?

r

;3 v1

CAKBA CAD IATD IAlA

CAD

PH 93-30 #

P 90-99 P 90-101 I’ 90-59A P W-58

Flow Flow Flow Flow Flow

CABA CAB IABA IATB IAT

P 90-36 PH 93-6 PH 93-39 P 90-44A P 90-19

How

Flow Flow Flow

Boulder

Flow

CAB

P 90-35

Flow

Flow

CAD

CAD

LW6#

WR FP

CAKBA

P 90-61

P 90-20 #

Flow Pluton Pluton Flow

CAA IATBDi IATBDi IATA

Flow

Boulder Boulder Boulder Flow Flow

Zamboanga Zamboanga Kabasalan Kabasalan

Kabasalan

Basilan is. Buug Pagadian

Mt Kaladis

Mt Kaladis

Lakewood

Lakewood

Ipil

Kiamba Maasin Maasin Kiamba

Mt Parker Talaguton Mt Matuium Malita Malita Malita Maasin Kiamba

Provenance i * 2 * It & & + k f & f +

1.38 0.17 0.23 0.18 0.22 0.90 1.25 0.425 1.24 0.40 2.12 3.95 10.86

11.91 14.94 16.89 IS.95

0.41 0.27 0.82 2.55 0.97 0.84 1.08 1.09 1.21 1.76 1.98 1.71 2.58 1.91 3.88 3.83 i * & f

+ f + + f i * & i_ f f & + * f f. 0.62 0.43 0.64 0.71

0.04 0.02 0.05$ 0.43 0.05 0.05 0.37 0.07 0.48 0.19 0.26 0.13 0.08 0.06 0.10 0.10

ZAMBOANGA

6.38 7.70 8.44 9.31 10.64 12.89 16.73 18.00 23.62 18.32 29.89 31.91 59.18

( + c)

Age (MaI

1.60 2.65 0.80 0.95

1.70

0.52 0.89 0.53 0.31 0.74

0.64

1.35 0.33 1.32

2.38

0.95 1.88 1.98 3.41 2.37 0.92 0.56 1.46 0.28 1.47 0.70 0.19 0.31

(wt %)

K&

1. (Continued)

6.16 12.81* 8.94 5.S3

0.38 0.21 0.35 0.27 0.41 0.36 0.22 0.22 0.20 0.50 0.34 0.20 0.62 0.46 2.13 2.10

1.96 4.68 5.40 10.44 8.15 3.84 3.03 8.51 2.15 8.73 6.80 2.02 6.01

“(‘Ar§$ (1O--7 cm3/g)

35.4 68.5 49.0 49.7

6.9 6.7 9.2 5.0 10.8 10.0 4.9 8.4 4.2 6.2 5.4 11.07 18.0 31.1 43.6 42.8

3.9 57.4 30.7 84.3 74.4 11.2 26.0 50.2 14.1 55.8 27.4 17.1 4.6

4UArQj: (“/)

3.37 1.57 1.13 1.57

1.01 1.oo 0.76 1.06 0.72 0.77 1.03 0.48 1.11 2.07 1.64 0.55 0.52 0.34 0.50 0.60

16.22 0.50 2.97 0.65 0.94 9.59 2.88 2.27 3.79 1.22 6.10 3.31 42.15

‘6Ar ( 10e9 cm’/g)

0.8011 0.8152 0.8465 0.8224

0.8069 1.0027

L.0040

0.8224 1.0014 0.8215 0.5143 0.8016 1.0081 0.8064 1.0022 0.8016 0.7037 1.0042 1.1734 0.8119

0.8018 0.5158 0.3014 1.0208 1.1097 0.5030 1.0331 0.6013 0.5413 0.3140 1.0227 1.0768 0.7028

Weight (g)

B B C D

1.58 0.59 2.21 3.26

A C C A A B A

1.23 2.58 2.12 0.39 0.18 1.53 1.22

1.84 1.46 3.36 0.98

B N B I3

A

B 0.91

0.81

A 0.4

B

C C C D B

C C

Freshnessll

3.04 1.80 1.35 2.98 3.04 2.12 4.66 1.29

LOT (wt %)

*WR, whole rock; Fp, feldspar; # adakite. jIAT (B, A, D, Di, G, mG), island arc tholeiire (basait, andesite, dacite, dioritc, gabbro, microgabbro); CA, c&-alkaline; CAK, high-K talc-alkaline; S, shoshonitic; DAB, back-arc basin. i4”Ar$ and 3”Ar are pertinent to the sample only; X4”Ar$ is pertinent to the whole experiment. $See Table 2 for corrected age. q/A, very fresh, no secondary minerals; B, moderately fresh ( < 3% modal secondary minerals); C, slightly altered (3-S% modal secondary minerals); D, moderately altered (5510% modal secondary minerals)

Pue-P&me B2999-5 B2998-4 B3000-6 I430 15-4

B3024-4 B3712 B3023-3 B3279-8 B3690-4 B3771-6 B3113-6 B3770-5 B3682-5 B3768-3

B3027-7 B3773-8 B3026-6 B3028-8 B3680-3

type

intrusive Dike

Sample

CABA CAD CABA CAKD CAKA CAG CAB CAD

Pre-Pliocene B3277-7 B3372-8 B3417-8 B2258-3 B2249-3 83390-8 B2250-4 B3293-6 B3294-7 B3389-7 B2213-1 B2214-2 B2391-4

Rock type?

PAR 69 PH 92-50 iMAT 73 MN0 89-21A MN0 89-21B PH 92-63 MN0 89-25 PH 92-105 # WR PP PH 92-102 # MN0 89-26A MN0 89-26B PH 92-107

Sample*

Lab. no.

Table

128

F. G. Sajona

et al.

-

11”OO’

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

25 26 27 28

Sample BLN29 BLN32 BLN 77 BLN106 BLNlO BLN92 PH 92-14 BLN51 8LN152 PH92-11 PH92-12 PH 92-5 PH 92-6 PH 92-23 LEY 90-37A LEY 90-378 LEY 90-37C LEY 90-37D PH 92-1 PH 88-02A PH 88-01 B PH 92-l 76 PH 92-18 PH 92-171 PH 92-l 73 PH 92-174 PH 92-172 PH 92-167

K-Ar AGE (Ma) 0.24 + 0.04

0.61

kO.15

1.39

+ 0.15

0.85

+ 0.18

55.23 47.21

t 1.99 2 2.16

Pana-on island

1.24 e 0.08 2.53 f 0.09

1.61

kO.09 124”30’

125”OO’

yru

Fig. 3. Composite geologic and sample location map of Biliran, Leyte and Pana-on islands modified from fold axes; E, Aurelio (1992). Key: A, Philippine fault; B, thrust fault; C, anticline axis, . D, undifferentiated Quaternary volcano; F, PNOC geothermal field; G, marshland; H, alluvium; I, Pleistocene limestone; J, Late Miocene to Early Pliocene sediments; K, Middle Miocene limestone; L, Late Oligocene to Early Miocene sediments; M, Eocene volcaniclastics; N, undifferentiated volcanic; 0, ophiolite.

San

31 -

.31 Ma \l

-9”35’

3 -W-

sampie 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

K-Ar Age (kh)

PK 92-44 PH92-37 MN0 88-46 PH 92-38 PK 92-28 PH92-30 PH 92-34 PK92-35 Q9568 PK 93-88 Pti92-36 t.WQ 8849 PK 93-90 PK 93-91 MN8 88-54

*Corrected

age

1.80” 2.31 t 0.24 0.21 0.09

+ 0.05 c 0.04

a.90* 13.25 f 0.65 2.79’ 3.37’ 17.16 t 0.36 (see

Table

2)

Fig. 4. Composite map of Surigao peninsula showing lithological units, structures and sample location. Key: A, caldera; B, Philippine fault; C, subsidiary faults; D, thrust fault; E, Late Pleistocene lava domes; F, Late Pleistocene pyroclastics; G, Early Pleistocene volcanics; H, Early Pleistocene elastic sediments; I, Early Pleistocene limestone; J, Pliocene volcanics; KY Middle Miocene turbidites; L, Early Middle Miocene formation; Late Cretaceous to Paleogene formation; N; Ultramafic basement. Index map: D, Dinagat island; EM, Eastern Mindanao; ADB, Agusan-Davao basin. Sources: Philippine ines and Geosciences Bureau (MGB) unpublished compilation maps; Tebar and Pagado 1990, Pubellier et al. 1993; NAMRIA topographic map P.C.G.S. 2535, 1989.

129

130

F. G. Sajona

Table 2. Results

and parameters

of age correction

Age (L) Ma WR Eastern Mindanao Q 90-68 Q 90-21 PH 92-37 MN0 88-49 PH 93-91 PH 92-80

1.08 0.31 2.21 2.91 3.78 6.46

f f + f f +

1.78 0.59 4.92 3.64 7.66 40.74

calculation for highly porphyritic feldspar ages

+ + + f + +

samples

with discordant

40Ar*cal (1 O-’ cm3/g)

% I&O

whole-rock

40Ar*cor (lo-’ cm’/g) WR

age (~3 Ma

Fp

WR

Fp

FpjWR

%KzO,

0.13 0.26 0.43 0.28 0.38 1.30

2.75 2.30 2.82 1.27 1.37 2.90

0.80 1.30 0.75 0.54 0.28 0.33

0.88 0.23 2.01 1.19 1.61 6.08

0.46 0.24 1.19 0.63 6.93 4.38

0.34 0.37 0.49 0.34 0.44 0.47

3.75 2.89 4.80 1.64 2.16 5.10

2.36 7.59

0.90 0.24 1.80 2.79 3.37 4.52

1.10 0.22 2.80

1.48

Daguma PH 92-109 PH 92-105

4.37 + 0.84 18.00 + 0.42

5.38 * 0.57 23.62 rfr 1.42

1.19 1.46

0.24 0.28

1.67 8.51

5.38 2.15

0.24 0.20

1.53 1.76

2.14 10.1

4.33 17.71

Zamboanga P90-20

0.82 + 0.07

2.55 + 0.43

1.35

0.33

0.35

0.27

0.17

1.56

0.37

0.72

t,,,, calculated age from actual analysis; WR, whole rock; Fp, feldspar; Fp/Wr, % KzOM, recalculated KzO content of rock minus feldspar; t,,, corrected age.

The central portion of Central Mindanao from Cagayan de Oro and Iligan in the north to Cotabato in the south (Fig. 6) is underlain by tectonic slivers of basement peridotites and undated volcanics overlain by Late Oligocene to Early Miocene sediments. They are blanketed by Late Miocene (NNll) elastic sediments and numerous Pliocene-Quaternary basaltic and andesitic lava flows. The latter are cut by E-W and ENE-WSW normal faults resulting in horst and graben structures that are also responsible for the Lake Lanao subsidence. A basaltic flow from Marawi yields a 2.31 + 0.11 Ma age, presently the oldest obtained from the young volcanics of Central Mindanao. Another sample from Iligan is much younger (0.16 + 0.24 Ma), while one specimen from Cotabato (P 90-9) dates back to 0.46 k 0.12 Ma. The eastern portion of Central Mindanao includes the Central Cordillera west of the Agusan-Davao Basin and the N-S string of Quaternary volcanoes from Camiguin island to Mt Apo. In the Central Cordillera, Late Oligocene to Early Miocene limestones overlie a basement of serpentinized peridotites and gabbroic/plagiogranitic outcrops in tectonic contact with metamorphosed volcanics and sediments (Quebral 1994). On its western flank, andesites dated at 19.86 i 0.36 Ma (MN0 89-20) and 16.32 f 0.85 Ma (MN0 89-12) are overlain by late Early Miocene limestones. Quaternary volcanoes west of the cordillera are mostly basaltic and pyroclastic in character. Several of them are considered active, e.g. Mts Hibok-Hibok and Vulcan in Camiguin island and Mts Kalatungan, Ragang and several minor cones in mainland Mindanao. In Camiguin island, field evidence suggest that volcanism started in its southern portion in Mt Butay and propagated northward to Mts Mambajao, Hibok-Hibok and Vulcan. A Mt Butay basaltic andesite is dated at 0.34 Ma (B002). An andesitic flow (MO 11) in Mt Mambajao gives a zero age (i.e. younger than 100 Ka). Historic eruptions are recorded for Mts Hibok-Hibok and Vulcan. In mainland eastern Central Mindanao, the oldest dated rock (MN0 89-17) at 1.15 + 0.27 Ma is a basalt from low-lying domes in Quezon. Younger ages ranging from 0.8 to 0.25 Ma are obtained for samples distributed from north (Mt Balingoan) to south (Mt Apo) without any apparent

modal proportion

of feldspar

and

Corrected

WR

FP 0.06 0.11 0.10 0.15 0.15 0.19

et al.

in whole rock;

chronological trend. Mts Kitanglad and Kalatungan consist of several E-W-orientated basaltic edifices parallel to the major structuration of the Lanao-Cotabato area and rare andesitic/dacitic adventive domes from which a sample (KL 143C) gives a zero age. The Mt Apo volcanic center, the highest summit (2990 m) in the Philippines, is composed chiefly of basaltic flows cut and overlain by more recent andesites in its northeastern portion.

Daguma-Sarangani Daguma range. This NW-SE-trending mountain range is bounded by the Cotabato basin to the northeast and the Celebes Sea to the southwest (Fig. 7) and cut by en echelon normal faults along its northeastern flanks. A weakly synclinally folded plateau 70 km long and 29 km wide is located in its center. Two Pliocene-Pleistocene volcanoes, Mt Blit and Mt Parker, are installed at the northwestern and southeastern ends of the range, respectively. Basement rocks are scarcely outcropping serpentinized ultramafics in tectonic contact with quartz mica schists and quartzites. They are unconformably overlain by a sequence of volcanic flows, pyroclastic breccia and indurated volcanogenic sediments, all considered to have Paleogene ages. An andesitic flow (PH 92-107) dates back to 59.18 + 10.99 Ma, the large uncertainty being probably due to the alteration suffered by this sample. Dioritic intrusives dated at 29.28 -t_ 2.12 and 3 1.95 f 3.95 Ma (Early Oligocene) cut the basement and are overlain by a volcanic and volcanogenic sequence from which a sample (MN0 X9-25) was dated at 16.73 + 1.25 Ma. Early to Middle Miocene limestones overlie the older units. At 10 km northwest of Kiamba (Fig. 7), the Paleogene basement is directly overlain by l&Ma-old andesitic to dacitic flows and pyroclastics (Table 1). An andesitic flow (PH 92-105) dated at 18.00 + 0.42 Ma gives a feldspar age of 23.62 + 1.42 Ma and a slightly younger corrected age of 17.7 Ma (Table 2). A pile of basic andesite flows and volcanic breccias dated at 6.38 f 1.42 Ma (PAR 69) underlies the Pliocene-Pleistocene Parker volcano.

Magmatism in Mindanao and Leyte (Philippines) Mt Parker, a 1.550-m-high volcano with a summital crater lake, is cut by normal faults affecting the northeastern flanks of the Daguma range (Fig. 7). An old volcanic phase emplaced massive andesitic lava flows and a younger activity resulted in the eruption of pyroclastics with subordinate volcanic flows (Bayon and Salonga 1993). A dacite dome just south of the crater lake probably represents the post-caldera phase. A dacitic plug which is attributed to the Mt Parker activity truncates the Late Miocene substratum north of the volcano. Outcrops of the lava flows are seldom fresh, probably due partly to the high porosity of the rocks. Two fresh boulders collected from the northwestern flank of the volcano are dated at 4.37 f 0.91 (PH 92-109) and 0.47 + 0.07 Ma (PH 92-108). Mt Blit, a still poorly studied volcanic center, has two main summits 1200-1300 m high and, like Mt Parker, is 125”55’

also affected by NW-SE normal faults. From very limited traverses in the area, Mt Blit appears to be built up primarily of 2.50 ) 0.16-Ma-old (PH 92-l 14) olivine basalt flows blanketed by abundant plyroclastic material. Rare dacitic flows, dated at 0.76 & 0.08 Ma (PH 92-l 17) do not display clear geological relationships with the basaltic pile. In addition, boulders of hornblende andesite dated at 1.06 $- 0.08 Ma (PH 92-l 15) are common along the volcano’s flanks and stream channels. Savangani peninsula. Situated ea.st of Daguma, it forms an asymmetric anticlinorium with a steeply grading eastern flank and a more moderately dipping western limb (Fig. 7). Its northwestern limit is overlain by the Quaternary Mt Matutum located opposite northeast of Mt Parker. The oldest rock unit consists of Early Miocene carbonates and elastic sediments that form the core of the anticlinorium, unconformably

126”OO’

Sample 44 45 46 47 48 49 SO 51 52 53 54 55 56 57

131

PH 93-82 PH 92-83 PH 92-82 PH 92-80 PH92-75 PH 92-77 Q9@21 PW 92-74 PH 92-73 Q9@22 PH 92-68 PH 92-l 5 PH 92-l 6 PH 93-81

‘Corrected

K-Ar Age (Ma) 31.79 * 2.19 12.02 f 0.30 47.16 i 1 so 3.22” 46.14 f: 1.12 11 .lO + 0.56 0.24’

12.35 18.09

‘. A cl I1,’ B tl

C

D

G

E

w

F

1

f 0.46 + 0.54

age (see Table

2)

Fig. 5. Composite map of North Davao mining area showing lithological units, structures and sample location. Key: A, fault; B, caldera; C, alluvium; D, Quaternary andesite/dacite domes; E, Pleistocene pyroclastics; F, Upper Miocene to Pliocene elastics; G, Late Oligocene to Early Middle Miocene limestone; H, Late Oligocene to Early Middle Miocene elastics; I, Eocene-Oligocene keratophyre flows, tuffs, volcanic wackes; .l; dioritic intrusives; K, Cretaceous arc basement. Index map: M, Mati; P, Pujuda peninsula. Sources of geological data: North Davao Mining Corp. unpublished map (1:50,000) and the present study.

F. G. Sajona et al.

132

sa 59 60 61 62 63 64 65

SAMPLE P 90-69 P 9O-7SA P 90-756 P 90-85 EAL 1 AMP 66 AMP 39 AMP 63

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

AMP 296 AMP 36 PH 93-52 PH 93-54 PH 93-55 PH 93-56 ALORAN PH 93-57 PH 93-58 TIP 1 TIP 4 TIP 5 TUOELA PH93.59 PH 93-67

K-AT 17.27 0.70 0.43

0.39

0.40 0.29 0.64

l\la

Age (Ma) t 0.69 r 0.49 * 0.12

al 62 a3 a4 as a6 a7 aa 69 90 91 92 93 94 95 96 97 98 99 loo 101 102 103

t 0.13

* 0.07 * 0.1 1 t 0.22

Fjb

SAMPLE PH 92-153 PH 93-68 PH 92-150 PM 92-151 PH 93-64 PH 93-73 PH 93-7s PH 93-74 PH 93-77 PH 93-76 P 90-7 P 90-9 PH 92-155 PH 93-69 PH 93-71 PH 92-149 KT PH 92-146 PH 92-145 PH 92-144 KT SaP KT2S KT17

K-Ar

2.31

0.46

a9c

(Ma)

+ 0.1

104 105 106 107 108 109 110 111 112 113 114 115 116 ii7 118 119 120 121 122 123 124 125 126

I

t 0.12

1

[xlc

[K\d

Fig.

SAMPLE KT S2J KT 52J6 KT 42J KL 162 KL 166P KL 167 KL 1438 KL143C PH 92.140 PH 92-141 KL la8 KL 2OSJ KL 126 KL aa MNOa9-16A MN0 a9-168 MN0 89-i 7 PH 92-135 PH 92-136 PH 92-134 MN0 89-12 MN0 89-20 PH 92-120

0.00

0.40 0.25 1.15

~9e

opposite

(b) 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 14s

c 0.00

t 0.05 z 0.07 2 0.27

SAMPLE PH 92.122 PH 92.123 PH 92-124 PH 92-12s PH 92-126 MN0 89.37 Am PH 92-158 PH 92-156 WY KINCGUITAN PCBAL4 TALISAYAN PH 92-164 PH 92-16s PH 92-166 Q 9O-60 MO41 6002

16.32 t 0.85 19.86 r 0.36 0.80 f 0.16

f

[]e

6-Caption

K-Ar

mg

r]h

K-Ar

&c

(Ma)

0.62

t 0.06

0.65 0.14 0.36

? 0.1 S * 0.10 t 0.12

0.00 0.34

2 0.03 Z 0.06

Magmatism

in Mindanao

and

Leyte

133

(Philippines)

Fig. 6. Composite map of Central Mindanao volcanic sector showing lithology, structures and sample C, Butuan city; GC, Gingoog city; DC, Davao City; CC, Cagayan de Oro city; IC, Iligan city; CoC, Cotabato city; DiC, Dipolog city. Volcanic centers: Am, Mt Ampiro; M, Mt Mahndang; Kt, Mt Kitanglad; Kl, Mt Kalatungan; R, Mt Ragang; H, Mt Hibok-Hibok; Ma, Mt Mambajao; B, Mt Balingoan; A, Mt Apo. Key: a, fault; b, thrust fault; c, anticlinal axis; d, synclinal axis; e, alluvium; f, PlioceneeQuaternary Iavas; g, Pliocene-Quaternary pyroclastics; h, Pliocene-Pleistocene sediments; i, Late Miocene-Pliocene 1, Oligocene-Miocene volcanics; m, sediments; j, Miocene volcanics; k, Oligocene-Miocene sediments; Paleogene sediments; n, undifferentiated volcanics; o, metamorphic basement; p, ultramafic basemenr. Source: Philippine Bureau of Mines (1963).

Sampie 46

PH 92-114

47

BLK,

K-Ar Age (Ma) 2.50 f 0.16

Sampk 167

PH 93-65

166

PH 93-87

PH 92.115

1.06

* 0.08

169

49

PH 92.116

0.76

* 0.08

170

PH 92.100

so

PH92-117

171

PH 92-9s

51

PH 92-102

t 8.32

172

MAT27

52

PH 92-105

I 7.70’

173

MATS7

4%

c 0.42

53

PH 92-107

59.18

t

54

HNO

69-2&h

29.89

2 2.12

IO.99

55

MN0

64268;

31.91.t

174

MAT61

17s

MAT77

3.95

176

PH 92.90

2 1.25

177

PH 92-92

MNOS¶-25

57

P” se-109

4.37

* 0.91

176

MAT79

56

PH $2-108

0.47

f 0.07

179

MAT73

59

P” 92.11,

180

MAT28

60

PAR60

181

PH 92.62

f. 0.04

2 0.00

1.83

c 0.07

0.00

f 0.00

6.44

t 0.26

PH 92.63

12.89

183

MNO

89.21A

9.31

MN0

89-216

10.64

165

PH 92.50

MA149

166

PH 92.47

PH 93.64

1.97

PH 92.59

PAR 50 PAR 69

63

PAR 79

64

PH 92-66

65 66

6.38 0.86

i

1.42

t 0.04

(Ma,

0.00

164

61 62

162

Age

0.85

MAT46

16.73

56

K-h

7.70

t 0.96 f 0.18 t 0.22 t 0.16

I\a El

b

e

::::: lzzl

f

*I L *-*** i Ezl

I

,

,

i

I \\C

9

-.-__ _.____ --.._ _-_.__ ---._ _.___. lIzI

I ‘x,d

h

t+*** I E”1

k

Fig. 7. Composite map of Daguma-Sarangani area showing lithology, structures and sample location. Volcanic centers: B; Mt Blit; M: Mt Matutum; P, Mt Parker. Key: a, fault; 5, thrust fault; c, anticlinal axis; d, synclinal axis; e, alluvium; f, Pliocene-Quaternary lavas; g, Pliocene-Quaternary pyroclastics; h, to Pliocene sediments; j, Miocene volcanics; k: Pliocene-Pleistocene sediments; i, Late Miocene Oligocene-Miocene sediments; 1, Oligocene intrusives; m, Paleogene sediments; n, undifferentiated volcanics; o, metamorphic basement; p, ultramafic basement. Source: Philippine Bureau of Mines (1963): Rayon and Salonga (1992).

134

F. G. Sajona et al.

overlain on opposite flanks by Late Miocene turbidites and on the northeastern flanks by late Middle to Late Miocene volcanics consisting of deeply weathered and faulted basalts and andesites. The latter are cut by relatively fresh 10.67.7-Ma-old andesites and dacites (Table 1) that appear to have been emplaced along normal faults affecting the older volcanics. Conglomerates overlying this unit consist mostly of altered volcanic clasts from which one relatively fresh microgabbroic specimen was dated at 12.89 IfI 0.96 Ma. Quaternary volcanics previously mapped along the eastern side of the peninsula (Philippine Bureau of Mines 1963) were not recognized. However, a N-S chain of Quaternary volcanic islands belonging to the Sangihe arc have been documented south of Sarangani (Morrice et al. 1983). A strip of Pliocene sediments flanking Sarangani to the west separates the peninsula from the Mt Matutum volcano (Fig. 7). Its substratum consists of (i) andesitic lava flows of probable Early to Middle Miocene age which are overlain by a Middle Miocene conglomerate containing clasts from the older volcanics, and (ii) a volcanic unit made up of andesitic flows dated at 8.44 + 0.26 Ma (MAT 73; Table 1) overlain by Pliocene elastics and Early Pleistocene limestone (Bayon and Salonga 1993). Mt Matutum is a 2380-m-high active volcano unaffected by normal faulting, unlike Mts Parker and Blit. Its summital crater is surrounded by hills and knobs of hornblende andesite and dacite flows and voluminous unconsolidated pyroclastic material blanketing the southern and western portions of the edifice. The radial drainage cuts deep juvenile gullies into the pyroclastics. Isolated plugs of hornblende andesite ranging in age from 1.5 to 0.85 Ma truncate the old volcanic substratum west and northwest of the crater area. Samples from the flanks of the volcano yield ages ranging from 1.83 Ma (MAT 57) to 0.17 Ma (PH 92-95), while a lava (MAT 77) from the crater area itself gives a zero K-Ar age. A charred wood within the pyroclastics dated at 2 Ka (‘“C) (Punongbayan, pers. comm.) probably represents the age of Matutum’s last eruption. Zamboanga

The portion of the Zamboanga peninsula east of Mts Malindang and Ampiro and northwest of the Celebes sea, forms a convex N-NW arc that extends from west of the Cotabato fault to a series of small volcanic and ultramafic islands from Basilan to Tawi-Tawi to the southwest (Figs 1 and 8). Basement rocks include metamorphosed continent-derived sediments, e.g. mica-schists, quartzites, phyllites and gneisses (Santos-Ynigo 1953; Pubellier et al. 1991a) similar to those found in Palawan and Mindoro, probably in tectonic contact with foliated serpentinized peridotites. Antonio (1972) described thin bedded sandstones and shales with minor carbonates dated back to Eocene, and Oligocene elastics and limestones with minor volcanic components. The most widespread Tertiary units are Burdigalian (NN5) calcareous mudstones and Langhian limestones that directly overlie the metamorphic basement. Undated dioritic plutons intrude Early to Middle Miocene sediments in the eastern axial portion of the peninsula (Antonio 1972). Lava flows in the east-central portion associated with the Middle Miocene sediments are dated at 18.95 + 0.71 Ma

(P 90-58) and 16.89 i: 0.64 Ma (P 90-59A). In the southwest portion and southern end of the Zamboanga peninsula, tabular andesitic flows and necks are dated at 14.94 + 0.43 Ma (P 90-101) and 11.91 &-0.62 Ma (P 90-99), respectively. A few pyroclastic units are associated with Late Miocene coal-bearing sediments. The recent arc volcanism in Zamboanga occurs only on the south side of its eastern portion (Fig. S), where tabular basaltic flows dated at 2.25 (average of two determinations) to 1.08 Ma (Table 1) overlie Late Pliocene to Early Pleistocene (NN19) elastic sediments. An andesite boulder resting upon Late Miocene sediments gives an age of 3.88 + 0.10 Ma (PH 93-30). Cinder cones and lava domes overlying the Middle and Late Miocene sediments in east-central Zamboanga and the Pliocene-Pleistocene basalts on the latter’s northernmost outcrops are dated at 1.0-0.7 Ma. The youngest K-Ar age (0.41 + 0.05 Ma) is that of a basaltic andesite flow (P 90-61) collected northwest of Ipil. Basilan island consists of several unexplored basaltic to basaltic andesitic volcanoes. A basaltic flow from the flanks of its central volcano is dated at 1.98 I 0.31 Ma (PH 93-39).

Time-related geochemical variations The general rock classification used in this paper is based on Pecerillo and Taylor’s (1976) KzO vs SiOz diagram for erogenic igneous rocks (e.g. Fig. 9a). Sajona et al. (1993, 1994) have shown that certain lava suites in Mindanao have peculiar trace element characteristics. They identified adakites, i.e. slab melts derived from subducted oceanic crust when it is young and hot (e.g. Kay 1978; Defant and Drummond 1990) or during the initiation of subduction (Peacock 1990; Sajona et al. 1993; Peacock et al. 1994). These rocks, which have SiO, 2 56%, Al,O, 2 15% and, often, high MgO (3-6%), Cr and Ni contents, are distinguished from typical island arc magmas in having elevated Sr (rarely < 400 ppm) and low Y ( 4 18 ppm) and HREE (e.g. Yb I 1.8 ppm). The adakites in Mindanao are sometimes associated with HFSE-rich basalts which Sajona et al. (1993, 1994) termed Nb-enriched basalts (NEB). Unlike typical arc lavas, these rocks have very slight positive or negative anomalies in Nb compared to elements of similar incompatibility such that their (La/Nb)n ratios are usually < 2 but rarely < 0.7. Leyte

and Eastern

Mindanao

Pre-Pliocene. The basalt and gabbro from the ophiolite in northeastern Leyte are K-poor rocks (KzO = 0.15-o. 16%) (Table 3) with high MgO (8%), Cr (300-370 ppm) and Ni (888105 ppm). While they have 24 times higher concentrations in large ion lithophile elements (LILE-Rb, Ba, K, Sr) than N-MORB, their (La/Yb)n ratios (0.70) are comparable to the latter. They display relatively low contents of high field strength elements (HFSE-Nb, Zr, Ti) such that normalized LILE/HFSE or LREEjHFSE (e.g. (La/Nb)n = 1.4-3.5) are intermediate between those of MORB and IAB. These characteristics are diagnostic of back-arc basin basalts (Saunders and Tarney 1984). Not much geochemical data are available for the pre-Pliocene arc magmatic rocks of Leyte. Based on major element

PH 93-8 PH93-5

19s

3

anlyses:

0.90

0.72’

2.25

3.55

K-Ar

1.42

(Ma)

‘Gorrected

* 0.071

I, 0.125

f

Age Sample

Table

P90-61B

P90-GlA

P90-598

P 90-S9A

ffl93-50

P9C-58

PH 93-30

P90-36

P90-35

Pti 9,3-22

P%!93-25

PH 93-l

P 90-448

P90-44A

PH 93-11

age (see

218

217

216

215

214

213

212

211

210

209

2%

207

206

205

204

8

2)

GULF

16.83

18.95

3.88

1.21

1.08

1.71

K-Ar

(Ma)

f 0.64

* 0.71

rt 0.10

i 0.68

f 0.52

f 0.13

Age

Sample

PH 93-3s P90-91

226

PH 93-38 Pt493.39 PH 93-40

231 23? 233

PH 93-37

PH 93-41

229 230

P90-99

228

PH 93-36

PH 93-34

224 22s 227

Pt: 33-33

P go-101

PH 93-32

PH 93-49

P90-61

223

222

221

220

219

LMY

(Ma)

f 0.62

i 0.43

f 0.049

Age

1.98~031

11.91

14.94

0.34

K-AI

Fig. 8. Geologic map of Zamboanga peninsula and Basilan island. Key: A, fault, dashed where inferred; B, thrust fault; C, antichal axis; 19, synclinal axis; E, Quaternary alluvial deposits; F, Recent massive lava domes and centers; G, Pliocene-Pleistocene volcanics (lava and pyroclastics); II, Pliocene-Pleistocene sediments; I, Late Miocene to Pliocene sediments; J, Late P, metkdmorphic Miocene volcanics; K, Early to Middle Miocene sediments; L, Early to Middle Miocene volcanics; M, Paleogene formation; N, Paleogene volcanics; 0, ultramafics; map of the Philippines (Pubellier et ul. 1993). (continental) basement. Sources: 1:1,OOO,OQOgeological map of the Philippines (Philippine Bureau of Mines, 1963); Neotectonic

of two

PH 93-l LW 6

201 202

&wage

PH 93-20 P9U21

199 200

P 90-20

PH 93-3

193 134

198

P90-!9

!92

PH93-10

PH 93-18

191

PH93-13

PH 93-6

190

196

PH9&17l?

189

197

PH9G17A

188

Sample

MORO

ILLANA

c u

F. 6. Sajona et al.

136

analyses by JICA-MMAJ-MGB (1986) the 20 Ma gabbro is talc-alkaline (CA). The ophiolitic rocks in Dinagat island and Pujada peninsula have been considered by Hawkins et al. (1985) as having back-arc basin character, while pillow basalts in Mati, on the east side of the Philippine fault (Fig. l), have island arc affinity. The Eocene-Oligocene igneous rocks from the present study area are all island tholeiites (IAT) (Fig. 9a) having typical arc-related trace element characters, i.e. enrichment in the LILE and light rare earth elements (LREE) relative to heavy (H)REE and HFSE (Fig. 9b). Their (Ba/La)n ratios, (0.82-2.36) as well as their (La/Yb)n, ratios (1.65-2.82) increase from the oldest to the youngest samples. The Miocene samples are either CA or high K talc-alkaline (CAK) exhibiting higher LILE concentrations than the older ones (Table 3). The 12-Ma andesites have low concentrations of Y (11.2-l .5 ) and HREE (e.g. Yb = 0.92-1.30 ) compared to the other

LEYTE

samples, although only PH 92-15 is Sr-enriched (Sr-925 ppm), such that it plots in the adakite field in Fig. 10a. Pliocene-Pleistocene. In Britain, high-alumina basalts and andesites (Al,O, = 19-20% are common, while basaltic andesites, andesites and dacites are encountered in Leyte and Pana-on (Fig. SC). The basalts and basaltic andesites are mostly CA, while the andesites are either CA or CAK. MgO does not exceed 4.5% and Fez03 generally decreases concomitantly with MgO. Except for the relatively low LILE concentrations of the basalts, all the samples exhibit relatively homogenous trace element patterns (Fig. 9d), e.g. (Ba/La)n = 3.34.4; (La/ Yb)n = 3-5 and negative anomalies in Nb (e.g. (La/Nb)n = 2.8-7.3) Ti and sometimes Zr, consistent with subduction-related magmas. Samples from Eastern Mindanao are mostly CA or CAK andesites to dacites (Fig. SC). With the exception of the 2.7-Ma-old CA andesite (MN0 8X-49) from Placer, all the Pliocene-Pleistocene andesites and dacites

- EASTERN

MINDANA

PRE-PLIOCENE 1000

al z ii !g 100

U

PH 92-16

e

PH92-15*

+

PH 92-77

--o-

PH 92-82

l',,,,,,,,,,,,,,,,' 45

50

55

60

wt.%

65

RbBa

70

KNbLaCeSrNd

P ZrEu

TiDy

Y ErYb

sio,

PLIOCENE-PLEISTOCENE 1000 Leyte C/s

45

50

55

60

65

70

Rb Ba K Nb La Ce Sr Nd P Zr Eu Ti Dy Y Er Yb

wt.% SiO, Fig. 9. (a) KzO vs SiOZ diagram of pre-Pliocene samples of Eastern Mindanao. (b) Trace element plots of selected samples from the pre-Pliocene rocks of Eastern Mindanao normalized to primitive mantle (Sun and McDonough, 1989). (c) K20 vs SiO, diagram of Pliocene-Pleistocene samples of Leyte and Eastern Mindanao. (d) Mantle normalized trace element plots of representative samples from the Pliocene-Pleistocene lavas of Leyte and Eastern Mindanao.

Magmatism

in Mindanao

are adakites with extreme depletions in Y and the HREE (e.g. SrjY up to 200; Fig lOa). Sajona et al. (1994) postulated a 1.X-Ma age for the initiation of adakite magmatism in Surigao. Additional field and analytical studies indicate that adakite volcanism started earlier at 34 Ma (Table I and 2). The youngest and only basaltic sample from Mt Pace’s summit is typically CA and has trace element pattern similar to those of the Leyte samples (Fig. 9d).

Pre-PEiocene. Among the Miocene samples, MN0 89-12 is IAT, while the other two (P 90-69 and MN0 89-20) are CAK (Fig. 1la). The CAK samples display more pronounced LILE and LREE enrichment compared to the IAT (Fig. llb). Both types exhibit HFSE depletions marked by negative spikes in Nb, P and Ti. Pliocene-Pleistocene. In general, two geochemically

distinct groups can be identified in Central Mindanao (Sajona el al. 1994): one composed of IAT and CA basalts to high-Si02 CAK lavas, which is called the CA group in this paper; and another which is more potassic and consists of low-Si02 CAK and shoshonitic (SH) rocks (Fig. 1Ic,e), here collectively called the SH group. Lavas from the CA group range in age from 2.5 Ma to > 1 Ka while those of the SH group were erupted during a more restricted time span at ca 0.8-0.3 Ma (Table 1). The SH group lavas were previously encountered only in Table 3. Geochemical

137

and Leyte (Philippines)

Central Mindanao (Sajona et al. 1994) but have also been recently discovered in ::he western portion (Fig. 11). In the western portion, the oldest (0.7-0.6 Ma) dated rocks include an adakite (P 90-75A) and a CA basaltic andesite (PH 93-59) (Fig. 1Id) sampled near the base of Mts Ampiro and Malindang, respectively. SH group (0.4 Ma) lavas are found mostly at higher elevations, while younger (0.3 Ma) CA group andesites occur on the eastern flanks of the volcanoes. SH group rocks are generally more magnesian compared to those of the CA group. The former contains up to 12% MgO, and up to 420 and 460 ppm Cr and Ni, respectively. ln general, lavas from this area are compositionally homogenous (e.g. (La/Yb)n = 2-3 and (Nb/La)n = 0.2-0.5) compared to those in the general and eastern portions of Central Mindanao. Lavas from the central portion all belong to the CA group, composed of IAT and CA high-Mg basalts and andesites (Fig. 1lc) with MgO contents from 4.5 to 8.5% and Cr concentrations of 100-400 ppm. Despite the high Si02-content of the andesites, they lack HzO-bearing minerals and even lavas with up to 58% SiOz contain modal olivine. LILE and H and HFSE depletion among the iavas are highly variable (e.g. (BajLa)n = 0.7-2.6; (La/Yb)n = 1.4-8; (La/Nb)n = 1.25-4.0) (Fig. Ild), such that some lavas are considered Nb-enriched. Some of the high-Mg andesites have elevated Sr/Y ratios of 3741 at

eastern

analyses of representative

rock samples from Leyte and Mindanao

PH 88.02A

2 PH 92-173

3 PH 92-82

4 PH 93-82

5 PH 92-16

6 PH 92-15

7 PH 92-28

8 MN0 89-12

9 PH 92-151

10 KL 142

55.23 1.99 48.80 1.19 15.08 10.08 0.13 8.22 11.13 2.77 0.15 0.05 2.25 99.85

2.53 0.09 57.00 0.81 17.90 7.25 0.09 2.35 7.10 3.49 2.32 0.25 1.43 99.99

47.16 1.58 53.40 0.89 17.75 12.10 0.06 3.23 7.30 3.42 0.15 0.31 1.19 99.80

32.27 0.78 62.80 0.44 16.80 5.50 0.12 2.38 6.30 3.70 0.50 0.15 0.67 99.36

18.09 0.54 61.65 0.59 16.95 6.12 0.13 2.04 5.71 3.91 1.54 0.18 0.94 99.76

12.35 0.46 61.50 0.44 16.95 4.35 0.10 1.95 5.15 4.75 2.72 0.20 1.92 100.03

0.21 0.05 59.10 0.65 18.25 5.47 0.11 3.48 5.80 4.90 2.05 0.25 0.67 99.73

16.32 0.85 55.30 0.87 18.18 7.46 0.16 3.85 8.24 3.04 0.65 0.30 1.60 99.65

2.31 0.11 55.80 1.03 16.30 7.84 0.10 5.71 7.43 3.45 1.14 0.22 1.51 100.53

0.52 0.04 56.20 0.13 20.10 4.96 0.13 1.72 6.00 4.60 3.75 0.52 0.99 99.70

64.75 0.39 16.80 3.93 0.08 2.24 4.60 4.28 1.71 0.17 0.57 99.52

44 450 3.6 15.9 24 550 20.5 101 1.45 4.4 27 2.8 2.42 23 11 9 17 262

2.7 42 2.05 5 14 290 12.5 65 1.2 4.3 24 2.6 2.18 28 3 3 5 265

8.9 100 1.6 6.1 15 339 10 15 0.7 2.7 16.3 1.5 1.55 14.2 9 29 13 125

53 420 3.25 8.55 20 295 13 17 1 3.8 24 2.5 2.45 12.2 2 2 14 130

39 825 5.15 11.1 22 925 11.5 23 0.8 1.8 11.2 1 0.92 8.9 12 33 11 114

36 428 3.65 14.55 28.5 780 17 85 0.95 2 11.8 1.2 0.98 13 33 78 16 185

20 132 3.2 9 n.d. 320 15.3 88 1.05 3.5 24 2.6 2.35 23 23 39 nd. 180

26.7 179 5.58 6.8 14 335 11 95 1.05 2.9 16.4 1.5 1.32 20 136 255 30 146

89 6&O 8.2 16.6 35 464 18.5 146 I.1 3.4 20.2 1.9 1.98 8.4 12 12 15 I30

43.6 327 3.8 10.3 22 565 10 50 0.65 1.7 10.3 1.1 0.95 9 27 36 13 72

1

Age (Ma) *g SiO, (wt%) TiO: AI0 FezO, MnQ MzQ CaO NazO KD PZOj LOI Total

11 KL 143c 0.00 0.00

De&&m limit 2.5 23 1.9 2.6

EmI

i.5

Nb La Ce Sr Nd Zr Eu DY Y Er Yb SC Ni Cr Co V

1 I 2 1l.d. 1 144 2 9 2 42 0.2 I.! 0.4 4.8 0.5 32 1 3.2 0.2 2.6 0.2 38 2 105 2 303 2 n.d. 3 250

Table 3-Continued

overleaf

138

F. C. Sajona

et al.

Table 3. continued

Age (Ma) +a Si02 (wt%) TiOz Al0 Fe0 MnO MgO CaO NazO LO PzO5 LO1 Total Detection Em) Nb La Ce Sr Nd Zr Eu DY Y Er Yb SC Ni Cr co V

12 PH 92-107

13 MN0 89-26A

14 PH 92-105

1.5 PH 92-50

16 PH 92-114

17 MAT 77

18 P 90-58

59.18 10.99 56.00 0.75 17.50 7.15 0.15 3.60 9.38 2.09 0.33 0.22 3.26 100.43

29.89 2.12 55.35 1.06 16.67 9.90 0.20 3.39 7.50 4.11 0.67 0.05 0.58 99.49

17.71* 0.42 63.25 0.51 17.15 4.63 0.08 2.19 5.00 4.70 1.36 0.19 1.29 100.35

7.70 0.18 65.00 0.48 15.50 5.00 0.10 2.12 5.80 2.32 2.03 0.12 1.80 100.27

2.50 0.16 49.50 1.24 16.20 11.70 0.15 7.56 9.32 3.33 0.43 0.23 0.84 100.50

0.00 0.00 63.25 0.56 16.60 4.55 0.08 2.90 5.30 4.40 1.66 0.18 0.22 99.70

18.95 0.71 58.80 0.73 18.20 6.80 0.14 2.76 7.88 3.23 0.77 0.25 0.98 100.54

33.5 274 1.6 6.5 17 687 8.5 66 0.65 1.4 8.1 0.7 0.7 8.1 11 8 11 104

19.3 358 0.35 4.4 11 273 6 33 0..6 2 13.1 1.2 1.38 20 8 13 12 187

4.3 94 7 8.2 16 370 11 80 0.15 3.5 22 7 1.7 25 180 305 49 198

25.5 308 3.9 8.4 18 745 11 110 0.8 2.2 12.7 1.5 1.15 13 28 35 14 116

18 175 2.8 11.05 25 429 15 78 0.85 2.7 18 2.3 1.72 21 15 24 17 192

19 90-:9A

20 P 90-35

21 P 90-20

22 P 90-61

16.89 0.64 67.80 0.44 14.60 3.72 0.06 1.43 5.20 2.98 0.66 0.16 3.36 100.41

1.08 0.52 49.50 1.62 15.04 11.43 0.38 6.68 9.20 2.97 0.48 0.26 2.58 100.14

0.72” 0.07 64.50 0.38 17.95 4.1 0.09 1.57 5.18 4.13 1.24 0.14 0.91 100.19

0.41 0.05 55.00 0.90 18.00 7.72 0.15 4.06 8.24 3.11 2.34 0.56 0.40 100.48

12 194 2.4 10.15 21 348 12 76 0.75 1.8 11 1.2 1 10 15 27 9 93

110 12 10 21 351 15 102 1.35 4.2 28 2.4 1.7 23 190 250 58 188

26 240

68 960 7.7 37.5 74 950 35 185 1.9 4.9 28 3.3 2.32 24 13 26 25 235

limit i.5 1 1 2 1 2 2 0.2 0.4 0.5 1 0.2 0.2 2 2 2 3

8 103 2.5 10.45 23 575 14 87 1.2 3.9 26 2.5 2.6 27 14 19 17 218

10 122 2.2 4.2 n.d. 198 11 22 1.1 4.4 30 3 2.8 32 4 3 n.d. 260

8

1

L

6.45 15 464 8.5 58 0.5 1.5 10 0.85 9.4 21 41 12 67

Leyte: 1, gabbro from ophiolite; 2, CAK andesite, Eastern Mindanao: 3, Eocene IAT basaltic andesite; 4, Oligocene diorite; 5, Middle Miocene diorite; 6, Middle Miocene adakite; 7, Pleistocene adakite. Central Mindanao: 8, Middle Miocene andesite; 9, Pliocene talc-alkaline andesite; 10, Pleistocene SH andesite; 11 Recent adakite. Daguma-Sarangani: 12, Paleogene andesite; 13, Oligocene diorite; 14, Middle Miocene adakite; 15, Late Miocene dacite; 16, Pliocene Nb-enriched basalt; 17, Recent adakite. Zamboanga: 18, Middle Miocene IAT andesite: 19, Middle Miocene adakite; 20, Pleistocene Nb-enriched basalt; 21, Pleistocene adakite; 22, Pleistocene CAK andesite. D.L., Detection limit; *, corrected age.

Y = 15-16 ppm, plotting just above the boundary overlap between the CA and adakite fields in Fig. lob. In general, lavas from eastern Central Mindanao are less magnesian compared to those from the western and central portions. The oldest dated rocks in this area (1.15 Ma) are CA. They are followed by SH group lavas (0.5-0.3 Ma) and younger (0.330 Ma) CA group rocks. Thus, they have variable K- and LILE-enrichment (e.g. (Ba/La)n = 1.45.7) (Fig. llf). (La/Nb)n ratios range from 1.5 to 5.0. The < 100 Ka parasitic cones along the flanks of the volcanoes are adakites with Sr/Y ratios from 40 to 60 (Fig. lob) and high Cr contents of up to 162 ppm. Figure 12 summarizes the variation in the K (and LILE) enrichment of the Central Mindanao lavas in the last 1.2 m.y. A slight increase of (K/La)n ratios from 1.2 to 0.5 Ma can be observed, followed by a drastic increase between 0.5 and 0.4 Ma. (K/La)n ratios abruptly diminish for lavas younger than 0.4 Ma, with values that are generally lower than those of pre-0.5 Ma rocks. Daguma-Sarangani Pre-Pliocene. The oldest dated rocks (50-30 Ma) in Daguma are IAT, while the 16-Ma-old basalt plots near

the limit between IAT and CA (Fig. 13a). Their spidergrams (Fig. 13~) all show enrichments in LILE compared to the HFSE (e.g. (Ba/Nb)n = l&6.5), with slight HREE/LREE enrichment (e.g. (La/Yb)n = 0.6 and 2.4). The l&Ma-old andesites are adakites with high Sr/Y (75-85) (Fig. 10~) together with low Y and HREE contents. The 6-Ma-old substratum of Mt Parker is typically enriched in LILE ((Ba/La)n = 1.6) and LREE ((La/Yb)n = 2.4), and slightly depleted in HFSE ((Nb/La)n = 0.7), and thus displays typical CA characteristics (Fig. 13d). The Miocene rocks from Sarangani are more LILEand LREE-rich than the Paleogene samples from Daguma (Fig. 13a,d). Some samples have HREE concentrations (13315 ppm) equivalent to those of adakites, but are relatively Sr-poor such that they plot within the limit between adakites and typical CA rocks in Fig. 10~. Pliocene-Pleistocene. Basalts, found only in Mt Blit, are IAT to CA (Fig. 13b) with moderate to high MgO contents (557%) together with high Cr (190-305 ppm) and Ni (74180 ppm). The IAT has a (La/Nb)n ratio of 1.2, similar to some Nb-enriched basalts found in Zamboanga and Central Mindanao (Sajona et al. 1993, 1994). Andesites plot near the boundary between the

Magmatism

in Mindanao

139

and Leyte (Philippines)

YTE-EASTERN MINDANAO

Adakite

> SiOz

t

2

56%

*

8o sio,

60

2

56%

40 20 0 10

15

25

20

30

35

40

5

10

15

20

25

30

35

4

Y rvm DAGUMA-SARANGANI

140

ZAMBOANG

,*n

120 0

01 Adakite

80 t &

60

5

10

15

25

20

30

35

40

Y wm

Y wm

Fig. 10. Sr/Y plots for andesites and dacites (SiOz 2 56%) from (a) Leyte and Eastern Wndanao, (b) Central Mindanao, (c) Daguma-Sarangani arc and (d) Zamboanga, showing the adakite and typical arc lava fields adapted from Defant et al. (1991). IAT and CA fields. The dacites are adakites, with Sr/Y = 89-131 (Fig. 10~) and (La/Yb)n = 12. Lavas from Mt Parker are CA andesites and dacites (Fig. 13b) which do not have very high MgO (1.3-3.5%) but have considerably high Cr (34153 ppm). Those of Mt Matutum are CA to CAK, have similar MgO (1.4-4.08%) and low to moderate Cr contents (761 ppm). Compared to their Late Miocene volcanic substratum, the young lavas display wide ranges of Y and HREE, such that in a Sr/Y vs Y diagram (Fig. lOc), some plot within the adakite field and others plot near the boundary between the adakite and the typical CA andesite fields. Zamboanga Pre-Pliocene. The Miocene magmatism is dominated by low-Ti CA basalts and basaltic andesites and lesser IAT, CA and CAK andesites and dacites (Fig. 14a) with MgO < 5%. Their trace element ratios are rather homogenous. For instance, (K/La)n ratios range from 1.4 to 1.9 in IAT and from 2 to 4 in CA and CAK lavas. Their (La/Nb)n ratios of 3 to 6 reflect their HFSE depletion (Fig. I4b). One dacite (P 90-59A) has slightly

elevated Sr/Y at low Y, and thus plots within the boundary between the adakite and normal CA fields (Fig. 10d). Pliocene-Pleistocene. The young volcanics are mostly IAT and CA basalts and basaltic andesites (Fig. 14~) with MgO up to 7.5%, high Cr contents up to 340 ppm and rather variable trace element ratios, e.g. (Ba/ La)n = 0.7-1.7, (La/Yb)n = 3.5-l 1.9. Their normalized trace elements plots (Fig. 14d) show weak negative or positive anomalies in Nb (e.g. (La/Nb)n = 0.5-2) and they are thus classified as NEB. The andesites and dacites (Fig. 14~) are all adakites which are less magnesian (MgO = O&3.9%) compared to those in Eastern and Central Mindanao. Their extreme depletions in Y and HREE are reflected in their Sr/Y ratios of 46-122 (Fig. IOd) and (La/Yb)n ratios of 6.3-17. The youngest (0.41 Ma) dated sample is a CAK basaltic andesite (Fig. 14~) that exhibits strong enrichments in the LILE and LREE (loo-60 times the primitive mantle) compared to the other basalts and basaltic andesites previously described (Fig. 14d). Thus., although it has a comparable Nb content (7.7 ppm) with the NE& it presents a strong negative Nb-anomaly.

140

F. 6. Sajona et al.

Discussion Pve-Pliocene magmatism: implications

sectors, together with the rest of the PMB, originated as an intra-oceanic arc in the PSP. In contrast to Rangin et aZ.‘s (1990) model, however, stratigraphic, structural, geochronological and geochemical data (Bellon et al. 1995) indicate that the major part of Luzon, except the Bicol region, was more probably autochtonous to EUR (Figs 1 and 16a). The earliest phase of arc magmatism

tectonic and paleokinematic

Leyte, Eastern and Central Mindanao. Paleokinematic reconstructions of Rangin et al. (1990) suggest that these

CENTRAL

MINDANAO

PRE-PLIOCENE 4

1000 Q) =

3

5 E

0 u” s

-b-D-

89-12 89-20

100

s j_ .-

2

‘C

>E 0.

5 1

10

: c?z

0

1

wt.% SiOz

PLIOCENE-PLEISTOCENE 1000

4

aJ 3

?

0 y”

; 2

100

_O-

PH 93-59

--f-ct-

90-75A’ PH 92-153

P ‘G .-

$

E

r

'i CL

1

10

sz :

45

50

55 wt.%

60

65

70

RbBa

KNbLaCeSrNd

P ZrEu

TiDy

Y ErYb

Si02

4

3

----i)-

PH 93-69

& *

92-120 92-134

0 y” 6

2

‘adakite

r 1

45

50

55

60

65

70

Rb Ba K NbLaCeSrNd

P ZrEu

TiDy

Y Er Yb

wt.% SiO, Fig. 11. KzO vs SiOZ diagrams for pre-Pliocene samples (a) and Pliocene-Pleistocene sample (c, e) from Central Mindanao, Mantle normalized trace elements plots for representative samples are shown in (b), (d) and (0. Data for the Camiguin island lavas are from P. Castillo (unpublished).

Magmatism in Mindanao

141

and Leyte (Philippines) -

12

iOt

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.i

1.;

ANW Pig. 12. Time-related

variations

of normalized

(K/La)n

recorded in Bicol is Late Jurassic (Geary and Kay 1989), and the last phase lasted until Late Oligocene to Early Miocene (Bellon et al. 1995) (see Table 4). Leyte, Eastern and Central Mindanao are correlated as the southern continuation of the Bicol terrane (e.g. Hawkins et al. 1985). However, as pre-Tertiary igneous rocks from Mindanao were not sampled comparisons with Bicol cannot be made. The oldest dated samples in the Mindanao PMB, Early Eocene and Early Oligocene, as well as their counterparts in the Halmahera arc (Sufni Hakim and Hall 1991), are all typical IAT to CA rocks (Fig. 15), while contemporaneous magmatism in Bicol is largely ad&tic (Bellon et al. 1995). Adakites have been interpreted as products of partial melting of subducted young (25 Ma) oceanic crust (e.g. Defant and Drummond 1990). Bellon et al. (1995) thus relate the Eocene to Oligocene magmatism in Bicol with the subduction of the then young Philippine Sea basin (e.g. Hilde and Lee 1984) (Fig. 16a). The difference in magmatic products between Bicol and Eastern Mindanao during this time would imply either (i) that the two regions evolved as distinct> and unrelated arc systems, an hypothesis presently not supported by structural data; or (ii) that they are parts of a single arc system beneath which young crust of the Philippine Sea subducted beneath Bicol only and not beneath Eastern Mindanao. This second hypothesis implies not only that Eocene-Oligocene magmatism in the PMB was related to a westward-dipping subduction (Fig. 16a), consistent with past and present subduction polarity of other west Pacific arcs (e.g. Palau-Kyushu, West Mariana and Mariana ridges), but also that the PMB was once an intra-oceanic arc outside the proto-PSP. A protoMolucca Sea would be the most viable locus of the ancient PMB. The young portion of the Philippine Sea basin could have been present only in the Bicol latitudes. This configuration is similar to the present tectonic setting in the eastern Pacific, where young portions of the Pacific ocean (e.g. Chile rise) are subducting beneath certain portions of the Americas thereby producing adakites, while normal CA rocks are produced where subduction involves old oceanic crust (e.g. Nazca plate) (Drummond and Defant 1990; Martin 1993). During the Early Miocene, magmatic activity stopped north and south of the PMB in Bicol (Bellon et al. 1995) and Halmahera (Sufni Hakim and Hall 1991). The cessation of magmatic activity in Bicol could be related to the collision of this area with eastern Luzon during this time (Fig. 1Bjb). Subduction continued only in Leyte

ratios of Central

Mindanao

lavas in the last I.2 m.y.

and Mindanao, producing more K and LILE-rich CA to CAK rocks dated at 20-16 Ma (Fig 15). Apparently, a short period of adakite production occurred at 12 Ma, followed immediately by normal CA volcanic activity dated at 1I Ma (PI-I 92-77). No candidate was found for a young crust that could have subducted and produced adakites during this time. Moreover, the short duration of adakite magmatism at 12 Ma is inconsistent with the hypothesis of continuous subduction of young crust. Peacock (1990) and Peacock et al. (1994) predicted that slab melting could occur during the initiation of subduction, regardless of the age of the subducting oceanic crust. Sajona et al. (1993) described Pliocene-Pleistocene adakites in Eastern Mindanao as an example of this process. re old crust is subducting, initial adakite produ of short duration would be followed by typical and more copious CA magmatism. A resumption of subduction could therefore explain the occurrence of the 12-Ma-old adakites. It is suggested that this reinitiation occurred when subduction polarity ch ged from west- to east-dipping in the late Early to iddle Miocene, giving birth to the Manila and the proto-Halmahera trenches (Fig. 16~). In fact, 16-11-Ma-old adakites occur in the Southern Sierra Madre in Luzon (Bellon et al. 1995) and might be linked to the start of the Manila trench subduction. As corollary, it would be during this period that Luzon became a part of the PMB, and the PMB, a part of the PSP. A possible cause of this subduction polarity reversal might be the combined effect of the Bicol and east Luzon collision and the docking of the Benham rise east of the PMB (Fig. 16b). Indeed, at DSDP sites 292 and 293 offshore eastern Phihppines, an important unconformity dated back to Early to Middle Miocene (N6-Nll) is attributed to the cessation of subduction east of Luzon (Karig 1975; Lew~isand Hayes 1983). This period also coincides with and probably was the cause of the important break in the northward drift or clockwise rotation of the PSI? at ca 25-15 Ma (Seno and Maruyama 1984). The 12-II-Ma magmatic episode, however, did not lead to the emplacement of major igneous units associated with Middle Miocene formations. This is further evidenced by abundant ea.rly Middle Miocene elastic and carbonate sediments deposited throughout Central and Eastern Mindanao (Pubellier et al. 1991a) and Leyte (Aurelio 1992). Magmatic activity has not been documented in the Late Miocene terranes. This paucity might res,ult from the collision of the Palawan

142

F. G. Sajona et al.

continental block with the PMB during this period (Fig. 16~) (e.g. Holloway 1981). Magmatism also appears to be lacking in Halmahera during most of the Miocene and its reactivation during the Late Miocene is interpreted by Sufni Hakim and Hall (1991) as the beginning of the eastward subduction of the Molucca Sea beneath the island. This subduction is therefore younger than the westward-dipping one along EUR (i.e. Sangihe trench), as supported by the shallower penetration of oceanic crust beneath Halmahera

compared to that in the Sangihe arc (Cardwell et al. 1980; Quebral 1994). Meanwhile, Central and Eastern Mindanao continued to approach Western Mindanao via left lateral displacement along a transform fault, a precursor of the Cotabato fault, connecting the Sangihe and Halmahera trenches (Fig. 16d) (Pubellier et al. 1991a). Daguma-Sarangani. Although the Daguma arc and the Sarangani peninsula have diverging orientations (Fig. 7), there is no structural evidence that the two arcs

DAGUMA-SARANG PLIOCENE-PLEISTOCENE

PRE-PLIOCENE 4

4s

60

5s

so

70

65

4s

so

60

5s

65

70

wt.% Si02

SiO2

wt.%

1000 MATUTUM c PARKER [ BUT

-G+ + * +

MAT 49’ PAR 79’ PH 92-117’ BLK I PH 92-113 ‘adakite

1;

(,

,

,

,

,

,

Rb Ba K NbLaCeSrNd

,

,

,

,

,

,

P Zr Eu Ti DyY

+

,

,

,’

e Rb E?a K Nb Lace

Er Yb

Sr Nd

P Zr Eu Ti Dy

Y Er Yb

PH 92-lOS*

*adakite

1i,,,t

,,,,,,,,,,, Rb Ba K NbLaCeSrNd

(

P Zr Eu Ti DyY

Er Yb

Fig. 13. K20 vs Si02 diagrams for pre-Pliocene samples (a) and corresponding trace element patterns (b, c) from Daguma-Sarangani. Similar diagrams are shown for the Pliocene-Pleistocene samples (d, e) on the opposite column.

Magmatism in Mindanao

143

and Leyte (Philippines)

ZAMBOANGA PRE-PLIOCENE

=-ki- PW-615 --G- P go-99 ‘-+- ? go-594%’

1

RbBaKNbLaCeSrNdPZrEuTiDyYErYb

wt.% 502

PLIOCENE-PLEISTOCENE

-o-

P90-35 P90-36

-e-

PW-61

-A-

"adakite

1 45

50

55

60

65

70

wt.% Si02

Rb Ba K Nb La Ce Sr Nd P Zr EL! Ti Dy Y Er Yb

Fig. 14. I&O vs SiOz diagrams for pre-Pliocene samples (a) and Pliocene-Pleistocene samples (c) from Zamboanga. Corresponding mantle normalized trace elements plots for representative samples are shown in (b) and (d), respectively.

once separated and brought together by arc-arc amalgamation. Paleotectonic reconstructions (Rangin et al. 1990) indicate that Daguma could have been the northern extension of the proto-Sunda-Sulawesi arc rimming the Celebes Sea basin and related to a NW-W-dipping subduction (Fig. 16a). Since it is admitted that the Celebes Sea rotated at least 60” counterclockwise from Eocene to Oligocene (Shibuya et al. 199 l), it implies that Daguma might have been a N-S-orientated east-facing arc that produced IAT rocks during its earlier evolution (Fig. 1.5). Apparently, the axis of magmatic activity shifted towards the east, probably due either to the withdrawal of Daguma from the trench or to an eastward jump of subduction as the arc rotated with the Celebes Sea. In the latter hypothesis, the reinitiation of subduction might have occurred in the Middle Miocene, producing adakites followed by typical 18-17-Ma-old CA lavas in southeastern Daguma (Fig. 15). As the oceanic crust (Molucca Sea) that subducted beneath the arc was probably old, slab melting occurred only briefly and was followed by typical GA to CAK magmatism produced at 16-6 Ma, were

forming the (Fig. 16c, d).

N-S

trending

Sarangani

peninsula

Zamboanga. The basement of the Zamboanga peninsula is considered as a portion of the EUR margin separated from the latter by the opening of the Suiu Sea (Fig. 16a, b). Based on paleontologic ages, Rangin and Silver (1991) consbdered that the Sulu Sea opened at ca 20 Ma and presented two alternative tectonic models: back-arc opening or independent marginal basin accretion analogous to the South China Sea model. Roeser (1991) proposed a 35-lo-Ma duration for the Sulu Sea spreading with a slow half-spreading rate of 0.6 cm/ yr based on sea-floor magnetic lineations. He identified the lo-Ma lineation as adjacent to the Sulu trench and the older lineations nearer to the Cagayan ridge. This asymmetry as well as the presence of a Miocene volcanic arc in Zamboanga implies that the Sulu Sea has subducted in the past along a proto-Sulu trench. The age of the old Zamboanga arc is constrained by paleontologic and geochronological data as Early to Middle Miocene (Table 1; Fig. 1.5). If Roeser’s (1991) model is admitted, the age of the oldest portion of the

events Mindanao

in Mindanao

and surrounding

areas Related

regional

events

Subduction east of Bicol also stopped due probably to the arrival of Benham rise east of the proto-Philippine trench (7,31). Start of Philippine Fault activity in Luzon at ca 18 Ma (17).

Bicol collided with Sierra Madre, stopping subduction beneath and associated magmatism in the latter (1). Obduction of the Northern Sierra Madre ophiolite (1).

Table 4-Continued

opposite

stopped

South China Sea and Parece-Vela basin spreading at ca 17 Ma (13,30). Volcanism along Cagayan ridge (19). Lack of volcanism in Halmahera (14).

Daguma rotated away from the proto-Sangihe trench, causing an eastward jump of subduction. Reinitiation of subduction produced adakite magmatism at 18 Ma followed immediately by normal CA volcanism. Proto-Sulu trench initiated at ca 20 Ma, producing CA and adakite lavas at 18-17 Ma, followed by normal CA volcanics in Zamboanga. CA volcanism continued in Leyte, Eastern and Central Mindanao due to continued subduction of the Philippine Sea on the southern portions of the PMB.

20-15 Ma

Opening of central basin of the South China Sea (30) probably as a result of extrusion tectonics of Indochina. Proto-South China Sea was probably being consumed along proto-Palawan trench. Accretion of the Parece-Vela basin (13). Philippine plate continued rotating clockwise until 25 Ma (8), when Bicol started colliding with Sierra Madre (1).

Except for one 29-Ma IAT sample in Daguma, there is a gap of geochronologic record in mindanao, although volcanic ashes are abundant in the 25-Ma sediment layer of the Celebes sea (27). Daguma continued its counter-clockwise rotation with Celebes Sea until at least Late Oligocene (20). Sulu Sea continued spreading (21). AgusanDavao basin started subsiding, separating Central from Eastern Mindanao (28,29).

Magmatism in Sierra Madre propagated northwards producing CA rocks (1,18). Cagayan Valley basin started subsiding at ca 28 Ma, rifting Central Cordillera from Sierra Madre (24,25,26).

30-20 Ma

Batholiths of CA character were emplaced in the Central Cordillera (1,16). Adakite magmatism continued in Bicol, probably producing the Paracale intrusion (1,15). Zambales ophiolite docked on the western edge of Luzon (17).

Collision of India with Asia (22). PMB rotated clockwise along with Philippine Sea basin (8). bpening of southwest portion South basin (23). Volcanic arc installed in the Palau-Kyushu ridge (13).

TAT magmatism continued in Daguma, emplacing diorites at ca 30 Ma (19). The arc rotated counter-clockwise with the Celebes Sea (20). Metamorphic basement of Zamboanga probably started rifting from Palawan-Mindoro metamorphic basement via opening of Sulu Sea (21). IAT plutonism continues in Eastern Mindanao.

IAT magmatism continued in the Sierra Madres, distal volcanics were deposited behind the arc, i.e. the future Central Cordillera (1,16,17,18). Dupax and Agno batholiths were emplaced. Adakite magmatism, probably related to subduction of young portion of the Philippine Sea basin, occurred as small intrusives in Catanduanes and, possibly, Polillo island (1,15).

of

metamorphic basement was a rifted fragment fringing a proto-South China sea

(5). Eastern Mindanao formed the southern continuation the PMB, producing IAT batholiths in Davao.

Zamboanga continental

40-30 Ma

Philippine Sea basin either formed part of the Pacific Ocean plate (6,7,8) or opened as a back-arc basin to the PMB (5,9,10,11). Pacific plate motion changed from northwest (12). Ryukyu volcanism from Late Cretaceous until ca 43 Ma (13). Mariana trench was installed at ca 4240 Ma to absorb stress generated by new Pacific plate motion (13). Elements of future Philippine archipelago moved in a general northward direction (5). IAT and CA magmatism in Haimahera arc (14).

West-dipping subduction created the Sierra Madres, producing batholithic intrusives of primitive island arc to IAT nature (1,2). Coastal batholith and Antipolo diorite (2) were emplaced. This activity occurred on a Cretaceous-Paleogene ocean rimming the Eurasian margin (proto-South Cina sea) (3). The Bicol region was a separate mature arc system producing evolved talc-alkaline magmas related to the subduction either of the Pacific or Tethys ocean (1,4). It represents the northern extension of the PMB.

Daguma arc was installed on the western edge of the Eocene Celebes Sea by subduction of proto-Molucca sea along a proto-Sangihe trench, producing IAT volcanics.

of chronological

SO-40 Ma

Luzon

Table 4. Synthesis

Adakites and Nb-enriched basalts are produced in Zamboanga and Daguma-Sarangani. Abundant CA volcanism in Leyte. Adakite production in Surigao and Davao starting ca 3 Ma. Copious CA to SH volcanism and sparse adakites in Central Mindanao were produced as post-collision volcanic activity.

Zamboanga and Daguma collided with Central and Eastern Mindanao (28); a portion of the Molucca sea slab detached and sank beneath the island (28,36,37). Subduction stopped along Sangihe Trench in Sarangani latitudes. Three subduction zones were instantaneously installed - the Philippine, Sulu and Cotabato trenches - as well as related arc volcanism. Philippine Trench and Philippine Fault propagated from north in Bicol to south in Leyte and Mindanao (35,29).

Installation of the Sarangani arc. Zamboanga arc activity stopped due to cessation of subduction along proto-Sulu Trench, probably reatted to collision of Palawan-Mindoro block with PMB in Late Miocene (3,5,30). Sulu Sea stopped spreading at ca 10 Ma (21). Minor volcanic activity in Central and Eastern Mindanao in the Middle Miocene due to cessation of subduction along the proto-Philippine Trench. Subduction polarity flipped, producing the proto-Halmahera Trench and adakites at 12 Ma in Davao. They were immediately followed by normal CA volcanism. This subduction stopped during Late Miocene due to collision of PMB with the Palawan-Mindoro block (3,5,30). PMB became part of the PSP. On-going Halmahera

arc-arc collision arcs (36).

between

Sangihe

and

Phihppine basin stopped spreading at ca 13 Ma (7). Volcanism stopped in Cagayan ridge as proto-South China Sea was completeiy consumed (5). Initiation of Halmahera Trench west of Halmahera arc in Late Miocene (14).

Holloway 198l; (4) Geary and Kay 1989; (5) Rangin ~1 al. 1990; (6) Uycda and Ben-Avraham 1972; (7) Hilde el 611.1977; (8) Wilde and Lee 1982; (II) Seno and Maruyama 1984; (12) Letouzey et al. 1988; (13) Meijer cf al. 1983; (14) Suf’m Hakim and Hall 1991; (15) David 1994; (18) Billed0 1994; (19) Bellon and Rangin 1991; (20) Shibuya et ai. 1991; (21) Roeser 1991; (22) Tapponier pi al. 1982; (23) Ru and Pigott 1977; (26) Caagusan 1981; (27) Pubellier et al. 1991a; (28) Pubellier et al. 1991b; (29) Quebral 1994; (30) Taylor and Hayes 1983; (31) Lewis (33) Barrier 1985; (34) Defant et al. 1991a; (35) Aurelio 192: (36) Silver and MOOR 1978; (37) Cardwell fr al. 1980.

Taiwan collided with north Luzon arc (33). Extinct spreading center of South China Sea started subducting beneath Luzon, producing adakites in Baguio, Lepanto (1) and Pinatubo. CA lavas are produced in the Macolod Corridor and in northern Luzon (34). Philippine Trench started subducting east of Bicol producing the young Bicol arc (35).

5-O Ma

(1) Bellon et a/. 1995; (2) Wolfe 1981; (3) 1984; (9) Rarig 1983; (10) Mrozowski et al. (16) Maleterre i989; (17) ~~nge~l~~~~1992; 1986; (24) Tamesis et nl. 1981; (25) Caagusan and Hayes 1984; (32) Schweller et al. 1983;

Subduction polarity in Luzon changed from west- to east-dipping, installing the Manila Trench. Corresponding magmatic activity initially produced Middle Miocene adakitcs in Southern Sierra Madre, followed by IAT and CA magma&m installed successively in the Central Cordillera (1). Southern Sierra Madre was decoupled from Central Cordillera due to the Philippine Fault activity (17). Uplift of Zambales ophioiite (32). Luzon became part of the PMB and the PSP.

15 5 Ma

;

*’ K

146

F. G. Sajona et al.

Sulu sea subducting beneath the arc during this time should be ca 15 Ma. Therefore, according to the model of slab melting by subduction of young and hot crust ( < 25 Ma; Defant and Drummond 1990) adakites should be expected to represent the main magmatic type in the old Zamboanga arc. It appears, however, that adakite production during this period was very limited and occurred only in a short time span ca 17 Ma, as the majority of our 18-1 l-Ma-old samples are typical CA lavas (Fig. 15). Tisseau and Tonnere (1995) numerically simulated non-steady-state thermal models of spreading ridges which show that at slow spreading centres, i.e. with a half-spreading rate of 1 cm/yr, high heat gradient (200-140°C km) is concentrated only in the first 10 km distance from the accretionary axis. At 40 km away from the spreading center, the heat gradient is only 35-50”C/km, and, predictably, lower at greater distances. Estimate of the half-spreading rate of Sulu is lower than 1 cm/yr (see above), and off-axis heat gradients could have been much less than 35-50”C/km. Therefore, the subduction of the leading edge of the Sulu Sea crust in the Early Miocene (Fig. 16b) might not have allowed slab melting to produce adakites for a long period, such that these rocks appeared only at the start of subduction. Subduction and volcanism stopped at 11 Ma (Fig. 15), almost coeval with the end of the Sulu Sea accretion at ca 10 Ma (Fig. 16d) (Rangin and Silver 1991). These latter events are probably related to the Palawan-PMB collision during late Middle to Late Miocene (e.g. Holloway 198 1). At this time, the Sulu Sea

I

Pliocene

I

Early -&

spreading ridge could have been adjacent to the extinct proto-Sulu trench. The Pliocene-Pleistocene

1

2

3

0 ARC THOLEIFTE

A HIGH-K

0 CALC-ALKALINE

?? POTASSIC

4 CALC-ALKALINE

5

magmatism

Leyte and Eastern Mindanao. The northward extension of the collision between the Sangihe and Halmahera arcs is considered to have occurred earlier in Mindanao ca 5 Ma ago (Pubellier et al. 1991a). An instantaneous response to this suturing was the major restructuration of the island’s margins, initiating several new subductions. Subduction along the Philippine Trench started at ca 4-5 m.y. ago as calculated from the rate of convergence along the trench and the length of subducting slab in Leyte (Aurelio 1992). Associated volcanism is highly variable in composition. In Leyte, all the young volcanics, ranging from basalts to dacites, are typically talc-alkaline. In Surigao and Davao, the dominantly andesitic to dacitic volcanism is adakitic, and, like in other regions where adakites are produced, they do not seem to have basaltic precursors. As the Philippine Trench is believed to be propagating from north to south (Quebral 1994), volcanism should follow this younging trend. However, the ages obtained from samples from Leyte to Davao overlap and no clear trend can be discerned. One possible indication for a younging trend is the more mature volcanoes and more abundant volcanic products in Leyte compared to those in Surigao and Davao, plus the fact that only Lake Leonard in Davao is considered active. The oldest adakite dated in

I

Miocene Mid

Oligocene

E

l--u--ii

I

Eocene

I

I

Late

j

I

0

subduction-related

Late

Early

Early

Late LEYTE

;

CENTRAi MINDkNAO

25

+ ADAKITE ?? Nb-ENRICHED

Late

-

:

F

15

I Paleocene

I

35 0

45

55

OPHIOLFE

BASALT

Fig. 15. Regional and time distribution of 40K-40Ar dated samples from Mindanao and Leyte modified from Sajona et al. (1994) to include new data. Samples from each area are arranged by order of decreasing ages for Paleocene-Miocene and Pliocene-Quaternary periods, respectively. Horizontal bars represent 1o incertitudes. D, Daguma, S, Sarangani.

t

Magmatism in Mindanao Davao (PI31 92-80) has been shown to contain a very significant amount of excess argon as evidenced by the highly discordant age of its feldspars (40 Ma) compared to the whole rock age (6.6 Ma). This might be due to excess Ar it inherited from Paleogene basement rocks that it intrudes. Even if the age correction resulted

and Leyte (Philippines)

147

to a significantly younger age of 4.5 could still be much younger, as the apparently very strong contamination of its feldspars raises the possibility that the other phenocryst phases present in the rock (e.g. amphibole) could likewise be contaminated.

Old PSP

/YYoung

Fig. 16a and &Caption

onevleaf

(40-35

Ma) PSP CRJS~~’

148

Fig. 16. Paloekinematic reconstructions pertinent to the tectonic evolution of the Philippine archipelago, inspired by and modified from Rangin et al. (1990). 1, subduction; 2, fault; 3, spreading center; 4, direction of relative plate movement; 5, Eurasian continental crust; 6, active volcanic are; 7, sedimentary depocenter. Abbreviations: SCS, South China sea; P, Palawan; PSCS, Proto-South China sea; SS, Sulu sea; Z, Zamboanga; CS, Celebes sea; Za, Zambales: L, Luzon; CS, Celebes sea; D, Daguma; S: Sulawesi; B, Bicol peninsula; N, Negros; EM, Eastern Mindanao; CM, Central Mindanao; PSP, Philippine sea plate; H, Halmahera; MT, Manila trench; Sa, Sarangani peninsula; ST, Sangihe trench; HT, Halmahera trench. Since the age of the Philippine sea crust subducting beneath this arc is Eocene (Hilde and Lee 1984) adakite production cannot be related with subduction of young crust but could be linked to the initiation of the Philippine trench subduction (Sajona et al. 1993). The youngest dated rock in Surigao (0.09 Ma) is a typical CA lava that could mark the end of adakite production. Adakites have not been found in Leyte possibly because volcanism started earlier there and initially produced low amounts of adakites are now buried beneath the more voluminous typical CA lavas.

Daguma-Sarangani. Shortly after the extinction of magmatism in Sarangani in the Late Miocene due to the initiation of collision between Central and Eastern Mindanao and the EUR block (Pubellier et al. 1991a), the Celebes Sea started subducting along the Cotabato trench. The corresponding magmatic activity produce first the Mt Parker and Mt Blit volcanoes in the Pliocene followed by the Pleistocene Mt Matutum. Among the early volcanic products are Nb-enriched basalts found only in Mt Blit, and adakites occurring in the three volcanoes (Fig. 15). Like the Philippine Sea basin,

Magmatism in Mindanao however the Celebes Sea crust is Eocene (Rangin and Silver 1991), and therefore is unlikely to preserve a high remanent heat. We therefore consider that the adakitic magmatism in Daguma-Sarangani reflects the initation of subduction along the Cotabato trench. Mt Parker, the older volcano, is considered extinct and lies ca 100 km away from the Cotabato trench, while Mt Matutum, which is active, is located ca 130 km away from the trench (Fig. 7). Deep seismic studies (Cardwell et al. 1980) reveal that the tip of the subducted portion of the Celebes Sea is located at a 130- km horizontal distance from the trench and at depth of no more than 75-85 km below Mt Matutum. This depth coincides with the amphiboliteeeclogite transition zone in the subducted slab, where garnet-bearing amphibolites, considered as the parent rock of slab melts, occur (Kay 1978; Drummond and Defant 1990; Peacock et al. 1994). From Mt Parker to Mt Matutum, there is an increase of K and LILE contents in lavas with increasing distance to the trench and as volcanism becomes younger-a feature that is observed in many arc systems (Gill 1981). Several possible mechanisms of LILE-enrichment in non-adakitic arc magmas, e.g. presence of old subcontinental mantle under arcs (Saunders et al. 1980; Varne 1985), contribution of enriched mantle in the source (Stern 1981; Morris and Hart 1983) and input of subducted sediment (Edgar 1980; Ellam et al. 1989), cannot be invoked in the case of the Parker-Matutum enrichment because adakites are supposed to derive from subducted slab melting and not from metasomatized mantle wedge melting. In addition to the K-enrichment from Mt Parker to Mt Matutum, a concomitant decrease of transition elements (i.e. Cr and Ni) was observed. Thus, the LILE-enrichment in the Mt Matutum lavas might result from the lower fusion degree of its slab source compared to that of Mt Parker volcanics. This could be related to the age of subduction. Indeed, as subduction matures, slab melting becomes increasingly difficult to realize (Peacock 1990; Peacock et al. 19941, umil such a time that instead of melting, it undergoes dehydration resulting in the metasomatism of the overlying mantle wedge which is the source of typical CA magmas. Zambonnga. The reinitiation of subduction in the Sulu Trench right after suturing in Central Mindanao produce mostly Nb-enriched basalts and lesser amounts of adakites (Fig. 15). Sajona et al. (1993, 1994) reported that the latter appears to be younger than the basalts based on field and K-Ar dating data. The 3.8-Ma-old adakite boulder, however, would suggest that the start of young adakite production in Zamboanga was more or less coeval with or even slightly predates the basalticbasaltic andesitic volcanism. Due to their very small volume, it is possible that the earliest adakite edifices were quickly removed through erosion, as indicated by the boulder sample. Adakites were emplaced near the trench relative to the basaltic volcanoes. This is consistent with the modelled tectonic configuration of their production (e.g. Drummond and Defant 1990) in which melting of the subducted slab occurs at a relatively shallow depth of 75-80 km. The Zamboanga adakites may result either from the initiation of the Sulu Sea subduction, or, alternatively, from the subduction of the remnant Sulu sea spreading center which could preserve a high heat flow. The latter hypothesis is supported by the younger magnetic lineations (Roeser 1991) and the

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higher heat gradient (up to 200”C/km; Hinz and Block 1990) measured near the Sulu trench compared to the same parameters towards the Cagayan ridge. On the other hand, the fact that the youngest dated rock in this region (0.4 Ma) is a CAK basaltic andesite (Fig. 15) possibly indicates that adakitic magmatism was in the process of being replaced by CA magmatism as in Eastern Mindanao. The origin of the Nb-enrichment in the basalts and basaltic andesites erupted south of the adakitic edifices is still enigmatic. Sajona et al. (1994) semi-quantitative evidence that the HFSE-e observed in these lavas might be due to the contribution of an enriched, QIB-type component in the source, or, alternatively, to the addition to highly depleted mantle of HFSE carried by slab melts that interacted with the mantle wedge. The association of adakites with HFSE-enriched lavas is commonly observed in other arcs (e.g. Panama--Defant et al. 1991b; Cascades-Leeman et al. 1990; Defant and Drummond 1993). The space and time links between the two rock types strongly suggest that adakites might play a role in the contribution of HFSE in the mantle. The post-collision magmatism in Crntval Mindanao indanao volcanism Two striking features of Central are its volume (it represents the biggest single volcanic field in the Philippines) and the wide variety of lavas erupted within a short time span of less than 3 Ma (Figs 12, 13 and 15). The base of this vo1cani.c field is composed of CA group lavas (2.5-O.&Ma-old) which are mostly basalts and basaltic andesites. SH group lavas overlie those of the CA group only in the western and eastern portions where the highest peaks of Central Mindanao occur (i.e. Mts Ampiro, Malindang, Kitanglad, Kalatungan and Apo). A younger generation of CA group andesite lavas either truncate the older units (e.g. in Mt Apo) or are emplaced in distinct volcanic centers (e.g. in the numerous small volcanoes in the southern central portion, and in Camiguin island). K and LILE enrichment of the lavas: therefore, vary with time, as shown in Fig. 12, The distinct distributions of the CA and SH group lavas emphasize the small-scale heterogeneity of the sub-Central Mindanao mantle. Further stratigraphic and analytical work, however, is needed to further constrain the space and time distribution of the two groups and their relationships to individual volcanic centers. Although the transition from one type of volcanism to the other in Central Mindanao is rather abrupt, it follows the general magmatic patterns documented in other post-collision areas, e.g. Anatolia, Turkey (Pearce et al. 1990; Yilmaz 1990) and the Roman volcanic province, Italy (e.g. Keller 1983; Beccaluva et al. 1991). As in these areas, the occurrence of recent volcanism in Central Mindanao has been attributed to the presence of a remnant subducted slab, i.e. the detached h/lolucca Sea slab detected seismically beneath the southern portion of the island (Fig. 2) (Cardwell et al. 1980; Pubellier et al. 19’91a;Quebral 1994). Sajona et al. (1994) attributed the wide range of trace element compositions of lavas produced in Central Mindanao to varying contributions of the slab, the heterogenously metasomatized mantle and, possibly, an enriched (01 mantle

source.

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Some lavas in the central and eastern portions of Central Mindanao are also enriched in HFSE, although to a lesser degree than observed among the Zamboanga basalts. The existence of associated adakitic rocks in Central Mindanao (Fig. lob) seems to reinforce the hypothesis of slab melt contribution in the HFSE-enrichment of the mantle. The origin of these adakites, however, is still problematic. Slab melting associated with the initiation of subduction or to the subduction of young crust cannot explain their occurrence in this sector primarily because subduction processes have been stopped by the collision event. Melting of magmatically underplated basalts (Atherton and Petford 1993) seems rather improbable because the underlying crust of Central Mindanao is probably not thick enough to allow melting at sub-crustal levels to occur (Sajona et al. 1993). Other possible explanations for the occurrence of adakitic rocks in Central Mindanao include the following. High-pressure fractionation. These rocks might be the result of deep-seated fractionation of minerals with a high KD for the HREE, i.e. garnet. However, in addition to the absence of garnet xenocrysts and phenocrysts, these acidic (Si02 = 59-68%) rocks have rather high elements concentrations in transition (Cr = 36 110 ppm; Ni = 3478 ppm) that could not be explained by extensive fractionation, but, instead, are consistent with models of andesite and dacite derivation from the partial melting of mafic crust (Kay 1978; Mahlburg Kay et al. 1993). Slab heating and melting due to mantle upwelling. Kay and Mahlburg Kay (1993) discussed the possibility of heating the subducted slab by mantle upwelling due to sinking of delaminated thickened lithosphere under the Altiplano-Puna plateau, Central Andes. They invoked this theory to explain the seismic gap along the subducting crust, and the presence of magmas ‘compositionally similar (to those) generated by melting of young, hot subducted oceanic lithosphere’. Although delamination is unlikely to occur beneath Central Mindanao, the detached Molucca Sea slab could be sinking, as the top of this slab is detected at deeper levels (75-150 km) south of Mindanao (7”N) than in the Molucca Sea (5-6”N) where it is still observed at the surface (Cardwell et al. 1980; Quebral 1994). Such a sinking process could cause hotter mantle material to upwell and heat the sinking slab. It could explain several features observed in Central Mindanao, i.e. the lack of seismic activity of the detached slab at shallow levels (100-200 km) at 8-9”N, the occurrence of adakites which would result from melting of the heated slab, and the production of talc-alkaline to potassic lavas having OIB-type geochemical signatures (e.g. enrichment in Nb, high La/Yb ratios) due to contributions of the upwelled mantle. Tectonic underplating. Seismic profiles along subduction zones (e.g. Rangin and Silver 1991; Silver et al. 1991) show that slivers of oceanic basement are being scraped off the subducting slab and incorporated on the hanging wall of the Benioff zone. It is suggested that this mechanism results in tectonic underplating in subduction zones, a process that might be more important in collision zones. Tectonically underplated mafic crust could contribute to crustal thickening without effecting uplift due to its denser nature compared to the arc crust. However, the lack of data on the upper mantle and lower

crust boundary in Central Mindanao, as well as the absence of heat flow studies in this area, makes this model difficult to assess. Furthermore, melting of basalts underplated beneath the crust is unlikely to play a role in the HFSE enrichment of the mantle except in a highly complex scenario where slivers of subducted ocean crust are superimposed with upper mantle material.

Concluding remarks In arc systems where subduction has been operating continuously for a long period (e.g. Sunda arc), a progressive increase in K- and LILE-contents can be generally correlated with the age of subduction and the distance of arc products from the trench (Bellon et al. 1989; Soeria-Atmadja et al 1994). However, such generalizations are not valid in more complex zones, as other tectonic events (e.g. collision, subduction polarity reversal, subduction breaks and reinitiations) disturb the above patterns. For instance, in Luzon, arc magmatism started with the production of primitive island arc and IAT rocks, followed by more K-rich CA and CAK magmas related to a west-dipping subduction from Early Paleogene to Early Miocene (Bellon et al. 1995). With the subduction polarity reversal in the Middle Miocene, magmatic products in the island turned back to IAT compositions and progressively evolved again to CA and CAK until the present. The structural evolution of Mindanao is still more complex than that of Luzon. Two arc systems developed separately perhaps since the pre-Tertiary times, displaying time-related variations known in other arc systems, i.e. evolution from IAT to CA and CAK magmatism as function of the maturation of the arcs. Their history is characterized by subduction breaks, arc polarity reversals and initiation and resumption of subduction. Adakitic magmatism may be correlated with these critical periods of Leyte and Mindanao’s history. Thus, their identification allows us to constrain the succession of these events. One of the most significant features of young magmatism in Mindanao is the LILE enrichment of magmatic products which follows the collision in Central Mindanao, similar to many cases recorded in other collision areas. The distribution of two groups of lavas in distinct although contiguous volcanic centers evidences the small-scale heterogeneity of the sub-Central Mindanao mantle. The eruption of compositionally different volcanic products in very short time spans is apparently cyclic: CA magmas were emplaced between 2.3 and 0.6 Ma, SW magmas between 0.5 and 0.4 Ma, and CA magmas again from 0.3 Ma to the present. The presence of relict portions of subducted crust beneath these regions is likely to play a role in the enrichment of the corresponding magmas. It also provides a possible source for young adakites in Central Mindanao. One general aspect that has to be further studied is whether OIB-type mantle components are involved in the enrichment process (e.g. Vollmer 1989) or whether mantle metasomatism beneath post-collision areas by slab-derived fluids and melts simply produces chemical characteristics equivalent to OIB enrichment. Although the role of an OTB-type source in the generation of HFSE-enriched lavas has been suggested, the association of these rocks with adakites not only in

Magmatism in Mindanao Mindanao but in other arcs beneath which young crust is subducting strongly suggests a petrogenetic link between these two magma types. The role of ascending adakitic magmas as metasomatizing agents in the mantle wedge may be an explanation for this association (Arculus 1994; Sajona et al. 1994). Acknowledgements--The authors thank the Philippine National Oil Company, the Manila Mining Corporation and the Western Mining Corporation for rock samples; Jean Claude Philippet for his invaluable technical assistance in K-Ar dating; and the Mines and Geosiences Sector of the Department of Environment and Natural Resources in Leyte and Mindanao for their cooperation. E. Mantaring and S. Laserna’s help in the field is highly appreciated

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