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Recognition, Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites, Dongargarh Supergroup, Central India ARTICLE in GONDWANA RESEARCH · JULY 2001 Impact Factor: 8.12 · DOI: 10.1016/S1342-937X(05)70351-8
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Gondwana Research, V 4, No. 3, p p . 519-527. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-93Z
GRiW;
Recognition, Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites, Dongargarh Supergroup, Central India Joydip Mukhopadhyay, Arijit Ray, Gautarn Ghosh, Rajkalpo A. Medda and Partho Pratim Bandyopadhyay Department of Geology, Presidency College, 86/1 College Street, Kolkata - 700 073, India, E-mail:
[email protected] (Manilscript received October 21,2000; accepted March 9,2001)
Abstract A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of hlgh-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies - A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A / B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.
Key words: Rheoignimbrite, Proterozoic, Dongargarh Supergroup, Bijli rhyolites, high-grade welding, pyroclastics.
Introduction Textural and structural studies of extensive sheets of silicic volcanics have been a major field of volcaniclastic research in recent years. The subject of interest in this field has gained ground from the fact that many of the silicic volcanics have been interpreted either as lava-flows or highly welded lava-like ignimbrites. Since the discovery of welded ignimbrites during 1960s there has been lot of controversy in recognition and origin of several extensive silicic sheets that closely resemble lava-flows without unequivocal vitroclastic texture. In ancient successions, these have been described as tuffolavas or clastolavas (Fisher and Schmincke, 1984, p. 228). However, with the emerging concept of grade in ignimbrites mainly depending upon their nature and degree of welding (Walker, 1983; Mahood, 1984), many silicic volcanic
successions have been reassessed in this line. The mechanism of emplacement and characteristics of these highly welded lava-like ignimbrites might differ significantly with those of non-welded particulate pyroclastic flows. Walker (1983) introduced the class ‘high-grade ignimbrites’ to those which show intense welding and near obliteration of vitroclastic texture, and ‘low-gradeignimbrites’to those flow deposits that are nonwelded or less welded with distinct particulate texture. On the basis of welding intensity of ignimbrites, Branney et al. (1992) have further extended the classification into: extremely high-grade ignimbrites (lava-like ignimbrites), high-grade ignimbrite (rheomorphic ignimbrite), moderate-grade ignimbrites that have both welded and non welded zones, and low-grade ignimbrites with no welding and distinct particulate nature. The grading parameter corresponds to variability of particle viscosity
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and yield-strength during ignimbrite emplacement, and provide much information for the nature and controls of the volcanic activity. Distinguishing these categories is a major challenge in the field of volcanology due to a spectrum of gradational characters between the lavas and welded ignimbrites. The problems in recognition are again compounded in deformed and metamorphosed terrains. Till date most of the records of high-grade ignimbrites come from Recent to sub-Recent volcanic provinces and from Phanerozoic successions. The facies characters of ignimbrite deposits from Precambrian metamorphosed successions are relatively poorly known. However, field and textural studies of ignimbrite deposits would likely to throw light on the nature and environmental settings of volcanic activities even from low-grade Precambrian mobile belt successions (cf., Henry and Wolff, 1992; Bhushan, 2000). The rhyolites and rhyodacites of the Paleoproterozoic Bijli rhyolites (ca. 2200 Ma) of the Dongargarh Supergroup (Sarkar, 1958; Gangopadhay and Ray, 1997; Neogi et al., 1996; Deshpande et al., 1990; Divakara Rao et al., 2000) offer an excellent opportunity to study the nature and emplacement mechanism of extensive highgrade rheomorphic rhyolitic ignimbrites. Though rhyolitic tuffs and ash deposits have been mentioned from this formation (Sarkar, 1958; Krisnamurthy et al., 1988a; Gangopadhyay and Ray, 1997), bulk of these silicic volcanics have been interpreted as lava flows. However, a reconnaissance revealed many variations in texture and organization of these rhyolites that could be interpreted in terms of the pyroclastic nature of these volcanics rather than lava flows (see also Pasayat, 1981). In this paper we attempt a detailed facies analysis of these rhyolitic pyroclastics and explore their implications for nature, mode of emplacement, possible source and environmental setting. The work is based on detailed facies analysis of the rhyolitic pyroclastics in a number of sections in the vicinity of the village of Salekasa (80°28' and 20"16'; toposheet no 64C/7, Survey of India). Textural and petrographic studies of about 30 selected samples were carried out.
I
Dongargarh Supergroup (Fig. 1) is a predominantly low-grade volcano-sedimentary Proterozoic succession in the Bhandara craton, central India. It forms a part of the Bhandara triangle with Sakoli and Sausar Series. Sarkar et al. (1981) divided the Dongargarh Supergroup into three groups such as, Amgaon, Nandgaon and Khairagarh, in the ascending order (Table 1). The Amgaon Group comprises migmatites, metasedimentary and metaigneous rocks of amphibolite
ll
Fig. 1. Geological map of Dongargarh Supergroup (modified after Sarkar, 1 9 5 8 ) . 1- Sijli rhyolites, 2-Pitepani volcanics, 3-Dongargarh granite, 4-Bortalao Formation, 5- Sitegota volcanics, 6-Karutola Formation, 7-Mangikhuta volcanics, Dashed lines - major faults. Table 1. Stratigraphic succession of Dongargarh Supergroup (after Sarkar et al., 1981; Deshpande et al., 1990). Chhatisgarh Supergroup
a
5 0 p:
Khairagarh Group
W
d
W
a 5 (A
d
Nandgaon Group
Geologic Setting
-
z 0 a Amgaon Group
Raipur Group Chandarpur Sandstone Unconformity-Khairagarh Orogenic phase Kotima Volcanics Ghogra Formation Mangikhuta Volcanics Karutola Formation Sitagota Volcanics (Intertrappean shale) Bortalao Formation Basal shale Unconformity Dongargarh Granite Pitepani Volcanics Sijli Rhyolites Amgaon Orogeny, metamorphism, and granitization, Quartz-sericite schist, feldspathic quartzite, garnet-epidote quartzite, hornblende biotite quartzite, quartz-feldspar biotite gneiss, hornblende schist, and amphibolite.
facies of metamorphism. Nandgaon Group includes Bijli rhyolites, Pitepani volcanics and the Dongargarh granite intrusive into the volcanics. The Khairagarh Group is a metasedimentary and metavolcanic assemblage, Gondwana Research, V. 4, No. 3,2001
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unconformably overlying the Bijli rhyolites and Dongargarh granite. Excepting the Amgaon Group the other two groups have mainly undergone a greenschist facies of metamorphism (Deshpande et al., 1990). Yedekar et al. (1990) and Jain et al. (1991) suggested that the metavolcanics and metasedimentary associations of the Nandgaon and the Khairagarh Groups represent subduction zone assemblage of the northward subducting Deccan protocontinent below the northern Bundelkhand protocontinent. Alternatively, based on the co-magamatic rhyolite-anorogenic granite assemblage together with bimodal rhyolite-basalt assemblage, a continental rift model has been proposed (Krishnamurthy et al., 1988a, 1990; Gangopadhyay and Ray, 1992,1997; Neogi et al., 1996).
The Bijli Rhyolites Petrology and age
The Bijli rhyolites are composed of thick stacks of rhyolitic volcanics unconformably overlying the gneissic rocks of Amgaon Group. The volcanic unit attains a maximum thickness of 4500m (Sarkar, 1958). These rocks are massive but often show laterally persistent flow layers. The rhyolitic volcanic unit contains lenses of fragmental rocks described as “rhyolite conglomerate” by Sarkar (1956) or “tuff and agglomerate” by Gangopadhyay and Ray (1997). The rhyolitic volcanics are dominantly fine-grained and are mineralogically similar, consisting of quartz and feldspar phenocrysts set in a recrystallized mosaic of quartz and feldspar with widely varying amount of chlorite, sericite, carbonates and Fe-oxides. These acid volcanics fall within rhyolite-dacite field of the TASdiagram and are peraluminous in composition (Gangopadhyay and Ray, 1997; their Fig. 5). A number of varieties in terms of content of phenocrysts such as (1) non-porphyritic type with phenocryst content less than 5%, (2) porphyritic rhyolites with phenocryst content 5 to 50% and (3) rhyolite porphyry containing more than 50% phenocrysts of quartz and feldspar (alkali feldspar and albite) have been recognized (Gangopadhyay and Ray, 1997). Sarkar et al. (1967, 1981) determined the wholerock isochron ages of rhyolite and associated granitoid rocks and found them to be 2180 k 25 Ma with initial 87Sr/86Srratioof 0.7057rt 0.0015 for rhyolite and 2270 f 90 Ma with initial 87Sr/86Srratio of 0.7092+ 0.0054 for granite, respectively. Krisnamurthy et al. (1988b) suggested a twelve-sample Rb/Sr age of 2462 f 25 Ma with an initial 87Sr/86Srratio 0.7054 rt 0.0014 for the rhyolites with relatively high MSWD, and 2503rt 35 Ma with an initial 87Sr/86Srratio of 0.7035+ .0017 for eight samples of rhyolite with reduced MSWD. Gondwana Reseauclz, V. 4, No. 3, 2001
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Facies Analysis of Rhyolitic Ignimbrites The rhyolite forms laterally discontinuous small hillocks forming a strike-parallel chain of ridges for several tens of kilometers. Individual mounds range from 60 to lOOm in height and 300 to 600m in width. The thickness: lateral extent ratio indicates these deposits as high aspect ratio ignimbrites (after Walker, 1983). Apparently in most instances the rhyolite lacks any particulate texture or vitroclastic texture, but there are considerable variations in internal character along and across strike, mainly reflected in the form of presence or absence of very well developed plane parallel banding (mm-scale) and some deformation features. However, very careful looks on weathered surface reveal vitroclastic textures that are nearly absent on homogeneous fresh surface. Besides this, at a number of profiles, there are beds of monolithologic breccias and monolithologic matrix-supported coarse ashto lapilli tuffs. Based on characters such as clast size, clast support, internal banding, lamination and vitroclastic texture, following facies have been identified,
Facies A: CZast-supported welded coarse ash tuff This facies is defined by a monolithologic brown to dark gray coloured massive rhyolitic rock rich in phenocrysts. A close examination in fresh and weathered surfaces reveals that the entire rock is made up of subrectangular coarse sand-sized clasts of rhyolite (Fig. 2). The margins of the clasts are predominantly welded nearly obliterating the vitroclastic texture. The clast size (long axis: short axis) varies between 3 x 0.5cm and 0.1 x O.lcm. Phenocrysts form about 1520% of the total rock and do not show any preferred orientation. The facies is primarily clast-supported with very low amount of interstitial matrix. Internally, it is massive but locally shows incipient crude lamination. Clasts other than
Fig. 2. Clast-supported welded coarse ash tuff (facies A). (Diameter of the coin is 2.55 cm).
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Fig. 3 . Photomicrograph (under crossed polars) of welded coarse ash tuff, note intense welding of clasts. (Scale bar: 20 pin).
Fig. 4. Photomicrograph (under crossed polars) of broken phenocryst with melt inclusion. (Scale bar: 20 pm).
rhyolite are conspicuously absent. The vitroclastic texture is more evident under microscope with low-illumination plane-polarized light. Clasts vary from sub-elliptical to bent rectangular in shape (Fig. 3 ) . Commonly, the boundaries are wavy or curved and some clasts tend to project into others along the zone of welding. Rinds of chlorite, biotite and iron oxides of dark brown colour define the welding margins that are best distinguished under low-illumination plane-polarized light. Phenocrysts of quartz, plagioclase and potash feldspar, and with euhedral to subhedral outlines occur in the interstices of the clasts. The extent of recrystallisation is overall similar although minor variation is observed locally. Phenocrysts are mostly broken and fractured with melt-inclusions, centered on radial fractures of the phenocrysts (Fig. 4). The boundaries of clasts are attenuated against phenocrysts. Phenocrysts have mostly embayed and irregular margins.
Facies B: Matrix-supported welded coarse ash tuff w i t h floating lapilli This facies is very similar to the facies A in composition. It is also a monolithologic rock made up of lapilli to sand sized rhyolitic clasts floating within highly welded monolithologic rhyolitic matrix (Fig. 5). Matrix portion constitutes more than 60% of the total volume. This facies lacks any internal stratification and is massive in appearance. The clasts have a wide size range from
Interpretation The prominent vitroclastic texture, observed in the hand specimen as well as under the microscope unequivocally suggests these deposits to be pyroclastics rather than lava flows (cf. Henry and Wolff, 1992). Abundance of broken, fractured and embayed phenocrysts, phenoclasts and phenocrysts with fractures radiating from melt-inclusions supports the explosive pyroclastic nature of these deposits. Massive, clast-supported vitroclastic texture further suggests their deposition from a high-density laminar viscous flow or a debris flow (e.g., Schmincke and Swanson, 1967; Fisher, 1979; Reedman et al., 1987; Wolff et al., 1989; Branney et al., 1992). Attenuated clasts, partial obliteration of interclast boundaries and deformation of clasts around or at the contact of phenocrysts suggest a high degree of welding and emplacement of a hot semisolid pyroclastic flow (cf. Sparks et al., 1999).
Fig. 5. Matrix-supported coarse ash tuff with floating lapilli. (Diameter of the coin is 1.9 cm).
5x2.8 cm to 1 . 80.5 ~ cin. the coarser clasts are sub-rounded, sub-elliptical and show a preferred orientation parallel to the basal surface. The matrix is made up of welded rhyolitic clasts very similar to the appearance of facies A. The rhyolitic sand sized grains in the matrix are extremely stretched and show near complete fusion of contacts. Phenocrysts constitute about 10 to 12 % by volume. Under microscope the clasts are more evident. The matrix is made up of clasts of sand-size grade rhyolite fragments with sub-elliptical to sub-rounded outline and .____
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Fig. 6. Photomicrograph (under crossed polars) of welded matrix [facies B). [Scale bar: 20 pm).
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Fig. 7. Flow-laminations in rheomorphic tuff (facies C), note planar to wavy nature of laminations and laminae deformation near the upper part. (Length of the pen is 12 cm).
curved margins (Fig. 6). Degree of welding of the clasts is largely similar to the facies A.
Interpretation This facies with vitroclastic texture comprising lapilli to coarse ash tuff is likely to be a product of pyroclastic flow. The pyroclastic nature is further supported from the fair abundance of broken phenocrysts, phenocrysts with melt-inclusions etc. Matrix-supported texture together with sub-parallel clast alignment suggests a viscous pyroclastic debris flow within laminar flow-regime (cf. Branney et al., 1992). Undeformed clasts with sharp margin emplaced in cryptocrystslline matrix are presumably derived from crater wall as cognate components. Presence of these clasts of cognate origin indicates a greater explosiveness during emplacement of these deposits in comparison to those of facies A. The matrix of highly bent and welded rhyolite fragments suggests that emplacement temperature was high enough as to emplace the clasts in subsolidus state (e.g., Branney et al., 1992).
Fig. 8. Eutaxitic to parataxitic texture in rheomorphic tuff. (Diameter of the coin is 2.2 cm).
Facies C: Thinlyflow-laminatedrheomorphic tuff This facies is characterized by uniformly flow-laminated rhyolite without any vitroclastic texture recognizable in hand specimen. The flow laminations, defined by mmscale recessive surfaces on weathered faces, are 1-3 mm thick, and planar to wavy in nature (Fig. 7). On fresh surface, laminae give an eutaxitic to parataxitic texture (Fig. 8). However, locally extremely stretched, folded and curved lenticular fiamme become prominent into differential lamina (Fig. 9). The flow laminations are commonly set into flow folds whose geometry and orientation changes rapidly within short distances. The flow are tight plunging to recumbent types and resemble slump folds of soft-sediment deformation Gondwana Research, V. 4, No. 3,2001
Fig. 9. Thinly flow laminated rheomorphic tuff showing extremely stretched, curved and folded clasts.(Scale bar: 3 cm).
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Under crossed polars this facies is predominantly a nearisotopic mosaic of lenticular clasts made up of cryptocrystalline quartz surrounded by somewhat coarser microcrystalline quartz rim that defines the clast boundaries and gives rise to avitroclastic texture (Fig. 12). The clasts are extremely stretched and nearly completely fused along their margins obliterating the vitroclsatic texture. The broken phenocrysts (phenoclasts) in parts appear separately surrounded by cryptocrystalline matrix.
Interpretation
Fig. 10. Flow folds in rheomorphic tuff. (Length of the pen is 12 cm).
Fig. 11. Second-generation clast in rheomorphic tuffs. (Diameter of the coin is 2.55 cm).
The cryptocrystalline areas are evidently devitrified fiammes with boundary-controlled recrystallized microcrystallinerims. The vitroclastic texture evident from the boundary-controlled recrystallisation of the extremely stretched and fused fiammes under microscope suggests a pyroclastic origin of these deposits. Presence of phenoclasts also supports this contention. Near obliteration of clast boundaries is likely to be caused by a very high degree of welding that almost generated a nonparticulate flow of agglutinated, low-viscosity fiammes during a laminar viscous flow (Schmincke and Swanson, 1967; Kobberger and Schmincke, 1999). The flow laminations presumably reflect original layers with differing crystallinity or vesicularity and a shear deformation or rheomorphism during emplacement. The flow-laminations thus may result from extreme attenuation of agglutinated fluidal particles with different textures. Flow folds and autobrecciation suggest a high viscosity flow and rheomorphic flow during and after emplacement of hot ignimbrite (cf. Chapin and Lowell, 1979; Wolff and Wright, 1981; Branney and Kokelaar, 1992).
Facies D: Autobreccia The flow-laminated facies (C) grades upwards to several meter thick zone of autobreccia. The autobreccia is composed of framework-supported to matrix-supported angular to sub-rounded blocks of flow-laminated rheomorphic tuff of facies C below (Fig. 13). The clast size varies between 21x 3.5 cm and 2.0 x 0.6 cm. Clasts include detached hinges of slump folds. The interstitial matrix is made up of sand-sized welded flow-laminated ash tuff.
Interpretation Fig. 12. Photomicrograph (under crossed polars) of rheomorphic tuff showing near isotropic devitrified clasts (C) with microcrystalline quartz rims (g).Note extremely stretched condition of clast. (Scale bar: 20 ,urn).
zones (Fig. 10). Locally, autobrecciation has generated secondaryclasts (Fig. 11). The phenocrysts are less abundant and also smaller in size than those on facies A and B.
The blocks and matrix of the autobreccia, solely madeup of facies C, unequivocally suggest their derivation from the underlying facies C. This unit resembles autobreccia in viscous obsidian block lava-flows and probably formed in a similar way, during emplacement, rheomorphism and degassing (cf. Branney et al., 1992). Where flow exerted a stress on the welded tuff that exceeded its tensile Gondwana Research, V: 4, No. 3,2002
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thick flow-laminated rheomorphic tuff in the middle (facies C) followed upwards by an upper 15m thick autobreccia (facies D). Contacts between successive facies are gradational. The massive ash-flow tuff (facies A) grades upwards to the well-laminated rheomorphic tuff through a transition zone. The upper part of the flowlaminated rheomorphic tuff shows intense flow-folding and local brecciation.
Facies sequence 2
Fig. 13. Matrix-supported autobreccia. Note: blocks of flow laminated rheomorphic tuffs.
strength, fracturing occurred forming blocks that rotated and fractured further as the flow continued.
This profile (Fig. 14b), from Sonartola village (Fig. l), consists of a basal 50 m thick matrix-supported welded coarse ash tuff with floating lapilli (facies B) overlain by a rather thin (about 20m) thinly flow-laminated rheomorphic tuff (facies C) showing excellent flow layering. This profile is basically composed of pyroclastic flow deposits.
Discussion
Facies Sequence
Eruption style
Facies sequence has been studied from two point profiles. Each profile gives an idea about the nature and emplacement of the pyroclastics and has been discussed below:
Volcanic plumes are generated when explosive eruptions discharge a mixture of pyroclasts and gas into atmosphere. These mixtures can, behave in two fundamentally different ways. They may either form convective plumes, which rise to great heights in the atmosphere, or they may collapse to form fountains at relatively low heights to produce flow of hot particles and gas as pyroclastic flows and surges. In a sustained eruption column, there are three subdivisions: gas thrust region, convective region and umbrella region, in ascending order (Sparks et al., 1997; p. 23 ). Low-height eruption columns commonly collapse in the basal gas thrust region due to the inability of gas mixture to rise buoyantly further above. A sustained explosive eruption in gas charged silicic magma develops stratospheric (high altitude) plumes referred as Plinian eruption. However, many monogenetic silicic small volcanoes can develop when partially degassed rhyolitic magma rises to the earth surface. These are typically either low-height column collapse types or spatter-fed fissure eruptive types. The variation in welding characters and gradation of eutaxitic flow-laminated rheomorphic tuffs from the more particulate flow deposits strongly suggests that the rhyolites in study area are pyroclastic in origin. Succession in sequence 1 and 2 (Fig. 14) with basal pyroclastic flow deposits to upper flow-laminated rheomorphic tuff units indicates variation in eruption style. The basal welded ash-flow tuff with abundant broken phenocrysts presumably reflects a gas-charged initial explosive phase. The ash-flow tuffs are products of eruption column collapse during the initial explosive phase. The pervasive
Facies sequence 1 The isolated hillock on the south of Salekasa (Fig. 1) railway level crossing gives an opportunity to study the facies sequence. The profile (Fig. 14a) includes a lower 45m thick-welded massive ash-flow tuff (facies A), 23m
Legend
@ - . Facies D Facies C Facies 0 Facies A
0
Cover
a
b
Fig. 14. Facies sequences measured in a) Salekasa (profile-1), b) Sonartola (profile-2). (Locations in Fig. 1).
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welding in both facies -A and B are likely to have resulted from low-height column collapse. In low-height explosive pyroclastic column the particles get insufficient freezing time and deposit as hot semisolid particles that get welded soon after deposition. The presence of cognate clasts in facies B presumably reflects a higher explosiveness than facies A during which early formed caldera wall rock also have been entrained in the column. The medial flow-laminated rheomorphic tuff (facies C) in both sequences resulted from still lower viscosity and higher temperature eruption that immediately followed the initial column-collapse episode (cf. Branney and Kokelaar, 1992; Branney et al., 1992). The very high-grade nature of welding suggests a high temperature of emplacement of very low viscosity particles. Agglutination of very high temperature silicic magma eruption has been invoked as causal mechanism for the origin of flowlaminated rheomorphic tuffs by many recent workers (e.g., Bonnichsen and Kauffman, 1987; Hausback, 1987; Branney and Kokelaar, 1992). The sequences with basal welded pyroclastic deposits presumably represent low-column height Plinian type eruption. The eruption was initiated with a low-gas charged eruption column that collapsed prior to attaining a convective buoyant lift stage. The initial violent phase was followed by quieter phase of extrusion of hot lowviscosity particles that had given rise to the flow laminated rheomorphic tuff unit. The autobreccia carapace developed near the top of the flow with higher rate of cooling after attaining a steeper flank slope.
Implications for high-grade welding Extremely high welding grade in the observed silicic volcanics may be attributed to very high temperature of emplacement. Several recent studies have shown that many intensely welded silicic rheomorphic ignimbrites had magmatic temperature between 800°C and 1100°C, much higher than common silicic rocks. The low-gas charged eruption, suggesting an anhydrous melting condition, can generate very low viscosity pyroclasts that can agglutinate and coalesce to form rheomorphic ignimbrites (Henry and Wolff, 1992). Similar high-grade of eruption of silicic magma may result from anhydrous melting of continental crust on a regional scale by massive intrusion of mafic magma into the lower crust (cf., Henry and Wolff, 1992). The nature of rhyolitic volcanics in the studied area points to such anhydrous crustal melting process. Similar ideas have also been put forward by several workers from geochemical studies of the Bijli rhyolites (Krishnamurthy et al., 1988a, 1990; Gangopadhya and Ray, 1992,1997; Neogi et.al., 1996). This idea is in contrary to the earlier proposals regarding the origin of these rhyolites as late differentiates from a
basaltic liquid in a subduction zone. Furthermore, the depositional setting for Bijli rhyolite was largely unknown due to lack of epiclastics with unequivocal sedimentary signature. The high-grade character of these pyroclastics in turn suggests eruption in a subaerial continental setting. The outcrop pattern together with the high-grade nature of ignimbrites are likely to indicate proximal vent facies associations (cf. Sparks et al., 1999) of small monogenetic rhyolitic volcanoes, presumably related to major fissure systems. A detailed mapping of the ignimbrite facies would only reveal the volcanic landscape and paleo-vent positions, and should be undertaken for mineral exploration in this area. Krishnamurthy et al. (1988a, 1990) suggested potential occurrences of uraninites and other uranium ore minerals along shear zones at the contact of rhyolites and basic effusives. However, any late stage hydrothermal vein-related uranium mineralization should be further explored after carefully locating the proximal vent ignimbrite associations.
Conclusions The rhyolitic silicic volcanics studied around Salekasa represent a spectrum of high-grade ignimbrites. The volcanics include welded, pyroclastic flow deposits, flow laminated to extremely high grade rheomorphic tuffs with autobreccia carapace. The high-grade of welding together with outcrop pattern of small monogenetic rhyolitic mounts point to low-column-collapse Plinian to very high temperature spatter-fed fissure eruptions of low viscosity pyroclasts from partially degassed magma chamber. The welding character also suggests a subaerial continental volcanism due to anhydrous melting of continental crust.
Acknowledgments We are thankful to Dr. S.K. Bhushan and one anonymous reviewer for their helpful review which considerably improved the final version of the Ms. We also acknowledge the Department of Geology, Presidency College, Kolkata, for providing UGC-SAP infrastructural facility.
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