Pulak Sengupta | Jadav[pur university (original) (raw)
Papers by Pulak Sengupta
Mineralogy and Petrology, 2009
The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characteriz... more The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite + clinopyroxene (mg# = 71–73) + grandite (16–21 mol% Adr) + quartz ± calcite, Type II: grandite (25–29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36–38 mol% Adr) + clinopyroxene (mg# = 55–65) + epidote + scapolite + calcite + quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO2 → wollastonite + V, 2. calcite + clinopyroxene + O2 → grandite + SiO2 + V, 3. scapolite + calcite + quartz + clinopyroxene + O2 → grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O2 → grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2 < 0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO2 and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (XCO2 < 0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al2O3 was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-XCO2 and isobaric T-XCO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9 ± 1 kbar and 750° ± 30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.
Precambrian Research, 2007
Kondapalle area, situated in the southern part of the Eastern Ghats Belt (EGB), expose an assembl... more Kondapalle area, situated in the southern part of the Eastern Ghats Belt (EGB), expose an assemblage of high grade rocks with protracted geological history of more than 1 billion years. Field and petrological studies in combination with the existing geochronological data indicate intermittent ...
American Journal of Science, 2002
The P-T-fluid histories of aluminous garnet؉clinopyroxene-bearing calc-silicate granulites are co... more The P-T-fluid histories of aluminous garnet؉clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO 2 -H 2 O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite-and wollastonitedominated reaction zones (scp؉cpx؎qtz,cal,qtz -wo؉cal؉cpx؎scp) developed between marble layers (cal؎cpx,wo,scp) and their host calc-silicate granulite (cpx؉pl؎qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz؉cal؉cpx؉scp؉grt؉v in the system CaO-FeO-Al 2 O 3 -SiO 2 -CO 2 it is demonstrated that at the ambient P-T-X fluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at "singular points" across which one or more phases change their sides with respect to garnet in a given reaction.
Journal of Southeast Asian Earth Sciences, 1996
Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofe... more Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures < 1000°C and were thus in thermal equilibrium with the country rock granulites during peak metamorphism. Comparison with experimental data on similar bulk compositions constrains the depth of emplacement of the magmas at 30-35 km. Geochemical characteristics indicate that the mafic magmas are essentially similar to continental flood basalts and have thus been generated in an extensional set-up. The apparent clockwise trajectory recorded in the Anakapalle granulites was produced by extension of the crust of near-normal thickness with concommitant basic magmatism.
Contributions To Mineralogy and Petrology, 1989
Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal gra... more Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields W Mg-MnGt(=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe≥0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+.
Journal of Metamorphic Geology, 2008
ABSTRACT A suite of garnet-wollastonite-scapolite-bearing calcsilicate granulites from the Easter... more ABSTRACT A suite of garnet-wollastonite-scapolite-bearing calcsilicate granulites from the Eastern Ghats has been investigated to document the controls of mineral reactions during the metamorphic evolution of the deep continental crust. The rocks studied show heterogeneity in modal mineralogy and phase compositions in millimetre-sized domains. Textural relations, and the compositional plots of the phases, established that the clinopyroxene exerts a strong influence on the formation and composition of garnet in the complex natural system. P-T estimates using the vapour-independent equilibria involving garnet define a near isobaric cooling path from c. 850C at c. 5.5–5.2 kbar. The deduced trajectory tallies well with the terminal segment of the overall retrograde P-T path construed from the associated rocks using well-calibrated thermobarometers. The ubiquitous occurrence of wollastonite and scapolite in the main calcsilicate body suggests low aCO2 during peak metamorphic condition. Fluid compositions constrained from mineral-fluid equilibria of the garnet-bearing assemblages show domainal variations as a function of the compositions of the solid phases, e.g. garnet and clinopyroxene. A quantitative log/CO2-log/O2 diagram has been constructed to depict the stability of the different calcsilicate assemblages as functions of the compositions and the behaviour of these fugitive species. The results of the mineral-fluid equilibria and the quantitative fluid/rock ratio calculations, in conjunction with the topological constraints, imply vapour-deficient meta-morphism in the rocks studied. It is argued that fO2 during peak metamorphism was monitored by the ambient fO2. Subsequently, during retrogression, different domains evolved independently, whereas the fluid composition was controlled by the mineral-fluid equilibria.
Journal of Metamorphic Geology, 2004
In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of... more In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of graphite-bearing calc-silicate granulites, veined by syenitic rocks, developed wollastonite-rich veins at 6–7 kbar and > 850 °C. During subsequent near-isobaric cooling wollastonite was replaced by calcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite with variable Al and F contents is present throughout the rock. Combining the compositional variation of titanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition of coexisting fluids and the mobility of Al exert a far greater control on the composition of titanite than pressure, temperature or the whole rock composition. Thermodynamically computed isothermal–isobaric logfO2– logfCO2 and logfF2– logfO2 grids in the systems Ca–Fe–Si–O–F (CISOF; calcite-free) and Ca–Fe–Si–O–F–C–H (CISOFV; calcite-present) demonstrate the influence of bulk rock and fluid compositions on the stability of the fluorite-bearing assemblages in diverse geological environments and resolve the problem of the stability of titanite in fayalite + fluorite-bearing rocks in the Adirondacks. The mineralogy of the studied rocks and the topological constraints tightly fix the logfO2, logfF2 and logfCO2 at −15.8, −30.6 and 4.1, respectively, at 6.5 kbar and c. 730 °C. Because of the similarity in the P–T conditions, the compositions of pore fluids in the fluorite-bearing assemblages of the Adirondacks and the Eastern Ghats Belt have been compared.
Journal of Metamorphic Geology, 2008
‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthit... more ‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum-, spinel- and sapphirine-bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite-sillimanite-cordierite gneisses of the Ihosy granulite unit in the NW of the Pan-African Bongolava-Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite-rich sediments or calcareous bauxites that were altered by Ca or Si infiltration-metasomatism prior to or coeval with the development of the anorthite-corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak-conditions around 800 °C and 6–7 kbar for the regional high-grade metamorphism and deformation in the NW part of the Bongolava-Ranotsara shear zone. The well-annealed granoblastic-polygonal textures indicate complete chemical and textural re-equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg-, Si- and K-bearing fluids into the anorthite-corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg-Al minerals, thereby producing corundum-, spinel- and sapphirine-bearing phlogopitites. Further advance of the resulting ‘residual’ Mg- and Si-bearing fluids into anorthite-corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava-Ranotsara ductile shear zone c. 530–500 Ma ago.
Contributions To Mineralogy and Petrology, 1991
A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statist... more A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe−Mg−Ca−Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe−Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW FeMgbi=1073±490 cal/mol. ΔH r and ΔS r for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.
Gondwana Research, 1999
In this paper, we compare tlie petrological histories of the Kemp Land Coast (east Antarctica) an... more In this paper, we compare tlie petrological histories of the Kemp Land Coast (east Antarctica) and Gokavaram area (Eastern Ghats), that were supposed to have been juxtaposed. The area around Gokavaram is dominated by different varieties of paragneisses (pelitic, quartzofeldspathic and calcareous composition) with relatively minor amounts of orthogneisses (mafic, enderbitic and granitic composition). The rocks were involved in three major phases of deformation and were finally affected by localised shear movement. On the basis of reaction textures, well preserved in high Mg-A1 granulites and calc-silicate granulites, and geothermobarometric data we deduce a polymetamorphic evolution of tlie rocks. Following an early M, metamorphism culminating at 9.2-9.4 kbar, > 950"C, the rocks cooled nearly isobarically down to 850°C. During a subsequent M, metamorphism, near isothermal decompression to 5-6 kbar occurred. This was followed by near isobaric cooling down to 600-650°C. M, is a weak ampliibolite facies overprint, largely restricted to late shears, which involved hydration as well. Available radiometric data from this area can be interpreted in terms of partial resetting of U-Pb systematics in older sphenes due to M, metamorphism at ca. 550 Ma. Despite tlie absence of sufficient isotopic data on tlie Eastern Ghats granulites, we document a remarkable similarity in the petrological history of the two supposedly erstwhile neighbours.
Journal of Petrology, 1999
A suite of quartz-and corundum-bearing metapelitic granulites, metamorphism occurred in the lower... more A suite of quartz-and corundum-bearing metapelitic granulites, metamorphism occurred in the lower crust (>8 kbar). Reported intruded by layered gabbronorite-pyroxenite-anorthosite at Kon-U-Pb cooling ages of monazite and allanite from a late pegmatite dapalle, Eastern Ghats Belt, preserves a multitude of reaction suggest the UHT metamorphism to be older than 1600 Ma. The textures involving oxide and silicate minerals that attest to several deduced P-T history and the absence of Grenvillian high-grade prograde and retrograde reactions. In the quartz-bearing associations, metamorphism in the study area provide important constraints on the reactions are: (a) biotite + sillimanite + quartz → garnet the configuration of East Gondwana, in particular on the continuation + liquid; (b) garnet + sillimanite → spinel (+ magnetite) + of the Napier-Rayner terrane boundary into the Eastern Ghats quartz; (c) Fe 2 TiO 4 + O 2 → ferrian ilmenite + magnetite; (d) Belt. reversal of reaction (b); (e) Fe 2 O 3 -rich ilmenite + plagioclase + quartz → Fe 2 O 3 -poor ilmenite + garnet + O 2 . Reactions in the corundum-bearing associations are: (f) Fe 2 TiO 4 + FeAl 2 O 4 + Belt, India; Indo-Antarctic terrane assembly O 2 → ferrian ilmenite + Fe 3 O 4 + Al 2 O 3 (in ilmenite); (j) garnet + corundum → spinel + sillimanite. To examine the paragenetic evolution of the metapelitic granulites, a petrogenetic grid for the KFMASH system at high temperatures and pressures, involving *
Journal of African Earth Sciences, 1996
Journal of Asian Earth Sciences, 2006
The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, e... more The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, eastern India, are folded into a series of large upright folds with variable plunges. The regional schistosity is axial-planar to the folds. The folds were produced by a second phase of deformation (D2) and were preceded by D1 deformation, which gave rise to isoclinal folds (mapped outside the study area) and the locally preserved, bedding-parallel schistosity. A shearing deformation during D2 was responsible for the sheath-like geometry of a major fold. The axial planes were curved by D3 warping. The first metamorphic episode (M1) of low-pressure type produced andalusite porphyroblasts prior to, or in the early stage of, D1 deformation. The main metamorphism (M2), responsible for the formation of chloritoid, kyanite, garnet and staurolite porphyroblasts, was late-to post-D2 in occurrence. The Staurolite isograd separates two zonal assemblages recorded in the highalumina and the low-alumina pelitic schists. Geothermobarometric calculations indicate the peak metamorphic temperature to be 550 8C at 5.5 kb. Fluid composition in the rocks before and during M2 metamorphism was buffered and fluid influx, if any, was not extensive enough to overcome the buffering capacity of the rocks. From M1 to M2, the P-T path is found to have a clockwise trajectory, that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting, followed by compressional deformation leading to loading and heating. q
Geological Society, London, Special Publications, 2003
... 2003; v. 206; p. 131-143 Geological Society, London, Special Publications Somnath Dasgupta an... more ... 2003; v. 206; p. 131-143 Geological Society, London, Special Publications Somnath Dasgupta and Pulak Sengupta ... Page 2. Indo-Antarctic Correlation: a perspective from the Eastern Ghats Granulite Belt, India SOMNATH DASGUPTA & PULAK SENGUPTA ...
American Mineralogist, 2009
... Razakamanana et al. 2000; Sengupta et al. 2004). In India, högbomite has been hitherto report... more ... Razakamanana et al. 2000; Sengupta et al. 2004). In India, högbomite has been hitherto reported only from four localities (Grew et al. 1987, 2003; Devaraju et al. 1981; Tsunogae and Santosh 2005). The origin of högbomite ...
Economic Geology, 1989
Fonomic Co/ogy Vol. 84, 1989, pp. 434-443 ... Mineral Reactions in Manganese Oxide Rocks: PTX P... more Fonomic Co/ogy Vol. 84, 1989, pp. 434-443 ... Mineral Reactions in Manganese Oxide Rocks: PTX Phase Relations ... SOMNATH DASGUPTA, PULAK SENGUPTA, 1 . K. BHATTACHARYA, M. MUKHERJEE, M. FUKUOKA*, H. BANERJEE, AND SUPRIYA ROY ...
Contributions To Mineralogy and Petrology, 2004
Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich peli... more Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich pelitic granulites is an important tool to unravel the P–T evolutionary history of high-grade rocks. In the Eastern Ghats Belt, India, several studies have been carried out on these fascinating aluminous granulites, and the results of these studies have revealed complex P–T–t histories (Dasgupta and Sengupta 1995; Sengupta et al. 1999; Rickers et al. 2001a, 2001b; Gupta et al. 1999; Dobmeier and Simmat 2002; Dobmeier and Raith 2003). In recent communication, Bhattacharya and Kar (2002) reported reaction textures from a suite of Mg-Al granulites from the Paderu area of the Eastern Ghats Belt. Combining the textural relations and thermodynamic calibration of some construed reactions, the authors have put forward a single phase metamorphic evolution of the area along a ‘clockwise’ pressure–temperature trajectory. Combining the petrological features from the Paderu area with those reported from the Chilka Lake complex, the authors proposed a general tectonic model for the entire Eastern Ghats Belt. Incidentally, the rocks in and around Paderu have been studied in some detail by several other workers (Lal et al. 1987; Mohan et al. 1997; Sengupta et al. 1997). The purpose of this comment is to demonstrate that the conclusions made in the paper are inconsistent with the petrological features described in the text. Further, the thermodynamic treatment used in the paper has serious errors in many places, and hence, is often in complete disagreement with the existing experimental data and theoretical analyses on the Mg-Al-rich assemblages. There are also significant problems arising from the poor quality of the analytical database. Unfortunately, the authors cite only a few published works (mostly their own) ignoring many other relevant studies from this belt (cited above). Our observations are organised according to the sections of the paper.
Ore Geology Reviews, 2011
The c.2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of d... more The c.2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6 m thick) within clinopyroxenite and highly calcic anorthosite (An 99-100 ) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at c.2.51 Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at c.2.50-2.45 Ga (granulite facies) and at 0.72-0.50 Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place. Chromite compositions show large variations (Cr# = 26-60, Mg# = 4-45, Fe 3+ # = 2-24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr# = 3-21; Mg# 45-61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe-Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean-Proterozoic boundary.
Journal of Geology, 1991
Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate ... more Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate rocks from the Anantagiri-Araku areas of the Eastern Ghats belt contain orthopyroxene, clinopyroxene, garnet, quartz, ilmenite, and plagioclase as major constituents. ...
Geological Journal, 1995
High Mg-Al granulites and calc-silicate granulites provide evidence for ultra-high temperatures o... more High Mg-Al granulites and calc-silicate granulites provide evidence for ultra-high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9–10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg-Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile-ilmenite or titanohaematite-ferrianilmenite. These high Mg-AI rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra-high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinel-cordierite-quartz and sapphirine-quartz and stabilization of the assemblages orthopyroxene-sillimanite-cordierite and spinel-quartz-sapphirine-garnet and (ii) the high Al2O3 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P-T path of evolution for the granulites. In calc-silicate granulites stabilization of nearly pure meionite and of the wollastonite-plagioclase-andradite-rich garnet, wollastonite-scapolite-grandite garnet-calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9–10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near-isobaric cooling followed by near-isothermal decompression. On the basis of the available evidence it appears that non-extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra-high temperature metamorphism on an anticlockwise path in the Eastern Ghats Belt.
Mineralogy and Petrology, 2009
The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characteriz... more The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite + clinopyroxene (mg# = 71–73) + grandite (16–21 mol% Adr) + quartz ± calcite, Type II: grandite (25–29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36–38 mol% Adr) + clinopyroxene (mg# = 55–65) + epidote + scapolite + calcite + quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO2 → wollastonite + V, 2. calcite + clinopyroxene + O2 → grandite + SiO2 + V, 3. scapolite + calcite + quartz + clinopyroxene + O2 → grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O2 → grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2 < 0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO2 and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (XCO2 < 0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al2O3 was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-XCO2 and isobaric T-XCO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9 ± 1 kbar and 750° ± 30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.
Precambrian Research, 2007
Kondapalle area, situated in the southern part of the Eastern Ghats Belt (EGB), expose an assembl... more Kondapalle area, situated in the southern part of the Eastern Ghats Belt (EGB), expose an assemblage of high grade rocks with protracted geological history of more than 1 billion years. Field and petrological studies in combination with the existing geochronological data indicate intermittent ...
American Journal of Science, 2002
The P-T-fluid histories of aluminous garnet؉clinopyroxene-bearing calc-silicate granulites are co... more The P-T-fluid histories of aluminous garnet؉clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO 2 -H 2 O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite-and wollastonitedominated reaction zones (scp؉cpx؎qtz,cal,qtz -wo؉cal؉cpx؎scp) developed between marble layers (cal؎cpx,wo,scp) and their host calc-silicate granulite (cpx؉pl؎qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz؉cal؉cpx؉scp؉grt؉v in the system CaO-FeO-Al 2 O 3 -SiO 2 -CO 2 it is demonstrated that at the ambient P-T-X fluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at "singular points" across which one or more phases change their sides with respect to garnet in a given reaction.
Journal of Southeast Asian Earth Sciences, 1996
Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofe... more Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures < 1000°C and were thus in thermal equilibrium with the country rock granulites during peak metamorphism. Comparison with experimental data on similar bulk compositions constrains the depth of emplacement of the magmas at 30-35 km. Geochemical characteristics indicate that the mafic magmas are essentially similar to continental flood basalts and have thus been generated in an extensional set-up. The apparent clockwise trajectory recorded in the Anakapalle granulites was produced by extension of the crust of near-normal thickness with concommitant basic magmatism.
Contributions To Mineralogy and Petrology, 1989
Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal gra... more Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields W Mg-MnGt(=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe≥0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+.
Journal of Metamorphic Geology, 2008
ABSTRACT A suite of garnet-wollastonite-scapolite-bearing calcsilicate granulites from the Easter... more ABSTRACT A suite of garnet-wollastonite-scapolite-bearing calcsilicate granulites from the Eastern Ghats has been investigated to document the controls of mineral reactions during the metamorphic evolution of the deep continental crust. The rocks studied show heterogeneity in modal mineralogy and phase compositions in millimetre-sized domains. Textural relations, and the compositional plots of the phases, established that the clinopyroxene exerts a strong influence on the formation and composition of garnet in the complex natural system. P-T estimates using the vapour-independent equilibria involving garnet define a near isobaric cooling path from c. 850C at c. 5.5–5.2 kbar. The deduced trajectory tallies well with the terminal segment of the overall retrograde P-T path construed from the associated rocks using well-calibrated thermobarometers. The ubiquitous occurrence of wollastonite and scapolite in the main calcsilicate body suggests low aCO2 during peak metamorphic condition. Fluid compositions constrained from mineral-fluid equilibria of the garnet-bearing assemblages show domainal variations as a function of the compositions of the solid phases, e.g. garnet and clinopyroxene. A quantitative log/CO2-log/O2 diagram has been constructed to depict the stability of the different calcsilicate assemblages as functions of the compositions and the behaviour of these fugitive species. The results of the mineral-fluid equilibria and the quantitative fluid/rock ratio calculations, in conjunction with the topological constraints, imply vapour-deficient meta-morphism in the rocks studied. It is argued that fO2 during peak metamorphism was monitored by the ambient fO2. Subsequently, during retrogression, different domains evolved independently, whereas the fluid composition was controlled by the mineral-fluid equilibria.
Journal of Metamorphic Geology, 2004
In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of... more In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of graphite-bearing calc-silicate granulites, veined by syenitic rocks, developed wollastonite-rich veins at 6–7 kbar and > 850 °C. During subsequent near-isobaric cooling wollastonite was replaced by calcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite with variable Al and F contents is present throughout the rock. Combining the compositional variation of titanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition of coexisting fluids and the mobility of Al exert a far greater control on the composition of titanite than pressure, temperature or the whole rock composition. Thermodynamically computed isothermal–isobaric logfO2– logfCO2 and logfF2– logfO2 grids in the systems Ca–Fe–Si–O–F (CISOF; calcite-free) and Ca–Fe–Si–O–F–C–H (CISOFV; calcite-present) demonstrate the influence of bulk rock and fluid compositions on the stability of the fluorite-bearing assemblages in diverse geological environments and resolve the problem of the stability of titanite in fayalite + fluorite-bearing rocks in the Adirondacks. The mineralogy of the studied rocks and the topological constraints tightly fix the logfO2, logfF2 and logfCO2 at −15.8, −30.6 and 4.1, respectively, at 6.5 kbar and c. 730 °C. Because of the similarity in the P–T conditions, the compositions of pore fluids in the fluorite-bearing assemblages of the Adirondacks and the Eastern Ghats Belt have been compared.
Journal of Metamorphic Geology, 2008
‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthit... more ‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum-, spinel- and sapphirine-bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite-sillimanite-cordierite gneisses of the Ihosy granulite unit in the NW of the Pan-African Bongolava-Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite-rich sediments or calcareous bauxites that were altered by Ca or Si infiltration-metasomatism prior to or coeval with the development of the anorthite-corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak-conditions around 800 °C and 6–7 kbar for the regional high-grade metamorphism and deformation in the NW part of the Bongolava-Ranotsara shear zone. The well-annealed granoblastic-polygonal textures indicate complete chemical and textural re-equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg-, Si- and K-bearing fluids into the anorthite-corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg-Al minerals, thereby producing corundum-, spinel- and sapphirine-bearing phlogopitites. Further advance of the resulting ‘residual’ Mg- and Si-bearing fluids into anorthite-corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava-Ranotsara ductile shear zone c. 530–500 Ma ago.
Contributions To Mineralogy and Petrology, 1991
A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statist... more A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe−Mg−Ca−Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe−Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW FeMgbi=1073±490 cal/mol. ΔH r and ΔS r for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.
Gondwana Research, 1999
In this paper, we compare tlie petrological histories of the Kemp Land Coast (east Antarctica) an... more In this paper, we compare tlie petrological histories of the Kemp Land Coast (east Antarctica) and Gokavaram area (Eastern Ghats), that were supposed to have been juxtaposed. The area around Gokavaram is dominated by different varieties of paragneisses (pelitic, quartzofeldspathic and calcareous composition) with relatively minor amounts of orthogneisses (mafic, enderbitic and granitic composition). The rocks were involved in three major phases of deformation and were finally affected by localised shear movement. On the basis of reaction textures, well preserved in high Mg-A1 granulites and calc-silicate granulites, and geothermobarometric data we deduce a polymetamorphic evolution of tlie rocks. Following an early M, metamorphism culminating at 9.2-9.4 kbar, > 950"C, the rocks cooled nearly isobarically down to 850°C. During a subsequent M, metamorphism, near isothermal decompression to 5-6 kbar occurred. This was followed by near isobaric cooling down to 600-650°C. M, is a weak ampliibolite facies overprint, largely restricted to late shears, which involved hydration as well. Available radiometric data from this area can be interpreted in terms of partial resetting of U-Pb systematics in older sphenes due to M, metamorphism at ca. 550 Ma. Despite tlie absence of sufficient isotopic data on tlie Eastern Ghats granulites, we document a remarkable similarity in the petrological history of the two supposedly erstwhile neighbours.
Journal of Petrology, 1999
A suite of quartz-and corundum-bearing metapelitic granulites, metamorphism occurred in the lower... more A suite of quartz-and corundum-bearing metapelitic granulites, metamorphism occurred in the lower crust (>8 kbar). Reported intruded by layered gabbronorite-pyroxenite-anorthosite at Kon-U-Pb cooling ages of monazite and allanite from a late pegmatite dapalle, Eastern Ghats Belt, preserves a multitude of reaction suggest the UHT metamorphism to be older than 1600 Ma. The textures involving oxide and silicate minerals that attest to several deduced P-T history and the absence of Grenvillian high-grade prograde and retrograde reactions. In the quartz-bearing associations, metamorphism in the study area provide important constraints on the reactions are: (a) biotite + sillimanite + quartz → garnet the configuration of East Gondwana, in particular on the continuation + liquid; (b) garnet + sillimanite → spinel (+ magnetite) + of the Napier-Rayner terrane boundary into the Eastern Ghats quartz; (c) Fe 2 TiO 4 + O 2 → ferrian ilmenite + magnetite; (d) Belt. reversal of reaction (b); (e) Fe 2 O 3 -rich ilmenite + plagioclase + quartz → Fe 2 O 3 -poor ilmenite + garnet + O 2 . Reactions in the corundum-bearing associations are: (f) Fe 2 TiO 4 + FeAl 2 O 4 + Belt, India; Indo-Antarctic terrane assembly O 2 → ferrian ilmenite + Fe 3 O 4 + Al 2 O 3 (in ilmenite); (j) garnet + corundum → spinel + sillimanite. To examine the paragenetic evolution of the metapelitic granulites, a petrogenetic grid for the KFMASH system at high temperatures and pressures, involving *
Journal of African Earth Sciences, 1996
Journal of Asian Earth Sciences, 2006
The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, e... more The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, eastern India, are folded into a series of large upright folds with variable plunges. The regional schistosity is axial-planar to the folds. The folds were produced by a second phase of deformation (D2) and were preceded by D1 deformation, which gave rise to isoclinal folds (mapped outside the study area) and the locally preserved, bedding-parallel schistosity. A shearing deformation during D2 was responsible for the sheath-like geometry of a major fold. The axial planes were curved by D3 warping. The first metamorphic episode (M1) of low-pressure type produced andalusite porphyroblasts prior to, or in the early stage of, D1 deformation. The main metamorphism (M2), responsible for the formation of chloritoid, kyanite, garnet and staurolite porphyroblasts, was late-to post-D2 in occurrence. The Staurolite isograd separates two zonal assemblages recorded in the highalumina and the low-alumina pelitic schists. Geothermobarometric calculations indicate the peak metamorphic temperature to be 550 8C at 5.5 kb. Fluid composition in the rocks before and during M2 metamorphism was buffered and fluid influx, if any, was not extensive enough to overcome the buffering capacity of the rocks. From M1 to M2, the P-T path is found to have a clockwise trajectory, that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting, followed by compressional deformation leading to loading and heating. q
Geological Society, London, Special Publications, 2003
... 2003; v. 206; p. 131-143 Geological Society, London, Special Publications Somnath Dasgupta an... more ... 2003; v. 206; p. 131-143 Geological Society, London, Special Publications Somnath Dasgupta and Pulak Sengupta ... Page 2. Indo-Antarctic Correlation: a perspective from the Eastern Ghats Granulite Belt, India SOMNATH DASGUPTA & PULAK SENGUPTA ...
American Mineralogist, 2009
... Razakamanana et al. 2000; Sengupta et al. 2004). In India, högbomite has been hitherto report... more ... Razakamanana et al. 2000; Sengupta et al. 2004). In India, högbomite has been hitherto reported only from four localities (Grew et al. 1987, 2003; Devaraju et al. 1981; Tsunogae and Santosh 2005). The origin of högbomite ...
Economic Geology, 1989
Fonomic Co/ogy Vol. 84, 1989, pp. 434-443 ... Mineral Reactions in Manganese Oxide Rocks: PTX P... more Fonomic Co/ogy Vol. 84, 1989, pp. 434-443 ... Mineral Reactions in Manganese Oxide Rocks: PTX Phase Relations ... SOMNATH DASGUPTA, PULAK SENGUPTA, 1 . K. BHATTACHARYA, M. MUKHERJEE, M. FUKUOKA*, H. BANERJEE, AND SUPRIYA ROY ...
Contributions To Mineralogy and Petrology, 2004
Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich peli... more Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich pelitic granulites is an important tool to unravel the P–T evolutionary history of high-grade rocks. In the Eastern Ghats Belt, India, several studies have been carried out on these fascinating aluminous granulites, and the results of these studies have revealed complex P–T–t histories (Dasgupta and Sengupta 1995; Sengupta et al. 1999; Rickers et al. 2001a, 2001b; Gupta et al. 1999; Dobmeier and Simmat 2002; Dobmeier and Raith 2003). In recent communication, Bhattacharya and Kar (2002) reported reaction textures from a suite of Mg-Al granulites from the Paderu area of the Eastern Ghats Belt. Combining the textural relations and thermodynamic calibration of some construed reactions, the authors have put forward a single phase metamorphic evolution of the area along a ‘clockwise’ pressure–temperature trajectory. Combining the petrological features from the Paderu area with those reported from the Chilka Lake complex, the authors proposed a general tectonic model for the entire Eastern Ghats Belt. Incidentally, the rocks in and around Paderu have been studied in some detail by several other workers (Lal et al. 1987; Mohan et al. 1997; Sengupta et al. 1997). The purpose of this comment is to demonstrate that the conclusions made in the paper are inconsistent with the petrological features described in the text. Further, the thermodynamic treatment used in the paper has serious errors in many places, and hence, is often in complete disagreement with the existing experimental data and theoretical analyses on the Mg-Al-rich assemblages. There are also significant problems arising from the poor quality of the analytical database. Unfortunately, the authors cite only a few published works (mostly their own) ignoring many other relevant studies from this belt (cited above). Our observations are organised according to the sections of the paper.
Ore Geology Reviews, 2011
The c.2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of d... more The c.2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6 m thick) within clinopyroxenite and highly calcic anorthosite (An 99-100 ) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at c.2.51 Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at c.2.50-2.45 Ga (granulite facies) and at 0.72-0.50 Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place. Chromite compositions show large variations (Cr# = 26-60, Mg# = 4-45, Fe 3+ # = 2-24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr# = 3-21; Mg# 45-61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe-Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean-Proterozoic boundary.
Journal of Geology, 1991
Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate ... more Mafic granulites occurring as conformable lenses within khondalite, leptynite, and calc-silicate rocks from the Anantagiri-Araku areas of the Eastern Ghats belt contain orthopyroxene, clinopyroxene, garnet, quartz, ilmenite, and plagioclase as major constituents. ...
Geological Journal, 1995
High Mg-Al granulites and calc-silicate granulites provide evidence for ultra-high temperatures o... more High Mg-Al granulites and calc-silicate granulites provide evidence for ultra-high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9–10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg-Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile-ilmenite or titanohaematite-ferrianilmenite. These high Mg-AI rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra-high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinel-cordierite-quartz and sapphirine-quartz and stabilization of the assemblages orthopyroxene-sillimanite-cordierite and spinel-quartz-sapphirine-garnet and (ii) the high Al2O3 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P-T path of evolution for the granulites. In calc-silicate granulites stabilization of nearly pure meionite and of the wollastonite-plagioclase-andradite-rich garnet, wollastonite-scapolite-grandite garnet-calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9–10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near-isobaric cooling followed by near-isothermal decompression. On the basis of the available evidence it appears that non-extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra-high temperature metamorphism on an anticlockwise path in the Eastern Ghats Belt.