Somnath Dasgupta

A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites

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 withWFeMgbi=1073±490 cal/mol. ΔHrand ΔSrfor 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.

Exhumation history of a section of the Sikkim Himalayas, India: records in the metamorphic mineral equilibria and compositional zoning of garnet

The exhumation history of pelitic migmatite samples from the High Himalayan Crystalline Complex (HHC) near the South Tibetian Detachment System in the Sikkim-Darjeeling section has been determined on the basis of thermo-barometric analyses, retrograde breakdown reactions and compositional zoning of garnet. The peak metamorphic condition is estimated to be ∼10.4 kbar, 800°C from thermo-barometric and phase equilibrium constraints. The observed retrograde breakdown of garnet to spinel and cordierite requires near isothermal and, hence, extremely rapid (∼15 mm/yr) exhumation up to the depth of ∼15 km. Numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower process, ∼2 mm/yr, up to at least ∼5 km depth, to lead to the development of the observed compositional zoning of garnet. The dramatic change of the exhumation velocity (Vz) might reflect a process of tectonic thinning followed by erosion and/or horizontal flow at shallow depth. Assuming that the Vz did not change significantly below 5 km depth, these results suggest that the HHC samples studied in this work exhumed from a depth of ∼34 km within ∼8 Ma.

India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India

We present zircon and monazite U-Pb data from ultrahigh-temperature (UHT) metamorphosed orthogneisses and paragneisses collected from key areas of the Eastern Ghats Belt, India. The results show contrasting tectonothermal histories in different isotopic domains of the Eastern Ghats Belt that were identified by previous workers. Of particular importance is the discovery of a ca. 1760 Ma event (concordia age) in the southern domain 1A, which is interpreted to be the age of an early UHT metamorphism event. This was followed by a second granulite-facies metamorphism event and partial melting at ca. 1600 Ma. This domain was presumably cratonized with India at around 1600 Ma. The record of the ca. 1760–1600 Ma events in domain 1A of the Eastern Ghats Belt allows us to speculate on modeling the Paleoproterozoic–Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and parts of India. The central part of Eastern Ghats Belt (isotopic domain 2), on the other hand, contains zircons showing inherited ages of ca. 1880–1700 Ma, with a concordant age group of ca. 1760 Ma. Moderately to strongly discordant ages in the time span of ca. 1600–1100 Ma in domain 2 are interpreted to be mixing ages as a result of strong overprint of a ca. 1030–900 Ma tectonothermal event(s) that affected this domain. An early UHT metamorphism event in this domain is inferred to have occurred at ca. 1030–990 Ma (chemical dating of included monazite grains). Zircon records the most pervasive tectonothermal event in this domain at ca. 980–900 Ma, which is correlative with the Rayner orogeny in East Antarctica as a part of the formation of Rodinia.

Indo-Antarctic correlation: a perspective from the Eastern Ghats granulite belt, India

Abstract Available lithological, petrological and geochronological data on the rocks of the Eastern Ghats Belt (EGB), which has a key role to play in any model of Indo-Antarctic correlation in Precambrian and Cambrian times, have been synthesized and interpreted. Longitudinal lithological subdivisions of the belt are mostly not supported by recent geochronological data, which rather suggest a fourfold division based on protolith ages of the ortho- and paragneisses. Mineral ages show prominent tectonothermal events at Mesoproterozoic, Grenvillian and Pan-African times. However, not all of the imprints of all the events are preserved in all the geochronological domains. In the most well-studied Domain II, three phases of metamorphism are recorded — an early ultra high-temperature metamorphism with an anticlockwise P-T path, a second granulite facies event at c. 1000 Ma, with a retrograde trajectory of near-isothermal decompression, and a third of amphibolite facies. The available information, however, is not conclusive as to whether or not the entire EGB experienced polyphase UHT metamorphism. The propagation of the Grenvillian orogenic front in the EGB is identified.

Petrology of an inverted Barrovian sequence of metapelites in Sikkim Himalaya, India: Constraints on the tectonics of inversion

The lesser Himalayan sequence in Sikkim exposes an unusually complete inverted Barrovian sequence of metapelitic rocks. A number of different bulk compositions are interlayered along the prograde sequence, providing an excellent natural laboratory for studying the controls of pressure, temperature and composition on stability of mineral assemblages. Using three different approaches for determining pressures and temperatures, and making use of the bulk chemical constraints, we show that all three methods yield consistent estimates of pressures and temperatures once the strengths and weaknesses inherent to each approach are critically evaluated. The metamorphic field gradient along two separate traverses are found to be positive (60 °C/kbar in east Sikkim and 70 °C/kbar in north Sikkim) - pressure as well as temperature increases continually up to the highest grade. The various bulk compositions allow different assemblages to be used to establish the coherence of the sequence. In combination these place tight constraints on the permissible tectonic mechanisms for the production of this inverted metamorphic sequence. Neither older hot iron type models, nor more recent models that invoke thrusting (post- or syn- metamorphic) as a mechanism of inversion can produce such a coherent package inverted in pressure as well as temperature. On the other hand, the observations are more consistent with the predictions of channel flow type tectonics. In particular, a recent geodynamic model of subduction-collision that takes into account high heat generation (as found in these Himalayan metapelites) and incorporates the effects of the resulting melting (reduction of viscosity and enhancement of buoyancy) on the tectonic evolution, predicts such inversion of coherent blocks as an unavoidable consequence. Such melting triggered inversion and exhumation as a coherent, fault bounded block explains many features of the inverted sequence in Sikkim and it is not necessary to invoke inversion by multiple thrusting events.

Stability of osumilite coexisting with spinel solid solution in metapelitic granulites at high oxygen fugacity

Abstract Experiments were carried out in the quartz-saturated part of the system KFMASH at hematitemagnetite buffer conditions in a piston-cylinder apparatus using synthetic biotite, K-feldspar, quartz, and sillimanite80-quartz20 gel. Natural garnet and sillimanite were used as seeds. Three bulk compositions were used: XMg = 0.81, 0.72, and 0.53, and the experiments were vapor absent. In the bulk composition with the highest XMg, dehydration-melting of biotite produced osumilite coexisting with spinel solid solution within the P-T window 7-8.5 kbar and 850-1000 °C. However, this mineral pair coexisted with cordierite at P < 7.5 kbar, and with sillimanite above 7.5 kbar. There is no appreciable change in the lower thermal stability of osumilite with lowering of XMg in the bulk composition, but both the upper pressure and upper thermal stabilities are reduced. With increasing pressure, osumilite + spinel breaks down to an orthopyroxene + sillimanite assemblage. There is no overlap of the stability fields of osumilite and sapphirine. Rather, sapphirine-bearing assemblages are produced at the expense of osumilite with increasing temperature. Sapphirine, however, did not appear in the bulk composition having XMg = 0.53. Instead, a wide stability field of spinel + cordierite + orthopyroxene + sillimanite is noted at P > 7 kbar and T > 950 °C. Garnet + orthopyroxene + sillimanite become stable at lower pressures with decreasing XMg in the bulk composition. The effects of additional components in natural assemblages, such as Zn in spinel and F and Ti in biotite, on the KFMASH equilibria are evaluated qualitatively. The experimental results were applied to six high-grade terranes, where osumilite + spinel coexistence at peak metamorphic conditions has been reported. The deduced P-T window for the osumilite + spinel association is entirely consistent with independent PT estimates for the natural occurrences. It is argued that osumilite + spinel assemblage should be characteristic of metamorphism of highly magnesian pelites at high fO₂ along a prograde path of high dT/dP.

Mixing behavior in quaternary garnet solid solution and an extended Ellis and Green garnet-clinopyroxene geothermometer

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 WMg-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+.

Very high density CO2 associated with ultrahigh-temperature metamorphism in the Eastern Ghats granulite belt, India

Spinel-bearing high Mg-Al granulites of the Vizianagram area in the Eastern Ghats granulite belt show textural features clearly establishing that the association spinel ss + quartz + Fe-Ti oxide solid solution ± sillimanite ± porphyroblastic orthopyroxene was stable during peak metamorphic conditions. Pressure-temperature (P-T) conditions estimated from both mineralogical thermobarometry and phase-equilibrium limitations indicate that the peak metamorphism occurred under ultrahigh-T conditions (>1000°C) at 8-9 kbar pressure. Retrograde P-T conditions of 750-800°C, 6-7 kbar are deduced from the compositions of coronal garnet and orthopyroxene, which have rims of spinel against quartz, indicating significant cooling with slight lowering of pressure. Quartz associated with the ultrahigh-T assemblage at Vizianagram contains ubiquitous single-phase carbonic inclusions as isolated clusters that belong to two categories. Group I shows extremely high density (homogenization temperature: −51±1.8°C; density 1.15 g/cm 3 ) and group II trapped relatively lower density fluids (homogenization temperature: −18.4±2.4°C; density 1.05 g/cm 3 ). The iso chores for group I inclusions pass through the peak metamorphic P-T conditions, whereas those for group II coincide with the P-T conditions of the formation of coronal garnet and orthopyroxene. Our study is the first report of very high density CO 2 associated with the Eastern Ghats granulite belt rocks and provides a strong case for the presence of CO 2 -rich fluids during ultrahigh-T metamorphism at lower crustal levels.

Mafic Granulites from the Eastern Ghats, India: Further Evidence for Extremely High Temperature Crustal Metamorphism

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. Orthopyroxene contains (100) lamellae of clinopyroxene, and clinopyroxene contains (100) orthopyroxene lamellae and (001) spindles of pigeonite. Reaction textures indicate formation of garnet through coupling of two reactions, viz. orthopyroxene + anorthite = garnet + clinopyroxene + quartz and anorthite + ilmenite -