Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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The granulite-gneiss terrain of Schirmacher region, East Antarctica includes mafic to acid granulites pervasively intruded by stringers, bands, lenses and stocks of felsic rocks. The late stage felsic rocks are the leucocratic rocks and are the focus of this paper. They have concordant to discordant relation with the associated felsic and other granulitic rocks of the region.

The fluid inclusions, show three stages of f1uid entrapment. The monophase CO₂ inclusions, with densities in the range of 0.996 to 0.811 g/cm³, show pressures of about 5.1 ± 0.6 kbars at inferred temperatures of 675 ± 25°C and indicate peak metamorphism after crystallization. Early stage deformed aqueous inclusions, however, represent initial crystallization. Entrapment of these inclusions was followed by carbonic aqueous inclusions, whose CO₂ densities range from 0.816 to 0.624 g/cm³ with increasing H₂O fluid entrapment occurring during the retrogressive stages. The increased concentration of aqueous fluids has resulted in biphase aqueous inclusions in trails. Their density and homogenization temperatures indicate a pressure of 2 ± 0.5 kbars.

The P-T trend observed from fluid inclusion studies is ref1ected from mineral equilibria studies. The GMBPQ assemblage of these rocks gave temperatures and pressures of 675 ± 25°C and 5.5 ± 0.3 kbars respectively. Further, the two-feldspar thermometry for these rocks record a reequilibrated temperatures of 444 ± 9°C. These P-T conditions of the leucogneisses suggest around 18 km of crust to have been removed, indicating > 50 km thick crust during the amphibolite facies metamorphism corresponding to geothermal gradients of around 37°C/km. A successive decrease in estimated P-T conditions along with density and compositional information suggests retrogression and compressional cooling path for these rocks.

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Detailed mineralogical studies of amphibolite to granulite facies transition zone rocks and granulite facies rocks in Dharmapuri district, Tamil Nadu, were carried out in order to know the gneiss-charnockite relations, nature of protolith, and metamorphic conditions.

The studies show that the rocks are of magmatic origin. and show a gradual decrease in modal composition of hornblende and biotite and gradual increase of hypersthene and iron oxide in a north to south traverse. Breakdown of hydrous minerals to anhydrous minerals is evident with the release of iron oxide suggesting progressive transformation of gneisses to charnockite, Biotite occurs in two generations, an earlier one is of massive type and breaks down to anhydrous minerals, while the latter one occurs as tabular flakes and likely to have formed during the waning stages of amphibolite to granulite facies metamorphism, and forms a zone of concentration just above the stage or CO₂ migration. The mineralogy of pyroxene, plagioclase, hornblende, biotite and iron oxides also suggests that the pressure-temperature conditions in the transition zone are lower than those of the granulite zone.

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The Wangtu Gneissic Complex (WGC) of Himachal Himalaya represents the basement rocks which remained more or less in its original realm. The unique feature of this complex is that the various phases of deformed and undeformed granitoids are exposed in the area. The complex has a peripheral zone of augen gneisses and core of non-foliated granitoids comprising of Coarse Porphyritic Granites (CPG) and Fine Grained Granites (FGG).

In the field the FGG rocks have sharp intrusive contacts with CPG rocks. At places they are entrapped or have protrusions into CPG rocks. Compositionally they represent syenogranites. They are peraluminous to meta-aluminous with Sr initial ratio of 0.705. The major and trace elements of these rocks show unimodal distribution pattern on different variation diagrams. They along with rare earth elements show marked fractionation trends, with a significant negative Eu anomaly. The rocks more or less define an isochron, representing Rb-Sr age of 1895±64 Ma.

The petrochemical studies favour a fractionated rather than multiple intrusions of magma. The studies suggest that fractional crystallisation of parental magma gave rise to early formed CPG rocks and the residual melts resulted in the crystallisation of FGG rocks. The last formed granitoids, the FGG rocks are interpreted as the final residue which crystallised rapidly after build-up and loss of fluids and volatiles which migrated to higher levels to form pegmatite phase in the aureol zone. The Rb-Sr systematics in the present case indicate that the granite have primary crystallisation melts derived from the deep crustal levels around 1895±64 Ma.

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Several granitic plutons were emplaced in the Lesser and Higher Himalayas of India during early Paleozoic. Jaspa pluton of Lahual region is one such pluton, where the granitoids are intrusive into the low-grade Haimantas made up of phyllitic and quartzitic rocks. The petrochemical studies of the Jaspa granitoids show two successive phases of intrusion. The early phase is dominated by Biotite-Muscovite Granite (BMG) of Ordovician age. Compositionally these rocks are similar to the biotite-granites of Shingo-La of Higher Himalayan range, which are around 50 km to the north of the BMG rocks. The younger phase is represented by tourmaline bearing leucogranites (TLG), which occur as minor phase generally along the marginal parts of the BMG rocks.

The BMG rocks in comparison to TLG rocks have relatively high concentrations of CaO, MgO, TiO₂, Fe₂O₃(T), Nb, Zr, Sr, Pb and Th and low concentrations of SiO₂, Na₂O, P₂O₅, Rb,Ga and U. They distinctly plot as two geochemical populations on different variation diagrams. The BMG rocks are characterised by enriched REE concentrations and show fractionating bends, while the TLG rocks have low REE concentrations and show less fractionated trends and depleted HREE pattern. The field, petrographic and geochemical studies suggest that the two phases of Jaspa pluton, the BMG and the TLG rocks were generated from the partial melting of the continental crust in which two chemically different source types were involved.

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The six point whole rock Rb-Sr isochron of the Tangmarg granite gives an age of 161 ± 5 Ma, with Sr ratio of 0.714. The small size of the pluton points to limited melt generated from a partial fusion 'event' of short duration. Such a melt is attributed to pressure release due to reactivation of the faults that were formed during Permo-Triassicrifting event.

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The Askot crystallines form a doubly plunging synformal belt and occurs as a detached crystalline belt or klippen in the vast sedimentary terrain lying between Central crystallines towards north and the Almora crystallines to the south. It is dominated by granite gneiss and augen gneiss, and also comprise of metapelites, migmatites and basic intrusives. In this paper, the geochemical studies of the granite gneiss and augen gneiss from the Askot crystallines, Kumaun Himalaya were carried out in order to understand their origin and evolution.

The granite gneiss is generally foliated, with less foliated and porphyritic variety seen in the core part. The K-feldspar shows Carlsbad twinning, while plagioclases show complex twinning. They show euhedral zircon and apatite along with titanite as accessory minerals. The granite gneiss is moderately evolved (Mg# ~50) and has granodiorite composition with metaluminous, calc-alkaline trends. They show higher concentration of Ti, Ca, Mg and low abundance of ΣREE (~165 ppm) in comparison to augen gneiss. They show volcanic arc signatures and compare well with Lateorogenic granites of Proterozoic times distributed world wide. These calc-alkaline granites appear derived from a Paleoproterozoic mafic/intermediate lower-crust reservoir probably involving arc magma underplating. Granite gneiss is also peraluminous with molar A/CNK>1.1, and the heterogeneity of granite gneiss can be explained with the precursor melts, experiencing assimilation during up-rise through crust or contamination of source itself involving sediments from the subduction zone.

The augen gneiss is more evolved (Mg# ~18) and show granite composition. They show megacrysts of perthites in a fine-to medium-grained matrix of feldspars and micas. The REE pattern of the augen gneiss shows much wide compositional variation (ΣREE ~171 ppm) than granite gneiss. It shows syn- to post-orogenic environment and derivation from the partial melting of an upper crustal source. Existing Rb-Sr isotopic data suggest that the granite gneiss defines an isochron age of ~1700-1800 Ma with a Sri ratio of ~0.71, while the augen gneiss defines an age of ~1300 Ma with much evolved Sr_i ratio (~1.65). The dominance of granite gneiss in the eastern Kumaun region suggests the production of heterogeneous granitic melts similar to those of Askot crystallines as an important event of crustal growth during Late Paleoproterozoic period in the region.

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