Geology Geochemistery and Mineralogy of Mafic-Ultramafic, Gneissic and cordierite Gneiss Enclaves of the south- eastern sector karnataka craton.

Abstract

This study attempts to decipher the relationships that may exist among the major crustal components in the Eastern Block of Karnataka Craton. Field traverses were undertaken to study the little-known enclaves of (a) mafic-ultramafic rocks and cordierite gneisses which may represent the Sargur Group, and (b) migmatitic gneisses possibly representing Peninsular Gneisses occurring within the younger granitic rocks of the Eastern Block. The geologic setting of the Kolar Greenstone Belt and the intrusive Patna granite seem to correspond to the Chitradurga schist belt representing the Dharwar Supergroup and the intrusive Chitradurga granite. The field observations supplemented with the published Rb/Sr whole-rock isochron ages for the migmatite gneiss, cordierite gneiss enclaves, and the granitic rock have enabled the establishment of the relationship between these crustal components. The chronostratigraphic relations between the major crustal components of the migmatite terrain of the south-eastern sector are summarised: Chronostratigraphy

AgeLithologic Units2600 MaHomogeneous granites of Bangalore2650 MaPatna and Closepet Granite; Kolar Greenstone Belt (Dharwar Supergroup); Migmatitic gneisses of Bangalore3010 MaCordierite-bearing gneisses (Sargur Group) This correlates with the lithostratigraphic relations provided for the Western Block. The pre-metamorphic history of the enclaves of the cordierite gneiss, mafic-ultramafic rocks, and migmatitic gneisses within the younger granitic rocks has been elucidated. The Sr-isotope data suggest that the amphibolites, migmatitic gneisses, and the cordierite-bearing rocks record a tectono-thermal metamorphic event around 3000 Ma. The peak metamorphic conditions are 5.2 kb and 710°C. This corresponds to a depth of about 25 km, indicating that the migmatite terrain presents a moderately deep section of the Archaean crust. The pre-metamorphic sedimentary origin of the major crustal components has been resolved with critical trends of chemical variation. The detailed geochemical investigations indicate that the cordierite-bearing gneisses are paragneisses in nature while the mafic-ultramafic enclaves are of igneous parentage. The migmatitic gneisses with high initial Sr-isotope ratios (0.7054) and REE abundance levels suggest that they are products of crustal anatexis. The granitic rocks from the migmatite quarries of Bangalore with initial Sr-isotope ratios not exceeding 0.7035 and highly fractionated REE patterns suggest that they are derived from an Archaean mantle source. On the other hand, the granitic rocks of Patna and Closepet with high REE abundance levels and high Sr-isotope ratios indicate that they are products of partial melting of Peninsular Gneisses. Mineral chemistry investigations of calcic amphiboles from mafic rocks, alkali feldspars from granitic rocks, and cordierites from cordierite gneisses were undertaken to understand the conditions of their formation. The calcic amphiboles investigated show a decrease in Si from 7.2 to 6.4, indicating the role of fluid pressure in their formation. Further, the narrow range of Al(IV)+Na(M4) values and increasing Al(VI)-Na(M4) values displayed by the amphiboles indicate the role of pressure rather than temperature during their formation. The alkali feldspars encountered in the granitic rocks of Bangalore show textural evidence for the triclinisation of untwinned orthoclases. X-ray analysis indicates the presence of both monoclinic and triclinic feldspars. The alkali feldspar from the pegmatite phase of these intrusive granitic rocks is highly triclinic. The pegmatites, being the end products of magmatic fractionation rich in hydrothermal fluids, develop only microclines of high triclinicity as revealed by the investigation of Parsons (1978). On the other hand, the textural and X-ray analyses of alkali feldspar from Patna and Closepet granites suggest that the K-feldspar in them is formed due to microclinisation of plagioclase. The detailed investigations of the cordierite minerals using chemical, optical, X-ray, and IR techniques reveal that the distortion index values do not depend entirely on the composition alone. The observed distortion indices in the cordierite grains do provide a measure for Al/Si order. The study of channel constituents in the cordierites has been meaningfully used as fugacity indicators in rocks. The present study reveals that one set of cordierite-bearing gneisses from the migmatite terrain is formed dominantly under H?O-rich conditions, while the other set associated with charnockitic rocks is formed under dominant CO? conditions, similar to the contemporaneous formation of Closepet granite and charnockites as reported by earlier investigators.