Petrology and geochemistry of the precambrian rocks around Ajagarh hills Agia Goalpara district Assam

Abstract

The Ajagarh hills area in Goalpara District, Assam predominantly consists of basic granulites, generally hornblendepyroxene and pyroxene-bearing variety. Quartzofeldspathic (occasionally migmatitic and garnetiferous variety), garnetiferous metapelites, scapolite- bearing calc-silicate gneisses and amphibolites (restricted to the northern part) and retrograded granulites (in the proximity of shear zones) are present in minor amounts. This thesis deals with petrology of the area, studied by means of petrographic, mineral-chemical and whole rock-chemical methods supplemented by geothermobarometric and fluid barometric data. The rocks are believed to represent a volcano-sedimentary sequence affected by high grade metamorphic event during earliest compressional deformational episode (F1 folding) as a consequence of emplacement of basic magma synchronous of F1 folding. Trace and REE elements study indicates that the earliest magmatic rocks which were completely amphibolites) possess a continental character and represent the basaltic melt derived from low melting of upper mantle. The petrographic data and the structural characteristics of the rocks show that the amphibolite facies assemblages in the north and granulites in the south, metamorphosed under one single deformational episode although, a gradual change in terms of metamorphic grade is evident from mineral paragenesis as well as from TiO2-content in biotite, Ca-content in plagioclase and edenite component of amphiboles. Electron probe analyses of coexisting minerals indicate an attainment of metamorphic equilibrium. P-T-PH20 conditions of metamorphic equilibration are estimated by means of different types of mineral geothermometers, geobarometers and fluid barometers in combination with a comparison of relevant experimental data. Resulting estimates for the peak metamorphic event are T-6500C, P-6 kbar (average) for amphibolite zone and T-7500C, P-7 kbar (average) for granulite zone with a variable PH2O. The retrogressed metamorphic event which preceded...