Differentiation of Late Archean Crust in the Eastern Dharwar Craton, Krishnagiri-Salem Area, South India

Abstract

The Late Archean crust south of Krishnagiri, Tamil Nadu, consists of tonalitic-trondhjemitic-granodioritic (TTG) gneisses with mafic and sedimentary enclaves, formed between 2.7 and 2.5 Ga and metamorphosed at amphibolite facies in the north to granulite facies in the south close to 2.5 Ga. Migmatization occurred at all grades, and numerous small granite bodies were emplaced near the amphibolite-to-granulite facies horizon. This nearly syn-accretion meta-morphism affected the entire crust and left a chemically differentiated section later exposed by uplift and erosion. Detailed chemical and petrographic study of >60 samples across a 90 km traverse provides evidence for an essentially unbroken crustal cross-section: Paleopressures range from 4 kbar (corresponding to 12-14 km paleodepth) in the north to 8 kbar (corresponding to 24-28 km paleodepth) to the south. Corresponding paleotemperatures vary between 650° and 800°C across the section. Mineralogic grade monitors, particularly increasing content of biotite, vary continuously southward and dominantly reflect progressive decrease of activity. The southward succession of index minerals in quartzfeldspathic rocks, hornblende clinopyroxene orthopyroxene garnet, is a consequence of continuously increasing dryness, temperature, and pressure. LIL elements Rb, Ba, K, and Th are continuously depleted from north to south. This pattern is a consequence of arrested upward movement of LIL elements and volatiles in granitic magmas and/or fluids during Late Archean metamorphism. There are many indications that a metamorphic fluid of low activity was an important agent in the crustal differentiation. ( 1 ) Thin lenses of granite and granodiorite are severely depleted in Rb in the biotite-rich charnockite zone. (2) Abundant synmetamorphic quartz veins containing orthopyroxene cannot be magmatic segregates but must have been deposited by fluids of low Because of extreme insolubility of silica in pure fluids, concentrated chloride/carbonate brines (essentially supercritical salt magmas) should be considered. An immiscible nearly pure phase, evidenced by fluid inclusions, is inadequate to account for the geochemical differentiation. (3) Pervasive networks of high-Ba potassium feldspar microveins in all of the Rb-depleted Shevaroy Hills charnockites are texturally in equilibrium with orthopyroxene and garnet and represent the deposits of migrating grain-boundary fluids exchanging alkalis with plagioclase. (4) Hematite-rich ilmenite and iron-poor orthopyroxene in these charnockites strongly suggest interaction with volatile oxidants including , and . The consistent mineralogical and geochemical profiles across the Krishnagiri-Salem traverse record the joint action of upward-migrating magmas and fluids in Archean crustal differentiation. Voluminous deep-crustal infusion of volatile- and alkali-rich basalts, liberated by remelting of enriched lithospheric mantle shortly after primary crustal accretion is hypothesized. We infer that processes of crustal evolution in the Late Archean may not have been much different from those of post-Archean times.