Abstract:The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses, middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons. The focus of the present study is mainly on basement TTG gneisses and a granite pluton (~ 240 sq km areal extent), to discuss crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase, lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende. Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized by higher SiO 2 , high Al 2 O 3 , and Na 2 O, low TiO 2 , Mg#, CaO, K 2 O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics of transitional TTGs in its high LILE, and progressive increase in K 2 O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate total REE contents with fractionated REE patterns (La/Yb N =17.73-61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/Yb N = 9.06-67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO, Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC.

In this paper we document widespread coeval felsic-mafic magma interaction and progressive hybridization near Gurgunta in the northern part of Eastern Dharwar Craton (EDC) where mafic magma pulses have injected into a 2.5 Ga granite pluton. The pluton contains voluminous pink porphyritic facies with minor equigranular grey facies. The mafic body shows compositional variation from diorite to meladiorite with hornblende as the chief mafic mineral with lesser clinopyroxene and biotite. The observed variation on binary diagrams suggests that granite was evolved by fractional crystallization. Chemical characteristics such as higher Al 2 O 3 and moderate to high CaO, Mg#, Ni, Cr, Co and V are interpreted by slab-melting. Mafic bodies show lower SiO 2 , Na 2 O and K 2 O; but higher CaO, Mg#, FeO, Cr, Ni and V; higher LREE with moderate to higher HREE which suggest their derivation from mantle. A major active shear zone has played an important role at the time of synplutonic mafic injection and hybridization process.Field evidences suggest that the synplutonic mafic body has injected into the crystallizing felsic magma chamber in successive stages. The first stage injection has resulted in extensive mixing and hybridization due to the liquidus state of resident felsic magma to which hot mafic magma was injected. However, progressive mixing produced heterogeneity as the xenocrysts started mechanically dispersed into hybrid magma. The second stage injection, after a time gap, encountered colder and viscous hybrid magma in the magma chamber, which inhibited free injection. As a consequence, the mafic magma spread into magma chamber as flows, producing massive mafic bodies. However, with the continued mafic pulses and the heat gradient, the viscosity contrasts of mafic magma and felsic magma were again lowered resulting in second stage mixing. This episode was followed by mingling when the granite was almost crystallized, but still viscous enough to accommodate lamellar and ribbon like mafic penetrations to produce mingling. The successive mixing and mingling processes account for the observed heterogeneity in the granite pluton.

Palaeoarchaean (3.38-3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450-1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite-tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46-42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94-16.18 wt.% MgO) extending into basaltic (7.71 -10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al 2 O 3 and (Gd/Yb) N ratios show that the Al-depleted komatiite magmas were generated at great depth (350-400 km) by 40-50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6-3.8 Ga.ARTICLE HISTORY

Brittle to semi-brittle shear zones near Gogalgatti, Mincheri and Yerjanti villages in the northern part of Eastern Dharwar craton show sporadic sulfide and gold occurrences. These shear zones are confined to a late Archaean (2.5 Ga) granitoid complex, consisting of TTG gneisses and granite plutons. Intense mylonitization, brecciation and fracturing are conspicuous in all the three shear zones. The mineralization is of disseminated type and the ore minerals are confined to fracture filling quartz veins. The veins are mainly parallel to the shear zones with minor veins showing cross- cutting relationship. The observed sulfide minerals in decreasing order of abundance are pyrite, chalcopyrite, arsenopyrite, tennantite and sphalerite with minor oxide minerals represented by specular hematite. Random chip sampling in Gogalgatti shear zone gave gold values from 0.52 to 0.65 ppm. However, similar sampling from Mincheri has not indicated any gold, though one of the polished sections of the samples from this area showed an exsolved gold globule within arsenopyrite. In the mineralized veins of these shear zones, primary inclusions of both type I (H2O) and type II (H2O-CO2) are more conspicuous. Secondary inclusions, though present, are mostly affected by deformation. Sulfide minerals are associated with moderate to high saline, type I fluids formed in the early stage of mineralization in all the three shear zones, whereas gold precipitated from high saline (28 -30 wt% NaCl eqvi. in Gogalgatti and ~29% wt% NaCl eqvi. in Mincheri) type II fluids in the later stages, in Gogalgatti and Mincheri shear zones. The temperature of fluid ascent has been determined to be 100 - 200° C in epithermal conditions of mineralization at a depth of 150 to 200 m. Granitoids, in which these shear zones are located, form the main source of hydrothermal fluids, which ascented and precipitated in the structural domains.

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