Mineragraphic studies of the chromites from Nuggihalli schist belt, Dharwar Craton, Southern India

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The chromites of Nuggihalli schist belt are part of layered ultramafic-mafic sequence. The chromite occurs as layered bodies, lenses and pods. Based on the field, petrography and mineragraphic characters the chromites have been classified into different types. The different configuration of chromites within the ultramafic-mafic rocks is related to multistage deformation processes of the schist belt and not to any magmatic processes. However, the chromite deposits of Nuggihalli schist belt are in general cumulates derived from Mg rich basaltic magma.

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Geochemical data are presented of an anorthosite dyke from Nuggihalli composed of calcic plagioclase (An70-80), amphibole, pyroxene and accessories magnetite, sphene and apatite. -T.R.
207Pb/206Pb ages of individual zircon grains from metasediments and tonalite trondhjemitic and granitic gneisses in the Nuggihalli schist belt, Dharwar craton are determined by an ion microprobe. Zircons identified in a metasedimentary rock have ages up to ∼3.2 Ga, suggesting an upper limit age of 3.3 Ga for the onset of sedimentation in this region. The ages of the gneissic protoliths are ∼3.1 Ga and they appear to be nearly contemporaneous with the metasedimentary protoliths. Our data and those reported previously are indicative of multiple emplacement of the gneissic precursors, and suggest an episodic evolution of the Dharwar craton over an extended period during the early Archean.
Siliceous-aluminous schists are common in the lower succession of the greenstone belts of India, Africa and other shield areas; they overlie basic-ultrabasic flows. The presence of associated bedded barites and secondary fuchsite quartzite convincingly corroborates their sedimentary origin. In the Dharwar craton they are found in the Sargur group as sillimanite-kyanite-staurolite-micaceous-corundum-quartz schists and in the Bababudan-Chitradurga groups as sericitic phyllites. The Sargur group metasediments vary in composition from highly Al (K + Ti) to Al (Mg + Fe + Ti + Cr + Ni)-rich varieties. They are depleted in Si and K-group elements like Rb and Sr, and enriched in Al, Ti, Mg, Fe, Cr and Ni compared with Bababudan-Chitradurga group metasediments and surrounding tonalitic gneisses. The evidence of a mixed source provenance (tonalites, trondhjemites, basalts and sediments) is preserved in the composition of the sediments of the Bababudan and Chitrudurga groups. A similar source is not expected to provide the material for the Sargur group sediments which require a source enriched in Al, Mg, Fe, Ca, Co, Cr and Ti, and depleted in Si and K-group elements. Archaean crust, made up of 60% low-K tholeiites, 30% high-Ti and Al anorthosites and 10% ultramafics like norite, trcctolite and peridotite, may give rise to such sediments. Data from the silicate planets indicate their identical initial evolutionary history. The lunar crust, whose evolution was arrested c. 4–3 by ago, is mainly made up of basalts and anorthosites. Therefore, it is possible that the results of the early exogenic process at the lunar type of earth's crust are preserved as metasediments in the lower parts of the supracrustals of great antiquity. Absence of primary quartzites in the early-middle Archaean, their relatively low to moderate abundance in the early Proterozoic and their prominence since the middle Proterozoic probaDly suggests that the change from an ANT-rich basaltic primordial crust to a quartz-rich granitic crust was between 4.0 ana 2.0 by ago.
Data on aspects of Archean shield geology are consistent with, although not necessarily indicative of, impact of extraterrestrial matter, possibly contemporaneous with impacts that affected the Moon about 4.1 to 3.8 × 109 yr ago. The isotopically oldest gneisses of several shield areas abound in xenoliths of ultramafic-mafic volcanic rocks, minor silicic volcanic rocks, and derived and (or) chemical sedimentary rocks. No confident oldest age limits were set on these xenoliths, which antedate the development of the true greenstone belts in these areas and are thought to represent relics of a once-widespread volcanic crust. This crust differed from modern oceanic crust in the following ways: (1) peridotitic, high-Mg, and silicic volcanic rocks were more abundant in the Archean crust than in the modern crust; (2) sea-floor spreading and crustal overturn in early Archean time should have resulted in the rapid development of large continents at that stage, contrary to observations; and (3) the apparent retention of a coherent parallel Archean tectonic pattern on Gondwanaland reconstructions renders large-scale plate movements, and thus sea-floor spreading, unlikely. Had the early Archean ultramafic-mafic crust been genetically related to impact, the recurrence of chemically similar volcanic rocks within greenstone belts up to about 2.6 × 109 yr ago may signify repetition and gradual waning of mantle activity initially triggered by such events. Evidence bearing on the impact theory may be gained by a search for shock metamorphic effects in the oldest agmatites and in fragments and detrital grains incorporated in the earliest sedimentary intercalations of volcanic sequences.