The enigmatic Archaean-Proterozoic transition: an exposed crustal cross-section in the Southern Granulite Terrane, Tamil Nadu, India

Abstract

The end of the Archaean, at 2.5 Ga, was marked by a major geochemical and geodynamic change in the evolution of the Earth-plate tectonics became dominant due to cooling of the Earth, mantle-derived magmas became less magnesian and their REEs more fractionated, crustal recycling became a factor in felsic magmatism, and there was a sharp increase in atmospheric oxygen and rates of weathering. Terranes preserving rocks formed during this transition are relatively rare, however. A notable exception is the Southern Granulite Terrane (SGT) of India, where an association of c. 2.5 Ga charnockites, gabbros, ultramafic complexes and felsic gneisses is exposed over an area of more than 25,000 km2. Through a close study of the major rock types throughout the area utilising field relationships, petrography, major and minor element geochemistry, U-Pb geochronology, Hf and O isotopes, fluid inclusions and P-T indicators it has been possible to address numerous burning questions of petrogenesis in this cross section of late Archaean to early Proterozoic crust.

The questions addressed in this thesis are regarding the petrogenesis of charnockite and its relationship to the other rock types in the region, the source of the different rock types and their geochemical and geochronological relationship and the later evolution of the terrane in terms of pressure and temperature conditions.

The U-Pb geochronology and P-T estimations show that the rocks in the SGT were mostly emplaced between c. 2.7 and 2.6 Ga, followed by their exposure to high pressure conditions at c. 2.5 Ga and subsequent decompression between c. 2.4 and 2.3 Ga.

Charnockites from the Kolli Hills, in the east of the SGT, have chemical and isotopic signatures indicative of formation at moderate to high temperatures and pressures from mantle sources. A major debate concerning the origin of massive charnockites can now be resolved- the various analytical techniques applied in this study point towards charnockitization of TTGs by CO2 fluxing at c. 2.5 Ga.

The Bhavani region, in the central SGT, contains abundant gabbro and minor granite. Hf isotopes are consistent with a mantle source, although slightly elevated zircon delta18O indicates some crustal involvement in the magma genesis. There is extensive evidence for decompression, late hydration and fluid fluxing. The source magma of the c. 800 Ma rocks in this region also evolved from the depleted mantle.

A layered complex in the Mahadevi region, consisting of mafic granulite with pyroxenite, felsic orthogneiss and BIF, preserves evidence of early Palaeozoic retrogression, but in addition evidence for mid Neoproterozoic (c. 800 Ma) high-grade metamorphism.

Large rafts of supracrustal rocks in younger granites from the Tiruchengode region, east central SGT, provided samples from the lower crust and further evidence for the mid Neoproterozoic event, as well as early Palaeozoic metamorphism. The mid Neoproterozoic calc-alkaline magmatism that produced those younger granites is a distinct event, unrelated to the Archaean rocks.

The picture emerging is of a complex Archaean crustal cross-section which is deformed and disaggregated. The major finding of this study is the particularly uniform Hf isotopic values in almost all the samples studied, suggesting similar source material for all the different rock types in the region. Considering the expanse of the study area, the complex petrogenetic history and the high P-T conditions, the uniformity of the Hf isotopic signature is remarkable. The study area is a juvenile crust with no involvement from Dharwar supracrustals. This suggests that huge volumes of magma were extracted over the Archaean-Proterozoic transition period. . Finally, the study area is not metamorphosed to granulite facies, but retrogressed from high-pressure conditions to upper-amphibolite facies. Show more