Metamorphism, Crustal Evolution and Amalgamation of Meso- to Neo-Archaean Greenstone-Granite Craton and Granulite-Gneiss Terrane in Rajasthan

Abstract

The studies in this extensive Meso- to Neo-Archaean cratonic part of the MC and granulite terrane of the SC of Rajasthan has documented two sequences of largely reworked and dismembered greenstones (Sequence-I and Sequence-II) which contain terranes of high grade granulite-gneiss. 25 K scale geological mapping in the MC-SC craton-terrane of BSG identified discrete lithological boundaries of the SC and various lithopackages of the MC which are shown to have been bound by prominent ductile shear or thrust zones having converging and diverging shear zone walls with independently developed tectonic grains, indicating amalgamation of different blocks at a later deformational (D2-F2 to probably D3-F3) event. The tectonostratigraphy of dismembered greenstone sequences, identified within the cratonic MC, initiate with Asan Group lithologies of magmatic mafic-felsic banded gneisses (Bimodal Gneiss) forming basement to the greenstones, followed tectonically upwards by ultramafic-mafic-chert-BIF (BMQ), migmatitic gneisses of individual protolith entities, mica schist, quartzite, calc gneiss and tuff (graphite schist) forming the lower Greenstone Sequence-I (called Sawadri Group; 3.2–2.8 Ga), and finally the tectonically overlying upper Greenstone Sequence-II (called Tanwan Group; 2.8 Ga) containing fuchsite quartzite-chert, mafic rocks as amphibolite, marble and calc gneiss, metagreywacke, sillimanite-staurolite-mica schist and gneiss. The greenstone sequences are metamorphosed up to amphibolite facies, and individual rock groups are tectonically bound and stitched together through discordant (NNE-SSW to NE-SW trending with high dip angle) regional ductile shear zones (50 m–0.5 km thick) or thrust zones of mylonitic to ultramylonitic rocks. Deformation, metamorphism and syn to post kinematic, polygenetic igneous complex consisting of tonalite-granodiorite, granite gneiss, porphyritic granite plutons and norite dykes (called Ran Igneous Complex; 2.5–1.72 Ga) reworked the greenstone sequences. The granulite gneiss terrane of the SC is represented by high grade metamorphosed package of pelitic granulite with interlaminated quartzite, calc granulites, intruded by enderbite-charnockite (now metamorphosed), two pyroxene granulite. A rapakivi bearing charnockite magma type (CMT) rock has intruded along major NNE-SSW trending transpressional shear zone bounding the high grade granulite gneiss terrane near Bhinai-Bandanwara area (Ajmer dist.). It is suggested that the range of lithologies deposited over the initial crust (AsanGroup) are fragments of a greenstone-belt type of sequence which was intruded and reworked by the granitic rocks of the Ran Igneous Complex, possibly derived from the partial melting and anatexis of the greenstone sequences itself during Neo-Archaean to Paleoproterozoic times (2.5 to 2.29–1.72 Ga). The amalgamation of the different greenstone sequences are explained by simple terrane accretion model wherein the forceful intrusive plutonic granitic bodies acted as stitching joins of craton-terrane parts. The complexly deformed and metamorphosed high grade granulite gneiss terranes occurring as tectonic wedges between greenstone-granite cratons in BSG is explained by deep crustal asymptotic ductile shear zones whereby the granulite gneisses were excavated from deeper levels of the crust.