Rb-Sr Isotopic Dating and Metamorphism of a Suite of Migmatite and Granites from Sandmata Granulite Complex, Rajasthan: An Evidence of Mesoproterozoic Thermal Event

Abstract

Sandmata Granulite complex (SGC) of Rajasthan of Western India is a multiply deformed, metamorphosed and migmatised granulite units representing a Mesoproterozoic collisional orogen. It consists of migmatised metapelites, calcsilicate granulites, pyroxene granulites and charnockitic intrusives at 1.7 Ga. The granulites mostly occur as dismembered, isolated enclaves in garnet bearing migmatites and quartzofeldspathic gneiss. Intrusion of small scale post-tectonic granite in granulite documents the last thermal activity in the area. At the eastern part of SGC, greenschist to lower amphibolite facies rocks of Manglawar Complex (MC) are exposed. The P-T conditions of garnet formation during metamorphism in migmatite, leptynite and quartzo-feldspathic gneiss are 780-850°C and 7–8 kbar. Retrograded biotite forming reactions occur at much lower temperature of 500 to 600°C and pressure of 3.5-4.5 kbar. The intrusive granite is weakly deformed, calc-alkaline to sub-alkaline, peraluminous in nature, having trace element signature of volcanic-arc to syn-collisional granite. The chondrite-normalised REE patterns indicate a garnet-free source. The whole-rock Rb-Sr age of 1715 ± 24 Ma deduced for the granite mark its emplacement age. Intrusive nature of granite suggests that the age of granulite metamorphism and migmatization should be >1715 Ma, which is much closer to the whole-rock Rb-Sr age of migmatite and quartzofeldspathic gneiss. The whole-rock Rb-Sr analysis of migmatite gives an age of 1735±150 Ma with an initial 87Sr/86Sr ratio of 0.704 which is interpreted as an age when the temperature of migmatization passed below the blocking temperature of Rb and Sr for biotite and plagioclase; an upper age limit of migmatization. SGC represents a major 1.9-1.7 Ga Palaeoproterozoic, tectonothermal event. The granulite facies metamorphism is succeeded by 1.7 Ga migmatisation, intrusion of calc-alkaline granite and emplacement of charnockite protolith. The whole granulite complex is subsequently cooled and later exhumed, and became juxtaposed with MC during a late major orogeny.