Structural relationships and Sr---Nd isotope systematics of polymetamorphic granitic gneisses and granitic rocks from central Rajasthan, India: implications for the evolution of the Aravalli craton

Abstract

This paper reviews the state of structural and geochronological knowledge of Precambrian cratonic rocks in part of central Rajasthan, India, and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area. The granitic rocks comprise plutons of two suites (BGC and Sendra suites), which intrude ortho- and paragneisses the Archean Banded Gneiss Complex (BGC) and its inliers, as well as the Proterozoic Delhi Supergroup. Nearly all the plutons show a foliation and a lineation, which are mostly the result of solid state deformation, although a magmatic component of strain is apparent in some of them. The intensity of these structural fabrics varies from weak to extremely strong. Samples of granitic rocks from the BGC suite yield ages from Sm---Nd isotopic data which scatter around a 2.83 Ga reference isochron [ε_Nd(t) of ~+4.1 ], with depleted mantle model ages ranging from 2.6 to 3.0 Ga. Scatter of the data suggests that the BGC is a composite group of gneisses, and confirms the antiquity of the BGC within this part of the Aravalli-Delhi orogenic belt. Rb---Sr isotopic data of the BGC suite lie on a 1.7 Ga reference isochron, indicating a thermal event at that time. Data from plutons of the Sendra suite, on the other hand, show some scatter but define a model II isochron with slope equivalent to an age of 0.966±250 Ga (initial ⁸⁷Sr/⁸⁶Sr of 0.70390±0.01889, MSWD-311). The scatter of data could reflect open system behavior during a subsequent thermal event (Malani volcanics), although independent evidence for such is lacking in the study area. Ages calculated from our Rb---Sr data are within the error of published results from previous workers. Structural observations on granitic rocks within the BGC inlier at Ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the Delhi Supergroup during the 1.7 Ga thermotectonic event contemporaneously with formation of D₁ structures in the wall rock, although the radiometric age of the latter structures is not well-constrained. Microtexture, mineralogy, and field relations of granitic rocks of the Sendra suite point to those plutons being emplaced into the Delhi Supergroup during the 0.966 Ga thermotectonic event late syntectonic relative to D₂ indicating that D₂ is ~0.96-1.0 Ga. D₃ represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling, probably between ~0.96 and 0.90 Ga. Further geochronological, geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central Rajasthan.