Neoproterozoic A-type granite magmatism in the Southern Granulite Terrane, India: Constrain on the Genesis and Basement- Cover relationship

Granitoids, being the most abundant lithologies of the Earth’s upper continental crust, are key source to study the crustal evolutionary history and the associated tectonic activities producing them. The Southern Granulite Terrane (SGT) comprises various granitoid intrusions during Neoproterozoic time that caused due to the subduction-accretion-collision processes during the Gondwana supercontinental assembly. This study aims to provide the origin, basement-cover relationship between the Sankari Granite (SG) and the Basement Hornblende-Biotite Gneisses (HBG) in the Namakkal block, Southern Granulite Terrane. A detailed field study suggests that the Sankari granite occurs as small to medium isolated hills and is mostly massive, leucocratic, pegmatoidal at some places. It also shows little deformation at the marginal area due to presence of shear zone. Field study suggests that SG intrudes into the basement HBG as the SG is seen in contact with the HBG where the HBG are getting migmatised. At some places, HBG can be seen as a caught-up and engulfed fragments within the SG. This evidence suggest that the HBG are older, and SG is younger. The migmatisation of HBG at the contact with SG suggests that SG is derived from the partial melting of the basement HBG.  The petrographic study suggests that Sankari granite is composed mainly quartz, k-feldspar, plagioclase with little amount of amphibole and biotite with minor accessory phases like calcite, apatite and Fe-Ti oxide. Mineralogically, it falls into monzo-granite, alkali feldspar granite to granite in the QAP diagram. A whole rock major element chemistry suggests that all the samples fall into the granite field and alkali to alkali-calcicferroan to peraluminous in nature. Trace element study suggests that SG is poorly enriched in REE (∑REE = 18.41–52.62 ppm) and show slight flat pattern with negative europium anomalies (EuN/Eu∗N = 0.75–2, av. 2.21) on average. It shows enrichment of Rb, Th, U and depleted Ti, Sr, P, Eu anomalies which is the characteristic of A-type granite (David and Chappell 1992). The Zircon U-Pb age of SG suggests it’s emplacement age of 559.1 ± 3.5 Ma (Glorie et al., 2014) which is younger than the HBG. The overall study suggests the emplacement of Sankari Granite, which is a A-type granite, during Neoproterozoic time by partial melting of basement Hornblende-Biotite Gneisses prior the amalgamation of early stage of Gondwana supercontinental assembly.