Mafic dykes at the southwestern margin of Eastern Ghats belt: Evidence of rifting and collision

Abstract

The southwestern margin of the Eastern Ghats Belt characteristically exposes mafic dykes intruding massif-type charnockites. Dykes of olivine basalt of alkaline composition have characteristic trace element signatures comparable with Ocean Island Basalt (OIB). Most importantly strong positive Nb anomaly and low values of Zr/Nb ratio are consistent with OIB source of the mafic dykes. K-Ar isotopic data indicate two cooling ages at 740 and 530 Ma. The Pan-African thermal event could be related to reactivation of major shear zones and represented by leuco-granite vein along minor shear bands. And 740 Ma cooling age may indicate the low grade metamorphic imprints, noted in some of the dykes. Although no intrusion age could be determined from the present dataset, it could be constrained by some age data of the host charnockite gneiss and Alkaline rocks of the adjacent Prakasam Province. Assuming an intrusion age of ∼ 1.3 Ga, Sr-Nd isotopic composition of the dykes indicate that they preserved time-integrated LREE enrichment. In view of the chemical signatures of OIB source, the mafic dykes could as well be related to continental rifting, around 1.3 Ga, which may have been initiated by intra-plate volcanism.

Magnetic fabrics in characterization of magma emplacement and tectonic evolution of the Moyar Shear Zone, South India

“…The South Indian shield, considered to be a composite continental segment with diverse petrological, geochemical, structural and metamorphic characteristics, has witnessed several phases of ultra-basic, basic, acidic and alkaline magmatism, mainly in the Proterozoic (Murthy, 1987;Ernst et al, 1995;Bhattacharya et al, 2010) and Late Palaeozoic eras (Radhakrishna, 2003;Radhakrishna et al, 2004). These intrusive bodies have been generally considered as post-tectonic (Rajesh, 2000), especially due to the absence of mesoscopic fabrics, obscured contact relations and multiphase reactivation history of the associated shear zones.…”