Proterozoic Mafic Dykes from the Southern Margin of Cuddapah Basin, India: Part 1 – Geochemistry and Petrogenesis

Abstract

A palaeomagnetic and 40Ar/39Ar study is reported from a suiteof 41 mafic dykes comprising a dense E-W trending swarm intruding Archaean granitic basement near the southern margin of the Proterozoic Cuddapah basin in southern India. Rockmagnetic studies define a range of mixed domain with dominant multi domain (MD) ferromagnetic carriers. Twenty four dykes exhibit stable behaviors to progressive demagnetisation and show that at least three magnetization events are represented within this swarm. Dyke groups are identified as RD (10 dykes, palaeopole at 72°S, 97°E dp/dm=4/7°), ND1 (5 dykes palaeopole at 34°N, 253°E, dp/dm = 9/12°), ND2 (4 dykes, palaeopole at 65°N, 286°E, dp/dm = 3/6°) and ID (4 dykes, palaeopole at 6°N, 336° E, dp/dm = 6/7°). These directions accord with earlier palaeomagnetic studies of the same swarm and 40 Ar/39Ar study yields apparent ages between 926 and 813 Ma for group RD and an age of 936 Ma for group ID. These age estimates are appreciably younger than sedimentation in the adjoining Cuddapah Basin and the directions of magnetisation contrast with directions in probable Palaeo-Mesoproterozoic dyke swarms from South Shield which typically have steeper inclinations. This evidence suggests that either the dykes record Early Neoproterozoic continental rifting with emplacement that did not migrate significantly upwards into the overlying supracrustal succession, or that they are of Palaeo-Mesoproterozoic age and overprinted during the Eastern Ghats (Grenville age) orogeny. The preservation of discrete magnetizations within the E-W swarm suggests that the former solution is more likely. An assessment of these and other palaeomagnetic poles from India assigned to the interval ~1,200–800 Ma shows a general accordance with apparent polar wander of the proto-Gondwana wing of the supercontinent Palaeopangea.