Palaeomagnetic and geochemical studies on dolerite dykes from Tamil Nadu, India

Abstract

Dolerite dykes of the South Arcot, North Arcot and Salem districts of Tamil Nadu can be broadly divided into four major groups depending on their remanent magnetic directions. They are: (a) D = 86, I = −35, K = 400, α 95 = 5, pole position at 189° E; 1° S; (b) D = 33, I = −48, K = 21, α 95 = 18, pole position at 220° E, 38° N; (c) D = 133, I = −75, K = 134, α 95 = 6, pole position at 54° E, 31° N; and (d) D = 294, I = −52, K = 18, α 95 = 23, pole position at 49° W, 13° N. VGP's of these dykes are in good agreement with the earlier Precambrian data available for Peninsular India, and their pole positions indicate that all these dyke events could be older than 1830 ± 200 Ma. Petrochemical studies suggest that the dykes are quartz-normative tholeiites and must have formed at shallow depths; their differentiation trends reveal that they were probably derived from two different magmatic sources. These dykes are transitional in character between an oceanic and a continental setting, which suggests that they intruded into thinned crust during an extensional event.