Proterozoic palaeomagnetism of the mafic dyke swarms in the high-grade region of Southern India

Abstract

Palaeomagnetic investigations have been carried out on four major dyke swarms (38 sites) in the high-grade metamorphic region of the south Indian shield. A total of 37 sites (243 samples) in four dyke swarms have yielded Precambrian palaeomagnetic vectors after progressive AC and thermal demagnetisation: (1) the Agali-Anaikatti swarm bordering the Nilgiri Massif to the south; (2) the Tiruvannamalai swarm to the interior of the Pondicherry coast; (3) the Dharmapuri swarm towards the west of Tiruvannamalai in the interior of Tamil Nadu; and (4) a few dykes from a swarm in north Kerala on the west coast. Ages of the palaeomagnetic vectors are estimated from available K---Ar isotope dates on samples from a number of sites from all swarms and Rb---Sr isotope results from the older Agali-Anaikatti dykes. The vectors and their corresponding ages are as follows: D=13.9°, I=27°, κ=94, α₉₅=26, POLE=75.7°N 153.2°E for the ~2000 Ma Agali-Anaikatti swarm (N=2 sites); D=359.8°, I=9.2°, κ=51, α₉₅=10, POLE=79.7°N 101.1°W for the 1800-1750 Ma Dharmapuri swarm (N=5 sites); D=217.2°, I=-50°, κ=103, α₉₅=24, POLE=51°N 130°E for the 1700-1650 Ma northern Kerala dykes (N=2 sites) and D=106.9°, I=−77.6°, κ=42, α₉₅=9.4, POLE=18.8°S 125.2°W for the ~1650 Ma Tiruvannamalai swarm (N=7 sites). These results in conjunction with other available good quality data have been used to compile an apparent polar wander curve of India for the Proterozoic Eon. The palaeolatitudes derived from the data show that the Indian Indian landmass was situated at mid-latitudes of the Northern Hemisphere during ~2000 Ma, at a low latitude near the Equator (~10°N) during ~1800-1750 Ma, close to the South Pole region (60°S) by ~1650 Ma, again near the Equator ~1000 Ma, near the North Pole (80°N) region at ~850-800 Ma, and moved to 10-30°N by 750 Ma.