PALEOMAGNETISM OF PRECAMBRIAN DIKE SWARMS IN THE HAROHALLI AREA, SOUTH OF BANGALORE, INDIA

Seventy-two oriented core samples from twelve sites in three dike swar ms in the Harohalli area, two of which have been dated radiometrically (Ikramuddin and Stueber, 1976) have been studied. Two old metadolerit e dikes were completely unstable and gave no consistent results. After detailed AF and thermal demagnetization, and principal component anal ysis, three sites in E-W quartz dolerite dikes in the group dated at 2 370 +/- 230 Ma gave consistent vectors: N=3, D=79.6, I=-82.3, k=191, a lpha(95),=9.0; pole 9.5 degrees S, 242.4 degrees C. One N.S. dike from this group is roughly reversed to the above. The fifth dike, an olivi ne dolerite, gave a completely distinct vector. Two of five alkali sye nite dikes, dated at 814 +/- 34 Ma gave a consistent vector: N=2, D=26 .9,I=83.0, k=1910, alpha(95)=5.7, pole 24.75 degrees N, 264.1 degrees E. The three E-W dolerite dike vectors correlate well with results fro m three other published dike studies, the mean of thirteen site vector s (presumed to date from similar to 2.4 Ga), yielding a pole at lat. 1 5.2 degrees N, long. 55.6 degrees E, alpha(95)=10.2.