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

Seventy-two oriented core samples from twelve sites in three dike swarms in the Harohalli area, two of which have been dated radiometrically (Ikramuddin and Stueber, 1976) have been studied. Two old metadolerite dikes were completely unstable and gave no consistent results. After detailed AF and thermal demagnetization, and principal component analysis, three sites in E-W quartz dolerite dikes in the group dated at 2370 +/- 230 Ma gave consistent vectors: N=3, D=79.6, I=-82.3, k=191, alpha(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 olivine dolerite, gave a completely distinct vector. Two of five alkali syenite 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 from three other published dike studies, the mean of thirteen site vectors (presumed to date from similar to 2.4 Ga), yielding a pole at lat. 15.2 degrees N, long. 55.6 degrees E, alpha(95)=10.2.