Palaeomagnetic and low-field AMS studies of proterozoic dykes and their basement rocks around Harohalli, South India

Palaeomagnetic and Low-field Anisotropy of Magnetic Susceptibility (AMS) re sults are reported from 8 unmetamorphosed dolerite dykes, and from one site each of the Closepet Granite (CG), the Peninsular Gneiss (PG) and the Char nockite (Ch) in the Harohalli area of the Dharwar craton. The AMS results e xhibit "normal" magnetic fabrics for three dykes. Rest of the dykes exhibit either "inverse" or "anomalous" magnetic fabrics, indicating the dominance of single domain (SD) uniaxial magnetite grains and a complex mixture of S D and multi-domain (MD) grains respectively. Almost similar AMS fabrics are exhibited by the Peninsular Gneiss, Closepet Granite and Charnockite. This magnetic fabric is in close conformity with the structural features of the terrain and indicates E-W compressional stress. Two dykes have yielded a m ean characteristic remanent magnetization (ChRM) direction (D = 130 degrees , I = -88 degrees, alpha(95) = 33 degrees N=2) which is similar to a previo us result from the same dyke swarm and the Tiruvannamalai dykes In the adja cent South Indian Granulite Terrain (SIGT). Thus we support the earlier sug gestion (Radhakrishna and Joseph, 1993) that no relative motion took place between the granite-greenstone terrain (GGT) and the SIGT, at least since t he emplacement of Tiruvannamalai dykes ca. 1600 Ma ago. The palaeomagnetic directions from the PG (D = 285 degrees, I = 43 degrees), CG (D = 300 degre es, I = 43 degrees), and Ch (D = 300 degrees, I = 43 degrees) are statistic ally indistinguishable and the corresponding pole positions fall close to t hose reported for 1100-1000 Ma old rocks. Thus partial remagnetization/stre aking in these sites occurred probably during the Eastern Ghats orogeny. It appears that the Eastern Ghats orogeny has penetrated selectively into the adjacent Dharwar craton.