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) results are reported from 8 unmetamorphosed dolerite dykes, and from one site each of the Closepet Granite (CG), the Peninsular Gneiss (PG) and the Charnockite (Ch) in the Harohalli area of the Dharwar craton. The AMS results exhibit "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 SD 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 mean characteristic remanent magnetization (ChRM) direction (D = 130°, I = -88°, α 95 =33°; N=2) which is similar to a previous result from the same dyke swarm and the Tiruvannamalai dykes in the adjacent South Indian Granulite Terrain (SIGT). Thus we support the earlier suggestion (Radhakrishna and Joseph, 1993) that no relative motion took place between the granite-greenstone terrain (GGT) and the SIGT, at least since the emplacement of Tiruvannamalai dykes ca. 1600 Ma ago. The palaeomagnetic directions from the PG (D = 285°, I = 43°), CG (D = 300°, I = 43°), and Ch (D = 300°, I = 43°) are statistically indistinguishable and the corresponding pole positions fall close to those reported for 1100-1000 Ma old rocks. Thus partial remagnetization/streaking 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.