Late Precambrian (850-800 Ma) palaeomagnetic pole for the South Indian shield from the Harohalli alkaline dykes: geotectonic implications for Gondwana reconstructions

Abstract

Palaeomagnetic results from nine sites of alkaline dykes in the Harohalli area, Dharwar craton, south India, provide the best palaeomagnetic pole for the Indian shield during 850-800 Ma. The mean palaeomagnetic direction is D=7.4°; I=81.5°; n=9; κ=46; α₉₅=7.7°) yielding a pole position at λ=28°S; φ=260°E (dp/dm=14/15°). The results for the Harohalli dykes, together with earlier results, define a motion of the Indian subcontinent between 1000 and 500 Ma and constrain an important geotectonic process between India and Africa during this period. While the 850-800 Ma Indian pole differs from the African pole for the same period in the Lotts and Rowley (1990) reconstruction, the Indian and African poles for 800-700 Ma and 650-500 Ma demonstrate excellent agreement. Thus, we suggest juxtaposition and collision between the African and the Indian continental nucleii along the Mozambique Belt (MB) to have occurred in the latest Precambrian (~800-750 Ma). Integration of our results with recent geochronological and petrological investigations in the then neighbouring Gondwana segments (India, Sri Lanka, Antarctica and Africa) helps to evaluate recently suggested geotectonic correlations between these fragments in the latest Precambrian. The south Indian granulite region extending up to the Bhavani shear zone could represent an independent Palaeoproterozoic terrane (SIGT) distinctive from the east coast region of India including the Eastern Ghat Mobile Belt (EGMB). The missing seafloor of the 'Mozambique Ocean'seems to have been consumed initially during the Late Precambrian (~800-750 Ma) by collision between Africa and the south Indian shield along the MB. The orogenic activity gradually migrated with time into the Highland/Southwestern Complex (HSWC) of Sri Lanka (~600-550 Ma) and then into the Lützow-Holm Complex (LHC) of Antarctica (~550-520 Ma) giving rise to final amalgamation of the continental blocks to form the Gondwana supercontinent.