500 Ma PAN-GONDWANA EVENT AND ITS TECTONIC SIGNIFICANCE : ANTARCTICA AND THE HIMALAYA

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Recent geochronological studies have revealed that an early Palaeozoic (around 500Ma) heating event is recognized in the Himalaya in addition to those in Tertiary age (Eocene and Miocene), late Proterozoic age (700-600Ma) and early Proterozoic age (2000-1800Ma). The early Palaeozoic event is mainly associated with granitic intrusions at the northern margin of the Indian continent, that is, in the Lesser Himalaya and Tethys Himalaya along the whole Himalaya from Sikkim in the east to Pakistan in the west (LE FORT et al. : Sci. Terre, Mem., 47,195,1986). These granites are mainly tourmaline-bearing mica leucogranites with or without cordierite and garnet, and have often changed into orthogneiss by later metamorphism in the Tertiary. Major element chemistry suggests the rocks to be S-type granites, and plots near minimum melting composition in the normative Qz-Ab-Or system. Chondrite-normalized REE patterns for the granites from the Nepal Himalaya and from the Yamato Mountains and Lutzow-Holm Bay, Antarctica show a similar pattern : (La/Lu)_<cn>=1.5-16 and clear negative Eu anomaly. In the Himalaya the sedimentary basin has been shifted from the Lesser Himalaya in the south to the Tethys Himalaya in the north in the early Palaeozoic age (HASHIMOTO et al. : Geology of the Nepal Himalayas, 257,1973). The Palaeozoic granitic intrusion appears to have occurred along the boundary zone between these basins, and to have been coeval with the shifting of the basin. Such early Palaeozoic orogenies with intensive acidic magmatism associated with slight metamorphism have been known to occur not only in East Gondwana (the Ross orogeny in Antarctica and the Adelaide orogeny in Australia) but also in West Gondwana (the Pan-African orogeny in Africa). Therefore, we propose to designate these orogenies in the whole of Gondwanaland as the Pan-Gondwana event which has been attributed to the final amalgamation of Gondwanaland in the early Palaeozoic age.
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500 Ma PAN-GONDWANA EVENT AND ITS TECTONIC SIGNIFICANCE : ANTARCTICA AND THE HIMALAYA

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