IGNEOUS ROCK BELTS IN THE HIMALAYAS AND THE GANDES ARC AND THEIR GENETIC MODEL
Jin Chengwei and Zhou Yunsheng
There are three great belts of igneous rocks in the Himalayas and Gandes Arc of southern and central Xizsang.The Himalayan intrusive acid rock belt mainly consists of granites and occasionally of granodiorite. From its regional distribution the belt is subdivisible into a south zone, the tourmaline-muscovite-granite zone of High Himalayas, and a north zone, the dimicaceous gneissic granite zone of southern Xizang Lower Ridge. The granites in both zones are characterized by a large amount of minerals rich in water and volatiles and high content of SiO2, AlO3 and (K2O+Na2O) in their chemical composition. The K-Ar age of the granite is 10-20 m.y. in the south zone and is 20-30 m.y. in the north one.The magmatic rock belt of Gandes Arc consists of many gigantic composite granite batholiths and extensive volcanic rocks. The radiometric dating of the granitic rocks gives three K-Ar age groups: 70-90 m.y. for quartz-diorite, 30-50 m.y. for granodiorite and hornblende-biotite granite, and 10-30 m.y. for dimicaceous granite. Chemical analysis of these rocks show that their K2O/SiO2 ratio changes within a range of 0.02-0.06, and remarkably increases from the Yarlung Zangbo Jiang northwards. The volcanic rocks show the same change in their composition as the granitic rocks. From south to north, they change from an alkali-lime series to meta-alkali series. Moreover, leucite basalt has been found in the northernmost part of this belt.The Gar-Yarlung Zangbo Jiang ophiolite belt consists of a set of rocks tectonically intensely deformed and intermixed with basic and ultrabasic rocks, spilite, tholeiite, radiotarian chert and flysch. The ultrabasic rocks, showing "cold intrusion" features, have higher ratio of Mg/Fe, and belong to Alpine magnesian type.The authors suggest that the Indian continent has been drifting towards north since Cretaceous and came into collision with the Eurasian continent in Eocene, then three genetically different types of magmatism may have taken place in the region studied: the first type caused by the subduction of the ocean crust of the Indian plate due to its underthrusting beneath the Eurasian plate and resulting in partial melting in the lower earth's crust under the Gandes Arc region; the second type appeared due to strong squeezing by the direct collision; and the third type resulted from a partial melting in the upper earth 's crust of the continental block, caused by the great thrust associated with the col'ision.. The present Gandes Arc belt was formed by overlapping of the first and third magmatism and the Gar Zangbo-YarlungZangbo Jiang belt——by overlapping of the first and second magmatism, whereasthe Himalayas belt is only a result of the third magmatism.