Seminar

The Chinese Altai experienced intensive mafic and felsic magmatism in the Middle Paleozoic. The Middle Paleozoic mafic rocks show wide ranges in Sr-Nd-Hf-Pb isotopes compositions due to widespread metasomatism in the lithospheric mantle source by hydrous melt and fluid from subducted slab. The large scale dehydration and melting of subducted slab suggest subduction of an oceanic ridge beneath the Chinese Altai in the Middle Paleozoic. The Middle Paleozoic mafic rocks were remelted under variable content of water to generate the I-type granites. The intense basaltic underplating also resulted in partial melting of sedimentary rocks, producing the strongly peraluminous S-type granites. The Middle Paleozoic felsic rocks all show significant Nd-Hf isotopic decoupling manifested by relatively high εHf(t) and low εNd(t) values. Such isotopic decoupling cannot be explained by the involvement of an ancient basement with elevated Lu/Hf ratios, but can be ultimately resulted from selective enrichment of Nd over Hf in the mantle due to the metasomatism by hydrous melts from subducted sediments. Thus, the Chinese Altai represents a magmatic arc which was made up of relatively juvenile materials in the Paleozoic. Most of the Middle Paleozoic granites have high Y contents and low Sr/Y ratios, suggesting that differentiation of the crust in the region mainly occurred within the amphibole stability field (<30 km) although the crust may have been thickened.