Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai, NW China: Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids

Abstract: Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai, and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447Ma to 368Ma with a climax at ca. 400Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280Ma, respectively. Positive ε Hf(t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421Ma old and also give positive ε Hf(t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period. Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400Ma and the above mentioned change of Hf isotopic composition in the magma source. [Copyright &y& Elsevier]