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. © 2011 Elsevier Ltd.