The origin of variable-delta O-18 zircons in Jurassic and Cretaceous Mo-bearing granitoids in the eastern Xing-Meng Orogenic Belt, Northeast China

摘要

This study reports new zircon U-Pb ages, Lu-Hf isotope data, and oxygen isotope data for Mesozoic Mo-bearing granitoids in the eastern Xing-Meng Orogenic Belt (XMOB) of Northeast China, within the eastern Central Asian Orogenic Belt. Combining these new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb ages with the results of previous research indicates that two stages of Mo-bearing granitoid magmatism occurred in the eastern XMOB, during the Early-Middle Jurassic (200-165 Ma) and the Early Cretaceous (ca. 111 Ma). The eastern XMOB also contains Mo-bearing granitoids with variable delta O-18 compositions that record variations in source oxygen isotopic compositions. Combining delta O-18 data with zircon U-Pb and Hf isotopic data provides evidence of the origin of these granitoids. Three types of zircon have been identified within these granitoids. Type 1 zircons formed during the Mesozoic and having high delta O-18 values (5.71-7.05 parts per thousand) that are consistent with the compositions of magmatic zircons from the Luming, Jiapigou, and Kanchuangou areas. These zircons suggest that the Mo-bearing granitoids were derived from a source containing supracrustal materials. The type 2 zircons have extremely low and heterogeneous delta O-18 values (4.64-4.89 parts per thousand) that are consistent with the compositions of magmatic zircons from the Jidetun and Fuanpu areas. These magmas were generated by the remelting of juvenile crustal material that was previously significantly modified by interaction with fluids. Type 3 zircons generally have mantle-like delta O-18 values (5.42-5.57 parts per thousand), with several zircons yielding higher delta O-18 values, suggesting that these intrusions formed from mantle-derived magmas that assimilated and were metasomatized by crustal material. Combining these geochemical data with the geology of this region indicates that the Mo-bearing granitoids were generated as a result of subduction of the Palaeo-Pacific Plate beneath the Eurasian continent.