Spatial-temporal relationships of late Mesozoic granitoids in Zhejiang Province, Southeast China: constraints on tectonic evolution

摘要

Late Mesozoic granitoids in South China are generally considered to have been generated under the Palaeo-Pacific tectonic regime, however, the precise subduction mechanism remains controversial. Detailed zircon U-Pb geochronological, major and trace element, and Sr-Nd-Hf isotopic data are used to document the spatiotemporal distribution of the granitoids in Zhejiang Province. Three periods of late Mesozoic magmatism, including stage 1 (170-145Ma), stage 2 (145-125Ma), and stage 3 (125-90Ma), can be distinguished based on systematic zircon U-Pb ages that become progressively younger towards the SE. Stage 1 granitic rocks are predominantly I-type granitoids, but minor S- or A-type rocks also occur. Sr-Nd-Hf isotopic data suggest that these granitoids were generated from hybrid magmas that resulted from mixing between depleted mantle-derived and ancient crust-derived magmas that formed in an active continental margin environment related to the low-angle subduction of the Palaeo-Pacific plate beneath Southeast China mainland. Stage 2 granitic rocks along the Jiangshan-Shaoxing Fault are predominantly I- and A-type granitoids with high initial Sr-87/Sr-86, low epsilon(Nd)(t), epsilon(Hf)(t) values and Mesoproterozoic Nd-Hf model ages. These results suggest that stage 2 granitoids were derived from mixing between enriched mantle-derived mafic magmas and ancient crust-derived magmas in an extensional back-arc setting related to rollback of the Palaeo-Pacific slab. Stage 3 granitic rocks along the Lishui-Yuyao Fault comprise mainly A- and I-type granitoids with high initial Sr-87/Sr-86 ratios, and low epsilon(Nd)(t) and epsilon(Hf)(t) values, again suggesting mixing of enriched mantle-derived mafic magmas with more ancient crustal magmas in an extensional back-arc setting, related in this case to the continued rollback the Palaeo-Pacific plate and the outboard retreat of its subduction zone.