Geochronology and petrogenesis of Cretaceous A-type granites from the NE Jiangnan Orogen, SE China

摘要

The Jiangnan Orogen is located at a key tectonic position along the junction between the Yangtze and Cathaysia blocks. We obtained detailed major and trace elements, whole-rock Nd + zircon Hf isotope data, and U-Pb age data from sev eral Mesozoic granites, including the Fuling (FL), Taiping-Huangshan (TH), Lingshan (LS), Sanqingshan (SQS), and Baijuhuajian intrusions in order to investigate their sources and petrogeneses related to extension in South China. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon from the FL, TH, SQS, and LS bodies yield Early Cretaceous ages of 124-135 Ma. These plutons are alkali-feldspar granites to syenogranites-monzogran-ites, and show A-type affinities. They have high K_2O and total alkali contents, and are enriched in rare earth elements (except for Eu), Zr, and other high-field-strength elements as well as high Ga/Al ratios, and are depleted in Ba and Sr. These granites are metaluminous to weakly peraluminous (ACNK from 0.81 to 1.27). The whole-rock sNd(T) values of-5.34 to -0.96 are coupled with zircon sHf(T) values (from -5.3 to +4.24), and all samples plot along the mantle array. Field observations, geochronology, geochemistry, Nd isotopic, and zircon Hf isotopic compositions suggest that they formed by the partial melt ing of Mesoproterozoic metamorphic basement, with input from juvenile, mantle-derived materials in the shallow (<30 km) crust at high temperatures (756-965°C). These melts underwent crystal fractionation of biotite, plagioclase, and K-feldspar. The upwelling of asthenosphere triggered partial melting of the metamorphic protolith in a back-arc or intra-arc rift set ting, reflecting rollback of the Pacific plate. Our research adds new geochronologic constraints on Cretaceous (135-120 Ma) A-type granites from the NE sector of the Jiangnan Orogen. Combined with previous research, we suggest that three main episodes of late Mesozoic extensional tectonism took place in South China: (1) 190-170 Ma (mainly inland), (2) 165-120 Ma (including 165-150 Ma in SE Shi-Hang, 135-120 Ma in NE Shi-Hang, and ~125 Ma in the Lower Yangtze River Belt), and (3) 100-90 Ma (coastal area), showing an oceanwards younging trend due to the subduction of the Palaeo-Pacific plate.