The Late Mesozoic tectonic evolution and magmatic history of west Zhejiang, SE China: implications for regional metallogeny
Abstract
The granitoids and related polymetallic mineralization in the Zhejiang Province at the southeast margin of the Yangtze Block in China provide an important window to evaluate metallogeny associated with convergent margin magmatism. Here, we present geochronological, geochemical, and isotopic data from the granitic rocks of west Zhejiang, to constrain the timing of transformation of the tectonic setting of this region from volcanic arc to intra-plate during Late Mesozoic and its bearing on regional metallogeny. The granitic rocks in west Zhejiang can be geochemically subdivided into two groups. The first group is characterized by relatively steep rare earth element (REE) patterns with slight Eu anomalies, high Sr, low Yb, and negative Nb–Ta–Ti (NTT) anomalies, indicating a volcanic arc environment with a thickened crust in a convergent setting. The second group is featured by flat REE patterns with prominent negative Eu anomalies, low Sr, high Yb, and weak NTT anomalies, suggesting an intra-plate extensional environment with a thin crust. The geochronology of granitic rocks in west Zhejiang, combined with ages of regional tectonic basins and nappe structures, constrains the timing of the tectonic transformation to be in the range from 150 to 140 Ma. Sr–Nd isotopic data and a positive correlation displayed by oxygen fugacity (fO2), and La/Sm and Ba/Th ratios (proxies of subducted sediments and slab dehydration fluids) suggest that the high oxygen fugacity is probably related to the melting of subducted sediments and slab dehydration. From 180 to 80 Ma, due to the increasing dip angle of the subducted Izanagi Plate, the volcanic arc belt migrated oceanward, leaving most of the interior of Zhejiang Province under an intra-plate environment where insufficient subducted components and upwelling mantle generated reduced magmas which were not favorable for Cu–Mo mineralization. Our model provides a plausible explanation for the absence of Cu–Mo porphyry deposits in the adjacent region of Zhejiang, Jiangxi, and Anhui provinces (Zhe-Gan-Wan region) after 140 Ma.