Geochemistry and zircon U-Pb ages of the Neoproterozoic Shimian plutons on the western margin of the Yangtze Block, Sichuan Province

摘要

The Shimian Neoproterozoic plutons are located on the western margin of the Yangtze Block and are important for reconstructing the tectonic environment of this region. We present in situ zircon LA-ICP-MS U-Pb geochronological and Hf isotopic analyses for the plutons, as well as whole-rock major- and trace-element compositions. The monzogranite and syenogranite yield concordant ages of 773.6 +/- 5.6 Ma and 812.2 +/- 1.8 Ma, respectively, which indicates that they formed during the Tonian of the Neoproterozoic. The samples have very similar geochemistry, with high SiO2(71.45-75.46 wt%) and Al2O3(12.38-14.36 wt%) contents. The samples have a geochemical characteristics typical of peraluminous A-type granites, including high Ga/Al and Rb/Sr ratios and elevated contents of high-field-strength elements. The samples also display high Rb/Sr (13.9-23.0) and Rb/Nb (12.5-19.2) ratios. The monzogranite and syenogranite yield positive epsilon(Hf)(t) values (+1.6 to +9.4 and + 5.2 to +9.8, respectively) and T(2DM)ages of 1.54-1.22 Ga and 1.51-1.21 Ga, respectively, which indicates that they were derived from partial melting of Mesoproterozoic crust or contamination with upper mantle. We propose that the syenogranite and monzogranite formed in a syn-orogenic extensional setting during the Tonian period and that continuous subduction was responsible for the Neoproterozoic magmatic activities.