Petrogenesis and tectonic implications of the Fushan moyite in the Fanchang area,Anhui Province-Evidence from zircon U-Pb age,geochemistry and Hf isotopes

The Fanchang area is one of the important components of the Middle-Lower Yangtze Valley metallogenic belt. The Fushan moyite is one of the most representative and important granitic intrusions. In this paper, LA-ICP-MS zircon U-Pb dating, petrographic observations and geochemical and Hf isotopic analysis were carried out for the Fushan moyite in the Fanchang area, with the emphasis placed on the analysis of petrogenesis and tectonic implications of the Fushan moyite. The Fushan moyite has a clear A-type geochemical signature, e.g., it is of metaluminous to weakly peraluminous nature (A/CNK=0.91~1.11) and has high SiO₂ (averagely 71.52%), total alkalis (Na₂O+K₂O=8.85%~9.60%), K*(K₂O/Na₂O=1.06~1.57), Fe*(TFeO/MgO=2.49~10.2), rare earth elements (ΣREE=278×10^-6~376×10^-6), Ga/Al ratio (2.90~3.31) and HFSE (Zr+Nb+Ce+Y=421.8×10^-6~655.0×10^-6). LAICP-MS zircon U-Pb dating yielded a concordant weighted mean ²⁰⁶Pb/²³⁸Pb age of 124.9±2.0Ma, which is similar to ages of other A-type granites in the Middle-Lower Yangtze Valley metallogenic belt, indicating that all A-type granites were formed in the Early Cretaceous. Zircons from this pluton yielded ε_Hf(t) values between -5.8 and -10.0, which, together with U-Pb age,(⁸⁷Sr/⁸⁶Sr)_I, δ¹⁸O and ε_Nd(t) values of the Fushan moyite, suggest the importance of juvenile material in the magma source, and the juvenile material was composed of the basaltic andesitic magma from enriched mantle lithosphere and minor crystalline metamorphic basement. These data, together with other accurate dating data of intermediate-acid plutons and volcanic rocks in the Middle-Lower Yangtze Valley metallogenic belt, support the peak time of crustal extension at ~125Ma in this area.