Geochronology, geochemistry and Hf isotopic composition of Late Cretaceous Laojunshan granites in the western Cathaysia block of South China and their metallogenic and tectonic implications

Abstract

Sn-, In- and W-rich granites are identified in Laojunshan area in the western Cathaysia block, South China, including coarse-grained two-mica monzogranite (CMG) and fine-grained two-mica monzogranite (FMG). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating of CMG and FMG are from 85.6 ± 0.8 Ma to 90.37 ± 0.7 Ma and 89.9 ± 1.4 Ma, respectively. All samples have relatively high total alkalis contents (5.74 wt% to 9.73 wt%) and Al saturation indices (0.95 to 1.79), indicating that most of granites belong to the strongly peraluminous granites. The granites are enriched in P, Rb, Cs, Sn, Li, Be, and U, and are depleted in Ti, Mg, Co, Ni, Sr, Ba, Zr, and Hf. The fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb, Sr, Ba and Eu concentrations. While rare earth elements were fractionated by accessory minerals such as apatite, monazite or ilmenite. These geochemical features suggest that most of rocks are highly fractionated S-type granites. The εHf(t) values of the zircon samples from CMG and FMG range from −12.88 to −1.19, with the two-stage Hf model ages ranging from 1.2 Ga to 1.9 Ga. The geochemical and isotopic data show that the Laojunshan granites are derived from a crystalline basement that has metamorphosed pelitic rocks from the Mes-paleoproterozoic continental crust. The upwelling of the asthenosphere caused by a strong lithospheric extension led to the partial melting of the metamorphosed pelitic rocks to form widespread Late Cretaceous S-type granites in the Laojunshan area. Lithospheric extension and Cretaceous magma in SE Yunnan are probably related to the Palaeo-tethys ocean crustal delamination, which subducted beneath the Indochina block.