Petrogenesis of the xiasai early cretaceous a-type granite from the Yidun Island arc belt, SW China: constraints from zircon U-Pb age, geochemistry and Hf isotope

Many Late Mesozoic granites distribute in the Yidun island arc belt, SW China. However, in situ zircon U-Pb ages and Hf isotopic data of these granites have rarely been reported so far. As lately reported granite from the central segment of the Yidun island arc belt, the Xiasai granite has a close genetic relationship with the Xiasai Ag-Pb-Zn polymetallic deposit. This paper presents (1) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age for the Xiasai granite to determine precisely the time of the magmatism, (2) geochemical and Hf isotope data for the Xiasai granite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS analyses of zircon from the Xiasai granite yielded a weighted mean 206Pb/238U age of 103±1 Ma (MSWD=0.5), indicating that it was emplaced in the Early Cretaceous. Petrography and chemical compositions of the granites show that they belong to high-K calc-alkaline series and are characterized by high silicon, enrichment of alkaline and iron but depletion in calcium and magnesium. Their SiO2 and K2O+Na2O contents are 72.94%-74.98% and 7.56%-8.08%, respectively. The A/CNK values vary from 1.06 to 1.10|, show a weak peraluminous affinity. The granitic rocks are enriched in high field strength elements (e.g. Zr and Hf) and large ion lithophile elements (e.g. U and Th) but depleted in Ba, Sr, P, and Ti. REEs are characterized by significant negative Eu anomalies (δEu=0.13-0.25) and exhibit right-inclined patterns with LREE enrichment but HREE depletion. Mineralogy and geochemistry of the rocks show an affinity to aluminous A-type granite, similar to other Late Cretaceous A-type granites such as the Rongyicuo, Lianlong and Ruorolong granites in the Yidun island arc belt. Zircon Hf isotopic compositions of the Xiasai granite are characterized by ΕHf(t) values varying from 2.7 to 0.6 and the two-stage model ages (TDM2) of 925 Ma to 1095 Ma, implying that different source materials have contributed to the magma genesis. Integrated geological, geochemical and isotopic data suggest that the Xiasai A-type granite is most likely generated via a two-stage process, including formation of parental magma by mixing of an asthenosphere-derived magma and a crustal-derived felsic magma in the deep crust, followed by intensive magmatic differentiation of the parental magma. These granites are likely to have been generated in a post-collisional extensional tectonic setting related to arc-continent collision during the Early Cretaceous. ©, 2014, Science Press. All right reserved.