Silurian A-type granitoids in the southern margin of the Tongbai-Dabieshan: Evidence from SHRIMP zircon geochronology and geochemistry

Abstract

SHRIMP U-Pb dating on magmatic zircons extracted from a riebeckite quartz syenite in the Huangyangshan pluton in Suizhou, the southern margin of Tongbai-Dabieshan yielded an age of 439±6Ma. According to the morphology and high Th/U ratios of the zircons, the age is interpreted as crystallization timing of the pluton. The Huangyangshan pluton is composed of peralkaline quartz syenite, alkaline granite and syenite. All of the rocks are characterized by high agpaitic index (A.I.=(Na+K)/Al, molar ratio) and Fe-number [FeOT/(FeOT+MgO)], low CaO and MgO contents, enrichment of high field strength elements (i.e. Nb, Zr, Ga, Y, Hf) and light REEs, evidently negative Eu anomalies and high Ga/Al ratios, which are consistent with anorogenic A-type granitoids. Being part of the South Qinling-South Dabieshan Paleozoic alkaline rock belt, the Huangyangshan A-type granitoid pluton results from the Paleozoic extension-initial rifting in the northern margin of the Yangtze Craton related to the opening of the eastern Paleo-Tethyan Ocean.