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[FeO_(T)/(FeO_(T)+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.
With the methods of single grain zircon U-Pb and hornblende-biotite Ar-Ar dating, a U-Pb zircon age (135.4(2.7) Ma of Liujiawa high Sr/Y granite and 40Ar-39Ar plateau ages (139.0(1.0) Ma and (125.3(0.2) Ma of hornblende and biotite respectively of Fenliupu high Sr/Y granite in the Dabieshan high- and ultrahigh-pressure metamorphic belt are yielded. Following the emplacement of the high Sr/Y granites, extension-related mafic intrusions and late granitoids with negative Sr and Eu anomalies and isotopic ages of 105-130 Ma were emplaced. Geochemical comparison between the two types of granitoids shows that, from early to late, the alkalinities, the K2O contents and the Sr/Y ratios as well as FeO/(FeO+MgO) ratios of the rocks increase, while the abundance of Sr reduces, suggesting that as time goes on the depth of crustal melting decreases. Rheological stratification of the crust due to early Cretaceous partial melting of crustal rocks resulted in crustal extension and deep-buried rock exhumation. Therefore, the high Sr/Y granite that appeared at ~135 Ma is the product of the tectonic transition from early shortening to late overall extension.
MA Changqian YANG Kunguang MING Houli LIN Guangchun
