Petrogenesis And Driving Mechanisms Of The Late Cretaceous Magmatism In Western Sichuan

Under the classical plate tectonics theory,magmatic activities are mainly developed at the the plate boundaries;however,the intracontinental orogenic processes and the corresponding magmatic responses away from the edge of the plates are still poorly understood,which is the key that restricts the“landing”of plate tectonics.In order to address this issue,this study reported the petrology,whole-rock major and trace elements,zircon U–Pb age and zircon Hf isotope data of the Late Cretaceous large granite batholiths in the Yidun terrane,western Sichuan.The Queershan batholith and the Haizishan batholith is primarily composed of biotite syenogranites/monzogranites with minor dark-colored mafic microgranular enclaves and xenoliths of country rocks in the margin of the batholith.The zircon LA-ICP-MS U–Pb dating results show that the Queershan batholith in the Changtai region in the north is ca.101 Ma,and the Haizishan batholith in the Xiangcheng region in the middle is ca.95 Ma;they constitutes the migration sequence of granitic magmatism from north to south together with a series of granitoid stcoks(Xiuwacu pluton,Hongshan pluton,Tongchanggou pluton,etc.)emplaced at 80–70 Ma in the Zhongdian region in the south.This southward migration trend is also shown by the variations of zircon Hf isotopes and emplacement depths of granite.The paleo-crust thickness variations tracked by whole rock Ce/Y and(La/Yb)_n ratios of the magmatic rocks in the Yidun terrane with corresponding geological records reveal that the Changtai region in the north was controlled by an extensional tectonic regime,while the Zhongdian region in the south is subjected compressional tectonic regime with the Xiangcheng region in the middle in a transitional state.Geochemically,the samples of the Queershan batholith in the Changtai region and the Haizishan batholith in the Xiangcheng region are very similar,and they both have the characteristics of high-silica,potassium-rich and peraluminous.In the term of source,after filtering highly evolved granite samples with the geochemical indicators like Nb/Ta,Zr/Hf,(Ga/Yb)_n,Rb/Sr,the relatively original samples of the Queershan batholith and the Haizishan batholith point to identical generating process,i.e.the muscovite dehydration melting reaction.Moreover,the calculated zircon saturation temperature results(750 ~oC)falls within the ideal range for muscovite dehydration melting reaction.For heating source,the heat from mantle is of little significance due to the absence of the mafic rocks in the Late Cretaceous,and the shearing heat is insufficient to provide heat for partial melting.On the other hand,the average heat production rate of samples from the Queershan batholith and the Haizishan batholith is as high as 6?W/m~3,which is an extremely high value for granites.Therefore,the accumulation of radioactive heat is an important heat source for generating the granite batholiths in the Yidun terrane.Considering contemporaneous convergences close to Yidun terrane,we propose that far-field stresses derived from hard indentation of Lhasa-Qiangtang collision exerted first-order control over the generation and migration of the Late Cretaceous granitic activities in the Yidun terrane.The diachronous scissor-like convergence of Lhasa and Qiangtang terranes commencing in the east and terminating in the west enables compressional stresses acting on the Yidun terrane to migrate from the east to the west with the corresponding extensional stresses migrating from the west to the east(i.e.from the north to the south for the current Yidun terrane).Therefore,the initial indentation on the east thickened the crust in the Zhongdian region in the south,but corresponding extensional stresses triggered the emplacement of granites in the Changtai region in the north.With the migration of stresses,granites sequentially emplaced in the Xiangcheng region and in the Zhongdian region.This reversed migration model well explains the temporal-spatial-isotopic variations of the Late Cretaceou granite belt in the Yidun terrane and sheds new light on the tectonic-magmatic evolution patterns in intracontinental orogenic belts.