Geochemical Characteristics And Petrogenesis Of The Caledonian Granite In Southwestern South China Block

The Qinjia,Damingshan and Yingqiao plutons are in the southwestern South China Block.They are produced by the late Caledonian magmatic activity.The Qinjia pluton is composed of medium-coarse granite and porphyritic granite,A/CNK=1.0~1.2,which is characterized by weak peraluminous-peraluminous S-I type genesis.The Damingshan pluton is comprised of porphyritic granite,A/CNK=1.29~1.60,which is an S type granite.The Yingqiao pluton is composed of gneiss granite,A/CNK=1.11~1.12,and it is an S type granite with weak peraluminous characteristic.The Qinjia pluton yield zircon U-Pb ages of 438.8~440.9 Ma with ?Hf(t)values from-2.9 to +6.1.And the intruding time of the diabase vein is about 397 Ma.The Damingshan and Yingqiao plutons yield zircon U-Pb ages of 431.8 Ma and 438.9 Ma,and ?Hf(t)values from-6.8 to +2.1,-12.8 to-6.1,respectively.The Qinjia and Yingqiao plutons are partial melts from metagreywackes,and Damingshan pluton are partial melts from metapelitic sources.TZr are calculated from 737 °C to 790 °C for Qinjia samples,from 748 °C to 841 °C for Damingshan samples,and 688 °C to 691 °C for Yingqiao samples.Results of zircon Ti temperature estimation are 685~692 °C,759 °C and 693 °C,respectively.Due to the addition of mantle derived materials,the diagenetic temperature of Qinjia and Damingshan plutons is high.We suggest that the Caledonian magmatic rocks are the product of the intracontinental orogeny rather than the oceanic crust subduction and the post-collision of Cathaysian and Yangtze Block.The peak age of high pressure metamorphism is 460~445Ma in South China Block,earlier than the granitoid formation period.We suggested that a period of thickening of the crust exists on the South China Block.Caledonian granitoids are the products of lithospheric delamination.According to the characteristics of magmatic rocks,palaeogeographic pattern and source of the sediment in the study area,we think that the former subduction of the Proto-Tethys ocean evolved into continental collision.And the force produced by the subduction and collision was transferred to the interior of South China Block,which made South China Block enter the intracontinental orogenic evolution stage,and the crust gradually thickened and led to the eclogite metamorphism in the lower crust.The density increases,resulting in the delamination of the lower crust and lithospheric mantle.The activity of diabase vein in 397 Ma of Qinjia pluton is related to the subsequent extension of the Paleo-Tethys and continental crust.