Petrogenesis and tectonic setting of the Gunzhongkou granite in the Helanshan Mountains,North China
The North China Craton(NCC)was formed by the assemblage of multiple terranes or micro-continents in the Paleoproterozoic.It is an important window for us to explore evolution history of the earth.At present,there is still controversy over the petrogenesis of the Paleoproterozoic S-type granite in the NCC Khondalite belt.In this paper,we have conducted petrological,chronological,and geochemical studies on the Kunzhongkou granite in the Khondalite belt,especially by using the zircon Hf-O multi isotope system to constrain its magma source,in order to obtain accurately its petrogenetic information.The results show that contents of SiO2,Na2O,K2O and Al2O3 of samples from the Gunzhongkou granite are 63.83％～67.95％,1.75％～2.75％,4.37％～6.20％,and 13.80％～16.76％,respectively,indicating that the Gunzhongkou granite belongs to a typical S-type granite.The U-Pb dating data of 30 zircon grains of the granite sample yielded an upper intercept age of 1914±26 Ma which is the emplacement age of the Gunzhongkou granite.Zircon grains of the sample have high eHf(t)values varying from-1.97 to+5.90,relatively high δ18O values varying from 6.40‰ to 10.46‰,relatively oldTDM1ages varying from 2415.72 Ma to 2088.70 Ma,and old TDM2 ages varying from 2672.74 Ma to 2187.88 Ma,indicating that the initial magma was originated from the partial melting of ancient crustal materials under the syn-collisional tectonic setting.Combined with published Hf-O isotope data of zircons of granites and sedimentary rocks in the study area,we found that the source nature of magmatic rocks had changed significantly at about 1950 Ma when the tectonic setting of the area was transformed from a plate subduction setting to a continental collision setting.Therefore,we have established a model for tectonic evolution of the area during 2200～1800Ma.The model reveals the collision orogenic process of the Ordos and Yinshan blocks and its response to the magmatism in depth,and will provide new constraints for restoring the Paleoproterozoic tectonic evolution history of the Khondalite belt and the NCC.
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