Geochemical features and tectonic setting of the Neoproterozoic granitic gneiss in Tianshui area on the northern margin of West Qinling

The Neoproterozoic granitic gneisses in Tianshui area are located in the Xinyang -Yuanlong ductile shear belt which is the boundary between the North Qinling orogenic belt and the North Qilian tectonic zone. Geochemistry, petrogenesis and tectonic setting of the granitic gneisses are discussed in this paper. The granitic gneisses are rich in SiO2 and Al2O3, with A/CNK being 1.104~ 1.389, showing the supersaturation of Al and Si and suggesting the features of typical strongly peraluminous S-type granite. The ∑REE range from 128.27×10-6 to 242.09×10-6. Chondrite-normalized REE patterns show a rightward inclination with medium negative Eu anomalies. The trace element geochemistry is characterized by positive anomalies of Cs, Rb, Ba, Th, U, K, La, strongly-depleted negative anomalies of Nb, Ta, Ti, Hf, and evident negative anomalies of Ba, P, Sr. The granitic gneisses have features of crust-derived and typical continent-continent syn-collisional granites. The Paleoproterozoic Qinling Group whose protoliths were immature psammites containing higher pelitic compositions probably served as the source rocks. The generation of granitic gneisses resulted from dehydration melting due to the breakdown of micaceous minerals, with the possible existence of some fractional crystallization. The granitic gneisses suggest a collision between the North Qinling micro-block and its adjacent block in the Early Neoproterozoic period. It is a response of the assembly of Rodinia in West Qinling.