Geochemistry,geochronology and geological implications of Permian quartz diorite from Caohu pluton in Beishan,Gansu
The Beishan region underwent collisional orogenesis during the Late Palaeozoic to Early Mesozoic,but previous scholars have held different views on the attribution of tectonic environments during these different geological periods.In particular,there is a contentious scientific debate about the precise timing of the transition from a compressional to an extensional environment in the Beishan region.The extensive development of Late Palaeozoic intrusive rocks in the Caohu area of the Beishan provides a research opportunity to explore this ques-tion.The LA-ICP-MS zircon U-Pb dating results indicate that the quartz diorite body was formed at(276.3±3.0)Ma,during late Early Permian.The rocks have an aluminium saturation index(A/CNK value)of less than 1.05,are quasi-aluminous and calc-alkaline in character,are enriched in light rare earths and large ionic lithophile ele-ments(e.g.Rb,Ba,Th and U)and deficient in high field strength elements(e.g.Nb and Ta),and are Cordil-lera I-type granites with magmas probably derived from a post-collisional extensional environment where lithospheric mantle magmatism led to thickening of the lower crust of the northern margin of the Dunhuang Massif.The granites are of mixed crustal-mantle origin and represent a late Early Permian extensional environment in the southern part of the Beishan area,where the magmas were partially melted and mixed with lower crustal material during uplift,providing a basis for further constraints on the eventual closure of the Palaeo-Asian Ocean in the Northern Mountains region.
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