The Forming Age and Geological Characteristics of the Metamorphic Intrusion of Hualong Rock Group in the Eastern Tectonic Belt of Nanshan,Qinghai

Hualong rock group is the basement rock of Nanshan tectonic belt in Qinghai province.Its formation age remains controversial.This paperstudied the gneissic granodiorite, which intruded into Hualong rock group, through petrology, chronology and petrogeochemistry.The research shows that the rock has high SiO₂ and Al₂O₃, and low K,P,Ti.The content of SiO₂ is 69.48%, of K₂O, 2.35%, of K₂O/Na₂O, 0.59,of (Na₂O+K₂O), 6.27%, of MgO, 1.32%, of Al₂O_3, 13.68%, belonging to mid-K calc-alkaline and prealuminous series.The content of rare earth elements is lower,∑REE being 253.77×10^-6.The rock is enriched in LREE and depeleted in HREE.The REE patterns show rightward incline and moderate negative Eu anomaly.Compared with the adjacent elements,the trace elements show enrichment of LILE(Th,U,Ba),evident depletion of K,P,Ti, and relative depletion of Rb,Sr.Zr and Hf were positive anomaly.The rock has the characteristics of A-typegranite with the forming age of(701±63) Ma.Combined with the geochemical characteristics and regional magmatic activity events, it is held that the gneissic granodiorite was generated in an extensional environment, being the magmatic activity of the Rodinia supercontinent cracking event in Qilian block.