Zircon U−Pb age, petrogeochemical characteristics of Paleoproterozoic granite in the Helishan area, Gansu Province, and its constraints for tectonic environment of the southern Alxa block

As the direct surrounding rock of Jinchuan nickel mine, the Longshoushan rock group distributed in the southern margin of Alxa block has great controversy on its formation age and tectonic background.This paper reports that the Paleoproterozoic granite body of Helishan, which intrudes into Longshoushan rock group, is newly discovered in the southern margin of Alxa block, which lithology is monzogranite, syenogranite and syenite. According to the petrogeochemical analysis, the Al₂O₃ / (CaO + Na₂O + K₂O) of Helishan granite is 0.91~1.06 and K₂O/Na₂O is 1.10~1.44, it belongs to potassium alkali calcium granite. The rock is relatively rich in large ion lithophile elements such as Rb, Ba, Th, K, etc., and lack in high field strength elements such as Ta, Nb, P, Zr, Hf, Ti, etc., which has the geochemical characteristics of arc magma and is formed in the magmatic arc tectonic environment of subduction zone. The zircon U−Pb dating result of LA−ICP−MS shows that the formation age of Helishan monzogranite is 1859±12 Ma and the intrusion occurred in the Late Paleoproterozoic, it reveals that the formation age of metamorphic strata of Longshoushan group is greater than 1.85 Ga, reflecting that the southern margin of Alxa block may be in the process of subduction in the Late Paleoproterozoic.