Petrogenesis of the Late Permian granodiorite in southern Dahuoluo area, Beishan, Gansu: Constraints from zircon U-Pb geochronology and geochemistry

This paper reports LA-ICP-MS zircon U-Pb ages and whole rock major and trace elements from granodiorite in southern Dahuoluo area of the Beishan region. Magma zircons from granodiorite yielded a weighted ²⁰⁶Pb/²³⁸U mean age of 259.7±1.5Ma. Granodiorite is characterized by SiO₂ ranging from 63.10% to 69.69%, Al₂O₃ ranging from 14.86% to 15.48%, 15.21% on average, MgO ranging form 0.96%~3.59%, 2.69% on average, strong enrichment of LREE, and (La/Yb)_N values ranging from 18.60 to 50.91. In addition, Eu anomaly is not obvious, with Eu/Eu^* values ranging from 0.83 to 1.06, Sr content is high (454×10^-6~862×10^-6) and Sr/Y ratio is 44.95~60.35, rocks show geochemical characteristics of adakite. Granodiorite is enriched in K and depleted in Na, and has high Cr, Ni and Mg^#(50.31~66.75). Combined with regional geological background, the authors hold that granodiorite was formed after the collision, as a product of both intraplate thermal upwelling extension and crust-mantle interaction. It is suggested that, in Late Paleozoic, the crust in Beishan area was in vertical increment, and the granites associated with the Permian and mantle-derived magmas were in the form of synchronic tectonic magmatic events.