Magmatism in the Early Triassic continental margin arc environment in the West Qinling mountains: Evidence from zircon U-Pb ages and geochemical characteristics of the Nazha pluton

The lithology of Nazha pluton in Hezuo, Gansu Province is granite porphyry. Through the study of rock geology, LA-ICP-MS zircon U-Pb age, and rock geochemistry, the genetic mechanism and tectonic significance of the Nazha pluton are explored, providing new evidences for the tectonic evolution of the Indosinian period in this region. The rock geochemical analysis of granite porphyry shows that the content of SiO₂ is 69.50%~73.48%, the content of TiO₂ is 0.008%~0.084%, the content of CaO is 0.22%~2.07%, the content of Na₂O is 2.96%~3.40%, the content of K₂O is 4.38%~4.84%, the content of Al₂O₃ is 14.74%~16.19%, the aluminum saturation index A/CNK value is 1.49 ~ 1.91(>1.1), and the Rittman index σ is 1.88~2.27. The result indicates the granite belonging to high potassium calc alkaline peraluminous S-type granite. Total rare earth element(ΣREE)content is 42.82×10^-6~62. 85×10^-6, LREE/HREE value is 16.55~24.19, light rare earth element is relatively enriched, and the δEu value is 0.79~1.41. High field strength elements P and Ti are relatively deficient, the Nb and Ta are slightly deficient, and large ion lithophile elements and light rare earth elements Th, Rb and K are relatively enriched. The geochemical characteristics of rocks show that the Nazha pluton originated from the partial melting of metamorphic sandstones and mudstones, with the high Mg^# value (67~77), and the low total rare earth element content, with some. The mantle-derived components added during diagenesis. LA-ICP-MS zircon U-Pb age (250.4±1.0 Ma) was obtained from granite porphyry and formed in Early Indosinian period. Combined with the regional geological background, the author believes that the Xiahe-Hezuo area in West Qinling region was in the continental margin arc environment of crustal and lithosphere thickening during the Early Indosinian period, and the Nazha pluton is the product of the magmatism in this stage.