Petrogenesis of Early Silurian Intrusive Rocks in Yeniujishan Area of South Qilian: Constraints from Geochemistry and LA-ICP-MS Zircon U-Pb Geochronology

In order to to constrain its petrogenetic and tectonic processes, the geochemistry and LA-ICP-MS zircon U-Pb dating of intrusive rocks from Yeniujishan area in South Qilian has been conducted in this paper. The intrusive rocks in this area are mainly composed of granodiorite, porphyritic granodiorite and monzonite granite. Geochemical data show these intrusive rocks are characterized by rich SiO₂ (64.69%~78.03%),Al₂O₃ (7.44%~14.16%), K(2.98%~5.23) and alkali(5.13%~7.92%), but low TiO₂ (0.55%~0.94%), FeO (1.52%~5.00%), CaO (1.11%~3.43%) and MgO(0.78%~2.49%). Their Rittmann indexes (δ) range from 0.91 to 2.89, while their aluminum saturation index (A/CNK) vary from 0.84 to 1.23, belonging to the calc-alkaline series, with the characteristics of metaluminous-weakly peraluminous rocks. These rocks have low ΣREE values (157.89×10^-6~299.15×10^-6), which are relatively enriched in LREE, large ion lithophile elements (Rb, K) and Th element. But, they are relatively depleted in HREE, while their Ce,Hf,Zr,Sm,Y and Yb values are obviously lower than the ones of mid-ocean ridge granitoids, with strong negative Eu anomaly (δEu=0.47~0.66). The geological and geochemical data suggest that these intrusive rocks are S-type granite, which was formed by the re-melting of crustal material. The discrimination diagrams for tectonic settings (w(Y)-w(Nb) and R1-R2) show that these granites are related to the plate collision of South Qilian orogenic belt. The U-Pb dating of porphyritic granodiorite yields an age of (443.9±2.1)Ma, indicting that these rocks were emplaced in Early Silurian. In combination with the comprehensive analysis of tectonic evolution, it is believed that these intrusive rocks in Yeniujishan area formed under the geodynamic background of subduction collision.