Geochronology, Geochemical Characteristics and Tectonic Significance of Qurugou Granodiorite in Western Section of West Qinling Orogen

Qurugou granodiorite is located in the western part of the western Qinling orogenic belt, and intrudes into the Lower Triassic Longwuhe formation. Field investigation demonstrates that the intrusion is mainly composed of off-white fine-grained massive granodiorite. Geochemical data shows that the granodiorites are high-K calc-alkaline series, characterized by high content of SiO₂ (66.08%—72.22%), K-rich (4.61%—5.01%) and quasi-aluminous ratio (A/CNK=1.04—1.11). The content of REE, LREE, HR EE and the ratio of LREE/HREE are respectively within the range of 99.41×10^-6—154.85×10^-6, 87.33×10^-6—143.67×10^-6, 11.08×10^-6—12.65×10^-6, and 7.23—12.85. The chondrite-normalized REE pattern shows right-dipped incline and weak negative Eu anomaly. The Qurugou granodiorites are obviously rich in LILE including Cs, Rb, Ba, Th, etc., and depleted in HSFE such as Nb, Ta, P, similar to magmatic arc or collision-type granite, belonging to high-K calc-alkaline I-type granites. The petrogenesis research shows that the source is the lower crust metabasalt, and the intrusionis the product of crust-mantle magma mingling, owing to the lithosphere delamination. According to the geochemistry and the trace element discrimination for tectonic interpretation, the granodiorites are considered to be the post-collision type. LA-ICP-MS zircon U-Pb dating reveals that the crystallization age of Qurugou granodiorites is 224.1±2.4Ma (MSWD=2.4), which indicates that the granodiorites formed in Late Triassic. Combining with regional geological data, it is considered that the Paleotethys in the western part of the western Qinling area closed in late Indosinian (224Ma), and entered into the post-collision tectonic evolution stage.