Different felsic rock assemblages can reflect magma source heterogeneity or different magmatic evolution processes. To determine the petrogenesis, crystallization conditions and geodynamic background of Mesozoic granitoids in the West Qinling Orogen, this study analyzed the Luchuba quartz diorite-granodiorite in the eastern West Qinling Orogen, through detailed field geological survey and petrographic, mineralogical, and geochemical analyses. We found that granite samples from the Luchuba pluton have 59.65%-67.36% SiO2, A/CNK=0.82-1.04, K2O/Na2O=1.11-1.74, and Mg#=47-53. The granites are meta-aluminous-weak peraluminous and high-K calcium-alkaline, among which granodiorite is I-type. The Luchuba plutonic rocks are enriched in Rb, Th, U, K, and Pb, and depleted in Nb, Ta, P, and Ti. The samples show moderately negative Eu anomalies (δEu=0.50-0.77) and steep LREE/HREE patterns ((La /Yb)N=9.43-30.37). The plagioclase An contents of the Luchuba pluton are 18 to 53, with some plagioclase grains showing oscillatory zoning. Amphibole is classified as Mg-amphibole, with some showing compositional zoning. The Mg/(Mg+Fe2+) value (0.88-0.91) and Ca content (17.80%-22.67 %) of the core are high, but the Al2O3 content is low, suggesting different crystallization conditions. The biotite Mg/(Mg+Fe2+) value (0.44-0.57) falls into the magnesia biotite range. The crystallization conditions of the Luchuba granitic rocks are featured by 736-795 ℃ zircon saturation temperature, 704-824 ℃ hornblende crystallization temperature, and 700-746 ℃ biotite crystallization temperature. The three thermometric results indicate a medium-temperature environment. The amphibole all-aluminum pressure gauge of the Luchuba pluton is 1.0-3.5 kbar (eqv. 6.8 km depth), the biotite all-aluminum pressure is 0.9-1.4 kbar (eqv. 4.1 km depth). The results of amphibole water content and oxygen fugacity meter show that the relative oxygen fugacity (ΔNNO) is 0.1-1.3, and the water content is 3.9%-6.3%. Accordingly, we argue that the Luchuba pluton was formed by partial melting of meta-mafic rocks dominated by amphibolite, along with the mantle-derived input as evidenced by the high Mg# and Cr-Ni contents. Integrating with published geological, geochronological, and geochemical data from the Triassic West Qinling Orogen, we conclude that the Luchuba pluton was formed under the tectonic transition from oceanic subduction to continent-continent collision.
