Petrogenesis of the Early Triassic Xichahe and Wulong plutons in the South Qinling Mountains and their tectonic significance

Abstract：The Early Triassic Indosinian Xichahe and Wulong plutons were emplaced into metamorphic rocks of the Mesoproterozoic Fuping Group in the South Qinling. A few mafic enclaves, which have a clear or obscure boundary with the host granodiorite, occur in the southern part of the Wulong pluton. Intensive petrographical, geochemical and Sr-Nd isotope studies indicate that the two plutons show some adakitic affinities：they are both strongly depleted in Yb and Y and enriched in Sr and Ba with higher Sr/Y and Y/Yb ratios and weak negative Eu anomalies. These patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionation mineral assemblage. The mafic enclaves have higher Mg (MgO=4.15-8.13%), Cr (14.8-371 μg/g) and Ni (20.0-224 μg/g), as well as higher LREE and LILE, with Nb/Ta ratios of 15.42-21.9. It is inferred that the underplated mantle magma was responsible for the generation of the mafic magmas. The two plutons have similar Sr and Nd isotopic compositions, with (87Sr/86Sr)i＝0.70445-0.70645, εNd=-1.50-3.62 and TDM=1.02-1.38 Ga. Based on the aforesaid study, combined with previous studies, the authors think that the underplating of mantle-derived magma to the crust-mantle boundary induced amphibole eclogite facies or granulite facies metamorphism and partial melting of the thickened lower crust, thus forming adakitic magma. The difference of the two plutons in contents of some trace elements was caused by different degrees of contamination with mantle materials, and their formation probably represents the breakoff of the subducted Mianlue oceanic slab during the Indosinian late-collisional orogenic stage in the Qinling area.