Magma mixing in the Xitian pluton of Hunan Province: Evidence from petrography, geochemistry and zircon U-Pb age

Abstract:The Xitian pluton area is located in the middle segment of the Nanling Mountains, and the Yanshanian granites mainly consist of porphyritic biotite monzogranites with many dark microgranular enclaves. These dark microgranular enclaves have plasticshape and magmatic structure, quench boundary, reverse veins and unbalanced textures, such as acicular apatite, quartz ocellar texture and K- feldspar zoning. These characteristics indicate magma mixing. Geochemically, these enclaves are poor in SiO2 and ALK, and the values of K, Fe, Mg, Ca and Ti in the enclave samples are higher than those in their host granites. The enclaves and their host rocks are rich in LREE, with LREE/HREE ratios ranging from 2.6 to 8.8, and display moderate to strong negative europium anomalies (δEu=0.09-0.74). The selected major oxides of enclaves and porphyritic biotite granite show a linear correlation in Harker diagrams. The enclaves and their host rocks display obvious features of magma mixing in the Co variant diagrams. The TFeO-MgO discrimination diagrams also suggest the magma mixing origin of the enclaves. The petrography and geochemistry also indicate magma mixing. The enclaves from the Xitian pluton were probably derived from quenching of mafic magma during its intrusion into felsic magma. The LA-ICP-MS single-zircons U-Pb dating demonstrate that the magmatic age is (150.04±0.52) Ma for the biotite monzogranites and (145.09±0.63) Ma for the enclave. The two ages are basically the same, suggesting that the mixingevent took place during the period of Late Jurassic.