Cognate Mafic Enclaves Formed by Cumulated Mush Convection in a Granitic Magma Chamber: Evidence from Mineral Chemistry of the Triassic Zhashui Pluton, Qinling Orogen

Mafic enclaves in granite are generally considered to represent coeval mafic melts that derived from metasomatized mantle, which can provide key information about crust-mantle interaction. Zircon U-Pb dating indicate that the mafic enclaves (219 ± 4.6Ma) display identical ages with the host monzogranite (214 ± 2.8Ma). Detailed whole-rock chemistry and zircon Lu-Hf works for the Triassic Zhaishui pluton reveal a cognate origin. Both of them share similar trace element geochemistry and zircon Lu-Hf isotopic compositions (monzogranite: εHf(t)= -0.99 to + 1.98; mafic enclaves: εHf(t)= -0.45 to + 3.26), suggesting common source region. However, detailed mineral chemistry reveals different crytallization condition between the mafic enclaves and host monzogranite. The amphibole in mafic enclaves reveal higher temperature and pressure (757℃, 2.65kbar) than those of the host monzogranite (733℃,1.96kbar). The temperature and pressure (587℃, 0.95kbar) of the Biotite in t mafic enclaves are consistent with that of the host monzogranite (599℃, 1.08kbar). Apatite in the mafic enclaves display slightly higher volatile contents (0.72 wt.%) than those of the apatite in the monzogranite (0.66 wt.%). These results indicate that the mafic enclaves represent early mafic accumulation in hybird granitic chamber, subsequent convection in magma chamber resulted in the formation of the mafic enclaves.