LATE TRIASSIC MONZOGRANITE AND ITS MAFIC ENCLAVES FROM THE CHAIJIAZHUANG PLUTON IN TIANSHUI AREA, WESTERN QINLING MOUNTAINS:Petrogenesis and Magmatic Mixing

The Late Triassic granitoid is widely distributed in the western Qinling-Dabie orogenic belt. The paper chooses the Late Triassic monzogranite and mafic enclaves from Chaijiazhuang pluton of western Qinling Mountains for detailed petrological and geochemical study. Acicular apatite and plagioclase megacrysts are developed in the mafic enclaves, suggesting the magmatic mixing process. The mafic enclaves are characterized by low SiO₂(60.27%-60.38%), high Mg# (54-55) and Nb/Ta ratio (14.8-16.6), which indicates that they were derived from the partial melting of subcontinental lithospheric mantle. Geochemically, the host monzogranite shows adakitic affinity with enrichment of Sr and Ba, depletion of Y and HREEs, high Sr/Y ratio (88-98) and Y/Yb ratio (13-15), hinting at the residue of garnet in the source area. Combining with previous researches, it is suggested that the monzogranite in the study area is the product of partial melting of thickened lower crust under the post-collision extension background caused by the Late Triassic subducting slab break-off in Western Qinling orogenic belt.