Petrographical and mineral chemical characteristics of garnet-bearing quartzofeldspathic xenolith in the Henglianghu Pilocene volcanic rock, northern Qiangtang terrane

This study is focussed on the largest Pliocene (3.2~2.5 Ma) volcanic rocks from the Henglianghu area in the north margin of the Qiangtang terrane. Detailed petrographical and mineral chemical work on the garnet- bearing quartzofeldspathic xenolith and phenocrysts revealed that melt in the deep crust (40~50 km) was trapped when the volcano erupted. The garnets in xenolith are characterized by extremely high Mg# (Mg2+×100/(Mg2++Fe2+)) values (41~46) which indicated they were formed under ultrahigh temperature metamorphism. Garnet- plagioclase geobarometer calculation for the xenolith yields 1. 18~1. 53 GPa corresponding to 39~50 km depth. Clinopyroxene- orthopyroxene geothermometers applied to the volcanic melt give temperature ranges from 950℃ to 1050℃. During ascending stage,volcanic melt interacted with the garnet along the xenolith fractures or mineral boundaries and transformed to orthopyroxene and plagioclase. Pressure estimated by garnet- orthopyroxne- plagiocalse geobarometers is 1. 30~1. 66 GPa (43~55 km). Pyroxene and plagioclase in the matrix exhibit distinct compositional zoning as the volcanic rock composition evolved. Our results not only first exhibit the texture and mineral chemical features of melt in the middle- deep crust of the northern Qiangtang terrane but also provide petrological constraints on the low S- wave and high anisotropy anomaly in this region. © 2022 Geological Society of China. All rights reserved.