Well‐preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Abstract

Ultra‐high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9 mm to analytical point 18) has decreasing XSps and increasing XPyr, while Segment II (from analytical point 18, 1 mm outward to the rim) has increasing XSps and XAlm and decreasing XPyr and XGrs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T path prior to the peak granulite metamorphism. The minimum P–T conditions are estimated to be 1.35 GPa and 850 °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6 GPa and 700 °C. The well‐preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T path is consistent with that of ultra‐high‐pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.