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.