METAMORPHIC EVOLUTION OF THE HAIZHOUQUN BLUESCHIST IN SUBEI, CHINA

The Subei Haizhouqun blueschists occur in the northeastern part of North Yangzi Block blueschist belt, which extends from the Haizhouqun region southwest to the Zhangbaling areas and across the Tanlu Fault northwest to the Dabie and Qingling areas. The typical mineral assemblage for hand samples is Mrb + Act + Ep + Chl + Phg + Ab + Qtz. Mineralogical studies indicate that amphiboles show strong zonation with sodic amphibole (magnesioriebeckite) in the core and sodic-calcic (winchite) or calcic amphibole (actinolite) at the edge. The micas are all phengites with high Si value. P-T (pressure-temperature) grid was obtained by using phase equiliria program (THERMOCALC V3.1) and thermodynamic data of Holland and Powell (1998) with activity models discussed in Holland and Powell (1998). The calculated grid not only limits the Subei Haizhouqun epidote-blueschist phase but also provides us a good tool to constrain the metamorphic P-T path. By combining P-T grid technique and other metamorphic P-T estimate methods, both peak and retrograde metamorphic P-T conditions were estimated, and they are 6.5-7.5 kbar and 350-420°C for peak metamorphism, and 4.2-5.0 kbar and 320-390°C for retrograde metamorphism. Based on these estimated metamorphic P-T conditions, we derived a clockwise retrograde P-T path, which is composed of two stages: (1) peak metamorphism stage and (2) blueschist retrograde stage. Consequently, the blueschists from Subei Haizhouqun of China have undergone high-pressure (HP) metamorphism and later uplift and exhumation.