TOP BOUNDARY OF THE DABIE UHPM ROCKS, CENTRAL CHINA

The top boundary of the ultra-high pressure metamorphic (UHPM) rocks i n the Dabie Mountains, central China, has been mapped. The results sho w that the top boundary of the UHPM rocks is occupied by flat-lying fe lsic gneiss, on which tonalitic gneiss rests. Petrological and mineral ogical evidence indicates a wide P-T gap across the boundary with an e stimated peak metamorphic P-T value of the UHPM rocks being >30 kbar a nd 800 +/- 50 degrees C, and that of the tonalitic gneiss around 11 kb ar and 800 degrees C. Their P-T-t paths are different. The former has a clockwise path and the latter is counterclockwise. This implies that the UHPM rocks at the bottom and the tonalitic gneiss on top have bee n structurally juxtaposed together. The top boundary, i.e. the contact between the UHPM rocks and tonalitic gneiss, shows characteristics of a ductile shear zone. Microscopically, feldspar and quartz in the she ar zone are characterized by ductile deformation. In larger scale, the top boundary is undulating. Shear sense in the top boundary is a two- way direction. Only the latest shear sense measured with c-axes of qua rtz show a consistent direction of top-to-the-north. The study provide s some constraints to the exhumation process of the Dabie UHPM rocks.