DIAMONDIFEROUS HIGH-PRESSURE METAMORPHIC ROCKS IN DABIE SHAN, ANHUI: MINERAL PARAGENESIS AND METAMORPHIC CONDITIONS
Xu Shutong,Su Wen,Liu Yican and Jiang Laili (Anhui Institute of Geology,Hefei,230001)
Eclogite bands and lenses, impure marble bands and very rare ultra-mafic lenses occur intercalated within the leucocratic gneisses. There are thousands of eclogite bands, lenses and nodules in various stages of preservation and ranging in size from a few centimetres to thousand metres. By the diagnostic mineral assemblages, eclogitic rocks are subdivided into ; (1) eclogite ; (2) garnet-pyroxenite; (3) garnet-low pyroxene-granulite and (4) garnet-peridotite. There are jadeite-bearing gneiss and lenticular jadeitite in the immediate country rock of eclogite and garnet-pyroxenite. Diamonds, coesite with its quartz pseudomorphs and, sometimes, jadeite occur in eclogite, garnet-pyroxenite or jadeitite, in which the mineral components are garnet, omphacite, diopside, rutile, kyanite , zoisite, epidote, titanite, biotite, chlorite, amphibole, diamond, coesite, plagioclase, quartz, phengite, glaucophane, barroisite, jadeite, lawsonite, calcite aragonite, with rare orthopyroxene and phlogopite. By the formation condi- tions and the relation between them, four generations of mineral parageneses are discerned: (1) Glau +Barr+Ep+Q+Ph; (2) Omph+Gt+Cs+Diam+J+Ar+Ru+Ky+Zo+Phl; (3) Di+Gt+Pl +Amph+Ti+Cc+Q; (4) Amph+Pl+Chl+Bi. These Show that there are four stages in the meta- morphic process of the diamond-bearing rocks in Dabie Shan. Average size of 40 discovered diamond grains is about 100μm, and some larger ones up to 300μm. Most diamond crystals are euhedra oc- tahedron and hexahedron, a few dodecahedron and tetrahedron. Diamonds in thin sections are identified by using X-ray diffraction and Raman spectrum microscopically. Diamonds extracted from artificial heavy sand are identified by X-ray with triaxial Gandolfi camera (designed by Dr. Zhang Hanging) The presence of the assmblage, Diam+Cs+J, indicates that diamonds and diamondiferous rocks are meta- morphic products under the pressure more than 4. 0 GPa and temperature above 900℃. The presence of four generations of mineral parageneses indicates that the continental rock slice containing eclogites once subducted from shallow level (where blue schist facies formed) to depth more than 120km (where diamond or eclogite facies formed) and returned to surface (where amphibolite and greenschist facies formed) finally. Diamonds from Dabie Shan is the second occurrence of the diamond of metamorphic origin after that from north Kazakhstan, USSR, whereas the mineral assemblage and rock association in Dabie Shan are the first one in the world.