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Formation of diamond requires high pressures and temperatures; thus, its presence in crustal metamorphic rocks has important implications for tectonic models of metamorphism in that burial of crustal rocks to great depths is required. Diamond in crustal rocks had been found only in Koktachev Massiv, northern Kazakhstan, Soviet Union (1). We have found a second occurrence of diamond of metamorphic origin, as shown by the presence of associated coesite and jadeite, in Dabie Shan, China. Dabie Shan forms the eastern part of Qingling orogen in central China, and is transected at the east end by Tan-Lu fault zone. Eclogite facies metamorphic rocks are distributed over an area of more than 1000 km sup 2 in Dabie Shan (Fig. 1). (Figure omitted) Rock types include eclogite; garnet-pyroxenite; and rare jadeitite bands, lenses, and nodules (with length from centimeters to hundreds of meters) intercalated with or enclosed within leucocratic gneisses and marbles. A few of the eclogite bands are intercalated with garnet-peridotite or enclosed in other ultramafic rocks. Diamonds are found as inclusions in garnets in these rocks.
More than 20 diamond crystals were found in polished thin sections. They all occur as inclusions in garnets. Most are 10 to 60 mum across, but some larger grains up to 240 mum were also found. Diamond crystals in thin section are typically euhedral octahedra (Fig. 2A), but a few are cubes (Fib. 2B) or dodecahedra. (Figures omitted) In addition, more than 20 diamond grains were extracted from rock samples for analysis. Most of these are cubes and octahedra (Fig. 2C), but a few are tetrahedra (Fig. 2D). (Figures omitted) These diamonds have diameters around 150 mum, although one is 700 mum. Features such as rouded (Fig. 2A), imbricated (Fig. 2C), and laminar or curved striated (Fig. 2, B and C) crystal faces are visible. (Figures omitted) Identification of diamond was made by optical microscopy at first, then by x-ray diffraction (Fig. 3A) and Raman spectroscopy (Fig. 3B) on a 0.1- to 0.3-mm-thick polished thin section. (Figures omitted) The slight deviation of the peak of the tested diamond in Fig. 3B (1310 to 1320 cm sup -1 ) from that of a given free diamond crystal (1330 cm sup -1 ) is believed to be due to the local...