HR: 1330h
AN: V22D-0617    [PDF]
TI: Diamond-free UHP garnet-clinopyroxene rock from Kumdy-kol in the Kokchetav Massif
AU: * Inoue, Y
EM: inoue@asagi.waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AU: Ogasawara, Y
EM: yoshi777@waseda.jp
AF: Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050 Japan
AB: Diamond-free garnet-clinopyroxene rock that looks like $'$skarns$'$ occurs at the Kumdy-kol in the Kokchetav Massif, northern Kazakhstan. This rock is distinguished from eclogites by lack of diamond and from the garnet-pyroxene rocks previously reported by the chemical compositions of garnet and clinopyroxene. It was unclear whether this garnet-clinopyroxene rock was a product of the Kokchetav UHP metamorphism or not. Recently, we found coesite exsolution needle/plates in titanite and K-feldspar/phlogopite needles or prisms in clinopyroxenes in this rock. These findings indicate that diamond-free garnet-clinopyroxene rock is also a product of UHP metamorphism. The descriptions in detail are as follows. Diamond-free garnet-clinopyroxene rock consists mainly of garnet (40 vol.%), clionopyroxene (45 vol.%), quartz (5 vol.%), amphibole ($<$5 vol.%) and K-feldspar ($<$ 5 vol. %) with minor amounts of titanite, calcite, plagioclase, chlorite and zoisite. Chemical compositions of garnet were plotted in two ranges: grossular-rich (Grs: 66.0-85.5, Pyp: 1.2-8.6, Alm: 21.4-26.8) and relatively pyrope-rich (Grs: 46.6-48.1, Pyp: 17.4-20.7, Alm: 30.6-32.7). Clinopyroxenes fell in the field from salite to ferrosalite. Amphibole, chlorite and zoisite are the secondary products. No diamond found. Tiny coesite needles (10-20 $\mu$m in longer dimension) and plates (10 $\mu$m in longer dimension) were found in titanite in the sample nos. XX-06, -16 and were considered to be exsolved from supersilicic titanite during exhumation stage according to their textural relations. The mode of occurrence of coesite in titanite is similar to that reported in calcite marble from the Kumdy-kol area (Ogasawara et al., 2002); however, the present garnet-clinopyroxene rock is clearly different from the calcite marble by a modal composition of calcite marble (calcite: 40 vol.%, K-feldspar: 30 vol.%, diopside: 20 vol.%) and by the chemical compositions of clinopyroxenes. Coesite was identified by the laser Raman spectroscopy. The strongest Raman band of coesite was detected at 521 cm$^{-1}$ with weak bands at about 119, 270 and 426 cm$^{-1}$. Clinopyroxene (0.3-2.0 mm in diameter) contains exsolved phases of K-feldspar and phlogopite in the sample nos. XX-04, -06, -16 and -22. These phases were identified by the laser Raman spectroscopy and an electron microprobe. K-feldspar shows needle shape (100 $\mu$m in longer dimension) or prismatic shape (20 $\mu$m) and the domain of host clinopyroxene near K-feldspar prism lacks needle-shaped K-feldspar and phlogopite. Such textures suggest that the exsolution of K$_{2}$O-bearing phases occurred at two stages. All these data described above showed diamond-free garnet-clinopyroxene rock had been subjected to UHP metamorphism together with other rocks, eclogite, gneisses and marbles. References: Ogasawara, Fukasawa and Maruyama (2002): {\it Am. Mineral.} {\bf87}, 454-461.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting