HR: 1330h
AN: V22D-0618    [PDF]
TI: Significance of Diamond-free Heterogeneous UHP Dolomitic Marble From the Kokchetav Massif
AU: * Aoki, K
EM: aoki@earth.edu.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: Local heterogeneity of fluid conditions during UHP metamorphism has been well demonstrated in a cm-scaled continuous sample of UHP marble (sample no. Y665) from the Kumdy-kol in the Kokchetav Massif. This banded carbonate rock sample consists of three zones; zone-A: dolomitic marble with abundant Ti-clinohumite, zone-B: dolomite marble, zone-C: dolomitic marble without forsterite and Ti-clinohumite. All three zones lack diamond. Zone-A consists of Ti-clinohumite, forsterite, diopside, Mg-calcite, dolomite, symplectite, and is characterized by the presence of Ti-clinohumite and by Ti-clinohumite-aragonite tie-line at the peak metamorphic condition. Diopside lacks lamella. Garnet is completely retrograded to symplectite (Di + Spl + Mg-Cal). Large-grained Ti-clinohumite shows clear compositional zoning of TiO$_{2}$ between core (1.5-1.8 wt.%) and rim (2.2-3.4 wt.%). Some Ti-clinohumite grains contain very thin needles with preferred orientations. Zone-B consists mainly of dolomite, diopside, phlogopite, symplectite (Di + Spl + Mg-Cal) with minor amount of Mg-calcite, and is characterized by the stability of dolomite-diopside tie-line and by lack of Ti-clinohumite and forsterite. No lamella was observed in diopside. Garnet is rarely preserved. Zone-C consists mainly of Mg-calcite, dolomite, diopside, garnet and phlogopite and is characterized by phlogopite lamellae in diopside that indicate high pressure conditions. Phlogopite lamellae in diopside were confirmed by the laser Raman spectroscopy (Raman bands: 681.3 cm$^{-1}$ and 357.9 cm$^{-1}$ ) and by microprobe analysis. Lack of Ti-clinohumite and forsterite also characterizes this zone. Garnet has been well preserved from the retrograde reaction. High MgCO$_{3}$ calcite (originally aragonite + dolomite) with large-grained exsolved dolomite lamellae occurs. All three zones have different bulk compositions each other; however, the tie-line relations cannot be explained by their bulk compositions. In zones C and B, diopside-dolomite tie-line was stable, whereas aragonite-Ti-clinohumite tie-line was stable in zone-A. Aragonite-clinohumite pair is stable at very low Xco$_{2}$ and diopside-dolomite pair has wide stability range in Xco$_{2}$ and is stable at higher Xco$_{2}$ than that of aragonite-clinohumite (Ogasawara et al., 1998). Such paragenetic relations observed in the banded carbonate specimen (no.Y665) suggest the local heterogeneity and the gradient of Xco$_{2}$ from zone A (low) to zone C (high) during UHP metamorphism. The infiltration of H$_{2}$O-rich fluid might occur in some layers of the marble to form the difference in the mineral assemblages during UHP metamorphism. References: Ogasawara, Zhang and Liou (1998): The Island Arc, 7, 82-97. Ogasawara, Ohta, Fukasawa, Katayama and Maruyama (2000) The Island Arc, 9, 400-416. Ogasawara, Fukasawa and Maruyama (2002): Am. Mineral. 87, 454-461.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting