HR: 1330h
AN: V22B-0580    [PDF]
TI: Mode of Occurrence, Chemistry, Equilibrium Domain, and Texture of Non-equilibrium Growth of Garnet and PT Evolution of Regional Metamorphism
AU: Banno, S
EM: banno.shohei@schist02.mbox.media.kyoto-u.ac.jp
AF: Dept.Geol.Kyoto Univ., Kitashirakawa Sakyo-ku, Kyoto, 606-8107 Japan
AU: * Kitamurs, M
EM: kitamura@mine.kueps.kyoto-u.ac.jp
AF: Dept.Geol.Kyoto Univ., Kitashirakawa Sakyo-ku, Kyoto, 606-8107 Japan
AB: The bell-shaped Mn distribution of garnet was noticed in the Sanbagawa belt in 1965. The sequence of mineral zones from low to high temperature of this belt, ascending as the chlorite, garnet and biotite zones, has been questioned as it owes to high Mn and high Ca contents of the pelitic rocks. However a trial pseudosection (Matsumoto and Banno to submit) shows that it is due to nothing but higher pressure of metamorphism. Garnet presumably formed maintaining surface equilibrium with chlorite, epidote and calcite. We confirmed Goto et al. (2002)_fs notion that the pelitic schists of the Sanbagawa schists in central Shikoku contain calcite by further examining the cores of 3 drill holes of 2000m long. It follows that garnet forms at lower temperature and pressure than hitherto considered as grossular is formed more easily by calcite-bearing assemblage than that free from it. Garnet from Sanbagawa pelitic schists shows, 1) different mineral zones have slightly different P-T paths, 2) Ca maximum in concentrically zoned garnet shows that the maximum pressure is reached in a middle grade of prograde metamorphism, 3) garnets that grew in a volume of 1cm3 or so have the same zoning pattern suggesting that the domain of equilibrium is fairly large. Thus garnet is an excellent tool to decipher the P-T path of the Sanbagawa regional Metamorphism. A few new types of garnet zoning formed by non-equilibrium crystallization have been found. The first type is the vicinal sector zoning that develops normal to crystal surface. The second type is concentric zoning with Mn-poor core from which XMn increases gradually to the smooth maximum in the midst of garnet (figure for Mn). After the maximum is reached XMn decreases to the rim where XMn is similar to the value of normal garnet porphyroblast. We define the _gnormal zoning_h for the crystal grows without, in extreme case, body diffusion under fractionation (e.g, plagioclase in igneous fractionation and garnet in prograde metamorphism) and reverse zoning vice versa. The reverse zoning mentioned above forms when the system is rapidly brought to multi-components reaction volume (Banno, to submit). Reverse zoning can be numerically simulated by using the non-equilibrium model with various initial conditions (Kitamura, to submit) to find optimum conditions to particular garnet. Garnets formed by these processes occur characteristically nearby the epidote amphibolite masses, retrograded eclogites. This suggests that those masses had higher temperature than that of the Sanbagawa schist to which they emplaced.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3620 Crystal chemistry
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting