HR: 16:44h
AN: V14A-04    [Abstracts]
TI: Re-interpretation of Progressive Metamorphism, Facies Series, P-T-t Path, and Exhumation Model for Global Collisional Orogenic Belts
AU: * Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Dept. of Earth and Planetary Sciences, Tokyo, 152 Japan
AU: Liou, J G
EM: liou@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305 United States
AB: Recent discovery of ultrahigh (UHP) minerals, and descriptions of geology, petrology, geochronology and geochemistry of the global collisional orogens leads us to re-interpret some basic concepts of orogeny. (1) A collisional belt consists of a thin tectonic slice with P-T maximum at structural intermediate. The UHP-HP unit is a thin slab up to a few km thick and bounded on the top and bottom by normal and reverse faults, respectively, and separated from the low-grade or low-P metamorphic rocks. (2) The underlying unit is thermally metamorphosed to form andalusite-sillimanite metamorphic rocks in some terranes by the tectonic intrusion of UHP-HP rocks. (3) Metamorphic facies series ranges from the greenschist/blueschist transition or the blueschist, through epidote-amphibolite, quartz- and zoisite-eclogite, to dry eclogite facies with a sharp kink point, indicating anti-clockwise P-T path. This facies series is consistent with the calculated P-T path by inclusion mineralogy in garnet combined with compositions of zoned garnet and paragenesis of mineral inclusions in zircon. (5) The extensive hydration at mid-crustal level obliterated the pre-existing UHP-HP mineralogy, except for mineral inclusions in garnet, zircon and omphacite. (6) Zoned zircons with UHP-HP minerals mantled by late-stage hydration stage with Barrovian minerals from the Kokchetav Massif are dated as ca. 30 m.y. older from that of hydration stage at rim, indicating a slow tectonic exhumation. (7) Combining above constraints, a tectonic extrusion model is most probable (Maruyama, 1990; Maruyama & Liou, 1994; Maruyama et al., 1996). (8) Previous interpretation (e.g., England and Thompson, 1984) of progressive metamorphism, facies series, P-T-t path are all quite different from those we summarized. Most of the observed assemblages exhibit the late-stage crustal metamorphism due to extensive hydration and retrograde recrystallization. The role of water must be re-evaluated to mask the progressive nature of metamorphism during subduction.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting