HR: 14:35h
AN: V42F-04    [PDF]
TI: Subduction-stage P-T path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction
AU: * Aoya, M
EM: f030402r@mbox.nagoya-u.ac.jp
AF: Nagoya University, Department of Earth & Planetary Sciences,Nagoya University, Nagoya, 464-8602 Japan
AU: Uehara, S
EM: Shin.Uehara@pet.hw.ac.uk
AF: Heriot-Watt University, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS United Kingdom
AU: Wallis, S R
EM: swallis@eps.nagoya-u.ac.jp
AF: Nagoya University, Department of Earth & Planetary Sciences,Nagoya University, Nagoya, 464-8602 Japan
AU: Enami, M
EM: enami@eps.nagoya-u.ac.jp
AF: Nagoya University, Department of Earth & Planetary Sciences,Nagoya University, Nagoya, 464-8602 Japan
AB: The Sambagawa belt in SW Japan is a subduction-type high-P/T metamorphic belt. Subduction-stage P-T paths of its constituent rocks are important because they directly constrain physical conditions of the EarthOs interior at the time exhumation of high-P/T metamorphic rocks became feasible. Although a few examples of subduction-stage P-T paths for the Sambagawa rocks have been recognized, these are limited to relatively low-pressure regions (~10 kbar). To augment these data the subduction-stage P-T path of the Kotsu glaucophane (Gln) eclogite is derived. The tectonic significance of the derived and previously determined P-T paths is further examined using a new thermal model. By using compositions of matrix minerals and rims of porphyroblastic garnet (Grt), the peak-T conditions of the Kotsu Gln eclogite have been estimated as ~20 kbar/ 600$\deg$C. However, the dP/dT of the P-T path leading to the peak-T conditions is unknown. Petrological studies focusing on inclusion minerals in Grt show: (1) albite is absent as inclusions within Grt; (2) acmite (Acm) component of cpx decreased during growth of Grt; (3) Tschermakite (Ts) component of amphibole decreased and Gln component increased during growth of Grt; and (4) Grt-Cpx thermometry shows a temperature increase during growth of Grt. Along with mineral textures observed in the matrix, the Gln-formation reaction can be determined as: 4Acm + 2Ts + 2quartz + H2O (R) 2Gln + 2epidote + hematite. P-T curve of this reaction always has a large positive dP/dT (>7.1 kbar/100 ?C) with the Gln stability field on the high-P/T side. To cross this reaction curve into the Gln stability field during a rise in temperature, the Kotsu eclogite must trace a very steep subduction-type P-T path. Compilation of previously obtained subduction-stage P-T paths for the Sambagawa rocks along with the P-T path of the Kotsu Gln eclogite shows that the series of subduction-stage P-T paths are not distributed on a straight line starting from the origin, but in a curve with dP/dT increasing with metamorphic pressure. In previous thermal models curved P-T paths of the same kind were predicted for subduction of a very young slab ($<5$ Ma) suggesting that the curved P-T paths will be formed just before oceanic-ridge subduction. A new model incorporating progressive approach of an oceanic ridge to a subduction zone shows that the series of the Sambagawa subduction P-T paths well fit the results for a setting where a ridge is close to being subducted at a slow rate relative to the subduction rate. The subduction P-T paths from the Sambagawa belt can, therefore, be regarded as a prophetic record of the subsequent ridge subduction. This suggests that exhumation of high-P/T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges.
DE: 3660 Metamorphic petrology
DE: 8130 Heat generation and transport
DE: 8150 Plate boundary--general (3040)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting