Paper No. 23
Presentation Time: 2:30 PM


KEYSER, William Mark, Department of Natural Resources and Environmental Studies, National Dong Hwa University, Hualien, 97401, Taiwan, TSAI, Chin-Ho, Department of Natural Resources and Environmental Studies, National Dong Hwa University, Hualien, 974, Taiwan and IIZUKA, Yoshiyuki, Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan,

The Chinshuichi area is one of two locations containing glaucophane in the high-pressure (HP)/low-temperature (LT) Yuli metamorphic belt of Taiwan. The area consists of metapsammitic and metapelitic rocks, which enclose blocks of serpentinite, metabasite and meta-plagiogranite interpreted as pieces of a dismembered and metamorphosed ophiolite (Liou, 1981; Lin et al., 1984). Recently we discovered a meta-plagiogranite containing aegirine-augite, various amphibole types, albite, phengite, rutile, titanite, epidote, biotite, chlorite and quartz. Aegirine-augite is often replaced by edenite and albite. Amphiboles display complex zoning from glaucophane core to pargasite mantle with winchite/actinolite rim, recording a blueschist-, epidote amphibolite-, to greenschist-facies transition. The existence of glaucophane implies a HP/LT subduction. Using pseudosection modeling, we estimate peak pressure-temperature conditions of 9-12 kbar at 360-420˚C for this rock type. Contrastingly, an epidote amphibolite showing an earlier relatively high-temperature metamorphic stage occurs in the same area. Amphiboles of this rock have brown magnesiohornblende/edenite cores with higher Ti-content decreasing towards green pargasite rims. Sub-micron needles of rutile are limited to amphibole cores whereas ilmenite is included in titanite, indicating a cooling process (Liu et al., 1996). Chlorite is a late-stage replacing amphibole. Compositional zoning in this rock type implies an epidote amphibolite-facies overprinted by a greenschist-facies. Amphiboles of this rock type tend to favor low-pressure (LP)/high-temperature (HT) conditions (Raase, 1974; Laird and Albee, 1981; Hynes, 1982). The coexistence of such contrasting rock types of differing metamorphic histories within the Chinshuichi area is similar to the glaucophane-containing Tamayen mélange in Juisui (Tsai et al., 2013) and is here interpreted as a tectonic mélange. These areas and other serpentinite/metabasite suites probably result from the accretion of an ongoing collision between the Luzon volcanic arc and the Chinese continental margin (Suppe, 1981; Ernst and Jahn, 1987). Our new thermobarometric constraint and rock types provide new insight into boundary conditions of a subduction and mélange-type formations in the Yuli belt.