CONTRASTING COMPOSITIONAL TRENDS OF AMPHIBOLES IN THE CHINSHUICHI AREA, YULI BELT, EASTERN TAIWAN: A TECTONIC MELANGE?

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.