AMPHIBOLE COMPOSITIONS AND TECTONIC IMPLICATIONS OF THE WANJUNG SERPENTINITE COMPLEX, YULI BELT, EASTERN TAIWAN

The Wanjung serpentinite complex mainly consists of serpentinite, mafic schist, and metagabbro. Rare metarodingite, serpentinized peridotite, and omphacite-bearing metabasite have also been reported (Lan and Liou, 1981; Yui and Lo, 1989; Beyssac et al., 2008). This complex and two other serpentinite-metabasite suites in Juisui (Tamayen) and in Yuli (Chinshuichi) have been interpreted as part of a dismembered and metamorphosed ophiolite, which represents foreign oceanic materials tectonically transported to the Yuli metamorphic belt (Liou, 1981; Lin, 1999). Recently we discovered a new occurrence in which omphacite-diopside bearing metabasite exists near the contact between serpentinite and mafic schist. The representative omphacite-rich rock contains omphacite, diopside, Mg-rich chlorite, zoisite, albite and titanite, but lacks quartz. Rare rutile and ilmenite are included in titanite. BSE imaging and EPMA mapping reveal that omphacite always contains patchy diopside, implying coexistence of these two phases (cf. Tsujimori, 1997). Mafic schist, metagabbro, and some ultramafic rocks contain amphibole and show compositional zoning. The amphibole core in mafic schist is barroisite, whereas that in metagabbro is edenite to magnesiohornblende. The amphibole rim in both meta-mafics is actinolite. A serpentine-rich “hybrid rock” contains amphibole with relict pargasite-edenite replaced by tremolite. One serpentinized metaperidotite contains olivine, talc, chromite, tremolite, and anthophyllite. The studied amphibole compositions of mafic rocks lie within the range of medium-pressure series on a Na/(Na+Ca) vs. Al/(Al+Si) diagram (Laird and Albee, 1981). Except the omphacite-bearing rock, metamorphic evolution from epidote-amphibolite (or amphibolite) facies to greenschist facies can be inferred for the mafic rocks. The Wanjung complex is a metamorphosed mélange but differs from the Tamayen and the Chinshuichi mélanges in metamorphic facies series, because glaucophane exists in the latter two but not in the former (Liou et al., 1975; Lin et al., 1984; Tsai et al., 2013). These three metamorphosed mafic-ultramafic complexes in the Yuli belt may have been of multiple origins and through different metamorphic histories.