REINTERPRETING HIGH-PRESSURE TECTONIC BLOCKS IN THE YULI BELT OF TAIWAN

High-pressure (HP) metamorphic rocks in Taiwan occur only within the Yuli belt, which is among the youngest blueschist belts worldwide (Ota and Kaneko, 2010). The Yuli belt consists mainly of pelitic and psammitic schists. Allochthonous mafic-ultramafic rock associations, mostly greenschists, albite epidote-amphibolite, and serpentinites, occur as discontinued blocks or "melanges" in the Fengtien, Wanjung, Juisui (Tamayen), and Yuli (Chinshuishi) areas. These exotic masses were previously interpreted as a “dismembered ophiolite” (Liou, 1981). Within some of these tectonic blocks, rare HP rocks, including Mn-rich garnet epidote-blueschist, garnet-free omphacitic metagabbro, and aegirine-augite/glaucophane-bearing metaplagiogranite, have been recently discovered in Tamayen, Wanjung, and Chinshuishi respectively (Tsai et al., 2013; Lan, 2011; Keyser et al., 2013). Although these mafic-ultramafic rocks appear to be exotic in origin relative to the metasediments of the bulk Yuli belt, our new pseudosection modeling results, combined with field and petrographic observations, indicate that the Tamayen and Chinshuishi tectonic blocks might have been isofacial with the surrounding garnet-phengite-bearing pelitic schists during HP metamorphism. However, unlike these two blocks, the Wanjung meta-mafic rocks contain relict diopside and relatively high-temperature calcic amphiboles, but no glaucophane. Some of the Wanjung mafic rocks show high Mg and Al contents in whole-rock composition, and a cumulate nature of protolith. Geochronological studies reveal late-Miocene metamorphic ages (Jahn et al., 1981; Sandmann et al., 2013) and a mid-Miocene protolith (magmatic) age of garnet epidote-blueschist samples from the Tamayen block (Chen et al., 2013). We reinterpret the Wanjung block as relics of a fore-arc mantle wedge and the Tamayen and Chinshuishi blocks as mixtures of subducted oceanic crust and sediments.