Geochemical and zircon U-Pb dating analysis of metamagmatic rocks constraint on the age and tectonic implication of Tananao Schist, Taiwan

The oldest rock of the Tananao Schist is exposed at the Backbone Range which consists of marble, schist, gneiss, and subordinate metabasites. According to the lithologic characteristics, the Tananao Schist can be divided into Tailuko belt and Yuli belt. The Tailuko belt consists of marble, gneiss, and subordinate scattered metabasite; the Yuli belt is composed of greenschist, serpentinite, meta-tuff, meta-gabbro, metabasite, and glaucophane-schist blocks which enclosed by spotted schist of the host rocks. The metamorphic belts were inferred as a Mesozoic mélange. It’s still controversial due to the difficulty of analyzing metamorphic rocks. In this study, we focus on the zircon U-Pb dating, geochemistry, and petrographic analysis of spotted schist, metabasite, meta-gabbro, and meta-tuff in order to constrain the formation and crystallization ages and interpret its tectonic setting. In Tailuko belt, we can divide metabasites to which intruded to marble, or which erupted to schist, and we use zircon U-Pb dating to figure out that metabasites intruded to marble occurred at 80-90 Ma. According to the geochemical characteristics, we figure out that the metabasites in Tailuko belt could be the same magma characteristic, for Th-Hf/3-Ta diagram and Zr/4-2Nb-Y diagram. It seems to interpret within-plate basalts. In Yuli belt, based on zircon U-Pb dating, the host rocks of spotted schist and the exotic blocks of meta-pyroclastic rock, meta-gabbro (16.5-14.5 Ma), and meta-basalt occurred at mid-Miocene. Geochemical characteristics for meta-basalt and meta-gabbro blocks show Ta-Nb-Ti depletion and LREE depletion in spider diagram occurring volcanic arc and N-MORB type affinities, respectively. Based on the results, Yuli belt have recorded magmatic rocks eruption and Yuli mélange process of Luzon arc. Early Miocene, the end of the South China Sea plate rifting, northwestern moving Luzon arc formed and erupted basalts incipiently, which captured Miocene sediments from the Cathaysia block, and deposited at forearc basin; the mid-Miocene, according to magmatic differentiation andesite erupted, and captured detrital zircon in sediments. Luzon arc continues to move northwest, while early igneous rocks of Luzon arc and Cenozoic sediments in the forearc basin thrusted together into the subduction zone; in mid-Miocene to Late Miocene , accretionary prism through rock metamorphism, and finally Luzon arc overthrusted the Eurasian continent, as part of the island of Taiwan, accretionary prism exposed to the surface during Penglai orogeny, formed Yuli metamorphic belt formed today.