Zircon U-Pb isotopic geochronology of Tongao schist in Taiwan and its geological significance

Located in the eastern part of Taiwan, the Tongao schist belongs to the northeast part of Tailuko belt and is composed of chlorite epidote schist, chlorite albite schist, mica quartz schist, quartz schist, and mica schist intercalated with graphite mica schist layer. Based on geological survey in combination with petrologic as well as major and trace element analysis, the authors hold that the original rocks were basalt, foidite or alkaline basalts and terrigenous clastic rocks. The rocks are mainly of alkaline series characterized by low Ba, high Sr and Pb, generally low total rare earth, and REE distribution patterns with features of LREE enrichment and no loss of europium. These features are similar to features of terrigenous rifting (magmatic) volcano-sedimentary rock combination. Zircons can be divided into three types, i.e., euhedral or sharply angular, subangular, and rounded ones. LAICP-MS U-Pb zircon ages have multiple peaks, in which there are four ²⁰⁶Pb/²³⁸U ratios; the minimum value is (95 ±3)Ma, representing euhedral zircons or sharply angular zircons and indicating late Cretaceous volcanic activity; the remaining three values are 118±2 Ma (Early Cretaceous), 160±5 Ma (Late Jurassic), and (250±3) Ma, Early Triassic), representing subangular and small euhedral zircons and indicating the magma age of corresponding source rock; there are two ²⁰⁷Pb/²⁰⁶Pb surface peak ages, i.e., 1838±41Ma and 2404±27Ma (Paleoproterozoic), representing rounded metamorphic zircons or magmatic zircon and indicating the age of Paleoproterozoic metamorphic basement. The Tongao schist was formed during the late Cretaceous in the Eurasian continental margin tectonic environment, being a product of volcanic-sedimentary activity.