Detrital zircon U-Pb geochronology constraint on the formation age of the Tananao Schist and tectonic implication, Taiwan

• Main Authors: Yi-Chang Huang, 黃奕彰
• Other Authors: Wen-Shan Chen
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/70825041457560846349

碩士 === 國立臺灣大學 === 地質科學研究所 === 103 === The Tananao Schist, the oldest rock, exposed at the Backbone Range which consists of marble, schist, gneiss, and subordinate scattered metabasites. According to the stratigraphy and structural analysis, the Tananao Schist in the ascending order can be divided into the Chiuchu Marble, the Kuyuan Schist, the Paiyang Schist, the Tienchang marble, and the Pilu Formation. Based on previous study, depositional age of the Chiuchu Marble is closed to 250±20 Ma(Late Permian). Using zircon U-Pb dating method, magmatic age of gneiss (I-type granite) yields 90- 86 Ma. Depositional age of schist formation was difficult to define because fossils were poorly-preserved. In present study, we try to use detrital zircon U-Pb dating to yield the maximum depositional age of schist. In this study, the youngest weight mean ages of the Paiyang Schist and Pilu Formation in western part of Tailuko belt are 180-135 Ma, which are older than the Kuyuan and Paiyang Schists exposed in eastern part of the Tailuko belt. Based on macroscopic structure of straitigraphy, microscopic structure analysis, and metamorphic facies, stratigraphy in western part of the Tailuko belt is younger than that in eastern part. Source of the Tarluko belt mainly derived from ignous rock of the Mesozoic basement in Taiwan Strait. Stratigraphy in western part of the Tailuko belt lacks younger detrital zircons because of reverse exhumation. The chemical composition of the metabasites of the Tailuko belt was suggested that they were relics of oceanic crust. But in our study, 90-80 Ma metabasite intruded into Chiuchu Marble probability represts for fractions of metabasites of the Tailuko belt might form in origin of continental crust. More detail chemical analysis and examinations are necessary in the future. Results as above mentioned, we explain tectonic setting of the Tananao Schist. Taiwan region was located on passive continental margin and deposited carbonates during late Permian period. The paleo-Pacific plate westward subducted to the Cathaysia Block along continental margin, and the carbonates are metamorphosed to marble since early Jurassic. The subduction related metamorphism is called the Tailuko orogeny. Clastic sediments were unconformably deposited on the Chiuchu Marble during early Cretaceous. At the same time, the island arc migrated from southeastern China to Taiwan Strait because of the retreating arc system. And then, I-type granite and mafic igneous rock intruded into marble and clastic sediments of Taiwan during 90-86 Ma. Afterwards, clastic sediments were deeply buried caused by continuing subduction and metamorphosed. The metamorphic event is known as the Nanao orogeny.