Detrital zircon geochronology of the Tananao Metamorphic Complex, Taiwan

Abstract:

The oldest rock of the Tananao Metamorphic Complex exposed at the Backbone Range which consists of marble, schist, and gneiss. According to the stratigraphy and structural analysis, the Complex in the ascending order can be divided into Chiuchu Marble, Paiyang Schist with Kuyuan Schist, and Pilu Formation. Based on Sr isotope dating, it is speculated that the formation age of Chiuchu Marble is closed to 250±20 Ma. Using the zircon U-Pb dating, the magnetic age of gneiss (I-type granite) yields 105- 86 Ma. The stratigraphic age of schist is difficult to define because of fossils are poorly-preserved. In our study, we use the detrital zircon U-Pb dating, by LA-ICPMS, to yield the maximum depositional age of the schist formation. We analyze fifteen samples in this study. The depositional age of the Kuyuan and Paiyang Schists would be not older than late Cretaceous, and the depositional age of the Pilu Formation may have formed in late Cretaceous and younger than Kuyuan and Paiyang Schists. Results from the above mentioned, we are interpreted as the tectonic setting of the Tananao Metamorpic Complex. The region was located on a passive continental margin that formed carbonate platform deposits during the late Permian period. The clastic sediments were unconformably deposited on Chiuchu Marble during the early Cretaceous. Then the paleo-Pacific plate subducted to the Cathaysia Block forming the granite at about 105-86 Ma. Afterwards, the clastic sediments were deeply buried caused by continuing subduction formed as schist rock. The metamorphic event is known as the Nanao orogeny.