利用鋯石鈾鉛定年探討南澳地區大南澳片岩之年代

Abstract

東澳－南澳地區的大南澳片岩位於中央山脈東斜面北部，出露岩性包含石英雲母片岩、石英片岩、黑色片岩、薄層大理岩、綠色片岩、角閃岩及花崗片麻岩。大南澳片岩至少歷經南澳運動與蓬萊造山運動，兩次的區域變質變形與斷層作用，使片岩之間的地層上下關係難以分辨，以及片岩中化石資料不足，難以得知片岩的地層時代。因此本研究利用鋯石鈾鉛定年，分析本區域石英雲母片岩、綠色片岩、變質燧石岩及花崗片麻岩，來訂定此地層的年代，以及探討地質構造環境。 本研究共分析24件標本，利用最年輕的碎屑鋯石的平均加權年齡結果，假設當時來源區域有同時代形成的火成岩，並受侵蝕搬運，在原岩為沉積岩的地層中，此最年輕的鋯石群年齡，可被認為岩層的沉積年代。東澳片岩、南澳嶺片岩及武塔片岩三個地層都有晚白堊紀晚期沉積的岩石及年代較老的岩石。最年輕的樣本的沉積年代75±2 Ma (20110810-14)，此年代可視為本區域混同層最年輕的年代。 晚中生代古太平洋板塊持續隱沒至華夏古陸，海洋地殼受擠壓斷裂，使不同年代成因的岩石混合在一起，如角閃岩、大理岩、變質燧石岩、變質再積性火山碎屑岩、石英雲母片岩、綠色片岩。因而，此地區的大南澳片岩認為屬於構造混同層環境，並持續至75 Ma之後。

The Tananao Schist exposed from Dongao to Nanao region is located in the northern part of the eastern limb of Central Range. The lithological characters include quartz-mica schist, metachert, black schist, thin layer of marble, green schist, amphibolite, and gneiss. The Tananao Schist experienced the Nanao Orogeny and Penglai Orogenies. It is difficult to identify the relationships between various schists due to the above-mentioned two orogenies buy reginal metamorphism. In addition, previous studies provided insufficient information of fossil records and dating in the Tananao schist; therefore, it is very difficult to diseriminal the stratigraphic sequerence of these schists. Accordingly, zircon U-Pb dating was used to analyze the quartz-mic schist, green schist, metachert, and gnesiss in this study and determine the stratigraphic age. This study can help us to unravel the tectonic environment of the Tananao schist in Dongao and Nanao region. We analyzed 24 samples and obtain the results of the detrital zircons U-Pb youngest age. In assumption, we could discover the youngest age of detrital zircon of the sedimentary rock in oringal, as if we could find simultaneously igneous rocks that eroded and replaced in the same age. The schists from Dongao Schist, Nanaoling Schist, and Wuta Schist all have some common features such as the sedimentary rocks from the late Cretaceous as well as some other aged. The youngest depositional age is 75±2 Ma among various layer ages. The age may be presented a tectonic mélange in origen. During the late Mesozoic, the Paleo-Pacific Plate subducted beneth to the Cathaysia Bolck. The Paleo-Pacific oceanic plate has been squeezed and ruptured, withing the subduction zone, all different layers from different ages then have been mixed together such as amphibolite, marble, metamorphic rock, meta-volcaniclastic rock, quartz-mica schist, and green schist. Hence, it is suggested that the schists was situated in a tectonic mélange until 75 Ma at least.