大南澳片岩太魯閣帶的碎屑鋯石鈾鉛定年研究

Detrital zircon U-Pb age study of the Tailuko belt in the Tananao Schist, eastern Taiwan

劉丞浩(Cheng-Hao Liu)

指導教授 ： 陳文山

國立臺灣大學/理學院/地質科學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02291

摘要

大南澳片岩於中央山脈東翼出露，台灣最古老的岩層，由各種片岩、大理岩及片麻岩組成。因受到多次變形與變質作用，岩層分佈與接觸關係極為複雜，且缺乏化石與定年資料，不易判釋岩層上下關係。一般認為大理岩時代為晚古生代，片岩的沉積年代為中生代中期。
碎屑鋯石年齡反映沉積物來源的原岩年代，沉積岩中最年輕的鋯石年齡可能與岩層年代相近或較老，本研究利用碎屑鋯石年齡討論中橫大南澳片岩(白楊片岩、谷園片岩)沉積年代。樣本主要採自中部橫貫公路太魯閣至天祥段、木瓜溪及和平溪口，採樣的岩石包括石英雲母片岩、綠色片岩、變質燧石、變質基性岩、硬綠泥石岩。
本研究共分析了34個樣本，各地層碎屑鋯石鈾鉛定年，最年輕鋯石單顆粒年齡：白楊片岩為80±2Ma；谷園片岩為80±2Ma；天長大理岩所夾之變質燧石為126±3Ma；九曲大理岩所夾之片岩為86±2Ma。最年輕鋯石群平均年齡：白楊片岩為110±4Ma；谷園片岩為110±3Ma；天長大理岩所夾之變質燧石為126±3Ma；九曲大理岩所夾之片岩為113±2Ma。由結果推測白楊片岩及谷園片岩原岩之沉積時期同為白堊紀早期或晚期。九曲大理岩所夾之片岩年代與大理岩年代相差甚遠，推測片岩受移位作用影響而夾入九曲大理岩之中。
白楊片岩及谷園片岩的年齡頻譜圖，有中生代、古元古代的年齡群組，可對比到華南構造運動及火成岩體分佈，顯示沉積物來源為華南地區。本研究覆蓋在九曲大理岩之上的片岩於白堊紀早期或晚期沉積，與二疊紀至三疊紀沉積的大理岩之間有相當大的年代缺失，因此，推測在片岩與大理岩之間有個不整合。
根據定年資料及華南地區花崗岩類岩石類型，推演台灣－福建地區在中生代是處於古太平洋向西隱沒至歐亞板塊的環境。

關鍵字

大南澳片岩 ； 碎屑鋯石 ； 鈾鉛定年

並列摘要

The oldest rocks exposed in Taiwan are the Tananao Schist that exposed along the eastern slope of the Central Range. It mainly consists of schist, marble, and metamorphosed granites. The protolith for marble is supposed to be late Permian, and the protolith for schist is middle Mesozoic. Because of lacking fossils and radiometric dating, the depositional age of schist was difficult to define.
U-Pb ages of detrital zircons represent crystallization age of sediment sources (terranes), and the youngest age may be close to or older than the depositional age. Therefore, we use the detrital zircon U-Pb dating, by using LA-Q-ICPMS, to discuss the maximum depositional age of the Schist Belt. Samples collected from Paiyang schist and Kuyuan schist along the Central Cross-Island highway between Tianxiang and Taroko, Mugua river, and Heping river are quartz-mica schist (n=10), metachert (n=13), green schist (n=5), Metabasite (n=4), and Chloritoid (n=2).
Results of this study show that (1) the youngest graphical age peak of samples is 109.5±2.5Ma, and the youngest single grain age of samples is 80±2Ma, which suggest the maximum depositional age of the schist should be not order than Cretaceous; (2) the deposition of marble took place in the Permo-Triassic, but deposition of schist took place in the Cretaceous, indicating that there is a hiatus between schist and marble; (3) I-type granites in Taiwan yield ages of 105-86Ma. Granitic plutons and depositional of the schist is at the same time, which suggest Taiwan-Fujian region is in a subduction environment of paleo-Pacific plate underneath the Eurasian plate during Mesozoic.

並列關鍵字

Tananao Schist ； Detrital Zircon ； U-Pb dating

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