利用變質火成岩岩石地球化學及鋯石鈾鉛定年探討大南澳片岩帶的年代與構造演化

Geochemical and zircon U-Pb dating analysis of metamagmatic rocks constraint on the age and tectonic implication of Tananao Schist, Taiwan

洪嘉佳(Chia-Chia Hung)

指導教授 ： 陳文山

共同指導教授 ： 朱美妃(Mei-Fei Chu)

國立臺灣大學/理學院/地質科學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201601985

摘要

大南澳片岩出露於脊樑山脈東翼，為台灣最古老的地層，根據岩相特性將大南澳片岩帶分為太魯閣帶以及玉里帶。太魯閣帶岩性主要為大理岩、片岩及片麻岩，變質基性岩則零星出露，而玉里帶岩性主要由黑色片岩、綠色片岩、石英雲母片岩組成，這些基底岩包裹著蛇紋岩、角閃岩、藍閃石片岩、變質基性岩、變質火山碎屑岩等岩塊。過去普遍認為玉里帶是中生代的混同層，近期研究方法進步，利用鋯石鈾鉛定年法應用在太魯閣及玉里帶中的變質基性火成岩，便於我們了解這些基性岩的岩漿年代，並且藉由岩石地球化學分析能更進一步解析形成火成岩噴的地體構造環境，嘗試解決大南澳片岩中變質基性岩的年代及環境，進而討論台灣的演化歷史。
本研究共分析31個地球化學樣本、22個鋯石樣本，根據野外產狀和地球化學結果顯示太魯閣帶的樣本為產狀不同的大陸地殼的板內玄武岩，岩石生成年代約為80-90 Ma；玉里帶樣本為混同層中出現的各式火成岩樣本，從地球化學結果來看這些樣本都具有島弧岩漿的Ta-Nb-Ti虧損，是為呂宋島弧初期生成時所噴發的岩漿，而具有單一年代的鋯石樣本約分布在16.5∼14.5 Ma，而含有繼承鋯石的火成岩樣本的頻譜為台灣中新世沉積岩地層的碎屑鋯石頻譜，顯示玉里帶中的變質火成岩岩漿形成時期應為中新世早－中期。
綜合上述結果，台灣東部地區於中新世地體構造演化記錄呂宋島弧初期岩漿噴發的火成岩及混同層形成的過程。中新世初期，南中國海板塊張裂結束，呂宋島弧形成後往西北方移動，島弧在噴發初期所形成的玄武岩噴發，捕獲來自華南板塊的中新世沉積物，並在前弧盆地中沉積；中新世中期，岩漿分異作用安山岩噴發，捕捉沉積物中的碎屑鋯石。呂宋島弧繼續往西北方移動，而這些呂宋島弧初期形成的火成岩因為逆衝作用進入隱沒帶，和新生代沉積物同時被捲進隱沒帶中；中中新世至晚中新世，增積岩體經過變質作用，最後在中新世晚期因為呂宋島弧上衝到歐亞大陸之上，成為台灣島的一部分，增積岩體也在造山過程中逐漸出露至地表，形成現今的玉里變質帶。

關鍵字

碎屑鋯石 ； 繼承鋯石 ； 岩漿鋯石 ； 鈾鉛定年 ； 大南澳片岩 ； 變質火成岩 ； 岩石地球化學

並列摘要

The oldest rock of the Tananao Schist is exposed at the Backbone Range which consists of marble, schist, gneiss, and subordinate metabasites. According to the lithologic characteristics, the Tananao Schist can be divided into Tailuko belt and Yuli belt. The Tailuko belt consists of marble, gneiss, and subordinate scattered metabasite; the Yuli belt is composed of greenschist, serpentinite, meta-tuff, meta-gabbro, metabasite, and glaucophane-schist blocks which enclosed by spotted schist of the host rocks. The metamorphic belts were inferred as a Mesozoic mélange. It’s still controversial due to the difficulty of analyzing metamorphic rocks. In this study, we focus on the zircon U-Pb dating, geochemistry, and petrographic analysis of spotted schist, metabasite, meta-gabbro, and meta-tuff in order to constrain the formation and crystallization ages and interpret its tectonic setting.
In Tailuko belt, we can divide metabasites to which intruded to marble, or which erupted to schist, and we use zircon U-Pb dating to figure out that metabasites intruded to marble occurred at 80-90 Ma. According to the geochemical characteristics, we figure out that the metabasites in Tailuko belt could be the same magma characteristic, for Th-Hf/3-Ta diagram and Zr/4-2Nb-Y diagram. It seems to interpret within-plate basalts.
In Yuli belt, based on zircon U-Pb dating, the host rocks of spotted schist and the exotic blocks of meta-pyroclastic rock, meta-gabbro (16.5-14.5 Ma), and meta-basalt occurred at mid-Miocene. Geochemical characteristics for meta-basalt and meta-gabbro blocks show Ta-Nb-Ti depletion and LREE depletion in spider diagram occurring volcanic arc and N-MORB type affinities, respectively.
Based on the results, Yuli belt have recorded magmatic rocks eruption and Yuli mélange process of Luzon arc. Early Miocene, the end of the South China Sea plate rifting, northwestern moving Luzon arc formed and erupted basalts incipiently, which captured Miocene sediments from the Cathaysia block, and deposited at forearc basin; the mid-Miocene, according to magmatic differentiation andesite erupted, and captured detrital zircon in sediments. Luzon arc continues to move northwest, while early igneous rocks of Luzon arc and Cenozoic sediments in the forearc basin thrusted together into the subduction zone; in mid-Miocene to Late Miocene , accretionary prism through rock metamorphism, and finally Luzon arc overthrusted the Eurasian continent, as part of the island of Taiwan, accretionary prism exposed to the surface during Penglai orogeny, formed Yuli metamorphic belt formed today.

並列關鍵字

Detrital Zircon ； Inherited zircon ； U-Pb dating ； Tananao Schist ； metabasite ； geochemical analysis

參考文獻

劉丞浩（2013）大南澳片岩太魯閣帶的碎屑鋯石鈾鉛定年研究。臺灣大學地質科學研究所學位論文，共142頁。

周賢元（2013）利用碎屑鋯石鈾鉛定年解析台灣中部始新統—上新統之沉積來源及剝蝕歷史. 臺灣大學地質科學研究所學位論文，共86頁。

邵文佑（2015）利用鋯石鈾鉛定年與鉿同位素組成探討台灣東部火成岩之岩石成因，臺灣大學地質科學研究所學位論文，共287頁。

黃奕彰（2015）利用碎屑鋯石鈾鉛定年探討大南澳片岩的地層年代與構造演化。臺灣大學地質科學研究所學位論文，199頁。

鄧嘉翔（2013）大南澳片岩鋯石的鈾鉛定年研究。國立中正大學地震研究所碩士論文，共147頁。