從台灣中部板岩帶之熱變質記錄探討台灣活動造山帶構造演化

Structural evolution of active Taiwan orogeny inferred from thermal metamorphic documentation of central Taiwan slate belt

吳俁(Yu Wu)

指導教授 ： 徐澔德 ； 李建成

國立臺灣大學/理學院/地質科學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201800497

摘要

台灣造山帶是由呂宋島弧碰撞歐亞板塊被動大陸邊緣所形成，此一典型造山帶地表出露之岩石大部分是由被動大陸邊緣基盤及其上沉積物堆疊變形而成。其中，雪山山脈原為始新世至中新世時被動大陸邊緣半地塹中的沉積物，於碰撞造山後加入造山楔中，成為現今的雪山山脈。若能了解雪山山脈的變質溫度紀錄，進一步推演物質之熱運動學歷史，對重建台灣造山帶的演化過程有很大的幫助。而一直以來都具有爭議的雪山山脈─脊梁山脈板岩帶邊界的性質，也能透過邊界兩側之變質溫度分布來判明。
因板岩帶地層中多富含碳質物，本研究將以碳質物拉曼光譜法(Raman spectroscopy of carbonaceous material, RSCM)來測量岩石在深埋及抬升變形中所經歷過的最高溫度(maximum temperature, TM)，於本研究中稱為RSCM-T。
本研究於前人工作資料較少雪山山脈南段及其相鄰脊梁山脈板岩帶中，以約2公里的間隔沿著垂直該區域岩層走向的剖面採集25個RSCM樣本，建立兩條橫跨雪山山脈南段與相鄰脊梁山脈板岩帶的RSCM-T剖面。由實驗所得RSCM-T剖面中可觀察出，本研究區域中地利斷層以東的內雪山山脈地層中所測得之RSCM-T皆相當接近，指示了內雪山山脈地層經歷了「褶皺後達到最高溫度」之變形模式；而在脊梁山脈板岩帶中，北、中、南段皆有RSCM-T向東增加之趨勢；此外也確認雪山山脈與脊梁山脈板岩帶之間應存在梨山斷層，將雪山山脈一側抬升；而內雪山山脈中可能還存在一沉積年代為漸新世晚期至中新世早期的板岩層。
分析台灣中部板岩帶熱變質紀錄後，本研究提出了雪山山脈南段之內雪山山脈以及梨山斷層其地層為主之變質、變形歷史，認為內雪山山脈地層先隨著被動大陸邊緣俯衝至造山楔下方，遭基底滑脫面之雙軌構造帶入台灣造山帶底部，並受到褶皺變形，接著被加溫達到TM，最終抬升至現今位置。

關鍵字

台灣造山帶 ； 雪山山脈 ； 梨山斷層 ； 變質溫度 ； 碳質物拉曼光譜(RSCM)

並列摘要

Taiwan orogenic belt is built by the collision between the Luzon arc of the Philippine Sea plate and the passive continental margin of the Eurasian plate. Rocks exposed in the orogenic belt were mostly deformed from the passive margin basement and cover series. Among the whole orogeny belt, the slate belt, which contains the Hsuehshan Range (HR) and the Backbone Range slate belt (BRSB), is composed of metamorphosed Eocene to Miocene sediments which experienced only the Neogene Taiwan orogeny after diagenesis at the passive margin. Characterizing the thermal metamorphic history of the slate belt is essential for reconstructing its geological evolution during the mountain building processes. The so far controversial contact relationship between HR and its juxtaposed BRSB can also be resolved by the thermal metamorphic documentation.
Since carbonaceous material is common in pelitic protolith of slates in the slate belt, the Raman spectrum of carbonaceous material (RSCM) measuring the rock maximum temperature(TM) is chosen for quantitative thermal metamorphic documentation. TM which measured from RSCM method is called RSCM-T in this research.
Previous studies have mostly focused on northern and central part of HR and presented different thermal metamorphic histories. Therefore, the target of this study is to obtain and analyze the thermal metamorphic documentation in the southern part of HR and the juxtaposed BRSB. 25 samples are taken from the Puli and Wushe area in Central Taiwan slate belt with a dense 2-km spacing, and two maximum temperature profiles are constructed. Results show that protolith of inner HR, which located on the eastern side of the Tili fault, had been folded while it reached maximum heating stage, and the eastward increasing trend of RSCM-T is found in the juxtaposed BRSB. Furthermore, the existence of Lishan Fault, which made its western side uplifted, is confirmed at the boundary between HR and BRSB. Furthermore, a slate formation which deposited in late Oligocene to early Miocene is proposed to be the youngest layer in the inner HR.
The evolution history of inner HR and Lishan Fault inferred from the result shows that the proto-inner HR strata were deformed and transferred from the passive margin into orogenic wedge through the basal accretion mechanism, and then achieved maximum heating.

並列關鍵字

Taiwan orogeny ； Hsuehshan Range ； Lishan fault ； thermal metamorphism ； Raman spectrum of carbonaceous material (RSCM)

參考文獻

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