台灣中央山脈東翼玉里帶東側及板岩帶變形歷史研究

Deformation history in the retrowedge during mountain building: A case study from the easternmost Yuli belt and slate belt in eastern Central Range of Taiwan

楊凱翔(Kai-Hsiang Yang)

指導教授 ： 陳于高

共同指導教授 ： 李建成(Jian-Cheng Lee)

國立臺灣大學/理學院/地質科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102709

摘要

本研究的目的是，為了更適切地了解台灣中央山脈東翼造山的過程，特別是在整個中央山脈發育的扇狀劈理中後楔(retro-wedge)的多期劈理發育及變形過程。近期的研究指出，中央山脈東翼的玉里帶發育了多期劈理，其中一期主要穿透性的劈理，可能是在隱沒過程中經歷溫壓峰值（Peak P/T）所形成，爾後在掘升(exhumation)過程中，造成了劈理倒轉的現象。但是玉里帶東側的板岩帶，似乎缺乏明確劈理倒轉的資料及證據。在本研究中，針對玉里帶的東側及板岩帶進行調查研究，主要沿著清水溪、鹿寮溪及鹿野溪進行野外調查，並採樣本進行薄片分析觀察，並結合近幾年發表鋯石鈾鉛定年與變質溫壓軌跡圖(P-T-t path)的資料，提供中央山脈東翼玉里帶及其上覆的東部板岩帶之構造演化的分析及解釋。
從野外露頭的觀察和薄片分析的結果，建立在多期劈理的形貌及疊置先後關係，我們將玉里帶與板岩帶分別賦予各自的變形期以及伴隨的變質作用：1）玉里帶D1、D2、D3及對應形成的劈理（S1、S2、S3）；2）板岩帶D1、D2’、Df及對應形成的劈理（S1、S1’、S2’、Sf）
以玉里帶基質中的石英雲母片岩而言，除了圍繞在一些超基性外來岩塊周圍片岩的劈理面外，在清水溪玉里帶D2變形期主要是「S1具向西伸向不對稱褶皺」伴隨「北東走向、向東傾、穿透性強的S2劈理（30°-40°）」；而鹿寮溪玉里帶東側D2變形期主要是「S1向東伸向不對稱褶皺」伴隨「北東走向、向西傾S2劈理（20°-40°）」，兩流域玉里帶的D2褶皺伸向相異。另外兩流域的玉里帶片岩局部可見D3變形期較微弱的次水平劈理S3發育。
玉里帶上覆的東部板岩帶，為中新世隱沒濁流岩中最年輕的地層。其變質度的範圍涵括了由片岩過渡到變質砂岩、千枚岩漸變至板岩。東部板岩帶D1變形期，呈現S0向西伸向褶皺或等斜褶皺與伴隨而生的S1劈理，走向以北東為主，與玉里帶D2變形期經歷的地體構造事件(T2)雷同，主要是東西向或西北西向的近水平擠壓作用。此外次生劈理也局部發育，例如近東西走向的直立褶皺與傾角近鉛直次生劈理S2’ （D2’變形期）。最後在造山掘升後期中的變形上，在近剪切帶的區域常可觀察到次生劈理Sf與近水平褶皺（Df變形期），劈理發育及位態與玉里帶S3相似，本研究解釋兩者經歷一樣的地體構造事件(T3)，是以鉛直向最大應力的作用。
薄片分析中，在清水溪玉里帶蛇紋岩塊體邊緣的片岩帶樣本中，觀察到S2面間有殘存的S1與石榴子石，以及石榴子石周圍新生成退變質之綠泥石。另外在鹿寮溪玉里帶片岩D3變形期樣本中，觀察到石英再結晶的形貌顯示為較低溫的膨脹再結晶作用（BLG），指示玉里帶D1¬-D2-D3時期為一持續降溫過程，至少在蛇紋岩塊體周遭的片岩如此。然而，板岩帶樣本中S1劈理發育情況與石英再結晶(BLG、SGR)皆顯示D1變形期較玉里帶D2變形期低溫。板岩帶之D2’與Df變形期的薄片樣本，也顯示最低溫的再結晶(BLG) ，顯示整個板岩帶不同變形期變質的溫度可能相當接近。
綜合構造分析，本研究提出以下中央山脈東翼的地體構造演化及伴隨之變形變質構造。玉里帶在隱沒過程中（T1，～15 － ～6Ma），濁流岩或傾瀉岩體產生褶皺 (F1) 並伴隨S1劈理發育（玉里帶D1變形期）。爾後在隱沒作用後期或掘升初期，受到菲律賓海板塊的貼近造成的聚合擠壓，持續的水平擠壓縮短或堆疊（T2，～6－2 Ma），在玉里帶D2變形中造成S1劈理的褶皺，並伴隨了S2劈理。同時上覆濁流岩（東板岩帶）也開始受第一期的變形變質作用（D1變形期），發育褶皺F1及板劈理S1。爾後兩者皆因持續的擠壓及可能的地殼回彈浮力而轉為抬升，開始返回地表之上，也代表開始降溫的過程。在持續的板塊聚合使地殼增厚及變薄過程中，玉里帶及板岩帶開始快速掘升（T3，2Ma -現今），由於造山過程中可能伴隨的垮塌作用或地殼變薄作用，在玉里帶產生D3變形期及板岩帶Df變形期，發育S3、Sf以及一系列的正斷層。

關鍵字

玉里帶 ； 東板岩帶 ； 多期劈理 ； 變形期 ； 正斷層 ； 中後楔形

並列摘要

The aim of this study is for better understanding the orogenic process in the eastern Central Range of Taiwan, especially the fan-shaped cleavage across a mountain range and the deformation process in the retro-wedge. In this study, I focused on field investigations along three river transects, including the Qingshui, Luliao and Luyeh Rivers, in the easternmost Yuli belt and slate belt. I also collected samples for thin-section analysis.
Based on multi-stage cleavage morphology and their superposition relationship, field observations and thin section analysis revealed multiple deformation phases and accompanying metamorphism, however, different patterns in the Yuli belt and the eastern slate belt：1）D1, D2, D3 in Yuli belt and cleavages (S1, S2, S3), respectively；2）D1, D2', Df in eastern slate belt and cleavage (S1, S1', S2', Sf), respectively.
In the Yuli belt, except for the cleavage surrounding exotic mafic rocks, the D2 deformation phase is mainly characterized by a S2 cleavage gentle dipping to east (30°-40°), associated with top-to-west shearing, west vergent folds in the quartz-mica schist in Qingshui River; whereas S2 cleavage is gentle dipping to west (20°-40°), associated with top-to-east shearing, east vergent folds in Luliao River. The two rivers show opposite vergence of D2 folds in the easternmost Yuli belt. In addition, the weak sub-horizontal cleavage S3 development can be found commonly in both rivers in the Yuli belt which I attributed as the D3 deformation phase.
In the eastern slate belt, it is mainly composed of metasandstone and slate. It shows original deposition layers S0 has deformed into top-to-west shearing, west-vergent folds or isoclinic folds, which were accompanying S1 cleavage in the D1 deformation phase, which is characterized by an east-west or northwest-southeast horizontal compression. In addition to S1 as the primary, penetrative cleavage set in slate belt, some secondary cleavages are also locally developed. Including 1) upright folds and sub-vertical secondary cleavage S2' (attributed to D2' deformation phase) and 2) sub-horizontal cleavage Sf associated with sub-horizontal fold and normal fault (attributed to Df deformation phase). The Df in slate belt is characterized mainly by vertical compression.
Thin section analysis from schist samples at the edge of the serpentinite block of Yuli belt in Qingshui river revealed that remnant of S1 layers and garnet crystals are observed between S2 layers. And retrograded chlorite was growing around the garnet crystals. The thin section samples of S3 cleavage in Yuli belt show that the quartz recrystallization morphology indicates BLG (Bulging recrystallization) type in Lulaio river. It thus implies that the D1-D2-D3 deformation phases is a cooling process, at least for the schist surrounding the serpentinite block in the Yuli belt.
For the samples in the slate belt, they revealed that the type of quartz recrystallization during S1 cleavage development (BLG, SGR) are indicating lower temperature than that during S2 development in Yuli belt. The S2' and Sf samples in the slate belt also show the recrystallization type with the lowest temperature (BLG).
I tentatively summarize tectonic evolution and its accompanying deformation and metamorphism of eastern Central Range of Taiwan as following. The Miocene turbidite/mélange of the Yuli belt and the eastern slate belt began to develop folds (F1) associated with S1 cleavage (Yuli belt D1 deformation phase) in Yuli belt during the subduction process (T1 tectonic event, ~15-6 Ma). As the Philippine Sea plate approaching characterized by crustal horizontal compression/shortening (T2 tectonic event, ~7-2 Ma), it resulted in F2 folding associated with S2 cleavage in the D2 deformation phase in the Yuli belt. At the same time, the upper part of the Miocene turbidite (Eastern Slate Belt) has begun to undergo deformation and metamorphism and has developed F1 folds and cleavage S1 (i.e., Slate belt D1 deformation phase). Later on during exhumation and also cooling process (T3 tectonic event, ~2 Ma-present), S3 (Yuli belt) and Sf (eastern slate belt), and a series of normal faults .

並列關鍵字

Yuli belt ； Eastern slate belt ； Multiple cleavage set ； Deformation phase ； Normal fault ； Retrowedge

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