| 摘要: | 在台灣造山帶中，板岩帶為被動大陸邊緣不同部位沉積物之深埋、隱沒、變質，最終掘升至地表之結果。由於板岩帶僅經由蓬萊造山運動變質而成，其相對單純的變質歷史相當適合作為重建此造山運動的研究對象。板岩帶可以依其地質年代、變形行為、變質度的落差，再劃分為西側的雪山板岩帶與東側的脊樑板岩帶。二者的邊界性質的釐清與探討不僅能為台灣造山架構提供更一步的剖析，板岩帶本身的熱變質溫度也能對物質在隱沒、變質並俯衝至增積楔下後，如何抬升並加入造山帶的變質歷程提供關鍵的資料。
本研究以拉曼碳質物光譜(Raman Spectroscopy of Carbonaceous Material, RSCM)進行板岩帶巔峰變質溫度(peak-T)的量測。RSCM為一高精度peak-T溫度計，其樣本間誤差值可低至10-15℃，對區域的peak-T變化能有更精準的結果。本研究區域以梨山為界，以北延伸至啞口一帶以較高空間解析度的採樣，界定出雪山-脊樑交界處peak-T有約60℃的陡降，此結果可歸因於梨山斷層的背衝作用，若以變質地溫梯度30℃/km進行計算，二者間的peak-T差異可換算為約2公里的垂直斷層落差，藉以判定此為雪山板岩帶與脊樑板岩帶二者間的斷層接觸，驗證了前人以梨山斷層作為二者接觸性質的理論。細部的peak-T變化及薄片資料也顯示出梨山斷層除了脆性的斷層錯移外，在深部也有韌性的剪切變形。此外，由於梨山斷層上盤屬於雪山板岩帶最頂部的地層，其資料不僅完整了內雪山山脈地層柱的peak-T序列，換算出來的深度也可幫助建立雪山板岩帶在巔峰變質度時的幾何樣貌，結果顯示出此區域由盆地沉積至巔峰變質時期有約30°的地層傾斜變形量。梨山以南至廬山，RSCM的採樣範圍及高程差較大以建立較為宏觀的脊樑山脈板岩帶的peak-T熱構造模型，進而解析出在巔峰變質後形成的向西伸向的變質等溫線構造。在與熱年代學的冷卻年代做比較後，將可重建出脊樑山脈板岩帶在經歷完巔峰變質度後，掘升至地表的過程中不同推覆體受到向西伸向變形的歷程。
除了RSCM所得出的peak-T結果外，本研究也結合了薄片微構造觀察及野外斷層擦痕資料，推論雪山山脈板岩帶及脊樑山脈板岩邊界的運動模式及變形歷史，最後提出台灣中部板岩帶的構造演化模型。
;In the Taiwan orogenic belt, the slate belt is metamorphosed from pelitic sediment depositing on the different architecture of passive continental margin and the basement of the South China Sea. Because the slate belt only underwent one-period orogeny, the Penglai Orogeny, it is suitable for the study of reconstruction of the mountain building. Based on the discrepancy in age of strata, style of deformation, and grade of metamorphism, it can be further divided into 2 units: Husehshan Range (HR) in the west and Backbone Range slate belt (BRSB) in the east. The feature of the boundary between them not only plays an integral role in the evolution of Taiwan mountain building history but their own thermal-metamorphic information also can insight the light of the pattern of how the material fluxed into the accretionary wedge.
The Raman Spectroscopy of Carbonaceous Material (RSCM) was applied to the junction zone between HR and BRSB in the central Taiwan slate belt to reveal the trend of peak metamorphic temperature (peak-T). RSCM is a high-precision thermometer of peak-T, and the uncertainty between samples of this method is as low as 10-15 °C, which can provide an insight into the subtle variation of peak-T. The research area extends from Lushan in the south to Yakou in the north. To the north of Lishan Town, intensive sampling enhances the spatial resolution of peak-T. The result exhibits about 60 °C drops of peak-T from HR to BRSB, and the vertical offset is about 2 km according to the 30 °C/km thermal gradient of the passive continental margin. Therefore, it is a fault named Lishan Fault that divides the slate belt into HR and BRSB. The detailed variation of RSCM temperature also indicates the brittle and ductile behavior of the Lishan Fault. Moreover, the data in the hanging wall of the Lishan Fault, which is the latest portion of HR, complete the peak-T in the top of HR, and that constrains the depth of HR in peak metamorphism which can be used to construct the model of HR evolution. The result reflects that the sediment sequence in the eastern Hsuehshan Trough had been deformed and tilted at ~30° before reaching peak metamorphism. In the range from Lushan to Lishan Town, a wide range of sampling is applied to the establishment of BRSB structural evolution. The constrain of the peak-T isogrades and documented thermochronological data reveal the how the material fluxed into the wedge and formed the BRSB.
Besides the results of peak temperature, the microstructure observation from the thin sections and the measurement of slickenside on the fault outcrop also are analyzed to discuss the deformational features and history in the research area. Concluding the result from different methods, the model of the structural evolution of the central Taiwan slate belt is proposed. |
