泰國西北部湄平剪切帶之熱定年學研究

Thermochronological Study on the Mae Ping Shear Zone, Northwestern Thailand

林郁伶(Lin Yu-Ling)

指導教授 ： 葉孟宛 ； 李通藝

國立臺灣師範大學/理學院/地球科學系/博士(2020年)

https://doi.org/10.6345/NTNU202000417

摘要

湄平剪切帶位於泰國西北部，呈現西北-東南走向，於前人文獻中曾被稱作王朝斷裂帶或藍山片麻岩。在新生代東亞碰撞-脫逸地體運動中，湄平剪切帶被認為是印支與滇緬馬蘇地塊往東南方向脫逸的南方邊界。為了驗證此脫逸模式的正確性，已有許多研究試著去釐清該剪切帶的起始年代與結束年代，但還是沒有確切的同剪切運動的年代，也缺乏詳細的構造分析以及跟年代的連結。因此本研究利用了鋯石鈾-鉛定年與鉿同位素分析、雲母氬-氬定點定年、鉀長石氬-氬階段加溫等定年法，結合了岩石學與構造分析以釐清所獲得年代與剪切事件的關係。為了釐清在剪切運動之前發生的地質事件，也研究了明顯遭湄平剪切帶錯移的清邁-臨滄火成/變質岩帶。
幸運地，本研究在清邁-臨滄帶的南端，即泰國半島的華欣-攀武里地區發現了兩處片麻岩與糜稜岩露頭，其含有502-498 Ma的岩漿鋯石，應是目前滇緬馬蘇地塊上最古老的變質基岩。經由比對其他辛梅利亞陸塊之鋯石鈾-鉛年代、鋯石鉿同位素、與古地理位置，顯示寒武紀時滇緬馬蘇地塊可能為岡瓦納大陸邊緣岩漿弧的一部分。
根據野外構造與顯微構造之觀察推論，湄平剪切帶應經歷了兩期左移塑性變形事件。由同剪切形成的淡色花岡岩白雲母氬-氬定點定年結果推測，第一期高角度左移剪切事件可能發生於白雲母紀錄的42 Ma之前。根據受第二期低角度左移運動剪切的眼球狀片麻岩之岩相觀察，顯示第二期左移剪切事件應發生在300-400 ℃之間，剛好與黑雲母的封存溫度相近，因此其黑雲母的氬-氬年代35-30 Ma應代表湄平剪切帶第二次左移剪切變形的年代。鋯石鈾-鉛定年結果顯示同一樣本以及鄰近的糜稜岩皆含有36-33 Ma的鋯石，其平均釷-鈾比值為0.5、鉿同位素為正值，可能跟清邁-臨滄帶北部的素帖山變質雜岩體中的糜稜岩40 Ma的鋯石一樣形成於張裂的環境。此外，剪切帶剖面西半側的副片麻岩樣本中都含有約500 Ma的碎屑鋯石，具有滇緬馬蘇地塊的特徵，並有約199 Ma的變質鋯石增生環。但剖面東半側的糜稜岩與受剪切的片麻岩則缺乏約500 Ma的鋯石，卻有220-200 Ma的火成鋯石，則顯示了素可泰島弧的特徵。故推測湄平剪切帶，至少在藍山片麻岩這個剖面上，應是個縫合帶，其構造演化史應比前人研究所認知的更為複雜。而在泰國東南地區格靈剪切帶與周邊火成岩的氬-氬定年研究中，利用單礦物雷射燒熔技術發現格靈剪切帶中有跟湄平剪切帶類似的黑雲母年代(35 Ma)，比周邊火成岩體年輕，值得利用氬-氬定點定年技術進一步研究其剪切年代。

關鍵字

湄平剪切帶 ； 藍山片麻岩 ； 王朝斷裂帶 ； 碰撞-脫逸地體運動 ； 辛梅利亞陸塊 ； 岡瓦納大陸 ； 素可泰島弧 ； 格靈斷層 ； 滇緬馬蘇地塊 ； 氬-氬定年 ； 鋯石鈾-鉛定年

並列摘要

The NW-SE trending Mae Ping shear zone (MPSZ, also known as Wang Chao fault zone or Lan Sang gneisses) in NW Thailand was considered as the southern boundary for the Indochina and northern Sibumasu terranes extruding toward SE during the Cenozoic escape tectonic. Many geochronology studies had been accomplished to constrain the shearing duration for MPSZ in order to verify the extrusion model. Nevertheless, no convincing age could represent the exact onset, syn-kinematic, or cessation of the strike-slip shearing since none of the dated minerals have been proved to record the syn-shearing age. Therefore, zircon U-Pb dating with Hf isotope analysis, in situ 40Ar/39Ar dating on micas, and 40Ar/39Ar furnace step heating on K-feldspar were combined with structural and petrographic analysis to decipher the geological significance of the dates on the shearing. In order to rule out the magmatism and metamorphism prior to shearing, the Chiang Mai-Lincang Belt, which was sheared and offset by MPSZ apparently, was also surveyed.
Luckily, the current oldest fragments of Sibumasu basement with zircon U-Pb age of 502-498 Ma were identified to have cropped out around the Hua Hin to Pran Buri area, where could be the south extension of the Chiang Mai-Lincang Belt in northern Thailand Peninsula. The correlation of U-Pb zircon age, Hf isotope with other Cimmerian terranes suggests that the Sibumasu terrane could have been a magmatic arc located at the margin of Gondwana in Cambrian.
The NW-SE trending Mae Ping shear zone (MPSZ, also known as Wang Chao fault zone or Lan Sang gneisses) in NW Thailand was considered as the southern boundary for the Indochina and northern Sibumasu terranes extruding toward SE during the Cenozoic escape tectonic. Many geochronology studies had been accomplished to constrain the shearing duration for MPSZ in order to verify the extrusion model. Nevertheless, no convincing age could represent the exact onset, syn-kinematic, or cessation of the strike-slip shearing since none of the dated minerals have been proved to record the syn-shearing age. Therefore, zircon U-Pb dating with Hf isotope analysis, in situ 40Ar/39Ar dating on micas, and 40Ar/39Ar furnace step heating on K-feldspar were combined with structural and petrographic analysis to decipher the geological significance of the dates on the shearing. In order to rule out the magmatism and metamorphism prior to shearing, the Chiang Mai-Lincang Belt, which was sheared and offset by MPSZ apparently, was also surveyed.
Luckily, the current oldest fragments of Sibumasu basement with zircon U-Pb age of 502-498 Ma were identified to have cropped out around the Hua Hin to Pranburi area, where could be the south extension of the Chiang Mai-Lincang Belt in northern Thailand Peninsula. The correlation of U-Pb zircon age, Hf isotope with other Cimmerian terranes suggests that the Sibumasu terrane could have been a magmatic arc located at the margin of Gondwana in Cambrian.
According to the observation of meso-structure and micro-structure, MPSZ should have experienced twice sinistral ductile shearing events. In situ 40Ar/39Ar dating of syn-shearing muscovite fish from leucogranite indicates that an order sinistral, high angle ductile shearing event of MPSZ may have occur before 42 Ma. In situ 40Ar/39Ar dating of biotite from a sheared gneiss, which may deform at 300-400 ℃ based on its petrology, indicates MPSZ experienced a second sinistral, ductile but low angle shearing event with thrust component between 35-30 Ma. This inference is also supported by igneous zircon U-Pb ages of 36-33 Ma from sheared augen gneiss, mylonite, and cataclasite in the MPSZ. These zircons with high Th/U ratios (~0.5) and positive εHf(T) value possibly resulted from the similar extension environment that formed the Doi Suthep complex at the northern part of Chiang Mai-Lincang Belt. Moreover, the zircon U-Pb dating reveals paragneisses from SW side of the Lan Sang gneiss transection has a 500 Ma peak with a metamorphic overgrowth event around 199 Ma, which indicates these paragneisses may have their origins from the Sibumasu terrane. While the NE side of Lan Sang gneiss transection has igneous zircons between ages of 220-200 Ma without any 500 Ma dates, this should be part of the Sukhothai arc. Therefore, MPSZ, at least at the Lan Sang gneiss section, is actually a suture zone between the Sibumasu terrane and Sukhothai arc. The evidence from structural, petrological, and geochronological analyses in this study suggest that the structural evolution of MPSZ is more complicated than previously thought. Preliminary 40Ar/39Ar single grain dating on the Klaeng fault and plutonic rocks nearby reveals similar biotite 40Ar/39Ar dates with MPSZ (35 Ma). Hence the correlation between the Klaeng fault and MPSZ would be an interesting topic for further study.

並列關鍵字

Mae Ping shear zone (MPSZ) ； Lan Sang gneiss ； Wang Chao fault zone ； collision-extrusion tectonics ； Cimmerian terrane ； Gondwana continent ； Sukhothai arc ； Klaeng fault ； Sibumasu terrane ； 40Ar/39Ar dating ； Zircon U-Pb dating

參考文獻

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