越南西北部變質沉積岩熱歷史初探，與對印支造山運動演化的隱示;Preliminary constrains on the thermal history of the metasedimentary rocks in northwest Vietnam: implications on Indosinian evolution

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Title:	越南西北部變質沉積岩熱歷史初探，與對印支造山運動演化的隱示;Preliminary constrains on the thermal history of the metasedimentary rocks in northwest Vietnam: implications on Indosinian evolution
Authors:	阮氏秋河;HA, NGUYEN THI THU
Contributors:	地球科學學系
Keywords:	Indosinian Orogeny;NW Vietnam;RSCM geothermometry;Indosinian Orogeny;NW Vietnam;RSCM geothermometry
Date:	2021-08-31
Issue Date:	2021-12-07 11:50:47 (UTC+8)
Publisher:	國立中央大學
Abstract:	早三疊世印支造山運動時期，越南西北部位於印度支那板塊與華南板塊的會聚板塊交界處，新生代喜馬拉雅-西藏造山運動時期又位於印度-歐亞大陸碰撞系的東南部.作為了解碰撞構造的重要區域，大多數已發表的作品主要集中在岩漿岩和紅河剪切帶；然而，人們對印支松馬縫和紅河剪切帶之間廣闊的變質沉積地形仍知之甚少。本研究應用地溫計方法碳質材料拉曼光譜（RSCM）推導出峰值溫度條件，並結合野外和微觀結構觀察以及地層信息來表徵和重建越南西北部的構造演化。從Yen Bai和Son La之間的塊狀板岩、大理石、泥質片岩和變質砂岩樣品中，共獲得了17個RSCM結果。初步結果表明，越南西北部變沉積岩的峰值溫度在 260 oC到 650 oC 以上的範圍內，可分為三個溫度範圍：最高溫度範圍從 Ta Khoa 地區超過 650oC；記錄的中溫範圍為 451 oC 至 545 oC，主要在 Nam Co 地區；最低溫度範圍為 260 oC 至 399 oC，主要在土樂和安白地區。根據 RSCM 結果，最高峰值溫度（超過 650 oC）出現在 Ta Khoa Dome 的核心，並逐漸降低到穹頂的兩側；它不受周圍二疊紀玄武岩系列的影響。因此，提出了兩種情況來解釋它：(a) 印支造山運動期間雙重疊層的隆升； (b) Ta Khoa 圓頂是在造山後坍塌和伸展過程中形成的變質核心複合體。中三疊世是印支造山前華南陸緣和古特提斯的最後沉積物，目前已在250oC左右變質。低表觀地溫梯度 (20 oC /km) 表明在折疊或變形過程中達到了峰值變質作用。我們認為，目前暴露的 13 公里厚的大陸邊緣沉積層序被折疊，同時下衝到 9 公里的深度，在那裡它記錄了峰值溫度，而（新元古代和中三疊世地層）頂部到底部之間的垂直距離減少了三分之一它的原始價值。一個模型表明印支造山運動分為三個階段：（1）造山前，古特提斯俯衝到印度支那地塊之下，那時華南大陸邊緣已經積累了約 13 公里厚的覆蓋沉積物； (2) 華南大陸邊緣併入造山系統並在增生棱柱下~9 km處下衝斷層時的同造山/高峰狀態； (3) 大陸邊緣岩石達到峰值溫度時的折返過程，被複式疊層折返。 ;Northwestern Vietnam lies on the convergent plate boundary between the Indochina plate and South China plate during the Indosinian Orogeny in the early-to-middle–Triassic, and again on the southeastern part of the India-Eurasia collision system during the Cenozoic Himalaya-Tibet Orogeny. Such an essential region in understanding collisional tectonics, most published works focused mainly on magmatic rocks and the Red River Shear Zone; however, the vast metasedimentary terrain between the Indosinian Song Ma Suture and the Red River Shear Zone remains poorly understood. This study applies the geothermometer method Raman spectroscopy of carbonaceous material (RSCM) to deduce the peak temperature condition, which is combined with field and microstructural observations as well as stratigraphy information for characterizing and reconstructing the tectonic evolution of northwestern Vietnam. A total of 17 RSCM results have been acquired from samples of massive slate, marbles, pelitic schist, and meta sandstone between Yen Bai and Son La. Preliminary results exhibit that the peak temperature of metasediment rocks in NW Vietnam falls in the range from 260oC to over 650oC, and can be categorized into three temperature ranges: the highest temperature range from over 650oC in Ta Khoa area; the middle-temperature range recorded from 451oC to 545oC mainly in the Nam Co area; and the lowest temperature range from 260oC to 399oC mainly in Tu Le and Yen Bai areas. According to the RSCM results, the highest peak temperature (over 650oC) is found in the core of Ta Khoa Dome, and it decreases to both limbs of the dome; it was not affected by the surrounding Permian basalt series. Thus, two cases were proposed to explain it: (a) uplift by duplex stacks during Indosinian Orogeny; (b) the Ta Khoa dome was a metamorphic core complex formed during post-orogenic collapse and extension. The Middle Triassic was the last deposits on the South China continental margin and the Paleotethys before the Indosinian orogeny, and now they have been metamorphosed around 250oC. The low apparent geothermal gradient (20oC/km) indicates peak metamorphism was achieved during folding or deformation. We suggest that the presently exposed continental margin sediment sequence of 13 km thickness was folded while underthrusting to 9 km depth where it recorded the peak temperature, while the vertical distance between top to bottom of the (Neoproterozoic and Middle Triassic formations) had reduced one third its original value. A model indicated that Indosinian Orogeny was proposed on three stages: (1) pre-orogenic when the Paleotethys subducted beneath Indochina block, by then South China continental margin had accumulated about 13 km thick of cover sediments; (2) syn-orogenic/peak state when the South China continental margin was incorporated in the orogenic system and underthrust at ~9 km beneath accretionary prism; (3) Exhumation when the rocks in continental margin achieved the peak temperature and were exhumed by duplex stacks.
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