| 摘要: | 越南西北部地區有著複雜的大地構造歷史，由南中國大陸與印支陸塊縫合構成，自中生代以來經歷了印支期與喜山期造山運動，是探討大陸碰撞造山與大陸地殼動力學的焦點區域。此區域廣泛出露前寒武至三疊紀所形成的沉積與火成岩序列，並均受到不同程度的變質作用；然而除了紅河斷裂帶與馬河縫合帶受到較多關注，其餘大範圍區域出露的岩石均缺乏變質作用的資料與約制，而包含變質度的岩石歷史是深入了解造山運動與對應構造的關鍵，也是討論現今喜山期運動於此區域造成構造變形的先決條件。本論文因此應用碳物質拉曼光譜地質溫度計(Raman spectroscopy of carbonaceous material, RSCM) 針對變質沉積岩中的含碳物質取得其所經歷最高溫度的估計，結合野外與顯微尺度的構造觀察，以及已發表之地層沉積架構與年代等資料，建立岩石的熱歷史演化，並以之探索此區域的變質分布與大地構造意涵。
本論文於越南西北部之奠邊省、安沛省與周邊區域採集了33個樣本進行分析。由RSCM法得到的岩石經歷最高溫度落在215至561度之間，最高溫樣本來自馬河縫合帶雜岩體中的新太古代Nam Co層。馬河縫合帶西南側印支陸塊區域的樣本最高溫度系統性地較低，顯示印支期運動是支配變質熱歷史的主要構造事件。南中國陸塊區域除未變質的侏儸系與更晚的陸相沖積層外，古古地中海閉合前最後沉積之三疊系沉積岩已被變質加熱超過200度，相當於被構造深埋於7公里深處(假設地溫梯度約每公里20度)；而自新元古代至三疊紀的層序則整體有約每公里地層厚度20度的熱變質梯度，指示其於印支期構造作用中岩層於最高變質階段前已有初始變形(對應於現有褶皺變形振幅約1/3)。詳細分析則發現，研究範圍內南中國陸塊的地層變質度有區域內的差異，大致可沿著Na Hieng斷層、Song Da斷層分為三個區塊以及Ta Khoa穹隆高溫變質區；依各區塊內的地層層序-變質溫度對應關係，自測得RSCM最高溫度樣本處可內外插草繪出等變質度線，從而建立西北越南的變質度分布草圖。南中國陸塊部分的熱變質紀錄顯示，此區域的岩層於中晚三疊紀、古古地中海盆關閉後，先隱沒/俯衝至約七公里深處並初始變形/褶皺，形成上述之熱變質梯度，爾後經掘升至地表形成目前之狀態。
;Northwest Vietnam contains numerous geological structures and exposes various sedimentary and igneous rocks from Precambrian to Triassic. Most exposed rocks here have obviously undergone regional metamorphism. The area has undergone two main events of continental collision: the Indosinian event and the India-Asia collision that happened during the Permian-Triassic and the Early Cenozoic, respectively. Northwest Vietnam lies on the convergent boundary between the Indochina block to the southwest and the South China block to the northeast with distinct geological characteristics. The closure of the Paleo-Tethys led to the collision between the Indochina and South China blocks, forming the Song Ma suture zone in the Triassic. Since the Early Cenozoic, Indian and Asian continents started to collide, and this collision caused several strike-slip faults.
Most existing works on structures and tectonics are focused on either meta-ophiolitic rocks along the Song Ma Suture Zone or the Red River Shear Zone, while metasedimentary rocks in between have not been thoroughly investigated. We therefore applied a geothermometer, Raman spectroscopy of carbonaceous material (RSCM) to estimate the peak temperature in rock history, which was integrated with field and microstructural observations as well as stratigraphy information, to reconstruct the thermal evolution of Northwest Vietnam. A total of 33 samples were collected across a section from Dien Bien province to Yen Bai province and neighboring locations. The temperature results from RSCM are in the range of 215 oC to 561 oC from Neoproterozoic to Triassic formations, with the highest RSCM temperature being found in the Neoproterozoic Nam Co formation of the Song Ma suture zone. The data also show a significant difference in thermal history between Indochina and South China blocks across the Song Ma Fault indicating that the Indosinian event is more likely to dominate the peak temperature state in the area. The Triassic sediments were the last deposits on the South China continental margin and the Paleo-Tethys with minimal diagenesis before the onset of the Indonesian orogeny. The Upper-Triassic formations were metamorphosed up to around 200 oC, implying that it was underthusted to ~7 km depth before exhumation (assuming a geothermal gradient of 30 oC /km). For the rest of the Neoproterozoic to Triassic succession, we found that the apparent thermal gradient calculated from RSCM thermometer is roughly 20oC/km through stratigraphy and RSCM temperature correlation, indicating that the apparent geothermal gradient has been reduced by about one-third of its original value in the whole study area by folding prior to peak metamorphism. The area is postulated that the currently exposed continental margin sediment sequence, which is roughly 16 km thick, was considerably folded while being underthrusted to ~7 km depth where it experienced peak metamorphism, while the vertical distance between the Triassic and the Neoproterozoic formations to produce the low apparent geothermal gradient, 20 oC/km was reduced to ~11 km among the anticline and syncline cores. The iso-grade map showing the metamorphic distribution and the corresponding major structures is proposed. Northwest Vietnam will be divided into two main zones along the Song Ma Fault, and three sub-zones with the Na Hieng Fault, and Song Da Fault as major structures along with the local metamorphic Ta Khoa zone. |
