西藏東久-米林剪切帶活動史之氬氬定年學研究
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2004
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東久-米林斷裂帶位於東喜馬拉雅構造結(Eastern Himalayan Syntaxis, EHS)與嘉黎剪切帶間，為一東北-西南走向的左移斷裂帶，其活動歷史對東亞地區的地體演化解釋，有著相當重要的角色。本研究採集斷裂帶上的糜嶺狀片麻岩、花崗岩及雲母片岩，利用40Ar/39Ar 階段加溫定年法，配合石榴子石-黑雲母地質溫度計及鉀長石多重擴散區間理論模擬，探討東久-米林斷裂帶的左移活動時間，以期對新生代以來東亞地體構造演化有進一步的了解。研究工作結果顯示，東久-米林變質岩體曾歷經500℃以上的變質作用，此變質作用將多數40Ar/39Ar 定年礦物中的同位素系統重置，重新紀錄斷裂帶活動時間。此外，由鉀長石多重擴散區間模擬的冷卻歷史可以觀察到三期以上的活動快速剝蝕抬升至地表的歷史。東久-米林斷裂帶熱事件活動的時間大約在17~8 Ma 間，而受東喜馬拉雅構造結抬升影響的活動年代發生在1 Ma 左右。綜合東亞地區各地體構造的活動時間及相關研究比較顯示，東久-米林斷裂帶的發生可能和東喜馬拉雅構造結擠入活動有關，並與嘉黎剪切帶為共軛斷層關係，且與西藏內部地殼變形的活動時期相當。
The Dongjug－Miling shear zone is located to the Eastern Himalayan Syntaxis and intersecting the Jaili fault to the north. It consists of a series of NE-SW aligned left-lateral faults. The Dongjug－Miling shear zone have played a considerably important role in the tectonic evolution of eastern Asia area. The samples analyzed were collected from the intensively sheared belts in the Dongjug－Miling fault zone. We carried out 40Ar/39Ar geochronological studies on granite and gneiss collected from the Dongjug－Miling fault zone. Furthermore, Gt-Bio geothermometer and K-feldspar multiple domain diffusion theory were also used to verify the timing for Dongjug－Miling activity and uplifting history of the Eastern Himalayan Syntaxis. As a result, the metamorphic terrain experienced metamorphism up to 500℃ and 5-6 kbar. Preliminary results indicated possibly three stages of deformation. The timing of the earliest deformation should be near the India-Eurasia collision occurred around the early Cenozoic. Then strike-slip activities started and were probably most active from early to middle Miocene and coincided with Jaili fault activities. The last uplifting stage is must likely during Pleistocene as a result of the pop-up of Namche Barwa.
The Dongjug－Miling shear zone is located to the Eastern Himalayan Syntaxis and intersecting the Jaili fault to the north. It consists of a series of NE-SW aligned left-lateral faults. The Dongjug－Miling shear zone have played a considerably important role in the tectonic evolution of eastern Asia area. The samples analyzed were collected from the intensively sheared belts in the Dongjug－Miling fault zone. We carried out 40Ar/39Ar geochronological studies on granite and gneiss collected from the Dongjug－Miling fault zone. Furthermore, Gt-Bio geothermometer and K-feldspar multiple domain diffusion theory were also used to verify the timing for Dongjug－Miling activity and uplifting history of the Eastern Himalayan Syntaxis. As a result, the metamorphic terrain experienced metamorphism up to 500℃ and 5-6 kbar. Preliminary results indicated possibly three stages of deformation. The timing of the earliest deformation should be near the India-Eurasia collision occurred around the early Cenozoic. Then strike-slip activities started and were probably most active from early to middle Miocene and coincided with Jaili fault activities. The last uplifting stage is must likely during Pleistocene as a result of the pop-up of Namche Barwa.
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西藏, 東久米林, 氬氬定年學, Tibet, Tungchiu-Milin