雅魯藏布江與伊洛瓦底江碎屑鋯石同位素示蹤研究

Abstract

自新生代喜馬拉雅造山帶形成以來，大地構造抬升與河流侵蝕堆積之間的交互作用，是影響當地地形的主要因素。為了對喜馬拉雅造山結與青藏高原東南部有更進一步的認識，本研究採集了雅魯藏布江與伊洛瓦底江沿岸的河流沉積物，並分選出其中的碎屑鋯石，利用北京離子探針中心的SHRIMP II進行原位的鈾鉛定年，並利用LAM-MC-ICPMS進行鉿同位素分析。結合本研究的分析結果與藏南主要岩體的鈾鉛定年與鉿同位素分析資料，可以更定量的檢視這些河流沉積物的來源區域─或者說是其「源岩」─的組成比率。 從河流的上游至南迦巴瓦峰以西的研究結果顯示，主要物源所佔的百分比會隨著不同流段而有顯著的改變；當Tethyan Himalayan sequences所佔的比例從80%降至小於50%之時，岡底斯岩體所佔的百分比卻從18%提高至50%左右。在河流進入大拐彎區域之前，岡底斯岩體所提供的鋯石約佔此處河沙鋯石中的40%左右，但到了墨脫地區卻又增加至50%。此外，在不同河段中所淘選出的岡底斯鋯石皆顯示了不同的鉿同位素特徵，表示沉積物的物源比例主要受控於鄰近區域的岩體。由此可見雅魯藏布江在青藏高原及東喜馬拉雅造山結處的侵蝕作用強烈，且為現今世界上最活躍的河川之ㄧ。 從緬甸伊洛瓦底江流域的晚中新世沉積岩中所分選出的碎屑鋯石裡的62顆已定年的鋯石中，有47顆的206Pb/238U年齡為白堊紀至古新世，且有24顆鋯石的eHf(T)值介於0至+16之間。不但此年齡的鋯石廣泛分布在拉薩地塊上的深成岩體中，且這樣的eHf(T)值只被報導在藏南岡底斯岩體的資料裡。本研究的結果支持在中新世時，這切穿岡底斯岩體的雅魯藏布江曾經匯入伊洛瓦底江中，且其演化應與嘉黎斷裂帶在中新世的活動高峰期有關。山嶽河川對大地構造所造成的影響可由南迦巴瓦構造結的抬升得到證實，此處的抬升所使布拉馬普特拉河的溯源侵蝕加劇且襲奪了雅魯藏布江，成為今日的東喜馬拉雅河流系統之濫觴。

The interactions among tectonic uplift, river erosion and alluvial deposition are fundamental processes that have shaped the landscape of the Himalayan-Tibetan orogen since its creation from early Cenozoic time. To understand these processes around the eastern Himalayan Syntaxis and southeastern Tibet, we conducted a study of riverbank sediments along the Yarlung-Tsangpo and Irrawaddy River. Detrital zircons separated from the sediments were subjected to in-situ U-Pb dating using SHRIMP II at the Beijing SHRIMP Center and Hf isotope measurements using LAM-MC-ICPMS. These results, together with the U-Pb ages and Hf isotope data that we have recently obtained for the Transhimalayan plutonic and surrounding basement rocks, allow a more quantitative examination of the provenance or “protosource” areas for the riverbank sediments. From the upper reaches of the River in the west to the Namche Barwa Syntaxis in the east, our results indicate that the percentage inputs from the major source provenances vary significantly, e.g., those of detrital zircons from the Gangdese batholith increase from ~18% to ~50% and those from the Tethyan Himalayan sequences decrease from ~80% to <50%. Before the River flows into the Big Bend gorge, the percentage of Gangdese-derived zircons lowers to ~40%, but it arises back to ~50% in the Muotuo area. Moreover, in each part of the River, these Gangdese-derived zircons show characteristic eHf(T) values that could be correlated to the neighboring batholithic values. Strong erosional processes are suspected to have been taking place in Tibet and the eastern Himalayan Syntaxis that, in turn, implies the Yarlung-Tsangpo and Brahmaputra River system to be one of the most dynamic mountain rivers on modern Earth. The first in situ Hf and U-Pb isotope analyses of detrital zircons from a Late Miocene sandstone in the Inner-Burma Tertiary Basin enable us to study the sedimentary source to sink relation and river system evolution around eastern Himalayas. Among 47 out of 62 dated zircons that exhibit Cretaceous and Paleogene 206Pb/238U ages, 24 grains have positive eHf(T) isotope values up to +16. Whilst zircons of such ages are common in the Transhimalayan plutons, those showing high eHf(T) values have been observed only in the Gangdese batholith, southeastern Tibet. Our results, therefore, validate the notion that by Late Miocene time the Yarlung-Tsangpo River, which cuts across the Gangdese batholith, drained into the Irrawaddy River. We attribute this river routing to the dextral Jiali faulting that was most active in the Middle Miocene. Subsequent reorganization of the mountain rivers was affiliated with uplift of the Namche Barwa Syntaxis, resulting from enhanced headward erosion of the Brahmaputra River that eventually captured the Yarlung-Tsangpo drainage to form the modern eastern Himalayan river system.