藏東南雅魯藏布江河流沉積物之雷射氬氬定年研究及其河系演化之隱示

Abstract

本研究嘗試尋找西藏高原地區Tsangpo River及Brahmaputra River之沉積物來源，並希望藉此了解河流演化構造機制。該河向東沿Yarlung-Tsangpo suture zone流經西藏東南區域，切過古生代至中生代的沉積地層與Trans-Himalayan深成及火成岩體，繞著南加巴瓦山結外圍轉向，再切過Lesser Himalaya和sub-Himalaya區域。 三個沉積物樣本分別沿著雅魯藏布江採集自29.35oN, 89.63 oE, 3780 m; 29.27 oN, 91.91 oE, 3559 m;和29.2 oN, 95.15 oE, 600 m。黑雲母、角閃石、鉀長石及白雲母碎屑在鏡下分離後進行雷射氬氬定年分析。分析結果與他人定年資料對比，指出鉀長石與角閃石樣本可能來自Tsangpo River北邊的Gangdese batholith，而黑、白雲母樣本以Himalaya變質岩來源為主。這些結果顯示，Tsangpo River的河道並未明顯改變。本研究同時也提供未來將相同技術應用於Irrawaddy River和Red River的基本參考架構。

The present study examines the source provenances of sediments of the Tsangpo River, the Tibetan part of the Brahmaputra River, and attempts to clarify its dynamics. The river drains the southeastern Tibetan plateau and flows eastward along the depression of the Yarlung-Tsangpo suture zone. On the plateau the river cuts and erodes Paleozoic to Eocene sedimentary formations associated with the Trans-Himalayan plutonic and volcanic rocks along the suture. The river takes a turn to the northeast near Pai, followed by an 180˚ Big Bend gorge that cuts the Greater Himalayan sequences around the Namche Barwa Syntaxis and forms a south-westerly course that enters Arunachal Pradesh in India where it is known as Siang or Dihang. The river then flows to the southeast, cuts through the Lesser Himalaya and sub-Himalaya, and enters the Brahmaputra plain at Pasighat. River sediment samples (ST140, ST118-B and DD-2) were collected from three localities at 29.35oN, 89.63 oE, 3780 m; 29.27 oN, 91.91 oE, 3559 m; and 29.2 oN, 95.15 oE, 600 m, along the Tsangpo River. Detrital grains of biotite, hornblende, K-feldspar, and muscovite were separated under microscope and subjected to laser 40Ar/39Ar total fusion analyses and 31 to 72 grains of each mineral were analyzed. Analytical results for K-feldspars can be summarized as: (1) ST140 shows 81% of Ar ages between 28 and 56 Ma, with a peak age at 39.5 Ma; (2) ST118-B has 63% of the ages between 36 and 58 Ma, with a peak age at 49.3 Ma; and (3) DD-2 yields Ar ages falling within a range between 26 and 82 Ma (83% between 26 and 58 Ma) with a peak at 43.6 Ma. Those for Hornblendes include: (1) ST118-B bears 74% from 20 to 48 Ma, with the peak at 37.4 Ma. (2) DD-2, with 84% ages are from 30 to 168 Ma, in this sample a more constrained peak age gives an interval from 49 to 121 Ma that accounts for 45% of the total grains analyzed, this sample shows multi-peak ages of 48.8 Ma, 75 Ma and 121.3 Ma. Biotite ages are only obtained for sample DD-2 that shows 86% between 8 and 40 Ma, with a peak age at 24.9 Ma. Muscovite ages obtained for DD-2 range between 8 and 40 Ma, with a peak age of 13.4 Ma. The laser step heating experiment on muscovite grain indicates a presence of many phases and the derived apparent plateau age of 13.6 ±0.1 Ma confirms the total fusion peak age. Comparing with the published age data for the basement rocks in the region of the Tsangpo River, the present data suggest that the most likely sources for K-feldspar and hornblende detrital grains are the Gangdese batholith, part of the Trans-Himalayan plutons (emplaced from ca. 120 to 40 Ma), cropping out in the north of the Tsangpo River. The biotite and muscovite grains in sample DD-2, by contrast, have a dominant Himalayan metamorphic origin related probably to the river’s focused erosion within the Big Bend gorge. These results suggest that the Tsangpo River in its way to the Big bend gorge did not experience significant change from its actual course, and corroborates the hypothesis of Brookfield (1998) that a paleo Tsangpo-Irrawaddy was flowing parallel to the three large rivers of Southeast Asia, the Yangtze, Mekong and Salween on the Tibetan plateau. In conclusion, the study results constitute a basis for future argon geochronology work on samples of the Irrawaddy and Red River to better constrain the supposed paleo Tsangpo-Irrawaddy and paleo Tsangpo-Red River.