| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 94 === 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.
|
|---|
