緬甸伊洛瓦底江與薩爾溫江流域碎屑鋯石研究

Abstract

印度-亞洲碰撞事件不但造成青藏高原的抬升與東南亞錯綜複雜的地形，也影響了當地主要河流的流域變化。伊洛瓦底江一般被認為曾經是位於青藏高原上的雅魯藏布江的下游；大約在中新世晚期，由於南迦巴瓦峰的抬升導致布拉馬普特拉河的溯源侵蝕進而襲奪雅魯藏布江，使伊洛瓦底江形成現今所見的流域。為了更了解亞洲地區的構造作用與河流侵蝕堆積和流域演化的相互關係，我們進行了緬甸伊洛瓦底江流域碎屑鋯石研究。本研究利用雷射剝蝕感應耦合電漿質譜術(LA-ICPMS)將沉積年代為白堊紀到中新世的沉積岩以及河沙分別進行鋯石的鈾鉛(U-Pb)定年與鉿(Hf)同位素分析。將分析結果與西藏南部、雲南西部及緬甸的主要花崗岩體(如岡底斯岩體)中岩漿鋯石的鈾鉛定年結果與鉿同位素數據作比對，探討伊洛瓦底江沉積物來源和東南亞大河的流域演化。本研究的數據顯示伊洛瓦底江的沉積物來源主要為西緬甸地塊上的Wuntho-Popa岩體，且有兩個週期的主要變化，發生在~55 Ma和~30 Ma。這兩個時間點的變化都是由物源集中變成物源分散，反映出印度-亞洲碰撞對河流發育的影響。另外一個重要的物源變化發生於中新世最早期，大約在20 Ma，此時在東北方的滇西-撣(Dianxi-Shan)岩體開始提供伊洛瓦底江沉積物源。而河沙的碎屑鋯石數據和中新世沉積岩的數據相似，說明了伊洛瓦底江從中新世以來流域沒有太大的改變，由此推斷伊洛瓦底江現今流域的雛形在中新世已經形成。本研究認為伊洛瓦底江的沉積物來源比較有可能來自於緬甸當地而非藏南地區，因此伊洛瓦底江流域可能從白堊紀晚期至今都是獨立發展的河系。 本研究另外報導一個採集自薩爾溫江口的河沙標本的碎屑鋯石鈾鉛定年與鉿同位素分析。薩爾溫江流域狹長，流經羌塘地塊南部、拉薩地塊東部與滇緬馬(Sibumasu)地塊。此標本的定年結果顯示有34% (n = 35/102)為中生代鋯石，大於250 Ma的鋯石則佔總分析量的62% (n = 63/102)。此外，中生代鋯石中約90% (n = 32/35)的鋯石有著較為富集的鉿同位素組成；其中18顆三疊紀(214 ± 3 Ma)的鋯石鉿同位素初始值很低(εHf(T) ＝ -5.8 to -20)，其最有可能的物源是當地的印支期岩體，如湄賓(Mae Ping)及臨滄(Lincang)岩體。薩爾溫江的碎屑鋯石數據明顯與伊洛瓦底江的碎屑鋯石數據不同，新生代鋯石所佔比例小，且有許多印支期鋯石存在。而大於250 Ma的河沙碎屑鋯石數據與滇緬馬地塊上兩個三疊紀砂岩碎屑鋯石數據相似，提供了滇緬馬地塊的岩漿事件與地殼演化的新證據：至少有五期岩漿活動，分別在約2.5 Ga、1.9 Ga、1.0 Ga， 650-500 Ma與213 Ma，其中第一期和第三期有重要的新生地殼形成。

The India-Asia collision has resulted in the uplift of the Tibetan plateau and complicated landscape changes in Southeast Asia since the early Cenozoic. It therefore influenced the river drainage patterns in the region. Being one of the major rivers in Southeast Asia, the Irrawaddy was probably the downstream of the Yarlu Tsangpo on southern Tibet that was captured by upward erosion of the Brahmaputra owing to uplift of the Namche Barwa syntaxis during perhaps Late Miocene time. To understand the interaction between Asian tectonics, drainage system evolution, river erosion and deposition, we conducted a detrital zircon study of the Irrawaddy River in Myanmar. Zircon separates from late Cretaceous to Miocene sedimentary rocks and riverbank sediments were subjected to in-situ U-Pb and Hf isotope measurements using LA-Q-ICPMS and LA-MC-ICPMS, respectively. These results, combined with zircon U-Pb and Hf isotope data available from major batholiths and related igneous rocks that crop out in southern Tibet (e.g., the Gangdese or so-called Transhimalayan batholiths) through western Yunnan and Myanmar, allow us to explore the sedimentary source-to-sink relation in the Irrawaddy and the drainage evolution in Southeast Asia. Our data identify two cycles of major changes in source provenance. The changes, occurring ~55 and ~30 Ma, both characterized with switching from more uniform to heterogeneous detrital sources, may have resulted from the tectonic forcing of the India-Asian collision. An additional marked change took place at ~20 Ma in the earliest Miocene, when abundant Cenozoic zircons with negative εHf(T) values started deriving from the Dianxi-Shan batholiths in northeast, which we attribute to development of the modern Irrawaddy drainage. Detrital zircon data from riverbank sands and Miocene samples are similar, suggesting that there has been insignificant change in the Irrawaddy drainage patterns since Miocene time. Our results suggest that the Irrawaddy sediments are more likely derived from local sources, not from Tibet or Himalaya. Thus, the Irrawaddy may have developed independently from other major rivers in Southeast Asia. In comparison, we report new U-Pb age and Hf isotope results of detrital zircons from the Salween River mouth. Being one of the major rivers in Southeast Asia sourced from the Tibetan plateau, the Salween is marked with a narrow drainage system that flows over the southern Qiangtang, eastern Lhasa and Sibumasu terranes. We performed in situ U-Pb and Hf isotope analysis using LA-ICPMS of 108 grains of detrital zircon separates from a riverbank sand sample from the Salween river mouth. The results show 37% (n = 38/102) of Mesozoic ages, and 62% (n = 63/102) of Paleozoic ages or older. Moreover, ~90% (n = 32/35) of the Mesozoic zircons have low Hf isotopic ratios or negative εHf(T) values. There are abundant (n = 18) Triassic zircons (214 ± 3Ma) that exhibit exclusively negative and low εHf(T) values (-5.8 to -20), derived most likely from the Indosinian granitoids such as the Mae Bing and Lincang batholiths exposed in the Sibumasu and Indochina terrane, respectively. These data, characterized by the scarcity of Cenozoic zircons and abundance of the Indosinian population, make the Salween river sands differ markedly from the Irrawaddy’s. As for the Paleozoic and older zircons, their U-Pb and Hf isotope patterns are similar to those of detrital zircons from two Triassic formations, i.e., the Shweminbon turbidite, Nanpandet area and Yewa mica schist, the Shan Scarp. They offer new constraints on the magmatic and crustal evolution of the Sibumasu terrane that, together with literature data, allow us to identify at least five stages of magmatic activities at ~2.5 Ga, 1.9 Ga, 1.0 Ga, 650-500 Ma and 213 Ma, with important mantle inputs or juvenile crustal growth at the first and third stages.