越南北部馬河三角洲岩心之碎屑鋯石鈾-鉛定年學研究

Abstract

馬河縫合帶(Song Ma Suture)位處華南地塊(South China Block)和印支地塊(Indochina Block)交界處，馬河三角洲(Song Ma Delta)接受了大量來自馬河縫合帶之沉積物。其中的碎屑鋯石應可記錄相關板塊構造事件，將碎屑鋯石年代和周圍構造單元年代比對，應有助於了解馬河縫合帶周遭的大地構造演化。故本研究透過對馬河三角洲五口鑽井岩心之碎屑鋯石，進行雷射剝蝕-感應耦合電漿質譜儀鈾-鉛定年，以製作年代頻譜，和周遭大地構造單元之年代頻譜以傳統視覺方法比對，並引入新興的相異性檢測法( 如 K-S 檢測或 Kuiper 檢測等) 與多維尺度化(Multidimensional scaling, MDS)圖等統計工具及離析(Unmixing)模型，藉以輔助年代特徵之量化比對，期望能了解馬河三角洲之沉積物源演化，並推論可能的大地構造歷史。研究結果表明，所有馬河三角洲樣本和周邊大部分印支地塊與華南地塊之大地構造單元相似，皆具有非常強烈的印支期造山事件主要峰值(300–200 Ma)以及強烈的加里東期造山事件次要峰值(490–400 Ma)，表明華南地塊與印支地塊可能於490–400 Ma 時，地理位置已經非常接近，使兩者擁有相似的物源；古特提斯海開啟之後，印支造山事件發生，兩地塊再次沿馬河縫合帶聚合，長山帶(Truong Son Belt)發育完成，將印支地塊與部分華南地塊等過去具有相似物源之地體分隔開，後續各自受當地較強的造山事件影響。根據前人文獻所發表的馬河三角洲鑽井岩心放射性碳同位素 14 碳定年結果，利用內插法粗略估算得到樣本所在地層年代介於中全新世到晚全新世之間；在距今(西元1950 年)大約6,700 年前，紅河或其支流直接流入馬河三角洲北部區域；到了距今約3,100 年前，紅河並未繼續供應沉積物至馬河三角洲，而使馬河上游成為馬河三角洲沉積物之唯一物源。然而，距今約 1,500 年前，馬河三角洲南方的藍江開始有沉積物輸送至馬河三角洲南部區域。故馬河三角洲北方與南方區域曾經分別受紅河及藍江影響，因受影響區域相對侷限，推論沿岸流造成影響較小，此影響主要來自河流直接流入；然離析模型結果表明，此影響亦不大，馬河三角洲沉積物之主要物源來自於其上游桑怒區(Sam Nua zone)。井位 LK5 中顆粒較粗的樣本所含鋯石有較大顆的傾向，年代有較輕的趨勢；而顆粒較細的樣本內含的鋯石則有較小顆的傾向，年代有較老的趨勢，顯示流體動力分化對礦物顆粒大小及年代可能的影響，然而，尚無法完全排除物源變化造成的效應。本研究導入的新興統計工具及離析模型等有良好的輔助功能，提供的結果具有一定可信度，同時使用多種工具相互驗證，配合傳統的視覺比較方法及對地質背景的了解，研究者能夠進行更貼近實際情況的推論，這些工具未來無疑是沉積物物源示蹤研究進展的重要推手。It is commonly believed that the Song Ma Suture is the boundary between the South China Block and the Indochina Block. Therefore, the Song Ma Suture is a valuable research target for better understanding the tectonic evolution of these two blocks. The Song Ma Delta collects a large amount of sediment from the Song Ma Suture, which contains abundant detrital zircons recording many plate tectonic events. Comparing the dating results derived from these zircons between the Song Ma Delta and the surrounding tectonic units can improve the understanding of the tectonic evolution around the Song Ma Suture. Accordingly, this study commences dating the detrital zircons collected from five drilling cores in the Song Ma Delta using the U-Pb method with the laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS), acquiring age spectra of zircons, which are then compared with those from surrounding tectonic units through visualization. Additionally, this study applies a newly developed Unmixing model and statistical tools, including dissimilarity measurement (such as the K-S test or Kuiper test) and Multidimensional Scaling, to aid in quantified comparison between age signaturesand improve the understanding of the provenance evolution of the Song Ma Delta and infer the possible tectonic history of the South China Block and the Indochina Block along the Song Ma Suture.Results indicate that all samples from the Song Ma Delta are similar to those from the surrounding tectonic units in the South China Block and the Indochina Block, which exhibit a dominant peak of the Indosinian orogeny (300–200 Ma) and a strong secondary peak of the Caledonian orogeny (490–400 Ma). This suggests that during 490–400 Ma, the South China Block might be close with the Indochina Block, resulting in a similiar provenance. After the opening of the Paleo-Tethys Ocean, these two blocks converged again along the Song Ma Suture during the Indosinian orogeny and built the Truong Son Belt, which separated the terranes sharing the similiar provenance in the Indochina Block and part of the South China Block. These terranes have been individually impacted by sequential local orogenic events.The stratigraphic ages, calculated through interpolation between radioactive 14C dating from previous studies, suggest that all samples collected by this study were deposited during the Middle to Late Holocene. Approximately 6,700 years before the present (1950 A.D.), the Red River or its distributaries directly flowed into the northern part of the Song Ma Delta. This continued until approximately 3,100 years before the present when the Red River ceased to feed sediment to the Song Ma Delta. Subsequently, the Song Ma upstream became the sole sedimentary source for the Song Ma Delta. However, approximately 1,500 years before the present, to the south of the Song Ma Delta, the Song Lam started to transport sediment to the southern part of the Song Ma Delta. The effect caused by the Red River and the Song Lam on the northern and southern parts of the Song Ma Delta, respectively, was locally confined, and the longshore currents could barely transport sediment to the Song Ma Delta. This suggests that such an effect mainly came from rivers rather than longshore currents transporting sediments. However, results from the Unmixing model indicate this effect was very weak and suggest that the major source of sediment for the Song Ma Delta is the upstream Sam Nua zone.Samples with coarser grains in the drilling core LK5 are composed of zircons with larger grain sizes, accompanied by younger ages. Conversely, samples with finer grains are composed of zircons with smaller grain sizes, accompanied by older ages. This implies that hydrodynamic fractionation might have impacted the grain size and age of minerals; however, the effect of provenance changing still cannot be ruled out.The novel statistical tools and the Unmixing model applied in this study show promising outcomes and provide robust and reasonable results. Testing these results against those obtained from different methods, along with a comparison to the traditional visulization method and a thorough understanding of the geological background, can undoubtedly assist researchers to make progresses in sediment provenance research in the future.