Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet
Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet
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DOI：
10.1144/gsl.sp.2003.218.01.11
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
J. Malpas;Mei‐Fu Zhou;P. Robinson;P. Reynolds
中科院分区：
文献类型：
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作者：
J. Malpas;Mei‐Fu Zhou;P. Robinson;P. Reynolds
摘要 藏南印度河-雅鲁藏布江缝合带标志着印度大陆与欧亚大陆拉萨地块的始新世碰撞。它的特点是，特别是在其中心部分，有一条东西向的蛇绿岩带和相关的海洋火山岩和沉积岩带，形成了许多结构上并置的地质地体。尽管许多地方受到构造破坏，但东部的罗布萨和泽东以及西部的日喀则附近存在几乎完整的蛇绿岩序列。在罗布萨，蛇绿岩层序被逆冲到罗布萨组的第三纪磨拉石矿床或冈底斯岩基的深成岩上。蛇绿岩地块以地幔层序为主，主要由方辉橄榄岩和含单斜辉石的方辉橄榄岩组成，并伴有丰富的豆状铬铁矿，被纯橄榄岩包裹。罗布萨蛇绿岩形成了洋内火山弧泽东地体的基底，该火山弧发育于中侏罗世和中白垩世之间。再往西，大竹曲和日喀则附近存在完整的蛇绿岩序列。这些蛇绿岩具有超俯冲带地球化学特征，但没有明显的火山弧发育。灵敏的高分辨率离子微探针 U-Pb 锆石分析得出大竹曲地块石英闪长岩的结晶年龄为 126 Ma。在蛇绿岩底部以混杂岩形式出现的大块角闪岩被认为是蛇绿岩仰冲过程早期产生的热液变质底的残余物。从这些岩石中分离出来的角闪石和黑云母的 Ar/Ar 地质年代学结果分别得出该事件的年龄为 80 Ma 和 90 Ma，该事件被认为是在印度大陆边缘进入大洋内俯冲带时发生的。新特提斯洋底剩余部分继续向北俯冲到欧亚大陆南缘之下，在拉萨地块南缘产生了冈底斯大陆弧，并导致始新世印度和欧亚大陆碰撞导致海洋最终闭合。
Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.