雅鲁藏布缝合带泽当段蛇绿岩的地球化学特征及构造意义

1、雅鲁藏布缝合带泽当段蛇绿岩的地球化学特征及构造意义        雅鲁藏布缝合带代表曾经存在于印度板块与亚欧板块之间的新特提斯古大洋残余。整个缝合带可分为西、中、东三段，泽当段属缝合带东段并由泽当蛇绿岩和一套岛弧岩石组合混杂组成。作为东段最典型的剖面之一，对泽当蛇绿岩的研究一直比较溥弱，其构造环境也存在争议。泽当地区不同于其他大多数仅以蛇绿岩为主体的缝合带地段，除了蛇绿岩，该区还保存着一套岛弧岩石，可能代表曾经于新特提斯洋内发育的岛弧。新特提斯的构造演化一直存在单一俯冲和双重俯冲的争论，关键问题就在于缺乏洋内俯冲带

2、和岛弧存在的证据。从这个角度来说，对泽当段缝合带及蛇绿岩的详细研究，不但有助于重建特提斯古洋盆格局，对进一步认识印度-亚欧板块的汇聚和碰撞过程也有重要意义。泽当蛇绿岩单元出露基本齐伞，包括地幔橄榄岩、辉长辉绿岩、块状和枕状玄武岩。对泽当蛇绿岩玄武岩进行的Sm-Nd同位素分析，得一等时线年龄为175+20Ma，代表了蛇绿岩的形成时代。玄武岩的Nd(t)值为+7.0+7.3，表明原始岩浆来源于强烈亏损的地幔源区。泽当蛇绿岩中的地幔橄榄岩以方辉橄榄岩为主，具有低Ti、Al，高Mg的岩石化学特征，REE表现为“U”型的分布型式，属典型的残余地幔橄榄岩。辉长辉绿岩样品都具有LREE亏损的左倾分布型式和E

3、u正异常，其微量元素绝对含量很低，表现出明显的Nb、Ta、Zr、Hf亏损和Cs、Rb、Sr、Ba、K富集。泽当蛇绿岩壳层熔岩根据地球化学特征可分为两组：玄武岩(接近N-MORB)和玄武岩(具IAT特征)。两组岩石均表现为平滑的左倾LREE亏损型分布和基本相似的微量元素分布型式，但玄武岩的元素丰度明显要高。玄武岩比玄武岩遭受到更强的俯冲作用影响，具有明显的Nb、Ta亏损。泽当蛇绿岩表现出岛弧和MORB的双重地球化学特征，属于SSZ型蛇绿岩，结合区域地质背景，推测其形成于俯冲带之上的弧前盆地。首次从泽当岛弧岩石组合中厘定出一套埃达克质英云闪长岩，是俯冲洋壳在角闪岩-榴辉岩相过渡带部分熔融的产物。这

4、一认识为新特提斯洋内俯冲带和岛弧的存在提供了重要证据。在前人研究的基础上，本次研究认为泽当地区新特提斯洋存在北向的洋内俯冲系统，泽当蛇绿岩和泽当岛弧均为洋内俯冲的产物。中侏罗世时，新特提斯洋内发育一条幔内型推覆韧性剪切带，印度板块和拉萨地块对大洋岩石圈的双向挤压诱发了洋内俯冲消减。俯冲板片向下消减作用的加强使得位于洋内俯冲带之上的大洋板块发生局部拉张，形成扩张脊。大量来自俯冲板片的水和大离子亲石元素进入亏损地幔楔，使之发生部分熔融，生成的岩浆沿着扩张脊在海底喷发，形成了新的SSZ型洋壳和洋盆，时代为175Ma左右。同时，早期发育的老洋壳在北面开始沿拉萨地块南缘俯冲消减，大约174Ma时在冈底斯

5、南缘生成了叶巴弧。由此形成了新特提斯洋的双重俯冲格局。在这期间，年轻的热的洋壳俯冲至75-85km深度左右时发生部分熔融并形成埃达克岩浆。俯冲板片的连续俯冲作用还导致了泽当蛇绿岩北侧洋内岛弧的形成，其活动时代约为156Ma。在晚白垩世，洋内俯冲系统仰冲到南面的印度板块之上，新特提斯洋洋盆停止扩张，转变为单俯冲格局。由于印度板块向北强烈推挤，沿拉萨地体南缘的俯冲消减活动更加强烈，火山岩浆活动十分广泛，进一步形成了大量中酸性的钙碱性岛弧岩浆岩，如桑日岛弧等。始新世时，印度板块和亚欧板块发生陆-陆碰撞，新特提斯洋关闭消亡，青藏高原开始隆升，缝合带及相邻地质体被后期构造运动改造成如今的面貌The Ya

6、rlung-Zangbo surture zone marks the Neo-Tethyan remnants existed between the Indian plate and the Eurasian plate. The suture zone can be divided into three parts, namely, the western, middle and eastern segments. Zedong suture located within the eastern segment, comprise a suit of melange made of Ze

7、dong ophiolite and island arc volcanic assemblage. Although the ophiolite has undergone complicated tectonic movements, its sequence is basically completed, composed mainly of peridotite, diabase and gabbro dyke swarms and pillowed and massive basalts.The Sm-Nd isochron age of basalts in Zedong ophi

8、olite is 175±20Ma, indicating that the ophiolite formed in the mid-Jurassic. The eNd(t) values of basalts range in +7.0-+7.3, suggesting a strong depleted mantle source and no crustal contamination.The mantle peridotite mostly consists of harzburgite, showing low Ti, Al, high Mg, and the "

9、U" type REE patterns, and belonging to the residual mantle peridotite. The gabbro and diabase all have LREE-depleted patterns and positive Eu anomaly. The rece elements abundance is low, exhibiting the obvious depletion of Nb, Ta, Zr, Hf and enrichment of Cs, Rb, Sr, Ba, K. According to the geo

10、chemical properties, the crustal lava of Zedong ophiolite can be divided into two groups: basalt I (close to N-MORB) and basalt II (is of IAT). Two groups of rocks all indicate smooth LREE-depleted REE patterns and similar distributions of race elements, and the element contents of basalt I are high

11、er. Basalt II underwent the stronger subduction than ophiolite I, exhibiting obvious Nb and Ta depletion.Zedong ophiolite shows the compound geochemistry characterisitics of island arc and MORB, belonging to the SSZ-type ophiolite. Considering the geological setting, we conclude it formed in the for

12、e-arc basin above the subduction zone.The subducted-type adakites in the Zedong segment of the Yarlungzangbo suture zone were discovered and identified for the first time. Zedong adakites were produced by the partial melting of young and hot subducted oceanic crust in amphibolite-eclogite transition

13、 zone and by going through the mantle wedge. This work presents new evidence for the intra-Tethyan subduction and the previous suggestion about the existence of intra-oceanic island arc within Tethys.Based on the former studies, a north directed intra-ocean subduction system once existed in Zedong T

14、ethyan ocean, and the Zedong ophiolite and island arc are all the production of intra-oceanic subduction. In mid-Jurassic, there was a ductile nappe shear zone developing in Neo-Tethyan ocean, and at the same time, the bidirectional compressing of Indian plate and Lhasa block on the oceanic lithosph

15、ere induced the intra-oceanic subduction. The reinforce of slab subduction resulted in the local extension of oceanic plate above the subduction zone and the formation of spreading ridges. Plenty of H_2O and LILE entered the depleted mantle wedge, led to the partial melting. Along the spreading ridg

16、es, the magma extruded on the sea floor and formed the new SSZ-type oceanic crust and basin(175Ma). Meanwhile, the old oceanic crust started to subduct toward the Lhasa block, and produced the Yeba-arc at the south edge of Gangdese. The Tethyan double subduction situation came into being. During thi

17、s period, the partial melting of young and hot subducted oceanic crust at 75-85km produced he adaktitic magma. The subduction also led to the formation of the Zedong intra-oceanic island arc in the north, which is active at 156Ma.At late Cretaceous, the Neo-Tethyan ocean basin stopped to spread and

18、turned to be the single subduction situation, and then the intra-oceanic subduction system obducted onto the Indian plate in the south. Because of the northward compressing of Indian plate, the subduction along the south edge of Lhasa block was stronger. The volcanic magma activities were extensive, producing abundant intermediate-acid calcium alkaline island arc igneous rocks. In Eocene, the Indian