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西秦岭-东昆仑造山带蛇绿岩及岛弧型岩浆岩的年代学和地球化学研究
中文摘要

中国大陆中部东—西走向的“中央造山系”是我国北方(以华北和塔里木为代表)和南方(以华南和羌塘为代表)两大陆块群碰撞拼合的结合部,其形成与原特提斯洋和古特提斯洋的演化密切相关。“中央造山系”东段是华北和华南陆块碰撞形成的秦岭—大别—条造山带;西段西秦岭以西撒开分为两支:北部的北祁连造山带和南部的昆仑造山带,中间夹有众多微陆块。查明秦岭—大别造山带与祁连—昆仑造山带及夹于其中的微陆块的衔接、拼合关系是中国大陆诸多陆块构造演化研究的关键问题,而西秦岭—东昆仑造山带正是东西衔接、转换的关键区域。“中央造山系”“秦—祁—柴—昆”地区沿各主要缝合带保存了大量与原特提斯洋和古特提斯洋构造演化有关的蛇绿岩和岛弧岩浆岩。精确测定区内出露的蛇绿岩和岛弧岩浆岩的年龄与地球化学性质,不仅为西秦岭和东昆仑造山带的演化,而且对查明中国中西部众多造山带与东部秦岭—大别造山带的衔接关系及特提斯洋的演化提供重要制约。前人研究成果及尚存问题分别是: ①东秦岭地区厘定出“商—丹”和“勉—略”两条重要的缝合带;早古生代华北陆块南缘的“商—丹洋”(属原特提斯构造域)发育有完整的“弧—盆”体系,晚古生代“勉—略洋”(属古特提斯构造域)的打开使南秦岭微陆块从冈瓦纳大陆分离。其中“商—丹”带向西如何延伸是秦岭构造研究的关键问题之一,前人依据地层对比提出的“商—丹缝合带西延与西秦岭天水—武山缝合带相连”的观点尚缺乏西秦岭地区蛇绿岩和岛弧岩浆岩的可靠地球化学和年代学数据。此外,“商—丹洋弧—盆”体系演化历史中的一些重要事件尚缺乏同位素年龄的制约,如“商—丹洋”洋壳的初始形成时代,二郎坪弧后盆的开启时代及“商—丹洋盆”闭合时代等。 ②西秦岭以西的“祁—柴—昆”地区存在众多与冈瓦纳陆块具有较强亲缘性的微陆块,它们彼此之间为早古生代缝合带所分割。新元古代—早古生代时期总体呈现原特提斯“多岛洋”的构造格局,本文工作开展之前积累的年代学数据显示该“多岛洋”的南部边界是“东昆中断裂带”,而更向南的“东昆南断裂带”因为仅有晚古生代蛇绿岩年龄数据报道,被认为属于古特提斯构造域。但西段布青山地区新发现的早古生代蛇绿岩暗示东昆南断裂带东段阿尼玛卿地区也可能存在尚未被发现的早古生代蛇绿岩,或者布青山地区与阿尼玛卿地区具有不同的演化历史。澄清这一问题涉及原特提斯“多岛洋”的南界问题,并需要对该带蛇绿岩开展更深入的年代学研究。 ③现代印度洋MORB表现出的特殊同位素特征是否继承自特提斯洋是地质学领域的热点问题。前人依据中国西部特提斯构造域(包括新特提斯、古特提斯和原特提斯)蛇绿岩均具有“印度洋MORB型同位素组成特征”判定现代印度洋MORB表现出的特殊同位素特征是从特提斯洋继承而来的。但是前人用于Pb同位素示踪的特提斯蛇绿岩中具MORB特征的玄武岩和辉长岩样品普遍存在不同于现代大洋MORB的元素Pb的正异常,因此,使用这些蛇绿岩Pb同位素示踪获得的“印度洋地幔大体上继承了特提斯洋地幔”的认识尚存在疑问。 ④随着古特提斯和新特提斯洋的闭合,中国东部和西部在中—新生代分别发生了若干大陆碰撞构造运动。中国东部大陆是三叠纪华北陆块和华南陆块碰撞形成。中国西部大陆是中—新生代羌塘、拉萨地块和印度陆块与古生代时已与华北陆块拼合的青藏高原诸多微陆块(从北向南包括中祁连、南祁连、柴达木、东昆北和东昆南等)接连发生碰撞拼合形成的。这东、西两套碰撞构造运动在“中央造山系”结合部——西秦岭和“祁—柴—昆”地区会发生什么构造响应是一个令人感兴趣的科学问题。 针对上述四个科学问题,本文对西秦岭—东昆仑造山带蛇绿岩和岛弧型岩浆岩及部分中生代构造—岩浆事件开展地球化学和年代学研究,获得的结果和新认识主要是: 1 西秦岭早古生代“天水—武山”洋及其“弧—盆”构造体系的年代学格架 本文对西秦岭地区沿天水—武山构造带及其邻近地区发育的蛇绿岩和岩浆岩开展了系统的地球化学和锆石SHRIMP U-Pb及角闪石⁴⁰Ar/³⁹Ar年代学研究,判明了它们各自形成的构造环境和时代。从而清晰地给出了西秦岭早古生代“天水—武山洋”及其弧—盆体系的年代学格架,支持“商—丹缝合带西延与天水—武山缝合带相连”的论断。 关子镇蛇绿混杂岩位于“天水—武山”构造带中部。本文识别出该蛇绿混杂岩包含了古洋壳型和古岛弧火成岩型两种类型:古洋壳型蛇绿岩样品不具有显著的Nb负异常(La/Nb)〓<1.5、LREE亏损或略富集,(La/Yb)〓<1.4、亏损的Nd同位素组成ε〓(t)=2.1~4.0,这些特征与MORB型变玄武岩类似;并首次在该区鉴别出洋中脊成因的“大洋斜长花岗岩类”。古岛弧型蛇绿岩样品具有显著的Nb负异常(La/Nb)〓=3.74、LREE显著富集(La/Yb)〓=17.1、富集的Nd同位素组成ε〓(t)=-6.9。锆石SHRIMP U-Pb定年结果为:古洋壳成因变辉长岩和变斜长花岗岩形成时代分别是534±9 Ma和517±8 Ma;古岛弧成因变辉长岩形成时代为489±10 Ma。变斜长花岗岩中角闪石的⁴⁰Ar/³⁹Ar坪年龄394.9±5.2Ma指示了关子镇蛇绿混杂岩经历角闪岩相变质作用的时代。 武山蛇绿混杂岩位于“天水—武山”构造带西部。该蛇绿混杂岩中的桦林沟辉长岩和鸳鸯镇辉长闪长岩样品LREE富集,(La/Yb)〓分别为7.1和10.8,Nb、 Ta、Ti显著亏损,显示它们形成于岛弧环境;锆石SHRIMP U-Pb年龄分别是440±5Ma和456±3。关子镇和武山岛弧岩浆岩样品的低ε〓(t)值(-3.8~-6.9)表明关子镇古岛弧和武山古岛弧都是华北陆块南缘的近陆古岛弧。 “天水—武山”断裂以北的秦岭群北侧,沿清水—红土堡一带出露有酸性和基性火山岩。它们的SiO₂含量变化于47.69%~50.80%和68.83%~70.14%,显示具有“双峰式”火山岩的特征。清水“陈家河群”“双峰式”火山岩中的玄武岩与红土堡玄武岩具有相似的痕量元素和Sr-Nd同位素组成。高放射成因Pb同位素组成,特别是高²⁰⁷Pb/²⁰⁴Pb比值表明这些玄武岩的形成与俯冲带有关。两件清水新城英安岩样品的锆石SHRIMP U-Pb年龄447±8 Ma和4484±8与己报道的红土堡玄武岩年龄443.4±1.7Ma接近。据此提出“陈家河群”火山岩形成于弧后盆初始拉张阶段,而不是前人建议的“形成于岛弧环境”。 综合上述新数据,提出如下的西秦岭早古生代“天水—武山”洋构造演化模型:“天水—武山”洋形成于早—中寒武世(534±9Ma~517±8 Ma),洋壳初始消减发生在晚寒武—早奥陶世(489±10Ma);武山地区存在晚奥陶(456±3Ma) —早志留世(440±5Ma)的两次洋壳消减事件;同时在448±8Ma时,清水—红土堡弧后盆开始拉张形成;在早—中泥盆世(394.9:55.2Ma)发生弧陆碰撞并引发变质作用。对比研究表明,本文建立的“天水—武山洋”及其弧—盆体系与东秦岭沿“商—丹”构造带发育的古大洋及其弧—盆体系之间存在很好的对应关系。因此,“商—丹”洋和“天水—武山”洋共同构成了华北陆块南缘早古生代“秦岭洋”,其演化历史可能反映了原特提斯部分边缘海的演化历史。 2 东昆南构造带存在晚震旦—早奥陶世的洋盆 首次在东昆南构造带花石峡—玛沁区段内发现具有晚震旦—早奥陶世锆石SHRIMP U-Pb年龄的古洋壳残片。形成于晚震旦世(555±59 Ma)的苦海辉长岩LREE略富集((La/Yb)N=5.15),高Ni(102 ppm)含量,Nb、Ta略亏损,这些特征类似于OIB;形成于早寒武世(535±510 Ma)的玛积雪山辉长岩LREE亏损((La/Yb)N=0.61),高Ni(160 ppm)含量,Nb、Ta不亏损,显示典型的MORB特征。而德尔尼闪长岩(493±6 Ma)LREE富集((La/Yb)N=52.64),Ni含量(18 ppm)低,Nb、Ta显著亏损,显示典型的IAB的特征,因此其年龄指示了古洋壳消减的时代。上述蛇绿混杂岩的定年结果与已报道的布青山早古生代蛇绿岩共同表明东昆南构造带存在不同于“勉—略”构造带的晚新元古代—早古生代演化历史。 东昆南构造带晚新元古代—早古生代岩浆事件的时代可与已报道的从北祁连到东昆中构造带的蛇绿岩和岛弧火山岩年龄数据对比。因此,该多岛洋的南部边界不应限定在早先认为的“东昆中断裂带”,而应该向南推到“东昆南断裂带”。 3 蛇绿岩变质过程中元素Pb的引入及对Pb同位素组成的影响——对“印度洋MORB型同位素特征”起源的制约 特提斯蛇绿岩的Pb同位素组成被用于判断印度洋地幔特殊的同位素组成是否继承自特提斯洋地幔,而我们首次明确指出MOR型蛇绿岩样品中存在Pb的正异常有可能影响Pb同位素对其地幔源区的示踪效果。为查明引起MOR型蛇绿岩Pb正异常的原因,本文对比样品采用不同预处理方法(酸淋洗与未淋洗)和不同测试方法(ICP-MS与同位素稀释法)分析所得元素Pb含量的差异,发现导致蛇绿岩样品元素Pb正异常的因素是:①ICP-MS分析流程的高本底Pb; ②地表作用过程中样品表面吸附的Pb;③变质流体作用引入的Pb。 样品的Nb/Pb(指示流体活动强度的指标)与δPb(指示Pb正异常程度的指标)负相关,表明MOR型蛇绿岩样品中的过量Pb与蛇绿岩构造侵位过程或其后变质流体的活动有关,蛇绿岩样品的实测Pb同位素组成是变质流体和未改造MORB相混合的结果。因此,本文只选择那些具有最小δPb的样品来进行蛇绿岩地幔源区的Pb同位素示踪。按照这种方法,重新分析获得的中国西部原特提斯、古特提斯和新特提斯洋MOR型蛇绿岩样品同位素数据,显示“印度洋MORB型同位素特征”大体上继承自特提斯洋地幔,这与前人的论断一致;但同时发现印度洋相比古特提斯洋,其地幔源区具有相对低的ε〓和高的²⁰⁶Pb/²⁰⁴Pb值,推测与古特提斯洋关闭过程中地壳物质再循环进入地幔有关。 4 西秦岭与“祁—柴—昆”地区对中国东、西部晚古生代—中生代大陆碰撞构造运动的响应——年代学记录 陆—陆斜向碰撞构造运动可导致平行活动陆缘的大型走滑断裂。清水新城糜棱岩化英安岩样品中黑云母⁴⁰Ar/³⁹Ar坪年龄355.2±2.6 Ma,指示了“陈家河断裂”走滑运动时代。武山北李家河剖面花岗质糜棱岩样品的锆石SHRIMP U-Pb年龄为951±13 Ma,表明原岩花岗岩形成于新元古代;而白云母⁴⁰Ar/³⁹Ar坪年龄226.84±2.2 Ma代表了这一花岗岩发生糜棱岩化作用的时代,表明“天水—武山断裂”在这一时期发生走滑运动。这两个构造年龄分别与“商—丹洋”和“勉—略洋”的闭合时代一致,表明这些陆缘断裂的走滑运动是华北陆块,北秦岭微陆块、南秦岭微陆块及华南陆块碰撞拼合作用的结果。因此,355Ma指示了南秦岭与北秦岭斜向碰撞的时代,而227Ma指示了华南与南秦岭斜向碰撞的时代。对于没有超高压变质岩出露的缝合带,本文观察到的陆缘走滑断裂发育时代与陆块拼合时代的一致性,为我们提供了一个判断陆块碰撞时代的有效方法。 由于南秦岭微陆块北缘的“天水—武山”断裂在中—晚三叠世发生右行走滑,而南缘的“勉—略”断裂发生左行走滑,因此南秦岭微陆块在三叠纪华北陆块和华南陆块碰撞过程中被“向西挤出”。该挤出构造传递的向西挤压力可导致西部古生代已拼合的“祁—柴—昆”地区发生引张。本文初步测定的柴北缘宗务隆蛇绿岩中辉长岩年龄233±9 Ma,指示了柴达木地块北缘一次三叠纪引张事件,这可能是对南秦岭微陆块三叠纪“西向挤出构造”的响应。而麻当碱性玄武岩的⁴⁰Ar/³⁹Ar坪年龄87.2±0.9 Ma则可能反映了西秦岭地区对青藏高原拉萨地块与羌塘地块晚白垩世碰撞事件的构造响应。 关键词:西秦岭山带 东昆仑造山带 蛇绿岩 岛弧型岩浆岩 年代学地球化学 特提斯

英文摘要

The east-west striking " Central Orogenic System (COS) " lain in the central China is considered to be the collisional zone between the Northern China blocks (represented by the North China Block and the Tarim Block) and the Southern China blocks (represented by the South China Block and the Qiangtang Block). The formation of the "COS" was closely related to the evolution of the Proto-Tethys and the Paleo-Tethys. In the eastern part of the "COS", there's the uniform Qinling-Dabie orogen which resulted from the collision between the North China Block and the South China Block, while to the west of the Western Qinling orogen, the "COS" has two major branches: i.e. the Qilian orogen in the north and the Kunlun orogen in the south, between which a lot of micro-continents are distributed. Trying to find out the joining relationship between the Qinling-Dabie orogen and the Qilian-Kunlun orogenic belts is a key project for understanding the tectonic evolution of China continental blocks. The Western Qinling orogen and the Eastern Kunlun orogen are the key areas to link up the Qinling-Dabie orogen and the Qilian-Kunlun orogens. Lots of ophiolites and island-arc-type igneous rocks, which are related to the evolution of the Proto-Tethys and the Paleo-Tethys, develop along those major suture zones in the area of "Qinling-Qilian-Qaidam-Kunlun" of the "COS". Accurate geochronological and geochemical studies for ophiolites and island-arc-type igneous rocks in these areas will provide crucial constraints not only on the evolution of the Western Qinling and the Eastern Kunlun orogens but also on the joining relationship between the Qinling-Dabie orogen in the east and those orogenic belts in the west, as well as on the evolution of the Tethys in the Central- Western China. Previous studies on such rocks and remained questions are summarized as following: ①Two main suture zones have been identified in the Eastern Qinling orogen: i.e. the "Shang-Dan" suture and the "Mian-Lüe" suture. An early Paleozoic arc-back arc basin system along the southern margin of the North China Block has been recognized in the "Shang-Dan Ocean" (belonged to the Proto-Tethyan domain), while the opening of the "Mian-Lüe Ocean" (belonged to the Paleo-Tethyan domain) split the South Qinling micro-continent from the Gondwana during the Late Paleozoic. How the "Shang-Dan" suture extends westward is a key question in the tectonic research on the Qinling orogen. Based on stratigraphic correlation, the Tianshui-Wushan suture in the western Qinling has been suggested to be the westward extension of the Shang-Dan suture. However, accurate geochemical and geo-chronological data for ophiolites and island-arc-type igneous rocks in the Western Qinling orogen have not been reported yet, which are critical to test such a view point. Furthermore, some important ages concerning the evolution history of the arc-back arc basin system in the "Shang-Dan Ocean" have not been well constrained, such as the age for the initial formation of the "Shang-Dan" oceanic crust, the age for the opening of the Ealangping back-arc basin and the times for the closure of the "Shang-Dan Ocean". ②To the west of the Western Qinling orogen, there are numerous micro-continentals in the area of "Qilian-Qaidam-Kunlun", which are geochemically similar to the Gondwana and separated from each other by several Early Paleozoic sutures. Therefore, the tectonic framework of this area is generally considered to be the Proto-Tethyan archipelagic ocean during the Late Neoproterozoic to the Early Paleozoic. According to previous geochronological data, the southern margin of such an archipelagic ocean was located in the central east Kunlun tectonic belt, while the southern east Kunlun tectonic belt, to the south, is considered to belong to the Paleo-Tethyan domain because only Late Paleozoic ages for the ophiolites in this belt have been reported. However, the recent discovery of the Early Paleozoic ophiolite in the Buqingshan area, located in the western section of the southern east Kunlun tectonic belt, suggests that either there is undiscovered Early Paleozoic ophiolite in the eastern section of this belt, i.e. A'nyemaqen area, or the evolution history of the Buqingshan area differed from the A'nyemaqen area. To clarify this question may help us to determine the location of the southern margin of the Proto-Tethyan archipelagic ocean, which needs further geochronological investigations on the ophiolites in the southern east Kunlun tectonic belt. ③It is a hot subject in geological society that whether the distinct isotopic features of MORB from the Indian Ocean has been inherited from the Tethys. It has been shown that "the Indian MORB-type isotopic signature" has been possessed by ophiolites form the Tethyan (including Proto-Tethys, Paleo-Tethys and Neo-Tethys) domain in the Western China. Thus, a hypothesis has been advocated by some scholars that the distinct Indian Ocean mantle domain may be largely "inherited" Tethyan mantle. However, contrasting with present-day MORB, ubiquitous enrichments of Pb exist in MORB-type basalts and gabbros from the Tethyan ophiolite suites. Therefore, the conclusion mentioned above, which is based on the Pb isotopes of the ophiolite samples with positive Pb anomaly, is still doubtful. ④Following the closure of the Paleo-Tethys and the Neo-Tethys, several continental collisions had taken place during Mesozoic and Cenozoic in Eastern China and Western China, respectively. The continent of Eastern China resulted from the collision between the North China Block and the South China Block in Triassic, while the continent of Western China was the product of the successive collisions between the blocks in Tibet plateau from the north to the south, such as Middle Qilian, South Qilian, Qaidam, North Kunlun and South Kunlun, which had been combined with the North China Block during the Paleozoic, and the Qiangtang Block, the Lhasa Block and the Indian Plate, which successively collided each other in the Masozoic and the Cenozoic. It is interested to know that what tectonic responses to those continental collision events developed in Eastern China and Western China, respectively, had happened in the junction part of the "COS", i.e. the area of Western Qinling and "Qilian-Qaidam-Kunlun". In order to answer the above four scientific questions, geochemical and geochronological studies on the ophiolites and island-arc-type igneous rocks, as well as some Mesozoic tectono-magmatic events in the Western Qinling orogen and the Eastern Kunlun orogen have been carried out. The following new findings and knowledge are achieved in this study. 1 Geochronological framework of the Early Paleozoic "Tianshui-Wushan" ocean and its arc-back arc basin system in the Western Qinling orogen The tectonic setting and ages of the ophiolite and igneous rocks developed along the Tianshui-Wushan suture and its adjacent area in the Western Qinling orogen have been determined based on comprehensive geochemical analyses and SHRIMP U-Pb dating of zircons as well as ⁴⁰Ar/³⁹Ar dating of hornblende. Thus, the geo-chronological framework of the Early Paleozoic "Tianshui-Wushan" ocean and its arc-back arc basin system in the Western Qinling orogen has been established, which supports the viewpoint that "the Tianshui-Wushan suture could be the westward extension of the Shang-Dan suture". The Guanzizhen ophiolite complex occurred in the central section of the Tianshui-Wushan tectonic belt. Two types of protolith have been distinguished for the samples from the Guanzizhen ophiolite complex, i.e. the paleo-oceanic crust and the paleo-island arc igneous rocks: samples related to the paleo-oceanic crust are characterized by LREE depletion or slight LREE enrichment ((La/Yb)〓<1.4) with no Nb negative anomalies in spider-diagram ((La/Nb)〓<1.5) and depleted Nd isotopes (ε〓(t)=2.1~ 4.0). Such geochemical features are similar to those of the MORB. Moreover, "oceanic plagiogranite" formed at the middle ocean ridge has been identified for the first time in the Guanzizhen area. In contrast, one gabbro sample related to the paleo-island arc is characterized by high LREE enrichment ((La/Yb)〓=17.1) with remarkable Nb negative anomalies in spider-diagram ((La/Nb)〓=3.74) and enriched Nd isotopes (ε〓(t)=-6.9). Zircon SHRIMP U-Pb dating yielded the ages of 534 ± 9Ma and 517 ± 8 Ma for meta-gabbro and meta-plagiogranite related to the paleo-oceanic crust, respectively, and an age of 489± 10 Ma for meta-gabbro related to the paleo-island arc. The ⁴⁰Ar/³⁹Ar plateau age of hornblende from the meta- plagiogranite is 394.9 ± 5.2 Ma, indicating the time of amphibolite-facies metamorphism. The Wushan ophiolite complex occurred in the western section of the Tianshui-Wushan tectonic belt. The samples from the Hualingou gabbro and the Yuanyangzhen gabbro-diorite are characterized by LREE enrichment ((La/Yb)〓=7.1 and 10.8, respectively) and remarkable Nb, Ta and Ti negative anomalies in spider-diagram, indicating they were formed in the island arc setting. The zircon SHRIMP U-Pb ages are 440 ± 5 Ma and 456 ± 3 Ma for the Hualingou gabbro and the Yuanyangzhen gabbro-diorite, respectively. Their low ε〓(t) values (-3.8 ~ -6.9) suggest that the Guanzizhen Paleozoic island arc and the Wushan Paleozoic island arc were epicontinental arcs developed on the southern margin of the North China Block. To the north of the Qinling Group developed on the northern side of the Tianshui-Wushan Fault, acidic and basic volcanic rocks occured in the area of Qingshui-Hongtubao. Their SiO₂ contents range from 47.69% ~ 50.80% and 68.83% ~ 70.14%, displaying characteristics of bimodal volcanic rocks. The geochemical and Sr-Nd isotopic features of the all basalts either from the "Chenjiahe Group" bimodal volcanic rocks at the Qingshui area or from the Hongtubao area are similar. Moreover, relatively higher radiogenic Pb isotopic compositions, especially higher ²⁰⁷Pb/²⁰⁴Pb ratios, suggest that the formation of these basalts were related to the subduction zone. Zircon SHRIMP U-Pb ages of 447 ± 8 Ma and 448 ±8 Ma for two dacite samples from Xincheng at the Qingshui area are consistent with the previously published age of 443.4±1.7 Ma for the basalt at the Hongtubao area. Thus, it's suggested that the "Chenjiahe Group" volcanic rocks were formed at the initial stage of a back-arc extension instead of the island arc setting suggested by previous studies. According to these new results, a model for the tectonic evolution of the "Tianshui-Wushan Ocean" in the Western Qinling orogen during the Early Paleozoic is proposed. The "Tianshui-Wushan Ocean" formed in the Early-Middle Cambrian (534 ± 9 Ma ~ 517 ± 8 Ma) followed by the initial subduction during the Late Cambrian to the Early Ordovician. Two subduction-related events took place at the Wushan area during the Late Ordovician (456 ± 3 Ma) to the Early Silurian (440 ± 5 Ma) together with the initial extension of the back-arc basin in Qingshui and Hongtubao area at 448 ± 8 Ma. Metamorphism resulted from the arc-continent collision happened during the Early-Middle Devonian (394.9 ± 5.2 Ma). The evolution history of the "Tianshui-Wushan" ocean and its arc-back arc basin system established in this study is well comparable to that developed along the "Shang-Dan" tectonic belt. Therefore, the "Shang-Dan Ocean" and the "Tianshui- Wushan Ocean" constitute, on the southern margin of the North China Block, the Early Paleozoic "Qinling Ocean", whose evolution history may reflect that of a marginal sea in the Proto-Tethys. 2 The Late Sinian to the Early Ordovician oceanic basin existed along the southern east Kunlun tectonic belt The fragments of paleo-oceanic crust with zircon SHRIMP U-Pb ages of the Late Sinian to the Early Ordovician are firstly observed in the Huashixia-Maqin section along the southern east Kunlun tectonic belt. The Kuhai gabbro, with the Late Sinian age (555 ± 9 Ma) is characterized by slight LREE enrichment ((La/Yb)〓=5.15), high Ni abundance (102 ppm) and slight depletion of Nb and Ta, which are similar to the features of the OIB, while the Majixueshan gabbro with the Early Cambrian age (535± 10 Ma) is characterized by LREE depletion ((La/Yb)〓=0.61), high Ni abundance (160 ppm) with no Nb or Ta negative anomalies, which are typical features of the MORB. On the other hand, the Dur'ngoi diorite with age of 493±6 Ma is characterized by LREE enrichment ((La/Yb)〓=52.64), low Ni abundance (18 ppm) and remarkable depletion of Nb and Ta, which are typical features of the IAB. So its age indicates the subduction time of the paleo-oceanic crust. The above results together with the Early Paleozoic ophiolite in the Buqingshan area reported previously document the existence of the Late Sinian to the Early Ordovician ocean in the southern east Kunlun tectonic belt, indicating that the southern east Kunlun tectonic belt has had an evolution history differing from the "Mian-Lüe" tectonic belt during the Late Neoproterozoic to the Early Paleozoic. The above dating results of the ophiolites occured along the southern east Kunlun tectonic belt are comparable to those reported ages for ophiolites and island-arc volcanic rocks from the Northern Qilian belt to the central east Kunlun tectonic belt. Thus, it is concluded that the southern margin of this Proto-Tethyan archpelagic ocean could not be limited in the central east Kunlun tectonic belt as suggested previously, but in the southern east Kunlun tectonic belt. 3 The introduction of Pb during metamorphism of ophiolites and its effect on the Pb isotopic compositions — implication for origin of the isotopic signature of the Indian Ocean MORB Pb isotopic compositions of the Tethyan ophiolites have been studied to test whether the distinct isotopic signatures of the Indian Ocean are inherited from the Tethys Ocean. However, we first point out that the positive Pb anomaly existed in MOR-type ophiolites may result in invalidation of Pb isotopes in tracing the sources of ophiolites. To determine the possible reasons causing the positive Pb anomaly in MOR-type ophiolites, the differences in Pb concentrations obtained using different pretreatment methods (acid leach vs. unleach) and different analytical methods (by ICP-MS vs. by isotopic dilution) are investigated. It shows that there are three major causes of the positive Pb anomaly observed in ophiolites, i.e. the high Pb blank during ICP-MS analyses, the adsorption of Pb during the surface processes, and the external Pb introduced by metamorphic fluids. The negative correlation between Nb/Pb (indicating the quantity of fluid flux) and 5Pb (indicating the degree of positive Pb anomaly) suggest that the excess Pb in MOR-type ophiolites may be related to the activity of metamorphic fluids during or after the tectonic emplacement of ophiolites. The Pb isotopic compositions of ophiolite samples are results of mixing between metamorphic fluids and unaltered MORB rocks. Therefore, in this study, only those samples with the lowest value of δPb in each ophiolite should be selected to constrain the Pb isotopic compositions of their mantle sources. Following this way, our new data obtained from the Proto-, Paleo- and Neo-Tethyan ophiolites in Western China suggest that most of the Indian MORB-type isotopic signatures are basically inherited from the Tethyan mantle, consistent with previous conclusion. However, the Indian Ocean mantle trends to have lower ε〓 values and higher ²⁰⁶Pb/²⁰⁴Pb ratios than the Paleo-Tethyan mantle, which could be a result of recycling of crustal materials during the closing of the Paleo-Tethys. 4 The tectonic responses in the Western Qinling and the "Qiliang-Qaidam-Kunlun" orogens to the continental collision events in Easten China and Western China during the Late Paleozoic to Mesozoic — Geochronological records Oblique collision between two continents may lead to large-scale strike-slip faults parallel to active continental margin. The ⁴⁰Ar/³⁹Ar plateau of biotite from the mylonite dacite at Xincheng in the Qingshui area gives an age of 355.2±2.6 Ma, indicating the time of strike-slip movement for the Chenjiahe Fault. Zircon SHRIMP U-Pb age of 951±13Ma for the granitic mylonite from the Lijiahe profile in the north of Wushan area indicates that the granitic protolith was formed during Neoproterozoic, while the ⁴⁰Ar/³⁹Ar plateau age of 226.8±2.2 Ma given by white mica from the same sample indicates the mylonitization time of the gneiss, suggesting a strike-slip movement for the Tianshui-Wushan Fault occurred at the Triassic. These two ages are just corresponding to the closure times of the "Shang-Dan Ocean" and the "Mian-Lüe Ocean", respectively, which suggest that these strike-slip movements on continental margin resulted from each oblique collision and suturing between the North China Block, North Qinling and South Qinling micro-continents. Thus, the age of 355Ma indicates the oblique collision between the South Qinling Block and the North Qinling Block, while the age of 227Ma indicates the oblique collision between the Yangtze Block and the South Qinling Block. To those suture zones without any ultrahigh-pressure metamorphic rocks, the consistency observed in this study between the formation ages for the strike-slip movement on continental margin and the times of continental matching provides us a valid means to constrain the times of continental collision. Because the Tianshui-Wushan Fault, on the northern margin of the South Qinling micro-continent, took place right-lateral strike-slip movement during the Middle-Late Triassic, while the "Mian-Ltie" Fault, on the southern margin of the South Qinling micro-continent, took place left-lateral strike-slip movement in the same time. It's suggested that the South Qinling micro-continent had undergone "Westward Extrusion" during the Triassic collision between the North China Block and the South China Block. The westward compression given by such a westward extrusion may cause an extension in the "Qilian-Qaidam-Kunlun"area, which had been assembled during the Early Paleozoic. The tentative dating result of 233±9 Ma for the gabbro from the Zongwulong ophiolite indicates an extensional event happened during the Triassic on the northern margin of the Qaidam Block. Such an extensional event could be the tectonic responses to the "Westward Extrusion" of the South Qinling micro-continent. Furthermore, the ⁴⁰Ar/³⁹Ar plateau age 87.2±0.9 Ma for the Madang alkaline basalt is considered as the tectonic responses in the Western Qinling area to the collision between the Lhasa Block and the Qiangtang Block in Tibet during the Late Cretaceous. Key words: the Western Qinling orogen, the Eastern Kunlun orogen, ophiolites, island-arc-type igneous rocks, geochronology, geochemistry, Tethys

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