KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 金沙江古特提斯洋志留-三叠纪演化及动力学过程 | |
| Alternative Title | The Silurian-Triassic evolution of Jinsha Paleotethys Ocean and geodynamics |
| 刘银 | |
| Subtype | 博士 |
| Thesis Advisor | 肖文交 |
| 2019-06-30 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Discipline | 工学博士 |
| Keyword | 金沙江古特提斯洋 洋岛玄武岩 洋中脊俯冲 微地体碰撞 Jinsha Paleotethys Ocean OIB ridge subduction microterrane collision |
| Abstract | 金沙江-哀牢山-松马缝合带代表的古特提斯洋是东古特提斯的重要组成部分，是青藏高原分隔南侧羌塘—思茅—印支地体和北(东)侧松潘-甘孜—扬子地体的重要分界线，保留了古特提演化的关键证据。作为东古特提斯洋的西延，金沙江古特提斯洋早期打开、俯冲、最后闭合时限以及伴随的石炭-三叠纪地球动力学机制，至今存在较大的争议。本文利用海山消减、洋中脊俯冲以及含微地体碰撞的俯冲模型为这些争议提供了有效约束和可行机制。第一，关于金沙江古特提斯洋早期打开、俯冲时限。本文在金沙江缝合带中获得晚志留-晚二叠世(422.4 ± 6.1, 336.4 ± 7.4, 272.3 ± 2.8 和 255.8 ± 3.4 Ma)具有OIB 地球化学特征的镁铁质岩块，结合前人发表的 236Ma OIB，这些晚志留-晚三叠世 OIB 岩块可以解释为一系列地幔柱成因的海山/大洋高原在消减增生过程中它们的碎片就拼贴位于增生楔的产物，最老的晚志留世 OIB 为金沙江古特提斯洋的存在提供了年龄上限。此外，在金沙江缝合带中获得中泥盆世(385Ma)具有陆缘弧特征的安山岩，代表了保存在该缝合带中最早的俯冲相关的岩浆记录，表明金沙江古特提斯洋的南向俯冲至少在中泥盆世已经开始。本文研究表明，金沙江古特提斯的演化经历了漫长的连续增生过程，该过程伴随一系列海山/大洋高原的增生就位，为青藏高原中部地壳生长机制提供了有力的新证据。第二，金沙江古特提斯洋的最后闭合时限。本文报道了松潘-甘孜地体中 6个花岗闪长岩-闪长岩样品的年代学、地球化学和锆石 Hf 同位素分析结果。三个石英闪长岩(214.9 ± 2.2, 211.0 ± 1.3, 205.8 ± 1.4Ma)和三个花岗闪长岩(210.8 ±2.7,206.7 ±3.3, 205.9 ±3.5Ma)锆石 LA-ICP-MS U-Pb 年龄介于 214.9-205.8Ma。这些样品地球化学上具有高镁安山岩特征，包括高 MgO (高达 9.1%)、Mg#(70.2%)、Al2O3,、Cr,、 Ni、低 TiO2 以及低 Sr、 Y，轻微-中等负 Eu 异常。野外可见捕获闪长质暗色包体，在具核边结构的锆石中可见晚三叠世的碎屑锆石(年轻至 208Ma)被捕获。Hf 同位素显示石英闪长岩和花岗闪长岩分别具有轻微-中等的负 εHf(t)值(-1.97 ~-5.76, -4.12 to ~ -6.72)以及不均一的 TDMC 年龄(1.38-1.61Ga, 1.51-1.68Ga)。结合松潘-甘孜杂岩中其它晚三叠世岩体的 Hf 同位素证据，发现 TDMC 年龄与东昆仑造山带中~240Ma 以及~440Ma 岩浆岩(松潘-甘孜杂岩的重要源区)高度吻合。表明这些高镁闪长岩很可能形成于消减的海相沉积物熔融形成的硅质熔体与地幔楔橄榄岩的反应。晚三叠世高镁安山岩类岩石从松潘-甘孜杂岩南段可以向西延伸至北羌塘地体，呈现出向东变老的线性带，结合北羌塘火山岩在线性带两侧截然不同的分布特征，本文认为在晚三叠世斜向汇聚导致了洋中脊俯冲，引发了具有不同消减角度的俯冲板片之间板片窗的打开。由于洋中脊消减明显早于最后闭合时限，因此最后闭合时限晚于 205Ma。同时指出板片窗东侧洋壳板片消减导致甘孜-理塘缝合带的形成，其延伸仅局限于板片窗东侧，而金沙江缝合带则可向西进一步延伸。第三，北羌塘地体石炭-三叠纪弧岩浆迁移机制。微地体是大洋板片地层的重要组分，在消减增生体系中扮演特殊的角色。 然而涉及到相同时期微地体碰撞和消减作用的岩浆记录是复杂的甚至是“矛盾” 的。本文获得一系列南北向弧岩浆年代学和地球化学证据，结合前人已发布的年代学数据，识别出北羌塘地体石炭-三叠纪期间的三期弧岩浆演化时段(312-252, 249-237, 236-202Ma)。其中，第二期(249-237Ma)弧岩浆在仅 12Ma 跨越了北羌塘地体中部到金沙江缝合带的巨大区域(~100km)，表明突然加快的金沙江板片回撤速率。值得注意的是，该时间段与早-中三叠世义敦微地体-羌塘地体碰撞(249-237Ma)十分吻合，暗示二者之间存在成因上的耦合关系。该耦合关系支持 249-237Ma 岩浆弧北向快速迁移是北侧金沙江古特提斯洋中的义敦微地体与羌塘地体碰撞导致俯冲板片快速回撤结果，而不是南侧龙木错-双湖洋北向俯冲的认识。而且，北羌塘-印支地体晚三叠世沉积岩碎屑锆石累积频率研究表明，早-中三叠世的初始碰撞仅局限于义敦地体至哀牢山缝合带之间(义敦微地体西部发生在~249Ma，思茅地体北部在~242Ma)，在西延(金沙江缝合带西段， <205Ma)和南延(松马缝合带， <230Ma)具有更年轻的碰撞时限。据此，本文提出金沙江古特提斯洋在早-中三叠世期间存在试图俯冲的义敦微地体与羌塘地体之间的局部碰撞模型。而且，存在微地体的俯冲模型是古老造山带中是普遍存在的，这种俯冲阶段发生的局部碰撞可以用来解释造山带中为什么会出现这种“早期的碰撞” 和“晚期的俯冲” 的矛盾现象。微地体俯冲机制在古老造山带拼合过程中，对岩浆弧迁移和成矿作用均扮演着不可替代的角色。 |
| Other Abstract | The Paleotethys Ocean represented by the Jinsha-Ailaoshan-Song Ma suture, asthe significant branch of Eastern Paleotethys Ocean and the boundary between theQiangtang—Simao—Indochina terranes to the south(west) and the Songpan-Ganzi—Yangtze terranes to the north(east), recorded the key evidences for the evolution ofPaleotethys Ocean. As the west extension of Eastern Paleotethys Ocean, however, thetiming of the early opening, subduction, final closure and Carboniferous - Triassicgeodynamics of the Jinsha Paleotethyan Ocean in central-northern Tibet have beenlong-standing debates. In this study, OIB (seamount/oceanic plateau) subduction,ridge subduction and microterrane-bearing subduction may provide potentialmechanisms employed to address or constrain these controversies.The first controversy is the timing of the early opening and initial subduction ofthe Paleotethys Ocean. This study presents four well-constrained U-Pb zircon ages(422.4 ± 6.1, 336.4 ± 7.4, 272.3 ± 2.8 and 255.8 ± 3.4 Ma) of mafic blocks in theJinsha Suture (JSS). These mafic blocks have geochemically oceanic island basalt(OIB) affinity. Their ages, together with previously published data (236Ma OIBs),vary from Late Silurian to Late Triassic, which may be interpreted as a series ofseamounts/oceanic plateaux that were accreted early in the subduction-accretionprocesses. The oldest OIB block suggests that the Jinsha Ocean have existed in LateSilurian. An andesite from theJSS showing continental arc characteristics yields azircon U-Pb weighted average age of 385.2 ± 6.6Ma, implying the initial subductionmay have occurred at the Middle Devonian. Our work suggests a long-livedcontinuous accretion, which also puts an earlier existence timing than previouslythought. Taking into account the above data and integrating with relevant publishedinformation, we favor a long-lived accretionary process accompanied withseamounts/oceanic plateaux emplacement, which provides robust new evidence forsubduction-related continental growth in the central Tibetan Plateau.The second debate is the timing of final closure of the Paleotethys Ocean. This study reports new geochronologic, geochemical and zircon Hf isotopic data of sixgranodiorite-diorites which intrude the Mid-Late Triassic Songpan-Ganzi complex(SGC). LA-ICP-MS zircon U-Pb analyses of three quartz diorites (214.9 ± 2.2, 211.0± 1.3, 205.8 ± 1.4Ma) and three granodiorites (210.8 ±2.7, 206.7 ±3.3, 205.9 ±3.5Ma)give a Late Triassic age range (214.9-205.8Ma). These granodiorite-diorite rocks haverelatively high MgO (up to 9.1%) and Mg# (up to 70.2), Al2O3, Cr, Ni, but low TiO2,showing high magnesium andesite/diorite (HMA) chemical signatures with low Srand Y contents, and slight-moderate negative Eu anomaly. Also, the dioritic enclavesand Late Triassic detrital zircons (as young as 208 Ma) were captured by studiedrocks. Hf isotope data show overall slight to moderate negative zircon εHf(t) values(-1.97 to -5.76, -4.12 to -6.72) and heterogeneous TDMC ages (1.38-1.61Ga,1.51-1.68Ga) for quartz diorites and granodiorites respectively, together with otherpublished Hf isotopic data in SGC, are in highly concert with those around ~240 and~440Ma magmas (as potential sources for SGC turbidites) in east Kunlun orogenicbelt. Taken together, the HMA magmas were probably derived from a reactionbetween subducted marine sedimentary siliceous melts and peridotites from a mantlewedge. The linear distribution of Late Triassic HMAs and their eastward-youngingtrend (from northern Qiangtang to southern SGC), in combination with the contrastingdistribution of volcanic rocks in the northern Qiangtang terrane, lead us to suggestthat a slab window opened in the downgoing oceanic crust with two diverse slabangles during oblique subduction. Accordingly, the final closure of the PaleotethysOcean was later than 205 Ma, and most likely in the end of the Late Triassic to EarlyJurassic.The third issue is the Permian-Triassic arc migration mechanism in northernQiangtang terrane. The microterranes are common components drifting on the oceaniccrust and play a specific role in the subduction-accretion system. However,superimposed geologic effects involving subduction and coeval microterrane collisioncan be complicated and even conflicting in fossil geologic records. Here we present a coupling event between Early-Mid Triassic microterrane collision and synchronousabruptly increased rollback velocity of subducting slab of Jinsha Paleotethys. Threeage intervals of magmatic arc (312-252, 249-237 and 234-202Ma) were recognized inthe North Qiangtang terrane during Carboniferous - Triassic period, of which thesecond interval, distributing in a relative large distance (~100km) from the trench, isin highly concert with the time period of local collision between North Qiangtangterrane and Yidun microterrane (249-237Ma). This coupling event suggests a rapidtrenchwards slab rollback/arc migration accompanied by microterrane-terranecollision and implies a geodynamics mechanism induced by southward subduction ofJinsha Paleotethys Ocean, rather than northward diving of Longmuco-ShuanghuOcean. Moreover, the sedimentary records further uncover a phenomenon that theEarly-Mid Triassic initial collision probably only occurred to South Jinsha-AilaoshanSuture segment (western Yidun at ~249Ma and northern Simao terrane at ~242Ma)while much later to its further extensions on either sides (<205Ma in West JSS and<230Ma in Song Ma Suture). In light of these factors, the local collision triggered byattempted subducting microterrane in a subduction system was proposed in thetectonic evolution of Jinsha Paleotethys orogen, which would be commonplace andcan be employed to account for a seemingly contradictory phenomenon that the earlydefined collision followed by a subsequent subduction scenario. Also, micorterranesubduction mechanism plays an indispensable role in arc migration and metallogenyduring the amalgamation process of ancient orogenic belts. |
| Subject Area | 地球探测与信息技术 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/15343 |
| Collection | 中国科学院新疆生态与地理研究所 研究系统 |
| Affiliation | 中国科学院新疆生态与地理研究所 |
| First Author Affilication | 中国科学院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | 刘银. 金沙江古特提斯洋志留-三叠纪演化及动力学过程[D]. 北京. 中国科学院大学,2019. |
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