【作者】 张修政；

【导师】 李才； 董永胜；

【作者基本信息】 吉林大学 ， 构造地质学， 2014， 博士

【副题名】来自香桃湖志留纪基性高压麻粒岩的岩石学、地球化学以及年代学证据

【摘要】 特提斯及环冈瓦纳大陆构造演化是目前地学界研究的前沿和热点。随着近年来区域地质工作的深入以及大量新资料的涌入，羌塘地区已经成为研究这一关键问题的理想地区之一。羌塘中部不仅保留了可能与原特提斯俯冲有关的岩浆活动，同时还出露与这一过程相关的早古生代蛇绿岩和中高级变质岩。然而，由于缺少关键性岩石类型，目前对于这一复杂演化过程的理解十分有限，直接制约了对羌塘乃至冈瓦纳北缘早期构造演化历史的认识。本文以迄今为止羌塘中部首例志留纪基性高压麻粒岩为研究对象，进行了系统的岩石学、矿物学、同位素年代学和地球化学研究，查明了高压麻粒岩及其围岩的野外地质特征，确定了高压麻粒岩的P-T-t轨迹及形成的构造环境，以高压麻粒岩为切入点，结合区域内早古生代蛇绿岩、古生物、古地磁资料以及其他大量相关地质事实，尝试对冈瓦纳北缘以及古特提斯洋（龙木错-双湖-澜沧江洋）早古生代构造演化历史进行探讨，在前人工作基础上初步构建了全新的构造演化模式，为进一步的相关研究工作提供了新的思路。香桃湖基性高压麻粒岩出露于羌塘西部拉顺-香桃湖蛇绿混杂岩带的东南部,与区域内蛇绿岩伴生产出，高压麻粒岩主要呈大小不一的透镜体产于其围岩斜长角闪片（麻）岩中，其围岩中常出露蛇纹石化橄榄岩块体，并发育大洋斜长花岗岩脉体，总体特征与南天山榆树沟高压麻粒岩类似，为一套遭受中高级变质作用改造的蛇绿岩残片。矿物学及变质作用研究表明，香桃湖基性高压麻粒岩经历了三期变质演化阶段：①峰期高压麻粒岩相变质阶段（M1），矿物组合为单斜辉石+斜长石+石榴石+石英，形成的温度压力条件为T=830~860°C，P=1.25~1.45GPa；②峰后期中压麻粒岩相阶段（M2），矿物主要呈蠕虫状或文象状后成合晶或冠状体分布于石榴石边部，主要矿物组合为紫苏辉石+斜长石±角闪石±磁铁矿，其形成的温度压力条件为T=810~830°C，P=0.65~0.85GPa；③晚期角闪岩相变质阶段（M3），矿物组合为斜长石+角闪石，形成的变质作用条件为T=590~650°C，P=0.62~0.85GPa。同位素定年结果表明，高压麻粒岩主期变质年龄为427~422Ma，且这一期变质锆石中包裹体矿物组合与峰期高压麻粒岩相变质矿物组合特征一致，代表了峰期变质作用（M1）的时代；峰后期中压麻粒岩相退变时代为392~389Ma，晚期角闪岩相退变发生在365~355Ma之间，残留的岩浆锆石中获得481±10和500±5Ma单颗粒锆石SHRIMP U-Pb年龄，可能代表了高压麻粒岩原岩结晶时代。围岩斜长角闪片麻岩峰期变质时代为Ca.391Ma，退变质时代为Ca.355Ma。地球化学研究表明，香桃湖基性高压麻粒岩具有与N-MORB稀土元素特征，同时强烈亏损Nb、Ta、Zr和Hf等高场强元素，显示了典型SSZ型蛇绿岩地球化学特征，且与缝合带内寒武纪-早奥陶世蛇绿岩特征一致；斜长角闪片麻岩则具有E-MORB地球化学特征，与区域内中晚奥陶世蛇绿岩特征极其相似。以上结果进一步表明香桃湖基性高压麻粒岩及其围岩为一套经历中高级变质作用改造的早古生代蛇绿岩残片。香桃湖基性高压麻粒岩具有顺时针近等温降压P-T-t轨迹，峰期高的变质温度压力条件指示其原岩（SSZ型蛇绿岩）在志留纪末期（427~422Ma）被带入40~50km地壳层次，暗示区域内至少在局部地区出现了短期的会聚型环境；在早泥盆世（392~389Ma）高压麻粒岩经历了一个快速折返的过程（近等温降压P-T轨迹），暗示可能由于俯冲板片的断离或造山带的垮塌，区域内动力学机制发生明显改变而进入伸展构造背景。根据本文研究成果，结合区域内蛇绿岩、岩浆岩、古生物和古地磁证据，初步构建了古特提斯洋（龙木错-双湖-澜沧江洋）早期构造演化模型，为进一步研究工作提供了新的思路，认为古特提斯洋可能经历了以下四个演化阶段：①早寒武世（530~517Ma），由于原特提斯向冈瓦纳北缘俯冲消减，古特提斯洋以弧后盆地的方式开启；②寒武纪-志留纪（517~432Ma），古特提斯洋慢速扩张，洋盆宽度有限，两侧代表性陆块中古生物面貌相似；③晚志留世-早泥盆世（427~410Ma），区域构造机制转变导致洋盆短暂收缩，局部地区在进一步的收缩过程中发生闭合或点碰撞，形成以香桃湖基性高压麻粒岩为代表的中高级变质岩石；④早泥盆世（392~389Ma），可能由于原特提斯俯冲板片断离导致冈瓦纳北缘整体进入大型伸展构造背景，香桃湖基性高压麻粒岩快速折返并发生了中压麻粒岩的退变，古特提斯洋先前闭合的局部洋盆亦在这一时期重新开启，而且整体进入了一个快速扩张阶段，在晚泥盆世-早石炭世大洋扩张达到最大规模。古特提斯洋可能在早石炭世发生了向北的初始俯冲，在中晚三叠世古特提斯洋闭合，南北羌塘碰撞形成统一的羌塘盆地并开始接受新一轮沉积。更多还原

【Abstract】 A recent study suggests that the central Qiangtang is a key locality, not only toinvestigate the evolution of the opening and closure of the Paleo-Tethys Ocean, butalso to study the tectonic evolution of the north margin of Gondwana in the earlyPaleozoic. In addition to rocks related to the Paleo-Tethys Ocean, geologic evidenceof early Paleozoic tectonism has been reported along the northern margin of SouthQiangtang near the Longmu Co-Shuanghu suture zone, including granitic gneiss,amphibolite gneiss (±garnet), and ophiolites.These early Paleozoic rocks indicate theoccurrence of earlier tectonothermal events during the evolution of the Paleo-TethysOcean.In this contribution we report on hitherto unknown Silurian HP basic granulitesfrom the Xiangtaohu area of central Qiangtang, northern Tibet. The purpose of thispaper is to give a detailed account of the petrological features, mineralogical data,whole-rock geochemistry, and U–Pb zircon ages for these HP basic granulites. Theresults are used to constrain the P–T-t paths and metamorphic evolution of the HProcks, which is critical to understanding the early Paleozoic tectonic history of theQiangtang terrane. Moreover, the new data allow for the evaluation of hypothesesregarding the evolution of the northeastern margin of Gondwana while providing a starting point for more comprehensive future studies.The Xiangtaohu HP basic granulites occur as blocks or lens-shaped bodies ofsizes ranging from tens of centimeters to several meters in diameter. The country rockof the HP basic granulite is mainly amphibolite gneiss.Detailed petrology and geochronology reveal a three-stage metamorphic historybased on inclusions, reaction textures, and garnet zoning patterns. Peakmetamorphism at830~860°C and1.25~1.45GPa (M1) is defined by high-Ca garnetcores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas oforthopyroxene+plagioclase±magnetite around garnet porphyroblasts indicate garnetbreakdown reactions at ca.810~830°C and0.65~0.85GPa (M2). Kelyphites ofamphibole+plagioclase around garnet formed during the cooling process at about590~650°C and0.62~0.82GPa (M3). These results help define a sequential P–T pathcontaining near-isothermal decompression (ITD) and near-isobaric cooling (IBC)stages.Identification of mineral inclusion assemblages in zircons dated by U–PbSHRIMP and LA–ICP–MS reveals peak HP metamorphism at ca.427~422Ma,subsequent near-isothermal decompression with associated retrograde reactions at ca.392~389Ma, and continued cooling at ca.360Ma. In situ zircon U-Pb LA-ICP-MSdating on the amphibolite gneiss reveals peak HP metamorphism at ca.391Ma andretrograde reactions at ca.355Ma.Geochemically, the HP basic granulites is tholeiitic and has low rare earthelement abundances (∑REE=11.08×10-6~24.93×10-6) and display light REE (LREE)depleted patterns with LaN/YbNvalues of0.21-0.77,with marked negative Nb, Ta, Zrand Hf anomalies. These features of HP basic granulites are comparable with those ofsupra-subduction zone (SSZ) type, whereas the country rock amphibolite gneissexhibit REE distribution patterns and trace element abundances similar to enrichedmid-ocean ridge basalts (E-MORB).The HP basic granulites record relatively high peak P–T conditions and aclockwise P–T path. A near-isothermal decompression path (segment from M1to M2)implies that protoliths of HP basic granulites experienced burial to depths of ca. 40~50km, followed by ca.20km of rapid erosional exhumation. This process likelyoccurred during collision along a convergent plate margin, followed by extensionalexhumation as the collisional orogen collapsed or slab break-off.Based on paleontological, paleomagnetic data, early Paleozoic tectonothermalevents and our new results, we propose a hypothetical tectonic model for the earlyPaleozoic evolution of Paleo-Tethys Ocean and northern margin of Gondwana.According to our model, the Paleo-Tethyan Ocean initially opened as a back-arc basinin the early cambrian as a result of the subduction of the proto-TethyanOceanlithosphere. The Xiangtaohu HP basic granulites formed as a result of locally closureof expanding Paleo-Tethys basin as the change of tectonic style from extension tocompression during the Middle Silurian to Early Devonian. Renewed subduction ofoceanic ridge(Proto-Tethys) and subsequent Silurian collision triggered slab break-off,leading to the reopening and expanding rapidly of the Paleo-Tethys ocean basin.更多还原