大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

对大别地区角闪岩相－麻粒岩相和榴辉岩相岩石进行较系统的地球化学研究,探讨了超高压俯冲变质及折返过程中元素的再分配行为,并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明,大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用,并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩Ｎｂ／Ｔａ比值升高,但后期的退变质作用使榴辉岩与其寄主围岩间的Ｎｂ／Ｔａ比值相对     

榴辉岩退变质过程中的微量元素地球化学行为：对CCSD主孔退变质榴辉岩的研究

对中国大陆科学钻探工程主孔榴辉岩退变质过程中的微量元素地球化学行为进行了研究。对退变质程度连续变化样品的不同部分的对比研究表明，流体作用下的退变质过程中大离子亲石元素（Cs、Rb、Ba、Sr、K、Th、U）和轻稀土元素表现出较大的活动性，重稀土元素和高场强元素变化相对较小。退变质后大离子亲石元素的显著增加和高场强元素、重稀土元素的轻微变化（甚至相对降低），表明与退变质作用有关的流体中的络阴离子含量很少，并不富集高场强元素和重稀土元素。退变质后总体上表现出的Si、大离子亲石元素和轻稀土元素的明显变化，表明外来流体参与了榴辉岩的退变质过程，带入和带出了一些元素。结合榴辉岩中单矿物微量元素组成以及前人对D^Mineral/Fluid的研究成果，对流体-榴辉岩作用形成的退变质分带（富石英条带→角闪岩→退变质榴辉岩→新鲜榴辉岩）的微量元素组成变化进行了详细研究。结果表明在流体作用下的榴辉岩退变质过程中，大离子亲石元素、轻稀土元素和高场强元素含量的变化除了受退变质流体性质的影响外，更大程度上取决于退变质过程中的矿物相（尤其是副矿物）的变化。     

北大别洪庙榴辉岩相岩石Sm-Nd年龄:峰期变质时代

前人工作认为北大别榴辉岩在榴辉岩相变质后,经历了麻粒岩相退变质作用,因此获得的Sm-Nd矿物等时线年龄代表了麻粒岩相变质时代.本文对北大别安徽洪庙百丈岩榴辉岩相岩石(辉石石榴石岩)的研究表明,该岩石经历了三叠纪超高压变质作用,经历了角闪岩相退变质.所研究样品在峰期变质之后是否经历了麻粒岩相退变质尚不能明确界定.结合矿物氧同位素体系平衡判据,辉石石榴石岩在峰期变质时达到了Sm-Nd同位素体系的均一化和平衡,并且在后期角闪岩相退变质中该同位素体系未出现明显的扰动,因此其Sm-Nd矿物等时线年龄[(225±14)Ma和(229±13)Ma]代表了榴辉岩相变质时代.同时,样品中矿物的同位素组成表明,在俯冲板块折返和退变质过程中,岩石受到外来和内部流体的不均匀作用,造成退变的单斜辉石在同位素组成上的不均一.     

荣成地区特别富^18O的榴辉岩的成因

荣成地区的M类榴辉岩特别富集18O,这样富18O的榴辉岩在大别山-苏鲁超高压变质带还尚未见报导。异常高的δ18O值表明M类榴辉岩与围岩-大理岩在变质过程中发生过强烈的氧同位素交换。稳定同位素、流体包裹体等证据揭示氧同位素交换可能发生在超高压岩石的折返过程中,由于叠加的麻粒岩相退变质作用使同俯冲的新元古代海相碳酸盐岩发生了去碳酸盐化作用,产生了富CO2的变质流体。这种退变质流体特别富18O,成为M类榴辉岩与围岩碳酸盐岩交换的媒介。所观测到的M类榴辉岩内矿物之间,以及榴辉岩与围岩大理岩之间都基本达到了高温下的氧同位素平衡。由于荣成地区各类榴辉岩记录的变质温度普遍高于大别山和苏鲁南段的榴辉岩,因此这一地区的榴辉岩在折返过程中一般都叠加有麻粒岩相和／或角闪岩相的退变质作用。退变质流体,特别是麻粒岩相退变质期间产生的富CO2流体,是造成这一地区M类榴辉岩有别于其它地区M类榴辉岩-特别富18O的根源。     

苏鲁榴辉岩中磷灰石的矿物学和微量元素地球化学

磷灰石是榴辉岩中最常见的副矿物之一,见证了高压-超高压变质岩从原岩形成、俯冲和折返所伴随的物理化学过程。为进一步揭示苏鲁超高压榴辉岩经历的物理和化学过程,我们对取自中国大陆科学钻探工程（CCSD）主孔岩心和苏北地表露头榴辉岩中的磷灰石进行了详细的岩相学分析和LA-ICP-MS原位微量元素分析。分析结果表明,在苏鲁榴辉岩中,磷灰石绝大多数是在超高压变质作用阶段重结晶生长的。未受退变质影响的磷灰石富含LREE和Sr元素,退变质作用促使磷灰石中活动性较强的LREE和Sr明显降低而HREE含量略微升高,并产生负Eu异常。磷灰石边部HREE的升高可能与折返过程中的升温作用和石榴子石分解有关,Eu负异常的产生可能还与退变质过程中发生了氧逸度f^O2的降低有关;结合前人对磷灰石中“出溶”现象的研究,提出榴辉岩磷灰石中的独居石“出溶体”很可能是磷灰石与富含NaCl和硅酸盐的退变质流体发生交代反应所致,磷灰石中硫化物“出溶体”的形成除了氧逸度降低的原因外,可能也与折返过程中发生的短期升温作用有关。超高压变质岩从进变质-峰期→早期退变质→角闪岩相退变质阶段,变质流体可能经历了氧化→还原→氧化状态的复杂变化。     

羌塘中部高压变质带的退变质作用及其构造侵位

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界.本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代.研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母40Ar-39Ar坪年龄为219±2Ma.高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表.     

北大别超高压榴辉岩的快速折返与缓慢冷却过程

岩石学研究表明,北大别超高压榴辉岩经过了超高压和高压榴辉岩相变质作用以及麻粒岩相叠加和角闪岩相退变质作用.其中,高压麻粒岩相和角闪岩相变质阶段形成的后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩在峰期超高压变质作用之后又经历了一个缓慢冷却过程.超高压岩石折返期间的缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压证据的重要原因.因此,本文为大别山不同超高压岩片的差异折返模型的建立提供了新的证据.     

中国大陆科学钻探工程主孔榴辉岩中金红石微量元素地球化学特征

本文利用电子探针分析了中国大陆科学钻探工程主孔各种类型榴辉岩中金红石的Nb、Cr和Zr含量。Zack等(2002)的金红石Nb-Cr图解表明榴辉岩的原岩均为镁铁质岩,但不同类型榴辉岩具有不同的地球化学特征,即:1金红石榴辉岩、石英榴辉岩、角闪岩和钛铁矿榴辉岩中金红石的Nb和Cr含量大致相同,主孔中上述榴辉岩中金红石的Nb、Cr含量与区域上小焦金红石矿区金红石榴辉岩中金红石的Nb、Cr含量基本相同。总体来讲,区域和主孔榴辉岩中金红石以低Nb为特征,反映它们的原岩为镁铁质岩石。2蓝晶石多硅白云母榴辉岩中金红石具最高的Nb和Cr含量,其Nb和Cr均值分别为720×10-6和712×10-6,多硅白云母榴辉岩中金红石比金红石榴辉岩、石英榴辉岩、角闪岩和钛铁矿榴辉岩中金红石富集Cr。利用Zack等(2004)提出的金红石地质温度计,计算得出金红石榴辉岩的金红石形成温度介于608~746℃,石英榴辉岩的金红石温度介于629~680℃,钛铁矿榴辉岩金红石的形成温度介于629~704℃,蓝晶石多硅白云母榴辉岩的金红石形成温度为600℃,角闪岩的金红石形成温度为629℃。一种可能的解释是,榴辉岩在折返过程中退变质作用明显,流体活动强烈,导致金红石中Zr扩散丢失,金红石中Zr含量不同程度地受到角闪岩相退变质过程中再平衡作用的影响,致使计算的温度偏低。     

大陆碰撞和超高压变质的化学地球动力学:来自中国大陆科学钻探的结果

利用中国大陆科学钻探工程连续取样的优势,对主孔200～4000m超高压变质岩进行了系统的地球化学研究,结果对大陆俯冲带变质的化学地球动力学提供了新的制约。目前取得的主要成果如下:(1)建立了200～4000m深度的超高压矿物氧同位素变化剖面,发现~(18)O亏损深度达3300m。结合大别-苏鲁造山带地表露头岩石达30000km~2的~(18)O亏损,表明扬子陆块北缘曾发生三维空间巨量物质(～100000km~3)的~(18)O亏损;(2)钻孔超高压变质岩的原岩形成于新元古代中期(～780Ma),属于双重双峰式岩浆活动的产物;原岩既有中元古代晚期新生地壳,也有古元古代中期古老地壳。这种双重双峰式岩浆活动出现在弧-陆碰撞带基础上发育的裂谷构造带,是对Rodinia超大陆开始裂解的响应;(3)高压榴辉岩相重结晶作用发生在216±3Ma,高压榴辉岩相重结晶作用持续的时间为1.9～9.3Myr或更短;(4)不同岩性接触带的矿物O同位素、H同位素、全水含量以及羟基含量的研究表明,退变质流体在同位素组成上是内部缓冲的,且退变质流体主要来自于名义上无水矿物中结构羟基和分子水的出溶。定量计算表明,这些出溶的结构羟基和分子水能为角闪岩相退变质作用提供足够的流体来源;(5)在不同岩性接触带,稳定同位素、水含量及岩相学上出现同步变化,表明不同岩性接触带可能是流体活动最剧烈的地方。对于远离榴辉岩-片麻岩接触带的榴辉岩,退变质流体主要来自榴辉岩内部名义上无水矿物中结构羟基和分子水的出溶。对于靠近接触带的榴辉岩,则可能主要受来自比榴辉岩更富水的片麻岩中矿物的脱水作用。根据相邻样品之间的距离、岩相学和δ~(18)O值,可以确定出不同岩性和相同岩性之间氧同位素不均一性的尺度为20到50cm,对应于大陆碰撞过程中流体活动的最大尺度;(6)对两个连续岩芯段的主量和微量元素剖面研究表明,在大陆俯冲带变质过程中,大离子亲石元素(LILE)和轻稀土(LREE)具有较高的活动性,而高场强元素(HFSE)和重稀土(HREE)是不活动的。这些元素的活动可能是由折返过程中大陆地壳的部分熔融导致的,因此指示了超临界流体的出现。折返过程中大陆地壳的这种部分熔融作用导致了不同板片组分之间的质量迁移,但是这种有效的迁移只发生在很小的尺度范围内,在岩相学接触带和裂隙带的局部开放系统中。     

中国大陆科学钻探（CCSD）高压超高压变质岩中石英脉流体包裹体初步研究

初步的岩相学和冷热台显微测温表明，CCSDHP—UHP岩石中石英脉和后期碳酸盐脉中含有3种流体包裹体：盐水溶液(NaCl-H2O)包裹体(Ⅰ类)、NaCl-CaCl2-H20(Ⅱ类)和N2-CH4纯气相(Ⅲ类)包裹体，其中Ⅰ类盐水溶液包裹体可进一步分为中高盐度盐水溶液包裹体(Ia)、中等盐度盐水溶液包裹体(Ib)和低盐度盐水溶液包裹体(Ic)，而Ⅲ类为CCSD中首次发现：Ia、Ib和Ⅱ型流体包裹体主要以原生或假次生形式赋存在榴辉岩的石英脉或石英颗粒中，在角闪岩相的片麻岩及其石英脉中均未观察到，显示它们可能主要被捕获于榴辉岩的减压重结晶或退变质阶段，而Ic型包裹体分布广泛，表明其主体可能是在超高压变质岩折返过程的最晚阶段捕获的；N2-CH4纯气相包裹体均为原生包裹体，主要呈孤立和小群状与Ia和Ib类包裹体分布于榴辉岩中条带状石英脉中，可能主要是在高压-超高压榴辉岩相变质条件下被捕获的；CCSD榴辉岩中石英脉的主体形成于板块折返有关的减压重结晶和退变质，而片麻岩中石英脉则主要来源于角闪岩相及其后期退变质作用；榴辉岩和片麻岩中石英脉流体包裹体的组成和地球化学特征存在明显区别，但它们各自相似于其寄主岩石中的石英颗粒中包裹体，说明CCSD中HP—UHP岩石在板块折返过程中释放出的变质流体没有经过大规模的迁移。     

苏鲁超高压变质带东北端多种成因类型变基性岩:来自岩石学、同位素年代学及地球化学属性的制约

苏鲁超高压变质带是扬子板块与华北板块在三叠纪俯冲-碰撞的产物。变基性岩是苏鲁超高压变质带内出露最广泛的岩石类型之一,研究其岩石学、年代学、地球化学属性及成因机制,对于揭示扬子板块与华北板块之间的俯冲-碰撞-折返的动力学过程具有重要的科学意义。以(退变)榴辉岩为代表的超高压变质岩石广泛出露在威海-荣成一带,少量出露在乳山地区。锆石LA-ICP-MS U-Pb定年结果显示,(退变)榴辉岩的原岩时代为792~760Ma,峰期榴辉岩相变质时代为243~226Ma,后期角闪岩相退变质时代为221~207Ma。非榴辉岩相变质的基性岩(麻粒岩和斜长角闪岩)主要出露在乳山地区,其原岩形成时代应不晚于古元古代(1939Ma),峰期麻粒岩相变质时代为1895~1870Ma,后期角闪岩相退变质时代为1848~1806Ma,与胶北地体变基性岩的原岩时代和变质时代十分相似。全岩地球化学研究结果表明,(退变)榴辉岩的原岩显示高Fe拉斑玄武岩的特点,根据其稀土和微量元素特征,可将(退变)榴辉岩进一步划分为A、B和C三组。在球粒陨石标准化稀土配分模式和原始地幔均一化蛛网图解上,A、B和C三组样品分别具有轻稀土弱亏损、轻稀土弱富集和轻稀土富集的特点。轻稀土富集或弱富集型(退变)榴辉岩的原岩地球化学性质与岛弧或大陆玄武岩相似,它们的源区可能与深部富集地幔或受流体交代的地幔楔存在密切的成因关系;而轻稀土亏损型(退变)榴辉岩的原岩可能来自于亏损地幔的部分熔融。由此可见,(退变)榴辉岩的原岩具有成因多样性的特点。乳山地区的基性麻粒岩和斜长角闪岩的原岩也具有高Fe拉斑玄武岩的地球化学属性,Al2O3与Mg O呈正相关变化,TiO_2、P_2O_5与MgO表现出一定程度的负相关性。绝大多数非榴辉岩相变质基性岩的球粒陨石标准化稀土配分模式和原始地幔均一化蛛网配分曲线具有微右倾或明显右倾的特点。上述特征表明,研究区绝大多数非榴辉岩相变质的基性岩原岩来自于富集地幔,少数来自于原始地幔或亏损地幔,并经历了斜长石和辉石的分离结晶以及不同程度的部分熔融过程。由此可见,乳山地区出露的非超高压变质基性岩的原岩具有与胶北地体(高压)基性麻粒岩相近的成因特点。岩石学、同位素年代学和地球化学特征的综合对比研究结果表明,在苏鲁超高压变质带东北端的威海-荣成-乳山地区,既存在与华北板块古老变质基底相关的变基性岩,也存在与华南板块北缘新元古代变质基底相关的超高压榴辉岩,表明三叠纪时期华北板块东南缘胶北地体的部分古老变质基底曾卷入到扬子板块与华北板块之间的俯冲-碰撞造山过程,随后与超高压岩石一起抬升折返,形成当今的构造混杂岩带。     

再论“大陆深俯冲和折返动力学”: 来自中国大陆科学群钻及苏鲁超高压变质带的制约

通过CCSD-MH、卫星孔的岩性-构造剖面和苏鲁造山带中榴辉岩-超镁铁质岩的产出、深俯冲/折返过程的岩石的塑性流变特征和变形序次的分析、俯冲-折返过程中流体作用及变质化学地球动力学对流变学行为的制约,以及韧性剪切作用形成的折返年代学时限,提出苏鲁超高压变质地体为面型深俯冲/折返杂岩带组成的穹形挤出推覆岩片、叠置在扬子陆块之上; 根据岩石变形微构造及组构的分析,重塑超高压变质岩石深俯冲阶段、折返早期、折返主期和折返后期的塑性流变;提出深俯冲的物质沿板块汇聚边界的多层隧道呈多重/分片样式“挤出”的折返模式,并认为在折返初期开始（230~220Ma）和折返主期（220~200Ma）形成的透入性韧性剪切是俯冲岩片挤出的重要机制;提出郯庐走滑断裂的形成对苏鲁高压/超高压变质地体演化的影响。     

北苏鲁荣成地区超高压变质带的形成与折返动力学

山东省荣成地区位于苏鲁高压-超高压变质带的东北部,以花岗质片麻岩、副片麻岩为主,夹有少量的榴辉岩、石榴辉石岩、麻粒岩、超基性岩、石英岩、大理岩和斜长角闪岩等,各类岩石的锆石中普遍含有柯石英包裹体,表明荣成地区岩石曾经历超高压变质作用过程。荣成地区区域构造格架表现为面理产状总体为NNE-SSW走向,向南转为由NE-SW走向,呈弧形展布,倾向SE或SSE,超高压变质岩石由一系列近平行的剪切岩片组成,岩片之间的分界线为一些韧性剪切带,是折返阶段角闪岩相-绿片岩相的产物。结合野外宏观变形现象、显微构造分析以及糜棱岩中石英的优选方位EBSD （electron backscatter diffraction,电子背散射技术）测量结果,可以判断北苏鲁荣成地区韧性剪切带主要形成于中、低温（550~350℃）条件,并具有由NW向SE“斜向正滑”的剪切指向。根据韧性剪切带中所含的榴辉岩透镜体的显微构造和绿辉石与金红石的优选方位的EBSD的研究,重塑超高压变质阶段的流变学特征：绿辉石和金红石都具有高温的滑移系组构模式。横穿苏鲁高压-超高压变质带的地震反射剖面揭示了苏鲁高压-超高压变质带呈厚10km以上的穹形板片,位于苏鲁高压-超高压变质板片下部的荣成及江苏刘山以韧性正断裂系列为主。Ar-Ar测年结果表明,发生这种伸展韧性剪切作用的时间在117~130Ma。位于板片上部的南苏鲁以韧性逆冲性断裂系列为主。基于以上各方面的研究,进一步验证了苏鲁超高压-超高压变质带折返动力学的挤出模式。     

大别超高压变质地体四道河地区岩石学研究

对四道河地区超高压变质岩剖面的研究分析显示,该剖面有3种岩石类型:榴辉岩类、片麻岩和面理化含榴花岗岩。榴辉岩具不同程度的退变质现象,呈透镜体状产出于斜长角闪岩、片麻岩和面理化含榴花岗岩中,原生矿物组合为石榴石、绿辉石、柯石英和金红石。榴辉岩退变为斜长角闪岩近于等化学系列;片麻岩在主量成分上与榴辉岩及其退变产物(斜长角闪岩)存在突变关系,但微量元素与榴辉岩有一定的相似性;面理化含榴花岗岩主量元素和微量元素地球化学特点为:富SiO2 、K2 O Na2 O和高价阳离子Ga、Y以及REE ,K2 O/Na2 O值低,贫Al、Ca、Mg、Ti、P ,结合构造环境、同位素及年代学资料分析,其应属于后碰撞造山A型花岗岩。基于以上认识推断:大陆板片俯冲至上地幔经历了超高压变质作用,表壳岩变质形成榴辉岩;当超高压变质岩石折返至中下地壳时发生了强烈的减压退变质作用形成斜长角闪岩,随后,与片麻岩及面理化含榴花岗岩一道从中下地壳向地表进一步折返,并一同经历了后期的变质变形作用。     

Ultrahigh-pressure texture inheritance during retrogression: Evidence from magnetofabrics in eclogites and ultramafic rocks (Chinese Continental Scientific Drilling project)

In order to contribute to a better understanding of exhumation related retrogression processes ultrahigh-pressure (UHP) mafic and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) in the Maobei eclogite body of the Sulu ultrahigh-pressure metamorphic belt in eastern China were studied for their magnetofabrics. Variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the borehole. A vein network of irregular shaped veins with a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles. SEM imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape-preferred garnet. SEM imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape-preferred garnet and that magnetite formed within a mesh texture related to serpentinization. Syn-serpentinization magnetite growth increased bulk susceptibilities, but reduced anisotropy. Maximum susceptibility axes from both eclogites and serpentinized garnet-peridotites trend N-S, i.e. parallel with the stretching lineation as defined by olivine and omphacite grains. This implies that the magnetic fabric mimics the UHP fabric and survived retrogression and that the fabric is inherited from the UHP stage. As a consequence, retrogression was not associated with substantial ductile deformation and the mafic-ultramafic Maobei body behaved as a rigid body within a ductile deforming quartzo-feldspathic matrix during exhumation. Internal strain is restricted to brittle fracturing, associated fluid circulation and vein formation facilitating retrograde reactions in the mafic-ultramafic rocks.     

苏鲁超高压榴辉岩中钾长石的超高压相

在苏鲁超高压榴辉岩中,石榴石和绿辉石包含多种含钾长石的显微多晶包襄体.确定其中的钾长石是否经历过超高压变质不仅有助于限定这些长英质包裹体的成因机制,而且有助于深化理解在超高压变质过程中陆壳岩石的物理和化学行为.榴辉岩B174样品采自于中国大陆科学钻探主孔岩心.该样品的石榴石含多种单相和多相多晶包裹体,其中包括钾长石和方解石单相包裹体、钾长石+石英和钾长石+方解石多晶包裹体.激光拉曼光谱分析揭示了部分钾长石残留有超高压钾长石相--钾质钡铝沸石,表明(1)钾长石至少经历了从钾质钡铝沸石向钾长石的退变质相变;和(2)榴辉岩中含钾长石多晶包裹体形成于超高压变质条件下.     

Retrogressive Microstructures:A Key to Post Collisional Uplift and Extension Tectonics of Ultrahigh-Pressure Metamorphic Rocks from Dabie Orogenic Belt,China

ThereisanunanimousunderstandingthattheUHPmetamorphisminDabie-SuluareaswasformedduringtheTri-assiccollisioneventbetweentheYangtzeandNorthChinacratons(Congetal.,l994$Zhouetal-,l996;Liouetal.,l994)-Eclogiteisthemostimportantrocktypeamongtheul-trahigh-pressure(UHP)metamorphicrocks.Accordingtotheiroccurrencethereare3typesofeclogites:typeIisen-clavesinultramaficintrusionsandasmembersinthelayeredmafic-ultramaficcomplex(Jahnl998);typeIispodsorlay-ersinthehighlymetamorphicsupracrustals(mainlying…     

Multiple partial melting events in the Sulu UHP terrane: zircon U–Pb dating of granitic leucosomes within amphibolite and gneiss

Thin layers and lenses of granitic leucosome are widely distributed within amphibolites, paragneisses and orthogneisses of the Sulu UHP terrane. They are parallel to, or cross‐cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre‐ to decimetre‐scale segregations. Variously migmatized rocks extend at least 350 km from SW Sulu (Maobei) to NE Sulu (Weihai), in a band at least 50 km wide. A combined study of mineral inclusions, cathoduluminescence (CL) images, U–Pb LA‐ICP‐MS dates, and in‐situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these UHP rocks. Most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited (igneous or metamorphic) cores or as new, euhedral crystals. The overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning and contain felsic mineral inclusions, such as Kfs + Pl + Qtz ± Ilm ± monazite (Mon). In contrast, the inherited igneous or metamorphic cores are rounded or irregular, contain low‐P or UHP mineral inclusions and show clear dissolution textures. These data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the UHP rocks. The REE patterns of the anatectic zircon show steep slopes from the HREE to LREE with strongly to moderately negative Eu anomalies (Eu/Eu* = 0.31–0.72) and pronounced positive Ce anomalies (Ce/Ce* = 6.8–26.5). Abundant U–Pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the Sulu UHP terrane. Anatectic zircon from 12 granitic leucosomes within amphibolites, paragneisses, and orthogneisses from Sulu UHP slices II and III yields consistent mean U–Pb ages of 219.0 ± 1.2 to 218.3 ± 1.6 Ma, 218.8 ± 2.0 to 217.3 ± 1.7 Ma and 218.2 ± 1.4 to 215.0 ± 1.5 Ma, respectively. In contrast, anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from Sulu UHP slice III records younger mean U–Pb ages of 151.9 ± 1.3 to 151.1 ± 1.8 Ma and 155.9 ± 1.8 to 153.7 ± 1.7 Ma, respectively. These data imply that the Sulu UHP terrane experienced two Mesozoic partial melting events. The first partial melting event (219–215 Ma) was probably associated with a Late Triassic granulite facies stage of ‘hot’ exhumation, whereas the second (156–151 Ma) is interpreted as the result of Middle‐Late Jurassic extension and thinning of the previously thickened crust of the Sulu UHP terrane. Both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks.     

Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: implications for rheology of subducted continental crust and origin of mantle reflections

Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth's surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallization-accommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The preservation of the relict crustal materials within the continental lithosphere may produce regionally extensive, strong, seismic reflections in the upper mantle. This may explain the origin of mantle reflections observed in many areas of the world.     

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835-1280 m), DFb (2010-2280 m) and DFc (2920-3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. ⁴⁰Ar/³⁹Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223-202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m}<a> and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N-S-striking foliation and a subhorizontal N-S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001)<a> is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.