【作者】 宗克清；

【导师】 刘勇胜；

【作者基本信息】 中国地质大学 ， 地球化学， 2010， 博士

【摘要】 微区地球化学(Microgeochemistry)研究被认为将主导今后的地球化学研究方向,它可以在单矿物微米尺度上揭示元素／同位素空间变化细节,为宏观地球化学研究提供制约。结合详细岩石学观察,对地质样品直接进行原位微区地球化学研究,是一种具有创新性的现代岩石地球化学研究方法。它使岩石学和地球化学在微米尺度上能够有机地结合在一起,为深入探讨地球化学动力学过程提供了可能。最近蓬勃兴起的激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)和激光剥蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICP-MS)为原位微区地球化学研究提供了原位、实时、快速、准确的分析方法。本研究利用LA-ICP-MS和LA-MC-ICP-MS相结合的方法对苏鲁超高压变质带榴辉岩中单矿物进行了详细的原位微区微量元素、同位素组成和锆石U-Pb年代学研究,结合利用微量元素和同位素分配和扩散实验研究成果进行的模拟计算,揭示了苏鲁超高压变质带不同地球动力学过程在单矿物微米尺度上的微量元素和同位素记录,探讨了深俯冲大陆地壳折返过程中的元素迁移、流体活动以及热力学过程。本研究主要取得以下四方面的认识：(1)榴辉岩中单矿物的微量元素组成比主量元素对化学和温度的变化更敏感,单矿物微量元素成分环带可以记录在岩石学和主量元素上无明显显示的复杂变质作用过程。利用LA-ICP-MS对中国大陆科学钻探工程(CCSD)主孔部分榴辉岩进行的单矿物微区微量元素空间剖面分析研究发现主量元素组成均一的石榴石和绿辉石具有明显的微量元素组成分带。石榴石边部表现出明显的中稀土元素(MREE)富集特征,绿辉石边部显著富集所有稀土元素(REE),磷灰石边部则富集重稀土元素(HREE)而低轻、中稀土元素。结合微量元素在榴辉岩各主要矿物中分配系数随温度、压力变化的实验研究,石榴石、绿辉石和磷灰石中的微量元素组成分带应反映榴辉岩在折返过程中可能经历的增温作用。利用石榴石和磷灰石中REE扩散系数进行的计算结果表明,榴辉岩增温作用所持续的时间可能非常短暂。该增温过程是苏鲁超高压变质岩经历部分熔融作用以及榴辉岩经历麻粒岩相叠加改造的可能原因。(2)磷灰石Sr同位素和微量元素成分环带是不同变质作用过程的良好记录器。利用LA-ICP-MS和LA-MC-ICP-MS对CCSD主孔中一件强退变多硅白云母榴辉岩中一粗粒(2mm×6.5 mm)磷灰石进行了详细的微区微量元素和Sr同位素组成研究。线扫描剖面分析和单点分析结果共同表明该磷灰石颗粒的微量元素和Sr同位素组成不均一。总体上Na、Sr、LREE、MREE、U、Th和Pb等元素具有从中心到两侧含量逐渐降低的特征。磷灰石主体部分HREE含量均一,但和石榴石紧密相邻的边部(-400μm)显著富集HREE。Na、Sr、LREE和MREE等元素在个别区域明显具有先升高、然后降低的变化规律。磷灰石最边部(<200μm)相对于核部区域显著高87Sr/86Sr比值。结合磷灰石中微量元素分配和扩散行为的实验研究以及磷灰石晶体化学特征,磷灰石微量元素组成变化应主要记录了磷灰石复杂的生长过程。Sr、LREE和MREE从核部到边部逐渐降低的总体特征指示磷灰石从中心向两侧的生长过程,而局部出现的次级先升高—然后降低的变化规律则反映了磷灰石经历的多次溶解和再生长作用。HREE含量均一的磷灰石主体形成于石榴石稳定存在的超高压变质作用阶段。和石榴石紧密相邻的磷灰石边部显著富集HREE的特征则是折返过程中短时增温作用导致石榴石释放HREE和／或退变质阶段石榴石分解释放HREE共同作用的结果。磷灰石最边部显著高87Sr/86Sr比值的特征则记录了角闪岩相退变质阶段多硅白云母的分解作用。(3)对岩石薄片中原位锆石直接进行U-Pb年龄和微量元素组成的同时测定既可以精确限定变质作用时间又可以制约变质锆石的形成环境。本研究建立了利用LA-ICP-MS在16μm激光斑束的条件下,对岩石薄片进行原位锆石U-Pb同位素和微量元素同时准确测定的方法,并利用该方法对一块取自CCSD主孔退变质榴辉岩岩石薄片中的锆石进行了系统研究。所研究岩石薄片中的锆石分为两类,一类锆石以包裹体的形式分布在新鲜的石榴石和绿辉石中,而二类锆石产于绿辉石周围的后成合晶中。其中,一类锆石的边部和少量二类锆石的幔部相对于核部显著低REE、Y、Nb和Ta,指示这些锆石区域可能同石榴石、金红石和磷灰石同时形成于超高压变质阶段,同时暗示这些锆石区域没有经历后期退变质作用的影响。这些薄片中原位锆石206Pb/238U的加权平均年龄(230±4 Ma,2σ)和已发表的利用矿物分选得到的含柯石英锆石的年龄(230±1 Ma,2σ)在误差范围内一致,说明苏鲁地区超高压变质作用的峰期年龄可能为-230 Ma。那些被后成合晶围绕或者贯穿的锆石区域可能受到了退变质作用的影响,给出了相对年轻的206Pb/238U加权平均年龄(209±4 Ma,2σ)。但是,这些退变质锆石区域同超高压变质锆石区域具有相似的REE组成,说明-209 Ma的锆石区域可能是角闪岩相退变质过程中流体改造的结果。在角闪岩相退变质流体改造的过程中,Pb发生了完全丢失,而REE成分没有被改变。(4)片麻岩部分熔融作用可能是诱发大陆俯冲带流体活动的重要原因。为了揭示苏鲁超高压变质带与片麻岩有关的流体活动,本研究对石英脉及其邻近的榴辉岩透镜体以及围岩片麻岩进行了系统岩石学和锆石年代学研究。变形石英脉分布在围岩片麻岩和榴辉岩透镜体之间。榴辉岩透镜体的角闪岩相退变质程度从边部到核部依次减弱。围岩片麻岩具有叶理化的结构特征,同时包含长英质脉体,这些岩石学特征指示围岩片麻岩可能经历了部分熔融作用。片麻岩锆石边部具有与熔体平衡的微量元素特征,如陡峭的HREE配分模式、高Hf含量和负Eu异常。片麻岩锆石边部给出的206Pb/238U加权平均年龄为219±3 Ma(2σ),明显小于大别-苏鲁超高压峰期变质年龄(-230 Ma)。因此,苏鲁超高压变质带片麻岩的部分熔融作用发生在折返的初期阶段。石英脉锆石边部确定的流体活动时间为219±2 Ma(2σ),与围岩片麻岩发生部分熔融的时间(219±3 Ma,2σ)一致。并且石英脉锆石边部的初始176Hf/177Hf比值显著低于榴辉岩透镜体锆石的初始176Hf/177Hf比值,但和附近片麻岩部分熔融形成的花岗岩脉锆石的初始176Hf/177Hf比值一致。说明石英脉以及相应的流体活动与围岩片麻岩的部分熔融有关。另外一方面,含角闪石并富集HREE的角闪岩锆石边部确定的角闪岩相退变质时间(217±5 Ma,2σ)与石英脉的形成时间一致。暗示与片麻岩部分熔融有关的流体活动可能引起了榴辉岩透镜体的角闪岩相退变质作用。基于以上观察,我们推测大陆俯冲带片麻岩在折返过程中经历的部分熔融作用可以诱发显著的流体活动,这是苏鲁超高压变质带石英脉形成以及榴辉岩经历广泛退变质作用的可能原因。更多还原

【Abstract】 It has been accepted that microgeochemistry would lead the direction of modern geochemistry. Microanalysis could reveal the spatial variations of trace element and isotope compositons of single mineral grain at the micron scale. Thus, geochemistry and petrology investigations can be integrated at the micron scale to decipher the geodynamic process. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), recently well developed, provide effective tools for in situ, real time, quick and accurate microanalysis of trace element and isotope compositions. In this work, in situ trace element and isotope compositions of mineral in eclogites from the Sulu ultrahigh-pressure (UHP) terrane were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Zircon U-Pb ages, trace element and Lu-Hf isotope compositions and trace element distribution and diffusion-based model calculation were combined to deciper the element mobility, fluid flow and thermal evolution during exhumation of the subducted continental crust, as summaried as following:(1) Trace element partitioning between metamorphic minerals is more sensitive than major elements to chemical and thermal changes of the eclogite, and trace element zoning of minerals could record complex metamorphic history. Trace element compositions of garnet, omphacite and apatite in UHP eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) project were in situ analyzed by LA-ICP-MS. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from the core to the rim in rare earth elements (REE). In particular, middle rare earth elements (MREE) in garnet, heavy rare earth elements (HREE) in apatite and all REE in omphacite increase from the core to the rim, respectively. Combination of dependence of trace element partition and diffusion coefficients on temperature and pressure and REE zonations in these minerals indicates that the trace element zoning in these minerals could record a short-lived heating event during exhumation. Such hot exhumation could have contributed significantly to the overprint of granulite facies metamorphism on the eclogites and the widespread partial melting in the Sulu UHP terrane.(2) Sr isotope and trace elment zoning of apatite could be a well recorder of the different UHP metamosphic stage of eclogite. Trace element and Sr isotope compositions of an apatite grain (-2mm×6.5mm) in a strongly retrograded eclogite from the CCSD-MH were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Both line-scanning and single spot analyses reveal that apatite is heterogeneous in trace element and Sr isotope compositions. Na, Sr, LREE, MREE, U, Th and Pb gradually decrease from the core to the rim. Remarkable secondary changes of Na, Sr, LREE and MREE occur in some local parts of the apatite. HREE are generally homogeneous in the main part of the apatite, but enriched in the outmost rim (-400 mm) close to garnet. The outmost rim (<200 mm) has the highest 87Sr/86Sr ratios. Combining the trace element partition and diffusion, and crystal-chemical characteristics of apatite, we suggest that the trace element variations reflect the complicated growth of apatite. The gradually decreasing from the core to the rim for Na, Sr, LREE, MREE, U, Th and Pb indicate apatite growth in the UHP metamorphism, while the secondary changes in local parts imply multi-stage dissolution and re-growth. The relatively homogeneous HREE compositions of the main part of the apatite suggest that apatite growth was mainly associated with the UHP metamorphism. HREE enrichments in the outmost rim of apatite could have resulted from garnet breakdown in amphibolite-facies retrograde metamorphism and/or HREE-releasing of garnet due to a short-lived heating during the exhumation. The high-87Sr/86Sr rim could indicate apatite growth coupled with phengite breakdown in the amphibolite-facies retrograde metamorphism.(3) In situ U-Pb dating and trace element analysis of zircons combined with a textural relationship investigation in thin section is a powerful tool to constrain the UHP stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the CCSD-MH in the Sulu UHP terrain. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average 206Pb/238U age of these zircon domains (230±4 Ma,2σ) agrees well with the published age of coesite-bearing zircon separates (230±1 Ma,2σ), suggesting that the peak UHP metamorphism in the Sulu terrain may have occurred at-230 Ma. Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average 206Pb/238U age of 209±4 Ma (2σ). These retrograde zircon domains have similar REE compositions to the-230 Ma UHP zircon domains. These observations imply that the-209 Ma zircon domains could have formed by fluid activity-associated alternations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REE in these domains.(4) Partial melting of the gneiss could have triggered the fluid flow in the continental subduction zone. In order to decipher the fluid activity in the Sulu UHP terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. The deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies metamorphic degree of the eclogite lens decreases from the rim to the core. The foliated gneiss contains felsic veins. Zircon rims from the gneiss are characterized by melt-related signatures in steep HREE patterns, high Hf contents and negative Eu anomalies and pool a weighted average 206Pb/238U age of 219±3 Ma (2σ), which is obviously younger than the peak metamorphic age of the Dabie-Sulu UHP terrane (-230 Ma). This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting during the initial exhumation stage.The formation age of the quartz vein (219±2 Ma,2σ) defined by zircon rims agrees well with the partial melting time (219±3 Ma,2σ) of the host gneiss. The initial 176Hf/177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217±5 Ma (2σ), overlaping with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced significant fluid flow during the exhumation stage, and thus contributed a lot to the extensive retrogression of eclogites in the Sulu UHP terrane.更多还原