共查询到19条相似文献，搜索用时 158 毫秒
1.
超高压岩石-脉体体系是认识俯冲带流体性质和行为的天然实验室.通过总结大别超高压变质带3个榴辉岩（角闪岩）-脉体体系的研究成果,探讨了大陆俯冲带变质流体的溶解-结晶过程和氧逸度变化规律以及流体对轻元素硼的迁移过程.对榴辉岩-复合高压脉体的研究发现超高压流体通过溶解矿物富集溶质组分,流体随后经历3期结晶过程,分别形成绿辉石-绿帘石脉、绿帘石-石英脉和蓝晶石-绿帘石-石英脉.绿帘石La、Cr和δEu值是判断结晶次序的关键指标.对榴辉岩-角闪岩-低压脉体研究表明大陆俯冲带低压变质流体的氧逸度明显高于高压-超高压变质流体.高氧逸度条件也导致一些反常矿物（如退变金红石）的生长.对含电气石榴辉岩-脉体研究揭示变质碳酸盐岩是大陆俯冲板片中重硼同位素的重要储库,其在汇聚板块边界的脱硼作用显著影响深部硼循环.上述研究成果为理解俯冲带变质流体演化和物质循环提供重要科学依据. 相似文献
2.
大陆俯冲带的深部流体及变质化学地球动力学
——以苏鲁超高压变质带为例 总被引：1，自引：0，他引：1
通过苏鲁超高压变质带的岩石学、矿物化学、地球化学和年代学研究,在大陆俯冲带深部流体与变质化学地球动力学方面取得了重要的创新性成果。研究证明大陆俯冲带的深部流体是高氧逸度、富硅酸盐的超临界流体,揭示出超高压变质极端条件下的流体－矿物(岩石)相互作用可以导致不活动元素发生溶解和迁移,可以导致金红石的Nb/Ta之间发生强烈的分异,提出俯冲到地幔深处的超高压榴辉岩是地球内部“隐藏”的超球粒陨石Nb/Ta比值的物质源区,与低球粒陨石Nb/Ta比值的物质源区大陆地壳和亏损地幔在化学成分上形成互补。 相似文献
3.
《吉林大学学报(地球科学版)》2015,(Z1)
<正>板块俯冲和折返过程中的流体活动是板块构造研究的一个重要组成部分,它不仅可以引起矿物反应,诱导岩石体系发生熔融,催化变质反应进行,还会使俯冲板块内部发生显著的流体迁移,引起元素活动和同位素变化(Zheng et al.,2012;van der Straaten et al.,2012;Zheng and Hermann,2014)。因此深入了解俯冲带深部的流体活动及其地球化学效应,不仅对于理解俯冲带内部的构造演化和深俯冲地壳岩片 相似文献
4.
《矿物岩石地球化学通报》2017,(2)
碳循环可以分为地球表层短周期的地表碳循环和地球内部长周期的深部碳循环。地球的碳90%以上是赋存在固体地球内部,因此深部碳循环研究对于探讨地表碳循环过程具有重要意义。本文较深入地探讨了俯冲带深部碳循环研究的现状和问题。目前俯冲带深部碳循环研究关键的科学问题包括:(1)俯冲带变质过程中含碳物质相的转变,(2)俯冲带脱碳机制,(3)俯冲带深部碳循环和地幔交代作用。俯冲带变质过程中含碳物质相的转变是深部碳循环研究的最基本问题,将是深部碳进一步研究的重点。俯冲带脱碳机制主要包括纯变质反应脱碳、流体溶解脱碳(流体渗透作用)、熔融作用脱碳和氧化还原反应脱碳4个方面,这是目前深部碳循环研究的前沿领域。俯冲带深部碳循环研究对于探讨地幔不均一性以及地幔交代过程都具有重要研究意义。 相似文献
5.
由板块俯冲引发的深部物质循环过程是地球内部的一级运行机制,主宰了地球从内到外的演化进程,是地球科学研究的重要前沿.俯冲带化学地球动力学研究不仅需要确定俯冲带地壳物质再循环的机制和形式,而且需要确定俯冲带动力来源和热体制及其随时间的变化.为了识别不同类型壳源熔/流体对地幔楔的交代作用、寻求板片-地幔界面反应的岩石学和地球化学证据、理解汇聚板块边缘地壳俯冲和拆沉对地幔不均一性的贡献,我们必须将俯冲带变质作用、交代作用和岩浆作用作为一个地球科学系统来考虑.板块俯冲带变质过程中发生一系列物理化学变化,这些变化不但是导致板块进一步俯冲的主要驱动力,同时也控制着释放的熔/流体组成和俯冲到地球深部的物质组成,对俯冲带化学地球动力学过程产生重要影响.地幔楔作为俯冲系统中连接俯冲盘和仰冲盘的关键构造单元,在地球层圈之间物质循环和能量交换等方面起着重要作用.造山带地幔楔橄榄岩直接记录了俯冲带多种性质的熔/流体交代作用,以及复杂的壳幔物质循环过程.俯冲带岩浆岩是大洋/大陆板块俯冲物质再循环的表现形式,这些岩石样品记录了俯冲带从深部地幔到浅部地壳的过程,也为认识地球深部物质循环提供了理想的天然样品.尽管国际上在俯冲带岩石学和地球化学领域针对地球深部过程的研究方面取得了多项重要进展,但由于研究工作缺乏密切的协同配合,包括俯冲带熔/流体的物理化学性质、俯冲带壳幔相互作用的机制和过程、俯冲带幔源岩浆活动的物质来源和启动机制以及深部地幔过程对地表环境的影响等许多关键科学问题尚未得到根本解决.将来的研究需要聚焦俯冲带物质循环这一核心科学问题,进一步查明俯冲带变质作用、交代作用、岩浆作用等过程的各自特征和相互联系,包括挥发性组分在地球深部的迁移过程及其资源和环境效应,着力考察研究相对薄弱的古俯冲带,阐明板块俯冲与地球深部物质循环之间的耦合机制. 相似文献
6.
俯冲带是地球上岩浆活动、高压—超高压变质作用、中深源地震、壳幔物质交换、元素循环和铜金大规模成矿的集中发生场所。富水流体不仅调控了这些地质作用,而且也深刻影响了全球C、S等挥发分的循环。当蚀变大洋岩石圈及上覆沉积物进入俯冲带中深部（15~300 km）,伴随由葡萄石- 绿纤石相至超高压榴辉岩相的递进变质作用,含水矿物在不同深度的分解造成流体释放为一连续过程。除极端高地温梯度环境之外,大多数俯冲带洋壳释放的流体为富水流体。但,俯冲带不同深度所产生的流体特征有明显差异。出露于全球造山带的高压—超高压变质地体保存了分凝体、脉体、水压致裂角砾岩等流体作用的有力证据,绿辉石、石榴子石、绿帘石等矿物中原生流体包裹体为流体的直接记录。在俯冲带中等深度（＜65 km）,流体是溶质含量很低的含卤化物水溶液,可含CO2-3、SO2-4、HS-等组分,其所含Si、Al、Ca、Mg、Fe、Na主量元素溶质相当于海水中固化物量的2~3倍,并具大离子亲石元素（LILE）、轻元素（B、Li）富集和高场强元素（HFSE）亏损的特点。当深度≥65 km时,流体转化为类似于超临界性质的溶液,含CH4、C2H6、H2S等挥发分,其所含主量元素溶质显著增加,微量元素除LILE等外,还负载相当量的HFSE和过渡族成矿元素。俯冲带65~100 km深度,为富水流体向超临界流体转变的区间。这种流体具有“亚超临界”性质,发生了氧化还原性质和元素溶解能力的渐变,并伴随硬柱石和角闪石的最终耗尽。高压变质岩石及相关脉体的O、Sr、Nd和金属同位素示踪研究表明俯冲带流体的源区多样,有蚀变基性洋壳、地幔橄榄岩和沉积物,并保存了海底热液蚀变作用的印迹。流体以脉冲方式沿网络状裂隙呈隧道式运移和传输,规模可达千米级,时间尺度在数月至数百年。目前通过高压—超高压变质岩的研究对俯冲带流体已经有了深入的了解,但展望未来,该领域依然有诸多争议和科学问题值得探索。 相似文献
7.
俯冲带是全球最大的物质循环系统,控制着硫(S)在地球内部圈层及表层的循环,影响着大气圈、水圈、生物圈、岩石圈的稳定性以及地球的宜居性。厘清S在俯冲带中的地球化学行为和循环特征对理解地球各储库的氧化还原状态、岩浆作用与演化、成矿物质聚集、以及地球大气成分等具有重要意义。本文首先总结了进入俯冲带之前的大洋岩石圈的S结构模型,对S在大洋板片中的分布状态和地球化学特征进行了系统归纳。随后,系统阐述了俯冲带高压-超高压变质岩记录的板片变质及脱水过程中硫的地球化学行为。岩石学研究表明俯冲板片中的S多以硫化物相存在,硫酸盐矿物在弧前深度就已被释放或分解。相较于熔体,俯冲带流体中S的溶解度更高,是运移硫的更有效方式。DEW模型计算结果显示,流体中S含量总体较低,但在俯冲板片~90km处其含量有一个峰值(浓度0.5%~1.0%)。岩相学证据、地球化学测试结果、磷灰石S近边吸收结构(S-XANES)特征以及模拟结果都显示俯冲深部流体中S多以HS^(-)及H_(2)S形式存在,不含大量的SO_(4)^(2-)及硫酸盐;中f_(S_(2))流体有利于S迁移出俯冲板片,从而促进俯冲带大规模S循环,而高f_(S_(2))流体在流-岩交换过程沿流体通道发生S的锁固作用而不利于俯冲带S循环。质量平衡计算显示全球俯冲带S输入通量为4.65×10^(13)g/yr,弧下深度板片S输出通量为2.91×10^(12)g/yr,板片-岛弧S循环效率仅6.3%。俯冲板片在弧下深度可能存在一个短暂高效的S释放窗口,释放流体的δ^(34)S值为-2.1±3.0‰。基于高压-超高压变质岩中硫化物的研究,初步厘清了俯冲板片中S的地球化学行为,首次从板片角度全面、定量地限定了俯冲带的脱硫通量、效率、种型和同位素特征,提出俯冲带循环的S不是岛弧岩浆的氧化剂,与岛弧环境的正δ^(34)S值也无直接因果联系,对解析俯冲带S循环和理解地球长期的S循环具有重要意义。最后,本文还展望了俯冲带S循环的未来发展方向,应在俯冲带流体氧化还原性质(硫酸盐的命运)、俯冲沉积物对S循环的制约、俯冲带环境下多硫同位素的分馏效应、S循环与其它挥发分(如C等)循环之间的耦合关系、地球历史上深部S循环等方向做出探索,更深入地理解俯冲带及全球S循环过程。 相似文献
8.
《吉林大学学报(地球科学版)》2015,(Z1)
<正>造山带橄榄岩以石榴二辉橄榄岩为主,还含有少量方辉橄榄岩、纯橄岩、易剥橄榄岩和辉石岩。石榴二辉橄榄岩记录丰富的变质演化和交代信息,是研究壳幔相互作用理想的样品。然而,为了准确地理解壳幔相互作用过程中熔/流体活动和元素迁移行为,除了要充分了解俯冲板片脱水/熔融过程,还要正确限定造山带橄榄岩的起源和初始的成分特征。石榴二辉橄榄岩由于遭受来自俯冲板片熔/流体的强烈改造和后期变质重结晶作用的影响,进入俯冲带之前的成分特征已经被不同程度改变;而纯橄岩是受到来自俯冲板片 相似文献
9.
俯冲带的流体地质作用 总被引：1，自引：0，他引：1
俯冲带是地球上流体活动最活跃的地区之一，也是流体由表层进入深部岩石圈的主要通道。流体活动对岩石成分、成岩作用、构造变形、变质作用、部分熔融和岩浆－火山作用，以及海洋生物－化学环境都有重要的控制和影响。本文回顾和总结了俯冲带流体地质作用研究的现状和进展，对古俯冲带流体地质作用的若干问题进行了探讨。 相似文献
10.
地壳交代作用是洋壳俯冲带板片-地幔界面的普遍现象,由于地幔楔样品的缺乏,其识别存在困难。而碰撞造山带广泛出露的地幔楔来源的造山带橄榄岩则是理想的研究对象。本文对大别-苏鲁造山带橄榄岩的已有研究成果进行了系统总结。这些成果表明这些橄榄岩在大洋俯冲向大陆碰撞转换的不同阶段经历了多期地壳来源流体的交代作用。地壳交代作用不仅改变了地幔楔橄榄岩的地球化学成分,而且导致了交代矿物的生长以及超镁铁质交代体的形成。这些交代体或作为同碰撞和碰撞后镁铁质火成岩的地幔源区,或将地壳组分传输到深部地幔,或释放流体交代大陆俯冲隧道中的俯冲陆壳。本文对地壳交代作用研究中存在的重要问题和解决思路提出了建议。 相似文献
11.
大陆碰撞过程中熔/流体的组成和演化是研究大陆深俯冲动力学的重要内容,而超高压岩石记录了大陆俯冲和折返过程中的熔/流体-岩石相互作用,因而是研究大陆碰撞过程中熔/流体组成和演化的天然实验室。大陆俯冲带高压/超高压变质矿物中多相固体包裹体作为熔/流体活动的直接记录,为我们提供了揭示超高压变质过程中熔/流体演化的重要制约。近年来,围绕超高压岩石中多相固体包裹体的形成时间、演化过程及其所反映的俯冲带超高压变质熔/流体的组成和性质,进行了大量的研究工作。超高压岩石中多相固体包裹体的发现,为理解峰期超高压变质流体的组成和演化提供了重要制约,同时也为研究俯冲板片-地幔楔界面的熔/流体交代作用提供了新的途径。本文从多相固体包裹体形成机制、结构形态特征、矿物化学成分及其地质地球化学意义等方面,对于超高压变质岩中多相固体包裹体的研究现状和存在的问题进行系统地总结和探讨,以期促进多相固体包裹体的岩石学和地球化学研究。 相似文献
12.
《International Geology Review》2012,54(10):1253-1277
ABSTRACTSeafloor subduction and subduction-zone metamorphism (SZM) are understood to be the very cause of both subduction-zone magmatism and mantle compositional heterogeneity. In this article, we compile geochemical data for blueschist and eclogite facies rocks from global palaeo-subduction-zones in the literature, including those from the Chinese Western Tianshan ultrahigh pressure (UHP) metamorphic belt. We synthesize our up-to-date understanding on how chemical elements behave and their controls during subduction-zone metamorphism. Although the compositional heterogeneity of metamorphic minerals from subducted rocks has been recently reported, we emphasize that the mineral compositional heterogeneity is controlled by elemental availability during mineral growth, which is affected by the protolith composition, the inherited composition of precursor minerals, and the competition with neighbouring growing minerals. In addition, given the likely effects of varying protolith compositions and metamorphic conditions on elemental behaviours, we classify meta-mafic rocks from global palaeo-subduction-zones with varying metamorphic conditions into groups in terms of their protolith compositions (i.e. ocean island basalt (OIB)-like, enriched mid-ocean ridge basalt (MORB)-like, normal [N]-MORB-like), and discuss geochemical behaviours of chemical elements within these co-genetic groups rather than simply accepting the conclusions in the literature. We also discuss the geochemical consequences of SZM with implications for chemical geodynamics, and propose with emphasis that: (1) the traditionally accepted ‘fluid flux induced-melting’ model for arc magmatism requires revision; and (2) the residual subducted ocean crust cannot be the major source material for OIB, although it can contribute to the deep mantle compositional heterogeneity. We also highlight some important questions and problems that need further investigations, e.g. complex subduction-zone geochemical processes, different contributions of seafloor subduction and resultant subduction of continental materials, and the representativeness of studied HP–UHP metamorphic rocks. 相似文献
13.
In situ Sr isotopic analyses of epidote: tracing the sources of multi-stage fluids in ultrahigh-pressure eclogite (Ganghe,Dabie terrane) 总被引：1，自引：0，他引：1
This study presents in situ strontium (Sr) isotope and Sr content data on multi-stage epidote crystals from ultrahigh-pressure (UHP) eclogites and omphacite–epidote veins therein at Ganghe (Dabie terrane, China), determined using LA-MC-ICP-MS. The Ganghe eclogites occur as lenses in mainly leucocratic UHP gneisses, and therefore, our data provide insights into the origin, composition, and transport scale of the discrete multi-stage fluids in UHP eclogites during the subduction and exhumation of a continental crust. Four textural types of epidote that record compositional and isotopic signatures of fluid at various metamorphic P–T conditions have been distinguished based on petrographic observations and compositional analyses. They are (1) fine-grained high-pressure (HP) epidote inclusions (Ep-In) in omphacite that define the earliest stage of epidote formation in the eclogite; (2) coarse-grained UHP epidote porphyroblasts (Ep-P) that contain omphacite with Ep-In inclusions in the eclogite; (3) fine-grained HP epidote in omphacite–epidote veins (Ep-V) as well as (4) the latest-stage epidote in disseminated amphibolite-facies veinlets (Ep-A), which crosscut the Ep-P or matrix minerals in the eclogite and HP vein. Both Ep-P and Ep-V crystals exhibit significant and complex chemical zonations with respect to the XFe (= Fe/(Fe + Al)) ratio and Sr content. In contrast to the varying Sr contents, Ep-In, Ep-P, and Ep-V have similar and narrow ranges of initial 87Sr/86Sr ratios (from 0.70692 to 0.70720 for Ep-In, from 0.70698 to 0.70721 for Ep-P, and from 0.70668 to 0.70723 for Ep-V), which are significantly different from those in Ep-A (from 0.70894 to 0.71172). The initial 87Sr/86Sr ratio of Ep-A is closer in value to the initial Sr isotopic composition of the gneisses (from 0.710790 to 0.712069) which enclose the UHP eclogite. These data indicate different sources of the eclogite-facies fluids and retrograde amphibolite-facies fluid in the Ganghe eclogites. The HP–UHP fluids responsible for the large amounts of hydrous minerals in the eclogites were internally derived and buffered. The omphacite–epidote veins were precipitated from the channelized solute-rich HP–UHP fluids released from the host eclogite. However, hydrated amphibolite-facies metamorphism during exhumation was mainly initiated by the low-Sr and high-87Sr/86Sr external fluid, which infiltrated into the eclogite from the surrounding gneisses. The eclogite-facies fluids in the Ganghe eclogites were locally derived, whereas the infiltration of the retrograde amphibolite-facies fluid from the gneisses required a long transport, most likely longer than 80 m. This study highlights that the in situ Sr isotopic analysis of multi-stage epidote can be employed as a powerful geochemical tracer to provide key information regarding the origin and behavior of various-stage subduction-zone metamorphic fluids. 相似文献
14.
高压-超高压变质岩石中石榴石的环带和成因 总被引：2，自引：2，他引：0
在俯冲带变质过程中,石榴石是高压-超高压变质榴辉岩和片麻岩的常见变质矿物。由于石榴石具有难熔和流体中的低溶解能力的特点,通常可以很好地保存下来,并且能够保留复杂的化学成分环带,以及不同类型的矿物或流体包裹体,为解释石榴石寄主岩石经历的变质演化历史提供了重要信息。石榴子石的主微量元素成分受控于很多因素,如全岩成分、变质的温压条件、控制石榴子石形成的相关变质反应、与石榴子石共生的矿物种类和成分等。因此,在利用石榴石探讨超高压变质的演化历史时,对石榴石进行系统的主要元素、微量元素、氧同位素以及矿物包裹体分析,以及相互间的成因关系。同时,对石榴石中的锆石或独居石包裹体并进行原位U-Pb定年和微量元素分析,可以为变质石榴石的形成时代提供直接的时间制约。深入研究超高压变质岩中石榴石的生长阶段,不仅可以为含石榴石寄主岩石的变质过程提供岩石学和地球化学证据,而且对于理解石榴石的形成机制、生长规律及其变质化学动力学过程具有重要的科学意义。 相似文献
15.
俯冲带流体作用的地球化学示踪 总被引：4，自引：1，他引：4
俯冲带流体包括俯冲带岩石脱水形成的水流体和熔融作用产生的熔体。俯冲带流体循环及伴随的化学作用是壳幔演化和物质交换的重要机制。岛弧火山岩地球化学研究可以示踪流体的性质、成分。造山带变质岩内高压脉是俯冲带流体活动的直接记录。通过高压脉及其主岩地球化学研究 ,可以示踪流体成分和来源。俯冲带水流体成分与流体释放深度有关 ,俯冲带深处 ( >5 0km)形成的水流体内溶质相当可观。水流体流动方式有大规模沟道式和小规模弥散式 ,与其相应的流体来源分别为外来的和内部的。俯冲带流体组分、流动方式、物质交换机制与俯冲带深度、热状态、物质结构的定量关系 ,以及矿物 /水流体间元素分配系数的高压实验可能将是俯冲带流体今后研究的重要课题。中国西天山高压变质带榴辉岩及蓝片岩中广泛发育高压脉 ,根据脉的矿物组合 ,高压脉可分成 4种类型。高压脉氧同位素研究表明 ,脉体与流体间氧同位素交换宏观与局部平衡 ,流体活动为大规模的 ;流体活动方式既有沟道式 ,又有渗透式 ;流体的δ18O值为 + 10 .8‰左右 ,流体来源于主岩经过海水低温蚀变的大洋玄武岩。西天山广泛发育的高压脉 (尤其是蓝片岩中的高压脉的发现目前在世界范围尚属首例 ) ,是研究古俯冲带流体作用良好的野外天然实验室。 相似文献
16.
《International Geology Review》2012,54(12):1055-1115
The Isua supracrustal belt (～3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out. Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics. 相似文献
17.
A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X CO2 and pressure-temperature (constant X CO2 ) diagrams were calculated to quantify the effects of CO2 in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X co2 <0.002. For X co2 >0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2 O-rich fluid and a more CO2 -rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites. 相似文献
18.
Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition 总被引：4，自引：0，他引：4
Matthias Konrad-Schmolke Patrick J. O'Brien Christian de Capitani Dennis A. Carswell 《Lithos》2008,103(3-4):309-332
In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks.Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones. 相似文献
19.
Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al2O3-Fe2O3 (+Na2O + SiO2+ H2O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the XFe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe3+ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions. 相似文献