高级变质岩中深熔作用的相平衡研究

深熔作用在高级变质岩中非常普遍并受到广泛关注。自20世纪90年代以来,随着变质相平衡研究的突破性发展,利用THERMOCALC程序和视剖面图方法可以定量研究固相线以上的熔体形成、熔体分馏和退变质反应。变质沉积岩中的熔融作用主要有三种机制饱和水固相线上的熔融、白云母脱水熔融和黑云母脱水熔融。在模拟泥质岩石的KFMASH体系和NCKFMASH体系中的相平衡计算表明,NCKFMASH体系中铁镁矿物的相平衡关系受KFMASH亚体系中矿物相平衡关系的控制,但KFMASH亚体系中固相线位置要比实际的高50~60℃。因此,模拟泥质岩石的固相线以上的相平衡关系最好在NCKFMASH或组分更多的体系中进行。相平衡研究表明麻粒岩相岩石的保存与熔体丢失有关;混合岩的形成过程包括部分熔融作用、不同程度熔体分凝与汲取和不同程度的逆反应和退变反应。     

基性岩高温-超高温变质作用与TTG质岩成因

变质基性岩在高温-超高温下部分熔融可以形成TTG(英云闪长质-奥长花岗质-花岗闪长质)质熔体,有关熔融反应机理、熔体地球化学特征以及太古宙TTG质岩石成因问题备受国内外学者关注。本文基于对相关实验岩石学研究的总结,结合基性岩高温-超高温相平衡的模拟计算,分析了变质基性岩(斜长角闪岩)深熔变质反应过程、P-T条件及其与TTG质岩石成因的联系。变质基性岩高温-超高温深熔作用主要受角闪石脱水熔融反应控制。在1.0GPa以下的无石榴石域,角闪石分解反应主要为:角闪石=单斜辉石+斜方辉石+斜长石+熔体(R1),该反应为多变滑动反应,以斜方辉石出现(800℃)和角闪石消失(1000~1100℃)为标志,其滑动温度范围超过200~300℃。实验岩石学确定的斜长角闪岩开始熔融或缺流体固相线大致相当于斜方辉石出现温度。实际上角闪石脱水熔融反应是从饱和水固相线开始的,反应为:角闪石+石英=单斜辉石+斜长石+熔体(R1a),开始有黑云母参与熔融反应,但该反应对熔体贡献有限。在1.0GPa以上的石榴石域,不同实验所确定的石榴角闪岩缺流体固相线温度主要介于800~900℃之间,固相线表现为正斜率、负斜率、或者为与压力无关的直线等不同结果。相平衡模拟计算表明在石榴石稳定域角闪石脱水熔融反应为较陡的负斜率,分为两部分:当有斜长石存在时,反应为角闪石+斜长石+石英=石榴石+单斜辉石+熔体(R2),低温部分有白云母、绿帘石参与反应。该反应从饱和水固相线(约630℃)开始,到角闪石消失(超过1000℃),滑动温度范围可超过400℃,跨越石榴角闪岩亚相与角闪高压麻粒岩亚相范围;在斜长石消失后角闪石脱水熔融反应为角闪石+石英=石榴石+单斜辉石+熔体(R2a),低温部分有绿帘石、白云母参与熔融反应,该反应从饱和水固相线(约650℃)开始,到角闪石消失(超过900℃),滑动温度范围可超过200~300℃。角闪石脱水熔融反应形成的无水残余物形成麻粒岩和榴辉岩,无水麻粒岩的峰期温度会超过1000℃,由于降温过程中的退变质演化,如超固相线下滞留熔体与残余物之间发生的深熔反应的逆反应,以及在亚固相线下离子交换反应,导致大多数麻粒岩只记录缺流体固相线组合与退变质温度。基性岩部分熔融的熔体成分取决于全岩成分、P-T条件及熔融程度。当熔融程度很低时(如小于5%)可形成富钾花岗质熔体,随着熔融程度增加,熔体成分可转变为奥长花岗质(如5%~20%)和英云闪长质(如大于20%),部分熔融的熔体成分受全岩成分影响很大,只有相对富钾的基性岩才能形成花岗闪长质到石英二长质熔体。太古宙TTG质岩石表现出富Sr、低Y、Yb、Nb、Ta、Ti以及稀土分馏程度高等地球化学特征,要求部分熔融压力较高,残余物中有石榴石(及金红石)存在。争论的焦点是部分熔融究竟发生在石榴角闪岩亚相(及角闪高压麻粒岩亚相),还是发生在榴辉岩相。对此,不同实验给出的不同结论应该与源岩地球化学特征不同有关。考虑到TTG质岩石的可能源岩如太古宙科马提岩和玄武岩地球化学特征的多样性,TTG质岩石本身地球化学特征上的差异也许不能完全指示熔融发生的P-T条件。综合实验岩石学和相平衡模拟结果,本文确定TTG质岩石是由基性岩在角闪石和石榴石共同稳定域由角闪石脱水熔融反应R2和R2a在角闪高压麻粒岩亚相和角闪榴辉岩亚相形成的,P-T条件为1.0~2.5GPa和800~1000/1100℃。角闪高压麻粒岩亚相相对应的地热梯度为15~25℃/km,角闪榴辉岩亚相对应的地热梯度为10~15℃/km。TTG质岩石形成的构造环境不能简单对应发生在显生宙的洋壳热俯冲带、碰撞造山带和洋底高原等。     

麻粒岩相变质作用与花岗岩成因-Ⅰ:变质泥质岩/杂砂岩高温-超高温变质相平衡

麻粒岩相岩石作为洞察下地壳的窗口一直备受重视。二十世纪九十年代以来麻粒岩研究的一个重要进展是利用变质相平衡的定量研究方法模拟岩石中所发生的深熔变质反应、熔体成分变化、及熔体丢失对变质矿物组合的影响等。本文利用KASH、NKASH和KFMASH等简单体系的相平衡关系,做出P-T投影图、组分共生图解和基于固定全岩成分的P-T视剖面图解,并结合有关实验岩石学结果,讨论了高温和超高温条件下变质泥质岩和杂砂岩中的变质熔融反应、矿物组合、全岩成分与P-T条件之间的相互关系。多数变质泥质岩和杂砂岩中饱和流体固相线熔融反应可利用NKASH体系中有水流体参与的熔融反应模拟,在没有外来流体注入时,这些反应可形成3mol%熔体。在不同体系中白云母脱水熔融反应型式及其P-T条件不同,如在NKASH和KFMASH体系中模拟计算的白云母脱水熔融反应与相应的实验结果相似,分别控制了白云母分解熔融的温度下限和上限;白云母的分解温度会随着其中Fe、Mg和Ti含量的增加而升高,也随着共生斜长石中钙长石组分增加而升高,泥质岩中白云母脱水熔融可以形成~10mol%熔体。在KFMASH体系中黑云母脱水熔融反应表现为4条单变反应,其理论计算的温度比实验模拟的结果低一些。在NCKFMASH体系或实际岩石中黑云母脱水熔融反应为滑动反应,如NCKFMASH体系中黑云母从其开始熔融到最后消失在泥质岩中可跨越~100℃,在杂砂岩中可跨越30~50℃。黑云母的稳定温度随着镁值升高而升高,其稳定上限受钛影响更大,黑云母脱水熔融可以形成超过30mol%~40mol%熔体。KFMASH体系中的相平衡模拟表明以出现斜方辉石+夕线石和假蓝宝石为特征的超高温组合易于出现于富镁泥质岩中,而对正常成分泥质岩在达到1000℃的超高温条件下,主要出现石榴石+夕线石(即夕线榴),该组合在更高温度反应形成假蓝宝石+尖晶石。利用饱和水固相线反应和白云母与黑云母分解反应可以更好地限定不同的变质相。如中压和低压条件下低角闪岩相和高角闪岩相的界限可利用NKASH体系中有水流体和白云母参与的熔融反应和亚固相线条件下的白云母分解反应限定;实验确定的泥质岩中黑云母开始熔融与消失的反应可分别用于限定高角闪岩相与(正常)麻粒岩相的界限,以及(正常)麻粒岩相和超高温麻粒岩相的界限。因此,从矿物组合角度,正常麻粒岩相可限定在黑云母开始熔融到完全消失的温度范围,超高温麻粒岩相可限定在黑云母消失(有石英存在)之后的温度范围。     

麻粒岩相变质作用与花岗岩成因-I:变质泥质岩/杂砂岩高温-超高温变质相平衡

麻粒岩相岩石作为洞察下地壳的窗口一直备受重视。二十世纪九十年代以来麻粒岩研究的一个重要进展是利用变质相平衡的定量研究方法模拟岩石中所发生的深熔变质反应、熔体成分变化、及熔体丢失对变质矿物组合的影响等。本文利用KASH、NKASH和KFMASH等简单体系的相平衡关系,做出P-T投影图、组分共生图解和基于固定全岩成分的P-T视剖面图解,并结合有关实验岩石学结果,讨论了高温和超高温条件下变质泥质岩和杂砂岩中的变质熔融反应、矿物组合、全岩成分与P-T条件之间的相互关系。多数变质泥质岩和杂砂岩中饱和流体固相线熔融反应可利用NKASH体系中有水流体参与的熔融反应模拟,在没有外来流体注入时,这些反应可形成<3mol%熔体。在不同体系中白云母脱水熔融反应型式及其P-T条件不同,如在NKASH和KFMASH体系中模拟计算的白云母脱水熔融反应与相应的实验结果相似,分别控制了白云母分解熔融的温度下限和上限;白云母的分解温度会随着其中Fe、Mg和Ti含量的增加而升高,也随着共生斜长石中钙长石组分增加而升高,泥质岩中白云母脱水熔融可以形成~10mol%熔体。在KFMASH体系中黑云母脱水熔融反应表现为4条单变反应,其理论计算的温度比实验模拟的结果低一些。在NCKFMASH体系或实际岩石中黑云母脱水熔融反应为滑动反应,如NCKFMASH体系中黑云母从其开始熔融到最后消失在泥质岩中可跨越~100℃,在杂砂岩中可跨越30~50℃。黑云母的稳定温度随着镁值升高而升高,其稳定上限受钛影响更大,黑云母脱水熔融可以形成超过30mol%~40mol%熔体。KFMASH体系中的相平衡模拟表明以出现斜方辉石+夕线石和假蓝宝石为特征的超高温组合易于出现于富镁泥质岩中,而对正常成分泥质岩在达到1000℃的超高温条件下,主要出现石榴石+夕线石(即夕线榴),该组合在更高温度反应形成假蓝宝石+尖晶石。利用饱和水固相线反应和白云母与黑云母分解反应可以更好地限定不同的变质相。如中压和低压条件下低角闪岩相和高角闪岩相的界限可利用NKASH体系中有水流体和白云母参与的熔融反应和亚固相线条件下的白云母分解反应限定;实验确定的泥质岩中黑云母开始熔融与消失的反应可分别用于限定高角闪岩相与(正常)麻粒岩相的界限,以及(正常)麻粒岩相和超高温麻粒岩相的界限。因此,从矿物组合角度,正常麻粒岩相可限定在黑云母开始熔融到完全消失的温度范围,超高温麻粒岩相可限定在黑云母消失(有石英存在)之后的温度范围。     

钠长花岗岩—H2O—HF体系相关系及含黄玉花岗质岩石的成因

在Ｐ＝１００ＭＰａ,ｔ＝８４０－４５０℃条件下,通过钠长花岗岩Ｈ２Ｏ－ＨＦ体系相关系实验获得;（１）随体系Ｆ含量的增加,固相线温度显著下降。（２）石英和黄玉的温度稳定域上限升高,碱性长石的稳定域上限降低;在Ｆ≤４％时,体系能在固线性之上结晶出典型的黄玉花岗岩矿物组合;在Ｆ＝６％时,体系能在固相线之上结晶出典型的黄玉云英矿物组合;（２）含氟浅色花岗质熔体具有能分异出极端富Ｆ残余熔体的趋势。     

麻粒岩相变质作用与花岗岩成因-Ⅱ:变质泥质岩高温-超高温变质相平衡与S型花岗岩成因的定量模拟

高温-超高温变质岩石的矿物组合及组构特点取决于不同的进变熔融反应,不同程度的熔体丢失以及不同程度的退变反应三种过程的综合效应。利用相平衡定量研究方法可以很好地模拟进变熔融反应的类型、P-T条件、熔体含量及其丢失行为、以及熔融过程中熔体与残余物的化学成分变化等,这对探讨高温-超高温变质作用过程以及花岗岩的成因非常重要。对平均泥质岩(APR)进行相平衡模拟表明变质泥质岩在等压(0.8GPa)升温熔融过程中可发生5种熔融反应:饱和流体固相线、白云母脱水熔融、黑云母熔融、钾长石-石榴石熔融和铝铁镁矿物熔融,后两种熔融反应主要发生在超高温条件下。减压过程中发生怎样的熔融反应受减压温度控制:在麻粒岩相(如850℃)减压可发生钾长石熔融、黑云母熔融和钾长石-石榴石熔融反应;在高角闪岩相(如750℃)减压主要发生白云母脱水熔融和钾长石熔融;在超高温麻粒岩相(如950~1000℃)减压主要发生钾长石-石榴石熔融和铝铁镁矿物熔融。熔体成分受熔融反应和P-T条件控制,如在高角闪岩相发生的饱和流体固相线和白云母脱水熔融可形成弱过铝的奥长花岗质和二长花岗质熔体;在麻粒岩相发生的黑云母熔融和钾长石熔融形成的熔体具有强过铝的二长花岗岩成分;在中压超高温发生的钾长石-石榴石熔融和铝铁镁矿物熔融形成强过铝的二长(钾长)花岗岩质熔体,可形成石榴石花岗岩;在低压超高温下发生的铝铁镁矿物熔融可形成堇青石花岗岩。除了极端超高温下的铝铁镁矿物熔融外,其它熔融反应都会使残余物的成分更贫硅,贫Na_2O和K_2O,富FeO和MgO,但Al_2O_3和Mg#基本不变。高温-超高温下发生深熔的岩石只记录降温过程形成的固相线组合,但固相线的类型与温度条件取决于熔体的丢失行为。在不丢失熔体或者获得熔体的岩石中,岩石最后只记录流体饱和固相线组合;发生熔体部分丢失的岩石会记录缺流体固相线组合,并且熔体丢失越多,缺流体固相线的温度越高;发生全部流体丢失的岩石可记录岩石所达到的最高温度。因此,在一个麻粒岩相区,甚至一个野外露头上不同部位的岩石记录不同的P-T条件。熔体丢失是导致使麻粒岩相组合在升温过程中发生超高温变质,在降温过程中得以部分保存的重要条件。发生部分熔融的高级变质岩中随着温度升高,熔体含量增加,会发生锆石分解,只有在降温过程中发生锆石结晶,因此,麻粒岩中新生锆石只记录降温过程到固相线及以后的年龄,一般不会记录麻粒岩相峰期时代。对泥质高压麻粒岩来说,如果经历ITD型变质演化,会发生递进减压熔融,变质反应易于达到平衡,但如果减压速度快并使岩石直接抬升到地壳浅部,会出现一些ITD型结构标志,如残留金红石、蓝晶石,或在石榴石周围出现堇青石的反应冠状体等,此时锆石记录的退变质年龄会与峰期变质年龄相差不大(如10~30Myr);但如果泥质高压麻粒岩减压至中、深地壳,受其中有滞留熔体影响易于发育IBC型结构特征,表现为麻粒岩组合被(中压)角闪岩相组合叠加,在泥质岩中出现黑云母+夕线石构成的暗色条带,或者出现退变白云母和含白云母的浅色体。在中、深地壳经历IBC过程的麻粒岩锆石记录的退变质年龄会与峰期年龄相差很大(如~100Myr)。高级变质岩中由于出现熔体使水流体活度降低,麻粒岩作为排除部分熔体的残余物,其水活度更低。从这一角度来说,水活度低是麻粒岩相变质作用的结果,而不是条件。某些麻粒岩区之所以出现多期麻粒岩相变质叠加受流体行为控制。在亚固相线下流体饱和岩石变质熔融作用从饱和水固相线开始,然后依次发生含水矿物的脱水熔融和无水矿物熔融,这一过程中流体是内部缓冲的,在麻粒岩相温度峰期形成一组平衡矿物组合,难以保留峰期之前的信息。而流体不饱和岩石(如已形成的麻粒岩或岩浆侵入体)变质作用受外部注入流体控制,与构造变形密切相关。如果发生两期麻粒岩相变质叠加变质,在强应变域会形成晚期麻粒岩组合;在弱应变域,会出现两期麻粒岩组合,其中晚期矿物表现为反应冠状体或细粒交生体;而在一些应变非常弱的区域,可能只保留早期矿物组合。     

公路隧道岩质和土质围岩统一亚级分级标准研究

目前实施的《公路隧道设计规范》（JTG D70－2004）对土质围岩分级、围岩亚级分级方法都没有进行研究,因此,不能满足隧道设计和施工的要求。通过对岩质围岩和土质围岩自稳性的大量室内试验、数值计算和理论分析,建立了公路隧道岩质围岩和土质围岩统一的亚级分级标准。研究结果显示：围岩自稳跨度是围岩自稳性的综合反映,围岩自稳跨度可分为长期稳定跨度、基本稳定跨度、暂时稳定跨度和不稳定跨度。研究表明,规范中岩质围岩的Ⅲ级、Ⅳ级和Ⅴ级分别依据自稳跨度可分为2个、3个和2个亚级;土质围岩尽管分为黏质土围岩、砂质土围岩及碎石土围岩,但按照各自的自稳跨度可以分别进行亚级划分。可见,以自稳跨度可以建立岩质围岩和土质围岩统一的亚级分级标准。     

高温高压下含水矿物对岩石熔点影响的实验研究

在温度约800—1300℃和压力1.0—3.5GPa下,对加入约2％水的钾质玄武岩和榴辉岩样品的熔融实验研究结果表明,两种岩石的固相线都明显低于干体系同类成分岩石熔融实验研究获得的固相线温度;其中前者由于相对富钾其熔点总体上又低于后者,与已有的研究资料一致。不同的是榴辉岩是随着压力的增大熔点温度增高,钾质玄武岩仅在1.5—2.5GPa和大于3.0GPa压力时其熔点随着压力的增大而增高,在2.5—3.0GPa压力范围内则相反。笔者认为这是由于钾质玄武岩在压力2.5GPa以下,存在着角闪石,2.5GPa以上存在金云母所致,二者矿物特定的成分决定了角闪石具有高于(或接近于)而金云母具有低于湿体系固相线的脱水温度;而含水榴辉岩实验的连续固相线特征则是其角闪石的脱水温度低于或接近含水条件的熔点温度所致。从而造成高压条件下岩石熔点的降低。因此,岩石圈中岩石的成分及其所决定的含水矿物类型和稳定温压条件是控制岩石固相线形式的重要因素,并可以很好地解释深部岩石的部分熔融和地震波低速带的成因。     

花岗岩体系Qz-Or-Ab-An-H_2O的初熔

本文采用石英和人工合成长石的混合物配制成的原料在P_(H2O)=2～15千巴的压力范围内进行了Qz-Or-Ab-H_2O体系初熔的反演。在无钙长石的碱性长石花岗岩体系Qz-Or-Ab-H_2O中,其熔融温度随着增压,从2千巴时的690℃降低到17千巴时的630℃。在花岗岩体系Qz-Or-Ab-An-H_2O中,随着钙长石含量的增加其固相线温度的升高是非常小的。与碱性长石花岗岩体系相比较,如果钠长石被斜长石An20(An40)代替,其固相线温度升高3℃(7°D)。碱性长石花岗岩体系和石英—钙长石—透长石组合(Qz-Or-An-H_2O体系)的固相线温度间的差别接近50℃。随着水压增高,斜长石及斜长石—碱性长石组合就变得不稳定,并分别被黝帘石+蓝晶石+石英和黝帘石+白云母-钠云母_(固溶体)+石英代替。并发现这些组合的压力稳定范围在600℃时介于6和16千巴之间。在高水压条件下(10～18千巴)。黝帘石-白云母-石英组合稳定到700和720℃。这个组合的固相线在透长石-黝帘石-白云母-石英混合物开始熔融温度以上10～20℃。为产生足够量的熔体,把变质岩变成岩浆岩状的岩石所需要水量只是非常少的。在层状混合岩的情况下证明,1％的水(或甚至更少)就足以使片麻岩局部转变为(岩浆状的)浅色部分,高级变质岩石或许是比较干的,而花岗岩质和花岗闪长岩质成分的重熔岩浆通常处于水不饱和状态。     

碳酸盐化泥质岩在6.0 GPa下的部分熔融行为研究

深俯冲碳酸盐化泥质岩的部分熔融行为研究是探索地球深部碳循环必不可少的方向之一,对地球深部物质循环、岩浆形成以及地幔化学成分不均一等过程起着不容忽视的作用。本文利用多顶砧大压机探索了6.0 GPa、800~1 600℃下碳酸盐化泥质岩的部分熔融行为,实验产物主要包括石榴子石、单斜辉石、柯石英、蓝晶石、碳酸盐矿物、多硅白云母以及熔体。碳酸盐矿物为方解石和菱镁矿,存在于6.0 GPa固相线以下的实验产物中。相对于同等压力下其它碳酸盐化体系,本文实验体系具有最低的固相线。部分熔融产生的熔体为硅酸盐熔体,且随着温度的升高,熔体比例逐渐增加,熔体成分也发生了明显的变化。     

Lower temperature stability limit of Mg cordierite in the range 1–7 kb water pressure: A redetermination

Hydrothermal experiments utilizing the seeding technique show conclusively that the lower temperature stability limit of Mg cordierite so far accepted (Schreyer and Yoder, 1964) represents metastable equilibria throughout the pressure range 1–7 kb. The stable equilibrium curve generally lies at somewhat higher temperatures and is not subdivided by an invariant point involving the phase pyrophyllite within this pressure range. Instead of pyrophyllite+andalusite+chlorite, hitherto assumed to be the breakdown assemblage of cordierite below 5 kb water pressure, the paragenesis Al silicate+chlorite+quartz was found to be stable over the entire pressure range investigated. The andalusite/kyanite transition, as determined by Richardson et al. (1969), is confirmed by a minor change of slope of the cordierite breakdown curve. Direct breakdown of cordierite into pyrophyllite-bearing assemblages must be limited to conditions below about 1 kb and 450° C. This gives indirect support to Kerrick's (1968) stability data on pyrophyllite, as opposed to those of Althaus (1969) and others. These new results offer an explanation for the common assemblage chlorite+andalusite (or kyanite)+quartz occuring in spotted slates, greenschists, and also as retrograde alteration products of natural cordierites.We thank Dr. K. Langer, Bochum, for partial analyses of some of the natural starting materials. Financial support of this work by Deutsche Forschungsgemeinschaft, Bad Godesberg, is gratefully acknowledged.     

A phenomenon in the configurational change of phase diagrams under constraints: Stability interchange of Mirror Symmetrical Configuration and its thermodynamic explanationSCIEI北大核心CSCD

实验或计算相图及野外观察表明,同一体系在不同但却相近的物理化学条件下常常会呈现出许多截然不同的相组合或经历截然不同的反应途径。为了分析这一现象的原因,本文作者计算了不同温度、压力条件下CaO-FeO-SiO_(2)-O_(2)体系(铁橄榄石、钙铁橄榄石、钙铁辉石、磁铁矿、钙铁榴石、赤铁矿、硅灰石)的氧逸度-硅活度[log f(O_(2))-log a(SiO_(2))]相图。结果表明,随着约束条件的变化,该体系所包含的两个不同一级多体系(由一个无变度组合加上一相所形成的子体系)的相图先后经历了构型转变;在每次转变中,同一个一级多体系中每条单变度线上的两个无变度点都会同时经历彼此靠近、重合于一点、再沿原来方向彼此远离的过程。因此,原来相对稳定的无变度点在新的条件下全部同时变为介稳,而原来介稳的无变度点在新的条件下全部同时变为相对稳定的点,其中每一构型中稳定部分与介稳部分的关系是互补的。因此,整个一级多体系的拓扑构型(包含稳定和介稳部分)呈现镜像对称。对于受约束条件下的多相、多组分体系,这种构型转变现象比较常见。它伴随着一系列相组合和反应的突变,因而可以影响相关地质过程的演化途径;而构型转变的条件可用来确定相组合突变所需要的条件变量的范围,其计算只涉及一级多体系(而不是整个体系)中的相。需要注意的是,在一级多体系中,某个“相对”稳定的无变度点在其它一级多体系中可能会变为介稳点。因此,一级多体系中“相对”稳定的无变度点在高级多体系中可能只有一部分依旧能够保持稳定。另外,受约束条件的变化常会导致新相的产生或已有相的消失,从而使得原有的一级多体系可能还未来得及发生相图构型转变就已经转变为一个新的体系。这种情况会大幅度地减少原有的一级多体系相图构型发生转变的机会。本文利用相图的拓扑分析理论对上述现象做了很好的热力学分析和解释。     

Low-temperature compatibility relations of the assemblage quartz-paragonite and the thermodynamic status of the phase rectorite

The reaction 3 Na-montmorillonite + 2 albite 3 paragonite + 8 quartz has been studied experimentally using starting materials composed of natural low albite, kaolinite and quartz. Rate studies at 2, 4 and 7 kb demonstrate that the reaction takes place at 335–315° C from lower to higher pressures. Attempts to reverse this reaction with runs lasting several months were without success. Comparison with pertinent data from natural mineral assemblages indicate that despite non-reversal, the data presented here may be very near to the true lower thermal compatibility limit of the assemblage quartz-paragonite.The above reaction becomes metastable beyond the upper pressure stability limit of the phase Na-montmorillonite; it is replaced here by another reaction 1 albite + 1 kaolinite 1 paragonite + 2 quartz + 1 H₂O, as suggested originally by Zen (1960). A P-T-grid showing possible compatibility relations of the assemblage quartz-paragonite is provided (Fig. 4). Perusal of natural assemblages belonging to the subsystem Na₂O-Al₂O₃-SiO₂-H₂O lends credence to this grid.In course of the rate studies reported here, various regular paragonite-sodium montmorillonite mixed-layer phases were encountered (Fig. 2); the 11 regular mixed-layer phase represents the synthetic analogue of the mineral rectorite (sometimes called allevardite), widely recorded from deep diagenetic and anchimetamorphic environments. Results of rate-studies (Fig. 3) suggest that the mixed-layer phases are all transient, metastable products obtained during the transformation of the albite-Na-montmorillonite assemblage to paragonite-quartz. As such, rectorite and related mixed-layer phases on the join montmorillonite-paragonite, are always less stable relative to the assemblage Na-montmorillonite-paragonite.     

Zur Stabilität des Ferrocordierits

Ferrocordierite with Fe⁺²/Fe⁺²+Mg=0,92 was synthesized as a single phase at 600° C and 1000 bars total pressure using cristobalite and natural monoclinic chloritoid as starting materials. Oxygen fugacities during synthesis were those given by the buffering power of the hydrothermal bombs. Pure ferrocordierite without Mg synthesized from various artificial starting materials was never obtained as a single phase, but coexisted with variable amounts of metastable hercynite+quartz. The ferrocordierites made in this study are orthorhombic with distortion indices Δ up to 0,26° as a function of pressure, temperature, and duration of the runs. The lattice constantsa ₀ = 17,234 and b₀ = 9,824 are considerably larger, c₀ = 9,298 smaller than those of pure Mg-cordierite; mean indices of refraction vary within the range 1,569–1,573±0,002. In successful breakdown experiments natural Fe-rich cordierite intergrown with quartz from Mujinazawa, Japan, as well as the most Fe-rich natural cordierite so far discovered, from Dolni Bory, Moravia, were used. Ferrocordierite is stable only above relatively high temperatures (450°–600° C depending on pressure), which are within the limits of determination identical to the lower stability limits of Mg-cordierite. In contrast to the latter phase, however, ferrocordierite becomes unstable at a yet undefined pressure between 6000 and 10000 bars. The stable breakdown products at low temperatures are chloritoid+quartz or - possibly only at very low pressures - assemblages with 7 Å-chamosite, which have been synthesized (metastably ?) over a more extended PT-range. Runs seeded with chloritoid + cristobalite did not yield critical evidence as to the more stable ,assemblage. Except for occasional small amounts of pyrophyllite there is considerable uncertainty concerning the aluminous phase coexisting with chamosite; kaolinite with all critical peaks covered by chamosite orγ-Al₂0₃ amorphous to X-rays may be present. At high temperatures and at a pressure of 10000 bars ferrocordierite broke down to assemblages containing an orthoamphibole, probably ferrogedrite. It is uncertain, however, whether these assemblages represent stable equilibrium, or whether they are metastable substitutes for other parageneses, e.g. almandite silimanite + quartz. Given the requisite bulk compositions and geothermal gradients ferrocordierite remains stable under static conditions to crustal depths of at least 20 km. ThusChinner's mechanism of differential breakdown of Fe-bearing cordierites with increasing pressures may operate in deep zones of regional metamorphism within orogenic belts, but appears unlikely in shallow posttectonic contact metamorphism, unless this metamorphism is caused by hot basic magmas producing temperatures above the incongruent melting of ferrocordierite. In contrast to the relations found for Mg-cordierite the formation of feroocordierite is very sluggish, the more so the higher the temperature and pressure. Intermediate metastable assemblages of other phases appear in its place. It seems possible that these unfavorable reaction kinetics are - opposed to thermodynamic equilibrium - in part responsible for the rare occurrence of Fe-rich cordierites in nature. The overlap of the stability fields of ferrocordierite and chloritoid as demonstrated in this study allows the stable coexistence of these two phases within a limited temperature range.     

Matrix analysis of metamorphic mineral assemblages and reactions

Assemblage diagrams are widely used in interpreting metamorphic mineral assemblages. In simple systems, they can help to identify assemblages which may represent equilibrium states; to determine whether differences between assemblages reflect changes in metamorphic grade or variations in bulk composition; and to characterize isograd reactions. In multicomponent assemblages these questions can be approached by investigating the rank, composition space (range) and reaction space (null-space) of a matrix representing the compositions of the phases involved. Singular value decomposition (SVD) provides an elegant way of (1) finding the rank of a matrix and detemining orthonormal bases for both the composition space and the reaction space needed to represent an assemblage or pair of assemblages; and (2) finding a model matrix of specified rank which is closest in a least squares sense to an observed assemblage. Although closely related to least squares techniques, the SVD approach has the advantages that it tolerates errors in all observations and is computationally simpler and more stable than non-linear least squares algorithms. Models of this sort can be used to interpret multicomponent mineral assemblages by straightforward generalizations of the methods used to interpret assemblage diagrams in simpler systems. SVD analysis of mineral assemblages described by Lang and Rice (1985) demonstrates the utility of the approach.     

Reactions in Amphibolite, Greenschist and Blueschist

Mineral assemblages in which chlorite [CHL], epidote [EPI],clinoamphibole [AMP], plagioclase [PLG] and quartz [QTZ] aremajor phases are characteristic of many low-grade mafic schists.The possible heterogeneous reactions in such an assemblage maybe separated into two types, exchange reactions and net-transferreactions. Only the latter alter significantly the modal proportionsof the minerals. A set of linearly independent reactions defines a reaction spaceof as many dimensions as there are independent reactions. Thespace defined by the net-transfer reactions alone is a sub-spacethat can be portrayed in three dimensions for the above assemblage.A procedure is presented herein that gives a set of independentreactions that may be taken as basis reactions for definingsuch a reaction space. All other reactions that can be writtenfor this assemblage, as well as observed whole-rock reactions,can be portrayed as vectors in these reaction spaces. Thesevectors connect the region (mineral facies) accessible to theabove assemblage. The whole-rock reactions of Laird (1980) relatinggreenschist, blueschist and various low-grade amphibolites fromVermont, provide informative examples, as do the whole-rockexperiments of Liou et al. (1974). Although reaction spaces apply to both equilibrium and disequilibriumassemblages the reactions selected as basis vectors correspondone-for-one to the chemical conditions for equilibrium thatmust obtain in any fully equilibrated assemblage. The set selectedis one that provides maximum sensitivity for geothermometric,geobarometric and geohygrometric purposes.     

Metamorphism of metapelites: calculations of equilibrium assemblages and numerical simulations of the crystallization of garnet

Abstract This work uses a simplified model of equilibrium to predict the assemblage sequence and compositional zoning in garnet that should result from prograde metamorphism of common bulk compositions of pelitic rocks. An internally-consistent set of model thermodynamic data are derived for natural mineral compositions from natural assemblages. Equilibrium assemblages can be calculated for pelitic compositions with excess quartz and either muscovite or K-feldspar at any pressure and water pressure. The compositions and abundances of phases in equilibrium assemblages can be calculated where the elements Mg, Fe and Mn are exchanged among phases. The prograde metamorphic assemblage sequences and the effects of pressure on assemblages, predicted by the simulation method presented here, are similar enough to natural observations to suggest that the simulations can be used to analyse natural equilibrium and growth processes. The calculated phase diagrams at moderate and high crustal pressures explain the mineral assemblage sequence produced by prograde metamorphism in common pelitic compositions. Garnet appears by continuous reaction of biotite and chlorite as the garnet-biotite-chlorite divariant field migrates toward higher Mg/Fe ratios over the bulk composition. Staurolite appears in common bulk compositions when garnet and chlorite become incompatible. An aluminum silicate phase can appear when staurolite and chlorite react. Staurolite breaks down at an extremum point to produce garnet. Continuous reaction of biotite and sillimanite causes growth of abundant garnet. The reaction sequence involving garnet, staurolite and aluminum silicates is probably different at low pressure, but the main reason that staurolite and garnet are rare is the restricted compositional range over which their assemblages exist. Andalusite appears by the divariant reaction of chlorite and cordierite appears at low temperature in low pressure assemblages for common bulk compositions by the extremumpoint breakdown reaction of chlorite. Compositional zoning of garnet and the systematic variation of biotite composition in metamorphic sequences indicate that garnet is probably fractionated during growth. Fractionation of garnet causes garnet-consuming, univariant reactions to become multivariant. The metastable persistence of garnet should reduce the abundance and stability range of staurolite. Fractionation of even small quantities of garnet should deplete the equilibrating bulk composition of Mn, but have little effect otherwise. The simulations show that the prograde assemblage sequence in pelitic rocks can be complex in detail, with some assemblages lasting over temperature intervals of only a few degrees. The major prograde reactions that release water are the breakdown of chlorite to form garnet at low grade and the breakdown of muscovite at high grade. The volume of water released by formation of garnet at high grade is also important. These reactions have the capacity to buffer water pressure. The density of anhydrous pelitic rock increases markedly when chlorite breaks down and by the continuous reaction forming garnet at high grade. The heat content is controlled principally by heat capacity and continuous reactions. Discontinuous reactions have little thermal buffering capacity. Simulations of garnet fractionation show that commonly-observed garnet zoning profiles can be formed by garnet growth in the assemblage garnet-biotite-chlorite in common bulk compositions. A reversal of Fe-zoning in garnet can occur when garnet resumes growth above staurolite grade in the assemblage garnetbiotite-sillimanite. Discontinuities in zoning profiles can be caused only by disequilibrium. The disequilibrium can be due to either metastable persistence during a hiatus in growth or to growth by irreversible reaction. Because the appearance of garnet is controlled by a continuous rather than a discontinuous reaction, the appearance of garnet is very sensitive to bulk composition. The early development of garnet is also sensitive to the pressure and water pressure of metamorphism. As a consequence the first garnet isograd is of limited thermometric value. Metastable persistence of kyanite and manite at high grades could reduce the abundance of garnet and allow biotite to persist. Metastable persistence would also limit the of cordierite formation.     

Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif,Central Alps)

Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re‐equilibration of the metastable magmatic assemblage induced shear zone widening at these P–T–X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P–T and chemical potential projections and sections show that the process of equilibration of the wall rocks (μ–μ path) occurs via the reactions: kf + cz + ab + bio + MgO + H₂O = mu + q + CaO + Na₂O and cz + ab + bio + MgO + H₂O = chl + mu + q + CaO + Na₂O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na₂O of ～100% and ～40% respectively, coupled with hydration and a gain of ～140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re‐equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re‐equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si‐content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite‐bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn‐deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.     

Evidence for melt migration enhancing recrystallization of metastable assemblages in mafic lower crust, Fiordland, New Zealand

A major arc batholith, the Western Fiordland Orthogneiss (WFO) in Fiordland, New Zealand, exhibits irregular, spatially restricted centimetre-scale recrystallization from two-pyroxene hornblende granulite to garnet granulite flanking felsic dykes. At Lake Grave, northern Fiordland, the composition and texture of narrow (<10–20 mm across) felsic dykes that cut the orthogneiss are consistent with an igneous origin and injection of melt to form orthogneiss migmatite. New U–Pb geochronology suggests that the injection of dykes and migmatization occurred at c . 115 Ma, during the later stages of arc magmatism. Recrystallization to garnet granulite is promoted by volatile extraction from the host two-pyroxene hornblende granulite via adjacent dykes and the patchy development of garnet granulite is left as a marker adjacent to the melt migration path. New mineral equilibria modelling suggests that a two-pyroxene hornblende assemblage is stable at <11 kbar, whereas a garnet granulite assemblage is stable at >12 kbar, suggesting that garnet granulite may have formed with <5 km crustal loading of the batholith. Although the garnet granulite assemblages signify that the WFO experienced high- P conditions, the very local nature of these textures indicates widespread metastability (>90%) of the two-pyroxene hornblende granulite assemblages. These results indicate the strongly metastable nature of assemblages in mafic lower arc crust during deep burial and demonstrate that the degree of reaction in the case of Fiordland is related to interaction with migrating melts.     

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

Due to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H₂O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H₂O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.