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超高温变质作用：以华北内蒙古土贵乌拉地区为例 总被引：13，自引：0，他引：13
超高温麻粒岩(富Mg-Al)是指温度高于900℃、压力为0.7~1.3GPa条件下形成的麻粒岩相变质岩,它记录了下地壳超高温极端变质作用的地质信息。富Mg-Al典型超高温矿物组合有:假蓝宝石+石英,尖晶石+石英,大隅石+石榴石,斜方辉石+夕线石+石英,高氟黑云母和钙镁闪石,刚玉+石英。目前世界上发现的超高温麻粒岩带(块)地区有非洲阿尔及利亚、南非、东南极、巴西中部、澳大利亚中部、印度南部和东南部等地。我们在华北克拉通北部内蒙古中南部地区孔兹岩区中发现了超高温麻粒岩,岩性主要为灰黑色条带状夕线石榴黑云片麻岩,其中含有尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英以及刚玉+尖晶石+石榴石等超高温矿物组合,指示温度达1000℃,压力超过1.0GPa的变质作用。独居石定年获得了(1927±11)Ma以及(1819±11)Ma两个峰期年龄,代表变质年龄。华北克拉通北部超高温麻粒岩的发现对研究华北克拉通与哥伦比亚超大陆的演化关系有重要意义。 相似文献
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内蒙古中南部古元古代高温型双变质带及其构造意义 总被引：6，自引：0，他引：6
内蒙古中南部集宁-丰镇-土贵乌拉地区发育了一条超高温变质岩带.通过对土贵乌拉地区超高温变质岩的矿物组合研究,发现其中包含了尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英的超高温标型矿物组合,并且在超高温变质作用区域内,经历了初始近等压冷却,随后近等温降压的逆时针退变质过程.已有的年代学数据表明,超高温变质作用峰期变质时代为1 920 Ma左右,降压退变质时代为1 850 Ma左右.在超高温变质岩带的东南地区,包括怀安-宣化一带,还出露了高压麻粒岩带.高压麻粒岩经历了近等温降压的顺时针退变质过程.年龄数据表明高压麻粒岩峰期变质时代为～1 900 Ma,退变质时代为1 850 Ma左右.内蒙古中南部超高温变质岩带和高压麻粒岩带相邻分布、产状相似且变质时代接近,共同构成了类似都城秋穗提出的双变质带;由于其温度偏高,故称为高温型双变质带,为世界上最古老的双变质带之一.该高温型双变质带可能反映了内蒙古中南部不同层次地壳物质在早前寒武纪碰撞作用中经历了P-T演化截然不同的变质作用,两条变质带在经历了各自的峰期变质作用后,作为一个共同的地体抬升.对内蒙古中南部古元古代高温型双变质带开展进一步研究,为认识华北克拉通早期构造背景、地壳热演化,特别是板块构造、不同地壳层次的构造作用提供了难得的科学机遇. 相似文献
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超高温变质作用的研究已成为世界上热门的地球科学领域之一,其中斜方辉石+夕线石+石英、假蓝宝石+石英、尖晶石+石英、大隅石+石英以及富铝斜方辉石等标型矿物组合和特征可以指示岩石经历了超高温变质作用(温度>900℃),这对查明地球深部下地壳的物质组成和构造演化具有重要意义。初次对产于世界上不同地区超高温麻粒岩尖晶石+石英共生等的标型矿物组合、不同赋存状态的尖晶石ZnO含量、XZn[Zn/(Mg+Fe2++Zn)]、XMg等进行分析和对比,认为不同地区不同赋存状态的尖晶石,其ZnO含量、XZn和XMg有所不同,但总的来说,超高温麻粒岩尖晶石中ZnO质量分数多数低于2.5%,但个别特殊的有高达近15%。尖晶石中XZn较低且尖晶石与石英共生,可以作为超高温变质的证据之一。通过对阿尔泰南缘超高温尖晶石-斜方辉石-石榴石麻粒岩中尖晶石成分及ZnO含量分析,得出与石英共生的尖晶石ZnO质量分数约为2.50%,且有与XFe成正比的变化趋势,而与钛铁矿共生的尖晶石ZnO质量分数约为1.85%,虽然两者含量稍有不同,但与世界上超高温麻粒岩中多数尖晶石ZnO含量相比都相近。结合其他特征矿物组合,可以确定该岩石中尖晶石与石英共生组合可以作为阿尔泰地区发生超高温变质作用的证据。通过对阿尔泰南缘与尖晶石-石英有关的变质反应期次划分等的初步研究,认为变质期次可划分为峰期变质和退变质两期,推测峰期的温度条件可达到1000℃,且该超高温麻粒岩可能经历了顺时针p-T演化轨迹。该超高温麻粒岩在阿尔泰造山带的出现,可能与古生代时期发生的古亚洲洋俯冲、西伯利亚板块和哈萨克斯坦—准噶尔板块碰撞造山作用过程有关。 相似文献
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华北克拉通两类早前寒武纪麻粒岩(HT-HP和HT-UHT)及其相关问题 总被引：35，自引：23，他引：12
华北克拉通广泛存在高级变质的早前寒武纪岩石,其中高温高压(HT-HP)麻粒岩和高温超高温(HT-UHT)麻粒岩近年来备受人们的关注,并将它们视为探讨早前寒武纪大陆演化的关键课题.HT-HP麻粒岩主要是含石榴石的基性麻粒岩,它们以透镜体或强烈变形的岩墙状出露于片麻岩中.HT-UHT麻粒岩主要是富铝的变质沉积岩系,俗称孔兹岩系,其中有含假蓝宝石和尖晶石等矿物组合,指示部分岩石的变质温度高于900～1000℃.本文探讨了这两类麻粒岩的出露状况、变质过程与变质历史、原岩与变质年龄.初步研究表明:①两类麻粒岩在变质的峰期温度和压力上有很大的重叠区间,都经历了一个近等温-略升温的降压变质;②两类麻粒岩很有可能在峰期和随后的降压变质阶段是同时的或有关联的;③HT-HP和HT-UHT麻粒岩的分布特征是线状或面状分布仍有待进一步查明;④高级变质的麻粒岩代表了华北克拉通的最下部地壳,它们变质的温压体系、岩石的刚性程度、分布特征、岩石组合以及抬升速率等具有与显生宙明显的不同.本文从而指出两类麻粒岩是否有成因联系是今后研究中非常值得注意的问题,深入研究将会对华北克拉通早期地壳演化、构造和动力学机制提供丰富的资料和证据. 相似文献
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华北克拉通基底主要构造单元变质作用演化及其若干问题讨论 总被引：77，自引：54，他引：23
华北克拉通基底可分为三个太古宙微陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个早元古宙活动带(孔兹岩带、华北中部带和胶-辽-吉带).这些构造单元具有不同的变质作用时间和P-T演化特征.东部陆块和阴山陆块晚太古宙基底岩系的变质作用发生在～2.5Ga,变质演化以等压冷却(IBC)逆时针P-T轨迹为特征,反映变质作用的成因与大规模地幔岩浆底侵有关.孔兹岩带主期变质作用发生在～1.95Ga,变质演化以近等温减压(ITD)顺时针P-T轨迹为特征,反映阴山陆块与鄂尔多斯陆块碰撞形成西部陆块的热构造过程.华北中部带变质作用发生在～1.85Ga,变质演化同样以近等温减压(ITD)顺时针P-T轨迹为特征,反映了西部陆块和东部陆块最终碰撞形成统一的华北克拉通基底的构造过程.早元古宙胶-辽-吉带变质作用表现‘双变质带'特征:西北带的北辽河群、老岭群和粉子山群的变质作用以中压顺时针P-T轨迹为特征,而东南带的南辽河群、吉安群和荆山群的变质作用以低压逆时针P-T演化为特征.华北克拉通基底变质作用演化地质图能更好地反映上述不同构造单元的变质作用演化特征.尽管岩浆弧、大陆裂谷和地幔柱模式都能解释东部陆块晚太古宙基底变质作用所具有的近等压冷却(IBC)逆时针P-T演化特征,地幔柱模式能够更合理解释东部陆块所存在的宽达800千米而时代近于相同的晚太古代火成岩带、大量科马提质超镁铁质岩石和双峰式火山岩、广泛发育的穹窿构造等.华北克拉通变质基底中具有石榴石-单斜辉石-斜长石-石英组合的高压基性麻粒岩和具有蓝晶石-钾长石组合的高压泥质麻粒岩的出露只局限在早元古宙华北中部带的北段和胶-辽-吉带的南端;这些高压麻粒岩形成在俯冲和陆-陆碰撞的构造环境中.西部陆块孔兹岩带含假蓝宝石麻粒岩是碰撞后(～1.92Ga)拉伸引发地幔岩浆底侵导致局部地带发生超高温(UHT)变质作用的产物. 相似文献
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茹尔群岛(又称赖于尔群岛)位于东南极普里兹构造带的东部边缘,是一个由太古宙和中元古代岩石组成的复合高级变质地体。中元古代岩石是含有富Fe-Al的含石榴子石-矽线石的费拉副片麻岩组合,经历了格林维尔和泛非两期变质作用。太古宙正片麻岩是含有富Mg-Al的含假蓝宝石的超高温泥质麻粒岩组合(梅瑟副片麻岩组合),主要由经历超高温变质作用的含假蓝宝石的泥质麻粒岩、富Mg的石榴子石-矽线石泥质片麻岩、斜方辉石-矽线石石英岩、含石榴子石镁铁质麻粒岩和钙硅酸盐麻粒岩等组成。其中,含假蓝宝石泥质麻粒岩中石榴子石变斑晶和矽线石集合体(蓝晶石假象)周围分别发育峰期后由假蓝宝石+斜方辉石和假蓝宝石+堇青石后成合晶组成的典型减压结构。含石榴子石镁铁质麻粒岩中石榴子石变斑晶周围则发育峰期后由斜方辉石+斜长石后成合晶组成的典型白眼圈减压结构。不同研究者得出了具有不同超高温峰期条件、峰期前及峰期后演化历史、不同形式的顺时针变质P-T轨迹。对超高温变质事件发生的时间和构造背景的认识也存在较大分歧,有认为超高温变质事件发生于格林维尔期(~1000 Ma)并与碰撞造山和弧岩浆作用有关,也有研究认为发生于泛非期(~590 Ma或~530 Ma)并与普里兹造山及冈瓦纳大陆聚合有关。因此,为理清该区超高温麻粒岩的变质演化历史和构造背景,需要对其进一步进行详细深入的矿物组合-变质结构分析、P-T轨迹重建及高精度的锆石-独居石U-Pb年代学研究,并进行区域上对比。 相似文献
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冀东地区新太古代麻粒岩相变质作用及其大地构造意义 总被引：8，自引：7，他引：1
冀东地区的早前寒武纪基底保留有太古宙克拉通普遍发育的"穹窿-龙骨构造",如卢龙-双山子表壳岩系呈近南北向带状分布于以TTG片麻岩为主构成的太平寨卵形构造域、迁安片麻岩穹窿和安子岭片麻岩穹窿之间,洒河桥线性构造带呈北东向切割太古宙构造线分布。在太平寨卵形域和洒河桥线性带中常见有零散分布的麻粒岩相表壳岩块体和古元古代变质基性岩墙。表壳岩块体包括基性和泥砂质麻粒岩和BIF型铁矿等,变质基性岩墙也发育高压麻粒岩相组合,变质锆石年龄为～1.81Ga。本文总结了近年来对该区麻粒岩的研究进展。太平寨卵形域中的基性麻粒岩以中粒二辉麻粒岩为主,有些样品的角闪石周围出现叠加变质形成的微粒矿物组合,个别样品见辉石周围发育石榴石冠状体,形成‘红眼圈’结构;利用稀土元素温度计确定二辉麻粒岩的峰期达到了～1000℃的超高温条件,麻粒岩中的锆石仅记录新太古代末期变质年龄(～2.50Ga),与周围TTG质岩石的结晶时间近于相同。洒河桥线性带中的基性麻粒岩以细粒高压麻粒岩组合为主,偶见中粒二辉麻粒岩残留,其峰期P-T条件分别为800～860℃/1.0～1.2GPa和950～1070℃/1.0GPa;麻粒岩中锆石主体记录新太古代末期变质年龄,但出现少量古元古代变质锆石(1.97～1.83Ga),石榴石-全岩Lu-Hf等时线年龄为1.77～1.78Ga。由此推测太平寨和洒河桥地区都经历了新太古代末期超高温麻粒岩相变质作用,又在古元古代晚期遭受了高压麻粒岩相差异性叠加,太平寨地区受叠加较弱,仍然保留太古宙的卵形构造,而洒河桥一带受叠加较强,形成了线性变形带。太平寨卵形域的泥砂质麻粒岩可识别出4阶段的变质矿物组合:包括包体组合、峰期组合、固相线(或最后)组合以及叠加组合。相平衡模拟表明固相线组合的P-T条件为870～890℃/～0.7GPa,峰期组合可达到1000℃/1.1GPa,石榴石中富钙斜长石包体指示麻粒岩早期经历了低压高温变质阶段。由此构建麻粒岩P-T轨迹为逆时针型,包括3个阶段:低压加热至超高温(AG-I),近等温升压至压力峰期(～1.1GPa)(AG-II),和峰后降压降温至固相线(AG-IIIa)以及在亚固相线下的降温降压过程(AG-IIIb)。锆石定年表明泥砂质麻粒岩原岩沉积年龄稍早于2.50Ga,变质年龄为2.48～2.50Ga。泥砂质麻粒岩的峰期变质条件和时代均与二辉麻粒岩一致,叠加组合的P-T条件与高压麻粒岩相似,反映泥砂质麻粒岩也受到了古元古代晚期变质作用影响。依据太平寨麻粒岩的逆时针型P-T轨迹,推测麻粒岩相变质作用包括如下构造过程:(i)AG-I指示表壳岩层受到后续地幔极高温岩浆喷发被埋深加热,或者受到下部TTG质岩浆海的加热过程;(ii)AG-II指示被加热的岩石(总伴有BIF铁矿层)被破碎并在在密度驱动下沉入岩浆海深部,达到下地壳深度;(iii)AG-IIIa/b对应沉入岩浆海深部的岩石伴随穹窿上升发生减压冷却的过程。很多地质观测和数值模拟研究表明太古宙克拉通的形成受太古宙特有的垂直构造体制控制,与太古代之后线性造山带的构造体制完全不同。 相似文献
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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 总被引：12，自引：12，他引：0
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。 相似文献
10.
内蒙古土贵乌拉孔兹岩带超高温变质作用:变质反应结构及P-T指示 总被引：6，自引：0，他引：6
内蒙古土贵乌拉地区分布了含石榴石花岗岩、夕线石榴黑云片麻岩以及二者以不同比例互层状组成的岩石,超高温变质岩以暗色含尖晶石堇青石夕线石榴黑云片麻岩条带或透镜体产于石榴石花岗岩和夕线石榴片麻岩中.基于[K]FMAS[H]系统以及岩石成因格子,认为超高温变质岩经历了三个阶段的变质:早期变质作用阶段,以同一石榴石颗粒中含有夕线石、尖晶石、石英等单相矿物包体为特征,表明变质作用进入到尖晶石 石英组合稳定域;峰期变质作用阶段,以尖晶石 石英、假蓝宝石 石英、斜方辉石 夕线石 石英三种超高温矿物组合为特征,表明峰期变质条件稳定在这三种矿物组合稳定域,指示变质温度高于1000℃;退变质阶段过程中,粗颗粒斜方辉石边部和核部Al含量的重新平衡,指示温度降低到950～970℃左右,表明了近等压冷却的退变质作用,随后由于快速抬升,发生了近等温降压的退变质作用,这个过程以各种反应边和后成合晶结构为标志,例如尖晶石与石英、假蓝宝石与石英之间有堇青石的反应边,石榴石外围有堇青石和斜方辉石的后成合晶.最后黑云母和斜方辉石分解为黑云母标志着高级变质作用的结束.内蒙古土贵乌拉地区超高温变质岩经历了逆时针的P-T演化特点. 相似文献
11.
Interpretation of reaction microstructures may provide constraints on the P–T path followed by rocks, with implications for the geodynamic evolution. Sapphirine generally occurs in diverse microstructures in ultrahigh-temperature (UHT) Mg–Al-rich granulites. Understanding multi-stage sapphirine formation processes and the resultant P–T path may provide insights into the cause of UHT metamorphism, which is otherwise under broad debate. Here, we investigate samples of UHT granulite containing two distinct types of sapphirine from the Dongpo locality in the Khondalite Belt, North China Craton, with the aim of understanding the processes of sapphirine formation and the metamorphic evolution of the host rocks. Petrographic observations show that early sapphirine, which occurs as coronas on spinel and as single porphyroblasts, formed together with biotite, sillimanite, and inclusion-rich garnet. Late symplectitic sapphirine along with fine-grained plagioclase and spinel plus plagioclase symplectites, formed by consumption of sillimanite, biotite, and garnet. Three pseudosections based on the bulk compositions of microdomains inferred to reflect spatially restricted equilibrium suggest that the rocks record near isobaric cooling (IBC) from ~980 to 830ºC at ~0.9 GPa for early sapphirine formation, and decompression and heating to ≤0.7 GPa and ~900ºC for late sapphirine formation. Our study in combination with other metamorphic P–T and age information reveals the common occurrence of IBC paths and long duration (c. 1.93 to 1.86 Ga) regional UHT metamorphism in the Khondalite Belt, North China Craton. Locally, this is followed by decompression–heating paths at c. 1.86 Ga. The Palaeoproterozoic UHT metamorphism with long-lived IBC path in the Khondalite Belt, North China Craton supports large hot orogen model in the amalgamation of this part in the supercontinent Nuna. 相似文献
12.
Ultrahigh‐temperature metamorphism in the Tuguiwula area,Khondalite Belt,North China Craton 
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下载免费PDF全文 In this study, sapphirine‐bearing granulites and sapphirine‐absent garnet–sillimanite gneisses from the Tuguiwula area in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to show a P–T evolution involving cooling at pressures of 8–9 kbar from >960°C to the solidus (~820°C) and late subsolidus decompression. This interpretation is based on the sequence of mineral appearance and thermodynamic modelling of phase equilibria. Sapphirine is observed to coexist with spinel within the peak assemblages. This observation conflicts with the traditional view that spinel generally appears prior to sapphirine and thus indicates pre‐Tmax compression. For ultrahigh‐temperature (UHT) metapelites at Tuguiwula, a clockwise P–T path may be more likely, which would be consistent with the clockwise P–T evolution of the extensive “normal” granulites (Tmax <900°C) and UHT granulites at other localities in the eastern segment of the Khondalite Belt. At Tuguiwula, for UHT metapelites with low bulk‐rock Mg/(Mg+FeT), the oxidation state/Fe3+ content is interpreted to be a significant factor in controlling the mineral assemblages. We find that these compositions tend to contain sapphirine under oxidized conditions but spinel (without sapphirine) under reduced conditions. This difference may account for the simultaneous presence of both sapphirine‐bearing UHT granulites and sapphirine‐absent garnet–sillimanite UHT gneisses at Tuguiwula. LA‐ICP‐MS U–Pb dating of metamorphic zircon in the UHT metapelites yields mean 207Pb/206Pb ages of c. 1.92 Ga (two samples), which are interpreted to record the timing of cooling of the UHT rocks to the solidus. The UHT metamorphism is interpreted to have been generated by mantle upwelling and emplacement of mafic magmas within a post‐orogenic setting. 相似文献
13.
Huatian Zhang Jianghai Li Shoujie Liu Wenshan Li M. Santosh Honghao Wang 《地学前缘(英文版)》2012,3(5):603-611
The Khondalite Belt within the Inner Mongolia Suture Zone（IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature（UHT） metamorphism.Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ.Petrological studies and thermodynamic modeling of the spinel+quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions.Spinel occurs in two textural settings:（1） high XZn（Zn/（Mg+FeⅡ+Zn）=0.071-0.232） spinel with perthitic K-feld-spar. sillimanite and quartz in the rock matrix;and（2） low XZn（0.045—0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks:（1） near-isobaric cooling from 975℃to 875℃around 8 kbar.represented by the formation of garnet porphyroblasts from spinel and quartz;and（2）cooling and decompression from 850℃.8 kbar to below 750℃.6.5 kbar,represented by the break-down of garnet.The spinel+quartz assemblage is considered to have been stable at peak metamorphisni.formed through the break-down of cordierite.indicating a near isothermal compression process.Our study confirms the regional extent of UHT metamorphisni within the IMSZ associated with the Paleoproterozoic subduction-collision process. 相似文献
14.
Stability of the assemblage sapphirine + quartz in Mg–Al-rich granulites implies ultrahigh temperature (UHT) condition of metamorphism but their direct contact is rarely preserved in natural rocks. The present study shows contrasting textural relations between sapphirine and quartz in different parts of the same occurrence of a Mg–Al-rich granulite, Eastern Ghats Belt, India. Textural data suggest stabilization of the assemblage sapphirine + quartz with orthopyroxene and cordierite during the metamorphic peak. Thermometric estimates yield temperature exceeding 950 °C for the stability of this assemblage. Most of such sapphirine grains (Spr1) are texturally separated from quartz and cordierite grains by double corona of sillimanite + orthopyroxene that results due to isobaric cooling during the post-peak stage. Sapphirine (Spr2) also forms a symplectic intergrowth with quartz and orthopyroxene at the fringe of coarse orthopyroxene. This textural feature can be explained by the breakdown of (Fe, Mg)-Tschermak components of orthopyroxene during the same isobaric cooling episode from UHT peak condition. The preservation of grain contact of this intergrown sapphirine and quartz can be attributed to a problem in reaction kinetics. In the other mode, sapphirine (Spr3) occurs with quartz with a thin skin of cordierite near a quartz vein. Such texture could result from isothermal decompression of the cooled crust. Alternatively and more possibly, cordierite could form from ingress of CO2–H2O rich fluid during terminal stage of cooling. Finally, sapphirine (Spr4) and quartz show direct contact close to the quartz vein. Direct contact of such sapphirine and quartz represents textural disequilibrium as this particular quartz is introduced as a vein much later than the peak metamorphism but prior to the major foliation-forming deformation. Coarse sapphirine and vein quartz, therefore, accidentally came in contact with each other and persisted metastably. Therefore, though coexistence of sapphirine and quartz is considered to be a strong evidence for ultrahigh temperature condition, care should be taken to decipher their stable coexistence. Different types of textural relations involving this mineral pair could originate in a single rock, probably in different stages of its metamorphic history. 相似文献
15.
假蓝宝石(Sapphirine)的矿物学特征及其在超高温变质作用研究中的应用 总被引：2，自引：1，他引：1
假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。 相似文献
16.
The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and
reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing
the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz,
spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel
crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage,
several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich
(12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene
> spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated
MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression
(up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may
have eventually resulted from exhumation of thickened crust. 相似文献
17.
Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of Xca (grt) isopleths. The Pmax (M2) is distinguished by the Xca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The Tmax metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques.LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga. 相似文献