冀西北石榴角闪二辉麻粒岩早期高压变质矿物组合的确定及其形成的温压条件——来自锆石中矿物包裹体的信息

采用激光拉曼、阴极发光和电子探针技术,确认冀西北石榴角闪二辉麻粒岩锆石中保存早期高压变质包体矿物组合:石榴石(Grt)+单斜辉石(Cpx)+斜长石(Pl)+石英(Qtz)+金红石(Rt)、单斜辉石(Cpx)+斜长石(Pl)+石英(Qtz)和石榴石(Grt)+单斜辉石(Cpx)+斜长石(Pl)+石英(Qtz)。其中少量锆石具有继承性锆石的核,而多数锆石则形成于高压麻粒岩相变质阶段。利用TWQ方法限定石榴角闪二辉麻粒岩锆石中所保存的高压包体矿物组合的变质温度条件为750～820℃,压力为1.07～1.40 GPa。该项研究成果对于如何识别高压麻粒岩以及深入研究其成因机制均具有重要的科学意义。     

大别山北部石榴二辉麻粒岩变质结构和水活度及其演化特征

通过对大别山北部石榴二辉麻粒岩岩相学、矿物地质温压计和热力学计算, 获得4个主要的变质演化阶段的矿物共生组合、温压条件和相应的水活度条件: (1) 榴辉岩相阶段(M₁), 以Cpx (含Jd) +Q +Ru +Gt组合为代表的残留矿物, 并呈包体的形式产于石榴石中, t=612~750℃; (2)麻粒岩相阶段(M₂), 以Opx+Cpx +Gt+Q +Ti+Mt矿物组合为特征, 其相应的t =837~887℃, p=1.03~1.25GPa, 水活度为0.718~0.799; (3) 角闪岩相阶段(M₃), 矿物组合为Cpx +Gt+Amp +Pl+Mt, t=530~660℃, p=0.85~0.95GPa, 其相应的水活度为0.2 3~ 0.2 4;和(4) 低角闪岩相阶段(M₄), 其形成的温压条件为t=495℃, p=0.5 6~ 0.70GPa, 相应的水活度为0.11~ 0.13.石榴二辉麻粒岩变质反应、变质结构、矿物组合及其演化, 不仅受控于形成时的温压条件, 而且与形成时体系中水活度的演化有着密切的成因关系.水活度的演化特征表明, 变质流体在变质作用过程中, 对变质反应温度起着一定的缓冲作用.      

喜马拉雅中段高压麻粒岩变质作用、地球化学与年代学

研究的高压麻粒岩发现于西藏亚东以北约40公里的(Zherger-La)、出露在藏南拆离系(STDS)主构造面下盘的高喜马拉雅结晶岩系中,是继喜马拉雅东西构造结的Nanga Barbat、Namjag Barwa和喜马拉雅中段Khatra & Marina地区、定结地区发现的榴辉岩或高压麻粒岩之后,在青藏高原上新近发现的高压麻粒岩.该麻粒岩呈岩片被包裹于花岗质片麻岩中.麻粒岩记录了两期变质作用,早期矿物组合为Grt+Cpx+Pl+Qz,属麻粒岩相变质产物,矿物成分分析显示早期矿物组合达到了平衡,并且没有表现成分扩散;后期矿物组合为Hbl+Pl+Bio或Opx+Pl,指示了较高温但相对压力较低的麻粒岩相退变变质作用,矿物成分分析和结构显示了退变作用没有达到变质平衡.显微结构可以观察到多组变质反应Grt+Cpx+Qtz=Opx+Pl,Grt+Qtz=Opx+Pl,Grt+Cpx+L=Hbl+Pl+Bio+Mt,和Cpx+L=Hbl+Mt.根据矿物平衡关系,利用Grt-Cpx温度计和Grt-Cpx-Pl-Qz压力计估算的早期变质作用温压为T=780～850℃,P=12～15kbar,相对应的地温梯度16℃～18℃/km.借用Hbl-Pl温度计和A1tot in Hbl压力计估算的晚期变质作用温压为T=730～760℃;P=4～6kbar,相当的地温梯度为38℃～50℃/km.变质作用P-T演化呈等温降压轨迹,指示麻粒岩地体从增厚(或俯冲)地壳到减薄增温(或部分熔融)地壳,进而被快速剥露地表的构造过程.初步的地球化学结果表明高压麻粒岩原岩可能相当于大陆拉斑玄武岩.麻粒岩锆石SHRIMP年代学有两组相对集中的年龄分别为98±5 Ma(5 spots)和17.0±0.3 Ma(13 spots).高压麻粒岩的两期变质作用的温度都在700℃以上,略高于锆石U-Pb同位素体系计时封闭温度,推断17 Ma是高压麻粒岩变质后发生折返,随高喜马拉雅结晶岩系剥露冷却的年龄;98.4Ma的测年结果被推测是高压麻粒岩原岩形成的年龄.在喜马拉雅山,高压麻粒岩记录了类似增厚地壳到减薄地壳的转变一方面可能是地壳深部作用机制的转变,另一方面,这种机制与喜马拉雅南坡巨大的降雨量和去顶作用有密切关系,意义重大.     

东喜马拉雅构造结南迦巴瓦岩群中的石榴辉石岩——印度大陆向欧亚板块之下俯冲至80～100km深度的证据

在东喜马拉雅构造结南迦巴瓦岩群中,石榴辉石岩呈透镜状产于麻粒岩相变质的长英质片麻岩和泥质片岩之中。石榴辉石岩主要由富铁铝榴石的石榴子石和透辉石组成,含少量的金红石、榍石和石英,不含斜长石和角闪石,是榴辉岩相高压变质作用的产物,其原岩相当于基性—超基性层状侵入体中的辉长岩。在高压岩石快速抬升的过程中叠加了麻粒岩相和角闪岩相退变质作用。石榴辉石岩峰期变质作用的温度和压力条件是800～900°C和2.6～2.8GPa,变质时代可能为50Ma。本研究成果,以及超高压变质岩在西喜马拉雅构造结和榴辉岩在珠穆朗玛峰地区的存在,表明整个喜马拉雅造山带,从西构造结到东构造结,都经历了古近纪的高压、超高压变质作用,证明印度板块向欧亚板块之下的俯冲深度至少达到了80～100km。     

喜马拉雅造山带中段亚东地区石榴角闪岩的成因：岩石学和锆石U- Pb年代学

喜马拉雅造山带中段出露的基性麻粒岩是理解印度大陆前喜马拉雅期演化历史和新生代碰撞造山作用的理想研究对象。本文对亚东多庆湖地区的石榴角闪岩进行了岩石学、全岩主微量元素地球化学和锆石U- Pb年代学研究,揭示了其原岩类型和新生代的变质作用过程。石榴角闪岩的原岩很可能为新元古代(~890 Ma)的玄武岩,具有E- MORB型岩石的地球化学特征。石榴角闪岩具有三期矿物组合：① 进变质矿物组合可能为石榴子石+角闪石+斜长石+钛铁矿+石英,即石榴子石核部及其中包裹体;② 峰期矿物组合为石榴子石+角闪石+斜长石+黑云母+石英,即石榴子石边部和基质矿物;③ 退变质矿物组合为角闪石+斜方辉石+斜长石+黑云母+石英,包括退变质域和石榴子石边部的后成合晶矿物。矿物温压计和相平衡模拟表明,石榴角闪岩进变质、峰期和退变质条件分别为609~621℃和0. 59~0. 65 GPa、805~845℃和0. 91~1. 04 GPa、825~840℃和0. 61~0. 68 GPa,经历了峰期高压麻粒岩相的变质作用。锆石U- Pb年代学研究表明,石榴角闪岩的峰期变质时间为34. 8~20. 3 Ma,退变质时间为18. 1~17. 7 Ma,可能经历了一个较长期的部分熔融过程。本文研究认为,亚东石榴角闪岩是印度板块向欧亚板块长期俯冲、地壳增厚成因的基性麻粒岩,原岩可能与Rodinia超大陆拼合相关;其以加热埋藏、近等温降压为特征的顺时针P- T轨迹指示了喜马拉雅造山带中段的大喜马拉雅岩系上部构造层位经历了长期持续的地壳增厚和高温麻粒岩相变质作用,以及早中新世(21~17 Ma)相对快速的减压抬升和随后(17 Ma之后)相对缓慢的折返至地表的演化过程。     

喜马拉雅东构造结高压麻粒岩PT轨迹、锆石U-Pb定年及其地质意义

喜马拉雅东构造结出露了一套基性高压麻粒岩,其峰期矿物组合为石榴石+单斜辉石+石英+金红石+斜长石,利用相平衡计算其峰期温压条件为904℃、1.37GPa,利用锆石U-Pb定年方法确定其变质年龄为20.7±2.3Ma。角闪斜方辉石麻粒岩为其第一阶段退变产物,其变质矿物组合为斜方辉石+角闪石+斜长石+石英+钛铁矿+磁铁矿,温压条件为压力小于0.6GPa,温度为720~760℃。角闪岩相退变矿物组合为角闪石+斜长石+石英+钛铁矿+磁铁矿,温度小于745℃,压力小于0.6GPa。在角闪斜方辉石麻粒岩中变质锆石获得的定年结果为9.38±0.22M,根据锆石中角闪石+斜长石+石英的矿物包体特征,确定该年龄代表角闪岩相退变质年龄。据此,确定了喜马拉雅东构造结基性高压麻粒岩的PTt轨迹为顺时针2阶段折返过程,即第一阶段发生在20Ma左右的由高压麻粒岩相到角闪岩相退变阶段,第二阶段发生在9Ma左右的从角闪岩相深度折返到地表的阶段,计算得到其折返速率分别为2.4mm/y和2.3mm/y,这2个阶段的折返与目前通常认为的青藏高原2个主要抬升阶段是基本一致的。     

喜马拉雅造山带核部的变质作用与部分熔融:亚东地区高压泥质麻粒岩的岩石学与年代学研究

喜马拉雅造山带核部的高喜马拉雅结晶岩系是印度大陆深俯冲到欧亚板块之下经历了高压变质作用的产物,记录了喜马拉雅造山带的形成与演化历史。本文对喜马拉雅造山带中段亚东地区高喜马拉雅结晶岩系中的泥质麻粒岩进行了岩石学和锆石U-Pb年代学研究,结果表明泥质麻粒岩经历了复杂的变质演化和部分熔融,可识别出三期变质矿物组合。早期进变质矿物组合为石榴石+斜长石+钾长石+黑云母+白云母+石英,峰期变质矿物组合为石榴石+斜长石+钾长石+黑云母+蓝晶石+石英,晚期退变质矿物组合为石榴石+斜长石+钾长石+夕线石+黑云母+白云母+石英。相平衡模拟表明,该泥质麻粒岩经历了高温、高压的峰期变质条件为800~835℃和12.8~14kbar,在进变质和峰期变质过程中经历了白云母和黑云母脱水熔融,所形成的熔体量至少为5%~8%。麻粒岩的晚期退变质条件为720~740℃和7.6~8.3kbar。这表明泥质麻粒岩经历了一条以高压麻粒岩相峰期变质和降温、降压退变质为特征的顺时针P-T轨迹。锆石U-Pb定年结果表明,麻粒岩相变质和深熔作用发生在28.5~17.0Ma。本研究表明高喜马拉雅结晶岩系的上部构造层位经历了高压麻粒岩相变质作用,而不是以前认为的以高温、低压变质作用为特征,并为喜马拉雅造山带构造演化的研究提供了新的见解。     

南秦岭勉略构造带高压基性麻粒岩变质作用及其锆石U-Pb年龄

勉县-略阳地区是勉略蛇绿构造混杂岩带的代表区段,本文在勉县北部徐家坪地区确定了主要矿物为Grt+Cpx+Pl和具有典型"白眼圈"反应结构的两类高压基性麻粒岩,分别对其进行细致的岩相学研究,并利用THERMOCALC3.33程序进行P-T视剖面图计算。一类高压基性麻粒岩的峰期矿物组合为Grt1+Cpx+Pl1+Qz,对应温压条件为T=800～860℃,P=12.4～14.6kbar,晚期退变质矿物组合为Grt2+Hbl+Pl2+Qz。另一类是具有典型"白眼圈"反应结构的高压基性麻粒岩,"白眼圈"结构中斜长石为富Na的钠-更长石,以此推断该高压基性麻粒岩早期矿物组合中含绿辉石,因此其变质峰期矿物组合可能为Grt1+Omp(?)+Qz或Grt1+Cpx(?)+Pl+Qz,对应温压条件分别为T=775～900℃,P>19.2kbar和T=750～850℃,P=16.5～19.8kbar;该岩石后期经历了以矿物组合Grt2+Opx+Hbl1+Pl1+Qz为代表的麻粒岩相及以Grt3+Hbl2+Pl2+Qz为代表的角闪岩相两期退变质作用。造成这两种高压基性麻粒岩峰期变质矿物组合及其温压条件存在差异的原因可能是岩石原始成分的不同。对高压基性麻粒岩及其中的浅色脉体分别进行了LA-ICP-MS锆石U-Pb年代学分析,得到高压基性麻粒岩214±11Ma的变质年龄及脉体215±5Ma的结晶时代,并结合锆石微量元素特征分析,认为214±11Ma的年龄值代表该高压基性麻粒岩角闪岩相的退变质时代,同时获得该高压基性麻粒岩原岩形成时代可能为477Ma。综合两件高压基性麻粒岩的P-T演化轨迹及变质时代,建立高压基性麻粒岩的P-T-t演化轨迹,据此反映秦岭造山带在印支期沿勉略构造带发生俯冲-碰撞造山过程。     

北大别饶拔寨石榴辉石岩P-T-t轨迹及其构造意义

P-T-t轨迹作为变质岩的重要研究方法之一，对揭示岩石的构造演化过程具有重要意义。北大别饶拔寨镁铁-超镁铁岩形成的构造环境和就位过程长期以来尚存争议。本文通过岩相学观察、矿物化学研究和温压计计算，揭示出饶拔寨石榴辉石岩经历了四个变质演化阶段：1）超高压变质阶段（M1）：主要根据石榴子石中金红石的出溶，单斜辉石中石英的出溶和磷灰石中不透明矿物的出溶，结合前人研究，认为饶拔寨石榴辉石岩经历过超高压变质阶段（≥2.5GPa）；2）高压麻粒岩相阶段（M2）：矿物组合为石榴子石（变斑晶）和单斜辉石（基质）+斜长石（基质），记录的温压条件为T=648~700℃，P=1.47~1.94GPa；3）中压麻粒岩相阶段（M3）：以石榴子石外围发育的主要由斜方辉石+斜长石组成的内圈"白眼圈"为特征，形成的温压条件为T=781~796℃，P=0.92~0.98GPa；4）角闪岩相阶段（M4）：以石榴子石变斑晶周围发育的外圈"白眼圈"为特征，其矿物组合为角闪石+斜长石的后成合晶，形成的温压条件为T=663~685℃，P=0.50~0.58GPa。石榴辉石岩的锆石SIMS U-Pb定年得到了3组不同的交点年龄，分别为208.1~202.1Ma、227.6Ma和817.7Ma。根据锆石包裹体中角闪石+斜长石的组合，推断208.1~202.1Ma代表了角闪岩相变质年龄的下限，227.6Ma则是高压麻粒岩相的下限年龄，而新元古代年龄（817.7Ma）与区域上的变质岩原岩年龄一致，可能代表了其原岩年龄。结合前人研究，饶拔寨石榴辉石岩记录了顺时针的P-T-t轨迹，揭示了板片俯冲（超高压变质）-碰撞-折返（降压升温过程，~227Ma）-抬升（降压降温过程，208~202Ma）的完整过程。     

西藏定结高压基性麻粒岩(退变榴辉岩)的变质P-T轨迹及构造意义

定结（Dinggye）位于藏南高喜马拉雅结晶岩系的中部,研究该区域麻粒岩的变质P-T轨迹对于理解青藏高原的碰撞和抬升过程至关重要.通过对该地区的高压基性麻粒岩（退变榴辉岩）的岩相学观察,确定了4期矿物组合:（1）峰期榴辉岩相矿物组合（M₁）由石榴子石（核部）+绿辉石（假象）+石英+金红石组成;（2）高压麻粒岩相矿物组合（M₂）主要由石榴子石（幔部）+单斜辉石+斜长石+钛铁矿+角闪石+黑云母组成;（3）中压麻粒岩相矿物组合（M₃）由石榴子石（边缘）+斜方辉石+斜长石+钛铁矿+黑云母组成;（4）角闪岩相矿物组合（M₄）主要由角闪石+斜长石组成.在NCFMASHTO体系下,用THERMOCALC软件对该高压基性麻粒岩进行了热力学模拟.结合传统温压计和平均温压计计算结果,求得M₂、M₃、M₄阶段的温压条件分别为786~826 ℃、0.78~0.96 GPa;798~850 ℃、0.71~0.75 GPa;610~666 ℃、0.51~0.60 GPa,这指示了一条以峰期后近等温降压（ITD）为特征的顺时针P-T轨迹.结合已有地质资料,表明定结高压基性麻粒岩（退变榴辉岩）是喜马拉雅碰撞造山的产物,峰期后经历了近等温降压的构造抬升过程.      

胶北南山口古元古代高压基性麻粒岩和钙硅酸盐岩的岩石地球化学特征探讨

胶北莱西古元古代的高压基性麻粒岩和钙硅酸盐岩的基本矿物组合分别为以铁铝榴石为主的石榴石-普通辉石-铁紫苏辉石和钙铝榴石-黝帘石-葡萄石-钠长石.矿物岩石学研究表明钙硅酸盐岩是由含石榴石高压基性麻粒岩经退变质和钙质交代作用形成.南山口高压基性麻粒岩记录了麻粒岩相变质作用前、麻粒岩相变质作用、退变质和钙硅酸盐岩化共同作用以及完全钙硅酸盐岩化的四个阶段的地质作用,其矿物组合分别为Cpx+ Pl+ Qtz(M1),Grt+ Cpx+ Rt+ Qtz(M2),Cpx+Pl+ Opx+ Ilm+ Mgt+ Ep(M3)和Grs+ Zo+ Prh+ Ab+ Cal(M4).微量元素研究表明,高压基性麻粒岩中大离子亲石元素Ba、Rb、K、Rb、Th富集,而高场强元素Nb、Zr、Ti、Y亏损,具有轻稀土富集的右倾型稀土配分曲线.稀土元素和微量元素配分图解显示了岛孤拉斑玄武岩的特征.主元素、微量元素的构造判别图解进一步分析表明高压基性麻粒岩及其钙硅酸盐岩的原岩形成于大陆边缘的岛弧环境.综合高压基性麻粒岩岩石学、元素地球化学特征认为,莱西高压基性麻粒岩的原岩是拉斑玄武岩质岩石,可能是形成于孤后扩张背景下基性的侵入岩或喷出岩.岩石形成以后,在胶-辽-吉带碰撞闭合过程中,经历了麻粒岩相变质作用,又在后来的抬升过程中经历退变质和钙硅酸盐岩化作用.     

P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south-east Inner Mongolia, China

Abstract The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.
A pre-peak stage of the first metamorphism at T = 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and 'protected'inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T = c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.
The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.     

A Clinopyroxene-Orthopyroxene-Plagioclase Symplectite Formed by Garnet Breakdown in Granulite Facies, Guaxupé, Minas Gerais, Brazil

In mafic granulites, garnet can form by reactions such as Opx + Pl = Cpx + Grt + Qtz; Opx + Pl = Grt + Qtz. As a result of isothermal decompression (ITD), garnet can then break down to a characteristic orthopyroxene-plagioclase symplectite. Mafic, iron-rich garnet-pyroxene granulite from the Guaxupé Massif has symplectite that formed by near-isothermal decompression, as a consequence of uplift of the granulite facies terrane. This symplectite was found to consist of vermicular clinopyroxene-orthopyroxene-plagioclase, with clinopyroxene clearly growing from the garnet that is breaking down, modal amounts of clinopyroxene being less than orthopyroxene. Electron probe analyses show clear differences between core (Cpx₁), rim, and symplectite clinopyroxene (Cpx₂). Considering also the presence of magnetite in the symplectite texture, garnet breakdown is thought to be better represented by a reaction such as Cpx₁ + Grt + O₂ = Cpx₂ + Opx + Pl +Mt + Qtz.     

Metamorphic evolution of the Río Santa Rosa Granulites,northern Sierra de Comechingones,Argentina

Highly anhydrous granulites from Río Santa Rosa in the eastern Sierras Pampeanas of Argentina occur as a thick lens surrounded by melt-depleted migmatites. Grt–Crd granulite composed of Qtz+Pl+Grt+Crd+Ilm±Spl±Ath±Phl is the dominant rock, whereas Opx–Grt granulite appears as discontinuous lenses in the center of the granulite body. Grt–Crd granulite includes blocks of metabasite that are relics of refractory lithologic beds interlayered in the supracrustal sequence. A distinct assemblage composed of Qtz, Pl, Grt, Crd, Opx, Spl, Crn, Sil, Bt, Phl, Ath, and Fe–Ti oxides in different combinations was generated in a reaction zone between Grt–Crd granulites and metabasites at peak metamorphism (850–900 °C and 7.6±0.5 kbar). The P–T trajectory of Grt–Crd granulites suggests an early prograde garnet-forming stage followed by nearly isothermal decompression that caused garnet breakdown. Melting and melt draining accompanying garnet growth was active during heating (to 900 °C) at intermediate pressures (∼7.6 kbar). Peak P–T estimates for Opx–Grt granulites are similar to those obtained with Grt–Crd granulites, which indicates that both granulites passed through the highest thermal stage. These results constrain the late evolution of Opx–Grt granulite to a garnet-consuming stage. Furthermore, they imply that garnet formation in Opx–Grt granulite happened at an early prograde P–T trajectory. Garnet growth in Opx–Grt granulite cannot result from heating at high pressure, which would lead to an apparent contradiction in the prograde P–T paths of the two granulites. This discrepancy may be solved by demonstrating that Opx–Grt granulite is the product of synmetamorphic mafic magmatism that was contaminated while cooling. The Río Santa Rosa granulites are inferred to have formed in a thickened crust in which mafic magmatic activity providing a local heat input.     

Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.     

Metamorphic Characteristics of Granulite Facies Rocks in the Laixi-Pingdu Area in Eastern Shandong Province

Abstract Granulite in eastern Shandong is mainly exposed in Laixi, Pingdu, Changyi and Anqiu, and the diagnostic mineral assemblage is Opx+Cpx+Hb+Pl ± Q ± Sea. The appearance of orthopyroxene and its coexistence with hornblende indicate that the reaction Hb+Q = Opx+Cpx+Pl+H₂O did not proceed completely and therefore these rocks belong to the amphibolite- granulite transition facies, i.e., belonging to hornblende-granulite subfacies. According to the data obtained from such geothermometers and geobarometers as Opx- Cpx, Opx- Hb, Cpx- Hb, Hb- PI, Sca- Pl and Fe- Ti oxides, it has been determined that the temperature of the main metamorphic stage was 720° – 810°C, the pressure 0.5 GPa and fo₂10^?15.5, showing a geothermal gradient of 41–46°C / km, and thus the rocks belong to “low-temperature” and low-pressure granulite facies.     

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

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。     

帕米尔高原沙克达拉穹窿变质作用及新生代下地壳演化

帕米尔高原从西到东展布的8个新生代变质穹窿构成帕米尔高原变质地壳的主体,沙克达拉穹窿是其中最大的一个。沙克达拉穹窿变质杂岩中石榴矽线石片麻岩峰期组合(Grt+Ky+Bi+Rt+Pl+Qz)变质作用温压条件为T约810 ℃/P约10 kbar, 石榴石单斜辉石基性麻粒岩峰期组合(Grt+Cpx+Rt+Pl+Qz)变质作用温压条件为T约824 ℃/P约16.3 kbar, 榴闪岩退变较强,其残留峰期组合(Grt+Pl+Hbl+ilm+Qz)变质作用温压条件为T约683 ℃～873 ℃/P约8.6～11.7 kbar。基性麻粒岩变质锆石的U-Pb年龄为19～35 Ma,反映了从晚始新世到早中新世帕米尔高原下地壳加热加厚过程。帕米尔穹窿的变质作用可以与高喜马拉雅结晶岩系类比,在新生代印度亚洲大陆碰撞过程中,帕米尔陆内各地体沿前新生代缝合带的陆内俯冲可能是帕米尔下地壳加厚的主要动因。     

Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the North Qaidam, Northern Tibet, NW China

Widespread evidence for ultrahigh‐pressure (UHP) metamorphism is reported in the Dulan eclogite‐bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K‐feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO₂ lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near‐isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high‐pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near‐isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub‐belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north‐western China.     

Petrology and metamorphism of khondalites from the Jining complex,North China craton

The basement of the North China craton (NCC) can be divided into eastern and western blocks separating the Trans-North China orogen on the basis of petrologic associations, structures, metamorphic processes, and isotopic ages. Aluminous gneiss khondalites occur in the western block, and record a clockwise metamorphic P–T history characterized by nearly isothermal decompression following peak metamorphism at ca. 1.3 GPa and 825°C. Four metamorphic stages are recognized based on mineral assemblages. The early prograde metamorphic assemblage contains Ky+Bt+Ms+Grt+Pl+Qtz. The peak metamorphic mineral assemblage is characterized by Grt+Sil+Bt+Kfs+Pl+Qtz and the formation of cordierite after garnet, leading to a retrograde assemblage of Grt+Sil+Crd+Pl+Kfs+Qtz. Garnet retrogrades to biotite and the formation of pervasive matrix muscovite define a final metamorphic stage, inferred at ca. < 0.6 GPa and 700°C. Quantified metamorphic stages and a related clockwise P–T path derived from pseudosection analysis in the KMASH system suggest collision of the north Yinshan block with the South Ordos block at 1.92 Ga, before final suturing of the entire NCC basement.