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1.
对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。  相似文献   

2.
石超  张泽明 《岩石学报》2007,23(12):3180-3200
中国大陆科学钻探主孔位于江苏东海县,苏鲁超高压变质带的南部.该钻孔的0～2050m深度获取了六种不同类型的榴辉岩和少量石榴石辉石岩岩心,它们是典型的基性超高压变质岩,为研究大陆深俯冲过程中的元素地球化学行为提供了非常好的样品.本文对各种超高压变质矿物的微量元素成分进行了系统的原位微区分析,结合全岩化学成分和矿物主量元素成分,深入地研究了超高压变质岩的微量元素赋存特征、分配规律、控制因素,及其对变质条件和流体.岩石相互作用的限定意义.结果表明,超高压榴辉岩中的LREE和Sr主要赋存在磷灰石、帘石和单斜辉石中,HREE赋存在石榴石中,Ba、Rb和Cs等LILE赋存在多硅白云母中,Ti、Nb和Ta等HFSE主要赋存在金红石、钛铁矿中,V、Sc、Co和Ni等元素大多赋存在石榴石和单斜辉石中.研究表明,全岩化学成分和矿物组成、及其含量的变化明显控制着超高压矿物的微量元素含量和分布形式.本研究也获得了如下重要的认识:超高压变质矿物之间的微量元素分配达到了化学平衡,并具有与地幔榴辉岩矿物之间类似的分配系数,表明榴辉岩的峰期变质温度很可能达到900℃～1000℃.部分高Ti和高Fe-Ti榴辉岩中的石榴石和绿辉石有明显的稀土元素成分环带,表明超高压变质岩经历了快速折返过程.金红石的Zr含量明显受到全岩成分和退变质作用影响,并不仅仅与形成温度有关,不是可靠的温度计.在超临界流体的作用下,榴辉岩中金红石的Nb、Ta发生了明显的分异,导致其Nb/Ta比值增大,由此推测俯冲到地幔深处的大量榴辉岩是地球内部高Nb/Ta比值的物质源区.在榴辉岩的不同程度退变质阶段,参与变质反应的流体具有不同的来源、成分和流体活动规模.  相似文献   

3.
华北地块东南缘中生代侵入杂岩中所含榴辉岩类包体中矿物的分析结果表明, 组成榴辉岩类包体的各矿物之间元素的分配在榴辉岩相变质阶段已经达到了化学平衡;石榴石中Al、Fe、Mn, 单斜辉石中Na、Mg、Ca, 角闪石中Na、Mg、Fe等主量元素含量与其全岩之间均呈良好的正相关, 受全岩成分制约;石榴石明显富集HREE、贫LREE, 单斜辉石相对富集LREE、MREE、贫HREE, 两者轻、重稀土含量呈相互消长的互补关系, 石榴石和单斜辉石中分别富集Y、Sr, Ga和过渡族元素Sc、Ti、V、Cr、Co等含量较高, 其含量受全岩成分控制;金红石中稀土元素的含量很少, Fe、Al、Mg、Ca、Mn、Nb、Ta、Cr和Zr含量相对较高, Rb、Sr、Ba含量极低, 部分微量元素(如Nb、Cr、Fe、V、W等)含量与其原岩有继承和对应关系;部分原生或/和退变角闪石的REE配分模式分别与单斜辉石和石榴石相似, 过渡族金属元素等的含量与石榴石、单斜辉石和全岩具有一定的相关性, 角闪石容纳Na、K、Sr、Rb、Ba等元素的能力较强, 其成分除受全岩成分的制约外, 石榴石、单斜辉石对其形成也有影响。榴辉岩类包体与其寄主中生代侵入杂岩中部分高价态/高场强元素(HFSE)和重稀土元素(HREE)呈相互消长的关系。榴辉岩的形成与扬子地块和华北地块之间的俯冲-碰撞作用有关。  相似文献   

4.
运用激光拉曼和阴极发光技术，配备电子探针测试，发现苏鲁地体地表露头和中国大陆科学钻探工程预先导孔CCSD-PP1和CCSD-PP2斜长角闪岩锆石中均保存以柯石英为代表的典型超高压矿物组合：柯石英 石榴石 绿辉石 金红石；柯石英 石榴石 绿辉石；柯石英 石榴石 绿辉石 多硅白云母 金红石 磷灰石；柯石英 绿辉石 金红石；柯石英 菱镁矿。该类矿物包体组合与苏鲁地体超高压榴辉岩的峰期矿物组合十分相似，表明斜长角闪岩可能是由超高压变质的榴辉岩在构造抬升过程中退变质而成。  相似文献   

5.
北秦岭官坡地区高压—超高压榴辉岩岩相学及变质作用研究   总被引：19，自引：2，他引：19  
北秦岭官坡地区的榴辉岩及含柯石英榴辉岩产在帮岭岩群的北侧，主要由绿辉石和石榴石组成，部分石榴石和绿辉石中含柯石英包体。此外还含有退变质的多硅白云母、角闪石、黝帘石和纳长石等矿物，根据变质矿物之间的替代关系及共生组合规律，榴辉岩退变质作用可划分为四个阶段，各阶段代表性矿物组合依次为：柯石英＋绿辉石＋石榴石；石英＋绿辉石＋石榴石；多硅白云母＋绿辉石＋石榴石＋石英；角闪石＋斜长石＋白云母＋黑云母。这四个  相似文献   

6.
CCSD主孔超高压榴辉岩金红石中的矿物包裹体研究   总被引：6，自引：0，他引：6  
金红石是榴辉岩中的主要含钛副矿物。中国大陆科学钻探工程（CCSD）主孔100～2000m岩心样品中，金红石榴辉岩、多硅白云母榴辉岩和蓝晶石榴辉岩中都程度不等地含有金红石。金红石既可以与其他矿物一起被石榴石、绿辉石等主要变质矿物包裹，也可以包裹其他矿物。本文利用电子探针技术，对CCSD所揭示的超高压榴辉岩的金红石中的矿物包裹体进行了鉴定和分析。结果显示，绿辉石、富铪锆石、高铝榍石、韭闪石和红闪石、斜黝帘石等矿物包裹体形成于榴辉岩相进变质至峰期变质阶段；随着超高压变质带快速折返，榴辉岩经受强烈的退变质作用，包括金红石、绿辉石在内的多种矿物都经受了退变质作用，与金红石共生的钛铁矿完全或者部分退变成含铁金红石和钛铁晶石。在退变的金红石中，还发现了透辉石＋斜长石后成合晶、低铝榍石、镁绿闪石等退变质矿物组合。  相似文献   

7.
对大别山太湖金河桥超高压榴辉岩作了矿物Sm-Nd内部等时线定年研究和激光氧同位素分析。石榴石+绿辉石Sm-Nd等时线给出了较低年龄210±3Ma,石榴石+金红石Sm-Nd等时线给出了较高年龄237±4Ma。岩相学观察发现,绿辉石具有角闪石退变质边。氧同位素分析表明,石榴石与绿辉石之间的氧同位素体系处于不平衡状态。据此,石榴石+绿辉石Sm-Nd同位素体系因退变质作用导致Nd同位素不平衡而给出不合理偏低年龄。较老的石榴石+金红石Sm-Nd年龄有可能指示了榴辉岩相前期阶段的时代,且在温度变质峰期没有使它们之间的Nd同位素再次均一化,它指示Nd在金红石中的扩散速率较慢,可能与石榴石相当。矿物对氧同位素测温得到,石英—石榴石对温度为695±35℃,石英—金红石对为460±15℃,与根据金红石U—Pb内部等时线估计的Pb扩散封闭温度470±50℃一致。对比表明,O在石榴石中的扩散速率与Nd相当或略低,而O和Pb在金红石中的扩散速率相近,且均比Nd快。  相似文献   

8.
侯信高  刘磊 《地质学报》2022,96(11):3834-3848
本文通过对来自大别山中部碧溪岭地区的3块含柯石英榴辉岩中石榴子石、绿辉石以及角闪石进行主量、微量元素分析，研究了榴辉岩中微量元素，特别是高场强元素在峰期变质作用阶段以及随后的角闪岩相退变质作用阶段的分配特征。榴辉岩中代表峰期变质作用的石榴子石与绿辉石颗粒的δEu呈现出相关性，线性拟合斜率为0.75，与前人结论相似，说明微量元素在它们之间的分配达到平衡。绿辉石颗粒中的Na2O含量(6.14%~7.92%)和硬玉组分含量(＞50%)较高，且通过石榴子石 单斜辉石地质温度计得到平均变质温度为T=699℃，表明这些绿辉石属于超高压榴辉岩相原生矿物。在超高压变质作用过程中，Zr (Kd=0.18~0.91)倾向于进入石榴子石，而Hf (Kd=0.60~3.92)相对于Zr更倾向于进入绿辉石。绿辉石中高场强元素(Zr)的含量与Mg、Fe2+含量之和呈正相关，说明Zr在绿辉石中占据八面体M1位置。Zr在绿辉石中的含量与硬玉组分含量呈负相关性，其原因为伴随着硬玉含量降低，绿辉石中M1 O键长缩短从而更适合Zr进入。而中阿尔卑斯地区幔源榴辉岩中绿辉石的硬玉含量较低(28.4%~42.8%)，碧溪岭地区榴辉岩中绿辉石的硬玉含量较高(44%~55%)。这解释了中阿尔卑斯地区幔源榴辉岩捕虏体中Zr倾向于进入绿辉石，而大别山地区榴辉岩中Zr倾向于进入石榴子石的原因，也说明了寄主矿物的化学成分是影响高场强元素分配行为的主要因素。具有明显转变关系的绿辉石与角闪石的微量元素组成表明，在角闪岩相退变质阶段，绿辉石中轻稀土元素与重稀土元素发生明显分异，其中轻稀土元素倾向于进入次生角闪石中，而重稀土元素则倾向于保存在绿辉石中。微量元素在角闪石与绿辉石之间虽然未达到平衡，但Zr、Ba倾向于进入角闪石而Sr倾向于保存在绿辉石中。  相似文献   

9.
常用于测定榴辉岩形成温度的有石榴石-绿辉石Fe-Mg配分温度计和石英-矿物对氧同位素温度计。最近的自然观察和实验测定发现，金红石中的Zr含量与温度之间存在线性关系，因此能够用于变质岩测温。本文首次将这三种温度计用于同一产地榴辉岩及其中的石英脉。对大别造山带黄镇低温超高压榴辉岩中金红石Zr含量的温度计算得到，产于矿物内部金红石Zr含量温度明显地高于粒间金红石Zr含量温度，产于矿物石榴石、绿辉石和黝帘石内部金红石Zr含量温度主要集中在528～589℃之间，而产于粒间金红石的温度主要集中在465～528℃之间。榴辉岩中金红石Zr含量最高的产于石榴石中，但是所计算的温度503～589℃仍然不同程度地低于榴辉岩形成温度670℃。石英脉中金红石Zr含量温度主要集中在465～528℃之间。石英-耐熔矿物对氧同位素温度主要集中在650～695℃之间，表明耐熔矿物石榴石、锆石和蓝晶石在该区榴辉岩中相对其它矿物来说保存最好，退变质作用最弱，因此其氧同位素温度与峰期超高压榴辉岩相变质奈件基本一致。而石英．易熔矿物对温度主要集中在450～510℃之间，与易熔矿物绿辉石、钠云母、斜黝帘石／黝帘石在榴辉岩中蚀变强烈一致，反映了角闪岩相退变质阶段的流体活动。石榴石-单斜辉石Fe-Mg配分温度结果分为三组：795～863℃、629～679℃和468～572℃，其中后两组与金红石Zr含量和石英-矿物对氧同位素测温结果具有可比较性，指示了榴辉岩相变质和角闪岩相退变质过程中的Fe-Mg交换平衡，而第一组温度明显高于已知的榴辉岩相变质温度，表明绿辉石后成合晶导致了部分石榴石与单斜辉石之间的Fe-Mg不平衡。榴辉岩折返过程中的流体活动可能是导致矿物之间元素和同位素扩散交换再平衡或不平衡的基本原因。粒内金红石Zr含量温度仍然不同程度地低于榴辉岩形成温度，可能说明其在进变质过程中形成后相对“孤立”，即使在峰期榴辉岩相条件下也不能与锆石之间达到Zr配分再平衡。粒间金红石Zr含量降低可能与金红石重结晶有关，结果导致它们与锆石之间的Zr配分平衡遭到破坏。  相似文献   

10.
杨经绥 Joliv.  M 《地质学报》2000,74(2):156-168
在青海都兰北东４０ｋｍ野马滩一带元古宙变质杂岩中,新发现走向近东西的榴辉岩;可分为南北两带,北带宽约５ｋｍ,南带宽３～５ｋｍ。北带榴辉岩相矿物组合为石榴石－绿辉石－多硅白云母－金红石,它们的退变质矿物主要为角闪石及少量高钠的斜长石类。而南带峰期变质矿物为石榴石－绿辉石－蓝晶石－金红石－黝帘石（？）,退变质矿物中则出现大量高钠质斜长石。带另一个明显的区别是北带铁铝榴石端元组分含量较南带的略高,而镁铝  相似文献   

11.
Trace element distribution in Central Dabie eclogites   总被引：16，自引：0，他引：16  
Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D REE Cpx/Grt decreasing with decreasing D Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000  相似文献   

12.
Zhang Zeming  Xu Zhiqin  Xu Huifen 《Lithos》2000,52(1-4):35-50
The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.  相似文献   

13.
The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1(1), (b) in the peak eclogite facies [stage M1(2)], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1(1) is constrained to 19–20 kbar and 640–660°C and M1(2) occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.  相似文献   

14.
Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.  相似文献   

15.
对中国大陆科学钻探工程主孔榴辉岩退变质过程中的微量元素地球化学行为进行了研究。对退变质程度连续变化样品的不同部分的对比研究表明，流体作用下的退变质过程中大离子亲石元素（Cs、Rb、Ba、Sr、K、Th、U）和轻稀土元素表现出较大的活动性，重稀土元素和高场强元素变化相对较小。退变质后大离子亲石元素的显著增加和高场强元素、重稀土元素的轻微变化（甚至相对降低），表明与退变质作用有关的流体中的络阴离子含量很少，并不富集高场强元素和重稀土元素。退变质后总体上表现出的Si、大离子亲石元素和轻稀土元素的明显变化，表明外来流体参与了榴辉岩的退变质过程，带入和带出了一些元素。结合榴辉岩中单矿物微量元素组成以及前人对D^Mineral/Fluid的研究成果，对流体-榴辉岩作用形成的退变质分带（富石英条带→角闪岩→退变质榴辉岩→新鲜榴辉岩）的微量元素组成变化进行了详细研究。结果表明在流体作用下的榴辉岩退变质过程中，大离子亲石元素、轻稀土元素和高场强元素含量的变化除了受退变质流体性质的影响外，更大程度上取决于退变质过程中的矿物相（尤其是副矿物）的变化。  相似文献   

16.
Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H2O, and Amp + Ab = Omp ± Czo + Qtz + H2O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H2O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.  相似文献   

17.
The Shirokaya Salma eclogite‐bearing complex is located in the Archean–Palaeoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz–amphibole, orthopyroxene and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages—(1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase+quartz+rutile+ilmenite±orthopyroxene—are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite and granulite facies metamorphism that occurred at 16–18 kbar, 750–800°C and 11–15 kbar, 820–850°C, respectively. The quartz–amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P–T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Palaeoproterozoic high‐grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite facies metamorphism in the Belomorian Province.  相似文献   

18.
吕增  王凯 《地球科学》2018,43(1):150-163
角闪岩是西南天山超高压变质带变基性岩的常见岩石类型之一.野外关系和矿物反应结构表明,大多数角闪岩是由榴辉岩或蓝片岩受到不同程度的钠长绿帘角闪岩相退变质叠加形成的.但对于一些平衡结构发育良好且孤立产出的角闪岩类型（如石榴角闪岩）仍缺乏系统的岩石学研究.本次从岩相学、矿物成分以及热力学模拟几个方面对哈布腾苏河下游地区超高压带内不含钠长石的石榴角闪岩开展了详细的工作.这些石榴角闪岩的主要矿物为绿色角闪石（钙质-钠钙质闪石）、帘石（黝帘石-绿帘石）和石榴石,三者总体积占80%~90%,明显有别于大多数由榴辉岩退变而成的含有钠长石变斑晶的石榴角闪岩.虽然这些角闪岩化学成分十分相近,都具有富钙贫钠和高的Mg/（Mg+Fe）比值,但在结构、构造和矿物组成等方面存在显著差异,据此将它们划分为两类.第一类角闪岩基质中不含石英,保存在变斑晶中的少量残余矿物组合为石榴石+绿辉石+硬柱石+蓝闪石+金红石,指示峰期硬柱石榴辉岩相变质条件,富钛矿物全部为金红石.第二类角闪岩强烈面理化,面理由绿色角闪石、绿帘石和绿泥石以及条带状石英集合体构成.石榴石粒度呈双峰式分布,粗粒比细粒低钙低锰.基质和包体中均未发现高压变质特征矿物绿辉石和蓝闪石.富钛矿物以榍石为主,金红石和钛铁矿仅存在于个别石榴石中.两类角闪岩的石榴石成分具有较大区分度,前者的钙含量较高而镁含量较低.P-T视剖面计算显示它们的峰期条件为480~520 ℃,30~33 kbar,均达到超高压范围,与哈布腾苏河下游及以西地区的榴辉岩相似,表明西南天山超高压变基性岩构成沿中天山南缘断裂延伸数十千米的独立地质单元,不存在所谓的俯冲隧道混杂现象.   相似文献   

19.
The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of 20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.  相似文献   

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