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1.
The New Caledonia ophiolite(Peridotite Nappe)consists primarily of harzburgites,locally overlain by mafic-ultramafic cumulates,and minor spinel and plagioclase lherzolites.In this study,a comprehensive geochemical data set(major and trace element,Sr-Nd-Pb isotopes)has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures,locally grading into protomylonitic textures.They exhibit a refractory nature,as attested by the notable absence of primary clinopyroxene,very high Fo content of olivine(91-93 mol.%),high Mg#of orthopyroxene(0.91-0.93)and high Cr#of spinel(0.44-0.71).The harzburgites are characterised by remarkably low REE concentrations(<0.1 chondritic values)and display"U-shaped"profiles,with steeply sloping HREE(DyN/YbN=0.07-0.16)and fractionated LREE-MREE segments(LaN/SmN=2.1-8.3),in the range of modern fore-arc peridotites.Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions(15%fractional melting),followed by hydrous melting in a subduction zone setting(up to 15%-18%).However,melting models fail to explain the enrichments observed for some FME(i.e.Ba,Sr,Pb),LREE-MREE and Zr-Hf.These enrichments,coupled with the frequent occurrence of thin,undeformed films of Al2 O3,and CaO-poor orthopyroxene(Al2O3=0.88-1.53 wt.%,CaO=0.31-0.56 wt.%)and clinopyroxene with low Na2 O(0.03-0.16 wt.%),Al2 O3(0.66-1.35 wt.%)and TiO2(0.04-0.10 wt.%)contents,point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subductionrelated melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic(-0.80<εNdi≤+13.32)and negatively correlate with Sr isotopes(0.70257≤87Sr/86Sr≤0.70770).Pb isotopes cover a wide range,trending from DMM toward enriched,sediment-like,compositions.We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.  相似文献   

2.
This Special Issue is in celebration of the career contribution of Ron Vernon. It arises from the ‘Ron Vernon Symposium’, which was held as part of the 15th Australian Geological Convention in Sydney in 2000. To date Ron has published more than one hundred scientific papers, several books and contributed widely to international conferences. He is the doyen in the multidisciplinary field of microstructures, which spans petrology and structural geology. However, Ron's range is phenomenal, and he has contributed to many aspects of metamorphism, magmatism and rock deformation. He has drawn on data from his work in these areas to contribute widely to understanding thermal regimes, crustal evolution and tectonics, as exemplified by his ideas about HTLP metamorphism and emplacement of granites. Ron has followed the scientific method in his work; it has led him to many innovative interpretations, some of which undermined established dogma. By example, Ron has been a positive role model for the younger generation. However, for the older generation, he has been something of a bête noire! The papers in this Special Issue cover the range from low‐ to high‐grade metamorphism, from metasomatism to melting, from microstructures to tectonics, and from fieldwork to experiments. It is our hope that in reading them you will take time to think about what has been achieved in the past 40 years, and use this as motivation for what remains to be done.  相似文献   

3.
ABSTRACT

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.  相似文献   

4.
蓝闪石+硬柱石是羌塘中北部红脊山地区首次发现的呈北西—南东走向展布的硬柱蓝闪片岩带中典型的低温高压变质矿物组合。蓝闪石呈浅蓝灰色,纤柱状;硬柱石无色,多呈团块状,较均匀地与蓝闪石混杂分布。电子探针分析表明,蓝闪石主要为铁蓝闪石和青铝闪石,硬柱石为较典型的硬柱石。硬柱蓝闪片岩呈(透镜状)构造岩块分布,与围岩呈断层接触,原岩主要为变质玄武岩及少量辉(长)绿岩、基性火山碎屑岩,围岩为变质砂岩、板岩和少量碳酸盐岩。红脊山硬柱蓝闪片岩带伴有构造混杂岩发育,二者共存构成红脊山地区北羌塘甜水河陆块与南羌塘查多岗日陆块之间的分界线。  相似文献   

5.
蓝闪石 硬柱石是羌塘中北部红脊山地区首次发现的呈北西-南东走向展布的硬柱蓝闪片岩带中典型的低温高压变质矿物组合.蓝闪石呈浅蓝灰色,纤柱状;硬柱石无色,多呈团块状,较均匀地与蓝闪石混杂分布.电子探针分析表明,蓝闪石主要为铁蓝闪石和青铝闪石,硬柱石为较典型的硬柱石.硬柱蓝闪片岩呈(透镜状)构造岩块分布,与围岩呈断层接触,原岩主要为变质玄武岩及少量辉(长)绿岩、基性火山碎屑岩,围岩为变质砂岩、板岩和少量碳酸盐岩.红脊山硬柱蓝闪片岩带伴有构造混杂岩发育,二者共存构成红脊山地区北羌塘甜水河陆块与南羌塘查多岗日陆块之间的分界线.  相似文献   

6.
刘敦一  简平 《地质学报》2004,78(2):211-217
应用北京SHRIMPⅡ,精确地测定了大别山双河硬玉石英岩中超高压变质锆石的中心、幔和壳的年龄。大部分锆石中心和幔的年龄集中于239~255Ma之间,加权平均值为243±1Ma(n=23)。壳和含矿物包体的晶域年龄集中在225~234Ma,加权平均值为228±2Ma(n=8)。这些锆石的中心、幔和壳都具有极低的Th/U(0.01~0.09)和REE总量,以及重REE平坦型的配分模式。因此,以锆石内部结构、矿物包体、Th/U值和REE地球化学特征为依据,本文提出了大别山超高压变质作用发生于243±1Ma,退变质作用发生于228±2Ma的新观点。  相似文献   

7.
原特提斯的消减极性:西昆仑128公里岩体的启示   总被引:5,自引:2,他引:5  
128公里岩体是西昆仑造山带中一个早古生代的花岗闪长岩体,长期以来一直是研究西昆仑构造演化的重要参考依据。然而由于区域地质资料的不足和研究手段的不同,对该岩体的形成年代、源区性质以及构造背景等方面还存在着不同的认识。本文试图通过地质年代学和地球化学方面的研究,明确128公里岩体的成岩时代和构造背景,进而制约西昆仑的早古生代构造演化。单颗粒锆石的U-Pb定年结果表明128公里岩体形成于471±5 Ma并含有可能形成于早期岩浆房或继承自源区的490 Ma左右的锆石。128公里岩体富Al_2O_3(15.7%~18.4%),Sr(470~864μg/g)和大离子亲石元素但相对亏损 高场强元素,相对富集轻稀土且具有低到中等的负铕异常(δEu=~0.7),显示出典型的Ⅰ型弧花岗岩特征。尽管其富集Al_2O_3、Sr、相对低的MgO含量和Y/Yb比值使其非常类似于埃达克岩,但其相对高的Yb(1.92~2.88μg/g)、Y(19.4~34.0μg/g)含量,低的Sr/Y(24.2~37.0)和Zr/Sm(7.3~21)比值以及相对高的初始Sr同位素组成(0.7075~0.7091)排除了消减板块在石榴石稳定区发生部分熔融的可能性。低的氧同位素组成(+5.7‰~+7.4‰)以及Sr-O同位素关系表明该岩体并非形成于地幔来源的岩浆与变质围岩间的同化混染。高的稀土含量、明显的稀土分馏以及相对高的Sr同  相似文献   

8.
俯冲带地震诱发机制:研究进展综述   总被引:4,自引:0,他引:4  
邵同宾  嵇少丞 《地质论评》2015,61(2):245-268
俯冲带作为地球循环体系的关键部位,具有构造活跃、地震多发以及地质条件复杂等特征。基于震源位置,俯冲带地震既可划分为板间和板内地震,也可分为浅源、中源和深源地震。俯冲带内的浅源地震包括板间地震和浅源板内地震,而中源和深源地震皆属于板内地震。在地球浅部,温度与压力低,浅源地震是由岩石发生脆性破裂或沿着先存断层发生不稳定摩擦滑移造成的。随着深度增加,温度和压力的增加使得流行于浅部的脆性和摩擦行为在无水条件下被强烈抑制,岩石从而表现为可抑制地震的韧性行为,使得中-深源地震的诱发机制有别于常规的脆性行为。随着研究的逐渐深入,人们了解到中源地震的诱发机制主要是脱水或与流体相关的致脆以及塑性剪切失稳,而深源地震的成因主要是相变致裂。然而,中-深源地震很可能是两种或两种以上机制共同作用的结果。例如,在中源深度既可能是流体相关的致脆导致脱水源区的脆性围岩产生地震,亦可能是脱水的蛇纹岩本身可能在流体孔隙压的作用下作粘滑滑移,而前者比后者更为重要。孕震带宽度大于"反裂隙模型"预测的亚稳态橄榄石冷核宽度的深源地震可能是由第一阶段的相变致裂和第二阶段的塑性剪切失稳诱发,而孕震带的实际宽度与预测宽度相当的深源地震则可能仅由相变致裂引起。只要过渡带内名义无水矿物中的结构水能释放出来,脱水致脆同样可能触发一些深源地震;而塑性剪切失稳不仅能在中-深源地震触发后的扩展阶段起着主导作用,而且还能单独触发一些中-深源地震,因此能够解释大多数反复发生的中-深源地震活动。  相似文献   

9.
Abstract. In the Kamuikotan zone, central Hokkaido, Japan, two distinct types of metamorphic rocks are tectonically mixed up, along with a great quantity of ultramafic rocks; one type consists of high-pressure metamorphic rocks, and the other of low-pressure ones. The high-pressure metamorphic rocks are divided into two categories. (1) Prograde greenschist to glaucophaneschist facies rocks derived from mudstone, sandstone, limestone, a variety of basic rocks such as pillow and massive lavas, hyaloclastite and tuff, and radiolarian (Valanginian to Hauterivian) chert, among which the basic rocks and the chert, and occasionally the sandstone, occur as incoherent blocks (or inclusions) enveloped by mudstone. (2) Retrograde amphibolites with minor metachert and glaucophane-calcite rock, which are tectonic (or exotic) blocks enclosed within prograde mudstone or serpentinite, or separated from these prograde rocks by faults. The K-Ar ages of the prograde metamorphic rocks (72, 107 and 116 Ma on phengitic muscovites) are younger than those of the retrograde rocks (109, 132, 135 and 145 Ma on muscovites, and 120 Ma on hornblende). The low-pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian (Tithonian) chert with the metamorphic grade ranging from the zeolite facies, through the greenschist (partly, actinolite-calcic plagioclase) facies to the amphibolite (partly, hornblende-granulite) facies. The low-pressure metamorphism has a number of similarities with that described for'ocean-floor'metamorphism. The tectonic evolution of such a mixed-up zone is discussed in relation to Mesozoic plate motion.  相似文献   

10.
11.
The Inner Carpathians comprise several distinct Neogene late-stage orogenic Pb–Zn–Cu–Ag–Au ore districts. The mineral deposits in these districts are closely related to volcanic and subvolcanic rocks, and represent mainly porphyry and epithermal vein deposits, which formed within short periods of time in each district. Here, we discuss possible geodynamic and structural controls that suggest why some of the Neogene volcanic districts within the Carpathians comprise abundant mineralization, while others are barren. The Neogene period has been characterized by an overall geodynamic regime of subduction, where primary roll-back of the subducted slab and secondary phenomena, like slab break-off and the development of slab windows, could have contributed to the evolution, location and type of volcanic activity. Structural features developing in the overlying lithosphere and visible in the Carpathian crust, such as transtensional wrench corridors, block rotation and relay structures due to extrusion tectonics, have probably acted in focusing hydrothermal activity. As a result of particular events in the geodynamic evolution and the development of specific structural features, mineralization formed during fluid channelling within transtensional wrench settings and during periods of extension related to block rotation.In the Slovakian ore district of the Western Carpathians, Neogene volcanism and associated mineralization were localized by sinistral, NE-trending wrench corridors, which formed part of the extruding Alcapa block. The Baia Mare ore district, in the Eastern Carpathians, reflects a transtensional wrench setting on distributed oversteps close to the termination of the Dragos Voda fault. There, mineralization was spatially controlled by the transtensional Dragos Voda master fault and associated cross-fault systems. The Golden Quadrangle Cu–Au ore district of the Southern Apuseni Mountains reflects an unusual rotated transtensional/extensional setting close to the termination of a graben system. There, fluid flow was probably localized by fault propagation at the inner tip of the graben system.The spatial and temporal evolution of the magmatism and its changing geochemical signature from (N)W to (S)E strongly suggests a link with the contemporaneous northeastward roll-back of the subducted slab and a progressive southeastward detachment during accelerating roll-back. This geodynamic evolution is further supported by the present-day overall and detailed mantle lithospheric density images, the present-day heat flow patterns, the crustal architecture and its interpreted evolution, and the spatial and temporal evolution of depocentres around the Carpathian arc. In contrast to all these features, the mineral deposits in the West Carpathians, East Carpathians and Apuseni Mountains are too synchronous with respect to their individual volcanic history and contrast too much with younger volcanics of similar style, but barren, in southeastern parts of the Carpathians to simply link them directly to the slab evolution. In all three districts, the presence of magmatic fluids released from shallow plutons and their mixing with meteoric water were critical for mineralization, requiring transtensional or extensional local regimes at the time of mineralization, possibly following initial compressional regimes.These three systems show that mineralization was probably controlled by the superposition of favourable mantle lithospheric conditions and partly independent, evolving upper crustal deformation conditions.In the 13 to 11 Ma period the dominant mineralization formed all across the Carpathians, and was superimposed on structurally favourable crustal areas with, at that time, volcanic–hydrothermal activity. The period may reflect the moment when the (upper part of the) crust failed under lithospheric extension imposed by the slab evolution. This crustal failure would have fragmented the overriding plate, possibly breaking up the thermal lid, to provoke intensive fluid flow in specific areas, and allowed subsequent accelerated tectonic development, block rotation and extrusion of a “family of sub-blocks” that are arbitrarily regarded as the Tisia–Dacia or Alcapa blocks, even though they have lost their internal entity.  相似文献   

12.
Centimetre‐ to decimetre‐wide quartz+calcite veins in schistes lustrés from Alpine Corsica were formed during exhumation at 30–40 Ma following blueschist facies metamorphism. The δ18O and δ13C values of the veins overlap those of the host schistes lustrés, and the δ18O values of the veins are much higher than those of other rocks on Corsica. These data suggest that the vein‐forming fluids were derived from the schistes lustrés. Fluids were probably generated by reactions that broke down carpholite, lawsonite, chlorite and white mica at 300–350 °C during decompression between c. 1400 and 800 MPa. However, the δ18O values of the veins are locally several per mil higher than expected given those of their host rocks. The magnitude of oxygen isotope disequilibrium between the veins and the host rock is inversely proportional to the δ18O value of the host rock. Additionally, calcite in some schists is in isotopic equilibrium with calcite in adjacent veins, but not with the silicate fraction of the schists. Locally, the schists are calcite bearing only within 1–20 cm of the veins. The vein‐forming fluids may have been preferentially derived from calcite‐bearing, high‐δ18O rocks that are common within the schistes lustrés and that locally contain abundant (>15%) veins. If the fluids were unable to completely isotopically equilibrate with the rocks, due to relatively rapid flow at moderate temperatures or being confined to fractures, they could form veins with higher δ18O values than those of the surrounding rocks. Alteration of the host rocks was probably inhibited by isolation of the fluid in ‘quartz‐armoured’ veins. Overall, the veins represent a metre‐ to hectometre‐scale fluid‐flow system confined to within the schistes lustrés unit, with little input from external sources. This fluid‐flow system is one of several that operated in the western Alps during exhumation following high‐pressure metamorphism.  相似文献   

13.
The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km?1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.  相似文献   

14.
The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar.The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening.Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone.However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.  相似文献   

15.
海南岛东部黄竹岭地区出露一套中浅变质岩系,其中夹变质火山岩。变质火山岩中存在2组不同成因的锆石,获颗粒锆石U-Pb一致曲线上交点年龄为527Ma±48Ma,可解释为火山岩生成年龄的下限,下交点年龄为230Ma±13Ma,为岩石后期经受变质热事件的年龄。据上述颗粒锆石U-Pb年龄,结合区域地质背景分析,认为变质火山岩的形成年龄介于498.9~(527±48)Ma之间,该套地层形成时代为早古生代。  相似文献   

16.
Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.  相似文献   

17.
邸英龙  曾令森  陈晶  高利娥  张立飞 《岩石学报》2021,37(11):3435-3444

方柱石是一种含挥发分的架状铝硅酸盐矿物,其成因一直有争议。对喜马拉雅造山带定结地区基性麻粒岩中方柱石及其出溶物的矿物化学和纳米结构分析表明:(1)方柱石为富S类型;(2)内部出溶矿物为陨硫铁,晶胞参数为a=b=0.5968nm,c=1.174nm,α=β=90°,γ=120°;(3)陨硫铁的(001)面平行于方柱石的(100)面。出溶前的方柱石在榴辉岩相条件下形成,其稳定压力上限与Fe含量呈正相关关系。在麻粒岩相作用时,无水条件下方柱石内的SO42-发生自氧化还原反应而产生出溶矿物和O2,反应方程式为:Fe2++SO42-=FeS+2O2。类似反应可能导致下地壳氧逸度高于中上地壳。方柱石的形成与分解记录了喜马拉雅造山带的形成过程,在大陆俯冲与折返过程中扮演重要角色,对地壳深部挥发分的调节起到了重要作用,并对地球深部氧逸度调节机制的研究开拓了新的思路。

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18.
The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H2O+CO2 fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H2O and 6.3 wt.% CO2 bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average Xgrs=0.52, Xpy=0.19) and the jadeite content of omphacite (Xjd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO2-rich (XCO2/XH2O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H2O (XCO2/XH2O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH2O1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H2O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.  相似文献   

19.
Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent-Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic-calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI+ Fe3++ Ti). These variations imply that structurally high rocks belong to a lower-pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.
Amphibole composition is dependent not only on structural position (i.e. P-T ), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe2++ Mn) and Fe3+/Fe2+. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.
Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.  相似文献   

20.
Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). 40Ar/39Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-1. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The εNd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (εNd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.  相似文献   

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