Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and Pressure   总被引：7，自引：0，他引：7  

J. ZHANG  H. W. GREEN 《Journal of Metamorphic Geology》2007,25(2):97-115

Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027–1427 °C, confining pressures of 2.5–3.5 GPa, and strain rates of 1 × 10^?5 s^?1 to 5 × 10^?4 s^?1. We obtained a power‐law creep for the high temperature and pressure deformation of a ‘dry’ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 10^{3.3 ± 1.0}, n = 3.5 ± 0.4, ΔE =403 ± 30 KJ mol^?1 and ΔV = 27.2 cm³ mol^?1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back‐scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear‐strain conditions, yielding a random LPO similar to that of non‐deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.  相似文献   

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea     

B. D. MONTELEONE  S. L. BALDWIN  L. E. WEBB  P. G. FITZGERALD  M. GROVE  A. K. SCHMITT 《Journal of Metamorphic Geology》2007,25(2):245-265

The D'Entrecasteaux Islands of south‐eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U–Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene–Pliocene (8–2 Ma) eclogite formation on these islands. Textural relationships and zircon–garnet rare earth element partition coefficients indicate that U–Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present‐day exposure of Late Miocene–Pliocene eclogites requires exhumation rates > 2.5 cm yr^?1.  相似文献   

Mineral oxygen isotope and hydroxyl content changes in ultrahigh-pressure eclogite–gneiss contacts from Chinese Continental Scientific Drilling Project cores     

Z-F. ZHAO  B. CHEN  Y-F. ZHENG  R-X. CHEN  & Y-B. WU 《Journal of Metamorphic Geology》2007,25(2):165-186

A combined study of mineral O isotopes and hydroxyl contents was carried out for the contacts between ultrahigh‐pressure eclogite and gneiss from main hole of the Chinese Continental Scientific Drilling Project in the Sulu orogen. While there is a large δ¹⁸O variation from ?8.3 to 7.3‰ for all minerals, different styles of mineral‐pair fractionation occur at the boundaries of different lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some samples of eclogite. This suggests that both eclogite and gneiss acquired their negative δ¹⁸O values by meteoric‐hydrothermal alteration of their protoliths at high temperatures before subduction, and that fluid‐assisted O isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite‐facies retrogression. Fourier Transform Infrared Spectroscopy analysis yielded H₂O concentrations of 50 to 1144 p.p.m. (by weight) for garnet and 139 to 751 p.p.m. for omphacite. The state of equilibrium or disequilibrium O isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales, indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation. The retrograde metamorphism results in mineral reactions and O isotope disequilibria between some of the minerals, but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the O isotope composition. A quantitative estimate suggests that a hand specimen (3 × 6 × 9 cm) of eclogite composed of 70% garnet and 30% omphacite can release 0.316 g water by the decompression exsolution of structural hydroxyl, which can form 14.4 g amphibole during exhumation. This provides sufficient amounts of water for the amphibolite‐facies retrogression.  相似文献   

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle   总被引：9，自引：0，他引：9  

S. G. SONG  L. F. ZHANG  Y. NIU  C. J. WEI  J. G. LIOU  G. M. SHU 《Journal of Metamorphic Geology》2007,25(5):547-563

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.  相似文献   

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China   总被引：1，自引：0，他引：1  

Y.-C. LIU  S.-G. LI  X.-F. GU  S.-T. XU  G.-B. CHEN 《Journal of Metamorphic Geology》2007,25(9):975-989

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.  相似文献   

The geology of the Morro Velho gold deposit in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil     

Diogenes Scipioni Vial  Ed DeWitt  Lydia Maria Lobato  Charles H. Thorman 《Ore Geology Reviews》2007,32(3-4):511

The Morro Velho gold deposit, Quadrilátero Ferrífero region, Minas Gerais, Brazil, is hosted by rocks at the base of the Archean Rio das Velhas greenstone belt. The deposit occurs within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites. Considering the temporal and spatial association of the deposit with the Rio das Velhas orogeny, and location in close proximity to a major NNW-trending fault zone, it can be classified as an orogenic gold deposit. Hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates, sulfides, chlorite, white mica±biotite, albite and quartz, as described in other Archean lode-type gold ores. Two types of ore occur in the deposit: dark gray quartz veins and sulfide-rich gold orebodies. The sulfide-rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies. The sulfide assemblage comprises (by volume), on average, 74% pyrrhotite, 17% arsenopyrite, 8% pyrite and 1% chalcopyrite. The orebodies have a long axis parallel to the local stretching lineation, with continuity down the plunge of fold axis for at least 4.8 km. The group of rocks hosting the Morro Velho gold mineralization is locally referred to as lapa seca. These were isoclinally folded and metamorphosed prior to gold mineralization. The lapa seca and the orebodies it hosts are distributed in five main tight folds related to F1 (the best examples are the X, Main and South orebodies, in level 25), which are disrupted by NE- to E-striking shear zones. Textural features indicate that the sulfide mineralization postdated regional peak metamorphism, and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed. Lead isotope ratios indicate a model age of 2.82 ± 0.05 Ga for both sulfide and gold mineralization. The lapa seca are interpreted as the results of a pre-gold alteration process and may be divided into carbonatic, micaceous and quartzose types. The carbonatic lapa seca is subdivided into gray and brown subtypes. Non-mineralized, gray carbonatic lapa seca forms the hanging wall to the orebodies, and is interpreted as the product of extreme CO₂ metasomatism during hydrothermal alteration. This dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite. The brown carbonatic and micaceous lapa secas are the host rocks to gold. These units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and/or volcanic rocks of varying composition from dacitic to andesitic, forming various types of schists and phyllites. The high-grade, massive sulfide orebodies occur at the base of the gray carbonatic lapa seca. Both disseminated mineralization and quartz veins are hosted by micaceous lapa seca. The data are consistent with a model of epigenetic mineralization for the lapa seca, from a hydrothermal fluid derived in part from the Archean basement or older crust material.  相似文献   

对山东高压-超高压变质带的几点看法——程裕淇院士野外考察记     

徐惠芬 《山东国土资源》1998,(4)

通过对山东高压超高压变质带的考察,初步认为：榴辉岩的最后隆升时代为燕山期;含硬玉石英岩＋榴辉岩＋（透辉石）大理岩组合说明表壳岩经历了高压超高压变质作用;该带晋宁期长英质片麻岩花岗岩广泛出露;在其北部麻粒岩和榴辉岩密切伴生。  相似文献   

榴辉岩中绿辉石的测定方法研究   总被引：3，自引：0，他引：3  

黄宝贵  曹文涛 《岩矿测试》1998,17(4):249-253

通过对某地榴辉岩中伴生的有用矿物的纯矿物物质组成的研究,发现Ｎａ可作为其中绿辉石的特征性成分,且Ｎａ２Ｏ含量与其纯度呈正相关。因此,推荐用测定Ｎａ２Ｏ量并以求得的换算系数计算榴辉岩中绿辉石含量的化学物相分析方法。经合成样品验证,方法的相对误差＜１．７％,用于生产样品分析,所得结果的重现性好,平均偏差＜０．５％。  相似文献   

扬子地块西南缘下震旦系火成岩系研究   总被引：9，自引：0，他引：9  

周朝宪  魏春生 《矿物学报》1998,18(4):401-410

研究区的天宝山组和苏雄组、开建桥组酸性、中酸性火山岩、火山碎屑岩同属下震旦系。它们及同期形成的花岗岩形成于后造山构造环境中,并非与裂谷活动有关。它们皆为S型,其源岩为太古界。  相似文献   

Metamorphic veins with kyanite, zoisite and quartz in the Zhu-Jia-Chong eclogite, Dabie Shan, China   总被引：13，自引：0，他引：13  

Daniele Castelli  Franco Rolfo  Roberto Compagnoni  & Shutong Xu 《Island Arc》1998,7(1-2):159-173

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