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1.
Pressure-Temperature Path Recorded in the Yangkou Garnet Peridotite, in Su-Lu Ultrahigh-pressure Metamorphic Belt, Eastern China 总被引:2,自引:0,他引:2
Chemical variations along with changes in microstructure ofthe principal constituent minerals make it possible to identifyat least four equilibrium stages in the evolution of the Yangkougarnet peridotite in the Su-Lu ultrahigh-pressure metamorphicbelt, eastern China: Stage Ia primary garnet lherzolitestage represented by coarse-grained (a few millimeters size)porphyroclastic aluminous pyroxenes + chromian spinel ±garnet; Stage IIan ultrahigh-pressure (UHP) stage definedby fine-grained matrix phases (0·10·3 mmsize) of garnet + extremely low-Al orthopyroxene + high-Na clinopyroxene+ chromite; Stage IIIa medium-pressure stage definedby fine-grained mineral aggregates (<0·10·2mm size) mainly composed of aluminous spinel + high-Al orthopyroxenein the matrix; Stage IVan amphibolite- to greenschist-faciesstage defined by poikiloblastic amphibole. Orthopyroxeneclinopyroxenethermometry and an empirical spinel barometer give temperaturesof around 800830°C and pressures of 1·22·9GPa for porphyroclasts of Stage I. Garnetorthopyroxene,garnetclinopyroxene and empirical spinel geothermobarometersgive relatively uniform PT conditions for the matrixgarnetorthopyroxeneclinopyroxenechromiteassemblage of Stage II ( 相似文献
2.
苏鲁芝麻坊超镁铁岩的岩石成因:大陆科学钻探卫星孔(CCSD-PP1)岩心研究 总被引:4,自引:0,他引:4
苏鲁超高压变质带(UHP)中的芝麻坊超基性岩体是一个不寻常的地幔岩块,由高度亏损的橄榄岩组成,因地幔交代作用而富集REE和LILE.岩体由交替出现的石榴石橄榄岩和不含石榴石橄榄岩层组成.2种类型的岩石具有许多共同的成分特征,并在相同的温压p-T条件下形成.岩石记录了数期地质事件,但没有进变质作用的证据.平衡矿物对计算出的p-T条件为6~7GPa.发生在中元古代的部分熔融的差异造成了互层状的2种岩石类型.橄榄岩中锆石220Ma的U—Pb同位素年龄记录了扬子板块和北中国板块碰撞,但此年龄也许并不代表岩石UHP变质峰期的年龄,而可能记录了俯冲带中地幔楔的碎块从深部折返时的年龄.我们认为石榴石可能不是由于俯冲变质作用形成,有可能是岩石从地幔深部上升过程中从斜方辉石中出溶而成。 相似文献
3.
KONZETT JURGEN; MILLER CHRISTINE; ARMSTRONG RICHARD; THONI MARTIN 《Journal of Petrology》2005,46(4):717-747
Olivine-rich rocks containing olivine + orthopyroxene + spinel+ Ca-amphibole ± clinopyroxene ± garnet are presentin the central ÖtztalStubai crystalline basementassociated with eclogites of tholeiitic affinity. These rockscontain centimetre-sized garnet layers and lenses with garnet+ clinopyroxene ± corundum. Protoliths of the olivine-richrocks are thought to be olivine + orthopyroxene + spinel dominatedcumulates generated from an already differentiated Fe-rich () tholeiitic magma that was emplaced into shallowcontinental crust. Protoliths of the garnet-rich rocks are interpretedas layers enriched in plagioclase and spinel intercalated ina cumulate rock sequence that is devoid of, or poor in, plagioclase.UPb sensitive high-resolution ion microprobe dating ofzircons from a garnet layer indicates that emplacement of thecumulates took place no later than 517 ± 7 Myr ago. Aftertheir emplacement, the cumulates were subjected to progressivemetamorphism, reaching eclogite-facies conditions around 800°Cand >2 GPa during a Variscan metamorphic event between 350and 360 Ma. Progressive high-P metamorphism induced breakdownof spinel to form garnet in the olivine-rich rocks and of plagioclase+ spinel to form garnet + clinopyroxene ± corundum inthe garnet layers. Retrogressive metamorphism at T 650680°Cled to the formation of Ca-amphibole, chlorite and talc in theolivine-rich rocks. In the garnet layers, högbomite formedfrom corundum + spinel along with Al-rich spinel, Ca-amphibole,chlorite, aspidolitepreiswerkite, magnetite, ilmeniteand apatite at the interface between olivine-rich rocks andgarnet layers at P < 0·8 GPa. Progressive desiccationof retrogade fluids through crystallization of hydrous phasesled to a local formation of saline brines in the garnet layers.The presence of these brines resulted in a late-stage formationof Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterizedby extremely high Cl contents of up to 3·5 and 6·5wt % Cl, respectively. KEY WORDS: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines 相似文献
4.
Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates? 总被引:3,自引:0,他引:3
Layers of Ca-rich garnetclinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the SuLu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards FeTi gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crustmantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped PT pathsfor the garnetclinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnetclinopyroxene rocks; SuLu terrane; UHP metamorphism 相似文献
5.
Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnoite, Malaita, Solomon Islands 总被引:1,自引:0,他引:1
A varied suite of mantle xenoliths from Malaita, Solomon Islands,was investigated to constrain the evolution of the mantle beneaththe Ontong Java Plateau. Comprehensive petrological and thermobarometricstudies make it possible to identify the dominant processesthat produced the compositional diversity and to reconstructthe lithospheric stratigraphy in the context of a paleogeotherm.PT estimates show that both peridotites and pyroxenitescan be assigned to a shallower or deeper origin, separated bya garnet-poor zone of 10 km between 90 and 100 km. This zoneis dominated by refractory spinel harzburgites (Fo9192),indicating the occurrence of an intra-lithospheric depletedzone. Shallower mantle ( 相似文献
6.
Vapor-Absent Melting of Tonalite at 15-32 kbar 总被引:13,自引:0,他引:13
The behavior of igneous continental crust during subductionis modeled by means of vapor-absent partial melting experimentson a tonalite, containing equal amounts of biotite and hornblende,at pressures of 1532 kbar. The experiments produce leucograniticmelts coexisting with garnet + omphacitic clinopyroxene + K-feldspar+ kyanite + quartz/coesite ± phengite ± zoisite.Experimental constraints and geometrical analysis of phase equilibriashow that the hydrous phases that control dehydration-meltingof tonalites in deep thickened continental crust and in theupper mantle are phengite and zoisite. The negatively slopingamphibole + quartz vapor-absent solidus characteristic of amphibolitesis largely suppressed in tonalites, because amphibole is eliminatedby water-conserving reactions that also consume K-feldspar andkyanite and produce phengite and zoisite. The temperature atwhich melt first appears in the experiments varies from <900°Cat 15 kbar, to 1000°C at 27 kbar, to <925°C at 32kbar. Moderate degrees of partial melting (2030%) yieldresidual assemblages with mantle-like densities but which canstill contain minor amounts of hydrous phases. Partial meltingof tonalitic crust during continental subduction can thus generateincompatible element-rich residues that would be able to remainin the mantle indefinitely, acting as long-term sources of metasomaticfluids. KEY WORDS: mantle; melting; metasomatism; tonalite; UHP metamorphism 相似文献
7.
Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism 总被引:1,自引:0,他引:1
The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone IIIII boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium 相似文献
8.
Discontinuous Melt Extraction and Weak Refertilization of Mantle Peridotites at the Vema Lithospheric Section (Mid-Atlantic Ridge) 总被引:1,自引:0,他引:1
BRUNELLI DANIELE; SEYLER MONIQUE; CIPRIANI ANNA; OTTOLINI LUISA; BONATTI ENRICO 《Journal of Petrology》2006,47(4):745-771
Melting processes beneath the Mid-Atlantic Ridge were studiedin residual mantle peridotites sampled from a lithospheric sectionexposed near the Vema Fracture Zone at 11°N along the Mid-AtlanticRidge. Fractional and dynamic melting models were tested basedon clinopyroxene rare earth element and high field strengthelement data. Pure fractional melting (non-modal) cannot accountfor the observed trends, whereas dynamic melting with criticalmass porosity <0·01 fits better the measured values.Observed microtextures suggest weak refertilization with 0·11%quasi-instantaneous or partially aggregated melts trapped duringpercolation. The composition of the melts is evaluated, togetherwith their provenance, with respect to the garnetspineltransition. Partial melts appear to be aggregated over shortbut variable intervals of the melting column. Deep melts (generatedwithin the garnet stability field at the base of the meltingcolumn) escape detection, being separated from the residuesby transport inside conduits or fractures. The temporal evolutionof the melting process along the exposed section shows a steadyincrease of mantle temperature from 20 Ma to present. KEY WORDS: mantle partial melting; abyssal peridotite; trace element; refertilization; Vema Fracture Zone 相似文献
9.
High-pressure Partial Melting of Mafic Lithologies in the Mantle 总被引:15,自引:2,他引:15
We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnetpyroxenethermal divide, defined by the enstatiteCa-Tschermakspyroxenediopside plane in CaOMgOAl2O3SiO2projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite 相似文献
10.
Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany 总被引:2,自引:2,他引:2
HAASE KARSTEN M.; GOLDSCHMIDT BJORN; GARBE-SCHONBERG C.-DIETER 《Journal of Petrology》2004,45(5):883-905
Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinelgarnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher 206Pb/204Pband 87Sr/86Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 3020 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting 相似文献
11.
Falloon Trevor J.; Green David H.; Danyushevsky Leonid V.; McNeill Andrew W. 《Journal of Petrology》2008,49(4):591-613
We have determined the near-solidus melt compositions for peridotiteMM-3, a suitable composition for the production of mid-oceanridge basalt (MORB) by decompression partial melting, at 1 and1·5 GPa. At 1 GPa the MM-3 composition has a subsolidusplagioclase-bearing spinel lherzolite assemblage, and a solidusat 1270°C. At only 5°C above the solidus, 4% meltis present as a result of almost complete melting of plagioclase.This melting behaviour in plagioclase lherzolite is predictedfrom simple systems and previous experimental work. The persistenceof plagioclase to > 0·8 GPa is strongly dependenton bulk-rock CaO/Na2O and normative plagioclase content in theperidotite. At 1·5 GPa the MM-3 composition has a subsolidusspinel lherzolite assemblage, and a solidus at 1350°C.We have determined a near-solidus melt composition at 2% meltingwithin 10°C of the solidus. Near-solidus melts at both 1and 1·5 GPa are nepheline normative, and have low normativediopside contents; also they have the highest TiO2, Al2O3 andNa2O, and the lowest FeO and Cr2O3 contents compared with higherdegree partial melts. Comparison of these near-solidus meltswith primitive MORB glasses, which lie in the olivine-only fieldof crystallization at low pressure, indicate that petrogeneticmodels involving aggregation of near-fractional melts formedduring melting at pressures of 1·5 GPa or less are unlikelyto be correct. In this study we use an experimental approachthat utilizes sintered oxide mix starting materials and peridotitereaction experiments. We also examine some recent studies usingan alternative approach of melt migration into, and entrapmentwithin melt traps (olivine, diamond, vitreouscarbon) and discuss optimal procedures for this method. KEY WORDS: experimental petrology; mantle melting; near-solidus; fertile peridotite; MORB 相似文献
12.
BERLY THOMAS J.; HERMANN JORG; ARCULUS RICHARD J.; LAPIERRE HENRIETTE 《Journal of Petrology》2006,47(8):1531-1555
Peridotites associated with pyroxenites (with rare olivine andspinel) are exposed on the islands of San Jorge and Santa Isabelin the Solomon Islands. Orthopyroxenite occurs in large outcrops(100 m2) whereas websterite and clinopyroxenite occur as layersand veins/dykes in peridotites. The bulk compositions of thepyroxenites are characterized by high Mg2+/(Mg2+ + Fe2+) (0·780·91)and low Al2O3 (<2·7 wt %). Low rare earth elementabundances are coupled with large ion lithophile element enrichmentsand positive Sr and Pb anomalies (primitive mantle-normalized)relative to adjacent rare earths. Temperatures of equilibrationfor the pyroxenites are between 950 and 1050°C. These relativelylow temperatures, combined with the occurrence of primary fluidinclusions, suggest that the pyroxenites formed by interactionof peridotite protoliths with an aqueous fluid. Bulk-rock andmineral compositions of the orthopyroxenites are similar tothose of mantle-derived pyroxenites, whereas the websteriteshave closer chemical affinity with crustal arc cumulates. Nevertheless,field relationships plus petrological, textural and geochemicalevidence are consistent with formation of all pyroxenite typesin supra-subduction zone mantle, resulting from metasomatismof peridotite by subducted Pacific Plate-derived fluid. Sucha setting for pyroxenite has not previously been reported indetail. We propose that these processes produce mantle pyroxenitewith compositions similar to crustal pyroxenite. KEY WORDS: mantle metasomatism; pyroxenite; supra-subduction zone 相似文献
13.
CORDIER CAROLE; CLEMENT JEAN-PHILIPPE; CAROFF MARTIAL; HEMOND CHRISTOPHE; BLAIS SYLVAIN; COTTEN JOSEPH; BOLLINGER CLAIRE; LAUNEAU PATRICK; GUILLE GERARD 《Journal of Petrology》2005,46(11):2281-2312
This study is based on a set of coarse-grained igneous rockscollected from two zoned plutons located in the central partof Tahiti Nui and Raiatea. The Ahititera pluton (central depressionof Tahiti Nui) comprises a great diversity of rocks, rangingfrom ultrabasic to felsic in composition. It shows a concentriczonation with nepheline-free rocks in its periphery and nepheline-bearingrocks in its central part. The Faaroa pluton (central depressionof Raiatea) is entirely mafic and includes only gabbros andtheralites. The two plutons have variable NdSr isotopicsignatures, especially the Ahititera rocks, which are subdividedinto three groups based on their mineralogy, geochemistry andisotope composition. The isotopic variability probably reflectslocal heterogeneities in the Society mantle plume. Petrographicand isotopic data have been used to define two magmatic suitesin Ahititera, identifiable from their degree of Si undersaturation.The evolution of the mildly Si-undersaturated suite is controlledby simple fractional crystallization, whereas the strongly Si-undersaturatedsuite requires additional H2O influx. The third isotopic groupincludes only theralites. The rare earth element (REE) compositionsof the mafic rocks from both plutons do not correlate with theirisotopic signature. The REE patterns of the most Si-undersaturatedrocks are systematically characterized by steeper slopes. Suchfeatures are also observed in lavas from seamounts located withinthe present-day hotspot area. It appears that REE concentrationsin Society lavas and intrusives are probably mainly governedby variable degrees of partial melting of a garnet-free mantlesource and are independent of their isotopic signature. KEY WORDS: cumulates; fractional crystallization; partial melting; French Polynesia; plutonic rocks; Society Islands; Tahiti; Raiatea 相似文献
14.
Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain) 总被引:4,自引:0,他引:4
Bodinier Jean-Louis; Garrido Carlos J.; Chanefo Ingrid; Bruguier Olivier; Gervilla Fernando 《Journal of Petrology》2008,49(5):999-1025
The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The asthenospherized part of the Ronda peridotitecontains abundant layered websterites (group Cpyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the layeredgranular subdomain and plagioclase-tectonitedomain) are on average more fertile than the residual, coarsegranular subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements 相似文献
15.
Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China 总被引:24,自引:0,他引:24
ZHENG JIANPING; GRIFFIN WILLIAM L.; O'REILLY SUZANNE Y.; YANG JINSUI; LI TIANFU; ZHANG MING; ZHANG RUTH Y.; LIOU JOHN G. 《Journal of Petrology》2006,47(11):2233-2256
Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457500Ma) kimberlites and Neogene (1618 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. PT estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m2 (Paleozoic) and 80 mW/m2 (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al2O3 and TiO2contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during MesozoicCenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassicCretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement 相似文献
16.
Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps) 总被引:1,自引:0,他引:1
Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (XMg =0·83) to rim (XMg = 0·60·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py2327, Alm4143,Grs2933) has two different compositional domains, onerelatively rich in Mg (Py2730) and the other rich inCa (Grs3538) with a low Mg (Py2025) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs4057,Py27, Alm4651). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached PTconditions of 550583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps 相似文献
17.
RIMMELE GAETAN; PARRA TEDDY; GOFFE BRUNO; OBERHANSLI ROLAND; JOLIVET LAURENT; CANDAN OSMAN 《Journal of Petrology》2005,46(3):641-669
The Menderes Massif and the overlying Lycian Nappes occupy anextensive area of SW Turkey where high-pressurelow-temperaturemetamorphic rocks occur. Precise retrograde PT pathsreflecting the tectonic mechanisms responsible for the exhumationof these high-pressurelow-temperature rocks can be constrainedwith multi-equilibrium PT estimates relying on localequilibria. Whereas a simple isothermal decompression is documentedfor the exhumation of high-pressure parageneses from the southernMenderes Massif, various PT paths are observed in theoverlying Karaova Formation of the Lycian Nappes. In the uppermostlevels of this unit, far from the contact with the MenderesMassif, all PT estimates depict cooling decompressionpaths. These high-pressure cooling paths are associated withtop-to-the-NNE movements related to the Akçakaya shearzone, located at the top of the Karaova Formation. This zoneof strain localization is a local intra-nappe contact that wasactive in the early stages of exhumation of the high-pressurerocks. In contrast, at the base of the Karaova Formation, alongthe contact with the Menderes Massif, PT calculationsshow decompressional heating exhumation paths. These paths areassociated with severe deformation characterized by top-to-the-eastshearing related to a major shear zone (the Gerit shear zone)that reflects late exhumation of high-pressure parageneses underwarmer conditions. KEY WORDS: exhumation; high-pressurelow-temperature metamorphism; multi-equilibrium PT estimates; Lycian Nappes; Menderes Massif 相似文献
18.
A mathematical approach is presented for the calculation ofthe major and trace element fractionation that is caused bygrowth of zoned garnet in metamorphic rocks. This approach isbased on textural and compositional parameters directly obtainedfrom natural examples. It takes into account the mode and compositionof all unzoned minerals, as well as the mode, crystal size distributionand zonation patterns of garnet grains of different sizes withina certain rock volume. These parameters can be used to fit functionsfrom which the amount of garnet fractionation at each step ofa garnet growth history can be calculated. The approach is testedfor two compositionally distinct domains within a single garnetbiotitegneiss sample from the Ruhla Crystalline Complex. This samplecontains unusual flat-top garnet grains with Y2O3-rich cores.It is shown that MnO, FeO and Y2O3 are extremely fractionatedduring garnet growth, but in different ways, and that MnO fractionationdoes not obey a Rayleigh function. To demonstrate the influenceof garnet fractionation on PT path estimates, quantitativephase diagrams in the model system Na2OK2OCaOMnOFeOMgOAl2O3TiO2SiO2H2Oare constructed by means of the computer software THERMOCALC.The good agreement between calculated and observed mineral assemblagesand garnet compositions for all fractionation steps indicatesthat the new approach can be used to infer detailed PTpaths, even for rocks that contain complexly zoned garnet grains.The results indicate that garnet growth in the metapelite underinvestigation occurred along a linear PT path from 470°Cand 2·7 kbar to 580°C and 8·5 kbar. The resultsalso show that garnet cores with high Y2O3 contents of about1 wt % nucleated over a temperature interval of c. 90°C,indicating that Y in garnet is relatively insensitive to temperaturechanges. KEY WORDS: garnet; fractionation; pseudosection; yttrium; THERMOCALC 相似文献
19.
Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, FeTi oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSrNdPb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The NdSrPbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotitespinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany 相似文献
20.
On the Initiation of Metamorphic Sulfide Anatexis 总被引:3,自引:0,他引:3
Mineral assemblages in common sulfide ore deposits are examinedtogether with phase relations to (1) investigate the pressuretemperatureconditions required for the onset of metamorphically inducedpartial melting involving economic minerals, and (2) place constraintson the amount of melt produced. Deposits that contain sulfosaltor telluride minerals may start to melt at conditions rangingfrom lowest greenschist facies to amphibolite facies. Depositslacking sulfosalt and/or telluride minerals may begin to meltonce PT conditions reach the upper amphibolite facies,if galena is present, or well into the granulite facies if galenais absent. The result is two broad melting domains: a low- tomedium-temperature, low melt volume domain involving meltingof volumetrically minor sulfosalt and/or telluride minerals;and a high-temperature, potentially higher melt volume domaininvolving partial melting of the major sulfide minerals. Epithermalgold deposits, which are especially rich in sulfosalt minerals,are predicted to commence melting at the lowest temperaturesof all sulfide deposit types. Massive PbZn (Cu)deposits may start to melt in the lower to middle amphibolitefacies if pyrite and arsenopyrite coexist at these conditions,and in the upper amphibolite facies if they do not. Exceptingsulfosalt-bearing occurrences, massive NiCuPGE(platinum group element) deposits will show little to no meltingunder common crustal metamorphic conditions, whereas disseminatedCu deposits are typically incapable of generating melt untilthe granulite facies is reached, when partial melting commencesin bornite-bearing rocks. The volume of polymetallic melt thatcan be generated in most deposit types is therefore largelya function of the abundance of sulfosalt minerals. Even at granulite-faciesconditions, this volume is usually less than 0·5%. Theexception is massive PbZn deposits, where melt volumessignificantly exceeding 0·5 vol. % may be segregatedinto sulfide magma dykes, allowing mobilization over large distances. KEY WORDS: sulfide melt; ore deposits; melt migration; metamorphism 相似文献