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981.
SATO HIROAKI; HOLTZ FRANCOIS; BEHRENS HARALD; BOTCHARNIKOV ROMAN; NAKADA SETSUYA 《Journal of Petrology》2005,46(2):339-354
High-temperaturepressure experiments were carried outto determine the chlorinehydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 4·6[Mg/(Mg + Fe)]hb at 23kbar and 800850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 19911995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·21·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano 相似文献
982.
NOHDA SUSUMU; KANEOKA ICHIRO; HANYU TAKESHI; XU SHENG; UTO KOZO 《Journal of Petrology》2005,46(3):505-522
We report Sr-, Nd- and Pb-isotopic compositions for the lavasof Mauritius, the second youngest volcanic island in the Réunionhotspot. The lavas of the Older Series (7·85·5Ma) have identical isotopic compositions (87Sr/86Sr = 0·70411to 0·70422,143Nd/144Nd = 0·512865 to 0·512854,and 206Pb/204Pb = 19·016 to 19·041) to those ofRéunion, where the center of volcanic activity is currentlylocated. The lavas of the Intermediate Series (3·51·9Ma) and Younger Series (0·700·17 Ma) areshifted to lower Sr-isotopic compositions (0·703640·70394,with 143Nd/144Nd = 0·512813 to 0·512948 and 206Pb/204Pb= 18·794 to 18·984). The Intermediate Series lavashave similar trace-element characteristics (e.g. ZrNb,BaY) to those of Rodrigues, in both cases requiring theinvolvement of an enriched mantle-like component in the mantlesource. During the volcanic history of Mauritius, the magmaslost the principal isotopic characteristics of the Réunionhotspot with time, and became gradually imprinted with the isotopicsignature of a shallower mantle source that produced the CentralIndian Ridge basalts. KEY WORDS: hotspot; isotopes; Mauritius; Réunion; trace element 相似文献
983.
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
984.
Extensive high-grade polydeformed metamorphic provinces surroundingArchaean cratonic nuclei in the East Antarctic Shield recordtwo tectono-thermal episodes in late Mesoproterozoic and lateNeoproterozoicCambrian times. In Western Dronning MaudLand, the high-grade Mesoproterozoic Maud Belt is juxtaposedagainst the Archaean Grunehogna Province and has traditionallybeen interpreted as a Grenvillian mobile belt that was thermallyoverprinted during the Early Palaeozoic. Integration of newUPb sensitive high-resolution ion microprobe and conventionalsingle zircon and monazite age data, and ArAr data onhornblende and biotite, with thermobarometric calculations onrocks from the H.U. Sverdrupfjella, northern Maud Belt, resultedin a more complex PTt evolution than previouslyassumed. A c. 540 Ma monazite, hosted by an upper ampibolite-faciesmineral assemblage defining a regionally dominant top-to-NWshear fabric, provides strong evidence for the penetrative deformationin the area being of Pan-African age and not of Grenvillianage as previously reported. Relics of an eclogite-facies garnetomphaciteassemblage within strain-protected mafic boudins indicate thatthe peak metamorphic conditions recorded by most rocks in thearea (T = 687758°C, P = 9·411·3kbar) were attained subsequent to decompression from P >12·9 kbar. By analogy with limited UPb singlezircon age data and on circumstantial textural grounds, thisearlier eclogite-facies metamorphism is ascribed to subductionand accretion around 565 Ma. Post-peak metamorphic K-metasomatismunder amphibolite-facies conditions is ascribed to the intrusionof post-orogenic granite at c. 480 Ma. The recognition of extensivePan-African tectonism in the Maud Belt casts doubts on previousRodinia reconstructions, in which this belt takes a pivotalposition between East Antarctica, the Kalahari Craton and Laurentia.Evidence of late Mesoproterozoic high-grade metamorphism duringthe formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics ofgranulite-facies metamorphism recorded from other parts of theMaud Belt. The polymetamorphic rocks are largely derived froma c. 1140 Ma volcanic arc and 1072 ± 10 Ma granite. KEY WORDS: Maud Belt; Pan-African orogeny; geochronology; PTt path, East Antarctica 相似文献
985.
The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. Peakpressuretemperature conditions of metamorphism were previouslyconstrained from associated high MgAl granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluidrockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivitycomposition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaOAl2O3FeOSiO2CO2O2in TaCO2fO2 space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO2 are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO2, however,behaves as a dependent variable of CO2 in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidationdecarbonation equilibria 相似文献
986.
Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents 总被引:20,自引:3,他引:20
Arc magmas and the continental crust share many chemical features,but a major question remains as to whether these features arecreated by subduction or are recycled from subducting sediment.This question is explored here using Th/La, which is low inoceanic basalts (<0·2), elevated in the continents(>0·25) and varies in arc basalts and marine sediments(0·090·34). Volcanic arcs form linear mixingarrays between mantle and sediment in plots of Th/La vs Sm/La.The mantle end-member for different arcs varies between highlydepleted and enriched compositions. The sedimentary end-memberis typically the same as local trench sediment. Thus, arc magmasinherit their Th/La from subducting sediment and high Th/Lais not newly created during subduction (or by intraplate, adakiteor Archaean magmatism). Instead, there is a large fractionationin Th/La within the continental crust, caused by the preferentialpartitioning of La over Th in mafic and accessory minerals.These observations suggest a mechanism of fractionation& foundering, whereby continents differentiate intoa granitic upper crust and restite-cumulate lower crust, whichperiodically founders into the mantle. The bulk continentalcrust can reach its current elevated Th/La if arc crust differentiatesand loses 2560% of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction 相似文献
987.
SHAW CLIFF S. J.; EYZAGUIRRE JIMENA; FRYER BRIAN; GAGNON JOEL 《Journal of Petrology》2005,46(5):945-972
Xenoliths record two distinct events in the mantle below theQuarternary West Eifel Volcanic Field, Germany. The first, duringthe Hercynian Orogeny, led to widespread formation of secondary,Ti-poor amphibole, clinopyroxene and phlogopite. The signatureof the second event, related to Quaternary volcanism, variesacross the field. At Dreiser Weiher and Meerfelder Maar, thisevent is characterized by amphibolephlogopiteclinopyroxeneveins, hosted in lherzolite and harzburgite xenoliths broughtto the surface by sodic olivine nephelinitebasanite suitelavas. These veins formed from crystallization of sodic magmathat flowed along fractures in the mantle. At Rockeskyller Kopf,Gees and Baarley, the Quaternary event is characterized by wehrlitexenoliths, many of which have phlogopiteclinopyroxeneveins, that were transported by potassic foid suite lavas. Wehrliteformed by reaction of lherzoliteharzburgite, with a largevolume of potassic magma that flowed along grain boundariesrather than in fractures. During reaction, orthopyroxene wasconsumed and secondary clinopyroxene, olivine and phlogopiteprecipitated. Veins formed in wehrlites only during periodicover-pressure events. The composition of the magmas parentalto the veins is similar to the lavas that carried the xenolithsto surface, indicating that the source of foid and olivine nephelinitebasanitesuite magma is domainal, as was the flow regime and magma flux. KEY WORDS: Eifel; mantle xenoliths; metasomatism; trace elements 相似文献
988.
DUGGEN S.; HOERNLE K.; VAN DEN BOGAARD P.; GARBE-SCHONBERG D. 《Journal of Petrology》2005,46(6):1155-1201
Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,SrNdPb-isotope analysis and laser 40Ar/39Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·24·8Ma), SiK-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·30·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the SiK-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. IzuBonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. SiK-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways 相似文献
989.
The Kyffhäuser Crystalline Complex, Central Germany, formspart of the Mid-German Crystalline Rise, which is assumed torepresent the Variscan collision zone between the East Avalonianterrane and the Armorican terrane assemblage. High-precisionUPb zircon and monazite dating indicates that sedimentaryrocks of the Kyffhäuser Crystalline Complex are youngerthan c. 470 Ma and were intruded by gabbros and diorites between345 ± 4 and 340 ± 1 Ma. These intrusions had magmatictemperatures between 850 and 900°C, and caused a contactmetamorphic overprint of the sediments at PT conditionsof 690750°C and 57 kbar, corresponding toan intrusion depth of 1925 km. At 337 ± 1 Ma themagmaticmetamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 711 km.This is inferred from a diorite, and conforms to PT pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 57 to 24 kbar at 690750°C.Subsequently, the metamorphicmagmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published KAr muscoviteage of 333 ± 7 Ma. With respect to PTDtdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; UPb dating; hornblende; garnet; Mid-German Crystalline Rise; PT pseudosection 相似文献
990.
Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basaltdacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO2 correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (r value). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t1/2), and was about 3 x 102m/year at t = 10 years and 1 x 102 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope 相似文献