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1.
Petrology and Petrogenesis of Cumulate Peridotites and Gabbros from the Marum Ophiolite Complex, Northern Papua New Guinea 总被引:1,自引:0,他引:1
The Marum ophiolite complex in northern Papua New Guinea includesa thick (34 km) sequence of ultramafic and mafic cumulates,which are layered on a gross scale from dunite at the base upwardsthrough wehrlite, lherzolite, plagioclase lherzolite, pyroxenite,olivine norite-gabbro and norite-gabbro to anorthositic gabbroand ferrogabbro at the top. Igneous layering and structures,and cumulus textures indicate an origin by magmatic crystallizationin a large magma chamber(s) from magma(s) of evolving composition.Most rocks however show textural and mineralogical evidenceof subsolidus re-equilibration. The cumulate sequence is olivine and chrome spinel followedby clinopyroxene, orthopyroxene and plagioclase, and the layeredsequence is similar to that of the Troodos and Papuan ophiolites.These sequences differ from ophiolites such as Vourinos by thepresence of cumulus magnesian orthopyroxene, and are not consistentwith accumulation of low pressure liquidus phases of mid-oceanridge-type olivine tholeiite basalts. The cumulus phases show cryptic variation from Mg- and Ca-richearly cumulates to lower temperature end-members, e.g. olivineMg9378, plagioclase An9463. Co-existing pyroxenesdefine a high temperature solidus with a narrower miscibilitygap than that of pyroxenes from stratiform intrusions. Re-equilibratedpyroxene pairs define a low-temperature, subsolidus solvus.Various geothermometers and geobarometers, together with thermodynamiccalculations involving silica buffers, suggest the pyroxene-bearingcumulates crystallized at 1200 °C and 12 kb pressureunder low fO2. The underlying dunites and chromitites crystallizedat higher temperature, 13001350 °C. The bulk of thecumulates have re-equilibrated under subsolidus conditions:co-existing pyroxenes record equilibration temperatures of 850900°C whereas olivine-spinel and magnetite-ilmenite pairs indicatefinal equilibration at very low temperatures (600 °C). Magmas parental to the cumulate sequence are considered to havebeen of magnesian olivine-poor tholeiite composition (>50per cent SiO2, 15 per cent MgO, 100 Mg/(Mg + Fe2+) 78) richin Ni and Cr, and poor in TiO2 and alkalies. Fractionated examplesof this magma type occur at a number of other ophiolites withsimilar cumulate sequences. Experimental studies show that suchlavas may result from ial melting of depleted mantle lherzoliteat shallow depth. The tectonic environment in which the complexformed might have been either a mid-ocean ridge or a back-arebasin. 相似文献
2.
The system peridotite-H2OCO2 serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H2O- and (H2O+CO2)- bearing vapors and with controlled hydrogenfugacity (fH2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. DecreasingfH2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of fH2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the fH2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr2O3 relative to the crystalline phases. High Mg/Mg+Fe)and Cr2O3 are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of XH2Ov > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K2O content of the startingmaterial (up to 1 wt. per cent K2O) results in increasing potassiumcontent of the amphibole (1 wt. per cent K2O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K20-poor (less than 1 wt. per cent K2O). Partial melts are olivine normative with XH2O 0.5, and initialliquids contain normative ol and ne at XH2O 0.4. The alkalinityof these liquids increases with decreasing XH2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high aCo2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle. 相似文献
3.
Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu-Bonin Arc, Japan 总被引:2,自引:0,他引:2
TAMURA Y.; TANI K.; ISHIZUKA O.; CHANG Q.; SHUKUNO H.; FISKE R. S. 《Journal of Petrology》2005,46(9):1769-1803
Basaltbasaltic andesite (<55 wt % SiO2) and daciterhyolite(6674 wt % SiO2) are the predominant eruptive productsin the Sumisu caldera volcano, IzuBonin arc, Japan. Themost magnesian basalt (8·5% MgO), as well as some ofthe other basalts, has a low Zr content (2025 ppm), andcannot yield basalts with higher Zr contents (2940 ppm)through fractionation and/or assimilation. The high- and low-Zrbasalts have different phenocryst assemblages, olivine, plagioclaseand pyroxene phenocryst chemistries, REE (rare earth element)patterns, and fluid-mobile element/immobile element ratios.Estimated primary olivine compositions are more magnesian (>Fo91)in the low-Zr basalts compared with those in high-Zr basalts(<Fo89). The low-Zr basalts contain up to 11 vol. % augite,but many high-Zr basalts are free of augite, which appears onlyin their more differentiated products. The low-Zr basalts areconsidered to be hydrous magmas in which olivine crystallizesfirst followed by augite and plagioclase, whereas the high-Zrbasalts are dry. The low-Zr basalts have higher U/Th ratiosthan the high-Zr basalts. We suggest that both dry and wet primarybasalts existed in the Sumisu magmatic system, each having differenttrace element concentrations, mineral assemblages and mineralchemistry. The lower contents of Zr and light REE and magnesianprimary olivines in the wet basalts could have resulted froma higher degree of partial melting (20%) of a hydrous sourcemantle compared with 10% melting of a dry source mantle. TheSr, Nd and Pb isotope compositions of the wet and dry basaltsare similar and are limited in range. These lines of evidenceindicate that a mantle diapir model might be applicable to satisfythe configuration of such a mantle source region beneath a singlevolcanic system such as Sumisu. KEY WORDS: degree of melting; hot fingers; isotopes; mantle diapir; mantle wedge 相似文献
4.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?745?7 wt. percent, Al2O3=1?6O8?21 wt. per cent, CaO=0?708?12wt. per cent,alk=0?100?90 wt. per cent and Mg/(Mg+Fe2+)=0?940?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite. 相似文献
5.
THOMPSON R. N.; GIBSON I. L.; MARRINER G. F.; MATTEY D. P.; MORRISON M. ANN 《Journal of Petrology》1980,21(2):265-293
The Palaeocene magnesian transitional basalts of the Main LavaSeries (SMLS) of Skye, NW Scotland, have concentration rangesof K, Ti, P, Rb, Sr, Ba, Nb, Ta, Zr, Hf, Th and light REE varyingby a factor of up to two at a given value of (FeO + Fe2O3)/(FeO+ Fe2O3 + MgO). Their chondrite-normalized REE patterns varywidely in slope and cuvature, with (Ce/Yb)N=2.24.7. Theabundances of Ti, P, Zr, Hf, Eu, Gd and Tb correlate negativelywith Si-saturation and are thought to be primary, reflectingvariable localized partial melting (5 per cent) for each magmabatch at about 60 km depth of a spinel-lherzolite upper mantle,leaving a lherzolitic residuum. Y and the heavy REE vary littlewith Si-saturation, due to their partial retention in residualmantle diopside. The large abundance ranges of Rb, Sr, Ba, Nb,Th, La, Ce and Nd in the SMLS basalts, uncorrelated with Si-saturation,may reflect local upper-mantle variability in the concentrationsof the ultra-incompatible elements beneath Skye, caused by thepre-Palaeocene extraction of small quantities of alkalic, incompatible-element-richmagma, such as formed the Permian lamprophyre dykes of westernScotland. The trace element data confirm major-element, least-squaresmodels, which show that fractional crystallization of SMLS magnesianbasalt to less-magnesian basalt residua involved the separationof 10 per cent olivine and 4 per cent plagioclase, whilst thefractionation of SMLS less-magnesian basalt to hawaiite occurredat about 35 km depth by precipitation of 8 per cent olivine,15 per cent plagioclase and 21 per cent aluminous sub-calcicaugite. The variation of Nb and Ta abundances in hawaiites,mugearites and low-Fe intermediate lavas suggests that theseelements partitioned strongly into liquidus titanomagnetitemicrophenocrysts. Zircon fractionation occurred during the finalstages of evolution of benmoreites and trachytes, the latterrepresenting the residuum of at least 90 per cent fractionalcrystallization of SMLS basalt magma. High-Ca, low-alkali olivine tholeiites of the Preshal Mhor magmatype occur near the top of the present lava field erosionalremnant and predominate in the dyke swarm transecting it. Theyhave low incompatible trace-element abundances and REE patternswith (Ce/Yb)N 0.6, similar to those of many mid-ocean ridgebasalts. Models attempting to explain the genesis and relationsof the contrasting SMLS and Preshal Mhor basalts by postulatingseparate mantle sources, arranged in plumes, blobs or layers,fail to account satisfactorily for: (1) the constant incompatible-elementratio ranges of all Skye basalt lavas, (2) the partial interstratificationof SMLS and Preshal Mhor basalts and (3) the appearance of PreshalMhor magmas at the climax of crustal extension in the dyke swarm.The contrasting REE patterns of SMLS transitional basalts andPreshal Mhor tholeiites, and the high Ca/Al, Ca/Na and Ca/Tiof the latter, can be explained if they were produced by furtherlimited partial fusion of the lherzolitic residuum (with a trappedmelt fraction) from SMLS genesis, leaving a final harzburgiticresiduum. The petrogenesis of the Skye lavas provides a localshort-timespan analogue of worldwide processes involved in thegeneration of mid-ocean ridge basalts. 相似文献
6.
JAHN BOR-MING; AUVRAY B.; BLAIS S.; CAPDEVILA R.; CORNICHET J.; VIDAL F.; HAMEURT J. 《Journal of Petrology》1980,21(2):201-244
Archean metavolcanic rocks from three greenstone belts (Suomussalmi,Kuhmo and Tipasjärvi) of eastern Finland have been subjectto a detailed geochemical study which leads to a discussionof their petrogenesis and the problem of compositional heterogeneityin the Archean mantle. Lithostratigraphically, the greenstonebelts are roughly divided into a lower and an upper volcanicsequence. Rocks of komatiitic and tholeiitic compositions arerestricted to the lower sequence, while andesitic tuffs, dacite-rhyodacitelavas and minor basalts of alkaline affinity occur in the uppersequence. All rocks from the greenstone belts have been subjectto regional metamorphism of the upper greenschist facies tothe lower garnet amphibolite facies. Consequently, the geochemicaldistinction of original magma types and the discussion of petrogenesishave relied heavily on the abundances of less mobile elements,such as TiO2, rare earth elements (REE), and some transitionmetals (e.g. Ni and Cr). Using all the possible discriminants of major element compositions,we have concluded that two general magmatic series that existin the lower volcanic sequence might be distinguished by theparameter of TiO2 content: the komatiitic series is characterizedby having TiO2 1.0 per cent and the tholeiitic series by 1.0per cent. The general series do not imply that a cogenetic relationshiplinked only by fractional crystallization exists in each series. Several magmatic types could be distinguished by their characteristicREE distribution patterns. In general, the komatiitic rocksshow flat HREE (heavy REE) and flat or depleted LREE (lightREE) patterns; the tholeiitic rocks show fractionated patternswith some degree of LREE enrichment, whilst the acidic rocksdemonstrate highly fractionated patterns with significant HREEdepletion. Model calculations indicate that: (1) the komatiiticand the tholeiitic series have no clear genetic relationship;(2) some basaltic komatiites (MgO < 12 per cent) could havebeen derived by crystal fractionation from a melt of peridotitickomatiite composition (MgO 30 per cent), but others requirevarious degrees of partial melting from the same or differentsource regions to account for their trace element abundances;(3) both partial melting and fractional crystallization haveinterplayed for the production of various rocks within the tholeiiticseries; (4) three different types of source materials are proposedfor all magmas from the lower volcanic sequence. All three typeshave the same initial HREE (about 2x chondrites) but differentLREE (from very depleted to 2x, flat) abundances; (5) volcanicrocks of the upper volcanic sequence must have originated atgreat depths where garnet remains in the residue after partialmelting and melt segregation. The recognition of the strongly LREE-depleted mantle sources,deduced from the REE patterns of peridotitic komatiites fromFinland, Canada and Rhodesia, may suggest that this depletionis a worldwide phenomenon, and that the Archean upper mantleis as heterogeneous in composition as the modern upper mantle.The causal effect of the depletion might be related to the generationof some contemporaneous LREE-enriched tholeiitic rocks, or morelikely, to contemporaneous or previous continental crust formingevents. 相似文献
7.
Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington 总被引:1,自引:1,他引:0
Small (<5 km2), lithologically diverse gabbro and dioritestocks make up 2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.72.4 wt% K20 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 927%melting of a garnet-free, LREE-enriched source (La/bN 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K2O (0.30.5 wt%) and La/bN (1.73.4),and high CaO (8.811.3 wt%) and Na2O/K2O (622).These traits suggest a source with more clinopyroxene and lowerLa/bN than the MKS source. Differences in Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures 10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element
*Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 2494847 or 3335752 Fax: (912) 3337389. e-mail: jtepper{at}valdosta.peachnet.edu 相似文献
8.
Basalts Generated by Decompressive Adiabatic Melting of a Mantle Plume: a Case Study from the Isle of Skye, NW Scotland 总被引:17,自引:13,他引:4
The primitive lavas of the Skye Main Lava Series (SMLS) arebasaltic rocks ranging from ne- to hy-normative, and definedas having MgO>7%. They have evolved by olivine(plus minorCr-spinel) fractionation from more picritic parental materialAn artificial data-set has been generated by normalizing allcompositions to 15% MgO by fractional addition of olivine, todefine compositional characteristics of primary magmas. Themost striking feature of the data-set is a very strong negativecorrelation between Si and Fe, as is seen in many oceanic alkalibasalt suites and in localized data-sets from mid-ocean ridgeswhen normalized for fractional crystallization. The SMLS dataare comparable to the compositions of equilibrium melts producedexperimentally by Hirose & Kushiro (Earth and PlanetaryScience Letters, 114, 477489, 1993) from the relativelySi- and Fe-rich starting composition HK-66. Estimates of depthsand temperatures of last equilibration of the SMLS magmas withtheir mantle source have been made, on the assumption that mantlemelting may have been an equilibrium process. On this basis,it appears that primary magmas, containing 1315% MgO,were generated by decompressive melting of abnormally hot mantle(estimated minimum mantle potential temperature, TP 1440C),associated with the Iceland plume. Melting was initiated inthe garnet stability field, and segregation is estimated tohave taken place over the pressure range 1836 kbar (60112-kmdepth) and a temperature range of 13901510 C. The PTtrajectory of segregation appears to coincide closely with estimatesof the solid + liquid adiabatfor mantle melting. Alkali basaltswere segregated from the greatest depths and olivine tholeiitesfrom higher levels, though the majority of magmas were derivedfrom near the top of the melting column. After segregation,magmas ascended to the surface, cooling at the rate of 3C/km,and were erupted in a comparatively narrow temperature rangeclose to 1200C. The suitability of HK-66 as a general modelfor mantle composition in ascending plumes is discussed, asmost within-plate primitive basalts in oceanic environmentsshare the same Fe-rich character as the SMLS, in which theycontrast with normal ridge-related magmas. Within-plate plumesmay perhaps tap mantle of abnormally high Fe/Mg, though thepossibility that compositional differences in plume-relatedand normal ridge-related magmas are generated by contrasts inprocess cannot yet be excluded. If, however, a relatively Fe-richreservoir exists deep within the mantle, and acts as the sourcefor plume-related magmatism, then ultimately a global positivecorrelation should exist between estimates ofTp and the Fe/Mgratio of the sources for individual suites.
Corresponding author 相似文献
9.
Archean Shoshonitic Lamprophyres of the Abitibi Subprovince, Canada: Petrogenesis, Age, and Tectonic Setting 总被引:7,自引:1,他引:6
Archean shoshonitic lamprophyre dikes are prevalent along majortranslithospheric structures that demark tectonostratigraphicterranes in the Abitibi greenstone belt of the Superior Province.The lamprophyres post-date volcanism, tonalitic batholiths,deformation, and metamorphism of the terranes, and are mostprominently developed in trans-tensional graben, where theyare associated with molasse sediments and an alkaline suiteof plutons, stocks, and trachytes. Mineralogically, the dikesare characterized by zoned phiogopite or hornblende phenocrystsand/or diopsidic pyroxene, restriction of feldspar to the groundmass,globular segregations of K-feldspar and calcite, olivine pilite,and accessory Ti-magnetite, Cr-spinel, apatite, titanite, andSr-rich calcite; crustal xenoliths are sporadically present. Compositionally, weakly altered primititive dikes have contentsof SiO2 (4148 wt.%), TiO2 (0611 wt.%), P2O5(041076 wt.%), Cr (258915 ppm), Co (3684ppm), Ni (159368 ppm), and Sc (1532 ppm), mg-numbers(7279), and K2O/Na2O ratios (1043) similar tothose of Phanerozoic shoshonitic lamprophyres. The primitivedikes are also characterized by extreme enrichments of K, Rb,Ba, Cs, U, and Th, enhanced light rare earth elements (REE),and fractionated REE patterns [Lan=33274; (La/Yb)n=1687].On mid-ocean ridge basalt (MORB) normalized plots the dikesshow coherent patterns, with (1)enrichment of K, Rb, and Barelative to Sr and LREE, (2) variable enrichments of Rb andBa relative to K, (3) troughs at TaNb and Ti, and (4)variable negative P and positive Sm anomalies. Compositionalvariations of lamprophyre suites within restricted areas areinterpreted to reflect melting of compositionally heterogeneoussources, variable degrees of assimilationfractional crystallization,and mixing of distinct batches of lamprophyric magmas. Primary18O values of the magmas are close to 63 as given by resistantpyroxene; these are 18O-relative to MORB, but comparable withPhanerozoic alkali basalts and lamprophyres. Mica, clinopyroxene,hornblende, and feldspar do not retain magmatic equilibriumfractionations for oxygen isotopes. A concordant UPbage of 26742 Ma was obtained from titanite, similar to theages of shoshonitic plutons in the same area. The lamprophyredikes possess a total range of Nd between 041 and 211(1),and define a distinct field in common with other late Archeanshoshonites on an f(Sm/Nd) vs. Nd plot. Pyroxenes yield a low87Sr/86Sr(0701102), whereas whole-rock Rb-Sr isotope systematicsare disturbed. Lamprophyres are not known from pre-27-Ga terranes. Their compositionand inferred geodynamic setting is consistent with an originin a depleted mantle wedge, enriched in large ion lithophileelements (LILE) and LREE during subduction by slab and sediment(low Sr/Nd) dehydration. Partial melting may have been triggeredby rebound and decompression that followed accretional collisionof two allochthonous greenstone terranes at a plate margin.The onset of shoshonitic magmatism at 27 Ga coincides withthe transition from tonalitetrondhjemitegranodiorite(TTG) dominated magmatism with high (La/Yb)n and low Yb (slabmelting) to mantle-wedge derived granites featuring lower (La/Yb)nand higher Yb (slab dehydration), owing to decreasing heat flow.Accretion of greenstone belts, and their buoyant harzburgiticroots, consolidated a thick subcontinental mantle lithosphereby 27 Ga, which was subsequently the source of Jurassic kimberlitesthat intruded the persistently reactivated Archean translithosphericstructures. 相似文献
10.
The system peridotite-H2O-CO2 serves as a simplified model forthe phase relations of mantle peridotite involving more thanone volatile component. Run products obtained in a study ofphase relations of four mantle peridotites in the presence ofH2O- and (H2O+CO2)-bearing vapors and with controlled hydrogenfugacity (fH2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. DecreasingfH2from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0?85 to approximately0?50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of fH2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the fH2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr2O3 relative to the crystalline phases. High Mg/(Mg+Fe)and Cr2O3 are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. 相似文献
11.
Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9monthinterval in 1982 83, some 305 106 m3 of mediumK,calcalkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An6075)and twopyrozene bearing andesites with 58% SiO2 to finalplagioclase (An8590), diopside, and olivine (Fo8590)bearing primitive magnesium basalts with 47% SiO2 Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of highMg parental melt. Theeruptive volumes of 35 106 m3 andesite, 120 106 m3 maficandesite, and 150 106 m3 basalt are consistent with the ideathat the 1982 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have 18O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with 18Oplagpxvalues of + 0.4 to + 0.6o, indicating internal Oisotopeequiliburium at temperature of 1100850 C. The magenesianbasalts have magmatic 18O/ 16O ratios similar to those of midoceanridge basalt, and the Oisotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by 18Orich crust during differentiation.Andesites and transitional mafic and sites have a more variableOisotope character, with laves and phenocrysts havingboth higher and lower 18O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 198283eruption. 相似文献
12.
FERRY JOHN M.; RUMBLE DOUGLAS III; WING BOSWELL A.; PENNISTON-DORLAND SARAH C. 《Journal of Petrology》2005,46(8):1725-1746
Progress () of the infiltration-driven reaction, 4olivine +5CO2 + H2O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m3 volume of rock. Mineral and stableisotope compositions record that XCO2, 18Ofluid, and 13Cfluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusiondispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusiondispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress 相似文献
13.
Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM2 at 1.6 Ga, M2 ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M2 grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:18O(carb) 1420%; metapelites: 18O 614%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(18O 513%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M2 contact metamorphism and fluidrockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing XCO2with increasing grade from <0.05 (greenschist facies) to0.71.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluidrock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology
*Corresponding author Fax: 61394791272. e-mail: geoisb{at}lure.latrobe.edu.au 相似文献
14.
Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China 总被引:21,自引:1,他引:20
ZHANG ZHAOCHONG; MAHONEY JOHN J.; MAO JINGWEN; WANG FUSHENG 《Journal of Petrology》2006,47(10):1997-2019
Picritic lava flows near Lijiang in the late Permian Emeishanflood basalt province are associated with augite-phyric basalt,aphyric basalt, and basaltic pyroclastic units. The dominantphenocryst in the picritic flows is Mg-rich olivine (up to 91·6%forsterite component) with high CaO contents (to 0·42wt %) and glass inclusions, indicating that the olivine crystallizedfrom a melt. Associated chromite has a high Cr-number (7375).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 16301690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected NdSrPbisotope ratios of the picritic and basaltic lavas are indistinguishableand cover a relatively small range [e.g. Nd(t) = 1·3to +4·0]. The higher Nd(t) lavas are isotopically similarto those of several modern oceanic hotspots, and have ocean-island-likepatterns of alteration-resistant incompatible elements. Heavyrare earth element characteristics indicate an important rolefor garnet during melting and that the lavas were formed byfairly small degrees of partial melting. Rough correlationsof isotope ratios with ratios of alteration-resistant highlyincompatible elements (e.g. Nb/La) suggest modest amounts ofcontamination involving continental material or a relativelylow-Nd component in the source. Overall, our results are consistentwith other evidence suggesting some type of plume-head originfor the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian 相似文献
15.
Mineral Chemistry, and major and trace element variations ofthe basalts from Klyuchevskoy, the world's most active islandare volcano, are most consistently explained by the persistenceof a non-steady state, erupting, recharging, and fractionatingmagma chamber in which fractionation of a parental high-MgObasalt melt produces high-Al2O3 basalt. Although fractionalcrystallization is the dominant controlling mechanism, periodicrecharge with a more primitive high-MgO basalt is also an importantprocess contributing to the chemical evolution of the magmas.Hybrid basalts are the mixed product of high-Al2O3 basalt rechargedwith high-MgO basalt. The lavas range in composition from high-MgO, low-Al2O3 ( 12wt. % MgO, 14 wt. % Al2O3) to high-Al2O3, low-MgO ( 18 wt. %Al2O3, 4 wt. % MgO). The high-MgO lavas are characterized byphenocrysts of olivine (cores FO9080 and rims FO8575)with chromite inclusions [Cr/(Cr + Al)0.7], clinopyroxene (Wo4642En4842Fs157),and the rare occurrence of orthopyroxene (En7270). Allthe phenocrysts are normally zoned and set in a groundmass ofplagioclase, pigeonite, clinopyroxene, magnetite, orthopyroxene.The high-Al2O3 basalts contain plagioclase (An8555),olivine (Fo8065), clinopyroxene (Wo4530En5038Fs2311), orthopyroxene (En7270) phenocrysts, that preserve bothnormal and reverse zoning in a groundmass of plagioclase, pigeonite,olivine, clinopyroxene, magnetite, orthopyroxene. Hybrid basaltshave characteristics of both high-MgO basalts and high-Al2O3basalts and preserve complicated normal-to-reverse, reverse-to-normal,and normally zoned phenocrysts. No hydrous minerals are presentin any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derivedfrom a magma chamber(s) located near the base of the Kamchatkacrust (pressures 0.50.9 GPa) and characterized by relativelyhigh crystallization temperatures, some in excess of 1150C.Under these conditions, the fractionation of a parental high-MgOmagma, produced principally from the melting of a fluid-fluxed,peridotitic mantle wedge, results in the production of a chemicallydiverse spectrum of basalts ranging from high-MgO, low-Al2O3to high-Al2O3, low-MgO basalt, traversing the relatively primitiveend of both the calc-alkalic and tholeiitic differentiationtrends. 相似文献
16.
Element Mobility During Incipient Granulite Formation at Kabbaldurga, Southern India 总被引:7,自引:2,他引:7
At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotitehornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700750?C and 57 kb. Extensive replacement of plagioclase (An1630)by K-feldspar through Na, CaK exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO2, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (LaN270; LaN/YbN= 520; EuN27; Eu/Eu* = 0.60.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (LaN200;LaN/YbN = 2080; EuN22; Eu/Eu* = 0.61.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (18O of 6.98.0 per mille for gneissesand charnockites; 13C of 8.5 and 6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of fossil carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition. 相似文献
17.
THOMPSON R. N.; OTTLEY C. J.; SMITH P. M.; PEARSON D. G.; DICKIN A. P.; MORRISON M. A.; LEAT P. T.; GIBSON S. A. 《Journal of Petrology》2005,46(8):1603-1643
The <80 ka basaltsbasanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New MexicoMexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·61·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K2O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging 87Sr/86Sr= 0·70308 (SD = 0·00004) and 143Nd/144Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a Os value of +13·6, typical forOIB. This contrasts with published 187Os/188Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negativeOs values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo89, has Na2O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; Tp 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at Tf 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (Tp 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; SrNdOs isotopes; petrogenetic modelling; thermal boundary layer 相似文献
18.
HARRIS CHRIS; MARSH JULIAN S.; DUNCAN ANDREW R.; ERLANK ANTHONY J. 《Journal of Petrology》1990,31(2):341-369
The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (SiO2 49?351?6 wt.%; MgO 5?16?6 wt.%),but the concentrations of certain incompatible elements (e.g.,Zr) vary by factors of approximately two or more. Most interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R0=0?70490?7065) may beexplained by 7% contamination by crustal material with an R0of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (18O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland. 相似文献
19.
The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility 总被引:1,自引:0,他引:1
HALAMA RALF; VENNEMANN TORSTEN; SIEBEL WOLFGANG; MARKL GREGOR 《Journal of Petrology》2005,46(1):191-217
The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcitesiderite carbonatite. Aegirineaugite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680910°Cand silica activities of 0·280·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalitemagnetitequartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCNYbCN = 1070). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicatecarbonate liquid pairs. Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has 18O values of 7·8 to 8·6and 13C values of 3·9 to 4·6. 18Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rockwater and mineralwater isotopefractionation to be 5·7 ± 0·4. Nd and COisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in 18O betweencalculated syenitic melts and carbonatites, and the overlapin Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite 相似文献
20.
RILEY TEAL R.; LEAT PHILIP T.; CURTIS MICHAEL L.; MILLAR IAN. L.; DUNCAN ROBERT A.; FAZEL ADELA 《Journal of Petrology》2005,46(7):1489-1524
A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New 40Ar/39Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO2 and Zr contents, and reinforced by rare earth element (REE),87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO2 and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO2 and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, 87Sr/86Sri 0·7035, Ndi 9). However, theyhave very high TiO2 (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high MgFe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO2Zr contents and are the most enriched magmagroup of the KarooAntarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENEWSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNESSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke 相似文献