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1.
Olivine tholeiites (8–10 wt. % MgO) from Krafla show significantcorrelations between major elements (notably Fe) and incompatibletrace elements. In particular, the samples with the highestFe contents are the most enriched in elements such as K, Ti,and light rare earth elements (LREEs). The observed trends cannotbe explained by fractional crystallization of olivine, plagioclase,or clinopyrox-ene from a single primary magma, nor are theylikely to result from crustal contamination. The simplest explanationfor the compositional variations is that they result from imperfectmixing of primary melts, produced at different levels in theupwelling asthenosphere, which later underwent olivine fractionation.Nd and Sr isotopic data hint at the possibility that some mixingbetween two (plume and non-plume) mantle sources may also berequired. The average olivine tholeiite composition is comparedwith the average compositions of melts, predicted from parameterizationsof melting experiments, produced from mantle with differentpotential temperatures. The predicted compositions were correctedfor fractional crystallization before the comparison was made.The data compare well with the predicted average compositionof melt from mantle with a potential temperature of {small tilde}1580C. Differences between the observed and predicted compositions(notably higher Fe and lower Na in the Krafla basalts) are ascribedeither to errors related to the modelling or to the effect oftemperature- and velocity-structure of the mantle plume beneathIceland. The average REE composition of the olivine tholeiiteswas then inverted to obtain the variation of melt fraction withdepth. The predicted melt fraction rises from 00 at a depthof {small tilde} 140 km (consistent with a potential temperatureclose to 1580 C) to a maximum value of {small tilde} 03 atthe surface. The predicted melt thickness ({small tilde}22 kmwhen corrected for fractional crystallization) is consistentwith geophysical estimates of crustal thickness.  相似文献   

2.
Melting of the Shallow Upper Mantle: A New Perspective   总被引：4，自引：3，他引：4  
Detailed examination of liquidus phase relationships in binaryand ternary joins of the CFMAS +Cr system has permitted a rigorousdetermination of the dry melting path of an initially fertilespinel peridotite composition resembling Bulk Silicate Earthor MORB-pyrolite. It is demonstrated that it is impossible tomodel mantle melting accurately using only one set of ratiosof phases entering the melt; this implies that the melting processis primarily controlled by solid solution rather than eutecticbehaviour. The proportions of phases entering a melt dependon whether a phase reacts and/or disappears from a system, andon the choice of the initial and final peridotite compositions.Four discrete domains in the melting regime of upper-mantleperidotites are distinguished, each characterized by differentphase melting coefficients, relating to the melting of: (1)lherzolites, (2) clinopyroxene-bearing harzburgites (i.e., free-clinopyroxene),(3) clinopyroxene-saturated harzburgites (i.e., clinopyroxenein solid solution in orthopyroxene), and (4) clinopyroxene-freeharzburgites (i.e., no clinopyroxene). The proposed non-linearfashion in which mantle lithologies melt explains the inadequacyof all previous models to reproduce the observed compositionsof upper-mantle peridotite melting residues. It is suggestedthat: (1) olivine and orthopyroxene will melt cotectically;(2) clinopyroxene and spinel will lose most of their aluminouscomponent after {small tilde}8% melting within the first 4 kb({smalltilde} 12 km) of ascent from the dry solidus; and that (3) clinopyroxenewill disappear completely from a MORB-pyrolite mantle after{small tilde}42% melting. Although such a number is significantlyhigher than that dictated by the position of the clinopyroxene-outcurves from peridotite isobaric equilibrium melting experiments({small tilde}22%), it is emphasized that the latter are a grossoversimplification of the natural melting process and are notequivalent to melting during adiabatic upwelling. It is concludedthat the commonly postulated disappearance of clinopyroxenefrom fertile peridotite compositions at {small tilde}22% meltingis greatly in error if melting in an adiabatically rising mantleis considered, thus providing an explanation for many unsuccessfulattempts by various authors to model the behaviour of transitionelements in sub-oceanic and supra-subduction-zone mantle andderivative magmas.  相似文献   

3.
Recent theoretical studies of rift tectonics have concludedthat their observed geophysical features, require that (1) extensionaffects a much wider zone of the underlying lithospheric mantlethan the crust; (2) early extension involves a comparativelywide zone that narrows with time. The Neogene evolution of thesegment of the Rio Grande rift between the Great Plains andColorado Plateau shows this theoretical pattern clearly. Thewidth of the crustal extension zone narrowed from {small tilde}170km in the Oligo-Miocene to {small tilde}50 km in the Pliocene.In contrast, both gravity and teleseismic studies indicate thatthe current width of the zone of thinned lithospheric mantle(ß = 2–3) beneath the rift is {small tilde}750km. To assess the contributions of lithosphere- and asthenosphere-derivedmelts to the magmatismassociated with the early phase of developmentof the Rio Grande rift, we have undertaken a 670-km geochemicaltraverse of Oligo-Miocene volcanism between latitudes 36 and38N. Our section is centered on the present-day axis of therift in the Espanola Basin. It extends from the Navajo volcanicfield, Arizona, to Two Buttes, SE Colorado, and intersects hypabyssalintrusions on the rift shoulders at Dulce, west of the rift,and Spanish Peaks to the east. We have sampled a diverse rangeof magma types that vary in composition from ultrapotassic toHy- and Ne-normative basalts. A geochemical profile along thistraverse shows a spatially symmetrical variation in elementand oxide ratios, such as Na2O/K2O and Ba/Nb, and also in Srand Nd isotope ratios. On the rift flanks and shoulders Oligo-Miocenevolcanism was dominated by K-rich mafic magmatism, whereas atthe rift axis tholeiitic and alkalic basalts with whole-rockcompositions similar to those of ocean-island basalts (OIB)were erupted. This symmetrical geochemical variation broadlyparallels the corresponding teleseismic lithosphere thicknessprofile and is a mirror image of the gravity profile. We interpret the OIB-type magmas at the rift axis as predominantlyasthenosphere-derived melts. These suggest that mantle upwelling,and melting by decompression, were occurring during the earlydevelopment of the Rio Grande rift The symmetrical variationof incompatible elements and isotope ratios in rocks about therift axis suggests that the sources of the K-rich mafic magmason the stable flanks and shoulders of the rift are not directlyrelated to the subduction of the Farallon plate: an asymmetricprocess. Instead, we propose that the K-rich mafic magmas onthe flanks and shoulders of the Rio Grande rift are derivedfrom the melting of a metasomatized layer in the lithosphericmantle during extension. *Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK  相似文献   

4.
The Ordovician Snowdon Volcanic Centre (SVC) of North Walescomprises a bimodal basalt–subalkaline/peralkaline associationemplaced around a caldera within a shallow marine environment.The tectonic setting was associated with closure of the LowerPalaeozoic Iapetus Ocean and cessation of ocean plate subduction.The SVC volcanic products include basaltic lavas and pyroclasticrocks, rhyolitic pyroclastic flow deposits, high-level intrusions,domes, and flows, together with reworked equivalents. A programmeof detailed field mapping, sampling, and chemical analysis hasbeen used to evaluate the structure and magmatic evolution ofthe SVC volcanic system. SVC basalts show a range in chemicalcharacteristics between volcanic arc type and within-plate,ocean island basalt (OIB) type. Subalkaline, silica-oversaturatedintermediate intrusions (icelandites) and five chemically distinctgroups of extrusive and intrusive subalkaline/peralkaline rhyolites(termed A1, A2, B1, B2, and B3) were emplaced during the evolutionof the SVC. This evolution was driven by material and thermalinput from basaltic magma. The SVC basaltic lavas were derivedas partial melts from a heterogeneous volcanic arc to OIB-typespinel lherzolite mantle and experienced up to 60% olivine gabbrofractionation during storage in sill networks in the sub-crustor lower crust. Some magma batches experienced further fractionalcrystallization ({small tilde}70%) and minor crustal contamination({small tilde}10%) to yield the icelandites. Trace element andNd isotope data do not favour an origin for the rhyolites bypartial or total fusion of likely crustal material, and thefive rhyolite groups are regarded as distinct homogeneous batchesof magma derived from varied basaltic magmas. The icelanditesand peralkaline rhyolites (group B3) result, respectively, from{small tilde}50% and {small tilde}80–90% zircon-free fractionalcrystallization of SVC basalts. The subalkaline rhyolites (groupsA1 and B1) result from {small tilde}80–90% fractionalcrystallization of subduction-related basalts similar to thoseof Ordovician basalts which pre-date the Lower Rhyolitic TuffFormation, and groups A2 and B2 were formed by mixing and homogenizationof A1, B1, and B3 magma batches. These data and interpretationsprovide the basis of a model for the complex evolution of asilicic magma system below the SVC caldera around the time ofcessation of Caledonian subduction in North Wales. Rhyolitemagma chambers were short lived and discontinuous; the largestwas probably disc shaped and was almost entirely evacuated duringa >60-km3 ash-flow eruption.  相似文献   

5.
Metamorphic isograds and time-integrated fluid fluxes were mappedover the 1500 km2 exposure of the Waits River Formation, easternVermont, south of latitude 4430'N. Isograds based on the appearanceof oligoclase, biotite, and amphibole in metacarbonate rocksdefine elongated metamorphic highs centered on the axes of twolarge antiforms. The highest-grade isograd based on the appearanceof diopside is closely associated spatially with synmetamorphicgranitic plutons. Pressure, calculated from mineral equilibria,was fairly uniform in the area, 7 1.5 kb; calculated temperatureincreases from {small tilde} 480C at the lowest grades in thearea to {small tilde} 575C in the diopside zone. CalculatedXco2f equilibrium metamorphic fluid increases from <0-03at the lowest grades to 0.2 in the amphibole zone and decreasesto 0.07 in the diopside zone. Time-integrated fluid fluxesincrease with increasing metamorphic grade, with the followingmean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclasezone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105;diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates twolarge deep-seated ({small tilde} 25-km depth) regional metamorphichydrothermal systems, each centered on one of the major antiforms.Fluid flowed subhorizontally perpendicular to the axis of theantiforms from their low-temperature flanks to their hot axialregions and drove prograde decarbonation reactions as they went.Along the axes of the antiforms fluid flow was further focusedaround synmetamorphic granitic intrusions. In the hot axialregion fluid changed direction and flowed subvertically outof the metamorphic terrane, precipitating quartz veins. Estimatesof the total recharge, based on progress of prograde decarbonationreactions, nearly match estimates of the total discharge, basedon measured quartz vein abundance, (2-10) x 1012 cm3 fluid percm system measured parallel to the axes of the antiforms. Withinthe axial regions fluids had lower XCO2 and rocks record greatertime-integrated fluxes close to the intrusions than at positionsmore than {small tilde} 5 km from them. The differences in bothfluid composition and time-integrated flux can be explainedby mixing close to the intrusions of regional metamorphic fluidsof XCO2/ with fluids from another source with XCO2{small tilde}0 in the approximate volume ratio of 1:2.  相似文献   

6.
The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km2) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and Fe–Ti oxides.The cryptic variation is restricted (plagioclase An81–51,olivine Fo85–66, clinopyroxene Wo43–41 En46–37Fs20–11) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0•2–0•4 mm) equigranular, thin(0•5–3 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo77–68 5)and plagioclase cores (An80–72) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}0•78. The average composition(47•7% SiO2, 13•3%MgO, 1•57% Na2O+K2O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system.  相似文献   

7.
Active or passive continental rifting is associated with thinning of the lithosphere, ascent of the asthenosphere, and decompressional melting. This melt may percolate within the partially molten source region, accumulate and be extracted. Two-dimensional numerical models of extension of the continental lithosphere–asthenosphere system are carried out using an Eulerian visco-plastic formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component (crust–mantle) and two-phase (solid–melt) system. Temperature-, pressure-, and stress-dependent rheologies based on laboratory data for granite, pyroxenite and olivine are used for the upper and lower crust, and mantle, respectively. Rifting is modelled by externally prescribing a constant rate of widening with velocities between 2.5 and 40 mm/yr. A typical extension experiment is characterized by 3 phases: 1) distributed extension, with superimposed pinch and swell instability, 2) lithospheric necking, 3) continental break up, followed by oceanization. The timing of the transition from stages 1) to 2) depends on the presence and magnitude of a localized perturbation, and occurs typically after 100–150 km of total extension for the lithospheric system studied here. This necking phase is associated with a pronounced negative topography (“rift valley”) and a few 100 m of rift flanks. The dynamic part of this topography amounts to about 1 km positive topography. This means, if rifting stops (e.g. due to a drop of external forces), immediate additional subsidence by this amount is predicted. Solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks, often observed at volcanic margins. After continental break up, a second time-dependent upwelling event off the rift axis beneath the continental margins is found, producing further volcanics. Melting has almost no or only a small accelerating effect on the local extension value (β-value) for a constant external extension rate. Melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift. This upwelling velocity is only weakly dependent on the rifting velocity. The melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks. Melting increases the topography of the flanks by 100–200 m due to depletion buoyancy. Another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension/subsidence. Modelled magma amounts are smaller than observed for East African Rift System. Increasing the mantle temperature, as would be the case for a large scale plume head, better fits the observed magma volumes. If extension stops before a new ocean is formed, melt remains present, and convection remains active for 50–100 Myr, and further subsidence is significant.  相似文献   

8.
Lower Crustal Xenoliths, Chinese Peak Lava Flow, Central Sierra Nevada   总被引：1，自引：0，他引：1  
An assemblage of pyroxenite, peridotite, and mafic granulitexenoliths contained in the toe of a 10 Ma trachybasalt flowremnant overlying Late Cretaceous granitoids indicates the presenceof a mafic-ultramafic complex beneath the Sierra Nevada batholith.Olivine-free pyroxenites that include orthopyroxenites, websterites,and clinopyroxenites are dominant. Primary igneous texturesare displayed by some pyroxenites, but commonly are masked byrecrystallization. Fe-rich harzburgites and lherzolites arerare. A few of the ultramafic xenoliths contain ovoid opaquepatches that are apparently pseudomorphs after garnet and havepyralspite garnet compositions. A pressure corresponding toa lower crustal depth of approximately 40 km has been determinedfrom two of these xenoliths using a garnet-orthopyroxene geobarometer.Abundant mafic granulites can be subdivided into those containing12 per cent or less A12O3 and chemically gradational with pyroxenitesand others containing more than 15 per cent A12O3 and showingconsiderable scatter on oxide variation diagrams. The high-aluminagranulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr,whereas low-alumina and ultramafic xenoliths have a wide rangeof 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicatean age for the complex roughly the same as that of overlyinggranitoid plutons. However, the granitoids have initial 87Sr/86Srratios intermediate between the high-alumina and ultramaficxenoliths, suggesting that they may have resulted from mixingof basaltic magma, represented by the ultramafics, and crustalmaterials, with subsequent crystal fractionation. The trachybasaltmay represent a partial melt of the ultramafic rocks.Rocks analogousto the Chinese Peak xenoliths are exposed in the Giles complexof central Australia, a series of several deformed layered maficand ultramafic intrusions, emplaced in a granulite facies terrain.Contemporaneous development of mafic-ultramafic complexes andthe Sierra Nevada batholith may explain the present day thick({small tilde} 50 km) crust in this region  相似文献   

9.
A suite of large and fresh peridotite xenoliths from a picritetuff deposit in the Cenozoic Vitim volcanic field, {small tilde}200km east of Lake Baikal, shows a continuous gradation from protogranularspinel through garnet–spinel to very abundant garnet peridotites.This includes composite nodules in which all these lithologiescoexist on the scale of a few centimeters. Garnet and many spinellherzolites are remarkably fertile in terms of their ‘basaltic’major element contents (CaO 30–37%, MgO 37–40%,Ca/Al=11, Cr/Al<013), whereas some garnet–spineland spinel peridotites are moderately depleted (Cr/Al 014–045).T estimates are 850–880C for the fertile spinel lherzolitesapparently brought up from shallow depths of 40–50 km.This contrasts with 980–1030C for depleted spinel peridotitesand 1000–1150C for the garnet-bearing peridotites forwhich equilibration pressures between 16 and 23 kbar are inferred.The data suggest that garnet and spinel peridotites coexistin the sub-Vitim mantle at a pressure of {small tilde}18 kbarover an interval of {small tilde}2 kbar, with the appearanceof garnet, and with the garnetto-spinel ratio in this transitionalzone primarily being controlled by bulk rock contents of Ca,Al, Cr, and Cr/Al ratios, in addition to P–T conditions. The Vitim peridotites show little evidence for metasomatic enrichment:they commonly show depletion of LREE compared with intermediateREE; this includes also rare amphibole-bearing veins. The fertilespinel and garnet lherzolites have very similar bulk rock majoroxide contents and REE distribution patterns; these featuresindicate a lack of significant chemical vertical mantle stratificationin that region. Garnet peridotites from Vitim show large differencesin modal and chemical composition from garnet peridotite xenolithsfrom Yakutian and South African kimberlites, suggesting distinctlithospheric mantle structure and composition in Archean cratonsand post-Archean mobile belts. * Present address: School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia  相似文献   

10.
The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO2-Al2O3-TiO2-Fe2O3-MgO-FeO-MnO-CaO-Na2O-K2O-H2O-CO2.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.  相似文献   

11.
Igneous rocks of broadly basaltic composition are widely distributedin the anorthosite-bearing Adirondack Highlands of New York.They constitute a mafic series of rocks that occurs on the marginsof the anorthosite series and up to 50 km away from the anorthosite.On an Sr reference diagram, the mafic series has an apparentinitial 87Sr/86Sr value of 0.7036, which is consistent with1100 Ma subcontinental mantle. The series was probably emplacedin the interval 1150–1100 Ma, during which time anorthositeand granitic magmas were also emplaced. The rocks were metamorphosedshortly thereafter ({small tilde} 1088 Ma) under upper-amphiboliteand granulite facies conditions. Individual bodies of maficrocks range in thickness from small enclaves and layers >>1 m thick to large lensoid masses several tens of meters thick.Despite deformation and metamorphic recrystallization, manyof the rocks retain chiHed margins, cross-cutting relationships,and relict igneous textures. Selected samples of the mafic series have been analyzed fortheir major and trace element compositions. Metamorphism didnot significantly alter the igneous geochemical relationships,and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb,Rb/Sr, and 87Sr/86Sr. The most primitive rocks of the seriesare silica-undersaturated gabbroic troctolites, and the moreevolved rocks are basaltic in composition. The mafic seriesas a whole has high abundances of A12O3, FeO, the light rareearth elements (LREE), and other incompatible trace elements.Even the most geochemically primitive compositions have highFeO contents. The Fe enrichment and Si depletion that are shownby chemically evolved compositions are consistent with a Fennertrend of fractionation. Low levels of normative di indicatethat high Fe is not a result of the extensive fractionationof cpx. The geochemical trends that are defined by the traceelements, including the REE, suggest the basaltic rocks maybe differentiates of a parental magma of gabbroic troctolitecomposition. The main compositional trend of the mafic seriescan be simulated by 61% crystallization of olivine and plagioclaseof a gabbroic troctolite, followed by 13% crystallization ofolivine, plagioclase, clinopyroxene, and titanomagnetite atthe final stages. The modelled ratio of olivine to plagioclasecrystallization changes from 1–8: 1 to 0–64: 1.These non-cotectic ratios may reflect a delay in the crystallizationof plagioclase relative to olivine, possibly as a result oflow nucleation rates. At later stages of differentiation, plagioclasewas more important in the crystallization of the series. Delayedcrystallization of plagioclase may also have resulted in thehigh A12O3 contents and enhancement of Eu and Sr relative toother trace elements at early stages of differentiation. The mafic series and the silica-saturated anorthosite seriestogether form an anorthosite-norite-troctolite (ANT) suite.More than one mantle composition may have been involved in generatingthe Adirondack mafic magmas. The rocks retain geochemical evidenceof a source that was depleted in basaltic components (cpx) butenriched in Fe, Ti, K, and the LREE. Previously documented evidenceof anticlockwise cooling paths (Bohlen, 1987) and of a regionalgravity high centered beneath the Adirondack region (Simmons,1964) suggests that of the continental crust by basaltic magmasmay have underplating of the continental crust by basaltic magmasmay have been an important feature of the tectonic evolutionofthe region. A model of mantle upwelling beneath thinning continentalcrust explains the geochemically hybrid nature of the maficseries magmas. It is also consistent with a tectonic settingof incipient or failed continental rifting, to which the generationof the anorthosites is commonly attributed.  相似文献   

12.
The convergent margin of western Mexico is uniquely characterizedby a volcanic front of lamprophyric and related lavas located{small tilde}70 km closer to the Middle America trench thanthe main axis of andesitic volcanism. This front, defined bysmall volcanic centers ranging in age from {small tilde}1 kato 3 Ma, contains several lava types: minette, absarokite, leucitite,spessartite, and kersantite, all without feldspar phenocrysts.Many of the lavas contain hydrous phenocrysts; they are enrichedin potassium and other incompatible elements, and they are moreoxidized relative to the andesitic suite of the main axis. Intimatelyassociated are flows of basaltic andesite of comparable volume.They range in composition from 53 to 58 wt.%SiO2, have 5–9wt.%MgO and contain phenocrysts of olivine, sparse augite, andvarying amounts of plagioclase. Their alkali contents are typicalof calc-alkaline varieties, with average Na2O and K2O concentrationsof 4?2 and 1?1 wt.% respectively. The basaltic andesites oftencontain olivine of unusually high forsterite content, reflectingcrystallization under oxidizing conditions, and they have oxygenfugacities up to 3?3 log units above the Ni-NiO buffer. Manifestationsof high water contents are (1) the ubiquitous occurrence ofgroundmass olivine rather than orthopyroxene, and (2) the suppressionof plagioclase as an early crystallizing phase. Both featuresreflect the role of water in reducing the activity of silicain the melt. The progressive influence of water during crystallizationis also seen in the continuum between the two intermediate lavatypes, basaltic andesite and kersantite, as plagioclase is suppressedand hornblende is stabilized in the phenocryst assemblage. Thus,despite the absence of hydrous minerals in the basaltic andesites,their phenocryst assemblages reveal the influence of substantialamounts of water, and thereby show a genetic link to the variouslamprophyric lavas.  相似文献   

13.
The Pb isotope compositions of amphiboles and clinopyroxenesin spinel peridotite and pyroxenite mantle xenoliths from theintra-plate Quaternary volcanic fields of the Eifel province(Germany) are strongly correlated with their Sr–Nd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and Sr–Nd–Pbisotope compositions similar to the depleted source of mid-oceanridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearingxenoliths from shallower depths (<45 km) provide evidencefor three temporally distinct episodes of mantle metasomatismin the subcontinental lithosphere: (1) aqueous fluids from anisotopically enriched (EM-like) mantle reservoir caused amphiboleformation during deformation in the shallow continental lithosphericmantle and may be subduction related, probably associated withthe last major tectonic event that influenced the area (Hercynianorogeny). (2) During a second phase of mantle metasomatism theEM-like lithospheric mantle was affected by melts from an ancient,HIMU-like (high time-integrated µ = 238U/204Pb) mantlesource. The HIMU-like component introduced by these fluids hada much more radiogenic Pb isotope composition than the asthenosphericsource of the widespread Cenozoic magmatism in Europe and maybe linked to reactivation of ancient subducted crustal domainsduring the Hercynian orogeny or to early Cretaceous deep-sourcedmantle plumes. (3) During a brief final stage the heterogeneouslyenriched EM–HIMU subcontinental lithosphere was locallymodified by basaltic melts migrating along fractures and veinsthrough the upper mantle as a consequence of the Cenozoic Eifelvolcanism. Although a DMM component is completely lacking inthe metasomatic fluids of the metasomatic episodes 1 and 2,the vein melts of episode 3 and the Cenozoic Eifel lavas requiremantle sources containing three end-member components (DMM–HIMU–EM).Thus, mobilization of the more depleted mantle material occurredat the earliest in the Tertiary, contemporaneously with thedevelopment of the extensive rift system and main melt generationin Europe. Alternatively, the variety of Sr–Nd–Pbisotope signatures of the metasomatic agents may have been producedby melting of isotopically distinct mantle domains in a heterogeneousuprising mantle plume. KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes  相似文献   

14.
Eocene igneous rocks from the Abrolhos Islands and surroundingsedimentary platform, offshore Brazil, 18?S, are largely Ti-richbasalt and diabase (4–6 wt.% TiO2), and cumulate rockssuch as wehrlite. Despite high Ti, incompatible-element abundancesare relatively low (e.g., K2O {small tilde} 1 wt.%; P2O5 0.5%; Zr 225 ppm; Rb 23 ppm; Ba 275 ppm); LREE enrichment yieldsLa/YbN {small tilde}8. Compared to other mafic rocks of theSouth Atlantic region, such as Mesozoic high-Ti dikes ({smalltilde}5 wt.% TiO2) and basalts (3–4 wt.% TiO2) of theSerra Geral (Paran?, southern Brazil) province, and high-Tibasalts ({small tilde}4 wt% TiO2) of some South Atlantic features(Walvis, southwest Indian ridge), Abrolhos basalts differ bylower incompatible-element concentrations and/or by isotopiccompositions that emphasize depleted characteristics (Sr–12;Nd 3) relative to bulk earth. Abrolhos isotopic compositionsdo, however, match those of some S. Atlantic islands (e.g.,Pb like those of nearby Trindade), and conform generally toDupal anomaly contours. Abrolhos high-Ti basalts can be modeled as liquids from about90% crystallization of parent picritic liquid emplaced nearthe base of the Brazilian crustal margin; no mantle geochemicalanomaly or special metasomatism are needed to account for theTi contents. Isotopic and trace-clement compositions (e.g.,Zr, Nb, Y) of the Abrolhos province suggest parentage in a mantlerepresenting a plume of bulk earth or ‘enriched’composition that interacted with overlying depleted mantle.  相似文献   

15.
The island of Tristan da Cunha, located at 37?S, 12?W in theSouth Atlantic, is the largest of a group of three islands,the others being Nightingale and Inaccessible. Tristan da Cunhacomprises a continuous series of alkaline lavas ranging in compositionfrom ankaramitic basanite through phonotephnte and tephnphonoliteto phonolite. Moderately porphyritic basanite is the dominantrock type ({small tilde}on the island. Major and trace element variations in the lavas describe well-definedtrends with increasing differentiation which are generally consistentwith control by fractional crystallization of phenocryst phases.None of the lavas can be considered to be primary in composition,mg-numbers range from 40 to 62, and covariation of certain minorand trace elements (e.g., Sr, Ba, P) suggests the presence ofat least two distinct fractionation trends. Sr, Nd, and Pb isotopicanalyses of a subset of the lavas confirm previously publisheddata for the island, but show a slightly greater range: 87Sr/86Sr= 0{dot}70495–0{dot}70517; 143Nd/144Nd = 0{dot}51259–0{dot}51247;206Pb/204Pb = 18{dot}47–18-{dot}74. Quantitative modelling of the compositional variations suggeststhat the ankaramitic basanites are partial olivine + clinopyroxene+ titanomagnetite (?minor plagioclase) cumulates, with {smalltilde}40% crystal accumulation being required to account forthe most porphyritic varieties. The range in composition frombasanite to phonotephrite can be accounted for by up to 50%fractional crystallization of clinopyroxene, olivine, titanomagnetite,and plagioclase, with minor apatite and, in some models, amphibole.Average proportions of these phases in the fractionate are Cpx40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionationis required in models involving the phonotephrites. The compositionsof the evolved tephriphonolites and phonolites are consistentwith extensive (up to 80%) fractional crystallization of aninitial basanitic magma with clinopyroxenc (1–4%), amphibole(17–23%), plagioclase (6–20%) alkali feldspar (0–13%),and titanomagnetite (4–6%)?minor apatite and sphene beingthe dominant fractionating phases. Inferred trace element and isotopic characteristics of the sourceregions of the Tristan lavas are distinct from those givingrise to the Walvis Ridge, Gough Island, or Discovery Seamountbasalts. Normalized trace element abundances of the Tristanlavas are more similar to those of Marion Island, whereas Sr-,Nd-, dnd Pb-isotopic ratios are most similar to nearby InaccessibleIsland lavas. If Tristan da Cunha is the present-day surfaceexpression of the upwelling mantle plume that previously gaverise to the Walvis Ridge, then the source material tapped bythe lavas is distinctly heterogeneous (or has changed with time)within the limits imposed by maintaining the general characteristicsof DUPAL-type mantle.  相似文献   

16.
Metamorphic Evolution of the Seward Peninsula Blueschist Terrane   总被引：3，自引：0，他引：3  
Blucschists and greenschists of the central Seward Peninsula(the Nome Group) crop out over an 8000 km2 area. The protolithpackage for the mid-Jurassic high P/T metamorphism consistedlargely of Cambrian, possibly also Precambrian, to Devoniansedimentary rocks (limestones, marls, pelites, carbonaceoussiltites, quartzites) of shallow water miogeoclinal origin,which maintained a coherent lithostratigraphy during metamorphismand concurrent high-strain, noncoaxial deformation. Several experimentally and empirically calibrated geothermobarometershave been integrated with textural and microstructural observationsin order to derive a pressure-temperature (P-T) path for themetamorphism of the Nome Group. These data, coupled with previouslyobtained Rb-Sr phengite-whole-rock isochron ages, yield a P-T-tpath showing a steep burial segment followed by a prolongedperiod ({small tilde}55 Ma) of isothermal decompression. Peakpressures, obtained from the jadeite content of clinopyroxenein rare, isofacial eclogites, and from the celadonite contentin phengite yield {small tilde}12 kb. Temperatures at this pressure,gained from garnet-clinopyroxene and microcline-plagioclasepairs, were 460?30?C. Decompression temperatures were essentiallythe same, as evidenced by calcite-dolomite and garnet-biotitethermometry. Uniformity of metamorphic conditions throughoutthe Seward Peninsula blueschist terrane coupled with the miogeoclinalnature of the protolith package indicate burial beneath a flat-lying(A-type) subduction zone. The derived P-T-t path is difficultto reconcile with one-dimensional thermal models of collisionalblueschists, where peak P and T are not generally coincident.  相似文献   

17.
Wolf volcano, an active shield volcano on northern Isabela Islandin the Galápagos Archipelago, has undergone two majorstages of caldera collapse, with a phase of partial calderarefilling between. Wolf is a typical Galápagos shieldvolcano, with circumferential vents on the steep upper carapaceand radial vents distributed in diffuse rift zones on the shallower-slopinglower flanks. The radial fissures continue into the submarineenvironment, where they form more tightly focused rift zones.Wolf's magmas are strikingly monotonous: estimated eruptivetemperatures of the majority of lavas span a total of only 22°C.This homogeneity is attributed to buffering of magmas as theyascend through a thick column of olivine gabbroic mush thathas been deposited from a thin, shallow (<2 km deep) subcalderasill that is in a thermochemical steady state. Wolf's lavashave the most depleted isotopic compositions of any historicallyactive intraplate ocean island volcano on the planet and haveisotopic compositions (except for 3He/4He) indistinguishablefrom mid-ocean ridge basalt erupted from the GalápagosSpreading Center (GSC) 250–410 km away from the peak ofinfluence of the Galápagos plume. Wolf's lavas are enrichedin incompatible trace elements and have systematic major elementdifferences relative to GSC lavas, however. Wolf's magmas resultfrom lower extents of melting, deeper melt extraction, and agreater influence of garnet compared with GSC magmas, but Wolfand the GSC share the same sources. These melt generation conditionsare attributed to melting in a thermal and mechanical boundarylayer of depleted asthenosphere at the margins of the Galápagosplume. The lower degrees of melting and extraction from deeperlevels result from a thicker lithospheric cap at Wolf than existsat the GSC. KEY WORDS: caldera; Galápagos; mush; partial melting; plume  相似文献   

18.
An investigation of a low- to high-grade transition area inDharmapuri district confirms a continuous section through aformer lower crust, with tonalitic gneisses predominating inthe upper part and enderbites in the lower part. Geothermometry and geobarometry show a prograde metamorphismof 5?5 ? 1?5 kb and 730 ?40 ?C in the transition zone near theorthopyroxene isograd in the north to {small tilde}8? 1?5 kband 775 ? 30 ?C in the south. The progressive increase in metamorphicgrade is demonstrated by the systematic change in mineral compositionsfrom tonalitic gneisses in the north to enderbites in the south.The pressure estimates record a depth of equilibration of mineralsat 14–23 km and 23–33 km in the intermediate andlower parts of the crust, respectively. The presence of suchrocks now at the surface of the continental crust ({small tilde}35 km) could be cited as evidence for this part of the EarlyProterozoic to Late Archean crust having been at least 50–68km thick. The P-T data in this area do not support the division into thewestern high-pressure and eastern low-pressure blocks of thelowland region suggested by earlier workers.  相似文献   

19.
Detailed mapping of Hercynian basement rocks exposed in theTrois Seigneurs Massif, Pyrenees, France, has demonstrated acontinuous metamorphic sequence developed in Palaeozoic peliticsediments, ranging from chlorite-grade phyllites, through andalusiteand sillimanite mica schists to a zone of migmatites and ultimatelya heterogeneous, peraluminous, biotite- and cordiente-bearinggranitoid (ranging in composition from biotite granite to quartzdiorite) at the deepest tectonic levels exposed. In additionto this ‘deep’ pluton, a syn-metamorphic leucogranitesuite forms pods and sills within the migmatites and mica schistsand a post-metamorphic, homogeneous biotite granodiorite intrudes(and superimposes a contact aureole on) the metasediments. Despitepost-metamorphic deformations, it is clear that the small ({smalltilde} 3 km) separation of low- and high-grade rocks impliesthe existence of very high temperature gradients (80–100?C km –1) during Hercynian metamorphism. Extensive meltingoccurred at {small tilde} 700 ?C at 10–12 km depth, indicatedby the metamorphic mineral assemblages and metamorphic reactionsoccurring in the mica schists. Whole rock XRF analyses of 50 rock samples, including all themain lithologies, indicate that leucogranite compositions areuniform and identical to those of migmatite leucosomes; theyare also close to the major-element composition of experimentallygenerated partial melts of pelitic rocks from the Trois SeigneursMassif. Taken with field relationships, this implies that allleucogranites were generated by partial fusion of pelitic material(< 40 wt. per cent) from the metamorphic sequence, with rapidremoval of the melt by segregation and intrusion to higher structurallevels. The deep biotite granite was probably generated by partialmelting and homogenisation of the same source material, withthe addition of a small magmatic component that was not derivedlocally from the pelites. The late granodiorite was not generatedby anatexis of pelitic material as observed in the metamorphicsequence, and was probably derived by melting of the lower crustat deeper levels than any contemporary exposure of Hercynianbasement in the Pyrenees. Petrological analysis of the metamorphic sequence suggests thatwater activity was externally buffered to high values throughoutthe ‘high-level’ anatexis observed in the TroisSeigneurs sequence. Evidence for this is provided by metacarbonateand metapelite mineral equilibria, by the sequence of metamorphicisograds and by their sharp definition. Moreover, ‘wet’melting conditions are required in order to generate the observedlarge quantities(> 40 wt. per cent) of granitic melt frompelitic material over the small (< 30 ?C) temperature increaseimplied by the section through the migmatite zone. Anatexisof pelitic metasediment was thus promoted by an influx of hydrousfluid into the melting zone. Stable-isotope studies suggestthat this influx was derived from the ground surface, allowingmelts to be continuously saturated as they were generated, andimplying that groundwater infiltration was primarily responsiblefor large-scale anatexis of metasediment at such shallow depths.  相似文献   

20.
Pyroxenes in a low-Ca boninite from a dyke show remarkable growthand overgrowth textures and chemical zoning, which are usedto infer crystallization mechanisms and kinetics. The rock comprisesnearly 50 vol.% fresh glass, phenocrysts and glomerocrysts ofclinoenstatite sensu lato ({small tilde}32 vol.%), microphenocrystsof orthopyroxene and Ca-rich clinopyroxene ({small tilde}17vol.%), and microlites of amphibole and oxides ({small tilde}3vol.%). Clinoenstatite, which inverted from protoenstatite,is skeletal with glass and chrome-spinel inclusions and thinovergrowth zones of Ca-rich clinopyroxene and amphibole; thecrystals are occasionally broken. Orthopyroxene is often skeletal,sometimes slightly resorbed with the same inclusions and overgrowthzones. Ca-rich clinopyroxene and amphibole occur mainly in pyroxeneovergrowth zones. Chrome-spinel in contact with glass is overgrownby magnetite-maghemite or amphibole. The rock is andesitic witha high mg-number of 0.78, and is quartz- and strongly hypersthene-and feldspar-normative. The glass is dacitic with an mg-numberof 0.15, strongly quartz- and feldspar-normative, and rich inwater ({small tilde}5.6wt.%). Strong chemical zoning occurs in all minerals, oscillatory zoningoccurring only in clinoenstatite and orthopyroxene. The mg-numberin clinoenstatite sensu lato ranges from 0.95 (clinoenstatitesensu stricto) to 0-68 (clinohypersthene), the largest rangeso far described, and from 0.88 to 0.43 in orthopyroxene. TheWo content of the former (0.21–1.56 with rare higher values)is considerably smaller than and does not overlap that of thelatter (1.36–4.84X Wo generally increasing with Fs. Thecores of both are chromian (up to 0–018 atoms per formulaunit), and Cr falls to or below the detection limit for Fs>25.The Fs-rich zones of clinohypersthene have {small tilde}2wt%A12O3 and (M6wt% TiO2, whereas those of orthopyroxene reachnearly 10 and 04 wt. %, respectively. Ca-rich clinopyroxenevaries from pigeonite through subcalcic augite and augite toferroaugite, the outer zones reaching 12wt.% A12O3 and 1–8wt% TiO2. Amphibole is magnesio-hornblende on pyroxene or tschermakitichornblende on oxide. The relative times of nucleation, growth, partial resorption,and fracturing of the crystals were determined from the texturesand the chemistry of the overgrowths. Protoenstatite and chrome-spinelnucleated from the melt and were overgrown in places by allsucceeding minerals. Orthopyroxene nucleated mainly from themelt, whereas the other phases occur only as overgrowths. Orthopyroxenemay show weak resorption at intermediate stages. Protoenstatitecrystals were occasionally broken, the relative times beingshown by the nature of the overgrowths. Minor-element concentrationsvary strongly with the growth stage, chromium soon being usedup in the melt and aluminium and titanium being strongly concentratedin the crystals during the last stages. Plagioclase and quartzare absent because of suppression of their nucleation in a water-richmelt on rapid cooling. Crystallization probably occurred intwo main stages, an early one of moderately fast cooling atlow undercooling probably in a shallow magma chamber (correspondingto the growth of the protoenstatite phenocrysts) and a laterone over a period of less than a day of faster cooling at higherundercooling after uprise and injection into the dyke (correspondingto the fracturing of protoenstatite and growth of the microphenocrystsand microlites).  相似文献   

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