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1.
Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas 总被引:4,自引:0,他引:4
PERINI GIULIA; FRANCALANCI LORELLA; DAVIDSON JON P.; CONTICELLI SANDRO 《Journal of Petrology》2004,45(1):139-182
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·710370·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% FeTi oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 1029%clinopyroxene + 1215% plagioclase + 4467% sanidine+ 24% phlogopite + 13% apatite + 710% FeTioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·708120·70948) were derived from a potassictrachybasalt by assimilationfractional crystallizationwith 2040% of solid removed and r = 0·40·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·710440·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different veinperidotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy 相似文献
2.
The Mesozoic to Early Cenozoic Magmatism of the Benue Trough (Nigeria); Geochemical Evidence for the Involvement of the St Helena Plume 总被引:2,自引:2,他引:2
COULON C.; VIDAL P.; DUPUY C.; BAUDIN P.; POPOFF M.; MALUSKI H.; HERMITTE D. 《Journal of Petrology》1996,37(6):1341-1358
The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of 40 Ar-39Ar ages: 147106 Ma, 9781 Ma and 6849 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=711.1; La/Nb = 0.771; 87Sr/86Sr; =0.70420.7065143Nd/144Nd;i = 0.51250.5127; 206Pb/204Pbi = 17.5918.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.66.8;La/Nb=0.530.66; 87Sr/86 Sri=0.70290.7037; 143Nd/144Ndi=0.51260.5129;206Pb/204Pbi = 18.5420.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume
*;Corresponding author. 相似文献
3.
Kistufell: Primitive Melt from the Iceland Mantle Plume 总被引:5,自引:2,他引:5
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (1015 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·550·58) indicative ofa trace element depleted source, and the SrNdPbisotopic ratios (87Sr/86Sr 0·703040·70308,143Nd/144Nd 0·5130580·513099, 206Pb/204Pb18·34318·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·316·8 R/Ra) combinedwith low 207Pb/204Pb (15·4215·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·391·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The 相似文献
4.
The South Auckland Volcanic Field is a Pleistocene (1·590·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·04·6wt %), Nb (729 ppm), and (La/Yb)N (3·47·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·37·9wt %), Nb (32102 ppm), and (La/Yb)N (1247). GroupA has slightly higher 87Sr/86Sr, similar Nd, and lower 206Pb/204Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high 238U/204Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources 相似文献
5.
PEATE D. W.; BAKER J. A.; BLICHERT-TOFT J.; HILTON D. R.; STOREY M.; KENT A. J. R.; BROOKS C. K.; HANSEN H.; PEDERSEN A. K.; DUNCAN R. A. 《Journal of Petrology》2003,44(2):279-304
We present elemental and isotopic (SrNdPbHfOsHe)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and 40Ar39Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high 3He/4He isotope ratios (R/RA 12·418·5),sub-chondritic 187Os/188Osi (0·1200·126),low 相似文献
6.
Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands 总被引:1,自引:0,他引:1
HOLM P. M.; WILSON J. R.; CHRISTENSEN B. P.; HANSEN L.; HANSEN S. L.; HEIN K. M.; MORTENSEN A. K.; PEDERSEN R.; PLESNER S.; RUNGE M. K. 《Journal of Petrology》2006,47(1):145-189
The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanitephonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinitephonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±FeTioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 190·03% MgO;the most primitive have 3746% SiO2, 2·57%TiO2 and are enriched 50200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineralmelt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 14% partial melting of a heterogeneous mantleperidotite source at depths of 90125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·52Ma) vary between two main mantle source components, one of whichis a young HIMU type with 206Pb/204Pb = 19·88, 7/4 =5, 8/4 0, 87Sr/86Sr = 0·7033 and 143Nd/144Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (20·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(206Pb/204Pb = 19·93, 7/4 = 5, 8/4 = 38,87Sr/86Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·30·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde 相似文献
7.
Tertiary Ultrapotassic Volcanism in Serbia: Constraints on Petrogenesis and Mantle Source Characteristics 总被引:5,自引:1,他引:4
PRELEVIC D.; FOLEY S. F.; ROMER R. L.; CVETKOVIC V.; DOWNES H. 《Journal of Petrology》2005,46(7):1443-1487
The Serbian province of Tertiary ultrapotassic volcanism isrelated to a post-collisional tectonic regime that followedthe closure of the Tethyan Vardar Ocean by Late Cretaceous subductionbeneath the southern European continental margin. Rocks of thisprovince form two ultrapotassic groups; one with affinitiesto lamproites, which is concentrated mostly in the central partsof the Vardar ophiolitic suture zone, and the other with affinitiesto kamafugites, which crops out in volcanoes restricted to thewestern part of Serbia. The lamproitic group is characterizedby a wide range of 87Sr/86Sri (0·707350·71299)and 143Nd/144Ndi (0·512510·51216), whereasthe kamafugitic group is isotopically more homogeneous witha limited range of 87Sr/86Sri (0·705990·70674)and 143Nd/144Ndi (0·512630·51256). ThePb isotope compositions of both groups are very similar (206Pb/204Pb18·5818·83, 207Pb/204Pb 15·6215·70and 208Pb/204Pb 38·7438·99), falling withinthe pelagic sediment field and resembling Mesozoic flysch sedimentsfrom the Vardar suture zone. The Sr and Nd isotopic signaturesof the primitive lamproitic rocks correlate with rare earthelement fractionation and enrichment of most high field strengthelements (HFSE), and can be explained by melting of a heterogeneousmantle source consisting of metasomatic veins with phlogopite,clinopyroxene and F-apatite that are out of isotopic equilibriumwith the peridotite wall-rock. Decompression melting, with varyingcontributions from depleted peridotite and ultramafic veinsto the final melt, accounts for consistent HFSE enrichment andisotopic variations in the lamproitic group. Conversely, themost primitive kamafugitic rocks show relatively uniform Srand Nd isotopic compositions and trace element patterns, andsmall but regular variations of HFSE, indicating variable degreesof partial melting of a relatively homogeneously metasomatizedmantle source. Geochemical modelling supports a role for phlogopite,apatite and Ti-oxide in the source of the kamafugitic rocks.The presence of two contrasting ultrapotassic suites in a restrictedgeographical area is attributable to the complex geodynamicsituation involving recent collision of a number of microcontinentswith contrasting histories and metasomatic imprints in theirmantle lithosphere. The geochemistry of the Serbian ultrapotassicrocks suggests that the enrichment events that modified thesource of both lamproitic and kamafugitic groups were relatedto Mesozoic subduction events. The postcollisional environmentof the northern Balkan region with many extensional episodesis consistent at regional and local levels with the occurrenceof ultrapotassic rocks, providing a straightforward relationshipbetween geodynamics and volcanism. KEY WORDS: kamafugite; lamproite; Mediterranean; Serbia; mantle metasomatism; veined mantle; petrogenesis 相似文献
8.
Trace Element and Sr-Pb-Nd-Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites 总被引:3,自引:0,他引:3
DAVIES G. R.; STOLZ A. J.; MAHOTKIN I. L.; NOWELL G. M.; PEARSON D. G. 《Journal of Petrology》2006,47(6):1119-1146
A phase of Mesozoic extension associated with the terminationof continental collision at the southern margin of the AldanShield produced ultrabasic lamproites in a discontinuous belt500 km long and 150 km wide. The lamproites, locally poorlydiamondiferous, were emplaced as dykes, sills and pipes. AllAldan lamproites have primitive chemical characteristics (e.g.MgO up to 22·7 wt %) and are ultrapotassic (K2O up to8·3 wt %) and peralkaline with K2O + Na2O/Al2O3 in therange 0·61·16. A distinctive feature ofthese rocks is their low TiO2 content (0·51·4wt %). Aldan lamproites are moderately light rare earth element(LREE) enriched with (La/Yb)N ranging from 10 to 47. Heavy rareearth element (HREE) abundances are lower than for all otherlamproites by up to a factor of five. Therefore, the combinedmajor and trace element characteristics of the Aldan samplesare not typical of other lamproite occurrences. Large ion lithophileelement concentrations are high (100800 x Primitive Mantle)but the high field strength elements (HFSE; Nb, Ta, Ti) plusTh and U display unusually low concentrations for rocks of thistype. The style of trace element enrichment recorded by theAldan Shield lamproites is comparable with that of subduction-relatedmagmatism. The Aldan lamproites have among the most extremeinitial isotopic ratios yet recorded from mantle-derived magmas;Ndi = 10·3 to 22·3, 87Sr/86Sri =0·70550·7079, Hfi = 7·6 to29·4 and 206Pb/204Pbi = 16·617·4.When interpreted in terms of multi-stage Pb isotope evolution,the Pb isotope data require fractionation from a Bulk Earthreservoir at 3·0 Ga and subsequent evolution with second-stageµ values between 6·4 and 8·0. The inferredArchaean age of the lamproite source is consistent with Nd andHf model ages, which range from 1·5 to 3·0 Ga.Aldan lamproites have Hf values that range from +3 to 7.Trace element and SrNdPbHf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative Hf as a result of metasomatism by slab-derivedhydrous melts that left rutilegarnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotopetrace element correlations and the lowREE contents. Although high-quality trace element data (e.g.HFSE) are not available for most lamproites, it appears thatmany of their source regions contain a component of recycledoceanic crust, possibly including subducted sediment. The sourcesof the Aldan and many other lamproites are distinct from oceanisland basalt mantle sources. This suggests that the long-termstorage of trace element enriched lamproite sources occurredin the sub-continental lithospheric mantle and not at depthwithin the convecting asthenosphere. KEY WORDS: potassic volcanism; isotope geochemistry; fluid enrichment 相似文献
9.
BATANOVA V. G.; PERTSEV A. N.; KAMENETSKY V. S.; ARISKIN A. A.; MOCHALOV A. G.; SOBOLEV A. V. 《Journal of Petrology》2005,46(7):1345-1366
Alaskan-type platinum-bearing plutons and potassium-enrichedmafic to ultramafic volcanic rocks are temporally and spatiallyassociated within the Late CretaceousPaleocene AchaivayamValaginskiiintra-oceanic palaeo-arc system, allochthonously present inthe Koryak Highland and Kamchatka Peninsula (Far East Russia).The compositions of the parental magmas to the Alaskan-typecomplexes are estimated using the Galmoenan plutonic complexas an example. This complex, composed of dunites, pyroxenitesand minor gabbros, is the largest (20 km3) in the system andthe best studied owing to associated platinum placer deposits.The compositions of the principal mineral phases in the Galmoenanintrusive rocks [olivine (Fo7992), clinopyroxene (13·5wt % Al2O3, 0·10·5 wt % TiO2), and Cr-spinel(515 wt % Al2O3 and 0·30·7 wt %TiO2)] are typical of liquidus assemblages in primitive island-arcmagmas in intra-oceanic settings, and closely resemble the mineralcompositions in the AchaivayamValaginskii ultramaficvolcanic rocks. The temporal and spatial association of intrusiveand extrusive units, and the similarity of their mineral compositions,suggest that both suites were formed from similar parental magmas.The composition of the parental magma for the Galmoenan plutonicrocks is estimated using previously reported data for the AchaivayamValaginskiiultramafic volcanic rocks and phenocryst-hosted melt inclusions.Quantitative simulation of crystallization of the parental magmain the Galmoenan magma chamber shows that the compositions ofthe cumulate units are best modelled by fractional crystallizationwith periodic magma replenishment. The model calculations reproducewell the observed mineral assemblages and the trace elementabundances in clinopyroxene. Based upon the estimated compositionof the parental magmas and their mantle source, we considerthat fluxing of a highly refractory mantle wedge (similar tothe source of boninites) by chlorine-rich aqueous fluids isprimarily responsible for both high degrees of partial meltingand the geochemical characteristics of the magmas, includingtheir enrichment in platinum-group elements. KEY WORDS: subduction; platinum-group elements; clinopyroxene; trace elements; fractional crystallization; Alaskan-type plutons 相似文献
10.
Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar 总被引:1,自引:0,他引:1
MELLUSO LEONE; MORRA VINCENZO; BROTZU PIETRO; TOMMASINI SIMONE; RENNA MARIA ROSARIA; DUNCAN ROBERT A.; FRANCIOSI LUIGI; D'AMELIO FOSCO 《Journal of Petrology》2005,46(10):1963-1996
The AntampombatoAmbatovy complex is the largest intrusionin the centraleastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km2. It has an 40Ar/39Ar incremental heating age of 89·9± 0·4 Ma and a UPb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO2 (2·2wt %) and total iron (14 wt % as Fe2O3), and moderate contentsof Nb (1011 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·70300·7037 and 0·512900·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·70370·7039 and 0·512710·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the maficultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically depleted componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; AntampombatoAmbatovy intrusion; Cretaceous; Madagascar 相似文献
11.
Gao Yongfeng; Hou Zengqian; Kamber Balz S.; Wei Ruihua; Meng Xiangjin; Zhao Rongsheng 《Journal of Petrology》2007,48(4):729-752
Major and trace element, SrNdPb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra YumcoXuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K2O andTiO2, low Al2O3 and Na2O contents, and also have high molarK2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow highly radiogenic 87Sr/86Sr (0· 71660·7363) and unradiogenic 143Nd/144Nd (0· 5117960·511962), low 206Pb/204Pb (18· 45918· 931),and elevated radiogenic 207Pb/204Pb (15· 673215·841) and 208Pb/204Pb (39· 55740· 058) ratios.On the basis of their geochemical and isotopic systematics,the lamproites in south Tibet have a distinct magma source thatcan be differentiated from the sources of potassic lavas inthe east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios(17· 1019· 84), extremely high Th/U ratios(5· 7013· 74) and distinctive isotope compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a northsouth-trendinggraben, indicates that the initiation of eastwest extensionin south Tibet started at 25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt 相似文献
12.
SHAW JULIA E.; BAKER JOEL A.; MENZIES MARTIN A.; THIRLWALL MATTHEW F.; IBRAHIM KHALIL M. 《Journal of Petrology》2003,44(9):1657-1679
Miocene to Recent volcanism in northwestern Arabia producedthe largest intraplate volcanic field on the Arabian plate (HarratAsh Shaam, Jordan). The chemically and isotopically diversevolcanic field comprises mafic alkali basalts and basanites.The magmas underwent limited fractional crystallization of ol± cpx ± plag and rare samples have assimilatedup to 20% of Late Proterozoic crust en route to the surface.However, there are subtle SrNdPb isotopic variations(87Sr/86Sr = 0·703050·70377, 143Nd/144Nd= 0·512970·51285, 206Pb/204Pb = 18·819·2),which exhibit marked correlations with major elements, incompatibletrace element ratios and abundances in relatively primitivebasalts (MgO >8·5 wt %), and cannot be explained byfractional crystallization and crustal contamination alone.Instead, the data require polybaric melting of heterogeneoussources. Semi-quantitative melt modelling suggests that thisheterogeneity is the result of small degree melts (25%)from spinel- and garnet-facies mantle, inferred to be shallowArabian lithosphere, that mixed with smaller degree melts (<1%)from a predominantly deep garnet-bearing asthenospheric(?) sourcewith ocean island basalt characteristics. The latter may bea ubiquitous part of the asthenosphere but is preferentiallytapped at small degrees of partial melting. Volcanism in Jordanappears to be the result of melting lithospheric mantle in responseto lithospheric extension. With time, thinning of the lithosphereallowed progressively deeper mantle (asthenosphere?) to be activatedand melts from this to mix with the shallower lithospheric mantlemelts. Although Jordanian intraplate volcanism is isotopicallysimilar to examples of Late Cenozoic volcanism throughout theArabian peninsula (Israel, Saudi Arabia), subtle chemical andisotopic differences between Yemen and Jordan intraplate volcanismsuggest that the Afar plume has not been channelled northwestwardsbeneath the Arabian plate and played no role in producing thenorthern Saudi Arabian and Jordan intraplate volcanic fields. KEY WORDS: asthenosphere; intraplate volcanism; Jordan; lithospheric mantle; SrNdPb isotopes 相似文献
13.
The Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central-Western Sector 总被引:1,自引:0,他引:1
The Cretaceous lava sequence and associated mafic dyke swarmin centralwestern Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordieriteorthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)n = 11·4 for tholeiites vs (La/Nd)n =0·651 for transitional rocks]. Initial (at 88Ma) 87Sr/86Sr and 相似文献
14.
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
15.
Yang Jin-Hui; Sun Jin-Feng; Chen Fukun; Wilde Simon A.; Wu Fu-Yuan 《Journal of Petrology》2007,48(10):1973-1997
A combination of major and trace element, whole-rock Sr, Ndand Hf isotope, and zircon U–Pb isotopic data are reportedfor a suite of dolerite dikes from the Liaodong Peninsula inthe northeastern North China Craton. The study aimed to investigatethe source, petrogenesis and tectonic setting of the dikes.Sensitive high-resolution ion microprobe U–Pb zircon analysesyield a Late Triassic emplacement age of 213 Ma for these dikes,post-dating the collision between the North China and Yangtzecratons and consequent ultrahigh-pressure metamorphism. Threegeochemical groups of dikes have been identified in the LiaodongPeninsula based on their geochemical and Sr–Nd–Hfisotope characteristics. Group 1 dikes are tholeiitic, withhigh TiO2 and total Fe2O3 and low MgO contents, absent to weaknegative Nb and Ta anomalies, variable (87Sr/86Sr)i (0·7060–0·7153),Nd(t) (– 0·8 to –6·5) and Hf(t) (–2·7to –7·8) values, and negative Hf(t) (–1·1to –7·8). They are inferred to be derived frompartial melting of a relatively fertile asthenospheric mantlein the spinel stability field, with some upper crustal assimilationand fractional crystallization. Group 2 dikes have geochemicalfeatures of high-Mg andesites with (87Sr/86Sr)i values of 0·7063–0·7072,and negative Nd(t) (–3·0 to –9·5)and Hf(t) (–3·2 to –10·1) values,and may have originated as melts of foundered lower crust, withsubsequent interaction with mantle peridotite. Group 3 dikesare shoshonitic in composition with relatively low (87Sr/86Sr)ivalues (0·7061–0·7063), and negative Nd(t)(–13·2 to –13·4) and Hf(t) (–11·0to –11·5) values, and were derived by partial meltingof an ancient, re-enriched, refractory lithospheric mantle inthe garnet stability field. The geochemical and geochronologicaldata presented here indicate that Late Triassic magmatism occurredin an extensional setting, most probably related to post-orogeniclithospheric delamination. KEY WORDS: mafic dike; asthenospheric mantle; lithospheric mantle; delamination; North China Craton 相似文献
16.
Basanite intrusions from the Early Cretaceous Erongo complex,Namibia, have compositions consistent with near-primary mantlemelts derived from a depth of at least 100 km. These rocks providea key reference for the mantle component(s) involved in breakup-relatedmagmatism in this region. Initial SrNdPb isotoperatios of the Erongo basanites and associated tephrites andphonotephrites (87Sr/86Sr = 0·704250·70465; 相似文献
17.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 7476 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830760°C, and 100200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·363·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·760·5wt % SiO2) formed by mixing of a primitive olivineplagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age stockpileof silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics 相似文献
18.
Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island 总被引:2,自引:0,他引:2
The effects of source composition and source evolution duringprogressive partial melting on the chemistry of mantle-derivedmid-ocean ridge basalt (MORB) melts were tested using a comprehensivegeochemical and SrNdPb isotopic dataset for fresh,magnesian basaltic glasses from the Miocene Macquarie Islandophiolite, SW Pacific. These glasses: (1) exhibit clear parentdaughterrelationships; (2) allow simple reconstruction of primary meltcompositions; (3) show exceptional compositional diversity (e.g.K2O/TiO2 0·090·9; La/Yb 1·522;206Pb/204Pb 18·7019·52); (4) preserve changesin major element and isotope compositions, which are correlatedwith the degree of trace element enrichment (e.g. La/Sm). Conventionalmodels for MORB genesis invoke melting of mantle that is heterogeneouson a small scale, followed by binary mixing of variably lithophileelement-enriched melt batches. This type of model fails to explainthe compositions of the Macquarie Island glasses, principallybecause incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pbisotope ratios vary non-systematically with the degree of enrichment.We propose that individual melt batches are produced from instantaneousparental mantle parageneses, which change continuouslyas melting and melt extraction proceeds. This concept of a dynamicsource combines the models of small-scale mantle heterogeneitiesand fractional melting. A dynamic source is an assemblage oflocally equilibrated mantle solids and a related melt fraction.Common MORB magmas that integrate the characteristics of numerousmelt batches therefore tend to conceal the chemical and isotopicidentity of a dynamic source. This study shows that isotoperatios of poorly mixed MORB melts are a complex function ofthe dynamic source evolution, and that the range in isotoperatios within a single MORB suite does not necessarily requiremixing of diverse components. KEY WORDS: mid-ocean ridge basalt; Macquarie Island; radiogenic isotopes; mantle; geochemistry 相似文献
19.
Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, FeTi oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSrNdPb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The NdSrPbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotitespinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany 相似文献
20.
Cenozoic, post-collisional, potassic and ultrapotassic igneousrocks in the North Qiangtang, SongpanGanzi and NorthKunlun terranes of the northern Tibetan Plateau are distributedalong a semi-continuous, eastwest-trending, volcanicbelt, which is over 1200 km in length. Spatially, there is aclose association with major strike-slip faults, thrust faultsand pull-apart basins. The ages of these magmatic rocks rangefrom 45 Ma to the present (the youngest known eruption occurredin 1951); they are shoshonitic, compositionally similar to K-richsubduction-related magmas, and range in SiO2 from 44 to 66 wt%. There is a relative enrichment of large ion lithophile elements(LILE) and light rare earth elements (LREE) in the most primitivemagmatic rocks (MgO >6 wt %) in the North Qiangtang terranecompared with those in the SongpanGanzi and North Kunlunterranes; correspondingly, the primitive magmas have higher87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in theNorth Qiangtang terrane than in the SongpanGanzi andNorth Kunlun terranes. The dominant factors that control thegeochemical characteristics of the magmas are an enriched asthenosphericmantle source composition, the degree of partial melting ofthis source, and the combined processes of crustal assimilationand fractional crystallization (AFC). Enrichment of the asthenosphereis considered to have occurred by incorporation of subductedsediments into the mantle wedge above a subducted slab of Indianlithosphere during IndiaAsia convergence. Continentallithospheric mantle, metasomatically enriched during earlierepisodes of subduction, may have also contributed a source componentto the magmas. Trace element modelling indicates that the mantlesource of the most primitive magmas in the North Qiangtang terranecontained higher amounts of subducted sediment (0·510%)compared with those in the SongpanGanzi and North Kunlunterranes (<2%). The degrees of partial melting required togenerate the primitive potassic and ultrapotassic magmas fromthe enriched mantle sources range from <0·1% to 15%in the three major basement terranes. Energy-constrained AFCmodel calculations show that the more evolved magmatic rocks(MgO <6 wt %) are the results of AFC processes in the middlecrust in the North Qiangtang terrane and the upper crust inthe SongpanGanzi and North Kunlun terranes. We proposethat the ultimate driving force for the generation of the post-collisionalpotassium-rich magmatism in north Tibet is the continuous northwardunderthrusting of the Indian continental lithosphere followingIndiaAsia collision. This underthrusting resulted inupwelling of hot asthenosphere beneath north Tibet, squeezedup between the advancing Indian lithosphere and the backstopof the rigid Asian continental lithosphere. Asthenospheric upwellingmay have also contributed to uplift of the northern TibetanPlateau. KEY WORDS: Tibetan Plateau; potassic and ultrapotassic magmatism; enriched asthenospheric mantle source; EC-AFC modelling; geodynamics 相似文献