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1.
Hafnium isotope and incompatible trace element data are presentedfor a suite of mid-ocean ridge basalts (MORB) from 13 to 47°Eon the Southwest Indian Ridge (SWIR), one of the slowest spreadingand most isotopically heterogeneous mid-ocean ridges. Variationsin Nd–Hf isotope compositions and Lu/Hf ratios clearlydistinguish an Atlantic–Pacific-type MORB source, presentwest of 26°E, characterized by relatively low Hf valuesfor a given Nd relative to the regression line through all Nd–Hfisotope data for oceanic basalts (termed the ‘Nd–Hfmantle array line’; the deviation from this line is termedHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature,present east of 32°E, characterized by relatively high Hfvalues and Lu/Hf ratios. Additionally, two localized, isotopicallyanomalous areas, at 13–15°E and 39–41°E,are characterized by distinctly low negative and high positiveHf values, respectively. The low Hf MORB from 13 to 15°Eappear to reflect contamination by HIMU-type mantle from thenearby Bouvet mantle plume, whereas the trace element and isotopiccompositions of MORB from 39 to 41°E are most consistentwith contamination by metasomatized Archean continental lithosphericmantle. Relatively small source-melt fractionation of Lu/Hfrelative to Sm/Nd, compared with MORB from faster-spreadingridges, argues against a significant role for garnet pyroxenitein the generation of most central SWIR MORB. Correlations betweenHf and Sr and Pb isotopic and trace element ratios clearly delineatea high-Hf ‘Indian Ocean mantle component’ that canexplain the isotope composition of most Indian Ocean MORB asmixtures between this component and a heterogeneous Atlantic–Pacific-typeMORB source. The Hf, Nd and Sr isotope compositions of IndianOcean MORB appear to be most consistent with the hypothesisthat this component represents fragments of subduction-modifiedlithospheric mantle beneath Proterozoic orogenic belts thatfoundered into the nascent Indian Ocean upper mantle duringthe Mesozoic breakup of Gondwana. KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean  相似文献   

2.
The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 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 fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCN–YbCN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate 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 rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope 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   

3.
The Palaeogene Kangerlussuaq Intrusion (50 Ma) of East Greenlanddisplays concentric zonation from quartz-rich nordmarkite (quartzsyenite) at the margin, through pulaskite, to foyaite (nephelinesyenite) in the centre; modal layering and igneous laminationare locally developed but there are no internal intrusive contacts.This is an apparent violation of the phase relations in Petrogeny'sResidua System. We propose that this intrusion is layered, gradingfrom quartz syenite at the bottom to nepheline syenite at thetop. Mineral and whole-rock major and trace element data andSr–Nd–Hf–Pb isotope data are presented thatprovide constraints on the petrogenesis of the intrusion. Radiogenicisotope data indicate a continuously decreasing crustal componentfrom the quartz nordmarkites (87Sr/86Sr = 0·7061; Ndi= 2·3; Hfi = 5·2; 206Pb/204Pbmeas = 16·98)to the foyaites (87Sr/86Sr = 0·7043–0·7044;Ndi = 3·8–4·9; Hfi = 10·7–11·1;206Pb/204Pbmeas = 17·78–17·88); the foyaitesare dominated by a mantle isotopic signature. The average Mg-numberof amphibole cores becomes increasingly primitive, varying from26·4 in the nordmarkites to 57·4 in the pulaskites.Modal layering, feldspar lamination and the presence of hugebasaltic xenoliths derived from the chamber roof, now restingon the transient chamber floor, demonstrate bottom-upwards crystallization.The intrusion cannot, therefore, have formed in a system closedto magmatic recharge. The lack of gneissic xenoliths in thenordmarkites suggests that most contamination took place deeperin the crust. In the proposed model, the nordmarkitic magmaformed during crustal assimilation in the roof zone of a large,silica-undersaturated alkali basaltic/basanitic, stratifiedmagma chamber, prior to emplacement in the uppermost crust.The more primitive syenites, terminating with foyaite at thetop of the intrusion, formed as a consequence of repeated rechargeof the Kangerlussuaq Intrusion magma chamber by tapping lesscontaminated, more primitive phonolitic melt from deeper partsof the underlying chamber during progressive armouring of theplumbing system. KEY WORDS: Kangerlussuaq; East Greenland; syenite; crustal contamination; magma mixing  相似文献   

4.
The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080).Quartz phenocryst 18O values indicate that the rhyolite magmashad 18O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with 18O values between4·8 and 6·2. The low-18O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ 18O magmavalues. These low-18O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low 18O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the 18O magma value. It is proposed that the low-18Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-18O magmas; partial melting  相似文献   

5.
The present work reports the first broad geochemical investigationof the recently discovered late Archean (2700 Ma) Skjoldungenalkaline igneous province (SAP) in southeast Greenland. Therocks studied range in composition from ultramafic to felsicand comprise pyroxenites, hornblendites, hornblende noritesand diorites, monzonites, syenites, and nephelinitic rocks andcarbonatites. Various lithologic units from the host Archeangneissic basement are also investigated. The magmatic rocksshow remarkably coherent major element, trace element, rareearth element (REE), and Sr and Nd isotope systematics, suggestinga petrogenetic relationship. The most important geochemicalfeatures are high normative proportions of nepheline, forsteriteand albite, low TiO2 (<15 wt %) and moderate FeO (total)(<12 wt %) contents, enrichments in large ion lithophileelements (LILE) and light rare earth elements both absoluteand relative to high field strength elements (HFSE) that displaylarge negative anomalies, and generally low to moderate abundancesof compatible elements. Field relations and REE and compatibleelement systematics among Skjoldungen rocks suggest that maficand ultramafic hornblende-rich samples may represent cumulatelithologies of the regional parental magma. On the basis ofmineral data, this is deduced to have had mg-number of 064,shoshonitic affinities (K2O15 wt %), been close to silica saturationand volatile rich. Major element, trace element and REE systematicsfurther suggest that felsic intrusions are related to the maficregional parental magma through extensive olivine, hyperstheneand hornblende fractionation. Lack of correlation between La/Yband other critical trace and REE ratios indicates that apatite,zircon and titaniferous minerals were not important cumulusphases at advanced stages of evolution. The measured Sm–Ndwhole-rock isochron age is 2716 23 Ma (2 error) [mean squareof weighted deviates (MSWD) = 14], whereas linear regressionof the Sr isotope data yields an age of 26047 Ma (2 error)(MSWD = 22•2). The age obtained by Nd isotopes is corroboratedby U–Pb zircon results (2698 7 Ma), suggesting thatthe Sm–Nd system remained closed since crystallization.By contrast, the 100 Ma younger age obtained by Sr isotopessuggests that the Rb–Sr system has been disturbed. Initial143Nd/144 Nd ratios span a narrow range corresponding to Nd(27Ga) =+074 to –109, whereas initial Sr values at 27Ga cover a comparatively larger interval from –10 to +20.Neither initial Nd nor initial Sr values conform to previouslysuggested mantle depletion curves and no meaningful correlationexists between Nd and Sr isotopes for the Skjoldungen magmaticrocks as a whole. Although compositionally heterogeneous, theanalyzed suite of samples from the host agmatitic basement isextremely homogeneous with respect to age, with TCHUR crustalresidence times around 2700–2800 Ma confirming limitedavailable isotopic evidence. Large-scale assimilation of Archeancrust or recycling of sediments derived from the local basementinto the mantle source fails to explain adequately negativeNb anomalies and low Nd signatures characteristic of the Skjoldungenintrusions. Rather, the nearchondritic isotopic compositionof Nd in the Skjoldungen samples together with the decoupledLILE and HFSE enrichment and slightly positive Sr values areconsidered to reflect characteristics of the mantle source ina subduction zone environment. The geodynamic site hosting theSkjoldungen province thus may be an early manifestation of modern-styleplate tectonics. KEY WORDS: Skjoldungen province; Greenland; Archean; alkaline igneous rocks; geochronology; geochemistry *Corresponding author. Present address: Ecole Normale Suprieure de Lyon, 46 AlLe d'Italie, 69364 Lyon Cedex 07, France  相似文献   

6.
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 (73–75).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 1630–1690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected Nd–Sr–Pbisotope 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   

7.
Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al2O3/TiO2 ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial 187Os/188Os of 0·1131 ± 0·0006(2), Os(I) = 0·1 ± 0·5. These data, coupledwith an initial Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment  相似文献   

8.
Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite,New England batholith, eastern Australia, are consistent withmagma mixing between two silicic melts, each derived from isotopicallydistinct sources. Although zircons from three zones within theWalcha Road pluton give a U–Pb crystallization age of249 ± 3 Ma, zircon populations from each zone have arange in Hf. Zircons from the mafic hornblende–biotitemonzogranite pluton margin and intermediate zones have Hf +5to +11, whereas those from the more felsic centre of the plutonhave Hf +7 to +16, representing a total variation of 11 Hfunits. The Lu–Hf depleted mantle model ages range from650 to 250 Ma, with the younger zircons present only in thefelsic pluton centre. The variation in Hf indicates the involvementof silicic melts from at least two sources, one a crustal componentwith a Neoproterozoic model age and the other a primitive mantle-derivedcomponent with model ages similar to the U–Pb crystallizationage of the pluton. The zircons reflect the isotopic compositionsof the different proportions of crustal-derived silicic melt,relative to mantle-derived silicic melt, between melt generationand final pluton construction. The Walcha Road pluton is consideredto have formed by incremental assembly of progressively morefelsic melt batches resulting from mixing, replenishment andcrystal–melt separation, with final pluton constructioninvolving mechanical concentration as zones of crystal mush.The zoned pluton and, more broadly, the Moonbi supersuite provideexamples of magma mixing by which the more silicic units havemore juvenile isotopic compositions as a result of increasingproportions of residual melt from basalt fractionation, relativeto crustal partial melt. KEY WORDS: Australia; granite magma mixing; zircon; zoned pluton; Hf isotopes  相似文献   

9.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination  相似文献   

10.
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 0·6–1·16. A distinctive feature ofthese rocks is their low TiO2 content (0·5–1·4wt %). 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 (100–800 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 =0·7055–0·7079, Hfi = –7·6 to–29·4 and 206Pb/204Pbi = 16·6–17·4.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 Sr–Nd–Pb–Hf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative Hf as a result of metasomatism by slab-derivedhydrous melts that left rutile–garnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotope–trace 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   

11.
The Ni-S System and Related Minerals   总被引：1，自引：0，他引：1  
The system Ni-S has been studied systematically from 200? to1, 030? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi3S2 (and a high temperature, non-quenchable Ni3?S2 phase),Ni7S6, Ni1–S4 Ni3S4, and NiS2. The geologic occurrenceof the minerals heazlewoodite (Ni2S2), millerite (ßSNi1-2S),polydymite (Ni3S4), and vaesite (NiS2) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556? C. This high-temperature phase(Ni3 has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600? C, and melts incongruently at 806??3? C.The ßNi7S6 phase inverts to Ni78 at 397? C6 when inequilibrium with Ni3S2, and at 400? C when in equilibrium withNiS. Crystals of Ni7S6 break down to Ni3-S2+NiS at 573??3?C.The low-temperature form of Ni1-S1 corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379??3?C, whereas Ni1-S with the maximum nickel deficiency invertsat 282??5OC. The Ni1-alphS-NiS2 solvus was determined to 985??3?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600?C but breaks down to Ni2-S2 and Ni1-S below797?C, whereas Ni1-S with 38.2 wt per cent sulfur melts congruentlyat 992??3?C. Vaesite does not vary measurably from stoichiometricNiS2 composition, and melts congruently at 1.007?5?C. Polydymitebreaks down to aNi-S? vaesite at 356??3?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991?C and over a wide compositionalrange.  相似文献   

12.
Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping 87Sr/86Sri ratios (0·70369–0·70662)and show a narrow range in initial Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has 13CPDB between –5·7 and –5,similar to primary mantle-derived carbonates, and 18OSMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (13CPDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology  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) 14–20%; metapelites: 18O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(18O 5–13%), 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 fluid—rockinteraction 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) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology *Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au  相似文献   

14.
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 diffusion–dispersion 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 diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress  相似文献   

15.
Isotopic results (Sr, Nd, Pb), as well as concentrations ofmajor and trace elements (REE) are reported for whole-rock samplesand mineral separates from the onland alkaline complex of Serrade Monchique (South Portugal) and the offshore alkali basaltvolcanic suite of Mount Ormonde (Gorringe Bank). These two geneticallyrelated alkaline complexes were emplaced at the east Atlanticcontinent–ocean boundary during the Upper Cretaceous,i.e. 66–72 m.y. ago. Taken together, Serra de Monchiqueand Mount Ormonde may be seen as one of the few examples ofwithin-plate magmatism that straddles the continent–oceanboundary. Major and trace element compositions fail to revealany significant differences between onland and offshore complexes.This is particularly true regarding less differentiated samples(mg-number 0.40) which show the same progressive and continuousenrichment of their trace element patterns, with no specificanomaly (e.g. negative Nb anomaly) being present in samplesfrom the onland complex. Initial Pb and Sr isotopic compositionsalso do not allow any distinction to be made between Serra deMonchique and Mount Ormonde samples. Initial Pb isotope ratiosare moderately high (19.1 < 206Pb/204Pb < 19.8; 207Pb/204Pb= 15.6) in both cases. Moreover, once the effects of Sr contaminationby seawater are taken into account and the most contaminatedsamples discarded using data from fresh clinopyroxene separatesand results of leaching experiments, the initial Sr isotopiccompositions of Mount Ormonde samples are found to be unradiogenic(87Sr/86Sr = 0.7031±1) and identical to those obtainedat Serra de Monchique (87Sr/86Sr = 0.7032±1). In contrast,a systematic mean difference of 2 Nd units is observed betweenSerra de Monchique [Nd(T) = +4.8] and Mount Ormonde [Nd(T) =+6.6] whole-rock samples. Surprisingly, a variation is alsoobserved at Mount Ormonde between the whole-rock samples andone of the two analysed clinopyroxene separates. Whereas MountOrmonde whole-rock samples invariably yielded Nd(T) = +6.6 (meanvalue), a value of +0.5 is obtained for one clinopyroxene separate,whereas another gives +6.0. The above geochemical and isotopicresults make it possible to assign respective roles to the asthenosphere,lithosphere and crust in the petrogenesis of Serra de Monchiqueand Mount Ormonde complexes. We propose that both complexesshare a common mantle source whose isotopic characteristicsare very similar to the source of oceanic island basalts. Continentalmantle lithosphere, already characterized isotopically by studiesof peridotite massifs within the Iberian peninsula, acts asa contaminant which is evident onland on the whole-rock scale,and also present offshore as discrete clinopyroxene xenocrysts.The continental crust appears to play no role in the petrogenesisof the Serra de Monchique alkaline rocks. KEY WORDS: alkaline complexes; continental lithosphere; isotope geochemistry; passive continental margin; within-plate volcanics  相似文献   

16.
A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H2O = mineral + H+, (sum of cations) + H2O = (sum of normative minerals of a rock)+ H+. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logPH2O = 0) and chemical activity of the metal ions ( log Mn+= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog H+ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.  相似文献   

17.
This study focuses on the origin of magma heterogeneity andthe genesis of refractory, boninite-type magmas along an arc–ridgeintersection, exposed in the Lewis Hills (Bay of Islands Ophiolite).The Lewis Hills contain the fossil fracture zone contact betweena split island arc and its related marginal oceanic basin. Threetypes of intrusions, which are closely related to this narrowtectonic boundary, have been investigated. Parental melts inequilibrium with the ultramafic cumulates of the PyroxeniteSuite are inferred to have high MgO contents and low Al2O3,Na2O and TiO2 contents. The trace element signatures of thesePyroxenite Suite parental melts indicate a re-enriched, highlydepleted source with 0·1 x mid-ocean ridge basalt (MORB)abundances of the heavy rare earth elements (HREE). InitialNd values of the Pyroxenite Suite range from -1·5 to+0·6, which overlap those observed for the island arc.Furthermore, the Pyroxenite Suite parental melts bear strongsimilarities to boninite-type equilibrium melts from islandarc-related pyroxenitic dykes and harzburgites. Basaltic dykessplit into two groups. Group I dykes have 0·6 x MORBabundances of the HREE, and initial Nd values ranging from +5·4to +7·5. Thus, they have a strong geochemical affinitywith basalts derived from the marginal basin spreading ridge.Group II dykes have comparatively lower trace element abundances(0·3 x MORB abundances of HREE), and slightly lower initialNd values (+5·4 to +5·9). The geochemical characteristicsof the Group II dykes are transitional between those of GroupI dykes and the Pyroxenite Suite parental melts. Cumulates fromthe Late Intrusion Suite are similarly transitional, with Ndvalues ranging from +2·9 to +4·6. We suggest thatthe magma heterogeneity observed in the Lewis Hills is due tothe involvement of two compositionally distinct mantle sources,which are the sub-island lithospheric mantle and the asthenosphericmarginal basin mantle. It is likely that the refractory, boninite-typeparental melts of the Pyroxenite Suite result from remeltingof the sub-arc lithospheric mantle at an arc–ridge intersection.Furthermore, it is suggested that the thermal-dynamic conditionsof the transtensional transform fault have provided the prerequisitefor generating magma heterogeneity, as a result of mixing relationshipsbetween arc-related and marginal basin-related magmas. KEY WORDS: Bay of Islands ophiolite; transform (arc)–ridge intersection; boninites; rare earth elements, Nd isotopes  相似文献   

18.
Re—Os and Sm—Nd isotopic data have been obtainedfor mafic and ultramafic cumulates from the 2700-Ma StillwaterComplex and associated fine-grained sills and dykes, so as tobetter constrain the geochemical characteristics of Stillwaterparental magmas and to trace the source(s) of the precious metalsthat have been concentrated in the J-M Reef, the major platinum-groupelement mineral deposit in the complex. Initial Os isotopiccompositions (187Os/188Os) for chromitites from the Ultramaficseries range from a radiogenic isotopic composition of 0.1321(Os = +21) for the platinum group element (PGE)-enriched B chromititeseam from the West Fork area to a near-chondritic isotopic compositionof 0.1069–0.1135 (Os=–2 to +4.1) for the PGE-poorG and H chromitite seams, respectively, near the middle of theUltramafic series. Osmium isotopic data for the PGE-rich B chromititeseam are generally isochronous with whole-rock and mineral datafor the J-M Reef (Os = + 12 to + 34). Re—Os isotopic datatherefore document a contrast between PGE-poor cumulates fromthe Ultramafic series and PGE-enriched cumulates from both theUltramafic series and the J-M Reef, suggesting that Os and probablythe other PGE were derived from at least two isotopically distinctsources. Moreover, these Re-Os isotopic characteristics correlatewith petrogenetic subdivisions of the Stillwater Complex basedon field mapping, petrology, REE geochemistry, and Sm—Ndisotope geochemistry. The data are best explained by mixingof two magma types, referred to as U-type and A-type magmas,with differing major element, trace element, and precious metalabundances and isotopic compositions. Although crustally contaminatedkomatiites can mimic the Os and Nd isotopic characteristicsof the U-type magma, the combination of low initial Os isotopicvalues (Os0) with low initial Nd isotopic values (Nd–1),high 207Pb/204Pb for a given 206Pb/204Pb (Wooden et al., 1991),and high (Ce/Yb)n ratios in U-type cumulates and fine-grainedsills and dykes is more consistent with the involvement of aRe-poor, but trace-element-enriched portion of the subcontinentallithospheric mantle in the petrogenesis of Stillwater U-typemagmas. However, the radiogenic initial Os isotopic compositionsof the J-M Reef and other portions of the intrusion with elevatedPGE concentrations suggest that A-type parental magmas incorporatedOs from radiogenic early Archaean crust. The relatively largerange in (Ce/Yb)n, Os, and Nd values suggests that mixing ofgeochemically distinct magmas may have been an important processthroughout the history of the Stillwater magma chamber. Magmamixing may then explain not only the PGE-enriched J-M Reef butalso the anomalous enrichment of the PGE in the B chromititeseam from the West Fork area and the variable values observedin other chromitite seams of the Ultramafic series. The intimateassociation of these magma types, derived from or modified inthe Archaean continental lithosphere, may then be crucial tothe formation of magmatic PGE mineral deposits.  相似文献   

19.
Tourmaline in the Martinamor antiform occurs in tourmalinites(rocks with >15–20% tourmaline by volume), clasticmetasedimentary rocks of the Upper Proterozoic Monterrubio formation,quartz veins, pre-Variscan orthogneisses and Variscan graniticrocks. Petrographic observations, back-scattered electron (BSE)images, and microprobe data document a multistaged developmentof tourmaline. Overall, variations in the Mg/(Mg + Fe) ratiosdecrease from tourmalinites (0·36–0·75),through veins (0·38–0·66) to granitic rocks(0·23–0·46), whereas Al increases in thesame order from 5·84–6·65 to 6·22–6·88apfu. The incorporation of Al into tourmaline is consistentwith combinations of xAl(NaR)–1 and AlO(R(OH))–1exchange vectors, where x represents X-site vacancy and R is(Mg + Fe2+ + Mn). Variations in x/(x + Na) ratios are similarin all the types of tourmaline occurrences, from 0·10to 0·53, with low Ca-contents (mostly <0·10apfu). Based on field and textural criteria, two groups of tourmaline-richrocks are distinguished: (1) pre-Variscan tourmalinites (probablyCadomian), affected by both deformation and regional metamorphismduring the Variscan orogeny; (2) tourmalinites related to thesynkinematic granitic complex of Martinamor. Textural and geochemicaldata are consistent with a psammopelitic parentage for the protolithof the tourmalinites. Boron isotope analyses of tourmaline havea total range of 11B values from –15·6 to 6·8;the lowest corresponding to granitic tourmalines (–15·6to –11·7) and the highest to veins (1·9to 6·8). Tourmalines from tourmalinites have intermediate11B values of –8·0 to +2·0. The observedvariations in 11B support an important crustal recycling ofboron in the Martinamor area, in which pre-Variscan tourmaliniteswere remobilized by a combination of mechanical and chemicalprocesses during Variscan deformation, metamorphism and anatexis,leading to the formation of multiple tourmaline-bearing veinsand a new stage of boron metasomatism. KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes  相似文献   

20.
This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have 18O values(6·1–8·4) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high 18O crustal material. In this regard, the 18O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower 18O values (average 6·3± 0·1) at given SiO2 contents relative to rockserupted from distal vents (>7·0 km; average 7·1± 0.1). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ± 0·2). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the 18O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high 18O continental crust (9·0) increasinglyhybridized by lower 18O (6·0) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source 18O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the18O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen  相似文献   

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