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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.
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   

3.
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   

4.
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   

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.
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   

7.
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   

8.
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   

9.
An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad 18O(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted 18O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also 18O depleted. l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock 18O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential 18O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic 18O(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have 18O(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower 18O(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. * Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada  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.
Major- and trace-element and Sr–Nd–Hf isotopic compositionsof garnet and clinopyroxene in kimberlite-borne eclogite andpyroxenite xenoliths were used to establish their origins andevolution in the subcontinental lithospheric mantle beneaththe central Slave Craton, Canada. The majority of eclogitescan be assigned to three groups (high-Mg, high-Ca or low-Mgeclogites) that have distinct trace-element patterns. Althoughpost-formation metasomatism involving high field strength element(HFSE) and light rare earth element (LREE) addition has partiallyobscured the primary compositional features of the high-Mg andhigh-Ca eclogites, trace-element features, such as unfractionatedmiddle REE (MREE) to heavy REE (HREE) patterns suggestive ofgarnet-free residues and low Zr/Sm consistent with plagioclaseaccumulation, could indicate a subduction origin from a broadlygabbroic protolith. In this scenario, the low REE and smallpositive Eu anomalies of the high-Mg eclogites suggest moreprimitive, plagioclase-rich protoliths, whereas the high-Caeclogites are proposed to have more evolved protoliths withhigher (normative) clinopyroxene/plagioclase ratios plus trappedmelt, consistent with their lower Mg-numbers, higher REE andabsence of Eu anomalies. In contrast, the subchondritic Zr/Hfand positive slope in the HREE of the low-Mg eclogites are similarto Archaean second-stage melts and point to a previously depletedsource for their precursors. Low ratios of fluid-mobile to lessfluid-mobile elements and of LREE to HREE are consistent withdehydration and partial melt loss for some eclogites. The trace-elementcharacteristics of the different eclogite types translate intolower Nd for high-Mg eclogites than for low-Mg eclogites. Withinthe low-Mg group, samples that show evidence for metasomaticenrichment in LREE and HFSE have lower Nd and Hf than a samplethat was apparently not enriched, pointing to long-term evolutionat their respective parent–daughter ratios. Garnet andclinopyroxene in pyroxenites show different major-element relationshipsfrom those in eclogites, such as an opposite CaO–Na2Otrend and the presence of a CaO–Cr2O3 trend, independentof whether or not opx is part of the assemblage. Therefore,these two rock types are probably not related by fractionationprocesses. The presence of opx in about half of the samplesprecludes direct crystallization from eclogite-derived melts.They probably formed from hybridized melts that reacted withthe peridotitic mantle. KEY WORDS: eclogites; pyroxenite xenoliths; mantle xenoliths; eclogite trace elements; eclogite Sr isotopes; eclogite Hf isotopes; eclogite Nd isotopes  相似文献   

12.
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   

13.
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   

14.
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   

15.
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   

16.
The Koperberg Suite comprises some 1700 small bodies of intrusiverocks largely composed of andesine anorthosite, biotite diorite,and leuconorite, norite and melanorite-hypersthenite; 30 mineshave been established in the O'okiep District in the cupriferousrocks of this anorthosite-charnockite kindred. The suite isintrusive into a sequence of granite gneiss and metavolcanicand metasedimentary rocks, and intrusive granite, that wereelevated to the granulite fades of regional metamorphism.TheSm-Nd model ages for the country rocks and the Koperberg Suiteare all 1700 Ma (TCHUR) and 2000 Ma (TDM) supporting a majorcrustforming event in this portion of Namaqualand at the endof Lower Proterozoic times. The granulite fades metamorphismin the O'okiep District is recorded by a Rb-Sr isochron ageof 1223 48 Ma on the Nababeep Granite Gneiss, and by (1197 15)-Ma-old inherited cores of zircons in the Koperberg Suite.The time of intrusion of the Concordia and Rietberg Granitesis believed to be reflected by their Rb-Sr whole-rock age of1105 24 Ma. The mean U-Pb age of 1029 10 Ma on individualzircon grains and zircon rims from the Koperberg Suite recordsthe time of its intrusion, and this is supported by the Sm-Ndwhole-rock age of 1022 42 Ma for the suite. Subsequent coolingand reheating events are recorded by the Ar-Ar ages of 800–850Ma for the Koperberg Suite, and of 500–550 Ma for thesuite and certain country rocks, respectively.An Nd value of-7,and its volume and composition, suggest a crustal-melt sourcefor the intrusive Concordia Granite. Moreover, the age-correctedhigh lSr (07061-07272) and low Nd (-9), and the high µ2(101), that characterize the Koperberg Suite also imply a crustalsource, and a model is presented for the generation of the majorpart of the suite by partial melting of granulites of overallintermediate (diorite) composition in the lower crust. Corresponding author  相似文献   

17.
Triclinic KFeSi3O8, iron-microcline, has been synthesized fromoxide mixes and by complete conversion of monoclinic KFeSi3O8,iron-sanidine. Iron-microcline is triclinic, C, a=8?68?0?01?, b=13?10?0?01, c=7?34?0?01, =90? 45'?10', ß=116?03'?10', =86?14'?10'. The optical properties (Na light) are:=1?585?0?002, ß=1?596?0?002, =1?605?0?002, 2V=85?(calc.), Xb, Z c=20??5?. A reversible phase transition betweentriclinic and monoclinic KFeSi3O8 occurs at 704??6? C at 2000bars total pressure. Iron-microcline is the low-temperaturepolymorph; no intermediate polymorphs were observed in eitherhydrothermal or dry heating experiments.  相似文献   

18.
The Wrangellia terrane of North America contains a large volumeof Middle to Late Triassic oceanic flood basalts which wereemplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopiccompositions reflect derivation from a plume source with Nd(T)+6 to + 7, 87Sr/86Sri0•7034, and 206Pb/204Pbi19•0.Major and trace element compositions suggest the Wrangelliaflood basalts (WFB) formed through relatively small degreesof partial melting at greater depths than estimated for otheroceanic plateaux such as Ontong Java. It appears that the WFBdid not form in a rifting environment, and that preexistingarc lithosphere limited the ascent and decompression meltingof the source plume. Rocks from the preexisting arc are stronglydepleted in high field strength elements (HFSEs) relative tolarge ion lithophile elements (LILEs), but the WFB are not.Assimilation of arc lithospheric mantle or crust was thereforegenerally minor. However, some contamination by arc componentsis evident, particularly in basalts erupted in the early stagesof volcanism. Minor isotopic shifts, to lower Nd(T) and 206Pb/204Pbiand higher 87Sr/86Sri, are accompanied by shifts in trace elementratios towards more arclike signatures, e.g. low Nb/Th and Nb/La.Arc contamination is greatest in the most evolved basalts, indicatingthat assimilation was coupled with fractional crystallization.A comparison of the WFB with other continental and oceanic floodbasalts reveals that continental flood basalts generally formthrough smaller degrees of melting than oceanic flood basaltsand that the contribution of material from the crust and litho-sphericmantle is significantly greater. KEY WORDS: oceanic flood basalts; Wrangellia terrane; petrogenesis; Sr-Nd-Pb isotopes *Corroponding author  相似文献   

19.
Geochemical and 40 Ar—39 Ar studies of the Malaita OlderSeries and Sigana Basalts, which form the basement of Malaitaand the northern portion of Santa Isabel, confirm the existenceof Ontong Java Plateau (OJP) crust on these islands. Sr, Nd,and Pb isotopic ratios of Malaita Older Series and Sigana lavasfall within limited ranges [(87Sr/86Sr)T= 0.70369–0.70423,ENd(T)= + 3.7 to +6.0, and 206Pb/204Pb = 18.25–18.64]virtually indistinguishable from those found in the three OJPbasement drill sites as far as 1600 km away, indicating a uniformhotspot-like mantle source with a slight ‘Dupal’signature for the world's largest oceanic plateau. Three chemicaltypes of basalts are recognized, two of which are equivalentto two of the three types drilled on the plateau, and one withno counterpart, as yet, on the plateau; the chemical data indicateslightly different, but all high, degrees of melting and slightvariation in source composition. All but one of the 40Ar-39Arplateau ages determined for Malaita Older Series and SiganaBasalt lavas are identical to those found at the distant drillsites: 121.30.9 Ma and 92.01.6 Ma, suggesting that two short-lived,volumetrically important plateau-building episodes took place30 m.y. apart. Aside from OJP lavas, three isotopically distinctsuites of alkalic rocks are present. The Sigana Alkalic Suitein Santa Isabel has an 40 Ar-39 Ar age of 91.70.4 Ma, the sameas that of the younger OJP tholeiites, yet it displays a distinct’HIMU‘ -type isotopic signature [206Pb/204Pb 20.20,(87Sr/86Sr) T 0.7032, Nd(T) 4.4], possibly representing small-degreemelts of a minor, less refractory component in the OJP mantlesource region. The Younger Series in southern Malaita has an40Ar-39Ar age of 44 Ma and isotopic ratios [Nd(T)=-0.5 to +1.0,(87Sr/86Sr)T =0.70404–0.70433, 206Pb/204Pb = 18.57–18.92]partly overlapping those of the ‘PHEM’ end-memberpostulated for Samoa, and those of present-day Rarotonga lavas;one or both of these hotspots may have caused alkalic volcanismon the plateau when it passed over them at 44 Ma. The NorthMalaita Alkalic Suite in northernmost Malaita is probably ofsimilar age, but has isotopic ratios [(87Sr/86Sr) T 0.7037,Nd(T) +4.5, 206pb/204pb 18.8) resembling those of some OJP basementlavas; it may result from a small amount of melting of agedplateau lithosphere during the OJP's passage over these hotspots.Juxtaposed against OJP crust in Santa Isabel is an 62–46-Maophiolitic (sensu lato) assemblage. Isotopic and chemical datareveal Pacific-MORB-like, backarc-basin-like, and arc-like signaturesfor these rocks, and suggest that most formed in an arc—backarcsetting before the Late Tertiary collision of the OJP againstthe old North Solomon Trench. The situation in Santa Isabelappears to provide a modern-day analog for some Precambriangreenstone belts. KEY WORDS: oceanic plateaux; Ontong Java Plateau; Solomon Islands; Sr-Nd-Pb isotopes; age and petrogenesis *Corresponding author.  相似文献   

20.
The petrogenesis of pyroxenite layers within the Beni Bouseraperidotite massif is investigated by means of elemental andNd-Sr-Pb-O-S isotope analyses. The light rare earth element(LREE) depleted nature of many of the pyroxenites, their widevariation in composition, and lack of correlation between incompatibleelements and fractionation indices preclude them from representingcrystallized melts from a peridotitic source. The physical characteristicsof the pyroxenites and their large (greater than a factor of20) range in Ni rule out partial melting as the cause of theirpetrological and geochemical diversity. Major and compatibletrace element geochemistry is consistent with formation of mostof the pyroxenite suite via high-pressure crystal segregationin magma conduits intruding the peridotites. These magmas crystallizedclinopyroxene, orthopyroxene, and garnet. The pressure of crystallizationis constrained to be above {small tilde}45 kbar from the presenceof graphitized diamonds in pyroxenite layers. Lack of correlationbetween fractionation indices and highly incompatible elementsand the wide variation in incompatible element abundances suggestthat the suite did not form from genetically related magmas.The presence of positive and negative Eu anomalies (Eu/Eu* =0•54–2•0) in pyroxenites which crystallizedat pressures much greater than the plagioclase stability field({small tilde} 45 kbar) suggests that the parental magmas originatedfrom precursors which formed in the crust. Oxygen isotope compositionsof coexisting minerals in the pyroxenites indicate high-temperatureequilibration but 18O values vary from +4•9 to + 9•3,ruling out their derivation from the host peridotites or othernormal mantle sources. The extreme O-isotope variation, togetherwith 34S values of up to + 13 in sulphides included within CPXstrongly suggests that the melts from which the pyroxenitescrystallized were derived from hydrothermally altered, subductedoceanic lithosphere. Extreme initial radiogenic isotope variationin the pyroxenites (Nd + 26 to –9 , 87Sr/86Sr 0•7025–0•7110,206Pb/204Pb 18•21–19•90) support such an originbut also require a component with ancient, high U/Pb and Th/Pbin their source to explain the high 7/4 and 8/4 values of somepyroxenites. This component may be subducted hemi-pelagic sediment.Further evidence for a sediment component in the pyroxenitesis provided by isotopically light carbon in the graphite pyroxenites(13C–16 to – 28). Parentdaughter isotopes in thepyroxenites are strongly decoupled, making estimation of formationages speculative. The decoupling occurred recently (<200Ma), probably as a result of partial melting associated withdiapiric upwelling and emplacement of the massif into the crustfrom the diamond stability field. This late partial meltingevent further depleted the pyroxenites in incompatible elements.The variably altered nature of the subducted protolith and complexhistory of trace element fractionation of the pyroxenites haslargely obscured geochemical mixing trends. However, Nd–Pbisotope systematics indicate that incorporation of the componentwith high U/Pb–Th/Pb occurred relatively recently (<200Ma) for some pyroxenites. Other pyroxenites do not show evidencefor incorporation of such a component and may be substantiallyolder. Tectonic, geophysical, and isotopic constraints indicateformation of the pyroxenites in the mantle wedge above a subductingslab during the Cretaceous. Physical and chemical evidence forhigh-pressure fractionation seen in most of the pyroxenitesprecludes them from simply representing ancient subducted oceaniclithosphere, thinned by diffusion. However, the petrologicaland isotopic diversity of the massif support the concept ofa ‘marble cake’ mantle capable of producing theobserved geochemical diversity seen in oceanic magmas. *Present Address: Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 Present address: Department of Geological Sciences, 1066 C.C. Little Building, University of Michigan, Ann Arbor, Michigan 48109  相似文献   

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