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Predominantly silica-oversaturated tholeiitic basalts spreadwidely across the northern East Greenland continental margin(73–76° N) at the onset of Tertiary magmatism. Thesehave been studied in two of the principal outcrop areas: Holdwith Hope and Wollaston Forland. In Hold with Hope very minoreruption of nephelinitic magma preceded the tholeiitic phase.The early tholeiites of northern East Greenland tend to be moresiliceous and aluminous and less iron and titanium rich thantheir more voluminous counterparts in the main East Greenlandprovince south of 70° N. Some, notably those of WollastonForland, approach ‘enriched MORB’ compositions.The early flood basalts (distinguished in the Hold with Hoperegion as the Lower Plateau Lava Series, LPLS) show limitedcompositional range. Such variation as is shown is attributedto low-pressure plagioclase—olivine—augite fractionation.Intrusion of quartz dolerite sills within and beneath the LPLSmay have constituted a separate but closely allied magmaticevent. The sills are slightly more enriched in incompatibleelements, have higher initial Sr isotopic ratios and show morepronounced Fe-enrichment trends. Conformably overlying the LPLS in the Hold with Hope area isa succession of incompatible element enriched basalts composingthe Upper Plateau Lava Series (UPLS). The more extremely enrichedlavas (anomalous UPLS) tend to define the base of the UPLS andrange from olivine tholeiites to analcime basanites. Togetherwith the more normal UPLS, these range from nearly aphyric tostrongly porphyritic varieties; the latter include highly magnesianoceanites and ankaramites. The normal UPLS ranges from picriticolivine tholeiites to quartz tholeiites. While orthopyroxenephenocrysts occur among the more differentiated basalts, pigeoniteappears wholly absent from the UPLS. Compositional variationamong the normal UPLS is ascribed to olivine + spinel fractionationjoined by clinopyroxene in compositions with less than 8 percent MgO. The UPLS basalts have incompatible element levelsand elemental ratios (e.g. Zr/Nb, Zr/Y, La/Y and Ti/V) thatdistinguish them from the earlier, more depleted tholeiites.Initial Sr isotope ratios vary widely, with high values indicativeof crustal interaction. A genetic model is proposed whereby the more depleted tholeiites,as exemplified by those of Wollaston Forland and the earliervolcanic products on Hold with Hope, arose from large-scalemelting of relatively infertile asthenosphere prior to the initiationof the Mohn's Ridge and continental separation, while the UPLSof the Hold with Hope region and isolated inland occurrencesof nephelinitic and basanitic magmatism were produced by meltingof K–Ti–P enriched facies of the heterogeneous mantlepresumed to have formed the lower part of the sub-Greenlandcontinental lithosphere. Basalts of the Vøring Plateauon the Norwegian continental shelf, may represent analoguesof the UPLS on the eastern Atlantic margins. 相似文献
2.
The Younger Giant Dyke Complex consists of a set of massivecoalescing dykes of Proterozoic age (c. 1170 Ma), resultingfrom intrusion of a suite of transitional olivine basaltic/hawaiiticmagmas in a continental rift setting. The suite, compositionallyrelated by low pressure (< 10 kb) olivine-plagioclase fractionation,is believed to have had a deeper level evolution dominated bypyroxene and possibly garnet fractionation. Slow cooling insitu of the interior parts of the dyke complex produced cumuliticsuites. Those exposed range from gabbroic to syenitic; residualbodies of riebeckite granite and, very subordinate, feldspathoidalsyenite were also generated. The basic magmas had notably lowfO2 values, leading to delayed magnetite and clinopyroxene precipitation,relatively iron-rich differentiates and some residual liquidsof pantelleritic composition. The basic magmas had high F/Clvalues and are inferred to have had low H2O contents. They werealso characterized by relatively high K/Rb and low 87Sr/86Srvalues; these characteristics imply a mantle source with highF/Cl but depleted in Rb relative to K and Sr. Basaltic magmasresponsible for (a) the preceding Older Giant Dyke Complex and(b) a suite of anorthositic xenoliths within the Younger GiantDyke Complex, are inferred to have been derived from separateprimary magma batches independent of those that yielded theYounger Giant Dyke Complex. The giant dykes are the highest-levelrepresentatives of a larger basic complex responsible for theextensive linear gravity ‘high’ in the Tugtutôq-Narssaqarea. 相似文献
3.
Schists from the Appalachian Orogen in south-east Vermont have undergone multiple phases of garnet growth. These phases can be distinguished by the trend and relative timing of f oliation i nflexion or i ntersection a xes (FIAs) of foliations preserved as inclusion trails in garnet porphyroblasts. The relative timing of different generations of FIAs is determined from samples containing porphyroblasts with two or three differently trending FIAs developed outwards from core to rim (multi-FIA porphyroblasts). Schists from south-east Vermont show a consistent pattern of relative clockwise rotation of FIA trends from oldest to youngest. Four populations or sets of FIAs can be distinguished on the basis of their relative timings and trends. From oldest to youngest, the four sets have modal peaks trending SW–NE, W–E, NNW–SSE and SSW–NNE. These peaks show that each of the four FIA sets has a statistically consistent trend at all scales across a 35×125 km area containing numerous mesoscopic and macroscopic folds. The FIAs of Set 4 are defined by inclusion trails that are continuous with matrix foliations, have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures. Conversely, Sets 1 to 3 are oblique to and pre-date most matrix foliations and folds. All four FIA sets occur in Siluro-Devonian rocks and must have formed in the Acadian Orogeny. The lack of statistically significant differences in the distribution of FIA trends across the study area and their consistent relative timings in multi-FIA porphyroblasts, despite a complex regional deformation history involving numerous phases of folding at all scales, suggest the porphyroblasts have not rotated relative to one another. The change in FIA trend with time resulted from rotation of the kinematic reference frame of bulk flow, possibly as a consequence of the reorganization of lithospheric plates responsible for Acadian orogenesis. Recognition of distinct generations of FIAs provides a means of distinguishing different phases of porphyroblast growth. Four periods of garnet porphyroblast growth occurred in the schists of south-east Vermont. This growth was heterogeneously distributed on the cm2–m2 scale. No single porphyroblast records all stages of growth, and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth. Factors that may have been responsible for switching porphyroblast growth on and off at this scale include: (i) subtle differences in bulk chemical composition; (ii) oscillating levels of heat, owing to the buffering effect of endothermic garnet-forming reactions; (iii) channelized infiltration of fluids with localized fluid buffering of bulk composition; and (iv) cyclic controls on the rates of diffusion and material transport of reactants, either by channelized fluid flow or by a changing pattern of microfracturing during foliation development. Consistency in FIA trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events, or even phases of metamorphism within a progressive tectonothermal event, along and across orogens. Using a consistent pattern of core to rim changes in FIA trend, multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished, allowing correlation of different phases of growth around and across macroscopic folds. The relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using FIA data and correlated around and across macroscopic folds. Conceptually, the paragenetic history preserved in each generation of porphyroblast growth, in the form of chemical zoning and the minerals in inclusion trails, could be combined to produce a more detailed P–T–t–deformation path than previously determined. 相似文献
4.
MACDONALD R.; UPTON B. G. J.; COLLERSON K. D.; HEARN B. C. HEARN Jr.; JAMES D. 《Journal of Petrology》1992,33(2):305-346
The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km3.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K2O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas. 相似文献
5.
DOWNES HILARY; MACDONALD RAY; UPTON BRIAN G. J.; COX KEITH G.; BODINIER JEAN-LOUIS; MASON PAUL R. D.; JAMES DODIE; HILL PETER G.; HEARN B. CARTER JR 《Journal of Petrology》2004,45(8):1631-1662
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette 相似文献
6.
Interaction between Continental Lithosphere and the Iceland Plume--Sr-Nd-Pb Isotope Geochemistry of Tertiary Basalts, NE Greenland 总被引：1，自引：0，他引：1
Volcanic rocks associated with Atlantic opening in northerneast Greenland (73–76N) form a 1-km thickness of basalticlavas located on the coast some 400 km north of the major basaltaccumulations of the Blosseville Coast (<70N). The LowerLava Series, which makes up the lower half of the sequence atHold with Hope and all of that at Wollaston Forland, is composedof homogeneous quartz tholeiites (5–8% MgO). These aremildly light rare earth element (LREE) enriched (La/YbN 2.060.45,1 S.D.) and show strong chemical and Pb-Nd-Sr isotopic similaritiesto Icelandic tholeiites. They are distinguished from Atlanticmid-ocean ridge basalt (MORB) in having less radiogenic Pb andNd, higher 8/4 and lower 7/4, and depletion in K and Rb relativeto other incompatible elements, and show no evidence of a MORBasthenosphere component in their source. A single nephelinitein the Lower Series has essentially similar isotopic characteristicsand K, Rb depletion. The tholeiites were derived from the hothead of the Iceland plume, which had spread laterally withinthe upper mantle, and represent large melt fractions (15–20%)from spinel-facies mantle combined with small melt fractions(2.2%) from the garnet facies. Pb isotopic data indicate thatthe Iceland plume contains no MORB asthenospheric component,and is therefore most unlikely to arise from enriched streaksin the convecting upper mantle. The K, Rb depletion is sharedwith the HIMU ocean islands, and suggests a similar origin forthe Iceland plume in subduction-processed oceanic crust. Therelatively low 206Pb/204Pb ratios, and near-MORB Sr-Nd isotopes,suggest that Iceland overlies an immature HIMU plume. The conformably overlying upper half of the Hold with Hope sequence(the Upper Lava Series) is extremely heterogeneous, being mainlyolivine and quartz tholeiites (4.5–9.5% MgO in inferredmelt compositions, and up to 27% in accumulative lavas), withoccasional undersaturated compositions. The latter are concentratednear the base of the Upper Series, and are associated with stronglyincompatible-element-enriched tholeiites. These enriched sampleshave La/YbN from 7.3 to 28.5, with most tholeiites 13, and theundersaturated rocks >23. They are isotopically heterogeneous,with a basanite resembling Icelandic compositions, and an alkalibasalt having much less radiogenic Pb and Nd. The bulk of theUpper Series tholeiites has a limited La/YbN range (4.7–7.3)but a wide range in isotope ratios, from almost Icelandic valuesto 87Sr/86Sr50=0.7100, 206Pb/204Pb50=18.7, and 143Nd/144Nd50=0.51247.This isotopic range is well correlated with SiO2, Ce/Pb, andK/Nb, in a manner suggesting crustal assimilation-fractionalcrystallization (AFC) relationships. The mantle-derived end-memberof the Upper Series is displaced to slightly less radiogenicNd than the Lower Series samples, perhaps through mixing witha small component from the subcontinental lithospheric mantle.A larger proportion of this melt was derived from garnet-faciesmantle than for Lower Series samples, and melt fractions weresmaller in both garnet and spinel stability fields. As isotopic compositions similar to those of Icelandic lavasare found in each of the three stratigraphic groups (Lower Series,basal enriched Upper Series, and normal Upper Series tholeiites),the Upper Series were derived from this mixed source, but stillhad a very dominant plume isotopic signature. The continuedpresence of a lithospheric ‘lid’ is indicated bythe smaller melt fractions in both garnet and spinel facies(0.01 and 0.1, respectively) than those responsible for theLower Series lavas. The thicker crust in the region allowedstagnation of the magmas in the plumbing system of a centralvolcano and consequent extensive accumulation, fractionation,and assimilation of crustal rocks. 相似文献
7.
Small hypabyssal intrusives of biotite pyroxenite and biotiteperidotite are described from part of theGardar alkaline province.The intrusives, composed essentially of diopside, olivine, phlogopiteand Fe-Ti oxides (and numerous accessory minerals) are inferredto have crystallized from silica-undersaturated ultramafic magmasof similar bulk composition. Crystallization occurred at anestimated depth of 3–4 km. Despite relatively rapid consolidation(probably attendant on a partial devolatilization) producingtypically fine-grained and sometimes porphyritic rocks, small-scaledifferentiation nevertheless occurred. The differentiates, asveins, pegmatoidal patches and roofing-facies tend to be olivine-poorphlogopite+ferriandiopside+ Fe-Ti oxide rocks, in which perovskite,andradite, apatite and sphene are significant components. Pyroxenecompositions in the ultramafites are consistent with the thesisthat the rocks formed from soda-deficient magmas in stronglyoxidizing conditions. Relative potassium enrichment (averageK2O: Na2O wt. % = 1.6) is expressed modally in the ubiquitouspresence of phlogopite and normatively by an average of 6.9%or and/or Ic. Despite compositional affinity with certain olivinemelilitites, melilite is absent, a fact that may be attributableto the comparatively high hydrostatic pressure (c. 1 kb) pertainingduring crystallization. Petrographic and geochemical evidencesuggests that the ultramafic magmas were residua after fractionationof forsteritic olivine from more magnesian magmas of kimberlitetype. It is proposed that the latter may in turn have been residuafrom eclogite fractionation of Fe-Ti rich primary magmas inthe upper mantle. 相似文献
8.
The Petrogenesis of Pliocene Alkaline Volcanic Rocks from the Pannonian Basin, Eastern Central Europe 总被引：12，自引：2，他引：10
EMBEY-ISZTIN A.; DOWNES H.; JAMES D. E.; UPTON B. G. J.; DOBOSI G.; INGRAM G. A.; HARMON R. S.; SCHARBERT H. G. 《Journal of Petrology》1993,34(2):317-343
Late Tertiary post-orogenic alkaline basalts erupted in theextensional Pannonian Basin following Eocene-Miocene subductionand its related calc-alkaline volcanism. The alkaline volcaniccentres, dated between 11•7 and 1•4 Ma, are concentratedin several regions of the Pannonian Basin. Some are near thewestern (Graz Basin, Burgenland), northern (Ngrd), and eastern(Transylvania) margins of the basin, but the majority are concentratednear the Central Range (Balaton area and Little Hungarian Plain).Fresh samples from 31 volcanic centres of the extension-relatedlavas range from slightly hy-normative transitional basaltsthrough alkali basalts and basanites to olivine nephelinites.No highly evolved compositions have been encountered. The presenceof peridotite xenoliths, mantle xenocrysts, and high-pressuremegacrysts, even in the slightly more evolved rocks, indicatesthat differentiation took place within the upper mantle. Rare earth elements (REE) and 87Sr/86Sr, 143Nd/144Nd, 18O, D,and Pb isotopic ratios have been determined on a subset of samples,and also on clinopyroxene and amphibole megacrysts. Sr and Ndisotope ratios span the range of Neogene alkali basalts fromwestern and central Europe, and suggest that the magmas of thePannonian Basin were dominantly derived from asthenosphericpartial melting, but Pb isotopes indicate that in most casesthey were modified by melt components from the enriched lithosphericmantle through which they have ascended. 18O values indicatethat the magmas have not been significantly contaminated withcrustal material during ascent, and isotopic and trace-elementratios therefore reflect mantle source characteristics. Incompatible-elementpatterns show that the basic lavas erupted in the Balaton areaand Little Hungarian Plain are relatively homogeneous and areenriched in K, Rb, Ba, Sr, and Pb with respect to average oceanisland basalt, and resemble alkali basalts of Gough Island.In addition, 207Pb/204Pb is enriched relative to 2O6Pb/204Pb.In these respects, the lavas of the Balaton area and the LittleHungarian Plain differ from those of other regions of Neogenealkaline magmatism of Europe. This may be due to the introductionof marine sediments into the mantle during the earlier periodof subduction and metasomatism of the lithosphere by slab-derivedfluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheralareas have incompatible-element patterns and isotopic characteristicsdifferent from those of the central areas of the basin, andmore closely resemble Neogene alkaline lavas from areas of westernEurope where recent subduction has not occurred. 相似文献
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