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MACDONALD R.; McGARVIE D. W.; PINKERTON H.; SMITH R. L.; PALACZ A. 《Journal of Petrology》1990,31(2):429-459
The Torfaj?kull volcano, Iceland's largest silicic centre, issituated close to the junction of the active, southerly propagatingEastern Rift Zone and the South Eastern Zone, an older crustalsegment. This paper provides major, trace, and some Sr isotopedata on postglacial (<10000 y) rocks, i.e., tholeiitic magmasof the Eastern Rift Zone and transitional basalts, icelandites,and rhyolites of the Torfaj?kull centre, and assesses the relationshipsbetween the magma types in terms of the development of the Icelandiccrust. Tholeiitic magmas from the Eastern Rift Zone are LILE-enrichedrelative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxenefractionation at low pressures. Compared with the tholeiites,Torfaj?kull transitional basalts show LILE/HFS enrichment andhigher (87Sr/86Sr)1 ratios. They define several magmatic lineagesand have equilibrated over a wide range of pressures. Both basalttypes were derived by very small degrees of partial meltingof compositionally similar mantle sources, the main differencebeing that the tholeiites were generated in the spinel Iherzolite,and the transitional basalts in the garnet lherzolite, stabilityfields, a conclusion previously reached by Meyer et al. (1985).The mantle sources may have contained LILE-enriched streaks. Low-pressure differentiation of Torfaj?kull transitional basaltproduced an iceiandite to sub-alkaline rhyolite sequence bycrystal fractionation, the rhyolites representing >90% crystallizationof parental basalts. The rhyolites were emplaced as nine separatelava fields, formed during 11 eruptive episodes. The compositionalrange within each field is limited, and, although similar, theranges define several magmatic lineages. Continued fractionationof plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zirconassemblages generated peralkaline rhyolites in certain post-glacialeruptions. Chemical variations in the deposits from the Hrafntinnuskerperalkaline eruption were related predominantly to alkali feldsparfractionation, and the melts were erupted from a zoned magmachamber. All postglacial volcanic rocks at Torfajokull havebeen mantle derived and thus represent new additions to theIcelandic crust. 相似文献
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Crustal Influences in the Petrogenesis of the Naivasha Basalt--Comendite Complex: Combined Trace Element and Sr-Nd-Pb Isotope Constraints 总被引：12，自引：0，他引：12
The bimodal Naivasha complex (central Kenya) comprises 2 suitesof transitional basalts and 7 chemostratigraphic groups of comendites.The early basalt series (EBS) predates the Group 1 comenditeswith the later series (LBS) erupted between Groups 5 and 6.Basalts from both suites are notable for their relatively radiogenic207Pb/204Pb isotope ratios which are higher than in the majorityof ocean island basalt (OIB, Zindler & Hart, 1986), and87Sr/86Sr ratios more radiogenic than basalts from northernKenya. Both basalt suites exhibit systematic trace element andisotopic variations which appear related to greater assimilationof Proterozoic amphibolite facies crust by the chemically moreevolved rock types. Their mantle source regions show evidenceof residual plagioclase and have a ‘Dupal’-likeOIB trace element and Pb-Sr-Nd isotope signature (Hart, 1984).A contribution from the sub-continental lithosphere is proposedin basalt genesis. The seven comendite groups have distinct trace element and isotopesystematics. Hydration of comendite glass causes significantchanges in Sr and Pb isotope ratios. In terms of their Sr-Ndisotope relationships the unaltered comendites could be derivedfrom the basalts by an assimilation-fractional crystallization(AFC) process dominated by the fractional crystallization offeldspars. However, the Pb systematics clearly demonstrate thatthe basalts and comendites are not part of a cogenetic suite.Chemical variations within individual comendite groups are predominantlythe result of fractional crystallization of the observed phenocrystassemblages (i.e. alkali feldspar dominated) and minor crustalinteraction. The majority of the chemical and isotopic differencesbetween Groups 1–7 cannot be explained by fractional crystallizationand appear to represent crustal melts derived from close tothe interface between Pan African basement and the overlyingMiocene-Holocene volcanoclastic rocks, at approximately 6 kmdepth (KRISP working group, 1987). Halogens play a fundamental role in the petrogenesis of thecomendites (Cl+F<1?7 per cent) permitting small degree meltsof low viscosity to be extracted from the crust and causingthe breakdown of minor phases e.g, zircon. These factors explainthe extreme enrichment of certain incompatible trace elements(Zr<2500, Nb<700) in the comendites and coupled with theretention of zircon in the source of the halogen poor comendites(Group 1<0?6 per cent Cl+F) result in notable fractionationamong the HFSE (Zr/Nb 1?5–5?5). Halogens may be concentratedin the source region from the surrounding crust by the presentlyactive hydrothermal system. Each of the chemostratigraphic comenditegroups is chemically distinct, implying that partial meltingof the heterogeneous crust is on a limited scale and that noextensive magma chambers exist beneath Naivasha. 相似文献
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CHARLES J. YONGE WILLIAM D. MACDONALD 《Boreas: An International Journal of Quaternary Research》1999,28(3):357-362
Perennial ice from caves on and to the east of the Canadian Great Divide yield δ18O and δD values, which are unusually high measurements when compared with the average precipitation for the region. Furthermore, these ice data fall below and along lines of lower slope than the Global Meteoric Water Line. To explain the observed relationships, we propose the following process. A vapour-ice isotopic fractionation mechanism operates on warm-season vapour when it precipitates as hoar ice on entering the caves. The subsequent fall of hoar to the cave floor through mechanical overloading, along with ice derived from ground-water seepage (with a mean annual isotopic composition), results in massive ice formation of a mixed composition. This mixed composition is what is observed in the characteristic relationships found here. Such findings suggest that a warm versus cold climate interpretation for ancient cave ice may be the opposite of that found in the more familiar polar and glacial ice cores. 相似文献
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JEFFREY A. SNYDER GLEN M. MACDONALD STEVEN L. FORMAN GENNADY A. TARASOV WILLIAM N. MODE 《Boreas: An International Journal of Quaternary Research》2000,29(4):261-271
A sediment core from Lake Yarnyshnoe-3 (69°04'N; 36°04'E), an emerged coastal lake from the tundra of the north-central Kola Peninsula, has been analyzed for fossil pollen and diatoms. The pollen record shows the Younger Dryas event marked by increasing Artemisia coupled with decreases in Poaceae, Cyperaceae and Salix at c. 10 700 to 10 000 BP. This core provides the first well-defined palynological record of the Younger Dryas event on the Kola Peninsula. Stomates from Pinus were recovered from the core interval between 8000 and 6000 BP. The stomates, coupled with elevated values of pine pollen, indicate that Pinus sylvestris grew near the arctic coastline of the central Kola Peninsula in the middle Holocene. However, the small number of stomates suggests that pines were not plentiful. The diatom record from the core reflects basin isolation from the sea and indicates additional limnological changes during the climate transition between c. 5000 and 4000 BP. The broadly similar climate and vegetation history on the north-central Kola Peninsula and in Fennoscandia demonstrates the propagation of late glacial and Holocene climate events from the North Atlantic region into the Eurasian Arctic. 相似文献
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DAVID I. M. MACDONALD 《Sedimentology》1986,33(2):243-259
Determination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back-arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse-grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in Tb and Tc divisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end-member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical. 相似文献
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MACDONALD R.; DAVIES G. R.; BLISS C. M.; LEAT P. T.; BAILEY D. K.; SMITH R. L. 《Journal of Petrology》1987,28(6):979-1008
The Recent (<15000 y) volcanic complex of southwest Naivasha,Kenya, consists of mildly peralkaline (comenditic) rhyolitedomes, lava flows, air fall pumices, and lake sediments, withminor, peripheral, basalts and hawaiites. The comendites areeither aphyric or sparsely porphyritic, few samples containing>5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon.Ferrohedenbergite and zircon are restricted to less peralkaline,and amphibole, aenigmatite, and biotite to more peralkaline,rocks. The comendites show unusually strong enrichment in Cs, F, Hf,Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletionin Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched,with large, negative Eu anomalies. Values of LIL/HFS elementratios, such as Th/Ta and Rb/Zr, are unusually high for peralkalinerhyolites, and are consistent with a substantial crustal componentin the comendites. Parameters such as LREE/HREE and Zr/Nb ratiosindicate that the Naivasha rhyolites represent several pulsesof closely related, but subtly different, magmas. Sanidine/glasspartition coefficients for Ba, Pb, Rb, Sr, U, and the REE arepresented for one specimen. Major and trace element modelling, and feldspar-rock relationships,show that closed system crystal fractionation cannot alone accountfor the overall compositional variations in the comendites.A model involving partial melting of variable crustal sourcerocks and migration of dissolved volatile-metal complexes maybe appropriate at Naivasha. 相似文献
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Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica 总被引：2，自引：0，他引：2
The Himalia Ridge Formation (Fossil Bluff Group), AlexanderIsland is a 2·2-km-thick sequence of Upper Jurassic–LowerCretaceous conglomerates, sandstones and mudstones, derivedfrom an andesitic volcanic arc and deposited in a fore-arc basin.The metamorphic and thermal history of the formation has beendetermined using authigenic mineral assemblages and vitrinitereflectance measurements. Metamorphic effects include compaction,pore-space reduction, cementation and dissolution and replacementof detrital grains by clay minerals (smectite, illite/smectite,corrensite and kaolinite), calcite, chlorite, laumontite, prehnite,pumpellyite, albite and mica, with less common quartz, haematite,pyrite and epidote. The authigenic mineral assemblages exhibita depth-dependence, and laumontite and calcite exhibit a strongantipathetic relationship. Detrital organic matter in the argillaceouslayers has vitrinite reflectance values (Ro) ranging from 2·3to 3·7%. This indicates considerable thermal maturation,with a systematic increase in reflectivity with increasing depth.There is good correlation of metamorphic mineral assemblageswith chlorite crystallinity and vitrinite reflectance values—allindicating temperatures in the range of 140 ± 20°Cat the top of the sequence to 250 ± 10°C at the baseof the sequence. The temperatures suggest a geothermal gradientof 36–64°C/km and a most likely gradient of 50°C/km.It is suggested that this higher-than-average gradient for afore-arc basin resulted either from rifting during basin formationor from a late-stage arc migration event. KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature metamorphism; vitrinite reflectance 相似文献
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