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Cordieritites and highly peraluminous granites within the ElPilón granite complex, Sierras Pampeanas, Argentina,were emplaced during a medium-P, high-T metamorphic event duringthe initial decompression of a Cambrian orogen along the southwesternmargin of Gondwana. Very fresh orbicular and massive cordierititebodies with up to 90% cordieritite are genetically associatedwith a cordierite monzogranite pluton and a larger body of porphyriticgranodiorite. The petrogenesis of this association has beenstudied using petrographical, mineralogical, thermobarometric,geochemical, geochronological and isotope methods. The graniticmagmas were formed by anatexis of mid-crustal metamorphic rocksformed earlier in the Pampean orogeny. The cordieritites appearat the top of feeder conduits that connected the source regionlocated at 相似文献
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Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana 总被引:10,自引:2,他引:8
New SHRIMP UPb zircon, RbSr whole-rock, and 40Ar39Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.UPb is the only reliable method for dating crystallizationin these rocks; RbSr is prone to hydrothermal resettingand ArAr is additionally affected by initial excess 40Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188178 Ma), V2 (172162Ma) and V3 (157153 Ma). The first essentially coincideswith the KarooFerrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; UPb; zircon 相似文献
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Origins of Large Volume Rhyolitic Volcanism in the Antarctic Peninsula and Patagonia by Crustal Melting 总被引:10,自引:1,他引:10
RILEY TEAL R.; LEAT PHILIP T.; PANKHURST ROBERT J.; HARRIS CHRIS 《Journal of Petrology》2001,42(6):1043-1065
Voluminous rhyolitic volcanism along the palaeo-Pacific marginof Gondwana was marked by three principal episodes of magmatism.The first of these ( V1) is essentially coincident with themain episode of KarooFerrar magmatism at 相似文献
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High precision 87Sr/86Sr analyses, together with determinationsof Rb, Sr, K2O, Na2O and, in a few cases, other elements, arereported for about fifty volcanic rocks (mainly basaltic) fromthe Atlantic Ocean basin. Results for dredged basalts from theReykjanes Ridge and Charlie Gibbs Fracture Zone, and an enstatite-forsteritebasalt from Kolbeinsey islet, support the general observationthat ocean-ridge tholeiites have uniformly low 87Sr/86Sr ratios(0.70294±4) and lithophile element contents comparedwith the most primitive basalts on ocean islands, includingthe Neovolcanic zones of Iceland, although progressive decreasein these quantities away from Iceland has not been confirmed.In contrast, the ocean island alkali basalts generally havehigher 87Sr/86Sr ratios (0.70334±5 for the Snaefellsnespeninsula of Iceland, 0.70343±4 for Jan Mayen, 0.70509±4for Tristan da Cunha and 0.70369±3 for Bouvetøya).The chief exception is Ascension Island, where volcanic rocksranging from alkali-olivine basalt to trachyte give a mean valueof 0.70284±4. The constancy of this ratio throughouteruptive sequences on any single island indicates that Sr-isotopecharacteristics are primary features. These variations, which are far outside analytical errors, areconsidered in the light of the geochemistry and isotope systematicsof ocean basalts in general. The implied isotopic (and lithophileelement) heterogeneities of the source regions have to be interpretedaccording to either equilibrium or disequilibrium melting models.The former, which is normally assumed, requires large-scale(domain) isotopic inhomogeneities within the mantle, which musthave existed over thousands of m.y. unless the Rb/Sr ratio ofextracted liquids is lower than that of the bulk source (aswould be the case if phlogopite were a residual phase). In thecase of disequilibrium melting, the inhomogeneities are reducedto the mineral scale, as observed in some studies of ultramaficnodules. It is shown that disequilibrium melting models couldgenerally account for the observed isotopic variations of oceanicrocks, although difficulties are again encountered unless phlogopiteis a stable residual phase. Evaluation of the relative importanceof these melting processes cannot be made at the present time. 相似文献
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Rubidium-strontium isotope analyses are reported for 90 whole-rocksfrom the Newer Gabbros and associated rocks of NE. Scotland.The Sr87/Sr86 ratio of the regional gabbro magma at Insch andBelhelvie underwent a progressive increase, in the former casefrom 0·703 to 0·712 during differentiation tosyenogabbros and syenites. This is ascribed to a process ofisotopic equilibration with country rocks of the Dalradian Series.The wider implications of a process by which the Sr87/Sr86 ratioof a basic magma can be increased in the crust without any othersigns of petrological contamination are considered. Cordierite-noritesand related rocks of the Haddo House and Arnage masses haveinitial Sr87sol;Sr86 ratios of 0·7200·730,essentially the same range as for the country rocks at the timeof intrusion. This is interpreted as support for the hypothesisthat such rocks were formed by the partial fusion of peliticsediments adjacent to the basic igneous sheet. 相似文献
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A Lateglacial archaeological site in the far north‐west of Europe at Rubha Port an t‐Seilich,Isle of Islay,western Scotland: Ahrensburgian‐style artefacts,absolute dating and geoarchaeology
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STEVEN MITHEN KAREN WICKS ANNE PIRIE FELIX RIEDE CHRISTINE LANE ROWENA BANERJEA VICTORIA CULLEN MATTHEW GITTINS NICHOLAS PANKHURST 《第四纪科学杂志》2015,30(5):396-416
The exact pattern, process and timing of the human re‐colonization of northern Europe after the end of the last Ice Age remain controversial. Recent research has provided increasingly early dates for at least pioneer explorations of latitudes above 54°N in many regions, yet the far north‐west of the European landmass, Scotland, has remained an unexplained exception to this pattern. Although the recently described Hamburgian artefacts from Howburn and an assemblage belonging to the arch‐backed point complex from Kilmelfort Cave have established at least a sporadic human presence during earlier stages of the Lateglacial Interstadial, we currently lack evidence for Younger Dryas/Greenland Stadial 1 (GS‐1) activity other than rare stray finds that have been claimed to be of Ahrensburgian affiliation but are difficult to interpret in isolation. We here report the discovery of chipped stone artefacts with technological and typological characteristics similar to those of the continental Ahrensburgian at a locality in western Scotland. A preliminary analysis of associated tephra, pollen and phytoliths, along with microstratigraphic analysis, suggest the artefacts represent one or more episodes of human activity that fall within the second half of GS‐1 and the Preboreal period. © 2015 The Authors. Journal of Quaternary Science published by John Wiley & Sons, Ltd. 相似文献
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Twelve new Sr-isotope analyses and seventeen new rare earthelement distribution patterns are reported for basalts fromIceland and the Reykjanes Ridge, together with Rb, Sr, Na2O,K2O, TiO2, and P2O5 contents. The samples were chosen to representthe widest range of basalt types known from the Iceland-ReykjanesRidge system. 87Sr/86Sr ratios range from 0.70291 ?4 to 0.70325?5 for tholeiitesand up to 0.70341 ?7 for alkali basalt. Rare earth elementsalso show a wide range of both total abundance and degree oflight-REE fractionation (chondrite-normalised Ce/Yb ratios of0.30 to 3.36 for tholeiites and up to 7.07 for alkali basalt).As found in previous studies of either Sr-isotope compositionor REE distribution, the ocean floor basalts from the southernportion of the Reykjanes Ridge have lower 87Sr/86Sr and CeN/YbNratios than most of the Icelandic basalts. However, some highlyMg-rich tholeiites from Theistareykir in northern Iceland andKj?lur in central Iceland also have among the lowest valuesfor these parameters and are indistinguishable in this respectfrom the ridge basalts. There is a very strong positive, linear,correlation between 87Sr/86Sr and CeN/YbN for all the tholeiitesincluding some up to 16 m.y. old, but this relationship doesnot hold for the alkali basalts which have proportionately farhigher CeN/YbN ratios. There is also a positive, linear, correlationbetween 87Sr/86Sr and Sr content, but not between 87Sr/86Srand 1/Sr. These relationships are found to be incompatible with disequilibriummelting of a single mantle source region, whether by variabledegrees of partial melting with different mineral stabilityconditions, or by removal of successive incremental melts. Itis certain that the data reflect relatively gross chemical heterogeneityin the upper mantle beneath Iceland, but the correlation withSr content apparently rules out simple binary mixing models(mantle-plume hypothesis). It is proposed that the heterogeneities result from establishmentof a lithophile element gradient during a single chemical fractionationevent in the upper mantle at least 100200 m.y. ago. Itis not possible at present to relate this geochemical gradientto known mantle structure. 相似文献
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