Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone   总被引：6，自引：1，他引：5  

JOHNSON  J. S.; GIBSON  S. A.; THOMPSON  R. N.; NOWELL  G. M. 《Journal of Petrology》2005,46(7):1309-1344

The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km³ of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. ⁸⁷Sr/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na₂O, P₂O₅, K₂O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]_n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting  相似文献   

Re-Os and Lu-Hf Isotope Constraints on the Origin and Age of Pyroxenites from the Beni Bousera Peridotite Massif: Implications for Mixed Peridotite-Pyroxenite Mantle Sources   总被引：2，自引：0，他引：2  

PEARSON  D. G.; NOWELL  G. M. 《Journal of Petrology》2004,45(2):439-455

A suite of pyroxenites from the Beni Bousera peridotite massif,northern Morocco, have been analysed for Re–Os and Lu–Hfisotopic compositions. Measured sections of the massif indicatethat pyroxenite layers make up between 1 and 9% by volume ofthe total outcrop. Clinopyroxenes from two Cr-diopside pyroxeniteshave unradiogenic Hf isotope compositions (  相似文献   

Trace Element and Sr-Pb-Nd-Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites   总被引：3，自引：0，他引：3  

DAVIES  G. R.; STOLZ  A. J.; MAHOTKIN  I. L.; NOWELL  G. M.; PEARSON  D. G. 《Journal of Petrology》2006,47(6):1119-1146

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/⁸⁶Sr_i =0·7055–0·7079, _Hfi = –7·6 to–29·4 and ²⁰⁶Pb/²⁰⁴Pb_i = 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   

Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions   总被引：10，自引：5，他引：10  

NOWELL  G. M.; PEARSON  D. G.; BELL  D. R.; CARLSON  R. W.; SMITH  C. B.; KEMPTON  P. D.; NOBLE  S. R. 《Journal of Petrology》2004,45(8):1583-1612

Kimberlites from Southern Africa, along with their low-Cr megacrysts,have unusual Hf–Nd isotopic characteristics. Group I andTransitional kimberlites define arrays trending oblique to,and well below, the Nd–Hf isotope ‘mantle array’,defined by oceanic basalts, i.e. they have negative  相似文献