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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 0·61·16. A distinctive feature ofthese rocks is their low TiO2 content (0·51·4wt %). 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 (100800 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 =0·70550·7079, Hfi = 7·6 to29·4 and 206Pb/204Pbi = 16·617·4.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 SrNdPbHf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative Hf as a result of metasomatism by slab-derivedhydrous melts that left rutilegarnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotopetrace 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
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Late Devonian Diamondiferous Kimberlite and Alkaline Picrite (Proto-kimberlite?) Magmatism in the Arkhangelsk Region, NW Russia 总被引:7,自引:4,他引:3
MAHOTKIN I. L.; GIBSON S. A.; THOMPSON R. N.; ZHURAVLEV D. Z.; ZHERDEV P. U. 《Journal of Petrology》2000,41(2):201-227
Widespread penecontemporaneous igneous activity affected NWRussia (the Kola Peninsula and adjoining areas to the SE aroundArkhangelsk) during the Late Devonian (360380 Ma). Magmatismvaries from tholeiitic basalts, erupted in the axial regionsof former Middle Proterozoic (Riphean) rifts, to strongly alkalinerock-types on and marginal to Archaean cratons. NNE of Arkhangelskkimberlites, olivine lamproites and alkaline picrites were emplaced;all these rock-types are diamondiferous to varying extents.Higher TiO2 (and also total Fe) distinguish predominantly mica-poorEastern Group kimberlites (TiO2 = 2·43·1wt %) and spatially associated alkaline picrites (TiO2 = 3·23·7wt %) from nearby micaceous Western Group kimberlites (TiO2= 0·81·1 wt %). Each rock-type also hasdistinctive rare earth element (REE) patterns, and 相似文献
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