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Transition between subduction and intraplate metasomatism as indicated by clinopyroxene and amphibole geochemistry in Kapfenstein lithospheric mantle xenoliths
No full textThe initial differentiation stages of the Middle Triassic magmas in the Southern Alps as recorded by clinopyroxenitic xenoliths from the Latemar area (Dolomites)
No full textEarly Paleogene alkaline magmatism in western Romania (Poiana Rusca): evidence for two different sources?
No full textSubduction-related and intraplate metasomatism in Kapfenstein lithospheric mantle (Styrian Basin, eastern Austria).
No full textMantle xenoliths from the Viliga volcanic field, N-E Siberia: evidence for a fertile subcontinental lithosphere.
No full textGeochemical and petrological signature of the fertile lithospheric mantle beneath the Viliga Volcanic field (NE Siberia) inferred from sp-lherzolite xenoliths investigations
No full textAmphibole genesis and adakite metasomatism in mantle xenoliths from Kapfenstein (Styria Basin, Austria).
No full textAsthenospheric signature in fertile spinel lherzolites from the Viliga Volcanic Field in Northeast Russia
No full textMantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the Okhotsk-Eolteka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical medium- to fine-grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/y-b)N = 0.48-1.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole-rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 1050-1160 °C and 15-21 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (2-6 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 2-9% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c. 87 Ma apparently caused a 'piecemeal' collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low 87Sr/86Sr and high 143Nd/144Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the 'piecemeal' collapse of the palaeo-Pacific plate. © The Geological Society of London 2008
Slab melt and intraplate metasomatism in Kapfenstein mantle xenoliths (Styrian Basin, Austria)
No full textAnhydrous and amphibole-bearing mantle peridotite xenoliths from Kapfenstein (Styrian Basin) have been studied with the aim of understanding both the processes responsible for amphibole formation and the nature of metasomatizing agents which affected this portion of lithosphere. This area of the Pannonian Basin underwent a subduction event which was followed after about 15 Ma, by alkaline intraplate magmatism. Primary clinopyroxene (cpx1) in four-phase lherzolite xenoliths is characterized by LREE-depleted to slightly LREE-enriched patterns. LREE-depleted cpx1 have low Th and U contents and Zr (and Hf) anomalies varying from slightly negative to positive. LREE-enriched cpx have high Th and U contents and remarkable positive anomalies of Zr and Hf. Primary clinopyroxenes in amphibole-bearing lherzolites present a comparable compositional variation from LREE (and Th, U, Zr, Hf)-depleted type to LREE (and Th, U, Zr, Hf)-enriched type. LREE-depleted cpx1, with strong negative Zr and Ti anomalies, are also recognized in the peridotite matrix of a composite sample cut by a large amphibole vein. Textural and geochemical evidence indicates that amphibole disseminated within the matrix grew at the expense of primary spinel and clinopyroxene, mimicking the trace element patterns of the latter. As a consequence, the geochemical features of amphibole vary in relation to those of clinopyroxene, from enriched to depleted. On the other hand, the composition of vein amphibole in the composite xenolith compares well with amphibole megacrysts and microphenocrysts, suggesting that it represents a fractionation product of alkaline melt that passed through the lithosphere. Two kinds of metasomatism, superimposed on a slightly depleted lithospheric mantle, were identified. A slab-derived melt (proto-adakite?) metasomatic agent was responsible for the first enrichment in Th, U, Zr and Hf observed in clinopyroxene, whereas an alkaline within-plate metasomatic agent caused the formation of the Nb (and Ta)- rich disseminated amphibole. The final process was the alkaline magmatism, which was responsible for the formation of the large amphibole vein and megacrysts. It is proposed that the Nb-poor and Nb-rich amphiboles record the transition between the suprasubduction slab melt-related and the intraplate alkaline metasomatism.
These geochemical features are consistent with a lithospheric portion enriched in slab melt components which was subsequently metasomatized by alkaline melt. Alternatively an asthenospheric uprising could have scavenged a previously slab melt-enriched region of the lithosphere. (C) 2006 Elsevier B.V. All rights reserved
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