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Enrichment processes in the Patagonian lithospheric mantle.

AuthorsMelchiorre, Massimiliano; Coltorti, Massimo; Gregoire, M.; Benoit, M.
KeywordsPatagonian lithospheric mantle
Issue Date26-Aug-2015
CitationMINERALOGIA - Special Papers, 43: 67-68 (2015)
AbstractSeveral mantle xenolith suites entrained in basalts from back arc mafic plateau in Patagonia were investigated. They appear to be affected by both depletion processes due to melt extraction and enrichment processes. These latter are related to metasomatic and refertilization melts that, migrating through the mantle, change its composition adding geochemical elements to the existing paragenesis or crystallizing new phases (mostly hydrated and clinopyroxene). The fluids responsible for the mantle enrichment have been linked to subduction (Stern et al. 1989; Kilian et al. 1998; Laurora et al. 2001; Kilian and Stern 2002; Rivalenti et al. 2007; Faccini et al. 2013), plume-related (Bjerg et al. 2005) geodynamics, and ascribed to silicate (Kempton et al. 1999; Stern et al. 1999) and carbonanititic (Gorring and Kay 2000) melts. Estancia Sol de Mayo (ESM) anhydrous mantle xenoliths from Central Patagonia were recently studied by Melchiorre et al. (2015). They are entrained in post-plateau alkaline lavas belonging to Meseta Lago Buenos Aires, characterized by a coarse-grained protogranular texture, devoid of any evident modal metasomatism and equilibrated at T of 1003¿1040 °C. Though no correlation between petrography of the mineral phases and their geochemistry has been observed, it is important to highlight the presence of two and three texturally different clinopyroxenes and orthopyroxenes: the first occurs as both protogranular (cpx1) and texturally related to spinel (cpx2); the second as large protogranular crystals with exsolution lamellae (opx1), as small clean and undeformed grains without exsolution lamellae (opx2) and finally as small grains arranged in a vein (opx3). Major element composition of cpx and opx of ESM highlights two different trends characterized by i) high Al2O3 content at almost constant mg# and ii) a slight increase in Al2O3 content with decreasing mg#. Cpx are LREE enriched and characterized by prominent to slightly negative Nb, Zr and Ti anomalies. Partial melting modeling using both major and trace elements indicates a melting degree between ~5% and ~13% (up to ~23% according to major element modeling) for lherzolites and between ~20% and ~30% for harzburgites (down to ~5% according to trace element modeling). La/Yb and Al2O3, as well as Sr and Al2O3 show negative correlation pointing to a refertilization event affecting the ESM lithospheric mantle domain. The presence of opx arranged in vein, as well as the similarity between the geochemical features of the ESM cpx and those from Northern Patagonia pyroxenites, point to a transitional alkaline/subalkaline melt as refertilizing agent. When comparing the ESM mantle xenoliths with those from Central and Northern Patagonian suites, similar trends can be recognized. This suggests that the Al2O3 enrichment through refertilization is a common process in the Patagonian lithospheric mantle, as also indicated by the widespread negative correlation between La/Yb - Sr and Al2O3. The most likely melt responsible for this event vary in composition from alkaline/subalkaline transitional to tholeiitic melts. In addition to this, another enrichment process can be envisaged for the Patagonian subcontinental lithospheric mantle, linked to alkaline metasomatism. Sr-Nd systematic performed on samples from several localities show a large range of both isotopic ratios. Mixing calculations allow to assess that up to 4% of an EM II component is needed to account for the most radiogenic samples.
Identifiersissn: 1899-8518
Appears in Collections:(ICTJA) Comunicaciones congresos
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