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Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia) |
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| Authors: | LAURORA ANGELA; MAZZUCCHELLI MAURIZIO; RIVALENTI GIORGIO; VANNUCCI RICCARDO; ZANETTI ALBERTO; BARBIERI MARIA ADELAIDE; CINGOLANI CARLOS A |
| Institution: | 1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI MODENA E REGGIO EMILIA, P.LE S. EUFEMIA, 19, I-41100 MODENA, ITALY 2DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA AND CNR CENTRO DI STUDIO PER LA CRISTALLOCHIMICA E CRISTALLOGRAFIA, VIA FERRATA, 1, I-27100 PAVIA, ITALY 3CENTRO DE INVESTIGACIONES GEOLÓGICAS, UNIVERSIDAD NACIONAL DE LA PLATA, 644 CALLE NO. 1, 1900 LA PLATA, ARGENTINA |
| Abstract: | Spinel-facies mantle xenoliths occur in a diatreme cutting throughthe Neogene Southern Patagonia Plateau at Gobernador Gregores(Santa Cruz Province, Argentina). This plateau is in a back-arcposition with respect to the Chile trench. Xenoliths differin their whole-rock composition from other South America occurrences,having higher CaO/Al2O3 ratios and, in some samples, TiO2 enrichment,whereas the Na2O/Al2O3 variation range is similar. Three assemblagescan be distinguished. Assemblage 1, in anhydrous protogranularlherzolites and harzburgites, contains clinopyroxene with adepleted major and trace element composition, indicating pre-metasomaticdepletion processes. This assemblage fully recrystallized toAssemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing)during a metasomatic episode. This causes clinopyroxene to acquiregeochemical characteristics often attributed to carbonate-meltmetasomatism. Noticeably, amphibole is markedly enriched inNb (up to 298 ppm), especially when depleted in Ti. A furtherevent, related to decompression during xenolith uplift to thesurface, induces closed-system (perhaps with the exception ofCO2 addition) disequilibrium melting of Assemblage 2, dominantlyof amphibole. It is found in pockets (where amphibole is a residualphase) consisting of Na–Si-rich glass and carbonate (Mg-richcalcite) drops, and in veins originating from the pockets (Assemblage3). Euhedral olivine, clinopyroxene and spinel crystallize onlyin the silicate glass. So do new, euhedral apatite crystalswhen glass is in contact with previous Assemblage 2 apatite.Textural evidence and comparison with experimental work suggestthat silicate glass and carbonates are the result of unmixingof a former homogeneous melt. Because of the different flowrates of carbonate and silicate melt, the xenoliths become enrichedin carbonate, which is found in the veins during their migration.Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusiveevidence of carbonatite metasomatism. This factor, and otherminor deviations from the expected results of carbonatite metasomatism,lead us to hypothesize an aqueous, Cl-rich fluid, possibly slabderived, as an alternative agent. Amphibole, resulting fromreactive porous flow of this agent in the mantle, could fullyexplain the observed geochemical features, as indicated by estimatesof its partition coefficients. KEY WORDS: carbonated xenoliths; Gobernador Gregores; LAM–ICP-MS; mantle metasomatism; silicate glass |
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