Xenoliths from Cerro de los Chenques (Patagonia): An example of slab-related metasomatism in the backarc lithospheric mantle
作者： Giorgio Rivalenti , Maurizio Mazzucchelli , Alberto Zanetti , Riccardo Vannucci , Claire Bollinger
DOI: 10.1016/J.LITHOS.2007.05.012
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摘要: Abstract The alkali basalts of the Cerro de los Chenques quaternary backarc volcano (200 km E volcanic arc) carry to surface abundant spinel-facies mantle xenoliths (lherzolites, harzburgites and dunites). clinopyroxene geochemistry indicates that pristine was a lherzolite with Depleted Mantle (DM) composition, recording either melting episodes triggered by infiltration metasomatic agent or only enrichment highly incompatible elements in those sectors where percolation occurred under decreasing fluid volume. Metasomatism operated originated garnet-bearing assemblages induced olivine dissolution, variations Sr Nd isotopic signatures (i.e. pre-metasomatic mantle: 87 Sr/ 86 Sr = 0.702712 143 Nd/ 144 Nd = 0.513495; highest metasomatism: Sr = 0.704234 Nd = 0.512870), increase LILE LREE, but not Nb Ti. resulting geochemical characteristics are similar observed arc magmas consistent sediments forming upper part subducted Nazca plate. Evidence for element fractionation host related reactive porous-flow processes lacking weak. granoblastic texture xenoliths, bearing no evidence deformation, rules out hypothesis formed subarc transported 200 km east reverse corner flow. Alternatively, it is suggested slab components were released as supercritical fluids beneath (at ∼ 6 GPa) phengite breakdown. Fluids flowed through, reacted with, hidden garnet-facies reached level. Reactions region led (La/Sm) N , Th, U, Nb, Pb while its initial (Sm/Yb)
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