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Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)

Abstract : Whole rock trace element and isotopic compositions of different HP-LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P-T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid-rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.
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Submitted on : Friday, April 12, 2013 - 10:44:34 AM
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Alifé El Korh, S.T. Schmidt, Torsten Vennemann, Michel Ballevre. Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France). Gondwana Research, Elsevier, 2013, 23 (3), pp.880-900. ⟨10.1016/⟩. ⟨insu-00812444⟩



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