Tectono-magmatic Interplay and Related Metasomatism in Gabbros of the Chenaillet Ophiolite (Western Alps)

The Jurassic Chenaillet ophiolite in the Western Alps consists of a gabbro-mantle associationexhumed to the seafloor through detachment faulting and partly covered by basaltic lavas. One ofthe Chenaillet gabbroic bodies includes mylonites that are transected by a network of felsic veins,thereby testifying to the interplay of ductile shearing and magma emplacement. The deformedgabbros preserve clinopyroxene porphyroclasts of primary magmatic origin, which are typicallymantled by amphibole (titanian edenite) and minor secondary clinopyroxene. Titanian edenite andsecondary clinopyroxene also occur as fine-grained syn-kinematic phases locally associated withfine-grained plagioclase. The felsic veins are made up of anorthite-poor plagioclase and minor titanianedenite. Geothermometric investigations document that the ductile gabbro deformation andthe crystallization of the felsic veins occurred at 765650 C and 800655 C, respectively. Withrespect to undeformed counterparts, the deformed gabbros are variably enriched in SiO2 and variablydepleted in Mg/(Mg þ Fe2þtot ) and Ca/(Ca þ Na). In addition, the deformed gabbros show relativelyhigh concentrations of incompatible trace elements such as rare earth elements (REE), Y, Zrand Nb. The felsic veins are characterized by low Mg/(Mg þ Fe2þtot ) and Ca/(Ca þ Na), high SiO2 andhigh concentrations of incompatible trace elements. Relict clinopyroxene porphyroclasts from thedeformed gabbros display a rather primitive, mid-ocean ridge-type geochemical signature, whichcontrasts with the trace element fingerprint of titanian edenite from both the deformed gabbrosand the felsic veins. For instance, titanian edenite typically has relatively high REE abundances,with chondrite-normalized REE patterns characterized by a pronounced negative Eu anomaly. Asimilar trace element signature is shown by secondary clinopyroxene from the deformed gabbros.Amphibole from both the deformed gabbros and the felsic veins displays high F/Cl values. Weshow that the SiO2-rich hydrous melts feeding the felsic veins were involved in the hightemperaturegabbro deformation and that melt-gabbro reactions led to major and trace elementmetasomatism of the deforming gabbros.