Fingerprinting stealth metasomatism in ophiolitic peridotites

Abstract

Mantle wedge peridotites are characterized by extraordinarily depleted compositions and their genesis is still controversial. In this study, we present microstructural observations and mineral chemical analyses of mantle harzburgites in the Lycian ophiolite of SW Turkey that reveal their formation in a suprasubduction zone mantle wedge. The textures of clinopyroxene in the harzburgites, particularly chromite-clinopyroxene symplectites and grains containing mineral inclusions, do not support a partial melting origin. Zoning textures and the presence of additional phases within the orthopyroxene porphyroclasts indicate that they underwent melt/fluid infiltration. Olivine crystals and interstitial grains are enclosed in orthopyroxene porphyroclasts, implying that the orthopyroxene grains crystallized by consuming olivine. Chromite grains in the harzburgites not only have zoned, but also contain a variety of mineral inclusions. Amphibole grains in some of the harzburgites also have compositions similar to those in the chromitites. Therefore, all mineral phases in the Lycian harzburgites have well-preserved metasomatic signatures that overprinted their partial melting features. The enrichment of fluid-mobile elements, such as Cs, Rb, U, and Pb in clinopyroxene and amphibole, coupled with their high Mg# values (92.8–95.6 and 93.7–96.4, respectively), suggest formation by Cr- and Mg-rich, hydrous metasomatic melts/fluids. The consistent compositions of clinopyroxene and chromite in the harzburgites and chromitites, together with the decreasing intensity of alteration and crystallization temperature of amphibole in the chromitite pods through dunites to harzburgites, imply that the hydrous melts/fluids were partially released from chromite surfaces during chromitite formation and/or from dehydration of the metamorphic sole, which likely attributed to metasomatism at different depths. Moreover, chromite-hosted inclusions in the harzburgites may reflect the reaction before solidification of an inclusion-rich zone possibly signifying a rapid increase of chromite-oversaturated melt. Positive correlations between Cr and Al in pyroxenes of ophiolites are widely preserved in metasomatic mantle peridotites from different tectonic settings and thus they provide a robust chemical proxy for the metasomatism. • Direct evidence of stealth metasomatism for harzburgites in the mantle section of the Lycian ophiolite; • The depleted Cr and Mg-rich hydrous melts/fluids are released from chromite surfaces or heating dehydration of the metamorphic sole; • The positive Cr and Al correlation of pyroxene suggest that metasomatism in the lithospheric mantle is widespread