The mantle section of the Careón Ophiolite (NW Iberian Massif): Exploring the mantle domain during Pangea assembly

The Careón Ophiolite is one of the best-preserved Devonian supra-subduction zone ophiolites (c. 395 Ma) exposed along the Variscan Orogen. The mantle section is comprised of harzburgites and metasomatic dunites/olivine-rich harzburgites. Dunites and olivine-rich harzburgites acted as focused conduits for melt transport through the main harzburgite section. The circulation and reaction of the melt through pre-existing harzburgite imposed major modal metasomatism, consisting of incongruent orthopyroxene dissolution, which produced major whole-rock SiO2-depletion and minor MgO-enrichment. Harzburgites and dunites/olivine-rich harzburgites show similar low-depletion in the less incompatible immobile elements, such as whole-rock Al2O3, TiO2, V, HREE, as well as in Cr#-Mg#-TiO2 compositions in the primary unreacted Mg-Al-rich spinel. This low-depletion is interpreted as a residue of moderate melting (10–18%) after partial melting by decompression in a MORB ridge without significant external H2O-input. Two types of cryptic refertilized REE patterns are identified based on LREE and HREE fractionations. The less-fractionated type is characteristic of the interaction between peridotite and MORB-like melts, whereas the more-fractionated type is related to the interaction with H2O-rich highly LREE-enriched IAT-like melts. The same partial melting throughout the entire mantle section, the existence of two distinct refertilization events and their spatial–compositional relationship with the Devonian IAT mafic crust, suggest that the mantle of the Careón Ophiolite was located in a Devonian forearc. This Devonian forearc developed above an older Cambrian oceanic back-arc lithosphere preserved at the margin of Gondwana. The relative stability of olivine compared to orthopyroxene during serpentinization, produced highly-serpentinized harzburgites and slightly-serpentinized dunites/olivine-rich harzburgites, indicating relatively high-temperature serpentinization. The composition of the serpentinization fluids and the low-temperature re-equilibration of pyroxenes determine the whole-rock CaO-depletion in harzburgites relative to dunites/olivine-rich harzburgites, which constitutes the major metasomatic change during serpentinization.