Primitive Arc Magmatism and Delamination: Petrology and Geochemistry of Pyroxenites from the Cabo Ortegal Complex, Spain

Mafic lithologies such as pyroxenites play a major role in the development of chemical heterogeneity in the upper mantle and in the recycling of elements between the deep Earth and its external envelopes. Pyroxenites are notably involved in the differentiation of primitive arc magmas whose genesis has had a major impact on continent formation in post-Archean times, but the rarity of exposures of deep arc sections limits our understanding of melt migration and melt-rock interaction. In the Variscan suture of north-western Iberia, the Herbeira massif of the Cabo Ortegal Complex represents a unique km-scale section of sub-arc mantle harzburgites, dunites and chromitites, and abundant pyroxenites. However, the processes responsible for their abundance and geochemical characteristics are not well established, notably due to a particularly complex tectonothermal history. This work combines new field and petrographic observations with the characterization of major-, trace-element and radiogenic-isotope (Sr, Nd, Hf and Os) compositions of these pyroxenites. After constraining the effects of secondary processes by studying small-scale heterogeneities, it is shown that the low-Al content of pyroxenes, the high abundance of compatible elements and the absence of plagioclase are related to primitive hydrous parental melts. Olivine websterites and clinopyroxenites preserving dunite lenses (type 1, and their foliated type-3 products) record the partial replacement of peridotites at decreasing melt/rock ratios, following intrusion of picritic melts at relatively low pressure (< 1.2 GPa), potentially within an arc root. Massive websterites (type 2) represent the final products of this reaction at higher melt/rock ratios, potentially as veins and dykes crystallized from boninitic melts differentiated from the initial picritic melts. Interaction between these Si-rich melts and dunites produced rare opx-rich websterites (type 4). Chromatographic reequilibration accompanied the late-magmatic crystallization of amphibole from migrating or trapped residual melts, producing a range of REE patterns from spoon-shaped to strongly LREE-enriched, consistent with increasingly unradiogenic Nd compositions. Particularly high CaO/Al2O3 and the selective enrichment of LILE/HFSE in Cabo Ortegal pyroxenites suggest the generation of their Ca-rich parental melts via low-degree, second-stage melting of a refractory lherzolite at < 2 GPa, following percolation of slab-derived fluids and/or carbonatite melts. Radiogenic-isotope systematics indicate the involvement of an old metasomatized mantle domain, suggesting the reworking of a continental margin during this arc magmatism, and of recycled sediments. Their geochronology suggests that Cabo Ortegal pyroxenites formed at around 500 Ma, which implies that previously reported ages around 390 Ma probably correspond to a metamorphic event. Pyroxenites and their host peridotites then recorded the development of sheath folds and mylonites following high-temperature deformation. We suggest that this episode corresponds to the delamination of the arc root associated with prograde metamorphism peaking at 1.6-1.8 GPa and 780-800°C. Exhumation of the arc root after its intrusion into a subduction zone was accompanied by hydration, as suggested by abundant post-kinematic amphibole, and by addition of S and mobilization of Re, as suggested by Os-isotope systematics. The Cabo Ortegal Complex may thus preserve a unique example of delaminated arc root, bearing evidence of the role of melt-peridotite interaction during the differentiation of primitive arc magmas at depth.