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

Research areas:
  • Romain Tilhac
  • Georges Ceuleneer
  • William L. Griffin
  • Suzanne Y. O’Reilly
  • Norman J. Pearson
  • Mathieu Benoit
  • Hadrien Henry
  • Jacques Girardeau
  • Michel Grégoire
Journal of Petrology
The genesis of primitive arc magmas has had a major impact on continent formation through time, but the rarity of exposures of deep arc sections limits our understanding of the details of melt migration and differentiation. Abundant pyroxenites are exposed within a 600 m thick section of arc-related mantle harzburgites and dunites in the Herbeira massif of the Cabo Ortegal Complex, Spain. We report a combination of field and petrographic observations with in situ and whole-rock geochemical studies of these pyroxenites. After constraining the effects of secondary processes (serpentinization, fluid or melt percolation and amphibolitization), we determine that the low Al content of pyroxenes, high abundance of compatible elements and the absence of plagioclase reflect melt–peridotite interaction and crystal segregation from primitive hydrous melts at relatively low pressure (\ly light REE (LREE)-enriched in type 2 and 3 pyroxenites. Particularly high CaO/Al2O3 ratios (2·2–11·3) and the selective enrichment of large ion lithophile elements (LILE) over high field strength elements (HFSE) in Cabo Ortegal pyroxenites suggest the generation of Ca-rich picritic–boninitic parental melts via low-degree, second-stage melting of a refractory lherzolite at \t that this episode corresponds to the delamination of an arc root owing to gravitational instabilities arising from the presence of abundant pyroxenites within mantle harzburgites. Retrograde metamorphism and hydration under amphibolite-facies conditions were recorded by abundant post-kinematic amphibole, which corresponds to the exhumation of the arc root after its intrusion into a subduction zone. The Cabo Ortegal Complex thus preserves a unique section of delaminated arc root, providing evidence for the significant role of melt–peridotite interaction during the differentiation of primitive arc magmas at depth.