Pyroxenite-Rich Peridotites of the Cabo Ortegal Complex (Northwestern Spain): Evidence for Large-Scale Upper-Mantle Heterogeneity

• Published in: Journal of petrology 1991, Vol.Special_Volume (2), p.135-154
• Main Authors: Girardeau, Jacques, Ibarguchi, Jose I. Gil
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/Special_Volume.2.135

Structural upper portions of the Cabo Ortegal complex display a section of composite upper mantle composed of a continuous sheet of pyroxenite (∼ 3 km long and 300 m thick), within spinel-bearing harzburgite of the Herbeira massif. The sheet contains an alternation of pyroxenite and minor dunite with scarce garnet-rich mafic horizons. These horizons (on a scale of millimetres to 3 m) are generally not zoned but may be locally enriched at the base in olivine, orthopyroxene, and/or spinel. From primary phase compositions it is shown that the peridotites have strong affinities to residual oceanic peridotite tectonites and that the pyroxenites may constitute magma segregates crystallized in dykes or sills intruded into the peridotite. The pyroxenite sheet exhibits a nearly lenticular shape which may be due to large-scale boudinage involving northwards shear of the massif. This deformation is well recorded by finely layered rocks near the base and top of the sheet which show abundant isoclinal folds, mylonitic shear bands, and normal faults at different scales. Peridotites and pyroxenites underwent a complex tectono-thermal history involving re-equilibration of primary phases at ∼800°C and 16·5 kb accompanying late- to syn-kinematic development of garnet after spinel in pyroxenites, followed by retrograde recrystallization under amphibolites facies conditions. Surrounding granulites and eclogites record similar metamorphic events. Accordingly, it is proposed that large-scale boudinage and high-pressure re-equilibration of the ultramafics was associated with tectonic imbrication of the crust and mantle in a subduction zone.