Petrografía y condiciones de formación de las rocas ultramáficas con granate asociadas a las eclogitas de Cabo Ortegal

Abstract

Garnet-bearing ultramafic rocks forming a decametric outcrop occur associated to the eclogites of the upper catazonal unit of the Cabo Ortegal Complex. These ultramafites are included within variably mylonitized amphibole-pyroxene gneisses located between the eclogite band on top and the so-called ‘banded gneisses’ formation below. They are harzburgites with orthopyroxene, olivine and amphibole as main phases and spinel, garnet and ilmenite as minor phases. Serpentine and chlorite are the main secondary minerals. The absence of pervasive deformational features, lack of anisotropy and poikilitic texture of pyroxene and amphibole are noteworthy and point to a possible magmatic/mantle origin of these rocks. However, a detailed petrographic study has shown the existence of discrete domains made of aggregates of minute pyrope-rich garnet, orthopyroxene and olivine with subgrain recrystallization. The mineralogical and textural features thus reveal a metamorphic origin for, at least part, of the present mineral association. Calculated P-T conditions for the orthopyroxene-garnet assemblages are ca. 760 °C and 19 kbar. The rocks studied bear important textural and, to some extent, mineralogical similarities to ultramafic rocks elsewhere within the complex along major tectonic contacts (e.g. Carreiro Zone of Tectonic Movement), although, in view of the relatively Mg-poor olivine and orthopyroxene compositions, an origin through recrystallization of serpentinite protoliths appears unlikely. Instead, an origin through partial recrystallization of mantle fragments related to the subduction and tectonic stacking processes that gave rise to the Complex is proposed