Petrology of eclogites and clinopyroxene-garnet metabasites from the Cabo Ortegal Complex (northwestern Spain)

Abstract High-pressure, subduction-related metamorphism is recorded in two structural units of the Cabo Ortegal complex of Spain: (i) an upper thrust unit (Concepenido-La Capelada) composed of granulites, eclogites and metaperidotites and (ii) a structurally underlying unit (Chimparra-Banded) formed of gneisses with minor metabasic intercalations. In the upper unit, a metamorphic episode of Lower Ordovician age caused the formation of eclogites from basic rocks with N-MORB compositions, while high-pressure, garnet-clinopyroxene granulites were formed from basic to intermediate protoliths with calc-alkaline affinities. Garnet replaces spinel in ultramafics of the same unit and may have originated simultaneously. The PT -conditions were c. 800°C at 13.5 kbar in the granulites, whereas similar temperatures and higher pressures ( > 17 kbar) prevailed in the eclogites. The migmatitic Chimparra and Banded gneisses contain boudins of garnet and omphacite-bearing rocks, with or without plagioclase. The basic protoliths of these rocks are similar to LREE-enriched basalts with alkaline affinities. The Silurian metamorphism in this gneissic unit took place at c. 700°C or lower and at pressures around 15 kbar. This metamorphism could have occurred when a volcano-sedimentary complex, formed in a thinned-crust environment, was subducted beneath an overlying active margin/volcanic arc complex. An ophiolitic unit, made up of the N- and T-type MORB metabasites of the Candelaria and Purrido-Pena Escrita Formations, underlies the gneisses. Metamorphism in this unit took place during the Early-Middle Devonian, mainly giving rise to amphibolite-facies associations. Sporadic coronitic metagabbros/dolerite and garnet-clinopyroxene metabasites indicate that amphibolite-granulite transitional conditions ( > 700°C, 8 kbar) were attained locally. Retrograde amphibolitic assemblages in the upper units are more or less coeval with this episode, which suggests that it may have been related to the cessation of obduction and the stacking of the overlying high-grade units.