High-pressure granulitic metamorphism of pre-Variscan metasomatic mafic rocks (Cabo Ortegal, NW Iberia)

In the Cabo Ortegal Complex, NW Iberia, the subduction-related Variscan (ca. 390 Ma) high-pressure granulites of the Bacariza Formation (Cambrian protolith age, ca. 500 Ma) commonly show banding/regions defined by varying mineral abundance of amphibole and plagioclase-quartz that overprint high-pressure high-temperature (HP-HT) garnet and Na-rich diopsidic clinopyroxene, implying retrogression-related replacements triggered by the infiltration of H 2 O-bearing fluids. Regions with varied amphibole+plagioclase and garnet+clinopyroxene abundances are similar in bulk-rock composition, however, denoting insignificant metasomatism during the post-peak infiltration of H 2 O-fluid. On the contrary, bands rich in carbonate (Mg-calcite) denote substantial changes in bulk-rock composition relative to the original magmatic protoliths. This study presents petrographic and geochemical data and thermodynamic modelling of metamorphosed banded metasomatic carbonate-bearing mafic granulites of the Bacariza Formation. The textural-mineral assemblage development of the rocks made of varied amounts of garnet, Na diopside, calcite, epidote, scapolite, plagioclase, amphibole, rutile, titanite and phengite, plus relict K-feldspar and chlorite, denotes pre-metamorphic CaO-K 2 O-CO 2 -SO 3 -H 2 O metasomatic transformations to varied extent and subsequent roughly isochemical granulitic HP–HT metamorphism characterized by garnet + Na diopside+calcite+epidote+titanite+SO 4 -CO 3 -rich scapolite±plagioclase in the more carbonated bands. U-Pb dating of texturally-peak titanite yielded 387.3 ± 1.4/5.8 Ma, interpreted as a cooling age near the peak of metamorphism, while dating of texturally-peak epidote yielded 373.2 ± 3.7 / 6.6 Ma of uncertain significance. Subsequently, these assemblages did not experience significant retrogression during exhumation and cooling, implying a limited influx of fluid. Hence, the localized metasomatism of the Bacariza Formation occurred prior to early Devonian Variscan HP-HT metamorphism due to infiltration of aqueous CO 2 -SO 3 -Ca-K rich fluids during the Cambrian volcanic-arc stage that formed the protoliths of this Formation or after its accretion to the margin of Gondwana, perhaps during the Ordovician rifting stage that affected this margin.