Petrology of an eclogite- and pyrigarnite-bearing polymetamorphic rock complex at Cabo Ortegal, NW Spain

At Cabo Ortegal, paragneisses are found in association with amphibolites, metagabbros, amphibolized eclogites, amphibolized (plagio) pyrigarnites, and serpentinized ultrabasic rocks. On the basis of petrographical and chemical evidence, their geological history was reconstructed as follows: Precambrian sedimentation of graywackes, was rapidly succeeded by intrusion of gabbroic sills and stocks and by emplacement of ultrabasic rocks. During an orogeny of supposed Precambrian age, the rocks in the area were subjected to mesozonal metamorphism in the upper part of the sequence and to catazonal metamorphism in the lower parts. Mesozonally-metamorphosed gneisses formed staurolitealmandine subfacies associations; their gabbroic inclusions were transformed into amphibolites. Catazonally-metamorphosed gneisses became partially anatexitic, and kyanite-almandine associations with or without biotite were formed in the unmelted portions (restite). Gabbro sills were altered into foliated eclogites (± a-zoisite ± kyanite ± carinthine) in the eastern part of the area and into non-foliated (plagio)pyrigarnite (high-pressure granulite) in its central and western parts. The ultrabasites were transformed into garnet peridotites. Evidence of isoclinal folding has been found in the eclogites and, less conspicuously, in the ultrabasic rocks. Renewed local intrusion of gabbro was followed by catazonal retrograde metamorphism (hornblende-clinopyroxene-almandine subfacies grade) in zones affected by pegmatoid injection or by penetrative deformation. Retrograde alterations could only be demonstrated for the mafic rocks from the appearance of greenish-brown hornblende or the formation of a new foliation. The newly-intruded gabbros were partly metamorphosed and acquired a flaser texture. After the Precambrian orogeny, gabbroic and granitic rocks once again intruded locally; the age of their intrusion is placed at 500 ± m.y. by analogy with similar intrusions in southern Galicia. These rocks were altered into schistose amphibolite and orthogneiss during the Hercynian orogeny; the metamorphic rocks were remetamorphosed and (re)folded under amphibolite facies conditions (kyanite-almandine-muscovite subfacies grade over the whole of the investigated area). In the paragneisses an assemblage of biotite + muscovite ± garnet ± kyanite was formed, while quartz and plagioclase recrystallized. Two consecutive phases of deformation caused isoclinal folds, followed in most cases by open folding. In the eclogites and in the (plagio)- pyrigarnites, the association blue-green hornblende + plagioclase + epidote ± garnet was formed secondarily after the catazonal associations. The eclogites were boudinaged; internal folding did not take place, and amphibolization was confined to the rims of boudins and pegmatoid injections as well as to narrow zones of penetrative deformation. More intensive tectonization (internal isoclinal folding, overprinted by open folds) caused the (plagio) pyrigarnites to be more extensively retrograded during the Hercynian orogeny. They were altered for the most part into laminated (garnet) amphibolites. The metagabbros were likewise altered into laminated (garnet) amphibolites, with textures ranging from mylonitic to gabbroic. In the ultrabasic rocks, garnet peridotites were replaced by amphibole or chlorite peridotites. Isoclinal as well as open folds were formed. During the Hercynian orogeny, the stratigraphie sequence was overturned in the eastern as well as in the western part of the area (formation of a ”mushroom-shaped” dome); and in the western part a zone of overthrust with isoclinally-folded blastomylonites, boudinaged ultrabasites, and microfolded amphibolites was formed. Post-Hercynian activities consisted mainly of ubiquitous hydrothermal action accompanied by local greenschist-facies retrogradations in most of the rocks and by extensive serpentinization in the ultrabasites. Normal faulting, resulting in mainly vertical block-movements, accompanied locally by brecciation or mylonitization, and small-scale emplacement of tonalitic and gabbroic dykes, also took place after the Hercynian orogenesis.