Amphibolites and other metamorphic mafic rocks of the blastomylonitic graben in Western Galicia, NW Spain: field relations and petrography

Within the strongly migmatized axial zone of the Hesperian massif in western Galicia a graben-like structure has been distinguished, characterized essentially by the presence of non-migmatic rocks that comprise orthogneisses with blastomylonitic textures, leucocratic gneisses, plagioclase-blastbearing paragneisses, pelitic schists, and numerous amphibolitic layers and lenses. In the southern and central part of the graben and at the borders in the north the majority of the amphibolites are metamorphosed mafic dike swarms that intruded in the Early Palaeozoic after the emplacement of biotite granites but before the intrusion of subalkaline and peralkaline granites. Few amphibolites are of sedimentary origin. The other amphibolitic rocks in the north are of inferred Proterozoic age and have a different appearance. They consist of retrograde eclogite facies mafites and garnet- and epidote-amphibolites that are typically associated with leucocratic gneisses and younger subalkaline orthogneisses. It is inferred that the northern part of the graben mainly represents a lower basement segment that underwent Precambrian and Early Palaeozoic catazonal metamorphism and subsequent retrogradation, while the central and southern parts represent higher basement levels of mesozonal metamorphic grade.