High-grade metamorphic rocks of the Mellid area, Galicia, NW Spain

This study concerns the petrology of the Mellid area, the SE portion of the outer zone of the Ordenes Complex which is one of the upthrusted Precambrian complexes in the axial zone of the Hercynian orogen in Galicia, NW Spain. An eugeosynclinal rock sequence is found containing units with different metamorphic evolutions. All units were affected by Precambrian tectonization and retrogressive metamorphism. This orogeny may be subdivided into three metamorphic and four deformation phases which caused definite changes in the mineralogical composition and the texture of the rock. The sequence of metamorphic phases, established in a granulite facies unit is as follows: the first phase of Precambrian metamorphism is characterized by the (hornblende-)granulite facies, more precisely the (hornblende-)clinopyroxene-garnet-sodic plagioclase subfacies of the kyanite-bearing granulite facies. At that time, PH2O must have been very low locally. The second and third phases were marked by the hornblende-clinopyroxene-garnet-sodic plagioclase subfacies and the amphibolite facies, respectively. The other units bear witness to lower grade metamorphic activities. The granulite facies unit comprises metamorphosed basic lavas, metapelitic rocks, garnet-bearing metagabbros and garnet-bearing peridotites. The metapelitic rocks (kyanite-garnet-orthoclase-sodic plagioclase-biotite) and the metamorphosed basic lavas (clinopyroxenegarnet-sodic plagioclase-amphibole) are described in detail. The latter rocks contain Ca-rich inclusions, displaying scapolite-bearing mineral assemblages. The inclusions can be ascribed to deuteric alteration or incipient metamorphism in the basic lavas prior to the granulite facies metamorphism. The other units contain metasedimentary rocks and granitic and granodioritic orthogneisses. Metamorphic conditions during the Hercynian orogeny did not go further than the lower amphibolite facies. Therefore, retrogradation of the Precambrian units continued but a clear conversion of the rock texture cannot be discerned. The most important Hercynian event in the Mellid area was the emplacement of an ophiolitic rock suite.