Basement evolution in the Northern Hesperian Massif. A preliminary survey of results obtained by the Leiden research group

Historical notes on Galician geology, and on the work of the Leiden University petrology team in particular, are first provided. This is followed by an introduction to the geology of Galicia with emphasis on its crystalline basement and upper mantle inliers. Six lithotectonic units are distinguished: 1) the Variscan granitic rocks and migmatites. 2) the Palaeozoic supracrustal rocks and dismembered meta-ophiolites, 3) the blastomylonitic graben between Malpica and Tuy, 4) the Lalin and Forcarey Units, 5) the Ordenes basin and its mesocatazonal peripheral belt, 6) the predominantly mafic-ultramafic catazonal complex at Cabo Ortegal. The supracrustal history of western Galicia in late Precambrian and early Palaeozoic times, as inferred from neighbouring areas, is briefly outlined and is compared with the igneous, tectonic and metamorphic evolution of the upper mantle and lower crustal rocks contained in the polymetamorphic basement complexes (1—4), which have sustained high-pressure and high-temperature metamorphism under a flow-folding regime prior to the Variscan orogeny. Several models proposed for the development of the basement complexes in the northern Hesperian Massif are briefly discussed. It is considered most likely that an early Palaeozoic rift system caused by mantle plume diapirism, and accompanied by deepseated thermal metamorphism, lower crustal recycling, updoming of the crust and incipient sea floor spreading, was closely followed by Variscan low-pressure metamorphism, migmatization and granite emplacement under an intermittently compressive and dilatational tectonic regime. Finally, the probability of a Precambrian orogenic crust in western Galicia is briefly explored.