Informe preliminar sobre los estudios de la petrografia del extremo occidental de Galicia

The extreme western part of Galicia around Cape Finisterre and Cape Toriñana, consists of pre-migmatic metamorphic rocks, partly migmatised, intruded by syn- and posttectonic granites. The pre-migmatic rocks are of sedimentary origin (biotite schists or gneisses) and of probable igneous origin (augengneisses). Locally the original schistosity has been destroyed by progressive migmatization (formation of hydrothermaloid, pegmatoid and granitoid phases). Two series may be distinguished: a, mica gneiss (or schist) — migmatic gneiss — nebulite — anatexitic granite and b, augengneiss — nebulitic augengneiss. The granites have been divided into the following types: the Dumbría granite (equivalent to the Barbanza granite of von Raumer, 1963) and the megacrystic alkalifeldsparbearing granites: the Finisterre, la Ruña and Mugía granites. The chemical composition, the triclinicity of the alkalifeldspar megacrysts and the distribution of sodium over plagioclase and alkalifeldspar (method of Barth, 1956) are discussed and presented in tables 1, 2 and 3 respectively. The Q-L-M and Q-Ab-Or values, and the variation diagram of the standard cell after Barth (1948, 1952), all calculated from the chemical analysis of the various types of rocks, are given in figures 2, 3 and 4 respectively.