Petrology, structure and geotectonic implications of glaucophane-bearing eclogites and related rocks from the Malpica—Tuy (MT) Unit, Galicia, northwest Spain

Abstract

The Malpica—Tuy (MT) unit is a narrow band of basement in Galicia (northwest Spain) composed of mica schists, paragneisses and calc-alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks. Garnet, omphacite, glaucophane/barroisite, rutile ± phengite ± paragonite ± kyanite ± zoisite developed in eclogites s.s. and eclogitic impure sandstones; garnet, phengite, rutile ± zoisite in the calc-alkaline orthogeneisses and garnet, phengite, rutile ± chloritoid ± kyanite in the metasediments. Biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures. Plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses. The minimum P— T conditions for the equilibrium of garnet—omphacite parageneses at the peak of the metamorphism were 600°C, 12 ± 2 kbar. The mineral chemistry of the orthogneisses and metasediments is compatible with these conditions. Furthermore, the structural data also suggest an in situ origin for the eclogites, i.e. the same as the metamorphism of the enclosing rocks. The crystalline area surrounding the MT unit is mainly composed of metasediments and anatectic granites. The T at the peak of the metamorphism was higher than 700°C, the P being of the order of 5 kbar. The two domains represent different parts of the Hercynian foldbelt tectonically juxtaposed. The juxtaposition is the result of complex tangential tectonics, three main phases of compressive deformation having been recognized. A first stage of subduction of a thinned crust, the materials of the MT unit, and subsequent obduction (development of hydrated minerals in eclogitic rocks) with emplacement of these materials over an already metamorphosed crustal area is suggested. The MT unit would represent a remnant of a Hercynian thrust nappe.