The Appinite-Migmatite Complex of Sanabria, NW Iberian Massif, Spain

Abstract

The Sanabria appinitic rocks and host migmatites form an unusual, non-peri-batholithic complex in which all the typical members of the appinite suite are present. It differs from most appinitic complexes in the deeper level of emplacement and the close temporal and spatial association with migmatites. Consequently, many in situ relationships that resulted from the invasion of mafic magma into a crustal anatectic zone are extremely well preserved. The complex shows unequivocal relations between members of the appinitic suite and between these and migmatites derived by anatexis of a gneissic formation (Ollo de Sapo gneiss). These relations point to derivation of monzodiorites and biotite diorites by hydrous basalt fractionation combined with fluid-assisted melting of the crustal rocks surrounding the appinitic intrusions. This hydrous basic magma may be derived from an enriched region of the mantle associated with subduction. Petrogenetic models have been tested using a combination of field relations and geochemical data. Despite the complexity of the processes involved, it is concluded that water played an important role in the petrogenesis of the intermediate and mafic magmas. Reaction between monzodiorite melts and the host migmatites was responsible for the generation of a range of intermediate rocks within the complex. The need for water to facilitate magma generation in both the mantle and the crust suggests that melting is linked with subduction. This interpretation has important implications because appinitic magmatism may be considered as indicative of subduction processes involved not only in the generation of the mafic end-members of the suite, but also in the generation of batholiths with which the appinitic rocks are spatially and temporally associated.