Las rocas básicas de Monte Castelo (La Coruña)

A gabbroic complex is found 30 km to the North of Santiago de Compostela, Galicia. It turns out to be a multiple intrusion of different sills with a North-South trend. The borders of the sills are indicated by the occurrence of biotite and garnet, and sometimes xenoliths were found. Reaction-equations are given to explain the appearance of garnetiferous norites at the expense of olivine-diopside gabbros caused by the contamination of SiOa and A1203 from the xenoliths. Each sill is differentiated. At the bottom the percentage of anorthite in plagioclase is higher and there is a higher content of olivine. Na, K, Ti, Al and Si increase and Ca and Mg decrease towards the top of each sill. This differentation pattern of the sills is obscured by later ”epidioritization”. Chemical analyses suggest the basic rocks to belong to the kindreds of tholeiitic or high-alumina basalts depending on their contamination with Al2O3 from the surrounding gneisses.