La diversite des basaltes

Abstract

Abstract By comparing the basalts from different areas, France and Japan in particular, two major groups can be distinguished--rhyolitic basalts and phonolitic basalts. Subdivisions recognized in each group represent the median points in the progressively changing suite of the basaltic lavas. The rhyolitic basalts are divided into four categories--tholeiites, high-alumina basalts, trachybasalts, and trachybasalts of the Puys chain. The phonolitic basalts are characterized by their poor silica content, giving rise to the crystallization of olivine and nepheline (basanites). This group includes the olivine-basalts of the Pacific and of Mont-Dore (France). Comparison of the mineral content of the basalts shows that they contain progressively larger amounts of dark minerals such as olivine and titano-augite in proportion to the diminishing silica content--a progression ending with the ankaramites, oceanites, and picrites. The theory that basalts are formed by the partial fusion of the peridotite mantle--those poor in silica at greater depths than the silica-rich--explains the continual variation in the chemical and mineralogical composition of the basalts studied. The shallower, rhyolitic magmas flow easily, dominating the larger volcanic complexes (Hawaii, Deccan, Mont-Dore, Cantal), while the deeper magmas, the basanites, form only rare batholiths of smaller proportions, as in Lorraine.