Origin of Primary Basalt Magmas and Classification of Basaltic Rocks

Abstract

The upper part of the mantle is assumed to be composed of peridotite represented by the inclusions in basaltic rocks. Compositions of some representative basalts including tholeiite, alkali olivine basalt, and nepheline basalt were subtracted from the average composition of the assumed mantle peridotite. These basalt magmas can be produced by partial melting of 2–9 per cent of the peridotite; the solid materials left after the partial melting still have peridotite mineralogy. The production of the various basaltic magmas from the single peridotite is possible only if various components of the silicate minerals of the peridotite enter into the liquid fraction in different proportions depending on the pressure and temperature of the melting. These components are Mg 2 SiO 4 and Fe 2 SiO 4 in olivine, and CaSiO 3 , MgSiO 3 , FeSiO 3 , NaFe 3+ Si 2 O 6 , NaAlSi 2 O 6 CaTiAl 2 O 6 , CaAl 2 SiO 6 , and MgAl 2 SiO 6 in pyroxenes. The above components were calculated from chemical analyses of basaltic rocks, and the rocks were classified into three groups: group A includes rocks with excess SiO 2 left after calculating the above components and with no Mg 2 SiO 4 and Fe 2 SiO 4 , group B includes those with no excess or deficient SiO 2 and therefore with Mg 2 SiO 3 , FeSiO 3 , Mg 2 SiO 4 , and Fe 2 SiO 4 , and group C includes those with deficient SiO 2 and without MgSiO 3 and FeSiO 3 . The only possible way of producing the magma of group A (tholeiite, high-alumina basalt, and some alkali olivine basalt) is by fusion involving incongruent melting of orthopyroxene. It is concluded that this magma is formed at shallow depths in the mantle, that of group B at intermediate depths, and that of group C (nepheline basalt) at greater depths. Even if the upper mantle is made up of eclogite, the same depth relation of the various basalt magma formations is obtained.