The basalt clan

The major element compositions of the various rocks that belong to the basalt clan are examined, and the basalt clan is defined as consisting of those volcanic rocks that contain between 44.0% and 53.5% silica. After examining both the diverse tectonic environments in which the basaltic rocks are found, and also the various petrographic suites to which they belong, or appear to belong, new chemical criteria were devised to divide them into the following groups: (1) low-potash basalt, (2) high-alumina basalt, (3) continental-flood basalt, (4) komatiitic basalt, (5) sodic basalt, (6) hawaiite, (7) phonolitic basalt, (8) potassic basalt, (9) trachybasalt, (10) leucitite, (11) sodic transitional basalt, and (12) potassic transitional basalt. The lunar mare-basalts are regarded as being transitional in their chemical character between the low-potash, ocean-floor basalts and the komatiitic basalts. While it was relatively easy to discover petrogenetic models that were able to account for the origin and evolution of the common sub-alkalic basalts, and even the sodic basalts, a variety of possible petrogenetic models had to be explored in order to account for the origin and evolution of the different basaltic rocks of the potash- and high-potash series.