Magmatism and tectonic settings

Mutual relationships of mobile lithospheric plates define two distinct tectonic environments each characterized by diagnostic magmatism. A compressional environment occurs at loci of plate convergence; quartz- and hypersthene-normative intermediate, acid and basic members of the calc-alkaline orogenic suite are emplaced above a dipping seismic zone. A tensional environment occurs mainly at loci of plate divergence, areas characterized by networks of deep fractures, offset by transform faults. Products of nonorogenic magmatism typically consist of abundant tholeiitic or alkali olivine basalt, possibly accompanied by rhyolite, trachyte, or phonolite (or plutonic equivalents); intermediate compositions are volumetrically insignificant. On a differentiation index versus frequency plot, orogenic suites define one maximum, nonorogenic suites two maximums. Orogenic rocks show no iron enrichment; nonorogenic suites do. Recognition of older igneous associations as orogenic or nonorogenic by employing chemical and mineralogical criteria can lead to inferences about tectonic environments during emplacement. Such criteria may provide a unique insight into regional tectonic evolution very difficult to deduce from structural investigations alone.