Potassic magmatism within Continental plates

Abstract

Most petrogenetic models assume that primary basic and ultrabasic magmas are generated by varying degrees of partial melting of fertile Iherzolite within the upper mantle (Ch. 3). The degree of partial melting and the depth of segregation of the magmas are considered to be the main variables in controlling the composition of the melt. Additionally, variations in the volatile content and mineralogy of the source mantle, and the extent of subsequent fractional crystallization and crustal contamination are invoked to explain the wide range of terrestrial basaltic magma compositions (Chs. 3 & 4). In the majority of tectonic settings which we have discussed thus far, an important characteristic of the primary basaltic magmas is that they contain significantly greater concentrations of Na₂O than K₂O on a weight per cent basis. Exceptions to this are the more potassic members of the subduction-related magmatic series (Chs. 6 & 7) and certain potassic oceanic-island suites (e.g. Tristan da Cunha; Ch. 9). However, even within these relatively potassic suites Na₂O is still greater than K₂O in the basaltic end-members. Rather more rarely, and almost totally restricted to within-continental plate-tectonic settings, basic and ultra-basic magmas are generated in which the content of K₂O exceeds that of Na₂O, often significantly so. Kimberlites are included within this category in addition to a range of highly potassic igneous rocks with exotic names and frequently equally exotic mineralogies, generated in a wide variety of tectonic settings.