ALKALIC IGNEOUS ROCKS OF BALCONES PROVINCE, TEXAS

In late Cretaceous time, subsilicic, alkalic magmas formed sills, laccoliths, plug-like bodies, small volcanoes, and a few dikes in Cretaceous sedimentary rocks of the Texas coastal plain along an arcuate trend approximately coinciding with the buried Ouachita structural belt. Transverse structural features apparently localized the igneous activity in two centers. In the larger center near Uvalde, Texas about half of the bodies are melilite-olivine nephelinite, a third olivine nephelinite and the remainder analcite phonolite, olivine basalt, and nepheline basanite, in order of decreasing abundance.Basaltic and nephelinitic magmas were the primary magmas from which all igneous rocks of the province were derived. Removal of olivine gave rise to a small range in chemical composition of the basaltic rocks, but not gradation toward silica undersaturation. Nephelinitic magmas with compositions near that at which normative calcium orthosilicate appears differentiated along two trends to form melilite-olivine nephelinite as one end product and through nepheline basanite to analcite phonolite as the other. These two trends arose independently, their courses being determined by the composition of the primary nephelinitic magma and the plane Fo-Di-Ne which has no piercing point at low pressure. © 1969 Oxford University Press.