Pegmatites Derived from Fractionation of a Melt: An Example from Pegmatites in the Owala-Kaikawala Area, Matale, Sri Lanka

Vertically and sub-vertically intruded syenite and granitic pegmatites, which carry many industrial minerals are found in the Owala-Kaikawala area in the central parts of Sri Lanka. The field setting, petrography and chemical composition of feldspars of these rocks illustrate that they have been derived from a single magmatic source formed by crustal anatexis. The enrichment of compatible elements in feldspars of the fluorite-bearing syenitic pegmatite and incompatible elements in granitic pegmatites can best be explained the chemical evolutions of the extremely fractionated melt. The initial fractionation of a volatile enriched, silica-undersaturated melt from the parental magma resulted into the fluorite-bearing syenitic pegmatite. The remaining melt that is rich in silica, fractionated again slightly and was emplaced into eastern parts of the area as granitic pegmatite. The fractionations of the fluid rich melt took place frequently until the crystallization of minerals.