A dynamic model for quartz-feldspar graphic intergrowths from granitic pegmatites in the southwestern Grenville Province

During the formation of U-, Th-, Mo-, REE- and Nb-bearing granitic pegmatites in this area in SW Ontario, conditions of crystallization were such that widespread graphic quartz-feldspar intergrowths were formed. The alkali feldspar-quartz intergrowth is characterized by angular rods of quartz that have an irregular inner interface and a planar outer interface with the host alkali feldspar. The quartz is interpreted as having nucleated epitactically on edges and corners of alkali feldspar crystals. Rapid growth, at or near volatile-saturated conditions, resulted in quartz saturation along the irregular melt-feldspar interface. Slow diffusion of Al and Si species (network formers) in the boundary-layer melt was probably the rate-controlling step for quartz saturation, which occurred along corners and edges, where the feldspar grew most rapidly. Diffusion-limited growth resulted in a buildup of SiO (sub 2) at the interface, producing oscillations from quartz-oversaturated to quartz-undersaturated conditions and thus the rhythmic quartz-feldspar intergrowths. Evidence of saturation in a volatile phase in these pegmatites indicates that water was a catalyst for feldspar growth and that lower a (sub H2O) in the melt decreased Si diffusivity at the crystal interface.