Coldwell alkaline complex adjacent to the north of Lake Superior in Ontario, Canada is an isolated plutonic body composed chiefly of syenite and gabbros and Archean metamorphic rocks intruded by above mentioned igneous rocks. A pegmatite swarm is developed in olivine gabbro, a member of the complex. Zonal distribution is found in most of pegmatites and is composed of three zones, that is, leucocratic fine-grained zone of anorthosite, melanocratic fine-grained zone of gabbronorite and pyroxenite and coarse-grained zone of gabbronorite. The mean grain size of the rock forming minerals in fine-grained zone is 0.27㎜ and that of coarse-grained zone 10.4㎜. An content of most plagioclases is in the range from An50 to An70 indicating that the pegmatite is mafic pegmatite. In a grain of plagioclase, An content decreases gradually from core to margin. Paragenetic sequence of main minerals in pegmatite is plagioclase, olivine, pyroxene, hornblende, biotite and opaque mineral from the earlier stage. Mineralogy in pegmatite is similar to that of the country rock, so it can be classified as simple and pure pegmatite. Fine-grained zone might be formed by rapid crystallization with high viscosity caused by rapid reduction in P_(H₂O) when coexisting vapor phase is suddenly released and raising the sotidus and liquidus temperatures abruptly. Coarse-grained zone is probably formed by high diffusion rate and low viscosity caused by depolymerization, formation of aqueous phase and decrease of solidus temperature. The rate of increase in viscosity caused by cooling of magma from the magmatic stage and high content of SiO₂ is ignorable compared with the rate of decrease in viscosity caused by concentration of water.

• 이혜임
• 정지곤
• Peter C. Jones