Petrology of Quaternary volcanics of the Mount Garibaldi map-area, southwestern British Columbia

Abstract

The petrology of a suite of volcanic rocks having limited age range and areal distribution but wide compositional differences is described. The mineralogy shows remarkably little relationship to bulk composition of the rocks. Cooling histories, particularly as influencing mineral zoning and degree of crystallization, and pressure histories are important in determining what minerals are present and what is the composition of these minerals. Rock classification based on chemical composition is hence more suited for correlation of these rocks than is one based on mineralogic composition. Petrologic characters and the volumetric distribution of rock types suggests the dif• ferentiation of a basaltic magma contaminated by granitic material, followed by gravitative selection during extrusion.