Mid-crustal plutons in the Coast Mountains, British Columbia, Canada: a physical and geochemical investigation of their origin and interaction with the host rock in the region.

Laura Kellogg and Maria Luisa Crawford

Bryn Mawr College

This study is based on a petrological and geochemical analysis of mid-crustal plutons, formed from molten rock, and surrounding host rock. It addresses the source for the plutons and the nature of their interaction with the host rocks. The study area is in the Coast Mountains Batholith near Prince Rupert, BC. Canada. The plutons formed in what was once the core of the Coast Mountains as a result of the Farallon/Kula plate subducting beneath the North American plate. Plutons in the study area range in age from 82 Ma to 53 Ma. They occur as large igneous bodies, about 12 km wide and as small plutonic screens in the host rock, less than a km wide. The large plutons are intermediate in composition and the small plutons can be separated into two groups; intermediate plutons similar to the large plutons and plutons of felsic composition. All intermediate plutons in the study area are gabbroic to dioritic in composition. The leucocratic plutons are tonalitic to granodioritic in composition. The intermediate plutons are proposed to have formed as a result of mafic magma mixed with crustal rock. Support for this is based on their mineralogical similarity to typical subduction related plutons and their trace element similarity to the host rock in the area. The leucocratic plutons are proposed to have formed purely from crustal melt. The host rock in the region is primarily pyroxene biotite gneiss, a more aluminum rich rock than the intermediate plutons. The host exhibits significant similarities to the leucocratic plutons both in its mineralogical and geochemical attributes, which supports the hypothesis that leucocratic plutons are a result of melted host rock. Post emplacement contamination of igneous material by the host rock is evident in a small young intermediate pluton and not as evident in an older small intermediate pluton. Evidence for the contamination is supported by a change in mineralogy and in trace element characteristics of igneous material at the edge of the pluton. The study reveals that the role of the continental crust is an important factor in orogenic processes, especially late in the magmatic history associated with subduction.