Distribution, nature, and origins of the 2400 BP eruption products of Mount Meager, British Columbia: linkages between magma chemistry and eruption behaviour

Abstract

The Bridge River Volcanic Assemblage comprises the eruptive products of a 2400 BP eruption of Mount Meager including airfall pumice, pyroclastic flows, lahars, and lava flow. There is also a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks are found underlying and overlying Bridge River Volcanic Assemblage deposits, but appear to be unrelated to the eruptive events. This report presents new units that were not previously recognized, new stratigraphic relationships, and new origins for some of the deposits. Rocks of the Bridge River Volcanic Assemblage are dacitic with phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. Sieve-textured plagioclase is pervasive in all Bridge River Volcanic Assemblage rock types. The presence of banded pumices suggests mingling of mafic and dacitic magma prior to eruption and perhaps represents the trigger mechanism for Recent volcanism. Pearce element ratio diagrams demonstrate that the Bridge River Volcanic Assemblage rocks record a chemical variation that correlates with mode of eruption. Pumices from airfall deposits and pumice blocks from pyroclastic flows are richer in water and tend to be more differentiated than do the Bridge River Volcanic Assemblage lavas. These slight chemical variations correlate with a change in eruptive style from explosive to extrusive.