Measuring host rock volume changes during magma emplacement

Abstract

We use a technique to evaluate volume strains in contact aureoles during magma emplacement based on geochemical mass balance calculations. Dynamothermal contact metamorphism adjacent to the Emigrant Gap composite pluton, Sierra Nevada, California, produced an ∼1 km wide thermal/structural aureole consisting of an outer andalusite±cordierite zone and an inner potassium feldspar±sillimanite zone. Mass balance calculations using Al as an immobile reference frame element in metapelites inside and outside of the aureole indicate that the percentage change in total rock mass between chlorite-grade rocks outside of the thermal effects of the pluton and the aureole is −11.1%±1.4%. Mass balance calculations indicate that Si was the major rock forming element depleted (∼−16%) from the aureole during contact metamorphism. Mass balance calculations and density measurements yield volume strains associated with mass transfer during contact metamorphism of −12.4%. These data, in conjunction with field relationships, are interpreted to suggest that (a) element mobility during contact metamorphism is not restricted to volatiles, and (b) volume losses within the contact aureole occur during magma emplacement and may contribute to the ‘space-making’ process during magma emplacement if intrusion spans the period necessary to engage hydrothermal circulation in the host rocks.