Measuring host rock volume changes during magma emplacement

We use a technique to evaluate volume strains in contact aureoles during ma gma emplacement based on geochemical mass balance calculations. Dynamotherm al contact metamorphism adjacent to the Emigrant Gap composite pluton, Sier ra Nevada, California, produced an similar to 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 o f the aureole indicate that the percentage change in total rock mass betwee n 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 (similar to- 16%) from the aureole during contact metamorphism. Mass balance calculations and density measure ments yield volume strains associated with mass transfer during contact met amorphism of -12.4%. These data, in conjunction with field relationships, a re interpreted to suggest that (a) element mobility during contact metamorp hism is not restricted to volatiles, and (b) volume losses within the conta ct aureole occur during magma emplacement and may contribute to the 'space- making' process during magma emplacement if intrusion spans the period nece ssary to engage hydrothermal circulation in the host rocks. (C) 1998 Elsevi er Science Ltd. All rights reserved.