One of the problems in explaining the emplacement of large plutons is how space is made for them within the Earth's crust. One proposed space-making mechanism is volume loss. Volume loss, through heat-and pressure-induced recrystallization within a pluton's aureole can produce reactions that are not isochemical, thus removing elemental mass from the surrounding rock. Presently, volume loss is not considered a significant mechanism for decreasing rock volume in contact metamorphic aureoles. The approximately 94 ± 2 Ma La Posta pluton is the largest of its type emplaced in the Peninsular Ranges batholith (PRb ). Along the northwestern margin of the La Posta is the Late Jurassic-Early Cretaceous Cuyamaca Reservoir gneiss (CRg). The CRg is one of a wide variety of rock types into which the La Posta intrudes. In order to assess element mobility and volume loss within the well-defined structural aureole of the La Posta pluton, 38 samples of the CRg were collected and analyzed for major, trace, and rare-earth elements. Elemental compositions for 18 samples collected from outside the aureole were compared with the compositions from 20 samples collected inside the aureole. Because of the constant-sum format of geochemical data, they are not useful in their conventional forms for assessing chemical similarities/differences between the rocks outside and inside the structural aureole of the La Posta pluton. Therefore, three different statistical methods were employed for evaluating the elemental data. These methods are the Bootstrap Algorithm of G. H. Girty, the SACD Method using the Matlab 5 Program, and the ISOCON Method. Results from these methods yield total changes in mass of -26%, -23%, and -20% respectively. In addition, the majority of the mass loss is due to decreases in Si02 and Al203. A significant amount of element mobility and depletion should be associated with evidence of volume loss. With the use of the mass-balance equation, measured sample densities, and geochemical concentrations, an estimated 21% to 26% of the CRg's volume was removed, in association with the emplacement of the La Posta pluton. Initial indications from point-count data suggest that the loss of feldspars, associated with the formation of the aureole, may account for this volume loss and differences in the CRg's composition inside and outside the structural aureole of the La Posta pluton.