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Description
The Residual Model assumes a heterogeneous starting protolith, and is based on interpreting bivariate plots of elemental concentrations. In the Residual Model, the investigator first establishes a relationship between two immobile species, 1 and k. Under the conditions of immobility during metamorphism, the ratio C 'k/C \ must remain constant. It then follows that 0 ' Ck ' C =-*C. (1) k c? , l where C 'k and C 'i represent the concentrations of elements k and i after metamorphism, and c0k and co i represent the concentrations of elements i and k before metamorphism. Differentiating equation (1) with respect to C'i yields a ' 0 ck_ ck -a, -c'! C . l l (2) Therefore, on a plot of Ck versus Ci, the slope of lines describing residual enrichment or dilution are given by C0k/C0 i• Ck Ci Shown in the above illustration are all possible concentrations of two ideal reference species, i and k. Initial protolith compositions are shown as the shaded circle. Two lines, emanating from the origin and tangent to the protolith field, define a wedge-shaped region, which under the constraints of element immobility, is an area m which all possible concentrations of i and k will lie following metamorphism of the protolith. Residual dilution occurs when the masses of other elements are increased. Residual enrichment occurs when the masses of other elements are decreased. If the mass of k is 3 increased, then the ratio C 'k/C'j also increases. Mass gain 1s shown schematically as a compositional array extending to the left of the wedge-shaped region. If species k moves out of the system, then the ratio C 'k/C'j decreases, and the resulting compositional array extends to the right of the wedge-shaped region. Forty one elements in 41 argillaceous samples were analyzed utilizing the principles of the Residual Model. The 41 samples are distributed from outside to inside the well defined contact metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton. The results of my work indicate that most, but not all, elements are generally immobile, as their masses appear to have remained relatively constant in going from outside to inside the aureole of the Emigrant Gap composite pluton. In contrast, Mn, V, and Pb display characteristics suggestive of severe mass gains, whereas Zr exhibits behavior indicative of mass loss. In addition, several other elements display evidence for modest element mobility. Based on these and other data, I conclude that element mobility was localized and nonuniform within the aureole, and may have been the result of variable fluid/rock ratios during the development of the aureole.
