Some thermal constraints on crustal assimilation during fractionation of hydrous, mantle-derived magmas with examples from central Alpine batholiths

erworben im Rahmen der Schweizer Nationallizenzen (www.nationallizenzen.ch)We provide a model for the fractional crystallization of hydrous mantle-derived magma to form calc-alkaline plutons, based upon mass balance for geological examples of fractionation sequences in the lower continental crust. This is complemented by a thermal model for the heat budget obtained from a projected phase diagram and thermodynamic data. Fractional crystallization (FC) and assimilation–fractional crystallization (AFC) paths have been calculated with these models and the mass ratio of assimilation to crystallization as a function of parent magma type and temperature, crustal rock fertility and temperature, and mechanism of assimilation, have been determined. When these results are combined with F (melt fraction) and r (ratio of mass assimilated/crystallized) values evaluated from geochemical data then new information, not available with the methods separately, can be deduced. This includes when and at what depth and temperature in the crust the assimilation took place, as well as the likely parent magma type and temperature of the assimilant. Our results are presented in simple graphical fashion to facilitate future studies that examine the evolution of individual calc-alkaline plutons and the mechanisms of crustal contamination, and to improve melt models involving hydrous magma in volcanic arcs and in the lower continental crust