Widely variable granite production from a restitic metasedimentary terrane by volatile redistribution

The amount of melt extracted from a metasedimentary source is a major parameter in quantitative models of crustal processes; however, quantification of melt volume is thwarted by the heterogeneity of the crust and a lack of estimates concerning the amount of fluid present. We observe that highly systematic trends in metasedimentary rocks allow treatment of protoliths as two component mixtures, which, coupled with mass balance and minimization by a residuals function, permit development of a general algorithm for quantification of melt loss and protolith composition based on the composition of the restitic rocks. The utilization of anhydrous compositions results in melt loss estimates independent of volatile phase, and particularly suitable for demonstration of the role of fluid. The application of this method to restites of the Mount Stafford complex in Australia reveals melt loss with unprecedent detail, and shows that melt loss from rocks of the same metamorphic grade was highly variable from negligible to 70 wt%. The melting mechanism in the complex changed from fluid saturated near the solidus, to fluid fluxed at higher temperatures, and fluid deficient in adjacent rocks. Heterogeneous melt productivity was caused by the redistribution of fluids generated by muscovite breakdown during heating. The overall melt productivity of mid-crustal melting was remarkably high and volume of extracted granitic melt perhaps was similar to the volume of restite.