Melt Loss Quantification from Restites of Heterogeneous Metasedimentary Protoliths Revealing Volatile Redistribution during Mid-Crustal Anatexis

Abstract The amount of melt lost from restitic rocks is a major parameter in numerical models of crustal processes. However, quantification of melt volume extracted is thwarted by the heterogeneity of the crust and a lack of estimates no the amount of fluid involved. The major element trends in many sedimentary suites allow treatment of protoliths as two component mixtures. This, coupled with a mass balance constrain and minimization by a residuals function, permit development of a general algorithm for quantification of both melt loss and exact protolith composition based on the composition of a restitic rock. The utilization of anhydrous compositions results in melt loss estimates independent of volatile phase, which are particularly suitable for elucidation of melting fluid regime. The application of this method to restites of the Mount Stafford complex in Australia provides a quantitative estimate of 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 % of the original rock mass. The quantification of uncertainties demonstrates that loss of low melt fraction could be unresolvable for individual samples, but uncertainty tends to decrease for highly restitic rocks. The melting mechanism in the Mount Stafford varied from fluid-fluxed to fluid-deficient in adjacent rocks. The heterogeneous melt productivity was likely caused by the redistribution of fluids generated by muscovite breakdown during heating. The overall melt productivity of mid-crustal melting was remarkably high and the volume of extracted granitic melt was perhaps similar on average to the volume of restite.