Experimental melts from crustal rocks: A lithochemical constraint on granite petrogenesis

Abstract

Many studies of experimental petrology have devoted to partial melting of crustal rocks. In order to provide lithochemical constraints on granite petrogenesis, this paper presents a compilation and synthesis of available experimental data for the major element compositions of felsic melts derived from partial melting of natural or synthetic materials in the compositional range of crustal rocks. The experimental melts are categorized into four types according to the species of hydrous minerals in starting materials: (I) amphibole-bearing; (II) amphibole- and biotite-bearing; (III) biotite-bearing; and (IV) biotite- and muscovite-bearing. If dehydration melting takes place at normal crustal conditions (P=5–10kbar, T≤1000°C), experimental melts are rich in SiO2 but poor in MgO+FeOT except those from amphibole-bearing sources. A comprehensive comparison of compositions between experimental melts and starting materials indicates that geochemical fractionation is variable for different major elements and their ratios. Source composition and melting temperature exert stronger controls on the compositional variations of experimental melts than pressure and fluid. By comparing the experimental melts with natural granites, the following insights into granite petrogenesis can be got: (1) while peritectic assemblage entrainment may be the dominant mechanism for the compositional variations of garnet/cordierite-rich S-type granites, fractional crystallization of diverse melts from heterogeneous metasedimentary precursors probably governs the compositional variations of garnet/cordierite-poor S-type granites; (2) relatively K2O-rich mafic to intermediate rocks are appropriate sources for calc-alkaline I-type granites. The compositional variations of calc-alkaline granites are jointly controlled by peritectic assemblage entrainment and subsequent fractional crystallization; (3) while dehydration melting at T>950°C is appropriate for the production of ferroan and alkali-rich granitic melts from intermediate magnesian tonalite or granodiorite, it is also possible for ferroan, alkali–rich and fluorine-rich granitic melts to be produced by dehydration melting of moderately magnesian mica–bearing materials at T≤900°C. Nevertheless, the low-T melts are more peraluminous than the high-T ones. Therefore, the composition of source rocks exerts the first-order control on the composition of granitic melts in closed systems. In addition, the dehydration melting of crustal rocks under different conditions is also responsible for variations in the composition of granites.