Assessing the effect of melt extraction from mushy reservoirs on compositions of granitoids: From a global database to a single batholith

Abstract

Abstract Mafic and ultramafic plutonic rocks are often considered to be crystal cumulates (i.e., they are melt-depleted), but such a classification is much more contentious for intermediate to silicic granitoids (e.g., tonalite, granodiorite, granite, and syenite). Whether or not a given plutonic rock has lost melt to feed shallower subvolcanic intrusive bodies or volcanic edifices has key implications for understanding igneous processes occurring within the crust throughout the evolution of the Earth. We use statistical analyses of a global volcanic and plutonic rock database to show that most mafic to felsic plutonic rock compositions can be interpreted as melt-depleted (i.e., most of the minerals analyzed are more evolved than their bulk-rock compositions would allow). To illustrate the application of the method to natural samples (from the Tertiary Adamello Batholith in the southern Alps), we estimate the degree of melt depletion using a combination of magmatic textures, bulk-rock chemistry, modal mineralogy, distributions of plagioclase composition (using scanning electron microscope phase mapping/electron microprobe analyses), and thermodynamic modeling. We find that melt depletion correlates with the magmatic foliation and is accompanied by bulk depletion in incompatible elements, low amounts of near-solidus minerals, and mineral compositions that are too evolved (i.e., depleted in Ca or Mg, depending on the mineral) to be in equilibrium with their bulk-rock chemistry. The analytical and modeling workflow proposed in this study provides a path to quantifying melt depletion in any plutonic samples. Most plutons cannot be assumed to represent liquids; degree of crystal-liquid separation must be a major consideration in their interpretation, whether or not cumulate textures are obvious. (Bacon and Druitt, 1988, p. 253) Igneous rocks, especially plutonic rocks, rarely represent quenched melts. (Miller et al., 2003, p. 529)