Processes in mushes and their role in the differentiation of granitic rocks

Abstract

The modification of magma through mixing or through assimilation of country rocks is commonly inferred from various chemical and isotopic features of magmatic rock suites and supported by a variety of field evidence, including features indicative of interaction between two magmas, disaggregation of solids in magma, and varied antecryst populations in volcanic and plutonic rocks. The increased recognition that mushes are important long-term reservoirs of magma, and that they play an important role in magmatic processes, suggests that the processes of magma mixing and country rock assimilation probably involves mushes. This paper investigates field evidence related to the behaviour of granitic mushes, in an attempt to understand their behaviour and assess their contribution to compositional variation in granitic rocks. Mushes, defined here as solids with interstitial melt, are permeable materials open to the invasion of extraneous melts. These can react with the crystalline framework and mix with or expel the original resident melt. This is the first step in either magma mixing or rock assimilation, and is recorded by ghost structures in migmatites, the loss of melt from a dyke to a mushy host rock, or patchy hybridization of microgranitoid enclaves (ME). The melt-filled porosity of mushes implies that they have an inherent physical weakness, which makes them subject to disaggregation and liquefaction through the combined effects of physical erosion, increased melt fraction, and pore pressure increase. As mushes disaggregate, solids may continue to react with their melt-rich surroundings. This is the second step in hybridization or assimilation, which tends to mask the history of exchange and the role of mushes. The history of granitic magmas starts with their extraction from a mushy migmatitic source or from a mushy cumulate, a process that controls their variable cargo of residual or cumulate crystals. These magmas may subsequently intrude, infiltrate and assimilate mushy wall-rocks or mushy xenoliths, or intrude, infiltrate and mingle with mushy mafic intrusions, forming mushy microgranitoid enclaves. The resulting magmas may themselves become temporary mushes, capable of being infiltrated by extraneous magmas before being liquefied and remobilized. Thus, mushes may play multiple and variable roles during the evolution of a single magma batch, facilitating magma mixing and wall-rock assimilation because mushes relax the energetic and physical constraints of these processes. Beyond providing opportunities for hybridization and assimilation, crustal mush columns provide multiple opportunities for magma batches to alternate between periods dominated by fractional crystallization and mush development, and periods dominated by remelting and mush remobilization, generating magmas with complex histories and varied geochemical signatures.