Melt segregation and magma interactions during crustal melting: Breaking out of the matrix

Granitic plutons contain chemically distinct sub-populations formed by source-level entrainment of the peritectic assemblages into the melts. Rapid evacuation at source levels and high ascent rates protect the magmas from wall-rock interactions during their tenure at source depths and during their ascent through cool upper crust. The existence of different types of granites, and hence of clearly defined chemically different magmas within plutons, dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move. Exposed deep-crustal sections are typically lithologically diverse, with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions. If the melts were required to segregate and accumulate into large volumes within their hot sources, especially by slow, gravity-driven melt percolation, it is very likely that they would be substantially modified by reaction with diverse source rocks. Thus, the processes mooted to occur in melting, assimilation, storage and homogenisation (MASH) or deep crustal hot (DCH) zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma, perhaps carrying evidence of extensive magma mixing. From various lines of evidence, we conclude that MASH and DCH zones may not exist and, even if they do, they cannot form the sources of most granitic magmas. The intrinsic heterogeneity in crustal source rocks, the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source-inherited chemical and isotopic heterogeneities. Rapid, semi-continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole-rock, trace-element chemistry and accessory mineral chemistry will be decoupled from major-element variations. This also means that a large source with low nominal fertility (due to low hydrous mineral content, for example) could still produce a substantial granitic pluton, by additions of individually small increments of magma.