What controls chemical variation in granitic magmas?

Clemens J.D. ; Stevens G. (2012)

Review

Consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations. This stems from a variety of model deficiencies, ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on, for example, isotope or trace-element data and major-element data. We contend that any model that fails any of these tests of internal consistency cannot be considered further. Thus, although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation, there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts. That primary mechanism is peritectic assemblage entrainment (PAE).We infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction, and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction. For those elements with low solubilities in granitic melts, PAE (in varying degrees), accompanied by co-entrainment of accessory minerals, is responsible for most of the primary elemental variation in granitic magmas. In contrast, the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way. The source is the primary control on granite magma chemistry; it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage. The apparent complexity in granitic rock suites is largely a consequence of these processes in the source. All other mechanisms contribute only as a secondary overlay. © 2012 Elsevier B.V.

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