The Importance of Residual Source Material (Restite) in Granite Petrogenesis

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Abstract
There is a great deal of evidence to support the hypothesis that many granites and related volcanic rocks contain significant amounts of crystals that did not crystallize from a melt. These crystals are residual from the partial melting of the source rocks (primary restitè) or result from the recrystallization of such residual crystals (secondary restite). Evidence for the presence of restite includes the occurrence of some rock inclusions that were relatively refractory fragments of the source, and the calcic plagioclase cores of comparatively uniform composition which occur in many granites. The presence of an ‘old’ zircon component in some I-type granites is best explained as being inherited from the source. Relatively mafic restite-bearing granite magmas can form at geologically reasonable temperatures in the crust and hence rocks of such composition do not have to be regarded as cumulates. The presence of more MgO in moderately felsic porphyritic volcanic rocks than can occur in liquids of that bulk composition at the known temperature of eruption, is best explained by the proposition that the phenocrysts are restite phases, rather than by the commonly held view that they precipitated from a melt at a substantially higher temperature. Some plutonic and volcanic rock suites of crustal derivation are chemically equivalent, but have a composition too mafic to represent melts at crustal temperatures this is compelling evidence that those rocks contain a substantial restite component and it shows that the plutonic rocks are not crystal cumulates. A significant restite component is an elegant way of overcoming the difficulties of nucleating crystals in unzoned porphyritic granite plutons. Likewise, we contend that many phenocrysts in volcanic rocks are restite minerals, rather than the result of nucleation at slow rates of cooling.

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