THE IMPORTANCE OF RESIDUAL SOURCE MATERIAL (RESTITE) IN GRANITE PETROGENESIS

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.Clearly there are granites that are more mafic and restite-free which have formed by the classical process of precipitation from a melt. However, examples of such granites are rare in the Lachlan Fold Belt of southeastern Australia. One such case is the Boggy Plain Supersuite and its associated felsic volcanic rocks. We compare this supersuite with two examples of the more typical restite-bearing case, the Bullenbalong Supersuite (S-type) and the Glenbog Suite (I-type), and their related volcanic rocks. Such a comparison shows that there are distinctive textural characteristics of granite suites containing restite and that the chemical variation trends throughout such suites are much more regular than those of restite-free suites.Partial melting in the crust of any rocks containing quartz, feldspars, and H 2O, either free or provided by the decomposition of one or more hydrous phases, produces a granitic melt in equilibrium with restite. Because of the high viscosity of felsic melts, the melt phase might not be extracted to form a body of completely molten magma. Instead, when the amount of melt becomes sufficiently high, the rigid restite framework is broken and the whole mass becomes a crystal-rich magma. Because of its relative buoyancy this magma may move up to intrude as a pluton, or to erupt at the surface. As a result of the high viscosity and yield strength of the melt, the separation of restite is inhibited, and much of it may remain to form part of the solidified granite or volcanic rock. According to the restite model, the composition of a rock formed in this way will be a function of the composition of the melt and restite, and of the degree of separation between the two. Under favourable circumstances, granite suites with compositions resulting from restite separation can be used to estimate the compositions of their source rocks and in this way they provide a window through which we can view the composition of the deeper crust. In the Lachlan Fold Belt, this has led tc the recognition of the I-type granites derived from older igneous rocks and the S-type granites produced from sedimentary source rocks that had a composition influenced by a previous episode of weathering. Within each of these two major groups there are numerous suites whose compositions reflect compositional variations of the source rocks on a finer scale.