THE INTERPLAY BETWEEN CRYSTALLIZATION, REPLENISHMENT AND HYBRIDIZATION IN LARGE FELSIC MAGMA CHAMBERS

While hybridized granitoid magmas are readily identifiable, the mechanisms of hybridization in large crustal magma chambers are so not clearly understood. Characteristic features of hybrid granitoids are (1) both the granitoid and included enclaves are commonly hybrids, as shown by mineralogy, geochemistry and isotopes; (2) mixing seen in zoned plutons and synplutonic dykes and enclaves occurred early; (3) zoned plagioclase phenocrysts commonly show very complex life histories of growth and dissolution; (4) mafic end-members in hybrids are commonly fractionated magmas and (5) stratification in subvolcanic granitoid magma chambers is not uncommon, and stratification has been identified in some deeper level plutons. Hybridization must overcome the tendency to form a stable stratification of dense mafic magma underlying less dense felsic magma. Experimental work with magma analogues and theoretical considerations reveal very severe thermal, theological and dynamical limitations on mixing: only very similar (composition, temperature) magmas are likely to mix to homogeneity, and only moderately silicic hybrids are likely to be produced. However, ''impossibly'' silicic hybrids do exist. Synchronous, interactive fractional crystallization and hybridization may provide a mechanism for hybridization of magmas, in the following manner. A mafic magma intrudes into the base of a stratified felsic magma and is cooled against it. Crystallization of the upper boundary layer of the mafic magma yields an eventually buoyant residual melt that overturns and mixes with an adjacent stratum of the felsic magma chamber. Subsequently, melt released by crystallization of this, now-hybrid zone mixes with adjacent, more felsic zones. Thus, a suite of hybrid magmas are progressively formed. Density inhibitions are overcome by the generation of relatively low density residual melts. As crystallization proceeds, later injections are preserved as dykes and enclaves composed of hybrid magma. In this process, only physically adjacent and dynamically-thermally similar magmas directly interact, and so may mix to homogeneity. Finally, not simply felsic and mafic endmembers mix, but a whole suite of ''intermediate'' endmembers participate, ranging from relatively mafic through to felsic pairs of magmas. Direct mixing between the primary magmas only occurs at the beginning.