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

While hybridized granitoid magmas are readily identifiable, the mechan isms of hybridization in large crustal magma chambers are so not clear ly understood. Characteristic features of hybrid granitoids are (1) bo th the granitoid and included enclaves are commonly hybrids, as shown by mineralogy, geochemistry and isotopes; (2) mixing seen in zoned plu tons and synplutonic dykes and enclaves occurred early; (3) zoned plag ioclase phenocrysts commonly show very complex life histories of growt h and dissolution; (4) mafic end-members in hybrids are commonly fract ionated magmas and (5) stratification in subvolcanic granitoid magma c hambers is not uncommon, and stratification has been identified in som e deeper level plutons. Hybridization must overcome the tendency to fo rm a stable stratification of dense mafic magma underlying less dense felsic magma. Experimental work with magma analogues and theoretical c onsiderations reveal very severe thermal, theological and dynamical li mitations on mixing: only very similar (composition, temperature) magm as are likely to mix to homogeneity, and only moderately silicic hybri ds are likely to be produced. However, ''impossibly'' silicic hybrids do exist. Synchronous, interactive fractional crystallization and hybr idization 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 b oundary 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 ov ercome 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 ad jacent and dynamically-thermally similar magmas directly interact, and so may mix to homogeneity. Finally, not simply felsic and mafic endme mbers mix, but a whole suite of ''intermediate'' endmembers participat e, ranging from relatively mafic through to felsic pairs of magmas. Di rect mixing between the primary magmas only occurs at the beginning.