REACTIVE BULK ASSIMILATION: A NEW PETROGENETIC PARADIGM

Bulk assimilation of small (mm to ~1km) crustal xenoliths - driven and (ultimately) masked by reactions during xenolith melting and magma crystallization - is an important mechanism for generating the large, chemically and isotopically mixed magmas that characterize continental magmatic provinces, especially continental arcs. Xenoliths containing mica or amphibole undergo dehydration melting reactions when incorporated into a host (e.g. basaltic) magma. The products of the reactions are mainly plagioclase, pyroxene, Fe-Ti oxides and hydrous melt. The physical integrity of the xenolith is compromised by partial melting and it begins to disintegrate as it moves through the host magma. Xenocrysts, including xenocrystic zircon, are liberated and mixed into the host magma at this stage. The cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions in which anhydrous minerals, especially pyroxenes, react with melt to yield amphiboles and micas. All anhydrous phases in the system, be they phenocrysts, xenocrysts, or crystals having a mixed signature will be subject to these reactions. Thus, late-crystallizing amphiboles and micas may incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation - much of the assimilant remains in solid form - compared to melt assimilation models. Among other things, this permits a substantially higher assimilation/crystallization ratio than is acceptable for melt-only models. In and of itself, a large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas. While the assimilation of low-density crust and concomitant fractionation provides the isostatic impetus for ascent, the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent. A substantial portion of the crust may simply become part of the magma as it ascends.