EXPERIMENTAL, GEOLOGICAL, AND GEOCHEMICAL CONSTRAINTS ON THE ORIGINS OF LOW-K SILICIC MAGMAS IN OCEANIC ARCS

The results of recent experimental and geochemical studies demonstrate that typical, low-K, island are dacites (IAD) and tonalites have liqu idus water contents below 6 wt %, while many are basalts, including ev olved, low-Mg high-alumina basalt (HAB; H2O greater than or equal to 4 wt %) and magnesian are tholeiites (H2O greater than or equal to 2 wt %) are water-rich. If these water contents are typical of are basalts , fractionation at pressures of 200 MPa or more would produce water-ri ch magmas with major element chemical characteristics unlike >99% of o bserved IAD. In light of this, plausible mechanisms for IAD genesis in clude (1) dehydration melting of amphibolitized are crust and (2) low- pressure (less than 200 MPa) fractionation or assimilation/fractional crystallization (AFC) (accompanied by devolatilization) of hydrous are basalts. In general, a partial melting origin is favored for tonaliti c plutons emplaced at pressures greater than or equal to 200 MPa, for bimodal suites where geochemistry rules out a genetic relationship bet ween the mafic and silicic end-members, and for IAD with isotopic char acteristics distinct from potential basaltic or andesitic parents. Evi dence for partial melting of amphibolite yielding IAD melts has been d ocumented in several ancient island are complexes, A fractionation ori gin is favored where there is isotopic homogeneity in a basalt-dacite system and for dacites having very low concentrations of incompatible trace elements. Bimodal magmatism in general and high concentrations o f incompatible elements in silicic magmas appear to favor a partial (b atch) melting origin over Raleigh fractionation but can also result fr om convectively driven batch fractionation processes (Brophy, 1998). A lthough both fractionation/AFC and partial melting may be legitimately invoked to explain dacitic magmatism in a given situation, a general model must also account for the absence of high-pressure fractionates of hydrous basalts. This observation seems to favor an important role for amphibolite melting in IAD genesis. It is also possible, however, that physical factors (e.g., neutral buoyancy) promote the formation o f basaltic magma chambers in the upper crust or that convective proces ses, enhanced by large temperature gradients in the upper crust, may f avor upper crustal over lower crustal fractionation.