THE VARIABLE ROLE OF SLAB-DERIVED FLUIDS IN THE GENERATION OF A SUITEOF PRIMITIVE CALC-ALKALINE LAVAS FROM THE SOUTHERNMOST CASCADES, CALIFORNIA

The compositional continuum observed in primitive calc-alkaline lavas erupted from small volcanoes across the southernmost Cascade are is pr oduced by the introduction of a variable proportion of slab-derived fl uid into the superjacent peridotite layer of the mantle wedge. Magmas derived from fluid-rich sources are erupted primarily in the forearc a nd are characterized by Sr and Pb enrichment (primitive mantle-normali zed Sr/P > 5.5), depletions of Ta and Nh, low incompatible-element abu ndances, and MORE-like Sr and Pb isotopic ratios. Magmas derived from fluid-poor sources are erupted primarily in the are axis and behind th e are, and are characterized by weak enrichment in Sr [1.0 < (Sr/P)(N) < 1.3], weak depletions in Ta and Nb, higher incompatible-element abu ndances, and GIB-like Sr, Nd, and Pb isotopic ratios. Fluxing the mant le wedge above the subducting slab with H2O-rich fluid stabilizes amph ibole and enriches the wedge peridotites in incompatible elements, par ticularly unradiogenic Sr and Pb. The hydrated amphibole-bearing porti on of the mantle wedge is downdragged beneath the forearc, where its s olidus is exceeded, yielding melts that are enriched in Sr and Pb, and depleted in Ta and Nh (reflecting both high Sr and Pb relative to Ta and Nb in the fluid, and the greater compatibility of Ta and Nb in amp hibole compared to other silicate phases in the wedge). A steady decre ase of the fluid-contributed geochemical signature away from the trenc h is produced by the progressive dehydration of the downdragged portio n of the mantle wedge with depth, resulting from melt extraction and i ncreased temperature at the slab-wedge interface. Inverse correlation between incompatible-element abundances and the size of the fluid-cont ributed geochemical signature is generated by melting of more depleted peridotites, rather than by significant differences in the degree of melting. High-(Sr/P)(N) lavas of the forearc are generated by melting of a MORE-source-Like peridotite that has been fluxed with a greater p roportion of slab-derived fluid, and low (Sr/P)(N) lavas of the are as is are produced by melting of an GIB-source-like peridotite in the pre sence of a smaller proportion of slab-derived fluid. This study docume nts the control that a slab-derived fluid can have on incompatible ele ment and isotopic systematics of are magmas by 1) the addition of inco mpatible elements to the wedge, 2) the stabilization of hydrous phases in the wedge, and 3) the lowering of peridotite solidi.