Abstract
New 187Os/188Os ratios of Quaternary Mount Adams volcanic rocks from the Cascade arc in southern Washington vary by .300% ( 187 Os/ 188 Os 5 0.165-0.564) and fall into high (.0.319) and low (0.166 to 0.281) groups of 187 Os/ 188 Os ratios that are sub- stantially more radiogenic than mantle values. These Os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high 187Os/188Os ratios but dif- fering ages, thus recording the process of crustal hybridization and homogenization. Os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction-zone en- vironments that reflect the magmatic history of the arc. Sr, Nd, Hf, and Pb isotope variations in this young, mafic arc complex are too small to allow such constraints.
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