9419018 Leeman This study addresses the relation between tectonic evolution of the Pacific Northwest and geochemical evolution of the mantle below the Cascadia volcanic arc. Cascade volcanism began at about 40 Ma following collision of a seamount/oceanic island chain (Oregon-Washington coast ranges) with the Paleogene subduction zone westward migration of subduction to its present location shortly after this event. Between that time and the present, there was a five-fold decrease in convergence rate as well as a systematic decrease in the age of the subducting plate as the Juan de Fuca ridge system approached the mainland. These variations should have resulted in progressive and predictable modifications of the subarc mantle. It is planned to test this subduction model in two ways: First the P.I.s plan to characterize the major and trace element and isotopic compositions of primitive pre-Quaternary lavas erupted throughout the history of the arc. this will be done in two temporal transects across the Cascade range where exposures are optimal and geologic mapping is available. Second, the P.I.s plan to complete a detailed petrologic study of variably metasomatized mantle xenoliths from the southern Washington Cascades. Some of these rocks have been strikingly modified by infiltrating potassic siliceous melts and CO2-rich fluids. Given the range of lithologies available, and assuming that the original lithosphere was oceanic, it will be possible to document directly certain aspects of subduction-related mass transfer fluxes. The mineral chemistry, textures, and fluid and melt inclusion characteristics of the xenoliths suite will be thoroughly documented to constrain the petrologic evolution of a portion of the subarc mantle.
