| This dissertation presents a comprehensive isotopic study of basaltic- to dacitic-composition orogenic/volcanic rocks from the Quaternary Cascade Arc located in northwestern United States. Strontium, Nd, Pb and Os isotope analyses were obtained for lavas from the Cascade Range in northwest U.S.A. Basaltic- to dacitic-composition volcanic rocks in the Cascade Arc have mantle-like Sr, Nd, O, and Pb isotope compositions, and provide little information about crustal interaction and modification of mantle-derived magmas. Osmium isotope compositions, however, are much more radiogenic than mantle values and have 187Os/188Os ratios that range from 0.1574 to 0.5637. These Sr, Nd, Pb and Os isotope analyses indicate that mantle-derived basalts have interacted significantly with a component which has radiogenic Os isotope compositions but mantle-like Sr, Nd, and Pb isotope compositions. Models of isotope compositions show that fractionated basalts to basaltic andesites emplaced in the lower crust over the last 5–10 Ma have mantle-like Sr, Nd, and Pb isotope compositions and elevated Os isotope compositions. Therefore, the Os isotope data suggest that mantle-derived magmas incorporated and recycled lower crustal material. Such a process is only recorded in the Os isotope compositions due to the extreme parent daughter fractionation of Re from Os that occurs during mantle melting and basalt fractionation, and is not recorded in the Sr, Nd, and Pb isotope data because of the lack of isotopic contrast between the mantle and lower crustal material.; The radiogenic Os isotope compositions in the Cascade Range lavas suggest that significant amounts of crustal recycling occurs in young convergent margin volcanic zones that is only preserved in the Os isotope data. Crustal growth models have previously interpreted mantle-like Sr, Nd, and Pb isotope compositions of volcanic rocks as an indication of no crustal recycling. The Os isotope data, however, suggest that these crustal growth models overestimate the rates of crustal growth because they do not account for recycling of lower crustal material into mantle-derived magmas. Osmium isotope data are interpreted to reflect interaction of mantle-derived magma with distinct age and isotopic domains in the lower crust that formed during magmatic episodes predating current (∼1 Ma) magmatism in the Cascades. |
