VARIATIONS IN 187OS/188OS AMONG DIVERSE MAGMAS INDICATE ASSIMILATION OF YOUNG AND ANCIENT CRUST ALONG THE CASCADE ARC

Patchwork crustal terranes and varying tectonic regimes along Cascadia affect the expression of subduction, including slab seismicity, eruptive styles, vent distributions, and magma compositions. Because Re-187 fractionates in to the crust during mantle melting and its daughter isotope Os-187 is highly compatible in mafic lithologies, 187Os/188Os in arc magmas are useful for tracking the age of crustal assimilants, particularly in mafic compositions where crustal contributions may otherwise be cryptic. Os isotopes have been examined in only a few locations along the Cascade Arc (Lassen Peak, Three Sisters, and Mt. Adams). The 187Os/188Os among erupted magmas at Lassen Peak and at Mt. Adams are variable (0.13 to 0.30 and 0.15 to 0.63, respectively; Borg et al., 2000; Hart, W.K. et al. 1997; Hart, G.L. et al. 2003; Jicha et al., 2009) relative to a more narrow range observed at the Three Sisters (0.141-0.159 for basalts and 0.166-0.186 for basaltic andesites). Variations among the Os isotopes mirror arc segmentation based on ranges in 87Sr/86Sr in primitive high-Mg magmas (Schmidt et al., 2008) that correlates well with distinct tectonic settings and crustal terranes. The isotopic segments are: 1) the dominantly calc-alkaline North Segment from Mt. Meager to Glacier Peak (87Sr/86Sr=0.7030-0.7036); 2) the Columbia Segment from Mt. Rainier to Mt. Jefferson that has produced more varied and high field strength-rich magmas correlated with the Siletz Terrane, an accreted oceanic plateau (87Sr/86Sr=0.7028-0.7037); 3) the extensional Central Segment from the Three Sisters to Medicine Lake, where magmas are more restricted in composition (87Sr/86Sr=0.7034-0.7038), and 4) the South Segment from Mt. Shasta to Lassen Peak, where high Ba/Ce and 87Sr/86Sr (0.7030-0.7044) suggest significant fluid addition. Higher values of 187Os/188Os in magmas from Lassen Peak and Mt. Adams likely result from the assimilation of ancient arc plutons beneath the South Segment and the enriched Siletz Terrane beneath the Columbia Segment. In contrast, 187Os/188Os of the basaltic andesites at the Three Sisters suggest they interacted with young mafic lithologies in the deep arc crust, probably related to extension within the central part of the arc since ~8 Ma.