Multiple Isotopic Components in Quaternary Volcanic Rocks of the Cascade Arc near Crater Lake, Oregon

Quaternary lavas and pyroclastic rocks of Mount Mazama, Crater Lake caldera, and the surrounding area have variable Sr, Nd, and Pb isotopic compositions. High-alumina olivine tholeiites (HAOT) have 87Sr/86Sr ratios of 0.70346–0.70364; basaltic andesite, 0–70349–0.70372; shoshonitic basaltic andesite, 0.70374–0.70388; and andesite, 0.70324–0.70383. Dacites of Mount Mazama have 87Sr/86Sr ratios of 0.70348–0.70373. Most rhyodacites converge on 0.7037. However, rhyodacite of the caldera-forming, climactic eruption has 87Sr/86Sr=0.70354 because of an admixed low-87Sr/86Sr component. Andesitic to mafic-cumulate scoriae of the climactic eruption, and enclaves in preclimactic rhyodacites, cluster in two groups but show nearly the entire 87Sr/86Sr range of the data set, confirming previously suggested introduction of diverse parental magmas into the growing climactic chamber. Pb and Nd isotope ratios display less variation (206Pb/204Pb= 18.838–18.967, 207Pb/204Pb=15.556–15.616, 208Pb/204Pb=38.405–38.619; εNd= +3.9 to +6.1) and generally covary with 87Sr/86 Sr ratios. Radiogenic isotope data from Crater Lake plot with published data for other Cascade volcanoes on isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources of primitive magmas to consist of depleted mantle and a subduction component, introduced in variable quantity to the depleted mantle wedge. Variable degrees of melting of this heterogeneous mantle, possibly at different depths, produced the diversity of isotopic compositions and large-ion lithophile element (LILE) abundances in primitive magmas. Trace element ratios do not indicate presence of an ocean island basalt (OIB) source component that has been reported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILE concentrations in evolved HAOT, where initial LILE concentrations were low. Contamination is more difficult to detect in the calcalkaline lavas because of their higher LILE concentrations and the small isotopic contrast with likely contaminants, such as mid- to lower-crustal rocks thought to be equivalents of igneous rocks of the Klamath Mountains and associated lower crust. Crustal assimilation appears to be required for calcalkaline rocks only by δ18O values, which vary from lows of +5.6 to + 6.0% in HAOT and primitive basaltic andesites to a high of +7.0% in dacite, a range that is too high to be explained by plagioclase-dominated closed-system fract