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 Sr-87/Sr-86 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 Sr-87/Sr-86 ratios of 0.70348-0.70373. Most rhyodacites converge on 0.7037. However, rhyodacite of the caldera-forming, climactic eruption has Sr-87/Sr-86=0.70354 because of an admired low-Sr-87/Sr-86 component. Andesitic to mafic-cumulate scoriae of the climactic eruption, and enclaves in preclimactic rhyodacites, cluster in two groups but show nearly the entire Sr-87/Sr-86 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 ((206)pb/Pb-204 = 18.838-18.967, Pb-207/Pb-204 = 15.556-15.616, Pb-208/Pb-204 = 38.405-38.619; epsilon(Nd) = +3.9 to +6.1) and generally covary with Sr-87/Sr-86 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 (GIB) 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 delta(18)O values, which vary from lows of +5.6 to +6.0 parts per thousand in HAOT and primitive basaltic andesites to a high of + 7.0 parts per thousand in dacite, a range that is too high to be explained by plagioclase-dominated closed-system fractional crystallization. Elevated delta(18)O values of differentiated lavas may be attributed to interaction with relatively O-18-rich, Sr-87-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption, and rarely in late Pleistocene eruptive units, have delta(18)O(pl) of - 3.4 to + 6.5 parts per thousand and delta(18)O(qz) of - 2.2 to + 8.0 parts per thousand but have Sr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g. Sr-87/Sr-86 approximate to 0.7037). Rb and Sr data for glass separates from granodiorites suggest that the source pluton is Miocene. Glass from granodiorite has Sr-87/Sr-86 ratios as high as 0.70617. Oxygen isotope fractionation between quartz, plagioclase, and glass indicates reequilibration of O isotopes at magmatic temperatures, after O-18/O-16 had been lowered by exchange with meteoric hydrothermal fluids. Unmelted granodiorite xenoliths from pre-climactic eruptive units have delta(18)O values that are consistent with onset of hydrothermal exchange early during growth of the climactic magma chamber. Assimilation of such upper-crustal granodiorite apparently lowered delta(18)O values of rhyodacites without significantly affecting their magmatic compositions in other ways.