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Calc-alkaline, shoshonitic, and primitive tholeiitic lavas from monogenetic volcanoes near Crater Lake, Oregon

Charles R. Bacon
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Abstract

Quaternary monogenetic volcanism in the High Cascades of Oregon is manifested by cinder cones, lava fields, and small shields. Near Crater Lake caldera, monogenetic lava compositions include: low-K (as low as 0·09% K 2 O) high-alumina olivine tholeiite (HAOT); medium-K. calc-alkaline basalt, basaltic andesite, and andesite; and shoshonitic basaltic andesite (2·1% K 2 O, 1750 ppm Sr at 54% SiO 2 ). Tholeiites have MORB-like trace element abundances except for elevated Sr, Ba, and Th and low high field strength elements (HFSE), and they represent near-primary liquids. They are similar to HAOTs from the Cascades and adjacent Basin and Range, and to many primitive basalts from intraoceanic arcs. Calc-alkaline lavas show a well-developed arc signature of high large-ion lithophile elements (LILE) and low HFSE. Their Zr and Hf concentrations are at least partly decoupled from those of Nb and Ta; HREE are low relative to HAOT. Incompatible element abundances and ratios vary widely among basaltic andesites. Some calc-alkaline lavas vented near Mount Mazama contain abundant gabbroic microxcnoliths, and are basaltic andesitic magmas contaminated with olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgO and compatible trace element contents that suggest only minor fractionation. There appears to be a compositional continuum between primitive tholeiitic and calc-alkaline lavas. Compositional variation within suites of comagmatic primitive lavas, both tholeiitic and calc-alkaline, mainly results from different degrees of partial melting. Sources of calc-alkaline primary magmas were enriched in LILE and LREE by a subduction component and contained residual garnet, whereas sources of HAOTs had lower LILE and LREE concentrations and contained residual clinopyroxene. High and variable LILE and LREE contents of calc-alkaline lavas reflect variations in fluid-transported subduction component added to the mantle wedge, degree of partial melting, and possibly also interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesites by fractionation of plagioclase+augite+magnetite+apatite ± orthopyroxene or olivine, commonly accompanied by assimilation. Many andesites are mixtures of andesitic or dacitic magma and a basaltic or basaltic andesitic component, or are contaminated with gabbroic material. Mingled basalt, andesite, and dacite of Williams Crater formed by multi-component, multi-stage mixing of basaltic andesitic magma, gabbro, and dacitic magma. The wide range of compositions vented from monogenetic volcanoes near Crater Lake is a result of the thick crust coupled with mild tectonic extension superimposed on a subduction-related magmatic arc.

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Charles R. Bacon. 1990. Calc-alkaline, shoshonitic, and primitive tholeiitic lavas from monogenetic volcanoes near Crater Lake, Oregon. https://doi.org/10.1093/petrology%2F31.1.135

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