Abstract

High Cascade volcanoes include all those formed along the Cascade trend during the Pliocene, Pleistocene, and Recent epochs. They have extruded lavas ranging from basalt to rhyolite, but dominantly andesite. The products of this intense period of volcanism were studied i n a 300-square-mile area surrounding and including Mt. Hood, Oregon. The object of the study was to obtain field and geochemical evidence bearing on the origin of andesitic magmas.

The High Cascade lavas of the Mt. Hood area were erupted onto a surface formed by the Miocene Yakima Basalt. Late Miocene activity was mild, producing a few scattered andesite volcanoes. These centers of activity shed epiclastic and pyroclastic debris into basins on each side of the emerging mountain belt. Volcaniclastic debris and interbedded flows accumulated to local thicknesses near 1400 feet before Pliocene time.

Early Pliocene activity was intense; andesite was erupted from several centers and locally accumulated to thicknesses of several thousand feet. A few basalt flows preceded the andesites and may be genetically related to the far more voluminous olivine andesite and pyroxene andesite. A shallow pluton of quartz-diorite was intruded into some of these lavas during the late part of this period.

Two late Pliocene volcanic centers erupted sequences of olivine basalt, olivine andesite, and pyroxene andesite. Other centers erupted voluminous andesite not associated with basalts.

The composite cone of the Mt. Hood volcano was built during late Pleistocene time. It is composed of olivine-, pyroxene-, and hornblende-dacite lavas and pyroclastic debris. Contemporaneous basalts and olivine andesites were erupted from many nearby vents, which are not directly connected with the Mt. Hood volcano. The main cone growth ceased before the Fraser Glaciation, but a plug-dome of hornblende andesite was extruded through the southern slope about 2000 years ago.

Other late Pleistocene activity yielded flows of olivine basalt and olivine andesite. Several domes and flows of hornblende dacite were also extruded.

Chemical analyses were obtained from 158 samples, for which relevant field relations are known. Plots of the Differentiation Index (normative Ab + An + Or + Qz) against lime, total alkalis, potash, and soda serve to characterize lavas from various eruptive centers. Andesites directly associated with high-alumina basalt contain significantly more potash than andesites not associated with basalts, such as those found at Mt. Hood.

Field relations coupled with chemical data lead to the following conclusions: (1) the Mt. Hood lava did not differentiate from high-alumina basalt magma; (2) the Mt. Hood lavas are probably not a single series of magmas related by differentiation, but may be a series of similar magmas generated through time by some process, such as supracrustal anatexis; (3) olivine basalts magmas do differentiate to form andesite; (4) all olivine basalts in the Mt. Hood area were differentiated or contaminated prior to eruption; and (5) several recognizably different magma series were erupted through time—some are related to a basalt parent; others, to a primary andesite.

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