Petrochemical study of the Clarno Group: Eocene-Oligocene continental margin volcanism of north-central Oregon
Novitsky-Evans, Joyce Marie
Rogers, John J. W.
Doctor of Philosophy
The Clarno Group is a calcalkaline volcanic association developed upon the American plate margin in central Oregon during Eocene-Oligocene time in response to eastward subduction of oceanic terrain beneath western North America.- This volcanic association is characterized by the presence of flows of andesite, basalt, quartz latite and rhyodacite, as well as andesitic and rhyodacitic tuff. Mudflows, agglomerates, lake beds, and volcanogenic sediments are intercalated with the flows and tuffs. Chemical analysis of major and minor elements in addition to Rb and Sr have been carried out on the flow rocks by X-ray fluorescence and, in the case of Na, by neutron activation techniques. The chemical character of the Clarno volcanics fits spatially into the overall chemical continuum of a major east-west calcalkaline province which extends from the Western Cascades of Oregon to western Montana. This province notably displays increasing K2%, K2/Na2 ratios, and the increasing abundance of differentiated rock types in an eastward direction normal to the continental margin. The Clarno volcanics are geographically located near the western margin of the calcalkaline province. Their proximity to the Early Tertiary continental margin is demonstrated by the juxtaposition of the Coast Range tholeiitic province and the calcalkaline province of the Western Cascades. Comparison of the Clarno volcanics with two volcanic provinces (Aleutian Islands and Andes Mountains) that are associated with presently active subducting margins, reveals a close chemical similarity between the Clarno and the Aleutian Island volcanic rocks. The composition of the Clarno volcanics, combined with tectonic and stratigraphic considerations, suggests that the suite formed on poorly developed continental crust; this crust may have been similar to that of the modern western Aleutians, having recently developed from a crust of oceanic character. Considering this, the present 4-kilometer-thick crust in central Oregon is an indication that crustal thickening and processes of continental development have been active on the western margin of the American plate. The Clarno volcanism is a manifestation of compressive plate interactions when the crust underlying the volcanic province was still sub-continental in character.