GEOCHEMISTRY AND PETROLOGY OF A THICK SEQUENCE OF CRESCENT BASALT IN THE DOSEWALLIPS RIVER VALLEY, OLYMPIC PENINSULA, WASHINGTON STATE

The remarkable Early to Middle Eocene volcanic sequence of the Crescent Formation exposed on the Olympic Peninsula of Washington State consists predominantly of tholeiitic basalts with minor basaltic andesites and sparse sedimentary interbeds. A composite section measured in the vicinity of the Dosewallips River Valley can be divided into an 8.4 km lower member (LC) composed chiefly of submarine basalts overlain by a 7.8 km upper member (UC) of subaerial flows. Major and trace element analyses have been done on 62 flows and sills from this section along with microprobe analyses of mineral compositions from nine of the samples. Ar-Ar ages range from 50.5 +/-1.6 Ma at the top of the subaerial section to 51.5 +/- 1.6 Ma at the base. A massive flow from the top of the submarine section was dated at 56.0 +/-1 Ma. However the lowermost flow in the section gave an age of 31.2 +/- 1.2 Ma.

Trace element analyses show that the UC is enriched (2-10x) in highly incompatible elements compared to MORB, especially Nb. The LC is enriched in the most incompatible elements (e.g. Th, Nb, Ta), but is otherwise similar to MORB. The transition to more enriched magmas occurs in the uppermost part of the LC, but the degree of enrichment is highly variable in the UC, indicating an interplay of two different sources.

Nearly all of the flows, except those in the uppermost part of the section, show evidence of recrystallization due to burial metamorphism, especially feldspars which are mainly albite or oligoclase. Metamorphic amphiboles can also be found in some flows of the lower section. However, relict clinopyroxenes ranging from augite to ferrosilite are present in all flows and have a differentiation trend similar to the Skaergaard Complex. Generally the LC flows are more differentiated than the UC.

Although the large volume of the extrusive material suggests a seamount origin, the chemistry of the flows appears to be more compatible with a rifted margin setting.