UNEXPECTED VERTICAL VARIATIONS IN METAMORPHISM WITHIN THE CRESCENT BASALT, DOSEWALLIPS RIVER VALLEY, OLYMPIC PENINSULA, WASHINGTON STATE

The Crescent Formation, a extensive stack of mainly tholeiitic basalt flows, is exposed on the Olympic Peninsula of Washington state. 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. No evidence was found for reverse faults to thicken the sequence, making the section a 16.2-km thick burial sequence.

Petrographic and mineral chemistry analyses reveal variation in metamorphic grade that does not increase monotonically with stratigraphic depth, despite the apparent homogeneity of the original basalts. Plagioclase alteration is variable and does not increase with depth: some feldspars have igneous compositions and are texturally pristine while others are albitized and altered to clays or saussurite. Clinopyroxenes are generally igneous except for a zone near the top of the LC, which also contains actinolite, epidote and quartz (otherwise ~absent from the section) and has been interpreted as a zone of greenschist facies metamorphism. Sparse edenitic amphiboles in one sample near the base of LC and one sample near the base of UC contain 2-7 weight percent Na₂O indicating high-pressure, low-temperature conditions through the middle and base of the sequence, while the upper regions (up to ~3 km below the top) contain prehnite-pumpellyite assemblages.

Bulk rock chemistry indicates a zone of substantial metasomatism that is situated just below the greenschist zone within the LC. Relative to the remainder of the LC, most oxides and elemental abundances within the zone are distinctly different. TiO₂, FeO_total, CaO, and Na₂O are more than one standard deviation away, and SiO₂, MgO, P₂O₅ and most trace elements are more than two. This is consistent with a cryptic fluid pathway which produced the greenschist zone by either delivering heat to the nearby rocks, moving them out of the blueschist facies. Overall, the data are consistent with a generally Sanbagawa facies series trend through the sequence, with a narrow zone of greenschist facies rocks developed via interaction with hot fluids.