GEOCHEMISTRY OF MAFIC ROCKS IN THE CHIWAUKUM SCHIST OF THE CASCADES CORE AND POSSIBLE CORRELATIVES: TECTONIC IMPLICATIONS FOR THE NORTH CASCADES

The Nason terrane, one of several amphibolite-facies terranes of uncertain age that comprise the Cascades core, consists largely of the Chiwaukum Schist (CS), a pelitic-semipelitic schist with subordinate amphibolites. Geochemical "fingerprinting" using trace element chemistry of the amphibolites has allowed us to infer possible tectonic settings for its protoliths and, in combination with lithologic data, to test possible correlations with other units in the Cascades core and lower-grade Northwest Cascades system that contain mafic rocks of presumed oceanic origin. Such correlations are, in turn, important elements of models for mid-Cretaceous thrusting.

Many of the CS amphibolites included in this study (16 samples) plot in or near the within plate basalt field (WPB) field on the Ti-Zr-Y discriminant diagram. The Ta-Hf-Th, Y-Cr, and Ti-V diagrams show a range in magmatic affinities, predominantly enriched mid-ocean ridge basalt and WPB.

The CS amphibolites are similar in their chemistry to mafic rocks of the Ingalls ophiolite (IO), which is interpreted as a suprasubduction zone ophiolite formed in a back-arc basin. The CS structurally underlies the IO along the Windy Pass thrust (WPT). Possible explanations for the similarity of magma types of CS and IO mafic rocks include one or more of the following: 1) during or before movement on the WPT, the ophiolite was imbricated in the CS, or blocks of Ingalls slid as olistoliths into the CS; 2) the Ingalls was continuous with Chiwaukum basement prior to thrusting; and 3) the IO and CS are not genetically related, but were formed in similar tectonic settings.

CS amphibolites are more variable in their chemistry than either the blueschist/greenschist facies Shuksan Suite (geochemistry by Joe Vance and colleagues) of the Northwest Cascades system or the Triassic basement to the Methow basin (Spider Peak Formation). Thus, geochemical data do not support proposed correlations (Monger, 1991; McGroder, 1991) of the CS with these units.

In contrast, meta-basites of the Twisp Valley Schist (Cascades core; amphibolite/metachert/marble) have the same MORB and WPB affinities as do the CS amphibolites. In addition to these geochemical similarities, both units have been interpreted as accretionary assemblages, but their associated sedimentary rocks are significantly different.