Subduction style magmatism in a non-subduction setting; the Colvilleigneous complex, NE Washington State, USA

Abstract

The Colville Igneous Complex is located within the Eocene MagmaticBelt of the North American Cordilleran interior. It straddles theUS-Canadian border in northeast Washington and southern BritishColumbia. The complex consists of three intrusive and two extrusivephases, the first extrusive phase being contemporaneous with thelatter two intrusive phases. As a consequence of subsolidusre-equilibration in the plutonic rocks, this study concentrates on thetwo, extrusive phases, the Sanpoil Volcanic Formation and the KlondikeMountain Formation. The Sanpoil Volcanic Formation consists ofandesites, dacites and rare trachyandesites (SiO (sub 2) = 55-70 wt %)exhibiting a slight decrease in total alkalis (Na (sub 2) O+K (sub 2)O) with increasing silica. The Klondike Mountain Formation consists ofbasalts, basaltic andesites, andesites, dacites and rhyolites (SiO(sub 2) = 51-75 wt %) with total alkalis increasing with increasingsilica. The calc-alkaline affinity of the rocks of the ColvilleIgneous Complex, coupled with the presence of a ""subduction signature""of enriched large ion lithophile elements (LILE) and depleted highfield strength elements (HFSE), has traditionally been attributed topetrogenesis in a subduction-related magmatic arc, the Challis Arc.New trace and rare earth element and isotopic data ( (super 87) Sr/(super 86) Sr (sub i) delta (super 18) O), however, suggest that thisexplanation is no longer tenable. We propose that the magmas of theSanpoil Volcanic Formation were generated by mid-crustal partialmelting of a mid-Proterozoic source and that the Klondike MountainFormation was formed by varying degrees of mixing between two distinctlate-Proterozoic lower-crustal sourced magmas and a mantle-derivedmagma. In all cases, the subduction or calc-alkaline signature wasinherited from the Proterozoic crustal sources. The only magmas thatcan confidently be attributed to a mantle source are the basalts ofthe Klondike Mountain Formation, which show no decoupling of the LILEand HFSE, i.e. no subduction signature, precluding the presence of asubduction slab beneath this part of the North American Cordilleraduring the Eocene. We propose the alternative model: that the ColvilleIgneous Complex formed as a result of decompression melting of crustand mantle during post-Laramide orogenic collapse of an overthickenedcrust.

58 samples from this reference