'Subduction style' magmatism in a nonsubduction setting: the Colville Igneous Complex, NE Washington State, USA

The Colville Igneous Complex is located within the Eocene Magmatic Belt of the North American Cordilleran interior It straddles the US-Canadian border in northeast Washington and southern British Columbia. The complex consist s of three intrusive and two extrusive phases, the first extrusive phase be ing contemporaneous with the latter hco intrusive phases. As a consequence of subsolidus re-equilibration in the plutonic rocks, this study concentrat es on the two extrusive phases, the Sanpoil Volcanic Formation and the Klon dike Mountain Formation. The Sanpoil Volcanic Formation consists of andesit es. dacites and rare trachyandesites (SiO2 = 55-70 wt %) exhibiting a sligh t decrease in total alkalis (Na2O + K2O) with increasing silica. The klondi ke Mountain Formation consists of basalts, basaltic andesites, andesites, d acites and rhyolites (SiO2 = 51-75 mt %) with total alkalis increasing with increasing silica. The calc-alkaline affinity of the rocks of the Colville Igneous Complex, coupled with the presence of a 'subduction signature' of enriched large ion lithophile elements (LILE) and depleted high field stren gth elements (HFSE), has traditionally been attributed to petrogenesis in a subduction-related magmatic arc, the 'Challis Arc' New trace and rare eart h element and isotopic data (87Sr/86Sr(i) epsilon(Ndb) delta(18)O), however suggesting that this explanation is no longer tenable. I Lh We propose tha t the magmas of the Sanpoil Volcanic Formation were generated by mid-crusta l partial melting of a mid-Proterozoic source and that the klondike Mountai n Formation was famed by varying degrees of mixing between two distinct lat e-Proterozoic lower--crustal sourced magmas mid a mantle-derived magma. In all cases, the subduction or calc-alkaline signature teas inherited from th e Proterozoic crustal sources, The only magmas that can confidently be attr ibuted to a mantle source are the basalts of the Klondike Mountain Formatio n, which show no decoupling of the LILE and HFSE, i.e. no subduction signat ure, the presence of a subduction slab beneath this part of the North Ameri can Cordillera during the Eocene. We propose the alternative model. that th e Colville Igneous Complex formed as a result of decompression melting of c rust and mantle during post-Laramide orogenic collapse of an overthickened crust.