GEOLOGY, GEOCHEMISTRY AND POSSIBLE TECTONIC AND STRUCTURAL DEVELOPMENT OF THE HELENA-HAYSTACK MÉLANGE, NORTH CASCADES, WASHINGTON STATE

The Helena-Haystack mélange (HHM) is one of several enigmatic mélanges located within the North Cascades, Washington State. The HHM consists of blocks of variably metamorphosed igneous and sedimentary rocks enclosed within a serpentinite matrix, and it has been dextrally displaced ~98 km along the ~N-S striking Eocene Straight Creek fault (SCF). These blocks have yielded Jurassic protolith ages (~160 Ma). Previous researchers suggest that the HHM was originally, in part, an ophiolite. The HHM has been further interpreted as a major suture between the Cretaceous thrust stacks of Paleozoic and Mesozoic oceanic and arc rocks of the Northwest Cascade System (NWCS) and the western and eastern mélange belts, structurally mixing these belts and other exotic terranes together. However, structural and P/T data suggest that the HHM may not have formed in this fashion and may be the uppermost thrust of the NWCS.

A large geochemical data set (n > 100) now exists for the HHM, and this can be used to test the interpretations for the generation of the mélanges. The HHM blocks on the western side of the SCF have within-plate basalt (WPB), normal and enriched mid-ocean ridge basalt (N-MORB & E-MORB), island-arc tholeiite (IAT), transitional IAT-N-MORB, and calc-alkaline (CA) affinities. One sample is a boninite and several samples are primitive arc basalts. The HHM blocks on the eastern side of the SCF predominantly have WPB and CA affinities, with N-MORB, IAT and E-MORB being less common. These extremely diverse geochemical affinities can be found in multiple tectonic settings, and most likely exclude the interpretation that the HHM formed in a single distinct oceanic environment. The HHM contains low-T blueschist-facies blocks in places, and thus could represent an oceanic suprasubduction zone, with an ocean island close to a spreading ridge, that mixed with an arc in a subduction zone. Alternatively, these affinities are broadly similar to several Middle to Late Jurassic oceanic terranes within the NWCS and elsewhere in the North American Cordillera; thus, imbrication of other terranes within the HHM during overthrusting may have occurred.