EOCENE LAMPROPHYRIC DIKES FROM NORTHERN IDAHO AND NORTHEASTERN WASHINGTON

Fifteen lamprophyric dikes (presumably of Challis-age) from northern Idaho and northeastern Washington were sampled for mineralogical and geochemical characterization. The majority of samples (11) occur directly east of the Purcell Trench fault, the eastern bounding fault of the Priest River complex (PRC), and intrude Mesoproterozoic metasediments of the Belt Supergroup. Two samples are from the western margin of the PRC, and one sample is just northwest of the Clearwater complex. The last sample is from a dike that intrudes amphibolite-grade orthogneiss within the western Idaho shear zone (WISZ) along the main Salmon River. The lamprophyres occur as fine-grained narrow dikes (<5 m width) and are undeformed. Kersantite and spessartite are the dominant rock types with minor minette. Phenocrysts (< 3.0 mm) of phlogopite, amphibole, and augite exhibit minor to moderate alteration to clays (and carbonate), with primary and secondary carbonate occurring in the groundmass. Some samples exhibit minor to moderate chloritic alteration. Accessory phases include apatite, rutile, magnetite and pyrite.

Geochemically, the dikes are calc-alkaline (shoshonitic) to alkaline, with SiO₂ contents ranging from 41.8-56.1 wt.%, and K₂O from 2.3-6.7 wt.%. Cr and Ni contents are 31-454 ppm and 28-215 ppm, respectively, with Mg#s ranging between .51-.73. LOI values range from 0.8-6.2%. Chondrite-normalized REE patterns are strongly fractionated with highly enriched LREE (La_n = 98-692) to moderately enriched HREE (Lu_n = 6-12). The WISZ sample is the least primitive lamprophyre (Mg# = .51) and exhibits an Ar-Ar date of 46.77 Ma. None of the other samples have been dated but are presumed to be post-Middle Eocene due to their undeformed nature and proximity to the Priest River complex. Geochemically (and geochronologically), these rocks correlate to mafic potassic rocks of the Yellow Lake area (south central British Columbia), along with lamprophyric dikes described from the Valhalla and Monashee complexes (southeast British Columbia), and mafic potassic rocks of the Montana alkaline province (Highwood Mountains).