Lamprophyre dikes of northern Idaho and northeastern Washington

Fifteen Paleogene lamprophyre dikes from northern Idaho and northeastern Washington were sampled for mineralogical and geochemical characterization. The lamprophyres occur as fine-grained narrow dikes and are undeformed. Kersantite and spessartite are the dominant rock types with lesser minette. Geochemically, the dikes are high-K calc-alkaline to shoshonitic, with SiO2 contents ranging from 42.5 to 56.1 weight percent, and K2O from 2.3 to 6.7 weight percent. Ni and Cr contents are 28 to 215 ppm and 31 to 454 ppm, respectively, with Mg#s ranging from 52 to 73. Chondrite-normalized REE patterns are strongly fractionated with highly enriched LREE (Lan = 98 to 692) to moderately enriched HREE (Lun = 6 to 12). One sample from within the western Idaho shear zone (WISZ) is the least primitive lamprophyre (Mg# = 52) and exhibits an 40Ar/39Ar date of 46.77 ± 0.26 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 early Eocene Priest River complex, one of the many metamorphic core complexes occurring in Pacific Northwest. Geochemically (and geochronologically), these rocks correlate to potassic volcanic rocks of the Yellow Lake area (southcentral British Columbia), along with lamprophyre dikes described from the margins of the Valhalla and Monashee complexes (southeast British Columbia), and mafic potassic rocks of the Montana alkaline province (Highwood Mountains). INTRODUCTION Lamprophyres are melanocratic to mesocratic igneous rocks, commonly exhibiting a panidiomorphic texture with mafic phenocrysts of dark mica and/or amphibole (Wooly and others, 1996). Lamprophyres may or may not have pyroxene or olivine, and the feldspars (and feldspathoids) are typically restricted as groundmass phases. Lamprophyre intrusions commonly occur as hypabyssal dikes and sills but can form shallow stocks or plugs. Chemically, lamprophyres are subdivided into three groups: calc-alkaline, alkaline, and ultramafic lamprophyres; see Rock (1987) for specific chemical criteria. Calc-alkaline lamprophyres are weakly silicasaturated to undersaturated, are typically melanocratic, and are rich in dark mica and/or amphibole. Calcalkaline lamprophyres are subdivided based on the dominant phenocryst versus groundmass (feldspar) phases with the four most common types being minette, kersantite, vogesite, and spessartite. The lamprophyres described in this study are high-K calc-alkaline to shoshonitic series, with kersantite and spessartite being the dominant types. Minette, although present, occurs in fewer numbers. Although lamprophyric rocks have been recognized and mapped within the Coeur d’Alene mining district (Shannon, 1920; Hobbs and others, 1965), no detailed study of lamprophyric rocks from northern Idaho outside the district has been completed. The mafic potassic rocks described here are similar both mineralogically and geochemically to lamprophyric occurrences in 1Science Department, Spokane Community College, 1810 Greene St, Spokane, Washington, 99217 USA 2Department of Geography and Geological Sciences, University of Idaho, 875 Perimeter Dr, MS 3022, Moscow, Idaho 83844, USA 3Department of Geoscience, University of Wisconsin, 1215 W Dayton St, Madison, Wisconsin 53706, USA 4Department of Environmental, Earth, and Atmospheric Sciences, University of Massachusetts Lowell, 1 University Ave, Lowell, MA 01854, USA