MAJOR- AND TRACE-ELEMENT CHARACTERISTICS OF ARCHEAN AND PALEOPROTEROZOIC BASEMENT ROCKS NEAR COEUR D’ALENE, IDAHO

The Cougar Gulch area near Coeur d’Alene, Idaho, is a newly recognized Paleoproterozoic to Archean basement occurrence located in the southern Priest River complex. Here, a structural culmination exposes deeper levels of the core complex infrastructure, similar to the Archean basement culmination in the northern portion of the complex near Priest River, Idaho. At Cougar Gulch, the basement rocks are composed of a variety of granitic orthogneisses and amphibolite, which are unconformably overlain by a graphite-bearing orthoquartzite. The orthoquartzite is in turn overlain by the Hauser Lake Gneiss (Mesoproterozoic, lower Belt Supergroup equivalent). The similarity of structure, metamorphic fabrics, and kinematics here and in the northern portions of the complex are consistent with the Cougar Gulch area being the southern continuation of the Spokane dome mylonite zone.

Neoarchean amphibolites (2.65 Ga) have been identified as part of the basement sequence. These amphibolites had a basaltic protolith and can be distinguished chemically from amphibolites found within the overlying Hauser Lake Gneiss, which are metamorphosed Mesoproterozoic Moyie sills. The Archean amphibolites have steeper REE slopes and consistently higher REE values. Protoliths of the Paleoproterozoic orthogneisses (1.87-1.86 Ga) are calc-alkaline, “I-type” syenogranites, monzogranites, and granodiorites, which exhibit subduction-related geochemical characteristics such as high LILE:HFSE concentrations along with characteristic depletions in Nb, Ta, P, Ti, and Eu. A second distinctive unit of orthogneiss, the Kidd Creek tonalite, is a high-Al tonalite, and exhibits TTG (tonalite-trondhjemite-granodiorite) chemical characteristics. The Kidd Creek tonalite has Sr/Y and La/Yb ratios, along with Y and HREE concentrations (no Eu anomalies) similar to Precambrian TTG compositions formed in subduction settings.