BASEMENT AMPHIBOLITES FROM THE SOUTHERN PRIEST RIVER COMPLEX NEAR COEUR D’ALENE, IDAHO

Three geochemically distinct groups of amphibolite have been identified in the basement culmination within the core of the Priest River complex near Cougar Gulch, south of Coeur d’Alene, Idaho. The variably deformed amphibolites occur as discontinuous bodies within Paleoproterozoic (1.85-1.87 Ga) granitic orthogneisses. Here, the crystalline basement is unconformably overlain by a graphitic orthquartzite, which is correlated to the graphitic Gold Cup orthoquartzite 65 km north at Priest River, ID. Near Priest River, amphibolite inclusions occur within the 2.65 Ga granodioritic Pend Oreille Gneiss.

The amphibolites at Cougar Gulch have basaltic protoliths but exhibit distinct geochemical characteristics. Group 1 amphibolites are Neoarchean (2.65 Ga) and exhibit enriched REE values (chondrite normalized) with moderately steep, negatively sloping LREE:HREE patterns. These Neoarchean amphibolites have distinctive negative anomalies in Nb, Ta, P, Ti, and Eu. The Group 2 amphibolites are not dated but have been geochemically correlated to amphibolites found within the overlying Hauser Lake Gneiss, and the Mesoproterozoic Moyie sills. The Group 2 amphibolites are less enriched than Group 1 and have relatively flat REE patterns with no negative Eu anomaly. Group 3 amphibolites (undated) are weakly enriched in REE’s (compared to Group 2) and show no negative Eu anomaly but have positive sloping (LREE:HREE) trends.

Two subrounded amphibolite inclusions (0.5-1.0 m) within the 2.65 Ga Pend Oreille Gneiss were sampled for geochemistry. One exhibits trace element characteristics similar to the Group 1 amphibolites at Cougar Gulch. The second appears to be correlative to the Group 2 Moyie sill type amphibolites at Cougar Gulch.

Group 1 amphibolites are interpreted as lower crustal inclusions within the Paleoproterozoic granites of the Cougar Gulch suite. Group 2 amphibolites are likely inclusions of "Moyie sill” feeder dikes that intruded pre-Belt basement at 1.47 Ga and were later tectonically dismembered and interleaved during core complex development. The undated Group 3 amphibolites are derived from a currently unrecognized source.