Basement, belt, and batholith: Bedrock geology of the Idaho Panhandle

Abstract

ABSTRACT This trip explores the three main bedrock geology elements in the Idaho Panhandle region and the Priest River metamorphic complex: basement gneisses, the Belt Supergroup (Purcell Supergroup in Canada), and the Kaniksu batholith. The Priest River metamorphic complex includes two areas with Precambrian basement gneisses, informally termed the Pend Oreille and Cougar Gulch complexes. The Pend Oreille complex contains a 2.67 Ga orthogneiss and a 1.58 Ga orthogneiss, the latter notable for falling in the “North American magmatic gap” (NAMG) age range. The Cougar Gulch complex is dominated by ca. 1.86 Ga orthogneisses with ca. 2.67 Ga amphibolites. Overlying the basement rocks in both areas are thin graphitic quartzites that have been correlated with the Neihart quartzite and thick paragneiss sequences that have been correlated with the Prichard Formation of the Belt Supergroup. The upper plate of the Priest River complex contains substantial exposures of only weakly metamorphosed Belt Supergroup. The Belt Supergroup is a very thick sequence of fine-grained clastic sedimentary rocks deposited in an intracontinental extensional setting between ca. 1.46 and 1.40 Ga. Its main formal divisions are the Prichard Formation, Ravalli Group, Piegan Group, and Missoula Group. The Prichard Formation consists of deep-water turbidites interlayered with contemporaneous mafic sills, whereas the other units of the Belt Supergroup accumulated in shallow-water environments. Both the Prichard Formation and Ravalli Group typically contain detrital zircons with ages in the NAMG that have long been interpreted to have originated on another previously connected continent block when North America was part of the Nuna supercontinent. The balance of evidence based on correlations with the basement units and study of Belt provenance strongly suggests that the Gawler craton of southern Australia was adjacent to this region in Nuna during deposition of early Belt units. The Kaniksu batholith forms the bulk of both the upper and lower plates of the Priest River metamorphic complex. New U-Pb zircon dating indicates that most of the batholith was constructed between 120 and 100 Ma with much less magmatism continuing to 72 Ma. The batholith is the southern end of the Omineca Belt of the Canadian Cordillera and is compositionally diverse, with isotope data suggesting at least three different crustal source components involved in its genesis. The batholith appears to have formed well to the rear of the Cretaceous Cordilleran arc front (i.e., the Coast Mountains batholith), and was thus probably not directly related to subduction but instead likely formed through episodic crustal thickening.