Geochronology of archean gneisses in the Lake Helen area, Southwestern Big Horn Mountains, Wyoming

Abstract

Abstract The RbSr and UPb methods were used to study gneisses in the 7 1 2 - minute Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite. A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial 87Sr/86Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source. A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The 87Sr/86Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.