EXTENDED HISTORY OF A 3.5-GA TRONDHJEMITIC GNEISS, WYOMING PROVINCE, USA - EVIDENCE FROM U-PB SYSTEMATICS IN ZIRCON

The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimen tary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically co mplex assemblage of lithotectonic units that include: (1) a strongly d eformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant U-Pb ages of 3. 5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this roc k: (1) an apparently protracted period of high-grade metamorphism and/ or intrusion of additional magmas at similar to 3.25 Ga; and (2) growt h of hydrothermal zircon at similar to 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacem ent of the gneiss. Although this latter event appears confined to area s along the BBMZ-MMP boundary, evidence of similar to 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughou t the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The lim ited occurrences of 2.8 Ga magmatic activity in the MMP suggest that i t had a controlling influence on late Archean magmatism as well.