Extended History of a 3.5 Ga Trondhjemitic Gneiss, Wyoming Province, USA: Evidence from U-Pb Systematics in Zircon


Mueller, P., Wooden, J., Mogk, D., Nutman, A. and Williams, I. (1996), Precambrian Research, 78: 41-52

Abstract


The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary 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 complex assemblage of lithotectonic units that include: 1) a strongly deformed 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 rock: 1) an apparently protracted period of high grade metamorphism and/or intrusion of additional magmas at ~3.25 Ga; and 2) growth of hydrothermal zircon at ~ 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ~3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on Late Archean magmatism as well.


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