MESOARCHEAN U-PB GEOCHRONOLOGY OF THE SOUTHERN MARGIN OF THE BIGHORN BATHOLITH, WYOMING

The Bighorn Mountains reveal one of the largest occurrences of Mesoarchean rocks in Laurentia. The northern part of the crystalline core is composed of the composite Bighorn Batholith, whereas the central and southern area of the range exposes older gneiss complexes as well as minor supracrustal rocks. We provide new high resolution LA-ICPMS U-Pb data on zircons sampled from tonalite, granodiorite, mylonite, migmatite, gneiss and pegmatite from the southern margin of the Bighorn Batholith (sample = 13; total zircons analyzed = 337) in the headwaters of the north fork of Paint Rock Creek. These rocks range from strongly foliated to massive but are difficult to subdivide into mappable units in the field because of their lithologic and structural similarities. Several mylonite zones (~10-meter-wide) that trend ~E-W and dip steeply are present in the study area. Three distinct age populations are evident: 2935, 2910, and 2890 Ma. Most samples have xenocryst zircons older than 3000 Ma and as old as 3500 Ma, which indicates the assimilation of older crust. Each of the three age populations reported here are older than the previously reported age of ~2850 age for the northern Bighorn Batholith. Our new ages align better with 2890, 2940, and 2950 Ma reported for the southern gneiss terrane. Two conclusions can be drawn from these data. First, the Bighorn Batholith, at least along the southern margin, contains phases at least 80 million years older than the northern phase of the body. Second, episodes of both intrusion and shearing took place in this area as the Bighorn Batholith was emplaced.