Paper No. 79-7
Presentation Time: 9:00 AM-5:30 PM
ARCHEAN GEOCHRONOLOGICAL AND DEFORMATIONAL FRAMEWORK OF THE SOUTHERN MARGIN OF THE BIGHORN BATHOLITH, WYOMING: SUCCESSIVE SYNPLUTONIC SHEARING BETWEEN 2960 AND 2840 MA
The Mesoarchean core of the Bighorn Mountains of Wyoming reveals a boundary between two separate terranes. The northern part of the crystalline core is composed of the ~2850 Ma Bighorn Batholith (BhB), an undeformed composite granitic intrusion, whereas the central and southern area of the range exposes older quartzofeldspathic gneiss complexes as well as minor supracrustal rocks. The boundary between the two regions is marked by a shear zone referred to here as the Upper Paint Rock Shear Zone (UPRSZ). Eight high-resolution U-Pb LA-ICPMS age determinations of granodiorite (4), migmatite leucosome (2), and mylonite (2) exposed just near Geneva Pass along the southern margin of the BhB reveal a >100-million-year history of magma intrusion and deformation. The earliest event was the intrusion of a coarse granodiorite between 2960-2935 Ma (and subsequently foliated). These rocks are cut by an east-trending 2932 Ma ultramylonite zone. A second magmatic event involved the emplacement of unfoliated granodiorite and tonalite between 2920-2894 Ma. The southern margin of the UPRSZ is marked by a ~20-meter-wide mylonite zone that dips 60-70 degrees to the north and has north side up kinematic indicators. The UPRSZ mylonite is 2860 Ma, and was active during the emplacement of the main body of the BhB exposed to the north. Late migmatite bodies intrude at 2839 Ma. The migmatite and early granodiorite each contain xenocrystic zircons as old as 3200 Ma, which indicates that the BhB assimilated some of the oldest rocks present in the Wyoming Archean province. Two conclusions can be drawn from these data. First, the BhB, at least along the southern margin, contains phases at least 100-million-years older than the main phase of the body. Second, episodes of both intrusion and shearing took place in this area as the BhB was emplaced. A comprehensive geologic map of the area has been completed, and samples for geochemical analysis are being processed; these data will aid in deciphering the origins of these magmatic events.