A GEOLOGIC MAP OF THE STAMPEDE MEADOWS QUADRANGLE, WYOMING: AN 800 MILLION YEAR RECORD OF ARCHEAN CRUSTAL EVOLUTION

The Stampede Meadows 7.5’ Quadrangle in the Archean Sacawee block of central Wyoming provides insights into the Paleoarchean, Mesoarchean, and Neoarchean evolution of the Wyoming craton. Excellent exposure enabled us to identify and map five Archean units, which correlate to different periods of tectonism: 1. Paleoarchean tonalitic and granitic gneisses, 2. Mesoarchean supracrustal rocks predominantly comprised of quartzite containing rutilated quartz, 3. Minor Neoarchean monzogranites with potassium feldspar porphyroblasts, 4. Neoarchean biotite-hornblende paragneisses that were partially melted, and 5. Granitic rocks of the Neoarchean Granite Mountains batholith. This EDMAP-funded mapping project focused on defining the Archean history of the quadrangle through field work, petrography, and isotopic and geochemical analyses.

Paleoarchean gneisses are mostly calcic and tonalitic, though granitic gneisses are also present. The 3.3 Ga gneisses are coeval with granitic gneisses elsewhere in the Sacawee block. Detrital zircon analysis of the Mesoarchean quartzite defines two zircon populations dating to 2.9 and 3.2 Ga, comparable to those in supracrustal sequences in the eastern Sacawee block. The monzogranite gneiss was dated at 2.73 Ga. The paragneiss contains 2.63 Ga leucosomes and detrital zircons with ages of 3.2, 2.9, and 2.7 Ga. All these units have negative initial εNd and Nd model ages between 3.6 and 3.8 Ga.

Deformation of these units occurred at ~2.64 Ga during accretion of the Sacawee block to the Wyoming province. This resulted in a northeast-trending fabric foliation with southwest-plunging lineations that locally become intense around blocks of lower strain.

The gneisses were intruded by 2.62 Ga alkali-calcic granitic rocks of the Neoarchean Granite Mountains batholith, which is essentially undeformed. This project identified several phases of the batholith. The granites and all Archean rocks in the map area show local evidence of epidotization at 2.5 Ga. Batholith samples have initial εNd near 0 and Nd model ages between 2.8 and 3.1 Ga. Our studies in the Stampede Meadows quadrangle suggest that crustal recycling dominated until the intrusion of the Neoarchean Granite Mountains batholith, which added components of mantle or younger crust to the southern margin of the Wyoming craton.