GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF ARCHEAN BASEMENT ROCKS FROM THE SHELL RESERVOIR QUADRANGLE, WYOMING: IMPLICATIONS FOR CRUSTAL EVOLUTION AND TECTONIC SETTINGS

The Wyoming Archean Province (WAP) in North America, forming the core of Laurentia, contains a variety of felsic intrusive rocks. These basement rocks are crucial for understanding the early Earth’s crustal evolution, given that Archean crusts comprised 70-80% granitoids. However, the evolution of these granitoids over time has altered their mineralogy and geochemical compositions, complicating the tracing of their source composition and tectonic setting. Geochemical analysis can potentially link granitoid composition to specific tectonic settings, but this remains challenging in Archean cratons due to overlapping geochemical signatures. Comprehensive interpretation models require geochemical data along with crystallization age, petrographic characteristics, and field relationships. In this study, we present geochemical analyses, thin section petrography, and detailed field observations of Archean granitoids exposed within the Shell Reservoir quadrangle in the northern Bighorn Mountains, Wyoming, USA. Our analysis of 48 granitoid samples reveals that most classify as granite and granodiorite on the QAP chart. These rocks are predominantly coarse-grained, with major minerals including quartz, alkali feldspar, and plagioclase, and minor minerals such as biotite, hornblende, muscovite, and sericite. Coarse-grained textures are common, with plagioclase feldspar exhibiting polysynthetic twinning and microcline feldspar showing tartan twinning. Myrmekitic textures, indicative of replacement features and hydrothermal alteration, are widespread. Major and trace element analyses of 10 samples indicate that all are peraluminous, suggesting crustal origin. The higher weight percentages of Na₂O + K₂O compared to FeO and MgO indicate a calc-alkaline composition. The Rb versus Nb+Y plot places all samples in the Volcanic Arc Granites (VAG) region. Nb and Zr depletion, along with Sr and Eu enrichment, suggests arc proximity and mixing with other sources during magma ascent. Consistent depletion in Heavy Rare Earth Elements (HREE) indicates a magma source was in equilibrium with garnet. Overall, these findings suggest that the magma source of these Archean granitoids originated in a volcanic arc produced by continental collision during Archean crustal evolution. This hypothesis awaits further validation through U-Pb zircon age data.