Petrogenesis of the 1.43 Ga Sherman Batholith, SE Wyoming, USA: a Reduced, Rapakivi-type Anorogenic Granite

The 1.43 Ga Sherman batholith, southeastern Wyoming, USA, shows extreme A-type petrochemical characteristics compared with other Mid-Proterozoic granite batholiths of North America. It consists of: (1) the Sherman granite, a coarse-grained biotite hornblende granite that locally contains fayalite and pyroxenes; (2) the Lincoln granite, a medium-grained biotite granite; (3) a porphyritic biotite hornblende granite that probably formed by interaction of granitic and mafic magmas; and (4) iron-enriched mafic dikes and pods. The ilmenite-series, metaluminous Sherman granite exhibits extreme values of FeOt/(FeOt+MgO) and is rich in K, REE, Nb and Y. It crystallized at temperatures exceeding 900°C and a pressure of ∼2.5 kbar, with water activity of 0.7 and Δlog fO2 of -0.1 to −0.5. The Lincoln granite, which is peraluminous and has less extreme A-type geochemical characteristics, crystallized at temperatures as low as 750°C and Δlog fO2 of around 0.5 units above FMQ (fayalite–magnetite–quartz). The rocks of the Sherman batholith are chemically equivalent to lavas from the Yellowstone hotspot. Like the Yellowstone magmas, the Sherman batholith probably originated by partial melting of underplated, mantle-derived mafic rocks.