Petrogenesis of the 1 center dot 43 Ga Sherman batholith, SE Wyoming, USA:A reduced, rapakivi-type anorogenic granite

The 1.43 Ga Sherman batholith, southeaster Wyoming, USA shows extreme A-typ e petrochemical characteristics compared with other Mid-Proterozoic granite batholiths of North America. It consists of: (1) the Sherman granite, a co arse-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 o f FeO'/(FeO' + MgO) and is rich in K, REE, Nb and Y. It crystallized at tem peratures exceeding 900 degrees C and a pressure of similar to 2.5 kbar, wi th water activity of 0.7 and Delta log fO(2) of -0.1 to -0.5. The Lincoln g ranite, which is peraluminous and has less extreme A-type geochemical chara cteristics, crystallized at temperatures as low as 750 degrees C and Delta log fO(2) of around 0.5 units above FMQ (fayalite-magnetite-quartz). The ro cks of the Sherman batholith are chemically equivalent to lavas from the Ye llowstone hotspot. Like the Yellowstone magmas, the Sherman batholith proba bly originated by partial melting of under-plated, mantle-derived mafic roc ks.