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

Abstract

The 1•43 Ga Sherman batholith, southeastern Wyoming, USA, origin that are found throughout the Proterozoic 'anorogenic' granite provinces of the southwestern USA, Adir-shows extreme A-type petrochemical characteristics compared with other Mid-Proterozoic granite batholiths of North America.It ondacks, eastern Canada, southern Greenland and the Fennoscandian Shield (Anderson, 1983).Phanerozoic consists of: (1) the Sherman granite, a coarse-grained biotite hornblende granite that locally contains fayalite and pyroxenes; (2) examples include Paleozoic granites of southeastern Australia (Collins et al., 1982) and of Corsica (Poitrasson et the Lincoln granite, a medium-grained biotite granite; (3) a porphyritic biotite hornblende granite that probably formed by interaction al., 1995), and the Mesozoic Nigerian Younger granites ( Jacobsen et al., 1958) and the White Mountain Magma of granitic and mafic magmas; and (4) iron-enriched mafic dikes and pods.The ilmenite-series, metaluminous Sherman granite Series (Foland & Allen, 1991).'Anorogenic granite' refers to plutons that are not associated with compressional exhibits extreme values of FeO t /(FeO t + MgO) and is rich in structures, and that were emplaced long after any known K, REE, Nb and Y.It crystallized at temperatures exceeding orogenic event.Anorogenic, or 'A-type' granites (Loiselle 900°C and a pressure of ~2•5 kbar, with water activity of 0•7 & Wones, 1979) are characterized by low H 2 O and O 2 and log fO 2 of -0•1 to -0•5.The Lincoln granite, which is fugacities along with high FeO t /(FeO t + MgO), K 2 O/ peraluminous and has less extreme A-type geochemical characteristics, Na 2 O and K 2 O contents.A-type granites typically are rich crystallized at temperatures as low as 750°C and log fO 2 of in incompatible elements, including rare earth elements around 0•5 units above FMQ (fayalite-magnetite-quartz).The (REE), Zr, Nb and Ta, but poor in Co, Sc, Cr, Ni, Ba, rocks of the Sherman batholith are chemically equivalent to lavas Sr, and Eu.Minimum magma liquidus temperatures are from the Yellowstone hotspot.Like the Yellowstone magmas, the 900-1000°C (Clemens et al., 1986; Creaser & White, Sherman batholith probably originated by partial melting of under- 1991).Some workers suggest these rocks are derived plated, mantle-derived mafic rocks.from melting of tonalitic or more felsic crust (Anderson & Cullers, 1978;Collins et al., 1982;Clemens et al., 1986;Creaser et al., 1991), whereas others relate the granites