Tholeiitic source for A-type granites; evidence from the northern Sherman Batholith, Southeast Wyoming, U.S.A.

Abstract

The Sherman batholith is unique among the Proterozoic A-type granites of the western and central U. S. because its northern portions intrude Archean, not Proterozoic, crust. The 1.44-1.43 Ga batholith was emplaced along the Cheyenne belt, the suture between Archean Wyoming province crust to the north and Proterozoic Colorado province crust to the south. The Archean-Proterozoic boundary dips to the south, so that the Sherman batholith north of the Cheyenne belt traversed only Archean crust during ascent and emplacement. Wyoming province Archean crust is isotopically distinct from 1.43 Ga mantle.The northern Sherman batholith includes both intrusive and hypabyssal facies. The Mule Creek lobe is composed mainly of medium-grained, equigranular fayalite granite, with small volumes of monzodiorite, fine-grained biotite granite dikes, high-alumina gabbro and monzodiorite. In the Richeau Hills, the Sherman batholith is represented by fine-grained to glassy hypabyssal rhyolite, monzodiorite, and high-alumina gabbro dikes. Mule Creek granite yielded a U-Pb zircon date of 1438.9+ or -2.7 Ma, and Richeau Hills rhyolite was determined by Harper (1997) to be 1436.8+ or -2.3 Ma., 5-10 m.y. older than the main units of the Sherman batholith south of the Cheyenne belt (Frost et al., J. Petrol., in press). Mule Creek and Richeau Hills rocks exhibit the most extreme iron-enrichment of the Sherman batholith, a reduced, rapakivi-type granite. Monzodiorite from the Mule Creek lobe crystallized at 910-920 degrees C and at oxygen fugacities around -0.5 below FMQ. The fayalite granites crystallized at 810-830 degrees C and similarly low oxygen fugacities. Initial E(Nd) for the Mule Creek plutonic rocks ranges from 0.55 to -3.2, initial (super 87) Sr/ (super 86) Sr is 0.702 to 0.714, and initial (super 207) Pb/ (super 204) Pb is 15.50-15.65. Nd mass balance calculations indicate that 70-90% of the Nd is derived from the mantle, and binary mixing calculations suggest that the proportion of the mantle-derived, tholeiitic endmember is 50-70%. Our data from the northern Sherman batholith, like those documented for compositionally equivalent high-K fayalite rhyolites at Yellowstone, support the hypothesis that reduced rapakivi-type magmas, represent juvenile material added to the crust in an extensional environment.