PETROGENESIS OF MESOPROTEROZOIC FERROAN GRANITES OF COLORADO AND WYOMING

Mesoproterozoic ferroan granites are a prominent feature of the Precambrian basement of Colorado and southeastern Wyoming. These granites, mainly 1.45-1.43 Ga in age, are part of a larger suite of ferroan granite and anorthosite that extends across the southwestern USA to Labrador and Baltica. In Colorado and Wyoming, ferroan granites intrude Proterozoic crust except at their northernmost extent where they intrude Archean crust. They include both metaluminous and peraluminous compositions, which have been ascribed to mantle and crustal sources, respectively. We evaluate this hypothesis with an examination of the Sherman batholith of northern Colorado and Wyoming, which includes the full spectrum of geochemical compositions. The Mule Creek granite in the north is relatively silica-poor, strongly ferroan, and alkalic to alkali-calcic. The Sherman granite is more siliceous, less strongly ferroan, and alkali-calcic. The Lincoln granite is most siliceous, alkali-calcic, and peraluminous. The Mule Creek granite formed by differentiation of reduced tholeiitic basaltic parental magmas at high pressure, conditions that delay plagioclase and oxide crystallization. As a result, Ca is removed by crystallization of clinopyroxene preferentially leaving a Na+K rich residual melt and Mg is removed preferentially over Fe, thereby producing an alkalic, ferroan granite. Intrusion of hot ferroan magmas promotes crustal melting, which is more pronounced in the Proterozoic Colorado province than in the refractory Archean Wyoming province. Partial melts of granitic and pelitic rocks are siliceous, strongly peraluminous, and mainly calc-alkalic. Variable assimilation of these crustal partial melts by alkalic ferroan magma produces the Sherman and Lincoln compositions, as is confirmed in the northernmost Sherman batholith where the Archean assimilant is isotopically distinct. We suggest that all the ferroan granites of Colorado and Wyoming ultimately are derived from tholeiitic parental magmas and that variable amounts of crustal assimilation account for the observed compositional spectrum.