Petrogenesis of two contrasting, late Archean granitoids, Wind River Range, Wyoming

Abstract

The Louis Lake batholith, the older unit, is an I-type granitoid that formed by partial melting of trondhjemitic to tonalitic protolith of Early Archean age. Melting and at least some crystallization took place at high pressure and water-saturated conditions. The chemistry of the batholith can be well approximated by a three-end-member mixing (unmixing) model in which an initial primitive granodioritic magma is differentiated by fractionation of two different solidus assemblages, possibly at two different pressures. The Bears Ears pluton, the younger unit, is a coarsely porphyritic, S-type granite that formed by partial melting of a peraluminous and petrologically evolved protolith of Early to Middle Archean age. Some time prior to magma formation, the protolith preferentially lost labile elements, such as uranium and alkali metals, probably in response to granulite-facies metamorphism. Crystallization (and possibly partial melting) took place at much lower pressures than those that existed for the Louis Lake batholith, and at least end-stage crystallization probably occurred at water-saturated conditions. Q-mode factor analysis of chemical data for the Bears Ears pluton shows that at least five end members are needed in any mixing model in order to approximate the chemical variations observed for the granite. Field relations and isotopic datamore » indicate that contamination with xenolithic material is common, and therefore, much of the chemical variability may be due to incorporation of country rock. Neither the Louis Lake batholith nor the Bears Ears pluton is associated with uranium deposits, and isotopic studies have shown that neither unit lost large amounts of uranium that could be trapped by adjacent sedimentary formations.« less