Origin of the Charnockites of the Louis Lake Batholith, Wind River Range, Wyoming

Abstract

The 2·63 Ga Louis Lake batholith, a calc-alkalic pluton exposed in Wind River Range of western Wyoming, consists of minor diorite, quartz diorite, granodiorite, and granite. At shallow structural levels the batholith is pyroxene free, but at deeper levels, all units of the batholith contain pyroxenes. On its northern margin the batholith was emplaced at P = 5-6 kbar, T = 775-800°C, fO2 at FMQ (fayalite-magnetite-quartz) + 1·5 to FMQ + 1·8, and aH2O ∼ 0·1. Along the southern margin of the batholith the emplacement pressure was ∼ 3 kbar. The batholith includes two compositional series. The peraluminous high-REE series is rich in K2O, and displays larger FeO1 / (FeO1 + MgO). The metaluminous low-REE series has less K2O and smaller FeO1 / (FeO1 + MgO). In the Boulder Canyon area, most of the rocks belong to the low-REE series. The Nd, Pb, and Sr initial isotopic compositions of both series vary and do not correlate with SiO2 abundance. Relatively constant Fe/Mg ratios over a wide range of SiO2 and the large variation in initial isotopic compositions indicate that the sources for the Louis Lake batholith include older Wyoming province crust- and mantle-derived mafic melts. The calc-alkalic nature of the Boulder Canyon charnockites indicates that there is no special C-type magma. Instead, charnockites will form in any rock type, if the water activity is low enough.