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 Opxand fayalite (Fay)-bearing felsic igneous rocks inin Wind River Range of western Wyoming, consists of minor diorite, clude opdalite (Opx granodiorite), enderbite (Opx tonalquartz diorite, granodiorite, and granite. At shallow structural levels ite), and mangerite (Opx monzonite, Le Maitre et al., the batholith is pyroxene free, but at deeper levels, all units of the 1989). Many of these rock names are not commonly batholith contain pyroxenes. On its northern margin the batholith used; the term charnockite (sensu lato) is typically applied was emplaced at P = 5–6 kbar, T = 775–800°C, fO2 at to any Opx-bearing granitoid, regardless of its bulk FMQ (fayalite–magnetite–quartz) + 1·5 to FMQ + 1·8, and composition. For example, the charnockite suite of south aH2O >0·1. Along the southern margin of the batholith the India includes enderbites, opdalites, and charnockites emplacement pressure was >3 kbar. The batholith includes two (Howie, 1955), the Nigerian suite includes charnockite compositional series. The peraluminous high-REE series is rich in and opdalite (Olarewaju, 1987), and that of Lofoten is K2O, and displays larger FeO/(FeO + MgO). The metaluminous associated with mangerites (Malm & Ormaasen, 1978). low-REE series has less K2O and smaller FeO/(FeO + MgO). Because the terms enderbite and opdalite are so rarely In the Boulder Canyon area, most of the rocks belong to the lowused, in this paper we use the term ‘charnockite’ for all REE series. The Nd, Pb, and Sr initial isotopic compositions of Opx-bearing granitic rocks. Where it is important to both series vary and do not correlate with SiO2 abundance. Relatively describe a specific rock type, we use Opx as a varietal constant Fe/Mg ratios over a wide range of SiO2 and the large modifer to the rock name (e.g. Opx granodiorite instead variation in initial isotopic compositions indicate that the sources of opdalite). for the Louis Lake batholith include older Wyoming province crustIn the 1980s workers in southern India recognized and mantle-derived mafic melts. The calc-alkalic nature of the areas where biotite-bearing granitic gneiss had been Boulder Canyon charnockites indicates that there is no special Cconverted to pyroxene-bearing gneiss along fluid pathtype magma. Instead, charnockites will form in any rock type, if ways ( Janardhan et al., 1982; Friend, 1985). This led to the water activity is low enough. the term ‘charnockitization’ (Srikantappa et al., 1985) and the concept that charnockite is a metamorphic, rather than an igneous rock. As a result, ‘charnockite’ has been used incorrectly by some workers as a synonym for