Highly alkaline lavas in a Proterozoic rift zone: Implications for Precambrian mantle metasomatic processes

Abstract

An occurrence of Proterozoic nephelinite and basanite lavas and pyroclastic rocks and associated phonolites indicates that the processes that generate modern alkaline magmas in intraplate settings were operative in the Early Proterozoic. These lavas occur near the top of a 1.9 Ga continental-margin sequence in the Cape Smith fold and thrust belt of northern Quebec. The lavas are classified as nephelinites, basanites, and phonolites on the basis of high field strength and rare earth element contents, although large ion lithophile elements, including alkalis, appear to have been strongly depleted by greenschist facies metamorphism and alteration. Certain major elements define trends consistent with low-pressure fractionation dominated by clinopyroxene, which is the only mafic phenocryst present in the lavas. The mafic and felsic lavas have identical 143 Nd/ 144 Nd ratios, consistent with consanguinity and a lack of contamination by older crust of the Superior province. Values for ϵ Nd (1.96 Ga) of +2 represent an enriched mantle source relative to +4 to +5 for the contemporaneous mid-oceanic-ridge basalt reservoir.