Geochemical and Nd isotopic compositions of the metasedimentary rocks in the La Ronge Domain, Trans-Hudson Orogen, Canada: implications for evolution of the domain

Abstract

Metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the Paleoproterozoic La Ronge Domain of the Trans-Hudson Orogen. Metasedimentary rocks in the Central Metavolcanic greenstone belt are low to intermediate in SiO 2 contents, have ϵ Nd values of +2.3 to +4.5, with two rare earth element (REE) patterns of (La/Yb) n <4 and >8.5, respectively. The predominant component, meta-arkoses, of the overlying McLennan Lake belt, have distinctly high K 2O and SiO 2 contents, high (La/Yb) n ratios (11–46), large negative Eu anomalies (Eu/Eu*=0.1–0.58) and uniformly low ϵ Nd values (−0.8 to +0.2). The MacLean Lake belt is comprised of a variety of metasedimentary gneisses. The volumetrically minor amphibole- and diopside-rich gneisses have high ferromagnesian element (Sc and Co) concentrations, high ϵ Nd values of +4.1 to +5.7, and flat REE patterns with no Eu anomalies. The predominant lithological unit in the MacLean Lake belt, the psammitic gneisses, is geochemically more variable. The mafic psammitic gneiss samples in the MacLean Lake belt and the metasedimentary rocks in the Central Metavolcanic belt are similar in trace and major element geochemistry and Nd isotope composition, whereas those more felsic psammitic gneisses are similar to the McLennan Lake meta-arkoses. Geochemical data, combined with lithological and geochronological evidence, indicate that sediments in the Central Metavolcanic belt were derived mainly from contiguous volcanic debris, with minor addition of felsic components from exotic sources. In contrast, the McLennan Lake meta-arkoses were derived from a distinct felsic source, whereas the psammitic gneisses in the MacLean Lake belt are mixtures of volcanic materials in the Central Metavolcanic belt and meta-arkoses-like felsic components. Nd isotopic data and ages of detrital zircon suggest that Proterozoic juvenile materials were dominant sources for metasedimentary rocks in the La Ronge Domain. Comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the Central Metavolcanic belt suggests that the felsic materials in the McLennan Lake and MacLean Lake belts could not have been derived predominantly from the plutonic rocks, implying more distal sources. Topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the MacLean Lake belt, to the dominance of a more distal source for the McLennan Lake meta-arkoses.