Continental tholeiitic mafic rocks of the Paleoproterozoic Hurwitz Group, Central Hearne sub-domain, Nunavut: insight into the evolution of the Hearne sub-continental lithosphere

Abstract

The Paleoproterozoic Hurwitz Group of the western Churchill Province is an erosional remnant of an areally extensive, predominantly shallow-water intracratonic basin comprised of four major sequences. Sequence 2, forming the central part of the stratigraphy, contains the Ameto Formation, a sequence of pillowed and massive basaltic rocks and associated gabbro sills termed the Happotiyik Member that are interlayered with subordinate deep-water mudstones, siltstones, and diamictites. Whole-rock geochemical data for the mafic rocks reveals a suite of homogeneous tholeiitic basalts with affinities to both continental and volcanic-arc tholeiites. Compatible trace elements and large-ion lithophile elements exhibit scattered behavior, whereas all high field strength elements show a systematic increase with Zr. The rocks are large-ion lithophile and light rare-earth element enriched, and have parallel primitive mantle normalized extended trace element patterns with prominent negative Nb, Ta, and Ti anomalies. εNd(t=2200 Ma) values for the rocks range from 0.0 to +0.8. The data indicate that the parental magmas were derived from a heterogeneous, predominantly depleted mantle source that included a minor metasomatically enriched component. Contamination by Neoarchean, juvenile silicic upper crust during ascent was minimal. We envisage that the rocks of the Happotiyik Member were generated from sub-continental lithospheric mantle that was stabilized immediately after formation of the ca. 2680 Ma, Neoarchean Central Hearne sub-domain. This enrichment occurred via metasomatic infiltration of subduction-derived fluids and melts into the overlying lithosphere. A wide range of Paleoproterozoic intra-continental mafic rocks in the western Churchill Province exhibit comparable geochemical and isotopic signatures that suggest an origin in the lithospheric mantle. These observations imply that the Hearne sub-continental lithospheric mantle has endured since the Neoarchean and likely persists today.