Profound pyroxenitic/megacrystic overprinting of the central Superior craton lithosphere recorded by sodic Cr-diopside xenocrysts from the Attawapiskat kimberlites, Ontario, Canada

Abstract

Diamonds mined from the Victor kimberlite (Ontario, Canada) grew at ~ 720 Ma in lherzolite-dominated substrates that themselves were generated ca. 400 My earlier by metasomatic overprinting of refractory harzburgites-dunites with proven Archean heritage. Since diamonds from Victor represent a restricted mantle sample (5.7 ± 0.2 GPa and 1129 ± 16 °C), we reanalyzed major- and trace-element compositions of 196 Cr-diopside xenocrysts from Victor to investigate the depth range, style and extent of ~ 1100 Ma-old overprinting in the lithospheric section. Our approach uses down-the-geotherm projection and visualization of geochemical variability. We find that a Si-Al-Ca-Na enriched andesitic to dacitic metasomatic agent, likely derived from eclogite, interacted with previously depleted high Cr/Al harzburgites-dunites, driving their bulk compositions towards lherzolites and pyroxenites. Our data provides support for (i) substantive preservation of (garnet-absent) refractory Cr-spinel harzburgites-dunites at T < 740 °C, (ii) profound overprinting of peridotitic substrates, thereby establishing a refertilized garnet ± spinel lherzolite ± pyroxenite assemblage over the lithospheric section from 600 to 1240 °C, (iii) relicts of partially overprinted refractory mantle at T ~ 1010 °C and T 1100–1150 °C, the latter coincident with lherzolite-hosted diamond mineralization, (iv) a discrete carbonatitic geochemical signal uniquely associated with diamond mineralization that resides, in part, in garnet wehrlite with refractory heritage, and (v) maximum metasomatic agent:substrate ratios over the 1000–1200 °C interval, producing high Na-Al, low-Cr clinopyroxene-garnet megacrysts. We infer that + 10 mm high-Cr clinopyroxene-ilmenite megacryst assemblages result from lower metasomatic agent:substrate ratios in the interval 860–1000 °C.