MULTI-STAGE MODIFICATION OF THE NORTHERN SLAVE MANTLE LITHOSPHERE: EVIDENCE FROM ZIRCON- AND DIAMOND-BEARING ECLOGITE XENOLITHS ENTRAINED IN JERICHO KIMBERLITE, CANADA

Abstract

The Jericho kimberlites are part of a small Jurassic kimberlite cluster in the northern Slave craton, Canada. A variety of dating techniques were applied to constrain the nature and age of two Jericho kimberlites, JD-1 (170·2 ± 4·3 Ma Rb - Sr phlogopite megacrysts, 172·8 ± 0·7 Ma U Pb eclogite rutile, 178 ± 5 Ma U-Pb eclogite zircon lower intercept) and JD-3 (173 ± 2 Ma Rb - Sr phlogopite megacryst; 176·6 ± 3·2 - Ma U - Pb perovskite), and all yielded identical results within analytical uncertainty. As there is no discernible difference in the radiometric ages obtained for these two pipes, the composite Rb - Sr phlogopite megacryst date of 173·1 ± 1·3 Ma is interpreted as the best estimate for the emplacement age of both Jericho pipes. The initial Sr isotope composition of 0·7053 ± 0·0003 derived from phlogopite megacrysts overlaps the range (0·7043-0·7084) previously reported for Jericho whole-rocks. These strontium isotope data, combined with the radiogenic initial 206Pb/204Pb ratio of 18·99 ± 0·33 obtained in this study, indicate that the Jericho kimberlites are isotopically similar to Group 1 kimberlites as defined in southern Africa. The Jericho kimberlites are an important new source of mantle xenoliths that hold clues to the nature of the Slave craton subcontinental mantle. A high proportion (30%) of the Jericho mantle xenolith population consists of various eclogite types including a small number (2-3%) of apatite-, diamond-, kyanite- and zircon-bearing eclogites. The most striking aspect of the Jericho zircon-bearing eclogite xenoliths is their peculiar geochemistry. Reconstructed whole-rock compositions indicate that they were derived from protoliths with high FeO, Al2O3 and Na2O contents, reflected in the high-FeO (22·6-27·5 wt %) nature of garnet and the high-Na2O (8·47-9·44 wt %) and high-Al2O3 (13·12-14·33 wt %) character of the clinopyroxene. These eclogite whole-rock compositions are highly enriched in high field strength elements (HFSE) such as Nb (133-1134 ppm), Ta (5-28 ppm), Zr (1779-4934 ppm) and Hf (23-64 ppm). This HFSE enrichment is linked to growth of large (up to 2 mm) zircon and niobian rutile crystals (up to 3 modal %) near the time of eclogite metamorphism. The diamond-bearing eclogites on the other hand are characterized by high-MgO (19·6-21·3 wt %) garnet and ultralow-Na2O (0·44-1·50 wt %) clinopyroxene. Paleotemperature estimates indicate that both the zircon- and diamond-bearing eclogites have similar equilibration temperatures of 950-1020°C and 990-1030°C, respectively, corresponding to mantle depths of 150-180 km. Integration of petrographic, whole-rock and mineral geochemistry, geochronology and isotope tracer techniques indicates that the Jericho zircon-bearing eclogite xenoliths have had a complex history involving Paleoproterozoic metamorphism, thermal perturbations, and two or more episodes of Precambrian mantle metasomatism. The oldest metasomatic event (Type 1) occurred near the time of Paleoproterozoic metamorphism (~1·8 Ga) and is responsible for the extreme HFSE enrichment and growth of zircon and high-niobian rutile. A second thermal perturbation and concomitant carbonatite metasomatism (Type 2) is responsible for significant apatite growth in some xenoliths and profound light rare earth element enrichment. Type 2 metasomatism occurred in the period 1·0-1·3 Ga and is recorded by relatively consistent whole-rock eclogite model Nd ages and secondary U-Pb zircon upper intercept dates. These eclogite xenoliths were derived from a variety of protoliths, some of which could represent metasomatized pieces of oceanic crust, possibly linked to east-dipping subduction beneath the Slave craton during construction of the 1·88-1·84 Ga Great Bear continental arc. Others, including the diamond-bearing eclogites, could be cumulates from mafic or ultramafic sill complexes that intruded the Slave lithospheric mantle at depths of about 150-180 km. © 2006 Oxford University Press.