Garnet pyroxenite and granulite xenoliths from northeastern Alberta: Evidence of ∼1.5 Ga lower crust and mantle in western Laurentia

Xenoliths from the Kendu ultramafic body in the Birch Mountains kimberlite field of northeastern Alberta present an unprecedented opportunity to document the lower crust and mantle beneath the Taltson magmatic zone, and in this region of western Laurentia. Mineralogical, geochemical and multi-isotope analyses were performed. The data show that representative garnet pyroxenite, mafic granulite and granulite xenoliths have temperature regimes of between 730 and 1130 °C with pressure estimates of 27–32 kbar, 19 kbar and 14 kbar, respectively.An important result of this work is the discovery of lower crust/mantle (>45 km) rocks that are significantly younger than the regions upper crustal rocks and tectonic assembly. Using the Sm–Nd geochronometer, an age of 1477 ± 10 Ma (mean square weighted deviate of 0.84) is obtained on whole rock, garnet and clinopyroxene fractions from a granulite xenolith. The Sm–Nd age is supported in a second isotopic system as whole rock and mineral separates from the granulite also yielded a Lu–Hf age of 1602 ± 150 Ma (mean square weighted deviate of 16). Errorchrons combining the granulite Lu–Hf and Sm–Nd data with those from garnet pyroxenite and mafic granulite xenoliths yielded ages that are within error to the ∼1477 Ma granulite isochron age. This lends support to a Mesoproterozoic event, and implies that mantle in this region also formed, or was influenced, during the Mesoproterozoic.The Mesoproterozoic age is approximately 220 Ma younger than major tectonism related to continental growth in this part of western Laurentia, and 520 Ma younger than the region's uppermost crustal rocks. The age discrepancy between lower crust/mantle and upper crustal rocks suggests the western margin of Laurentia be modelled by geological processes that could include an under-thrusted terrane, or a crust–mantle reaction at the expense of a thickening crust and underplating. Perhaps the most intriguing chronological link is with widespread Mesoproterozoic intracratonic anorogenic magmatism that was prominent throughout the continental United States at ca. 1450–1475 Ma representing one of the largest genetically connected magmatic provinces on Earth.