[Show abstract][Hide abstract] ABSTRACT: 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 27–32kbar, 19kbar and 14kbar, respectively.An important result of this work is the discovery of lower crust/mantle (>45km) rocks that are significantly younger than the regions upper crustal rocks and tectonic assembly. Using the Sm–Nd geochronometer, an age of 1477±10Ma (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±150Ma (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 ∼1477Ma 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 220Ma younger than major tectonism related to continental growth in this part of western Laurentia, and 520Ma 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. 1450–1475Ma representing one of the largest genetically connected magmatic provinces on Earth.
Precambrian Research 03/2010; 177(3):339-354. DOI:10.1016/j.precamres.2010.01.006 · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We characterized single sulfides in eclogite and pyroxenite xenoliths from the Diavik kimberlites (central Slave Craton, Canada) with regard to their petrography, major-element composition and Re–Os isotope systematics. Together with trace-element and Re–Os isotope compositions of whole rocks, these data allow identification of the major Re–Os host phases and provide constraints on the origin(s) of sulfides in these samples.The majority of sulfide minerals contain 8 to 28 at.% Ni, with intragranular sulfides having on average significantly lower contents (~ 6 at.%) than intergranular sulfides (~ 12 at.%). These high Ni-sulfides are not in equilibrium with an eclogitic assemblage and were likely introduced from a peridotitic source subsequent to eclogite formation. In contrast, their Re–Os abundances and Re/Os ratios (average ~ 825 ppb, 190 ppb and 10, respectively) overlap those of primary eclogitic sulfides. These conflicting compositional characteristics may document open-system disequilibrium processes accompanying the introduction of sulfides into eclogites. The general association of high 187Os/188Os with high 187Re/188Os of sulfides in three low-temperature eclogite xenoliths suggests that the addition is not young. In contrast, sulfides in a high-temperature eclogite plot on a ~ 90 Ma errorchron with radiogenic initial 187Os/188Os, perhaps indicative of young introduction of sulfides from a deep enriched source.Sulfides in a single pyroxenite xenolith have Ni, Re and Os contents intermediate between pristine eclogitic and peridotitic sulfides, and correlated Re–Os isotope systematics defining an age of 1.84 ± 0.14 Ga with a radiogenic 187Os/188Osi (0.16 ± 0.01). The age and 187Os/188Osi are identical to those obtained for eclogitic sulfide inclusions in diamonds from Diavik, thus supporting a link between eclogite and pyroxenite formation.Several eclogite and pyroxenite whole rocks show evidence for addition of secondary sulfides, but many plot on Paleoproterozoic Re–Os age arrays – particularly so at low Re/Os – coincident with previously determined ages using Lu–Hf and Pb–Pb techniques. They may represent sulfide-poor varieties that did not suffer secondary sulfide addition and that may be best suited to yielding meaningful Re–Os ages.