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Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous-Tertiary kimberlite intrusions have been used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both xenoliths and concentrates lie near a 35 mW/m2 conductive model at T <= 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to >= 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ~60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the deeper layer contains 15-20% harzburgite and 80-85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate that the shallow layer is more magnesian (Fo92-94) than the deeper layer (Fo91-92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections. The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2·6-2·7 Ga. The deeper layer is interpreted as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2·6 Ga; evidence includes a high proportion of the superdeep inclusion assemblage (ferropericlase-perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2·6 Ga post-tectonic granites. Proterozoic subduction from east and west may have modified the cratonic root, mainly by introduction of eclogites near its base. [ABSTRACT FROM PUBLISHER]

The Cretaceous Prairie Creek lamproites of southern Arkansas intrude Proterozoic crust near the boundary between the 1.5--1.3 Ga Granite-Rhyolite Province and the 1.3--1.0 Ga Grenville orogen. They carry xeno crysts and rare xenoliths derived from the subcontinental lithospheric mantle (SCLM) and the deep crust. U-Pb age dating of groundmass perovskite in the Prairie Creek lamproites gives a poorly constrained Cretaceous age. U-Pb dating and in situ Sr and Nd isotope data show that perovskite micro nodules in the Twin Knobs #2 lamprophyre are ca. 600 Ma old, and may represent samples of rift-related alkalic magmas derived from a juvenile mantle . A lithologic section constructed from the mantle-derived xeno crysts shows a moderately depleted SCLM that has experienced a high degree of melt-related metasomatism, especially in the depth range 150 to 140 km. In situ Re-Os analysis of sulfide grains in the xenoliths yields model ages ranging up to 3.4 Ga, with major peaks at 1.4--1.5 Ga and 200--300 Ma. Early Paleo protero zoic model ages appear to reflect mixing between residual Archean high-Os sulfides and later low-Os sulfide melts. These data suggest that the SCLM beneath the Prairie Creek area formed in Archean time, and has been progressively refertilized by a series of magmatic events, which appear to correlate in time with events in the overlying crust. The Archean SCLM sampled by the lamproites may represent the mantle root of the Sabine microcontinent, which lies mainly to the south of the lamproite field and is recognizable in seismic tomography as a feature with higher shear-wave velocity (Vs) (100--175 km depth). Seismic tomography also shows several blocks with high Vs beneath the Grenville province to the east, which may represent other microcontinental blocks. These findings suggest that the growth of individual continents is significantly affected by the accretion of older microcontinental blocks, and that the extent of early continental crust therefore may be greater than generally estimated. [ABSTRACT FROM AUTHOR]

The nature of tectonic processes on the early Earth is still controversial. The scarcity of high-pressure metamorphic rocks such as eclogite (the high-pressure equivalent of basalt) in Archean cratons has been used to argue that plate tectonics did not operate until Earth had cooled to a critical point, perhaps around the 2.5 Ga Archean-Proterozoic transition. However, eclogites occur as meter- to kilometer-sized lenses enclosed in Archean gneisses of the Belomorian Province of the Fennoscandian shield. Geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros, troctolites, and Fe-Ti oxide gabbros. Greenschist facies mineral parageneses are enclosed in prograde-zoned eclogite garnets, and peak metamorphic conditions define an apparent thermal gradient (12-15 °C/km), consistent with metamorphism in a warm Archean subduction zone. We show here that these eclogites represent the oldest known high-pressure metamorphic rocks. U-Pb dating and Hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2.87 Ga for the Uzkaya Salma eclogite; a 2.70 Ga age for the Shirokaya Salma eclogite is interpreted as the age of a granulite facies overprint. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9-1.8 Ga) orogeny. These new data imply that plate tectonic processes operated at least locally in late Mesoarchean time. The adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce Archean tonalite-trondhjemite-granodiorite-type magmas. [ABSTRACT FROM AUTHOR]