Kimberlite dykes (age 600 Ma: Larsen and Rex, 1992) from the Archeaen craton and its border zone in West Greenland contain xenocrysts of disaggregated mantle
material and/or mantle xenoliths. The content of trace elements (Ni, Cu, Zn, Ga, Sr, Y, Zr) in the peridotitic garnet xenocrysts obtained by proton microprobe analysis provide important information about metamorphic processes in and the geochemistry of the lithospheric mantle in West Greenland. The data can also be use to construct the local paleogeotherm ("Garnet Geotherm": Ryan et al., 1995). When the informations from many single garnet grains are referred to the geotherm the result is a stratigraphic section through the lithospheric mantle at 600 Ma, with informations on rock type, metamorphic processes and temperature with depth.
The paleogeotherms defined by garnet concentrates from kimberlites from the Sukkertoppen area within the Archeaen craton and the Sarfartoq area (the Archeaen/Proterozoic border zone) follow a 36 mW/m2 and a 38 mW/m2 conductive model, respectively. The base of the depleted lithosphere, as defined by the upper T-limit and the deepest occurrence of Y-depleted garnets, lies at depths of 210-220 km in the Sukkertoppen area, and at depths of 200-210 km in the Sarfartoq area.
The lithostratigraphic column in boths areas is dominated by lherzolite with lesser amounts (lenses?) of harzburgite and wehrlite. In the Sukkertoppen area harzburgite constitutes up to 25 % of the lithosphere at depths of 110-130 km, increasing to 30 % at depths of 180-200 km. Wehrlite constitutes 5-10 % of the lithosphere at depths of 150-180 km. In the Sarfartoq area harzburgite constitutes up to 30 % at depths of 80-110 km and maximum 5 % further down through the stratigraphic column to the base of the lithosphere at 210 km. Wehrlite comprises < 5 % through the entire lithosphere.
The depth distribution of metasomatic processes shows, that in both sections the upper parts (down to 120 km) consisting of depleted lherzolites and harzburgites, are only little influenced by magmatic (high Zr, Y, TiO2) infiltration and phlogopite-related (high Zr, Zr/TiO2, low Y) metasomatism. In the depth range of 120-150 km a stronger influence of phlogopite-related metasomatism is seen, especially in the lithosphere beneath the Sarfartoq area. This type of metasomatism decreases towards the base of the lithosphere and is replaced by melt-related metasomatism, probably caused by rising asthenosphere derived melts.
The trace element content in garnet concentrates from kimberlites from the Sukkertoppen and Sarfartoq areas, and geothermobarometric results, show that the paleogeothermal conditions and the thickness of the lithosphere beneath the Archeaen craton and its border zone were very much identical at the time of the kimberlite intrusion. Distinct differences in the lithostratigraphic sections exist, but lherzolites and harzburgites from both areas are influenced by identical processes. However, the lithospheric mantle beneath Sarfartoq has undergone a much larger degree of phlogopite related metasomatism. The coeval Sarfartoq carbonatitic complex (Larsen and Rex, 1992) probably originated in this metasomatised mantle.
Larsen, L.M. & Rex, D.C., Lithos 28, 367-402 (1992).
Ryan, C.G., Griffin, W.L. & Pearson, N., J. Geophys. Res. submitted (1995).