GEMOC ARC National Key Centre

THE LITHOSPHERE BENEATH THE LAC DE GRAS AREA, SLAVE CRATON, CANADA: A XENOLITH STUDY

Norman J. Pearson¹, William L. Griffin^1,2, Buddy J. Doyle³, Suzanne Y. O'Reilly¹,
Esmé  van Achterbergh¹ and Kevin Kivi³

1. GEMOC Macquarie,
2. CSIRO Exploration and Mining,
3. Kennecott Canada

The composition, structure and thermal state of the lithosphere beneath the Lac de Gras area in the Slave Craton have been determined from a suite of mantle-derived xenoliths.  The xenolith studies form the basic element of 4-D lithospheric mapping and are an essential complement to interpretation of the garnet xenocryst data (Griffin et al., 1998).  The xenoliths have been brought to the Earth's surface in several generations of kimberlites ranging in age from 47-75 Ma  (Davis and Kjarsgaard, 1997).  The majority of the xenoliths in this study come from kimberlite pipes DO-18, DO-27 and A154S and have been recovered during the crushing stage of processing of the kimberlite.

Several lithological groups have been recognised in the xenolith sample population: lherzolites (ol+opx+cpx+grt±crt); harzburgites (ol+opx+grt±crt); dunites (ol±grt±crt); wehrlites (ol+cpx+grt±crt); websterites (opx+grt±cpx±ol±crt); garnet clinopyroxenites (grt+cpx); eclogites (cpx+grt±rut±ky); granulites (plag+cpx+grt±opx). Garnet lherzolites and harzburgites are fine to coarse grained (<1mm to >1cm), with microstructures ranging from equigranular, porhyroclastic to mylonitic.  The absence of modal cpx is used to distinguish harzburgite from lherzolite, but the majority of harzburgite garnet compositions are lherzolitic (G9) and indicate coexistence with cpx.  Subcalcic garnets are abundant in the garnet cocnentrate (Griffin et al., 1998) and Boyd and Canil (1997) have analysed sub-calcic garnets in harzburgite xenoliths from the Grizzly Pipe, north west of A154.

The websterites and eclogites can be subdivided on the basis of mineral compositions: the websterites include a high-Cr group (grt Cr2O3 1.62-8.30 wt%) and a low-Cr group (grt Cr2O3 < 1.5 wt%); 2 types of eclogites are distinguished using the CaO content of garnet (CaO < 7 wt% and CaO 8.5-13 wt%).  Modal variations and mineral compositions indicate that gradations exist between the eclogites with low-Ca garnet and the low-Cr websterites (opx-eclogites).

Geothermobarometry on the peridotite and websterite xenoliths produces a P-T array with a non-uniform geothermal gradient.  At T < 900°C the P-T estimates fall near a 35 mWm-2 conductive model geotherm, whereas at T between 900 and 1250 °C the locus of P-T points shifts toward a 40 mWm-2 geotherm.  This 'stepped' geotherm is unlike the 'kinked' geotherm that characterises a number of cratonic xenolith suites (e.g., Lesotho). The offset in the xenolith paleogeotherm and gross compositional changes in the garnet concentrate (Griffin et al., 1998) defines two compositionally distinct layers, with a boundary at ~900°C.  Comparison of the  sheared high-T grt lherzolites with those from the Kaapvaal and Siberian cratons indicates a number of similarities that imply metasomatism by asthenosphere-derived melts. However, the occurrence of undeformed high-T (1200-1250°C) xenoliths requires a minimum lithosphere thickness of * 200 km.

Projection of T estimates for the eclogites to the geotherm places this group of xenoliths in the deeper layer.  A bimodal distribution of T for the 2 types of eclogites provides evidence for stratification of the deep layer, with the low-CaO grt eclogites concentrated in the upper part  and the high-CaO grt eclogites mainly occuring in the lower part.

High olivine/orthopyroxene ratios combined with high average olivine Fo contents in the peridotites in the shallow layer confirms the ultra-depleted nature of the shallow layer predicted from the garnet concentrate data (Griffin et al., 1998). These same features distinguish the peridotite xenoliths from other Archean xenoliths and imply that the mantle beneath the Lac de Gras area differs from the lithosphere beneath other Archean cratons.

References
Boyd, F.R. & Canil, D. 1997.  Peridotite xenoliths from the Slave Craton, Northwest Territories.  Abstracts Goldschmidt Conf., pp. 34-35.
Davis, W.J. & Kjarsgaard, B.A. 1997.  A Rb-Sr isochron age for a kimberlite from the recently discovered Lac de Gras field, Slave province, northwest Canada.  Jour. Geol. 105, pp. 503-509.
Griffin, W.L., Doyle, B.J., Ryan, C.J., Pearson, N.J., O'Reilly, S.Y., Davies, R., Kivi, K. and van Achterbergh, E., 1998.  Lithosphere Structure and Mantle Terranes:  Slave Craton, Canada.  Jour. Petrol., subm.