CHEMICAL AND ISOTOPIC COMPOSITION OF THE LOWER CRUST BENEATH THE MEGUMA LITHOTECTONIC ZONE, NOVA-SCOTIA - EVIDENCE FROM GRANULITE FACIES XENOLITHS

Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinct groups of xenoliths occur. Both groups contain quartz + feldspar + biotite + Fe - Ti oxides + sulfides. In addition, group A contains garnet + graphite +/- [aluminosilicates + spinel +/- sapphirine (hight Al2O3 sub-groups A1 and A2)] or [clinopyroxene + sphene + apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other ''undepleted'' granulite-facies rocks. Low P-T sedimentary assemblages of quartz-chlorite - clay minerals - calcite - albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO > 13 wt%) sub-group with high transition-metal contents, and low-MgO (MgO < 9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from epsilon-Nd(t) = -2.56 to -0.80 and Sr-87/Sr-86(t) = 0.7046 to 0.7182 for t = 370 Ma. For group B these values are + 1.45 to + 5.33 and 0.7028 to 0.7048, respectively. Model ages (T(CHUR)) are correspondingly low and are tightly constrained (544 +/- 52 Ma). These ''young'' ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (= Avalon), and an older allochthonous Meguma Group upper crust.