Age, geochemistry and Sm-Nd isotopic signature of the 0.76 Ga Burin Group: Compositional equivalent of avalonian basement?

The ca. 760 Ma Burin Group occurs in a well-exposed 60 km long northeasterly trending belt on the Burin Peninsula in the Avalon Zone of the southeastern Newfoundland Appalachians. This lithologically distinctive, tectonically bounded succession is characterized by low grade massive and pillowed basalts, abundant gabbro and diabase dykes and sills, mafic pyroclastic and epiclastic rocks and stromatolitic-carbonate-bearing rocks. The Burin Group preserves the oldest known magmatic event in the Avalon terrane, but its tectonic evolution and its relationship to the voluminous 640-570 Ma arc-related magmatism that typifies Avalonia is uncertain. Geochemical analyses confirm that the basalts are predominantly low-K tholeiites. They are characterized by high ratios of LIL/HFS elements, and display depletion to slight enrichment in LREE. Sm-Nd isotopic data reveal that most basalts have juvenile compositions, with epsilon(Nd) values similar to contemporaneous depleted mantle, indicating that high LIL/HFS elemental ratios were probably due to coeval subduction which contaminated the mantle source. Other basalts have lower epsilon(Nd) values, and the negative correlation of epsilon(Nd) With La/Sm, together with a positive correlation of epsilon(Nd) with Sm-147/Nd-144 suggest that their isotopic signatures have been modified by a Mesoproterozoic older oceanic crust or mantle into which the Burin Group mafic magmas were emplaced. The isotopic signature of the Burin mafic rocks is similar to that inferred for the source of the main phase of Avalonian magmatism. These data, together with paleocontinental reconstructions for ca. 760 Ma, suggest that the Burin Group is a local representative of ensimatic arcs within the Panthalassa-type ocean that surrounded Rodinia, possibly as a far-field response to the breakup of Rodinia. Although its low metamorphic grade precludes it being the basement from which Avalonian magmas were extracted, the Burin Group may be representative of the geochemical and isotopic composition of that basement. (C) 2008 Elsevier B.V. All rights reserved.