Abstract

The Liscomb Complex comprises Late Devonian intrusive rocks (principally peraluminous granite) and medium- to high-grade metamorphic rocks (“gneisses”) that collectively are hosted by low-grade (greenschist facies) metasediments of the Cambro-Ordovician Meguma Group. The conventional view that these “gneisses” contain high-grade mineral assemblages and represent basement rocks has recently been challenged, and indeed, some of the rocks previously mapped as gneisses, particularly metapelites, have isotopic compositions resembling the Meguma Group. Amphibole-bearing enclaves in the Liscomb plutons, however, are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the Popes Harbour lamprophyre dyke, south of the Liscomb area. Metasedimentary enclaves with Meguma isotopic signatures can contain garnets with unzoned cores (implying high temperatures) that host high-grade minerals (prismatic sillimanite, spinel, and (or) corundum) and are enclosed by retrograde-zoned rims. These features are interpreted here as having formed during and following the attainment of peak temperatures related to Liscomb magmatism. The amphibole-bearing meta-igneous rocks described here contain cummingtonite or hornblendic amphibole and occur as enclaves in granodioritic to tonalitic plutons. They are mineralogically, texturally, and isotopically distinct from Meguma metasediments and at least some of the plutonic rocks that enclose them, so remain the most likely candidate for basement rocks in the Liscomb Complex.

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