Geochemistry, U-Pb geochronology and Nd-Hf isotopes of leucocratic dykes in the Cape Spencer area, southern New Brunswick, Canada: insights into the Alleghanian orogeny in the northern Appalachians

Abstract Contrary to the southern Appalachians, where Alleghanian magmatism is widespread and well documented, the expressions of magmatism in the Canadian Appalachians are limited. In this study, a suite of leucocratic dykes from the Cape Spencer area in southern New Brunswick, Canada, were investigated to determine the nature, timing and source of these magmas using zircon and monazite U-Pb geochronology, whole-rock geochemistry and Nd-Hf isotopes. An LA-ICP-MS U-Pb monazite Alleghanian age of 273.7 ± 1.3 Ma obtained for these dykes constitutes a new example of magmatism in the northern segment of the orogen, where significant strike-slip movement and reheating have been the primary markers of the Alleghanian Orogeny. These metaluminous leucocratic dykes are enriched in light rare elements, U and Th; depleted in high-field strength elements (HFSE; Nb, P, Ti); and have slight negative Europium anomalies [(Eu/Eu*) N = 0.72–0.95]. All the dykes samples have negative εNd(t) values (−9.76 to −5.7), negative εHf(t) values (−1.8 to −1.0) and Mesoproterozoic Nd depleted-model ages (T DM = 1371–1618 Ma). The geochemical and isotopic characteristics suggest that the dykes were formed by the partial melting of lower crust that assimilated Meguma metasedimentary rocks and/or Avalonian sedimentary rocks, following terminal subduction of the Rheic Ocean and thermal re-equilibration during the Alleghanian orogeny. The effects of the closure of the Rheic Ocean in the oblique collision between composite Laurentia and Gondwana were, to a certain extent, accommodated along the Minas Fault Zone, where magmatism and regional fluid flow were concentrated.