Geochemistry of metavolcanic rocks of the Ross Island and ingalls head formations, Grand Manan Island, New Brunswick

Abstract

Southeastern Grand Manan Island consists of explosive and effusive volcanic rocks of the Ross Island and Ingalls Head formations that include syn-eruptive sediments and hyaloclastites. Lavas are commonly amygdaloidal and/or microporphyritic with phenocrysts of feldspar and, more rarely, pyroxene or quartz set in a glassy to microcrystalline groundmass. They are compositionally similar to orogenic andesites with elevated TiO2 (up to 1.5%). The andesites are divisible in two groups that differ in CaO, Na2O, P2O5, Cr, and Sr contents, and in the enrichment of Ba relative to Rb and Th. Among the chemically similar andesite samples, those with higher MgO also contain higher amounts of incompatible elements. Unsually for such rocks, there is no evidence for a significant role of feldspar in the evolution from andesite (61% SiO2) to dacite (66% SiO2). A sample from a mafic dyke cutting the Ingalls Head Formation contains high FeO (>12% vs. ≤9% in the host rocks) and TiO2 (>2%). The incompatible element pattern of the dyke sample exhibits troughs at Nb-Ta and Sr similar to those shown by the andesite and dacite samples, but not at Ti. The geochemical characteristics of andesite and the time-space-compositional relationship of the dyke with the host andesite are considered to reflect the origin of these rocks by partial melting of hydrous mafic crust under moderate to low pressure (<8 kbar). The melts may have formed in equilibrium with olivine + orthopyroxene + clinopyroxene ± amphibole ± plagioclase ± apatite ± Fe-Ti oxide. Some of the volcanic rocks resemble late Cambrian to late Ordovician rocks of the New River belt on the northwestern margin of the Avalon composite terrane.