Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians

Abstract The voluminous, bimodal, Silurian Topsails igneous suite consists mainly of ‘A-type’ peralkaline to slightly peraluminous, hypersohnis to subsolvus granites with subordinate syenite, onzonite and diabase, plus consanguineous basalts and highsilica rhyolites. ∈Nd(T) values from the suite range from −1.5 to +5.4; most granitoid components exhibit positive ∈Nd(T) values (+1.1 to +3.9). Granitoid initial 87Sr/86Sr and most δ18 O values are in the range expected for rocks derived from mantle-like protoliths (0.701–0.706 and +6 to +80/‰). Restricted 207Pb/204Pb variation is accompanied by significant dispersion of 206Pb/204Pb and 208Pb/204Pb. Superficially, petrogenesis by either direct (via fractionation from basalt) or indirect (via melting of juvenile crust) derivation from mantle sources appears plausible. Remelting of the granulitic protolith of Ordovician are-type granitoids can be ruled out, because these rocks exhibit negative ∈Nd(T) and a large range in 207Pb/204Pb. Geochemical and isotopic relationships are most compatible with remelting of hybridized lithospheric mantle generated during arc-continent collision. A genetic link is suggested among collision-related delamination or slab break-off events and emplacement of ‘post-erogenic’ granite suites. A-type granites may recycle previously subducted continental material, and help explain the mass balance noted for modern arcs. However, they need not represent net, new, crustal growth.