Geochemical and isotopic (O Nd, Pb and Sr) constraints on a type granite petrogenesis based on the topsails igneous suite, Newfoundland Appalachians

The voluminous, bimodal, Silurian Topsails igneous suite consists mainly of 'A-type' peralkaline to slightly peraluminous, hypersolvus to subsolvus granites with subordinate syenite, monzonite and diabase, plus consanguineous basalts and high-silica rhyolites. epsilon(Nd)(T) values from the suite range from -1.5 to +5.4; most granitoid components exhibit positive epsilon(Nd)(T) values (+1.1 to +3.9). Granitoid initial Sr-87/Sr-86 and most delta(18)O values are in the range expected for rocks derived from mantle-like protoliths (0.701-0.706 and +6 to +8 parts per thousand). Restricted Pb-207/Pb-204 variation is accompanied by significant dispersion of Pb-206/Pb-204 and Pb-208/Pb-204. 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 arc-type granitoids can be ruled out, because these rocks exhibit negative epsilon(Nd)(T) and a large range in Pb-207/Pb-204. 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-orogenic' 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.