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 main ly of 'A-type' peralkaline to slightly peraluminous, hypersolvus to su bsolvus granites with subordinate syenite, monzonite and diabase, plus consanguineous basalts and high-silica rhyolites. epsilon(Nd)(T) valu es from the suite range from -1.5 to +5.4; most granitoid components e xhibit positive epsilon(Nd)(T) values (+1.1 to +3.9). Granitoid initia l 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 p arts per thousand). Restricted Pb-207/Pb-204 variation is accompanied by significant dispersion of Pb-206/Pb-204 and Pb-208/Pb-204. Superfic ially, petrogenesis by either direct (via fractionation from basalt) o r indirect (via melting of juvenile crust) derivation from mantle sour ces appears plausible. Remelting of the granulitic protolith of Ordovi cian arc-type granitoids can be ruled out, because these rocks exhibit negative epsilon(Nd)(T) and a large range in Pb-207/Pb-204. Geochemic al and isotopic relationships are most compatible with remelting of hy bridized lithospheric mantle generated during arc-continent collision. A genetic link is suggested among collision-related delamination or s lab break-off events and emplacement of 'post-orogenic' granite suites . A-type granites may recycle previously subducted continental materia l, and help explain the mass balance noted for modern arcs. However, t hey need not represent net, new, crustal growth.