Journal of Petrology
Joseph. B. Whalen, George A. Jenner, Frederick J. Longstaffe, Francine Robert
and Clement Gariepy
ABSTRACT
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. eNd(T) values from the suite range from -1.5 to +
5.4; most granitoid components exhibit positive eNd(T) values (+1.1 to +3.9).
Granitoid initial 87Sr/86Sr and most d18O values are in the range expected for
rocks derived from mantle-like protoliths (0.701 to 0.706 and +6 to +8o/oo).
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 appear
plausible. Remelting of the granulitic protolith of Ordovician arc-type granitoids
can be ruled out, because these rocks exhibit negative eNd(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 `postorogenic' 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.
KEY WORDS:
A-type granites; juvenile crust; isotopes; Newfoundland
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