Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia
occurred during three time
intervals: Cambrian–early Ordovician, late Ordovician–early
Silurian and middle–late Devonian. In the
Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited
to the middle Ordovician.
Geochemical data show that most of these volcanic rocks are bimodal,
within-plate suites. Initial εNd signatures
range from +5.4 to −1.9 in the rhyolites and +6.8 to +2.7 in the
basalts, a difference attributable to the
absence or presence, respectively, of a significant crustal component.
The data and regional tectonic settings
of the Avalon and Meguma terranes suggest that the volcanism was generated
in three different within-plate
settings: (1) Cambrian–early Ordovician volcanism related to
thermal decay of late Proterozoic arc magmatism
during transtensional deformation; (2) middle Ordovician–early
Silurian volcanism during sinistral telescoping
between Laurentia and Gondwana where extensional bends in the
Appalachians produced rifting;
and (3) Devonian volcanism resulting from lithospheric delamination
during dextral transpression and telescoping.
In each setting, active faults served as conduits for the magmas. Nd
isotopic data indicate that the
source of the Palaeozoic felsic volcanic rocks is isotopically
indistinguishable beneath southern and northern
Nova Scotia and did not substantially change with time. This crustal
source appears to have separated from
the mantle during the Proterozoic, a conclusion consistent with the
hypothesis that the Palaeozoic rocks in
Nova Scotia were deposited upon a late Proterozoic oceanic–cratonic
volcanic arc terrane. The Nd data, when
combined with published faunal, palaeomagnetic and U–Pb
isotopic data, suggest that the Avalon Terrane
was peripheral to Gondwana off northwestern South America during
Neoproterozoic and early Palaeozoic times.