Abstract

Volcanic groups of the Central Mobile Belt of the Newfoundland Appalachians have previously been subdivided into "early" and "late" arc sequences, separated in time by a quiescent Caradocian stage defined in some areas by fossil-bearing black shales.New U–Pb zircon ages of graphic and 473 ± 2 Ma for rhyolites of the Buchans and Roberts Arm groups, respectively, show them to be correlative early Ordovician sequences. These ages serve to refute both the previous Rb–Sr whole-rock isochron ages of 447 Ma and the idea that these groups were "late arc" sequences. These new ages corroborate evidence from late Arenig – early Llanvirn conodonts in the Buchans Group and calibrate this fossil occurrence.A new U–Pb zircon age of 479 ± 3 Ma from plagiogranite of the Mansfield Cove Complex immediately west of the Roberts Arm Group shows that this plutonic body is only slightly older than the adjacent volcanic rocks and not Hadrynian as previously supposed. Local field relationships suggest this body may represent part of a disrupted ophiolite. It is coeval with the ophiolitic Annieopsquotch Complex along a strike to the south and may form part of a belt of rocks derived from early Ordovician ocean floor that is discontinuously exposed along the western boundary of the Buchans – Roberts Arm Belt.Zircons from rhyolite at the northeast termination of the Tulks Hill volcanics, part of the extensive Victoria Lake Group, give an age of graphic. This dated sequence contains limestone previously dated as Llanvirn–Llandeilo by conodonts. This part of the group is therefore younger than the Buchans Group, and the designations "early" and "late" arc are not appropriate. The thrusting that juxtaposed these groups is no longer constrained to be of Silurian age but could have been middle to late Ordovician. Precambrian zircons included in the Victoria Lake Group rhyolite could have been incorporated from associated sedimentary rocks and suggest that the group may have formed in a tectonic setting transitional between oceanic and continental and received detritus from several sources.

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