DETRITAL ZIRCON GEOCHRONOLOGY IN NORTHERN APPALACHIAN AVALONIAN TERRANES: INSIGHTS INTO BASIN DEVELOPMENT AND TERRANE JUXTAPOSITIONS

Avalonian terranes of the northern Appalachian orogen preserve a complex history of arc-related igneous activity and Ediacaran - Cambrian sedimentation. In Avalonian southeastern Cape Breton Island, redbeds of the Cambrian Bengal Road Formation that overlie ca. 620 Ma rhyolite of the East Bay Hills Group contain major populations of zircon grains with ages of ca. 570 – 620 Ma and ca. 650 – 690 Ma, as well as sparse older ages. These Cryogenian-Ediacaran ages are consistent with derivation from volcanic and associated plutonic units documented in the Stirling (ca. 670-680 Ma, East Bay Hills (ca. 620 Ma) and Fourchu (ca. 575 Ma) groups of the region, but the prominent peak at 590-610 Ma has not yet been matched by ages from now-exposed volcanic and plutonic units. Similarly, no rocks have been dated with igneous crystallization ages matching the 640-660 Ma peaks, although those ages form prominent peaks in some ca. 620 Ma plutonic units, where they are interpreted to represent inheritance.

Four samples from the overlying Cambrian Sgadan Lake Formation in MacCodrum Brook contain sparser populations of zircon ages in the range 600 – 650 Ma, but their largest populations of zircons have ages of 540 – 550 Ma, with smaller younger populations around 530 Ma and older at 580 Ma. The youngest population ca. 520 Ma. These young populations are similar to those in samples from the Bengal Road Formation elsewhere in southeastern Cape Breton Island in which the youngest ages are between 520 Ma and 530 Ma. These results show that Cambrian units have similar source areas and cannot be readily distinguished from each other by detrital zircon data alone.

Comparisons with detrital zircon signatures in Cambrian sedimentary units from Avalonia in Newfoundland, the Antigonish Highlands, and southern New Brunswick show wide variation in signatures, both in the range of Ediacaran-Cambrian ages and in the abundance of Mesoproterozoic-Archean ages. This absence of systematic variation further demonstrates that presence of older grains should not be used to interpret the position of Avalonia during the Cambrian or the timing of its rifting from Gondwana. If Ganderia and Avalonia were temporarily juxtaposed by ca. 550 Ma as proposed in recent tectonic models, then zircon derived from Ganderia may have contributed to the prominent zircon age peaks at ca. 550 Ma.