U–PB zircon geochronologyof proterozoic and paleozoic rocks, North Islesboro, coastal Maine (USA): Links to West Africa and Penobscottianorogenesis in Southeastern Ganderia?

Abstract

Within the Ganderian inlierof Penobscot Bay, coastal Maine, the Islesboro fault blockoccupies a central position between the St. Croix terraneof continental affinity and, to the east, the Ellsworth terraneofoceanic affinity. New field, petrographic, geochemical, and U–Pb LA-ICP-MS geochronological dataon detrital and magmatic zircon grains constrain the provenance and transfer historyof these terranes from Gondwana to the Appalachian marginof Laurentia.on North Islesboro, the Coombs Limestone and Hutchins Island Quartzite (new name), intruded by E-MORB amphibolite, constitute a newly recognized local inlierof Proterozoic basement. Together with the nearby Seven Hundred Acre Island Formation, these mature, carbonate-rich strata record depositionon a low-latitude passive margin. Abundant detrital zircon grains in the Hutchins Island Quartzite, allolder than ca. 1.8 Ga, have a predominant population at ca. 2.0 Ga and a small peak between ca. 2.8 Ga and 2.4 Ga, an age spectrum strikingly similar to thoseof both the Paleoproterozoic Taghdout Quartzite in Morocco,on the West African craton, and basement rocks from Georges Bank,offshore Massachusetts. Theoverlying Neoproterozoic–Cambrian Islesboro Formation records a second periodof extension (interstratified EMORB greenstone) synchronous with accumulationof interbedded siliciclastic and carbonate sediment, prior to recumbent folding. At the baseof the moderately deformed Turtle Head Cove (new name) cover sequence, immature greywacke has a youngest zircon populationof ca. 515 Ma, large late Neoproterozoic populations (ca. 624 Ma and 678 Ma), a small peak at 1.2 Ga, a moderate numberof ca. 1.5 Ga to 2.0 Ga grains, and a few Late Archean grains. Compared with many similar Ganderian age spectra reported from Vermont to New Brunswick, which are all consistent with a source in either the Amazonianor West African cratons, this new age spectrum most closely resembles those from quartzites in the Grand Manaan and Brookville terranesof coastal New Brunswick. Significantly, exotic blocks lithologically indistinguishable from Proterozoic strataon Islesborooccur in the St. Croix terrane within a Lowerordovician black shale mélange at the baseof the Penobscot Formation, suggesting that the St. Croix terrane, Islesboro block, and Ellsworth terrane were initially juxtaposed by Penobscottian thrusting prior to the Middleordovician. Subsequently, the Islesboro block was isolated between the bounding post-Silurian, pre-Late Devonian Turtle Head and Penobscot Bay dextral strike-slip faults. Along the North Islesboro fault, a fault-bounded lensof foliated pyritic felsic volcanic and volcaniclastic rock, dated at ca. 372 Ma and containing Devonian to Archean detrital zircons, records late Paleozoic deformation recognized previously in coastal New Brunswick but not in Penobscot Bay.