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Avalonia is the largest accreted crustal block in the Appalachian orogen and comprises a collection of late Neoproterozoic volcano-sedimentary sequences that are overlain by a Palaeozoic platformal sedimentary succession. Detrital zircons from the Conception Group are dominated by 570-620 Ma ages and contain a significant component generated by erosion of coeval igneous arc-volcanic rocks. Overlying samples from the Cuckold and Crown Hill formations are dominated by Neoproterozoic populations with ages between 600 and 650 Ma and are interpreted to be derived from the underlying calc-alkaline arc-plutonic rocks. Early Palaeozoic platform units are dominated by c. 620 Ma zircons with lesser Mesoproterozoic and Palaeoproterozoic zircons. The range of detrital zircon ages is inconsistent with a West African provenance and suggests that Avalonia originated along the Gondwanan margin of the Amazon craton. The influx of Mesoproterozoic and Palaeoproterozoic detritus in the Avalonian platform suggests a major change in tectonic regime. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean. The Redmans Formation is interpreted to represent the rift-drift transition of the Rheic Ocean, which imposes important constraints on the palaeotectonic evolution of Avalonia. Supplementary material: U-Pb isotopic data of LA-ICP-MS analysis of detrital zircons are available at

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The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the westernmost of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia.U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ± 0.8 Ma) and overlying Castine Volcanics (503.5 ± 2.5 Ma) are Middle Cambrian in age. Two tholeiitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava fl ows and as sills in both units, has depleted La and Ce ([La/Nd] N = 0.53-0.87) values, fl at heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantlenormalized diagrams. In contrast, type Tb-2 basalt, present only in the Castine Volcanics, has slightly enriched LREE ([La/Yb] N = 1.42-2.92) values and no Th or Ta anomalies. Both basalt types have strongly positive ε Nd (500) values (Tb-1 = +7.9-+8.6; Tb-2 = +5.6-+7.0) and relatively enriched Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.037-19.784; 207 Pb/ 204 Pb = 15.531-15.660; 208 Pb/ 204 Pb = 37.810-38.817). The basalts have compositions transitional between recent normal and enriched midocean-ridge basalt, and they were probably derived by partial melting of compositionally heterogeneous asthenospheric mantle. Two types of rhyolite also are present. Type R-1 rhyolite, which mostly occurs as tuffs interlayered with basalt in the Ellsworth Schist, is calc-alkaline and characterized by relatively low REE, Zr, and Hf contents, enriched LREE ([La/Yb] N ~3-6), positive Th and negative Ta anomalies, ε Nd (500) values near zero (+0.5 to −0.9), and relatively unradiogenic Pb isotope values ( 206 Pb/ 204 Pb = 18.845; 207 Pb/ 204 Pb = 15.625; 208 Pb/ 204 Pb = 38.626). The data suggest that R-1 rhyolite magma was likely derived by mixing of basalt with melts from a relatively depleted crustal source. Type R-2 rhyolite, which mostly occurs as lava fl ows and domes in the Castine volcanics, is tholeiitic and characterized by enriched REE with fl at patterns ([La/Yb] N = 1-2.5), moderate negative Eu anomalies (Eu/Eu* = 0.3-0.5), enriched Th, small negative Ta anomalies, and ε Nd (500) (+5.8-+7.5) and Pb isotope ( 206 Pb/ 204 Pb = 19.175-19.619; 207 Pb/ 204 Pb = 15.605-15.649; 208 Pb/ 204 Pb = 38.834-38.851) values that overlap those of the tholeiitic basalts. The data suggest that R-2 rhyolite magma was derived by the partial melting of hydrothermally...