Regional isotope patterns of Cambro-Ordovician granitods from the Wilson Terrane
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The Nashoba Terrane is fault-bounded and lies between the Avalon Terrane immediately to the east and the Acadiandeformed Merrimack Belt (Kearsarge-Central Maine Synclinorium-KCMS) to the west. Recent U/Pb age determinations and geochemical data from metavolcanic and intrusive rocks within the terrane have provided a new perspective on the tectonics of this terrane and the role it played in the evolution of the eastern margin of the Appalachian orogen during the mid-Paleozoic. The Fish Brook Gneiss, previously interpreted as Precambrian basement, has yielded a Cambro-Ordovician U/ Pb abraded zircon crystallization age of 499 ± 6/-3 Ma. This date constrains the age of the metavolcanic and metasedimentary rocks of the terrane to the interval between late Cambrian
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Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal εHf(t) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and εHf(t) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and εHf(t) values, and also include early Mississippian zircons with highly evolved εHf(t) signatures.
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Late Devonian extinction
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Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c . 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia. Supplementary material: U–Pb and Lu–Hf data tables are available at www.geolsoc.org.uk/SUP18557 .
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Isotope Geochemistry
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During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian margin until c . 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen. Supplementary material: Details of sample coordinates, analytical procedure and U/Pb detrital zircon analytical data are available at www.geolsoc.org.uk/SUP18739 .
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Abraded zircons from the basal rhyolitic tuff member of the White Rock Formation, which disconformably overlies the Cambrian-Early Ordovician Meguma Group, have yielded a nearly concordant U-Pb age of 442+ or -4 Ma interpreted as the age of extrusion. This age straddles the approximately 443 Ma Ordovician-Silurian boundary. Abraded zircons from the Brenton granite lie on a chord with an upper intercept age of 439+4/-3 Ma, which is interpreted to be the age of intrusion, thus supporting its inferred subvolcanic nature. On the other hand, monazite from the Brenton pluton yielded a nearly concordant analysis with a ^207^ Pb/ ^206^ Pb age of 380+ or -3 Ma, which is interpreted to be the time of the low pressure, high temperature metamorphism. Using these new data, the following observations suggest that the Meguma and Avalon terranes were neighbors during the Silurian-Early Devonian: (1) both have latest Ordovician-earliest Silurian, bimodal, subaerial, alkalic-tholeiitic, rift-related, volcanic rocks with Nd signatures indicating a similar continental basement source; (2) both show a similar progression of depositional environments: subaerial in the earliest Silurian, progressively deeper-water marine strata in the Llandovery and Wenlock, switching to gradually shallowing marine environments in the Ludlow, and reverting to subaerial in the Pragian; and (3) both contain Rhenish-Bohemian Early Devonian fauna. Furthermore, the southeast to northwest transition from an offshore sandbar to a beach sand in the Silurian White Rock Formation suggests the presence of land to the north, now recognized as Avalonia. These conclusions support published suggestions that the Siluro-Devonian successions in the Meguma and Avalon terranes form part of the overstep sequence that extends across most of the northern Appalachians. Published data indicate that Avalonia was adjacent to Gondwana in the Neoproterozoic, that it separated from Gondwana in the Early Ordovician, and was accreted to eastern Laurentia in the Late Ordovician-Early Silurian. On the basis of the data and correlations presented here, we suggest that the Meguma Terrane travelled with Avalonia. This is consistent with the absence of a phase of deformation between the Cambro-Ordovician Meguma Group and the Silurian White Rock Formation.
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The Ordovician–Silurian meta-sedimentary rocks of the Southern Uplands–Down–Longford terrane were deposited at the Laurentian margin and structurally emplaced in an accretionary prism during subduction of Iapetus Ocean lithosphere. Within the terrane, a Central Belt consists of repeated structural tracts in which Ordovician–Silurian mudstone and sandstone (Moffat Shale Group) are overlain by Silurian greywacke (Gala Group). A Moffat Shale Group sandstone and Gala Group greywacke sandstones from Co. Monaghan, Ireland were sampled in order to examine, for the first time, detrital zircon spectra with a view to elucidating sediment provenance and tectonic evolution of this sector of the Laurentian margin. Detrital zircon ages from Gala Group greywacke sandstones include variable abundance of Precambrian zircons with ages typical of Laurentia, but are dominated by two Ordovician peaks at c. 462 Ma and c. 474 Ma. These Ordovician ages equate to magmatism in the local Laurentian margin in Ireland. The age spectra are distinct from those of equivalent rocks of the Southern Uplands of Scotland. There is no evidence from detrital zircon ages that a Gondwana-derived terrane had accreted to the Laurentian margin before or during sedimentation. Zircons from a Moffat Shale Group sandstone include a middle Cambrian component that may be derived from Ganderia. Supplementary material: The complete U–Pb data table is available at http://doi.org/10.6084/m9.figshare.c.3270848
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The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic–early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm.
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Laurentia
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Nd isotopic data are presented for rock units in four terranes within the traditional Avalon Zone of southern New Brunswick. Initial ε Nd values for igneous rocks within the Caledonia terrane range from -1.6 to +4.6, whereas values for sedimentary rocks range from -8.4 to +3.6. A granite within the Kingston terrane has ε Nd (438 Ma) = +4.0. Nd isotopic compositions for the Kingston and Caledonia terranes are similar to those of the Mira terrane in Cape Breton Island, the Antigonish Highlands of Nova Scotia, and plutonic rocks of eastern Newfoundland. Each of these regions may be a dismembered part of a single terrane, the Avalon terrane sensu stricto. Initial ε Nd values for rocks from the Brookville terrane range from -1.3 to +2.8. The Coverdale anorthosite within the Brookville terrane has a present day ε Nd value of -12.1 and a depleted mantle model age of 1.3 Ga, similar to Mesoproterozoic anorthosites in Laurentia. Clastic sedimentary rocks in the Green Head Group have ε Nd (750 Ma) values of -2.0 and -10.9. Viewed as a whole, the Brookville terrane is isotopically more evolved than the Caledonia terrane. Initial ε Nd values for rocks in the New River terrane range from -2.9 to 0.0. The Nd isotopic composition of the Brookville and New River terranes are thus similar to one another and have isotopic character similar to that of the Bras d'Or terrane of Cape Breton Island. It is suggested that all three regions belong to a single terrane (Bras d'Oria).
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