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The Coldbrook Group and related plutons in the Caledonian Highlands of southern New Brunswick contain voluminous late Ediacaran silicic rocks formed in a magmatic event not recognized in other parts of Avalonia in the northern Appalachian orogen. To better constrain the age and origin of these rocks, we used U–Pb zircon dating by laser ablation inductively coupled plasma mass spectrometry to check for older inherited zircon and obtain trace element data, followed by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-TIMS) to obtain precise dates. Four silicic samples were dated from the Coldbrook Group, one from the Bonnell Brook pluton, and, for comparison, a felsic lithic-crystal tuff sample from the older arc-related Broad River Group. Overlapping CA-TIMS dates of 551.57 ± 0.23, 551.38 ± 0.24, and 551.70 ± 0.20 Ma for samples from the lower, middle, and upper Coldbrook Group, respectively, and 551.71 ± 0.19 Ma for granite from the Bonnell Brook pluton show that these units crystallized in 760000 years or less, consistent with a super-eruption event. Rhyolite from the uppermost unit of the Coldbrook Group yielded a younger date of 549.18 ± 0.09 Ma, but the large extent of that unit is consistent with the possibility of a second younger super-eruption. The felsic lithic-crystal tuff sample from the Broad River Group yielded a date of 615.48 ± 0.16 Ma, consistent with previously published dates from that group and associated plutons. Differences in zircon chemistry between the Broad River Group sample and the late Ediacaran samples are consistent with the contrasting subduction-related vs within-plate extensional tectonic settings as suggested by previous studies of whole-rock petrological characteristics of the two age groups. [ABSTRACT FROM AUTHOR]

The MacCodrum Formation is a classical "lower" Cambrian unit in southeastern Cape Breton Island stratigraphy, described since the 1800s. The age of this formation and its correlation with other Avalonian Cambrian units in eastern Newfoundland and southern New Brunswick have remained uncertain through numerous revisions. Here we present U-Pb CA-TIMS ages from an ash bed in the basal part of the MacCodrum Formation in its type-section on MacCodrum Brook that fix the maximum time of deposition at 531.86 ± 0.34 Ma. Organic-walled microfossils sampled throughout the MacCodrum Formation type-section yield acritarch taxa identifying the Asteridium–Comasphaerdium Zone, whereas the first acritarchs of the Skiagia–Fimbriaglomerella Zone appear in the overlying Canoe Brook Formation in other sections. The radiometric age and acritarch zonation place the MacCodrum Formation in the upper Fortunian, Cambrian Stage 2. Among trace fossils in the MacCodrum Formation, the meandering trace fossil Didymaulichnus dailyi comb. nov. is of particular note and morphologically identical to the type material from the lower part of the Ratcliffe Brook Formation in New Brunswick. The new radiometric and biostratigraphic data presented here provide the first firm constraints on the age of the MacCodrum Formation and enable more precise correlation with sections in southern New Brunswick and eastern Newfoundland. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ganderia's provenance in Amazonia. • Cambrian rifting and scissor-like opening of Rheic Ocean in Penobscot backarc region. • Complex Cambrian to Tremadocian interaction with East Avalonia along the margin of West Gondwana. • Diachronous accretion to Laurentia over time span of ca. 50 my. • Tectonic elements show marked non-cylindricity. West and East Ganderia in the northern Appalachians and Caledonides, respectively, represent a Gondwanan superterrane situated along the Tornquist margin of Amazonia prior to Furongian drift into the Iapetus Ocean, which opened the Rheic Ocean from west to east. The ocean-facing Penobscot arc-backarc system was established by 515 Ma in West Ganderia. A correlative arc formed at ca. 480 Ma in East Ganderia. In West Ganderia, the Tremadocian Penobscottian orogeny involved closure of the Penobscot backarc basin. Tremadocian Monian tectonism in East Ganderia was mainly related to oblique accretion to East Avalonia and the Megumian Cymru terrane. Penobscottian and late Floian Monian orogenesis led to termination of Early Ordovician arc magmatism, probably due to shallow subduction of buoyant oceanic lithosphere. Early to Middle Ordovician arc-backarc systems were erected on Penobscottian-Monian modified West and East Ganderia. The active edge of West Ganderia accreted diachronously to peri -Laurentia between 475 Ma and 455 Ma, followed by Wenlock to Ludlow Salinic accretion of the inboard Gander margin through closure of the intervening backarc basin. In the Caledonides, East Ganderia and East Avalonia accreted to Laurentia during the correlative Wenlock Scandian orogeny. The Ordovician to Silurian tectonic evolution of Ganderia was markedly non-cylindrical with pronounced partitioning of Salinic-Scandian convergence. Pridoli to Lochkovian closure of the Acadian seaway in the northern Appalachians led to Acadian accretion of West Avalonia to composite Laurentia. Shallow Early Devonian underthrusting of West and East Avalonia beneath Laurentia produced widespread Acadian tectonism and voluminous Early Devonian Acadian magmatism. The Appalachian Meguma terrane formed part of Megumia, which probably formed originally adjacent to East Avalonia and West Africa. The Meguma terrane accreted dextrally to Laurentia during and after the late Emsian to Famennian Neoacadian orogeny, mainly driven by outboard subduction of the Rheic Ocean. No correlative terrane docking took place in the Caledonides. [ABSTRACT FROM AUTHOR]

40Ar/39Ar in situ UV laser ablation of white mica, Rb–Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by 40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb–Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle–Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The 40Ar/39Ar spot age spectra also display relict age peaks at 477–465 Ma, 439 Ma and 420–428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician–Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean. [ABSTRACT FROM PUBLISHER]

To better understand geodynamic processes related to the assembly of various belts of volcanic–sedimentary rocks in the Avalonian Mira terrane of SE Cape Breton Island (protolith ages 680–560 Ma) we investigated metamorphic processes in 20 white-mica-bearing mafic and felsic metavolcanic rocks. The felsic metavolcanic rocks are foliated and partly sheared with blastoporphyritic relict fabric and contain the assemblage phengite–epidote–chlorite–albite–K-feldspar–quartz–titanite ± stilpnomelane ± calcite ± ilmenite. In contrast, many mafic metavolcanic rocks are relatively undeformed and display relict porphyritic and amygdaloidal textures. They contain the assemblages epidote–chlorite–albite–quartz–titanite ± phengite ± pumpellyite ± prehnite ± calcite ± K-feldspar and actinolite–epidote–chlorite–albite–quartz–titanite ± phengite ± calcite ± K-feldspar. Heterogeneous metamorphic overprinting is indicated by local relicts of magmatic clinopyroxene, magnetite and plagioclase. Metamorphic minerals formed by local precipitation in clusters and are due to continuous nucleation of very low-grade phases during pulses of variably pervasive fluids, which were released during intense dehydration at 250–300°C. Nucleaction rate dominated over growth rate at these conditions. Potassic white mica in both mafic and felsic rocks is mostly phengite with a wide compositional range (3·11–3·41 Si a.p.f.u.). Maximum Si contents are typically between 3·30 and 3·41 a.p.f.u. P–T pseudosections were calculated for the range 200–400°C and 1–7 kbar. The peak metamorphic assemblages occupy P–T fields consistent with the position of isopleths for corresponding maximum Si contents in white mica. Peak P–T conditions in the Mira terrane samples lie within a narrow range of 3·5 ± 0·4 kbar and 280 ± 30°C in samples representing all of the assembled volcanic–sedimentary belts. The derived peak metamorphic conditions suggest syn-collisional burial to 11–14 km depth and a low metamorphic geotherm of 20–25°C km–1. Under these conditions subsequent strike-slip deformation is attributed to the final assembly of magmatic arc slices to form the crust of the Mira terrane. [ABSTRACT FROM PUBLISHER]

Abstract: Cashes Ledge igneous suite in the central Gulf of Maine is represented by 10 granitic and two felsic tuff samples collected from bedrock outcrops using the submersible Alvin in 1971–1972 and archived at the Woods Hole Oceanographic Institute. Laser ablation ICP-MS analyses of zircon grains yielded crystallization ages of 414.9±1.1Ma and 399.7±1.5Ma for two alkali feldspar granite samples, 407.0±1.9Ma for a syenogranite sample, and 384.4±2.3Ma and 383.9±1.6Ma for two felsic tuff samples. The samples contain iron-rich mafic minerals, including aegirine-augite, grunerite/ferroedenite, and annite. Most of the samples are alkaline to slightly peralkaline, with high concentrations of SiO2, Y, Zr, Nb, and REE, strong negative Eu anomalies, and positive epsilon Nd values (1.8 to 3.7). The suite resembles part of a belt of similar Silurian–Devonian rocks with ages between 426 and 370Ma now recognized in the central part of Avalonia in southeastern New England. They formed in a long-lived, likely extensional regime linked to subduction and subsequent complex transcurrent motions among Ganderia, Avalonia, and Meguma, culminating in the closure of the Rheic Ocean. [Copyright &y& Elsevier]