Avalonia Research Articles

Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

Abstract Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of ∼5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation–inductively coupled plasma–single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or ductilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370–360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegmatites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370–355 Ma in age. All metapelitic paragneisses contain foliations and lineations that are synkinematic with retrograde garnet + K-feldspar → sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360–330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400–370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375–370 Ma, leading to advection of heat to the lower continental crust and attendant magmatic and metamorphic responses over the time span 370–355 Ma.

Probing the Structure of the Crust and Mantle Lithosphere beneath the Southern New England Appalachians via the SEISConn Deployment

AbstractThe eastern margin of North America has been affected by a range of fundamental tectonic processes in the geologic past. Major events include the Paleozoic Appalachian orogeny, which culminated in the formation of the supercontinent Pangea, and the breakup of Pangea during the Mesozoic. The southern New England Appalachians exhibit a particularly rich set of geologic and tectonic structures that reflect multiple episodes of subduction and terrane accretion, as well as subsequent continental breakup. It remains poorly known, however, to what extent structures at depth in the crust and lithospheric mantle reflect these processes, and how they relate to the geological architecture at the surface. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) was a deployment of 15 broadband seismometers in a dense linear array across northern Connecticut. The array traversed a number of major tectonic boundaries, sampling across the Laurentian margin in its western portion to the Avalonian terrane at its eastern end. It also crossed the Hartford rift basin in the central portion of the state. The SEISConn stations operated between 2015 and 2019; data from the experiment are archived at the Incorporated Research Institutions for Seismology Data Management Center and will be publicly available beginning in 2021. A suite of imaging techniques is being applied to SEISConn data, with the goal of providing a detailed view of the crust and mantle lithosphere (including discontinuities, seismic velocities, and seismic anisotropy) beneath the southern New England Appalachians. Results from these analyses will inform a host of fundamental scientific questions about the structural evolution of orogens, the processes involved in continental rifting, and the nature of crustal and mantle lithospheric deformation during subduction, terrane accretion, and continental breakup.

Formation of Anorthositic Rocks within the Blair River Inlier of Northern Cape Breton Island, Nova Scotia (Canada)

Abstract Rocks from the Blair River inlier of Northern Cape Breton Island (Nova Scotia, Canada) have been correlated with either the Grenville basement of eastern Laurentia or the accreted Avalon terrane. Additional zircon U-Pb dates of spatially associated anorthositic dykes (425.1±2.2 Ma) and a metagabbro (423.8±2.5 Ma) from the Fox Back Ridge intrusion of the Blair River inlier reveal Late Silurian emplacement ages. Their contemporaneity suggests that they may be members of a larger intrusive complex. The anorthositic rocks have high Eu/Eu∗ values (>2.5), and bulk compositions are similar to the mineral compositions of labradorite (An50-70) and andesine (An30-50). The metagabbro is compositionally similar to alkali basalt and does not seem to have been affected by crustal contamination (Nb/U>24; Th/NbPM≤1.1) although it was metamorphosed. The high Tb/YbN (1.8-1.9) ratios suggest that the parental magma of the metagabbro was derived from a garnet-bearing peridotite. Fractional crystallization and mass balance calculations indicate that the anorthositic rocks can be derived by mineral accumulation from a mafic parental magma similar in composition to the metagabbro of this study. The Late Silurian ages suggest that the rocks were emplaced into the Avalon terrane after the closure of the Iapetus Ocean but before Early Devonian (415-410 Ma) sinistral transpression.

Avalonian arc-to-platform transition in southeastern New England: U-Pb geochronology and stratigraphy of Ediacaran Cambridge “argillite,” Boston Basin, Massachusetts, USA

High-precision CA-TIMS 206Pb/238U zircon dates clarify the age and tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by ∼50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 ± 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 ± 0.20 Ma ash bed from the Mystic Quarry in Somerville, Massachusetts lies near the top of an approximately 5350 m thick, dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960 tunnels and from mapping logs obtained from tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 ± 0.16 Ma aplite sill intruding argillite ∼800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing strata of the lower Cambrian Weymouth Formation located south of Boston that overlies the Cambridge Formation without obvious break in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous after 584 Ma, the depositional interval would exceed 40 million years. An estimated ∼20 Ma depositional hiatus seems more likely because the base of the Cambridge Formation appears to define a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595 to 584 Ma Roxbury Conglomerate and the 597 to 593 Ma Lynn-Mattapan Volcanic Complex. Cambridge deposition set in once arc activity in more northerly “West” Avalonian terranes extending through Atlantic Canada to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic volcanic textures and thin basaltic flows with soft sediment contacts are present in argillite of the City Tunnel Extension, and whole rock major element and REE compositions reveal mixed terrigenous and volcanic components deposited under marine conditions throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560 to 550 Ma Coldbrook Group in New Brunswick9s Caledonia terrane.

A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales

Fossils of trilobites and hyolithids were first identified in the Llanberis Slate of Penrhyn Quarry, Bethesda, North Wales, in 1888, and were reviewed in 1950, leading to the assignment of an early Cambrian (Stage 3, series 2) age. Recent collections from the Upper Green Slate Member of the Llanberis Slate include organisms associated with Burgess-Shale-type Konservat Lagerstätten. Despite low - grade metamorphism of the deposit, a substantial diversity of taxa is recognisable, including sponges, non - trilobite arthropods, vetulicolians, putative sipuncula and other worms. The value of Lagerstätten is the insight they provide into the composition of infaunal and epifaunal communities, including weakly - sclerotised organisms whose identity may only be inferred where the taphonomy is restricted to biomineralised elements or ichnofossils. Exceptional preservation from Cambrian localities including the Burgess Shale of British Columbia and the Maotianshan Shale of Southern China, have provided a benchmark against which formerly unidentified material can be assessed. This fauna is now known to occur World - wide, but good examples from the Avalonian terrane, comprising Southern Britain and Eastern Canada in Cambrian times, have not previously been identified.This is the first record of a Burgess Shale - type fauna in the Cambrian strata of Wales, where it represents the oldest comprehensive early Cambrian biota from the historical type area on which the Cambrian System was originally based. Many of the organisms described also represent the first records from North Wales, and, in an international context, it is also the first record of a Cambrian Burgess Shale - type assemblage from Avalonia.

Crustal evolution of peri-Gondwana crust into present day Europe: The Serbo-Macedonian and Rhodope massifs as a case study

The continental crust of Europe west of the Trans-European Suture Zone (TESZ), is mostly a collage of Peri-Gondwana Cadomian and Avalonian terranes that were formed on the late Neoproterozoic active margins of the supercontinent, rifted from it during consecutive opening of oceanic basins, and variably involved in Ordovician, Carboniferous (Variscan), and Cretaceous-Eocene (Alpine) orogenies. While in many localities it is possible to study isolated portions of the European geologic history, few sites can hold evidence for the entire record in a relatively small area. The Balkan terranes of the Serbo-Macedonian (SMM) and the Rhodope massifs, which contain Neoproterozoic Cadomian remnant basement, and were involved in multiple Phanerozoic phases of magmatism and metamorphism, provide an ideal locality in which we can study the entire crustal evolutionary history of Europe (west of the TESZ). Here we present new zircon U-Pb-Hf and rutile UPb data from basement rocks of the SMM and Rhodope as well as of beach placers from mouths of the large rivers draining the Balkan towards the Aegean Sea (Strymon, Nestos and Evros Rivers). Basement igneous rocks of the SMM and Circum-Rhodope were dated to the late Neoproterozoic (588 Ma), middle Ordovician (ca. 460 Ma), Carboniferous (300 Ma), Permo-Triassic (254 Ma), Paleocene (65 Ma) and Miocene (ca. 23 Ma), demonstrating the episodic nature of crustal evolution in this terrane assemblage. Hf-in zircon shows these intrusions evolve from negative εHf(t) values in the Ordovician towards positive values in the Paleocene. Zircons in beach placer samples also show distinct age peaks that follow the pulses of igneous activity in the source terranes, with the addition of Jurassic (ca. 150 Ma) zircons. The UPb system in rutiles from the beach placer samples was mostly reset, and records the final metamorphic phase during Alpine orogeny (35 Ma), with some rutiles preserving Eo-Alpine (80 Ma) ages. In contrast to the igneous samples, detrital zircons from the beach placer samples follow a trend bound by evolution lines leading to Hf-TDM model ages between 0.6 and 1.6 Ga. As a whole since the late Neoproterozoic the Balkan crust was the subject of repeated reworking in a series of magmatic/orogenic events. Some extent of juvenile igneous addition on this evolving Cadomian substrate brings about the shallow rise in εHf(t) values we observe in the igneous basement rocks. The end result observed in the Balkan is of a continental crust comprising a Neoproterozoic peri-Gondwana Cadomian core, reworked in multiple episodes of Phanerozoic orogenies, and augmented to some extent by variable mantle derived input. We argue that this mode of crustal origin and evolution is also applicable to many peri-Gondwana terranes of Western Europe.

Derivation of the Early Carboniferous Wedgeport pluton by crystal fractionation of a mafic parental magma: a rare case of an A-type granite within the Meguma terrane (Nova Scotia, Canada)

AbstractThe Cambrian–Ordovician metasedimentary rocks of the Meguma terrane (Canadian Appalachians) were extensively intruded by silicic plutons during Middle Devonian to Early Carboniferous times. Syn-plutonic but volumetrically minor mafic-ultramafic intrusions were also emplaced. In most localities, the silicic plutons and mafic-ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources. The Attwoods Brook gabbronorite of SW Nova Scotia yielded an in situ zircon weighted-mean 206Pb–238U age of 357.9 ± 3.3 Ma that is within the uncertainty of the age of the neighbouring Wedgeport pluton (357 ± 1 Ma). The Wedgeport pluton is a rare example of a mantle-derived, peraluminous A-type granite within the Meguma terrane. The similar ages and Nd isotopes of the Attwoods Brook gabbronorite (εNd(t) = +1.1 to +4.0) and Wedgeport pluton (εNd(t) = +2.1 to +3.3) suggest the two intrusions are petrogenetically related. Fractional crystallization modelling demonstrates that a parental magma similar to the Attwoods Brook gabbronorite can produce residual silicic liquids that resemble the granites of the Wedgeport pluton, indicating that they could be members of the same intrusive complex. The emplacement of the gabbronorite and Wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the Maritimes Basin that produced the Fountain Lake continental flood basalts and A-type granites of the Cobequid Highlands within the Avalon terrane. It is possible that the Early Carboniferous rocks of SW Nova Scotia are related to the rifted-related magmatism within the Maritimes Basin.

Constraining the geotherm beneath the British Isles from Bayesian inversion of Curie depth: integrated modelling of magnetic, geothermal, and seismic data

Abstract. Curie depth offers a valuable constraint on the thermal structure of the lithosphere, based on its interpretation as the depth to 580 ∘C, but current methods underestimate the range of uncertainty. We formulate the estimation of Curie depth within a Bayesian framework to quantify its uncertainty across the British Isles. Uncertainty increases exponentially with Curie depth but this can be moderated by increasing the size of the spatial window taken from the magnetic anomaly. The choice of window size needed to resolve the magnetic thickness is often ambiguous but, based on our chosen spectral method, we determine that significant gains in precision can be obtained with window sizes 15–30 times larger than the deepest magnetic source. Our Curie depth map of the British Isles includes a combination of window sizes: smaller windows are used where the magnetic base is shallow to resolve small-scale features, and larger window sizes are used where the magnetic base is deep in order to improve precision. On average, the Curie depth increases from Laurentian crust (22.2±5.3 km) to Avalonian crust (31.2±9.2 km). The temperature distribution in the crust, and associated uncertainty, was simulated from the ensemble of Curie depth realizations assigned to a lower thermal boundary condition of a crustal model (sedimentary thickness, Moho depth, heat production, thermal conductivity), constructed from various geophysical and geochemical datasets. The uncertainty in the simulated heat flow field substantially increases from ±10 mW m−2 for shallow Curie depths at ∼15 km to ±80 mW m−2 for Curie depths >40 km. Surface heat flow observations are concordant with the simulated heat flow field except in regions that contain igneous bodies. Heat flow data within large batholiths in the British Isles exceed the simulated heat flow by ∼25 mW m−2 as a result of their high rates of heat production (4–6 µW m−3). Conversely, heat refraction around thermally resistive mafic volcanics and thick sedimentary layers induce a negative heat flow misfit of a similar magnitude. A northward thinning of the lithosphere is supported by shallower Curie depths on the northern side of the Iapetus Suture, which separates Laurentian and Avalonian terranes. Cenozoic volcanism in Northern Britain and Ireland has previously been attributed to a lateral branch of the proto-Icelandic mantle plume. Our results show that high surface heat flow (>90 mW m−2) and shallow Curie depth (∼15 km) occur within the same region, which supports the hypothesis that lithospheric thinning occurred due to the influence of a mantle plume. The fact that the uncertainty is only ±3–8 km in this region demonstrates that Curie depths are more reliable in hotter regions of the crust where the magnetic base is shallow.

Probabilistic Surface Heat Flow Estimates Assimilating Paleoclimate History: New Implications for the Thermochemical Structure of Ireland

AbstractRegions where surface temperature has increased since past glaciation events, such as Ireland, underestimate the heat output of the Earth unless paleoclimate corrections are applied. We apply probabilistic techniques to quantify the uncertainty of 22 paleoclimate‐corrected heat flow estimates in Ireland, which assimilate multiple surface temperature histories associated with 130 ka of glacial oscillation in the British Isles. Heat flow values increase by ∼15 mW/m2 after a paleoclimate correction and provide new insights into the thermochemical structure of the lithosphere. The heat flow regime is broadly delineated by the Iapetus Suture Zone that separates Laurentian to the north and Avalonian terranes to the south (mean surface heat flow of 73 ± 14 and 65 ± 14 mW/m2, respectively). The degree to which heat‐producing elements are partitioned into the uppermost crust is described by the differentiation index of a heat flow province. From Bayesian inversion, we determine that radiogenic elements are substantially more differentiated in the uppermost crust of Laurentia (DI=2.8 ± 1.4) than Avalonia (DI=1.5 ± 1.3), despite a moderately enriched lower crust (0.8 ± 0.3 μW/m3). This is facilitated by a thin yet highly radiogenic layer in the uppermost crust of Laurentia (3.9 ± 1.8 μW/m3). Extrapolating these results across the British Isles and Newfoundland suggests that heat‐producing elements have been more successfully reworked into the upper crust to the north of the Iapetus Suture Zone during continental accretion between Laurentia and Avalonia.

Post-accretionary exhumation of the Meguma terrane relative to the Avalon terrane in the Canadian Appalachians

The accretion of the Avalon and Meguma terranes to the Laurentian margin was a major event in the development of the Appalachian orogen. The Minas Fault Zone delineates the boundary between these terranes. A strongly lineated and foliated granite intrusion (Kelly Brook pluton) exposed along the West River St. Mary's Fault intruded into Meguma Supergroup metasedimentary rocks. Well-developed C-S fabrics characterize the pluton with muscovite aligned in the S fabric, and a shallow (~10°) westerly plunging quartz stretching lineation. New U-Pb (zircon, LA-ICP-MS) geochronological data yield a crystallization age of 375.0 ± 4.6 Ma for the Kelly Brook pluton that is indistinguishable from other Late-Devonian intrusive rocks in the Meguma terrane. Petrographic data for muscovite indicate that it grew prior to deformation. A 100% plateau 40Ar/39Ar age of 369.0 ± 1.2 Ma on a single muscovite grain constrains the time of cooling of the granite to approximately 450 to 420 °C. Apatite grains also demonstrate pre- to syn-deformational fabrics and yield a U-Pb (LA-ICP-MS) age of 361.2 ± 5.6 Ma indicating the temperature of the granite was still above 350 °C at this time. Earliest Tournaisian fossils in the unconformably overlying Horton Group strata indicate that the granite was exposed by ca. 359 Ma. Exhumation rates of the Kelly Brook pluton and the Meguma terrane from approximately 9.5–13.3 km depth from ca. 375 Ma to surface exposure at ca. 359 Ma are 0.06 to 0.08 cm yr−1. Estimates for the Upper Devonian regional geothermal gradient in the northern Meguma terrane are high between 39.5 °C km−1 and 67.8 °C km−1, gradients that are common in extensional tectonic environments such as the Basin and Range province. An apatite fission-track age of 215 ± 14 Ma indicates Mesozoic reburial and reheating of the granite prior to opening of the Atlantic Ocean.

The Pocologan metamorphic suite of southern New Brunswick, Canada: New constraints on age and conditions of medium- to high-pressure metamorphism on the Ganderian margin of the Rheic Ocean

Geologically diverse fault-bounded crustal slices form the present-day southern margin of Ganderia in southern New Brunswick. One such slice, the Pocologan metamorphic suite (PMS), was previously shown to contain high-pressure metamorphic rocks and interpreted as part of the forearc of the adjacent Silurian Kingston arc terrane. In this study, the protolith age of metasedimentary rocks in the PMS has been constrained by U-Pb dating of detrital zircon to less than about 470 Ma. The age spectrum is dominated by 500 to 600 Ma ages and sparse older ages, typical of other Lower Paleozoic sedimentary components of Ganderia. An orthogneiss sample yielded a U-Pb concordia age of 433.5 ± 0.7 (2σ) Ma, interpreted as the igneous crystallization age and further supporting a link to the Kingston arc. Electron-microprobe dating of monazite from mica-schist samples yielded a mean age of 388.3 ± 2.8 (2σ) Ma, an older population at about 417 Ma, and older detrital grains at 450 Ma. Pseudosection modeling and quartz-in-garnet barometry showed that garnet cores in these mica-schist samples formed at 7 kbar and 550 °C; similar modeling combined with phengite geobarometry showed a second event at similar pressure but higher temperature. This event was followed by pressure decrease to 5 kbar and slight cooling, when biotite and staurolite porphyroblasts formed. Mylonitization started at the peak of the second metamorphic event and ended during early exhumation. The P-T and monazite age data suggest that the PMS was metamorphosed in the Early Devonian by burial to middle-lower crustal levels during the collision of Avalonia with Ganderia and metamorphosed again about 30 million years later, also at middle-lower crustal levels, during dominantly transcurrent tectonic activity. During this time different crustal slices were juxtaposed to form the PMS and probably the entire fault-bounded Ganderian and Avalonian terrane assembly in southern New Brunswick.

Detrital U-Pb and 40Ar/39Ar geochronology of the Connecticut Valley-Gaspé trough, southern Quebec and northern Vermont – Transitional tectonism from Salinic to Acadian orogenic cycles

Detrital ages constrain the timing of deposition, along-strike paleo-geographic variations and the nature of erosional sources in sedimentary basins and can be used to constrain the transition between orogenic cycles. The Connecticut Valley-Gaspé (CVG) trough is a major Upper Ordovician-to-Devonian sedimentary basin that extends for >1000 km in the Northern Appalachians and was deposited during the Salinic and Acadian orogeneses. In Quebec and northern New England, the CVG trough is host to the Gaspé Belt and correlative units of the Connecticut Valley sequence (CVS). In southern Quebec, the Gaspé belt sedimentation occurred during late Silurian-to-Early Devonian crustal extension related to the Salinic orogeny. This study reports detrital U/Pb zircon and 40Ar/39Ar muscovite ages from the Ayers Cliff and Compton formations, Quebec and the correlatives Waits River and Gile Mountain formations, Vermont. Detrital zircon ages suggest that proximal early Silurian-to-Devonian erosional sources were submerged during sedimentation, which is consistent with an extension-related marine transgression marked by the Ayers Cliff and Waits River formations. The lack of Ediacaran zircons in the Ayers Cliff Formation contrasts with the Waits River Formation, suggesting that the nature of erosional sources varied during deposition of the lowermost part of the basin. Stratigraphically higher Compton and Gile Mountain formations have similar detrital zircon provenance suggesting homogenization of detrital contributions. This is attributed to the encroachment of Avalonian terranes and ongoing erosion, probably during uplift of Laurentia and Avalonian terranes leading to foreland basin development and deposition of the uppermost part of the Compton and Gile Mountain formations which yielded youngest mean ages of ca. 413 Ma and ca. 401 Ma, respectively. Low-grade metamorphic conditions allow the preservation of detrital 40Ar/39Ar muscovite ages which highlight the contribution of erosional sources that experienced Grenvillian, Taconian and Salinic metamorphism.

Cadomian metasediments and Ordovician sandstone from Corsica: detrital zircon U–Pb–Hf constrains on their provenance and paleogeography

The island of Corsica, which belongs to the southern Variscan realm, was detached from southern France in the Tertiary. Alongside Alpine and Variscan edifices, it carries outliers of Neoproterozoic metasediments and Lower Paleozoic siliciclastics including Ordovician sandstone and conglomerates as well as Hirnantian diamictites in its NW sector. We investigated the U–Pb–Hf of detrital zircons of basement metasediments and overlying Ordovician sandstone and channeling conglomerate to constrain their provenance, the timing of their deposition, and to deduce the late Ediacaran to Ordovician paleogeography. The youngest detrital zircons in the metasediments are 0.55–0.53 Ga indicating their maximum age of deposition is Late Ediacaran to Early Cambrian, thus classifying the Corsica basement metasediments as Cadomian. The U–Pb analyses revealed that a preponderance of the detrital zircons in the basement micaschist and quartzite portray Neoproterozoic ages concentrating between 0.55 and 0.65 Ga. This is partly consistent with derivation from Pan-African terranes of north Africa yet the presence of detrital zircons younger than ~ 0.6 Ga indicates significant input from Cadomian magmatic arcs that resided within or at the margin of the Cadomian basin in which the metasediments of Corsica were deposited. eHf values of the Ediacaran zircons varies between samples, indicating the provenance comprised both, juvenile arcs and magmatic arcs that involved various degrees of mixing with old crustal components. The Hf-TDM ages of many of the Ediacaran-aged zircons point to a plausible involvement of Meso-Paleoproterozoic crust in the generation of these Cadomian arcs. The presence of small but fairly distinguished populations of Mesoproterozoic-aged (1.0–1.6 Ga) as well as 2.0–2.2 Ga and 2.4–2.6 Ga detrital zircons in the Cadomian metasediments farther indicates the presence of such crust in the provenance. Although Mesoproterozoic detrital zircons are usually considered the hallmark of Avalonian terranes, the presence of Hirnantian glacial sediments at the Corsican sequence indicates it resided in the vicinity of Gondwana. We therefore postulate that the Pre-Neoproterozoic zircons have sourced from exotic crustal vestiges that were entrained and accreted within the Cadomian realm itself. The transition into the overlying Ordovician sandstone and conglomerate marks a major change in the provenance, possibly pointing to lateral motions along the strike of the peripheral Cadomian domain. The youngest concordant detrital zircon in the Ordovician (“Ciuttone”) sandstone yielded an age of 0.48 Ga. The detrital zircon ages define an overwhelming concentration at 0.55 Ga, indicating the source of the Ordovician sandstone was cut off from the Gondwana hinterland and that sand was exclusively derived from a latest Ediacaran arc. In view of the sharp detrital zircon age peak, the Corsica Ordovician sandstone cannot be straightforwardly correlated with Armorican sandstone because the detrital zircon spectra of the latter are generally broader, indicating derivation from various sectors of the North Gondwana crust. eHf(t) values of the 0.55 Ga zircons are mostly positive and the corresponding TDM ages at 0.7–1.2 Ga indicating derivation from a juvenile island arc. While TDM ages of this type are common for late Ediacaran Avalonian rocks, the presence of Hirnantian diamictite in Corsica further substantiates that the aforementioned 0.55 Ga island arc evolved in the peripheral Cadomian realm. As a whole, the U–Pb–Hf zircon data from the Corsica sequences reveal the presence of juvenile Cadomian arcs alongside Cadomian arcs that recycled ancient (pre-Neoproterozoic) crust. Along strike variations of this type are known from the Japanese islands, in line with the peripheral Cadomian orogeny being an ancient analog of a Western-Pacific type plate boundary.

Granitoid plutons in peri-Gondwanaan terranes of Cape Breton Island, Nova Scotia, Canada: new U-Pb (zircon) age constraints

Granitoid plutons are a major component of pre-Carboniferous rocks in Cape Breton Island and knowledge of the time and tectonic setting of their emplacement is crucial for understanding the geological history of the island, guiding exploration for granite-related economic mineralization, and making along-orogen correlations. The distribution of these plutons and their petrological characteristics have been used in the past for recognizing both Laurentian and peri-Gondwanan components in Cape Breton Island, and for subdividing the peri-Gondwanan components into Ganderian and Avalonian terranes. However, ages of many plutons were assumed on the basis of field relations and petrological features compared to those of the relatively few reliably dated plutons. Seventeen new U–Pb (zircon) ages from igneous units reported here provide enhanced understanding of the distribution of pluton ages. Arc-related plutons in the Aspy terrane with ages of ca. 490 to 475 Ma likely record the Penobscottian tectonomagmatic event recognized in the Exploits subzone of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Arc-related Devonian plutonic activity in the same terrane is more widespread, continuous, and protracted (445 Ma to 395 Ma) than previously known. Late Devonian magmatism in the Ganderian Aspy terrane is similar in age to that in the Avalonian Mira terrane (380 to 360 Ma) but the tectonic settings are different. In contrast, magmatic activity in the Bras d’Or terrane is almost exclusively arc-related in the Late Ediacaran (580 to 540 Ma) and rift-related in the Late Cambrian (520 to 490 Ma). The new data support the terrane distinctions previously documented.

The Jeffers Brook diorite–granodiorite pluton: style of emplacement and role of volatiles at various crustal levels in Avalonian appinites, Canadian Appalachians

Small appinite plutons ca. 610 Ma outcrop in the peri-Gondwanan Avalon terrane of northern Nova Scotia, with different structural levels exposed. Field mapping shows that the Jeffers Brook pluton is a laccolith emplaced along an upper crustal thrust zone, likely in a dilational jog in a regional dextral strike-slip system. The oldest rocks are probably mafic sills, which heated the area facilitating emplacement of intermediate magmas. Cross-cutting relationships show that both mafic and intermediate magmas were supplied throughout the history of pluton emplacement. The modal composition, mineral chemistry, and bulk chemistry of gabbro, diorite, tonalite, granodiorite, and granite have been studied in the main plutonic phases, dykes, and sills, and mafic microgranular enclaves. As with the type appinites in the Scottish Caledonides, the pluton shows evidence of high water content: the dominance of hornblende, locally within pegmatitic texture; vesicles and irregular felsic patches in enclaves; and late aplite dykes. Analyzed mafic microgranular enclaves are geochemically similar to larger diorite bodies in the pluton. Tonalite–granodiorite is distinct from the diorite in trace-element geochemistry and radiogenic isotopes. Elsewhere to the east, similar rocks of the same age form vertically sheeted complexes in major shear zones; hornblende chemistry shows that they were emplaced at a deeper upper crustal level. This implies that little of the observed geochemical variability in the Jeffers Brook pluton was developed within the pluton. The general requirements to form appinites are proposed to be small magma volumes of subduction-related magmas that reach the upper crust because of continual heating by mafic magmas moving through strike-slip fault pathways and trapping of aqueous fluids rather than venting through volcanic activity.

Geological relationships and laser ablation ICP-MS U-Pb geochronology of the Saint George Batholith, southwestern New Brunswick, Canada: implications for its tectonomagmatic evolution

The Late Silurian to Late Devonian Saint George Batholith in southwestern New Brunswick is a large composite intrusion (2000 km2) emplaced into the continental margin of the peri-Gondwanan microcontinent of Ganderia. The batholith includes: (1) Bocabec Gabbro; (2) equigranular Utopia and Wellington Lake biotite granites; (3) Welsford, Jake Lee Mountain, and Parks Brook peralkaline granites; (4) two-mica John Lee Brook Granite; (6) Jimmy Hill and Magaguadavic megacrystic granites; and (6) rapakivi Mount Douglas Granite. New LA ICP-MS in situ analyses of six samples from the Saint George Batholith are as follows: (1) U-Pb monazite crystallization age of 425.5 ± 2.1 Ma for the Utopia Granite in the western part of the batholith (2) U-Pb zircon crystallization ages of 420.4 ± 2.4 Ma and 420.0 ± 3.5 Ma for two samples of the Utopia Granite from the central part of the batholith; (3) U-Pb zircon crystallization age of 418.0 ± 2.3 Ma for the Jake Lee Mountain Granite; (4) U-Pb zircon crystallization age of 415.5 ± 2.1 Ma for the Wellington Lake Granite; and (5) U-Pb monazite crystallization age of 413.3 ± 2.1 Ma for the John Lee Brook Granite. The new geochronological together with new and existing geochemical data suggest that the protracted magmatic evolution of the Late Silurian to Early Devonian plutonic rocks is related to the transition of the Silurian Kingston arc-Mascarene backarc system from an extensional to compressional tectonic environment during collision of the Avalonian microcontinent with Laurentia followed by slab break-off.

Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA

We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.

The agnostoid arthropod Lotagnostus Whitehouse, 1936 (late Cambrian; Furongian) from Avalonian Cape Breton Island (Nova Scotia, Canada) and its significance for international correlation

AbstractNew and archival collections from the Chelsey Drive Group of the Avalon terrane of Cape Breton Island, Nova Scotia, Canada, yield late Cambrian trilobites and agnostoid arthropods with full convexity that contrast with compacted, often deformed material from shale and slate typical of Avalonian Britain. Four species of the agnostoid Lotagnostus form a stratigraphic succession in the upper Furongian (Ctenopyge tumida–Parabolina lobata zones). Two species, L. ponepunctus (Matthew, 1901) and L. germanus (Matthew, 1901) are previously named; L. salteri and L. matthewi are new. Lotagnostus trisectus (Salter, 1864), the type species of the genus, is restricted to compacted material from its type area in Malvern, England. Lotagnostus americanus (Billings, 1860) has been proposed as a globally appropriate index for the base of ‘Stage 10’ of the Cambrian. All four species from Avalonian Canada are differentiated clearly from L. americanus in its type area in Laurentian North America (i.e., from debris flow blocks in Taconian Quebec). In our view, putative occurrences of L. americanus from other Cambrian continents record very different species. Lotagnostus americanus cannot be recognized worldwide, and other taxa should be sought to define the base of Stage 10, such as the conodont Eoconodontus notchhpeakensis.

Similar crustal evolution in the western units of the Adrar Souttouf Massif (Moroccan Sahara) and the Avalonian terranes: Insights from Hf isotope data

The Adrar Souttouf Massif is located at the western margin of the West African Craton and consists of several NNE–SSW trending units. Of them, the two westernmost have been interpreted to be linked with the Avalonian terrane assemblage and Meguma, respectively. New Hf isotopic data corroborates the Avalon correlation but has no impact one way or another on the possible Meguma connection, as there is no Hf data available from the latter. The obtained pattern of εHf(t) values vs. zircon age of the likely Avalonia related Oued Togba unit is similar to published data from Avalonia. Zircons of this unit show characteristic patterns of crustal mixing at 0.7 to 1.3Ga and 1.75 to 2.25Ga, while juvenile crust was likely formed around 0.6 to 0.75Ga, from 1.2 to 2.2Ga, and between 2.5 and 3.2Ga. The zircons of the Sebkha Gezmayet unit reveal crustal mixing for the entire Palaeozoic and Neoproterozoic, from 2.05 to 2.11Ga, and 2.8 to 2.9Ga. Juvenile crust formation is interpreted to have occurred from 0.5 to 0.7Ga, at around 2.1Ga, and at ca. 2.9Ga. As Mesoproterozoic zircons are abundant in the likely Avalonia-like Oued Togba unit, but uncommon at the West African Craton, their origin has to be found elsewhere. A comparison of available Hf data from Amazonia and Baltica, the two potential source cratons of Avalonia, shows similarities but is hampered by the lack of available data from Amazonia. Finally, a few grains from both units have Eoarchaean model ages. Among similar grains from other peri-Gondwanan terranes, they give indication of partial recycling of Eoarchaean crust in the vicinity of the northwestern West African Craton.

Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician-Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometers of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.