Northern Appalachian Orogen Research Articles

Bayesian frameworks for integrating petrologic and geochronologic data

Constraining the absolute time and duration of geologic processes is one of the great challenges and goals in Earth sciences. Increasingly, the integration of geochronologic constraints with petrologic information is being qualitatively applied to understanding the timescales of metamorphic, igneous, tectonic, and fluid-related processes. Many rocks and geochronometers preserve relative age constraints such as compositional zoning or cross cutting relationships. This prior information can be formalized in a Bayesian statistical framework to generate a probabilistic posterior chronology. As we show here, these “age-sequence” models can enhance precision on geochronologic dates and rates and insight into tectonic models. Bayesian modeling of complex, concentrically, zoned monazite from the northern Appalachian orogen was used to develop a detailed temperature-time history through the Acadian (∼405–395 Ma) and Neoacadian (∼380–350 Ma) orogenies with significantly reduced uncertainties (40–70 %). Modeling of zoned monazite from a southern Trans-Hudson orogen granulite yielded durations of 0.5+9/-0.4 Ma and 20+5/-8 Ma for biotite-dehydration melting and suprasolidus conditions, respectively. The relatively short intervals of heating and peak conditions are consistent with a back-arc tectonic setting. A complementary approach, Bayesian change point detection, provides a framework to constrain the timing of compositional changes that can be linked with metamorphic reactions. Applying this approach in the northern Appalachian orogen demonstrates contrasting durations of low-Y monazite crystallization (∼10 vs ∼30 myr) in regions with different pressure-temperature histories. Compositionally distinct monazite domains can be linked with garnet stability, which provides a key constraint on tectonic models. Bayesian statistical analysis represent a powerful tool that can be widely applied to refine the absolute time and duration of geologic processes. A more objective and reproducible set of interpretations are produced by this more formal, although not necessarily complex, statistical analysis.

Ages and structural relationships in the Ganderian central Cape Breton Highlands, Nova Scotia, Canada

Structural complexity of the Cape Breton Highlands is a key problem in reconstructing tectonic events in the northern Appalachian orogen. A new U–Pb thermal ionization mass spectrometry age of 428.53 ± 0.16 Ma for metarhyolite in the Calumruadh Brook Formation shows that volcanic and sedimentary rocks were deposited before collision of the Aspy and Bras d'Or terranes along the Eastern Highlands shear zone. A new U–Pb laser ablation zircon age of 394 +6/−4 Ma confirms that peak metamorphism in the Middle River complex continued during convergence linked to late stages of the Acadian orogeny. The compressive tectonic environment evolved into a transpressional system after initial collision in the late Silurian and caused a repeated pattern of imbrication of units in the Aspy terrane in the hanging wall in the collision. The shear zones bounding the geological units are curvilinear and have south-directed kinematics, imbricating units and transporting higher grade rocks over lower grade rocks, and moving plutons upward relative to their host rocks during and shortly after intrusion. The vergence of imbrication is parallel to the direction of transpressional movement on the main Eastern Highlands shear zone. This geometry is present in Ordovician–Silurian rocks and repeated in Devonian plutonic rocks, indicating that the overall transpressional tectonic setting was a long-lived feature of the orogen. The shear zones localized late syn- to post-deformational plutons that intruded at ca. 375–370 Ma. By the latest Devonian, emplacement of the ca. 363 Ma Margaree and related plutons marked the beginning of extension in the central Cape Breton Highlands.

Tonian rift successions in Newfoundland, Canada: a window to late tectonic events in the Mesoproterozoic Laurentian margin

Basement rocks in the Humber terrane of the Appalachian Orogen record the last stages in the history of the Mesoproterozoic Laurentian margin in Canada. These stages were revealed by recent work in the East Pond Metamorphic Suite on the western Baie Verte Peninsula, Newfoundland, where two Tonian bimodal volcaniclastic-sedimentary successions were recognized (Pine Pond successions). The older, ca. 980 Ma succession, contains detrital igneous zircon and titanite (ca. 1160–1057 Ma) presumably derived from the Mesoproterozoic Laurentian margin, while the younger, ca. 950 Ma succession, contains 980 Ma detrital igneous zircon and titanite. Although metamorphosed to eclogite facies during the assembly of the Appalachian Orogen, the successions preserve protolith features and geochemical data that indicate melt likely originated in an extensional setting. The new ages integrated with geochemical and Sm–Nd isotopic data suggest that the felsic volcaniclastic units of the Pine Pond successions are related to 975–950 Ma granitic plutons in the Pinware terrane of the eastern Grenville Province in southeastern Labrador. These new data solidify a previous interpretation that the Pine Pond successions were deposited at the continental apex of the Asgard Sea and that the Pinware terrane intrusions are a part of this event. Furthermore, these new Tonian ages for rift-related strata call into question the interpretation of Ediacaran depositional ages for clastic sequences in the northern Appalachian Orogen, with youngest detrital zircons that are Tonian, and show that the tectonic evolution of the Mesoproterozoic Laurentian margin in Canada is more complex than previously known.

Late Devonian syntaxis in the Northern Appalachian orogen

Abstract The pre-accretionary shapes of cratonic margins form successions of promontories and re-entrants inherited from the rifting of supercontinents. In accretionary orogens, the extent of deformation related to a collision with a continent characterized by an irregular margin is obfuscated through the partitioning of deformation along pre-existing structures. In the Northern Appalachians, the extent of the deformation related to the oblique collision of the Meguma terrane with the composite Laurentian margin is disputed. Herein, we use a framework based on modern collisional settings to investigate the Late Devonian to Mississippian deformation inboard of the Avalonia–Meguma boundary and evaluate the regional tectonic setting. We combine published shear zone kinematic interpretations, deformation ages and regional 40 Ar/ 39 Ar cooling ages with structural interpretation of aeromagnetic and gravimetric depth slices covering the Northern Appalachians. We find that the deformation related to the collision of the Meguma terrane, attributed to the Neoacadian orogeny, has a larger structural footprint than previously documented. While this deformation is partitioned in multiple structures in the Canadian Appalachians, northern New England is characterized by rapid crustal deformation, high palaeoelevation and fast erosional exhumation, similar to modern syntaxis structures.

Petrochronologic constraints on inverted metamorphism, terrane accretion, thrust stacking, and ductile flow in the Gneiss Dome belt, northern Appalachian orogen

AbstractGneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U‐Th‐Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P‐T‐t) paths for each unit in the gneiss dome belt tectono‐stratigraphy. The structurally lowest units, Laurentia‐derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana‐derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high‐pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome‐mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P‐T‐t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn‐collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn‐convergent orogen parallel ductile flow dramatically modified earlier accretion‐related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn‐collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.

Termination of the Ganderian Cambrian–Ordovician Miramichi terrane in east-central Maine, northern Appalachian orogen, USA

The Ganderian Cambrian–Ordovician Miramichi terrane narrows in east-central Maine and terminates at the junction of faults that separate it from the mostly Silurian Central Maine/Aroostook–Matapedia basin (CMAM) to the northwest and Fredericton trough to the southeast. The terrane was emergent after Middle Ordovician recumbent folding and shed sediment to both adjacent depocenters. Its boundary faults are the youngest deformation events and play important roles in its termination, but do not by themselves explain it. The presence of distinctive CMAM strata southeast of the northwest boundary fault indicates that the first step in developing the current relationships was an episode of hitherto unrecognized late Silurian eastward thrusting. In the northern (Danforth) segment of the terrane, intermediate facies CMAM strata were thrust onto their Miramichi source rocks. The thrust sheet was deformed by Acadian upright folds, then dissected by dip-slip offset along boundary and internal faults prior to intrusion of the 409 ± 2 Ma Skiff Lake pluton. Subsequent erosion isolated a remnant of the thrust sheet as the Dill Hill klippe, its allochthonous CMAM strata isolated among Miramichi rocks. The southern (Greenfield) segment experienced similar events, but current relationships are different and timing of the late-stage faults is not well constrained. Allochthonous CMAM strata may have overridden the Miramichi terrane completely, so that a remnant of distinctive CMAM strata is now exposed east of the Miramichi terrane in fault contact with rocks of the Fredericton trough. The entire Greenfield segment is interpreted as a fault block exposed within the thrust sheet.

U-Pb zircon ages from metasedimentary and plutonic rocks in the Bras d'Or terrane of Cape Breton Island, Nova Scotia, Canada: insights into the Ediacaran–Cambrian tectonomagmatic evolution of Ganderia

The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620–600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma.

Precise U–Pb zircon dates from silicic super-eruptions during late Ediacaran extension in the Avalonian Caledonia terrane of southern New Brunswick, Canada

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 760 000 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.

Tournaisian volcanism associated with transtensional basin development in western Newfoundland during the amalgamation of Pangea

During the Carboniferous, the assembly of the supercontinent Pangea periodically reactivated strike-slip faults and shear zones that transect the northern Appalachian Orogen. These strike-slip deformation zones represent crustal-scale zones of structural weakness that have long-lived and complex kinematic histories. Their periodic reactivation controlled the evolution of the transtensional sedimentary basins of the region. The continuous interplay between the lithospheric plates, their subcontinental lithospheric mantle, and surficial processes (i.e. oceanic/riverine, biological, atmospheric, gravitational, erosional) controlled the development of the Deer Lake Basin in western Newfoundland. The Saltwater Cove Formation of the Deer Lake Basin is a late Tournaisian lacustrian-deltaic succession interrupted by minor eruption of basalt flows and pyroclastic deposits. The basalts have E-MORB (enriched-mid ocean ridge basalt) to OIB (ocean island basalt) chemical affinities with LREE enriched profiles, lack negative Nb and Ti anomalies and εNd(t=350 Ma) values between +2.8 to +6.4. Pyroclastic deposits include mafic lapilli tuff and intermediate tuff, and have compositions similar to CAB (continental arc basalt), having steep LREE relative to HREE patterns, negative Nb and Ti anomalies, and εNd(t=350 Ma) ranging from −6.1 to −1.0. Geochemical variation within the mafic rocks is explained by an E-MORB source variably influenced by crustal contamination processes. Lithofacies analysis coupled with geochemical and isotopic data support their eruption in a transtensional, intra-orogenic basin setting that formed during the assembly of Pangea. Their moderately primitive chemistry supports the interpretation that faulting, associated with transtensional basin development, facilitated asthenosphere upwelling via edge-driven convection.

Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA

The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes.

Protracted intra- and inter-pluton magmatism during the Acadian orogeny: evidence from new LA-ICP-MS U-Pb zircon ages from northwestern Maine, USA

Devonian granitoid plutons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410–405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10–15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380–365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plutons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination.

Suprasubduction zone ophiolite fragments in the central Appalachian orogen: Evidence for mantle and Moho in the Baltimore Mafic Complex (Maryland, USA)

AbstractSuprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultramafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics—including highly depleted bulk-rock trace element patterns and high Cr# of spinel—characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the “layered ultramafics” that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean–derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike.

Age and tectonic setting of Neoproterozoic granitoid rocks, Antigonish Highlands, Nova Scotia, Canada: implications for Avalonia in the northern Appalachian orogen

The oldest rocks in the Avalonian Antigonish Highlands of northern mainland Nova Scotia, Canada, are late Neoproterozoic (>618 Ma) volcanic and sedimentary rocks of the Georgeville Group intruded by gabbroic/dioritic to granitic plutons. New U–Pb zircon ages presented here for five samples from plutons intruded into the James River and (or) Keppoch formations of the Georgeville Group have ages ranging from ca. 615 to 604 Ma. They have petrological characteristics of expanded calc-alkalic “Andean-type” suites but are compositionally biased toward evolved high-silica compositions and are interpreted as evolved I-type granites. They were emplaced at shallow depths and some were likely comagmatic with felsic volcanic components of their host rocks. These plutons are younger and show less varied Sm–Nd isotopic compositions than most plutonic rocks formed in the early Ediacaran “main arc phase” elsewhere in Avalonian terranes in the northern Appalachian orogen, although they are similar in age to plutons in southeastern New England and in the Bass River and Jeffers blocks of the Cobequid Highlands, Nova Scotia. The Jeffers block of the Cobequid Highlands appears to be most similar to the Antigonish Highlands but both areas record a Neoproterozoic history less protracted than in other parts of Avalonia.

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.

Petrology, age, and tectonic setting of the rapakivi-bearing Margaree pluton, Cape Breton Island, Canada: evidence for a Late Devonian posttectonic cryptic silicic-mafic magma chamber

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U–Pb (zircon) ages of 363 ± 1.6, 364.8 ± 1.6, and 365.5 ± 3.3 Ma, respectively, consistent with field and petrological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm–Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen.

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

AbstractThe Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.

Detrital zircon signatures in Precambrian and Paleozoic sedimentary units in southern New Brunswick – more pieces of the puzzle

Southern New Brunswick consists of a complex collage of fault-bounded belts of Late Neoproterozoic igneous and metamorphic rocks, Early Paleozoic sedimentary, metamorphic and igneous units, and overlying Carboniferous sedimentary rocks. The area also contains the boundary between the Avalonian and Ganderian terranes as interpreted in the northern Appalachian orogen. New detrital zircon ages reported here provide improved understanding of depositional ages and provenance of diverse Neoproterozoic to Carboniferous rocks in this complex area. Detrital zircon data from samples with Neoproterozoic maximum depositional ages indicate a dominantly Gondwanan provenance with a strong influence from the Amazonian craton. However, quartzite from The Thoroughfare Formation on Grand Manan Island contains dominanly 2 Ga zircon grains, consistent with derivation from the West African Craton. The age spectrum is similar to that from the Hutchins Island Quartzite in the Isleboro block in Penobscot Bay, Maine, strengthening the previously proposed correlation between the two areas. Cambrian samples also show prominent peri-Gondwanan provenance with strong influence from Ediacaran to Early Cambrian arc magmatism. The maximum depositional ages of these samples are consistent with previous interpretations of Cambrian ages based on fossil correlations and field data. A Carboniferous sample from Avalonia shows a significant contribution from Devonian magmatism as the youngest detrital component, although its depositional age based on field relationships is Carboniferous. The results exemplify the need to integrate multiple datasets in making interpretations from detrital zircon data.

New U–Pb age constraints on the geological history of the Ganderian Bras d’Or terrane, Cape Breton Island, Nova Scotia

The northern Appalachian orogen preserves evidence of a complex history of amalgamation of terranes with both Laurentian and Gondwanan affinities. The Ganderian Bras d’Or terrane of central Cape Breton Island is not well represented elsewhere in the orogen and its relationship to other Ganderian terranes is enigmatic, particularly with respect to its pre-Neoproterozoic history. The Boisdale Hills and Kellys Mountain areas contain the oldest metamorphic rocks in the Bras d’Or terrane. Quartzite units in the Boisdale Hills have detrital zircon populations with ages ranging from 3.2 to ca. 1 Ga. Paragneiss units from the Kellys Mountain area contain Meso- to Neoproterozoic detrital zircons, in which the youngest grains indicate that the maximum depositional age is <600 Ma. The detrital zircon populations of rocks from both areas are consistent with Gondwanan provenance for the protoliths, most likely the Amazonian craton. New U–Pb dates for subduction-related dioritic to granodioritic plutons in the Boisdale Hills yielded ages of 560 to ca. 540 Ma. Sedimentary, bimodal volcanic and plutonic rocks from the Bourinot belt in the Boisdale Hills and related plutons in the Kellys Mountain area have ages of ca. 510–490 Ma and are interpreted to have formed during extension related to separation of Ganderia from Gondwana. The southeastern Bras d’Or terrane preserves rocks formed in Pan-African subduction zones on a former passive margin of Gondwana as well as rocks formed during the initial stages of rifting of Ganderia from Gondwana and the subsequent opening of the Rheic Ocean.

Anticosti foreland basin offshore of western Newfoundland: Concealed record of northern Appalachian orogen development

AbstractThe Anticosti Basin, largely hidden beneath the Gulf of St. Lawrence, includes foreland basin successions that record multiple tectonic events associated with the Ordovician to Devonian evolution of the northern Appalachian orogen. Due to the lack of well ties and minimal onshore exposure, geophysical data must be used in mapping the offshore stratigraphy. Outcropping geologic boundaries are tied to magnetic lineaments that parallel stratigraphy. These lineaments are correlated with reflections on seismic profiles in order to interpret the subsurface. Seismic isochron maps for successive basin development episodes display differences in geometry, implying that orogenic loading varied through time. The geometry and subsidence rates recorded by the Middle Ordovician Goose Tickle Group imply that it formed in a pro‐arc setting associated with loading during arc‐continent collision that was most intense in the northern Newfoundland Appalachians. The geometry and subsidence recorded by the overlying Long Point Group imply pro‐arc loading by Taconian allochthons in the Québec segment of the orogen. Diachronous subduction polarity reversal along the margin placed the Long Point Group in a combined retro‐arc and pro‐arc setting, comparable to that experienced by parts of the north Australian margin at the present day. The uppermost Silurian to Lower Devonian Clam Bank Formation and Lower Devonian Red Island Road Formation represent foreland basin successions associated with the later Salinian and Acadian orogenies. Their consistent thickness implies a broad, shallow basin, suggesting that the lithosphere was cooler and stronger than during earlier subsidence, and are consistent with a retro‐arc setting.

Tonian Fe-Ti-P ferronorite and alkali anorthosite in the northern Appalachian orogen, southern New Brunswick, Canada: Amazonian basement in Ganderia?

The Lower Coverdale plutonic suite is located in the Ganderian Brookville terrane of southern New Brunswick, Canada. It does not outcrop at surface but has been intersected in 11 drill holes at depths of 100–200 m below unconformably overlying Carboniferous sedimentary rocks, and extends to a depth of at least 1100 m. No host rocks are present in the drill core, which consists entirely of plutonic and meta-plutonic rocks. The dominant rock types are anorthosite, mela-anorthosite, and ferronorite with layers of ilmenite and ilmenite-apatite rocks. They are intruded by abundant undated mafic dykes and minor ca. 390 Ma felsic dykes. A ferronorite sample yielded a Tonian age of 975.5 ± 2.3 Ma, interpreted to be the igneous crystallization age of the plutonic suite. A mela-anorthosite sample yielded a similar but less precise age of 979 ± 16 Ma, likely also the igneous crystallization age. The anorthosite has characteristics of alkali anorthosite massifs, including andesine (An30-35) commonly with exsolution lamellae of K-feldspar. The Lower Coverdale suite is interpreted to have been emplaced during orogen-scale extension following crustal thickening associated with the Grenvillian orogeny. The Lower Coverdale anorthosite and ferronorite have more isotopically evolved signatures (higher 206Pb/204Pb and 207Pb/204Pb and lower ∊Nd) compared to typical Laurentian Grenville-age plutons. We infer from these data that the Lower Coverdale plutonic suite originated in the Amazonian part of the Grenville orogen during the early stages of Tonian breakup of Rodinia. It was then incorporated into Ganderia before accreting to Laurentia as part of the Brookville terrane during the Pangean phase of supercontinent assembly.