Avalon Zone Research Articles

Carbonate production and reef building under ferruginous seawater conditions in the Cambrian rift branches of the Avalon Zone, Newfoundland

AbstractThe characterization of carbonate production on rift basins is critical for understanding the nucleation and demise of reefs in tectonically active areas. A new petrographic and mineralogical analysis of Cambrian strata from the Avalon Zone in Newfoundland, based on scanning electron microscopy – back‐scattered electron detector and Raman spectrometer analyses, facilitates recognition of several episodes of Terreneuvian–Miaolingian carbonate production and associated precipitation of ironstone and phosphorite. These distinct units mainly developed on uplifted rift shoulders and basaltic lava palaeoreliefs, and reflect amalgamated high‐energy events, interrupted by scouring discontinuities (diastems) commonly lined by phosphatized and ferruginized microbial crusts. Mud‐mounds, in contrast, nucleated under calm conditions episodically punctuated by high‐energy episodes, where scattered thromboid structures occur as both clotted textures and distinct calcimicrobes. Precipitation of hematite/goethite versus chamosite couplets, both occluding primary porosities and replacing interlaminae and cortices of oncoids and coated aggregates, point to marine substrates close to the Fe‐redox boundary. Upwelling of phosphate‐rich ferruginous hydrothermal waters contributed to the precipitation of ironstone and phosphate interbeds. Ferruginous waters related to penecontemporaneous hydrothermal activity, reflected by the record of synsedimentary fissuring and stockwork ore bodies, were delivered to confined rift‐related horst‐and‐graben settings, largely controlled by the development of specific Cambrian carbonate and associated ironstone facies. The influence of ferruginous waters necessarily affected the record of climatically sensitive evaporitic pseudomorphs, reefs/mounds and phosphorites, which are then not suitable criteria to discriminate palaeolatitude, as demonstrated by a comparison of low‐latitude to middle‐latitude margins fringing Baltica, and the Avalonian and Atlas – Ossa‐Morena – Northarmorican rift transects of West Gondwana.

Stratal evolution and provenance of a precambrian delta in a tectonically active terrane: The lower Signal Hill Group, Avalon Zone, Newfoundland

Sedimentologic and provenance studies of the syn-orogenic Ediacaran lower Signal Hill Group (Gibbett Hill and Quidi Vidi formations) in the Avalon Peninsula, Newfoundland, Canada, were conducted to test structural and depositional models related to the enigmatic late Ediacaran (ca. 575–550 Ma) Avalon orogenesis. Two facies associations corresponding to delta front and delta plain environments are identified from five stratigraphic sections. The presence of an unconformity between these facies near the location of a previously proposed thrust-related paleo-high and changes in sedimentary patterns reflecting shoreline regression support the inference of syn-sedimentary uplift and reveals coeval base level fall. Framework petrography, detrital geochronology, and detrital heavy mineral analysis indicate sediment sourcing from hinterlands of West Avalonian igneous and metamorphic basement. Changes in provenance between the delta front and delta plain facies coeval with regression of the Signal Hill delta suggest the thrusting of volcanic cover sequences over already exhumed plutonic sources and the exhumation of deeper/higher-grade metamorphic rocks in the hinterland. Detrital zircon U-Pb maximum depositional ages of the delta front and delta plain facies yield 557 ± 9 Ma and 556 ± 22 Ma, respectively, constraining this episode of West Avalonian hinterland exhumation, wedge-top foreland basin deformation, and forced regression to ca. 556 Ma. The regional significance of this late Ediacaran (ca. 565–538 Ma) foreland basin to tectonic models of West Avalonia’s transition from an arc setting to a Paleozoic rifted platform is not currently understood, but it may represent a retro-arc basin formed either as part of a coherent West Avalonian block overlying a subducted spreading ridge or on a subsidiary West Avalonian terrane prior to and during terrane collision preceding deposition of the Paleozoic overstep sequence.

Late-magmatic processes in the St. Lawrence Granite: Implications for fluorite mineralization

The St. Lawrence Granite (SLG), located in the Avalon Zone of the Appalachian Orogeny, is a peralkaline to metaluminous granite, best known for its association with extensive, and economically significant, fluorite deposits. Genetic models for the fluorite mineralization have been proposed, but major gaps and misconceptions still remain, such as the lack of fluorite in the country rocks, the source of Ca to form fluorite and the locally abundant calcite in the fluorite veins and the granite. Petrography, geochemistry and electron microprobe analysis revealed new details on the late-magmatic processes in the granite that played a significant role in fluorite mineralization.The SLG intruded at shallow levels and comprises several phases, each having different textures, mineralogy, and geochemistry. Most of the fluorite deposits are associated with the East lobe (E lobe), which is the only peralkaline and one of the most intensely altered phases of the granite. Alteration and fluorite mineralization are the results of devolatilization of the granite. The volatiles included F, Cl, CO2 and water. Early exsolution of a F-rich vapour, followed by a F-rich acidic fluid led to the fluorite mineralization. In the later stages, a F-poor and CO2-rich fluid was left and was unable to leave the granite, migrated along the upper contact of the granite and accumulated in the cupolas resulting in late carbonatization and extensive autometasomatism.Exsolution of a F-rich vapour, coupled with the shallow depth of intrusion of the SLG, allowed the F to leave the granite leading to fluorite mineralization in the country rocks, as demonstrated by the AGS fluorite deposit, which is partially hosted in sedimentary rocks overlying the granite; this encourages further exploration in the surrounding rocks. The source of the Ca is from the alteration of primary Ca-bearing feldspar and amphiboles in the granite, implying the lack of spatial correlation between fluorite mineralization and Ca-containing country rocks. This is significant for pursuing further exploration for fluorite in the St. Lawrence area, and elsewhere.

Characteristics and genesis of phyllosilicate hydrothermal assemblages from Neoproterozoic epithermal Au-Ag mineralization of the Avalon Zone of Newfoundland, Canada

The Avalon Zone hosts well-preserved examples of high- and low-sulfidation epithermal Au-Ag mineralization and associated phyllosilicate alteration. Using examples from the Hope Brook and Hickey's Pond high-sulfidation systems (HSS) and the Heritage low-sulfidation system (LSS), this study presents new data on the mineralogical and chemical characterization of phyllosilicates associated with epithermal Au-Ag deposits. Electron probe microanalysis and X-ray diffraction studies revealed that the HSS examples contain Fe muscovite ((K0.62–0.79Na0.10–0.37Ca0.00–0.01)Al1.31–1.72(Mg0.01–0.08Fe0.05–0.19)(Al1.00–1.24Si2.76–3.00O10)(OH)2), paragonite ((K0.05–0.30Na0.78–1.02Ca0.00–0.01)Al1.66–1.84(Mg0.00–0.01Fe0.02–0.04)(Al1.01–1.11Si2.89–2.99O10)(OH)2), Al clinochlore ((Mg2.42–2.65Fe1.62–1.78)(Si2.57–2.69Al1.31–1.43O10)(OH)8), pyrophyllite, kaolin minerals, illite-smectite (I-Sme) and chlorite-smectite (C-Sme). Conversely, the LSS examples contain Fe-Mg muscovite ((K0.44–0.76Na0.00–0.02Ca0.00–0.01)Al1.55–1.82(Mg0.17–0.31Fe0.05–0.12)(Al0.55–0.71Si3.29–3.45O10)(OH)2), Al clinochlore ((Mg2.17–2.56Fe1.59–2.01)(Si2.71–2.92Al1.08–1.29O10)(OH)8), and I-Sme and C-Sme intermediates. Chlorite geothermometry indicated that HSS and LSS assemblages crystallized at average temperatures of 261 °C and 145 °C, respectively. Mica and chlorite from HSS and LSS are defined by Tschermak substitution and high-temperature polytypism (2M1 and IIb, respectively). In the HSS examples, one or more pulses of hot, acidic fluid reacted with host rocks generating an envelope of advanced argillic alteration (Fe-Ms, Kln, Pg, Prl) around a vuggy silica core. This is bounded by a zone of argillic alteration (Fe-Ms, Kln), with sericitic/chloritic alterations (Fe-Ms, Al-Clc) developed in distal parts of the systems. In the LSS, near-neutral hydrothermal fluids led to the formation of broad phyllic/chloritic zones (Fe-Mg Ms and Al-Clc). A low temperature overprint with 1M mica, Ib clinochlore ((Mg2.48–2.74Fe1.58–1.73)(Si3.05–3.15Al0.85–0.95O10)(OH)8), kaolinite and Sme-poor I-Sme and C-Sme is attributed to late waning stage hydrothermal activity and/or weathering. A close paragenetic link was established between Fe-Mg muscovite and Ag mineralization at Heritage, as well as between Fe-Mg muscovite and bladed calcite, suggesting boiling as the precipitation mechanism. The present study provides guidance on how phyllosilicate assemblages have the potential to discern HSS versus LSS mineralization during early prospection in terranes with limited exposure, and to contribute to the reconstruction of their hydrothermal record.

Cambrian syn‐rift tectonic pulses at unconformity‐bounded carbonates in the Avalon Zone of Newfoundland, Canada

AbstractIn the Avalon Zone of SE Newfoundland, several Terreneuvian to Cambrian Series 2 carbonate bodies occur associated with paraconformities and onlapping geometries. Their stratigraphic discontinuities and related gaps represent syn‐rift episodes characterized by sharply tilted and downfaulted blocks and deposition of chaotic megabreccia beds and conglomerates on neighbouring footwall areas. Microbial and shelly carbonate production nucleated on tectonically unstable palaeohorsts, where stromatolitic crusts and mats commonly reflect the onset of hiatal stratigraphic diastems, and mud‐mounds the episodic establishment of calm conditions. Short‐term uplift episodes of rift shoulders led to unroofing of the Neoproterozoic basement (yielding granitoid clasts and input of reworked exotic clasts and allochthonous fossils from underlying Cambrian strata), localized subaerial exposure and karst features, and the development of synsedimentary fracture networks that provided favourable conduits and pathways for hydrothermal fluids. These carbonate beds host vein networks of carbonate‐hosted polymetallic sulphide‐sulphate‐oxide mineral associations (Pb–Cu–Fe–barite stockworks). The mixture of parautochthonous and allochthonous bioclastic assemblages points to the record of event‐concentration strata, which may clarify the long stratigraphic ranges of some involved microfossils. The Cambrian Avalonia rift and the offshore to basinal rift sector of the neighbouring Atlas‒Ossa‐Morena rift, preserved in the Moroccan northern High Atlas and Coastal Meseta, share common temperate‐water carbonate facies and build‐ups, lacking ‘subtropical’ indicators such as archaeocyathan‐microbial reefs, ooidal shoals or evaporitic pseudomorphs. These diagnostic facies and minerals occur only in relatively stable, shallow‐water rift branches, such as those preserved in the Anti‐Atlas and the Ossa‐Morena Zone (Iberian Massif).

Geochemistry of the highly evolved Sn-W-Mo-bearing Mount Douglas Granite, New Brunswick, Canada: Implications for origin and mineralization

The Late Devonian (368 ± 1 Ma) post-orogenic peraluminous Mount Douglas Granite, located in southwestern New Brunswick, Canada, forms the eastern part of the Saint George Batholith. The batholith was emplaced following the accretion of the Gander and Avalon zones of the northern Appalachians during the Neoacadian Orogeny. The Mount Douglas Granite is divided into three units, Dmd1, Dmd2, and Dmd3, that formed by progressive high degrees of fractional crystallization. The two most fractionated units, i.e., Dmd2 and Dmd3, are locally associated with greisen/sheeted veins-related endogranitic Sn-W-Mo mineralization. Whole-rock δ18O values of +6.0 to +7.3‰, high initial 87Sr/86Sr ratios of 0.70550 to 0.71665, slightly positive εNd(368 Ma) values (+0.3 to +1.1), and Pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile Avalonian crust, contaminated by supracrustal rocks.Petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas. This is supported by estimated zircon saturation temperatures that decrease from Dmd1 (747–826 °C) → Dmd2 (733–817 °C) → Dmd3 (729–816 °C). The estimated crystallization pressure based on normative quartz and feldspar (albite + orthoclase) contents also decreases with increasing silica content, from 2.3 kbar (~7.7 km) in Dmd1 to 0.3 kbar (~1 km) in Dmd3.Whole-rock geochemical characteristics of this system show that unit Dmd3 is the most highly evolved unit; it has the highest SiO2 (avg. 76.4 wt.%.), LILE contents (e.g., Li, Rb, Cs), Rb/Sr (mean = 42), certain HFSE (Ta, Th, U) including Y (≤138 ppm), and REE, and has the most pronounced negative Eu anomalies (avg. Eu/Eu* = 0.08) and the lowest K/Rb (70–127), Nb/Ta (mean = 4.9), and Zr/Hf (mean = 23.5). Therefore, unit Dmd3 with the highest content of incompatible elements is the most prospective for Sn, W, Zn, Bi, and U mineralization, and then Dmd2.

Volcanological and environmental controls on the Snowdon mineralization, North Wales, UK: A failed volcanogenic massive sulfide system in the Avalon Zone of the British Caledonides

The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide accumulations are absent in the caldera. This is attributed to the shallow water depths, which promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the tectonic instability of the caldera and the high energy, shallow marine environment would have limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26±0.35Ma) and top (454.42±0.45Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. The absence of massive sulfide mineralization is consistent with the general paucity of economic VMS deposits in the Avalon Zone. Despite the highly prospective geological setting this study further illustrates the importance of volcanic facies mapping and associated paleo-environmental interpretations in VMS exploration.

Late Neoproterozoic arc-related magmatism in the Horse Cove Complex, eastern Avalon Zone, Newfoundland

The Horse Cove Complex (HCC) is a recently recognized late Ediacaran swarm of mafic-to-felsic dykes that occurs in the Newfoundland Avalon Zone. It is focused along the extrapolated trace of the To...

Paleomagnetic study of the late Neoproterozoic Bull Arm and Crown Hill formations (Musgravetown Group) of eastern Newfoundland: implications for Avalonia and West Gondwana paleogeography1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

A paleomagnetic study of subaerial volcanic rocks and associated siltstones of the Ediacaran Bull Arm Formation in the Avalon Zone of Newfoundland revealed a stable bipolar, hematite-borne primary remanence supported by positive conglomerate, contact, and fold tests. Mean remanence directions in two distal areas (Bonavista and Argentia) are similar, indicating a low paleolatitude position of Avalonia at ∼570 Ma. Redbeds of the overlying ∼550 Ma Crown Hill Formation also carry a primary bipolar hematite-borne remanence with moderate inclination, indicating that Avalonia remained at low to medium paleolatitudes through the end of the Ediacaran. Combining our results with previously published paleomagnetic data of Avalonia suggests moderate-scale drift of Avalonia through low southern paleolatitudes through the latter half of the Ediacaran, providing a paleogeographic context for the development of the first complex metazoan life.

Provenance and paleodrainage patterns of Upper Jurassic and Lower Cretaceous synrift sandstones in the Flemish Pass Basin, offshore Newfoundland, east coast of Canada

Upper Jurassic to Lower Cretaceous synrift sandstones from two industry exploratory wells in the Flemish Pass Basin were studied for provenance using heavy mineral proxies. These sandstones were deposited during the North Atlantic rifting stage, at which time rifting intensified between Iberia and the Grand Banks. Several heavy mineral methods were used, including in-situ U-Pb geochronology and morphological and chemical analysis of detrital zircons, chemical discrimination of detrital tourmalines, and heavy mineral ratios.Clastic bed-load material entering the northern Flemish Pass Basin during the Tithonian to Berriasian was connected to transport systems reaching west, as far as 400 to 500 km (249–311 mi). Source areas to the west included igneous rocks and associated cover sequences of the Avalon zone, as well as the magmatic rocks, metasedimentary rocks, and associated cover sequences present in the Central mobile belt. The eastward-directed input of bed-load material into the basin is considered to have resulted in the concentration of sandstone units along its western margin, with deteriorating reservoir grade toward the east. Given the regional extent of drainage systems, it is likely that similar constraints apply to equivalent sandstones in the adjacent East Orphan Basin.By the Valanginian–Barremian, paleodrainage orientations had switched and bed-load material began to enter the Flemish Pass Basin from more proximal areas to the southeast with the Avalon zone basement.Overall, no evidence exists to support sediment sourcing from the Iberian margin or the Flemish Cap-Galicia Bank continental fragment to the east, and material from these areas is instead interpreted to have been shed into the incipient Atlantic Ocean or Bay of Biscay. Also, no evidence exists to support sourcing from Grenville basement farther to the west, implying that a drainage divide may have existed somewhere between central and western Newfoundland, west of which material was presumably being transported to the south or southwest.

Cambro-Ordovician paleogeography of the Southeastern New England Avalon Zone: Implications for Gondwana breakup

Paleomagnetic measurements and U-Pb geochronology of alkalic igneous rocks intruding the Ediacaran Cambridge Argillite and the Lower Cambrian Weymouth Formation north of Boston at Nahant, Massachusetts, yield a well-constrained paleopole. The oldest intrusive phases are a syenodiorite sill and a granite dike with 206 Pb/ 238 U crystallization ages of 490.19 ± 0.90 Ma and 489.03 ± 0.82 Ma, respectively. This sequence along with undated mafic sills penetrating both the argillite and Weymouth strata are cut by 488.53 ± 0.81 Ma gabbro locally associated with 488.48 ± 0.79 Ma syenite forming a steep-sided intrusive plug. Ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable A magnetic component with a mean direction of D = 278.5°, I = −76.7° (A 95 = 3.9°, N = 10). This is considered to be the primary magnetization direction because it passes both fold and reversals tests. The corresponding paleopole at 320LE, 34°N (A 95 = 7.2°, N = 10) places the Southeastern New England Avalon Zone at a southerly paleolatitude of 65° slightly removed from the African margin of Gondwana and suggests that bimodal Nahant magmatism is linked with latest Cambrian–Early Ordovician rifting of West Avalonia to form the Rheic Ocean. Stable B and C magnetic components in Nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track Late Silurian through Carboniferous segments of the North American apparent polar wander path, indicating docking of West Avalonia by mid-Paleozoic time.

Age, geochemistry and Sm–Nd isotopic signature of the 0.76 Ga Burin Group: Compositional equivalent of Avalonian basement?

The ca. 760 Ma Burin Group occurs in a well-exposed 60 km long northeasterly trending belt on the Burin Peninsula in the Avalon Zone of the southeastern Newfoundland Appalachians. This lithologically distinctive, tectonically bounded succession is characterized by low grade massive and pillowed basalts, abundant gabbro and diabase dykes and sills, mafic pyroclastic and epiclastic rocks and stromatolitic-carbonate-bearing rocks. The Burin Group preserves the oldest known magmatic event in the Avalon terrane, but its tectonic evolution and its relationship to the voluminous 640–570 Ma arc-related magmatism that typifies Avalonia is uncertain. Geochemical analyses confirm that the basalts are predominantly low-K tholeiites. They are characterized by high ratios of LIL/HFS elements, and display depletion to slight enrichment in LREE. Sm–Nd isotopic data reveal that most basalts have juvenile compositions, with ɛ Nd values similar to contemporaneous depleted mantle, indicating that high LIL/HFS elemental ratios were probably due to coeval subduction which contaminated the mantle source. Other basalts have lower ɛ Nd values, and the negative correlation of ɛ Nd with La/Sm, together with a positive correlation of ɛ Nd with 147Sm/ 144Nd suggest that their isotopic signatures have been modified by a Mesoproterozoic older oceanic crust or mantle into which the Burin Group mafic magmas were emplaced. The isotopic signature of the Burin mafic rocks is similar to that inferred for the source of the main phase of Avalonian magmatism. These data, together with paleocontinental reconstructions for ca. 760 Ma, suggest that the Burin Group is a local representative of ensimatic arcs within the Panthalassa-type ocean that surrounded Rodinia, possibly as a far-field response to the breakup of Rodinia. Although its low metamorphic grade precludes it being the basement from which Avalonian magmas were extracted, the Burin Group may be representative of the geochemical and isotopic composition of that basement.

Late Neoproterozoic paleogeography of the Southeastern New England Avalon Zone: Insights from U-Pb geochronology and paleomagnetism

Integrated geochronologic and paleomagnetic investigation of the arc-related Lynn and Mattapan volcanic complexes around Boston, Massachusetts, establishes a Neoproterozoic paleopole for the Southeastern New England Avalon Zone. Four new U-Pb zircon dates obtained using isotope dilution and thermal ionization mass spectrometry (ID-TIMS) methods range from 597.4 ± 1.5 Ma to 595.7 ± 1.6 Ma, in agreement with published Lynn-Mattapan results of ca. 596 Ma. The mean direction calculated from the A component of magnetization at nine Lynn-Mattapan sites has a declination (D) of 319.9° and an inclination (I) of 57.0° (α95 = 7.4°); the paleopole based on this mean direction is 206°E, 60°N. These results pass both reversals and fold tests and are regionally consistent among lithologies that include basalt, andesite, rhyodacite, and rhyolite, which represent both volcanic and subvolcanic phases. Most sites also contain stable B and C magnetic components with directions that suggest Ordovician to Permian remagnetizations. A broadly comparable A component direction with D = 109.0° and I = −63.4° (α95 = 9.4°, n = 11 samples) was obtained from one site in the Squantum Member of the Roxbury Conglomerate (maximum age of 593 Ma) overlying the volcanic sequence. The Lynn-Mattapan results indicate that southeastern New England occupied a mid-latitude position at ca. 595 Ma, similar to those reported from Avalonian terranes in maritime Canada. The most likely peri-Gondwanan position for Avalonia consistent with these paleomagnetic data lies off the West African margin. Using poles from the East Sahara craton (Egypt) and the Rio de la Plata craton (Brazil) has permitted reconstruction of West Gondwana without recourse to Laurentian proxies.

Spindle-shaped Ediacara fossils from the Mistaken Point assemblage, Avalon Zone, Newfoundland

The Mistaken Point assemblage of the Ediacara fossils is dominated by rangeomorphs with homologous fractal branching elements. The most distinctive are the fusiform fossils, herein named Fractofusus misrai n.gen., and n.sp., and Fractofusus andersoni n.gen., and n.sp. Although endemic to the Newfoundland portion of the Avalonia terrane, they dominated deep sea-floor communities below the photic zone, in the mid-Ediacaran Period (ca. 575–560 Ma). Their biological affinities remain uncertain, but their architecture suggests a phylogenetic position near the base of the Metazoa.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - I. Results from a seismic refraction profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - II. Results from a seismic reflection profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Polycyclic evolution of the Late Neoproterozoic basement in the Hermitage Flexure region (southwest Newfoundland Appalachians): New evidence from the Cinq-Cerf gneiss

The Cinq-Cerf gneiss represents one of the oldest tectonic elements yet documented within the “Avalonian” basement complex of the Hermitage Flexure region of the Newfoundland Appalachians. This gneiss is a composite unit that includes both supra-crustal and granitic gneisses, the latter having an igneous crystallization age of 675 + 12/−11 Ma. The timing of the gneiss-forming event (D 1 polyphase deformation) is constrained by the protolith age of the orthogneiss and the 584 + 7/−6 Ma crystallization age of granodiorite intrusive into gneiss. The emplacement of the granodiorite is coeval with the previously dated 585 Ma eruption of felsic tuff within nearby low-grade stratified elements of the pre-Silurian basement, north of the Grand Bruit Fault Zone (GBFZ), adding support to models of a Late Neoproterozoic basement-cover relationship between the gneiss and the low-grade volcano-sedimentary sequence. Younger Neoproterozoic magmatism in the gneissic basement is recorded by emplacement of 557 + 14/−5 Ma gabbro and the intrusion of 547 + 2/−7 Ma diorite dykes. The latter have a recrystallized tectonic fabric that hosts 448 + 9/−3 Ma titanite. Peak Appalachian-cycle overprinting of the Cinq-Cerf gneiss occurred in mid- to late Silurian time, and commenced with high-temperature deformation (D 2) of the gneiss, associated with synkinematic intrusion of Western Head granite at 431.5 ± 1 Ma. Mafic dyke swarms that intrude the gneiss and the granite contain 420 ± 3 Ma titanite. This age reflects new growth during Silurian retrogression and mylonitization of the mafic dykes. The D 3 mylonitization is widespread and linked to thrusting of the Cinq-Cerf gneiss along the GBFZ during the 420–418 Ma closure of the northerly adjacent Silurian volcano-sedimentary cover basin. New data bearing on the Late Neoproterozoic record of the basement block of the western Hermitage Flexure further demonstrates similarities in magmatic, stratigraphic and metallogenic record documented in Avalonian rocks of east Newfoundland's Connaigre, Burin and Avalon peninsulas, pointing to a Neoproterozoic linkage with the rocks in these areas. The early to mid-Paleozoic history of the Hermitage Flexure basement indicates that, during that time, the basement block evolved as a terrane separate from the Avalon Zone (sensu stricto).

The Appalachian peri-Gondwanan realm: a palaeogeographical perspective from the south

Abstract The Appalachian peri-Gondwanan realm (APGR) is an extensive tract of exotic Neoproterozoic-early Palaeozoic crustal blocks that occupies the eastern flank of the orogen. Traditionally, southern APGR elements have been correlated with those of the northern Appalachians on the basis of gross geological similarities. Most palaeogeographical reconstructions of the APGR are based on data from the northern Appalachians; consequently in these reconstructions, southern APGR elements are viewed commonly either as being affiliated spatially with those of the north or ignored. However, emerging data from two southern Appalachian crustal blocks give new insights into the palaeogeography of the APGR. The Smith River allochthon may be a part of the APGR on the basis of recently obtained U-Pb monazite and staurolite ages that are apparently incompatible with a Laurentian origin. The allochthon and possibly adjacent terranes, appear to have followed a palaeogeographical track independent of other APGR elements. The Carolina zone is recognized as peri-Gondwanan in origin on the basis of its (i) gross geological evolution, (ii) fossil fauna and (iii) tectonic history. Mid-Palaeozoic regional kinematic patterns suggest that Carolina and its commonly held northern counterpart, the Avalon zone, travelled together on the same lithospheric plate, but their contrasting tectonic histories suggest that they formed along different margins of this plate. These interpretations lead to a new model for middle Palaeozoic interactions of the APGR with Laurentia.

MORPHOMETRIC ANALYSIS OF THE EDIACARAN FROND CHARNIODISCUS FROM THE MISTAKEN POINT FORMATION, NEWFOUNDLAND

Charniodiscus is a leaf-shaped Ediacaran (terminal Neoproterozoic) fossil with a worldwide distribution, but the scarcity of complete specimens has previously hindered evaluation of its taxonomy and ecology. The presence of hundreds of complete (fronds with stem and disc attached) Charniodiscus specimens from the Avalon Zone of Newfoundland has allowed for detailed morphometric analysis of Charniodiscus specimens and permits determination of characteristics which vary with growth (e.g., stem length, frond width, and disc diameter) versus those that reflect taxonomic differences (e.g., number of primary segments, presence of a distal spine, shape ratios). This has led to the recognition of three species of Charniodiscus in the Mistaken Point biota, including numerous specimens of two new taxa, C. procerus n. sp. and C. spinosus n. sp., and rare specimens of the Australian species C. arboreus. C. procerus n. sp. and C. spinosus n. sp. represent similar, yet ecologically distinct forms of upper-level filter feeders with diverging feeding strategies in order to reduce the competition for resources. Ratio plots and principal components analyses (PCAs) confirm the existence of five (possibly six) morphologically distinct species of Charniodiscus worldwide.

COMPOSITION OF BIOTITE FROM GRANITIC ROCKS OF THE CANADIAN APPALACHIAN OROGEN: A POTENTIAL TECTONOMAGMATIC INDICATOR?

Biotite, the dominant ferromagnesian mineral in Paleozoic granitic rocks of the Canadian Appalachians, has been analyzed with an electron microprobe (wavelength dispersion) for major elements and by 57Fe Mossbauer spectroscopy. We sampled a wide variety of rock types, ranging from gabbro, diorite, syenite to granite, but by far mostly granitic ( sensu lato ). The most pronounced variations are in total Al contents and Fe/(Fe + Mg) values. In the biotite quadrilateral (annite –siderophyllite –phlogopite – eastonite), biotite from A-type granites of the Humber and Avalon zones in Gaspe (Quebec) and New Brunswick is characterized by low mean Al contents, ~1.15 atoms per formula unit ( apfu ), and variable Fe/(Fe + Mg) values in the range 0.4 to 0.9. In the granites of the Notre Dame arc of the Dunnage zone in Newfoundland, biotite has moderate mean values of Al (~1.40 apfu ) and Fe/(Fe + Mg) (~0.58). In granites of the Gander zone of New Brunswick and Newfoundland, biotite has a mean Fe/(Fe + Mg) value of 0.6 and shows a pronounced trend of increasing total Al (1.05 to 1.75 apfu ), confirming significant contributions of aluminous supracrustal material to the magmas, either by assimilation or anatexis. Finally, in granites of the Meguma zone, derived entirely from metasedimentary material, biotite exhibits a remarkable increase in total Al (1.30 to 2.00 apfu ) and considerable iron-enrichment [Fe/(Fe + Mg) in the range 0.4 to 1], with compositions nearing the siderophyllite end-member. The biotite from most zones plots on or above the NNO buffer, indicating moderately oxidizing conditions, whereas that from the Meguma zone plots mainly between the QFM and NNO buffers, implying fairly reducing conditions during crystallization. Assuming a reasonable range of crystallization temperatures of 750 to 900°C, oxygen fugacities ranged from 10−10 to 10−16.9 bars during crystallization. The composition of biotite reflects primarily the nature of the host magmas. It cannot readily be used for tectonomagmatic characterization of these rocks without the aid of other types of data.