Avalon Composite Terrane Research Articles

U-Pb Geochronology of Devonian Granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for Hotspot Melting of a Neoproterozoic Source.

U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.

The Distribution, Style, and Intensity of Alleghanian Metamorphism in South‐Central New England: Petrologic Evidence from the Pelham and Willimantic Domes

Alleghanian metamorphism in south‐central New England is characterized by Late Pennsylvanian, $$\mathrm{kyanite}\,+\mathrm{muscovite}\,+\mathrm{staurolite}\,$$ grade, heterogeneous reconstitution of migmatitic, $$\mathrm{K\,feldspar}\,+\mathrm{sillimanite}\,$$ grade, pelitic gneisses previously metamorphosed during the Silurian to Devonian Acadian orogeny. Reconstitution resulted from strain within an approximately 5‐km‐thick zone of ductile deformation between the underlying Late Proterozoic Avalon Terrane, exposed in the cores of the domes, and cover rocks of the Putnam‐Nashoba, Merrimack, Kearsarge–Central Maine, and/or Bronson Hill Terranes. This style of metamorphism developed along the Avalon‐Acadian cover terrane boundary and is exposed in the Pelham and Willimantic Domes. Pressure‐temperature (PT) conditions in the cover rocks, based on conventional thermobarometry, oxygen isotope thermometry, and petrogenetic grid constraints, evolved from $$6\pm 1$$ kbar at 800°–825°C during Acadian metamorphism to $$6.5\pm 1$$ kbar at 600°–650°C during Alleghanian reconstitution. Based on the similarity in metamorphic style and intensity, structural setting, and timing of reconstitution, it may be inferred that the distribution of Alleghanian metamorphic mineral assemblages in south‐central New England outside the Avalon Composite Terrane is largely controlled by the proximity to a regional, essentially horizontal, midcrustal ductile shear zone. The formation of Alleghanian metamorphic mineral assemblages along the boundary zone accounts for the paucity of Alleghanian mineral assemblages in an area presently exposing mainly Acadian‐age assemblages. Prograde Alleghanian $$\mathrm{kyanite}\,+\mathrm{muscovite}\,+\mathrm{staurolite}\,$$ metamorphism within the boundary zone is contemporaneous with Late Pennsylvanian clastic sedimentation, and subsequent uplift and denudation is contemporaneous with prograde Barrovian metamorphism, in the Narragansett basin to the southeast. Metamorphism, denudation, and relatively rapid cooling documented for the Avalon Composite Terrane and Narragansett basin may thus be linked to Late Paleozoic tectonism in terranes to the northwest.

Geochronology and geochemistry of the Pola de Allande granitoids (northern Spain): their bearing on the Cadomian-Avalonian evolution of northwest Iberia

The Pola de Allande pre-Variscan tonalite-granodiorite plutons are located in the Narcea Antiform, at the boundary zone between the Cantabrian and west Asturian Leonese zones of the Iberian Variscan belt. These granitoids were intruded into a Neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by Variscan thrust-related shear deformation. U-Pb laser ablation inductively coupled plasma - mass spectrometry (ICP-MS) dating of zircons from two plutons yielded concordant ages of intrusion of 605 ± 10 and 580 ± 15 Ma. To the authors' knowledge, this is the first reported U-Pb Cadomian-Avalonian age for igneous rocks in this section of the Iberian Variscan belt. These intrusions are coeval with the main episode of dominantly calc-alkaline magmatic activity related to Cadomian-Avalonian subduction. Major and trace element composition of the granitoids is characteristic of I-type high-K calc-alkaline granitoids generated in continental arc settings, and are comparable to those of coeval granitoids in other areas of the Cadomian-Avalonian belt. Sr and Nd isotopic signatures indicate that the genesis of the Pola de Allande granitoids involved either mixing of mantle melts of Cadomian extraction with an older enriched crust (Eburnean-Icarthian, i.e., ca. 2 Ga old crust) or melting of a mafic infracrustal protolith with a Grenville age (ca. 1.2 Ga) mantle extraction. The Neoproterozoic sediments, hosting the Pola de Allande granitoids and present in large areas of northwest Iberia, may represent the back-arc basin of the subduction complex in which the Avalon composite terrane constituted the main magmatic arc.

U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton Island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after ~977 Ma, extended through a period of volcanic activity between ~694 and 637 Ma, and ended before the ~629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between ~694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between ~612 and 607 ± 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at ~553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-arc basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic arc following cessation of subduction, respectively. The combination of ~700-550 Ma arc magmatism and ~1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico.

Birth of the Avalon arc in Nova Scotia, Canada: geochemical evidence for ∼700–630 Ma back-arc rift volcanism off Gondwana

Central Cape Breton Island in Nova Scotia, Canada, is host to ∼700–630 Ma felsic and associated mafic volcanic rocks that are relatively rare in other parts of the Avalon Composite Terrane, occurring elsewhere only in the Stirling Block of southern Cape Breton Island and in parts of eastern Newfoundland. The mafic rocks of central Cape Breton Island are typically intraplate tholeiitic basalts generated by melting of a garnet-bearing mantle source. They lack a continental trace element and εNd imprint although they were emplaced on continental crust; they resemble oceanic island basalts. Contemporaneous volcanism in the Stirling Block is calc-alkaline and formed in a volcanic arc setting. In the absence of evidence for an intervening trench complex or suture, it may be inferred that the central Cape Breton tholeiites formed in a back-arc setting relative to the Stirling Block. This rifting may represent the initial stages of separation of an Avalonian arc from western Gondwana. The arc rifted further between ∼630–610 Ma when the younger Antigonish-Cobequid back-arc basin formed. Subsequently, the extensional arc became convergent, telescoping the back-arc basin. Northwestward migration of calc-alkaline arc magmatism may be related to shallowing of the associated Benioff zone through time.

40Ar/39Ar ages of detrital muscovite within Lower Cambrian and Carboniferous clastic sequences in northern Nova Scotia and southern New Brunswick: implications for provenance regions

Detrital muscovite from lowermost Cambrian sequences exposed in the Avalon Composite Terrane in Nova Scotia and New Brunswick record 40Ar/39Ar plateau ages of ca. 625–600 Ma. These are interpreted to date times of cooling in source areas. The regional distribution of coarse-grained detrital muscovite in Lower Cambrian rocks of Avalonian overstep sequences suggests a source region of dimensions considerably larger than any presently exposed in Appalachian segments of the Avalon Composite Terrane. Late Proterozoic tectonic reconstructions locate the Avalon Composite Terrane adjacent to northwestern South America, thereby suggesting a possible source within Late Proterozoic PanAfrican – Brasiliano orogens. Detrital muscovite from clastic sequences of the proximally derived, Lower Carboniferous (Tournaisian) Horton Group and the more distal Upper Carboniferous (Westphalian D – Stephanian) Pictou Group in Nova Scotia records 40Ar/39Ar spectra that define plateau ages of ca. 390–380 Ma (Horton Group) and and ca. 370 Ma (Pictou Group). Finer grained fractions from samples of the Horton Group display more internally discordant age spectra defining total-gas ages of ca. 397–395 Ma. A provenance for the finer muscovite may be found in southern Nova Scotia where Cambrian–Ordovician turbidites of the Meguma Group display a regionally developed micaceous cleavage of this age. The ca. 390–380 Ma detrital muscovites probably were derived from granite stocks presently exposed in proximal areas of northernmost Cape Breton Island. A more distal source for the ca. 370 Ma detrital muscovites in the Pictou Group is suggested by its original extensive distribution, although a local, possibly recycled, source may also have been present. The presence of only 400–370 Ma detrital muscovite suggests a rapidly exhumed orogenic source with characteristics similar to those of crystalline rocks presently exposed in the Cape Breton Highlands and (or) the Meguma Terrane.

550–580 Ma magmatism in Cape Breton Island (Nova Scotia, Canada): the product of NW-dipping subduction during the final stage of amalgamation of Gondwana

Cape Breton Island is host to widespread magmatic activity dated between ∼ 550 and 580 Ma. In the south, its products include the Capelin Cove pluton and host Coastal Volcanic Belt; both are composed of bimodal, tholeiitic-calc-alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system. Their trace element and Nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber. In contrast, the Indian Brook and Kerrs Brook plutons and Price Point volcanic rocks in central Cape Breton Island have higher contents of strongly incompatible trace elements and resemble modern arc-related calc-alkaline rocks emplaced on a moderately thick continental crust. Comparison with available data for other 550–580 Ma igneous rocks across Cape Breton Island indicates that they were produced above a NW-dipping subduction zone. This convergence was synchronous with the final stage of globalscale orogenic activity that culminated with the final amalgamation of Gondwana. Available detrital zircon ages in late Neoproterozoic (∼ 650-545 Ma) sediments in the Antigonish Highlands (Avalon Composite Terrane) suggest derivation from the Amazon craton. It is proposed that the Avalon Composite Terrane formed off northwestern South America during the late Neoproterozoic.

Silurian orogenesis in the western Penobscot Bay region, Maine

New 40Ar/39Ar mineral ages from rocks collected west of Penobscot Bay, Maine, indicate this region was regionally deformed, metamorphosed to amphibolite facies conditions, and intruded by plutons in Silurian times rather than in the Devonian as previously assumed. Disturbed hornblende age spectra, along with the presence of some Devonian felsic plutons and extensive retrograde metamorphic textures do suggest, however, that these rocks were subsequently affected by low-grade Devonian thermal events. In sharp contrast, rocks west of the Sennebec Pond thrust fault, a major tectono-stratigraphic boundary in this region, lack a significant Silurian tectono-thermal signature, and instead record the effects of intense Devonian deformation and high-grade regional metamorphism. The data suggest the two regions experienced very different pre-Devonian histories and were most likely juxtaposed by the Sennebec Pond thrust fault in latest Silurian to Early Devonian time. Rocks now exposed east of the Sennebec Pond fault probably occupied much higher structural levels during Devonian orogenesis and were not subjected to the same intense Devonian deformation and metamorphism as those rocks now found to the west of this structure. The Silurian tectonism now recognized in this region bears striking resemblance to events of similar age recorded along the northwest margin of the Avalon composite terrane throughout much of Atlantic Canada. This greatly extends the zone of Silurian orogenesis in the northern Appalachians and requires that previous models of New England middle Paleozoic tectonism be significantly revised.

Kinematics of the Rockland Brook Fault, Nova Scotia: Implications for the interaction of the Meguma and Avalon terranes

The Cobequid-Chedabucto fault system of northern mainland Nova Scotia represents the surface expression of the Avalon-Meguma terrane boundary, but because it is exposed at high crustal levels in the Cobequid Highlands, the fault system provides little information as to the kinematic relationships of the two terranes in this area. In the eastern Cobequid Highlands, the Rockland Brook Fault (RBF) is exposed within the more deeply eroded highlands massif and juxtaposes units of widely varying ages and lithologies. Therefore, this fault is better suited to define the nature and timing of fault movement associated with Avalon-Meguma terrane interaction. In several large Carboniferous plutons along the length of the RBF, and in previously deformed Precambrian rocks, mylonitic foliation orientations are predominantly east-west trending and mineral lineations plunge southeast. Kinematic indicators such as minor fold vergence, porphyroclast systems, asymmetric boudins, shear-band fabrics, and preferred recrystallization orientations indicate dextral shear. These data are taken to infer that the central section of the RBF is dominated by dextral strike-slip motion. Transpression occurs locally where the RBF curves into restraining bends. Kinematic data in these bends indicate top to the northwest thrusting. At the easternmost extent of the RBF, high-level brittle normal faults predominate in the locally extensional environment. The timing of RBF movement is constrained only by the ca 360 Ma granite bodies which it deforms and by the Westphalian sedimentary rocks which are affected by only the latest stages of movement. These kinematic data are consistent with previously published kinematic models for the interaction of the southern margin of the Avalon Composite Terrane with the Meguma Terrane in mainland Nova Scotia. These models suggest that regional dextral shear was accompanied by localized components of transpressional thrusting, wrench tectonism, and small-scale sedimentary basin development during Devonian to Carboniferous terrane interaction.

Detrital zircon geochronology of a conglomerate in the northeastern Cape Breton Highlands: implications for the relationships between terranes in Cape Breton Island, the Canadian Appalachians

Cape Breton Island has been interpreted as consisting of four zones of pre-Carboniferous rocks, but the relationships among them are controversial. To help resolve the controversy, we have dated detrital zircons from a conglomerate (part of the Cheticamp Lake Gneiss) in the Aspy terrane in the northeastern Cape Breton Highlands using the U–Pb method. The following ages were obtained: 462 ± 2 Ma (Middle Ordovician); ~492–488 Ma (6 ages; Early Ordovician); 552 ± 3 Ma (latest Precambrian–Early Cambrian); 620 ± 13 and 687 ± 4 Ma (Cadomian); and 809 ± 17, 1423 ± 10, 1462 ± 12, 1605 ± 14, 1644 ± 4, and 1911 ± 5 Ma (Proterozoic). The Middle Ordovician age sets a maximum age limit for deposition of the conglomerate, and supports an Ordovician–Silurian age for the Cheticamp Lake Gneiss. The Early Ordovician, latest Precambrian–Early Cambrian, and Cadomian ages match published ages from the Bras d'Or terrane (and its correlatives) and the Mira terrane (and its correlatives), and indicate provenance of the conglomerate from both terranes. They also indicate that the Bras d'Or and Mira terranes had been connected by the time of deposition of the conglomerate. The combination of the Cadomian and the Proterozoic ages is typical of parts of South America, supporting a suggestion that the Avalon Composite Terrane was derived from South America.

Structural and 40Ar/39Ar mineral age constraints for the tectonothermal evolution of the Green Head Group and Brookville Gneiss, southern New Brunswick, Canada: Implications for the configuration of the Avalon composite terrane

AbstractIn the late Precambrian Avalon composite terrane of the Canadian Appalachians, the local juxtaposition of Avalonian successions against gneiss complex–platformal metasedimentary rock associations of uncertain relationship to the Avalonian overstep sequence has raised important questions about the configuration of the composite terrane. Typical of this relationship is the juxtapostion of Avalonian arc‐related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and metacarbonate–quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of the predominantly greenschist facies Green Head Group accompanied development of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow in carbonate rocks and the development of a regional foliation was followed by NW‐directed shortening and the local development of a penetrative axial planar fabric that intensifies towards the shear zone. Associated structural elements suggest regional dextral transpression, consistent with the metamorphic contrast across the shear zone. Steeply plunging east–west folds may record younger, sinistral movement on associated NE–SW faults. Deformation coincident with metamorphic culmination in the Brookville Gneiss produced a gneissic foliation that was later deformed to produce widespread minor folds of sheath‐like geometry. These folds are best developed proximal to the shear zone where they locally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kinematic indicators within the gneiss are predominantly dextral.36Ar/40Ar versus 39Ar/40Ar isotope‐correlation ages recorded by metamorphic hornblende suggest that regional cooling of the Brookville Gneiss through ca. 500°C occurred at ca. 540 Ma, providing a minimum age for initial deformation and concomitant metamorphic culmination in the gneiss. 40Ar/39Ar plateau ages for metamorphic muscovite suggest cooling through ca. 375°C at ca. 500–520 Ma, providing a minimum age for progressive deformation in both lithotectonic sequences. Low temperature age discordance in the muscovite spectra suggest partial rejuvenation in the mid‐ and late Palaeozoic.Protracted Cambrian tectonothermal activity in the Brookville assemblage contrasts with the Avalonian tectonostratigraphic record of the Caledonia assemblage in which late Precambrian arc‐related packages are overstepped by Cambrian–Ordovician shallow marine strata. Significant Cambrian separation between the two assemblages is therefore suggested, despite Precambrian similarities in their tectonothermal evolution. Separation as a consequence of terrane dispersal is suggested, and may imply a significant rearrangement of the Avalon composite terrane at this time. Final juxtaposition of the two assemblages pre‐dates their shared late Palaeozoic rejuvenation, and may correspond to an earlier, mid‐Palaeozoic thermal overprint correlated with tectonothermal activity accompanying accretion of the Avalon and outboard Meguma terranes to more inboard tectonic elements of the northern Appalachians.

40Ar/39Ar whole-rock phyllite ages from late Precambrian rocks of the Avalon composite terrane, New Brunswick: evidence of Silurian–Devonian thermal rejuvenation

Several variably deformed and metamorphosed, late Precambrian volcanic–sedimentary successions have been recognized within the Avalon composite terrane exposed in the Caledonian Highlands of southern New Brunswick. Whole-rock samples of metasedimentary phyllite and phyllitic metatuff from the oldest (ca. 600–635 Ma) Avalonian succession display similar, internally discordant 40Ar/39Ar age and apparent K/Ca spectra. Intermediate-temperature gas fractions were experimentally evolved solely from very fine grained, cleavage-aligned white micas. These yield apparent ages between ca. 430 and 410 Ma, and are interpreted to closely date a static Late Silurian – Early Devonian thermal rejuvenation.Evidence for a Silurian – Devonian thermal event has not been previously documented in Avalonian rocks of the Caledonian Highlands (Caledonia assemblage). However, a thermal overprint of similar age (ca. 400 Ma) is recorded by metamorphic muscovite in high-grade gneisses and platformal metasedimentary rocks (Brookville assemblage), which are in tectonic contact with the low-grade Caledonia assemblage. These potentially correlative thermal overprints may provide minimum age constraints on the juxtaposition of these contrasting tectono-stratigraphic assemblages, which are likely to have been palinspastically separate tectonic elements during the earliest Paleozoic.

Geochemistry and tectonic discrimination of Late Proterozoic arc-related volcaniclastic turbidite sequences, Antigonish Highlands, Nova Scotia

The Late Proterozoic (ca. 618–610 Ma) Georgeville Group of northern mainland Nova Scotia lies within the Avalon Composite Terrane and consists of subgreenschist- to greenschist-facies mafic and felsic volcanic rocks overlain by volcaniclastic turbidites that were deposited in an ensialic basin within a rifted volcanic arc. Geochronological data indicate that the volcanic and sedimentary rocks are coeval. The geochemical and isotopic signatures of the sedimentary rocks are attributed to erosion of the coeval Avalonian volcanic rocks that flank the basin and are consistent with synorogenic deposition. There is no evidence of significant chemical contribution from Avalonian basement.Knowledge of the tectonic setting facilitates the testing of published geochemical discriminant diagrams for clastic sedimentary rocks. Discrimination diagrams using ratios such as K2O/Na2O and Al2O3/(CaO + Na2O) give inconclusive results, probably due to elemental mobility during secondary processes. Plots involving MgO, TiO2, and Fe2O3 detect the chemical contribution of mafic detritus, give much tighter clusters of data, and plot between Aleutian- and Cascade-type arc-derived sediments, suggesting a moderate thickness of continental crust beneath the arc.The arc-related signature of the Georgeville sedimentary rocks is clearly recognizable on ternary plots involving inter-element ratios of high field strength elements (e.g., Ti–Y–Zr, Nb–Y–Zr, and Hf–Ta–Th) in which the samples plot as mixing trends between mafic and felsic end members. Diagrams of this type may have widespread application to tectonic discrimination of sedimentary rocks because in most suites these ratios are relatively insensitive to sedimentary and metamorphic processes.

Appalachian basement and its intrusion by Cretaceous dykes, offshore southeast Nova Scotia, Canada

Aeromagnetic data collected between eastern Nova Scotia and southern Newfoundland provide new information about the offshore extension of the Avalon and Meguma terranes. A zone of short-wavelength anomalies that delineates Scatarie Ridge extends westward to the Late Proterozoic Fourchu Group in southeastern Cape Breton Island and eastward towards the Burin Peninsula of Newfoundland, suggesting that both regions belong to the same tectono-stratigraphic province of the Avalon composite terrane. A different zone of short-wavelength, discontinuously lineated anomalies at the northern edge of the Canso Ridge correlates with amphibolite-facies metamorphic rocks of the Meguma terrane on the Canso Peninsula, interpreted as an exhumed deeper metamorphic level of the Meguma terrane at its boundary with the Avalon terrane. The S-shaped pattern of long linear magnetic trends, characteristic of lower grade Meguma rocks on the southern flank of the Canso Ridge, indicates plastic deformation of the Meguma terrane during the Acadian orogeny when emplaced against the rigid Cape Breton Island block indentor. Analogous patterns occur off western Nova Scotia, suggesting little strike-slip motion occurred between the Meguma and Avalon terranes since the Acadian orogeny.Late Proterozoic rocks on Scatarie Ridge are intruded by Cretaceous diabase dykes. The diabase is alkaline with a within-plate geochemical signature, similar in composition to basalt flows in the Orpheus half-graben. A depleted-mantle model age TDM (Nd) of 731 Ma, εNd = +6.5, suggests that the magma was sourced from a lithospheric mantle reservoir involved in Late Proterozoic magmatic activity. Aeromagnetic data interpretation confirms the distribution of Cretaceous basalt flows and sills within Mesozoic sedimentary strata of the Orpheus half-graben previously outlined by seismic methods but was unable to differentiate between Proterozoic and Mesozoic intrusive rocks where the Proterozoic rocks lay near to the ocean floor.

Synthesis of Palaeozoic deformational events and terrane accretion in the Canadian Appalachians

Plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion, with some deformation associated with transform/transcurrent movements. Deformation associated with subduction varies between two end members: (1) where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes, a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over-riding plate; and (2) where continent-continent collision is occurring, a wide surface zone of accretionary deformation starts synchronously or with limited diachronism. Palaeozoic deformational events in the Canadian Appalachians correspond to narrow diachronous events in the Ordovician and Silurian, whereas Devonian, Carboniferous and Permian deformational events are widespread and broadly synchronous. Along the western side of the Canadian Appalachians, the Taconian deformational event starts diachronously throughout the Ordovician and corresponds to the north-north-west accretion of the Notre Dame, Ascot-Weedon, St Victor and various ophiolitic massifs (volcanic arc and peri-arc terranes) over cratonic North America. Within the eastern half of the Central Mobile Belt, the Late Cambrian-Early Ordovician Penobscotian deformational event corresponds to the ?south-easterly accretion of the Exploits subzone (various volcanic are and peri-arc terranes) over the Gander Zone (?continental rise). In the centre of the orogen, the Late Ordovician-Silurian Beothukan deformational event corresponds to the south-easterly accretion of the Notre Dame over the Exploits-Gander subzones. Along the south-eastern side of the Central Mobile Belt, the Silurian Ganderian deformational event corresponds to the north-north-east, sinistral transcurrent accretion of the Avalon Composite Terrane (microcontinent) over the Gander-Exploits zones. Along the south-eastern half of the orogen, the Late Silurian-Middle Devonian Acadian deformation event corresponds to the westerly accretion of the Meguma terrane (intradeep or continental rise) over the Avalon Composite Terrane. Affecting the entire orogen, the Late Devonian, Carboniferous and Permian, Acadian-Alleghanian deformational events correspond to the east-west convergence between Laurentia and Gondwana (continent-continent collision).

40Ar/39Ar Mineral Ages from the Southern Cape Breton Highlands and Creignish Hills, Cape Breton Island, Canada: Evidence for a Polyphase Tectonothermal Evolution

$^{40}Ar/^{39}Ar$ ages for hornblende and muscovite from plutons and metamorphic country rocks in the southern Cape Breton Highlands and Creignish Hills, together with previously published U-Pb, Rb-Sr and $^{40}Ar/^{39}Ar$ ages, document a complex tectonothermal history that included (1) episodes of plutonic activity at ca. 600, 565-560, 555, 500-490-(?470), and 410-395 Ma (all of which cooled through 500-400°C within ca. 20 m.y.); and (2) deformation in the late Precambrian (>550 Ma) and the Late Silurian-Middle Devonian (415-380 Ma). The latter was associated with evelopment of a positive flower-like structure during maintenance of medium-pressure, greenschist-amphibolite facies, regional metamorphic conditions that may be traced northward along the length of the Cape Breton Highlands. It may be correlated with the Kingston Complex and associated mylonite zone exposed in the Avalon Composite Terrane of southern New Brunswick. Northward it merges with a major high-grade, poly deformed belt within the Gan...

Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia

Late Precambrian volcanic–sedimentary belts in the Mira (Avalon) terrane of southeastern Cape Breton Island display differences in rock types, petrochemistry, and age, showing that they did not form contemporaneously above a single northwest-dipping subduction zone, as proposed in earlier models. The oldest rocks are 680 Ma mafic and felsic flows and tuffs, and abundant, mainly tuffaceous, sedimentary rocks in the Stirling belt. They are interpreted to have formed in a trough within or peripheral to a volcanic-arc complex. Northwest of the Stirling belt, the East Bay Hills, Coxheath Hills, and Sporting Mountain belts consist of ca. 620 Ma mafic to felsic subaerial pyroclastic rocks and flows and contemporaneous dioritic to granitic plutons. Both volcanic and plutonic rocks are calc-alkalic to high-K calc-alkalic suites, formed in a continental margin volcanic arc. A correlative 620 Ma plutonic suite intruded the western margin of the Stirling belt, suggesting that subduction may have been toward the present southeast. The ca. 575 Ma Coastal belt, located southeast of the Stirling belt, is significantly younger than the other belts and appears to represent a less evolved calc-alkalic to low-K continental margin volcanic-arc and intra-arc basin formed above a northwest-dipping subduction zone. These various volcanic–sedimentary belts were juxtaposed by lateral movements along major faults in the late Precambrian to form this part of the Avalon composite terrane. Subduction-related, calc-alkalic magmatism at ca. 620 Ma was apparently widespread throughout the Avalon terrane of the northern Appalachian Orogen. However, ca. 680 Ma magmatism like that in the Stirling belt has been documented elsewhere only in the Connaigre Bay Group of Newfoundland. Circa 575 Ma and younger subduction-generated igneous activity like that in the Coastal belt has been recognized in southern New Brunswick, but alkaline magmas were forming in extensional regimes in other areas of the Avalon terrane at that time.

40Ar/39Ar Amphibole Ages from the Kingston Complex, New Brunswick: Evidence for Silurian-Devonian Tectonothermal Activity and Implications for the Accretion of the Avalon Composite Terrane

The Kingston complex of southern New Brunswick, Canada, is a regional, NE-trending belt of mafic and felsic dikes bordered by dextral ductile shear zones. The complex separates two contrasting late Precambrian-early Paleozoic tectonostratigraphic assemblages near the NW margin of the Avalon composite terrane. Rhyolitic dikes within the complex record an early Silurian crystallization age. Dike orientations suggest that emplacement of the complex accompanied sinistral movement interpreted to reflect accretion of the Avalon composite terrane to cratonic North America. Mafic dikes within the complex display greenschist to epidote-amphibolite facies metamorphic assemblages that developed during dextral transpression. Amphibole concentrates from seven metamorphosed mafic dikes record Late Silurian to Early Devonian (ca. 416-390 Ma) vs. isotope correlation ages, interpreted to date closely the dextral transpressive deformation. Together with other recent age data, the results suggest that regionally extensive Silurian-Early Devonian tectonothermal events are recorded along the inboard margin of the Avalon composite terrane. The data also provide well-defined constraints for the transition from sinistral to dextral movement recorded along many major northern Applachian faults. In Maritime Canada, this transition is likely to have been associated with a kinematic reversal that occurred between initial Silurian accretion of the Avalon composite terrane and Silurian-Devonian docking of the outboard Meguma terrane.

Tectonic significance of the Late Proterozoic Economy River gneiss, Cobequid Highlands, Avalon Composite Terrane, Nova Scotia

A 734 ± 2 Ma U–Pb (zircon) age for the Economy River orthogneiss, Coboquid Highlands, Nova Scotia, is interpreted as being representative of a regionally extensive ca. 820–660 Ma event that is recorded in many parts of the Late Proterozoic – Early Cambrian Avalon Composite Terrane and the Gondwanan margin. The geochemistry of the gneiss is consistent with an arc environment. Although the gneiss may represent part of the sialic basement to the terrane, field relationships indicate that some of the basement is significantly older. The date may provide a minimum age for the platformal sedimentary rocks (Gamble Brook Formation) that the orthogneiss intruded and thus help constrain the Late Proterozoic paleogeographic position of Avalon relative to Gondwanaland.

Tectonic implications of 40Ar/39Ar mineral ages from late Precambrian – Cambrian plutons, Avalon composite terrane, southern New Brunswick, Canada

Within the Avalon composite terrane exposed in southern New Brunswick, late Precambrian, low-grade volcanic–sedimentary sequences are juxtaposed against late Precambrian gneisses (Brookville Gneiss) and older platformal metasedimentary rocks (Green Head Group) along the Caledonia Fault. Both assemblages host petrographically similar suites of calc-alkalic dioritic and granodioritic plutons. Those intruding volcanic–sedimentary sequences (Caledonia terrane) record ca. 615–625 Ma crystallization ages typical of arc-related magmatism throughout the Avalon composite terrane. However, 40Ar/39Ar age data from stocks intruding gneisses and platformal metasedimentary rocks (Brookville terrane) suggest significantly younger crystallization ages.36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende are interpreted to closely date postmagmatic cooling within six plutons: Fairville Granite (547 ± 1 Ma); French Village Quartz Diorite (539 ± 2 and 537 ± 1 Ma); Rockwood Park Granodiorite (529 ± 2 and 523 ± 3.5 Ma); Musquash Granite (526 ± 2 Ma); Milkish Head Granite (Red Bridge pluton, 520 ± 1.5 Ma); Lepreau Diorite (Talbot Road pluton, 519 ± 2 Ma and Hansen Stream pluton, 518 ± 1.5 Ma. A hornblende isotope correlation age of 530 ± 2 Ma from penetratively foliated amphibolite within the French Village Quartz Diorite suggests that the magmatic activity was locally accompanied by ductile shear. Muscovite within granitic pegmatite in the Brookville Gneiss records a 40Ar/39Ar plateau age of 510 ± 1 Ma interpreted to date final phases of associated magmatic activity.Arc-related magmatism extending into the Cambrian contrasts with the characteristic tectono-stratigraphic record in the Avalon composite terrane where late Precambrian igneous rocks are overstepped by Cambrian–Ordovician shallow-marine strata with only a local and minor record of rift-related volcanic activity. Although the Brookville terrane shows affinities with the Avalon composite terrane during the late Precambrian, the 40Ar/39Ar age data suggest that it was isolated as a distinct tectono-stratigraphic element by the Early Cambrian.