Meguma Terrane Research Articles

Orogenesis and Basin Development: U-Pb Detrital Zircon Age Constraints on Evolution of the Late Paleozoic St. Marys Basin, Central Mainland Nova Scotia.

The St. Marys Basin, along the southern flank of the composite Late Paleozoic Magdalen Basin in the Canadian Appalachians and along the Avalon-Meguma terrane boundary, contains Late Devonian-Early Carboniferous continental clastic rocks of the Horton Group that were deposited in fluvial and lacustrine environments after the peak of the Acadian orogeny. SHRIMP II (Geological Survey of Canada) data on approximately 100 detrital zircons from three samples of Horton Group rocks from the St. Marys Basin show that most of the zircons have been involved in a multistage history, recycled from clastic rocks in the adjacent Meguma and Avalonian terranes. Although there is a minor contribution from Early Silurian (411 Ma) and Late Devonian suites (ca. 380-370 Ma), Neoproterozoic (ca. 700-550 Ma) and Paleoproterozoic (ca. 2.0-2.2 Ga) zircon populations predominate, with a minor contribution from ca. 1.0-, 1.2-, and 1.8-Ga zircons. Published U-Pb single-zircon analyses on clastic sedimentary rocks indicate that the Meguma and Avalon terranes have different populations of detrital zircons, sourced from discrete portions (Amazonian and West African cratons) of the ancient Gondwanan margin. Both terranes contain Neoproterozoic and Late Archean populations. The SHRIMP data, in conjunction with published sedimentological and geochemical data, indicate that the Horton Group basin-fill sediments are largely the result of rapid uplift and erosion of Meguma terrane metasedimentary and granitoid rocks immediately to the south of the St. Marys Basin during the waning stages of the Acadian orogeny. Regional syntheses indicate that this uplift occurred before and during deposition and was a consequence of dextral ramping of the Meguma terrane over the Avalon terrane along the southern flank of the Magdalen Basin.

Pre-Mesozoic basement rocks offshore Nova Scotia, Canada: New constraints on the accretion history of the Meguma terrane

Basement samples from offshore wells and geophysical data show that the Meguma terrane forms the basement of Mesozoic–Cenozoic sedimentary basins as far as 200 km offshore Nova Scotia. The data provide constraints on Avalon-Meguma terrane interaction during middle to late Paleozoic events in the Appalachian orogen. The basement consists of metasedimentary rocks intruded by late Paleozoic granitoid plutons, principally magnetite-bearing granodiorites, which contrast with more peraluminous ilmenite-bearing Devonian granodiorite and granite onshore. Offshore tonalites have a subduction-related trace element signature that is similar to the signature in minor Devonian gabbro and tonalite intrusions on land. Nd, Sr, and Pb isotopes suggest three sources for Meguma terrane plutons: (1) 2 Ga basement (possibly Saharan Shield) and sediments derived therefrom—this basement may be present beneath the southwestern Scotian Shelf; (2) Late Proterozoic underthrust Avalonian basement and basement autochthonous to the Meguma terrane; and (3) melts from mantle last remobilized in the Late Proterozoic Pan-African orogeny. Seismic reflection profiles show Meguma terrane thrust northward over a wedge of Avalon basement extending 30 km south of the surface suture. There is no evidence that the Devonian tonalites are directly related to subduction. The lack of either ophiolites or Paleozoic pelagic sediment suggests that convergence of the Avalon and Meguma terranes was achieved principally by strike-slip motion resulting from low-angle continent-continent convergence, with only a minor thrust component. The widespread Devonian-Carboniferous igneous activity of the amalgamated Meguma and Avalon terranes was a consequence of regional extension, which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material.

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Geochemical and isotopic data from the clastic rocks of the Namurian Lismore Formation in mainland Nova Scotia identify key episodes of tectonic activity during the development of the Maritimes Basin in Atlantic Canada. The Lismore Formation forms part of the Mabou Group and is an upward-coarsening 2500 m thick fluvial sequence deposited in the Merigomish sub-basin along the southern flank of the Maritimes Basin. Based on stratigraphic evidence, the Lismore Formation can be divided into upper and lower members which reflect variations in depositional environment and paleoclimate. The geochemical and isotopic data may also be subdivided into two groupings that primarily reflect varying contributions from accessory phases, clay minerals, or rock fragments. This subdivision occurs 115 m above the base of the upper member. The data from the lower grouping (group A) show an important contribution from underlying Silurian rocks, with a relatively minor contribution from Late Devonian granitoid rocks from the adjacent Cobequid Highlands and possibly metasedimentary rocks from the Meguma Terrane to the south. The data from the upper grouping (group B) reveal a more important contribution from the Cobequid Highlands granitoid rocks. This variation in geochemistry is thought to constrain the age of renewed motion and uplift along the faults along the southern flank of the Maritimes Basin and, more generally, suggests that geochemical and isotopic data of continental clastic rocks may help constrain the age of tectonic events that influence deposition of basin-fill rocks.

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.

Precise U-Pb dating of Meguma basement xenoliths: new evidence for Avalonian underthrusting

U-Pb zircon and monazite dates for granulite-facies basement xenoliths from the Popes Harbour dyke on the eastern shore of Nova Scotia provide strong evidence that the Meguma terrane overlies Avalonian basement. Slightly discordant (1.6%), "facetted" zircons from a mafic granulite indicate a minimum crystallization age of ~629 Ma, with near-concordant (0.7%) rounded zircons suggesting a maximum age for the last metamorphic event affecting the zircons at ~575 Ma. Two near-concordant (-0.9 to 0.4%) monazite fractions from a pelitic granulite indicate a major metamorphic disturbance at 378 ± 1 Ma, ~10 Ma prior to dyke entrainment and coincident with Meguma regional metamorphism. Projections from 378 Ma through four highly discordant (15-42%) metapelite zircon fractions give provenance ages between 880 and 1050 Ma and two others project to maximum ages of ~1530 Ma. Unlike Meguma sediments which lack Grenvillian-age (~1 Ga) detrital zircons and are dominated by ~2000 Ma detrital zircons, these dates indicate a dominantly Grenvillian-age provenance for the pelitic xenoliths. The "Avalonian" igneous, metamorphic, and provenance ages from the xenoliths suggest the Meguma rests on Avalonian basement. Because Avalonian sediments need a Grenvillian provenance and Meguma sediments lack such a source but require a [Formula: see text]2.0 Ga component missing in the xenoliths, it seems unlikely the Meguma was deposited on Avalonian crust. Thus the dating places on firmer footing the suggestion from earlier structural, seismic, and geochemical work that the Meguma structurally overlies Avalonian terrane. Thrusting occurred between the time of earliest Meguma deformation (~400 Ma) and intrusion of the xenolith-bearing dyke (~370 Ma).

Geophysical and structural signatures of syntectonic batholith construction: the South Mountain Batholith, Meguma Terrane, Nova Scotia

SUMMARY The Late Devonian South Mountain Batholith is a very large (7000 km2) composite peraluminous granitoid complex situated within the Meguma Terrane of the northern Appalachians. It is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380‐370 Ma during the Acadian Orogeny, i.e. during the collision of Gondwana with the eastern margin of North America. A significant geophysical and geological database makes the South Mountain Batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen. Gravity models reveal the plutons have flat or gently dipping floors at approximately 7.0 km depth and aspect ratios >651. They are underlain by deeper (>10 km) elongate northeast‐southwest-trending roots that may indicate magma feeder zones. Dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale. The very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement; space was mostly created by vertical displacements of country rocks. The data are consistent with a laccolithic model for syntectonic batholith assembly. The laccolithic plutons may have been emplaced at the base of the Meguma Supergroup metasedimentary rocks, suggesting a maximum thickness of approximately 7.0 km for the supracrustal rocks in the Meguma Terrane.

Fuzzy Weights of Evidence Method and Its Application in Mineral Potential Mapping

This paper proposes a new approach of weights of evidence method based on fuzzy sets and fuzzy probabilities for mineral potential mapping. It can be considered as a generalization of the ordinary weights of evidence method, which is based on binary or ternary patterns of evidence and has been used in conjunction with geographic information systems for mineral potential mapping during the past few years. In the newly proposed method, instead of separating evidence into binary or ternary form, fuzzy sets containing more subjective genetic elements are created; fuzzy probabilities are defined to construct a model for calculating the posterior probability of a unit area containing mineral deposits on the basis of the fuzzy evidence for the unit area. The method can be treated as a hybrid method, which allows objective or subjective definition of a fuzzy membership function of evidence augmented by objective definition of fuzzy or conditional probabilities. Posterior probabilities calculated by this method would depend on existing data in a totally data-driven approach method, but depend partly on expert's knowledge when the hybrid method is used. A case study for demonstration purposes consists of application of the method to gold deposits in Meguma Terrane, Nova Scotia, Canada.

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

40Ar/39Ar dating of ribbon-textured veins and wall-rock material from Meguma lode gold deposits, Nova Scotia: implications for timing and duration of vein formation in slate-belt hosted vein gold deposits

Results of 15 40Ar/39Ar age spectra for whole-rock argillite samples collected from within and adjacent to veins from eight Meguma gold districts in the Meguma Terrane of southern Nova Scotia are presented. The samples give excellent plateau ages (i.e., concordance of plateaux, correlation, and integrated ages) that range from ca. 379 to ca. 403 Ma, and there is no apparent correlation of age with bulk rock composition (K2O, Al2O3) or sampling localities. In addition, apparent ages for samples from a district are similar within analytical error, except for one deposit, and there is no difference for samples from within ribbon-textured veins versus samples collected outside of the gold districts (i.e., in areas free of quartz veining). The results of this work compare well to previous whole-rock 40Ar/39Ar dating of Meguma Group samples and we concur with previous workers that the range in dates (i.e., 380-410 Ma) reflects diachronous cooling of the area through the intracrystalline retention temperature for argon in mica (i.e., approx. 300-350°C). However, with respect to the vein samples, there is a marked difference between the 40Ar/39Ar ages of vein-hosted whole-rock samples and hydrothermal minerals (amphibole, muscovite, biotite) from the same deposits previously dated, which indicates that whole-rock samples have retained their metamorphic ages and have not been reset by the later hydrothermal event responsible for vein formation, despite being incorporated within the high-temperature fluids (approx. 400-450°C). This discrepancy in ages indicates that the wall rocks and veins were in thermal disequilibrium, as the vein temperatures were well above that required to cause diffusion of argon out of mica phases within the whole-rock samples, and implies therefore that the fluids must have been derived from depth. The results have the following important implications for models of vein formation: (i) vein formation was rapid and is consistent with models of hydrofracturing due to fluid overpressure, and (ii) the vein-forming fluids were derived from depth and cannot have been produced by a lateral secretion processes whereby fluids and gold are derived from the Meguma Group.

A review of the mesothermal gold deposits of the Meguma Group, Nova Scotia, Canada

The gold province of the Meguma Terrane in Nova Scotia contains several classic examples of turbidite-hosted mesothermal gold deposits. There are over 300 gold occurrences and deposits hosted by the Cambro–Ordovician Meguma group rocks in southern Nova Scotia, Canada. The Meguma Group consists of two formations: (1) a lower, sand-dominated flysch (Goldenville Formation) and (2) an upper shaley flysch (Halifax Formation). Subdivision of the Goldenville Formation into several map units is leading to a better understanding of the stratigraphic constraints on the gold mineralization. Although detailed stratigraphic investigations in all of the major gold-producing areas is still ongoing, it is evident that the Steve's Road–Beaverbank and the Mt. Uniacke stratigraphic intervals contain most of the major past gold producers in the province. The Meguma Group rocks are variably deformed into upright gently to moderately doubly-plunging folds with multiple cleavages. The strata are metamorphosed from greenschist to amphibolite facies and are subsequently intruded by ca. 370 Ma peraluminous granites and minor mafic intrusions. The gold deposits can be divided into three main types: (1) high grade (approximately 15 g/t Au) narrow gold-bearing quartz veins; (2) low-grade (0.5–4 g/t Au) slate/argillite hosted; and (3) low-grade meta-sandstone hosted. A combination of two or more of the above can also occur. Almost all of the historic production (47 million g Au) has come from the high-grade veins within 200 m of surface. A variety of vein types occur although most of the gold produced came from bedding concordant, stockwork and fissure veins. Isotopic investigations and dating of the vein minerals indicate that the mineralization was related to migration of metamorphic fluids from multiple reservoirs in the waning stages of the Acadian Orogeny. Disseminated low-grade gold, with minor electrum and associated intermetallic compounds and metal alloys have recently been recognized in host rocks at several of the former gold mining areas in Nova Scotia. The presence of disseminated gold in combination with high-grade vein mineralization greatly enhances the economic viability of gold deposits in the Meguma Group. The association of disseminated gold and narrow vein-hosted deposits has important exploration implications for similar mesothermal gold districts elsewhere.

Syn-Acadian emplacement model for the South Mountain batholith, Meguma Terrane, Nova Scotia: Magnetic fabric and structural analyses

The anisotropy of magnetic susceptibility and the structural geology of the Late Devonian South Mountain batholith (Meguma Terrane, Nova Scotia) are used to characterize its emplacement and structural evolution. The South Mountain batholith is the largest peraluminous granitoid complex within the Appalachian orogen. It is a composite batholith made up of stage 1 granodiorites and monzogranites and stage 2 leucomonzogranites and leucogranites. The anisotropy of magnetic susceptibility was used to map the biotite petrofabric at the scale of the batholith. The biotite fabric pattern is similar to the pattern of folding and shearing of the feldspar megacryst foliation, documented at the outcrop scale. In the stage 1 plutons, the magnetic foliation is deformed into a girdle about the horizontal, northeast-southwest—trending magnetic lineation, which is parallel to regional fold axes and extension lineations in the country rocks. In the stage 2 plutons, a horizontal northeast-southwest– to east-west–trending magnetic lineation is also present, but the horizontal orientation of the magnetic foliation is widely preserved. The folding and shearing of the foliations and the horizontal northeast-southwest–trending lineation that is pervasive throughout the South Mountain batholith are interpreted to record Acadian tectonic deformation of the plutons as they crystallized. Acadian deformation continued during the latest stages of crystallization, resulting in the localization of mineral deposits and greisens along shear zones, faults, and joints. A model is presented for the syntectonic emplacement of the South Mountain batholith as a laccolithic complex.

Biostratigraphie et paléobiogéographie du Siluro-Dévonien de la zone de Meguma (Nouvelle-Écosse, Canada)

Studies of fossils collected from the Nictaux–Torbrook and Bear River synclines (Nova Scotia) allow a reexamination of the Siluro-Devonian stratigraphy of the Meguma terrane. The location of this terrane relative to the main paleocontinents of the circumatlantic domain during this period is discussed. Wenlockian, Ludlovian, and Pridolian biostratigraphic horizons have been dated. The benthic fauna of the Torbrook Formation are assigned here to the Lochkovian, Pragian, and Lower Emsian. The Pridolian fauna shows northern European affinities. Thus, the Meguma terrane probably belonged to the North Atlantic domain, as for the Avalon terrane, and occupied a southern position in the "Euramerica" plate during this period. The Devonian fauna belongs to the "old world realm" and shows north Gondwanian and Rhenish affinities. As early as the Lochkovian, species known from both the Rhenish and north Gondwanian domains are recognized in Meguma, as well as others reported so far only from northwestern Africa, Iberian Peninsula, and (or) Armorican Massif. The presence of Rhenish fauna confirms the postulated location for the Meguma terrane during the Upper Silurian since this fauna is representative of southern Baltica marginal areas in Europe. On the other hand, the presence of north Gondwanian fauna implies close relationships with western European margin of Gondwana. The lack of a wide oceanic gap separating north Gondwana and Euramerica can explain the faunal exchanges during Lower Devonian times between western Europe and easternmost Appalachian province despite the presence of a physical barrier.

The margins of Avalonia

During Cambrian and earliest Ordovician times, Avalonia was an area forming an integral part of the huge Gondwanan continent, probably along the northern margin of Amazonia, until in early Ordovician (late Arenig or Llanvirn) time it split off from Gondwana, leaving a widening Rheic Ocean to its south. Today, its southern margin with Gondwana extends northeast from east of Cape Cod, Massachusetts, through Nova Scotia north of the Meguma terrane, and thence below sea level to the south of Newfoundland. On the eastern side of the present Atlantic, the southern margin may separate southwest Portugal from the rest of the Iberian Peninsula; it can be traced eastwards with more certainty from the south Cornwall nappes to a line separating the Northern Phyllite Belt (on the southern margin of the Rhenohercynian terrane) and the Mid-German Crystalline High. There is no certain evidence of Avalonian crust to the northeast of the Elbe Line. The northern margin of Avalonia extends westwards from south of Denmark to the British Isles, where it merges with the Iapetus Ocean suture between Scotland and England. Traced westwards, it crosses Ireland and reappears in northern Newfoundland to the east of New World Island, where it may follow the trace of the Dog Bay Line and the Cape Ray Fault. Recent work suggests that the northern margin of Avalonia may clip the northern tip of Cape Breton Island in Nova Scotia, and then enter the North American mainland at the Bay of Chaleur; it may then be traced from north and west of the Popelogan and Bronson Hill arcs to Long Island Sound near Newhaven, Connecticut. The Cambrian to Devonian faunas reflect the history of Avalonia: initially they were purely Gondwanan but, as Ordovician time proceeded, more genera crossed firstly the Tornquist Ocean as it narrowed between Avalonia and Baltica to close in latest Ordovician and early Silurian times, and secondly the Iapetus Ocean, so that by the early Silurian most of the benthic shelly faunas, apart from the ostracods, were the same round the adjacent margins of all three palaeocontinents.

Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions

Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian–early Ordovician, late Ordovician–early Silurian and middle–late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial εNd signatures range from +5.4 to −1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian–early Ordovician volcanism related to thermal decay of late Proterozoic arc magmatism during transtensional deformation; (2) middle Ordovician–early Silurian volcanism during sinistral telescoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and telescoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic–cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U–Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times.

An isotopic (C, O, Sr) study of vein gold deposits in the Meguma Terrane, Nova Scotia; implication for source reservoirs

Vein quartz, carbonate, and tourmaline from 19 Meguma gold deposits in the Meguma terrane of Nova Scotia have been analyzed for stable (delta 18 O, delta 13 C) and radiogenic ( 87 Sr/ 86 Sr) isotopes in order to assess the nature and origin of the vein-forming fluids. As with other mesothermal gold provinces, the Meguma gold deposits are well suited to such a study because carbonate is the next most abundant phase after quartz in these mesothermal lode gold deposits. All vein types have been sampled for quartz and carbonate and, in addition, all compositional and textural varieties of carbonate have been sampled. Vein quartz is of uniform isotopic composition with most 18 O values between 10.2 to 17.6 per mil (avg ca. 14 per mil), except for one deposit (West Gore Sb-Au) where values go to 19.4 per mil. There is no systematic variation for quartz within a deposit or position within the Meguma Group stratigraphy. Vein carbonate 18 O values range from 11.8 to 27.5 per mil, with most in the 13 to 16 per mil range. The relatively 18 O-enriched carbonates reflect exchange with low-temperature fluids based on analyses of quartz-carbonate pairs that indicate disequilibrium fractionation (Delta (sub quartz-carbonate) < or =0); this is best illustrated by the strong negative correlation between delta 18 O carbonate and Delta (sub quartz-carbonate) . The 18 O water is estimated at 10+ or -3 per mil for a temperature of vein formation of 350 degrees to 400 degrees C and using the appropriate mineral-water fractionation equations. Whereas delta 18 O water values partly overlap the field for magmatic fluids, the values are wholly consistent with a metamorphic fluid, and it is considered unlikely that a primary magmatic fluid signature has been substantially modified due to wall-rock influences given that mesothermal gold systems are sites of high fluid/rock ratios. Vein carbonate has delta 13 C values of -13.1 to -25.9 per mil, but a slight negative correlation between delta 13 C carbonate and Delta (sub quartz-carbonate) suggests that the primary values lie in the range -20 to -25 per mil. Thus, the delta 13 C values indicate a reduced, biogenic source for the carbon. Oxidation of the reduced carbon, as indicated by CO 2 in fluid inclusions, may have occurred via hydrolysis of graphite or dissolution of carbonate minerals, both of which occur in the wall-rock lithologies of the Meguma Group, the latter of which has the appropriate isotopic composition. The initial 87 Sr/ 86 Sr of the vein fluid, estimated from the analyses of 52 vein carbonates (17 deposits) and four tourmalines (three deposits), ranges from 0.70118 to 0.72284 and within deposits considerable variation is observed. There is insufficient data to quantify the extent of the low-temperature overprint which has modified the C and O isotope data, although it is likely that some influence is present. Nevertheless, the data cannot be reconciled by a source confined exclusively to the Meguma Group, which suggests, therefore, involvement of another reservoir(s). The isotopic heterogeneity can be explained by variable amounts of contamination of a primary fluid with radiogenic Sr derived from Meguma Group lithologies by interaction along the fluid path or at the site of vein formation concomitant with wall-rock alteration; as discussed above, a dominantly magmatic source is not considered feasible. This fluid source is suggested to be within the structural basement to the Meguma Group, and the Liscomb gneisses are the favored source based on the combined results of the Sr isotope data presented herein and previously published Pb isotope data. Collectively, the data indicate that a primary fluid of metamorphic origin has had its isotopic signature variably modified due to interaction with different reservoirs. The most affected isotopic systems are C and S (based on earlier work on delta 34 S values) which are abundant as graphite carbonate and sulfides in the Meguma wall rock, respectively. The range in 87 Sr/ 86 Sr values of the fluid also reflects contamination, but this was quite variable. The uniform delta 18 O water value for the fluid indicates that this was the least affected isotopic system, except for the later exchange of carbonate at low temperatures.

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.

Variscan accretionary complex of northwest Iberia: Terrane correlation and succession of tectonothermal events

The allochthonous terranes of northwest Iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the Variscan collision. Assuming that the existing ophiolites represent the suture formed by the closure of the Rheic ocean, the units in the footwall to the suture correspond to the outer edge of the Gondwana continental margin, and the units in the hanging wall are interpreted as fragments of the conjugate margin, represented by the Meguma terrane. This correlation establishes a precise link between circum-Atlantic terranes, and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded. Following the amalgamation of Avalon to Laurentia, the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between Laurentia and Gondwana.

Silurian Tectonics of Western Avalonia: Strain-Corrected Subsidence History of the Arisaig Group, Nova Scotia

The Arisaig Group is a Silurian succession of predominantly shallow marine clastic sediments overlying volcanics, exposed in northern mainland Nova Scotia. Sediment accumulation provides a record of the subsidence of western Avalonia during the interval when terranes were being accreted within the Canadian Appalachians. To calculate the amount of subsidence, one must correct the measured thicknesses for the effects of tectonic strain. Deformed fossils on bedding surfaces indicate strain ratios mainly between 1.2 and 1.6. An empirical porosity-depth relationship is used to correct for compaction. The subsidence curves are then adjusted to allow for variations in water depth and eustatic sea level. The resulting curves show significant variations in subsidence rate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian time, with deeper-water sedimentation during an early Ludlovian eustatic high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidence rate occurred during deposition of the Pridolian Stonehouse Formation. This episode was accompanied by a rapid increase in sediment supply, and a change in paleocurrent flow from southwest to northwest. The rapid Pridolian subsidence probably resulted from oblique collision between Avalonia and the Meguma Terrane to the south.

U-Pb geochronology of Late Palaeozoic plutons, Cobequid Highlands, Nova Scotia, Canada: evidence for Late Devonian emplacement adjacent to the Meguma-Avalon terrane boundary in the Canadian Appalachians

The Cobequid Highlands in the Canadian Appalachian orogen lie within Avalonia adjacent to the Meguma Terrane. U-Pb (zircon) data show that the age range of voluminous bimodal plutonism in the highlands is from 358 to 363 Ma (late Devonian). This age range is much narrower than that previously suggested by Rb/Sr geochronology and confirms that the Cobequid Highlands preserve the youngest large-scale plutonic event in the Canadian Appalachians. Late Palaeozoic tectonic history of the Appalachian orogen is profoundly influenced by predominantly dextral motion on the Avalon-Meguma terrane boundary. This age of plutonism is coeval with previously published 40Ar/39Ar (muscovite) plateau ages derived from shear zones in the Meguma terrane adjacent to the terrane boundary, which is defined by the Minas fault zone. The NNE trending structural grain of the Appalachian orogen is disturbed in this area by the E-W Minas fault zone and pluton emplacement may have been associated with motion along this terrane boundary.

Mylonite to megabreccia: Tracking fault events within a transcurrent terrane boundary in Nova Scotia, Canada

Transcurrent terrane boundaries commonly evolve into long-lived faults that preserve little evidence for early docking events. A remarkable exception is exposed at Clarke Head along the Appalachian Meguma terrane boundary in Nova Scotia, Canada, where a Late Carboniferous fault megabreccia contains Devonian (369 Ma: U-Pb zircon) granulite-grade mylonite fractured by veins filled with Visean amphibole (ca. 335 Ma: Ar-Ar). This fractured mylonite was later mixed with Early Carboniferous sedimentary rocks during megabrecciation (ca. 315–310 Ma). These three fault events are reflected in the tectonostratigraphic record. Devonian (ca. 370–360 Ma) transpressional terrane docking ramped Meguma up against Avalonia and shed clastic detritus across the fault system. The Visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface. The late Namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation. The Clarke Head megabreccia therefore preserves an episodic late Paleozoic fault history spanning some 55 m.y. during convergence between Laurentia and Gondwana and the assembly of the Pangean supercontinent.