Meguma Terrane Research Articles

Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone

U–Pb detrital zircon data from the Baixo Alentejo Flysch Group in the South Portuguese Zone show significant age differences between formations. The Visean Mértola Formation and Serpukhovian to early Bashkirian Mira Formation are dominated by zircons in the 316–388 Ma age range, whereas the late Bashkirian to late Moscovian Brejeira Formation is dominated by zircons with an age range of 498–687 Ma. Detrital zircons spanning an age range of 0.9–1.1 Ga are present in the Brejeira Formation but are absent in the Mértola and Mira formations. Detrital zircon ages of the Mértola and Mira formations indicate provenance from an extra-basinal source (Ossa–Morena Zone) with a minor intra-basinal contribution (South Portuguese Zone). The abundant presence of detrital zircon with age ranges of 500–750 and 0.9–1.1 Ga in the Brejeira Formation suggests a sediment source from the Avalon–Meguma terranes with limited recycling from the SW Portugal Domain. The different inferred source areas for the Baixo Alentejo Flysch Group formations are attributed to the presence of a forebulge that was formed in Mid-Visean times during the foreland phase of the South Portuguese Zone. The forebulge acted as a physical barrier separating sub-basins that accumulated the Mértola–Mira and Brejeira sediments respectively.

Does the Meguma Terrane Extend into SW England?

Does the Meguma Terrane Extend into SW England?

Stratigraphy, provenance and tectonic setting of the Lumsden Dam and Bluestone Quarry formations (Lower Ordovician), Halifax Group, Nova Scotia, Canada

<p><span style="font-family: 'Minion Pro','serif'; font-size: 12pt; mso-fareast-font-family: Cambria; mso-bidi-font-family: 'Times New Roman'; mso-ansi-language: EN-CA; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;">Cambrian to Ordovician metamorphosed clastic sedimentary rocks of the Meguma terrane have no correlatives elsewhere in Atlantic Canada but are similar to successions in North Wales. In the Meguma terrane, the Cambrian Goldenville Group, dominated by sandstone, is overlain by the Halifax Group, consisting mainly of fine-grained slate and siltstone. Within the Halifax Group widespread Furongian black slate units are overlain by greyer units with rare Early Ordovician fossils, assigned to the laterally equivalent Bear River, Feltzen, Bluestone Quarry, Lumsden Dam and Glen Brook formations. The type section of the Bluestone Quarry Formation, here defined, is on Halifax Peninsula, where four constituent members are recognized; the type section of the Lumsden Dam Formation is here defined in the Lumsden Dam region near Wolfville. Detrital zircons extracted from a sample of the Lumsden Dam Formation show a range of ages similar to those displayed by the underlying Goldenville Group, including abundant Neoproterozoic zircon representing Avalonian or Pan-African sources, and a prominent group of peaks between 1.95 and 2.2 Ga, probably representing sources in West Africa. A sample from the Glen Brook Formation east of Halifax shows a similar distribution. In contrast to the correlative Welsh successions, no influx of Mesoproterozoic zircon is seen in Early Ordovician samples, suggesting that, if the two basins were in close proximity in the Cambrian, they had diverged by the Early Ordovician, possibly as a result of strike-slip motion along the margin of Gondwana.</span></p>

Effects of fluid flow, cooling and deformation as recorded by40Ar/39Ar, Rb–Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)

Abstract40Ar/39Arin situUV laser ablation of white mica, Rb–Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb–Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle–Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The40Ar/39Ar spot age spectra also display relict age peaks at 477–465 Ma, 439 Ma and 420–428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician–Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

Precambrian to Ordovician sedimentary basins of Wales display contrasting histories across major NE-striking fault systems. Northeast of the Menai Strait Fault System the Monian Supergroup is a deformed succession of mainly metaclastic rocks. Within the fault system the Arfon Basin contains sandstone and slate overlying Neoproterozoic volcanic rocks. South of the fault system, in the Harlech Dome of the Welsh Basin, a distinctive succession of clastic rocks has been correlated with those of the Meguma Terrane in the northern Appalachian orogen, together comprising the domain Megumia. A detrital zircon analysis from Cambrian sandstone in the Harlech Dome is consistent with this correlation. However, the Early Ordovician Dol-cyn-afon Formation higher in the succession and the overlying Late Ordovician Conway Castle Sandstone show a more diverse provenance, consistent with derivation from the Monian Supergroup. Cambrian sandstone from the Llanberis Slates Formation in the Arfon Basin shows a distinct provenance dominated by a Neoproterozoic source. None of the samples analyzed contains Laurentian detritus. These results suggest that the Welsh Basin was juxtaposed with the Monian Supergroup and its Precambrian substrate along the Menai Strait Fault System by the Tremadocian, and indicate that the Iapetus Ocean remained open at least until the Silurian. The Cambrian detrital zircon record from the Arfon Basin does not show clear similarity to the Monian Supergroup, nor to the Welsh Basin and adjacent Midland Platform, indicating that the basin was isolated from these sedimentary sources within the fault system. The juxtaposition of these terranes probably took place during strike-slip to transpressional tectonism close to the northern margin of Gondwana during the Early Ordovician.

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

The Partridge Island block is a newly identified tectonic element in the Saint John area of southern New Brunswick, located south of and in faulted contact with Proterozoic and Cambrian rocks of the Ganderian Brookville and Avalonian Caledonia terranes. It includes the Lorneville Group and Tiner Point complex. The Lorneville Group consists of interbedded volcanic and sedimentary rocks, subdivided into the Taylors Island Formation west of Saint John Harbour and West Beach Formation east of Saint John Harbour. A sample from thin rhyolite layers interbedded with basaltic flows of the Taylors Island Formation at Sheldon Point yielded a Late Devonian – Early Carboniferous U–Pb (zircon) age of 358.9 +6/–5 Ma. Petrological similarities indicate that all of the basaltic rocks of the Taylors Island and West Beach formations are of similar age and formed in a continental within-plate tectonic setting. West of Saint John Harbour, basaltic and sedimentary rocks of the Taylors Island Formation are increasingly deformed and mylonitic to the south, and in part tectonically interlayered with mylonitic granitoid rocks and minor metasedimentary rocks of the Tiner Point complex. Based on magnetic signatures, the deformed rocks of the Tiner Point complex can be traced through Partridge Island to the eastern side of Saint John Harbour, where together with the West Beach Formation, they occupy a thrust sheet above a redbed sequence of the mid-Carboniferous Balls Lake Formation. The Tiner Point complex includes leucotonalite and aegirine-bearing alkali-feldspar granite with A-type chemical affinity and Early Carboniferous U–Pb (zircon) ages of 353.6 ± 5.7 and 346.4 ± 0.7 Ma, respectively. Based on similarities in age, petrological characteristics, alteration, iron oxide – copper – gold (IOCG)-type mineralization, and deformation style, the Partridge Island block is correlated with Late Devonian – Early Carboniferous volcanic–sedimentary–plutonic rocks of the Cobequid Highlands in northern mainland Nova Scotia. Deformation was likely a result of dextral transpression along the Cobequid–Chedabucto fault zone during juxtaposition of the Meguma terrane.

A petrological and geochronological study of a 360 Ma metallogenic event in Maritime Canada with implications for lithophile-metal mineralization in the Canadian Appalachians

The occurrence of ca. 360 Ma greisen-related molybdenite mineralization in the Clayton Hill biotite–muscovite leucomonzongranite (<1 km2) is unusual for the southwestern Meguma terrane of the Maritime Appalachians, where 380 Ma granite-related Sn mineralization dominates. The presence of extensive hornfelsing and occurrence of mineralized (Sn–W–Cu–Zn) skarns in the area suggests a larger intrusion occurs at depth. The intrusion and mineralization are constrained to 361 Ma based on concordant Re–Os (molybdenite) and 40Ar/39Ar (muscovite) ages, which equates to other ca. 360 Ma granites in the area, hence magmatism of this age is more widespread than previously considered. The intrusion is highly evolved (74 wt.% SiO2), metaluminous (A/CNK = 1–1.05; A/CNK = molar ratio [Al2O3/(CaO + Na2O + K2O)]) and depleted in cafemic and transition elements. Rare-earth elements (∑REE = 106–23 ppm) are strongly fractionated with light rare-earth elements (LREE) enriched (La/LuN = 10), and negative Eu anomalies. Biotite is Fe-rich (Fe/(Fe + Mg) = 0.75), and both biotite and muscovite are F deficient. These geochemical features indicate affinity to the other 360 Ma granites and suggest a similar petrogenesis with some A-type affinity. Mineralization occurs as molybdenite-bearing greisens and rusty miaroles lined with pyrite and enrichment in Cu–As–Bi–Au. Stable isotopes (O, S) indicate greisen formation at 450 °C from fluids with δ18OH2O = 7‰ and δ34SH2S = 2‰–3‰, which suggests an A- or I-type magmatic reservoir rather than a peraluminous magma. These data indicate the presence of a widespread magmatic event that is 20 Ma younger than the 380 Ma Sn-base metal event in southern Nova Scotia (e.g., East Kemptville). That this event is the same age as the world class Mount Pleasant Mo–Sn–W–Zn–In deposit of southern New Brunswick indicates additional potential for significant granite-related mineralization in this part of Atlantic Canada.

The provenance of Middle Jurassic sandstones in the Scotian Basin: petrographic evidence of passive margin tectonics 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Geological Survey of Canada Contribution 20110319.

The tectonic and geomorphological evolution of the Scotian margin and its hinterland is poorly known between Late Triassic rifting and the Early Cretaceous progradation of major deltas. This study determined sedimentary provenance of Middle Jurassic Mohican Formation sandstones from three wells using heavy minerals and mineral chemistry. Indicator minerals such as xenotime, altered ilmenite, and varietal types of garnet and tourmaline are similar to those in Hauterivian–Barremian sandstones in the western Scotian Basin, which are almost exclusively derived from the Meguma terrane. The wells adjacent to the Canso Ridge have more zircon and less ilmenite, indicating a greater contribution of polycyclic reworking, but with an ultimate source in the Meguma terrane. Zircon and ilmenite were likely derived in part from Carboniferous sandstones in eastern mainland Nova Scotia and Cape Breton Island. Any river drainage from the inboard terranes of the Appalachians either was diverted through the Fundy Basin or entered the easternmost Scotian Basin, where the Mohican Formation is 5.5 km thick, along the linear continuation of the southwest Grand Banks transform. Such sediment did not reach the Canso Ridge, suggesting that the Cobequid–Chedabucto fault zone in Orpheus graben was not a significant physiographic feature. This tectonically controlled paleogeography in the Middle Jurassic is quite different from that during active rifting in the Late Triassic – Early Jurassic. Middle Jurassic quiescence was followed in the Tithonian – Early Cretaceous by renewed tectonic uplift associated with rifting of Grand Banks from Iberia and Labrador from Greenland.

Detrital muscovite geochronology and the Cretaceous tectonics of the inner Scotian Shelf, southeastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin. 2Geological Survey of Canada Contribution 20120142.

Geochronology of detrital muscovite from the Lower Cretaceous of the central Scotian Basin has shown predominant supply of Meguma terrane detritus, including muscovite with Alleghanian (mid-Carboniferous to Permian) ages from the inner Scotian Shelf. In this study, 87 detrital muscovite grains from five wells in the eastern Scotian Basin, representing depositional ages from Tithonian to Early Albian, were dated by single-grain 40Ar/39Ar techniques. Previous work shows that the provenance of this sediment was different from wells in the central Scotian Basin. Compared with the central Scotian Basin, the muscovite age populations from the eastern Scotian Basin have more (20% versus 7%) pre-Devonian ages, derived from inboard terranes of the Appalachians. In both the central and eastern Scotian Basin, most old grains are found in the Tithonian and the Aptian–Albian, with fewer in the Valanginian and almost none in the Kimmeridgian and the Hauterivian–Barremian. These data suggest rapid unroofing of the Meguma terrane in the Kimmeridgian and Hauterivian–Barremian and greater supply of sediment from Newfoundland to the eastern Scotian Basin in the Tithonian and Aptian–Albian. Compared with the central Scotian Basin, which has most ages in the range 340–300 Ma, the muscovite age populations from the eastern Scotian Basin have a higher proportion of 420–340 Ma and in particular 300–260 Ma ages. Such late Alleghanian overprinting of argon ages is very rare on land in Nova Scotia, but in the Maritimes Basin, this was the time of uplift and erosion of strata.

Detrital zircon geochronology and polycyclic sediment sources, Upper Jurassic – Lower Cretaceous of the Scotian Basin, southeastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin.

Sources of Tithonian–Albian sediment in the Scotian Basin are interpreted from detrital zircon geochronology to test previous hypotheses about the sources and pathways of sediment to thick deltaic successions that are important hydrocarbon reservoirs. Sediment provenance influences reservoir quality, but also provides information on tectonism during rifting of the North Atlantic Ocean. More than 760 zircons were dated by laser ablation U–Pb methods from nine offshore wells and one borehole on land and were characterized by external morphology, internal zoning, and Th/U ratio. A Meguma terrane source to the LaHave Platform was confirmed by peaks in detrital zircon abundance at 550–650 Ma, 1.0–1.2 Ga, and ∼2.1 Ga. Samples from the Sable Subbasin show a large peak in detrital zircon abundance at ∼1050 Ma, with lower peaks from 400–650, ∼1480, ∼1650, ∼1860 Ma and 2.7 Ga, characteristic of inboard Appalachian terranes of Laurentide affinity. Many late Paleozoic to Neoproterozoic zircons are euhedral or subhedral, and apparently first cycle, as are a few older zircons that indicate transport from the rising rift shoulder in southern Labrador as far north as the Makkovik Province (∼1860 Ma). About half the zircons are rounded and polycyclic. Samples from the Abenaki Subbasin are similar, but late Paleozoic to Neoproterozoic zircons are rare and ∼40% of the Mesoproterozoic zircons are subhedral, implying a different Laurentide source through the Humber valley. Euhedral–subhedral unzoned zircons yielded two groups of Cretaceous dates: ∼105 Ma from the Cree Member, and ∼120 Ma from the Missisauga Formation.

Cambrian-Ordovician acritarchs in the Meguma terrane, Nova Scotia, Canada: Resolution of early Paleozoic stratigraphy and implications for paleogeography

Organic-walled microfossils (acritarchs) provide age constraints for the previously poorly dated Cambrian and Lower Ordovician turbiditic Goldenville and Halifax Groups in the northern Appalachian Meguma terrane of Nova Scotia and address controversies about stratigraphy and provenance. The oldest exposed formation of the ∼8-km-thick Goldenville Group contains the trace fossil Oldhamia , which suggests an age of late Early Cambrian, whereas the coticule-bearing uppermost formation yielded an acritarch species consistent with Middle Cambrian (Epoch 3) age. The conformably overlying ∼5-km-thick Halifax Group includes basal pyritiferous units that yielded a Late Cambrian (Furongian) assemblage of acritarch species, providing further confirmation that the underlying manganese-bearing formations are of Cambrian Epoch 3 age, and not Ordovician as recently claimed. Overlying nonpyritiferous formations contain the Early Ordovician graptolite Rhabdinopora flabelliformis flabelliformis and acritarchs of similar age. Samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later Tremadocian and Floian. The new fossil data confirm a gap in age of ∼30 m.y. between the Halifax Group and the overlying Silurian to Lower Devonian Rockville Notch Group. The new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the Meguma terrane. They also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north Wales, with which correlation has been proposed previously based mainly on lithological similarities.

Silurian magmatism in eastern Senegal and its significance for the Paleozoic evolution of NW-Gondwana

Submarine basalt and trachyte of the Nandoumba group occur in eastern Senegal within the Bassarides branch of the Mauritanides orogen. The unit forms part of the parautochthonous domain which is stacked between underlying Neoproterozoic to Paleozoic foreland and overlying Variscan nappes. The crystallisation age of the volcanic to subvolcanic rocks has been determined by U–Pb single zircon SHRIMP method at 428±5.2Ma whereas zircon xenocryst ages vary from 500 to 2200Ma. The shape of the xenocryst grains document proximal Neo- and Paleoproterozoic and distal Mesoproterozoic provenance areas for assimilated sediments. This is compatible with the Paleoproterozoic Birimian basement and Neoproterozoic cover rocks nearby whereas an origin from the Amazonian craton could be assumed for distal Mesoproterozoic zircons.Geochemical and Sm–Nd isotope whole rock analysis show that basalts of the Nandoumba group are similar to modern transitional to alkaline volcanic lavas in intraplate settings. Those basalts have a deep mantle source with a great contribution of a recycled mantle component such as EM1 and/or EM2. The basalts resemble in their composition those from the Meguma terrane of Nova Scotia which are of similar age suggesting a common source and therefore connection of Meguma with Gondwana during this period. Review of circum-Atlantic Silurian magmatism indicates ongoing fragmentation of NW-Gondwana that started in Cambro/Ordovician times.

40Ar–39Ar ages for detrital white mica in Meguma terrane, Nova Scotia, Canada: implications for provenance of the Goldenville and Halifax groups

Single-grain 40Ar/39Ar ages are reported for detrital white mica, along with conventional step-heating data for whole rocks, from low-grade metasandstone samples from the Goldenville, Halifax, and Rockville Notch groups in the Meguma terrane of southern Nova Scotia. The majority (166) of single grains from 11 samples yielded ages between ca. 615 and 350 Ma, and the remaining 12 grains yielded ages between ca. 1900 and 870 Ma. The late Neoproterozoic–Paleozoic age distributions are consistent with derivation of sediments from the rapidly uplifted flanks of an active rift, where partial to complete resetting of white mica ages occurred at ca. 520–500 Ma, preceding sediment deposition. The ca. 615 Ma ages may be relics of the original detrital white mica that existed in the source rocks prior to the rifting event. Ages from the Upper Silurian White Rock Formation appear to reflect this same ca. 520–500 Ma event, suggesting that sediments in the White Rock Formation were recycled from the Goldenville and Halifax groups. The older Precambrian ages are inferred to represent white mica in the source region, likely Amazonia. The whole-rock age spectra are discordant, with pronounced age gradients and no well-defined age plateaus. Initial gas released from five of the samples at low laboratory extraction temperatures (ca. 450–500 °C) yielded ages of ca. 260–300 Ma, not seen in the single-grain data, whereas gas released at the highest extraction temperatures yielded ages in the range ca. 510–530 Ma, possibly reflecting the principal result obtained from the single-grain data.

Probing the composition of unexposed basement, South Portuguese Zone, southern Iberia: implications for the connections between the Appalachian and Variscan orogens

Geochemistry and Sm–Nd and U–Pb (magmatic zircon) isotope data from a postcollisional batholith that crosscuts the allochthonous South Portuguese Zone (SPZ) of southern Iberia suggest that the basement is compositionally more juvenile than the exposed upper crust. The SPZ is an allochthonous terrane of the late Paleozoic Variscan orogen. The oldest exposed units in the SPZ are Late Devonian continental clastics, and as a result, the origins of the SPZ are unknown. Multifaceted inherited zircon cores from a granitoid batholith (Sierra Norte Batholith, SNB) reveal Neoproterozoic (ca. 561–647 Ma) and Mesoproterozoic ages (ca. 1075 – ca. 1116 Ma). Granitoid samples are characterized by εNd values ranging from +1.4 to –9.6 and model ages ca. 0.76–1.8 Ga. Conversely, the exposed Late Devonian clastics of the SPZ are characterized by more negative εNd values (–7.5 to –10.4). Taken together, U–Pb and Sm–Nd data indicate the lower crust that melted to yield the SNB was (i) Neoproterozoic (ca. 560–650 Ma) to Mesoproterozoic (ca. 1.0–1.2 Ga) in age, (ii) was not compositionally similar to the overlying Devono-Carboniferous continental detritus but was instead more juvenile, with model ages between ca. 0.9–1.2 Ga. This unusual relationship is similar to the relationship between the relatively juvenile basement and ancient upper crust documented in the exposed portion of the Meguma terrane in the northern Appalachians, which paleogeographic reconstructions show was immediately outboard of southern Iberia in the Late Devonian.

TEXTURE AND COMPOSITION OF GARNET AS A GUIDE TO CONTAMINATION OF GRANITOID PLUTONS: AN EXAMPLE FROM THE GOVERNOR LAKE AREA, MEGUMA TERRANE, NOVA SCOTIA

The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps 19–74 ) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps 22–71 ) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions ( e.g. , sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps 21–52 ), whereas type-3B crystals have a Mn-rich rim (Sps 15–21 ). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions ( e.g. , apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps 4–11 ) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps 8–16 ) was partially resorbed and overgrown by a Mn-rich rim (Sps 11–26 ). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.

Varietal heavy mineral analysis of sediment provenance, Lower Cretaceous Scotian Basin, eastern Canada

The origin of a major increase in sand supply in the Early Cretaceous of the Scotian Basin, offshore eastern Canada, has been investigated by analysis of heavy mineral varieties. The chemistry of detrital minerals from the Scotian Basin was compared with that of minerals from potential source rocks, sampled from older sedimentary successions, river sands and glacial tills on land. Heavy mineral varieties were established based on > 1000 high quality electron microprobe (EMP) analyses and used to classify heavy mineral types by Mineral Liberation Analysis (MLA), which uses a rapid scanning electron microscope particle analyser. Heavy mineral varieties were compared between the Cretaceous quartz arenites and subarkoses and samples from potential source areas. Different chemical varieties of the same mineral group have different grain size distributions, so that their relative modal abundances may be influenced by hydraulic sorting, but modal abundance of different varieties of tourmaline, garnet and spinel provides useful information. Widely used provenance-sensitive mineral ratios are difficult to interpret because of the abundance of polycyclic sources. Comparisons of the shape parameters of the individual mineral types are also useful for provenance identification. Aspect ratio in elongate grains, such as tourmaline and zircon, is a measure of polycyclic reworking, but is influenced by the initial shapes of minerals from different source rocks. Uniformity of varietal heavy minerals over a large area and stratigraphic range, and the covariance of spinel and zircon with quite different crystalline sources, indicate predominance of polycyclic sources to the Central Scotian Basin, probably principally from Carboniferous and Lower Paleozoic sandstones, in part sourced from western Newfoundland ophiolites. Heavy mineral varieties in the West Scotian Basin can be closely matched to sources in the adjacent Meguma Terrane of the Appalachians. All the data show that the sands of the Scotian Basin were derived from reworking of crystalline basement of the Appalachian orogen and its Paleozoic sandstone cover, uplifted during rifting of Iberia from the Grand Banks. There is no evidence of a significant supply of sand to the Scotian Basin from a proto-St Lawrence River.

An oxygen isotope study of two contrasting orogenic vein gold systems in the Meguma Terrane, Nova Scotia, Canada, with implications for fluid sources and genetic models

Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408 Ma; Dufferin, 380 Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their δ18Oquartz signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average δ18O values of +15.2 ± 0.9‰ (n = 16) for The Ovens and +15.7 ± 0.6‰ (n = 12) for Dufferin. Using an average δ18O value of +15.4‰ for the two deposits, the corresponding δ18OH2O values, calculated for 400°C and 350°C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4 ± 0.2‰ and +10.2 ± 0.2‰, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30 Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in δ18OH2O values for deposits when arranged in their relative stratigraphic position such that δ18OH2O values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy.

Cambrian successions of the Meguma Terrane, Nova Scotia, and Harlech Dome, North Wales: dispersed fragments of a peri-Gondwanan basin?

Abstract: The Meguma Terrane of Nova Scotia and the Harlech Dome of North Wales preserve similar sedimentary successions of Cambrian age. Both successions comprise a thick succession of early Cambrian sandstone turbidites, overlain by early to middle Cambrian alternating mud-rich and sand-rich units in which manganese is concentrated in two stratigraphic intervals. Above these, both successions comprise anoxic, organic-rich turbidites, shallowing upward into paler, more bioturbated Tremadocian mudstone with Rhabdinopora . Within the limited constraints of the available biostratigraphic and geochronological data, major changes in environment occurred synchronously in the two successions. Both successions show much greater similarity to each other than to adjacent successions on ‘Avalonia'. A detrital zircon analysis from the Rhinog Formation, low in the Harlech Dome succession, reveals distinct clusters of ages around 537 Ma and 2.0–2.1 Ga. A close similarity to analyses from the Meguma Terrane suggests proximity between the two terranes on the margin of Gondwana during the Cambrian Period. We suggest the term Megumia for the palaeogeographical domain that included the two successions, which was dispersed during subsequent Appalachian and Caledonian movements. These observations suggest that Megumia may have separated terranes previously regarded as parts of Avalonia. Supplementary material: U/Pb detrital zircon analytical data are available at http://www.geolsoc.org.uk/SUP18431 .

The evolutionary significance of a Lower Cambrian trace-fossil assemblage from the Meguma terrane, Nova Scotia

We report a diverse assemblage of trace fossils from the Lower(?) Cambrian High Head member of the Church Point Formation near Yarmouth, Nova Scotia. Based on the presence of sharp-based beds, load casts, and flute casts, along with other sedimentological criteria, the strata are interpreted to have a turbidite origin. Other characteristics, including wrinkle structures and high fidelity of trace-fossil preservation, suggest that the inter-episode biotope was influenced by the presence of biomats. The trace-fossil assemblage includes (in approximate order of abundance) Planolites , Helminthopsis , Oldhamia , Chondrites , Gordia , Cladichnus , Psammichnites gigas , Treptichnus , Phycodes , Lorenzinia , Palaeophycus , and Teichichnus . Trace fossils are increasingly abundant upwards in the section, but they are ultimately mitigated by an increase in (inferred) sedimentation rates. The trace-fossil assemblage, which bears many similarities to other Lower Cambrian locales (e.g., Puncoviscana Formation of northwest Argentina, Chapel Island Formation of southeastern Newfoundland), and the distribution of ichnofossils strongly support an earliest Cambrian age for the studied strata. More importantly, evolutionary trends can be interpreted from the trace-fossil assemblage, namely (1) an increase in size and diversity of animals in deep water; (2) an expansion of motile strategies used to coexist with biomats; and, (3) increasingly complex mining strategies below the sediment-biomat interface.

Early Jurassic magmatism on the northern margin of CAMP: Derivation from a Proterozoic sub-continental lithospheric mantle

Recent studies have shown that the sub-continental lithospheric mantle (SCLM) can either underlie regions for a long period of time or be replaced during delamination or plume magmatism. The occurrence and timing of mantle replacement may be identified by comparing the Sm-Nd isotopic signature of suites of basaltic rock within the same terrane over a range of time. In mainland Nova Scotia, the early Jurassic North Mountain Basalt (NMB) is widely recognized as the northernmost representative of the Central Atlantic Magmatic Province (CAMP), but it could also have been derived from the underlying SCLM (Avalonian) mantle. The Sm-Nd isotopic data from the NMB yield εNdt values (t = 200 Ma) ranging from + 0.1 to − 2.7. These values are significantly lower than the contemporary depleted mantle and indicate derivation from the underlying Proterozoic SCLM, rather than from the CAMP plume. Comparison with older mafic magmatic events indicates that the SCLM is isotopically indistinguishable from that inferred to yield Ordovician and Devonian mafic magmas in the Meguma terrane and Neoproterozoic to Devonian magmas in the Avalon terrane. These data indicate that the same SCLM underlay mainland Nova Scotia from Neoproterozoic to Jurassic times, and that Paleozoic tectonic events did not significantly detach the mantle from the overlying crust.