Avalon Zone Research Articles

Volcanic rocks in the Narragansett basin, southeastern New England: petrology and significance to early basin formation

A suite of volcanic rocks in the northwest corner of the Narragansett basin includes at least four basalt flows, two rhyolite flows, and associated pyroclastic rocks. The volcanics are interbedded with non-marine, sedimentary rocks within the lower part of the Wamsutta Formation and, until recently, were considered Pennsylva- nian in age. An age of about 373 Ma has been reported for the rhyolite, based on U/Pb geochronology (Thompson and others, 1999). The basalt flows are typically 1 to 2 m thick and marked by pillows, sediment dikes, and magma/sediment commingling features. The two younger flows contain intergranular to subophitic clinopyroxene and sparse phenocrysts of plagioclase and pseudomorphed olivine in a pilotaxitic ground- mass. The two older flows are similar but contain no pyroxene. The rhyolite flows, from 3 to 20 m thick, are characterized by subhorizontal quartz seams that represent cooling cracks or zones of preferential vesiculation filled with late-stage quartz. Both rhyolite flows contain phenocrysts of anorthoclase in a granophyric, devitrified ground- mass with relict perlitic features. Identification of pyroclastic deposits beneath one rhyolite flow indicates that extrusion was preceded by explosive activity. Major and trace elements indicate that the volcanics are mildly alkaline, and geochemical trends suggest the basalt and rhyolite originated by partial melting of different sources, followed by limited fractional crystallization. Apparent restriction to early basin sediments suggests that the volcanics reflect a rifting event associated with the formation of the Narragansett basin. Though it is now apparent that the basin was active well before the Pennsylvanian, it is not clear whether the rift event was related to the extensional environment that prevailed through much of the Paleozoic, producing the alkalic plutonism prominent in this part of the Avalon Zone. Similarities to Devonian-Carboniferous bimodal suites in the Maritimes basin of Canada suggest possibly analogous origins.

Nd isotopic characteristics of terranes within the Avalon Zone, southern New Brunswick

Nd isotopic data are presented for rock units in four terranes within the traditional Avalon Zone of southern New Brunswick. Initial εNd values for igneous rocks within the Caledonia terrane range from -1.6 to +4.6, whereas values for sedimentary rocks range from -8.4 to +3.6. A granite within the Kingston terrane has εNd(438 Ma) = +4.0. Nd isotopic compositions for the Kingston and Caledonia terranes are similar to those of the Mira terrane in Cape Breton Island, the Antigonish Highlands of Nova Scotia, and plutonic rocks of eastern Newfoundland. Each of these regions may be a dismembered part of a single terrane, the Avalon terrane sensu stricto. Initial εNd values for rocks from the Brookville terrane range from -1.3 to +2.8. The Coverdale anorthosite within the Brookville terrane has a present day εNd value of -12.1 and a depleted mantle model age of 1.3 Ga, similar to Mesoproterozoic anorthosites in Laurentia. Clastic sedimentary rocks in the Green Head Group have εNd(750 Ma) values of -2.0 and -10.9. Viewed as a whole, the Brookville terrane is isotopically more evolved than the Caledonia terrane. Initial εNd values for rocks in the New River terrane range from -2.9 to 0.0. The Nd isotopic composition of the Brookville and New River terranes are thus similar to one another and have isotopic character similar to that of the Bras d'Or terrane of Cape Breton Island. It is suggested that all three regions belong to a single terrane (Bras d'Oria).

Nd isotopic characteristics of terranes within the Avalon Zone, southern New Brunswick

Nd isotopic characteristics of terranes within the Avalon Zone, southern New Brunswick

Geophysical studies of the structure of the Appalachian orogen in the Atlantic borderlands of Canada

Results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the Appalachian orogen in eastern Canada. Wide-angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins. High-velocity lower crust, attributed to underplating, is found below the former Laurentian continental margin in Newfoundland and below the Magdalen basin. Potential field data are used to trace the surface zones of the orogen from the northeast Newfoundland shelf to Cape Breton, but extrapolation to New Brunswick and Quebec is unclear because of late Paleozoic basin development. The central mobile belt of the orogen is only a few tens of kilometres wide in southwest Newfoundland and Cape Breton, but broadens substantially to around 200 km elsewhere. Reflection images show a strong deep-crustal fabric that runs along the orogen, with a margin that crosses into the Avalon zone in southern Newfoundland but coincides with the Avalon-Gander zone boundary elsewhere. The fabric formed during mid-Silurian continental collision and (or) during postorogenic collapse. Variation in fabric pattern and metamorphic grade, tightening of structures towards southwest Newfoundland and Cape Breton, and voluminous plutonism in southern Newfoundland are all in accord with maximal crustal thickening followed by erosion and isostatic readjustment in southwest Newfoundland and Cape Breton, and relatively little in northeast Newfoundland and its adjacent shelf. Reflection fabrics in the upper crust appear to be detached from those in the deeper crust; this is attributed to rheological contrast across the base of a quartz-rich upper crust.

Geology of the Hope Brook Mine, Newfoundland, Canada; a preserved late Proterozoic high-sulfidation epithermal gold deposit and its implications for exploration

The Hope Brook gold mine is a high-sulfidation epithermal gold deposit hosted by Late Proterozoic rocks of the northern Appalachian Avalon zone. It is located within the Whittle Hill Sandstone, which is intruded by a Late Proterozoic quartz-feldspar porphyry sill-dike complex of the Roti Intrusive Suite. The deposit (45 t Au) is enclosed within a zone of hydrothermal alteration >3 km long and up to 400 m wide. This zone of alteration is characterized by extensive advanced argillic alteration with pyrophyllite, kaolinite, andalusite, and alunite, which is developed mostly in the structural hanging wall of the ore zone, and two main stages of massive silicic alteration. The first, a buff-colored stage of massive silicic alteration, extends for > or = 3 km laterally away from the deposit. It constitutes a barren to weakly auriferous unit and most likely results from the pervasive acid leaching of the original host(s). The second stage of gray massive silicic alteration is spatially coincident with gold mineralization and is characterized by vuggy quartz. The mineralization consists of several percent of pyrite and lesser amounts of chalcopyrite and bornite and some tennantite with local traces of enargite. In addition to Au and Cu, there is a very limited amount of other metals in the deposit, although it has anomalously high amounts of Sb, Bi, Pb, and As.The ages of altered (pre- and late ore) quartz-feldspar porphyry and of an unaltered (postore) intermediate dike cutting altered rocks bracket the age of mineralization-alteration between 578 and 574 Ma. This temporally and genetically links mineralization-alteration and Roti Intrusive Suite plutonism. Despite a strong spatial relationship between a major fault zone (Cinq Cerf fault zone) and gold mineralization, the Hope Brook deposit bears no genetic relationship to major ductile deformation. The deposit was formed approximately 150 m.y. prior to the development of this fault zone.The existence of Hope Brook, along with a few Mesozoic (Pueblo Viejo), Paleozoic (Temora), Neoproterozoic (Brewer), and Paleoproterozoic deposits (Enaasen), highlights the potential for discovery of such Au-Cu intrusion-related deposits in older terranes. Despite their emplacement at high levels in the crust, the potential for preservation of such high-sulfidation deposits in older terranes may be high if tilting and/or burial took place early in the geologic evolution of the host volcano-plutonic arc.Although the nature and distribution of the initial buff silicic stage of alteration are critical indications of a large-scale hydrothermal system, the ore at Hope Brook is only localized in rocks displaying the gray-colored second silicic stage of alteration. Consequently, the recognition of the two stages of massive silicic alteration was critical in understanding the genesis of the Hope Brook deposit and constitutes an important exploration tool in discriminating between potentially mineralized and barren zones.

Palaeozoic Palaeogeography in Hercynian and Appalachian Belts by Sm-Nd Isotopic Studies of Shales

The Sm-Nd isotopic studies of shales give important informations about the crustal growth and the provenance of the sediments. Because shales integrate the diversity of eroded rocks and because the Sm-Nd isotopic system is left undisturbed by surface processes, the Nd isotopic initial composition reflects an average of the different mechanically eroded sources. This composition reflects a balance between juvenile inputs from the mantle by volcanism and reworked inputs from the continental crust. However, the Sm-Nd isotopic signal is a biased average in which pre-existing sediments and young mountains appear to be over represented. The Nd isotopic initial composition can be expressed by depleted mantle model ages (TDM). Assuming that similar TDM values can characterize a common tectonic domain, comparisons between data from several areas can give new palaeogeographical constraints. First, we test this assumption for the Hercynian belt and we expand it to the Appalachian belt in eastern Canada. Our data from the Hercynian belt concern Sm-Nd isotopic data on 46 shales from the Montagne Noire (southern French Massif Central). The sedimentary record, which comprises one of the most complete Palaeozoic sequences in Europe, was sampled from Early Cambrian to Early Jurassic (560 to 180 Ma). This sequence is interrupted by two hiatuses. The first is situated in Middle Ordovician (490 to 450 Ma) and the second during Carboniferous (330 to 300 Ma). We analysed volcanogenic materials and lavas in order to identify possible source materials. During the studied time span, most of the TDM's are around 1.6 Ga. We observe some rapid TDM changes toward older ages at 520 and 270 Ma and toward younger ages at 300 Ma. These variations are related to different sedimentary settings and to the volcanic event (300 Ma) detected in a continental basin. Our results indicate that in any case, younger TDM do not reflect large Palaeozoic juvenile inputs related to orogenic processes in the south of the French Massif Central. We compare our data with others published data from Brittany (Medio North Armorican Domain) and from Spain (Central Iberian Domain and Cantabrian Domain). The similar TI~M calculated for these areas from Early Ordovician to Early Carboniferous (490 to 330 Ma) suggest that these three areas belonged to the same palaeogeographic entity at least for this time span. This contradicts models that propose there is an oceanic suture between Brittany and Spain and between the two Spanish zones. Conversely, our observations are in agreement with palaeontological data indicating that Brittany and Spain were adjoining during this time span. On the other hand, because the Montagne Noire has younger TI~M from 490 to 400 Ma compare to Brittany and Spain, we propose that this area probably undergone a different tectonic evolution during this time span. The Appalachian belt was created by several accretion events of exotic terranes to the North American shield (Laurentia) during the Palaeozoic. The origin, the timing and the link between the accreted terranes are a matter to debate. In order to give new constraints, we analysed shales from the Dunnage zone, which represent eroded products from the Laurentia, and shales from accreted terranes: the Avalon and Meguma zones. For the Dunnage zone, we observe a rapid TDM shift toward younger ages near 440 Ma. This result is interpreted as an evidence for the accretion of Avalon to the Laurentia around that time.

Tectonic and paleogeographic significance of Late Ordovician conodonts in the Canadian Appalachians

Conodonts of Late Ordovician age are well represented in Laurentian parts of eastern Canada, but are rare in the Appalachian orogenic belt. They are known from the Anticosti Basin, the Matapedia Belt, and the Exploits Subzone of the Dunnage Zone. Two new discoveries are described: one from unnamed strata on Markey Brook, west-central New Brunswick, and one from the Goss Point Formation (Avalon Zone, southwestern New Brunswick). Faunas from the Anticosti Basin are of undoubted Midcontinent Faunal Region affinity. Those from the Matapedia Belt are highly mixed both paleoecologically and provincially, containing elements of shallow- and deep-water Midcontinent affinity and elements typical of the Atlantic Faunal Region. Faunas from the Exploits Subzone and Markey Brook are also mixed, suggesting endemic faunas of Atlantic affinity with local influx of Midcontinent faunas from Laurentia. The Goss Point Formation, previously believed to belong to the Mascarene Group (a Silurian cover sequence on the Avalon Zone), also yields mixed faunas. Provincial mixing of all faunas east of the Anticosti Basin suggests that Late Ordovician faunas around Iapetus Ocean may have been more homogeneous than earlier Ordovician faunas. Therefore, the Iapetus Ocean may have been smaller or current patterns may have changed to permit trans-Iapetan migration. A strong global pattern of provincialism for Late Ordovician conodonts is recognized and a new Australasian Province is proposed. Conodonts permit identification of a previously unrecognized Ordovician volcano-sedimentary succession that has economic and tectonic implications for the region.

Iapetus Ocean floor stuffed into a suture zone: xenolith Nd isotopic evidence for Dunnage-equivalent basement in central Newfoundland

Granulite-facies xenoliths from Late Jurassic alkaline lamprophyres may represent basement to the Dunnage Zone in north-central Newfoundland (Notre Dame Bay area). At 143 Ma the xenoliths had positive εNd values between 0.9 and 4.7. They give Nd depleted mantle model ages around 700 Ma and have trace element and major element compositions reminiscent of oceanic arc-related intermediate volcanic and sedimentary rocks. Their positive εNd values and associated "young" Nd model ages argue against their representing Grenvillian crust. Similarly, Gander Zone basement to the east produced granitic rocks with strongly negative εNd values unlike those of the xenoliths. Positive εNd values for Avalonian granites indicate that the xenoliths could represent Avalon-type basement; however, there are 100–200 km of Gander and Dunnage zone rocks between the xenoiith locality and the Avalon Zone. Early orogenic volcanic rocks and some late orogenic to postorogenic granitic rocks in the central to northern portion of the Gander Zone have positive εNd values, consistent with extraction from a depleted mantle at the same time as material forming the xenoliths. Similarities between the xenolith chemistry and that of early orogenic (Cambrian) arc-related intermediate volcanic rocks of the Dunnage Zone indicate that the xenoliths and basement in the Notre Dame Bay area are composed of Iapetus Ocean floor relics (volcanic or volcanic-rich sedimentary rocks) stuffed into a collisional suture zone during ocean closure.

Geochemistry of the granitoid plutons of the Brookville terrane, Saint John, New Brunswick, and implications for development of the Avalon Zone

The Brookville terrane forms a fault-bounded region, underlain mainly by plutonic and high-grade metamorphic rocks, flanked on one side by a typical Avalonian terrane (Caledonia terrane), and separated from a typical Avalonian terrane (Mascarene terrane) on the other side by the Silurian Kingston Dike Complex.
 
 Mafic plutons of the Brookville terrane were derived from a source similar to that of the continental margin basalts of the Coldbrook Group of the Caledonia terrane. Some sialic plutons were probably derived from mixtures of this basaltic component with a component similar to bulk crust. Major element data for most of the plutons fall on smooth curves on Harker diagrams, suggesting that crystal fractionation is the major process controlling the evolution of individual plutons. Trace element modelling indicates that this fractionation was largely due to the removal of hornblende, plagioclase, and alkali feldspar.
 
 The presence of A2~type granites of similar age (ca. 550 Ma) in the Caledonia terrane (Bonnell Brook), Brookville terrane (Fairville), and Mascarene terrane (Lingley) suggest that these terranes were amalgamated by 550 Ma. Subsequent magmatism in the Brookville terrane was characterized by emplacement of plutons typical of continental margin magmatism. This period of magmatism ended with the emplacement of A2-type plutons, suggesting that magmatism in the Brookville terrane ceased during post-subduction extension.
 
 RÉSUMÉ
 Le terrane de Brookville forme une région, délimitée par des failles, qui repose principalement sur des roches plutoniques et des roches métamorphiques à haute teneur, accolée d'un côté à un terrane avalonien caractéristique (terrane de Caledonia) et séparée, de l'autre côté d'un terrane avalonien caractéristique (terrane de Mascarene) par te complcxe de filons parallèles siluricn de Kingston.
 
 Les plutons mafiqucs du terrane de Brookville sont provenus d'une source semblable à celle des basaltes de la marge continental du groupe de Coldbrook du terrane de Caledonia. Certains plutons sialiques sont probablement provenus de mélanges de ce composant basaltique avec un composant analogue à une masse de croute. Les données sur les principaux éléments de la majority des plutons décrivent des courbes égales et régulieres sur les diagrammes de différenciation, ce qui laisse supposer que la cristallisation fractionnée constitue le principal processus régissant l'évolution de chacun des plutons. La modélisation des éléments traces révèle que cette cristallisation était grandement due au retrait de hornblende, de plagioclase et de feldspath alcalin.
 
 La présence de granites de type A-2 d'âige similaire (vers 550 Ma) dans te terrane de Caledonia (Bonnell Brook), le terrane de Brookville (Fairville) et le terrane de Mascarene (Lingley) permet de supposer que ces terranes se sont fusionnés vers 550 Ma. Le magmatisme subsequent dans le terrane de Brookville a été caractérisé par l'intrusion de plutons typiques d'un magmatisme de la marge continentale. Cette période de magmatisme a pris fin avec une intrusion de plutons de type A-2, ce qui amène à penser que le magmatisme à l'intérieur du terrane de Brookville a cessé au cours de l'extension postérieure à la subduction.
 
 [Traduit par la rédaction]

The position and nature of the Gander–Avalon boundary, southern New Brunswick, based on geochemical and isotopic data from granitoid rocks

In southern New Brunswick, the Gander–Avalon boundary is obscured by boundary-parallel faults and various cover sequences. Siluro-Devonian granites, which intrude unequivocal Gander or Avalon rocks, display exclusively negative (−1.9 ± 1.0) and positive (+1.9 ± 0.7) εNd(T) signatures, respectively. Such contrasting Nd isotopic signatures, combined with other geochemical differences between plutons, are potentially valuable tools for terrane analysis. Nine small Devonian plutons intruding the boundary zone fall into contrasting geochemical groups with (La/Lu)N <4 and >4. The former are topaz-bearing granites, while the latter are volcanic-arc-type granites. Except for one pluton, with an εNd(T) signature of –2.0, εNd(T) values range from –0.4 to +0.7, spanning the gap between "type" Avalon and "type" Gander plutons. These results suggest the plutons sampled either (i) stratigraphically overlapping or tectonically interleaved Gander and Avalon basement rocks, or (ii) a distinct basement source beneath the boundary zone. Our results demonstrate that the Gander–Avalon boundary in southern New Brunswick is not a simple throughgoing crustal fault, and that the Gander and Avalon zones are underlain by different continental basement blocks. Comparison with results from Newfoundland and Nova Scotia suggests that these basement blocks are continuous throughout the Canadian Appalachians.

Chronology of Avalonian events on Presqu'île du Cap Miquelon (Le Cap), Saint-Pierre et Miquelon (France)

U/Pb geochronology and field studies on Presqu'île du Cap Miquelon demonstrate that amphibolite-grade metamorphism, ductile deformation, and subsequent plutonism in this part of the Appalachian Avalon Zone are of Proterozoic age. Clear, crosscutting relationships between a dated granite dyke (part of the Cap Blanc granite) and foliated, amphibolite-grade metasedimentary and metaigneous rocks provide a younger age limit of [Formula: see text] Ma for tectonism. Proterozoic titanite and hornblende ages indicate that no significant younger thermal overprint occurred such as those recorded in nearby southern Newfoundland. Deformation and metamorphism of the country rocks and intrusion of the Cap Blanc granite are at least 30 Ma older than the onset of deposition of the youngest Proterozoic volcano-sedimentary successions in adjacent insular Newfoundland. Possible equivalents of these younger sequences occur near Cap Blanc, and contain what may be interpreted as clasts of granite, dated at 620 ± 3 Ma. The Miquelon metamorphic sequence is unique with respect to the southern Newfoundland Appalachians in representing a significant area of Proterozoic migmatite, not observably affected by tectonothermal events after 580 Ma.

Neoproterozoic rocks of the Newfoundland Avalon Zone

A ca. 15 km thick succession of volcanic and sedimentary Neoproterozoic rocks are exposed within the Avalon Terrane of Newfoundland. Petrologic and geochemical study of volcanic suites and sedimentological analyses indicate that most deposition took place in ensialic rift basins and associated continental arc settings. On the Avalon Peninsula, over 7500 m of volcaniclastic strata include initial submarine fan and slope deposits, which pass upward into a prograded wedge of deltaic and fluvial deposits. This flysch-to-molasse transition, and associated folding and volcanism, mark the Neoproterozoic Avalonian Orogeny. Post-orogenic erosion was followed by deposition of extensive Cambrian siliciclastic deposits. Recent U Pb zircon dating of many of the Neoproterozoic volcanic units provides some constraints on both intra-Avalon and global correlations. More precise dates are needed to improve correlation and better constrain the geologic history of this region. Glaciogenic diamictites of the Gaskiers Formation exposed on the Avalon Peninsula may eventually also prove useful for global correlation. Biostratigraphic study of the latest Neoproterozoic-Lower Cambrian deposits has resulted in ratification of the Precambrian-Cambrian Boundary Stratotype in an exposure on the Burin Peninsula. Older Neoproterozoic rocks contain well-preserved Ediacara-type fossils and a low-diversity suite of microfossils including acritarchs and carbonaceous filaments, the latter of which have little biostratigraphic utility. Aside from work on the Precambrian-Cambrian boundary section, little or no chemostratigraphic, magnetostratigraphic or sequence stratigraphic work has been completed on the Neoproterozoic of the Newfoundland Avalon Terrane.

Shear zones within the Bloody Bluff fault zone: Analysis and tectonic implications

The Bloody Bluff fault zone, which divides the New England Avalon zone and Nashoba zone, contains at least two shear zones that are within Avalonian rocks. The Rice Road shear zone (sinistral, strike-slip) affects the Westboro Formation and is intruded by the 630 Ma Dedham Granite. The Rice Road shear zone, and equivalent pre-granite mylonites appearing in drill cores, parallel the terrane boundary, and may have controlled the later mylonitization. The Nobscot shear zone (dextral, strike-slip) is a prograde shear zone cutting a granite assumed to be related to the surrounding 630 Ma plutons. Similar shear zones have been seen cutting Late Proterozoic plutons in the New England Avalon zone, and represent a series of en echelon strike-slip shears. The Burlington mylonite zone (shear sense equivocal) is part of the terrane boundary. This is a retrograde shear zone that forms the southeastern border of the Wolfpen lens, a lenticular body of sheared and altered metamorphic and intrusive rock that has been assumed to be part of the New England Avalon zone. Microstructural characteristics indicate that the Burlington mylonite zone was active after the Nobscot shear zone. In particular, quartz in the Nobscot shear zone was dynamically recrystallized by a combination of grain boundary migration and rotation recrystallization processes, thought to occur during shearing at upper-greenschist conditions. In contrast, quartz in the Burlington mylonite zone was recrystallized predominantly by rotation recrystallization, indicating lower-greenschist, retrograde, deformation. The two shear zones are too close for these differences to be a result of a simple thermal field gradient. While mineral assemblages in most of the study area indicate no metamorphic grade higher than upper-greenschist temperatures, the Wolfpen lens contains amphibolites with assemblages formed at temperatures above the oligoclase isograd, indicating mid-amphibolite facies metamorphism. As metamorphic contrast is one of the key features differentiating the Nashoba zone from the New England Avalon zone, the Wolfpen lens cannot be assumed to be part of Avalon. It may be a small block of rocks of intermediate grade between the two terranes.

Sm–Nd isotopic geochemistry of Precambrian to Paleozoic granitoid suites and the deep-crustal structure of the southeast margin of the Newfoundland Appalachians

In the Eastern Central Mobile Belt of the Newfoundland Appalachians, late Precambrian basement inliers have εNd from −3 to +2, but Cambro-Ordovician metasedimentary rocks have initial εNd below −7. This region is inferred to have an "inverted" crustal residence structure, which influenced subsequent Appalachian-cycle magmatism. Ordovician and Silurian granitoid suites have εNd of −8 to −2, bracketing both basement and cover, but peraluminous, "S-type" granites have the lowest εNd. Devonian granites have initial εNd values from −5 to +1, and low εNd is associated with peraluminous character. These Paleozoic granites show geographic trends, with lowest εNd values in areas where metasedimentary rocks are abundant. They are suggested to contain anatectic material from both Precambrian basement and metasedimentary cover, but some "I-type" suites probably also include a mantle-derived component. In the adjacent Avalon Zone, Precambrian plutonic suites mostly have εNd from +1 to +6, but there are negative εNd values (−8 to −4) in the westernmost Avalon Zone. Devonian plutonic suites mostly have εNd from +2 to +5. Thus, the Precambrian crust of the Avalon Zone is largely "juvenile," except at its westernmost edge. Contrasts across the Eastern Central Mobile Belt–Avalon Zone boundary, defined by the Dover–Hermitage Bay fault system, indicate a major, crustal-scale structure, and suggest an isotopically distinct "central block" beneath the central Appalachian Orogen, rather than a simple extension of "Avalonian" crust. Similar geographic–isotopic patterns have been reported in Nova Scotia and New Brunswick, suggesting that this pattern represents a first-order deep-crustal subdivision of the northern Appalachian Orogen.

Structural evolution of the Gander-Avalon terrane boundary: a reactivated transpression zone in the NE Newfoundland Appalachians

In the Appalachian Orogen of NE Newfoundland, the Gander and Avalon zones are separated by the 1–2 km wide, dextral, Dover Fault Shear Zone. This structure overprints sinistral transpressive deformations that affect broad regions either side of the Gander-Avalon boundary and is post-dated by ?Alleghenian (or younger) brittle dextral faults that are not thought to record large regional displacements. In ?Lower Palaeozoic sedimentary rocks of the Gander Lake Subzone, a steeply dipping sinistral shear zone (up to 20 km wide) was the focus of syn-tectonic sillimanite grade metamorphism, migmatization and granitic plutonism. In lower greenschist facies late Precambrian sedimentary and volcanic rocks of the Avalon Zone, early sinistral displacements were focused into a high strain zone ( c . 2km wide) along the boundary. Although developed at different metamorphic grades, these early sinistral transpressive shear zones can be correlated and are thought to record the main phase of sinistral-oblique accretion and regional orogenesis during the mid-Silurian ( c. 415-435 Ma). The switch to dextral displacements forming the Dover Fault Shear Zone appears to have initiated during later stages of syntectonic plutonism in the Gander Lake Subzone, suggesting that it may also be Silurian in age. No evidence links the Gander and Avalon zones prior to sinistral transpression. Sinistral fabrics overprint flat-lying structures in the Gander Lake Subzone that are thought to be related to mid-Ordovician accretion of the superjacent Exploits (Dunnage) arc-ophiolite terrane to the west. The Gander and Dunnage allochthons were probably accreted to the Laurentian margin prior to the sinistral-oblique collision of Western Avalonia during the Silurian and they overlie the suture that separates Laurentia from the Central Lower Crustal Block of Avalonian affinities. The Gander-Avalon boundary in NE Newfoundland would then form the steeply-dipping surface expression of a collision-linked sinistral shear zone that allowed lateral decoupling of the Avalon and Central Lower Crustal Blocks during or following oblique collision. Along the margin of the Gander Lake Subzone, the shear zone-hosted belt of sheeted granites and melting increasingly became the focus of deformation during the later stages of sinistral transpression. This suggests significant rheological softening of the crust that would account for the ability of transpressive terrane boundaries to accommodate large displacements. Furthermore, the estab-lishment of a thermal anomaly would have a long-lived weakening effect that could explain why such terrane boundaries are prone to reactivation.

Geochemical and Isotopic Characteristics of Granitoids of the Avalon Zone, Southern New Brunswick: Possible Evidence for Repeated Delamination Events

Nd, O, and Pb isotope data from southern New Brunswick suggest that 625 to 360 Ma granitoid plutons contain a major juvenile component and small amounts of Middle Proterozoic or older crustal material. Plutons fall into four major age groups: 600-630, 540-560, 422-430, and 367-396 Ma. The first and third groups can be related to subduction in other parts of the Appalachian orogen, but most of the second and fourth magmatic pulses have lithologic associations and chemistry not readily reconciled to subduction. The timing and volume of magmatism, presence of mafic end-members, and juvenile signatures in the plutons suggest that crustal delamination may have been involved. Little evidence exists for extensive Middle Proterozoic or older continental crust beneath the Avalon Zone, which consists mainly of Late Proterozoic continental margin arc material intruded by younger granitoid plutons that include a major mantle component. This differs from contiguous Gander and Meguma Zones in which plutons contain a major volume of recycled old (>1.5 Ga) crust. A region with a greater old crustal component in the northwest part of Avalon Zone (Brookville Terrane) may represent a link to the enigmatic "Central Crustal Block" beneath Gander Zone.

LITHOPROBE East onshore-offshore seismic refraction survey —constraints on interpretation of reflection data in the Newfoundland Appalachians

Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.

The youngest Paleozoic plutonism of the Newfoundland Appalachians: U–Pb ages from the St. Lawrence and François granites

The posttectonic St. Lawrence and François granites have long been regarded as the youngest Paleozoic plutonic suites of the Newfoundland Appalachians. Their U–Pb ages of 374 ± 2 and 378 ± 2 Ma, respectively, define a Middle to Late Devonian magmatic event. Mid-Carboniferous magmatic and (or) rifting events, suggested on the basis of earlier Rb–Sr dating of the St. Lawrence Granite, are not supported by these new data. Both granites intrude major ductile and brittle structures that were active during the early Paleozoic, and they provide a younger age limit for major tectonic activity in the Avalon Zone and on the south coast of Newfoundland. The granites may correlate with plutons of closely similar age reported from the Aspy and Mira terranes of Cape Breton Island, Nova Scotia.

A Silurian U–Pb age for the Cape St. Mary's sills, Avalon Peninsula, Newfoundland, Canada: implications for Silurian orogenesis in the Avalon Zone

Mafic sills from Cape St. Mary's on the Avalon Peninsula of Newfoundland give an U–Pb baddeleyite age of 441 ± 2 Ma. This age corresponds with the earliest ages recorded for the climactic Silurian orogenic event that dominantly affected rocks of the Central Mobile Belt in Newfoundland. The age is consistent with but in no way necessitates that the Avalon and Gander zones were juxtaposed during the Silurian. Because sills tend to form in poorly lithified and undeformed sedimentary rocks, it is unlikely that Cambrian sediments hosting the sills were affected by Ordovician orogenic events that strongly affected central Newfoundland. Negative Nb and Ti anomalies on mid-ocean-ridge basalt normalized diagrams show that the sill geochemistry is consistent with formation in a transpressional tectonic environment. Mafic magmas clearly associated with the Silurian event share these chemical and tectonic affinities. Thus both the age and geochemical data are consistent with but do not require a link between the Gander and Avalon zones during the Silurian. If the two zones were joined prior to the Silurian then the Avalon must have been distal to both the Ordovician and Silurian orogenic activity. Further, considerable post-Silurian movement would have had to occur along the bounding Hermitage–Dover fault to account for contrasts in the intensity of metamorphism, plutonism, and deformation between the Gander and Avalon zones.

Geochemistry of Upper Cambrian-Lower Ordovician black shale along a northeastern Appalachian transect

Samples of Upper Cambrian to Lower Ordovician black shale, collected in a transect extending across the Canadian Appalachians from the Humber Zone in Quebec, the Gander and Avalon Zones in New Brunswick, to the Meguma Zone in Nova Scotia, were analyzed for 40 elements, including rare earths. These geochemical data lend support to previously established plate-tectonic models of the region. They also provide information on provenance and depositional environments of sediments that are too fine grained for more traditional mineralogical and sedimentological studies. Generally high Al 2 O 3 contents, high Al/Ti values, and steep rare-earth-element (REE) distribution patterns are consistent with a continental-margin depositional setting for shale from the Humber and Meguma Zones on opposite sides of the Iapetus ocean. Generally higher K 2 O contents, lower La/Th values, and lower absolute REE abundances distinguish shale of the Humber Zone deposited on the Laurentian margin from that deposited in the Meguma Zone on the Gondwanan margin. High La/Th values indicate a similar Gondwanan source for shale from both the Meguma and Avalon Zones. Shale from the Gander Zone contains less Al 2 O 3 at increasing distances from its boundary with the Avalon Zone. Low Al/Ti and less fractionated REE distribution patterns suggest a greater component of volcanic detritus in shale of the Avalon and Gander Zones. The presence of distinctive Balto-Scandian signatures (high U, V, and Mo) in shale of the Avalon and northwestern Gander Zones is possibly related to deposition in isolated peri-Gondwanan back-arc basins during a highstand of sea level.