ABSTRACT Detrital zircon U-Pb geochronology has been widely used to constrain the pre-Carboniferous geography of the European and, to a lesser extent, the Moroccan Variscides. The latter have been generally considered as part of a long-lasting passive margin that characterized northern Gondwana from Ordovician to Devonian time, and was subsequently involved in the late Paleozoic Variscan orogeny. We report detrital zircon ages for three Early to Late Ordovician samples from the Beni Mellala inlier in the northeastern part of the Western Moroccan Meseta in order to discuss the temporal evolution of the sources of sediments in this region. The detrital zircon spectra of these samples, characterized by two main populations with mean ages of 630–610 Ma and 2170–2060 Ma, are typical of Cambrian–Devonian rocks from the Moroccan Variscides and confirm their link to the West African craton. A minor Stenian–Tonian population (peak at ca. 970 Ma) suggests the influence of a distant and intermittent NE African source (Sahara metacraton), which was probably interrupted after Ordovician time. Our data support previous interpretations of the Moroccan Meseta (and the entire northern Moroccan Variscides) as part of the northern Gondwana passive margin. The main sources of these sediments would have been the West African craton in the western regions of the passive margin (Moroccan Meseta and central European Paleozoic massifs), and the Arabian-Nubian Shield and/or Sahara metacraton in the eastern areas (Libya, Egypt, Jordan, central and NW Iberian zones during Paleozoic time), where the 1.0 Ga detrital zircon population is persistent throughout the Ordovician–Devonian time span.

ABSTRACT West Avalonia is a composite terrane that rifted from the supercontinent Gondwana in the Ordovician and accreted to Laurentia during the latest Silurian to Devonian Acadian orogeny. The nature and extent of West Avalonia are well constrained in Nova Scotia, New Brunswick, and Newfoundland, Canada, by U-Pb detrital zircon data and/or isotope geochemistry of (meta)sedimentary and igneous rocks. The southeastern New England Avalon terrane in eastern Massachusetts, Connecticut, and Rhode Island has generally been interpreted as an along-strike continuance of West Avalonia in Canada, but the ages and origins of metasedimentary units along the western boundary of the Avalon terrane in Massachusetts and Connecticut remain poorly constrained. In this study, new detrital zircon U-Pb and Lu-Hf laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data from three samples of metasedimentary units along the western boundary of the southeastern New England Avalon terrane in Connecticut and Massachusetts were compared with existing data to test whether these metasedimentary units can be correlated along strike. The data were also compared with existing detrital zircon U-Pb and εHf data in New England and Canada in order to constrain the extent and provenance of West Avalonia. The maximum depositional age of two of the three detrital zircon samples analyzed in this study, based on the youngest single grain in each sample (600 ± 28 Ma, n = 1; 617 ± 28 Ma, n = 1) and consistency with existing analyses elsewhere in the southeastern New England Avalon terrane, is Ediacaran, while that of the third sample is Tonian (959 ± 40 Ma, n = 4). Detrital zircon analyses of all three samples from this study showed similar substantial Mesoproterozoic and lesser Paleoproterozoic and Archean populations. Other existing detrital zircon U-Pb data from quartzites in the southeastern New England Avalon terrane show similar Tonian populations with or without Ediacaran grains or populations. Most published detrital zircon U-Pb data from (meta)sedimentary rocks in West Avalonia in Canada yielded Ediacaran youngest detrital zircon age populations, except for a quartzite unit within the Gamble Brook Formation in the Cobequid Highlands of Nova Scotia, which showed a Tonian maximum depositional age, and otherwise a nearly identical detrital zircon signature with rocks from the southeastern New England Avalon terrane. All samples compiled from the southeastern New England Avalon terrane and West Avalonia in Canada show main age populations between ca. 2.0 Ga and ca. 1.0 Ga, with major peaks at ca. 1.95, ca. 1.50, ca. 1.20, and ca. 1.00 Ga, and minor ca. 3.1–3.0 Ga and ca. 2.8–2.6 Ga populations. The εHf ( t ) values from the three samples yielded similar results to those from West Avalonia in Canada, suggesting that both regions were derived from the same cratonic sources. The εHf ( t ) values of all West Avalonian samples overlap with both Amazonia and Baltica, suggesting that there is a mixed signature between cratonic sources, possibly as a result of previous collision and transfer of basement fragments between these cratons during the formation of supercontinent Rodinia, or during subsequent arc collisions.

ABSTRACT Synthesis of the Ordovician Taconic orogeny in the northern Appalachians has been hindered by along-strike variations in Laurentian, Gondwanan, and arc-generated tectonic elements. The Dashwoods terrane in Newfoundland has been interpreted as a peri-Laurentian arc terrane that collided with the Laurentian margin at the onset of the Taconic orogeny, whereas along strike in New England, the Moretown terrane marks the leading edge of peri-Gondwanan arcs. The peri-Laurentian affinity of the Dashwoods terrane hinges on the correlation of its oldest metasedimentary rocks with upper Ediacaran to Lower Ordovician rift-drift deposits of the Laurentian Humber margin in western Newfoundland. Here, we report U-Pb dates and trace-element geochemistry on detrital zircons from metasedimentary rocks in the southern Dashwoods terrane that challenge this correlation and provide new insights into the Taconic orogeny. Based on age and trace-element geochemistry of detrital zircons analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), we identified ca. 462–445 Ma sedimentary packages with a mixed provenance consisting of Laurentian, Gondwanan, and arc-derived Cambrian–Ordovician sources. These deposits overlap in age with Upper Ordovician strata of the Badger Group of the Exploits subzone, which also contain Laurentian detritus. We infer dominantly east-directed transport of Laurentian detritus from the Taconic collision zone across a postcollisional arc–back-arc complex at ca. 462–455 Ma followed by dominantly west-directed transport of detritus from the Red Indian Lake arc at ca. 455–445 Ma. Our analysis of zircon inheritance from Dashwoods igneous rocks suggests that 1500–900 Ma Laurentian crystalline basement of the Humber margin is an unlikely source of Dashwoods inherited zircon. Instead, a more cosmopolitan Laurentian inheritance may be best explained as sourced from subducted Laurentian sediment. Our results demonstrate that the sampled metasedimentary units from the southern Dashwoods terrane do not correlate with rift-drift strata of the Humber margin as previously proposed, nor with the basement of the Moretown terrane; yet, these Middle to Upper Ordovician successions suggest the potential for an alternative plate-tectonic model in which the Taconic orogeny may have been initiated by collision of Gondwanan arc terranes that closed the main tract of the Iapetus Ocean along the Baie Verte–Brompton Line.

ABSTRACT The Avalon terrane of southeastern New England is a composite terrane in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia, and New Brunswick, Canada, based on rock types and ages, U-Pb detrital zircon signatures of metasedimentary rocks, and Sm-Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ca. 647–643 and ca. 745–733 Ma age populations and minor zircon up to ca. 3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane, or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, Massachusetts, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, Massachusetts, and as far west as Warwick, Rhode Island, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or it may have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.

ABSTRACT The Paleozoic plate boundary zone between Laurussia and Gondwana in western Pangea hosts major magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization. Individual mineral deposits represent the results of the superposition of a series of exogenic and endogenic processes. Exogenic processes controlled (1) the enrichment of the ore elements in sedimentary protoliths via residual enrichment during intense chemical weathering and via climatically or tectonically controlled redox traps, (2) the spatial distribution of fertile protoliths, and, thus, eventually (3) the spatial distribution of mineralization. Endogenic processes resulting in metamorphism and crustal melting controlled the mobilization of Sn-W, Au, and U from these enriched protoliths and, thus, account for the age distribution of Sn-W and Au mineralization and U-fertile granites. It is the sequence of exogenic and endogenic processes that eventually results in the formation of mineralization in particular tectonic zones. Whereas the endogenic processes were controlled by orogenic processes during the assembly of western Pangea itself, the exogenic processes were linked to the formation of suitable source rocks for later mineralization. The contrasting distribution of magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization on the Laurussia and Gondwana sides of the plate boundary zone reflects the contrasting distribution of fertile protoliths and the contrasting tectonic situation on these margins. The Laurussian margin was an active margin during most of the Paleozoic, and the distribution of different mineralization types reflects the distribution of terranes of contrasting provenance. The Gondwanan margin was a passive margin during most of the Paleozoic, and the similar distribution of a wide range of different metals (Sn, W, Ta, Au, and U) reflects the fact that the protoliths for the various metals were diachronously accumulated on the same shelf, before the metals were mobilized during Acadian, Variscan, and Alleghanian orogenic processes.

ABSTRACT Granitoid batholiths dominated by felsic to intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here, we reviewed the age and lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced in the Northern Highlands, Grampian and Connemara terranes of the Caledonide orogen of Scotland and Ireland. The Northern Highlands terrane consists mostly of Neoproterozoic metasedimentary rocks of the Moine Supergroup and is located north of the Great Glen fault. The Grampian terrane also consists of Neoproterozoic metasedimentary rocks (Dalradian Supergroup) and is located south of the Great Glen fault in both Scotland and Ireland. Amphibolite-facies metasedimentary rocks in the Connemara terrane are correlated with the Dalradian Supergroup, and the terrane is bounded by splays of the Highland Boundary and Southern Uplands faults. These three terranes were intruded by Silurian–Devonian mafic and felsic to intermediate plutonic rocks that display field evidence for mingling and mixing and have a similar range (between ca. 437 and 370 Ma) in emplacement ages. This range implies they were intruded during and after the late Caledonian Scandian orogenic event that resulted from the mid- to late Silurian collision of amalgamated Avalonia and Baltica with Laurentia and the final closure of the Iapetus Ocean. Our review supports the contention that the Great Glen fault represents a major compositional boundary in the Silurian lithosphere. Felsic to intermediate plutons that occur north of the Great Glen fault are more enriched in light rare earth elements and Ba-Sr-K compared to those to the south. Isotopic compositions of these late Caledonian plutonic rocks on both sides of the Great Glen fault indicate that metasomatism and enrichment of the subcontinental lithospheric mantle beneath the Northern Highlands terrane occurred just prior to emplacement of late Caledonian plutons. Within the same terrane, mafic and felsic to intermediate rocks display similar trace-element and rare earth element concentrations compatible with models implying that fractionation of a mafic magma played an important role in generating the felsic to intermediate magmas. The onset of slab failure magmatism may have been diachronous along the length of the collision zone. If so, slab failure may have propagated laterally, possibly initiating where promontories collided.

ABSTRACT Three Silurian basin fills, the Llandovery–Wenlock Croagh Patrick and Killary Harbour–Joyce Country successions and the Ludlow–Pridoli Louisburgh–Clare Island succession, overstep the tectonic contacts between elements of the Grampian (Taconic) accretionary history of the Caledonian-Appalachian orogeny in western Ireland. New U-Pb detrital zircon data from lower strata of these Silurian rocks provide insight into basin evolution and paleogeography. The shallow-marine Croagh Patrick succession unconformably overlies the Clew Bay Complex and the northern part of the Ordovician South Mayo Trough. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, with few younger grains. Up to 13% of the grains form a cluster at ca. 950–800 Ma, which is younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting; these may be recycled grains from Dalradian strata, derived from distal Tonian intrusions. The Killary Harbour–Joyce Country succession overlies the structural contact between the Lough Nafooey arc and the Connemara Dalradian block and records a transgressive-regressive cycle. Four samples of the Lough Mask Formation show contrasting age spectra. Two samples from east of the Maam Valley fault zone, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source, likely in north Mayo. One sample also includes 8% Silurian grains. Two samples from west of the fault overlie Dalradian basement and are dominated by Ordovician grains. Circa 450 Ma ages are younger than any preserved Ordovician rocks in the region and are inferred to represent poorly preserved arc fragments that are exposed in northeastern North America. Cambrian to late Neoproterozoic grains in association with young Ordovician ages suggest derivation from a peri-Gondwanan source in the late stages of Iapetus closure. The Louisburgh–Clare Island succession comprises terrestrial red beds. It unconformably overlies the Clew Bay Complex on Clare Island and is faulted against the Croagh Patrick succession on the mainland. The Strake Banded Formation yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician–Silurian ages. Although the basin formed during strike-slip deformation along the Laurentian margin in Ireland and Scotland, sediment provenance is consistent with local Dalradian sources and contemporaneous volcanism. Our results support ideas that Ganderian continental fragments became part of Laurentia prior to the full closure of the Iapetus Ocean.

ABSTRACT The Seve Nappe Complex in the Scandinavian Caledonides records a range of peak metamorphic conditions and timings. To better understand pressure-temperature-deformation-time differences throughout the complex and possible tectonic scenarios, metamorphosed mafic rocks within the Tarfala Valley of the Kebnekaise Massif (Sweden) were investigated using integrated petrologic and geochronologic techniques. Thermodynamic modeling of two samples using domainal and whole-rock compositions integrated with mineral chemistry, mineral textures, and titanite and zircon U-Pb geochronology constrained a portion of the pressure-temperature ( P-T ) path. Peak metamorphic conditions of 590–660 °C and 9.7–10.5 kbar were followed by near-isothermal decompression or a subsolidus clockwise P-T path. Amphibolite units in the valley record retrograde conditions at 450–550 °C at less than 7.5 kbar, although mineral modes and textures are most consistent with pressures <4 kbar. The majority of titanite growth occurred due to the introduction of hydrous fluids during cooling and following exhumation to midcrustal levels. U-Pb ages of retrograde titanite define a spread from ca. 480 to 449 Ma, and the oldest age is interpreted to constrain the timing of retrogression following exhumation. This interpretation is supported by a U-Pb zircon crystallization age of 481 ± 7 Ma for a metamorphosed intermediate to felsic synkinematic dike hosted in one of the amphibolite units. These results indicate that the Kebnekaise region records Early Ordovician deformation and metamorphism that was of lower grade compared to other Seve Nappe Complex locations to the south. The tectonic history of these rocks includes metamorphism and exhumation during the Cambrian–Ordovician pre-Scandian event, followed by thrusting of the Seve Nappe Complex and neighboring rocks onto Baltica during the Silurian Scandian orogeny.

ABSTRACT The Scandinavian Caledonides formed during the continental collision between Baltica and Laurentia. During the collision, a complex nappe stack was thrust over the Baltican continental margin. The orogen can be subdivided into segments based on architectural differences within the Scandian nappes. The southern and central segments of the orogen link up in the Gudbrandsdalen area in south-central Norway. Alpine-type metaperidotite-bearing metasedimentary complexes occur in the southern and central segments and can be traced continuously along the strike of the orogen from one into the other segment. Traditionally, these units have been assigned to different tectono-stratigraphic levels, one below the Middle Allochthon and one above the Middle Allochthon. Here, we trace the Alpine-type metaperidotite-bearing units from Bergen to Esandsjøen and show that these units exhibit a common geologic and metamorphic history, consistent with the metaperidotite-bearing units representing a single tectonic unit. We suggest that the metaperidotite-bearing units can be used as a “marker level” to revise the tectono-stratigraphy of the Gudbrandsdalen and adjacent areas. The tectono-stratigraphic revisions imply that the Scandian nappe stack consists of seven tectono-stratigraphic levels that can be traced throughout the southern and central segments of the Scandinavian Caledonides. Moreover, the revision of the tectono-stratigraphy and new U-Pb geochronology data also suggest a revision of the timing of the succession of tectonic events leading up to the Scandian continental collision. The available evidence indicates that Baltica-derived tectonic units collided with the Iapetan/Laurentian subduction complexes as early as ca. 450 Ma. The initial collision was followed by in-sequence nappe formation of Baltican-derived units, which occurred contemporaneously with the opening of a marginal basin in the upper plate. After the arrival of thick, buoyant, unthinned Baltican crust at the trench, the main zone of convergence stepped outboard, the marginal basins closed, and those basins were thrust out-of-sequence over the previously assembled nappe stack.

ABSTRACT A Silurian–Devonian metamorphic core complex has recently been recognized in northwest Spitsbergen, on the northwest corner of the Barents Shelf at the junction between the Atlantic and Arctic oceans. The associated Keisarhjelmen detachment, a major, ductile-brittle fault zone, is 200–500 m thick and has a map trace >150 km. A top-to-the-north transport direction is parallel to the axis of a large-scale, shallowly north-plunging, detachment corrugation. This detachment zone separates overlying faulted Silurian–Devonian aged cover strata from underlying migmatitic rocks in the core. The detachment shows a diverse array of fault and metamorphic rocks with structural ascent, ranging from sheared migmatite, mylonite, ultramylonite, foliated cataclasite, pseudotachylite, and breccia. Footwall post-kinematic granitic intrusions occurred shortly prior to, and likely during, deposition of the older cover strata. Variably deformed, syn-kinematic granitic sheets and veins within the detachment zone are considered coeval. Thin sections show significant grain size reduction, porphyroclasts, and well-developed composite fault surfaces. Relict garnet sigma porphyroclasts associated with chlorite and sericite indicate retrogression. Feldspar porphyroclasts show significant sericite alteration, undulose extinction and limited recrystallization low in the detachment, and brittle deformation throughout. Quartz deformation textures and grain size vary considerably within and between samples. Deformation during retrogression continued into the brittle realm with the development of thick foliated cataclasites, fault breccias, and local pseudotachylites concentrated at the top of the detachment. Biotite in particular shows grain size reduction, concentration along C-surfaces, and shredding and redistribution, suggesting it played a significant role in both ductile and brittle faulting. Veins, micro-veins, and fluid inclusion planes are ubiquitous throughout the detachment, indicating substantial fault-related fluid flow. Given existing geochronologic and P-T (pressure-temperature) data from the basement rocks of the area, the kinematics, retrogression, and ductile-brittle transition are consistent with exhumation of a core complex developing by orogen-parallel extension associated with transtension during the Late Silurian and Early to Middle Devonian in northwest Spitsbergen. Remaining questions include how this core complex connects with coeval plate-scale strike-slip faults in Svalbard, and its relationship to mainland Norwegian core complexes and Devonian basins to the south.

ABSTRACT The Scandinavian Caledonides have a complex latest Proterozoic–Early Devonian history, but they were finally assembled during the Silurian–Devonian (Scandian orogeny) collision between Baltica and Laurentia. Their dominant structural components are the Lower (Baltican margin), Middle (Baltican and farther outboard), Upper (Iapetan arcs), and Uppermost (Laurentian margin) Allochthons. This study examined the Blåhø Nappe, a complex unit of metamorphosed, intensely deformed igneous and sedimentary rocks assigned to the Middle Allochthon. Metamorphic grades are regionally amphibolite facies, but granulite- and eclogite-facies rocks are locally found. Although most metamorphic ages span a range from Middle Ordovician to Devonian, Blåhø eclogite and other high-pressure rock ages are exclusively Scandian. We analyzed 95 samples of Blåhø Nappe metamorphosed igneous rocks, which were mostly mafic rocks, composed of a minor arc-derived set and a major set transitional between arc and depleted to enriched mid-ocean-ridge basalt (MORB), a range characteristic of back-arc basins. Historically, the Blåhø Nappe has been assigned to the Seve Nappe Complex, the upper part of the Middle Allochthon as mapped in western Sweden and easternmost Norway. In contrast to the Blåhø Nappe, eclogites and other high-pressure rocks in the Seve Nappe Complex have yielded exclusively pre–Scandian orogeny Cambrian and Ordovician ages. Additionally, post–mid-Proterozoic igneous rocks of the Seve Nappe Complex are overwhelmingly dike swarms that were emplaced during the latest Proterozoic breakup of Rodinia, which have rift and MORB-type chemical signatures rather than arc and back-arc signatures, as has the Blåhø Nappe. We hypothesize that the Blåhø Nappe precursors formed on the upper plate, above a west-directed, late Cambrian to Ordovician subduction zone off the Baltican margin. Subduction of the Baltican margin, and possibly rifted fragments on the lower plate, produced the older Seve Nappe Complex eclogites and thrust the Blåhø and Seve Nappe Complex materials onto Baltica. This left the Blåhø Nappe and Seve Nappe Complex precursors on the lower plate during Scandian subduction and collision with Laurentia, allowing exclusively Scandian eclogite formation in the Blåhø Nappe. The Blåhø Nappe and Seve Nappe Complex thus seem to have distinct origins and should not be correlated with one another.

ABSTRACT Southeastern New England is largely composed of Ediacaran granitoid and related volcanic rocks formed during the main phase of arc-related magmatism recorded in West Avalonian lithotectonic assemblages extending through Atlantic Canada to eastern Newfoundland. In situ Lu-Hf analyses presented here for zircons from the Dedham, Milford, and Esmond Granites and from the Lynn-Mattapan volcanic complex show a restricted range of εHf values (+2 to +5) and associated Hf- T DM model ages of 1.3–0.9 Ga, assuming felsic crustal sources. The most evolved granites within this suite lie in a belt north and west of the Boston Basin, whereas upfaulted granites on the south, as well as the slightly younger volcanic units, show more juvenile Hf isotopic compositions. Similar inferences have been drawn from previously published Sm-Nd isotopic signatures for several of the same plutons. Collectively, the isotopic compositions and high-precision U-Pb geochronological constraints now available for southeastern New England differ in important respects from patterns in the Mira terrane of Cape Breton Island or the Newfoundland Avalon zone, but they closely resemble those documented in the Cobequid and Antigonish Highlands of mainland Nova Scotia and New Brunswick’s Caledonia terrane. Particularly significant features are similarities between the younger than 912 Ma Westboro Formation in New England and the younger than 945 Ma Gamble Brook Formation in the Cobequid Highlands, both of which yield detrital zircon age spectra consistent with sources on the Timanide margin of Baltica. This relationship provides the starting point for a recent model in which episodic West Avalonian arc magmatism began along the Tonian margin of Baltica and terminated during diachronous late Ediacaran arc-arc collision with the Ganderian margin of Gondwana.

ABSTRACT Forty-three new U-Pb zircon ages from metasedimentary and igneous rock units throughout the Cobequid Highlands of northern mainland Nova Scotia, Canada, provide new insights into the Neoproterozoic evolution of this long-enigmatic part of Avalonia in the northern Appalachian orogen. Contrasts in ages and rock types resulted in the identification of fault-bounded Neoproterozoic assemblages of units forming the Bass River, Jeffers, and Mount Ephraim blocks. In the Bass River block, quartzite, metawacke, and minor calc-silicate rocks and marble (Gamble Brook Formation) with a maximum depositional age of 945 ± 12 Ma are associated with subaqueous mafic volcanic rocks, siltstone, and ironstone (Folly River Formation) and intruded by 615–600 Ma calc-alkalic subduction-related dioritic to granitic rocks of the Bass River plutonic suite. The contrasting Jeffers block forms most of the Cobequid Highlands and consists mainly of intermediate to felsic volcanic, epiclastic, and minor plutonic rocks. The western and eastern areas of that block yielded ages mainly ca. 607–592 Ma for both volcanic and plutonic rocks, whereas the central area has ages of ca. 630–625 Ma from both volcanic and plutonic rocks and inheritance in overlying Devonian conglomerate. The Mount Ephraim block forms the eastern part of the highlands and includes possible ca. 800 Ma quartzofeldspathic, semipelitic and pelitic gneiss and schist of the Mount Thom Formation, ca. 752 Ma volcanic arc rocks of the Dalhousie Mountain Formation and related 752–730 Ma gabbroic/dioritic to granitic plutons of the Mount Ephraim plutonic suite and Six Mile Brook pluton, as well as ca. 631 Ma granitoid rocks of the Gunshot Brook pluton. The pre–750 Ma high-grade regional metamorphism and deformation and 752–730 Ma subduction-related magmatism recorded in the Mount Ephraim block were previously unrecognized in Avalonia. Evidence from zircon inheritance and Sm-Nd isotopic data in igneous units suggests linkages among these now-separate areas, and comparison with other parts of Avalonia in the northern Appalachian orogen suggests similarity to southeastern New England.

ABSTRACT Voluminous bimodal volcanic rocks of the Silurian (ca. 422–420 Ma) Dickie Cove Group in the Ganderia domain of northern New Brunswick, Canada, are subaerial units that were deposited in an extensional setting, with the mafic types corresponding to continental tholeiites. Felsic rocks are rhyolites with calc-alkaline affinities. They exhibit geochemical characteristics that are typical of A2-type felsic magmas, such as enrichments in the incompatible elements Zr, Nb, and Y, as well as high FeO*/(FeO* + MgO) and Ga/Al ratios. Their ε Nd ( t ) values are positive (+0.7 to +3.4) but lower than those of the associated basalts. Saturation thermometry has yielded average zircon crystallization temperature estimates for the rhyolites that are well above 900 °C. The geochemical data indicate that the felsic melts were likely sourced from heterogeneous Neoproterozoic lower crust and generated by dehydration melting triggered by heat derived from underplated mafic magma. Parent melts of the rhyolites underwent fractional crystallization in a complex magma chamber prior to eruption. The Nd isotopic data suggest that the lower crust of Ganderia is similar to that of Avalonia in northern mainland Nova Scotia, and that the two microcontinents shared a common Neoproterozoic history and origin as continental blocks rifted from neighboring parts of Gondwana. The tectono-magmatic setting of the Dickie Cove Group volcanic rocks is interpreted as being related to Pridolian, post-Salinic relaxation and slab breakoff, which generated volcanism initially constrained within the Chaleur zone of the Chaleur Bay synclinorium, a large domain of the northern Appalachians. This was followed later in the Pridolian by extensional collapse and widening of the area of magmatic activity, which then prograded into the Tobique zone farther to the southwest.

ABSTRACT Avalonia and Ganderia are composite microcontinental fragments in the northern Appalachian orogen likely derived from Gondwanan sources. Avalonia includes numerous Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran–Ordovician cover sequence of mainly siliciclastic rocks. We characterized the nature of the basement on which these arcs were constructed using zircon grains from arc-related magmatic rocks in Atlantic Canada that were analyzed for their Lu-Hf isotope composition. The majority of zircon grains from Avalonia are characterized by initial 176 Hf/ 177 Hf values that are more radiogenic than chondritic uniform reservoir, and calculated crust formation Hf T DM (i.e., depleted mantle) model ages range from 1.2 to 0.8 Ga. These data contrast with those from Ganderia, which show typically positive initial εHf values and Hf T DM model ages that imply magmatism was derived by melting of crustal sources with diverse ages ranging from ca. 1.8 to 1.0 Ga. The positive distribution of initial εHf values along with the pattern of Hf T DM model ages provide a clear record of two distinct subduction systems. Cryogenian–Ediacaran magmatism is interpreted to have resulted from reworking of an evolved Mesoproterozoic crustal component in a long-lived, subduction-dominated accretionary margin along the margin of northern Amazonia. A change in Hf isotope trajectory during the Ediacaran implies a greater contribution of isotopically evolved material consistent with an arc-arc–style collision of Ganderia with Avalonia. The shallow-sloping Hf isotopic pattern for Paleozoic Ganderian magmatism remains continuous for ~200 m.y., consistent with tectonic models of subduction in the Iapetus and Rheic Oceans and episodic accretion of juvenile crustal terranes to Laurentia.

ABSTRACT The Baie Verte Line in western Newfoundland marks a suture zone between (1) an upper plate represented by suprasubduction zone oceanic crust (Baie Verte oceanic tract) and the trailing continental Notre Dame arc, with related upper-plate rocks built upon the Dashwoods terrane; and (2) a lower plate of Laurentian margin metasedimentary rocks with an adjoining ocean-continent transition zone (Birchy Complex). The Baie Verte oceanic tract formed during closure of the Taconic seaway in a forearc position and started to be obducted onto the Laurentian margin between ca. 485 and 476 Ma (early Taconic event), whereas the Birchy Complex, at the leading edge of the Laurentian margin, was subducted to maximum depths as calculated by pseudosection techniques (6.7–11.2 kbar, 315–560 °C) by ca. 467–460 Ma, during the culmination of the Taconic collision between the trailing Notre Dame arc and Laurentia, and it cooled isobarically to 9.2–10.0 kbar and 360–450 °C by 454–449 Ma (M 1 ). This collisional wedge progressively incorporated upper-plate Baie Verte oceanic tract rocks, with remnants preserved in M 1 high-pressure, low-temperature greenschist-facies rocks (4.8–8.0 kbar, 270–340 °C) recording typical low metamorphic gradients (10–14 °C/km). Subsequently, the early Taconic collisional wedge was redeformed and metamorphosed during the final stages of the Taconic cycle. We relate existing and new 40 Ar/ 39 Ar ages between 454 and 439 Ma to a late Taconic reactivation of the structurally weak suture zone. The Taconic wedge on both sides of the Baie Verte suture zone was subsequently strongly shortened (D 2 ), metamorphosed (M 2 ), and intruded by a voluminous suite of plutons during the Salinic orogenic cycle. Calculated low- to medium-pressure, low-temperature M 2 conditions in the Baie Verte oceanic tract varied at 3.0–5.0 kbar and 275–340 °C, with increased metamorphic gradients of ~17–25 °C/km during activity of the Notre Dame arc, and correlate with M 2 assemblages in the Birchy Complex. These conditions are associated with existing Salinic S 2 white mica 40 Ar/ 39 Ar ages of ca. 432 Ma in a D 2 transpressional shear zone and synkinematic intrusions of comparable age. A third metamorphic event (M 3 ) was recorded during the Devonian with calculated low-pressure, low-temperature conditions of 3.2–3.8 kbar and 315–330 °C under the highest metamorphic gradients (23–30 °C/km) and associated with Devonian–early Carboniferous isotopic ages as young as 356 ± 5 Ma. The youngest ages are related to localized extension associated with a large-scale transtensional zone, which reused parts of the Baie Verte Line suture zone. Extension culminated in the formation of a Middle to Late Devonian Neoacadian metamorphic core complex in upper- and lower-plate rocks by reactivation of Baie Verte Line tectonites formed during the Taconic and Salinic cycles. The Baie Verte Line suture zone is a collisional complex subjected to repeated, episodic structural reactivation during the Late Ordovician Taconic 3, Silurian Salinic, and Early–Late Devonian Acadian/Neoacadian orogenic cycles. Deformation appears to have been progressively localized in major fault zones associated with earlier suturing. This emphasizes the importance of existing zones of structural weakness, where reactivation took place in the hinterland during successive collision events.

ABSTRACT Early Ordovician to late Permian orogenies at different plate-boundary zones of western Pangea affected continental crust derived from the plates of North America (Laurentia), Europe (East European Craton including Baltica plus Arctida), and Gondwana. The diachronic orogenic processes comprised stages of intraoceanic subduction, formation and accretion of island arcs, and collision of several continents. Using established plate-tectonic models proposed for different regions and time spans, we provide for the first time a generic model that explains the tectonics of the entire Gondwana-Laurussia plate-boundary zone in a consistent way. We combined the plate kinematic model of the Pannotia-Pangea supercontinent cycle with geologic constraints from the different Paleozoic orogens. In terms of oceanic lithosphere, the Iapetus Ocean is subdivided into an older segment (I) and a younger (II) segment. Early Cambrian subduction of the Iapetus I and the Tornquist oceans at active plate boundaries of the East European Craton triggered the breakup of Pannotia, formation of Iapetus II, and the separation of Gondwana from Laurentia. Prolonged subduction of Iapetus I (ca. 530 –430 Ma) culminated in the Scandian collision of the Greenland-Scandinavian Caledonides of Laurussia. Due to plate-tectonic reorganization at ca. 500 Ma, seafloor spreading of Iapetus II ceased, and the Rheic Ocean opened. This complex opening scenario included the transformation of passive continental margins into active ones and culminated in the Ordovician Taconic and Famatinian accretionary orogenies at the peri-Laurentian margin and at the South American edge of Gondwana, respectively. Rifting along the Avalonian-Cadomian belt of peri-Gondwana resulted in the separation of West Avalonian arc terranes and the East Avalonian continent. The vast African/Arabian shelf was affected by intracontinental extension and remained on the passive peri-Gondwana margin of the Rheic Ocean. The final assembly of western Pangea was characterized by the prolonged and diachronous closure of the Rheic Ocean (ca. 400–270 Ma). Continental collision started within the Variscan-Acadian segment of the Gondwana-Laurussia plate-boundary zone. Subsequent zipper-style suturing affected the Gondwanan Mauritanides and the conjugate Laurentian margin from north to south. In the Appalachians, previously accreted island-arc terranes were affected by Alleghanian thrusting. The fold-and-thrust belts of southern Laurentia, i.e., the Ouachita-Marathon-Sonora orogenic system, evolved from the transformation of a vast continental shelf area into a collision zone. From a geodynamic point of view, an intrinsic feature of the model is that initial breakup of Pannotia, as well as the assembly of western Pangea, was facilitated by subduction and seafloor spreading at the leading and the trailing edges of the North American plate and Gondwana, respectively. Slab pull as the plate-driving force is sufficient to explain the entire Pannotia–western Pangea supercontinent cycle for the proposed scenario.

New analytical and field techniques, as well as increased international communication and collaboration, have resulted in significant new geological discoveries within the Appalachian-Caledonian-Variscan orogen. Cross-Atlantic correlations are more tightly constrained and the database that helps us understand the origins of Gondwanan terranes continues to grow. Special Paper 554 provides a comprehensive overview of our current understanding of the evolution of this orogen. It takes the reader along a clockwise path around the North Atlantic Ocean from the U.S. and Canadian Appalachians, to the Caledonides of Spitsbergen, Scandinavia, Scotland and Ireland, and thence south to the Variscides of Morocco.