Abstract Special Publication 542 is a tribute to the remarkable career of J. Brendan Murphy and features 32 articles by 134 authors from 17 different countries; a testament to the high-profile and far-reaching influence of Brendan's work. The topics are wide-ranging in accord with Brendan's diverse research interests, but fall into three broad categories that encompass Brendan's main fields of influence: (i) supercontinents and the supercontinent cycle, including reconstructions and modelling, (ii) orogenesis and terranes, with a focus on the Appalachian–Variscan and Central Asian orogenic belts and the oceans with which they are associated and (iii) magmatism and magmatic processes, with an emphasis on the geochemistry and isotopic compositions of magmas in arc and rift settings. Like Brendan's own research, the scope of the papers spans the globe from Canada to China and ranges from regional field-based studies to conceptual global analyses. All of the articles, however, are focused on unravelling some critical aspect of geology or aimed at clarifying some crucial geologic process. Hence they also share a theme common to Brendan's many contributions in emphasizing the importance of process-oriented research.

Abstract Accurate palaeogeographical reconstructions of past supercontinents are necessary to test models of supercontinent cyclicity, secular variation in plate tectonics and mantle geodynamics. However, numerous factors limit our ability to reconstruct past supercontinents as is evident in the ongoing debates regarding the construction phase and geometry of Earth's most recent supercontinent, late Paleozoic to early Mesozoic Pangaea. An important factor in these debates, and a focus of our study is how best to palinspastically restore, in section and in plan-view, the orogenic belts along which supercontinents were stitched together. We utilize the orogenic belts spanning Baffin Island, Greenland and Fennoscandia that are inferred to record the assembly of the Paleoproterozoic supercontinent Columbia to test the accepted reconstruction of the Nuna core of Columbia. We show that as reconstructed in the Nuna model, each of Baffin Island, Greenland (with some complications) and Fennoscandia are characterized by an older cratonic backstop that gives way to younger accretionary complexes toward the inferred oceanic domain that lay to the south and which is inferred to have closed by subduction beneath northeastern Laurentia–Fennoscandia during Columbia assembly. This southward transition from cratonic backstop to accretionary orogen is a hallmark of upper plates in modern convergent plate margins and is consistent with the construction of the Columbia supercontinent, including its Nuna core, through plate tectonic processes, and provides a broad validation of the Nuna reconstruction and hence for Columbia as a whole. Map view curvature of the cratonic backstops is restricted to long wavelength, open bends consistent with the Archean crust having been characterized by significant lithospheric strength. The more accretionary southern portions of Nuna are, however, characterized by sinuous orogens that developed by oroclinal bending of formerly more linear belts, significantly complicating their palinspastic restoration and rendering detailed correlation of juvenile orogenic belts across Nuna problematic.

Abstract Laurentia, core of the North American continent, is surrounded by Neoproterozoic to Cambrian rifted margins. This led to early suggestions that it was located within a Neoproterozoic supercontinent, Rodinia. Recent models of Precambrian palaeogeographical development also point to a ‘Laurentia-centric’ Rodinian supercontinent. Before plate tectonics, the geometry of continental margins, comparison of cratonic interiors and sedimentary covers, and orogenic piercing points were employed to postulate the geography of Phanerozoic Pangaea. Marine studies have subsequently demonstrated that the results were remarkably accurate. Absent in situ Precambrian oceanic crust, the same lines of evidence are employed here to reconstruct Rodinia, together with others unavailable at that time. A strong case can be made for the former juxtaposition of the Pacific margins of Laurentia and East Antarctica–Australia approximately as proposed in the 1990s, even though the precise match remains elusive. The Atlantic margin is likely to have rifted from Baltica, Amazonia and other South American cratons along the Grenvillian orogenic suture in the early Paleozoic, although the suture itself makes accurate reconstruction difficult. A piercing point and ‘tectonic tracer’ can be used to position the Kalahari craton and Coats Land crustal block of Antarctica off the present southern margin of Laurentia and contemporaneous large igneous provinces point to Siberia being located off the Arctic margin. Hence Laurentia does appear to be the ‘Key’ to Rodinian palaeogeography even though the exact geometric fit to its surrounding cratons remains to be refined.

Abstract The core of the Rodinia supercontinent has long been considered to have consisted of three cratons – Baltica, Laurentia and Amazonia – amalgamated along the late Mesoproterozoic Sveconorwegian, Grenville and Sunsas orogens. In recent years, however, it has become increasingly clear that the metamorphic and magmatic evolution of the Sveconorwegian orogen is inconsistent with a collisional model. Although geological data alone do not rule out proximity to Rodinia, palaeomagnetic data indicate significant latitudinal separation of Baltica and Laurentia during supercontinent assembly. In this contribution, we briefly review two recently proposed and mutually exclusive tectonic models for the Sveconorwegian orogeny and present a compilation of previously published and new chemical and isotopic data. A lack of crustal thickening throughout much of the orogen and few if any changes in lower-crustal sources and melting conditions between 1.3 and 0.9 Ga suggest that the western part of the Sveconorwegian orogeny represents a change from a dominantly extensional to a compressional back-arc regime, but without a significant change in overall tectonic setting. This orogenic evolution is incompatible with amalgamation into Rodinia and suggests that Baltica may have been isolated until the Silurian Caledonian orogeny.

Abstract Rodinia break-up with late Ediacaran rifting defined a NE Laurentia triple junction (New York Promontory–Ottawa–Bonnechere aulacogen (OBA)–Quebec Reentrant). Rifting persisted to c. 510 Ma. The oldest passive-margin shelf units (Forestdale Marble and Moosalamoo Phyllite) underlie a sandstone (Cheshire) commonly regarded as the oldest passive unit. Late Dyeran–Middle Cambrian rifting led to the oldest OBa sedimentation and formed the Franklin Basin (NW Vermont). Cambrian–Darriwillian shelf–slope facies are linked eustatically – not Taconic Orogeny onset. Onlap and shelf carbonates are coeval with black slope mud; and lowstand shelf unconformities with green, oxic slope mud. Early–middle Dyeran eustatic change defined slope units: (1) Browns Pond Formation dysoxic–anoxic (d–a) interval with debrite cap (Holcombville Member, new); (2) Middle Granville Formation Oxic Interval (new); and (3) lower Hatch Hill Formation d–a interval. Our analysis leads to two controversial conclusions: (i) the existence of the Dashwoods and other micro-continental blocks due to hyperextension is not supported by cover sequences linking Laurentia to proposed Dashwoods areas (i.e. Green Mountains) and an arc origin of the type Dashwoods; and (ii) ‘Hawke Bay Event(s)’, widely interpreted as Cambrian global regressive event(s), is a local highstand systems tract facies with shelf sand bypass onto the Hatch Hill Formation slope in its NE Laurentia type region.

Abstract Dashwoods is a composite peri-Laurentian terrane in Newfoundland and forms the basement to the Early Ordovician to Silurian Notre Dame arc. The southern part of Dashwoods is characterized by paragneiss that is intruded by Early Ordovician to Late Silurian plutons and affected by polyphase Taconic to Salinic deformation and high-grade metamorphism. The crystalline basement of Dashwoods is not exposed and pre-Middle Ordovician paragneiss is investigated herein to constrain the provenance of Dashwoods. SHRIMP U–Pb zircon analysis of the paragneiss yielded metamorphic rims ranging from c. 500 to 395 Ma and abundant detrital grain cores ranging from c. 1853 to 546 Ma. The presence of abundant Tonian dates differentiates Dashwoods from the adjacent Humber Margin in Newfoundland, and Hebridean and Grampian terranes in the British Isles. The detrital provenance of Dashwoods is most similar to the Baie Verte Margin in Newfoundland, and Tyrone Complex and Dalradian Supergroup in Ireland. These data suggest that Dashwoods and the Baie Verte Margin originated near the Rockall promontory and were subsequently emplaced outboard of the Humber Margin by Ordovician to Carboniferous motion along the Baie Verte–Brompton Line.