Arenig Times Research Articles

Arc–continent collision and the formation of continental crust: a new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

Abstract: Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian–Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U–Pb zircon dating yields ages of 493 ± 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 ± 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc–continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian–Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc–continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

An Early Ordovician (Finnmarkian?) foreland basin and related lithospheric flexure in the Scandinavian Caledonides

Early Ordovician (479 - 455 Ma) graywackes overlying Cambrian passive margin successions in the Baltica palaeocontinent-related Lower AUochthon of the Scandinavian Caledonides document early Caledonian tectonic activity. Such rocks occur from the Jamtland area (Sweden) northwards along the eastern Caledonian margin, and imply a wide regional extent of a foreland basin in Early Ordovician times. This foreland basin was subsequently incorporated into a fold-and-thrust belt during the final Caledonian, Scandian tectonic episode (c. 425 - 390 Ma). Based on published cross section data, this late deformation is restored. The results and other available information are compiled into a palaeogeographic map and sections on the geometry and evolution of the Early Ordovician foreland basin. Graywacke/turbidite sedimentation (Follinge Formation) started in western (internal) areas already in Early Arenig times with a main phase from Llanvirn to Late Caradoc (c. 472 - 461 Ma). There, the Follinge Formation rests with an erosional unconformity on older beds. Towards east, however, it overlies successively younger beds related to a carbonate domain. It is suggested that the areas of the sub-turbidite erosional unconformity represent the early location of a flexural forebulge, which subsequently migrated eastwards towards the margin of the carbonate domain of palaeocontinent Baltica. The restored foreland-basin geometry is compared with numerical models in order to derive some characteristics of the flexure of the foreland lithosphere. Time and lithospheric constraints make it possible to test the viability of the restoration and of foreland basin models. Relative to the available lithospheric strength data, the width of the basin is too large, the depth relatively small, and the large wavelength of the flexure is difficult to explain with simple orogenic loading. Graywacke sedimentation occurred after Finnmarkian (515 - 475 Ma) HP metamorphism, perhaps as a consequence of exhumation and consequent loading of the Baltican margin. Graywacke sedimentation ended at the time of Jamtland phase (460 -440 Ma) HP metamorphism and relatively deep water conditions. It is speculated that eclogitization of crustal material produced an additional load, which caused a relatively large down flexure of the lithosphere.

Billingen (Lower Arenig/Lower Ordovician) acritarchs from the East European Platform and their palaeobiogeographic significance

Billingen (Lower Arenig/Lower Ordovician) sediments of the St. Petersburg region, northwest Russia and the Leba area, northern Poland of the East European Craton yield acritarch assemblages, which are largely homogenous though displaying minor compositional differences that probably reflect a gradient from inner to outer shelf environments. Comparison with coeval acritarch microflora from the Yangtze Platform, South China, shows an overall similarity between Baltoscandian and South Chinese phytoplankton. The widespread uniformity in the fossil microphytoplankton may be related to the extensive global ‘evae’ sea-level transgression, which characterized the Billingen time. This suggests that during the Tremadoc through early Arenig times, acritarch assemblages displayed essentially an undifferentiated cold-water and oceanic character along the whole margin of Perigondwana in the South, as well as on the South Chinese and Baltic platforms, at middle latitudes (Mediterranean oceanic Realm). Despite this overall similarity, however, some typical taxa of the high-latitude Mediterranean Province (Arbusculidium, Coryphidium and Striatotheca) occur in South China, but are absent in Baltica. This discrepancy is explained as caused by differences in climatic and physiographic conditions that prevailed at the two palaeocontinents at this time. The inferred pattern of oceanic circulation during the Lower Ordovician is consistent with the palynological evidence of a prevailing warmer climate in Baltica than in South China, although the two palaeocontinents occupied the same palaeolatitudinal position.

The Hølonda Porphyrites, Norwegian Caledonides: geochemistry and tectonic setting of Early–Mid-Ordovician shoshonitic volcanism

Petrological and geochemical data denote that the Late Arenig–Early Llanvirn Hølonda Porphyrites belong to the subduction-related shoshonite association. Original compositions ranged from absarokite through shoshonite to latite, and their very evolved nature signifies a mature arc aYnity in an active continental margin setting. Much of the geochemical variation can be interpreted in terms of fractional crystallization followed by a varying admixture of cumulus phases to the ascending melt and accumulation of phenocrysts within the magmatic feeder system. Their eruptive products covered an area which was at least 20–25 km across, forming local islands with carbonate-rich shelves in a shallow to deeper marine environment. The immediate substrate to the volcanosedimentary ‘Hølonda basin’ is the Løkken Ophiolite, which formed in a primitive oceanic arc/marginal basin setting some 10–20 Ma prior to the shoshonitic magmatic event. Arc–continent collision led to emplacement of the oceanic, arc and marginal basin crust onto the continental margin in pre-Late Arenig times. Strong uplift of the obducted oceanic plate occurred in response to the buoyancy effect of the underthrusting, less dense, continental margin. This is interpreted to have been followed by a reversal of subduction polarity and formation of a steeply dipping subduction zone against the continental margin lithosphere, and was succeeded by the very evolved Hølonda Porphyrite shoshonitic magmatism in volcanic islands along this submerged continental margin.

“Fauna cámbrica” versus “fauna paleozoica” en el Ordovícico temprano del Oeste de Argentina: Interacción entre provincialismo y ambiente

The limestones of the San Juan Formation (Upper Tremadoc-Lower Llanvirn) in the Precordillera basin bear a group of communities dominated by sessile-suspension feeder organisms, including mainly brachiopods and sponges. The broadly contemporaneous siliciclastic Acoite (Lower-Middle Arenig) Formation in the Northwestern basin yield a succession of trilobite dominated communities. According to the global model of the three evolutionary faunas the former associations fit in the “Palaeozoic Fauna” and the latter could be referred to the “Cambrian Fauna”. Comparative analysis of structure and function of both types of communities, taking in account tiering strategies and the development of different guilds, demonstrates similar values of dominance diversity and richness as well as number of occupied guilds and tiers. Besides, trilobite dominated communities extend along different bathymetric ranges in the shelf environment -from offshore to the shoreface and even are represented in estuarine settings- without significant shifts in higher taxa proportions, showing a pattern of ecological stasis through time. Possible explanations for this pattern are derived from the ecological properties underlain by individualistic community histories, governed by the paleogeographic position of both areas during the Arenig times. We conclude that the three evolutionary faunas concept could be applied to equatorial and subequatorial areas. Cold water Gondwanan and peri-Gondwanan associations could be included in a new model of evolutionary faunas. For this purpose further tests in other Ordovician Gondwanan communities would be necessary.

Ordovician magnetostratigraphy: Llanvirn-Caradoc limestones of the Baltic platform

SUMMARY Palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the Baltoscandian Ordovician carbonates. After removing a ubiquitous PermoCarboniferous (287 f 14 Ma) remagnetization between 200 and 500 C, a LlanvirnCaradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550-580C. Three reversed (SE, down) and three normal (NW, up) antipodal polarity intervals have been recognized. A primary/early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons. Primary magnetizations are resident in detrital/biogenic or early diagenetically formed single- and pseudo-single domain magnetite phases and subordinate early diagenetic.pigmentary haematite. The recognition of a primary remanence within these well-dated Ordovician carbonates has the following important tectonic and magnetostratigraphic consequences. (1) Accurate time-calibration of the Baltic APW path implies that rapid counterclockwise rotation took place in late Tremadoc and Llandeilo times. The ArenigLlanvirn epochs are characterized by a 'still stand'. Baltica occupied intermediate to high Southerly latitudes during the early Ordovician (Tremadoc-Llanvirn). Systematic northward drift is recognized in post-Llanvirn times. (2) A time-calibrated Ordovician reversal stratigraphy is proposed. Presently available data suggest the geomagnetic field was predominantly reversely polarized during Tremadoc and Arenig times. Two normal polarity zones of short duration are identified within mid-Llanvirn and mid-Llandeilo strata. Discontinuities within the succession may mask other short-period events. Late Llandeilo to mid-Caradoc times were then characterized by a normal polarity field.

The Ordovician history of the Iapetus Ocean in Britain: new palaeomagnetic constraints

New late Tremadoc—early Arenig palaeomagnetic results from SW Wales imply that S Britain (part of Eastern Avalonia) occupied a southerly latitude of c . 60°S in early Ordovician times. When combined with Scottish Ordoviaan palaneomagnetic data, which indicate a 15°S latitude, the results indicate that the British sector of the Iapetus Ocean reached n latitudinal width of c . 5000 km in Tremadoc—Arenig times, which was reduced to c . 3000 km by Llanvirn—Llandeilo (mid-Ordovician) times. The new data resolve two previous controversies in Palaeozoic palaeogeography. First, the high southerly palaeolatitude links Avdonia to Gondwana, marginal to W Africa, thus reconciling codlicting reconstructions based upon either palaeomagnetic or faunal/facies evidence alone. Second, reliable Llanvim palaeomagnetic data imply that Avlonia had rifted northwards by Arenig time, whereas Armorica remained proximal to northern Africa throughout the Ordovician. The combined data therefore establish that Avdonia and Armorica formed separate micro-continents when rifting from Gondwana.

Relations between conodont provincialism and the changing palaeogeography during the Early Palaeozoic

Abstract World-wide distribution patterns of conodont species during the Cambrian, Ordovician, and Silurian are assessed quantitatively using a Coefficient of Similarity (CS) formula. The validity of the idea is examined that there is a correlation between CS values and the geographic distance between the sample points, and that changes in CS values may, at least in part, reflect plate motions. To minimize effects of local palaeoecological control and the varying degree of exploration of the faunas, well known faunas are used from comparable depositional environments on the several continental plates. Conodont provincialism can be traced back to Upper Cambrian time, when there was an initial differentiation into low-latitude, warm-water (Midcontinent Faunal Region) and high-latitude, colder-water (Atlantic Faunal Region) faunas. The Lower Ordovician is characterized by great taxonomic diversification as well as striking provincial differentiation. Based on regional distribution patterns, it is possible to recognize a North American Interior Province, a Mediterranean Province, a North Chinese Province, a Siberian Province, a Baltic Province, and an Australian Province during Arenig time. A similar lateral differentiation prevailed during Llanvirn to lower Caradoc time. It is difficult to recognize a close correlation between CS values and inferred distances of plate separations in the Cambrian and Lower and Middle Ordovician. Unlike some megafossil groups that markedly decreased in provincialism during the Upper Ordovician, the conodont faunas continued to exhibit pronounced lateral differentiation despite the fact that the Baltic, Siberian, and North American plates were then relatively closely together in the equatorial zone. Near the Ordovician-Silurian boundary (probably in the uppermost Ordovician) there was a global, quite conspicuous turn-over in the conodont faunas. During that time, the taxa characteristics of the Atlantic Faunal Region disappeared and apparently, the ancestors of virtually all Silurian taxa are to be found in the Midcontinent Faunal Region. The Silurian conodont faunas have a cosmopolitan character although there are minor local differences that may be attributed to local environmental control. During Silurian time, the plates that have produced the best conodont faunas (North America, Baltica, Siberia) were all in the equatorial zone, and a general uniformity in environmental conditions may be one of several reasons behind the cosmopolitan character of the Silurian conodont faunas. Many Lower Palaeozoic conodont species were capable of crossing water bodies of oceanic dimensions, and the distribution of many taxa seems to have been more closely controlled by water temperature and other ecological parameters than by potential migration barriers related to size of water bodies and emerged continental blocks. Nevertheless, because of their pronounced latitudinal differentiation during the Ordovician, the conodont faunas may be used as tools to decipher some of the positions of continental plates.

Palaeomagnetic studies within the Ballantrae Ophiolite; southwest Scotland: magnetotectonic and regional tectonic implications

The Early Arenig Slockenray Formation within the Ballantrae Ophiolite, southwest Scotland, displays a multivectorial magnetisation structure. Two components (S and M) are identified delineated by differing blocking temperature/ coercivity spectra. Component S is removed around 200°C/10 mT, and is regarded to be of recent viscous origin. Component M forms the characteristic formation magnetisation and resides in both magnetite and haematite. Extensive sampling of all exposed lithologies reveals an (in situ) non-Fisherian distribution of the characteristic magnetisation defining an envelope from SE moderate positive to SW shallow negative directions. A negative infra-formation conglomerate test identifies this component as a pervasive overprint. A second conglomerate test performed in the overlying Benan Conglomerate of Llandeilo age, reveals dispersely directed magnetisation with a stability range equivalent to that of component M. This field test therefore defines a maximum remagnetisation window of 30 million years for the characteristic remanence. “Hard” viscous magnetisations are identified both in the Benan Conglomerate and at some sites within the Slockenray Formation. Structurally corrected site mean results from the Slockenray Formation define a non-Fisherian distribution and form a small circle partial arc centred on a vertical axis (NW moderate positive to SW moderate positive directions). A combined palaeomagnetic fold and fault test suggests that acquisition of component M pre-dates both folding and faulting. The resulting palaeolatitude of remanence acquisition (28.8°S) implies a tectonic position close to the southern Laurentian margin for the Ballantrae ophiolite in Arenig times.

Tectonic controls of Ordovician arc and marginal basin volcanism in Wales

Ensialic destructive plate margin volcanism persisted in Wales from late Tremadoc to Caradoc times. Late Tremadoc uplift and erosion in NW Wales is attributable to onset of subduction and development of a frontal ridge system (fore-arc). South-east of this, in the vicinity of the Harlech Dome and in SW Wales, more localized uplift was associated with the development of arc volcanoes at a volcanic front. In North Wales the arc volcanism occurred in a regime of E-W crustal extension and was centred on a deep crustal discontinuity. During Arenig times the arc terrane was transgressed. Subsequently, marine conditions, with local and temporary emergence, persisted through Arenig to Caradoc episodes of marginal basin volcanism. While there was widespread and persistent subsidence in Wales during development of the marginal basin, the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence. The grabens mark the sites of deep and steep crustal discontinuities (‘fractures’), one of which controlled the earlier arc volcanism. Changing sites of volcanism in Wales can now be seen as due to transfer of active extension from one fracture to another. There is evidence that the fractures were present by Tremadoc times, and they almost certainly formed before development of the Cambrian marine rift-basin. They are most likely to have been strike-slip faults or shear zones, and their pattern in Wales indicates that they may represent part of a complicated deep-crustal strike-slip duplex system. The volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults (flower structure) which channelled magmas through the cover during extension across the fractures. The fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during Late Caledonian compression and transpression. In North Wales the volcanically active grabens trended broadly N-S, oblique to the exposed NE-SW basement tectonic grain, and dominantly E-W extension seems to have persisted throughout the Ordovician volcanism. While there is considerable uncertainty concerning Ordovìcian plate tectonic configurations and displacements, the extension is best explained by invoking a N-S convergence vector between the oceanic and over-riding microcontinental plates, with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin. The model highlights the suspect nature of the relationship between the Welsh Ordovician terrane and others to the north.

U/Pb geochronology of the Coney Head Complex, Newfoundland

The Coney Head Complex, White Bay, Newfoundland, comprises allochthonous plutonic rocks and deformed equivalents emplaced over the continental margin during the Ordovician Taconic Orogeny. A U/Pb zircon age of 474 + 2 Ma for tonalite considered as having formed by partial melting in an island arc, indicates that it crystallized in Arenig time. A microgranite dated by U/Pb (zircon + titanite) at 432 ± 2 Ma cuts the tonalite Inherited Proterozoic zircon in the microgranite indicates that the rock formed from magma generated in part by melting of continental crust or sediments derived from such crust. These relationships indicate that the microgranite was emplaced in Silurian time, after Ordovician transport of the tonalite above a continental substrate.

Lower Palaeozoic convergent plate margin volcanism on Bømlo, SW Norway, and its bearing on the tectonic environments of the Norwegian Caledonides

The island of Bømlo presents a geological section through a Caledonian convergent plate margin. The Lykling Ophiolite and the overlying ensimatic are sequence of the Geitung Unit were emplaced onto the Baltic Shield, probably in Arenig times. After a period of deep erosion, the Basin and Range type volcanics of the Siggjo Complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism, possibly due to the successful subduction of a spreading ridge. At the top of the mainly sedimentary Ashgill/Llandovery Vikafjord Group, the Basin and Range type volcanism resumed. This was probably preceded immediately by the calc-alkaline, subaerial volcanism at the base of the Langevåg Group, which passes stratigraphically upwards into basin-fill lithologies such as submarine volcaniclastics, cherts, turbidites and submarine greenstones of chemistry compatible with the early stages of ensialic back-arc basin development. The stratigraphy and volcanism are discussed in context with recent views on the rest of the Norwegian Caledonides.

A conspectus of Scandinavian Caledonian ophiolites

Summary Dismembered and fragmented ophiolite assemblages constitute important elements of the metamorphic allochthon of the Scandinavian Caledonides. Ophiolites recognized to date range in age from probable Vendian to Middle Ordovician and occur at two principal tectonostratigraphic levels. Field criteria, with support from geochemical data, permit a classification of many of the ophiolite complexes into two fundamental groups. Group I is characterized by a generally well-developed pseudostratigraphy, MORB as well as IAT and WPB petrochemistry, plagiogranites at higher levels, and a conformable cap of oceanic sediments with local ocean or immature arc volcanism. The group is representative of either a major ocean or evolved marginal-basin regime. Evidence points to their obduction, and initial internal deformation, in the Finnmarkian orogenic event in pre-Middle Arenig time. Group II complexes are younger, of late Arenig to Llanvirn and possibly Llandeilo age, and show a poorer-defined pseudostratigraphy. They are laterally and vertically intercalated with either siliciclastic sediments from a contemporary magmatic arc or arcs, and are considered as having been generated in a restricted marginal-basin setting. Their prime tectonic deformation, and translation within nappes, is essentially of Silurian age. Group I ophiolites were also further dissected and deformed during this main Caledonian event.

Ordovician marginal basin development in the central Norwegian Caledonides

Summary In the Caledonides of central Norway, volcano-sedimentary successions, of the Lower and Upper Hovin and Horg Groups (Ordovician to ?Lower Silurian), unconformably overlie a variable substrate which includes fragmented oceanic crust and ensimatic immature arc assemblages initially deformed and metamorphosed in earliest Ordovician times. In western areas, remnants of a mid-Ordovician, evolved island-arc complex comprise a volcanic-plutonic suite of clear calc-alkaline affinity. To the E, in the Trondheim district, continental margin-type basic to andesitic volcanics, and sediments of comparable age, denote accumulation in a fault-dissected, back-arc marginal basin characterized by marked facies variations and bipolar sediment dispersal patterns. Sediment composition reflects the nature of the contemporaneous volcanism, although an input of metamorphic detritus from a continental margin source can also be determined. Thick, massive to pillowed tholeiitic greenstones, mainly of OFB chemistry, occur in several areas and some contain thick sheeted-dyke units, gabbro complexes and subordinate plagiogranites. These ophiolitic assemblages appear to occur at two stratigraphic levels within the Lower Hovin Group, reflecting separate phases of crustal thinning and oceanic lithosphere accretion. Mega-breccias and an olistostrome may relate to abortive ophiolite displacement or intra-basinal movements in late Arenig times. During the Caradoc and Ashgill, and possibly in earliest Silurian times, continental margin-type acidic volcanism developed widely, particularly in the S, while in central and northern parts of the basin, turbidites and resedimented conglomerates, derived from the basin margins, show SW transport in the deeper-water axial trough. Although there is strong evidence that the marginal basin and arc lay at the edge of the continental plate (Baltoscandia) on the SE side of the Iapetus, with SE subduction, this is not supported by the faunas in the lowest parts of the Ordovician successions, which are largely of North American affinity. A compromise model involving an elongate microcontinental plate within the Iapetus is invoked.

Petrology and geochemistry of post-obduction dykes of the Ballantrae complex, SW Scotland

ABSTRACTDoleritic intrusions known to post-date the obduction of the Ballantrae complex during Arenig times record changing magma provenance during the cooling and serpentinisation of obducted peridotite. There are two groups of dolerites with different petrographical and geochemical characteristics.The earlier emplaced group, which is subordinate, is characterised by amphibole formed under low metamorphic facies conditions and the virtual absence of Fe–Ti oxides. Chemically these rocks are of primitive tholeiitic character and are similar to modern island-arc basalts. They were derived from a strongly depleted mantle source region. The source region of the later emplaced group, which is predominant, was much less depleted. Clinopyroxene and plagioclase, much of it albitised, are the dominant minerals. Fe–Ti oxides also are common and there are pronounced chilled margins and well-preserved ophitic textures. Although showing mildly alkaline tendencies, these rocks originated from a tholeiitic parental magma of “within-plate” type.Both groups show evidence of rodingitisation and associated alteration related to serpentinisation of the peridotite. During the metasomatic activity, only some elements were mobile, while Al2O3, total Fe, MgO, TiO2, Cr, Ni, Nb, Y and Zr remained almost constant. Assessment of original magma type and geotectonic environment, and demonstration that both groups of doleritic rocks show the products of fractionation, has relied heavily on data for the apparently immobile elements and on petrographical study which identified those rocks least affected by alteration.

Basic magmatism in Connemara, Ireland: evidence for a volcanic arc?

It is found by the use of Pearce & Cann diagrams that basic magmatism during Dalradian sedimentation in Connemara was of a fundamentally different type to that which led to later synorogenic basic intrusions. The Dalradian magmatism is of a transitional within-plate to divergent plate margin type whereas the synorogenic intrusions are of volcanic arc type. It is suggested that the synorogenic basic intrusions of Connemara, Tyrone and NE Scotland represent the roots of a volcanic arc that developed by Arenig times over a northerly directed subduction zone.

Tectonostratigraphic development of the Trondheim region Caledonides, Central Norway

The metamorphic allochthon of the central Norwegian Caledonides comprises a complex of discrete nappes of metasediments and igneous rocks ranging in age from probable Svecofennian through Vendian to Silurian. This southeastward-translated allochthon overlies a thin cover of autochthonous Vendian to Cambrian sediments deposited upon a crystalline Precambrian basement, and is superseded by late-orogenic, intermontanebasinal sediments of latest Silurian to Middle Devonian age. Stratigraphical sequences in higher allochthonous units are floored by oceanic tholeiitic basalts with rare, subjacent sheeted-dyke and gabbro units, considered as fragments of an ophiolite assemblage which suffered initial eastward transport in pre-Middle Arenig times, an important orogenic event which is well represented in northern and southwestern Norway. The overlying Ordovician—Silurian sequences, disturbed by episodic parorogenic events, embrace a variety of sedimentary facies from shallow-water carbonates to deep-marine terrigenous turbidites and include both island arc and marginal basin lavas and intrusives. Polyphase Middle Silurian metamorphism and deformation resulted in a complex telescoping and dissection of the Lower Palaeozoic rocks and their Precambrian substrate, with nappe translation in the order of several hundred kilometres. Folding and thrusting of Old Red Sandstone molasse sediments attests to continuing tectonism well into Devonian times.

An early paleozoic plate tectonic model of Newfoundland

We assume initial eastward subduction off the coast of the Grenvillian continent, terminated by collision of continent and subduction zone, with westward obduction of oceanic allochthons commencing in Arenig time. This regime was succeeded by oblique westward subduction beneath “proto-Newfoundland”. A projection of the Avalonian continent collided with the subduction zone in Llanvirn time, leading first to eastward obduction of oceanic material, and subsequently to rotation and deformation of the continental rise prism. Eventual continent-continent contact brought subduction to an end. Subsequent deformation, metamorphism and plutonism resulted from thermal and isostatic adjustments in the collapsed continental rise prisms. In addition to explaining many of the geological features of Newfoundland, the model predicts (1) a 30° clock-wise rotation of southeastern Newfoundland relative to northwestern Newfoundland in mid-Ordovician time, and (2) about 50 km of left-lateral displacement on the Lobster Cove Fault.

ACRITARCH DATING OF ARENIG VOLCANISM IN THE LAKE DISTRICT

SUMMARY A volcanic sequence cropping out in the south-western Lake District has been dated as mid-Arenig by acritarchs of probable upper D. extensus Zone age. This is the oldest episode of volcanic activity identified in the Lake District and shows that the local Ordovician island arc system was active during Arenig times.

U–Pb ages of some crystalline rocks from the Burlington Peninsula, Newfoundland, and implications for the age of Fleur de Lys metamorphism: Discussion

In a recent paper, Mattinson (1977) reports some U-Pb zircon data from the Burlington Peninsula, Newfoundland, and proposes a synthesis of the isotopic data from this area. His synthesis, albeit tentative, is as follows (the geological units are shown in Fig. 1): (1) ophiolite formation -500-510 Ma ago (basal part of the Snooks Arm Group), followed by deposition of sediments in Arenig time; (2) emplacement of the Cape St. John Group and the Cape Brule porphyry -475 Ma; (3) intrusion of the Burlington granodiorite, Dunamagon granite, and Seal Island Bight syenite -440 Ma ago; and (4) resetting of some mineral ages in the Burlington granodiorite by an event -340 Ma ago. Since the Burlington granodiorite, Seal Island Bight s yenite, and Dunamagon granite are all either preor syn-kinematic intrusions, Mattinson also suggests that both deformation and metamorphism in the Burlington Peninsula had begun by -440 Ma, but not prior to 475 Ma. Mattinson's (1977) data represent a welcome , contribution to the understanding of the evolution I of the region, but we do wish to dispute some of his I findings. To begin with, we take strong exception to what we consider to be a misrepresentation by Mattinson of our Rb-Sr data, which we documented in a preliminary report to the Newfoundland Department of Mines and Energy (Bell and Blenkinsop 1975). Not only does the omission of our inferences give a distorted and unrealistic picture of our findings, but it denies us any credit for earlier drawing conclusions similar to Mattinson's regarding the Cape St. John volcanics. To set the record straight, we quote from our report (the quotation refers only to the Cape St. John volcanics):