Central Mobile Belt Research Articles

Variations in style of volcanism and massive sulfide deposition in Early to Middle Ordovician island-arc sequences of the Newfoundland Central Mobile Belt

The Newfoundland Central mobile belt records the Early Paleozoic birth, development, and destruction of the Iapetus ocean. Following opening of the ocean in the Cambrian, a compressional plate margin developed in the Early Ordovician and an island arc was formed above an east-dipping subduction zone. Volcanism was active at this plate margin from late Tremadocian to early Caradocian, at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch.The pre-Caradocian island-arc volcanic sequences of the Central mobile belt are host to numerous volcanogenic massive sulfide occurrences. Both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt. In the northern part of the belt (Notre Dame Bay area), submarine, dominantly mafic, volcanic rocks host pyritic Cu-Zn deposits associated with local felsic domes. These sequences pass southward into the Victoria River area, where volcanic rocks are characterized by laterally extensive felsic pyroclastic sheets which host polymetallic Zn-Pb-Cu deposits. In southern Newfoundland (the Hermitage flexure area), partially emergent, preponderantly felsic volcanic rocks host Pb-Zn-Ag massive sulfide mineralization formed during the waning stages of volcanism.Lead isotope ratios in massive sulfide deposits in the Hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north. This, coupled with geologic evidence, suggests that these sequences formed under the influence of sialic crustal material. It is inferred that the Early-Middle Ordovician island-arc volcanism in the Newfoundland Central mobile belt occurred in both oceanic and continental margin settings. The compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings.Time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear. All Early to Middle Ordovician volcanic rocks in central Newfoundland may have formed from contemporaneous volcanic activity. If this is the case, simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin. Alternatively, the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin.The massive sulfide deposits of the pre-Caradocian island arc exhibit lead isotope ratios which are distinctly more radiogenic than those in the Buchans, Roberts Arm, and post-Caradocian Cutwell Groups. This appears to support geologically based inferences that the latter sequences are not part of the main phase of island-arc activity in central Newfoundland.

Paleomagnetism and orogenic history of the Botwood Group and Mount Peyton Batholith, Central Mobile Belt, Newfoundland

This is the first palcomagnetic result from the Paleozoic Botwood Central Zone of Newfoundland. Two Lower Paleozoic rock units were sampled: the Botwood Group (rhyolite and red sandstone) at 16 sites and the Mount Peyton Batholith (dioritic and granitic phases) at 12 sites. Two phases are recognized in this intrusion: dioritic (420 Ma) and granitic (380 Ma). Four distinct magnetization directions were observed during the study. In the Lawrenceton Formation (Bot-wood Group, rhyolite) the direction obtained after tilt correction (180°, + 47°) gives a pole at 125°E, 13°N, probably of Lower Silurian age. The overlying Wig Wam Formation of the Botwood Group (red sandstone) was magnetized during a reversal. Detailed demagnetization techniques (thermal, chemical, and alternating field) were used to separate the normal and reversed directions in composite magnetizations; with a mean direction of 194°, + 28°, the pole (100°E, 25°N) obtained after tilt correction is related to a Silurian age. Two magnetizations were found in the Mount Peyton Batholith: the first one (pole at 68°E, 15°S) is associated with the dioritic phase with an age of 420 Ma. The second one (pole at 125°E, 63°S) is associated with the granitic phase at 380 Ma.

The age of the Roberts Arm Group, north-central Newfoundland

Felsites from the Roberts Arm Group have yielded a Rb–Sr isochron age of 447 ± 7 Ma, with an initial 87Sr/86Sr of 0.7063, and a genetically related granite pluton has yielded an isochron age of 464 ± 13 Ma, with an initial ratio of 0.7064. The Mansfield Cove oceanic plagiogranite complex, which is in fault contact with the Roberts Arm Group, has yielded a minimum zircon 207Pb/206Pb age of 594 ± 10 Ma. This plagiogranite, and basaltic rocks of the Hall Hill complex which partly surround it, thus provide a basement of suitable age upon which the Roberts Arm Group may have been deposited. The age of this basement corresponds to that of the opening of the lapetus Ocean.The stratigraphic age of the Roberts Arm Group, indicated by these ages, is Middle or possibly Late Ordovician. The volcanics were deposited either before or slightly after the Caradocian black shales of the Notre Dame Bay region. In the former case, Ordovician volcanism around western Notre Dame Bay ceased at about the time of obduction of the allochthons of western Newfoundland. In the latter, volcanism of the Roberts Arm Group marks a north to westward shift of volcanism within the central mobile belt of Newfoundland that occurred in late Middle or early Late Ordovician time. If the Buchans Group is of Silurian age and correlative with the Roberts Arm Group then this lithostratigraphic unit is probably diachronous, younging to the south.

Teleseismic reflections and the Newfoundland lithosphere

Travel-time residuals for teleseismic PP waves reflected under Newfoundland are consistent with a crust towards the western edge of the Central Mobile Belt at least 10 km thicker than in adjacent regions. The thicker and faster crust probably overlies an area of the upper mantle that is up to about 0.5 km s−1 faster than normal for P waves, down to at least 100 km depth. The higher seismic velocities may be due to anisotropy in a former oceanic upper mantle. The low number of reflections observed from the eastern margin of the Mobile Belt could be due to asymmetry in the ray paths due to dipping structure, such as a fossil subduction zone, near the PP reflection point.

The geochronology of some granitic bodies from eastern Newfoundland and its bearing on Appalachian evolution

Whole-rock Rb–Sr isochron ages from nine granitic plutons that intrude the Central Mobile Belt and the Avalon Zone of Newfoundland are consistent with the view that most of the magmatic activity for this segment of the Appalachian Orogen is related to the Acadian Orogeny. Most of the granites are between 400 Ma and 340 Ma old, although two from the Avalon Zone are significantly different. One, the foliated Swift Current granite, is late Cambrian – early Ordovician (500 ± 30 Ma), whereas the other, the peralkaline St. Lawrence granite, is Carboniferous (315 ± 5 Ma).Several foliated granites from the Gander Zone of the Central Mobile Belt, formerly considered to be older than Mid-Ordovician, are Silurian or Devonian in age. Five megacrystic biotite granites have ages ranging between 420 ± 20 Ma and 340 ± 10 Ma, and a leucocratic granite, the Middle Ridge Pluton, gives an age of 370 ± 15 Ma. None of the granites from the Gander Zone is older than 420 ± 20 Ma, and geochronological evidence for a Precambrian basement in the Central Mobile Belt has still to be found.The initial 87Sr/86Sr ratios show no correlation with age, although the initial ratio of 0.722 for the St. Lawrence granite is much higher than the ratios of between 0.704 and 0.709 for the other granites; this high ratio is consistent with derivation of the granite by partial melting of the continental crust.The isotopic data impose certain constraints on models proposed for the evolution of Newfoundland, the most important being: (i) extensive Devonian magmatism; (ii) intense post-400 Ma cataclasis; (iii) generation of petrographically similar granites at different times; and (iv) Carboniferous magmatism.

Geochronological evidence of Hercynian activity in Newfoundland

A RECONSTRUCTION of the continental blocks for the central and North Atlantic has led to the widespread acceptance of the continuity of the Palaeozoic orogenic belts on both sides of the Atlantic. A reconstruction of the land masses during the Carboniferous and Permian shows the Variscan Belt of Europe, the Mauritanides of Morocco, and the Southern Appalachians as being continuous1. Although the Hercynian Front has now been recognised in New Brunswick2 the situation in Newfoundland is unclear. We suggest, on the basis of the geochemical data outlined in Table 1, that the continuation of these belts can be interpolated between New Brunswick and south-west England to include at least the Central Mobile Belt and the Avalon Zone of Newfoundland. We find the earlier correlation by Holmes3 particularly attractive in view of our new isotopic data, a correlation which extends the Hercynian Front westwars from south-west England and Wales into Canada, where it passes along the Cabot Fault into the New Brunswick seaboard and the Bay of Fundy (Fig. 1). Cherkis et al.4 link the Charlie Gibbs Fracture Zone to the Hercynian Front on both the European and North American continents, and have recently offered new evidence for the emergence of the front in Newfoundland5. The intersection of the Charlie Gibbs Fracture Zone and the continental shelf of Newfoundland lies about 100km east of the Cabot Fault, so that their delineation of the front lies further to the south than the one outlined here.

Age relationships along the Hermitage Bay–Dover Fault System, Newfoundland

WE present here geochronological evidence that parts of the Central Mobile Belt and the Avalon Zone of Newfoundland have been contiguous since the Ordovician or earlier, and that some time after about 4 Myr, significant movement took place along part of the boundary.

Geochronology of eastern Newfoundland

WHOLE-ROCK Rb–Sr isochrons from granitoid rocks intruded into the easternmost part of the Central Mobile Belt and the Avalon Platform of Newfoundland indicate that the bulk of the plutonic activity took place during Devonian and Carboniferous times. Of the nine intrusions that were dated, only one, a Cambrian granite from the Avalon Platform, was significantly older.

A probable glacio-marine deposit of Late Ordovician—Early Silurian age from the north central Newfoundland Appalachian Belt

SummaryConglomerate beds occur in an Upper Ordovician—Lower Silurian clastic sequence on the northeastern side of the Newfoundland Central Paleozoic Mobile Belt. They contain scattered pebbles and cobbles dispersed in a finely laminated sandstone and siltstone matrix. Laminations are generally 5–20 mm thick and the clasts 10–300 mm across. The laminations are locally disrupted by the clasts. These laminations have subsequently been tectonically flattened into augen around the clasts and locally disrupted by boudinage. It is concluded that these conglomerate beds represent icerafted glacio-marine deposits interbedded with turbidites which were probably deposited in a locally restricted marine environment. The deposits are interpreted on the basis of scattered fossil localities as being of Late Ordovician–Early Silurian age. They are compared with deposits of similar age elsewhere in the North Atlantic region.

Possible Cryptic Suture in South-west Newfoundland

THE concept of an important fault, the Cabot Fault, cutting across western Newfoundland was proposed by Wilson1 who interpreted it as a great sinistral fault and correlated it with the Great Glen Fault system in Scotland. There is, however, a second large fault in south-west Newfoundland, the Cape Ray Fault, the importance of which has previously been unrecognized. This fault separates two entirely different pre-Ordovician gneissic terrains which, when traced north-eastwards from Port aux Basques (Fig. 1) form the eastern and western margins of the Newfoundland Central Mobile Belt. This belt contains Lower Ordovician rocks in its central part which are interpreted as remnants of an old ocean floor, the proto-Atlantic1–5. The Cape Ray Fault thus effectively cuts out this ocean toward the south and juxtaposes its eastern and western margins. It may therefore represent a cryptic suture along which complete closure of the proto-Atlantic took place.

Pre-Ordovician Polyphase Structure in the Burlington Peninsula of the Newfoundland Appalachians

THE Appalachian System in Newfoundland can be subdivided into three tectonostratigraphic divisions characterized by differences in Early Palaeozoic depositional and structural development (Fig. 1). The Central Mobile Belt1, characterized by thick variably deformed late Precambrian sedimentary and volcanic rocks, is flanked by more stable platformal regions2,3 underlain by thinner, generally less deformed sequences of similar age. On the Western Platform these rocks unconformably overlie a gneissic Grenvillian basement. The Central Mobile Belt in the northern part of the island can be further subdivided into an axial region of sedimentary and volcanic rocks of dominantly Ordovician and Silurian age, which are generally moderately deformed and incipiently metamorphosed in contrast to the marginal crystalline belts (Fig. 1) of poly-deformed metasedimentary and metavolcanic rocks of upper greenschist to low amphibolite facies, which contain large recumbent folds4. These rocks have been subjected to polyphase deformation and moderate grade metamorphism before deposition of adjacent Lower Ordovician rocks in the west5,6 and Middle Ordovician rocks in the east7.

Geology of the Gullbridge Copper Deposit, Newfoundland: Volcanogenic Sulfides In Cordierite–Anthophyllite Rocks

The Gullbridge copper deposit occurs in a folded and regionally metamorphosed sequence of Upper Ordovician mafic volcanic and tuffaceous rocks in the Central Mobile Belt of Newfoundland. The ore consists of chalcopyrite, pyrrhotite, and pyrite in a tabular zone of cordierite–andalusite–chlorite schist that is surrounded by cordierite–anthophyilite rock. Regional metamorphism to greenschist or amphibolite fades preceded and accompanied regional de formation that produced tight folds with a strong axial plane schistosity (S1) in the host and country rocks. Contact metamoprhism related to nearby plutons superimposed the cordierite, andalusite, and anthophyllite mineral assemblages upon S1. This was followed by a second deformation and by chloritization of the ore zone. Finally the ore body underwent some minor mineralogical and structural modifications. From various field and laboratory evidence the sulfides, and possibly the associated magnesium alteration, are interpreted as volcanogenic. The Gullbridge deposit is similar to a number of other copper sulfide deposits in cordierite anthophyilite host rocks in North America and Scandinavia.

The Hermitage Flexure, the Cabot Fault, and the Disappearance of the Newfoundland Central Mobile Belt

The Central Mobile Belt of the Appalachian System in Newfoundland varies in width across the island from 130 mi on the northern coast to about 40 mi on the southern coast. Granitic gneisses, metasediments, amphibolites, and mylonites that form the eastern margin of the belt in the north are involved in a major “S” shaped structure, the Hermitage Flexure. This structure brings them into proximity with the present western margin of the belt in southwestern Newfoundland. There the belt is apparently truncated by the Cabot Fault. Assuming a constant width for the Central Mobile Belt during the developmental stages of the geosyncline, then its southward thinning and partial disappearance can be explained first by open folding of the belt around the Hermitage Flexure and then by westward truncation by left-lateral (sinistral) displacement along the Cabot Fault.

MOBILE PROVINCES AND THE PROBLEMS OF THEIR NOMENCLATURE

This is a critical review of the nomenclature of geosynclines, offering for discussion certain terms for mobile (geosynclinal-folded) provinces of the world. Discussion is limited essentially to post-Proterozoic structures, concentrating on mobile segments of the crust, as arranged in belts, on a planetary scale. These include the Mediterranean and Pacific mobile belts, the Central Asian mobile belt and the Ural-north Tien Shan and circum-Arctic mobile belts.--M. Russell.