Bay Of Islands Ophiolite Complex Research Articles

The structure of sheeted dikes and associated rocks in North Arm massif, Bay of Islands ophiolite complex, and the intrusive process at oceanic spreading centers

The structures of North Arm Mountain ophiolitic basalt and diabase indicate that these formed as a frozen lid to an underlying magma chamber in a system analogous to present-day mid-ocean spreading systems. Volcanics consist of pillow lavas, flows, and breccias. Volcanics grade downward into sheeted dikes by increase in dike frequency across a transition commonly less than 50 m thick. Sheeted dikes, subparallel throughout, dip away from the paleospreading axis, located to the west of present ophiolite position as indicated by chilled margin bias, implying rotation of the lid down and away from the axis. One-way chilling statistics, dike thickness, and dike intrusion sequence analyses all point toward a relatively narrow but complex zone of dike intrusion. The mean measured dike thickness is 0.90 m. Diabase contains zones of fracturing subparallel to dike trends, interpreted to reflect oceanic fissuring and faulting. These commonly terminate downward in foliated gabbro and (or) lineated amphibolite. Diabase grades into gabbros across a complex transition wherein (1) dikes grade into gabbro by gradual decrease in dike frequency, (2) dikes grade abruptly into gabbro, and (3) gabbro intrusions truncate and stope overlying diabase.

The structure of sheeted dikes and associated rocks in North Arm massif, Bay of Islands ophiolite complex, and the intrusive process at oceanic spreading centers

The structure of sheeted dikes and associated rocks in North Arm massif, Bay of Islands ophiolite complex, and the intrusive process at oceanic spreading centers

Ultramafic intrusions in the Lewis Hills Massif, Bay of Islands Ophiolite Complex, Newfoundland: Implications for igneous processes at oceanic fracture zones

The Coastal Complex of western Newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone. Within the Coastal Complex, a suite of peridotite bodies and associated basaltic dikes occur. The least-deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them. Intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers. These processes are believed to be characteristic of ridge-transform intersections or “leaky” transform regions. Two petrologically distinct types of peridotite have been identified. One type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks, and these lithologies may be interpreted as differentiates produced in the evolution of typical Mid-Ocean Ridge Basalts (MORBs). The second type of peridotite, although lithologically similar to the first, has relatively high CaO/Al 2 O 3 ratios and relatively low TiO 2 and Na 2 O. This second type of peridotite cannot be simply related to the genesis of typical MORBs. Chilled margins and fine-grained dikes along the margins of some of these peridotite bodies are high-magnesium basalts.

Amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust: Bay of Islands, Newfoundland

In the Bay of Islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed. Higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering. The resulting shear bands are narrow and composed either of mylonitic amphibole-bearing gabbros or of highly foliated amphibolites. The amphibole composition and nature, and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450°C. This shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle. A second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge, occurred at lower temperatures ranging from 500 to 300°C. The surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450°C. All metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis.

Mineral chemistry of ultramafic cumulates from the North Arm Mountain Massif of the Bay of Islands ophiolite: Evidence for high‐pressure crystal fractionation of oceanic basalts

The mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the North Arm Mountain massif, Bay of Islands ophiolite complex, is not consistent with crystal‐liquid fractionation of a primitive mid‐ocean ridge basalt at low pressures. The appearance of clinopyroxene and orthopyroxene having very high Mg numbers [100 × Mg/(Mg + Fe)= 85 to 92] and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine (Fo86–92) suggest that moderate‐ to high‐pressure phase relationships are applicable. We propose on the basis of mineral chemistry, crystallization sequences, and structural relationships that the basal ultramafic cumulates on the North Arm massif represent the products of high‐pressure (>10 kbar) polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid‐ocean spreading center. These basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km. After their formation, the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid‐ocean ridge.

Tectonic structures related to thrusting of ophiolitic complexes: the White Hills Peridotite, Newfoundland

The White Hills Peridotite and its underlying metamorphic rocks represent the basal portion of a partly eroded ophiolitic complex. At the contact with the metamorphic rocks, the peridotites display low-temperature–high-stress mylonitic textures, which grade upwards to fine-grained porphyroclastic textures indicative of higher temperatures and lower stresses. Petrological and structural evidence indicates that the mylonitic peridotites and underlying metamorphic rocks have been deformed during the oceanic thrusting of the lithosphere. The analysis of the main tectonic structures observed in the peridotites shows that the complex has been thrust northwestward along a southeast-dipping thrust plane. The data are compared with those obtained from the Bay of Islands ophiolite complex.

Lateral heterogeneity in the seismic structure of the oceanic crust inferred from velocity studies in the Bay of Islands ophiolite, Newfoundland

Received 1981 June 26; in original form 1980 October 17 Summary. In the Bay of Islands ophiolite complex of Western Newfoundland, Lower Ordovician crust and upper mantle are exposed in four massifs. Two of these, the Blow-Me-Down and North Arm massifs, contain complete or nearly complete ophiolite successions ranging from clastic sediments at the top, through pillow basalts, sheeted dykes, gabbros, and peridotites at the base. The detailed velocity structure of the North Arm massif has been reconstructed from compressional and shear wave velocity measurements to confining pressures of 6 kbar and low pore pressures for 125 cores collected from $3 sites of known stratigraphic level and is compared with the previously determined seismic structure of the Blow-Me-Down massif. Within the upper 2km of North Arm, compressional wave velocities increase from 5.0 to 7.0 km s-' and shear wave velocities increase from 2.8 to 3.8 km s-'. Velocities at this level in the Blow-Me-Down massif are higher due to the presence of pumpellyite. In both massifs, the top of layer 3 occurs within the sheeted dyke complex at a depth of approximately 1.5 km and marks the transition from greenschist to amphibolite facies metadolerites. High level gabbros and plagiogranites at depths of between 2 and 4 km are responsible for velocity inversions. In North Arm, Poisson's ratios at this depth are relatively high (0.3 1-0.34); whereas in Blow-Me-Down, abundant quartz-rich plagiogranites give rise to low Poisson's ratios (0.25-0.27). The lowest crustal unit in North Arm, which consists primarily of gabbro and amphibolite, has average compressional and shear wave velocities of 7.0 and 3.8 km s-', respectively. Within this section (4-6 km) the velocities decrease slightly with increasing depth in North Arm, but increase abruptly at 5.5 km depth in Blow-Me-Down. In the North Arm massif, the transition from crust to mantle is rather gradual due to the presence of a relatively thick mixed zone of gabbro and ultramafics.

Reconstructed seismic velocity structure of the Lewis Hills Massif and implications for oceanic fracture zones

The Lewis Hills Massif of the Bay of Islands Ophiolite Complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere. The structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the Bay of Islands complex. Results of detailed field mapping, petrography, and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere. Velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region. Although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail, from the perspective of the present study it is suggested that, in general, seismic properties differing from that of ‘normal’ oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones. In these regions, lower crustal as well as upper mantle anisotropy is expected. The Moho might be expected to shallow due to the presence of abundant ultramafic cumulates, resulting in crustal thinning, and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics. In oceanic lithosphere which has passed or is passing through a transform fault domain, enhanced mantle anisotropy might occur. The middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks, which grade upward into serpentinites and highly fractured gabbros, diabases, and basalts with relatively low velocities. It is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones.

Magma chamber profiles from Bay of Islands ophiolite complex

Magma chamber profiles from Bay of Islands ophiolite complex

Magma chamber profiles from Bay of Islands ophiolite complex (reply)

Magma chamber profiles from Bay of Islands ophiolite complex (reply)

An estimate of the Nd isotopic composition of Iapetus seawater from ca. 490 Ma metalliferous sediments

Well-preserved metalliferous sediments and pillow basalts of Lower Ordovician age (ca. 490 Ma) have been studied in an attempt to specify the Nd isotopic composition of Iapetus seawater. Initial 143Nd/ 144Nd ratios of the pillow basalts are indistinguishable from published initial ratios for the 505-Ma Bay of Islands ophiolite complex and are within the anticipated range for MORB-type basalts 500 Ma ago. Metalliferous sediments occur both interstitial to basalt pillows and as well-developed sedimentary accumulations. The initial 143Nd/ 144Nd ratios for the non-interstitial variety range from 0.511851 to 0.511712 (ε Nd = −2.7to−5.4) and are considered to provide an estimate of 143Nd/ 144Nd in Iapetus seawater. The interstitial metalliferous sediments show evidence for a significant basalt-derived Nd component. Although volcanic activity occurred at the margin of Iapetus essentially contemporaneous with the formation of the metalliferous sediments, it is clear that arc-type volcanic material was not a major source of Nd in Iapetus seawater. Rather the source of Nd was from continental regions with a similar average age to those supplying material to the present-day Atlantic Ocean.

Magma chamber profiles from the Bay of Islands ophiolite complex

Models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle. Evidence from the Bay of Islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber, but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir. Coherent, well defined and systematically oriented, large-scale, arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the Bay of Islands complex. These layering patterns seem to outline the shape of a large, continuously evolving, steady-state magma chamber that extended along a ridge axis and ended at a transform fault intersection.

A parallochthonous group of sedimentary rocks unconformable overlying the Bay of Islands ophiolite complex, North Arm Mountain, Newfoundland

This paper reports and documents the existence on the North Arm Mountain massif of a parallochthonous group of sedimentary rocks that overlie the Bay of Islands ophiolite complex with marked angular unconformity. Some of these rocks are those previously regarded as oceanic sediments lying conformably on top of the igneous rocks of the ophiolite complex, and some were mapped as part of the igneous rocks themselves. The name Crabb Brook Group is proposed for this sedimentary sequence and three new formation names are also put forward. Massive sedimentary breccias directly overlie and contain clasts from all units of the ophiolite complex. Evidence of intense predepositional weathering of the clasts and the underlying bedrock is common. Bedded shales and their disrupted equivalents in olistostromes containing sedimentary and igneous blocks overlie the breccias. Two fossil localities within the shales yield acritarchs of Llanvirnian age and inarticulate brachiopods probably not older than this. A small remnant of coarse red arenite caps the section. The sequence appears to overlie some thrust faults and to be cut by others. It is tightly folded on a large scale along with the thrusts and the ophiolite complex. We interpret the whole folded stack, including some underlying allochthonous sediments, to be truncated by gently inclined thrusts formed in the later stages of allochthon emplacement. The lithologies, facies, structural and tectonic relationships, and fossil age of this sedimentary group indicate that it is parallochthonous in the context of the regional geology. It was deposited on the Bay of Islands ophiolite complex during its obduction and cannot therefore be used to infer the tectonic environment of the spreading ridge and transform fault that formed the Bay of Islands complex.

Nd and Sr isotopic study of the Bay of Islands Ophiolite Complex and the evolution of the source of midocean ridge basalts

Two Sm‐Nd internal isochrons for pyroxene gabbros of the Bay of Islands Ophiolite Complex give well‐defined ages of 508 ± 6 m.y. and 501 ± 13 m.y. with initial 143Nd/144Nd of εNd = +7.7 ∓ 0.1 and εNd = +7.5 ∓ 0.2, respectively. Total rock samples from pillow basalts, sheeted dikes, trondhjemites, hornblende gabbros, pyroxene gabbros, and an orthopyroxenite layer from the harzburgite give initial εNd in the range from +6.5 to +8.1 with an average value of +7.6. The initial 87Sr/86Sr obtained on a pyroxenegabbro is εSr = −19.3 ± 0.3, which is typical of oceanic samples. However, the initial 87Sr/86Sr within the different phases of the complex is found to be highly variable (∼52 ε units) and shows the effect of sea water alteration. The εNd values demonstrate a clear oceanic affinity for the Bay of Islands complex and support earlier interpretations made on the basis of structure and geochemistry. The magnitudes of the initial εNd values (+7.6) are somewhat smaller than for typical present‐day midocean ridge basalts (MORB) (+10). This is most likely due to differential evolution over the past 0.5 aeon of the oceanic mantle relative to the bulk earth. The observed shift is quantitatively what should be expected for a simple single‐stage evolution. For a model with a single differentiation event at time TD to produce the depleted mantle, both Sm‐Nd and Rb‐Sr data for MORB and the Bay of Islands Complex give TD ≈ 1.8 aeons. This age is, however, interpreted as the mean age of the MORB source and does not refer to a unique event. The total time for producing this source by a uniform process would be of the order of 3.6 aeons. The Nd isotopic signature of oceanic crust is clearly present in this Paleozoic ophiolite and suggests that typical high‐εNd reservoirs are sources of oceanic crust through the Phanerozoic. This implies relatively rapid turnover between oceanic crust and mantle sources and a good mixing of oceanic mantle for the past 1.0 aeon, including contributions from recycled continental materials. These data indicate that the major distinction between oceanic basalts and continental basalts observed in recent rocks is also preserved through the Phanerozoic. These isotopic differences clearly imply a long time distinction between the magma sources of these basalt types. It should be possible to apply the distinctive εNd values of oceanic crust and mantle to identify old obducted oceanic segments on continental blocks.

Comment on ‘The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, an exposure of oceanic crust and upper mantle’ by Matthew H. Salisbury and Nikolas I. Christensen

Comment on ‘The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, an exposure of oceanic crust and upper mantle’ by Matthew H. Salisbury and Nikolas I. Christensen

Reply [to “ Comment on ‘The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, an exposure of oceanic crust and upper mantle’”

Reply [to “ Comment on ‘The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, an exposure of oceanic crust and upper mantle’”

Seismic anisotropy in the oceanic upper mantle: Evidence from the Bay of Islands Ophiolite Complex

Olivine fabrics in 17 field‐oriented ultramafics and mafics from three widely‐spaced traverses in the Bay of Islands Ophiolite Complex, Newfoundland, display a remarkably uniform symmetry in which the olivine a crystallographic axes are aligned subprependicular to the sheeted dikes and the b and c axes lie within the plane of the sheeted dikes. The ultramafics studied consist entirely of tectonites; any olivine formed at the ridge crest by cumulus processes has since been re‐oriented by translation gliding and/or syntectonic recrystallization. Deformation has extended from the ultramafics into the overlying gabbro, which suggests that in many oceanic regions the deepest levels of layer 3 consist of gabbroic tectonites. Compressional wave velocities computed from these petrofabrics display 5–6% anisotropy in the plane of the Mohorovičić discontinuity, with Vp fast parallel to the direction of spreading inferred from dike orientations. Since this pattern is identical to that observed for the oceanic upper mantle, it is concluded that the Bay of Islands Complex is a segment of oceanic crust and upper mantle. Shear wave velocity contours calculated from the same fabrics indicate that the upper mantle is nearly isotropic in terms of the maximum shear wave velocity, , but that the difference in velocity, ΔVs, between shear waves of orthogonal polarization traveling in the same direction may be sufficiently large parallel to the intersection of the Mohorovičić discontinuity and the sheeted dikes to allow detection of two distinct shear wave arrivals.

The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, An exposure of oceanic crust and upper mantle

Although the ophiolites are widely recognized as segments of oceanic crust emplaced on land, direct correlation between the ophiolites and the oceanic crust has proven difficult owing to the near absence of common criteria on which to base a comparison; the ophiolites are defined petrologically, while the oceanic crust is defined largely in terms of seismic structure. To bridge this gap the seismic velocity structure of a traverse through the Blow‐Me‐Down massif of the Bay of Islands ophiolite complex, Newfoundland, has been reconstructed in detail from values of compressional (Vp) and shear (Vs) wave velocity measured in the laboratory under conditions of hydrostatic confining pressure and water saturation thought to approximate conditions in the oceanic crust through oriented samples collected from 60 closely spaced sites of known stratigraphic level. The velocity structure thus determined is indistinguishable from that of normal oceanic crust: The uppermost velocity unit in the massif consists of 0.5 km of prehnite‐pumpellyite facies metabasalt with Vp ≤ 5.70 and Vs ≤ 3.10 km/s, underlain by 0.8 km of greenschist facies pillow basalts and brecciated dikes with Vp ≤ 6.20 and Vs ≤ 3.35 km/s. Between 1.3 and 6.4 km, in a thick unit composed of metadolerite sheeted dikes underlain by coarse‐grained metagabbro grading downward through pyroxene and troctolitic olivine gabbro, Vp and Vs increase from 6.75 and 3.75 km/s near the top to 7.40 and 3.90 km/s near the base, respectively. This increase is gradational, except at 5.3 km, where a step increase in Vp to 7.40 km/s marks an increase in olivine content. A sharp velocity inversion in Vp, caused by quartz‐rich late differentiates, is found in the upper levels of this unit between 2.8 and 3.3 km. The deepest level of the complex, composed of ultramafics, is characterized by values of Vp and Vs of 8.4 and 4.9 km/s, respectively. A comparison of the seismic velocity structure and petrology of the traverse across the Blow‐Me‐Down massif with oceanic seismic structure suggests that at many sites in the ocean basins, (1) layer 2 consists of prehnite‐pumpellyite and greenschist facies pillow basalts and brecciated dikes, (2) the layer 2–3 boundary separates greenschist facies metabasalts and brecciated dikes at the base of layer 2 from epidote‐amphibolite facies sheeted dikes at the top of layer 3, (3) layer 3 consists of metadolerite sheeted dikes underlain by metagabbro, pyroxene gabbro, and troctolitic olivine gabbro, (4) the 7.4‐km/s basal layer observed in sonobuoy studies consists of interlayered olivine gabbro, troctolite, and plagioclase peridotite, (5) the Mohorovičić discontinuity represents a relatively sharp transition from gabbro to dunite and peridotite, and (6) pronounced, but laterally discontinuous, velocity inversions may be present at the base of layer 2 below the relatively high velocity prehnite‐pumpellyite facies metabasalt level and at intermediate levels in layer 3 in association with late differentiates.

Coastal Complex, western Newfoundland: An Early Ordovician oceanic fracture zone

Oceanic crust and mantle adjacent to fracture zone extensions of ridge-ridge transform faults consist of two strips on either side of and parallel with the fracture zone. One strip with a transform-fault deformation history is welded to a younger strip that has not had a transform history. Observations of oceanic fracture zones and theoretical models indicate that both strips have complex petrologic and structural histories and relationships quite different from oceanic crust and mantle generated at ridge segments away from fracture zones. Geologic relationships predicted by our simple models are matched in a remarkably precise way in ophiolite assemblages in western Newfoundland. The Coastal Complex, a hitherto enigmatic, varied, and complex assemblage of sedimentary and ultramafic and mafic igneous rocks is believed to have acquired its complexity during movement through a Late Cambrian ridge-ridge transform domain and past a ridge termination. The Bay of Islands Ophiolite Complex is, in one region, in direct continuity with the Coastal Complex and is believed to have originated on the nontransform side of the ridge termination past which the Coastal Complex was moving. This Late Cambrian–Early Ordovician fracture zone may have nucleated the medial Ordovician obduction site, the younger, higher Bay of Islands Complex riding across the older, lower Coastal Complex side and, locally, carrying with it and preserving strips of the older fracture zone assemblage. The high frequency of fracture zones along some accreting plate margins in modern oceans suggests the likelihood that ophiolite complexes will contain parts of oceanic crust and mantle preserving a fracture zone history. This likelihood is further enhanced if fracture zones nucleate obduction zones. Many published accounts of ophiolite complexes, particularly in the Alpine system of Europe and the Middle East, include descriptions of petrologic and structural relationships that accord well with relationships observed in the Coastal Complex and also may have been developed in oceanic fracture zones.

K–Ar ages of amphiboles from the Bay of Islands ophiolite and the Little Port Complex, western Newfoundland, and their geological implications

Over 20 K–Ar ages and one 40Ar/39Ar incremental heating analysis of amphiboles from the Bay of Islands ophiolite complex and the Little Port Complex, western Newfoundland, are reported. Amphiboles were obtained from granitic rocks and amphibolites which outcrop in the northern part of the Little Port Complex, and from the sheeted dykes and gabbros and from the basal metamorphic aureole of the ophiolite. The results indicate that obduction of the ophiolite took place approximately 470 m.y. ago and that movement, associated with the emplacement of the Humber Arm allochthon, ceased after 455 m.y. BP. It is postulated that initial argon in amphiboles taken from the basal metamorphic aureole of the ophiolite has resulted in some anomalously high ages.Since the stratigraphic and paleontological relationships in the Bay of Islands are are well known, the dates for obduction and final movement represent additional geochronological reference points in the lower to middle Ordovician time scale.