Bay Of Islands Ophiolite Complex Research Articles

Osmium Isotope Heterogeneity of the Upper Mantle: Evidence From the Bay of Islands Ophiolite Complex, Newfoundland

AbstractUnderstanding and determining the composition and evolution of the upper mantle is valuable to unravel Earth's evolutionary geodynamics. The compositions of oceanic basalts are inevitably biased by the preferential melting of fusible mantle components and the pooling of melts from multiple sources. Here we present whole rock and mineral chemistry as well as Re‐Os isotopic systematics of the Table Mountain ophiolitic mantle massif from the Bay of Islands ophiolite complex, complemented by thermodynamic models of melt extraction. The Table Mountain lherzolites were formed by moderate degrees (5%–15%) of shallow polybaric melting. We divide the harzburgites into two types. Type I harzburgites represent residues of high‐pressure melting, and Type II harzburgites appear to have been further modified by second‐stage flux melting above a subduction zone induced by slab sediment‐derived aqueous fluids. The Table Mountain peridotites display highly variable “initial” 187Os/188Os485 Ma isotope compositions, including 0.1252–0.1253 for the Type I harzburgites, 0.1217–0.1236 and 0.1192–0.1198 for both lherzolites and Type II harzburgites. These Os isotopic variations cannot be generated by different degrees of partial melting of a homogenous fertile mantle immediately prior to obduction, and hence must have been inherited from ancient melting events. The Table Mountain lherzolites with inherited ancient melting signals are of particular significance because they demonstrate that the record of earlier melting events is not limited to ultra‐refractory harzburgites but is also present even in relatively fertile lherzolites. Our observations underscore the ubiquity of ancient melting signatures in the entire compositional spectrum of the depleted upper mantle.

Synchronous formation of the ‘forearc’ Bay of Islands ophiolite and its basal high-temperature metamorphic sole constrained by U–Pb zircon ages

The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex (BOIC) in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature (P-T) metamorphic gradient from HT-MP granulite to LT-LP greenschist facies. It incorporates mafic volcanic/plutonic protoliths mixed with pelagic, hemi-pelagic and coarser epiclastic sedimentary protoliths. New LA-ICP-MS U–Pb concordia ages, trace elements, and Ti-in-zircon geothermometry for ∼250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported. Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma, respectively, that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite, which formed at a supra-subduction zone (SSZ) forearc spreading center. A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected ∼30 m below the peridotite contact of the Table Mountain massif. Zircon crystals analyzed have similar size and morphologies, subparallel rare earth element (REE) variation patterns, and steep heavy REE-enrichments ((Lu/Gd)cn >20), significant positive Ce anomalies (dominantly >5) and slight positive to dominantly negative Eu anomalies (1.2 – 0.4). Zircon shows Th/U mean values of 0.37 – 0.48 with little to no rim to core variation. Minimum Ti-in-zircon mean crystallization temperatures range from ∼764 – 787 °C. These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites. Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge, that is, a consequence of the newly-formed BOIC forearc spreading center extending from the back arc to a triple junction along the westward- (or paleo-northward) verging trench of the Notre Dame arc. The early HT sole formation age at ca. 489–488 Ma is long prior to initiation of obduction at ca. 470 Ma and long after initiation of subduction beneath the paleo-northward verging Notre Dame peri-Laurentian arc at ca. 514 Ma. This indicates Newfoundland sole ages of the BOIC and St. Anthony Complex are correlated with the age of SSZ spreading, but not necessarily subduction initiation because previously existing and self-sustaining subduction was ongoing. Sole ages are then not correlated with the younger age of obduction-related orogenic events (e.g., proposed Taconic I and II) in the Newfoundland Appalachians.

Lithium isotopic and fluid mobile trace element systematics of the Bay of Islands altered forearc upper to lower ophiolitic crust

Bulk rock Li isotope and select fluid mobile element traverses through the crustal section of the Late Cambrian Bay of Islands Ophiolite Complex (BOIC) in the Taconic Humber Arm Allochthon in the Newfoundland Appalachians are presented to examine the extent and conditions of alteration at different pseudo-stratigraphic levels in the ancient forearc oceanic crust that was obducted during the Middle Ordovician.N-MORB normalized enrichment factors (EFs) calculated for Li and fluid mobile elements (FMEs) show variable extents of enrichments or depletions from shallow to deep oceanic crustal sections in the BOIC that are consistent with a broad range of alteration-related δ7Li values from −2.97 to +20.69 ‰ (vs. unaltered MORB range of +1.5 to +6.1 ‰ with a mean of +3.4 ± 1.4 ‰). EFs and fluid-rock reaction modeling for Li contents and δ7Li values indicate alteration dominantly controlled enrichments and depletions of Li isotopic and FMEs systematics through all crustal levels of the BOIC ophiolite. Alteration fluids are dominated by modern seawater-like δ7Li composition rather than the recently proposed Cambrian seawater that is based on ancient carbonate shell compositions.BOIC basaltic samples proximal to the York Harbor Mine (YHM), similar to the classic mafic Cu-Zn Cyprus-type volcanic massive sulfide (VMS) deposits, show light δ7Li values of −2.97 to +1.59 ‰, but highly enriched Li contents (14.0–40.7 μg/g). Modeling results indicate that these YHM basalts were altered by metal-rich magmatic fluids, in part, derived from deeper plagiogranite intrusions near the base of the sheeted dikes that are possibly admixed with seawater-derived hydrothermal fluids below the VMS deposit. Magmatic volatile exsolution may be associated with the crystallization of directly underlying subvolcanic silicic intrusions, resulting in over-pressuring and fracturing of the carapace that may assist in localizing the high temperature (400–200 °C) upflow zones of the seafloor hydrothermal system.The BOIC layered gabbroic rocks also show evidence of variable extent of alteration as reflected by both Li content (EF both >1 and < 1) and δ7Li composition (+0.79 to +9.47 ‰), likely indicating high-temperature amphibolite (800–600 °C within the ductile deformation field) to lower temperature greenschist facies paragenesis, with down-temperature complex retrograde petrogenetic lineages. Mylonites and ductile shear zones commonly show heavier δ7Li values between +5.47 and + 18.83 ‰. We suggest that mylonite zones and coarser-grained high-strain zones allowed near-axis fluid penetration at high to lower temperatures that extended to the basal gabbroic sections in the crust during ongoing igneous activity. Shear zones were associated with rheologic weakening and initial permeable pathways for hydrothermal fluids. Shallow on-axis and deep off-axis hydrothermal circulation likely occurred at a relatively fast-spreading rate, which may account for the significant shallow to deep crustal heat extraction as predicted by the multiple sill model of lithospheric cooling.

A new concordia age for the 'forearc' Bay of Islands Ophiolite Complex, Western Newfoundland utilizing spatially-resolved LA-ICP-MS U-Pb analyses of zircon

Advancements of zircon U-Pb dating techniques using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide an opportunity to precisely constrain the age of the Bay of Islands Ophiolite Complex (BOIC) in the Newfoundland Appalachians. New LA-ICP-MS U-Pb dates are presented for a large population of relatively simple magmatic zircon grains from trondhjemites using selective domain analyses and a 3% discordance filter in five samples collected from a single ophiolite massif, with four of the samples from a single pluton. All samples were derived from plagiogranite plutons intruded just below the gabbro-sheeted dike contact of the Blow Me Down Mountain massif, the massif sampled for three of the four prior single sample ID-TIMS age studies of the ophiolite. A highly consistent series of LA-ICP-MS concordant ages from the five samples collected yield a composite concordia age of 488.3 ± 1.5 Ma, which we propose for the revised igneous age of the ophiolite massif. This new age for the BOIC is based on 127 individual LA-ICP-MS zircon analyses and is slightly older and with lower uncertainty than the previous consensus age. We review previous pioneering ID-TIMS U-Pb dates, ages, intercept assumptions, and uncertainties for the BOIC in light of this new formation age. Legacy ages and uncertainties differ somewhat when data are reduced with modern recommended techniques and without geological interpretations that fix lower intercepts. The ca. 488.3 Ma BOIC age indicates its formation along a younger supra-subduction forearc trench-orthogonal spreading center that rifted the older Coastal Complex forearc ophiolitic assemblages reported to be formed at ca. 500-508 Ma. This rifted forearc is analogous to many modern ophiolitic forearcs. The BOIC age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites, indicating temporal BOIC spreading center linkages within the Baie Verte Oceanic Tract.

Reply to Discussion on ‘The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland’ Journal , 170, 2013, pp. 715–722

Zagorevski & van Staal (2014) question our model (Dewey & Casey 2013) in which we invoked forearc suprasubduction zone (SSZ) spreading to form the Bay of Islands Ophiolite Complex (BOIC) as part of the Notre Dame arc of the Appalachian–Caledonian Orogen. We interpret the formation of the BOIC to have occurred by spreading proximal to a ridge–trench–trench (R–TR–TR) triple junction. Our model explains (1) the formation of the BOIC within 15–20 myr of obduction, (2) the associated subcretion of the metamorphic sole shortly after formation during subduction of the ocean lithosphere bordering Laurentia, (3) subsequent oblique ophiolite encroachment to the Laurentian passive margin above a west-facing subduction zone as part of the Notre Dame forearc–arc system, and (4) the final obduction with the high-angle palaeo-ridge geometry relative to the Laurentian margin, with the emplacement initiated in the Late Arenig (Dapingian) during the Taconic Orogeny. We respond to the statements and questions posed by Zagorevski and van Staal that contest our model, which we further clarify and develop. Zagorevski and van Staal conclude, mistakenly, that we stated that the formation of the c . 489–484 Ma BOIC was associated with ‘intra-oceanic subduction initiation’ by the conversion of a ‘ridge–transform boundary’ in the Early Ordovician. They imply that we proposed a typical mid-ocean ‘intra-oceanic’ spreading centre environment for the origin of the BOIC that is linked to subduction initiation on a transform. On the contrary, in our fig. 1, p. 716, we proposed that the BOIC formed by sea-floor spreading adjacent to a pre-existing older lithosphere (Karson & Dewey 1978; Casey & Dewey 1984; Cawood & Suhr 1992; Kurth-Velz et al . 2004), represented by the earlier Middle–Late Cambrian Coastal Complex (CC) and Lush’s Bight (LB) oceanic tract, that we interpret as SSZ forearc. The CC–LB …

The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland

The early Ordovician ( c . 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician ( c . 470 Ma). The subduction zone was nucleated on an oceanic transform–fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a suprasubduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform–fracture zone–subduction zone system and NE of a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of mid-ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the greenschist-facies, obducted, oceanic and continental margin rocks trend WSW (the obduction direction).

Hydrothermal circulation and metamorphism in crustal gabbroic rocks of the Bay of Islands ophiolite complex, Newfoundland, Canada: evidence from mineral and oxygen isotope geochemistry

Abstract The gabbroic crust of the Ordovician Bay of Islands ophiolite complex formed in an island-arc setting near the North American continental margin. Detailed structural studies on the North Arm Mountain massif provide us with a scheme of syn-oceanic deformation events recorded in the crust. During a first transtensional stage, which generated gabbroic rocks, sheeted dykes and lavas, the temperature of formation of amphiboles in the gabbroic unit fell with time in three steps from 880–745 °C, to 790–680 °C and to 550–500 °C. The Ti, Na and Al IV contents of amphiboles decreased, whereas the Si activity of the fluid increased with time. The first amphibole to form has typical mid-ocean ridge basalt δ 18 O VSMOW indicating equilibration with a magmatic fluid or evolved seawater at low fluid/rock ratio. Lower δ 18 O values for some amphiboles (0–2.5‰) indicate the circulation of large volumes of seawater. The lowest δ 18 O values are found in the inner part of the shear zones, which channelled deep infiltration of seawater into the gabbroic unit. During brittle deformation, infiltration of low-temperature seawater produced prehnite, carbonate and quartz veins, and plagioclase with high δ 18 O. This study documents that the hydration of ophiolitic crust in the Bay of Islands ophiolitic complex occurred mainly along pre-obduction oceanic structures in an intraoceanic setting.

Origin of mineral chemical stratification in the mantle section of the Table Mountain massif (Bay of Islands Ophiolite, Newfoundland, Canada)

Mineral chemical data have been collected from the 6 km thick mantle section of the Table Mountain massif (Bay of Islands Ophiolite Complex, Newfoundland). The samples chosen represent the geochemical background, i.e. exclude samples from the vicinity of bands, dykes and pods of dunites and pyroxenites within the host harzburgites and (rare) lherzolites. Three large mineral-chemical domains can be distinguished as a function of depth below the crustal gabbros. The intermediate mantle section (2–5 km beneath the gabbros) has a very depleted geochemical pattern with Cr# [ 100 Cr ( Cr + Al) ] in spinel > 44, low Ti. The basal kilometer of the mantle section shows a steep geochemical gradient towards less depleted mineral chemistries (Cr# as low as 10). Both the intermediate and basal mantle rocks fit into a progressive partial melting trend. The top of the mantle section (0–2 km below the gabbros) shows Cr# between 30 and 44, but also higher Ti and ferric iron in spinel, and Na in clinopyroxene. Microstructural evidence for melt-related clinopyroxene is locally preserved and, in one area, plagioclase lherzolites occur. The topmost mantle section is considered to have been affected by melt infiltration. The geochemical variation is larger than elsewhere, suggesting that melt-infiltration, or at least the process of trapping of melt, was heterogeneous. In most melt-infiltrated harzburgites, plagioclase is absent. Other peridotites show good microstructural evidence for melt-infiltration but little chemical evidence to substantiate it. Both features are discussed and can be explained by a variety of processes demonstrating the complexity of melt-infiltration.

Generation and obduction of ophiolites: Constraints from the Bay of Islands Complex, western Newfoundland

Constraints provided by the Bay of Islands ophiolite complex (BOIC), western Newfoundland, suggest that its generation, as well as its obduction, is related to collision of the Dunnage Zone island arc system with the irregular continental margin of eastern North America. The constraints include the supra‐subduction zone setting of the BOIC; its extensional environment of generation; the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the North American continental margin; and the restriction of the BOIC, as well as other Appalachian ophiolites and associated Taconian allochthons, to reentrants in the North American margin. During the early phases of collision of the Dunnage arc with the North American margin, subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation. The first develops through trench rollback, leading to ophiolite generation in the overriding arc plate outboard of the reentrant, whereas the second develops in a transtensional environment with spreading centers offset by strike‐slip faults which propagate through the overriding plate away from the indenter promontory. The configuration of the BOIC spreading ridge normal to the continental margin, and the direction of initial obduction parallel to the margin, favor generation of the ophiolite in an overall strike‐slip regime. With complete subduction of the oceanic remnant, extension in the overriding plate ceases, and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin. Diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the Dunnage Zone, and for termination of arc volcanism along the North American margin.

High seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern Canada

A deep seismic reflection profile on the continental rise off eastern Canada shows strong dipping reflections, apparently from within lower oceanic crust, over a region of limited lateral extent. The origin of these and similar reflections described elsewhere is uncertain. This paper reports new seismic-refraction results from the region of these reflections. They appear to be associated with a layer of P-wave velocity of about 7.4 km s −1 or higher, while the surrounding crust has a velocity ∼ 7 km s −1. The Moho, with velocity 8.1 km s −1, lies just below the reflectors, and the total crustal thickness is around 8 km. The crust is interpreted to be oceanic rather than continental, and the high velocities of the dipping reflectors are comparable to those of the mafic/ultramafic crust-mantle transition zone in the Bay of Islands ophiolite complex.

A possible explanation of gravity anomalies over mid‐ocean ridges

Gravity fields over the crests of slow spreading mid‐ocean ridges are characterized by small amplitude free air gravity anomaly lows (30–70 mGal) with wavelengths of 40–60 km, flanked by smaller gravity highs. In general, the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until, at fast spreading ridges (&gt; 5 cm/yr), free air gravity highs (10–20 mGal) are observed. Previous explanations of these anomalies, which involve uncompensated ridges, thermally expanded ridges and elastic plate compensation models, are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest. In particular, the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid‐ocean ridge basalts, which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more. In addition, similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers. Geometrical models of mid‐ocean ridges derived from detailed structural and petrologic studies of the Bay of Islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates. Modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly: the ridge topography, the crustal level magma chamber, the configuration of the isotherms near the ridge crest, and a low‐density, partially gabbroic cumulate root extending vertically into the mantle beneath the ridge. Systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers. Major contributions are found to come from the topography and from the lowdensity root, which broadens with slower spreading rates, resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate. At extinct spreading centers, contributions from the magma chamber, isotherms, and topography gradually diminish with time once the ridge ceases to spread, whereas the low‐density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field. Seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the Bay of Islands ophiolite and the gravity analysis presented here.

Olivine fabrics in the Bay of Islands Ophiolite: implications for oceanic mantle structure and anisotropy

Petrofabric analysis of oriented ultramafic and mafic rock samples from six traverses representing all four massifs of the Bay of Islands ophiolite complex, Newfoundland, indicate that the ultramafic rocks are tectonites displaying fabrics consistent with high-temperature plastic flow on the olivine (010) [100] and (0kl) [100] slip systems. The fabric orientation is uniform in three of the four massifs but varies between massifs, suggesting differential rotation before or during emplacement. Within North Arm Mountain, the olivine a axes are aligned approximately perpendicular to the sheeted dikes in both the ultramafic tectonites and the overlying gabbroic tectonites. In Blow Me Down Mountain, the olivine a axes in the gabbros are perpendicular to the dikes, but they are parallel to them in the ultramafic rocks. It is concluded that the ultramafic rocks on Blow Me Down Mountain were rotated 90° during emplacement or that local decoupling and rotation occurred between the crust and upper mantle prior to emplacement. Within the Lewis Hills, the olivine fabrics rotate and weaken near the shear zone in the center of the massif. A second deformation, perhaps associated with low-temperature plastic flow, appears to have obliterated the fabric patterns still observed in the ultramafic rocks to the east.

Serpentinization of cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite

Partially serpentinized dunites and wehrlites comprise the bulk of the cumulate ultramafic unit at the North Arm Mountain massif of the Bay of Islands ophiolite complex, Newfoundland. In a suite of 59 dunites and werhlites from the base of the unit, the serpentinized portions consist of lizardite + chrysotile + brucite + (accessory) magnetite. The ratio of ( lizardite + chrysotile) to brucite = ~8:2 ( weight percent) . Petrographic observations show that most serpentinization occurred at the expense of olivine; only limited amounts of clinopyroxene were serpentized. An estimated volume increase of 32% accompanied serpentinization of the peridotites. Reconstructions of the primary modal proportions of wehrlites (made taking this volume increase into account) contain an average of 6% more clinopyroxene and 6% less olivine than do modal reconstructions that ignore the volume increase. Mass balance calculations provide no clear evidence for appreciable metasomatism of Al 2O 3, CaO, FeO, MgO, or SiO 2 during Serpentinization. The presence of brucite, the evidence that most serpentinization occurred at the expense of olivine, and the lack of appreciable metasomatism, suggest that the primary reaction that controlled serpentinization of the peridotites is: 2Mg 2SiO 2 + 3H 2O ⇌ Mg 3Si 2O 5(OH) 4 + Mg(OH) 2 . olivine added serpentine brucite

MIneralogic variation in a layered ultramafic cumulate sequence at the North Arm Mountain Massif, Bay of Islands ophiolite, Newfoundland

In order to investigate magmatic processes operative within magma chambers underlying oceanic spreading centers, a suite of 59 samples was collected over an 8‐m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the North Arm Mountain Massif in the Bay of Islands ophiolite complex, Newfoundland. Interlayered dunites and wehrlites are present in the 8‐m section. Low abundances of incompatible elements (Ti, Y, Zr) and other evidence suggest that these rocks originally formed as adcumulates. Olivine compositions are in the range Fo84.5–89.9 and clinopyroxene Mg # (= (Mg/Mg+Fe2+) × 100) range from 89.7 to 93.3. Spinel compositions are highly variable, and Mg # and Al2O3 contents of spinels have been modified by subsolidus reequlibration with olivine and clinopyroxene. Cryptic variation patterns defined by the Mg # of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging ∼1 m. The cryptic regressions are believed to record influxes of fresh, relatively unfractionated magma into a magma chamber or conduit. The gradual decrease in the Mg # of olivine and clinopyroxene and the decrease in olivine Ni contents between successive cryptic regressions indicate that ∼10% crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks. In some cases, the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes (from wehrlite to dunite), suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma. In other cases, inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas. Comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on North Arm Mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive (higher MgO/FeO) than the liquids that crystallized to form the base of the unit. This feature is an indication of the complexity of processes that may operate in magma chambers underlying present‐day oceanic spreading centers.

The very depleted nature of certain primary mid-ocean ridge basalts

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na 2O and TiO 2. These very depleted primary magmas have 0.60 to 1.50 wt.% Na 2O and 0.10 to 0.50 wt.% TiO 2 compared to MORBs, which typically have > 1.90% Na 2O and >0.60% TiO 2. Evidence for these depleted primary magma compositions is obtained from megacrysts in MORBs, from glass inclusions within these megacrysts, and from the highly calcic plagioclases (An 91–96) and depleted clinopyroxenes (Na 2O mostly between 0.10 and 0.35) in certain abyssal peridotites. Cumulate ultramanfi and gabbroic rocks from the North Arm Mountain Massif of the Bay of Islands ophiolite complex show a progressive increase in the Na 2O and TiO 2 abundances in clinopyroxene crystals with stratigraphic height in the ophiolite. The use of mineral-liquid distribution coefficients and cumulate mineral compositions indicate that the liquids from which these minerals crystallized had 0.10 to 0.20 wt.% TiO 2 and 0.60 to 0.80 wt.% Na 2O for the lowermost cumulate ultramafic rocks, with TiO 2 and Na 2O abundances of liquids increasing progressively to normal MORB abundances during crystallization of higher-level gabbroic cumulates. These data clearly demonstrate that primary basalts that are very depleted in Na 2O and TiO 2 can differentiate to form residual magmas that are indistinguishable from MORBs.

Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands Ophiolite Complex

Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition and internal structure of map‐scale (kilometers across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity‐depth functions of the crust/mantle (“Moho”) transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure. The crust/mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick. The mafic/ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long. These units (megalenses) may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid‐state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units. Substantial relief and slopes of up to at least 12° over lateral distances of about 10 km occur on the top of the crust/mantle transition zone as well as other geologic unit contacts. Although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes, many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.

Magnetic properties of the Bay of Islands ophiolite suite and implications for the magnetization of oceanic crust

Rock magnetic properties, opaque mineralogy, and degree of metamorphism were determined for 101 unoriented samples from the North Arm and Blow‐Me‐Down massifs of the Bay of Islands ophiolite complex, Newfoundland. The weathered and metamorphosed extrusive basalt samples have a weak, secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin. The initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation. The magnetic intensity of the gabbroic samples increases as the degree of alteration increases, with the highly altered upper metagabbros having an average intensity of 3×10−3 emu/c3. Because magnetization of the metagabbro samples is related to nonpervasive, variable alteration, these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies. A compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer, roughly 600 m thick, with no contribution from a deeper source layer recognizable from these Bay of Islands data.

Cryptic mineral-chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland

Summary The compositions of cumulus olivine, orthopyroxene, and clinopyroxene from a detailed traverse through the upper portion of the cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite complex have been determined. The compositions of olivine (Fo 91.5–83.6 ), clinopyroxene (100 × Mg/(Mg + Fe) = 94.2–87.5) and orthopyroxene (100 × Mg/(Mg + Fe) = 90.8–85.1) within this traverse, when compared to the compositions of minerals that crystallize from oceanic basalts at various pressures, suggest that these cumulate ultramafic rocks crystallized from basaltic liquids at high pressures (&gt;10 kb) in a subaxial magma conduit system. The mineral compositions, when plotted against stratigraphic height within the complex, define eleven different cryptic units over a ‘stratigraphic’ thickness of 136 m. The boundary of each of these cryptic units is marked by an abrupt discontinuity in mineral chemistry from somewhat fractionated to very primitive compositions, suggesting that batches of magma passing through the conduit system do not intermix with other batches of magma undergoing crystallization in the system.

Reconstruction of the geometry of accretion during formation of the Bay of Islands Ophiolite Complex

Two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the Bay of Islands region of southwestern Newfoundland, Canada. The first, called the Coastal Complex (CC), is exposed as a long, linear belt along the west coast. It consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history. The second, called the Bay of Islands Complex (BOIC), crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history. An autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs, indicating that each evolved along a single ridge‐transform system. The internal structure of both the BOIC and the CC is consistent with the above interpretations. The NE‐SW structural trends in the CC transform are oriented approximately parallel to the sea‐floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the BOIC. One‐way chilling statistics from the sheeted diabase dike unit, growth directions of dendritic crystals along inclined comb layers in the plutonic section, and the sense of shear along the CC transform all indicate that the BOIC was accreted on the NE side of a NW‐SE trending spreading center (present geographic reference frame). Juxtaposition of the BOIC with the NE‐SW trending CC occurred at a ridge‐transform intersection. The transform exhibited a left‐lateral offset. Magma chamber profiles have been identified in the layered gabbroic sections of the BOIC by using the patterns outlined by the attitudes of fine‐scale igneous layering. These patterns depict arcuate structures which appear to define roughly a cross‐sectional view of a systematically oriented half‐cylinder; the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts. Thus we believe that a large cylindrical‐shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial‐plane at the time of formation of the BOlC. The CC marks the abrupt truncation of this long, linear magma chamber. Petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section, crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30‐km depth beneath the crustal level magma chamber. After formation, these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence, then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted. A geometric and tectonic model is presented for the accretion process responsible for formation of the Bay of Islands Ophiolite.

P−T conditions during emplacement of the Bay of Islands ophiolite complex

A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P−T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle. Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole. The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite.