North Arm Mountain Massif Research Articles

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.

Formation of chromitites by assimilation of crustal pyroxenites and gabbros into peridotitic intrusions: North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada

The lowermost crust of the North Arm Mountain massif, Bay of Islands ophiolite, is dominated by peridotitic intrusions that assimilated and reacted with host gabbros, pyroxenites and peridotites. Cr‐spinel is a common accessory and locally forms chromitite layers, schlieren, or rims. Thermobarometric calculations and fractionation models suggest that the Cr‐spinels formed at oxidation states between QFM and QFM+1.5. Postcumulus and subsolidus diffusional exchanges with silicate minerals increased the Fe/Mg ratio of Cr‐spinels at North Arm but cannot explain the diversity of Cr‐spinel Cr/Al. Nor can closed or open‐system fractional crystallization of a single parental magma in cotectic proportions generate the required range of Cr/Al in Cr‐spinel. Either a variety of primary magmas was present or the Cr/Al ratio of the melt was perturbed by assimilation of host rocks, with assimilation inducing crystallization of spinel in greater than cotectic proportions. Outcrop‐scale structures and phase relationships suggest that high‐Cr/Al chromitites that are associated with contacts between intrusive peridotites and host pyroxenites are by‐products of the incongruent dissolution of pyroxenes into a Cr‐spinel‐saturated magma. Aluminous chromitites occur higher in the section, in association with hybrid feldspathic peridotites and gabbros that formed by disaggregation of feldspathic cumulates into peridotitic intrusions. These rocks commonly have disseminated anhedral Cr‐spinels and may also contain thin chromitite schlieren, both with low Cr/Al spinels. The disseminated Cr‐spinels fill dissolution pits in feldspar xenocrysts, implying that they formed by incongruent dissolution of feldspar. The H2O needed to stabilize the amphibole included in the low‐Cr/Al Cr‐spinels was probably derived from the reactant gabbros, many of which were previously affected by subseafloor alteration. Locally, chromitites and disseminated Cr‐spinels have higher concentrations of Ti, V, Fe2+, and Fe3+, which we attribute to assimilation of gabbros rich in Fe‐Ti oxides. The composition of Cr‐spinels in chromitites at North Arm depends more on the nature of the minerals being assimilated than on the composition of the magma.

The lower crust of the Bay of Islands ophiolite, Canada: Petrology, mineralogy, and the importance of syntexis in magmatic differentiation in ophiolites and at ocean ridges

The lower crust of the North Arm Mountain massif (Bay of Islands ophiolite) is composed of interfolded, porphyroclastic‐textured peridotite, pyroxenite, and gabbro, belonging to several differentiation series. Low‐Ti pyroxenes, high An/Fo, abundant orthopyroxene, and high spinel Cr/Al suggest arc affinities. Discordant contacts and inclusion relationships show that many peridotites are synkinematic sills. Some host rocks were altered to sulphide‐bearing, greenschist‐grade assemblages prior to intrusion. Most feldspathic peridotites and melagabbros are hybrids formed by disaggregation of host gabbros into primitive intrusions. Many pyroxenites and chromitites are by‐products of incongruent dissolution reactions. Much of the pyroxene component of peridotites and olivine pyroxenites is xenolithic. Synmagmatic shear augmented the efficiency and extent of hydridization and assimilation reactions. Syntexis (the melting and dissolution of xenocrysts) was as important as fractional crystallization in generating petrological diversity and controlling melt evolution. Synmagmatic extension at spreading ridges could produce similar results, and so account for many enigmatic features of mid‐ocean ridge basalts (xenocrysts of anorthite and aluminous spinel, excess chlorine in basalts, and the pyroxene paradox). If syntexis is an important ridge‐axis process, then assuming that fractional crystallization alone controls melt evolution may generate incorrect calculated parental melt compositions, and so lead to erroneous conclusions about mantle sources and processes.

Interactions between melt and upper-mantle peridotites in the North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada: Implications for the genesis of boninitic and related magmas

Upper-mantle tectonites forming the basal part of the North Arm Mountain (NAM) massif ophiolitic sequence are dominantly refractory harzburgites, grading upward to Iherzolites near the contact with the overlying crust, and downward to coarse-grained lherzolite close to the metamorphic sole. The tectonites are cut by numerous pyroxenitic dykes ranging from orthopyroxenite, to websterite, to clinopyroxenite, and containing variable amounts of olivine, spinel and disseminated FeNi-sulphides. Microprobe traverses were executed from the core to the rim of several pyroxenite dykes, through the wallrock peridotites and into distal host peridotites. Wallrock peridotites recorded complex mineral modal and chemical variations suggesting that major dissolution-precipitation reactions occurred between minerals of the wallrock peridotites and intrusive melts, involving CrAl and MgFe solid solution exchanges between olivines, pyroxenes, spinels and migrating melts, and selective dissolution of clinopyroxene from wallrock peridotites into migrating melts. There is a general trend of modal clinopyroxene increase and FeAl enrichment among minerals of the pyroxenites along the mantle section, from deeper orthopyroxenites toward websterites and clinopyroxenites located near the crust-mantle interface. This suggests that clinopyroxene-bearing intrusions crystallized from the most evolved members of magmas related to the pyroxenites. Fractionation and interaction processes could both have led NAM migrating melts to Al 2O 3 and FeO enrichment, and CaO enrichment leading towards saturation in clinopyroxene. Magma compositions that would be in equilibrium with intro-mantle intrusive pyroxenites show similarities in composition and evolutionary trends with western Pacific boninites and high-Mg# andesites. If this interpretation is correct then the NAM ophiolitic massif would bear an intraoceanic fore-arc component. We suggest that modern boninites may partly have inherited their unusual magma chemistry by similar interactions and continuous re-equilibration with upper-mantle rocks during their ascent through the mantle wedge above a subduction zone.

Oceanic crust as a reactive filter: Synkinematic intrusion, hybridization, and assimilation in an ophiolitic magma chamber, western Newfoundland

Research Article| January 01, 1993 Oceanic crust as a reactive filter: Synkinematic intrusion, hybridization, and assimilation in an ophiolitic magma chamber, western Newfoundland Jean H. Bédard Jean H. Bédard 1Geological Survey of Canada, Centre Géoscientifique de Québec, 2700 Einstein, C.P. 7500, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Jean H. Bédard 1Geological Survey of Canada, Centre Géoscientifique de Québec, 2700 Einstein, C.P. 7500, Ste-Foy, Québec G1V 4C7, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (1): 77–80. https://doi.org/10.1130/0091-7613(1993)021<0077:OCAARF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jean H. Bédard; Oceanic crust as a reactive filter: Synkinematic intrusion, hybridization, and assimilation in an ophiolitic magma chamber, western Newfoundland. Geology 1993;; 21 (1): 77–80. doi: https://doi.org/10.1130/0091-7613(1993)021<0077:OCAARF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Ultramafic and gabbroic rocks dominate the lower crust of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland. Synkinematic sills (≤50 m thick) make up a large proportion of the crust. There is no evidence for large, long-lived magma chambers. Peridotitic intrusions assimilated and reacted with their hosts. Chromitites formed by incongruent dissolution of pyroxene and feldspar. Pyroxenite formed through pore-scale hybridization between invading pyroxene-saturated magmas and partial melts of gabbroic hosts. Most feldspathic peridotites and olivine gabbros display textures suggesting that they are mixtures of primitive magmas and partially solidified gabbroic cumulates. The crust has acted as a reactive filter: the chemical evolution of primary magmas owes as much to assimilation and reaction with older cumulates as it does to fractional crystallization. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cumulate Recycling and Crustal Evolution in the Bay of Islands Ophiolite

The lower crustal section of the North Arm Mountain Massif of the Bay of Islands Ophiolite is composed of interlayered gabbroic and ultramafic cumulates. Despite pervasive ridge-related deformation, key contact relationships are preserved in low-strain domains or late-kinematic intrusions. Angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts: olivine gabbro, gabbro, gabbro-norite, norite, and Fe-oxide-amphibole gabbros. The ultramafic intrusions are composed of dunite, wehrlite, harzburgite, lherzolite, and olivine websterite. Clino- to ortho-pyroxenite reaction rims line many peridotitic/gabbroic contacts. The voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros. Fine-grained harzburgitic sills crosscut the foliation of wehrlitic host rocks, with orthopyroxenite reaction rims at many of the contacts. Medium- to coarse-grained harzburgitic cumulates are important locally in the lower crust. Where pyroxenite reaction rims are detached or absent, the interaction between the hot, intruding, primitive magma and host gabbroic cumulates led to the formation of hybrid olivine gabbro, troctolite, and feldspathic peridotite. Feldspars in these hybrids occur as embayed grains and clots (anorthositic or gabbroic), commonly with discontinuous aluminous Cr-spinel rims. Aluminous chromitite schlieren are commonly found within hybrid feldspathic peridotite bodies. High-Cr spinel (Cr# = Cr/Cr + Al >0.5) layers are associated with lower crustal and upper mantle orthopyroxenite schlieren. The coronitic textures and spatial associations suggest that chromitites in ophiolites are by-products of the assimilation of gabbro (Cr# <0.5) and pyroxenite (Cr# >0.5) by invading primitive magmas.

Mineral compositions of plutonic rocks from the Lewis Hills Massif, Bay of Islands ophiolite

The Lewis Hills massif is interpreted as having been formed at an oceanic fracture zone (FZ) prior to its obduction. The compositions of minerals in the residual mantle and cumulate rocks of the massif are similar to those in other massifs of the Bay of Islands ophiolite and those in comparable rocks from the oceanic basins. The residual mantle rocks consist of olivine (Fo89.9–91.7, Ni0 = 0.37 ‐ 0.46%), orthopyroxene (100*Mg/[Mg+Fe] ranges from 90.6 to 91.8), spinel (100*Cr/[Cr+Al] = 43.7 ‐ 57.4 and 100*Mg/[Mg+Fe2+] = 51.2 ‐ 67.7), and rare clinopyroxene. These minerals have compositions similar to those of the most depleted abyssal peridotites. The cumulate units are mixtures of olivine (Fo88.8‐78.0, Ni0 = 0.32 ‐ 0.08), plagioclase (An88.4‐68.8), clinopyroxene (100*Mg/[Mg+Fe] = 93.0 ‐ 75.6) and Cr‐Al spinel (100*Cr/[Cr+Al] = 36.0 ‐ 8.8 and 100*Mg/[Mg+Fe2+] = 73.7 ‐ 50.0). The ultramafic cumulates appear to have crystallized at moderate to high pressures and the gabbroic rocks at low pressures. Differences between the magmatic processes recorded in the Lewis Hills massif during its formation adjacent to a FZ and those inferred for the North Arm Mountain massif while it formed more distantly from the FZ are (1) the more depleted nature of the mantle near the FZ, (2) the higher Ti02 contents of diabase dikes near the FZ, (3) the greater abundance of feldspathic ultramafic rocks near the FZ, and (4) the absence of orthopyroxene as a cumulate phase near the FZ.

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.

Major element variations in basalts and diabases from the North Arm Mountain Massif, Bay of Islands ophiolite: Implications for magma chamber processes at mid-ocean ridges

The major element compositions of aphyric diabases from the North Arm Mountain Massif range from primitive basalts to highly evolved ferrobasalts. From modeling of these compositional variations, it is clear that the diabases have undergone highly variable (~ 10 to ~ 90%) increments of crystallization at low pressures. The dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine, plagioclase, and clinopyroxene, the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks. The projection of these compositions onto pseudo-liquidus phase diagrams suggests that the primary magma(s) from which this North Arm suite was produced were generated by melting at 10–25 kbar (or greater) pressure. There is no indication in the major element variations of these diabases of the operation of high-pressure crystal factionation processes, which is not surprising because low-pressure fractionation processes will effectively mask evidence for earlier high-pressure processes. It is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete, fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition.

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.

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.

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 (&gt;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.

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.