The geochemical effects of olivine slurry replenishment and dolostone assimilation in the plumbing system of the Franklin Large Igneous Province, Victoria Island, Arctic Canada

The Neoproterozoic (~723–716 Ma) Franklin Large Igneous Province exposed on Victoria Island in the Canadian Arctic is comprised of a sill-dominated magma plumbing system overlain by the coeval Natkusiak flood basalts. We have investigated three sections, separated by a total of >50 km of distance, of a sill (the Fort Collinson Sill Complex) emplaced just above a prominent sedimentary marker unit. The sill is characterized by a basal olivine-enriched layer (OZ: up to 55 % olivine) and an upper gabbroic unit. The observed diversity of olivine compositions in the OZ implies that bulk-rock MgO versus FeO arrays reflect accumulation of a heterogeneous olivine crystal cargo. We suggest that the OZ was formed as a late olivine slurry replenishment in a partially crystallized gabbroic sill, propagating for over 50 km along strike. This interpretation is consistent with Pb-isotope data, which show that at least three geochemically distinct magmas were emplaced into the Fort Collinson Sill Complex. The OZs exhibit a gradual westward evolution toward more Fe-rich bulk compositions. This is best explained by progressive mixing of the replenishing olivine slurry with a resident gabbroic mush during westward flow. Pb-isotopic signatures suggest that magmas near the inferred conduit feeder assimilated small amounts (<10 %) of dolostone country rock, which may have locally buffered olivine compositions to high-Fo contents

Assembly of the Annieopsquotch Accretionary Tract, Newfoundland Appalachians: Age and Geodynamic Constraints from Syn‐Kinematic Intrusions

The Annieopsquotch Accretionary Tract (AAT) comprises several ophiolites and arc‐back‐arc igneous complexes that were accreted to the Dashwoods microcontinent during the Ordovician Taconic orogeny. The Lloyds River Fault Zone, which separates the AAT from the Dashwoods microcontinent, yielded 40Ar/39Ar hornblende ages of ca. 470 Ma. The fault zone was intruded syn‐kinematically by the shoshonitic Portage Lake monzogabbro and the Pierre’s Pond suite, which gave U/Pb zircon ages of Ma plus Ma and Ma, respectively. The Otter Pond granodiorite intruded syn‐kinematically into the Otter Brook Shear Zone, which separates the Annieopsquotch ophiolite belt from the structurally underlying ophiolitic Lloyds River Complex. It yielded a U/Pb zircon age of Ma. The Buchans arc and its continental basement were accreted to the Lloyds River Complex prior to 468 Ma. Syn‐kinematic plutons have arc affinity, with εNd ranging between −0.9 and −6.8, and are coeval with the adjacent Notre Dame Arc. Our data thus suggest the majority of the AAT was accreted to the Dashwoods microcontinent by 468 Ma, when consanguineous, dominantly arclike plutons intruded within the AAT and adjacent Notre Dame Arc. The Portage Lake monzogabbro and Otter Pond mafic suite are more mafic than Notre Dame Arc plutons of similar age because of their intrusion into the thin, mafic crust of the AAT and ascent along shear zones. Our data indicate the formation and subsequent accretion of ophiolites and arc‐back‐arc complexes occurred within a very short time span (5–10 Ma). The sources of AAT syn‐orogenic magmatism are diverse and include melting of subarc mantle during slab breakoff, lithospheric mantle, and lower crust. The Ordovician Appalachian margin of Laurentia grew by the accretion of oceanic terranes and intrusion of mantle‐derived magma. Recycling of continental crust by rifting and subsequent collision played an important part of the tectonic evolution of the AAT

Sulfide immiscibility induced by wall-rock assimilation in a fault-guided basaltic feeder system, Franklin Large Igneous Province, Victoria Island (Arctic Canada)

The Southern Feeder Dike Complex is part of the Franklin Large Igneous Province (LIP), exposed in the Minto Inlier of Victoria Island in the Canadian Arctic. Previous field and geochemical studies on the Franklin LIP considered its igneous rocks to be prospective for Fe-Ni-Cu mineralization. The Southern Feeder Dike Complex comprises a series of NW-SE-trending gabbroic intrusions and sedimentary hosts. Field and textural relationships show that the Complex intrusions were emplaced contemporaneously with Neoproterozoic normal faulting. Faulted contact zones correspond to prominent first derivative magnetic lineaments. Gabbroic dikes have intrusive contacts against brecciated country rock, and diabasic microxenoliths in basaltic matrices indicate multiple intrusive/brecciation events. Intrusive breccias are commonly overprinted by hydrothermal greenschist facies assemblages, with calcite + pyrite veins filling open spaces between breccia fragments. Late dikes emplaced into these heterogeneous breccias contain disseminated globular and net-textured sulfides suggesting that sulfide immiscibility was triggered on a local scale by assimilation of local wall rock. This inference is supported by elevated δ34S values of sulfides in these dikes, consistent with assimilation of country rocks. Wall-rock assimilation would have been facilitated by fault-related brecciation and cataclasis, which would expose extensive xenolith surface areas to fresh magma. Gossanous and meter-scale semimassive sulfide showings associated with dikes and sills located upsection from the Southern Feeder Dike Complex suggest that immiscible sulfide liquids may have been flushed downstream (or upsection) during replenishment of composite dike systems. Fault-mediated melt ascent along northwest-southeast faults has been documented elsewhere in the Minto Inlier, providing equivalent opportunities for wall-rock assimilation and consequent triggering of sulfide immiscibility and sulfide melt redistribution. The evidence preserved in the Complex confirms the Fe-Ni-Cu potential of the Franklin LIP and informs current models of ore deposit formation in conduit-type magmatic plumbing systems

U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge

New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206Pb/238U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges

Melt–rock reaction in the lower oceanic crust and its implications for the genesis of mid-ocean ridge basalt

Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host rock, and olivine is notably absent as a chadacryst despite being abundant in the host rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such reactions are enriched in Al2O3 and MgO and depleted in CaO and SiO2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al2O3–MgO–SiO2 relationships of MORB may result from melt–rock reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–rock reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB

Dynamics of accretion of arc and backarc crust to continental margins: inferences from the Annieopsquotch accretionary tract, Newfoundland Appalachians

The Annieopsquotch accretionary tract comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the Laurentian margin of Iapetus during Middle to Upper Ordovician. Geological relationships suggest that the constituent terranes of the Annieopsquotch accretionary tract initially formed outboard of a peri-Laurentian Dashwoods microcontinent in an extensional arc, but occupied a lower plate setting with respect to Dashwoods during accretion. Metamorphic mineral assemblages indicate that the terranes were underplated beneath the composite Laurentian margin at depths ranging from ~ 3 km up to > 18 km. We infer the accretion of the terranes to be controlled by the brittle–ductile transition in the hydrated crust. The decoupling of brittle from ductile crust resulted in very high aspect ratios of the terranes, which comprise thin (< 5 km) but very large (up to 25 × 250 km) slabs of supracrustal arc rocks and ophiolite crust. Arc basement and ophiolitic mantle are not preserved and were either underplated at a greater depth or subducted and recycled back in the mantle. The accreted crust forms a reasonable approximation to bulk continental crust requiring little post-accretionary modification; hence, the accretion of arc–backarc complexes which occupy a lower plate setting can form an important mechanism for creation of new continental crust required to balance crustal loss at convergent margins

Mantle melting, melt transport, and delivery beneath a slow-spreading ridge: the paleo-MAR from 23°15′N to 23°45′N

Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an ∼300–400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large ∼1 km+ thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust–mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt–rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep. The average Kane transform peridotite is a lherzolite with 5·2% Cpx, whereas that from the main core complex is a harzburgite with only 3·5% Cpx. As the area corresponds to a regional bathymetric low, and the crust is apparently thin, it is likely that most residual mantle along the MAR is significantly more depleted. Thus, harzburgitic and lherzolitic ophiolite subtypes cannot be simply interpreted as slow- and fast-spreading ridges respectively. The mantle peridotites are consistent with a transform edge effect caused by juxtaposition of old cold lithosphere against upwelling mantle at the ridge–transform intersection. This effect is far more local, confined to within 10 km of the transform slip zone, and far smaller than previously thought, corresponding to ∼8% as opposed to 12·5% melting of a pyrolitic mantle away from the transform. Excluding the transform, the overall degree of melting over 3 Myr indicated by the peridotites is uniform, ranging from ∼11·3 to 13·8%. Large variations in composition for a single dredge or ROV dive, however, reflect local melt transport through the shallow mantle. This produced variable extents of melt–rock reaction, dunite formation, and melt impregnation. At least three styles of late mantle metasomatism are present. Small amounts of plagioclase with elevated sodium and titanium and alumina-depletion in pyroxene relative to residual spinel peridotites represent impregnation by a MORB-like melt. Highly variable alumina depletion in pyroxene rims in spinel peridotite probably represents cryptic metasomatism by small volumes of late transient silica-rich melts meandering through the shallow mantle. Direct evidence for such melts is seen in orthopyroxenite veins. Finally, a late hydrous fluid may be required to explain anomalous pyroxene sodium enrichment in spinel peridotites. The discontinuous thin lower crust exposed at Kane Megamullion contrasts with the >700 km2 1·5 km+ thick Atlantis Bank gabbro massif at the SW Indian Ridge (SWIR), clearly showing more robust magmatism at the latter. However, the SWIR spreading rate is 54% of the MAR rate, the offset of the Atlantis II Fracture Zone is 46% greater and Na8 of the spatially associated basalts 16% greater—all of which predict precisely the opposite. At the same time, the average compositions of Kane and Atlantis II transform peridotites are nearly identical. This is best explained by a more fertile parent mantle beneath the SWIR and demonstrates that crustal thickness predicted by simply inverting MORB compositions can be significantly in error

The origin of mafic-ultramafic bodies within the northern Dashwoods Subzone, Newfoundland Appalachians

Mafic-ultramafic bodies of possible ophiolitic origin are widespread within the northern part of the Dashwoods Subzone of the Newfoundland Appalachians. The bodies fall into two types; the first consists of relatively undeformed, fresh, layered gabbroic bodies, which contrasts with the second that consists of metamorphosed ultramafic lithologies. Two of the largest gabbroic bodies yielded U-Pb zircon ages of 432.4±1.0 and 429.9±1.2 Ma. Their Silurian age refutes an ophiolitic origin, and, along with their hydrous nature and fresh character, indicates that they formed in the widespread Early Silurian igneous event that affected both the Dashwoods and Notre Dame subzones. Our new data further substantiate the bimodal character of the Silurian event. The ultramafic bodies are inferred to be of ophiolitic origin. Those that occur in the Dashwoods subzone are interpreted to be related to the Lushs Bight oceanic tract, whereas those in the Lloyds River Fault Zone are correlated with the Annieopsquotch ophiolite belt

"Moist MORB" axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin

The Oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid-oceanic ridge basalt (MORB). Its internal structure is regarded by many as being directly comparable to ocean lithosphere from the East Pacific Rise. However, there has long been controversy over the geodynamic setting in which the ophiolite formed, and the extent to which the analogy can be drawn, because the MORB-like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites. To some, this implies that the entire ophiolite formed above a subduction zone; others maintain that it formed at a true open-ocean MOR, and that the water required to generate the arc-like magmas derived from descending near-axis hydrothermal fluids or from ancient subduction. A popular compromise posits that the axial suite formed at a true MOR and the later magmatism documents the initiation of obduction. We test these models by reexamining the “MORB-like” character of the early axial lavas and dikes. We show that fractionation trends require the presence of water at concentrations significantly higher than any open-ocean MORB; instead, trends are identical to those of backarc basin and intraoceanic forearc volcanics. By showing that the entire ophiolite formed in a hydrous system, we rule out all models in which the Oman ophiolite was generated at an open-ocean MOR; instead, it formed at a submarine spreading center above a (probably newly initiated) subduction zone

The structure and geochemistry of the gabbro zone of the Annieopsquotch ophiolite, Newfoundland: implications for lower crustal accretion at spreading ridges

The Annieopsquotch ophiolite exposes a c. 5.5-km-thick section of tholeiitic gabbros, sheeted dykes and pillow basalts. Based on the along-strike consistency in thickness of the major crustal units, and lack of significant throw on spreading-related normal faults, the Annieopsquotch ophiolite is interpreted to have formed at an intermediate- to fast-spreading ridge. The upper c. 400 m of the gabbro zone is composed of massive, texturally heterogeneous gabbros with compositions that approach those of the sheeted dykes and basalts. Below this is c. 1.6 km of 10–30-m-thick planar intrusive sheets or sills. The lowermost part of the gabbro zone is composed of gabbroic rocks with relict troctolite and troctolitic gabbro enclaves, which are veined and partly replaced by gabbro and pyroxenite. Sill contacts within the central, sill-dominated part of the gabbro zone are sub-parallel to boundaries between the major ophiolite lithostratigraphic units. The upper and lower contacts of individual gabbro sills may have finer grain sizes, indicating that the intrusions cooled from the top and bottom. Locally well-preserved comb structures (crescumulates) indicate downward growth, supporting a sill interpretation. The sills are composed of weakly or un-deformed plagioclase+clinopyroxene ±olivine cumulates. Trace element modeling suggests that the parental magmas of these cumulates had compositions very similar to the overlying sheeted dykes and basaltic lavas, as do dykes emplaced within the gabbro zone. Model liquids calculated from the gabbroic sills generally become more evolved up-section, indicating that magma evolution in the Annieopsquotch ophiolite was dominated by fractionation in lower crustal conduits, below the level of a putative axial melt lens (AML). The model liquids, sheeted dykes and basalts preserve a similar, wide range of compositions, which may indicate that aggregation, homogenization and fractionation in an AML was inefficient. Similar intra-conduit fractionation of mantle-derived melts might also contribute to MORB evolution at mid-ocean ridges