Jean H. Bédard

A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids

Abstract Whole-rock trace-element analyses can be used in conjunction with partition coefficients and mineral modes to calculate a liquidus-temperature equilibrium distribution of trace elements among the constituent minerals of cumulate rocks. Once the concentration of a trace element in a given mineral phase is known, then the trace-element concentration in the liquid that was in equilibrium with this mineral can be calculated if certain assumptions are made about the temperature and major-element chemistry of the liquid. The effects of a trapped melt fraction (TMF) are accounted for by: (1) including in the calculation a phase (the trapped melt) with a mineral/melt partition coefficient of 1; and (2) adjusting the rocks mode to reflect removal of the TMF. The method yields results comparable to those given by mineral separation or in situ analysis for trace elements.

Evidence for forearc seafloor-spreading from the Betts Cove ophiolite, Newfoundland: oceanic crust of boninitic affinity

Abstract The Ordovician Betts Cove ophiolite of Newfoundland has a well-developed cumulate sequence, in which is rooted a sheeted dyke complex that grades up into pillow lavas. Dominant chromite + olivine + orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas. Dykes and lavas have phenocrysts of olivine + high- Cr Al chromite ± orthopyroxene ± low-TiO2 clinopyroxene. They have high SiO2 and MgO contents, and depleted U-shaped trace-element profiles indistinguishable from those of Bonin Islands boninites. Field data imply that cumulates, dykes and lavas all are comagmatic, while geochemical and mineralogical data indicate that all are of boninitic affinity. Since boninites are only found in forearcs, this implies that the Betts Cove ophiolitic crust formed in a forearc. Since the entire oceanic crustal section at Betts Cove is of boninitic affinity, then this implies that true seafloor-spreading can initiate in forearc.

Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discriminant diagrams

When dealing with ancient subalkaline volcanic rocks, the alkali – total iron – magnesium (AFM) diagram is of limited use in assigning a tholeiitic versus calc-alkaline affinity because these eleme...

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

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.

Disequilibrium mantle melting

Abstract The attainment of equilibrium between basalt and residual peridotite depends on the relative magnitudes of melt segregation rate and solid residue re-equilibration rate. Garnets and pyroxenes (and perhaps spinels) require at least 10 3 years to equilibrate with melt by volume inter-diffusion. Percolation velocities of melt in porous (1–10%) mantle rocks are of the order of 10 −4 to 10 −8 cm/s. If the melt moves by homogeneous percolation, then a column of peridotite matrix several kilometers in height must interact with the melt for an equilibrium partitioning of elements to be achieved. If melt segregation also involves flow through dykes, as seems likely, then the time available for equilibration is even shorter, and disequilibration melting is to be expected. By varying the time melt and residues remain in contact, variable equilibration will ensue. This can produce crossing REE profiles in basalts derived by identical degrees of melting from identical sources. Basalt-depleted residues from disequilibrium melting have fractionated REE profiles. This provides as alternative to “cryptic” metasomatism for the enigmatic decoupling of major and trace elements that is observed in many mantle peridotite nodules.

Fault-mediated melt ascent in a Neoproterozoic continental flood basalt province, the Franklin sills, Victoria Island, Canada

The Neoproterozoic Franklin large igneous province on Victoria Island, Canada, is characterized by continental flood basalts and a sill-dominated feeder system. Field relationships indicate that fault-guided transfer zones allowed magma to jump up-section to form higher-level intrusions. Where sills connect to dikes and magmas moved up-section, roof and wall rocks are characterized by wide and intense contact-metamorphic haloes, consistent with throughflow of magma. The geometric constraints suggest that conduits may have opened episodically and then closed when magma pressure waned. The episodic nature of conduit opening events can explain the pulsed ascent of crystal slurries, and may also play a role in the deposition of Ni-sulfides.

Evidence for protracted High Arctic large igneous province magmatism in the central Sverdrup Basin from stratigraphy, geochronology, and paleodepths of saucer-shaped sills

Field evidence, map compilation, geochemistry, geochronology, and potential field data document six intervals of Cretaceous magmatism in the central Sverdrup Basin. These are: (1) Hauterivian (ca. 130 Ma) volcaniclastic deposition in the lower Isachsen Formation; (2) 126.6 ± 1.2 Ma (U-Pb zircon) gabbroic intrusion; (3) 120.8 ± 0.8 Ma (U-Pb baddeleyite) diabasic intrusion; (4) 105.40 ± 0.22 Ma (U-Pb detrital zircon) pyroclastic deposition at the top of the Invincible Point Member, Christopher Formation; (5) upper Albian (ca. 103 Ma) pillow and hydroclastic breccia in the upper Christopher Formation; and (6) uppermost Albian (ca. 101 Ma) volcanic breccia and scoria in the Hassel Formation. Whole-rock geochemical data show that these magmatic rocks are similar to previously documented High Arctic large igneous province tholeiitic basalts, but analyses of fresh glass in tuffs reveal evolved ferroandesite to dacite compositions not recorded in whole-rock data. Approximate ages of saucer-shaped sills inferred from the relationship of sill width to depth of emplacement suggest at least three intervals of sill emplacement between 130 and 120 Ma. The new data show that volcanism in the Sverdrup Basin was of greater spatial extent, and that magmatism occurred more frequently, than was previously recognized. Comparison of the new central Sverdrup Basin data and interpretations with other data sets from the Sverdrup Basin, Svalbard, and Franz Josef Land suggests that High Arctic large igneous province magmatism occurred over a more extended period of time in the central Sverdrup Basin than in other regions.

Neoarchean disaggregation and reassembly of the Superior craton

The southern and western Superior craton of Canada (SWSC) is widely considered to be a tectonic collage accreted from north to south by multiple coeval subduction zones. We propose an alternative non–plate tectonic scenario where SWSC continental fragments are not exotic but derived by partial disaggregation of a heterogeneous older (Superior I) craton in response to a mantle overturn event that started at ca. 2780 Ma. During overturn, radial mantle outflow stretched and disaggregated the lithosphere to create the concentric Neoarchean fabric of the Superior craton, the southern part of which (SWSC) broke up into ribbon continents separated by oceanic tracts. Neoarchean calc-alkaline magmas record interaction between plume-related magma and older crust. A change in the mantle flow field at ca. 2720 Ma caused southward drift of the Northern Superior cratonic block as a result of mantle traction on its lithospheric keel, and SWSC terranes accreted to its leading edge.

Evidence for Regional‐Scale, Pluton‐Driven, High‐Grade Metamorphism in the Archaean Minto Block, Northern Superior Province, Canada

The Archaean Minto Block of the Superior Province is dominated by foliated tonalite, trondhjemite, granite, granodiorite, enderbite, and clinopyroxene‐ or orthopyroxene‐bearing hornblende‐tonalite; most partly retain igneous microtextures. Outcrop‐scale structures such as melt‐filled shear zones and deformed intraplutonic dikes imply that deformation was synmagmatic. Compositions of epidote (pistachite 25–30), garnet, and muscovite ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape