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When dealing with ancient subalkaline volcanic rocks, the alkali-total iron-magnesium (AFM) diagram is of limited use in assigning a tholeiitic vs. calc-alkaline affinity because these elements are often mobile during alteration and metamorphism. Classification diagrams using immobile trace elements are preferable, but need to be tested and optimized on unaltered rocks.To this end, a geochemical database containing over a thousand presumed unaltered subalkaline volcanic samples from young oceanic arcs was assembled. The data were classified using both

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Mapping of the Eastern Layered Series (ELS) of the Rhum ultrabasic complex on the northern flank of Hallival shows that peridotite and allivalite (troctolite or gabbro) layers are laterally discontinuous and vary both in thickness and lithology. Peridotite generally has sharp upper and lower contacts against the allivalites, which sometimes cut across the layering in the allivalite. Reaction, dissolution and hybridization effects between peridotite and allivalite are developed locally. Some troctolite layers terminate as isolated, fingered blocks in peridotite. There are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs. They are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them. We propose that many of the peridotite layers in the ELS formed as thick sills of picritic magma emplaced into a partly solidified, layered troctolite complex. The stratiform gabbros of the ELS are heterogeneous, layered rocks that commonly contain relicts of troctolite and anorthosite. Wavy (metre-scale) contacts between gabbro and troctolite cut across pre-existing grain-size, modal and rhythmic layering with little disruption. These metasomatic gabbros mimic the textures, grain-size and rhythmic layering of their troctolitic protoliths. We propose that many of the ELS gabbros formed as a result of interaction between porous troctolites and a low-temperature basaltic melt. Residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism.The Eastern Layered Series (ELS) of the Rhum ultrabasic complex is composed of alternations of thick layers (tens of metres) of peridotite and feldspar-rich allivalite. Allivalite is a local term for conformable feldspathic rocks (troctolite and gabbro). Harker (1908) argued that this macro-layering resulted from alternating injections of two magma types that had differentiated at depth. Modern experimental petrology and phase equilibria indicate that these rocks are not liquid compositions but crystal cumulates. As a consequence, Harker's sill-complex model has been neglected since the study by Brown (1956).Brown (1956) defined fifteen major peridotite-allivalite alternations in the ELS. The absence of systematic differentiation with stratigraphic height, in contrast to the Skaergaard intrusion, led him to propose that these repetitions of peridotite and allivalite layers were the result of periodic replenishment by primitive basalt. ,The replenishment model has been accepted by everyone since, although opinion has swung to the view that the replenishing magmas were picritic rather than basaltic (Donaldson 1975;Gibb 1976;Huppert & Sparks 1980;Tait 1985).In Brown's (1956) basaltic replenishment model a peridotite-allivalite cyclic unit reflects closed-system fractionation. Peridotites (olivine cumulates) accumulate from mafic parental basalt. The magma evolves until feldspar joins olivine on the liquidus and troct...

Field evidence, map compilation, geochemistry, geochronology, and potential fi eld 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.