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TitleSyntexis and the genesis of lower oceanic crust
AuthorBédard, J H; Berclaz, A; Hébert, R; Varfalvy, V
SourceOphiolites and oceanic crust: new insights from field studies and ocean drilling program; by Dilek, Y (ed.); Moores, E M (ed.); Elthon, D (ed.); Nicolas, A (ed.); Geological Society of America, Special Paper 349, 2000 p. 105-119,
Alt SeriesGeological Survey of Canada, Contribution Series 1999049
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf
ProvinceNewfoundland and Labrador
NTS12G/01; 12G/08; 12B/15; 12B/16
AreaBay of Islands
Lat/Long WENS -59.0000 -58.0000 49.5000 48.7500
Subjectsigneous and metamorphic petrology; ophiolites; magmas; magmatism; sills; gabbros; igneous rocks; dykes; North Arm Mountain Massif; Bay of Islands Ophiolite; syntexis
Illustrationssketch maps; photographs; ternary diagrams
AbstractWe propose that the term ''syntexis'' be revived and expanded to encompass all of the processes, mechanical and chemical, by which magmas react with and assimilate their host rocks. Synkinematic sills (#50 m thick) make up the dominant proportion of the crust of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland. Field, textural, and geochemical evidence implies that at North Arm, most near-monomineralic rocks (chromitites, anorthosites, and pyroxenites) are products of syntectic assimilation reactions, and a substantial proportion of wehrlites, feldspathic peridotites, and olivine gabbros are syntectic hybrids. Chromitites formed by incongruent dissolution of pyroxene or feldspar into olivine 1 chromite-saturated melts. Such reaction-derived chromites associated with gabbroic protoliths are systematically more aluminous than those associated with pyroxenitic protoliths; this fact suggests that the composition of the Crspinel in crustal chromitites at North Arm is controlled by the nature of the phases being assimilated, not by the composition of the magma. Pyroxenite and anorthosite reaction rims are interpreted to have formed through pore-scale hybridization between invading primitive magmas and partial melts of gabbroic hosts. Most feldspathic peridotites and olivine gabbros display textures suggesting that they are mixtures of invading primitive magmas and disaggregated gabbroic hosts and do not represent simple cotectic cumulates. The data from North Arm suggest that the lower ophiolitic (and oceanic) crust may act as a reactive filter and that the chemical evolution of primary magmas owes as much to assimilation and reaction with older cumulates as it does to fractional crystallization.

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