| Title | Ophiolitic magma chamber processes, a perspective from the Canadian Appalachians |
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| Author | Bédard, J H |
| Source | Layered Intrusions; by Charlier, B (ed.); Namur, O (ed.); Latypov, R (ed.); Tegner, C (ed.); 2015 p. 693-732, https://doi.org/10.1007/978-94-017-9652-1 15 |
| Image |  |
| Year | 2015 |
| Alt Series | Earth Sciences Sector, Contribution Series 20130156 |
| Publisher | Springer Netherlands |
| Document | book |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf |
| Province | Newfoundland and Labrador |
| NTS | 2; 3; 12 |
| Lat/Long WENS | -60.0000 -52.0000 52.0000 46.0000 |
| Subjects | tectonics; igneous and metamorphic petrology; ophiolites; igneous rocks; magmatic rocks; magmatism; continental crust; crustal movements; crustal studies; intrusive rocks; Annieopsquotch Ophiolite; Bay
of Islands Ophiolite Complex; Betts Cove Ophiolite |
| Illustrations | location maps; photographs; cross-sections; plots |
| Released | 2015 05 19 |
| Abstract | Oceanic/ophiolitic magma chambers differ from continental layered mafic-ultramafic intrusions because magmatism is synchronous with extensional tectonism in a submarine environment. Because oceanic
ridges continuously extend, new magma formed by decompression melting of the upwelling mantle constantly arrives beneath the ridge axis. Arriving magma commonly ponds at the base of the crust, or forms sills where favourable crustal structures
(faults, shear zones, older sills) are encountered. A sheeted sill architecture for the middle and lower oceanic crust is probably common. Many monomineralic facies (anorthosite, chromitite, pyroxenite) in ophiolites form as reaction rims between
newly emplaced primitive magma and evolved host cumulates as a result of incongruent dissolution or mixing across phase boundaries. When deformation is broadly distributed through the crust (Bay of Islands ophiolite), many previously-emplaced rocks
experience high-temperature ductile shear that straddles the solidus. Consequently, modal cumulate layering is not always produced by sequential crystallization/accumulation or crystal sorting against a cooling surface or floor, but may form by
transposition and tectonic repetition of partly-solidified intrusions, hosts and reaction facies. Syn-magmatic deformation triggers and activates mixing between intra-cumulate intrusions and incompletely consolidated host rocks to create a range of
hybrid facies, few of which have cotectic phase proportions. Cumulates affected by penetrative deformation tend to have lower trapped melt fractions (5–10?%) than those unaffected by shear (20–30?%), suggesting that shear pumping actively expells
pore melt from the deforming matrix. Percolation of primitive to residual melt through a deforming cumulus framework has the potential to mobilize incompatible elements and transform chemical signatures (Annieopsquotch ophiolite). Cumulates in the
Betts Cove ophiolite are not penetratively deformed, and show well-developed size-graded cumulate beds, some with basal load structures, indicating an origin as gravity deposits. These types of cumulates may form in subsiding, fault-bounded
‘trap-door’ chambers. Graded harzburgitic cumulate beds are intercalated with bedding-parallel pyroxenite sheets that merge with discordant pyroxenite dykes, suggesting that they are bedding-parallel melt segregation veins that fed residual melt into
fault-guided conduits, allowing expelled pore melt to be evacuated efficiently from within the thick pile of compacting cumulates. |
| Summary | (Plain Language Summary, not published)
The nature of magma chambers beneath ocean ridges, and as preserved in ophiolites (on-land equivalents) is reviewed. Oceanic magma chambers differ from
continental intrusions because they are commonly affected by the continuous extension and submarine setting of ocean ridges. The implications for rock evolution are discussed. The common synmagmatic deformation helps squeeze melt out of the pores in
the crystallizing mass. |
| GEOSCAN ID | 292818 |
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