Ophiolitic Magma Chamber Processes, a Perspective from the Canadian Appalachians

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.