Origin and emplacement of the ophiolite suite: Appalachian ophiolites in Newfoundland

Abstract

The ophiolite suite is probably generated by axial plate accretion at oceanic ridges, and by diffuse slow spreading in marginal basins behind and within island arc complexes. The dunite/harzburgite component may represent the highest parts of a depleted upper mantle shell, about 16 km thick, from which a basaltic partial melt (approx. 30%) has been withdrawn to form the gabbros, dike swarms, and pillow lavas. Ophiolites have complex internal igneous, structural, and metamorphic relationships that are probably related to processes involved in their generation at ridges and in marginal basins, having no significance in terms of processes within the orogenic belt in which they are finally emplaced. Varying plate accretion rates may have a great influence on the structural and metamorphic patterns of ophiolites. The autochthonous sedimentary cap of ophiolites developed by axial accretion may, rarely, consist of continental margin non-volcanic flysch and salt, but usually comprises chert, argillite, and deep-water limestone: that of ophiolites generated in marginal basins probably consists, in addition to these facies, of andesitic flysch derived from adjacent island arcs. Ophiolites are probably emplaced, in orogenic belts, at consuming plate margins either beneath and behind oceanic trenches (subduction zones) or by thrusting onto continental margins (obduction zones) when a continental margin meets a subduction zone. Metamorphic relationships within and around ophiolite complexes are numerous, probably reflecting processes involved in both genesis and emplacement, and may involve (1) thermal and hydrothermal alteration at the site of origin, (2) blueschist metamorphism in subduction zones, (3) regional high-temperature metamorphism in island arcs, (4) high-temperature aureoles developed around peridotites injected into oceanic crust at time of origin, into the transition region between marginal basins and island arcs or into the regionally metamorphosed assemblages of island arcs, (5) garnet-amphibolites developed along the sole thrusts of obducted peridotites, which may represent transported high-temperature arc metamorphics, oceanic crust, or metamorphic rocks generated during obduction, (6) rodingites developed during low-temperature peridotite diapirism and serpentinization on ridge flanks or during ophiolite emplacement, and (7) amphibolites and eclogites, in subduction melanges, representing transformed oceanic crust ripped up from the downgoing plate beneath trenches. Obducted ophiolites often indicate a narrow time span between origin and emplacement, possibly due to the opening and closing of marginal basins. Evolving ridge/transform/trench and trench/trench/trench triple junctions have great significance for developing diachronous events associated with ophiolite emplacement along continental margins and island arcs. Early Ordovician ophiolites in the Newfoundland Appalachians have complex and variable structural relationships with rocks of the Paleozoic continental margin. Immense obducted ophiolite slices carry transported garnet-amphibolites, and are believed to have developed in a marginal basin behind early Paleozoic island arcs on the northwestern (present) margin of a proto-Atlantic Ocean.