MAGMA CHAMBERS IN OPHIOLITES

Historically, cumulate layering orientation was used to define magma chamber shape in ophiolites, implying large (~10 km), funnel-like or semi-cylindrical chambers. Some ophiolites (Betts Cove) have undeformed cumulate sections with thick (~500m) ultramafic to mafic macro-cycles that may indicate a fairly large open chamber. Ultramafic cumulates have poikilitic pyroxene and feldspar, and geochemical models indicate retention of 20-30% intercumulus melt. Pyroxenite dyke and sill swarms rooted in the cumulate peridotite are probably intercumulus melt escape channels. The uppermost macrocycle has rhythmically-layered (~2m), size-graded orthopyroxenite-websterite-gabbronorite beds that may record expulsion of this melt onto the floor of a higher-level chamber. In contrast, mapping in the North Arm massif (Bay of Islands) shows that most mafic cumulates (4-5 km) related to seafloor spreading are deformed, so that layering orientation cannot be used to define the chamber shape. At the top (~2-3 km), folded, layered, evolved gabbros with diorite-trondhjemite pods are generally separated from the sheeted dykes by shear zones. In one place gabbros feed up into the sheeted dykes through a ~250m wide paleo-vertical mylonite zone where porphyritic diabase dykes grade into porphyroclastic gneisses. In the east the entire crust is made up of evolved gabbros. In the west, the evolved gabbros are underlain by ~2 km of olivine gabbro and troctolite (mostly) cumulates emplaced as sills <25m thick. The youngest sills are undeformed and crosscut older foliated troctolite sills, which in turn crosscut isoclinally folded hybrid gabbro-troctolite-anorthosite complexes. At each stage, intrusions react with and assimilate host rocks. These sill-cumulate complexes form lenses ~500m x 200m bounded by senestral extensional mylonites, with deformation extending from granulite to greenschist grades. Trondhjemite infills of breccias and extensional shear veins record expulsion of residual or anatectic melts into deformation zones. Paleo-horizontal shear zones channeled residual melts and fluids and may contain abundant Fe-Ti-oxides. Melt moved laterally in response to pressure gradients generated by heterogeneous crustal extension; i.e. lithospheric necking zones suck out the evolved melt from the gabbroic mush.