STRAIN PARTITIONING AND SHEAR ZONE LOCALIZATION ASSOCIATED WITH REACTIVE MELT MIGRATION IN MANTLE PERIDOTITES OF THE RED HILLS MASSIF, DUN MOUNTAIN OPHIOLITE BELT, NEW ZEALAND

The Red Hills peridotite, part of the Dun Mountain Ophiolite Belt (south island, New Zealand), forms a compositionally layered and structurally heterogeneous massif comprised largely of harzburgite and dunite; the latter forming bands that increase in abundance toward the west of the complex. In the center of the massif, abundant clinopyroxenite dikes and a discontinuous plagioclase foliation crosscut earlier formed fabrics in the host harzburgites, defining a broad central zone of plagioclase lherzolite. Based on field and microstructural relationships, these plagioclase lherzolites are interpreted to have formed by reactive melt migration accompanying decompression of the massif to the plagioclase stability field (i.e.<8 kbar), thus trapping partial melts during subsequent low-pressure cooling.

In the upper section of the plagioclase-bearing lherzolite layer, numerous clinopyroxenite dikes are deformed into dextrally asymmetric, tight to isoclinal folds. Magmatic plagioclase defines a spaced foliation that is localized around the dikes, suggesting the two melts result from related focused and diffuse melt transport and that emplacement of clinopyroxenite dikes occurred while the peridotite massif had passed into the plagioclase lherzolite field. The plagioclase foliation is axial planar to the folds of the dikes, also suggesting deformation accompanied melt infiltration. Near the base of the plagioclase-bearing layer, the plagioclase foliation is perpendicular to the layer boundary of the lherzolites. However, towards the west of the complex, this foliation rotates into parallelism with the layer boundary and the intensity of the tectonite foliation becomes more pervasive. We interpret these structural patterns as representative of a massif-scale shear zone that accommodated top-to-the-N shear between the western, dunitic part of the massif and the eastern, harzburgite-dominated lithospheric mantle. Shear zone initiation likely occurred during initial exhumation of the massif, facilitated by the migration of late reactive partial melts resulting in strain localization.