MELTING AND MELT SEGREGATION IN THE MANTLE WEDGE ABOVE A SUBDUCTION ZONE - EVIDENCE FROM THE CHROMITE-BEARING PERIDOTITES OF THE MIYAMORI OPHIOLITE COMPLEX, NORTHEASTERN JAPAN

Chromite-bearing peridotites of the Ordovician Miyamori ophiolite complex exhibit spatial mineralogical variations on scales ranging from several centimeters to a few kilometers. The largest variations correspond to the entire structure of the complex, which features a layered zone of interstratified harzburgite, wehrlite, and various pyroxenites sandwiched between zones of unlayered harzburgite and dunite containing only minor pyroxenite bands. All zones exhibit the same deformation microstructures, tabular equigranular to porphyroclastic textures, and strong mineral aggregate lineation. Harzburgite from the unlayered zones is characterized by olivine values of 100Mg/(Mg + Fe)=91-93.5 and chromite values of 100Cr/(Cr + AL + Fe3+)=40-75. These variables exhibit a positive correlation, which is typical of harzburgites and lherzolites from the basal units of ophiolites and from xenoliths in alkali basalts and kimberlites. The harzburgite is therefore interpreted as a residue from partial melting in the mantle. By contrast, harzburgite in the interlayered zone features olivine values of 100Mg/(Mg + Fe)=88-92 and chromite values of 100Cr/(Cr + Al + Fe3+)=40-60, and in this case the variables tend to show a negative correlation in any given locality and they partly overlap data from the intercalated wehrlite and dunite. The harzburgite of the layered zone is interpreted as residual mantle that reacted with evolved melts that then crystallized as wehrlite and dunite. The harzburgite in the unlayered zones is more refractory than that in the layered zone, even after removing effects of reaction. This difference can be explained either by enhanced partial melting and melt extraction in the unlayered zones, possibly owing to the preferential introduction of a water-rich fluid, or by melt segregation from the unlayered zones and transfer to the layered zone in response to a piezometric pressure gradient and compaction of a solid residual matrix. Mineralogical evidence suggests that evolved melts migrated through conduits formed in the layered zone by fracturing or diapirism.