Petrology of peridotite xenoliths from Iraya volcano, Philippines, and its implication for dynamic mantle-wedge processes

Peridotite xenoliths entrained in calc-alkaline andesites from the Iraya volcano, Philippines, were petrologically examined to constrain the nature of the mantle-wedge materials and processes. They can be classed into two types: C-type (coarse-grained type) and F-type (fine-grained type) peridotites. C-type peridotites are mostly coarse gained (olivine, similar to1 mm across) harzburgites with porphyroclastic to protogranular textures but include subordinate dunites. F-type peridotites are fine-grained (olivine, similar to60-70 mum across). Secondary orthopyroxenes that replace olivine and sometimes show radial (spherulitic) aggregation are very common in F-type peridotites and, subordinately, in C-type peridotites, in which the total amount of orthopyroxene increased in volume. Fine-grained olivine in F-type peridotites characteristically has minute glass and chromian spinel inclusions. Mineral chemistry is clearly different between the two types of peridotite: olivine is around Fo(91-92) and Fo(89-91) in G type and F-type peridotites, respectively. The Cr/(Cr + Al) atomic ratio (Cr number) and Fe3+/(Cr + Al + Fe3+) atomic ratio of chromian spinel are 0.2-0.3 and < 0(.)1, respectively, in C-type peridotites, and 0(.)4-0(.)7 and around 0(.)1, respectively, in F-type peridotites. The secondary orthopyroxenes are appreciably lower in Al2O3, Cr2O3 and CaO than the primary ones. A textural transition from C-type to F-type peridotites can be observed; coarse olivine becomes recrystallized into fine grains through subgrains that preserve the previous coarse texture. The C-type harzburgites are similar in mineral chemistry to arc-type harzburgites, e.g. mantle xenoliths from the Japanese island arcs, and may represent samples of the sub-arc lithospheric mantle. The C-type harzburgites beneath the Iraya volcano may have been strained and deformed during oblique subduction of the South China Basin. A silicate melt rich in SiO2, H2O and Fe, possibly derived by fractional crystallization from a primitive arc magma, assisted the recrystallization of the C-type peridotites to the F-type peridotites with metasomatic chemical modification. Oblique subduction is common in arc-trench systems, suggesting that F-type peridotite formation may be common within the mantle wedge.