Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes

Abstract

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 classified into two types: C-type (coarse-grained type) and Ftype (fine-grained type) peridotites. C-type peridotites are mostly coarse-grained (olivine, 1 mm across) harzburgites with porphyroclastic to protogranular textures but include subordinate dunites. Ftype peridotites are fine-grained (olivine, 60---70 mm 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 Fo91---92 and Fo89---91 in C-type and F-type peridotites, respectively. The Cr/(Cr þ Al) atomic ratio (Cr number) and Fe3þ/(Cr þ Al þ Fe3þ) atomic ratio of chromian spinel are 0 2---0 3 and 50 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.