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Journal of Petrology | Volume 45 | Number 2 | Pages 369-389 | 2004
© Oxford University Press 2004; all rights reserved
Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes
1 DEPARTMENT OF EARTH SCIENCES, FACULTY OF SCIENCE, KANAZAWA UNIVERSITY, KANAZAWA 920-1192, JAPAN
2 INSTITUTE OF GEOSCIENCES, FACULTY OF SCIENCE, SHIZUOKA UNIVERSITY, SHIZUOKA 422-8529, JAPAN
* Corresponding author. Telephone: 81-(0)76-264-5724. Fax: 81-(0)76-264-5746. E-mail: ultrasa{at}kenroku.kanazawa-u.ac.jp
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 F-type (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. F-type peridotites are fine-grained (olivine, 60–70 µm 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 + 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.
KEY WORDS: mantle wedge; peridotite; metasomatism; Iraya volcano; Philippines
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