[S-MP46_28PM1] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-09] The relationship between microstructures and metasomatism preserved within coarse granular peridotites derived from Kaap

*Daiki TOMITA¹, Katsuyoshi MICHIBAYASHI¹, Ikuo KATAYAMA², Tsuyoshi KOMIYA³ (1.Institute of Geosciences, Shizuoka University, 2.Department of Earth and Planetary Systems Science, Hiroshima University, 3.Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo)

Keywords:kimberlite, peridotite, garnet, olivine, craton, crystal-fabrics

Kimberlite was generated in deep upper mantle (70-250km) beneath craton and subsequently ascended to surface rapidly. Peridotite xenoliths, which were entrained by kimberlite, record composition and texture formed in upper mantle beneath the craton. We studied coarse granular peridotites obtained from Kimberley pipe, South Africa, as they have a few studies in terms of microstructural development, presumably because of very coarser grains. We performed mineral crystal-fabric analyses of the coarse granular peridotites in order to understand the structure of the cratonic lithosphere. The peridotites consist mostly of olivine and orthopyroxene with clinopyroxene, garnet and a minor amount of spinel and phlogopite. The crystallization of clinopyroxene appears to be associated with melt metasomatism, whereas that of phlogopite could be associated with hydration metasomatism. Garnet grains occur commonly with kelyphite consisting of fine-grained orthopyroxene, clinopyroxene and spinel, indicating that these peridotites could have been uplifted above the phase boundary between garnet peridotite and spinel peridotite stability fields. Although both foliation and lineation are not commonly identified because of coarse granular texture, olivine crystal fabrics are characterized by a single maximum of [010] with single maxima or weak girdles of [100] and [001]. We found that the intensities of olivine and orthopyroxene crystal-fabrics are correlated to the modal composition of clinopyroxene and phlogopite. It suggests that the melt metasomatism weakened crystal-fabrics, whereas the hydration metasomatism intensified crystal-fabrics. As a consequence, the metasomatism could result in the development of different types of microstructures in the peridotites and may weaken the craton lithosphere.