HR: 0830h
AN: V31E-0984    [PDF]
TI: Petrology Of Peridotitic Xenoliths From The Orapa Kimberlites, Botswana
AU: * Uehara, Y
EM: yuehara@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo, Meguro, 1528551
AU: Komiya, T
EM: tkomiya@goe.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo, Meguro, 1528551
AU: Hirose, K
EM: kei@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo, Meguro, 1528551
AB: Xenoliths in kimberlites provide a wide window into the underlying crust and upper mantle and, with the assist of detailed petrological and geochemical study, can help us understand the characteristics of the deeper mantle. The diamondiferous Orapa kimberlites intruded into a mobile belt between Kaapvaal and Zimbabwe craton about 90Ma in the period of upper Cretaceous. The apparent tectonic setting is different from other diamondiferous kimberlites because most diamondiferous kimberlites intruded into Archean Cratons. To investigate the deep mantle beneath the mobile belt between Kaapvaal and Zimbabwe cratons, we have studied xenoliths, especially those of peridotitic, from Orapa kimberlites. The total number of the samples from Orapa kimberlites reaches 2200. In these samples, mantle-derived xenoliths include peridotites, pyroxenites, eclogites and some megacrysts of garnet or pyroxene or ilmenite. The proportion of peridotites is approximately 60%. Previous studies of the xenoliths from Orapa kimberlites were limited to the eclogitic ones because of the lack of peridotitic xenoliths. In this study, we focus on the petrology of new collection of peridotitic xenoliths. Most of the peridotites are represented by garnet lherzolites and garnet harzburgites as well as spinel lherzolites and spinel harzburgites. In the triangle of the axes of olivine, orthopyroxene and clinopyroxene, most of the modes of peridotitic xenoliths are plotted in harzbugitic area. Almost all are coarse-grained, but some show intensely deformed texture. In some cases, signatures of metasomatic reactions are existed and contain phlogopite, amphibole, rutile, ilmenite and diopside. These metasomatized peridotitic xenoliths form a metasomatic sequence from garnet peridotite (GP), garnet phlogopite peridotite (GPP) to phlogopite peridotite (PP). The Fo-value of peridotites, especially of lherzolites and harzburgites, ranges from 90% to 94%. Orthopyroxenes and clinopyroxenes are enstatitic, ranging from En90 to En95, and diopsidic, from 38wt% to 52wt% in Wo-value, respectively. On the other hand, garnets consist of both G9 and G10 and rang between 66% and 78%in pyrope component. Calculated temperature-depth relations estimated by tow pyroxene geothermometer and orthopyroxene-garnet geobarometer shows a well-developed correlation between the textures and the P-T conditions. Namely, the peridotites with deformed texture have higher temperature-depth signature than the case with coarse texture. This correlation is comparable to the data of the peridotitic xenoliths from other kimberlites in Kaapvaal craton. The highest P-T condition is around 1300 degC and 60kbars, estimated from a peridotitic xenolith with deformed texture. The compositional characters and P-T conditions indicate the presence of thick lithospheric mantle beneath Orapa. The unique tectonic setting at Orapa kimberlites is probably only at the superficial one. Depending on the new data, we constructed one of schematic cross-sections of the mantle beneath Orapa, on the basis of rock types and P-T relations at several depths.
DE: 1015 Composition of the core
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 9305 Africa
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting