HR: 1340h
AN: V33E-1498    [Abstracts]
TI: Peraluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: Plagioclase-rich cumulates re-equilibrated at uppermost mantle and crustal conditions?
AU: * Ulianov, A
EM: alexey.ulyanov@unine.ch
AF: University of Neuchatel, Institute of Geology, Rue Emile-Argand 11, Neuchatel, CH2007 Switzerland
AU: Kalt, A
EM: angelika.kalt@unine.ch
AF: University of Neuchatel, Institute of Geology, Rue Emile-Argand 11, Neuchatel, CH2007 Switzerland
AU: Pettke, T
EM: thomas.pettke@erdw.ethz.ch
AF: ETH Zentrum, Institute of Isotope Geology and Mineral Resources, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AB: Basanites of the Chyulu Hills volcanic field, Kenya, contain a suite of meta-igneous peraluminous spinel-garnet olivine websterite, Mg-Al sapphirine-bearing and Ca-Al hibonite-bearing granulite xenoliths. The websterites are the most mafic and magnesian members of this sequence. The Mg-Al sapphirine-bearing granulites are more Si- and Al-rich and less magnesian. They consist of ortho- and clinopyroxene, corundum, spinel, sapphirine, sillimanite, plagioclase and garnet. The Ca-Al hibonite-bearing granulites are most enriched in Si and Al. They are dominated by clinopyroxene and plagioclase and may contain hibonite closely associated with spinel, mullite, sapphirine and sillimanite. Hibonite, which is very rare in terrestrial rocks, is the earliest mineral in the crystallization sequence. All rocks are poor in REE, HFSE and Cr and enriched in LILE and Ni. They follow a magmatic fractionation trend and form a cumulate sequence finally equilibrated in the range of uppermost mantle (websterites) and crustal (granulites) depths. The websterites could have formed by high-pressure metamorphism of low-pressure troctolite-like cumulates and appear similar to some websteritic lithologies from exhumed high-pressure ultramafic complexes (Kornprobst, 1990; Morishita et al., 2003). The final P-T conditions for most websterites correspond to 920-1000 C / 17-22 kbar, whereas the final equilibration in granulites occurred at ca. 600-740 C / <8 kbar, the pressure limit being defined by the stability field of sillimanite. The geodynamic interpretation of spatially close granulite terranes (Moeller et al., 1998) coupled with the petrological evidence suggest that the studied rocks were metamorphosed and may have formed in the environment of a Pan-African active continental margin. Moeller, A., Mezger, K., Schenk, V. (1998). Journal of Petrology 39, 749-783. Kornprobst, J., Piboule, M., Roden, M., Tabit, A. (1990). Journal of Petrology 31, 717-745. Morishita, T., Arai, S., Gervilla, F., Green, D. (2003). Geochimica et Cosmochimica Acta 67, 303-310.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting