Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Barton, J. M. Articles by Gerya, T.V. Search for Related Content GeoRef GeoRef Citation

Mylonitization and decomposition of Garnet: Evidence for rapid deformation and entrainment of Mantle Garnet-Harzburgite by Kimberlite Magma, K1 Pipe, Venetia Mine, South Africa

Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa. e-mail: jmb{at}na.rau.ac.za

T.V. Gerya

Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow district, 142432, Russia. e-mail: taras{at}iem.ac.ru Institut für Geologie, Mineralogie und Geophysik, Fakultät für Geowissenschaften, Ruhr-Universität Bochum, 44780 Bochum, Germany. e-mail: taras.gerya{at}ruhr-uni-bochum.de

Sheared and unsheared xenoliths of garnet-harzburgite from the K1 Venetia kimberlite pipe (South Africa) are composed primarily of olivine and orthopyroxene (partially replaced by serpentine, magnetite and chlorite) and abundant porphyroblasts of garnet with kelyphitic rims of orthopyroxene, clinopyroxene and spinel. Mylonitized, orthopyroxene defines a mineral elongation lineation whereas bent orthopyroxene and sigmoidal garnet with wings of orthopyroxene define consistent senses of motion within individual xenoliths. The kelyphite surrounding both deformed and undeformed garnet grains is not deformed and appears to have resulted from the inversion of garnet and olivine to orthopyroxene, clinopyroxene and spinel, corresponding to the isograd separating garnet peridotite from spinel peridotite. Subsequent hydration around rims and along fractures within xenoliths resulted in the formation of serpentine, magnetite and chlorite from olivine and orthopyroxene. This hydration apparently resulted from reaction with the entraining kimberlite magma. The preservation of mylonite rather than its annealing and recrystallization to coarse-grained rocks at high temperature strain free mantle conditions requires quenching shortly after mylonite formation. This preservation taken with the presence of undeformed kelyphite around garnet grains implies that mylonitization and entrainment of garnet-harzburgite into kimberlite magma and kelyphite formation occurred during very rapid magma ascent.