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Journal of Petrology | Volume 44 | Number 5 | Pages 789-813 | 2003
© Oxford University Press 2003
Partial Melting of Metapelitic Rocks Beneath the Bushveld Complex, South Africa
1 LABORATORY FOR CRUSTAL PETROLOGY, DEPARTMENT OF GEOLOGY, UNIVERSITY OF MARYLAND, COLLEGE PARK, MD 20742, USA
2 DEPARTMENT OF GEOLOGY, SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG 2050, SOUTH AFRICA
3 DEPARTMENT OF EARTH SCIENCES AND VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES (VIEPS), LA TROBE UNIVERSITY, BUNDOORA, VIC. 3086, AUSTRALIA
4 SCHOOL OF GEOSCIENCES, MONASH UNIVERSITY, CLAYTON, VIC. 3800, AUSTRALIA
Telephone: 301-405-8181. Fax: 301-314-9661. E-mail: timj{at}geol.umd.edu
Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high- and low-aH2O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H2O-rich volatile phase derived from underlying, dehydrating rocks. At lower grade (T < 700°C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (<1 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3–4%), preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H2O. Subhorizontal volatile-phase flow was concentrated within thin (
10 mm) metapsammite horizons that are preserved as stromatic quartz–sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz–sillimanite veins and along the axial planar schistosity. Closer to the contact (T > 725°C), volatile-phase-absent, biotite-consuming melting and melt extraction produced coarse-grained garnet–cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.
KEY WORDS: anatexis; Bushveld Complex aureole; H2O; melt segregation; migmatite
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