Implications of the revised oxygen isotope and cation exchange thermobarometry for the amphibolite-granulite transition of the Southern Marginal Zone, Limpopo Belt, South Africa

Détails

ID Serval
serval:BIB_561EF85334EB
Type
Article: article d'un périodique ou d'un magazine.
Collection
Publications
Titre
Implications of the revised oxygen isotope and cation exchange thermobarometry for the amphibolite-granulite transition of the Southern Marginal Zone, Limpopo Belt, South Africa
Périodique
South African Journal of Geology
Auteur(s)
Vennemann T.W.
ISSN-L
0371-7208
Statut éditorial
Publié
Date de publication
1998
Peer-reviewed
Oui
Volume
101
Pages
123-142
Langue
anglais
Résumé
For the granulite facies rocks of the Southern Marginal Zone (SMZ) of
the Limpopo Belt, minimum peak metamorphic temperatures of 800 +/- 70
degrees C are estimated by quartz-garnet oxygen isotope thermometry.
These temperatures are in agreement with temperatures expected for these
rocks by anhydrous melting experiments of biotite. Fe-Mg
garnet-orthopyroxene, garnet-cordierite, and garnet-biotite exchange
thermometry provide lower estimates of peak metamorphic temperatures
which are best explained by variable resetting during retrogression.
Similarly, the oxygen isotope fractionations between quartz,
plagioclase, and biotite have been reset. For the orthoamphibole
gneisses found south of the previously proposed retrograde
orthoamphibole isograd, best minimum peak temperature estimates of 760
+/- 50 degrees C are given by quartz-garnet and quartz-orthoamphibole
oxygen isotope thermometry in rocks containing orthoamphibole
corresponding to the top of the anthophyllite-gedrite solvus, but lower
temperatures (similar to 660 degrees C) in orthoamphibole gneisses with
at least two distinct coarse-grained orthoamphiboles. The revised
thermometry, the orthoclase content in plagioclase, and the Ti- and
F-content of biotite are consistent with a regional prograde metamorphic
temperature gradient between the orthoamphibole gneisses and the
granulites. It is possible that at least some orthoamphibole is prograde
in origin. Average pressures for garnet-plagioclase-orthopyroxene-quartz
barometry are 6 +/- 1.5 kbar, which are about 1.5 kbar lower than those
for garnet-cordierite barometry (7.5 +/- 1 kbar) in the granulites, as
well as preliminary garnet-grunerite (gedrite)-plagioclase-quartz
barometry in the orthoamphibole gneisses (7.5 +/- 1 kbar). Low peak
pressures of the garnt-plagioclase-orthopyroxene-quartz barometer may be
related to the removal of significant amounts of melt, and hence albite
component from these rocks. Overall similarities in the oxygen isotope
fractionations between quartz, plagioclase, and biotite, as well as the
Fe-Mg partitioning of contact garnet-biotite pairs between granulite
facies rocks and orthoamphibole gneisses also suggest that the
retrograde thermal history of these rocks was very similar.
Retrogression, including fine-grained orthoamphibole found to replace
cordierite and orthopyroxene, is compatible with the release of small
amounts of aqueous fluids from crystallizing residual anatectic melts at
temperatures close to the vapour present granite solidus. No terrane
wide infiltration of CO2-rich fluids nor faster cooling of the
orthoamphibole gneisses due to obduction onto granite-greenstone crust
is required to account for the formation of retrograde orthoamphibole.
Création de la notice
29/09/2012 16:23
Dernière modification de la notice
20/08/2019 14:10
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