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Trace element constraints on mid-crustal partial melting processes – A garnet ionprobe study from polyphase migmatites (Damara orogen, Namibia)

By
C. Jung
C. Jung
1
Fachbereich Geowissenschaften, Philipps Universität Marburg, 35032 Marburg, FRG
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S. Jung
S. Jung
2
Max-Planck-Institut für Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, FRG
3
Department Geowissenschaften, Mineralogisch-Petrographisches Institut, Universität Hamburg, 20146 Hamburg, FRG, Email: stefan.jung@mineralogie.uni-hamburg.de
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E. Hellebrand
E. Hellebrand
2
Max-Planck-Institut für Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, FRG
4
SOEST – Department of Geology and Geophysics, University of Hawaii, USA
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E. Hoffer
E. Hoffer
1
Fachbereich Geowissenschaften, Philipps Universität Marburg, 35032 Marburg, FRG
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Published:
October 01, 2010

Trace element abundances in garnet from a polyphase migmatite were measured by secondary ion mass spectrometry (SIMS) in order to identify some of the effective variables on the trace element distribution between garnet and melanosome or leucosome. In general, garnet is zoned with respect to REE, in which garnet cores are enriched by a factor of 2–3 relative to the rims. For an inclusion-rich garnet from the melanosome, equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Inclusion-poor garnet from the same melanosome located in the vicinity of the leucosomes shows distinct enrichment and depletion patterns for REE from core to rim. These features suggest disequilibrium between garnet and the host rock which, in this case, could have been an in-situ derived melt. This would probably indicate a period of open-system behaviour at a time when the garnet, originally nucleated in the metamorphic environment reacted with the melt. In addition, non-gradual variation in trace element abundances between core and rim may suggest variable garnet growth rates. Inclusion-free garnet from the leucosome, interpreted to have crystallised in the presence of a melt, has a small core with high REE abundances and a broad rim with lower REE abundances. Here, crystal-liquid diffusion-controlled partitioning is a likely process to explain the trace element variation.

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GSA Special Papers

Sixth Hutton Symposium on The Origin of Granites and Related Rocks: Proceedings of a Symposium held in Stellenbosch, South Africa, 2- 6 July 2007

John D. Clemens
John D. Clemens
Department of Earth Sciences, University of Stellenbosch, South Africa
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Colin Donaldson
Colin Donaldson
EESTRSE Editor:
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Carol D. Frost
Carol D. Frost
Guest Editors:
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Alexander F.M. Kisters
Alexander F.M. Kisters
Guest Editors:
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Jean-François Moyen
Jean-François Moyen
Guest Editors:
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Tracy Rushmer
Tracy Rushmer
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Gary Stevens
Gary Stevens
Guest Editors:
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Geological Society of America
Volume
472
ISBN print:
9780813724720
Publication date:
October 01, 2010

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