Upper-amphibolite facies partial melting of paragneisses from the Epupa Complex,NW Nambia,and relations to Mesoproterozoic anorthosite magmatism

Brandt, S. and Klemd, R. (2008) Upper-amphibolite facies partial melting of paragneisses from the Epupa Complex,NW Nambia,and relations to Mesoproterozoic anorthosite magmatism Journal of Metamorphic Geology, 26 (9). pp. 871-893. DOI 10.1111/j.1525-1314.2008.00793.x.

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Abstract

The evolution of the mineral assemblages and P-T conditions during partial melting of upper-amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P-T-X phase diagrams in the Na(2)O-CaO-K(2)O-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper-amphibolite facies low- and medium-pressure metapelites, which occur in many high-grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite-sillimanite-quartz assemblages were replaced via melt-producing reactions by cordierite-bearing assemblages. In a Southern Zone, they were replaced via melt-producing reactions by garnet-bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt-Sil-Crd-Bt peak assemblages. Peak-metamorphic conditions of 700-750 degrees C at 5.5-6.7 kbar in the Southern Zone and of similar to 750 degrees C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back-reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite-sillimanite and/or biotite-muscovite intergrowths. Upper-amphibolite facies metamorphism and partial melting (c. 1340-1320 Ma) in the rocks of the Southern Zone of the Orue Unit, which underwent probably near-isobaric heating-cooling paths, are attributed to contact metamorphism induced by the coeval (c. 1385-1319 Ma) emplacement of the Kunene Intrusive Complex, a huge massif-type anorthosite body. The lower-pressure metapelites of the Northern Zone are interpreted to record contact metamorphism at an upper crustal level.

Document Type: Article
Keywords: contact metamorphism geothermobarometry metapelite migmatite phase diagrams nw namibia intrusive complex reaction textures kunene complex western angola pelitic rocks pb ages u-pb garnet granulites
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1111/j.1525-1314.2008.00793.x
ISSN: 0263-4929
Date Deposited: 12 Jan 2012 05:58
Last Modified: 08 Oct 2012 10:14
URI: http://eprints.uni-kiel.de/id/eprint/15829

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