Ultrahigh-temperature metamorphism from an unusual corundum plus orthopyroxene intergrowth bearing Al-Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa

Journal article

Authors/Editors

Rajesh, Hariharan

Research Areas

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Publication Details

Author list: Belyanin GA, Rajesh HM, Sajeev K, Van Reenen DD

Publisher: Springer

Place: NEW YORK

Publication year: 2012

Journal: Contributions to Mineralogy and Petrology (0010-7999)

Journal acronym: CONTRIB MINERAL PETR

Volume number: 164

Issue number: 3

Start page: 457

End page: 475

Number of pages: 19

ISSN: 0010-7999

eISSN: 1432-0967

Languages: English-Great Britain (EN-GB)

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Abstract

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite +/- A quartz, Al-rich orthopyroxene (similar to 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite +/- A quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as similar to 1,050-1,100 A degrees C, and similar to 10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path).

Keywords

Al-Mg granulite, Antiperthite, Clockwise P-T path, high-Al orthopyroxene, Limpopo Complex, Orthopyroxene plus sillimanite +/- quartz, Southern Marginal Zone, Spinel plus quartz, ultrahigh-temperature metamorphism

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