Ultrahigh-temperature metamorphism of the Archean Limpopo Belt and its thermal effect on the adjacent low-grade Zimbabwe Craton, southern Africa.

Tsunogae, T., Nabara, A., Fukui, T., Harada, H., Mzvanga, W., and Mugumbate, F. (2001)
AGSO Geoscience Australia, Record 2001/37, 362-364.


Abstract

The Limpopo Belt in southern Africa is an example of late Archean (ca. 2.6-2.7 Ga) high-grade terrane located between Zimbabwe and Kaapvaal Cratons. The belt is subdivided into three zones on the basis of lithological and structural characters; the Northern Marginal Zone (NMZ), the Central Zone (CZ), and the Southern Marginal Zone (SMZ). The boundaries of the three zones are defined by major tectonic breaks as shear zones. The NMZ of the Limpopo Belt is characterized by the presence of granulite-facies mineral assemblages such as garnet - sillimanite - opx - cordierite - K-feldspar - quartz in pelitic gneiss, opx - cpx - plagioclase - quartz in mafic gneiss, opx - cpx - hornblende in ultramafic gneiss, and K-feldspar - quartz - plagioclase - opx in granitic gneiss (charnockite and enderbite). Dominant isothermal decompression texture of garnet + sillimanite + quartz -> cordierite in pelitic gneiss of the NMZ indicate that the zone followed a clockwise P-T path. Although the thermal peak of the NMZ has been estimated as about 800-850°C at 8 kbar using conventional geothermobarometry and phase equilibria (e.g. Rollinson, 1989; Tsunogae et al., 1992, Kamber et al., 1995), the P-T conditions may indicate a cooling event because of later mineral re-equilibrium during high-T retrograde metamorphism. In this study we newly identified much higher metamorphic temperature from opx-bearing leucocratic gneiss and pelitic gneiss of the zone. Our results suggest that the Limpopo Belt suffered the ultrahigh-temperature (UHT) crustal metamorphism (Harley, 1998).

Geophysical, structural, and geochronological studies of the northern end of the Limpopo Belt indicate that the NMZ thrusted onto the granite-greenstone terrane of the Zimbabwe Craton at ca. 2.6 Ga (James, 1975; Stuart & Zengeni, 1987; Mkweli et al., 1995). It is probable that the thrusting of the "hanging wall" Limpopo Belt may have increased temperature and pressure condition of the “foot wall” Zimbabwe Craton. We therefore examined petrological and mineralogical characteristics of metavolcanic and metasedimentary rocks of the Buhwa-Mweza Greenstone Belt which is located in the southern edge of the Zimbabwe Craton adjacent to the Limpopo Belt.

UHT metamorphic condition of the Limpopo Belt

Leucosomes associated with granulites of the NMZ are generally subdivided into three dominant mineral assemblages. They are: (1) quartz, K-feldspar, and plagioclase, (2) quartz, K-feldspar, plagioclase, and garnet, and (3) opx, K-feldspar, plagioclase, and quartz. Types (1) and (2) leucosomes are present as layers of several cm to 1 m in thickness in various lithologies of the NMZ. They are generally parallel to the regional foliation of the host granulites. Type (3) leucosome is, however, different from the other types because it is present only in enderbitic and mafic gneisses. It occurs as irregular lenses or pods occasionally oblique to the foliation of matrix gneisses. The type (3) leucosome studied in this study is characterized by a mineral assemblage of opx, mesoperthite, and quartz. Minor ilmenite, rutile, apatite, and magnetite are also present. The opx is coarse-grained and euhedral in shape suggesting crystallization from melt. As the occurrence of the type (3) leucosome is restricted to gneiss with intermediate to mafic compositions, we regard the leucosome as a product of partial melting of enderbite or mafic granulite during high-grade metamorphism.

In order to obtained a maximum temperature from the rock, we applied the ternary feldspar geothermometry of Fuhrman & Lindsley (1988) to our mesoperthites following the method of Hokada et al. (1999). The results show a temperature range of 900-950°C. Such high-temperature condition is also supported by high-Al contents in opx (up to 7.2 wt.% Al2O3 and XMg=0.65, which corresponds to a minimum temperature of 920°C at 7.5 kbar) coexisting with sillimanite, quartz, and garnet (Alm57Pyr39Grs2Sps2) in pelitic granulite of the NMZ. High fluorine content of biotite in the pelitic granulite (up to 1.2 wt.%) may also support the evidence of the high-temperature event. Similar high-Al opx (up to 9.2 wt.% Al2O3, which corresponds to a minimum temperature of 1000°C at 8 kbar) is also present in garnet - opx - sillimanite - quartz pelitic granulite in the SMZ of the Limpopo Belt in South Africa. Our results therefore suggest that both the NMZ and SMZ suffered similar UHT event during late Archean.

Metamorphic P-T condition of the Buhwa-Mweza Greenstone Belt

The Buhwa-Mweza Greenstone Belt is a zone of 100 km in length and 15 km in width elongated parallel to the thrust boundary between the NMZ and the Zimbabwe Craton (Worst, 1962; Stagman et al., 1978). The greenstone belt is particularly important because the belt is located in the southern end of the Archean Zimbabwe Craton. It is composed of various greenstone lithologies such as mafic and ultramafic schist, phyllite, banded iron-formation, and meta-quartzite. Ages of protolith formation and metamorphism of the Buhwa-Mweza Greenstone Belt are inferred by Fedo et al. (1995) as ~3.0 Ga and 2.9-2.5 Ga, respectively. Although metamorphic conditions of the craton is generally regarded as greenschist to amphibolite facies, we identified a significant increase in metamorphic grade within the greenstone belt toward the Muponjani Shear Zone (Rollinson & Blenkinsop, 1995) which is defined as a boundary between the Zimbabwe Craton and the Limpopo NMZ. The mineral assemblage of mafic schist changes from epidote - hornblende - plagioclase - quartz in the north to hornblende - garnet - cummingtonite - plagioclase - quartz toward south. The hornblende - garnet - plagioclase - quartz assemblage in the southern part of the greenstone belt suggests a P-T condition of 580°C at 5 kbar In the southern end, approximately 0.5 km north of the Muponjani Shear Zone, the remnant of greenstone metabasite shows the highest-grade assemblage of hornblende - cpx - plagioclase - quartz. The metamorphic temperature abruptly increases up to 750°C near the shear zone. An increase in grossular content toward the rim of subhedral to euhedral garnet in mafic schists and phyllites from the southern part of the greenstone belt suggests a pressure increase as well as temperature probably because of the loading of the NMZ.

Discussion

Our results indicate that the NMZ of the Limpopo Belt suffered >900°C UHT metamorphism during late Archean. Although the UHT metamorphism of the Limpopo Belt has been inferred from the sapphirine-quartz equilibrium in aluminous granulite from the CZ in South Africa (Schreyer et al., 1984), this is the first quantitative evidence from marginal zones. We therefore suggest that the peak metamorphic temperature of the NMZ was probably much higher than we have estimated previously. Such regional UHT terrane has been identified from several localities: the Napier Complex of East Antarctica, the Eastern Ghats belt of India, the Highland Complex of Sri Lanka, Labwor Hills of Uganda, Wilson Lake of Canada, In Ouzzal of Algeria, the Aldan Shield of Siberia, and the Kontum Massif of central Vietnam. The Limpopo Belt is therefore regarded as one of the oldest UHT terrane in the world. The mechanism of such UHT metamorphism and source of heat in Archean lower crust remain interesting topics to be addressed in our future studies.

The significant increase in P-T condition of the Buhwa-Mweza Greenstone Belt toward the boundary with the NMZ implies a loading of the UHT Limpopo Belt onto the Zimbabwe Craton and subsequent heating. The event is probably related to the late Archean continent-continent collision between the Zimbabwe and Kaapvaal Cratons (e.g. Roering et al., 1992).

Acknowledgements

TT thanks Geology Department of the University of Zimbabwe, Geological Survey Department of Zimbabwe, and Sandawana Mine Co. Ltd. for support of field work in 1996, 1997, and 1999. Prof. T. Miyano, Dr. K. Hisada, Dr. S. Tojo, and Mr. Y. Kaneko are acknowledged for their discussion and field assistance.

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