Metamorphic P-T trajectories from the Northern Marginal Zone of the Limpopo Belt, Zimbabwe.

Tsunogae, T., Miyano, T., and Ridley, J. (1990)@In: J.M. Barton, Jr. Eds. Extended Abstracts, The Limpopo Belt: A Field Workshop on Granulites and Deep Crustal Tectonics, RAU, Johannesburg (South Africa), 136-139.


Abstract

Metamorphic P-T trajectories of the Northern Marginal Zone (NMZ) in the Limpopo Belt. Zimbabwe. were investigated on the basis of mineralogic and petrologic data from the eastern (Renco and Triangle) and western (Makado and West Nicholson) parts of the zone. The studied areas are mainly composed of leucocratic to mesocratic, granitic gneisses and minor pelitic and mafic rocks which are intensely deformed and highly metamorphosed. The rocks are subdivided into five lithologies and 14 representative mineral assemblages as listed in Table l.

Although rocks in the NMZ and Central Zone (ZC) around Triangle are similar in lithology and mineral chemistry. the metamorphic grade in each zone appears to decrease towards the west where muscovite-quartz schist occur and garnet-bearing rocks are less common. This is also supported by lowering of anorthite molecule in plagioclase in mafic granulites and charnockites from east to west. The anorthite molecule for mafic granulites in the eastern part shows An60 to An70, but An40 to An50 in the western part (Fig. l). The molecule for amphibolite from the low-grade Zimbabwe Craton (ZC) is An34 to An38. Similar lowering is also observed in charnockites from the NMZ. An50 to An52 in the eastern part and An42 to An48 in the western part (Fig. 2).

Regardless of decreasing metamorphic grade to the west, charnockite and enderbite are abundant in both the eastern and western parts. They are characterized by anhedral orthopyroxene and/or clinopyroxene partly surrounded by hornblende and/or biotite. However, these rocks are much less abundant in the Southern Marginal Zone (SMZ) than in the NMZ and CZ (Du Toit et al., 1983). which is an important difference in lithology between the NMZ and SMZ.

Metamorphic P-T conditions for garnet bearing granulites from the eastern part of the NMZ were estimated using available geothermobarometries. Cordierite and orthopyroxene in these rocks show no evidence of hydration. Peak metamorphic conditions of about 750°C and 7.5 kbar, estimated from assemblage garnet(core)-orthopyroxene-plagioclase-quartz is followed by retrograde event of about 650°C and 4 kbar, estimated from assemblage garnet(rim)-cordierite-sillimanite-quartz (Fig. 3). The P-T trajectory was constructed using mineral assemblages in pelitic granulite from southwest Renco Mine. The trajectory appears to indicate a possible style of normal clockwise path. Pelitic granulites in the NMZ and SMZ (especially Bandelierkop Quarry) show many similarities in mineral assemblage, chemistry, and P-T trajectory as well as in lithological and structural characters, suggesting that the two zones may have been subjected to similar metamorphic evolution.

Du Toit. M. C.. Van Reenen. D. D. & Roering. C. (1983). Some aspects of the geology, structure and metamorphism of the southern marginal zone of the Limpopo metamorphic complex. Af . 8. 121-142.

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Table 1