Metamorphism as a clue to the presence of discrete crustal blocks in the Southern Marginal Zone of the Limpopo Belt.

Tsunogae, T., van Reenen, D.D., and Miyano, T. (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), 140-142.


Abstract

Peak and retrograde P-T conditions reflected in pelitic granulites throughout the Southern Marginal Zone (SMZ) of the Limpopo Belt were re-investigated using most recently available geothermobarometry. The SMZ is sub-divided into several crustal blocks (A. B, C) by high grade shear zones which are located north of the retrograde orthoamphibole isograd (van Reenen et al., 1987) (Fig. l). All the studied samples have similar assemblages which include garnet, biotite, orthopyroxene, quartz, plagioclase (An25-An35), cordierite, and aluminosilicate(s). Accessory minerals are apatite, zircon, rutile, ilmenite, and magnetite. Reaction textures including garnet are common in all the samples, and these textures are related to two different reactions:

(i) garnet + aluminosilicate + quartz = cordierite

This reaction, which does not involve hypersthene, is recognized by textures in which garnet is surrounded by cordierite. It appears to be more prominent in metapelites in the eastern part of the SMZ.

(ii) garnet + quartz = cordierite + hypersthene.

This reaction appears to be best developed in metapelites from the central and western part of the SMZ, and is recognized by symplectic intergrowths of cordierite and hypersthene surrounding garnet.

Garnet in all these samples have a similar composition with a Mg/(Mg+Fe) ratio (core) which varies from 0.47 to 0.49 in block A, from 0.40 to 0.44 in block B, and from 0.33 to 0.39 in block C.

Assemblages used to estimate maximum P-T conditions are garnet-orthopyroxene (thermometry) and garnet-orthopyroxene-plagioclase-quartz (barometry). Maximum calculated P-T conditions for each crustal block is a function of the distance from the shear zone which bounds each block to the south. These P-T conditions decreases towards the shear zones as follows: from 680-740°C at 5.8-6.8 kb in the north to 630-680°C at 5.0-5.5 kb in the south in block A; from 750-780°C at 6.4-8.0 kb in the north to 640-660°C at 5.5-6.0 kb in the south in block B; and from 780-800°C at 6.5-7.0 kb in the north to 640-670°C at 6.0-6.7 kb in the south in block C (Figs. 2a, 2b). The retro-grade conditions in the same areas are relatively constant and are estimated to vary from 600 to 720°C based on the assemblage garnet(rim)-cordierite, while the pressures vary from 3.5 to 6 kb based on the assemblage garnet(rim)-cordierite-aluminosilicate-quartz. The metamorphic evolution of each individual block can be explained by clockwise P-T paths. It is important to note that the suggested steep retrograde path, calculated from garnet (rim)-cordierite-aluminosilicate (M2) assemblages is not in agreement with the isobaric cooling path based on the presence of kyanite as a M3 retrograde phase.

The observed variation of maximum P-T conditions suggests that the thermal history of the SMZ is dependent on differential uplift of discreet crustal blocks. Difference in the P-T conditions between different blocks appear to be controlled by the depth at which the blocks suffered peak metamorphism. The irregular distribution pattern for pressure in block C suggests the possible presence of additional shear zone(s) within this block.

Van Reenen, D.D., Barton, J.M., Jr., Roering, C., Smit, C.A., and van Schalkwyk, J.F. (1987) : Deep crustal response to continental collision: the Limpopo Belt of southern Africa. Geology, 15, 11-14.

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