Structural and petrographic in the Chisamba and Kabwe areas, Central Zambia : implications for the late proterozoic Mwembeshi dislocation zone
Date
2013-01-18
Authors
Katongo, Crispin
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Abstract
In the Chisamba area, some lithologies hitherto mapped as quartzites and dolomitic marbles are quartz mylonites and interbanded calc-silicate, marble and amphibolites, respectively. Both meso-and microstructural fabrics in the Chisamba area indicate that deformation associated with shearing was predominantly crystal-plastic under low temperature conditions consistent with greenschist facies metamorphism, while deformation in the Kabwe area was predominantly cataclastic.
Mesoscopic and microscopic shear sense indicators are used for the first time to deduce sense of shear in the Mwembeshi Dislocation Zone (MDZ). Micro-textures such as asymmetric porphyroclasts, oblique grain shape-fabrics, S-C fabrics and mica-fish consistently indicate dextral strike-slip shear in the Chisamba area, whilst a sinistral shear sense is predominant in the Kabwe area. The plunging stretching lineation in the Chisamba area indicates similar oblique northeast thrust transport as that of the Lufilian Arc to the north. Amphibolite facies assemblages were downgraded to greenschist facies assemblages during shearing in the Chisamba and Kabwe areas. Quartz and, in some instances feldspar, in both areas displays annealing textures, which are attributed to a major thermal episode associated with syntectonic magmatism, related to the MDZ. Breakdown of garnet to plagioclase and hornblende in amphibolites in the Chisamba area, indicating decompression, is considered to be related to exhumation during Lufilian thrusting. The garnet-amphibole geothermometer indicates temperatures between 400°C and 600°C for the decompression reaction involving garnet. The lithologies of the Chisamba Formation in the Kabwe area and Chunga Formation in the Chisamba area are continuous across the MDZ and are therefore considered to be one and the same Formation. The Lusaka dolomite is not juxtaposed against basement rocks along the MDZ but instead oversteps them. Amphibolite to eclogite facies metamorphic grades attained in both the Lufilian and Zambezi mobile Belts on either side of the MDZ suggest that, there is no metamorphic break across the zone. Fluid inclusions in quartz veins from the Chisamba area consists of H2O-CO2 +Halite, H20-Halite, and low saline inclusions, whilst those from the Kabwe area comprise H2O-Halite, and low saline complex inclusions. P-T trapping conditions in the Chisamba area range from 200°C-350°C temperature; at pressure between 2 and 3 Kbars, consistent with greenschist facies grades. Fluid inclusions in the Kabwe area were trapped at relatively higher temperatures, ranging from 350°C-400°C.
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Petrofabric analysis