Home About us Contact
|
Home About us Contact | ||
|
|
||
Limpopo Belt (limpopo + belt)
Selected AbstractsGravity evidence for a larger Limpopo Belt in southern Africa and geodynamic implicationsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002R. T. Ranganai Summary The Limpopo Belt of southern Africa is a Neoarchean orogenic belt located between two older Archean provinces, the Zimbabwe craton to the north and the Kaapvaal craton to the south. Previous studies considered the Limpopo Belt to be a linearly trending east-northeast belt with a width of ,250 km and ,600 km long. We provide evidence from gravity data constrained by seismic and geochronologic data suggesting that the Limpopo Belt is much larger than previously assumed and includes the Shashe Belt in Botswana, thus defining a southward convex orogenic arc sandwiched between the two cratons. The 2 Ga Magondi orogenic belt truncates the Limpopo,Shahse Belt to the west. The northern marginal, central and southern marginal tectonic zones define a single gravity anomaly on upward continued maps, indicating that they had the same exhumation history. This interpretation requires a tectonic model involving convergence between the Kaapvaal and Zimbabwe cratons during a Neoarchean orogeny that preserved the thick cratonic keel that has been imaged in tomographic models. [source] Pro- and retrograde P,T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implicationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2004A. Zeh Abstract Interpretations based on quantitative phase diagrams in the system CaO,Na2O,K2O,TiO2,MnO,FeO,MgO,Al2O3,SiO2,H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P,T path. That path displays a prograde P,T increase from 600 °C/7.0 kbar to 780 °C/9,10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P,T decrease to 600 °C/4 kbar. The data used to construct the P,T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al-rich garnet,biotite,cordierite,sillimanite,K-feldspar,plagioclase,quartz,graphite gneiss (2) a K-poor, Al-rich garnet,biotite,staurolite,cordierite,kyanite,sillimanite,plagioclase,quartz,rutile gneiss, and (3) a K, Al-poor, Fe-rich garnet,orthopyroxene,biotite,chlorite,plagioclase,quartz,rutile,ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro- and retrograde P,T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983). [source] P,T conditions of decompression of the Limpopo high-grade terrane: record from shear zonesJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2001C. A. Smit Abstract The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high-grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low-strain gneisses, bound by an imbricate system of kilometre-wide deep crustal shear zones characterized by the presence of high-strain gneisses (,primary straight gneisses'). These shear zones developed during the decompression stage of this high-grade terrane. Low- and high-strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low-strain gneisses as a uniform orientation of relict Al-rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg-rich garnet porphyroblasts. This rare fabric formed at >,820 °C and >,7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630,2670 Ma) is well developed in high-strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid-shaped reaction textures with oriented cordierite,orthopyroxene symplectites formed after the early M1 Mg-rich garnet porphyroblasts, and syn-decompression M2 pencil-shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near-isobaric cooling at 750,630 °C and 6,5 kbar. The symplectites and pencil-shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low-strain gneisses from thrust sheets show similar M2 decompression cooling and near-isobaric cooling reaction textures that formed within the same P,T range, but under low-strain conditions, as shown by their pseudo-idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite,orthopyroxene symplectites. The presence of similar reaction textures reflecting low-strain conditions in gneisses from thrust sheets and high-strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz,plagioclase,biotite matrix developed at <,600 °C, as the result of post-decompression shearing under epidote,amphibolite facies conditions. [source] | ||||||||||