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Phase relationships in grunerite–garnet-bearing amphibolites in the system CFMASH, with applications to metamorphic rocks from the Central Zone of the Limpopo Belt, South Africa
Authors:A. ZEH,T. J. B. HOLLAND,&   R. KLEMD
Affiliation:Mineralogisches Institut, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany (); Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK; Rand Afrikaans University, Geology Department, Johannesburg, South Africa
Abstract:A petrogenetic grid in the model system CaO–FeO–MgO–Al2O3–SiO2–H2O is presented, illustrating the phase relationships among the minerals grunerite, hornblende, garnet, clinopyroxene, chlorite, olivine, anorthite, zoisite and aluminosilicates, with quartz and H2O in excess. The grid was calculated with the computer software thermocalc , using an upgraded version of the internally consistent thermodynamic dataset HP98 and non‐ideal mixing activity models for all solid solutions. From this grid, quantitative phase diagrams (PT pseudosections) are derived and employed to infer a PT path for grunerite–garnet‐bearing amphibolites from the Endora Klippe, part of the Venetia Klippen Complex within the Central Zone of the Limpopo Belt. Agreement between calculated and observed mineral assemblages and garnet zonation indicates that this part of the Central Zone underwent a prograde temperature and pressure increase from c. 540 °C/4.5 kbar to 650 °C/6.5 kbar, followed by a post‐peak metamorphic pressure decrease. The inferred PT path supports a geotectonic model suggesting that the area surrounding the Venetia kimberlite pipes represents the amphibolite‐facies roof zone of migmatitic gneisses and granulites that occur widely within the Central Zone. In addition, the PT path conforms to an interpretation that the Proterozoic evolution of the Central Zone was controlled by horizontal tectonics, causing stacking and differential heating at c. 2.0 Ga.
Keywords:thermocalc    CFMASH    grunerite    Limpopo Belt    petrogenetic grid
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