CHEMOGRAPHIC RELATIONSHIPS lN SAPPHIRINE-BEARING ROCKS OF THE LIMPOPO BELT, SOUTHERN AFRICA

D. ACKERMAND, R K. HIERD, B. F. WINDLEY

Resumo


Silica-deficient metasedimentary rocks in which sapphirine coexists with several minerais in various paragenesis occur in lenses or as thin layers within the Messina Layered lntrusion. Following the intrusion of some tholeiitic dykes, the Messina Layered Intrusion was emplaced into a sequence of supracrustal rocks (~ 3.2 Ga). The intrusion consists of poorly exposed chromite layered anorthosites, leucogabbros and gabbros. After the intrusion of some granitic rocks there was a widespread period of deformation and a high-grade metamorphism (~1.9 Ga). The average XMg of all minerals in the silica-deficient metasedimentary rocks is rather high and varies in the folowing order: cordierite (~ 0.96) > enstatite (~0.96) > chlorite (~ 0.94) >> phlogopite (~ 0.93) > sapphirine (0.96-0.86) > kornerupine (~ 0.90) > gedrite (~ 0.85) > spinel (0.92-0.80). Other minerals are corundum, sillimanite, relict kyanite and plagioclase. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al203-SiO2(H20) system are consistent with the following sequence of main reactions: (1) enstatite + corundum → cordierite + sapphirine; (4) sapphirine + sillimanite → cordierite + corundurn; (13) kornerupine → cordierite + sapphirine + enstatite; (17) sapphirine + enstatite → cordicrtte + spinel; (20) sapphirine → chlorite + corundum + spinel. These selected reactions illustrate the movement path from the PT peak elose to 10 kb/800°C to the minimum observable at 3.5 kb/700°C as indicated by experimental data for the pure system MgO-Al203-Si02-(H20). The protolith of this material was possibly sedimentary, with a bulk composition close to the composition of kornerupine and probably was composed of chlorite, pyrophyllite and alunite.


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