Partial melting of pelitic rocks in the Mahalapye Plutonic Block, Eastern Botswana: Evidence for 2 Ga low-P-high-T migniatization in the Limpopo granulite complex

The silica-depleted pelitic migmatites from the Mahalapye Plutonic Block (MPB), located in the southwestern part of the Central Zone of the Limpopo granulite complex, preserve textural features reflecting the former presence of melt. The rocks consist of melanocratic and leucocratic parts with various mineral assemblages, commonly including K-feldspar and plagioclase: Spl + Bt + Crd, Grt + Bt + Crd and Bt + Crd (in melanocratic parts) and Opx + Bt + Qtz and Bt + Crd² (in leucocratic parts). Phase relations in the partial melting of pelitic rocks were extended to quartz-absent equilibria in this study. Garnet (assemblage Grt + Bt + Crd + Kfs) in melanocratic parts was produced through the quartz-absent reaction, Bt + Crd + M = Grt + Kfs + Fl, at low a_H2O^Fl. On the other hand, orthopyroxene-bearing leucocratic parts resulted from the interaction of a segregated melt with solid-phase, Bt + M = Opx + Crd + Kfs + Fl, at high a_H2O^Fl during cooling. The above equilibria and the published geochronological studies show that a low-P-high-T reworking (mainly thermal) event could have occurred in the Mahalapye area at 2.0 Ga, overprinting the ∼2.6 Ga granulite-facies gneisses that widely occur throughout the Central Zone of the Limpopo granulite complex.