Tectonometamorphic evolution of the Chewore Inliers: Partial re-equilibration of high-grade basement during the Pan-African Orogeny

The Chewore Inliers are isolated outcrops of the Zambezi Mobile Belt within the Mesozoic Lower Zambezi Rift Valley in Zimbabwe. Detailed mapping has recognized four terranes: the Zambezi Quartzite, Granulite and Ophiolite Terranes. Apart from the Ophiolite Terrane, all are dominated by supracrustal gneisses with concordant granitic orthogneiss units of 1071 +/- 8 and 1083 +/- 8 Ma age. These terranes experienced low-P-high-T metamorphism (M-1) terminated by isobaric cooling at 945 +/- 34 Ma. M-1 assemblages of sillimanite-spinel-garnet, garnet-orthopyroxene and two-pyroxene mafics are recorded in the Granulite Terrane, and conditions of formation were 4.4 +/- 1.7 kbar and >800 degrees C. M-1 mineral parageneses and associated ductile deformation structures dominate the Granulite Terrane, but M-1, mineral parageneses are only preserved as sillimanite-spinel inclusions in garnet cores in the other terranes. The Zambezi, Quartzite and Ophiolite Terranes were almost totally recrystallized during reworking in the M-2 metamorphic cycle. M-2 metamorphism accompanied NE over SW directed transport during Pan-African orogenesis of the Zambezi Belt at 524 +/- 16 Ma. Average peak M-2 conditions, calculated wing THERMOCALC V2.0b, were 7.9-8.6 kbar and 590 +/- 95 degrees C, 630 +/- 95 degrees C and 717 +/- 95 degrees C from the south and north Zambezi Terranes and Quartzite Terrane, respectively. M-2 involved a clockwise P-T path from the chloritoid stability field with matrix assemblages crystallized in the kyanite-staurolite field or at the kyanite-sillimanite transition, and near-isothermal decompression occurred through the peak of metamorphism into the sillimanite field. In contrast, the Granulite Terrane was incorporated within the Zambezi Belt as a thrust-bound slab and experienced only miller structural reworking during M-2. Granulite Terrane samples within 2 km of the basal thrust margin preserve MI mineral assemblages but these minerals were chemically re-equilibrated without recrystallization during M-2 at conditions of 5.6 +/- 1.5 kbar and 631 +/- 100 degrees C. Granulite Terrane samples were totally recrystallized in shear zones at the margin of this terrane. These samples equilibrated at conditions identical to the peak of M-2 at 7.7 +/- 1.9 kbar and 590 +/- 110 degrees C. I;he re-equilibrated and recrystallized sample sets define two points on the clockwise P-T path experienced by the Granulite Terrane during further burial and reworking in the Pan-African Orogeny, and are consistent with the M-2 P-T Path documented for the other terranes.