Journal of Petrology, Volume 39, Issue 7: July 1 1998.

Tectonometamorphic Evolution of the Chewore Inliers: Partial Re-equilibration of High-grade Basement during the Pan-African Orogeny

BEN GOSCOMBE¹, R. ARMSTRONG² AND J. M. BARTON³

¹GEOLOGICAL SURVEY OF ZIMBABWE, P.O. BOX CY210, CAUSEWAY, ZIMBABWE ²RESEARCH SCHOOL OF EARTH SCIENCE, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T., AUSTRALIA ³GEOLOGY DEPARTMENT, RAND AFRIKAANS UNIVERSITY, P.O. BOX 524, JOHANNESBURG, REPUBLIC OF SOUTH AFRICA

RECEIVED JULY 10, 1997; REVISED TYPESCRIPT ACCEPTED FEBRUARY 2, 1998

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 (M₁) terminated by isobaric cooling at 945 ± 34 Ma. M₁ 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°C. M₁ mineral parageneses and associated ductile deformation structures dominate the Granulite Terrane, but M₁ 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 M₂ metamorphic cycle. M₂ metamorphism accompanied NE over SW directed transport during Pan-African orogenesis of the Zambezi Belt at 524 ± 16 Ma. Average peak M₂ conditions, calculated using THERMOCALC V2.0b, were 7·9-8·6 kbar and 590 ± 95°C, 630 ± 95°C and 717 ± 95°C from the south and north Zambezi Terranes and Quartzite Terrane, respectively. M₂ 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 minor structural reworking during M₂. Granulite Terrane samples within 2 km of the basal thrust margin preserve M₁ mineral assemblages but these minerals were chemically re-equilibrated without recrystallization during M₂ at conditions of 5·6 ± 1·5 kbar and 631±100°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 M₂, at 7·7 ± 1·9 kbar and 590 ± 110°C. The 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 M₂ P-T path documented for the other terranes.

Keywords: equilibrium thermodynamics;geochronology;metamorphism; P-T paths; reworking; Pan-African Orogeny

Pages 1347-1384

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