P-T conditions of decompression of the Limpopo high-grade terrane: record from shear zones

The Southern Marginal Zone of the late Archean Limpopo Belt of southern Afr ica is an example of a high-grade gneiss terrane in which both upper and lo wer crustal deformational processes can be studied. This marginal zone cons ists of large thrust sheets of complexly folded low-strain gneisses, bound by an imbricate system of kilometre-wide deep crustal shear zones character ized by the presence of high-strain gneisses ('primary straight gneisses'). These shear zones developed during the decompression stage of this high-gr ade terrane. Low- and high-strain gneisses both contain similar reaction te xtures that formed under different kinematic conditions during decompressio n. Evidence for the early M1/D1 metamorphic phase (>2690 Ma) is rarely pres erved in low-strain gneisses as a uniform orientation of relict Al-rich ort hopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg-rich garnet porphyroblasts. This rare fabric formed at > 520 degreesC and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630- 2670 Ma) is well developed in high-strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction texture s that formed synkinematically during sheer deformation: M2 sigmoid-shaped reaction textures with oriented cordierite-orthopyroxene symplectites forme d after the early M1 Mg-rich garnet porphyroblasts, and syn-decompression M 2. pencil-shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near-isobaric cooling at 750-63 0 degreesC and 6-5 kbar. The symplectites and pencil-shaped garnet are orie nted parallel to the shear fabric and in the stretching direction. Low-stra in gneisses from thrust sheets show similar M2 decompression cooling and ne ar-isobaric cooling reaction textures that formed within the same P-T range , but under low-strain conditions, as shown by their pseudo-idioblastic sha pes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite-orthopyroxene symplectites. The presence of similar re action textures reflecting low-strain conditions in gneisses from thrust sh eets and high-strain conditions ill primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (<2637 Ma) in unhydrated mylo nites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz-plagioclase-biotite matrix developed at < 600 degreesC, as the result of post-decompression shearing under epidote-a mphibolite facies conditions.