Deformation characteristics, kinematic analysis, and formation ages of gneissose Palala granite in the Palala Shear Zone, southern boundary of the Central Zone, Limpopo Belt, South Africa. (March 2021)

• Record Type: Journal Article
• Authors: Liang, Chenyue; Liu, Xiaojing; Kramers, Jan D.; Zheng, Changqing; Hu, Pengyue; Zhang, Qian
• Is Part Of: Journal of African earth sciences. Volume 175(2021)
• Publication Date: 2021-03
• Subjects: Palala granite -- EBSD analysis -- Zircon U–Pb dating -- Geochemistry -- Quartz dynamic recrystallization -- Limpopo belt; Earth sciences -- Africa -- Periodicals; Earth sciences -- Middle East -- Periodicals; Geology -- Africa -- Periodicals; Geology -- Middle East -- Periodicals; Sciences de la terre -- Afrique -- Périodiques; Sciences de la terre -- Moyen-Orient -- Périodiques; Géologie -- Afrique -- Périodiques; Géologie -- Moyen-Orient -- Périodiques; Earth sciences; Geology; Africa; Middle East; Periodicals; Electronic journals; 556.05
• DOI: 10.1016/j.jafrearsci.2020.104092
• Languages: English
• ISSNs: 1464-343X

Abstract

Abstract: The Palala Shear Zone (PSZ) is a c. 13 km wide part of a vertical, crustal-scale lineament that forms the southern boundary of the polymetamorphic high grade Central Zone of the Limpopo Belt in Southern Africa. We have studied a set of strongly sheared samples of Palala granite, taken at the southern edge of the shear zone at the Palala River transect, in order to determine the deformation patterns and evolution history of the PSZ as well as the age and petrogenesis of the granite. We studied the characteristics of macro- and microscopic deformations, analyzed the finite strain and quartz electron backscatter diffraction (EBSD) fabrics, and estimated the rheological parameters of these mylonitic gneisses. We also carried out zircon U–Pb dating and geochemical analysis. The former yielded a weighted mean 207 Pb/ 206 Pb age of 2053.5 ± 9.9 Ma which is within error identical to the age of the Bushveld Igneous Complex (BIC). The Palala granite is an alkaline A-type ferroan granite very similar in major- and trace element chemistry to the voluminous and well-studied Nebo Granite, considered to be a final differentiation product of the BIC magma. The deformed samples have uniform subhorizontal lineation and exhibit steady-state microstructures. The quartz CPO's in a non-coaxial deformation regime are promoted by pervasive dynamic recrystallization with a dominant prismatic slip and rhomb slip system. The main high-grade dextral shearing along the PSZ in the study area is associated with this high-medium temperature deformation at 500–600 °C. The strain measurement indicates that the strain types of all these SL-type and L = S-type tectonites were a flattening strain within a steady-state simple shear-dominant ductile deformation. Rheological parameter estimations suggest differential stress within the range of 15.67–17.50 MPa with the paleo-strain rate in a relatively high range of 3.97 × 10 −12 to 1.06 × 10 −13 S −1 . Together with published research, these data impose a constraint on the initiating of the transpression stage of a Paleoproterozoic intracontinental orogenic event of the Limpopo Belt. The main ductile deformation was the product of an NNW-SSE directed shortening that produced flattening strain and horizontal lineations along dextral sense shear zones, and resulted from NNW-SSE compression of the area between the Zimbabwe and Kaapvaal cratons. This high-level ductile deformation and mylonitization occurred after the emplacement of the Palala Granite at 2.05 Ga, and is thought to have taken place at medium crustal levels under upper greenschist to low amphibolite facies conditions. A late low-temperature deformation phase might be related to low-temperature basal slip which is identified by our EBSD analysis. Highlights: The deformed Palala Granite with a formation age of 2053.5 ± 9.9 Ma is an alkaline, ferroan A-type granite. It belongs to the Lebowa suite of granites as an ultimate fractionation of the Bushveld Igneous Complex magma. All SL-type and L=S-type tectonites were flattening indicating a steady-state simple shear-dominant ductile deformation. A dextral shearing apparently resulting from the NNW-SSE transpressive collision of the Zimbabwe and Kaapvaal cratons.