Role of granite intrusions for the formation of ring structures in granulite complexes: Examples from the Limpopo belt, South Africa

Abstract

Ring structures are cylindrical or sheath folds with concentrically distributed beds, often with granites in the cores. This paper reports structural, petrographic, and petrological evidence for four such structures from the Central Zone of the Limpopo complex, which were formed during granulite exhumation in Neoarchean time (event D2/M2). It was demonstrated that the orientation of linear and planar elements in the rocks are practically identical and independent of the position within the Central Zone. All existing measurements (hundreds for the four structures) project within the same fields in stereograms. The fold axes plunge SW at an angle of ∼40° within the whole area of the Central Zone of the Limpopo complex. This implies that the rocks were metamorphosed and deformed during event D2/M2, which is typical of the Neoarchean stage of the development of the Limpopo granulite complex. Local mineral equilibria and fluid inclusions were studied in a series of key rocks, and P-T paths were derived for them. A gravitation mechanism was substantiated for the ascent of granulites and accompanying granite bodies. The structure of ring complexes was evaluated on the basis of various erosion sections. It was shown that stocklike granite bodies occur at the base of each ring structure. Petrochemical and structural data were used to demonstrate that the granites (2627 Ma) had been derived by the complete or partial melting of the lower parts of the section of Neoarchean (2651 Ma) country rocks. The upwelling of a less dense granite magma synchronously with the exhumation resulted in the helical squeezing of the overlying gneisses. This led to the concentric arrangement of beds and development of a ring structure, a sheath fold containing a granite core in some sections. A preliminary numeric 2D model is considered for the ascent of a granite diapir accompanied by the downwelling of colder and denser country rocks. A better understanding of this process can be gained by 3D numerical simulation.