The petrogenesis of orbicular granites in the Diana’s Pool area, Zimbabwe

Abstract

Abstract This paper assesses a poorly studied outcrop of orbicular granite from the Matopos granite batholith in the Diana’s Pool area, Zimbabwe. Historical samples from Diana’s Pool exhibit closely packed orbicules in a granitic matrix. They are 9 to 14 cm in diameter and are characterised by different types of orbicules containing coarse-grained felsic cores, fine-grained and alternating ferromagnesian and feldspathic shells, and a coarse-grained matrix. The orbicules are generally spherical to ellipsoidal in shape, however, some appear to be abraded and deformed. The compositions and grain sizes of cores and the matrix are comparable. Both the matrix and the cores are medium- to coarse-grained and dominated by plagioclase (the matrix probably in slightly lesser proportions), microcline (in variable proportions, and seemingly absent in some cores), quartz, biotite and accessory hornblende and magnetite. Contrary to the cores and matrix, shells are fine-grained and exhibit polygonal textures. In addition to this, the shells are dominated by biotite and magnetite; however, they do not contain hornblende. Plagioclase shows an almost complete overlap of An contents, x¯ = 26 ± 2.3 (core), x¯ = 24 ± 0.9 (shell) and x¯ = 25 ± 2.0 (matrix). Biotite composition in the shells is significantly less magnesian (x¯ = 16 ± 2.4) than in core (x¯ = 27 ± 2.2) and matrix (x¯ = 25 ± 2.2), whose compositions overlap. Average initial 87Sr/86Sr ratios from plagioclase in all the analysed shells (x¯ = 0.70226) are slightly more radiogenic than in the matrix (x¯ = 0.70193) and cores (x¯ = 0.70187). Cores are autoliths, which are plagioclase-rich, cumulate, or rim fragments reworked by new magma inputs or injections. Heterogeneous nucleation leading to the formation of orbicular shells around the cores is attributed to adiabatic decompression of magma pulses ascending in dykes, leading to superheating and resorption of early solids, and volatile exsolution, inducing undercooling, supersaturation, and shell crystallisation. The coarse-grained matrix crystallised later, after the orbicules formed, creating the groundmass and locking the orbicules in place. The deformation of shells and cores suggests that the orbicules continued to evolve in the presence of a melt (matrix material). As part of the Matopos Hills World Heritage Site, the Diana’s Pool orbicular granites present a unique and noteworthy petrogenesis, which should be preserved as part of the region’s important geoheritage.