Pyroxene-bearing low- and high-HREE TTGs from the northeastern margin of the Kaapvaal Craton, southern Africa: Implications for Archean geodynamics

Archean tonalite-trondhjemite-granodiorite (TTG) rocks are generally amphibole- and/or biotite-bearing. Pyroxene-bearing granitoids of TTG affinity are less commonly reported. This study evaluates the range in composition and petrogenesis of pyroxene-bearing TTGs from a high-grade terrane along Archean cratonic margin. The Southern Marginal Zone high-grade terrane along the northeastern margin of the Kaapvaal Craton exposes prominent Mesoarchean pyroxene-bearing granitoids. The present study describes two groups of the granitoids in the Commissiedraai-Hugomond areas. The tonalite-trondhjemites with Fe-rich orthopyroxene (X_Mg = 0.46–0.49) dominate and occur as massive homogenous and banded varieties, the latter likely due to anatectic overprint. Enclaves of metabasites occur in both varieties. The darker layers of the banded variety are identical in mineralogy, texture and chemistry to the massive dark greenish-grey rocks. A less dominant group of clinopyroxene-rich massive tonalites occur in the Hugomond area. Clinopyroxene exhibit prominent orthopyroxene exsolution, with a pre-exsolution composition of X_Mg = 0.61–0.63. The spatially associated orthopyroxene-bearing and clinopyroxene-rich rocks are compositionally connected by a two-pyroxene-bearing variety with Mg-rich orthopyroxene (X_Mg = 0.56–0.59). No textures of pyroxene formation after earlier Fe[sbnd]Mg minerals occur in the studied rocks, whereas textures of pyroxene destabilization are indicative of late magmatic interaction with a residual melt. Pyroxenes in all rocks are primary magmatic. Biotite-quartz intergrowths and amphibole after pyroxenes are late magmatic in origin. Small garnet crystals occur locally in the orthopyroxene-bearing tonalite-trondhjemites and are related to metamorphic/anatectic overprint. The rocks are affected by late alkali metasomatism close to shear zones. Geochemical characteristics of the orthopyroxene-bearing tonalite-trondhjemites are comparable to low-HREE TTGs, while that of the clinopyroxene-rich tonalites are similar to high-HREE TTGs. The proposed petrogenetic model considers the formation of both groups of pyroxene-bearing granitoids via remelting of mafic sources in a thickened plume-related oceanic plateau. The high temperatures of melting, low water activity and oxidized conditions likely resulted in the formation of charnockitic assemblages, and is supported by phase equilibria modelling. Shallow level melting of a MORB-like source produced clinopyroxene-rich high-HREE TTG. Subsequent melting of a slightly enriched-MORB-like source occurred at relatively greater depths forming orthopyroxene-bearing TTG with low-HREE characteristics. The two-pyroxene-bearing rocks likely resulted from the mingling-mixing of the high- and low-HREE TTG end members. Implications of the results on Archean geodynamics are discussed together with a comparative exercise with similar rocks from other Archean terranes.