Nature and timing of polymetamorphic, anatectic and metasomatic events in mafic granulite in Archean high-grade terranes: Scenario from Central Zone of the Limpopo Complex, southern Africa

Abstract

Mafic granulite in Archean high-grade terranes are highly sensitive to reworking and overprint processes. Understanding the nature and timing of different events requires a careful approach with a combination of methods. This study presents field, petrographic, mineralogical, PERPLE_X phase equilibria modelling, U-Th-Hf-trace-REE-zircon and ⁴⁰Ar/³⁹Ar-amphibole systematics on mafic granulites from the Verbaard area in Central Zone of the Limpopo Complex, southern Africa. The mafic granulites are interlayered and infolded conformably with the oldest meta-supracrustal rocks including quartzites, meta-ironstone and migmatitic metapelites and are intruded by c.2.63–2.62 Ga granitoids. The granoblastic mafic granulites exhibit mineral assemblages representing various metamorphic transformations including minor partial melting during two metamorphic stages related to the Neoarchean (M1) and Paleoproterozoic (M2). The earliest M1 assemblage (Grt) + Opx + Cpx + Pl + Qz + Ilm formed at 830–850 °C, >6.5 kbar, and was transformed to Opx + Cpx + Amp + Pl + Qz + Ilm during the Neoarchean decompression to c.6.5 kbar and subsequent cooling. The M2 assemblage (Grt) + Opx + Cpx + Amp + Pl + Qz + Ilm + Mag formed at < 800 °C, c.5.5 kbar during the Paleoproterozoic overprint. CL-image, Lu-Hf isotope and trace-REE characteristics of zircons distinguish the effect of two metamorphic events at 2618 ± 11 Ma (M1) and 2057 ± 25 Ma (M2), both involving anatectic components. In contrast to the Neoarchean zircon cores, the Paleoproterozoic rims/newly grown grains show lower HREE abundance, suggesting their growth/overgrowth in equilibrium with garnet. The radiogenic Hf isotope compositions of both groups of zircons are explained by local subsolidus mineral reactions rather than an input of the juvenile material. The rocks show effects of late metasomatic alteration that is variably reflected in zircons in higher LREE concentrations and a flatter LREE profile. The c.1.95 Ga age is preserved in the Paleoproterozoic zircons with transgressive recrystallization structure, and supported by ⁴⁰Ar/³⁹Ar amphibole geochronology. The results highlight the textural and corresponding elemental signatures to look out for delineation of multiple overprint events involving metamorphism, anatexis and metasomatism in Archean mafic granulites.