Evidence for Internally Derived Fluids and Closed-System Retrogression Constrained by Monazite and Apatite U–Pb and Sm–Nd Isotopes, Southern Limpopo Belt, South Africa

Abstract Fluid-driven retrogression of high-grade metamorphic rocks is a key process influencing elemental redistribution and the chemical evolution of the crust. In the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo Belt, the transition from granulite- to amphibolite-facies conditions offers a unique opportunity to evaluate trace and rare earth element (REE) mobility, and behaviour of the Sm–Nd and U–Pb isotopic systems. This setting also raises critical questions about the timing of hydration and the sources of fluids involved. Mineral textural and assemblage differences between the northern and southern SMZ record contrasting retrograde histories. In the northern domain, the granulite-facies assemblage (quartz + plagioclase + garnet + orthopyroxene + cordierite) is well preserved, with limited retrogression marked by localized replacement of only cordierite by fine-grained kyanite, biotite, and orthoamphibole. In contrast, the southern domain shows extensive fluid-driven retrogression, with orthopyroxene completely replaced by orthoamphibole and cordierite and garnet partially overprinted by coarse-grained amphibolite-facies assemblages, including retrograde biotite, orthoamphibole, kyanite, and a second generation of garnet. In both domains, graphite intergrown with the retrograde assemblages indicates a mixed H2O-CO2 fluid and a rock-dominated system. We use garnet, monazite, apatite, and amphibole chemical compositions from retrogressed metapelites to constrain fluid sources and trace element mobility during dissolution–precipitation reactions driven by fluids linked to the retrogression of former granulites. Garnet REE patterns reveal two distinct populations described as Grt1 (Eu/Eu* = 0.26–0.36) and Grt2 (Eu/Eu* = 1.51–1.69). Monazite displays relatively homogeneous light rare earth elements (LREE) patterns with variable heavy rare earth elements (HREE) patterns and negative Eu anomalies (Eu/Eu* = 0.20–0.38) between individual samples. Apatite also displays two distinct REE patterns, one relatively enriched in REE (ΣREE = 1054–3009 μg g−1) and another depleted in REE (ΣREE = 141–159 μg g−1). Monazite U–Pb ages of c. 2742–2707 Ma suggest the amphibolite-facies assemblages developed during cooling from the Neoarchean granulite-facies event. Apatite U–Pb analyzes yielded younger ages of c. 2050 Ma. Monazite, amphibole, and apatite from individual samples align along an Sm–Nd isochron of 2684 ± 51 Ma (2s; n = 21; MSWD = 2.6) with an initial 143Nd/144Nd ratio of 0.50894 ± 0.00006 (2s), which is interpreted as mineral crystallization and amphibolite-facies retrogression during evolution of the c. 2.7 Ga Neoarchean granulite-facies metamorphism. The fluids responsible for retrogression were internally derived via fluid saturation of crystallizing in situ granitic melts. Reaction of this H2O-dominated fluid with graphite likely produced a mixed H2O-CO2 composition. Thus, the rock-dominated retrogression cycles carbon from metasediments into the fluid phase, then reprecipitates graphite as decreasing fluid H2O-CO2 ratios drive anhydrous silicates reactions to form orthoamphibole and biotite. These interpretations are supported by the closed-system behaviour of the Sm–Nd isotopic system. Initial Sr isotope ratios (87Sr/86Sri = 0.7130) exclude a mantle fluid source. These processes suggest a key mechanism controlling trace element mobility and isotopic resetting during continental crust metamorphic evolution, in which apatite provides crucial insights into the early stages of metamorphism, offering a complementary approach for reconstructing the evolution of polymetamorphic terranes.