Unravelling fluid-rock interaction in the hydrated zone of the Southern Marginal Zone of the Limpopo Belt - South Africa: a geochemical investigation based on U-Pb geochronology and Sm-Nd isotope composition of monazite and apatite in metapelites

The behaviour of rare earth elements (REE), U-Pb, Sm-Nd, and Sr-Sr isotope compositions in accessory minerals provides a unique opportunity to track fluid sources and investigate trace element mobilization during fluid-assisted metamorphism. Metapelites of the Southern Marginal Zone (SMZ), Limpopo Belt (Bandelierkop formation), South Africa, experienced a ca. 2.7 Ga granulite-facies event in which the rocks underwent anatexis. The southern portion of the SMZ contains dispersed fragments of retrogressed metapelites with ubiquitous amphibolite facies assemblages including biotite, orthoamphibole, kyanite and a 2^nd generation of garnet replacing cordierite, equilibrated under 660-600ºC and ≥ 0.6GPa. The hydrated metapelites contain graphite intergrown with the retrograde assemblages, indicating a mixed H₂O-CO₂ fluid and a rock-dominated system. However, the source of the fluids that caused the retrogression remains unclear. Previous studies suggested that hydrating fluids were originally internally derived from the crystallizing in-situ melts, in sediments containing biogenic graphite, or that an externally derived CO₂ and H₂O-bearing fluid infiltrated the metapelites through shear zone systems. For the latter, some studies have proposed that this occurred during uplift of the granulite directly after Neo-Archean peak metamorphism, while others have used rutile U-Pb ages of ca. 2.1 Ga to argue for Paleoproterozoic retrogression.

This study investigated the geochemistry of garnet, apatite, and monazite from the hydrated zone metapelites to understand the origin of the fluids. Garnet trace elements show two distinct populations described as Grt1 (Eu/Eu*=0.36) and Grt2 (Eu/Eu*=1.55). Monazite shows relatively homogeneous REE pattern for distinct samples with a slight variation in HREE and negative Eu anomaly (Eu/Eu*=0.20-0.38). As for apatite the REE pattern is variable and distinct within and between samples (Eu/Eu*=0.36-0.37). 2741 Ma to 2707 Ma U-Pb monazite ages suggest that the amphibolite-facies assemblages are mainly related to the ca. 2.7 Ga granulite-facies event. U-Pb apatite dating yielded younger ages ranging from ca. 2057 Ma to 2047 Ma. Sm-Nd isochron of apatite yielded an initial ¹⁴³Nd/¹⁴⁴Nd (0.50950 ± 0.00100; 2s; n = 21; MSWD = 4.6) equivalent, within uncertainties, to the monazite initial ¹⁴³Nd/¹⁴⁴Nd (0.50882 ± 0.00030; 2s; n = 30; MSWD = 0.81). Monazite and apatite preserved its primary 2.7 Ga Sm-Nd isotope signature, but the U-Pb apatite system was reset at ca. 2.05Ga by solid-state diffusion. Thus, apatite reacted in the presence of a disequilibrium fluid in the Neo-Archean, as evidenced by the REE chemical variation, but did not experience dissolution/reprecipitation processes.  

We propose that the retrogressed zone of the SMZ experienced a Neo-Archean peak granulite-facies followed by amphibolite-facies retrogression, in which an internally derived fluid interacted with the metapelites. This is supported by published Sm-Nd bulk rock compositions that follow the same evolution trend as these samples, indicating a closed system history in the retrogressed zone of the SMZ. The rocks also record a Paleoproterozoic, lower-temperature, amphibolite-facies re-heating event, responsible for the resetting of the apatite. Initial ⁸⁷Sr/⁸⁸Sr = 0.7130±0.0014 (2s; n = 19; MSWD = 13) indicates a continental origin for the fluids that crystallized the apatite.