Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

Abstract

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7–2.3, La/Gdchond. = 0.6–1.7, Sm/Luchond. = 0.9–1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5–2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 ± 62 (n = 12, MSWD = 1.5) and 3396 ± 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 ± 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and εHf values for zircon of −2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (μ142Nd = −2.3 ± 2.2 to +1.7 ± 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.