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

Bolhar, Robert; Wilson, Allan; Tusch, Jonas; Pettke, Thomas; Hasenstab-Dübeler, Eric; Fischer-Gödde, Mario; Anhaeusser, Carl (2023). 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. Chemical geology, 626, p. 121427. Elsevier 10.1016/j.chemgeo.2023.121427

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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 emplacement age (zircon, titanite) of 3.25 Ga and Hf 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 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 (142Nd =
-2.3±2.2 to +1.7±2.1) are not resolvable from the modern mantle value, indicating that 142Ndenriched
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 most
possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous
Ca-metasomatism as documented by exposed rodingites.

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