A petrochronology window into near-surface fluid/rock interaction within Archaean ultramafic-mafic crust: Insights from the 3.25 Ga Stolzburg Complex, Barberton Greenstone Belt

Abstract

The Stolzburg Complex is one of several early Archaean ultramafic-mafic layered complexes situated along the northern edge of the Barberton Greenstone Belt on the eastern Kaapvaal Craton. In order to constrain timing and mode of emplacement, as well as post-emplacement processes affecting this igneous complex, we present whole rock trace element compositions as well as isotopic (U-Pb, O, Hf) and REE-Ti concentration data for zircon and titanite from three representative meta-gabbroic samples from the central and western portions of the complex. Zircon (SHRIMP) and titanite (LA-MC-ICPMS) analyses yield precise and identical U-Pb ages of 3247 ± 3 Ma, 3252 ± 16 Ma and 3259 ± 6 Ma (2SE), respectively. Cathodo-luminescence imaging of zircon reveals remnant, faint oscillatory and convolute zoning that is cross-cut by luminescent, irregular fracture fillings. Zircon crystals from one meta-gabbro sample yield oxygen isotopic compositions (δ18O) of <1 to +6‰, which are uncorrelated with the initial Hf isotopic compositions (εHf) of 0 to +3 epsilon units. Zircon REE systematics are highly variable across crystal domains and characterized by enriched light and medium REE. Micro-textural evidence of post-emplacement modification, along with zircon δ18O and trace element systematics, are indicative of pervasive but heterogeneous/irregular overprinting involving high-T hydrothermal fluids (>250 °C) soon after magma emplacement. Some recent metamictization may have also contributed to the low δ18O component identified in the magmatic zircon, but is difficult to disentangle from hydrothermal effects. Based on zircon trace element systematics, the gabbros appear to have formed in an oceanic setting, consistent with the lack of whole rock geochemical evidence for assimilation of more evolved felsic crustal material. Considering analytical uncertainty, zircon εHf values are relatively uniform and point to derivation of the mafic magma from a Depleted Mantle source. Titanium-in-zircon temperatures (assuming αSiO2 = 1.0, αTiO2 = 1.0) range from 840 to 1020 °C. Although such apparent crystallization temperatures do neither constrain magma petrogenesis nor tectonic setting uniquely, they match temperatures recorded by modern mid-oceanic gabbros that experienced extensive hydrothermal alteration. Thus, the meta-gabbros of the ca. 3.25 Ga Stolzburg Complex provide a rare record of fluid-rock interaction at high temperatures within mafic crust on an ancient seafloor.