a b s t r a c tThe Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-ray fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon Formation over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204 Pb-corrected 206 Pb/ 238 U and 207 Pb/ 235 U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon Formation provide temporal context for deposition. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation.While the Mendon and Weltevreden Formations are in part age-correlative and have similar lithologies, they do not appear to be genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the Barberton Greenstone Belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early tectonics to more modern, subduction-related styles.

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The paucity of komatiitic pyroclastic rocks in Archean greenstone sequences is commonly interpreted to refl ect the low volatile contents of komatiitic liquids and the mechanical diffi culties in explosively erupting highly fl uid ultramafi c magmas. Here we report the presence of multi ple layers of ultramafi c pyroclastic debris in 3481-3243 Ma rocks of the Onverwacht Group in the Barberton greenstone belt, South Africa. In the northern part of the belt, layers of serpentinized tuff are interbedded with komatiites and komatiitic basalts and constitute 30%-40% of the stratigraphic column, including sections of tuff as thick as 60 m. They retain elevated MgO contents and incompatible element abundances consistent with derivation from komatiitic liquids. In the southern part of the belt, more than 80 individual beds of silicifi ed and carbonated tuffs, most <1 m thick, are associated with komatiite, komatiitic basalt, and tholeiitic fl ow rocks. These tuffs contain abundant accretionary lapilli, attesting to their pyroclastic origins, and ratios of immobile elements, such as Al, Ti, Th, Nb, Zr, and the heavy rare earth elements, that are most compatible with those of associated Barberton komatiites. These deposits appear to refl ect the explosive interaction of high-Mg melts with dominantly external water.

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