Abstract

The paucity of komatiitic pyroclastic rocks in Archean greenstone sequences is commonly interpreted to reflect the low volatile contents of komatiitic liquids and the mechanical difficulties in explosively erupting highly fluid ultramaficmagmas. Here we report the presence of multiple layers of ultramafic 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 silicified and carbonated tuffs, most <1 m thick, are associated with komatiite, komatiitic basalt, and tholeiitic flow 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 reflect the explosive interaction of high-Mg melts with dominantly external water.

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