Abundant pyroclastic komatiitic volcanism in the 3.5–3.2 Ga Barberton greenstone belt, South Africa

Abstract

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.

Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa

“…This is consistent with their sheet-like geometry, wide areal distribution, and fi ne grain size (Lowe, 1999c;Thompson Stiegler et al, 2008). The low viscosity and low volatile content of komatiitic magmas favor effusive eruptions, so it seems likely that comagmatic fl ows accompanied these pyroclastic explosions.…”

“…The few other felsic volcaniclastic deposits are thin (<10 cm), fi ne-grained, and have yet to be found interbedded with komatiitic tuffs. By contrast, komatiitic pyroclastic units are present throughout the Hooggenoeg, Kromberg, Mendon, and Weltevreden Formations, and they have been divided into two groups (Thompson Stiegler et al, 2008). South of the Inyoka fault, fi ne-grained komatiitic debris has been silicifi ed due to early Si metasomatism (Lowe and Byerly, 1986;Duchac and Hanor, 1987;Lowe, 1999b), while coarser volcaniclastic material was typically cemented and partially replaced by carbonate (Lowe, 1999b …”

“…8B). It contains more than a dozen 2-to 60-m-thick tuff units that are laterally and vertically discontinuous due to faulting and possible original bed lenticularity (Thompson Stiegler et al, 2008). Many of the fl ow rocks display upper spinifex zones refl ecting their extrusive origin (Cooper, 2008).…”

“…In the Onverwacht Group, they include ultra-Al-depleted (Al 2 O 3 /TiO 2 = 3-6), Al-depleted (Al 2 O 3 /TiO 2 = 9-15), Alundepleted (Al 2 O 3 /TiO 2 = 16-32), and Alenriched (Al 2 O 3 /TiO 2 = 44-65) komatiites. A comprehensive geochemical data set is published in Thompson Stiegler et al (2008.…”

“…These pyroclastic deposits comprise a signifi cant portion of the sedimentary rocks (40%-50%) within the volcanic portion of the sequence (Thompson Stiegler et al, 2008), and understanding their origins is crucial to the reconstruction of early terrestrial environments and to the development of the belt. The sedimentology of many of these tuff units has been discussed by Reimer (1975), Lowe and Knauth (1978), Stanistreet et al (1981), Lanier and Lowe (1982), Heinrichs (1984), and Lowe (1999c), but with few exceptions (Lowe, 1999b;Ransom et al, 1999), there has been limited consideration of their physical volcanic origins.…”