Subaqueous to subaerial Archean ultramafic phreatomagmatic volcanism, Kromberg Formation, Barberton Greenstone Belt, South Africa

Abstract

Thick mafic and ultramafic volcanic sequences that characterize Archean greenstone belts are made up largely of massive and pillowed flow rocks; corresponding pyroclastic rocks are less common. Widespread mafic volcaniclastic deposits of the middle member of the Kromberg Formation (K2) in the Barberton Greenstone Belt are an exception. Their outcrops in the Onverwacht anticline and the adjoining Kromberg syncline expose a well-preserved tephra blanket more than 30 km in extent and as much as 1 km thick. On the west limb of the Onverwacht anticline, where the tephra sheet is thickest, it includes from base upward: 0-100 m of fine-grained tuff (division K2v-a); 300-1,000 m of massive to current-structured lapillistone and lapilli tuff (K2v-b); and a thin capping chert composed of silicified, commonly currentworked ash and dust (K2c). The mineralogy and composition of all rocks have been changed by early low-temperature alteration and later metamorphism. However, relict mineralogy, pyroclast textures, and trace-element chemistry suggest that the pyroclastic debris of K2v-b were derived from mafic to ultramafic magmas.The thickest section of pyroclastic debris on the west limb of the Onverwacht anticline represents the fill of a phreatomagmatic explosion crater formed by the removal of as much as 300 m of the underlying Buck Reef Chert. The lower part of the 1,000-m-thick fill consists largely of coarse lapillistone containing accidental blocks of Buck Reef Chert. This debris was deposited mainly by coarse subaqueous sediment flows but may include pyroclastic fall deposits, both deriving material from a subaqueous eruption column. Initial water depth is estimated to have been 700 m or less. Shoaling of the vent and the eventual transition to subaerial eruptions are suggested by upward transitions in the volcaniclastic pile into shallow-water, currentstructured lapilli tuff deposits in the upper part of the sequence. The chert cap, K2c, is composed of fine-grained silicified ash and accretionary lapilli deposited under very shallow water to subaerial conditions.

Environmental Controls on Photosynthetic Microbial Mat Distribution and Morphogenesis on a 3.42 Ga Clastic-Starved Platform

“…The basin facies is succeeded locally by a second interval of black-and-white banded chert. The BRC is capped by a unit of mafic and ultramafic tuffs and flows Ransom et al, 1999).…”

High resolution tephra and U/Pb chronology of the 3.33–3.26Ga Mendon Formation, Barberton Greenstone Belt, South Africa

“…Pyroclastic volcanism involving komatiitic magma was initially thought unlikely due to the low viscosity and low volatile content of komatiitic liquid. Recent field and geochemical studies (Ransom et al, 1999;Thompson Stiegler et al, 2008, 2011, however, have shown that komatiitic eruptions could be highly explosive. Eruptions were probably phreatomagmatic based on the lack of volatiles in the magmas and the juvenile nature of pyroclasts.…”

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

“…K2 consists of 0-100 m of deeply weathered, fi ne-grained tuff overlain by 300-1000 m of komatiitic basalt lapilli tuffs and lapillistones (Ransom et al, 1999). The latter includes the fi ll of a nearly 2-km-wide and 100-to 200-m-deep pit crater on the western limb of the Onverwacht anticline.…”

“…The discussion here focuses on the well-preserved, upper 20-30 m of lapilli tuffs in the lower Kromberg Formation. We then summarize the textures and origins of K2 based mostly on the work of Ransom et al (1999).…”

“…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. Our objectives are to investigate the mechanisms that enabled the fragmentation of these low-viscosity ultramafi c liquids and to explore the frequency, distribution, and environmental signifi cance of explosive komatiitic eruptions during formation of the Barberton belt.…”