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

Research Article| May 01, 2011 Fragmentation and dispersal of komatiitic pyroclasts in the 3.5–3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa M. Thompson Stiegler; M. Thompson Stiegler † 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA †Current address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. E-mail: melaniet@lsu.edu Search for other works by this author on: GSW Google Scholar Donald R. Lowe; Donald R. Lowe 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Gary R. Byerly Gary R. Byerly 2Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA Search for other works by this author on: GSW Google Scholar Author and Article Information M. Thompson Stiegler † 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Donald R. Lowe 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Gary R. Byerly 2Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA †Current address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. E-mail: melaniet@lsu.edu Publisher: Geological Society of America Received: 01 Oct 2009 Revision Received: 30 Apr 2010 Accepted: 25 Jun 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (5-6): 1112–1126. https://doi.org/10.1130/B30191.1 Article history Received: 01 Oct 2009 Revision Received: 30 Apr 2010 Accepted: 25 Jun 2010 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation M. Thompson Stiegler, Donald R. Lowe, Gary R. Byerly; Fragmentation and dispersal of komatiitic pyroclasts in the 3.5–3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa. GSA Bulletin 2011;; 123 (5-6): 1112–1126. doi: https://doi.org/10.1130/B30191.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The 3.5–3.2 Ga Onverwacht Group of the Barberton greenstone belt, South Africa, contains thick sequences of komatiitic tuff, lapilli tuff, and lapillistone representing proximal to distal volcanic settings. Volcaniclastic komatiites in the southern part of the belt include: (1) areally extensive sheets of silicified, massive to normally graded ash and accretionary lapilli interpreted as pyroclastic fall deposits, and (2) thick, widespread, carbonatized beds of lapilli and minor ash deposited by subaqueous sediment flow and pyroclastic fallout. Lithofacies 1 clasts are aphyric and poorly to nonvesicular, implying rapid quench rates and fragmentation primarily through magma-water contact. Low water/magma ratios, initial melt fracturing through minor volatile exsolution, and high magma fluxes were probably the main factors that enabled the production of buoyant eruption columns and widespread ash distribution. These tuffs were generated during vent shoaling in water that was probably <10 m deep and were preferentially preserved in distal settings on komatiitic platforms. Lithofacies 2 contains both coarse, blocky microphyric and porphyritic grains indicative of low-energy hydroclastic fragmentation and fluidal, ragged clasts formed during vigorous subaqueous explosions. These were deposited mainly in proximal to medial settings prior to vent emergence. In the northern part of the belt, 2–60-m-thick sections of serpentinized, fine-grained komatiitic tuff were deposited by pyroclastic fallout into subaqueous environments and were widely reworked by moderate- to low-energy currents. Komatiites throughout the Onverwacht Group were produced in multivent volcanic fields in marine settings that were starved of clastic input, making explosive komatiitic volcanism one of the main sediment-generating processes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.