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

Melanie Thompson Stiegler, Donald R. Lowe and Gary R. Byerly
Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa
Geological Society of America Bulletin (May 2011) 123 (5-6): 1112-1126

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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 123
Serial Issue: 5-6
Title: Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa
Affiliation: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA, United States
Pages: 1112-1126
Published: 201105
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 106
Accession Number: 2011-044438
Categories: StratigraphyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2011101
Illustration Description: illus. incl. strat. cols., 2 tables, geol. sketch maps
S27°00'00" - S25°00'00", E30°00'00" - E31°30'00"
Secondary Affiliation: Louisiana State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201125
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