HR: 1340h
AN: V13E-0587    [Abstracts]
TI: The Pioneer Ultramafic Complex of the Barberton Greenstone Belt, South Africa
AU: * Cooper, M R
EM: mcoope3@lsu.edu
AF: Louisiana State University, Dept. of Geology and Geophysics, Baton Rouge, LA 70803 United States
AU: Byerly, G R
EM: gary@geol.lsu.edu
AF: Louisiana State University, Dept. of Geology and Geophysics, Baton Rouge, LA 70803 United States
AU: Lowe, D R
EM: lowe@pangea.Stanford.edu
AF: Stanford University, Geological and Environmental Sciences Dept., Stanford, CA 94305 United States
AU: Thompson, M E
EM: melanie9@stanford.edu
AF: Stanford University, Geological and Environmental Sciences Dept., Stanford, CA 94305 United States
AB: The 3.55-3.22 Ga Barberton Greenstone Belt is an approximately 100km x 30km northeast trending, isoclinally folded, volcanic and sedimentary succession surrounded by intrusive granitic rocks. It is perhaps Earth's best preserved mid-Archean supracrustal sequence and also among the most magnesian, making it an ideal location for studying compositionally distinct rocks of the Archean, such as komatiites. The Pioneer Ultramafic Complex has been interpreted as a komatiitic intrusion but we argue that it is a sequence of layered komatiitic flows and interbedded tuffs correlative with other komatiitic extrusive units of the 3.29 Ga Weltevreden Formation, the uppermost formation of the Onverwacht Group. The Pioneer Ultramafic Complex contains at least 900m of section in the study area, including at least 5 flow sets, with individual flows up to 100 m thick, sections of tuff up to 100m thick and additional thinner tuff units. The base of the sequence is in fault contact with the Sawmill Ultramafic Complex, which is similar to and perhaps correlative with the Pioneer. The top of the sequence is bounded by the Moodies Fault and slightly younger sedimentary rocks of the Fig Tree and Moodies Groups. Typical flows of the Pioneer have highly serpentinized olivine-rich cumulate bases, fresh olivine bearing peridotitic lithologies in central portions, and increasing pyroxene content, pyroxene size, and elongation of grains toward the flow tops. Three of the five flows are capped with random and/or oriented spinifex layers. The tuffs within this and other layered ultramafic complexes of the Barberton Greenstone Belt are mostly fine grained, slaty serpentinites that were previously interpreted as bedding horizontal zones of shearing. However, rare preservation of angular and vesicular lapilli, and more commonly cross-stratification in finer grained layers, provide strong evidence that these layers represent tuffs. High chromium and other trace element contents suggest they are komatiitic tuffs likely co-magmatic with the interbedded komatiitic lava flows. Compositions of fresh olivines range between 91 to 93 percent forsterite, indicating a komatiitic melt composition. In addition to olivine phenocrysts, fresh chromite, orthopyroxene, pigeonite, and augite are all present as smaller intercumulus crystals or microphenocrysts. The pyroxenes have Mg numbers up to 89 and Al/Ti ratios approximately 10-15. The latter are consistent with the Al/Ti ratios of 20-30 found within the komatiites and tuffs analyzed thus far. These ratios indicate the flows belong to the aluminium undepleted group of komatiites. The rock and mineral chemistry of these flows allow us to determine melt compositions and explore correlations and relationships with other komatiitic flows and layered ultramafic complexes of the Barberton Greenstone Belt. Field studies of these flows help characterize an Archean igneous complex believed to represent shallow marine deposition of komatiitic tuffs and coeval emplacement of thick vertically differentiated komatiitic flows.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
DE: 3641 Extrusive structures and rocks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005