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The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa

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dc.contributor.author Sandstå, Nils Rune
dc.contributor.author Robins, Brian
dc.contributor.author Furnes, Harald
dc.contributor.author De Wit, Maarten
dc.date.accessioned 2011-02-21T09:02:03Z
dc.date.available 2011-02-21T09:02:03Z
dc.date.issued 2011
dc.identifier.citation Contributions to Mineralogy and Petrology 162(2): 365-377 en_US
dc.identifier.issn 0010-7999
dc.identifier.issn 1432-0967
dc.identifier.uri http://dx.doi.org/10.1007/s00410-010-0601-4
dc.identifier.uri http://hdl.handle.net/1956/4518
dc.description.abstract Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows. en_US
dc.publisher Springer en
dc.rights Copyright The Author(s) 2010. This article is published with open access at Springerlink.com en_US
dc.rights.uri http://creativecommons.org/licenses/by-nc/2.5/ en_US
dc.subject Varioles en
dc.subject Pillow lava en
dc.subject Plagioclase spherulites en
dc.subject Archaean lava en
dc.subject Barberton en
dc.subject Greenstone Belt en
dc.subject Hooggenoeg en
dc.title The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa en_US
dc.type Journal article en
dc.type Peer reviewed en
dc.subject.nsi VDP::Mathematics and natural science: 400 en_US
dc.type.version publishedVersion en_US


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Copyright The Author(s) 2010. This article is published with open access at Springerlink.com Except where otherwise noted, this item's license is described as Copyright The Author(s) 2010. This article is published with open access at Springerlink.com

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