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

Bergen Open Research Archive

Show simple item record Sandstå, Nils Rune eng Robins, Brian eng Furnes, Harald eng De Wit, Maarten eng 2011-02-21T09:02:03Z 2011-02-21T09:02:03Z 2011 eng
dc.identifier.citation Contributions to Mineralogy and Petrology 162(2): 365-377 en_US
dc.identifier.issn 0010-7999 eng
dc.identifier.issn 1432-0967 eng
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 eng
dc.rights Attribution-NonCommercial CC BY-NC eng
dc.rights.uri eng
dc.subject Varioles eng
dc.subject Pillow lava eng
dc.subject Plagioclase spherulites eng
dc.subject Archaean lava eng
dc.subject Barberton eng
dc.subject Greenstone Belt eng
dc.subject Hooggenoeg eng
dc.title The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa eng
dc.type Journal article eng
dc.type Peer reviewed eng
dc.subject.nsi VDP::Mathematics and natural science: 400 eng
dc.rights.holder Copyright The Author(s) 2010. This article is published with open access at
dc.type.version publishedVersion eng
bora.peerreviewed Peer reviewed eng
bibo.doi eng

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