Show simple item record

dc.contributor.authorSandstå, Nils Runeeng
dc.contributor.authorRobins, Brianeng
dc.contributor.authorFurnes, Haraldeng
dc.contributor.authorDe Wit, Maarteneng
dc.date.accessioned2011-02-21T09:02:03Z
dc.date.available2011-02-21T09:02:03Z
dc.date.issued2011eng
dc.identifier.citationContributions to Mineralogy and Petrology 162(2): 365-377en_US
dc.identifier.issn0010-7999eng
dc.identifier.issn1432-0967eng
dc.identifier.urihttp://hdl.handle.net/1956/4518
dc.description.abstractExceptionally 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.publisherSpringereng
dc.rightsAttribution-NonCommercial CC BY-NCeng
dc.rights.urihttp://creativecommons.org/licenses/by-nc/2.5/eng
dc.subjectVarioleseng
dc.subjectPillow lavaeng
dc.subjectPlagioclase spheruliteseng
dc.subjectArchaean lavaeng
dc.subjectBarbertoneng
dc.subjectGreenstone Belteng
dc.subjectHooggenoegeng
dc.titleThe origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africaeng
dc.typeJournal articleeng
dc.typePeer reviewedeng
dc.subject.nsiVDP::Mathematics and natural science: 400eng
dc.rights.holderCopyright The Author(s) 2010. This article is published with open access at Springerlink.com
dc.type.versionpublishedVersioneng
bora.peerreviewedPeer reviewedeng
bibo.doihttp://dx.doi.org/10.1007/s00410-010-0601-4eng
dc.identifier.doihttp://dx.doi.org/10.1007/s00410-010-0601-4


Files in this item

Thumbnail

This item appears in the following Collection(s)

Show simple item record

Attribution-NonCommercial CC BY-NC
Except where otherwise noted, this item's license is described as Attribution-NonCommercial CC BY-NC