The Lower Zone–Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

Boorman, S.; Boudreau, A. E.; Kruger, F. J.

doi:10.1093/petrology/egh011

Cited by 140 publications

(74 citation statements)

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“…However, several studies have demonstrated the significance of residual melt migration in cumulates (Irvine 1980;Tait and Jaupart 1992;Bédard et al 1992;Meurer and Meurer 2006), whereby an interstitial melt is expelled from a framework of cumulus phases or a "crystal mush". For example, Boorman et al (2004) suggested a compaction-driven melt migration model to explain the modest increase in plagioclase abundance, associated with the transition from the Lower Zone to the Critical Zone of the Bushveld Complex.…”

Section: Electronic Supplementary Materialsmentioning

confidence: 99%

“…A feasible mechanism to allow for trapped liquid to be lost during late stages of cumulate solidification could be compaction-driven melt migration (Meurer and Boudreau 1996;Boorman et al 2004). The two phlogopite-rich samples, however, demonstrate that evolved trapped liquid in these systems can also be gained and not only lost: trapped liquid estimations derived from plagioclase and clinopyroxene modes in orthopyroxenite sample NG2-180 suggest 30 vol% and 33 vol% trapped liquid, respectively, whereas late stage minerals account for 50 vol% of the mode.…”

Section: Melt Migrationmentioning

confidence: 99%

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Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Karykowski

2017

Contrib Mineral Petrol

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by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.

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Section: Electronic Supplementary Materialsmentioning

confidence: 99%

Section: Melt Migrationmentioning

confidence: 99%

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Karykowski

2017

Contrib Mineral Petrol

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“…McKenzie 2011) or diffusive processes such as dissolution-reprecipitation that result in the complete recrystallization of primary igneous microstructures (e.g. Boorman et al 2004). Although we have presented evidence in support of a thin mushy layer that is unlikely to have exerted a significant load, we need to consider other indicators to establish whether or not the observed deformation may have been caused by viscous compaction.…”

Section: What Caused the Deformation In The Rustenburg Layered Suite?mentioning

confidence: 99%

“…We conclude that these adcumulates experienced almost no viscous deformation by dislocation creep. Boorman et al (2004) argued that viscous compaction, achieved by diffusive processes such as dissolution-reprecipitation and recrystallization, was a dominant process in Critical and Lower Zone. We do not have detailed geochemical information for our samples that can be used to demonstrate the preservation of original growth faces and compositional zoning (as described by Holness et al 2017b), and therefore cannot make a full assessment of the extent to which diffusive processes may have acted in the Rustenburg Layered Suite.…”

Section: What Caused the Deformation In The Rustenburg Layered Suite?mentioning

confidence: 99%

The thickness of the crystal mush on the floor of the Bushveld magma chamber

Holness

Cawthorn

Roberts

2017

Contrib Mineral Petrol

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The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.

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“…l-10c-e) has been attributed to processes such as crystal accumulation and fractionation , compaction and compaction-driven recrystallisation (e.g. Boorman et al, 2004), mixing of crystal populations or magmas (e.g. Higgins & Roberge, 2007;Jerram et al, 2003), textural coarsening (= Ostwald ripening) (e.g.…”

Section: Gt Gmentioning

confidence: 99%

Mécanismes de solidification des magmas basaltiques :

Vinet¹

2010

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PAR NICOLAS VINET MÉCANISMES DE SOLIDIFICATION DES MAGMAS BASALTIQUES : ÉTUDE QUANTITATIVE TEXTURALE ET GEOCHIMIQUE DES LAVES DU VOLCAN KILAUEA, HAWAII NOVEMBRE 2010 11 RÉSUMÉLe volcan Kilauea, Hawaii, est probablement le système magmatique basaltique actif le plus étudié sur Terre, et représente donc un site privilégié pour l'étude des processus de solidification basaltique en milieu naturel. Une meilleure compréhension de la solidification magmatique est d'importance majeure dans le raffinement de modèles expliquant le dynamisme des chambres magmatiques, et son étude détaillée est susceptible de grandement améliorer notre connaissance de l'évolution globale des systèmes magmatiques. Dans ce contexte volcanique, les lacs de lave offrent une rare opportunité d'étudier directement la solidification magmatique et peuvent être considérés, en première approximation, comme des analogues superficiels de petites chambres magmatiques.Le but premier de ce doctorat est de déterminer et quantifier les principaux processus de solidification magmatiques à l'oeuvre dans la genèse des basaltes tholéiitiques. Ce travail s'articule autour du minéral olivine comme composant central. Dans les deux premiers chapitres, l'approche est double, texture et géochimie, mais l'emphase porte sur l'aspect textural dont l'analyse de la distribution de la taille des cristaux (CSD) est la composante phare. Ce travail a été réalisé sur les laves produites par les éruptions de 1969 (Mauna Ulu) et 1959 du volcan Kilauea. L'étude des coulées de lave produites par l'éruption du Mauna Ulu permet de mieux comprendre les processus actifs de solidification dans tout le système magmatique superficiel (la "tuyauterie") de l'édifice. L'étude du lac de lave Kilauea Iki renseigne quant à elle sur la solidification en système semi-fermé en sub-surface. Dans un dernier temps, il est question d'évaluer plus en détail l'influence de la déformation magmatique sur la structure interne des divines, et de la quantifier, en utilisant une technique in situ récente de micro-diffraction des rayons X. Chacun des trois chapitres de cette thèse est un article publié ou destiné à la publication dans une revue scientifique internationale.L'article 1 présente les résultats en éléments majeurs et traces (roche totale), les compositions de l'olivine, et les CSDs de 11 échantillons de laves du Mauna Ulu. Les variations chimiques en roche totale sont interprétées comme étant partiellement produites par addition d'olivine dans le système magmatique. Les profiles CSD suggèrent qu'au moins deux populations d'olivines interviennent : (1) une population d'âges 3-40 ans, caractérisée par une faible densité de "gros" cristaux et des pentes CSD relativement faibles ; et (2) une population d'âges 1,5-15 ans, marquée par une forte densité de petits cristaux et des pentes CSD plus fortes. La gamme de compositions de l'olivine suggère que ces cristaux se sont formés à partir de magmas différents, probablement reliés par crystallisation fractionnée. La présence d'olivines déformées de toutes ...

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The Lower Zone–Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

Cited by 140 publications

References 60 publications

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

The thickness of the crystal mush on the floor of the Bushveld magma chamber

Mécanismes de solidification des magmas basaltiques :

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