Types of Igneous Cumulates

Wager, L. R.; Brown, G. M.; Wadsworth, W. J.

doi:10.1093/petrology/1.1.73

Cited by 542 publications

(203 citation statements)

References 3 publications

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“…Rhum has had a central role in developing cumulus theory (Brown, 1956;Wager, Brown & Wadsworth, 1960). Several papers in this volume illustrate that Rhum continues to inspire imaginative new research into the origin of layered rocks.…”

Section: R S J Sparks and Othersmentioning

confidence: 99%

“…The theory of Wager, Brown & Wadsworth (1960) was based on the concept of crystal settling and ideas concerning postcumulus processes originally conceived by Hess (1939Hess ( , 1960. At the orthocumulate extreme they envisaged rapid accumulation of crystals which trapped interstitial liquid.…”

Section: Cumulus Theorymentioning

confidence: 99%

“…The cumulus crystals were thus able to maintain chemical communication with the main magma reservoir by diffusion of components through the interstitial melt, and thus could grow at constant composition until virtually all the pore space was eradicated. Wager, Brown & Wadsworth (1960) recognized the difficulty of static diffusion being a viable mechanism deep in the cumulate pile and argued that adcumulus growth would be most effective in the uppermost layer of the cumulate pile. These ideas assumed that compaction was unimportant (Wadsworth, this volume) and that the consolidation of cumulate piles was essentially a cementation process.…”

Section: Cumulus Theorymentioning

confidence: 99%

“…Despite the uncertainties that currently exist on the origin of these rocks, the cumulus classification survives. This is because Wager & Deer (1939), Wager, Brown & Wadsworth (1960) and Jackson (1961) recognized that many of the rocks from layered intrusions consist of an initial crystal framework with interstitial pore material. Those rocks with a substantial proportion of pore material can be termed orthocumulates.…”

Section: Introductionmentioning

confidence: 99%

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Postcumulus processes in layered intrusions

et al. 1985

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-During the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values ( < 1 %) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

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Section: R S J Sparks and Othersmentioning

confidence: 99%

Section: Cumulus Theorymentioning

confidence: 99%

Section: Cumulus Theorymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Postcumulus processes in layered intrusions

et al. 1985

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“…Various possibilities exist, including: in situ or bottom growth (for example, McBirney and Noyes, 1979;McCarthy and Cawthorn, 1983;McCarthy, 1980 and1981;Kruger and Smart, 1987); simple gravitative accumulation in the absence of convection currents (e.g. Wager and Brown, 1968); deposition by currents bearing crystals in suspension (e.g. Irvine, 1987); and flotation of crystal suspensions (e.g.…”

Section: Summary and Discussionmentioning

confidence: 99%

Unusual textures and structures associated with a magnetitite layer in the Bushveld Complex: a contribution to the adcumulus debate

Butcher

Merkle

1991

Mineral. mag.

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In the Brits area of the Bushveld Complex, the stratigraphically lowermost magnetitite layer of the Upper Zone is underlain by an anorthosite containing discrete, euhedral magnetite crystals, measuring 0.1-0.3 mm in diameter, which form thin layers (1 mm-1 cm thick) and laminae (commonly one grain in thickness). Discordant relationships between laminae and layers indicate a consistent younging direction upwards towards the contact with the overlying magnetitite. Our interpretation is that these crystals are cumulus and have escaped modification by postcumulus overgrowth. They also record, in places, the morphology of the floor onto which they accumulated. Their preservation is largely attributed to adcumulus growth of plagioclase before, during and after, accumulation, but may also indicate that there was only a limited supply of magnetite to the floor, thus precluding grain contact and prohibiting grain enlargement. The results of a study of compositional variations across individual plagioclase grains suggest that the presence of magnetite within the crystallising mush hindered the development of an ideal plagioclase adcumulate.

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Geochemical character and tectonic environment of ultramafic to mafic cumulate rocks from the Tekirova (Antalya) ophiolite (southern Turkey)

2006

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The Upper Cretaceous Tekirova (Antalya) ophiolite derived from the southern branch of the Neotethys is located at the southwestern part of the Antalya Complex along the Tauride belt in southern Turkey. It comprises harzburgitic tectonites, ultramafic to mafic cumulates, isotropic gabbro, sheeted dykes, volcanics and associated sedimentary rocks. The cumulate rocks are located mainly along the Ç iral-Tekirova section to the south and at Doyran to the north. Several isolated blocks of cumulate rocks are also observed in the north-south-trending Gödene zone of the Antalya Complex. The ultramafic and mafic cumulate rocks are represented by wehrlite, lherzolite, olivine clinopyroxenite, olivine gabbronorite, olivine gabbro, gabbronorite and gabbro. The order of crystallization in the cumulates is olivine (Fo 88-76 ) AE chromian spinel ! clinopyroxene (Mg# 92-76 ) ! orthopyroxene (Mg# 86-70 ) ! plagioclase (An 97-84 ). The major and trace element geochemistry of the plutonic rocks suggests that the primary magma generating the Tekirova (Antalya) ophiolite is compositionally similar to those observed in modern island arc tholeiitic sequences. The presence of highly magnesian clino-and orthopyroxenes along with the absence of plagioclase in the ultramafic cumulates suggest their formation as a result of high-pressure crystal fractionation (c. 10 kbar). In contrast, the mafic cumulates in the upper crustal level show evidence of a lower-pressure (2-2.5 kbar) environment within the same tectonic setting. Therefore, it is suggested that the cumulate ultramafic rocks were probably precipitated as a result of in situ crystallization processes along the walls of a conduit system extending downward from a crustal level magma chamber to mantle depths of approximately 30 km. After their formation, the ultramafic cumulates were transported to a shallow-level magma chamber where the remainder of the plutonic section (mafic cumulates) was formed at a lower pressure environment.

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Types of Igneous Cumulates

Cited by 542 publications

References 3 publications

Postcumulus processes in layered intrusions

Postcumulus processes in layered intrusions

Unusual textures and structures associated with a magnetitite layer in the Bushveld Complex: a contribution to the adcumulus debate

Geochemical character and tectonic environment of ultramafic to mafic cumulate rocks from the Tekirova (Antalya) ophiolite (southern Turkey)

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