scholarly journals Formation of the Rustenburg Layered Suite by assimilation – batch crystallization (ABC) and – fractional crystallization (AFC)

2020 ◽  
Author(s):  
Zhuosen Yao ◽  
James Mungall ◽  
M Jenkins
Keyword(s):  
Magma Chamber ◽  
Fractional Crystallization ◽  
Bushveld Complex ◽  
Mineral Deposits ◽  
Ultramafic Rocks ◽  
Crystal Mush ◽  
Magma Chambers ◽  
Batch Crystallization ◽  
Magma Chamber Processes ◽  
Second Stage

Abstract The Rustenburg Layered Suite (RLS) of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. The layers are widely assumed to result from fractional crystallization from a melt-dominated magma chamber. We derive compositions of all units of the RLS with thermodynamic models of assimilation of crust by a komatiitic parent magma. Ultramafic U-type cumulate layers represent crystal mush produced by single stages of assimilation-batch crystallization (ABC). Anorthositic (A-type) magma mushes emerged by a second stage of batch crystallization during ascent of melts supernatant to mid-crustal U-type cumulates. Only the ferrobasaltic magma of the Upper and Upper Main Zone formed via classical assimilation-fractional crystallization (AFC) and ponded in a melt-dominated magma chamber that subsequently evolved by fractional crystallization. Mineral deposits associated with reversals between mafic and ultramafic layers, hitherto attributed to magma chamber processes, might form in small intrusions entirely lacking melt-dominated magma chambers.

scholarly journals The Rustenburg Layered Suite formed as a stack of mush with transient magma chambers

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Zhuosen Yao ◽  
James E. Mungall ◽  
M. Christopher Jenkins
Keyword(s):  
Magma Chamber ◽  
Fractional Crystallization ◽  
Bushveld Complex ◽  
Mineral Deposits ◽  
Ultramafic Rocks ◽  
Layered Intrusions ◽  
Crustal Assimilation ◽  
Magma Chambers ◽  
Magma Chamber Processes ◽  
Lower Crustal

AbstractThe Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. It has long been regarded as a textbook result of fractional crystallization from a melt-dominated magma chamber. Here, we show that most units of the Rustenburg Layered Suite can be derived with thermodynamic models of crustal assimilation by komatiitic magma to form magmatic mushes without requiring the existence of a magma chamber. Ultramafic and mafic cumulate layers below the Upper and Upper Main Zone represent multiple crystal slurries produced by assimilation-batch crystallization in the upper and middle crust, whereas the chilled marginal rocks represent complementary supernatant liquids. Only the uppermost third formed via lower-crustal assimilation–fractional crystallization and evolved by fractional crystallization within a melt-rich pocket. Layered intrusions need not form in open magma chambers. Mineral deposits hitherto attributed to magma chamber processes might form in smaller intrusions of any geometric form, from mushy systems entirely lacking melt-dominated magma chambers.

scholarly journals The Creation and Evolution of Crystal Mush in the Upper Zone of the Rustenburg Layered Suite, Bushveld Complex, South Africa

2019 ◽  
Vol 60 (8) ◽  
pp. 1523-1542 ◽  
Author(s):  
Z Vukmanovic ◽  
M B Holness ◽  
M J Stock ◽  
R J Roberts
Keyword(s):  
South Africa ◽  
Magma Chamber ◽  
Simple Shear ◽  
Bushveld Complex ◽  
Immiscible Liquid ◽  
Textural Analysis ◽  
Crystal Nucleation ◽  
Crystal Mush ◽  
Gravitational Loading ◽  
Crystal Accumulation

Abstract The Upper Zone of the Rustenburg Layered Suite of the Bushveld Complex contains the world’s largest Fe–Ti–V ± P deposit and formed from the last major injection of magma into the chamber. Quantitative textural analysis of Upper Zone rocks was undertaken to constrain the processes operating during mush formation and solidification, focussing on horizons with the greatest density contrast to isolate the effects of gravitational loading. We examined three magnetitite layers, together with their underlying and overlying anorthosites. The similarity of microstructures in anorthosites above and below the dense magnetitite layers suggests that the rocks were not affected by viscous compaction driven by gravitational loading. The magnetitite cumulate layers formed by crystal accumulation from a mobile crystal slurry dominated by the Fe-rich conjugate of an unmixed immiscible liquid. We suggest a new mechanism of crystal nucleation in deforming crystal-rich systems, driven by undercooling caused by cavitation as grains slide past each other during simple shear. We propose that the super-solidus deformation recorded in these rocks was caused by prolonged regional subsidence of the magma chamber at Upper Zone times.

The geochemistry of mafic and ultramafic from the Archaean greenstone belts of Sierra Leone

1983 ◽  
Vol 47 (344) ◽  
pp. 267-280 ◽  
Author(s):  
H. R. Rollinson
Keyword(s):  
Sierra Leone ◽  
Magma Chamber ◽  
Source Region ◽  
Double Diffusion ◽  
Ultramafic Rocks ◽  
Magma Chambers ◽  
Basaltic Rocks ◽  
Ocean Ridges ◽  
Different Depths ◽  
Greenstone Belts

AbstractThe Archaean (c. 2800 Ma) ultramafic rocks in eastern Sierra Leone cut basalt lavas and are mostly olivine-rich cumulates either iron-rich (Fo85–86) and derived from a basaltic or picritic parent, or more magnesian (Fo92–93) derived from an ultramafic melt with c. 18–25 wt. % MgO. In central Sierra Leone the ultramafic rocks are lavas predating tholeiitic basalts.The basalts show a wide variation in Zr/Y, suggesting that garnet was present in the source region of some of these rocks but not others. This implies that melting took place at different depths in the mantle. The REE evidence for basaltic rocks in the upper part of the Nimini belt succession suggests that they were derived from a mantle source region which had already suffered melt extraction. Ti/Zr ratios in the basaltic rocks are also variable and individual belts define different trends on a Ti vs. Zr plot implying that the basaltic rocks evolved in geographically distinct magma chambers. It is likely that the basaltic rocks evolved from a parental liquid with Ti/Zr = 90 via shallow level crystal fractionation. The source region for these rocks therefore had a lower than chondritic Ti/Zr.There are two possible models for the basaltic and ultramafic magmas in the Sierra Leone greenstone belts. First that the ultramafic and basaltic liquids were derived from mantle diapirs of differing size, but originating in the same region of the mantle. Ultramafic liquids were produced in small diapirs, which store large melt fractions, and basaltic liquids in larger diapirs which segregate larger melt fractions. A second model is based upon the double diffusion process suggested for magma chambers at mid-ocean ridges and involves a transient magma chamber from which basalts, derived from parental ultramafic liquids, are erupted, with ultramafic liquids rising directly to the surface when the magma chamber is frozen. The available data does not discriminate between these two models.

scholarly journals Drilling through the largest magma chamber on Earth: Bushveld Igneous Complex Drilling Project (BICDP)

2015 ◽  
Vol 19 ◽  
pp. 33-37 ◽  
Author(s):  
R. B. Trumbull ◽  
L. D. Ashwal ◽  
S. J. Webb ◽  
I. V. Veksler
Keyword(s):  
Magma Chamber ◽  
Bushveld Complex ◽  
Igneous Complex ◽  
Scientific Drilling ◽  
Origin And Evolution ◽  
Magma Chamber Processes ◽  
Crustal Stresses ◽  
Bushveld Igneous Complex ◽  
Felsic Volcanic ◽  
Drilling Project

Abstract. A scientific drilling project in the Bushveld Igneous Complex in South Africa has been proposed to contribute to the following scientific topics of the International Continental Drilling Program (ICDP): large igneous provinces and mantle plumes, natural resources, volcanic systems and thermal regimes, and deep life. An interdisciplinary team of researchers from eight countries met in Johannesburg to exchange ideas about the scientific objectives and a drilling strategy to achieve them. The workshop identified drilling targets in each of the three main lobes of the Bushveld Complex, which will integrate existing drill cores with new boreholes to establish permanently curated and accessible reference profiles of the Bushveld Complex. Coordinated studies of this material will address fundamental questions related to the origin and evolution of parental Bushveld magma(s), the magma chamber processes that caused layering and ore formation, and the role of crust vs. mantle in the genesis of Bushveld granites and felsic volcanic units. Other objectives are to study geophysical and geodynamic aspects of the Bushveld intrusion, including crustal stresses and thermal gradient, and to determine the nature of deep groundwater systems and the biology of subsurface microbial communities.

Granites, dynamic magma chamber processes and pluton construction: the Aztec Wash pluton, Eldorado Mountains, Nevada, USA

2004 ◽  
Vol 95 (1-2) ◽  
pp. 277-295 ◽  
Author(s):  
Brian E. Harper ◽  
Calvin F. Miller ◽  
G. Christopher Koteas ◽  
Nicole L. Cates ◽  
Robert A. Wiebe ◽  
...  
Keyword(s):  
Magma Chamber ◽  
Debris Flows ◽  
Mafic Magma ◽  
Fractional Crystallisation ◽  
Accessory Mineral ◽  
Magma Chamber Processes ◽  
Second Stage ◽  
Crystal Accumulation ◽  
Melt Segregation ◽  
Aztec Wash Pluton

ABSTRACTThe mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber.Tilting has exposed a 5-km section through the GZ and adjacent portions of the HZ. A porphyry is interpreted as a remnant of a chilled roof zone that marks the first stage of felsic GZ intrusion. Subsequent recharging by felsic and mafic magma, reflected by repeated cycles of crystal accumulation and melt segregation in the GZ and emplacement of mafic flows in the HZ, rejuvenated and maintained the chamber. Kilometre-scale lobes of mafic HZ material were deposited as prograding tongues into the GZ during periods of increased mafic input. Thus, they are lateral equivalents of the cumulate GZ granites with which they interfinger. Conglomerate-like units comprising rounded, matrix-supported intermediate clasts in cumulate granite are located immediately above the lobes. These ‘conglomerates’ appear to represent debris flows shed from sloping upper surfaces of the lobes. Thus, the GZ can be viewed as comprising distal facies, remote from the site of mafic recharging in the HZ, and the HZ as comprising proximal facies.Elemental chemistry suggests that the GZ cumulate granites represent a second-stage accumulation from an already evolved melt, and that coarse, more mafic, feldspar+biotite+accessory mineral ± hornblende rocks trapped between mafic sheets in the HZ are the initial cumulates. Fractionated melt accumulated roofward and laterally, and was the direct parent of the ‘evolved’ GZ cumulates. The most highly fractionated, fluid-rich melts accumulated at the roof.

Identification of Anorthite-enriched Plagioclase Antecrysts in the Bushveld Complex, South Africa

2019 ◽  
Vol 60 (6) ◽  
pp. 1109-1118 ◽  
Author(s):  
R Grant Cawthorn
Keyword(s):  
South Africa ◽  
Standard Deviation ◽  
Magma Chamber ◽  
Bushveld Complex ◽  
Alternative View ◽  
Layered Intrusions ◽  
Constant Composition ◽  
Magma Chambers ◽  
Anorthite Content ◽  
Deep Boreholes

Abstract The origin of cumulate grains in layered intrusions is actively debated. Earliest views assumed that all grains grew in the now-exposed magma chamber. An alternative view is that some grains were injected from deeper magma chambers (never to be exposed). Such grains have been called antecrysts. In this model upward reversals in the anorthite content of plagioclase grains in anorthosite-bearing sequences have been considered to indicate such processes, and are considered to represent the bases of cycles. Data from two deep boreholes in the upper half of the Bushveld Complex permit testing of such ideas. Careful inspection shows that anorthosites (over 45 in one core and 12 in another) do not show an increase in their anorthite contents relative to their immediate footwall samples. Further, all examples of cycles (where enough closely spaced samples are available) in one borehole show that there is a slow upward increase in the anorthite contents over tens of metres and several samples, and that anorthosite does not occur at the base of such reversals, inconsistent with injection and accumulation of a slurry of grains with constant composition. Multiple analyses of many grains in a single sample show a typical standard deviation of ±1·5% An. However, a very few samples from both boreholes show a much larger standard deviation. Examination of every single analysis from one core shows that there are rare, isolated grains with a much higher anorthite content (±5%) than the average, rarely more than one per sample (out of 10–20 analyses). It is perfectly possible that these grains are indeed antecrysts. They are not located specifically in anorthosite samples, but can occur in rocks with any proportion of plagioclase. Based on 3000 analyses they constitute of the order of 1% of the total analysed population. The injection of magma may have occurred, but its entrainment of slurries of plagioclase is not consistent with these data.

scholarly journals Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms

2019 ◽  
Vol 45 (3-4) ◽  
pp. 117-135
Author(s):  
Stephen A. Prevec
Keyword(s):  
Magma Chamber ◽  
Bushveld Complex ◽  
Magma Mixing ◽  
Layered Intrusion ◽  
Ore Deposits ◽  
Magmatic Fluid ◽  
Magma Chamber Processes ◽  
Ore Minerals ◽  
Gravity Driven ◽  
Conventional Cooling

SUMMARYThe Bushveld Complex has continued to serve as the basis for study into the fundamental nature of petrological processes for layered intrusion formation and for oxide and sulphide hosted Platinum Group Element (PGE)–Cu–Ni ore deposits. These studies have included discoveries in terms of the physical extent of Bushveld magmatism, both laterally and internally. Lateral variations in the mafic to ultramafic Rustenburg Layered Suite of the Northern Lobe of the complex have also revealed petrologically distinctive Upper Critical Zone equivalent rocks (the so-called Flatreef) with enhanced contamination and mineralization traits that reflect a transition between Eastern and Western Lobe equivalent stratigraphy and Platreef-style complexity. Traditional magma mixing models have been re-examined in light of radiogenic isotopic evidence for crustal involvement early in the chromite precipitation or formation process, combined with evidence for associated heterogeneous fluid contents, cryptic layering profiles, and textural evidence. A wide variety of alternative ore-genesis models have been proposed as a consequence. The fundamental mechanics of magma chamber processes and the existence of the magma chamber as an entity have been called into question through various lines of evidence which have promoted the concept of progressive emplacement of the complex as a stack of not-necessarily-quite-sequentially intruded sills (with or without significant quantities of transported phenocrysts), emplaced into variably crystallized and compacted crystal-liquid mush mixtures, modified by compaction-driven late magmatic fluid (silicate and aqueous) activity. Alternatively, petrological and geochemical observations have been used to discount these interpretations in favour of more conventional cooling and gravity-driven accumulation of silicate and ore minerals in a large, liquid-dominated system.RÉSUMÉLe complexe de Bushveld a demeuré à la base d’études sur la nature fondamentale des processus pétrologiques de formation d’intrusions litées et des gîtes des éléments du groupe platine (ÉGP)-Cu-Ni hébergés dans les oxydes et les sulfures. Ces études ont comporté des découvertes sur l’étendue physique, à la fois latérale et interne, du magmatisme de Bushveld. Les variations latérales de la suite stratifiée et mafique à ultramafique Rustenburg du lobe nord du complexe ont également révélé des roches équivalentes pétrologiquement distinctes de la zone critique supérieure (le communément désigné Flatreef) avec des traits de contamination et de minéralisation accrus qui reflètent une transition entre la stratigraphie équivalente des lobes est et ouest et la complexité de type Platreef. Les modèles traditionnels de mélanges magmatiques ont été réexaminés à la lumière de preuves isotopiques radiogéniques indiquant une implication de la croûte au début du processus de précipitation ou de formation de la chromite, combinées à des preuves de contenu fluide hétérogène associé, de profils de litage cryptique et de preuves texturales. Ainsi, une grande variété de modèles alternatifs de genèse de minerai a été proposée. La mécanique fondamentale des processus de la chambre magmatique et l'existence de la chambre magmatique en tant qu'entité ont été remises en question au moyen de divers éléments de preuve qui ont mis en avant le concept de mise en place progressive du complexe sous forme d'un empilement non-nécessairement séquentiel de sills injectés (avec ou sans quantités significatives de phénocristaux transportés) mis en place dans des mélanges de bouillie cristaux/liquide à cristallisation et compaction variable, modifiés par une activité tardive de fluide magmatique (silicaté et aqueux) induite par la compaction. Alternativement, des observations pétrologiques et géochimiques ont été utilisées pour écarter ces interprétations en faveur d'un processus plus conventionnel de refroidissement et d’accumulation de minérais silicatés et minéralisés induite par la gravité dans un vaste système à dominance liquide.

PETROCHEMISTRY AND MAGMA CHAMBER PROCESSES OF THE PERUVIAN COASTAL BATHOLITH GRANITES IN THE NORTHERN PART OF THE AREQUIPA SEGMENT

2016 ◽  
Author(s):  
Ana Maria Martinez ◽  
◽  
Benjamin L. Clausen ◽  
Scott R. Paterson ◽  
Vali Memeti
Keyword(s):  
Magma Chamber ◽  
Magma Chamber Processes

scholarly journals Editorial for the Special Issue “Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes”

Minerals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 102
Author(s):  
Silvio Mollo ◽  
Flavio Di Stefano ◽  
Francesca Forni
Keyword(s):  
Magma Chamber ◽  
Igneous Rocks ◽  
Eruption Dynamics ◽  
Special Issue ◽  
Magma Chamber Processes ◽  
Magmatic Processes ◽  
Compositional Variations

This Special Issue of Minerals collects seven different scientific contributions highlighting how magma chamber processes and eruption dynamics studied either in the laboratory or in nature may ultimately control the evolutionary histories and geochemical complexities of igneous rocks [...]

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