scholarly journals Dating the Bushveld Complex: Timing of Crystallization, Duration of Magmatism, and Cooling of the World’s Largest Layered Intrusion and Related Rocks

2021 ◽  
Author(s):  
James S Scoates ◽  
Corey J Wall ◽  
Richard M Friedman ◽  
Dominique Weis ◽  
Edmond A Mathez ◽  
...  
Keyword(s):  
Bushveld Complex ◽  
Layered Intrusion ◽  
Temporal Correlation ◽  
Critical Zone ◽  
Granitic Rocks ◽  
Rock Types ◽  
Igneous Provinces ◽  
Zircon Crystallization ◽  
Igneous Activity ◽  
Duration Relationship

Abstract The Paleoproterozoic Bushveld Complex, including the world’s largest layered intrusion and host to world-class stratiform chromium, platinum group element, and vanadium deposits, is a remarkable natural laboratory for investigating the timescales of magmatic processes in the Earth’s crust. A framework for the emplacement, crystallization, and cooling of the Bushveld Complex based on integrated U-Pb zircon-baddeleyite-titanite-rutile geochronology is presented for samples of different rock types from the Bushveld Complex, including ultramafic and mafic cumulates, mineralized horizons, granitic rocks from the roof, and a carbonatite from the nearby alkaline Phalaborwa Complex. The results indicate that (1) the Bushveld Complex was built incrementally over a ∼5 million-year interval from 2060 Ma to 2055 Ma with a peak in magma flux at c.2055–2056 Ma, (2) U-Pb zircon crystallization ages do not decrease in an uninterrupted systematic manner from the base to the top of the intrusion indicating that the Bushveld Complex does not represent the crystallized products of a single progressively filled and cooled magma chamber, and (3) U-Pb rutile dates constrain cooling of the intrusion at the level of the Critical Zone through ∼500 °C by 2053 Ma. The c.2060 Ma Phalaborwa Complex (pyroxenite, syenite, carbonatite + Cu-Fe-phosphate-vermiculite deposits) represents one of the earliest manifestations of widespread Bushveld-related magmatism in the northern Kaapvaal craton. The extended range and out-of-sequence U-Pb zircon dates determined for a harzburgite from the Lower Zone (c.2056 Ma), an orthopyroxenite from the Lower Critical Zone (c.2057 Ma), and orthopyroxenites from the Upper Critical Zone (c.2057–2060 Ma) are interpreted to indicate that the lower part of the Bushveld Complex developed through successive intrusions and accretion of sheet-like intrusions (sills), some intruded at different stratigraphic levels. Crystallization of the main volume of the Bushveld Complex, as represented by the thick gabbroic sequences of the Main Zone and Upper Zone, is constrained to a relatively narrow interval of time (∼1 million years) at c.2055–2056 Ma. Granites and granophyres in the roof, and a diorite in the uppermost Upper Zone, constitute the youngest igneous activity in the Bushveld Complex at c.2055 Ma. Collectively, these results contribute to an emerging paradigm shift for the assembly of some ultramafic-mafic magmatic systems from the conventional “big tank” model to an “amalgamated sill” model. The volume-duration relationship determined for magmatism in the Bushveld Complex, when compared to timescales established for the assembly of other layered intrusions and more silica-rich plutonic-volcanic systems worldwide, is distinct and equivalent to those determined for Phanerozoic continental and oceanic flood basalts that constitute large igneous provinces. Emplacement of the 2055–2060 Ma Bushveld Complex corresponds to the end of the Lomagundi-Jatuli Event, the largest magnitude positive carbon isotope excursion in Earth history, and this temporal correlation suggests that there may have been a contribution from voluminous Bushveld ultramafic-mafic-silicic magmatism to disruptions in the global paleoenvironment.

Layered Granitic Rocks at Chebucto Head, Halifax County, Nova Scotia

1975 ◽  
Vol 12 (3) ◽  
pp. 456-463 ◽  
Author(s):  
T. E. Smith
Keyword(s):  
Grain Size ◽  
Nova Scotia ◽  
Granitic Rocks ◽  
Textural Characteristics ◽  
Rock Types ◽  
Flow Sorting

Structural, mineralogical and textural characteristics of some layered granitic rocks are described. The layers result from the segregation of minerals into bands and lenses of contrasting color and grain size. They were formed near the roof of the pluton prior to final crystallization by flow sorting during episodic shearing generated during intrusion. Comparison of the mineralogy and textures of the layered rocks with those of the main rock types of the pluton shows that the differentiation of the pluton as a whole took place after the formation of the layers by interaction of felsic components concentrated in residual liquids with earlier formed crystals.

scholarly journals Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa

2015 ◽  
Vol 56 (6) ◽  
pp. 1229-1250 ◽  
Author(s):  
Ilya V. Veksler ◽  
David L. Reid ◽  
Peter Dulski ◽  
Jakob K. Keiding ◽  
Mathias Schannor ◽  
...  
Keyword(s):  
South Africa ◽  
Bushveld Complex ◽  
Critical Zone ◽  
Crystal Mush ◽  
Electrochemical Processes

Re-Os isotope and Pd/Ru variations in chromitites from the Critical Zone, Bushveld Complex, South Africa

1999 ◽  
Vol 63 (6) ◽  
pp. 911-923 ◽  
Author(s):  
Tom E. McCandless ◽  
Joaquin Ruiz ◽  
B.Ivan Adair ◽  
Claire Freydier
Keyword(s):  
South Africa ◽  
Bushveld Complex ◽  
Critical Zone ◽  
Os Isotope

La viabilité des bétons du Québec: le rôle des granulats

1986 ◽  
Vol 13 (1) ◽  
pp. 12-24 ◽  
Author(s):  
Jean Bérard ◽  
Richard Roux
Keyword(s):  
Key Words ◽  
Silica Gel ◽  
Cement Paste ◽  
Chemical Reactions ◽  
Mechanical Deformation ◽  
Granitic Rocks ◽  
Concrete Aggregate ◽  
Coarse Aggregates ◽  
Rock Types ◽  
Petrographic Study

The petrographic study of numerous concrete structures in Quebec has revealed the extent of postconstruction chemical reactions. Those reactions between the aggregates, which form 70% of the volume of the concrete, and the cement paste result in early deterioration and undesirable mechanical deformation of the structures. From this study, it appears that the coarse aggregates can react in three different ways: (a) by peripheral reaction, which is the case for massive granitic rocks, (b) by bulk swelling, as illustrated by the Potsdam orthoquartzite, and (c) by formation of silica gel veinules within the aggregate, as shown by some Trenton limestone. Considering these three mechanisms, it is easy to understand why certain aggregates react rapidly, that is within a few months, whereas others show signs of distress only after scores of years. Using the three rock types cited above, we have also tried to evaluate existing concrete standards and their capability to detect the most important pathologies encountered in concrete. Key words: concrete, aggregate, alkalis, disintegration, expansion, testing, durability.

The oxygen isotope composition of the surface crystalline rocks of the Canadian Shield

1978 ◽  
Vol 15 (11) ◽  
pp. 1773-1782 ◽  
Author(s):  
Yuch-Ning Shieh ◽  
Henry P. Schwarcz
Keyword(s):  
Isotope Composition ◽  
Crystalline Rocks ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Baffin Island ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Northern District ◽  
Basic Rocks

The average 18O/16O ratios of the major rock types of the surface crystalline rocks in different parts of the Canadian Precambrian Shield have been determined, using 47 composite samples prepared from 2221 individual rock specimens. The sampling areas include Baffin Island, northern and southwestern Quebec, Battle Harbour – Cartwright, northern District of Keewatin, Fort Enterprise, Snowbird Lake, Kasmere Lake, and Saskatchewan, covering approximately 1 400 000 km2. The granitic rocks from the Superior, Slave, and Churchill Provinces vary only slightly from region to region (δ18O = 6.9–8.4‰) and are significantly lower in 18O than similar rock types from the younger Grenville Province (δ = 9.2–10.0‰). The sedimentary and metasedimentary rocks have δ18O = 9.0–11.7‰ and hence are considerably lower than their Phanerozoic equivalents, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the original sediments. The basic rocks in most regions fall within a δ18O range of 6.8–7.6‰, except in northern and southwestern Quebec where the δ-values are abnormally high (8.5–8.9‰). The overall average 18O/16O ratio of the surface crystalline rocks of the Canadian Shield is estimated to be 8.0‰, which represents an enrichment with respect to probable mantle derived starting materials by about 2‰.

Shattered crust: how brittle deformation enables Critical Zone processes beneath southern Africa

2021 ◽  
Author(s):  
T. Dhansay
Keyword(s):  
Water Movement ◽  
Critical Zone ◽  
Upper Crust ◽  
Water Drainage ◽  
Structural Mapping ◽  
Groundwater Aquifers ◽  
Igneous Provinces ◽  
Tectonic Zones ◽  
Further Development ◽  
Earth’S Critical Zone

Abstract The delicate interplay of various Earth’s systems processes in the Critical Zone is vital in ensuring an equilibrium across the different spheres of life. The upper crust forms a thin veneer on the Earth’s surface that is defined by an interconnected network of brittle structures. These brittle structures enable various Earth System processes. Increased anthropogenic interactions within the very upper crust have seemingly resulted in a growing number of negative natural effects, including induced seismicity, mine water drainage and land degradation. Brittle structures across South Africa are investigated. These structures include various fractures and dykes of different ages and geodynamic evolutions. The orientation of these structures is compared to the underlying tectonic domains and their bounding suture zones. The orientations corroborate an apparent link between the formation of the brittle structures and the tectonic evolution of the southern African crust. Reactivation and the creation of new structures are also apparent. These are linked to the variability of the surrounding stress field and are shown to have promoted magmatism, e.g., Large Igneous Provinces, and the movement of hydrothermal fluids. These fluids were commonly responsible for the formation of important mineral deposits. The preferred structural orientations and their relationship to underlying tectonic zones are also linked to fractured groundwater aquifers. Subsurface groundwater displays a link to structural orientations. This comparison is extended to surficial water movement. Surface water movement also highlights an apparent link to brittle structures. The apparent correlation between these Earth’s systems processes and the interconnectivity developed by brittle structures are clear. This highlights the importance of high-resolution geological and structural mapping and linking this to further development of the Earth’s Critical Zone.

A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust

2021 ◽  
Author(s):  
Rais Latypov ◽  
Sofya Chistyakova ◽  
Richard Hornsey ◽  
Gelu Costin ◽  
Mauritz van der Merwe
Keyword(s):  
Bushveld Complex ◽  
Earth’S Crust ◽  
Magma Chambers ◽  
Crystallization Sequence ◽  
Igneous Provinces ◽  
Research Concept ◽  
History Of ◽  
Mineral Compositions ◽  
Earth's Crust ◽  
Chamber Floor

Abstract Several recent studies have argued that large, long-lived and molten magma chambers1–10 may not occur in the shallow Earth’s crust11–23. Here we present, however, field-based observations from the Bushveld Complex24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces26. This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’1–10 remains a viable research concept for some of Earth’s magmatic provinces.

Le massif granitique de Kongolo (Cameroun)

1950 ◽  
Vol S5-XX (4-6) ◽  
pp. 121-124
Author(s):  
J. Gazel
Keyword(s):  
Chemical Analysis ◽  
West Africa ◽  
Granitic Rocks ◽  
French West Africa ◽  
Igneous Activity

Abstract The alkaline granitic rocks of the Kongolo massif in Cameroun differ from other granites of the region. They represent post-tectonic igneous activity and exhibit striking analogies with younger granites of French West Africa and Nigeria. A chemical analysis is appended.

Determination of the characteristics of crystalline rocks by field experiments: a review

1986 ◽  
Vol 319 (1545) ◽  
pp. 139-156 ◽  
Keyword(s):  
Radioactive Waste ◽  
Field Experiments ◽  
The United States ◽  
Crystalline Rocks ◽  
Granitic Rocks ◽  
Rock Types ◽  
Basement Rocks ◽  
Deep Boreholes ◽  
High Level ◽  
Underground Research Laboratory

Crystalline rocks, particularly granitic rocks and basalts, are one of the principal rock types under consideration as a potential host rock for a high-level radioactive waste repository. Permeability in such rocks is related to discontinuities of various scales, and the quantification and prediction of groundwater flow within both the fractures and the intact rock between the fractures is the major goal of field experiments. The Canadian Underground Research Laboratory is unique in that the hydrogeological conditions within a large volume of rock surrounding the experimental shaft are being monitored before, during and after excavation and the results compared with model predictions. In Switzerland twelve deep boreholes are being drilled to over 1000 m to investigate crystalline basement rocks beneath a cover of sediments. The Stripa Mine in Sweden has hosted a major experimental programme including heater tests to stimulate the thermal effect of radioactive waste and hydrogeological tests at various scales down to individual fractures. The United States of America, the United Kingdom, France and Finland have also embarked on major experimental programmes. Continuing research is needed, with an emphasis on field experiments and research in underground rooms, to provide the data on which detailed risk assessments can be based.

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