Abstract Three dolerite dykes (Small, 3.2 cm; Middle, 13.5 cm; Thick, 50 cm) from Torsholma Island, SW Finland, reveal distinctly different internal zonation that becomes more complicated with increasing thickness of the dykes. The Small Dyke shows a systematic inward increase in normative Pl (An+Ab+Or) and a decrease in normative An (100*An/(An+Ab)), whole-rock MgO, Mg number (100*Mg/(Mg+Fe total )), TiO 2 , K 2 O and Zr. The Middle Dyke exhibits the same compositional pattern at the margins, while the centre is distinguished by an abrupt increase in normative Pl and An, whole-rock Sr and Mg number. From the margins inwards, the Thick Dyke displays first a compositional pattern identical to that observed in the Small Dyke and the margins of the Middle Dyke. This is followed by a region where whole-rock MgO, Mg number and normative An start increasing inwards, while the transition to the centre of the dyke is characterized by a compositional pattern similar to that in the centre of Middle Dyke. The origin of chemical zonation in these dykes is attributed to the operation of three independent physico-chemical processes, namely: the Small Dyke formed exclusively by progressive changes in the composition of inflowing magma; the Middle Dyke by changes in composition of inflowing magma (margins) and concentration of plagioclase and olivine phenocrysts by flow differentiation (centre); the Thick Dyke by changes in composition of inflowing magma (margins), in situ cumulate growth against dyke sidewalls (middle) and flow differentiation (centre). Systematic changes in these processes and, as a result, in internal chemical zonation, likely take place in response to crystallization of magma under less supercooled conditions with increasing dyke thickness. A comprehensive geochemical study of the internal zonation of small mafic dykes worldwide is required to develop a complete understanding of the processes operating in mafic dykes.
Abstract We describe an impressive ~55 m high outcrop from the Pilanesberg Platinum Mine open pit, located in the North-Western Bushveld Complex. The outcrop exposes the complete two-dimensional structure of three Merensky Unit potholes that cut several metres down into the underlying footwall anorthosites. The transgressive field relationships are interpreted to have resulted from thermochemical erosion of the footwall rocks by new pulses of magma replenishing the chamber and resulting in incremental growth of the Bushveld Complex.
The vertical growth rate of basaltic magma chambers remains largely unknown with available estimates being highly uncertain. Here, we propose a novel approach to address this issue using the classical Skaergaard intrusion that started crystallizing from all margins inward only after it had been completely filled with magma. Our numerical simulations indicate that to keep the growing Skaergaard magma chamber completely molten, the vertical growth rate must have been on the order of several hundreds to a few thousands of meters per year, corresponding to volumetric flow rates of tens to hundreds of cubic kilometers per year. These rates are several orders of magnitude higher than current estimates and were likely achieved by rapid subsidence of the floor rocks along faults. We propose that the Skaergaard is a plutonic equivalent of supereruptions or intrusions that grow via catastrophically rapid magma emplacement into the crust, producing totally molten magma chambers in a matter of a few months to dozens of years.
Abstract A recent re-interpretation of the Bushveld Complex and other layered intrusions as stacks of randomly emplaced, amalgamated sills is mostly fuelled by finding of zircon ages that are not getting progressively younger from the base upwards, as expected from a classical model for the formation of layered intrusions. Rather, they display several reversals from older to younger ages and vice-versa with moving up-section through the layered intrusions. Here, we show that the reported zircon ages are at odds with the relative ages of rocks as defined by cross-cutting relations in potholes of the Bushveld Complex. This indicates that interpretation of the zircon isotopic data as the emplacement age of the studied rocks/units is incorrect, making a new emplacement model for layered intrusions baseless. This conclusion is further buttressed by the phase equilibria analysis showing that regular cumulate sequences of layered intrusions are not reconcilable with a model of randomly emplaced sills. In this model, the late sills are free to intrude at any stratigraphic position of the pre-existing rocks, producing magmatic bodies with chaotic crystallization sequences and mineral compositional trends that are never observed in layered intrusions. There are thus no valid justifications for the re-evaluation of the current petrological model of the Bushveld Complex and other layered intrusions as large, long-lived and largely molten magma chambers. A fundamental implication of this analysis is that the current high-precision U-Pb TIMS ages from layered intrusions are inherently unreliable on the scale of several million years and cannot therefore be used for rigorous estimations of the timing of crystallization, duration of magmatism, and cooling of these intrusions.
Abstract An emerging and increasingly pervasive school of thought is that large, long-lived and largely molten magma chambers are transient to non-existent in Earth’s history 1–13 . These ideas attempt to supplant the classical paradigm of the ‘big magma tank’ chambers in which the melt differentiates, is replenished, and occasionally feeds the overlying volcanoes 14–23 . The stratiform chromitites in the Bushveld Complex – the largest magmatic body in the Earth’s crust 24 – however, offers strong contest to this shifting concept. Several chromitites in this complex occur as layers up to 2 metres in thickness and more than 400 kilometres in lateral extent, implying that chromitite-forming events were chamber-wide phenomena 24–27 . Field relations and microtextural data, specifically the relationship of 3D coordination number and grain size, indicate that the chromitites grew as a 3D framework of touching chromite grains directly at the chamber floor from a melt saturated in chromite only 28–30 . Mass-balance estimates dictate that a 1 to 4 km thick column of this melt 26,31,32 is required to form each of these chromitite layers. Therefore, an enormous volume of melt (>1,00,000 km3) 24,25 must have been involved in the generation of all the Bushveld chromitite layers, with half of this melt being expelled from the magma chamber 24,26 . We therefore argue that the very existence of thick and laterally extensive chromitite layers in the Bushveld and other layered intrusions strongly buttress the classical paradigm of ‘big magma tank’ chambers.
Research Article| July 01, 2017 A triple S-shaped compositional profile in a Karoo dolerite sill—Evidence of concurrent multiple fractionation processes R.G. Cawthorn; R.G. Cawthorn * 1School of Geosciences, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa *E-mail: grant.cawthorn@wits.ac.za Search for other works by this author on: GSW Google Scholar S.Y. Chistyakova; S.Y. Chistyakova 1School of Geosciences, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa Search for other works by this author on: GSW Google Scholar R.M. Latypov; R.M. Latypov 1School of Geosciences, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa Search for other works by this author on: GSW Google Scholar V.S. Kamenetsky; V.S. Kamenetsky 2School of Earth Sciences, University of Tasmania, Locked Bag 1362, Launceston, Tasmania, Australia Search for other works by this author on: GSW Google Scholar L.V. Danyushevsky L.V. Danyushevsky 2School of Earth Sciences, University of Tasmania, Locked Bag 1362, Launceston, Tasmania, Australia Search for other works by this author on: GSW Google Scholar Geology (2017) 45 (7): 603–606. https://doi.org/10.1130/G38917.1 Article history received: 23 Dec 2016 rev-recd: 07 Mar 2017 accepted: 07 Mar 2017 first online: 28 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R.G. Cawthorn, S.Y. Chistyakova, R.M. Latypov, V.S. Kamenetsky, L.V. Danyushevsky; A triple S-shaped compositional profile in a Karoo dolerite sill—Evidence of concurrent multiple fractionation processes. Geology 2017;; 45 (7): 603–606. doi: https://doi.org/10.1130/G38917.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Thick dolerite sills show a range of vertical geochemical variation trends attributable to various processes during slow crystallization. We have identified chemical parameters in a 169-m-thick sill from the Karoo igneous province in South Africa that define three different lower crossover levels (maximum or minimum concentrations) creating S-shaped variation trends. The crossover level for whole-rock MgO is at 20 m height (due to mechanical sorting of olivine); the anorthite content of plagioclase is at 52 m (due to addition of primitive magma); and that of the incompatible trace elements is at 75 m (due to different proportions of early formed grains to trapped liquid). Each process can operate independently and concurrently, leading to their maximum effects occurring at different levels in the intrusion. The independence of these processes and the triple S-shaped geochemical profiles have not been recognized before in any mafic-ultramafic sills. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Several recent studies have argued that large, long-lived and molten magma chambers 1–10 may not occur in the shallow Earth’s crust 11–23 . Here we present, however, field-based observations from the Bushveld Complex 24 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 km 3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history 25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces 26 . 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.
Most of the world's economically-viable platinum deposits occur as 'reefs' in layered intrusions - thin layers of silicate rocks that contain sulphides enriched in noble metals. There are two contrasting magmatic hypotheses for their formation. The first suggests accumulation through gravity-induced settling of crystals onto the magma chamber floor. The alternative argues for in situ crystallization, i.e. upward growth from the floor. Here we report on our discovery of the Merensky Reef in the Bushveld Complex that occurs on subvertical to overturned margins of depressions in a temporary chamber floor. Such relationships preclude crystal settling and demonstrate that the reef crystallized in situ. This finding indicates that platinum deposits can grow directly at the chamber floor, with immiscible sulfide droplets sequestering ore-forming noble metals from strongly convecting silicate magmas. Our model also provides evidence for the paradigm that argues for magma chambers being masses of nearly crystal-free melt, which gradually loses heat and crystallizes from the margins inward.
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