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

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.

Chromitite layers require the existence of large, long-lived, and entirely molten magma chambers

“…The conclusion is further supported by the remarkable homogeneity of Sr isotopes over an interval of more than 2.5 km of the Upper Zone 56 , which indicates a melt column thickness in the chamber being that thick or even thicker 57,58 . This is in contrast with the recent assertion, mostly based on out-ofsequence geochronology 9,13 , that depicts this giant complex as a stack of thin crystal-rich sills 9,11,13 . Field relationships indicate, however, that zircon isotopic ages in these studies were almost certainly misinterpreted 59 .…”

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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][2][3][4][5][6][7][8][9][10][11][12][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][15][16][17][18][19][20][21][22][23] .

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“…This paradigm envisages a magma chamber as a large body of the molten, long-lived, and slowly fractionating magma ('a big magma tank') enclosed in crustal rocks [14][15][16][17][18][19][20][21][22][23] . In recent years, this classic view of a magma chamber garnered increased scrutiny from many geoscientists who contend that such long-lived and largely molten 'big tank' magma chambers are either very short-lived or never existed in Earth's history [1][2][3][4][5][6][7][8][9][10][11][12][13] . For instance, some igneous petrologists maintain that tectonic processes that create open spaces in the crust at any tectonic setting are so slow that the liquid-dominated magma bodies of any composition can hardly form 1,2 .…”

“…The transcrustal systems contain only small melt lenses that are produced by compaction 3,7 or tectonic destabilization 4,8 of the crystal mush and exist for only a very short period of time before accumulating and erupting as lavas on the Earth's surface [1][2][3][4][5][6][7][8] . Yet another group of mafic plutonists, influenced by out-of-sequence zircon geochronological data 9,10,13 , have proposed that mafic plutons do not require the existence of large magma chambers 12 . These are rather produced as a stack of randomly-emplaced sills, with successive crystal-rich pulses often invading pre-existing cumulates [9][10][11][12][13] .…”

“…Yet another group of mafic plutonists, influenced by out-of-sequence zircon geochronological data 9,10,13 , have proposed that mafic plutons do not require the existence of large magma chambers 12 . These are rather produced as a stack of randomly-emplaced sills, with successive crystal-rich pulses often invading pre-existing cumulates [9][10][11][12][13] . In stark contrast to these 'anti-magma-chamber' approaches, we argue here that the existence of large magma chambers is indicated by laterally extensive layers of chromite-rich cumulates, which require many times their own volume of magma to supply the key component, chromium (Cr).…”