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 an ∼5 Myr interval from 2060 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 Myr) 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 with 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.