Establishing the age and duration of magmatism in large open-system layered intrusions from the high-precision geochronology of the Neoarchean Stillwater Complex and Paleoproterozoic Bushveld Complex

The Neoarchean Stillwater Complex (Montana, USA) and the Paleoproterozoic Bushveld Complex (South Africa), two of the world’s largest layered intrusions, have been cornerstones for the study of magmatic processes in the Earth’s crust. Mafic layered intrusions are natural laboratories for assessing the emplacement, crystallization, and cooling mechanisms of mantle-derived basaltic magmas. Most layered intrusions do not yet have robust geochronological frameworks from the base to the top of their stratigraphic successions. Zircon is recognized as a relatively common accessory mineral in the Stillwater and Bushveld intrusions and crystallized from highly fractionated interstitial melt at near-solidus temperatures (980-720°C). High-precision geochronologic frameworks established for both intrusions by U-Pb zircon dating, combined with trace element and hafnium isotope compositions of zircon, reveal extended durations of magmatism (3-5 million years) and non-stratigraphic or out-of-sequence ages for both intrusions. Dating of platinum group element deposits in both intrusions (J-M Reef, Stillwater; Merensky Reef, Bushveld) indicates that they are intrusion-wide time markers that crystallized synchronously over large distances (>300 km, Bushveld). The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for assessing the timing and duration of mineralization processes in layered intrusions worldwide. Zircon from a thick anorthosite horizon in the Stillwater Complex has been identified as a reference material for U-Pb geochronology of Archean rocks (>2.5 Ga) and fills an important gap in the geologic timescale for the application of precise and accurate U-Pb geochronology. Collectively, the dating results indicate that both the Stillwater Complex and Bushveld Complex do not represent the products of progressively crystallized magma chambers but instead formed as stacks of amalgamated sills representing repeated injections of magma at different stratigraphic levels. These conclusions call into question current concepts regarding the origin of layered intrusions and challenge us to rethink our understanding of the timescales of magma processes throughout Earth history.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat