Zircon geochronology constraints on the geometry and solidification timescales of the Dufek layered mafic intrusion, Antarctica

Abstract

Abstract The ~ 182 Ma Dufek Intrusion of Antarctica, associated with the Ferrar Dolerite Province, is a large layered mafic intrusion that is estimated to be similar in size to the 8–9 km thick Bushveld Complex of South Africa. Similar to the Bushveld Complex, the Dufek Intrusion displays a consistent stratigraphic progression of fractional crystallization all the way to the roof of the intrusion and contact with overlying granophyre. In this study, we test competing hypotheses for the ‘big tank’ vs. ‘out-of-sequence emplacement’ models for layered intrusions by presenting high precision CA-ID-TIMS zircon U–Pb ages, zircon εHf, and zircon trace element compositions of 10 samples from across the two outcrop exposure regions (the upper Forrestal Range and lower Dufek Massif) that represent the Dufek Intrusion stratigraphy. Our results suggest that the majority of zircon found in the Dufek Intrusion, and by inference many other layered mafic intrusions, is crystallized from the interstitial melt within the cumulate mush and cannot be used to reconstruct emplacement style or high temperature crystallization of the original magma. Cooling durations to the 800°C isotherm recorded by the zircons are consistent with 1D thermal modeling of conductive cooling of a single 6–7 km thick magma chamber emplaced in the shallow crust, but cannot rule out their formation as two separate intrusions as previously proposed on the basis of geophysics. Given the similarity of ages, geochemistry, and outcrop dip, we propose that the Forrestal Range and Dufek Massif exposures originally formed as a single magma chamber, but are now separated by a fault buried beneath the Sallee Snowfield, with limited missing stratigraphy between them.