Constraints on the Cretaceous thermal event in the Transantarctic Mountains from alteration processes in Ferrar flood basalts

K–Ar and 40Ar/39Ar incremental-heating analyses on apophyllite formed during hydrothermal alteration of volcanic rocks from the Ferrar Supergroup in North Victoria Land, Antarctica, provide strong evidence for hydrothermal events during mid-Cretaceous time. A last event has been dated at 96.7±0.6 Ma. Variable older ages between 112 and 125 Ma are interpreted as mixed ages of hydrothermal events or may be caused by disturbances of the Ar–Ar system. The Rb–Sr isotope system of the apophyllites is not applicable to dating because a large portion of the Sr is radiogenic and because of Rb-mobility in the crystal structure. Secondary mineralogy suggests a temperature for alteration between 300° and 400°C. Assuming a normal thermal gradient, this temperature implies a burial depth of about 10 km. However, there is no evidence for such a burial of the Ferrar flood basalts. Therefore, an elevated thermal gradient in mid-Cretaceous time in combination with circulating fluids is proposed for the origin of the alteration phenomena.