Geochronology of a Late Archaean flood basalt province in the Pilbara Craton, Australia: constraints on basin evolution, volcanic and sedimentary accumulation, and continental drift rates
Abstract Eleven high precision (±2–5 million years) SHRIMP zircon U–Pb ages have been obtained from felsic rocks within a single stratigraphic section of late Archaean volcanic and sedimentary rocks in the east Pilbara of Western Australia. The stratigraphic succession (Nullagine and Mount Jope Supersequences in sequence-stratigraphic terminology, Fortescue Group in lithostratigraphic terminology) is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional, rift-related environment between about 2772 and 2715 Ma. The geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy. Field mapping, geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs. While flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession, felsic volcanism is now known to have been active periodically through each geotectonic cycle. The succession covers a time period of about 57 million years. The lower ∼1400 m of a thick (∼1700 m) clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years. In contrast, a younger ∼150 m thick cogenetic tuff-basalt unit accumulated in less than 3 million years, and others probably accumulated at similar rates, comparable to those of Phanerozoic flood basalts. Unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time-gap of more than 10 million years. The unconformity-based stratigraphic framework, the new geochronology and palaeomagnetic studies [J. Geophys. Res. 108 (2003) B12, 2551, EMP 2-1 to 2-21] have been combined to determine a possible late Archaean continental drift rate for one part of the succession, implying a period of motion as fast as or up to five times faster than any known from the Phanerozoic.