PALEOGEOGRAPHY AND HIGH-PRECISION GEOCHRONOLOGY OF THE NEOARCHEAN FORTESCUE GROUP, PILBARA, WESTERN AUSTRALIA

Abstract

While rates of Phanerozoic plate movements and magnetic field reversals have been well studied, little is known about such phenomena on early Earth. The ca. 2.8 – 2.7 Ga Fortescue Group on the Pilbara craton in Western Australia has been recognized as a well-preserved sequence of Archean rift volcanics thought to derive from a flood basalt province, and may have been moving rapidly across the globe at two different intervals in its depositional history. We present the results of a magnetostratigraphic study integrated with high-precision U-Pb ID-TIMS geochronology aiming to quantify rates of cratonic motion and provide a continuous time series for changes in Pilbara paleogeography during these two rapid intervals, at ~2.77 and 2.72 Ga. We provide six new or updated high-quality paleomagnetic poles for inclusion in databases tracking Precambrian cratonic motion. During the craton ’ s largest geographic displacement at ~2.77 Ga, we resolve a minimum drift rate of 23 ± 20 cm/a if there was substantial rotation of the Pilbara craton along with translational motion, and a more rapid minimum estimate of 64 ± 23 cm/a if the motion was dominated by translation; these estimates exceed both Mesoarchean and most modern rates of plate motion. We provide a new high-precision U-Pb zircon age of 2721.23 ± 0.88/0.88/6.9 Ma for the Tumbiana Formation stromatolite colony, which developed as the Pilbara craton drifted from 51.5 ± 7.0 ◦ to 32.1 ± 5.7 ◦ paleolatitude. Although the Fortescue Group has been considered an early prototype of large igneous provinces, it was emplaced over a longer duration than its Phanerozoic counterparts and does not fit at least one definition of a large igneous province (LIP). But as a potential prototype of LIP magmatism, the Fortescue succession chronicles eruptive dynamics, rapid paleogeographic changes, and a series of robustly determined magnetic field reversals during the Neoarchean.