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

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. The Fortescue Group appears to have been emplaced during two intervals of potentially rapid motion of the Pilbara craton. We present the results of a magnetostratigraphic study integrated with high-precision U-Pb CA-ID-TIMS zircon geochronology aiming to quantify rates of cratonic motion and provide a continuous time series for changes in Pilbara paleogeography during these two intervals, at ~2.77 and 2.72 Ma. We collected paleomagnetic samples from 75 sites and 4 geochronology samples within a detailed context of at total of 3.5 km of stratigraphic sections measured at the dm scale. We provide 5 new or updated high-quality paleomagnetic poles for inclusion in databases tracking Precambrian cratonic motion. We resolve a minimum drift rate of 65.0±23.8 cm/a during the craton’s largest geographic displacement at ~2.77 Ma, which exceeds both Mesoarchean and modern rates of plate motion. We provide a new high-precision U-Pb zircon age of 2721.57±0.64 Ma for the Tumbiana Formation stromatolite colony, which developed as the Pilbara craton drifted from 52.8±8.7° to 32.2±8.9° paleolatitude. Although the Fortescue Group presents significant differences from its Phanerozoic counterparts and would not fit the definition of a large igneous province (LIP), it could be considered an early prototype of LIP magmatism.