Turmoil before the boring billion: Paleomagnetism of the 1880–1860 Ma Uatumã event in the Amazonian craton

Abstract

The Uatumã silicic large igneous province (SLIP) has covered about 1,500,000km2 of the Amazonian craton at ca. 1880Ma, when the Columbia/Nuna supercontinent has been assembled. Paleomagnetic and geochronological data for this unit were obtained for the Santa Rosa and Sobreiro Formations in the Carajás Province, southwestern Amazonian craton (Central-Brazil Shield). AF and thermal demagnetizations revealed northern (southern) directions with high upward (downward) inclinations (component SF1), which passes a ‘B’ reversal test, and is carried by magnetite and SD hematite with high-blocking temperature. This component is present on well-dated 1877.4±4.3Ma (U-Pb zrn - LA-ICPMS) rhyolitic lava flows, providing the SF1 key paleomagnetic pole (Q=6) located at 319.7°E, 24.7°S (A95=16.9°). A second southwestern (northeastern) direction with low inclination (Component SF2) was obtained for a well-dated 1853.7±6.2Ma (U-Pb zrn - LA-ICPMS) dike of the Velho Guilherme Suite. This component also appears as a secondary component in the host rhyolites of the Santa Rosa Fm and andesites of the Sobreiro Fm at the margins of the dike previously dated. Its primary origin is confirmed by a positive baked contact test, where a Velho Guilherme dike crosscuts the 1880Ma andesite from the Sobreiro Formation. The corresponding SF2 key pole is located at 220.1°E, 31.1°S (A95=5°) and is classified with a reliability criterion Q=7. The large angular distance between the almost coeval (difference of ~25Ma) SF1 and SF2 poles implies high plate velocities (~39.3cm/yr) which are not consistent with modern plate tectonics. The similar significant discrepancy of paleomagnetic poles with ages between 1880 and 1860Ma observed in several cratons could be explained by a true polar wander (TPW) event. This event is the consequence of the reorganization of the whole mantle convection, and is supported by paleomagnetic reconstructions at 1880Ma and 1860Ma and also by geological/geochronological evidence.