A Mid-Paleoproterozoic Apparent Polar Wander Track for the Piedra Alta Terrane, Rio de Plata Craton): Paleogeographic and Geomagnetic Implications

RAPALINI, A.E.; MARTÍNEZ DOPICO, CARMEN; SÁNCHEZ BETTUCHI, LEDA; FRANCESCHINIS, PABLO R; MILANESE, FLORENCIA N.

Chęciny

Congreso; 16th Castle Meeting New Trends on Paleo, Rock and Environmental Magnetism; 2018

The Paleoproterozoic is a fascinating period of Earth history. Inception of plate tectonics,first supercontinents, inner core nucleation, eukaryote appearance, atmospheric greatoxigenation event and other first order global processes likely ocurred during this period.Reliable and well-dated paleomagnetic data from the Paleoproterozoic is scarce, whichhampers global paleogeographic and geodynamic models. The characteristics of the EarthMagnetic field in such old times are also known very schematically due to data paucity. Inorder to contribute to a better understanding of the Paleoproterozoic paleogeographic, geodynamicand geomagnetic evolution, a systematic paleomagnetic study is being carried outon a series of numerous late to post-tectonic igneous bodies of mid-Paleoproterozoic ageexposed in the Piedra Alta terrane of Uruguay, which is considered as the core of the Rio dela Plata craton (e.g., et al. 2018 and references therein). First results from thisresearch were published by Rapalini et al. (2015) who presented a schematic apparent polarwander path for this craton for the late Rhyacian to the early Orosirian, based on three paleomagneticpoles obtained from three of these plutons. They also suggested that whencompared with coeval data from the Sao-Francisco, Guyana and West Africa cratons it supportedan unorthodox configuration of the hypothetical Atlantica continent. Available U-Pb(both SHRIMP and LA-ICPMS) from several of these igneous bodies strongly suggest thatthey were intruded in a relatively short period between ca. 2.1 and 2.05 Ga. Further paleomagneticresults on several other plutons are presented. After stepwise AF and/or thermaldemagnetization, consistent characteristic magnetic components were isolated from the Cu--Cerro Albornoz granites (2.086 Ga), Carreta Quemada Gabbro (2.086 Ga), ArroyoGraniteand GM4 pluton. Our results permit to compute individual paleomagnetic poles from each body that together with those already published from Mahoma (2.1 Ga), Isla Mala(2.076 Ga) and Soca (2.056 Ga) plutons are distributed along a simple track composed of-Paleoproterozoic but experiencing a fast displacement and rotation. Only the Tia Josefa poleseems to be an outlier of this single track. Most bodies show a unique polarity, either normalor reverse, which, according to their position along the track, suggest at least five reversalsof the Earth Magnetic Field during that time span and a dominant dipolar field. Since all reliableradiometric datings are U-Pb, precise magnetization ages depend on the cooling ratesof these bodies. Few thermo-barometric determinations suggesting shallow intrusive levelsand old Rb-Sr ages on these igneous bodies falling in the range 1.95 2.1 (with large uncertaintiesbetween 50 and 75 Ma) point to a relatively fast cooling. Comparison with coevalpoles from other blocks in South America and Africa support the unorthodox Atlantica reconstructionand point to magnetizations ages somewhat 20 to 30 Ma younger than U-Pbages. Ar-Ar datings on amphibole are under way to better constrain the magnetization agesof these plutons