Paleomagnetic Pole for the Neoproterozoic dikes of the Nico Pérez terrane (Uruguay) and the Apparent Polar Wander Path (APWP) for the Río de la Plata craton

ANA LOSSADA; AUGUSTO RAPALINI; LEDA SÁNCHEZ BETUCCI

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2010 vol. 51

0006-6729

Neoproterozoic basic dikes intrude the Paleoproterozoic basement rocks of the Nico Pérez terrane,in central Uruguay. They are sub-vertical, trend broadly E-W, are up to 10 meters thick and can befollowed in the field for tens to hundreds of meters. The general texture is porphyritic withphenocrysts of plagioclase, inmerse in an intersertal groundmass consisting of plagioclase,clinopyroxene and intersticial glass. Opaque minerals represent a minor constituent. Samples arefresh, with very low grade of alteration and no evidence of metamorphism. The age of the dikes ispoorly established, having been reported one K/Ar data that indicates an age of 581±13 Ma.One hundred and five (105) oriented samples, distributed into seven (7) sites, were collected in thesurroundings of Zapicán (Lavalleja department, Uruguay, 33º31´S, 55º56´W). Anisotropy of magneticsusceptibility (AMS) results suggests subhorizontal flow direction, while isothermal remanent magnetization (IRM) acquisiton curves indicate a ferrimagnetic phase as the carrier of magnetization.Samples were submitted to AF and thermal desmagnetization, with typical desmagnetization steps of0, 3, 6, 9, 12, 15, 20, 25, 30, 35, 40, 50, 60, 80, 100 and 120 mT, and 0, 100, 150, 200, 250, 300, 350,400, 450, 500 and 550ºC. All samples presented high stability of magnetization with unblockingtemperatures close to 550ºC, which indicates the presence of magnetite or low-Ti titanomagnetite asthe principal mineral carrier of the magnetization. Consistency of the isolated magnetic componentswas found in five sites, three of which showed normal and two reverse polarities. The mean of fivevirtual geomagnetic poles (VGPs) is located at 9.9ºS, 262.7ºE (A95=14.1º). The position of thispreliminary paleomagnetic pole for the Nico Pérez terrane is consistent with the apparent polarwander path (APWP) for the Río de la Plata (RP) craton for the Ediacaran-Cambrian interval, and itslocation on the curve suggests an age for the intrusion at sometime between 580 and 530 Ma,consistent with the scarce geochronologic data. This PP is also consistent with coeval poles from otherGondwanan blocks of ca. 550Ma, suggesting that RP and several other plates were already assembledin a Gondwana arrangement by that time.Over one hundred and thirty oriented samples from thirteen (13) sites were recently collected in thesame region for a more detailed paleomagnetic study. These and an Ar-Ar dating of these basalts,which is under way, will help in confirming or modifying these preliminary paleomagnetic results