Further evidence of lower Permian remagnetization in the North Patagonian Massif, Argentina International

TOMEZZOLI, R.; RAPALINI, A.E.; LOPEZ DE LUCHI, M.G.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Año: 2010 vol. 51 p. 163 - 164

0006-6729

The Paleozoic evolution of Patagonia is bracketed between two hypothesis either Patagonia is an accreted crustal block that collided with Gondwana or it is an autochthonous part of South America and Gondwana. A paleomagnetic study on accurately dated (472-476 Ma) Early Ordovician undeformed granitoids exposed in the eastern (41.5°S, 65.0°W) margin of the North Patagonian Massif was carried out, with the main goal of constraining the Early Paleozoic paleogeographic and tectonic evolution of Patagonia and its relations with Gondwana. About one hundred specimens were processed, from ten sites on the granites plus two sites on the quartzites and sandstones of the unconformably overlying Early Devonian Sierra Grande Formation. Seven out of the twelve sites, all in the granitoids, provided consistent directions of characteristic remanent magnetization  Moderate to fissural post-magmatic hydrothermal alteration affected the granitoids and led to the crystallization of magnetite and hematite after biotite.  Structural correction of paleomagnetic data from plutons was applied only at a few sites, from bedding attitudes of the Devonian or Tertiary sedimentary rocks. The paleomagnetic study failed to obtain the primary remanence. The well defined strong remanent magnetizations associated with high unblocking temperatures indicate its probably acquisition controlled by circulating high temperature fluids which developed a pervasive to localized hydrothermal alteration which led to the formation of magnetite and/or hematite after biotite. Thermal demagnetization allowed the isolation of a reversed magnetic component, consistent with a magnetization probably acquired during the Kiaman reverse superchron.  A paleomagnetic pole (PP) was computed from the mean site directions. The position of this PP (011°E, 65.0°S; A95=12°, K=24.5) on the apparent polar wander path of South America suggests that this magnetization was acquired during the Early Permian.