Further evidencies of lower Gondwanic Permian remagnetization in the North Patagonian Massif, Argentina

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

GONDWANA RESEARCH

ELSEVIER SCIENCE BV

Año: 2010 p. 1 - 15

1342-937X

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:none; mso-layout-grid-align:none; punctuation-wrap:simple; text-autospace:none; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> The southwestern Gondwanaland margin was located between the South America and the Patagonia during the late Palaeozoic. A dispute has taken place in the recent years that argues whether Patagonia is an accreted crustal block that collided with Gondwana during the paleozoic times or is an authochtonous part of South America. Widespread Late Paleozoic igneous activity in northern Patagonia, commonly known as North Patagonian Massif, (NPM) may provide significant scientific targets to test these hypotheses. A paleomagnetic study of the Punta Sierra granite and related granitoids exposed in the eastern margin of the area (41.5°S, 65.0°W) was carried out, with the main goal of obtaining reliable paleomagnetic poles to constrain the paleogeographic and tectonic evolution of Patagonia and the southern boundary of Gondwana. These rocks have recently yielded U-Pb crystallization ages between 472-476 Ma. About one hundred specimens were processed, comprising ten sites on the granite and two sites on the quartzites and sandstones of the Early Devonian Sierra Grande Formation, with five cores each. Thermal demagnetization allowed the isolatation of a reversed magnetization, consistent with magnetization probably acquired during the Kiaman reverse superchron. Structural correction of paleomagnetic data from plutons was applied only at a few sites, from bedding attitudes of the Devonian or Tertiary sedimentary rocks. Seven out of twelve sites, in the granites, provided consistent directions of characteristic remanent magnetization. A paleomagnetic pole (PP) was computed from the mean site directions. The position of this PP on the apparent polar wander path of South America is at: 011°E, 65.0°S; A95=12°, K=24.5 suggesting that this magnetization was acquired during the Early Permian. This result is consistent with other coeval poles from South America. A remagnetization during the Early Permian has been already reported on some outcrops of the sedimentary Devonian Sierra Grande Fm. in the same area. Our data suggest that the remagnetization was pervasive and affected the Ordovician granitoids as well. It is noteworthy that many South American Lower Permian PP were derived from syntectonic magnetizations (e.g. Ponón Trehue; Alcaparrosa, Tunas I PP; Cochico PP; Río Curacó PP; and Sierra Chica PP, among others) along the Gondwanides belt. This deformation assigned to theSan Rafaelic orogenic phase dated at approximately 290 Ma, have been linked to remagnetization of a regional scale mainly recognized in the western areas of Argentina. Time coincidence of remagnetizations suggests that there could be a causal link for all of them including those in the NPM. The similar position of the PPs all along the Gondwana margin with the Punta Sierra PP located in the Nordpatagonian terrane evidences the proximity between both margins as part of the same continent block for the upper Paleozoic time.