Remagnetized limestones and dolostones from the Upper Cambrian La Flecha Formation, La Rioja province, Argentine Precordillera

Abstract

Representative rock-magnetic experiments and petrographic studies on remagnetized limestones and dolostones from the Upper Cambrian La Flecha Formation (Quebrada La Angostura, Precordillera of La Rioja, Argentina) carrying a pre-tectonic Permo-Triassic secondary magnetization are reported in order to improve the understanding of the pervasive Sanrafaelic overprint that reset the palaeomagnetic record of most of the Early Cambrian to Mid-Ordovician units of the Argentine Precordillera and the San Rafael Block. The main carrier of the remagnetization is determined as fine-grained magnetite of chemical origin, though three additional ferromagnetic phases were identified (pyrrhotite, hematite and goethite). A power-law model fitted to hysteresis ratio data (Mrs/Msvs. Bcr/Bc) defined an almost identical trend as that of the limestones from the Middle-Ordovician Ponón Trehué Formation (San Rafael Block, Mendoza province, Argentina) which were affected by the same regional event. Magnetic fabrics revealed by anisotropy of magnetic susceptibility and of anhysteretic remanence show mostly a pre-tectonic origin and, in some cases, a secondary overprint probably related to recent formation of magnetic minerals.