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/Ms vs. Bcr/Bc) defined an almost identical trend as that of the limestones from the Middle-Ordovician Ponon ´ Trehu´e 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.