The crystalline fabrics in the border of the Pingo-Pingo pluton and in its country rocks. Cordón de Lila, Northern Chile

Abstract

The Ordovician Pingo-Pingo pluton (northern Chile) develops in its northern border a weak magmatic fabric, characterized by oriented amphibole and biotite. Due to this weak aspect, the fabric was analyzed by means of Anisotropy of Magnetic Susceptibility (AMS). Twenty sites were sampled, distributed in N-S profiles, which included the northern margin of the pluton, its metamorphic aureole, and the country rock. The AMS results show that the magnetic signal is produced by the ferromagnetic fraction: magnetite crystals included in mafic minerals (amphibole and biotite). The magmatic and magnetic foliations are concordant for the two first lithologies and are oriented subparallel to the contact between the pluton and its country rock, results that are consistent with an oblate ellipsoid. We interpret that the magnetic fabric on the metamorphic aureole reflects a local deformation that is directly related to the flattening fabric at the border of the pluton developed during the intrusion. Likewise, the lineation plunges generally to the pluton’s interior (23-63°) and seems to be consistent with a northward emplacement by lateral inflation. The latter is reinforced by the development of an E-W-oriented km-scale folding system accompanied by a vertical axial planar cleavage in the country rock, from which a ~30% of shortening during pluton emplacement is deduced.