Structural and magnetic fabric study of the Marimanha granite (Axial Zone of the Pyrenees)

Abstract

The structural and magnetic fabric study of the Marimanha granite, Axial Zone of the Pyrenees, provides new data to characterize the zonation and the internal structure of the pluton. The Marimanha granite intrudes Cambro-Ordovician clastic rocks and Silurian-Devonian limestones and slates. The zonation of the low field magnetic susceptibility, consistent with the petrological zonation of the igneous body, indicates a concentric arrangement of rock types, with more basic compositions at the external areas. This pluton is characterized by a low susceptibility, and rock-magnetic studies indicate a majority of ''paramagnetic'' samples. Magnetic foliations strike parallel to petrographic contacts and to contours of zonation of magnetic susceptibility, and show a dominant NE-SW strike, steeply dipping towards the North. Locally, in the northern border of the pluton foliation directions become perpendicular to petrographic contacts and depict sigmoidal trajectories. Magnetic lineations are characterized by the predominance of NE-SW trends with shallow plunges to the NE and SW. These foliations and lineations are parallel to the slight elongation of internal petrographic zonation. Magnetic fabric within the granitoid body and internal elliptical shape of petrographic zonation, suggest an intrusion contemporary with a transpressional regime and NNW-SSE shortening, syntectonic with the late stages of the Variscan orogeny. These results are in accordance with anisotropy of magnetic susceptibility studies of others plutons in the Pyrenees. To explain the origin of the Marimanha granite we propose magma ascent through faults at depth and emplacement by ballooning in situ at the rheological boundary between Cambro-Ordovician and SiluroDevonian metasediments.

Anisotropy of magnetic susceptibility of the Pyrenean granites

“…Currently, studies on AMS have been completed in 21 plutons, and cover almost all Pyrenean massifs (25). These studies combined have provided evidence of the syntectonic character of the granite bodies, and have allowed a new emplacement model under a dextral transpressive regime to be proposed, related to the D2 Variscan deformation phase (Antolín-Tomás et al, 2009;Auréjac, Gleizes, Diot, & Bouchez, 2004;Evans et al, 1997Evans et al, , 1998, Gleizes, Leblanc, Santana, Olivier, & Bouchez, 1998, Gleizes, Leblanc, Olivier, & Bouchez, 2001Gleizes et al, 2006;Izquierdo-Llavall et al, 2012;Leblanc et al, 1996;Román-Berdiel et al, 2004, 2006. The main structures observed in the granites are magmatic fabrics and localized shear zones with late-Variscan and Alpine ages (Carreras & Cires, 1986;Román-Berdiel et al, 2004).…”

On the reliability of AMS in ilmenite-type granites: an insight from the Marimanha pluton, central Pyrenees

“…This magmatic body was chosen for three main reasons: (1) the different facies observed in a relatively small geographic area, (2) the weak nature of the deformation, previously documented by Antolín et al (2009) by means of AMS and structural analyses and (3) its relatively small grain size that allows for a reliable orientation datasets to be obtained from standard-size thin sections. To test the correspondence between the shape anisotropy of the carriers of AMS and the actual AMS in rocks with different biotite content, we carry out a multidisciplinary study on samples from the paramagnetic Marimanha granitoid (ilmenite type) in the Pyrenees with minerals of weak SPO and crystallographic preferred orientation (CPO).…”

“…Results from the work by Antolín et al (2009) obtained from the study of 494 standard cylindrical specimens (2.5 cm in diameter and 2.1 cm long) distributed in 62 sites throughout the pluton, characterize the magnetic susceptibility as low (from 28 to 467 × 10-6 SI with an average of 203 × 10-6 SI). Those selected 11 samples cover the five groups in which sites are subdivided, depending on their bulk magnetic susceptibility and petrographic description (Antolín et al 2009). However, high-field experiments performed in an AC/DC SQUID cryogenic magnetometer to check the percentage of ferromagnetic minerals point to solely paramagnetic and diamagnetic behavior in the 11 analysed samples (fig.…”

“…Additional acquisition of isothermal remanent magnetization (IRM) and thermal demagnetization of three axes IRM (method of Lowrie et al 1990) indicate the presence of low coercivity ferromagnetic phases such as iron sulphides (or Ti-rich magnetite) or little quantity of magnetite (samples 20 of Antolín et al's (not in this paper) and 21 equivalent to CA11). Magnetic foliations show a concentric pattern, and magnetic lineations have preferred orientations between NNW-SSE and NE-SW with shallow plunges (Antolín et al 2009). 5 in Antolín et al 2009).…”

“…However, high-field experiments performed in an AC/DC SQUID cryogenic magnetometer to check the percentage of ferromagnetic minerals point to solely paramagnetic and diamagnetic behavior in the 11 analysed samples (fig. The orientation of the magnetic fabric, the structural data, the geometry of the granitoid and petrographic contacts together with the relationship between the countours of zonation of the bulk magnetic susceptibility with the host-rock structure suggest the intrusion contemporary with a transpressive regime in the late stages of the Variscan orogeny (Antolín et al 2009). Those selected 11 samples cover the five groups in which sites are subdivided, depending on their bulk magnetic susceptibility and petrographic description (Antolín et al 2009).…”