Analysis of S-C magmatic structures through magnetic fabric investigation at the Arroio do Silva Pluton, southernmost Brazil

The study of plutonic rocks is an important tool for unravelling the tectonic conditions of any area. The Arroio do Silva Pluton (ASP), in southernmost Brazil, comprises a syenite-monzonite-diorite association from Neoproterozoic post-collisional setting. Despite being well characterized with petrological and geochemical data, this association of rocks lacks a deeper understanding of its structural and geophysical aspects. ASP maps show four distinct neighbouring bodies, but concordant structural data suggest that they represent one single body in sub-surface. Regional structural analysis reveals a steeply-dipping, NW-striking magmatic foliation in three of the four ASP outcropping areas, with no indication of linear structure. Although foliation demonstrates a constant structural pattern for most areas on a regional scale, variations of the planar fabric are also observed, indicating a more complex internal geometry for the pluton. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are largely used in Earth Sciences to determine the magnetic fabric consistent with the macroscopic foliation and lineation. This work presents the results of AMS and AARM studies in a key outcrop of the ASP, aiming to compare structural and magnetic fabrics. The structures along the approximately 140m-long section were interpreted as a magmatic S-C pair. Heterogeneous progressive shearing along the NW-striking subvertical C-foliation leads to alternate zones where the S-foliation is intact and zones where it is progressively displaced. When not affected by C-planes, the S-foliation is sub-horizontal and marked by aligned 010 faces of K-feldspar. A total of 124 specimens (36 drills) were obtained from 7 sites distributed along the outcrop, located in one of the ASP central bodies. A magnetic mineralogy investigation was conducted by hysteresis loops, thermomagnetic and IRM acquisition curves. Magnetic susceptibility ranges from 0.4 to 13.3 x 10^-3 SI. Shape parameter (T) ranges from -0.581 to 0.473, and anisotropy degree (P) ranges from 1.055 to 1.107. As positive values of T (triaxial/oblate) are associated with the sub-horizontal foliation, and negative values (triaxial/prolate) with foliation interpreted as C-planes, the structural interpretation of zones where shearing was not effective preserving foliation S is coherent when compared to the AMS scalar data. No solid-state deformation is observed in ASP, so P values are not expected to vary significantly. Hysteresis loops, thermomagnetic and IRM curves point to magnetite as the main magnetic carrier. Magnetic and magmatic foliations are predominantly concordant, with minor variations. Magnetic lineation plunges at shallow angles in both structures. In the portions where sub-horizontal foliation is dominant, lineation plunges NW, while in the regions where shear was effective, magnetic lineation plunges NNW-N. This leads to the interpretation of dextral kinematics. The field structural data, together with magnetic data, indicate that the ASP registers a local transcurrent dextral shearing in the magmatic stage. From the observation of similar magnetic mineralogy between the structures, together with the absence of solid-state deformation, it is proposed that the ASP records a late stage of the transpressive event, as described for older units of this area.