Magnetic fabrics of the neoproterozoic piquiri syenite massif (Southernmost Brazil): Implications for 3D geometry and emplacement

摘要

The study of magnetic fabrics and rock magnetic properties, together with geological and structural mapping, was carried out in a syenite pluton to investigate its shape and emplacement history. The Piquiri Syenite Massif (PSM) is an alkaline pluton which exhibits S > > L magmatic fabric and is interpreted to be part of the last Neoproterozoic post-collisional magmatic episodes in southernmost Brazil. Thermomagnetic curves, hysteresis data and coercivity spectra obtained from representative samples of different facies in the massif reveal that magnetic susceptibility is dominated by ferromagnetic minerals, especially magnetite. Magnetic fabric data were determined by using Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Anhysteretic Remanence (AARM). Both fabrics are coaxial, and the parallelism of AMS and AARM tensors in more than 84 % of the sampled sites rules out the possibility of significant effects of Single Domain (SD) crystals. The magnetic foliation is concordant with the magmatic foliation field measurements, both parallel to pluton contacts, with high, inward dip angles. The magnetic lineation shows distinct but related behaviour from one facies to another. It is dominantly subvertical in the marginal fades rocks and plunges at moderate to shallow angles in the main facies. It is sub-horizontal in the quartz-syenites and plunges at shallow angles in the granitic rocks. Oxidizing conditions determined from the study of magnetic mineralogy leads to challenge former interpretation of in situ differentiation and crystallization and points to the multi-intrusive character of the pluton. Field relations such as fragments of marginal facies rocks found within the main facies rocks, which are in turn intruded by quartz-syenites, together with the general absence of contact metamorphism except near the marginal facies, lead to interpret that a sequence of magmatic pulses have built up the pluton. Thus, a first magmatic pulse may have heated the host rocks and resulted in the marginal facies which was followed by the next pulses to form the main fades and the quartz-rich varieties, therefore constructing the pluton from outside inwards.