A influência da mineralogia e da geoquímica na susceptibilidade magnética de granitos de tipo-A: o caso do Complexo Morro Redondo, Sul do Brasil

Abstract

The A-type granitic plutons that form the Graciosa Province, in southeastern and southern Brazil, especially the Papanduva and Quiriri Plutons, focus of this work, have proved to be important sources of information on the relationship among the magnetic susceptibility (K) and the chemical (alkaline vs. subalkaline) and mineralogical signatures of granitic rocks. These plutons, along with contemporary volcanic rocks constitute the Morro Redondo Complex that outcrops between the PR and SC states. The Papanduva Pluton is composed of peralkaline alkali-feldspar granites, with aegirine and arfvedsonite as the main mafic minerals, whereas the Quiriri Pluton comprises slightly peraluminous syeno-monzogranites, with biotite ± amphibole as the typical mafic phases. The opaque minerals are ilmenite (Papanduva) and magnetite (Quiriri). In granitic rocks, the factors controlling the K intensity include geochemistry, mineralogy and even post-magmatic events (hydrothermalism, weathering, deformation). This work aims to characterize the intensity of the magnetic susceptibility in the Papanduva and Quiriri granites, as well as to investigate the influence of the mineralogy, composition (bulk-rock and mineral chemistry), and hydrothermal alteration and weathering on the observed K variations. Individual K measurements in hand samples and outcrops reveal K values mainly between 19 – 2490 x 10-6[SI], with an average of 239 x 10-6 [SI] for the Papanduva Pluton. Some anomalously higher values around 6272 x 10-6 [SI] are locally registered in samples with large amounts of post-magmatic magnetite. On the other hand, K values for the Quiriri Pluton are mainly between 173 – 6910 x 10-6[SI], with an average of 2633 x 10-6 [SI]. This distinctive contrast favors the use of the magnetic susceptibility as an auxiliary geological mapping tool. In the Papanduva Pluton, K is positive correlated with Fe3+, Mn and Na, and it is influenced by the paramagnetic mineralogy, especially sodic clinopyroxenes. In these minerals, an increment in Fe3+ and alkalis contents towards crystalline rims (i.e. more oxidizing and peralkaline conditions) is followed by increasing K intensities. Contrarily, magnetic susceptibility in the Quiriri Pluton is essentially controlled by the ferrimagnetic mineralogy, namely magnetite. Moreover, post-magmatic events (hydrothermalism, weathering) may have played an additional influence on the K intensities in the Quiriri granites.