Gravimetric inversion and three-dimensional geological modelling of the Piquiri Syenite Massif, southern Brazil

Abstract

The Pelotas Batholith corresponds to the eastern margin of the Dom Feliciano Belt in southernmost Brazil. It comprises multiple intrusions formed by successive tectonic-magmatic processes during the Brasiliano / Pan-African Cycle. One of these intrusive igneous bodies is the Piquiri Syenite Massif (PSM), recently described as a multi-intrusive body formed by three successive pulses, dated by LA-MC-ICP-MS (U-Pb in zircon), with the oldest pulse located at the margins and the youngest in the centre of the intrusion. Pulse 1 (609.3 ± 1.5 Ma) comprises fine- to medium-grained equigranular syenite to quartz syenite and alkali-feldspar quartz syenite, with colour index M′15–30. Pulse 2 (603.4 ± 3.9 Ma) comprises medium- to coarse-grained equigranular alkali-feldspar syenite and alkali-feldspar quartz syenite, with colour index M′ 5–15 and fragments of Pulse 1 varieties. Pulse 3 (588.8 ± 3.1 to 583.2 ± 1.8)comprises medium- to coarse-grained inequigranular quartz syenites, with colour index M′ 2–10, containing fragments of pulses 1 and 2 varieties. As defined by the AMS data, the PSM magnetic fabric is concordant with the magmatic fabric, parallel to the outer edges of the body, dipping towards the centre. However, the construction and emplacement of the body have been the subject of different interpretations. The main objective of this work is to relate the general morphology of the massif and the behaviour of the pulses within the massif with the processes of formation and ascent of the magmas that have built it. For this, terrestrial geophysical surveys were carried out to obtain gravimetric data, with which the Bouguer Anomaly map was generated. Modelling by gravimetric inversion was carried out in nine profiles using tools from the Oasis Montaj software, which was the basis for constructing a three-dimensional geological model using the Leapfrog Geo software. Associating the geological model with the magmatic and magnetic foliations, including the magnetic lineation, it was possible to determine a change in behaviour between the youngest and the two oldest pulses related to the ascension process. In addition, the body’s general shape, with the greatest depths located in the eastern region, allowed us to relate the rise of magma to a structure in the region.