Emplacement Dynamics of the Alkaline Volcanic and Subvolcanic Rocks of Trindade Island, Brazil: an Anisotropy of Magnetic Susceptibility (AMS) Study.

Trindade Island is located at around 1.200 km from the Brazilian coast, composed of lava flows, intrusions and pyroclastic rocks of alkaline, SiO₂ undersaturated nature, formed between 3.9-0.25 Ma. An AMS study was performed in the island to contribute for understanding the processes involved in magma transportation and emplacement dynamics during its formation. Sixteen paleomagnetic sampling sites were set during fieldwork, of which 7 correspond to phonolitic necks and a melanephelinitic dyke that form the island basement and 9 to melanephelinitic 'a'a flows. A total of 160 oriented cylinders and 503 specimens were obtained. Magnetic mineralogy was investigated through petrography, scanning electron microscopy analyses (SEM), thermomagnetic curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORC). AMS measurements were obtained using the MFK1–FA – Multi-Function Kappabridge (AGICO) susceptibilimeter and interpreted with ANISOFT 5 (AGICO). Anisotropy of anhysteretic remanent magnetization (AARM) was run for selected sites using the LDA3-AMU1 (AGICO) with a peak AF of 100 mT and a bias field of 100 μT. Mean magnetic susceptibility (K_m) values are lower to the phonolites when compared to the melanephelinites. The degree of anisotropy (P’) vs. shape parameter (T) shows an oblate AMS fabric for the phonolitic necks, oblate/prolate fabric for the melanephelinitic lavas and prolate for the dyke. Results suggest a dominant low-coercivity magnetic mineral phase (low-Ti magnetite), normal fabric (multi-domain grains) and sub-vertical magnetic foliation for the phonolitic necks, implying in a nearly vertical magma motion during the emplacement of these bodies. The melanephelinitic 'a'a lavas and dyke present at least two magnetic mineral phases of high and low-coercivity (hematite, maghemite and titanomagnetite), and normal or inverse fabrics (pseudo-single domain or single domain grains), with mean magnetic foliation and lineation suggesting lavas flowed towards NE and the dyke was N-S oriented and vertically emplaced. Data indicates that the subvolcanic bodies were emplaced vertically during the initial stages of island formation and then lavas were mostly conditioned to northeast by the previous formed relief.