Emplacement dynamics of alkaline volcanic and subvolcanic rocks in Trindade Island, Brazil

Abstract

Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanent magnetization (AARM) are tools to characterize flow structures and emplacement conditions of volcanic and subvolcanic bodies, directly related to magnetic fabrics. An AMS and AARM study was performed to contribute to the understanding of the processes involved in magma transportation and emplacement dynamics of the volcanic and subvolcanic rocks of Trindade Island, and their implications on the evolution of the volcanic field. Trindade Island is located in the South Atlantic Ocean, at 1260 km to the east of the Espírito Santo state coast in Brazil. The island is composed of lava flows, intrusions and pyroclastic rocks of alkaline, SiO2 undersaturated nature, forming five geological units aged 3.9–0.25 Ma. During fieldwork, 17 sampling sites were established in the phonolitic necks and melanephelinitic dyke of the oldest unit (Trindade Complex) and in the melanephelinitic ‘a'a flows of the youngest units (Morro Vermelho, Valado and Paredão Volcano formations), totaling 160 oriented mini-cores and 504 rock specimens. The analysis of petrofabric, magnetic mineralogy, AMS and AARM diagrams suggests the dominance of a low-coercivity magnetic mineral phase, represented by low-Ti titanomagnetite/titanomagnetite in the phonolitic necks and melanephelinitic dyke. These bodies display normal fabric (multi-domain and vortex grains) and sub-vertical/high-angle magnetic foliation and lineation. The melanephelinitic ‘a'a lavas present at least two magnetic mineral phases of high and low-coercivity (titanomagnetite, maghemite and hematite), with normal, intermediate or inverse fabrics (vortex or single domain grains). Mean magnetic foliation and lineation suggest that lavas flowed to NE. Data interpretation indicates that the subvolcanic bodies were emplaced vertically during the initial stages of island formation, followed by lavas flowing to the northeast. The orientation of the dyke and lava feeders along NNW-SSE leads to the conclusion that the tensional field during the evolution of volcanism remained the same, with NW direction for σ during approximately 4 My. This study is an example for the understanding of physical processes in the evolution of world oceanic islands.