Intrusion mechanism of tabular intrusive bodies of subhorizontal discordant emplacement of the Cabo Frio Island and the neighbour areas, State of Rio de Janeiro, Brazil.

Abstract

This article discusses intrusion mode and emplacement mechanism of the Early Cenozoic tabular intrusive bodies of the areas around the Cabo Frio Island, State of Rio de Janeiro, Brazil. In the studied area, three generation of tabular bodies are found: The first generation composed of lamprophyre; the second one constituted by trachyte with deuteric alteration; the third one made up of phonolite without deuteric alteration. The second generation bodies show transition from subvertical intrusion to subhorizontal emplacement: On the outcrops of low altitude, they are subvertical-discordant; at a middle height, oblique-concordant; at the top, subhorizontal-discordant. The subhorizontal intrusion is not originated from host rock structure density and unable to be justified by the traditional model of magma fracture-fill along pre-existing weak zone. By the way, the model of hydraulic fracturing caused by magma pressure can explain the above-mentioned intrusion mode. The transition from vertical to horizontal intrusion takes place at the depth in which the σ3 changes from horizontal to vertical. However, the emplacement site of the trachytic bodies, about 3 km, is too deep for this phenomenon. The intrusion and consequent volume expansion of the Cabo Frio Island syenitic body and low density of the syenitic magma could have reduced the σ vertical of the areas around the syenitic body, which made possible the subhorizontal intrusion of the trachytic bodies. Keywords: dyke, sill, weak zone, horizontal intrusion, fracture-fill, hydraulic fracturing, Cabo Frio Island.