Características geológicas, litológicas e petrográficas do Complexo Alcalino Intrusivo de Itaúna, São Gonçalo, RJ, com atenção especial no seu modo de posicionamento

This article presents field studies and microscopic observations of the felsic alkaline rocks of the Itauna Intrusive Complex, Sao Goncalo, State of Rio de Janeiro, Brazil. The rock body is constituted mainly by phonolitic and nepheline syenitic rocks and locally by volcanic breccia. The phonolitic main body is intruded by syenitic dykes of metric width. The volcanic breccia takes place only within a small area of 20 x 30 m. The clasts are composed entirely of massive trachytic or phonolitic rock of variable size, form 1 cm to 1 m, even within the same outcrop. The large clasts are semi-rounded and small ones are angular. Pisolite, bomb-sag structure, and vesicular clasts are not confirmed. No granulometric sorting and normal grading of the clasts are observed. The matrix shows welded structure with steeply dipped secondary flowage. The above-mentioned restricted occurrence area, steep secondary flowage planes, and variable clast size indicate that the pyroclastic rock is not tephra or pyroclastic flows, but vent-filling tuff breccia forming a subvolcanic conduit.