The pyroclastic dyke and welded crystal tuff of the Morro dos Gatos alkaline intrusive complex, State of Rio de Janeiro, Brazil

Abstract

The present article reports the geologic, lithologic, and petrographic characteristics of the pyroclastic dyke and phenocryst-rich welded crystal tuff of the Morro dos Gatos alkaline intrusive body, State of Rio de Janeiro, Brazil. The intrusive complex has an extension of 1.2 x 0.8 km occupying an area of 0.5 km². It is constituted mainly by monzonite and trachyte. At the south flank of the massif, there exits a pyroclastic dyke and at the northwest flank, an intrusive welded crystal tuff. The pyroclastic dyke is 40 to 80 cm wide and intrudes into the basement porphyritic granite. This rock is constituted by welded tuff breccia with large amount of lapilli-size angular lithic fragments of trachyte. Along the contact plane of this dyke, there is an intercalation of the trachyte. The crystal tuff is characterised by abundant alkaline feldspar phenocrysts and mineral fragments of alkaline feldspar, quartz, biotite, and amphibole, showing a crystal matrix-supported texture. Large phenocrysts are aggregations of idiomorphic alkaline feldspar crystals with a general size of 2 mm and a maximum one of 6 mm. The matrix is composed mainly of mineral fragments of less than 1 mm in general size. The size-frequency diagram for the mineral fragments suggests that they have been originated from an explosive eruption. The matrix is strongly welded and completely devitrified showing a cryptocrystalline texture. The crystals were concentrated by sorting with the help of the volcanic gas ascending in the volcanic conduit during a continuous and effusive phase of the eruption. The limited special distribution of the pyroclastic rocks and small outcrop size indicate that they are not constituent of pyroclastic flow deposits, but of subvolcanic conduit and fissure.