Subvolcanic vent-filling welded tuff breccia of Cabo Frio Island, State of Rio de Janeiro, Brazil (Brecha soldada de preenchimento de conduto subvulcânico da ilha do Cabo Frio, RJ)

Abstract At the southwestern corner of Cabo Frio Island, State ofRio de Janeiro, Brazil, a pyroclastic body is present in an areaof 500 x 600 m, intruding into the host orthogneiss withsubvertical contact. Between the orthogneiss and pyroclasticbody, there is a trachyte intercalation, 5 to 10 m wide. Thepyroclastic rocks have a clast-matrix supported structure andthe clast size ranges from 1 mm to 40 cm. Larger than 10 cmclasts are semi-rounded and those that are less than 5 cm areangular. They are composed mainly of trachyte, subordinatelyof orthogneiss, and eventually of welded tuff. The texture isheterogeneous and no grain-size sorting or volcanic layeringof the clasts was observed. Microscopic observations haverevealed hydrothermal alteration featured by calcitedissemination, alkaline feldspar sericitization, and mafic mineraldecomposition. The matrix is filled by angular fragments ofalkaline feldspar, quartz, plagioclase, and opaque minerals,smaller than 0.6 mm. The existence of the welded tuff clastsindicates that explosive eruptions took place repeatedly. Thesubvertical intrusive contact, small exposure area, roundedclasts, heterogeneous clast size, and absence of volcanic layeringindicate that the pyroclastic rock is subvolcanic vent-fillingwelded tuff breccia, and not a constituent of a subaerial eruptivedeposit.