Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Rio Bonito


Occurrence number: 
Rio de Janeiro
Longitude: -42.6, Latitude: -22.67

Rio Bonito is a zoned stock of some 28 km2 emplaced in Precambrian gneisses and granulites near to the Tangua and Saorinho intrusions (Fig. 2_206). Slightly undersaturated to saturated pulaskites, umptekites and quartz syenites surround and grade into a central core of nepheline syenites, in which sodalite and analcime occur. Amphiboles of the katophorite-Mg-katophorite series are of greater abundance than aegirine/aegirine-augite and biotite in the outer parts of the intrusion, but pyroxene predominates in the nepheline syenites. The colour index decreases inwards and upwards. Dykes of trachyte, phonolite and minette cut the intrusion and surrounding gneisses. There are rare magmatic breccias.

Fluorite-rich veins occur.
68 and 70.5 Ma by K-Ar on alkali feldspar from nepheline syenite (Amaral et al. 1967).
AMARAL, G., BUSHEE, J., CORDANI, U.G., KAWASHITA, K. and REYNOLDS, J.H. 1967. Potassium-argon ages of alkaline rocks from southern Brazil. Geochimica et Cosmochimica Acta, 31: 117-42. LIMA, P.R.A. dos S. 1976. Geologia dos macicos alcalinos do estado do Rio de Janeiro. Semana de Estudos Geologicos, Itaguai: 205-59. VALENCA, J.G. and KLEIN, V.C. 1984. Complexos alcalinos situados a leste da Baia de Guanabara, Rio de Janeiro. Anais Congresso Brasileiro de Geologia, 33: 5317-33
Fig. 1_204 Distribution of alkaline rocks around Rio de Janeiro. and Fig. 1_206 Saorinho, Rio Bonito and Tangua (after Lima, 1976, p. 220).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith