Alkaline Rocks and Carbonatites of the World

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Ilha Do Monte De Trigo

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Occurrence number: 
025-00-095
Country: 
Brazil
Region: 
Sao Paulo
Location: 
Longitude: -45.78, Latitude: -23.87

The island of Monte de Trigo is 1.2x1.3 km, 90% of which is foyaite and the rest melatheralite with minor olivine gabbro, jacupirangite, teschenite and essexite. The foyaite consists of alkali feldspar, nepheline (2-11%), barkevikite, aegirine-augite and a little biotite, sodalite or analcime. The theralites are dominated by olivine (4-28%), augite (44-75%) and plagioclase (7-15%), but nepheline may reach 15% and there is variable amphibole and biotite. Dykes of microfoyaite are abundant, but phonolites, rhomb porphyries, lamprophyres and shonkinites are also represented.

Age: 
80 Ma by K-Ar on foyaite and rhomb porphyry (Amaral et al. 1967); 79.8 Ma by K-Ar on biotite from rhomb porphyry (Bushee, 1974).
References: 
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. BUSHEE, J.M. 1974. Potassium-argon ages of some alkaline rocks from southern Brazil.Thesis (unpublished), University of California, Berkeley. 145 pp. COUTINHO, J.M.V. and MELCHER, G.C. 1973. Levantamento geologico e petrografico na Ilha do Monte de Trigo (litoral norte de Sao Paulo, Brasil). Revista Brasileira de Geociencias, 3(4): 243-56
Fig. 215 Ilha do Montao de Trigo (after Coutinho and Melcher, 1973, Fig. 1).
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