stripes
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.
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 Potassium-argon ages of some alkaline rocks from southern Brazil.Thesis (unpublished), University of California, Berkeley. 145 pp.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
