Alkaline Rocks and Carbonatites of the World

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Pao De Acucar (Fecho Dos Morros)


Occurrence number: 
Mata Grosso do Sul
Longitude: -57.93, Latitude: -21.53

This intrusion forms prominent steep-sided hills on either side of the Paraguay River (see Paraguay No.2 Cerro Boggiani). It is surrounded by Recent sediments, but has not been mapped in detail. It consists principally of nepheline syenite (foyaite) with aegirine-augite, biotite, amphibole and sometimes sodalite. A pyroxene syenite is thought to form the northern part of the mass, but has not been mapped. Essexite, shonkinite, phonolite and bostonite have also been described.

K-Ar on biotite from nepheline syenite gave 236±9 Ma and alkali feldspar 205±7 Ma (Amaral et al. 1967), while a K-Ar determination on phonolite gave 209±11 Ma (Comte and Hasui, 1971). Biotite from nepheline syenite gave 239.3 and 235.9 Ma and alkali feldspar 205.0 and 206.7 Ma, also by K-Ar (Bushee, 1974).
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 Potassium-argon ages of some alkaline rocks from southern Brazil.Thesis (unpublished), University of California, Berkeley. 145 pp. COMPTE, D. and HASUI, Y. 1971. Geochronology of eastern Paraguay by the potassium-argon method. Revista Brasileira de Geociencias, 1: 33-43. ECKEL, E.B., MILTON, C. and SULSONA, P.T. 1959. Geology and mineral resources of Paraguay - a reconnaissance. Professional Paper, United States Geological Survey, 327: 1-110. PUTZER, H. and BOOM, G. van den, 1962. Uber einige verkommen von alkaligesteinen in Paraguay. Geologisches Jahrbuch, 79: 423-41
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