Alkaline Rocks and Carbonatites of the World

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Pao De Acucar (Fecho Dos Morros)

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Occurrence number: 
025-00-070
Country: 
Brazil
Region: 
Mata Grosso do Sul
Location: 
Longitude: -57.93, Latitude: -21.53

This intrusion forms prominent steep-sided hills on either side of the Paraguay River (see 130-00-002 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.

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 Potassium-argon ages of some alkaline rocks from southern Brazil.Thesis (unpublished), University of California, Berkeley. 145 pp.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