Alkaline Rocks and Carbonatites of the World

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Itauna

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Occurrence number: 
025-00-080
Country: 
Brazil
Region: 
Rio de Janeiro
Location: 
Longitude: -43.05, Latitude: -22.77

Itauna covers 6 km2 and comprises a number of incomplete but concentric rings and a central plug. It is emplaced into Precambrian gneisses. Pseudoleucite syenites and microsyenites are intruded by a central core of pseudoleucite phonolite. There is also one extensive outcrop and several smaller ones of breccias containing xenoliths of the other rock types. The pseudoleucite phonolite contains, as well as pseudoleucites reaching several centimetres in diameter, rare crystals of nosean and hauyne. Analyses of pseudoleucites indicate them to have unusually potassic compositions (Valenca and Edgar, 1979). Dykes of pseudoleucite tinguaite, phonolite and trachyte occur within the intrusions.

Age: 
K-Ar on feldspar from syenite gave 67.8±2.3 Ma, feldspar from microsyenite 49.8±1.8 Ma and phonolite (whole rock) 61.2±1.9 Ma (Lima, 1976).
References: 
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. 1974. Geologia e petrografia do macico alcalino de Itauna Sao Goncalo - E. do Rio de Janeiro. Anais Congresso Brasileiro de Geologia, 28(1): 427-31. VALENCA, J.G. and EDGAR, A.D. 1979. Pseudoleucites from Rio de Janeiro State, Brazil. American Mineralogist, 64: 733-5
Fig.204 Distribution of alkaline rocks around Rio de Janeiro. Fig. 207 Itauna (after Lima, 1976, p. 217).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith