Alkaline Rocks and Carbonatites of the World

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Guariba

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Occurrence number: 
025-00-014
Country: 
Brazil
Region: 
Amazonas
Location: 
Longitude: -60.1, Latitude: -8.78

This irregularly shaped intrusion, approximately 5x2.5 km, consists principally of syenites but with granites and trachytes/phonolites also occurring. The syenites comprise essentially perthite, a little oligoclase, rare quartz, biotite, plentiful hastingsite and rarer riebeckite or arfvedsonite. The granites are apparently not peralkaline, but the trachytes, which form dykes and included blocks, contain phenocrysts of K-feldspar, abundant aegirine and cancrinite, which is considered to be primary, while fine micaceous patches are thought to be after nepheline. Rare violet fluorite occurs and carbonate is also present.

Age: 
Rb-Sr on four whole rocks gave 1109-1395 Ma and a K-Ar determination on cancrinite trachyte 872±10 Ma (Leal et al. 1978).
References: 
LEAL, J.W.L., SILVA, G.H., SANTOS, D.B. dos, TEIXEIRA, W. LIMA, M.I.C. de, FERNANDES, C.A.C. and PINTO, A.do C. 1978. DNPM. Projeto Radambrasil. Folha SC.20 Porto Velho. Levantamento de Recursos Naturais, Geologia, 16: 19-184
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith