Alkaline Rocks and Carbonatites of the World

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Macusani

stripes

Occurrence number: 
131-00-004
Country: 
Peru
Region: 
Puno
Location: 
Longitude: -70.5, Latitude: -13.83

Two large nepheline syenite intrusions cover 100 km2. They lie within volcanics of the Mitu Group, which they metamorphose. Two much smaller masses occur some 20 km to the south. The main syenite is a coarse rock of K-feldspar, nepheline, biotite, barkevikite, augite/aegirine, sodalite and analcime. Finer grained facies are sometimes enriched in amphibole, pyroxene and biotite. Along the margins a fine grained phonolitic facies occurs and contains geodes with crystals of barkevikite, aegirine/aegirine-augite and albite. Along the northeastern margin of the intrusions an intense mylonitization affects the syenites, producing a gneissose fabric.

Age: 
Biotite separated from nepheline syenite gave 180 Ma by K-Ar (Stewart et al., 1974)
References: 
LAUBACHER, G. 1978. Estudio geologico de la region norte del Lago Titicaca. Boletim, Instituto de Geologia y Mineria, 5, Serie D: 1-122. STEWART, J.W., EVERNDEN, J.F. and SNELLING, N.J. 1974. Age determinations from Andean Peru - a reconnaissance survey. Bulletin of the Geological Society of America, 85: 1107-16
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith