Macusani

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.