Sombreretillo

This complex is intruded into cherty and shaly limestones of Cretaceous age, and comprises a western intrusion of about 4x3 km joined by a syenite to an eastern intrusion of about 6x4 km. The western intrusion consists predominantly of gabbroic rocks of labradorite, some containing nepheline and perthite, and titanaugite/aegirine which is partly altered to a number of minerals including riebeckite. There is a little pyroxenite in which aegirine-augite sometimes rims the titanaugite. About 25% of the intrusion is diorite which at one locality is reported to contain nepheline. The eastern intrusion consists principally of nepheline syenite cut by numerous microsyenite dykes. The syenite is composed mainly of perthite with up to 15% nepheline, sometimes a little analcime and sodalite, augite with green sodic rims, biotite and an amphibole; quartz-bearing varieties are apparently also present. Aplite and pegmatite dykes close to the western intrusion contain up to 5% eudialyte, and eucolite has also been identified.