Colima

The composite Colima volcano lies at the western end of the Mexican Volcanic Belt and to the south of the Colima graben; it is historically the most active volcano in Mexico. Eruptive products are predominantly andesitic, but a number of relatively recent cinder cones to the north of the main volcanic centre include seven that are alkaline. The cones consist of bedded ash, scoria and bombs but lava flows also occur. Many of the rocks are referred to by Luhr and Carmichael (1981, p. 130) as basanites but they contain only normative and not modal nepheline. Leucite basanites do contain modal leucite and analcime is present in rocks referred to as minettes and monchiquites. All rock types contain olivine with chromite and augite followed in the basanites by rare labradorite, titanomagnetite and glass. The leucite basanites contain andesine-anorthoclase-sodic sanidine, leucite, titanomagnetite, apatite and usually Ti-F-rich phlogopite, while in the minettes phlogopite occurs as a phenocryst and groundmass phase with apatite, labradorite-anorthoclase-sodic sanidine, analcime and titanomagnetite. A detailed account of the chemistry of the rocks and minerals and their petrogenesis will be found in Luhr and Carmichael (1981). Older alkaline lavas outcrop in the graben which runs north from Colima and Allan and Carmichael (1984) have described a "phlogopite-kalsilite-ankaratrite" from about 25 km north of Colima. Phlogopite phenocrysts lie in a groundmass of diopsidic pyroxene, nepheline, kalsilite, titanomagnetite, phlogopite, analcime, zeolites and glass. Mineral and rock analyses are given. Age Based on their morphology the cinder cones have estimated ages from 1500 years to greater than 20 000 years (Luhr and Carmichael, 1981, Table 1). The alkaline lavas cropping out in the graben north of Colima give K-Ar dates from 4.6 to 1.25 Ma (Allan and Luhr, 1982).