The Colima volcanic complex,Mexico:III

A 34 meter section of ash- and scoria-fall units has been studied on the upper NE flank of the active Volcán Colima. Charcoal and soil horizons are restricted to the topmost 12 m. Nine ¹⁴C dates show a smooth progression from 235 years near the surface to 8,300 years at 10 meters depth, indicating a post-Pleistocene accumulation rate of 1.3 m/1,000 years at 6 km from the vent. This figure allows an estimate of the magmatic eruption rate for fallmaterial of 0.31 km³/1,000 years, less than 15% of Colima's lava eruption rate. An unusually thick and coarse-grained scoria-fall horizon at about 4 m depth in the section appears to have been produced by the caldera-forming eruption of Colima some 4,300 years ago.The majority (40) of 46 analyzed scoria and ash horizons are typical Colima hornblende-andesites with an average SiO₂ content of 58%, nearly identical to the scoriae of Colima's 1913 pyroclastic flows. The scoriae are significantly more basic than Colima's andesitic lavas, which average 61% SiO₂. The six remaining scoria horizons are anomalously alkalic and rich in incompatible elements. Five coarse alkaline scoria layers occur in sequence just below the 8,300 year level. They show progressive upward increases in K, P, Ba, Sr, Zr, La, Ce, and related elements, culminating in a phlogopite-bearing scoria horizon.Over the last 20,000 years or so, a group of cinder cones erupted 20–35 km to the north of Volcán Colima, producing basic alkalic magmas including basanites and phlogopite-bearing minettes. The alkaline scoriae of the studied sections probably record pre-eruptive injections of minette magma into the subvolcanic, calc-alkaline system of V. Colima. The age, composition, and mineralogy of the alkaline scoriae are consistent with this interpretation. Least squares mixing models suggest 60 wt.% minette component in the phlogopite-bearing horizon.