The Colima Volcanic complex,Mexico |
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| Authors: | James F Luhr Ian S E Carmichael |
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| Institution: | (1) Dept. of Geology and Geophysics, University of California, 94720 Berkeley, California, USA |
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| Abstract: | Volcán Colima is Mexico's most historically active andesitic composite volcano. It lies 150 km north of the Middle America Trench at the western end of the Mexican Volcanic Belt, closer to the trench than any other composite volcano in Mexico. Since its earliest reported eruption in 1576, V. Colima has evolved through three cycles of activity. Each cycle culminated in a major ashflow eruption, halting activity for 50 or more years. The last major ashflow eruption occurred in 1913. Andesitic block lava eruptions in 1961–1962 and 1975–1976 marked the inception of activity in a fourth historical cycle which may also terminate with a major ashflow eruption in the early part of the next century.Major and trace element analyses of whole rock samples and all constituent phases are presented for a suite of nine post-caldera hornblende and olivine-andesites. The suite includes samples from Colima's four major eruptions since 1869, spanning the last two eruptive cycles. Colima's post-caldera andesites are poor in K and other incompatible elements (Ti, P, Zn, Rb, Y, Zr, Ba, La, Yb, Hf, Th, and U) as may be characteristic of near trench andesites. From the 1913 ashflow eruption through the fourth cycle andesites, there have been increases in whole rock abundances of Si, Ba, and Cs, and decreases in Ti, Fe, Mg, Ni, Cr, and Sc. Crystal fractionation models can closely reproduce major element variations in the post-caldera suite, but systematically fail to predict sufficient concentrations of the compatible trace elements Cr, Ni, and Zn. Anomalous enrichments of compatible trace elements in Colima's andesites probably reflect simultaneous crystal fractionation and magma mixing in the subvolcanic system.Estimated pre-eruptive temperatures range from 940 °–1,000 ° C in the hornblende-andesites and 1,030 °–1,060 ° C in the olivine-andesites. Pre-eruptive magmatic water contents of 1.0–3.6 wt.% are calculated for the hornblende-andesites; the phenocryst assemblage of the olivine-andesite is calculated to equilibrate at 1,000 bars with 0.8% H2O.Orthopyroxenes and certain clinopyroxenes in all pre-1961 samples are reversely zoned, with relatively Mg-rich rims. The most pronounced Mg-rich rims occur in the olivine-andesites and are thought to reflect pre-eruptive magma mixing, involving a basic, olivine+/-clinopyroxene-bearing magma. In addition to their normally zoned pyroxenes, the post-1961, fourth cycle andesites display a number of other features which distinguish them from earlier post-caldera hornblende-andesites of similar bulk composition. These include: (1) higher total crystal contents, (2) lower modal hornblende contents, (3) higher calculated pre-eruptive silica activities, and (4) lower calculated pre-eruptive water contents. These features are all consistent with the interpretation that the fourth cycle andesites were less hydrous prior to eruption. The slight Mg-rich pyroxene rims in pre-1961 hornblende-andesites may record late-stage, pre-eruptive increases in magmatic water content, which act to raise magmatic f
O
2 and Mg/Fe+2 ratios in the melt and in all crystalline phases. The fourth cycle andesites apparently did not experience a strong, pre-eruptive influx of water, resulting in lower magmatic water contents and normally zoned pyroxenes. |
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