Petrology and geochemistry of stage-I andesites and dacites from the caldera wall of Vol can Colima, Mexico

Abstract

Major and trace element concentrations and modes are reported for 10 andesitic and dacitic lavas from stage-1 activity at Volcan Colima. Nine of these samples were collected from the walls of the summit caldera, formed by one or more major, Mount St. Helens-type collapse events during the Holocene. These data are contrasted with data for lavas erupted during stage II, following formation of the caldera, and arc also discussed in the light of all available analyses for the southward-younging volcanic chain that runs from Volcan Cintaro in the north, through Nevado de Colima, to Volcán Colima in the south. Both the stage-I and stagc-11 lavas of Volcán Colima are significantly richer in Si02 than contemporaneous seoriae erupted during the Holocene. Stage-lavas range to higher Si02 values than stagc-11 lavas, however, a pattern that is also evident in the evolution of magma compositions with time at Nevado. The lavas from Volcán Cantaro are relatively enriched in K20, Sr, La, Ce, and Sm compared to all samples from stage-II activity at Yo lean Colima, but four of the stage-i lavas discussed in this paper also show these enrichments. Compared to the stage-11 Volcán Colima lavas, those from both stage I and from Volcan Cantaro have lower Yb and Lu abundances and higher La/Yb and Sr/Yb values. These features probably reflect a relatively greater role for residual garnet in the source regions for the earlier magmas, which would act to retain the heavy rare earth elements. The lavas from Volcán Cantaro and the stage-I lavas from Voldn Colima also show relatively lower Rb/Sr values than the stagc-U lavas. This difference could reflect a shift with time away from a mantle source containing amphibole or any other mineral that could retain Rb upon melting. The preferred interpretation for the transition to higher Yb, Lu, and Rb/Sr and lower La/Yb and Sr/Yb following caldera formation at Volcán Colima is a shift in the relative contributions of source components to the magmas, with a decrease in melting of the garnet-amphibole-bearing subductcd slab, and an increase in melts derived from the overlying mantle wedge.