Minette lavas and associated leucitites from the western front of the Mexican Volcanic Belt: petrology, chemistry, and origin

Abstract

During the late Pliocene, K-rich minette and leucitite lavas erupted in the western Mexican Volcanic Belt near the town of Los Volcanes, a region which is located much closer to the Middle America Trench than the main line of currently active andesite stratovolcanoes. During this period the tectonic regime in western Mexico was highly complex due to the simultaneous occurrence of active subduction of the young Rivera Plate, and rifting caused by crustal extension. Most of these basic lavas contain phenocrysts of phlogopite, augite and apatite, along with microphenocrysts of leucite and Fe-Ti oxide. Olivine is absent from all but two of the flows: one an olivine leucitite, and the other a felsic minette. The phlogopite phenocrysts and high whole rock Fe2O3/FeO ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities. All of these rock types are highly enriched in the large ion lithophile and light rare earth elements, with Sr≤5100 ppm, Ba≤4800 ppm and Ce≤330 ppm. They are also mildly enriched in the high field strength elements (e.g. Zr 260–700 ppm) and display the strong relative enrichment of the LILE over the HFSE that is characteristic of magmas erupted in convergent margins. Consideration of high pressure phase equilibria in the Mg2SiO4-CaMgSi2O6-KAlSiO4-SiO2 system shows that the minettes from this region are not related through fractional crystallization to the more MgO-rich, olivinebearing minettes which have erupted in other parts of the western Mexican Volcanic Belt during the Quaternary. This conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models. Instead, the data suggests that these high-K magmas were derived from a source region which consists predominantly of phlogopite, clinopyroxene and apatite, and which has formed through hydrous enrichment of the subarc mantle in response to subduction.