Alkaline Lavas in the Volcanic Front of the Western Mexican Volcanic Belt: Geology and Petrology of the Ayutla and Tapalpa Volcanic Fields

Abstract

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in the INTRODUCTION volcanic front of the western Mexican Volcanic Belt (WMVB) In a widely accepted model of subduction zones, fluidcontain a wide variety of alkaline volcanic rocks, rather than only fluxed mantle peridotite begins to melt at a depth of calc-alkaline rocks as found in many continental arcs. There are >100 km, and some of this melt rises to form the volcanic three principal rock series in this region: an intraplate alkaline series front of the arc (e.g. Davies & Stevenson, 1992; Arculus, (alkali basalts and hawaiites), a potassic series (lamprophyres and 1994). As a result, magma erupted at the volcanic front trachylavas), and a calc-alkaline series. Phlogopite-clinopyroxenite can provide information about conditions in the sub-arc and hornblende-gabbro cumulate xenoliths from an augite minette mantle and the nature of the fluids and melts being lava flow have orthocumulate textures. The phlogopite-clinopyroxenite released and generated there. The great diversity of mafic xenoliths also contain apatite and titanomagnetite and probably volcanic rocks produced in the western Mexican Volcanic formed by accumulation of minerals fractionated from an augite Belt (WMVB) (e.g. Allan et al., 1991) is due in part to minette more primitive than the host. The intraplate alkaline series different amounts and types of fluids fluxing the mantle is probably generated by decompression melting of asthenospheric wedge. However, the majority of the diverse basic mantle as a result of corner flow in the mantle wedge beneath the magmas have erupted within tectonic depressions, makarc. Alkaline magmas may be common in the WMVB as a result ing it unclear whether the unusually diverse compositions of prior metasomatism (during Tertiary Sierra Madre Occidental are an intrinsic characteristic of the subduction magmagmatism) of the Mexican sub-arc mantle. Generation of the more matism or have been amplified by extension. Lavas from evolved andesites and dacites of the calc-alkaline series is due to the volcanic front of the WMVB have been studied in either combined assimilation and fractional crystallization (AFC) the Tepic, San Sebastian, Los Volcanes, and Mascota or magma mixing. The preponderance of alkaline and hydrous lavas regions in the north (Nixon et al., 1987; Verma & Nelson, in this region demonstrates that these lava types are the norm, rather 1989; Righter et al., 1995; Carmichael et al., 1996) and than the exception in western Mexico, and occur in regions that are also in the Sayula and Colima regions in the south (Luhr not necessarily associated with active rifting. & Carmichael, 1981; Allan & Carmichael, 1984).