Alkaline lavas in the volcanic front of the Western Mexican Volcanic Belt: Geology and petrology of th Ayutla and Tapalpa volcanic fields

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in the volcanic front of the western Mexican Volcanic Belt (WMVB) contain a wide variety of alkaline volcanic rocks, rather than only calc-alkaline rocks as found in many continental arcs. There are three principal rock series in this region: an intraplate alkaline series (alkali basalts and hawaiites), a potassic series (lamprophyres and trachylavas), and a calc-alkaline series. Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenoliths from an augite minette lava flow have orthocumulate textures. The phlogopite-clinopyroxenite xenoliths also contain apatite and titanomagnetite and probably formed by accumulation of minerals fractionated from an augite minette more primitive than the host. The intraplate alkaline series is probably generated by decompression melting of asthenospheric mantle as a result of corner flow in the mantle wedge beneath the arc. Alkaline magmas may be common in the WMVB as a result of prior metasomatism (during Tertiary Sierra Madre Occidental magmatism) of the Mexican sub-arc mantle. Generation of the more evolved andesites and dacites of the calc-alkaline series is due to either combined assimilation and fractional crystallization (AFC) or magma mixing. The preponderance of alkaline and hydrous lavas in this region demonstrates that these lava types are the norm, rather than the exception in western Mexico, and occur in regions that are not necessarily associated with active rifting