Primitive calc‐alkaline and alkaline rock types from the Western Mexican Volcanic Belt

Abstract

Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calc‐alkaline magmas. Both suites include relatively rare “primitive” types, characterized by high Mg#, Cr, and Ni contents. Primitive hypersthene‐normative basalts, parental to the calc‐alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic‐front calc‐alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic‐front locations also contain hydrous minerals. These nepheline‐normative suites include basanites, phlogopite‐bearing minettes, and other hornblende‐bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB‐normalized elemental plots, all lamprophyres show patterns similar to the calc‐alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements. The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc‐alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc‐alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite‐rich veins cutting asthenospheric peridotite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through‐crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc‐alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock.

Description of the Geology of the Tecolotlan Graben, Jalisco, Mexico

“…Minette lavas are rare in the volcanic record elsewhere, but here they are commonly found in a NW-trend along the front of the Trans Mexican volcanic belt (Fig. 1) including San Sebastian (Lange and Carmichael, 1990), Mascota Maria and Luhr, 2008), Los Volcanes (Wallace and Carmichael, 1992), Tapalpa (Righter and Rosas-Elguera, 2001), and north of Volcan Colima (Maria and Luhr, 2008;Luhr et al, 1989;Crummy et al, 2014). They form a lineament parallel to the trench and to the TepicZacoalco graben (Fig.…”

Present state of knowledge and new geochemical constraints on the central part of the Mexican Volcanic Belt and comparison with the Central American Volcanic Arc in terms of near and far trench magmas

“…The new discriminant function-based tectonic discrimination diagrams Agrawal et al, 2008;Verma and Agrawal, 2011) (Hofmann and Feigenson, 1983;Hofmann et al, 1984;McKenzie and O'Nions, 1991;Ormerod et al, 1991;Feigenson and Carr, 1993;Maaløe, 1994;Velasco-Tapia andVerma, 2001a, 2013;Feigenson et al, 2003;Verma, 2004Verma, , 2006. Basic magmas with characteristics of primitive, little-differentiated magmas are present throughout the MVB (e.g., Luhr et al, 1989;Luhr, 1997;Wallace and Carmichael, 1999;Verma, 2000aVerma, , 2002Verma, , 2004, as well as in the LTVF (Verma et al, 1993a;Nelson et al, 1995;Verma, 2006). The geochemical data from the MVB have been subjected to inverse modeling.…”

Petrogénesis ígnea de la Faja Volcánica Transmexicana

“…Los productos de actividad volcánica intraplaca constituyen ~5% del volumen total de rocas volcánicas emplazadas en el sector (Ferrari et al, 2001). Estas rocas coexisten espacial y temporalmente con rocas calcialcalinas y, en algunas localidades, con rocas alcalinas potásicas (Luhr et al, 1989b;Righter et al, 1995;Righter y Rosas-Elguera, 2001), aunque por lo general tienden a ser más abundantes hacia las porciones más alejadas de la trinchera. Las localidades de vulcanismo intraplaca que han sido descritas hasta la fecha en el sector occidental (Figura 15) se encuentran en la zona costera de Nayarit, al oeste de Tepic (Righter et al, 1995); en el graben de Tepic-Zacoalco, incluyendo al área del volcán Sangangüey (Nelson y Carmichael, 1984); en el graben San Pedro-Ceboruco (Ferrari et al, 2003b;Petrone et al, 2003), en el graben Plan de Barrancas-Santa Rosa y la región al noreste de Guadalajara (Moore et al, 1994); dentro del bloque Jalisco en el graben Amatlán de Cañas (graben de Atenguillo) (Righter y Carmichael, 1992;Ferrari y Rosas-Elguera, 2000); y fi nalmente en la región de Ayutla (Righter y Rosas-Elguera, 2001).…”