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Sierra De Chichinautzin Volcanic Field (Mexico) Turkish Journal of Earth Sciences Turkish J Earth Sci (2013) 22: 32-60 http://journals.tubitak.gov.tr/earth/ © TÜBİTAK Research Article doi:10.3906/yer-1104-9 Magmatic processes at the volcanic front of Central Mexican Volcanic Belt: Sierra de Chichinautzin Volcanic Field (Mexico) 1, 2 Fernando VELASCO-TAPIA *, Surendra P. VERMA 1 Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, Ex-Hacienda de Guadalupe, Carretera Linares-Cerro Prieto km 8, Linares, N.L., 67700, Mexico 2 Departamento de Sistemas Energéticos, Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Privada Xochicalco s/n, Col. Centro, Temixco, Mor., 62580, Mexico Received: 18.04.2010 Accepted: 27.05.2011 Published Online: 04.01.2013 Printed: 25.01.2013 Abstract: The Sierra de Chichinautzin (SCN) volcanic field is considered one of the key areas to understand the complex petrogenetic processes at the volcanic front of the Mexican Volcanic Belt (MVB). New as well as published major- and trace-element and Sr and Nd isotopic data are used to constrain the magma generation and evolution processes in the SCN. From inverse and direct modelling, combined 87Sr/86Sr and 143Nd/144Nd data, and use of multi-dimensional log-ratio discriminant function based diagrams and other geological and geophysical considerations, we infer that mafic magmas from the SCN were generated by partial melting of continental lithospheric mantle in an extensional setting. Inverse modelling of primary magmas from the SCN further indicates that the source region is not depleted in high-field strength elements (HFSE) compared to large ion lithophile elements (LILE) and rare-earth elements (REE). The petrogenesis of evolved magmas from the SCN is consistent with the partial melting of the continental crust facilitated by influx of mantle-derived magmas. Generally, an extensional setting is indicated for the SCN despite continuing subduction at the Middle America Trench. Key Words: geochemistry, subduction, extension, multi-dimensional discrimination diagrams, isotopes, inverse modelling, direct modelling 1. Introduction et al. 2007a). It has also been called a large igneous The theory of plate tectonics has provided a framework province (LIP, Sheth 2007). It comprises more than 8000 for the study of the different styles and geochemical individual volcanic structures, including stratovolcanoes, characteristics of past and present igneous activity (Stock monogenetic cone fields, domes and calderas (Robin 1996; Kearey et al. 2009). At least four distinct tectonic 1982). Uniquely, the MVB is oriented at an angle of about environments have been established in which magmas 15–20° with respect to the Middle America Trench (MAT, may be generated. These are: (a) destructive plate margin Figure 1, Molnar & Sykes 1969). In particular, in the central setting (island and continental arcs), (b) continental intra- MVB (C-MVB) continuing subduction of the Cocos plate setting (extensional and rift zones), (c) oceanic oceanic plate under the North American continental intra-plate setting (ocean islands), and (d) constructive plate and the subalkaline character of most of the lavas, a plate margin setting (mid-ocean ridges and back-arc classic subduction-related magmatic arc model has been spreading centres). However, despite deviations from the suggested as appropriate. However, several geological, conventional rigid plate hypothesis (vertical motions, geophysical and geochemical features of the C-MVB pose deformation in plate interiors or limitations on the sizes problems with this simple model and have motivated of plates; Stock 1996; Keith 2001), an unambiguous a debate about the magma genesis and origin of this petrogenetic-tectonic model, though very much needed, is controversial magmatic province (e.g., Shurbet & Cebull difficult to establish in tectonically complex zones, such as 1984; Márquez et al. 1999a; Verma 1999, 2000, 2002, 2004, the Mexican Volcanic Belt (MVB, Figure 1). 2009; Sheth et al. 2000; Ferrari et al. 2001; Ferrari 2004; The MVB is a major province, about 1000 km long and Blatter et al. 2007; Mori et al. 2009). 50–300 km wide, of Miocene to present-day volcanism A basic problem of the subduction hypothesis is related in southern Mexico (e.g., Robin 1982; Gómez-Tuena to the lack of a well-defined Wadati-Benioff zone (Pacheco * Correspondence: [email protected] 32 VELASCO-TAPIA and VERMA / Turkish J Earth Sci Figure 1. Location of the Sierra de Chichinautzin (SCN) volcanic field at the volcanic front of the central part of the Mexican Volcanic Belt (MVB). This Figure (modified from Verma 2002) also includes the approximate location of the Eastern Alkaline Province (EAP), Los Tuxtlas Volcanic Field (LTVF), and Central American Volcanic Arc (CAVA). Other tectonic features are the Middle America Trench (MAT, shown by a thick blue curve) and the East Pacific Rise (EPR, shown by a pair of dashed-dotted black lines). The traces marked by numbers 5 to 20 on the oceanic Cocos plate give the approximate age of the oceanic plate in Ma. Locations of Iztaccíhuatl (I) and Popocatépetl stratovolcanoes (P; from which crustal xenoliths were analysed by Schaaf et al. 2005), are also shown. Cities are: PV– Puerto Vallarta, MC– Mexico City, and V– Veracruz. & Singh 2010 and references therein). The volcanic front of (2009) assumed a slab temperature model to interpret the the C-MVB is about 300 km from the MAT (Verma 2009) seismic data and inferred tomography and thermal state of whereas, in spite of numerous attempts and a very dense the Cocos plate, meaning that the results from this circular seismic network, the subducted Cocos plate is seismically argument, especially the thermal regime, would depend poorly defined beyond the Pacific coast of Mexico and can directly on the basic assumptions. Futhermore, these only be traced to about 40 km depth at a distance of about authors ignored the geochemical and isotopic constraints 240 km from the trench (Pacheco & Singh 2010). Thus, for basic magmas from the C-MVB (e.g., Verma 1999, the presence of the subducted slab can only be inferred 2000, 2002, 2004; Velasco-Tapia & Verma 2001a, b). from the MAT up to about 60 km away from the C-MVB Similarly, Pérez-Campos et al. (2008) did not take into volcanic front. Recently, subhorizontal subduction has consideration these geochemical and isotopic constraints been inferred by Pérez-Campos et al. (2008), Husker & in their geological interpretation of the seismic data. Davis (2009), and Pacheco & Singh (2010) from seismic The diminution or even cessation of arc-related data obtained from a dense network. The quasi-horizontal volcanism observed in the south-central Andes has been subduction and a very shallow subducted slab (at most at related to subhorizontal subduction of the Nazca plate about 40 km depth; Figure 5 in Pacheco & Singh 2010) are (Kay et al. 1987; Martinod et al. 2010). Steeper subduction not thermodynamically favourable conditions for magma angles are commonly observed in many arcs (Doglioni generation (Tatsumi & Eggins 1993). Husker & Davis et al. 2007; Schellart 2007). For example, average slab 33 VELASCO-TAPIA and VERMA / Turkish J Earth Sci dip angles in the Tonga, Kermadec, New Hebrides and 2001; Velasco-Tapia & Verma 2001a, b), or to detachment Marianas arcs vary from about 50° to almost 90° (Schellart of the lower continental crust (Mori et al. 2009). 2005). In this context, the Sierra de Chichinautzin volcanic Unlike the south-central Andes and in spite of field (SCN, Figure 2; Márquez et al. 1999a, b; Wallace the peculiarities of subhorizontal subduction and an & Carmichael 1999; Velasco-Tapia & Verma 2001a, b; undefined Benioff zone, widespread volcanism occurs Meriggi et al. 2008) represents one of the key areas in along the entire MVB. Extrapolation of the subducted which to study the origin and evolution of the magmatism Cocos plate to greater depths, without any solid seismic within the MVB for the following reasons: (1) the SCN evidence, was proposed to overcome this problem (Pardo marks the front of the central MVB (Figure 1) and, if the & Suárez 1995; Pérez-Campos et al. 2008), although this volcanism is related to subduction, the geochemistry of all solution has already been criticized in the literature (Sheth rocks should display clear relationships with the subducted et al 2000; Verma 2009). The slab is imagined to be broken Cocos plate (see Verma 2009); (2) 14C age determinations and to plunge vertically into the mantle and, interestingly, of palaeosols and organic matter interbedded between it is done artificially, without any direct seismic evidence, SCN volcanics have always given ages younger than 40,000 after bringing it close to the volcanic front of the C-MVB years (Velasco-Tapia & Verma 2001a) and consequently, (Pérez-Campos et al. 2008; Husker & Davis 2009). the processes related to the origin of magmas could still In a magnetotelluric study of southern Mexico (two- be active beneath this area; (3) the geochemical and Sr, dimensional inversion) by Jödicke et al. (2006), fluid Nd, and Pb isotopic composition of the descending slab release from the subhorizontal subducted Cocos plate and is known in this part of the trench from previous studies consequent partial melting of the crust beneath the MVB (Verma 2000); (4) new multi-dimensional tectonic were inferred to explain the volcanism. Several questions discrimination diagrams based on log-ratio transformed remain to be answered, such as the inadequacy of a two- variables with statistically
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