Geochemical and petrological insights into the tectonic origin of the Transmexican Volcanic Belt

    Geochemical and petrological insights into the tectonic origin of the Transmexican Volcanic Belt Arturo G´omez-Tuena, Laura Mori, Susanne M. Straub PII: DOI: Reference:

S0012-8252(16)30463-9 doi: 10.1016/j.earscirev.2016.12.006 EARTH 2356

To appear in:

Earth Science Reviews

Received date: Revised date: Accepted date:

20 April 2016 7 December 2016 8 December 2016

Please cite this article as: G´omez-Tuena, Arturo, Mori, Laura, Straub, Susanne M., Geochemical and petrological insights into the tectonic origin of the Transmexican Volcanic Belt, Earth Science Reviews (2016), doi: 10.1016/j.earscirev.2016.12.006

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Geochemical and petrological insights into the tectonic

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origin of the Transmexican Volcanic Belt

Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230,

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Arturo Gómez-Tuena1*, Laura Mori2 and Susanne M. Straub3

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Mexico

Facultad de Ingeniería, Universidad Nacional Autónoma de México, Mexico City 04510,

Lamont Doherty Earth Observatory of Columbia University, Palisades NY 10964, U.S.A.

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Mexico

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*Corresponding author: [email protected]

Abstract The Transmexican Volcanic Belt (TMVB) is the magmatic expression of one of the most complex convergent margins on the planet, and as such constitutes a prime location for testing emerging hypotheses on arc magma genesis and its influence on continental crust formation. By coupling an extensive geochemical and petrological database with an improved stratigraphic and geophysical framework, in this contribution we will examine

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ACCEPTED MANUSCRIPT the compositional diversity of the TMVB from the perspective of changes in subduction zone geometry and crustal thickness, as well as within the context of more subtle tectonic

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processes such as lithospheric foundering, slab detachment, fore-arc subduction erosion,

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crustal relamination and diapiric exhumation. We will illustrate that the compositional variability of mafic magmas across the region is an inherited characteristic of a

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geochemically enriched pre-subduction background mantle wedge, which has been variably

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overprinted by diverse chemical fluxes released from the slab at different thermal conditions. We will argue that the volumetrically dominant intermediate magmas in Mexico

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—from andesite to dacite and even some rhyolite— represent primary melts from hybrid slab and mantle sources, with no perceptible compositional influences from the overlying

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continental crust. These interpretations depart from conventional models that invoke intracrustal differentiation and contamination of basalt to create intermediate magmas, and

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Keywords

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therefore have important implications to understanding the genesis of global continents.

Subduction, mantle, continental crust, arc magmatism, Mexico, Transmexican Volcanic Belt.

1. Introduction The Transmexican Volcanic Belt (TMVB) is one of the most compositionally diverse magmatic arcs on Earth and as such constitutes a prime location for testing emerging

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ACCEPTED MANUSCRIPT hypotheses on arc petrogenesis and tectonics, with implications to our current understanding of the construction and evolution of continents. Like in any other continental

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arc, calc-alkaline andesites are the most ubiquitous byproduct, but the TMVB has a

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surprising diversity of mafic and felsic volcanism that encompasses a great number of the known terrestrial varieties: from olivine tholeiites to Na-alkaline hawaiites and K-enriched

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minettes, and from metaluminous and peralkaline to peraluminous and even some rare

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trondhjemitic rhyolites. Despite the enormous advances that have been achieved in the stratigraphic, petrologic, geochemical, and geophysical characterization of the Mexican

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convergent margin, many of the same questions continue to linger: how can we explain the variety of rock types? Is there an underlying genetic link among them? And what does the

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diversity and distribution of these rocks tell us in terms of the tectonic origin and evolution of the convergent margin? This contribution attempts to examine these questions from the geochemical and petrological perspective; sustained upon an updated geochemical database

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and a much improved stratigraphic, geologic and geophysical framework.

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The present manuscript represents the latest in a series of reviews on the TMVB and the Mexican convergent margin published over the past decade (Ferrari et al., 2012; GómezTuena et al., 2007b; Manea et al., 2013), to which the reader is referred for a deeper understanding on the evolution of ideas. Most of the argumentation will be based on the compositions of mafic and intermediate rock types because these are more appropriate to decipher the dynamics of the deep mantle, and because most of the recent literature has been focused on their origin and evolution. Much less attention has been given to felsic volcanism since the last series of reviews, making these rocks a fertile ground for future generations to attend.

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ACCEPTED MANUSCRIPT 2. The Mexican Convergent Margin The geologic and geodynamic evolution of the Mexican convergent margin has been the

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subject of so numerous studies over the past half century that any modern review of literature has to be highly selective. The most important aspects relevant to the present

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study will be briefly summarized here, but the reader is referred to the most recent reviews and references therein for a more comprehensive treatment of these subjects (Ferrari et al.,

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2012; Gómez-Tuena et al., 2007b; Manea et al., 2013).

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The TMVB has been constructed in a tectonic and geologic framework that is, in many aspects, unique on Earth. The Mexican subduction zone comprises two independent

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oceanic plates, Cocos and Rivera, which currently converge against the North American Plate with different geometries, velocities, and relative motions (Fig. 1). Both oceanic plates are in turn the youngest offsprings of the ancient Farallon plate, as it was

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progressively consumed by the convergent margin (Atwater and Stock, 1998; Engebretson et al., 1985; Lonsdale, 2005). As a result, all of the parameters that govern the thermal

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structure of the subduction zone vary along strike of the arc, with western Rivera sector being one of warmest convergent systems on Earth (Ferrari et al., 2012; Syracuse et al., 2010). The virtual disappearance of seismic hypocenters below 60 km depth prevented earlier seismologists the clear definition of a Wadati-Benioff zone beyond the fore-arc region (Pardo and Suárez, 1993, 1995), but more recent high-density seismic deployments confirmed that subduction of the young Rivera plate is very steep whereas subduction of Cocos becomes flat to the east (Pérez-Campos et al., 2008; Yang et al., 2009). As long suspected by pioneer researchers (Menard, 1978; Nixon, 1982; Pardo and Suárez, 1993,

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ACCEPTED MANUSCRIPT 1995; Urrutia-Fucugauchi and Böhnel, 1987), such an unusual geometric configuration of the subducted plates is responsible for the displacement of the volcanic front more than 300

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km away from the trench, the reason for the striking obliquity between arc and trench, and

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indeed the underlying rationale for naming the volcanic belt as Transmexican (Fig. 1). Accurately located seismic hypocenters (Pardo and Suárez, 1993) and recent seismic

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tomographies (Yang et al., 2009) have shown that the Rivera plate dips with a 50° angle,

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reaching about 140 km depth below the active volcanic front and up to 320 km underneath the more magmatically productive rear arc (Fig. 1). In contrast, seismic hypocenters (Pardo

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and Suárez, 1995), together with receiver function analyses of teleseismic waves and tomographic images (Chen and Clayton, 2009; Husker and Davis, 2009; Kim et al., 2010;

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Pérez-Campos et al., 2008) indicate that flat subducting Cocos underplates the continental crust for nearly 280 km from the trench before bending into the mantle at a ~75° dip angle, just a few kilometers before the first appearance of arc-front volcanoes south of Mexico

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City (Fig. 1). The Cocos plate continues its descent into the mantle for the entire length of

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the TMVB, down to the mantle transition zone, where it suddenly appears truncated at about 500-550 km depth (Husker and Davis, 2009; Pérez-Campos et al., 2008). A different kind of ―slab window‖ seems to be developing just north Colima volcano, and underneath Guadalajara City, due to the divergent motion at depth between Rivera and Cocos (Yang et al., 2009). Whether these truncations of the slab have an effect on mantle dynamics and the origin of magmatism are questions that will be further explored below. The TMVB is presumably emplaced upon a highly heterogeneous continental crust that varies in age, thickness and composition (Fig. 2). And yet the exact nature of basement rocks has long been a matter of speculation because of the lack of exposures and the very few crustal xenoliths that have been recovered. Gravimetric and seismic studies 5

ACCEPTED MANUSCRIPT (Geolimex-Group, 1994; Molina-Garza and Urrutia-Fucugauchi, 1993; Pérez-Campos et al., 2008; Suhardja et al., 2015; Urrutia-Fucugauchi and Flores-Ruiz, 1996) have shown that

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the crust underneath the western sector is relatively thinner (~35 km) and apparently

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younger, essentially constituted by the plutonic and volcanic arc assemblages of Cretaceous to Paleocene age of the Jalisco Block (Köhler et al., 1988; Valencia et al., 2013); which are

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overlain to the north by the thick ignimbrite successions of the late Oligocene to early

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Miocene Sierra Madre Occidental (SMO) (Duque-Trujillo et al., 2014; Ferrari et al., 2013). Crustal thickness slightly increases underneath the central sector of the belt (~40 km), the

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basement of which is apparently dominated by Triassic to Cretaceous oceanic arc volcanic and sedimentary sequences of the Guerrero composite terrane (Centeno-García et al., 2011),

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and later intruded by late Paleogene granitoids (Ortega-Gutiérrez et al., 2014a). While some authors have entertained the possibility that the western and central sectors of the arc could conceal a relatively older basement (Schaaf et al., 1995), recent U-Pb ages and

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geothermometry evidence of crustal xenoliths from Valle Santiago maars (Fig. 2) indicate

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that the lowermost granulitic crust is no older than Upper Cretaceous and possibly as young as Holocene (Ortega-Gutiérrez et al., 2014b). Therefore, to date, no direct evidence exists for an ancient continental crustal basement underneath the western and central parts of the TMVB. A sharp N-S trending increase of crustal thickness is observed to the east of latitude 101°W, probably coincident with a major crustal boundary that separates the mostly Mesozoic Guerrero terrane to the west from the much older Paleozoic to Precambrian high-grade metamorphic complexes of the Mixteco and Zapoteco terranes to the east (Ortega-Gutiérrez et al., 1994). High-grade metasedimentary and metaigneous xenoliths have been found in two locations to the east of this crustal boundary (Pepechuca (Pep) and Chalcatzingo (Chal), 6

ACCEPTED MANUSCRIPT Fig. 2), and even if the metamorphic assemblages and geothermometry appear to be consistent with deeply buried —and possibly Precambrian— terranes (Elías-Herrera and

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Ortega-Gutiérrez, 1997; Ortega-Gutiérrez et al., 2008, 2011, 2012), direct isotopic ages

lithostratigraphic sequences exposed in southern Mexico.

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have not been obtained to confirm their direct association to the well characterized

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Crustal thickness sharply decreases to the east of Citlaltépetl volcano (Pico de Orizaba) at

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the easternmost sector of the arc (35-20 km), possibly also in accordance with a major crustal boundary separating the Zapoteco and Cuicateco terranes (Ortega-Gutiérrez et al.,

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1994; Pérez-Gutiérrez et al., 2009b) from the Paleozoic granitoids of the Maya terrane (Weber et al., 2012). A Paleozoic basement has long been suspected to exist underneath

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this area based on stratigraphic correlations and a few exploratory wells (Torres et al., 1999), and even if peridotitic xenoliths are often found in mafic alkaline lavas from this sector (Gómez-Tuena et al., 2003), no deep crustal enclaves have been reported thus far,

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and therefore the specific nature of the lowermost continental basement for this sector

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remains largely unknown.

The Mexican subduction zone was considered for many years as a classic example of an accretionary plate margin (Karig et al., 1978; Moore et al., 1979; Shipley et al., 1980), but different lines of evidence have now recognized it as one of the most erosive convergent margins on the planet (Clift and Vannucchi, 2004; Keppie et al., 2012; Morán-Zenteno et al., 1996). The sharp truncation of the continental margin (Schaaf et al., 1995), the very steep trench slope (>10°), the rapid subsidence of the fore-arc and the consequent trench retreat (Clift and Vannucchi, 2004; Mercier de Lépinay et al., 1997), as well as the gradual migration of magmatism towards the continental interior during the late Oligocene and early Miocene (Garduño-Monroy and Gutiérrez-Negrín, 1992; Manea et al., 2013), are all 7

ACCEPTED MANUSCRIPT strong indications that a large part of the Mexican fore-arc has been tectonically removed. The specific mechanisms and timing of fore-arc removal are still a matter of debate (Keppie

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et al., 2012; Pindell et al., 1988; Ross and Scotese, 1988), but the paleobathymetric

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evidence indicates that the trench has been retreating at a rate of at least 1 km/Ma since the early Miocene, while the thermobarometry of coastal plutons also indicate unroofing of a

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10–20 km-thick crust during the same period (Ducea et al., 2004b; Keppie et al., 2012;

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Morán-Zenteno et al., 1996). Since there is no evidence of sediment accumulation in the Middle American trench facing the arc (Manea et al., 2003), it is clear that all eroded

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continental material has been subducted.

3. The Transmexican Volcanic Belt (TMVB) The TMVB is the youngest magmatic expression of a long history of eastward subduction

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that has affected the Mexican territory at least since the late Jurassic (Ducea et al., 2004a;

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Pérez-Gutiérrez et al., 2009a). Arc magmatism has been a continuous phenomenon ever since, but the locus of magmatism has migrated widely, presumably as a response to reconfigurations of the tectonic plates and fluctuations in subduction zone parameters (Ferrari et al., 2012; Gómez-Tuena et al., 2003; Gómez-Tuena and Carrasco-Núñez, 2000; Manea et al., 2013). In this context, defining the geographic limits and specific timings of individual magmatic arcs are tasks that serve the purpose of visualization, but that not necessarily coincide with the natural conditions. Over the past two decades, it has been customary to consider that the individualization of the TMVB as a distinct geologic entity initiated during the early and middle Miocene (2313 Ma), as a result of a gradual counterclockwise rotation of the magmatic arcs that formed 8

ACCEPTED MANUSCRIPT the Sierra Madre del Sur (SMS; Morán-Zenteno et al., 2007) and Sierra Madre Occidental (SMO, Ferrari et al., 2007). The earliest volcanic manifestations of the TMVB are found in

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the Sierra de Angangueo-Mil Cumbres in Michoacán State (Gómez-Vasconcelos et al.,

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2015; Hernández-Bernal et al., 2016) and to south of Mexico City in the Tepoztlán Formation (Lenhardt et al., 2010) and Chalcatzingo (Gómez-Tuena et al., 2008), roughly

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around the same areas that are now covered by the Quaternary monogenetic fields of

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Michoacán-Guanajuato (MGVF) and Chichinautzin (CVF) (Fig. 3a). And yet volcanic and plutonic rocks of early and middle Miocene age are also found much farther south, in the

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Tehuantepec Isthmus of Oaxaca (Martínez-Serrano et al., 2008; Molina-Garza et al., 2015), as well as farther to the west in Nayarit (Duque-Trujillo et al., 2014) and in the Comondú

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group of Baja California (Bryan et al., 2014). Volcanic rocks in these latter areas have been considered as the youngest manifestations of the SMS and SMO, respectively, albeit they largely correspond to the same transitional geodynamic setting that formed the initial stages

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of the TMVB proper. Integrating all these volcanic segments into a larger tectonic context

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will be a worthwhile endeavor for future researchers. Following these initial stages, and during the entire Miocene, magmatism gradually migrated to the north in the eastern and central sectors of the TMVB (Fig. 3a), eventually forming the stratovolcano complexes of Palo Huérfano-La Joya-Zamorano (PH-LJ-Z) in Querétaro at 12-10 Ma (Mori et al., 2007; Pérez-Venzor et al., 1996; Valdéz-Moreno et al., 1998), the Cerro Grande volcano in Puebla at 11-10 Ma (Gómez-Tuena and CarrascoNúñez, 2000), and the plutonic to subvolcanic units of Palma Sola in Veracruz at 16-11 Ma (Ferrari et al., 2005b; Gómez-Tuena et al., 2003). Interestingly, no vestiges of volcanism have been found in the central and western sectors of the TMVB for this period of time, even if plate convergence along the western margin of Mexico was not interrupted. Indeed, 9

ACCEPTED MANUSCRIPT SMO-related silicic volcanism in this region abruptly ended at ca. 22 Ma, and was followed by an extended hiatus in effusive activity that lasted for a period of ~10 million years

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(Ferrari et al., 2002). Volcanism subsequently resumed in the late Miocene (Fig. 3a) as a

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widespread mafic igneous province —dubbed by some as a small-scale continental flood basalt event (Mori et al., 2009)— that extruded more than 3,800 km3 of tholeiitic to calc-

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alkaline mafic plateau lavas distributed within an estimated area of up to 15,500 km2

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(Ferrari et al., 2000a). Published ages of the mafic burst range from 11.5 to 8 Ma with no clear migration pattern between 105°W and 101°W and mostly within the Los Altos de

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Jalisco plateau (Fig. 3a), indicating an almost simultaneous extrusion of mafic lavas with an eruption rate of ~1.66 km3/ka (Ferrari, 2004; Mori et al., 2009). A compositionally similar,

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but slightly younger (11-7 Ma) and less voluminous mafic episode extends further east to the Querétaro area (Ferrari, 2004; Mori et al., 2007), while sporadic mafic volcanism of mostly intraplate-like character becomes gradually younger to the east, reaching the Gulf of

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Mexico in the Palma Sola area between 7-4 Ma (Ferrari, 2004; Ferrari et al., 2005b;

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Gómez-Tuena et al., 2003).

By the end of the Miocene and into the Pliocene (Fig. 3b), magmatism started to migrate towards the trench and became more evolved. Voluminous caldera-forming eruptions generated widespread rhyolitic ignimbrites in the eastern and central sectors of the arc (Aguirre-Díaz et al., 1997; Aguirre-Díaz and López-Martínez, 2001; Aguirre-Díaz and McDowell, 2000; Cantagrel and Robin, 1979; Ferrari et al., 1991; Pasquaré et al., 1991; Pradal and Robin, 1994), whereas mostly effusive silicic and bimodal (silicic and mafic) volcanism was emplaced in the western sector (Allan, 1986; Ferrari et al., 2000b; Frey et al., 2007; Gilbert et al., 1985; Righter and Rosas-Elguera, 2001; Rossotti et al., 2002).

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ACCEPTED MANUSCRIPT The most recent episode of the TMVB, starting in the Pliocene and extending to the present times, bears witness of an extraordinary compositional and volcanological diversity. It is

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during this episode that the largest monogenetic fields of the Michoacán-Guanajuato

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(MGVF) (Hasenaka and Carmichael, 1985) and Chichinautzin (CVC) (Bloomfield, 1975; Siebe et al., 2004) were formed, and the period of time in which the most emblematic

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Mexican strovolcanoes were constructed (Fig. 3c). Monogenetic volcanism is widespread

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along the TMVB, however, and can be found punctuating almost any region of the arc since the late Pliocene, with cinder cones that are often aligned with the regional tectonic

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structures (Alaniz-Alvarez et al., 1998; Cebriá et al., 2011a; Connor, 1990). In the western sector, large stratovolcanoes are also arranged inside extensional structures that either run

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parallel to the trench behind the volcanic front, like San Juan, Sangangüey, Ceboruco, and Tequila (Ferrari and Rosas-Elguera, 2000), or perpendicular to it like Colima (Allan, 1986). Further to the east, all active stratovolcanoes are located at or close to the volcanic front,

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together forming an alignment that is oblique by ~16° to the trench. Some of the most

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prominent and active stratovolcanoes, like Colima, Popocatépetl and Citlaltépetl (Pico de Orizaba), are the at the southern end of a N-S alignment of otherwise compositionally similar stratovolcanoes (Alaniz-Alvarez et al., 1998). Others like Tancítaro, Xinantécatl (Nevado de Toluca) and Malinche appear isolated, albeit they are always surrounded by monogenetic volcanoes.

4. Geochemical Diversity It has long been recognized that among the global spectrum of arcs, the TMVB is relatively enriched in incompatible trace elements (Plank and Langmuir, 1988; Turner and Langmuir, 11

ACCEPTED MANUSCRIPT 2015a): it contains higher Na2O and K2O and more fractionated rare earth element (REE) patterns when compared to arcs emplaced on thinner crusts, such as the island arcs of the

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western Pacific (Fig. 4). As in any other continental arc emplaced on a thick continental

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basement, the most common magmatic suite is sub-alkaline, or more strictly calc-alkaline, in which intermediate andesites of medium potassium constitute the dominant lithology.

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Silica distribution and alkali contents of analyzed rocks in the Mexican arc are thus not

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very different from those observed in the Andean arc, for instance, but they are slightly higher than in thin-crusted arcs which often contain a larger proportion of mafic volcanics

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(Fig. 4d). To date, many hypotheses have been put forth to account for these first-order compositional differences among global arcs, with none of them mutually exclusive: (1) is

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the background mantle wedge below continental arcs like the TMVB intrinsically more enriched in incompatible elements? (Chin et al., 2014; Turner and Langmuir, 2015b); (2) is the enrichment controlled by an overall lower extent of mantle melting due to a thicker

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lithosphere? (Plank and Langmuir, 1988; Turner and Langmuir, 2015a), (3) is it because

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magmas become stalled and contaminated due to the existence of a thicker continental crust? (Annen et al., 2006; Hildreth and Moorbath, 1988), (4) or is it because the slab contributions transfer a larger amount of incompatible trace elements due to higher slab temperatures (Cooper et al., 2012; Gómez-Tuena et al., 2011; Kessel et al., 2005), (5) subduction of a thicker sediment column (Plank, 2005), (6) or because of the effects of subduction erosion (Kay et al., 2014; Straub et al., 2015)? As it will become clear below, the TMVB has been no stranger to these discussions. Calc-alkaline andesites dominate the landscape, but the TMVB also contains at least two other distinctly different magmatic series that depart from this classic rock type, namely Kalkaline and Na-alkaline volcanics (Figs. 4 and 5). Some low-K arc tholeiites have also 12

ACCEPTED MANUSCRIPT been found within the late Miocene mafic episode (Fig. 3a, Mori et al., 2009), but since these rocks become transitional to more typical medium-K calc-alkaline compositions they

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will not be treated as a separate series for simplicity. The alkaline varieties are not

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particularly volumetric, nor do they appear throughout the geologic history of the TMVB, but they have received a disproportionate attention in the literature because they appear to

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be keys for understanding the origin and evolution of the arc as a whole. For these reasons,

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in the following sections, we will concentrate in their geologic, petrologic and geochemical characteristics and their relationship to the more typical calc-alkaline andesites that

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constitute by far the largest magmatic output of the volcanic belt. Figure 4 shows a distinction among the major three magmatic series based on classic

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compositional subdivisions (Ewart, 1982; Le Bas et al., 1986; McDonald and Katsura, 1964). In keeping with the historical norm, these rock series will be plotted as a background in subsequent geochemical diagrams. It is important to emphasize, however, that this

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formal classification scheme is not always useful to the interpretations, since the trace

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elements and isotopic compositions of the volcanic rocks in Mexico display important distinctions that may not be registered by the major elements. This classification problem has encouraged the introduction of several informal names to groups of rocks that may depart from their formal designations, but that are nonetheless useful from the petrogenetic point of view. For these reasons, and as will be further elaborated below, in this contribution we will follow the unconventional approach of naming an important group of rocks as "intraplate-like", not because they are easily distinguished from the rest by their alkaline contents, but because of their departure from the trace-element and isotopic characteristics that are commonly found in arc environments. For the sake of clarity, the geochemical diagrams will also include a set of representative examples of the different 13

ACCEPTED MANUSCRIPT rock series discussed in the text, but the reader is advised to take these only as guidance

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because compositional boundaries among the series are extremely difficult to establish.

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4.1. Intraplate-like volcanic rocks

As mentioned before, intraplate-like rocks from the TMVB are difficult to categorize using

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the classic geochemical classification systems. The most primitive varieties are mainly Na-

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alkaline, but several rocks that could be formally classified as potassium-alkaline and even a few calc-alkaline rocks also display similar trace element and isotopic compositions, and

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therefore they have been often grouped together regardless of their conventional classification. For these reasons, rocks of this kind are sometimes better defined as high-Nb or high-TiO2 (Gómez-Tuena et al., 2011; Straub et al., 2013), because it is the relative

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enrichment of high-field strength elements (HFSE) with respect to large ion lithophile elements (LILE) and light rare earths (LREE) what clearly distinguishes these rocks from

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the rest of the sequences (Figs. 5a and 6a). Given that these peculiar chemical compositions are unusual for a convergent margin, rocks of this kind have also been dubbed as ocean island basalt (OIB)-type, or as we have chosen here: intraplate-like rocks (Gómez-Tuena et al., 2003; Luhr, 1997; Márquez et al., 1999; Verma, 2000). Intraplate-like rocks started to appear in the TMVB during the latest Miocene to early Pliocene, and have continued to erupt sporadically throughout the belt ever since (Fig. 3). Being essentially restricted to cinder cones and fissural lava flows, these rocks constitute a relatively small volume when compared to calc-alkaline successions, but their presence is significant towards the rear arc, especially in the eastern and western sectors of the belt (Fig.

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ACCEPTED MANUSCRIPT 7). Nonetheless, rocks of this kind are also found at the volcanic front in some locations, like the Chichinautizin volcanic field, where they constitute a N-S alignment of cinder

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cones that is nearly equidistant to the neighboring Xinantécatl (Nevado de Toluca) and

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Popocatépetl stratovolcanoes (Straub et al., 2014; Wallace and Carmichael, 1999). Likewise, in the western sector of the TMVB, intraplate-like cinder cones are often found at

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the foot of large andesitic stratovolcanoes like Sangangüey, Tequila and Ceboruco (Gómez-

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Tuena et al., 2014a; Nelson and Livieres, 1986; Petrone et al., 2003; Petrone, 2010). In addition, a few large cinder cones have erupted intraplate-like rocks either slightly before or

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contemporaneously to calc-alkaline lavas (Carrasco-Núñez et al., 2005; Straub et al., 2013). Therefore, while there seems to be some geographic and structural control on the

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distribution of intraplate-like rocks, it is important to note that they almost always appear in close space-time association with volcanoes that are more typical of a convergent margin. Most primitive intraplate-like volcanic rocks are mostly trachybasaltic to trachyandesitic

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(hawaiites and mugearite) that are distinguished from other alkaline varieties by their

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higher TiO2/K2O ratios (Fig. 5a and 7). Displaying variable MgO contents and Mg numbers (Mg#) at relatively constant SiO2 (Figs. 5b), some of these rocks also appear to follow a nearly tholeiitic differentiation trend characterized by an incipient enrichment of FeOtot (Fig. 5c). The incompatible trace element contents and HFSE/LILE ratios of these rocks are usually high, but they are also quite variable, and often appear transitional to those of more abundant calc-alkaline successions, thus making it difficult to define a precise boundary between them (Fig. 6a). Most primitive varieties are also water-poor, with H2O contents that range between 0.5 and ~3 wt.% that correlate strongly with HFSE/LILE ratios (Cervantes and Wallace, 2003; Cooper et al., 2012; Díaz-Bravo et al., 2014; Johnson et al., 2009). The Sr-Nd isotopic compositions of these rocks largely overlap with the rest of the 15

ACCEPTED MANUSCRIPT rock series, but locally they tend to display different Sr isotope ratios at equivalent Nd isotopic compositions than the nearby calc-alkaline rocks: lower in the Chichinautzin

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volcanic field of the central sector (Straub et al., 2015) but higher in the Tepic-Zacoalco rift

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of the western sector (Gómez-Tuena et al., 2014a). Nevertheless, the most distinguishing isotopic feature is their radiogenic Pb values that overlap the compositions of other

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intraplate-like rocks in Mexico at high HFSE/LREE ratios (Fig. 8c). Taken together, these

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isotopic characteristics somehow resemble a mantle source with moderately high- composition (238U/204Pb; Zindler and Hart, 1986), which is currently referred as FOZO

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in the modern nomenclature of mantle components (Stracke et al., 2005).

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4.2. Potassium-alkaline volcanic rocks If intraplate-like volcanic rocks are unusual for a convergent margin, potassic rocks are perplexing. In stark contrast to what is observed in most magmatic arcs that increase their

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K2O contents with distance from the trench (i.e. the K–h relationship; Dickinson, 1975), K-

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alkaline volcanic rocks in Mexico have erupted exclusively at the volcanic front during the past 4 million years (Fig. 7, Lange and Carmichael, 1991). They are most abundant in the western sector of the arc, forming monogenetic volcanic fields that have been locally constructed inside small grabens (e.g. San Sebastián, Mascota, Atenguillo, Tapalpa; Lange and Carmichael, 1991). All together, these monogenetic fields become gradually younger towards the northwest, defining the so-called Jalisco Volcanic Lineament which runs parallel to the Middle American Trench (Bandy et al., 2001; Ownby et al., 2008). And yet monogenetic volcanoes similar to those found along this lineament have also erupted very recently (Carmichael et al., 2006) at the northwestern flanks of Colima volcano (Fig. 7). Potassic volcanism becomes much less abundant towards the central and eastern sectors of

16

ACCEPTED MANUSCRIPT the arc, but some potassic cinder cones have been identified in the Michoacán-Guanajuato volcanic field (Hasenaka and Carmichael, 1987; Luhr and Carmichael, 1985) and the Valle

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de Bravo–Zitácuaro region (Blatter et al., 2001; Gómez-Tuena et al., 2007a). Although it

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was believed that K-alkaline volcanism completely disappeared towards the eastern sector (Gómez-Tuena et al., 2007b), a few isolated potassic cinder cones have been discovered

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recently in Chichinautzin volcanic field (Straub et al., 2015) and in the Serdán-Oriental

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basin (Becerra-Torres, 2014).

K-alkaline rocks in Mexico have also received many names depending on the author‘s main

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discipline and objectives. Petrologists often classify them by their petrographic IUGS recommended names as lamprophyres (Le Maitre, 2002), due to the common presence of

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hydrous minerals (e.g. phlogopite, amphibole) and absence of feldspar phenocrysts: minettes, hornblende-lamprophyres, absarokites, and spessartites (Carmichael et al., 1996; Luhr, 1997). Geochemists usually name them on the basis of the total alkali vs. SiO2

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diagram as potassic trachybasalts, trachyandesites or phonotephrites (Fig. 4). Rocks with

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similar chemical characteristics have also been regarded as high-La to highlight the strong enrichment in LREE (Gómez-Tuena et al., 2007a; Straub et al., 2015), which is perhaps one of their best distinguishing features. In this work, we will be using the more profane term K-alkaline, or simply potassic, because rocks with contrasting mineral assemblages often display similar chemical compositions and the LREE contents are too variable and overlapping to the rest of the sequences. In terms of the major elements, K-alkaline rocks from the western sector of the arc are distinguished from the rest of the magma series by their overall higher and positively correlated concentrations of K2O and P2O5 that are often accompanied by very low Al2O3 contents (Gómez-Tuena et al., 2011; Luhr, 1997). With SiO2 abundances <52 wt.% and 17

ACCEPTED MANUSCRIPT Mg#> 60, potassic rocks from the western sector are usually some of the most mafic rocks found all across the belt, and they are also easily distinguishable from the mafic intraplate-

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like rocks by their much lower HFSE/LILE(LREE) ratios (Figs. 5 and 6). The very few

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potassic cones from the central and eastern sectors of the arc are much less enriched in K2O and P2O5 and tend to be more evolved than their western counterparts. They often classify

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within the limit of the calc-alkaline field, and therefore are much more difficult to

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distinguish in terms of the major elements (Blatter et al., 2001; Gómez-Tuena et al., 2007a; Straub et al., 2015).

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The incompatible trace element contents are overall much higher in these rocks when compared to the rest of series in the TMVB, but by being enriched in the LILE with respect

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to the HFSE, their trace elements patterns are otherwise typical of arc magmas (Fig. 6). Interestingly, while the most incompatible trace elements and especially the Light Rare Earth Elements (LREE) could become enriched by more than 2 orders of magnitude when

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compared to mid-ocean ridge basalt (MORB), the heavy REE (HREE) are less variable and

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even lower than those of a typical MORB. Again, the few potassic cones from the central and eastern sector are overall much less enriched in trace elements, but most of them display a somehow unusual negative Zr-Hf anomaly that is very seldom observed in potassic cones from the west. The trace element contents of potassic rocks are extremely variable, but the isotopic compositions are fairly homogeneous, overlapping mostly with each other in terms of Sr, Nd, and Pb isotopes regardless of their location (Fig. 8). If anything, the Sr isotopic compositions of rocks from the eastern sector tend to be slightly higher at equivalent Nd and Pb isotopes than rocks from the west, albeit their isotopic compositions overlap with those observed in nearby calc-alkaline rocks. Interestingly, the virtually identical isotopic 18

ACCEPTED MANUSCRIPT compositions with respect to nearby calc-alkaline volcanoes is in clear contrast to what is observed in calc-alkaline and potassic volcanoes from Italy, for instance, where the potassic

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varieties are often shifted to more enriched isotopic values (Peccerillo, 2005).

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4.3. Calc-alkaline volcanic rocks

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Calc-alkaline volcanic rocks are by far the most abundant in the TMVB and they have been emplaced throughout its entire geologic history. While a few of these rocks could be strictly

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classified as low-K arc tholeiites (Mori et al., 2009), the vast majority of them belong to the classical medium-K to high-K calc-alkaline series (Figure 4b). The most mafic varieties are

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always found in monogenetic volcanoes, or less commonly forming plateau lavas fed by fissures; whereas more evolved compositions usually erupt from a large variety of vents: from cinder cones to composite volcanoes, such as stratovolcanoes, domes, and calderas.

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The largest volumes of calc-alkaline and tholeiitic basalts in the TMVB were emplaced in the late Miocene, during the ‗small-scale continental flood basalt event‘ that formed the so-

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called Los Altos de Jalisco plateau and the Querétaro Volcanic Succession (Ferrari et al., 2000a; Mori et al., 2007, 2009) (Fig. 3a). And yet, after this rather unusual event, true calcalkaline primitive basalts (< 52 wt.% SiO2) became surprisingly rare throughout the TMVB. So much so that only one calc-alkaline basaltic cinder cone seems to exist at the volcanic front of the western sector (Gómez-Tuena et al., 2011; Luhr, 1997), and just a couple of others have been found close to Ceboruco volcano towards the rear arc (Gómez-Tuena et al., 2014a; Petrone, 2010). Calc-alkaline basaltic cinder cones are slightly more common in the large monogenetic fields of the central and eastern sector (MGVF, CVC, SerdánOriental and Palma Sola; Gómez-Tuena et al., 2003; Hasenaka and Carmichael, 1987;

19

ACCEPTED MANUSCRIPT Negendank et al., 1985; Straub et al., 2013), albeit the most primitive ones often merge and become transitional with the intraplate-like mafic varieties, in occasions during the same

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eruptive cycle (Carrasco-Núñez et al., 2005; Straub et al., 2013).

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The calc-alkaline rock series is therefore almost entirely dominated by basaltic andesites to dacites, rocks that typically erupt from large stratovolcanoes. Interestingly, and despite the

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large variability in SiO2, the Mg# of most intermediate calc-alkaline rocks remain

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consistently high, close to equilibrium with mantle values (~60), only dropping to less than 50 in highly evolved rhyolites at >70 wt.% SiO2 (Fig. 5b). As often observed in calc-

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alkaline series worldwide (Gill, 1981; Reubi and Blundy, 2009), linear trends against silica are observed for the rest of the major elements (e.g. Fig. 5d).

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The trace element patterns of intermediate calc-alkaline rocks are also remarkably homogeneous, showing the high LILE/HFSE ratios that are typical of arc magmas (Fig. 6c). Calc-alkaline andesites are obviously much less enriched in incompatible trace elements

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than K-alkaline rocks, especially in terms of the LREE, but they share the overall lower

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HREE contents when compared to MORB (Fig. 6c). This translates in overall higher Sr/Y and La/Yb ratios in some andesites, a characteristic akin to the so-called adakites worldwide (Fig. 9; Defant and Drummond, 1990). It is interesting to note however, that higher Sr/Y and La/Yb values are usually observed in K-alkaline mafic varieties, and also in the very rare Miocene trondhjemites from Chalcatzingo that marks one the earliest magmatic stages of the TMVB (Fig. 3a; Gómez-Tuena et al., 2008). The isotopic compositions of calc-alkaline rocks mostly overlap with the rest of the sequences, but data are more dispersed than their alkaline counterparts (Fig. 8b). Pb isotopes form a tight positive correlation between MORB and an upper crustal end-member with radiogenic Pb regardless of location, but available Sr and Nd isotopic data from large 20

ACCEPTED MANUSCRIPT stratovolcanoes display interesting differences along strike of the arc (Fig. 10). Excluding for the moment the extremely variable late Miocene stratovolcanoes (PH-LJ-Z; Mori et al., 87

Sr/86Sr, lower

143

Nd/144Nd) are

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2007), the most enriched isotopic compositions (higher

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found in the easternmost Citlaltépetl volcano (Cai et al., 2014; Schaaf and Carrasco-Núñez, 2010). Albeit internally variable, isotopes gradually become more depleted towards the

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west in a stepwise fashion, going from Citlaltépetl to Malinche, Popocatépetl and

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Xinantécatl (Cai et al., 2014; Martínez-Serrano et al., 2004; Schaaf et al., 2005; SosaCeballos et al., 2015). Interestingly, the Sr and Nd isotopes become gradually more

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enriched further to the west of Xinantécatl, with the notable exception of Colima volcano which plots slightly off-trend at relatively higher Nd isotopic compositions (Gómez-Tuena

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et al., 2014a; Hochstaedter et al., 1996; Lassiter and Luhr, 2001; Luhr, 1997, 2000; Nelson, 1980; Valdez-Moreno et al., 2006; Verma and Luhr, 2010a; Wallace and Carmichael, 1994). Trends are more difficult to recognize using the entire dataset, but it is nonetheless

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important to observe that the most primitive calc-alkaline rocks (> 5 wt% MgO) can be

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almost as isotopically enriched as andesites that erupt from nearby stratovolcanoes.

5. Igneous Petrogenesis and Tectonics 5.1. On peridotites and pyroxenites: the story of olivine Seismic studies (Kim et al., 2010; Pérez-Campos et al., 2008) and peridotite xenoliths entrained in arc front and rear arc magmas (Blatter and Carmichael, 1998a; Gómez-Tuena et al., 2003; Luhr and Aranda-Gómez, 1997) provide direct evidence for the existence of a peridotitic mantle wedge beneath the TMVB. Consequently, TMVB primary magmas are

21

ACCEPTED MANUSCRIPT thought to be basaltic, or possibly basaltic-andesitic if mantle melting occurs at elevated water contents (Blatter and Carmichael, 1998b, 2001; Carmichael, 2002; Moore and

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Carmichael, 1998). However, the recent discovery of ubiquitous ‗high-Ni olivines‘ in the

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eastern and western TMVB (e.g. Díaz-Bravo et al., 2014; Straub et al., 2008, 2011) provide a new and different perspective on the origin and composition of primary mantle melts.

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‗High-Ni olivines‘ are olivines that have higher Ni abundance at a given forsterite content

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than is feasible for olivines that crystallize from melts produced by the partial melting of peridotite (e.g. MORB olivines). High-Ni olivines were first reported in intraplate

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environments by Sobolev et al. (2005), who linked their formation to melting of an olivinefree mantle source made of secondary ‗reaction‘ pyroxenite lithologies, which in turn forms

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by the infiltration of silicic partial melts from deep-seated eclogite enclosed within the peridotite mantle (Sobolev et al., 2005, 2007). By addition of SiO2 and CaO, new ‗reaction‘ orthopyroxenes and clinopyroxenes form by olivine consumption. Reaction pyroxenes are

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Ni-rich as they inherit the Ni from olivine, which is the major repository of mantle Ni.

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Because pyroxenites are more fusible than peridotite, but have lower partition coefficients for Ni (KdNi) than olivine, they melt preferentially to produce Ni-rich melts that crystallize high-Ni olivines upon cooling at shallower crustal levels. Because forsteritic (Fo) olivine is a common —and often the only— phenocryst in monogenetic basaltic to andesitic TMVB magmas and can also be found in some basalticandesites and andesites from stratovolcanoes (e.g. Popocatépetl; Sosa-Ceballos et al., 2015; Straub et al., 2008) it provides a perfect means for testing the pyroxenite reaction hypothesis in an arc setting where the siliceous input may come from the subducted slab (Ammannati et al., 2016; Bryant et al., 2010; Conticelli et al., 2015; Prelević and Foley, 2007; Rowe and Tepley, 2016). Remarkably, high-Ni olivines are common in all three 22

ACCEPTED MANUSCRIPT major rock series from the TMVB, whereby the most Ni-rich olivines with up to 5,0006,000 ppm Ni at Fo80-94 occur in the calc-alkaline and potassic magmas (Fig. 11a). Olivine

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Ni contents from intraplate-like magmas are generally lower, with values similar to MORB

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in rocks from the western sector (~2600 ppm Ni at Fo90; Díaz-Bravo et al., 2014) and slightly more elevated (about 4000 ppm Ni at Fo80-90) in rocks from the Chichinautzin

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volcanic field of the eastern sector (Fig. 11b; Straub et al., 2008, 2011).

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The high Ni of the TMVB olivines is significant, as it cannot be reconciled with models of Ni enrichment other than the Sobolev et al. (2005) reaction model. Indeed, while the

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KdNioliv/melt is linked to melt composition (Hart and Davis, 1978; Wang and Gaetani, 2008) and melt temperature (Matzen et al., 2013), neither effect is large enough to reproduce the

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TMVB olivine Ni maxima unless the original background mantle is much richer in Ni than the average mantle peridotite. On the other hand, quantitative modeling confirms that the maximum Ni in the TMVB olivines can be reproduced by the reaction model starting with

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the average Ni contents of the upper mantle (Straub et al., 2008, 2011). In Figure 11, the

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field labeled ‗peridotite‘ represents olivine in equilibrium with a range of melts produced from fertile to refractory mantle (with equilibrium olivines of Fo89 to Fo93, and Ni=1,7003,700 ppm). The refractory mantle is created by stepwise melt extraction from fertile mantle, assuming up to 22% melt loss to the point of clinopyroxene exhaustion. The field labeled ‗pyroxenite‘ illustrate the composition of olivines in equilibrium with melts formed from reaction pyroxenite after transformation of the peridotite, based on experimental Ni, Fe and Mg olivine/melt partitioning data (Beattie, 1993, p. 199; Hart and Davis, 1978; Roeder and Emslie, 1970) [for details see Straub et al., 2008, 2011]. Thus, given an inherent variability of Ni and Mg# in the mantle source slightly in excess to the range assumed in the model, the modeled Ni enrichment matches well the observed data. 23

ACCEPTED MANUSCRIPT Olivines with the highest Ni (>5,000 ppm) would represent partial melts of pure pyroxenite lithologies, while those with lower Ni contents may either represent later-crystallizing

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olivines in melts with progressively lower Ni, or olivines that crystallized from mixtures of

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partial melts from pyroxenite and peridotite lithologies. Here the model agrees with the perception that the intraplate series come from a mantle region that is less pervasively

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transformed by the slab flux, with the most pristine intraplate-like rocks of the western

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sector representing partial melts of almost pure peridotite mantle (Díaz-Bravo et al., 2014). Such melts also seem to be tapping a more fertile source than that of the calc-alkaline and

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potassic series, which could come from a mantle region that may be locally depleted by

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previous events of melt extraction (Straub et al., 2011).

5.2. On ridge jumps, rifts, mantle plumes, slab windows and rollbacks The existence of intraplate-like rocks in continental arc settings like the TMVB has always

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been a puzzle for geoscientists. Lacking the chemical characteristics that are widely expected in a convergent margin, the tendency among authors has been to assign

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exceptional explanations to their origin, based upon the long-held premise that rock compositions by themselves must hold their very own tectonic connotation. Most of them do, of course, but it is also true that tectonic boundaries are intrinsically dynamic systems and that interpretations extracted from the chemistry of magmatic byproducts alone are prone to be erratic at best, if not completely mistaken. As summarized above, Mexico is an excellent example of how complex tectonic systems can become in a relatively short period of time; but perhaps it is precisely in these capricious parts of the world where we can extract a much deeper understanding of the intricate relationship between magmatism and tectonics.

24

ACCEPTED MANUSCRIPT Earliest reports of intraplate-like rocks in the western sector of TMVB recognized their close association to extensional faulting, and prompted some authors to suggest an origin

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related to an eastward ―jump‖ of the East Pacific Rise (EPR) into the continent (Allan et al.,

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1991; Luhr et al., 1985). This process was held responsible for the rifting of the Jalisco Block; in a similar way as the Baja California Peninsula was detached from the Mexican

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mainland (see also Frey et al., 2007). Likewise, an extensional tectonic regime was

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proposed for the existence of an ―Eastern Alkaline Province‖ that has been migrating southwards along the coast of the Gulf of Mexico since the Oligocene, and which

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intersected the eastern sector of the TMVB proper in the late Miocene (Demant and Robin, 1975; Robin, 1976). A later study by Moore et al. (1994) suggested that intraplate-like

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rocks could be associated to a mantle plume that impacted the western sector of the arc in the late Miocene. The work of Márquez et al., (1999) revisited the plume hypothesis and extended its influence eastward as a result of rifting and upper plate motion. The rifting

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hypothesis was further elaborated by Verma (2002), who not only emphasized the

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importance of tectonic extension for the generation of intraplate-like volcanism, but also for the origin of the rest of the volcanic successions through mixing and crustal contamination, thus neglecting the very existence of an arc for the entire TMVB. Other authors have incorporated intraplate-like volcanism into the dynamic of plate convergence, either associating it to the migration and replenishment of enriched mantle from the rear arc (Luhr, 1997; Wallace and Carmichael, 1999), to the infiltration of deeper and hotter asthenospheric mantle as a consequence of slab rollback (Ferrari et al., 2001; Gómez-Tuena et al., 2003), or through a slab window that resulted from a slab detachment event (Ferrari, 2004).

25

ACCEPTED MANUSCRIPT Intraplate-like rocks have been clearly at the core of the discussions for the past four decades, but regardless of the explanations we choose to embrace, there seems to be

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widespread agreement that at least the most primitive and pristine intraplate-like basalts in

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Mexico must come from a mantle region that has not been extensively modified by the subduction agents. Some crystal fractionation must have occurred during magma ascent

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through the upper plate (Gómez-Tuena et al., 2011; Righter and Rosas-Elguera, 2001), as

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attested in the variable Mg# at fairly constant SiO2 (Fig. 5b), but interaction with the continental crust appears to be minimal or inexistent in most cases, given their almost

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identical compositions to some OIB that had no continent to traverse (e.g. Socorro Island offshore western Mexico; Fig. 6a). Their isotopic compositions strongly support a mantle

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origin too, but Pb isotopes clearly reveal that their source must also be different to the upper mantle that generates MORB, and also distinct from the source being tapped by the rest of the magmas in the TMVB that otherwise carry a significant arc signature (Fig. 8c).

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Absent crustal or slab contributions, their higher

206

Pb/204Pb ratios must then be indicative

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of a long time-integrated isotopic ingrowth of a mantle source with a slightly higher μ than the MORB mantle (Díaz-Bravo et al., 2014; Luhr, 1997). While a possibility exists that such an enriched source resides in the lithospheric mantle (Niu et al., 2011; Pilet et al., 2008), spinel-to-plagioclase-bearing mantle xenoliths collected all across Mexico appear to be too heterogeneous at the local and regional scale (Luhr and Aranda-Gómez, 1997) to generate basaltic lavas that are remarkably alike, regardless if they are emplaced nearly a thousand kilometers apart, across different lithospheric boundaries (Fig. 2), or even in different tectonic settings. Indeed, one must not forget that volcanic rocks with these same geochemical signatures are not exclusive of the TMVB: they are found as scattered cinder cones in an enormous area of the Mexican Basin and 26

ACCEPTED MANUSCRIPT Range province (Aranda-Gómez et al., 2007; Pier et al., 1989), in the extended margins bordering the Gulf of California (Ferrari et al., 2013), in the Pacific Ocean Islands of

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Socorro and Isabel (Bohrson and Reid, 1995, 1997; Housh et al., 2010), and even in

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abandoned spreading centers off the western coast of Baja California (Tian et al., 2011). Similar rocks are also obviously found beyond Mexico, in the rear arc of Central America

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(Hoernle et al., 2008) as well as in the Galapagos Islands (Harpp and White, 2001). The

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evidence thus indicates that the isotopically enriched mantle source of intraplate-like volcanic rocks in Mexico is most likely asthenospheric, and most certainly not exclusive of

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the TMVB, as it seems to occupy a regional area of enormous proportions that has been able to melt in virtually any kind of tectonic setting.

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The enriched isotopic nature of a mantle source like the high-μ, FOZO and the rest of the mantle flavors have been traditionally associated to melting of high-grade metamorphic assemblages or as secondary melt/rock reaction products, such as eclogite or pyroxenite,

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that reside as discrete heterogeneities in the peridotitic mantle for billions of years

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(Hofmann and White, 1982; Niu et al., 2011; Pilet et al., 2008; Stracke et al., 2005; White, 1985; Zindler and Hart, 1986). Surprisingly, Ni contents in olivines from the most primitive and pristine intraplate-like magmas of the western sector of the TMVB are virtually identical to olivines from MORB (Fig. 11b), strongly suggesting that crystallization occurred from a typical peridotite melt (Sobolev et al., 2005). Olivines of intraplate-like rocks from the Chichinautzin Volcanic Field in the eastern sector extend to high Ni contents (Fig. 11b), suggesting contributions from a pyroxenite source instead (Straub et al., 2008, 2011), but their host magmas are not as pure intraplate-like as the ones in the west (Fig. 6a), while their Pb isotopic compositions approach those of more typical calc-alkaline volcanics (Fig. 8c). The olivine evidence from Mexico thus indicates that mantle 27

ACCEPTED MANUSCRIPT pyroxenitization has to be recent and subduction-related (Díaz-Bravo et al., 2014; Straub et al., 2008, 2011), while the enriched source of the background mantle wedge is ancient and

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entirely hosted in peridotite (Díaz-Bravo et al., 2014).

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If the mantle source of intraplate-like volcanic rocks is a peridotite that acquired its isotopic identity through billions of years of radioactive decay, and has been residing underneath

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Mexico and the Pacific basin for tens of millions of years, then there seems to be no reason

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to regard the appearance of intraplate-like volcanic rocks in the TMVB as an infiltration of an extraneous mantle, either by a ―jump‖ of the spreading center, a mantle plume, or

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through the opening of a slab window. The only conditions that have to be met in order to observe the pristine character of such a mantle source is that it melts at its lowest extents,

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under nearly anhydrous conditions, and with almost no influence from the slab-derived chemical agents or the continental crust (Díaz-Bravo et al., 2014; Gómez-Tuena et al., 2003, 2011; Luhr, 1997). Thus, while ridge jumps, mantle plumes and slab windows may very

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well exist, none of these tectonic processes are compulsory circumstances that can

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unequivocally account for the petrogenesis of these rocks. As outlined by Díaz-Bravo et al. (2014), the upper mantle below Mexico must be residing close to partial fusion temperatures, so that very small decompression disturbances can be conducive to melting of its most fusible portions. Decompression melting could occur by intra-arc extension and rifting, of course, but mantle upwelling is also intrinsic to the dynamic of convergence, as dry mantle has to be continuously dragged from the rear arc in order to replenish the mantle that descends by viscous coupling along the sinking slab (Fig. 12). Back-arcs are examples of this process taken to the extent of creating new ocean basins (Langmuir et al., 2006), but several continental margins develop more subtle extensional rear arcs where less volumetric intraplate magmatism is not uncommon (Hickey-Vargas et 28

ACCEPTED MANUSCRIPT al., 1986; Hoernle et al., 2008; Kay et al., 1994). Upwelling of a dry mantle from the rear arc could be restricted during periods of slab flattening, as the mantle wedge is being

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―squeezed‖ backwards or sideways (Manea et al., 2013), but it can become especially

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important, and perhaps even enhanced, during periods of slab rollback. It is thus not surprising that intraplate-like magmas in Mexico did not erupt during the earliest

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evolutionary stages of the arc when volcanism was migrating to the north due to gradual

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flattening of the slab (Manea et al., 2013; Mori et al., 2007).

Slab rollback may therefore be the simplest explanation for the Mexican case, as it also

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accounts for upper plate extension, convection-driven replenishment of a compositionally similar but dryer mantle from the rear arc, low extents of melting due to the existence of a

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thick lithosphere, the preferential eruption of intraplate-like volcanics at the rear arc, and the gradual migration of arc volcanism towards the trench. Within this context, and as will be discussed below, the close relationship that exists between Na-alkaline volcanism and

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typical arc volcanoes must then speak of the pathways followed by subduction agents

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within the mantle wedge, and of the petrologic reactions that transform it during their transit to the surface.

5.3. On mantle veins and supercritical fluids Intraplate-like volcanic rocks best reflect the petrologic and geochemical characteristics of the background mantle wedge, but the rest of the rock sequences in TMVB have compositions that attest for strong modifications of this mantle source by extraneous chemical agents coming either from the subducted slab and/or the overlying continental crust. Distinguishing the specific sources and mechanisms governing these modifications

29

ACCEPTED MANUSCRIPT has been an extremely difficult task, and even if the proposed models may not apply for every single volcano all along the TMVB, many important advances have been achieved

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over the past couple of decades that a consensus on a general framework could possibly be

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reached.

With Mg# that are close to equilibrium with mantle values at relatively low SiO2,

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potassium-alkaline volcanoes in Mexico are the most primitive of all, and therefore they

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best reflect the inherited characteristics of a mantle wedge that has been strongly modified at depth by some kind of slab-derived chemical agent (Gómez-Tuena et al., 2011; Luhr,

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1997). Calc-alkaline volcanoes share many of the same ―subduction features‖ such as the high LILE/HFSE ratios, but they usually have higher SiO2 and lower absolute abundances

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of incompatible trace elements, while mostly preserving a relatively high Mg# up to dacitic and sometimes even rhyolitic compositions (Fig. 5). As will be discussed below, these very simple characteristics are inconsistent with a genetic relationship governed by crystal

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fractionation of a single primitive magma, even if considering a significant amount of

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contamination with the overlying continental crust (Carmichael et al., 1996; Carmichael, 2002; Lassiter and Luhr, 2001; Luhr, 1997; Savov et al., 2008; Wallace and Carmichael, 1989).

The petrologic relationship between mafic calc-alkaline and K-alkaline rocks in the TMVB has been traditionally explained following a classic vein-plus-wall-rock melting relationship described by Foley (1992a), and refined by several authors over the years for the Mexican case (Carmichael et al., 1996; Luhr et al., 1989; Luhr, 1997). In this model, potassic magmas would derive from low extents of melting of a strongly metasomatized mantle source rich in phlogopite, amphibole, pyroxene and apatite; whereas calc-alkaline mafic magmas would form by the incorporation of peridotitic lithologies at larger extents of 30

ACCEPTED MANUSCRIPT melting. Thus, the positively correlated K2O and P2O5 contents displayed by these rocks have been interpreted as a melting relationship, controlled by the continuous breakdown of

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phlogopite and apatite-rich veins that are enclosed in an otherwise depleted mantle

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peridotite. As such, the highly enriched and primitive potassic lamprophyres from western Mexico were regarded by Luhr (1997) as the ―essence‖ of subduction, as they best reflect

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the most pristine effects of the slab-derived chemical agents.

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The concept of a modally metasomatized veined mantle enjoys widespread agreement among researchers, in Mexico and the world over (Conceição and Green, 2004; Foley,

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1992a; Peccerillo, 2005). In detail, however, several important aspects that are intrinsic to the vein model remain unclear, especially those regarding the exact nature of the

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metasomatic agents involved in their formation, as well as their very restricted geographic distribution at the current volcanic front (Fig. 7). Some authors have suggested that these metasomatic veins could be an inherited feature of previous subduction events that have

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affected the Mexican upper mantle at least since the Jurassic (Hochstaedter et al., 1996;

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Lange and Carmichael, 1991; Righter and Rosas-Elguera, 2001), but this model seems inconsistent with the overall similar Sr and Os isotopic compositions of contemporaneous potassic and calc-alkaline volcanoes (Lassiter and Luhr, 2001), and is also at odds with their preferential eruption almost exclusively at the volcanic front (Blatter et al., 2001; Gómez-Tuena et al., 2011). It thus seems more reasonable to assume that if these veins exist, they are most likely being created recently by the release of metasomatic aqueous fluids/melts from the subducted slab (Gómez-Tuena et al., 2011; Luhr, 1997; Vigouroux et al., 2008). And yet more recent studies have shown that some of the peculiar trace element features displayed by potassic magmas in western Mexico, such as their unusually high and 31

ACCEPTED MANUSCRIPT correlated Nb/Ta, Rb/Ta and La/Ta ratios (Fig. 13), are not entirely in agreement with a melting relationship of phlogopite-and-apatite-rich veins (Gómez-Tuena et al., 2011). In

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fact, fractionations between trace elements like Nb and Ta are unusual for a convergent

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margin since they are largely considered as fluid-immobile (McCulloch and Gamble, 1991) and therefore entirely controlled by the mantle (Pearce et al., 2005). Phlogopite could

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potentially host and fractionate these elements during melting (Adam and Green, 2006;

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Green et al., 2000), but this mineral should also be the main host for elements like Rb, and therefore its presence and continuous breakdown would not be able to produce a significant

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enrichment of Rb over a less compatible element like Ta (e.g. high Rb/Ta ratios), while inducing a simultaneous fractionation of the Nb/Ta ratios (Fig. 13). Hence, unless the

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HFSE are hosted and fractionated by a cryptic and as yet unrecognized titaniferous mantle phase, phlogopite could not remain stable during melt extraction of potassium-alkaline magmas (Fig. 13). The existence of apatite controlling the large enrichments of LREE

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suffers from similar complications, because metasomatic apatite can only be formed if very

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large quantities of insoluble elements like P and LREE are transferred to the mantle by the subduction flux.

The trace element evidence appears to be in conflict with a vein-plus-wall-rock melting relationship, but it could be explained by considering that the unusual trace element fractionations do not occur during mantle melting but as a natural consequence of the metamorphic and dissolution reactions affecting potassic, titaniferous and REE-bearing phases in the subducting slab (Gómez-Tuena et al., 2011). Indeed, titaniferous phases are unlikely to remain stable during melting of peridotites (Ryerson and Watson, 1987), but rutile is a major host of titanium and HFSE in basaltic eclogites, as well as a common accessory mineral in high-grade metamorphic sediments (Zack et al., 2002), and its 32

ACCEPTED MANUSCRIPT presence as residual phase during melting can exert a significant control on the abundance and fractionation of the HFSE (Avanzinelli et al., 2009; Green, 1995). Likewise, phengite

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and allanite and/or monazite are the main carriers of LILE and LREE in deeply subducted

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lithologies (Hermann, 2002; Hermann and Spandler, 2008; Schmidt et al., 2004; Skora et al., 2015; Skora and Blundy, 2012; Spandler et al., 2003), and their dissolution and

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potential breakdown must play an important role on the release and fractionation of these

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elements at increasing metamorphic grades (Avanzinelli et al., 2012; Klimm et al., 2008; Plank et al., 2009). The highly enriched nature of potassium-alkaline magmas in Mexico,

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and their unusual trace element fractionations, could then be related to deep and hot slab melts or supercritical fluids, extracted from the slab during the breakdown of minerals like

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phengite, monazite and allanite; with rutile and garnet being the only major residual phases (Gómez-Tuena et al., 2011).

Experimental evidence indicates that deep slab melts or supercritical fluids would likely be

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water-rich and granitic in composition (Schmidt et al., 2004), and therefore they would

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readily react or re-equilibrate with mantle peridotites depending on the ambient temperature and the melt/peridotite ratio (Zanetti et al., 1999). Such an interaction is probably best documented in the Finero peridotite of the Ivrea section of the Alps, where secondary phases such as pyroxene and indeed phlogopite and apatite were formed due to reequilibration of slab-derived silica-rich melts with a mantle harzburgite (Zanetti et al., 1999). Interestingly, the pyroxenitic veins in Finero are almost as mafic as the peridotites themselves (i.e. silica-deficient pyroxenites; Kogiso et al., 2004) probably because the interaction occurred at temperatures slightly above the peridotite hydrous solidus and the proportion of melt to peridotite remained relatively low (Zanetti et al., 1999). Such a mafic and pyroxene-rich lithology from Finero thus represents an appropriate source for creating 33

ACCEPTED MANUSCRIPT mafic and potassium-rich melts that would crystallize high-Ni olivines upon cooling (Fig. 11). Nevertheless, and even if it could be inevitable that these secondary metasomatic

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phases would form by melt/peridotite interactions like in Finero, they should also be the

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ones that contribute preferentially to the formation of K-alkaline mafic melts, not only because of their relatively lower solidi compared to peridotite (Kogiso et al., 2004), but also

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because the trace element correlations in Figure 13 would not be preserved otherwise. In

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other words, a strongly metasomatized mantle wedge could very well be the ultimate source of potassium-alkaline magmas in Mexico and worldwide (Carmichael et al., 1996; Foley,

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1992a), but the main reason it comes into existence is through the interaction with soluterich supercritical fluids or melts released from the slab at very high-grade metamorphic

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conditions (Schmidt et al., 2004). As such, the ―essence‖ of subduction, elegantly expressed by Luhr (1997) for the Mexican potassic lamprophyres, could be just one of the most

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extreme flavors available in the subduction spectrum.

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5.4. On hot zones and slab melts The widely accepted vein-plus-wall-rock melting relationship also embraces calc-alkaline magmas, or at least their most mafic varieties, through an increasing contribution of peridotitic wall-rocks at higher extents of melting (Carmichael et al., 1996; Foley, 1992a). And yet, as mentioned before, true calc-alkaline basalts are rare occurrences in the postPliocene TMVB, even if calc-alkaline andesites are the most abundant rock type. Hence, if the Mexican andesites are derivative liquids from basalt, it is only fair to wonder why the parental basalts remain almost always hidden from view while equally mafic alkaline volcanoes so often erupt. Even more confusing is the fact that during the period of time in

34

ACCEPTED MANUSCRIPT which tholeiitic and calc-alkaline basalts did erupt in high volumes, such as during the late Miocene mafic burst that formed the Los Altos de Jalisco plateau (Mori et al., 2009),

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andesitic stratovolcanoes did not form in conjunction. Clearly, something must have

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changed in the magma generation and/or storage conditions between these evolutionary stages that deserve further investigation. But before explaining the rather unusual case of

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the late Miocene mafic burst, let us first explore the origin of the most emblematic Mexican

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stratovolcanoes and their possible relationship to the tectonic setting. On the basis of experimental and modeling approaches, several workers have demonstrated

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that crystal fractionation of basalt is unable to explain the high Mg# of most Mexican andesites, even if considering significant contamination with the continental crust (Gómez-

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Tuena et al., 2007a; Kelemen et al., 2003; Reubi and Blundy, 2008; Straub et al., 2011). Indeed, crystal fractionation involving olivine, pyroxene, plagioclase, or even amphibole and phlogopite —the most common mineral assemblages observed in thin sections and

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predicted by fractionation models— should invariably produce sharp decreases in Mg#

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without significantly modifying SiO2, create curved or inflected liquid lines of descent in the major element variation diagrams, and promote a sharp increase of the most incompatible trace elements in the derivative liquids. Surprisingly enough, none of these features are observed in the TMVB dataset as whole (Fig. 5 and 6), neither are they recognized in specific volcanic fields or within individual volcanoes (Gómez-Tuena et al., 2007a, 2014a; Mori et al., 2007; Rasoazanamparany et al., 2016; Reubi and Blundy, 2008; Straub et al., 2013). Crystallization coupled with crustal contamination has been suggested in some cases (e.g. Chesley et al., 2002) —the world-famous Parícutin volcano being the most emblematic example (Cebriá et al., 2011b; Wilcox, 1954) —; but even in there, more recent data have revealed a much more complex evolution than previously thought, so that 35

ACCEPTED MANUSCRIPT the very concept of crustal contamination has come under question (Rowe et al., 2011). These observations should not be used to argue that crystal fractionation or crustal

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interactions do not exist; the presence of xenoliths, xenocrysts and phenocrysts are obvious

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reminders that they do. Rather, the evidence indicates that these processes alone do not seem to be governing the petrologic and geochemical variability of the arc as a whole. In

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fact, it has been long recognized that one of the most important petrologic mechanisms

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recorded in the Mexican andesites is not crystal fractionation, but magma mixing (Nixon, 1988a; Straub and Martin-Del Pozzo, 2001). Mixing could be an effective homogenizer of

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mafic and felsic magmatic end-members and can explain the linear arrays of the major elements, the petrographic disequilibrium textures and mineral zonations that are almost

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universally found in andesites, and even possibly the explosiveness of some volcanic eruptions (Sosa-Ceballos et al., 2012). Furthermore, since rhyolitic melts have very low Mg and Fe contents, their incorporation into mantle-derived mafic magmas at moderate

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amounts will not alter significantly the Mg# of the mixture, and thus can potentially explain

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the variable SiO2 contents at fairly constant Mg# (Fig. 5). Mixing is thus largely accepted as a fundamental petrologic mechanism for andesite formation in Mexico and worldwide (Gómez-Tuena et al., 2014b). Nevertheless, it has been extremely difficult to recognize the exact nature of the end-members involved in the mixture, their primary origin, and the precise location where the mixing process occurs. Some of the most influential models on arc magmatism hold that the hybrid nature of andesites are derivative features created from mantle-derived basalts during magma storage and transport within the upper plate crust, in a complex process of melting, assimilation, storage and homogenization ((Eichelberger, 1978; and the MASH zones of Hildreth and Moorbath, 1988). In these models, the chemical imprints of intermediate arc magmas are 36

ACCEPTED MANUSCRIPT largely acquired in deep crustal ―hot zones‖ (Annen et al., 2006), where melting and assimilation of pre-existent crust could potentially play a significant role in governing the

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trace element and isotopic diversity of arc magmas. Contributions from a pre-existent crust

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appear to be especially strong in thick-crusted arcs like the Andes, for instance, where the isotopic compositions and trace element enrichments correlate strongly with simple

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parameters like crustal thickness (Hildreth and Moorbath, 1998).

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Interestingly, and just as in the Andes, the isotopic compositions of andesitic arc volcanoes in Mexico vary strongly along strike of the arc (Fig 10). The isotopic range displayed by

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each stratovolcano is highly variable, but they are generally more enriched than MORB or than intraplate-like volcanoes from the rear arc having the weakest subduction signatures. If

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the compositions of these latter magmas are taken as representative of the pre-existent background mantle wedge, then the isotopic variations displayed by the stratovolcanoes clearly indicate that a compositional overprint from a pre-existent continental crust must

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exist. And yet, unlike the Andes, the isotopic compositions of the Mexican volcanoes do

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not simply correlate with crustal thickness, neither with the nature and composition of the underlying crustal basement. For instance, while the most enriched isotopes are found in Citlaltépetl (Pico de Orizaba) volcano that most likely sits on a Precambrian crust (Fig. 2), the crustal thickness in this region is at least 10 km thinner than below Popocatépetl volcano, which rests over an equally evolved and possibly even Precambrian basement as well (Ortega-Gutiérrez et al., 2012). The continental crust is clearly younger and thinner below the central and western sectors of the belt, but the isotopic compositions of volcanoes of these sectors can be as enriched as in Popocatépetl, with only Colima volcano being a puzzling isotopic outlier to the radiogenic side in the Nd isotopes. Interestingly as well is the fact that some of the most heterogeneous and enriched isotopic compositions are 37

ACCEPTED MANUSCRIPT actually found in the Late Miocene stratovolcanoes of Querétaro (PH-LJ-Z; Mori et al., 2007), which most likely traversed the isotopically depleted Cretaceous oceanic arc

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successions of the Guerrero terrane (Freydier et al., 1996), a crustal basement that should

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be in any case similar to the one below the much isotopically depleted Xinantécatl volcano (Nevado de Toluca).

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While perhaps not as predictable as in the Andes, the isotopic evidence in Mexico clearly

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indicates that a crustal component must be recycling and even melting during the formation of andesitic volcanoes. And yet many different lines of evidence indicate that this is not

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occurring during intrusion and storage in the upper plate crust as the MASH or the ―hot zones‖ models suggest. The fact that mafic cinder cones surrounding large stratovolcanoes

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can be isotopically enriched themselves (Fig. 10) is an obvious first indication that the isotopic imprint must be acquired at depth, in the mantle wedge, and most likely through slab-derived contributions. Of course it could be argued that none of these cinder cones

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erupt truly primitive basaltic mantle melts, so that a small amount of assimilated crust could

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obscure the pristine mantle features. It is in fact true that calc-alkaline basalts almost never erupt. And yet the strongest evidence supporting the idea that the isotopic enrichment should be essentially slab-derived comes from the recently discovered decoupling between the isotopic compositions of oxygen and He in olivine phenocrysts from the Chichinautzin volcanic field and Popocatépetl volcano (Straub et al., 2015). Indeed, Figure 14 shows that while the He isotopes remain mantle-like (4He/3He Ra = ~7) going from basalt to andesite, the oxygen isotopes are all overwhelmingly crustal (δO18 >6‰), and in fact correlate with host-rock SiO2 and the rest of the radiogenic isotopes. Since He in an ancient crustal basement ought to be extremely radiogenic too, the isotopic decoupling strongly indicates that the crustal imprint should be an inherited characteristic of the melt source, and not a 38

ACCEPTED MANUSCRIPT feature acquired during magma cooling and crystallization within the continental crust. Radiogenic He is introduced into the subduction zone along with sediments and eroded

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crustal fragments, but due to its higher diffusivity, He could be easily lost from the

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subducted lithologies at relatively shallow depths and is unlikely to be transferred in significant quantities to the mantle source of arc volcanoes (Bach and Niedermann, 1998).

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An important implication from the isotopic evidence above is that most of the chemical

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attributes of all these melts, including the SiO2 enrichments, should also be essentially slabderived and not crystal fractionation or contamination features as so often assumed (Straub

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et al., 2013). In other words, andesites, and not only basalts, appear to be primitive mantle melts, with a primary origin that should be intimately linked to slab-derived contributions.

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The idea of Mexican andesites being direct mantle extracts was initially conceived on the bases of experimental phase equilibria (Blatter and Carmichael, 2001; Carmichael, 2002; Moore and Carmichael, 1998; Wallace and Carmichael, 1994), but the exact mechanism

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governing their formation has been under extensive debate for over a decade. Some authors

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have proposed that melting of peridotite under water saturated conditions could create andesites directly from the mantle (Blatter and Carmichael, 2001; Weber et al., 2012), but this is only possible if the temperatures and pressures of last equilibration are kept in a very narrow window of conditions (< 1,000 °C and 1 GPa) that are unlikely to be met all across the TMVB. Other authors have shown that at least some Mexican andesites, including both, monogenetic volcanoes (Gómez-Tuena et al., 2007a), as well as large stratovolcanoes like Palo-Huérfano-La Joya-Zamorano (Mori et al., 2007), Tancítaro (Cavazos-Tovar, 2006) and Xinantécatl (Cai et al., 2014; Martínez-Serrano et al., 2004), can be the result of melt/rock interactions between slab-derived siliceous melts and the mantle wedge, a process that is akin to the formation of so-called adakites and high-Mg# andesites (Defant 39

ACCEPTED MANUSCRIPT and Drummond, 1990; Kelemen et al., 2003). The slab-melt hypothesis in Mexico was further supported by the discovery of some exotic trondhjemitic rocks with strong garnet

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signatures and MORB-like radiogenic isotopes (Figs. 8 and 9), compositions that are only

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very rarely found outside the Archaean realm (Gómez-Tuena et al., 2008). And yet not all intermediate rocks in Mexico have the high Sr/Y or the depleted isotopic compositions that

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are indicative of melting of MORB-like eclogites, neither do these geochemical signals

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seem to correlate with other parameters like crustal thickness (Fig. 10) or the predicted thermal structure of the subducted slab (Ferrari et al., 2012). Andesitic stratovolcanoes

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from the western sector of the arc are remarkable in this regard, since they show some of the lowest Sr/Y ratios despite being constructed in one of the warmest subduction zones on

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Earth (Cooper et al., 2012; Ferrari et al., 2012; Syracuse et al., 2010), and in close association with potassium-alkaline magmas that, as shown above, presumably formed by the influx of deep and hot slab-melts or supercritical fluids.

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The isotopic evidence is sufficiently robust in showing that the continental crust must be

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recycling during the formation of most Mexican andesites (Fig. 10). Yet this is not occurring in the upper plate during storage and crystallization, but deep in the subduction zone by the re-melting of subducted crustal lithologies. These may be in the form of oceanic sediments, or most likely of eroded continental fragments ablated by the intense process of subduction erosion that, as shown above, has affected the Mexican continental margin since at least the early Miocene (Clift and Vannucchi, 2004; Morán-Zenteno et al., 1996). Indeed, recent isotopic and trace element evidence from the Chichinautzin volcanic field, Popocatépetl and Xinantécatl volcanoes in eastern sector of the TMVB indicate that coastal and offshore crustal plutons are the dominant recycled component in basalts to dacites with very little, if any, contribution from oceanic sediments (Straub et al., 2015). 40

ACCEPTED MANUSCRIPT Once down-dragged into the mantle along with the oceanic crust, eroded continental rocks are subject to prograde metamorphism so that the composition of fluids or melts that are

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released from them will be dependent on the mineral assemblages that become stable at

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different thermal conditions. Experimental petrology has shown that fluids or melts extracted at shallow depths must be compositionally different from those extracted further

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down, and therefore should impart different geochemical signatures to the source of

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volcanic rocks (Kessel et al., 2005). For instance, Figure 13 shows that potassic rocks from the volcanic front display high Nb/Ta and Gd/Yb ratios, whereas andesitic stratovolcanoes

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are more variable but generally follow an opposite trend. As shown above, the high Nb/Ta and Gd/Yb ratios can be readily explained by the presence of residual rutile and garnet, but

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experimental petrology has shown that the only important mineral phase that can create the opposite effect in these trace element ratios is amphibole (Davidson et al., 2007; Foley et al., 2002). And yet, since amphibole is strongly dependent on pressure (< 2.5 GPa, Poli and

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Schmidt 2002), and can only remain stable as a peritectic phase if melting occurs in the

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presence of excess water (Gardien et al., 2000; Reichardt and Weinberg, 2012), the thermal conditions for andesitic magma generation have to be very different from those of potassium-alkaline magmas. Thus, what is melting and recycling below all Mexican arc volcanoes could very well be similar in terms of bulk compositions (i.e. a subduction mélange), what change are the specific thermal conditions upon which melting and recycling occur: extremely deep and under fluid-absent conditions for the potassic magmas, but much shallower and in the presence of free fluids for calc-alkaline andesites (GómezTuena et al., 2011, 2014a). In summary, the petrologic and geochemical evidence indicate that the hybrid nature of andesites should be mostly acquired at mantle depths through the reaction of felsic slab41

ACCEPTED MANUSCRIPT derived melts with mantle peridotites. As outlined by Straub et al. (2011), this process likely creates discrete segregations of silica-deficient and silica-excess pyroxenites in the

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mantle wedge (Kogiso et al., 2004), that upon remelting could produce a broad spectrum of

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primitive high Mg# basaltic, andesitic, dacitic and possibly even rhyolitic liquids. Some of these melts could ascend rapidly to the surface suffering little secondary modifications

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(Blatter and Carmichael, 1998a; Gómez-Tuena et al., 2008), but most of them would likely

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degas, stall and mix with other melts during transit through the upper plate, creating the textural and geochemical arrays that are typical of intermediate rocks worldwide. While it

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is entirely possible that magma chambers below stratovolcanoes could eventually receive an influx of true basaltic magma (e.g. Crummy et al., 2014), the existence of a large body

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of parental basalt at depth is not really required, as the same textural and compositional characteristics could be achieved by mixing of various batches of compositionally similar andesites (Smith et al., 2009; Wallace and Carmichael, 1994) or even solely by degassing-

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induced crystallization (Frey and Lange, 2011). As such, the paucity of intermediate melt

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inclusions in andesitic rocks —and most importantly the almost complete absence of mafic ones— is probably not because of a dearth of true intermediate liquids as recently suggested (Kent, 2014; Reubi and Blundy, 2009), but perhaps because the plumbing systems below andesitic volcanoes are only rarely nourished by basalts. The more felsic melt inclusions in andesites could simply represent trapped residual liquids derived from a primarily andesitic melt that has cooled, degassed and crystallized to a significant extent in a shallow reservoir (Crabtree and Lange, 2011; Moore and Carmichael, 1998).

42

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5.5. On slab diapirs

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We have shown above that, in a broader picture, the most mafic post-Pliocene rocks are highly potassic at the front and intraplate-like towards the rear arc (Fig. 7). This geographic

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distribution could be simply explained if the slab-derived fluxes concentrate near the trench,

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imprinting stronger ―subduction signatures‖ to arc front volcanoes, while they become less influential towards the rear arc where decompression melting of a dryer upper mantle

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dominates. And yet this simple explanation becomes more complicated when calc-alkaline volcanism is taken into account, because at least five andesitic stratovolcanoes have been

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erupting within the rear arc of the western sector of the TMVB for the past 1 million years (Ceboruco, San Juan, Sangangüey, Tepetiltic and Tequila) (Frey et al., 2013; Gómez-Tuena et al., 2014a; Lewis-Kenedi et al., 2005; Luhr, 2000; Nelson, 1980; Wallace and

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Carmichael, 1994). Despite being constructed in a tectonic scenario in which the subducted slab lies at >300 km depth (Fig. 1), all these volcanoes erupt monotonous andesites with

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petrographic and major element compositions that are almost identical to those found in stratovolcanoes at the active volcanic front like Colima or Popocatépetl, which otherwise rest on top of a subducted slab that does not exceed ~100 km depth. Melting of the slab under amphibolite facies conditions could be a reasonable explanation for Colima volcano, which is closer to the trench and over a relatively shallow slab. It is also possible that subduction of the Rivera Fracture Zone right below Colima volcano (Fig. 1) may have played a role in providing a larger than usual amount of fluids for amphibole to remain stable after melt extraction (Gómez-Tuena et al., 2011; Manea et al., 2014). And yet melting of amphibolite-facies rocks is unlikely for stratovolcanoes further to the east where

43

ACCEPTED MANUSCRIPT no clear influence from fracture zones exists, and entirely unreasonable for stratovolcanoes at the rear arc simply because the subducted Rivera slab lies at depths that are at least 250

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km below the stability field of amphibole (Poli and Schmidt, 2002). Moreover, if potassic

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magmatism is related to melts or supercritical fluids extracted from the slab at extremely high-grade metamorphic conditions, how can they be delivered exclusively at the volcanic

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front and in close proximity to andesitic volcanoes that register the influence of a much

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lower-grade subduction flux?

A solution to this conundrum strongly depends on the specific transport pathways followed

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by the metasomatic agents from the slab to the mantle wedge. Most models assume that material transport occurs in a nearly vertical fashion, so that the slab thermal conditions

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underneath the arc could have a direct influence in the chemical compositions of volcanoes at the surface (Cooper et al., 2012). And yet, as shown above, the chemical compositions of volcanoes in the TMVB do not bear an obvious relationship with the predicted thermal

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structure of the subduction zone (Ferrari et al., 2012): higher temperature slab-fluxes are

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recorded at the volcanic front than at the rear arc of the western sector, whereas andesites with the highest Sr/Y ratios are not located above the slab with the highest geotherm. Importantly as well is that in some locations, andesitic stratovolcanoes have been constructed in close proximity to intraplate-like rocks which otherwise show a negligible influence from the subduction agents. If most of the geochemical features of these rocks are acquired in the mantle source, then the upper mantle below Mexico has to be not only extremely heterogeneous, but also one in which very sharp boundaries must exist. That is, instead of pervasively permeating the entire mantle wedge in the form of ―hydrous curtains‖ (Tatsumi and Eggins, 1995) with a predictable composition that depends on the slab‘s thermal structure (Poli and Schmidt, 2002), slab fluxes in Mexico are apparently 44

ACCEPTED MANUSCRIPT being delivered as discrete and compositionally distinct parcels, that in occasions can even be entirely surrounded by a pristinely dry peridotite. The evidence from the TMVB thus

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clearly indicates that the slab fluxes do not always follow a vertical path and that alternative

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mechanisms of slab recycling must exist.

The very action of subduction relies on the contrasting densities of colliding lithospheres,

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but it has been recognized that most of the lithologies involved in subduction mélanges

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have positive buoyancies with respect to mantle peridotites, even when buried to ultra-high pressure conditions (Behn et al., 2011; Currie et al., 2007; Hacker et al., 2011). Pelagic and

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terrigenous sediments, as well as quartzo-feldspathic eroded continental fragments oppose the strongest resistance to being dragged into the mantle, and even basalts are known to

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become buoyant if fragmented and surrounded by less dense hydrous or felsic materials (Little et al., 2011; Martens et al., 2012). Numerical simulations have also shown that mixtures of hydrous mantle, and partially molten basalts and sediments, can rise buoyantly

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through a network of conduits and diapirs that develop spontaneously at the slab-mantle

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interface as Rayleigh-Taylor thermomechanical instabilities (Gerya, 2011; Gerya et al., 2004; Gerya and Yuen, 2003). Diapiric exhumation of subducted lithologies and their channelized flow within the mantle wedge could potentially play an important role for arc magmatism in Mexico (GómezTuena et al., 2014a; Straub et al., 2015), and in the construction of global continents (Kelemen and Behn, 2016; Marschall and Schumacher, 2012), because diapirs like these carry all the necessary ingredients to form andesitic magmas upon direct partial melting in the upper mantle (Castro et al., 2010). Once detached from the slab, diapirs will be at the mercy of local mantle convection (Fig. 15): they can ascend fast and even propagate laterally, or be dragged down depending on their size and relative buoyancy (Gerya, 2011). 45

ACCEPTED MANUSCRIPT Large and highly buoyant diapirs may be incorporated into the hot core of the mantle wedge, heat up by diffusion, dehydrate and most inevitably undergo extensive partial

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melting at relatively shallow pressures due to their lower solidi (Katz and Rudge, 2011;

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Marschall and Schumacher, 2012). Voluminous hydrous andesites feeding stratovolcanoes can potentially be formed by flux melting of the recycled crustal lithologies, triggered by

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the temperature-dependent decomposition of minerals like chlorite (Grove et al., 2009;

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Pawley, 2003), which likely comprise a significant volume of rising diapirs (Marschall and Schumacher, 2012). Smaller diapirs can detach at a later stage, or be dragged down to much

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greater depths, and remain solid for long if most of their water is carried in minerals like phengite (Hermann and Spandler, 2008; Poli and Schmidt, 2002; Schmidt et al., 2004).

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Since the complete breakdown of minerals like phengite necessitates temperatures >1,000 °C, and these are at least 200 °C higher than those calculated for the slab surface below the arc front (Ferrari et al., 2012), it is clear that potassic magmas cannot come from

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the slab surface either. These most likely form by fluid-absent melting of smaller slab

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diapirs located in the mantle wedge where temperatures as high as 1,300 °C can easily be reached (Gómez-Tuena et al., 2014a).

5.6. On slab detachments and lithospheric drips The volumetric late Miocene mafic burst that formed the Los Altos de Jalisco plateau is volcanologically and compositionally unusual when compared to the typical rock successions found in the rest of the TMVB and to continental magmatic arcs in general (Ferrari et al., 2000a; Mori et al., 2009). And not surprisingly, explanations about its origin have been various and controversial. On one side are models that advocate an external

46

ACCEPTED MANUSCRIPT disturbance of the upper mantle, either in the form of a mantle plume (Márquez et al., 1999; Moore et al., 1994), or by the infiltration of deeper and hotter asthenospheric mantle

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through a slab window that opened due to the collision of the East Pacific Rise to the

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continental margin (Ferrari, 2004). On the other side is a model suggesting that the mafic volcanic burst was the consequence of a sudden lithospheric drip (Mori et al., 2009), related

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to a gradual engrossment and densification of the continental lithosphere during the

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generation of the Sierra Madre Occidental —one of the largest silicic provinces on Earth— and later enhanced by the stagnation of magmas at the base of the crust during a ~10

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million years long period in which subduction was uninterrupted but no volcanism was registered.

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While conceptually distinct, all models coincide in that the unusual mafic burst has to be related to an extraordinary influx of mantle melts, the level of which is difficult to explain by normal subduction processes alone. In the external disturbance models, whether by a

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plume or through a slab window, mantle melting should have occurred as a response of an

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upwelling asthenosphere, that not only brought a hotter and compositionally distinct mantle into the system, but that also conductively heated a pre-existent and likely subductionmodified mantle wedge (Ferrari, 2004). In contrast, melting during the hypothetical lithospheric drip likely occurred by mantle upwelling around the foundering instability, as well as by the injection of hydrous fluids or melts released from the sinking materials (Fig. 16; Mori et al., 2009). In principle, both kinds of models could produce mafic volcanism with similar characteristics, albeit conductive heating is generally considered an extremely slow and inefficient process for magmatism, bringing protracted volcanism over tens of millions of years at very low eruption rates and extents of melting (Turner et al., 1996). As thoroughly elaborated by Mori et al. (2009), these characteristics are, in effect, opposite to 47

ACCEPTED MANUSCRIPT what is observed in Los Altos de Jalisco plateau, which records large extents of melting and volumetric mafic volcanism at much higher extrusion rates. On the other hand, lithospheric

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downwelling can potentially generate voluminous magmatism in a much shorter period of

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time (Elkins-Tanton, 2007), but it will likely promote a significant crustal uplift that is not markedly registered in the area.

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Additional evidence in support or against any of these models is largely circumstantial, and

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relies on a series of indirect observations and assumptions that are difficult to confirm independently. For instance, support for the slab detachment hypothesis comes from recent

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seismic tomographic images, which clearly portray a truncation of the subducted Cocos plate close the mantle transition zone below northeastern Mexico (Husker and Davis, 2009;

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Pérez-Campos et al., 2008). No direct seismic evidence for the proposed foundered block exists, albeit the available tomography for the western and central sectors of the TMVB (Yang et al., 2009) does not reach the area underneath the Los Altos de Jalisco plateau

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where the foundered lithosphere could possibly reside. And yet, it should never be

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forgotten that tomography can only reveal an instantaneous snapshot of the Earth‘s internal structure, but that unfortunately it tells us little about its internal motion (Foulger et al., 2015). In other words, the slab may very well be broken at depth —and a foundered block may be found— but a present-day tomography cannot reveal when and how this truncation occurred and much less explains the origin of volcanic episodes that occurred millions of years in the past. Support for a lithospheric drip similarly depends on indirect geochemical and petrological arguments, such as the deep (>40 km) fractionation of primitive basalts in the garnet stability field and their concurrent contamination with felsic and upper-crustal-like lithologies that appear to reside at those depths. Mori et al. (2009) suggested that such an 48

ACCEPTED MANUSCRIPT unusual fractionation and contamination relationship indicates that the expected maficultramafic arc lower crust should be entirely missing in western Mexico. Interestingly, the

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petrology of deep-crustal granulitic xenoliths in the MGVF (Ortega-Gutiérrez et al., 2014b),

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just southeast of Los Altos plateau, is also consistent with this interpretation, as it indicates that underplating and fractionation of more recent intraplate-like mafic magmas not only

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occurs at ~30 km depth, but that it also provided the necessary heat for the granulitization

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of Paleocene granites that also reside at those depths. The absence of a thick lithospheric mantle root underneath the TMVB is also in accordance with the overall geophysical

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evidence (Gérault et al., 2015; Suhardja et al., 2015).

Despite the enormous advances, understanding the origin of the late Miocene mafic burst

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may still lie ahead of us. One of the keys to the dilemma could be hidden in that 10 million years long period of time following the waning of SMO in which no volcanism has been found despite continuous subduction. Whether this hiatus was caused by an overthickened

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crust (Mori et al., 2009) or by the subduction of an anomalously young slab (Ferrari et al.,

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2012) remains a matter of almost pure speculation, since magmas are known to erupt in even the most thickened crustal conditions, while subduction of young slabs like Rivera or Juan de Fuca has clearly not prevented volcanism to be extremely prolific in western Mexico and Cascadia. A more likely mechanism that could potentially shutdown magmatism is flat subduction, as in Peruvian Andes (Antonijevic et al., 2015); and in fact, a similar mechanism occurred in the eastern part of the TMVB where magmatism was extinguished closer to the trench while gradually migrating to the north (Fig. 3). Flat slab could also have played a role in eroding the continental lithospheric mantle and even parts of the lower crust, but by being significantly denser than the overlying continent, an eclogitic basaltic slab will inevitably sink, either by gradually rolling back, as in the eastern 49

ACCEPTED MANUSCRIPT part of the TMVB, or perhaps even more suddenly by breaking and foundering, in a similar way as envisaged in the lithospheric drip model. A mechanism like this could potentially

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help to reconcile some of the observations, but it is also clear that unless independent data

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are obtained, and the entire geological, geochemical and geophysical evidence is accounted for in a holistic explanation, the origin of a volcanic anomaly like the late Miocene mafic

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burst of the TMVB will remain a fascinating enigma.

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6. Concluding remarks

Convergent margins are extremely complex systems and the classic sketches that we

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incessantly draft in classrooms and in scientific articles are unable to bring justice to their enormous convolutions. Igneous rocks are obviously a reflection of this complexity and by being one of the very few tools at our disposal to explore Earth‘s deepest reaches, the

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compositional information extracted from them has been of large importance to understand

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Earth‘s internal motion. And yet geochemistry is not an infallible recipe, and rock compositions cannot be used to sustain a tectonic argumentation unless the petrologic character of their source and the conditions of melt generation and subsequent evolution are reasonably well understood. What we have learned so far from the TMVB is that rock compositions can change significantly during a relatively short geologic history (~20 m.y.), and that volcanism can be very prolific or completely extinguished, even if plate convergence has been continuous the whole time. Extraordinary explanations are always tempting and indeed attractive when rock compositions depart from ordinary, but it is also true that more simple changes in subduction parameters, like slab geometry, can exert a strong influence in the internal motion of the mantle wedge, as well as in the composition 50

ACCEPTED MANUSCRIPT and delivery mechanisms of the slab-derived chemical agents, which in turn could account for the petrologic diversity.

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Intraplate-like rocks in the TMVB have been the subject of intense debate and scrutiny, but

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the petrologic and geochemical evidence indicate that the only condition that has to be met to create them is that an ordinary peridotite melts at its lowest extents and under nearly

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anhydrous conditions. Dry melting of peridotite could seem counterintuitive for most arcs

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where hydrous melting is supposed to pervade, but geodynamic models have shown that most volcanic arcs are located above a broad region extending from the volcanic front

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towards the back-arc in which temperature approaches or even exceeds the anhydrous solidus of peridotite (Schmidt and Poli, 1998). Decompression melting of an upwelling dry

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mantle is not only expected under these circumstances but may even be a necessary requirement that governs arc front locations (England and Katz, 2010). Consequently, the fact that in Mexico some of these almost pristine anhydrous melts are able to erupt in close

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association with other more typical arc magmas seems to speak more about the transport

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pathways followed by the subduction agents rather than about processes that are extraneous to convergent margins. Potassic rocks from the TMVB are at the opposite end of intraplate magmas, as they record the most extreme conditions of melt generation in arcs. Evidence suggest that these highly enriched magmas should be intimately linked to melting of the subducted materials under very high-grade (>1,000 °C) and possibly even fluid-absent metamorphic conditions, where phases like phengite, monazite and allanite should be completely consumed to the melt. These high-grade metamorphic conditions are unlikely to be met directly at the slab surface below the arc, even if the western subduction zone is widely regarded as one of warmest on Earth (Syracuse et al., 2010). Melting and recycling of the subducted materials should 51

ACCEPTED MANUSCRIPT therefore occur within the mantle wedge itself, where melt transformation and the formation of secondary metasomatic veins rich in phlogopite and pyroxene may be an

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inevitable consequence of melt-rock reactions (Foley, 1992b; Zanetti et al., 1999). And yet

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trace element constraints indicate that these phases should have a very short residence and be consumed to the melt in order to form highly potassic magmas. Nonetheless, since

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metasomatic phlogopite can remain stable up to 4 GPa (Kushiro et al., 1967), the striking

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existence of a potassic volcanic front in Mexico, and of potassic rocks in global arcs, could be related to the very steep subduction angle that allows a highly metasomatized mantle to

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melt at greater depths, and not necessarily a direct consequence of a warmer slab geotherm (Gómez-Tuena et al., 2011). Indeed, it is interesting to note that potassic volcanism in the

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Aeolian arc erupts above one of the coldest subduction zones on Earth (Pasquale et al., 2005), but that it shares the very steep angle that characterizes Rivera plate. In contrast, the Juan de Fuca plate is as young as Rivera and among the warmest in the world (Hacker et al.,

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2003), but differs significantly from Rivera in having a much gentle dip angle below

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Cascadia, a continental arc that is entirely devoid of potassic magmatism (Hildreth, 2007). Calc-alkaline andesitic stratovolcanoes in Mexico are not particularly different from those of other arcs, having a bulk composition that is almost a perfect match to that of global continents (Gómez-Tuena et al., 2014a, 2014b). Just as in every other continental arc, the Mexican andesites clearly represent hybrid magmas made of mixtures between mantle and pre-existent crust. Whether such a mixture occurs during storage and ascent through the overriding plate or through recycling via subduction remains one of the most controversial open questions for geoscientists, as it has important implications for our current understanding of the solid Earth geochemical cycle. Clearly, many uncertainties remain and a broad consensus is yet to be reached, but the mounting evidence from Mexico 52

ACCEPTED MANUSCRIPT summarized above indicates that the vast majority of intermediate rocks, such as those emanating from large stratovolcanoes, should have acquired most of their chemical

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character within the mantle wedge with no significant secondary fractionation, and without

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a perceptible influence from the overlying continental crust. Andesites from the TMVB are therefore direct mantle extracts. The enrichment in silica and the isotopic deviations to

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enriched values should in turn be acquired through melting of subducted materials in the

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form altered oceanic crust, sediments and eroded continental crustal fragments, that are naturally introduced into the mantle as a highly heterogeneous subduction mélange. The

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high Mg# and Ni contents in olivines also bear witness that these melts should induce an intense petrologic transformation of the mantle wedge into secondary pyroxene-rich

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lithologies. While it was generally considered that subducted materials should mainly undergo dehydration reactions and that melting could only occur in the warmest subduction zones with the consequent formation of adakites (Kelemen et al., 2003; Peacock et al.,

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1994), more recent experimental data and models of global volcanism have shown that slab

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melting should be the rule rather than the exception in most if not all subduction zones (Hermann and Spandler, 2008; Klimm et al., 2008; Plank et al., 2009; Turner and Langmuir, 2015b). In other words, subducted materials always melt and adakites are not unequivocal diagnostics of slab melting as the evidence from Mexico presented above clearly demonstrates. The existence of slab diapirs, made of highly heterogeneous subduction mélanges that melt in the core of the wedge at relatively shallow depths and in the presence of free fluids, is a possible explanation for creating primitive andesitic melts with trace element signatures that could be different from classic adakites, but that otherwise derive from melting of the subducted components.

53

ACCEPTED MANUSCRIPT Finally, the evidence described above clearly indicates that no single tectonic, thermal or compositional parameter accounts for the distinctively enriched character of the TMVB

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when compared to global arcs (Fig. 4). Intraplate-like magmas indicate that large portions

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of the peridotitic mantle wedge underneath Mexico are intrinsically enriched, and that they have been so for an extended period of geologic time. And yet the enriched nature of such a

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source is only visible when mantle melting occurs at its lowest extents, and becomes easily

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obscured at larger melt fractions or when overprinted by even the smallest inputs from the subducted slab. Crustal thickness may play a key role in controlling lower extents of mantle

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melting below Mexico in comparison to the thin-crusted arcs of the western Pacific, as it displaces the melting regime to relatively higher pressures and lower temperatures (Turner

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and Langmuir, 2015b). In addition, the relatively warmer thermal structure of the Mexican subduction zone and its unusual geometry have also been considered as important parameters governing the compositions of arc volcanoes, as they could potentially scale to

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the quantity and composition of fluids or melts released from the subducted materials at

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different P-T conditions (Gómez-Tuena et al., 2003, 2011; Mori et al., 2007). In effect it has been shown that arcs above warm slabs are on average more enriched in incompatible trace elements than those above cooler subduction regimes (Cooper et al., 2012). And yet most of the quantitative thermochemical models published to date do not recognize that highly enriched potassic volcanoes exist in arcs with very contrasting thermal structures, such as the TMVB and Aeolian arc, neither do they take into account that most of the elemental recycling could occur within the wedge itself and thus under different thermal conditions than those found at the slab surface. Lastly, like in most continental arcs, melting and recycling of the pre-existent continental crust is also unequivocally supported by the isotopic evidence (Fig. 10), albeit this appears to be mostly occurring deep in the 54

ACCEPTED MANUSCRIPT subduction zone and to a lesser extent during magma ascent through the overriding crust. As such, processes like subduction erosion and crustal recycling by relamination and

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diapirism would need to be taken into consideration by future researchers studying global

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arc volcanism.

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Acknowledgments

We are extremely grateful to all our co-workers and students for their important

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contributions to the various aspects of this work: Ofelia Pérez, Carlos Ortega, Manuel Albarrán, Gabriela Hernández, Eduardo Becerra, Alma Vázquez, Rosaisela Leija, Beatriz

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Díaz, Felipe Rodríguez, José Cavazos, Nelly Rincón, Mattia Parolari, Ana Tavera, Eli Sánchez, David Castillo, Fernando Ortega, Martin Tanner, Alex LaGatta, Georg Zellmer, Dario Tedesco, Steve Goldstein, Charlie Langmuir and Yue Cai. The present manuscript

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benefited greatly from the thorough and insightful suggestions of two anonymous reviewers.

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Editorial handling by Prof. C. Doglioni is also appreciated. This work has been supported by CONACyT (grants 239494 and 39785) and PAPIIT-UNAM (grants IN103907 and IN107810) to A. Gómez-Tuena; DGAPA-UNAM (grant IB100912-2) to L. Mori; and by NSF (grants EAR-07-38707 and EAR-12-2-0481) to S. M. Straub. This contribution is part of a CONACyT group initiative on the Tectonics of Mexico (grant CB164454) led by F. Ortega-Gutiérrez.

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I.K., Rocholl, A., Sigurdsson, I.A., Sushchevskaya, N.M., Teklay, M., 2007. The Amount of Recycled Crust in Sources of Mantle-Derived Melts. Science 316, 412– 417. doi:10.1126/science. 1138113 Sobolev, A.V., Hofmann, A.W., Sobolev, S.V., Nikogosian, I.K., 2005. An olivine-free mantle source of Hawaiian shield basalts. Nature 434, 590–597. doi:10.1038/nature03411 Sosa-Ceballos, G., Gardner, J.E., Siebe, C., Macías, J.L., 2012. A caldera-forming eruption ~ 14,100 14C yr BP at Popocatépetl volcano, México: Insights from eruption dynamics and magma mixing. J. Volcanol. Geotherm. Res. 213–214, 27–40. doi:10.1016/j.jvolgeores.2011.11.001 Sosa-Ceballos, G., Macías, J.L., García-Tenorio, F., Layer, P., Schaaf, P., Solís-Pichardo, G., Arce, J.L., 2015. El Ventorrillo, a paleostructure of Popocatépetl volcano: insights from geochronology and geochemistry. Bull. Volcanol. 77, 1–20. doi:10.1007/s00445-015-0975-2 Spandler, C., Hermann, J., Arculus, R., Mavrogenes, J., 2003. Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes. Contrib. Mineral. Petrol. 146, 205–222. doi:10.1007/s00410-003-0495-5 Stracke, A., Hofmann, A.W., Hart, S.R., 2005. FOZO, HIMU, and the rest of the mantle zoo. Geochem. Geophys. Geosystems 6, Q05007. doi:10.1029/2004GC000824 Straub, S., LaGatta, A., Martin-Del Pozzo, A.L., Langmuir, C.H., 2008. Evidence from high Ni olivines for a hybridized peridotite/pyroxenite source for orogenic andesites from the central Mexican Volcanic Belt. Geochem. Geophys. Geosystems 9, doi:10.1029/2007GC001583. Straub, S., Martin-Del Pozzo, A.L., 2001. The significance of phenocryst diversity in tephra from recent eruptions at Popocatépetl Stratovolcano (central Mexico). Contrib. Mineral. Petrol. 140, 487–510. Straub, S.M., Gómez-Tuena, A., Bindeman, I.N., Bolge, L.L., Brandl, P.A., EspinasaPerena, R., Solari, L., Stuart, F.M., Vannucchi, P., Zellmer, G.F., 2015. Crustal recycling by subduction erosion in the central Mexican Volcanic Belt. Geochim. Cosmochim. Acta 166, 29–52. doi:10.1016/j.gca.2015.06.001 Straub, S.M., Gomez-Tuena, A., Stuart, F.M., Zellmer, G.F., Espinasa-Perena, R., Cai, Y., Iizuka, Y., 2011. Formation of hybrid arc andesites beneath thick continental crust. Earth Planet. Sci. Lett. 303, 337–347. doi:10.1016/j.epsl.2011.01.013 Straub, S.M., Gómez-Tuena, A., Zellmer, G.F., Espinasa-Perena, R., Stuart, F.M., Cai, Y., Langmuir, C.H., Pozzo, A.L.M.-D., Mesko, G.T., 2013. The Processes of Melt Differentiation in Arc Volcanic Rocks: Insights from OIB-type Arc Magmas in the Central Mexican Volcanic Belt. J. Petrol. 54, 665–701. doi:10.1093/petrology/egs081 Straub, S.M., Zellmer, G.F., Gómez-Tuena, A., Espinasa-Pereña, R., Pozzo, A.L.M., Stuart, F.M., Langmuir, C.H., 2014. A genetic link between silicic slab components and calc-alkaline arc volcanism in central Mexico. Geol. Soc. Lond. Spec. Publ. 385, 31–64. doi:10.1144/SP385.14 Suhardja, S.K., Grand, S.P., Wilson, D., Guzman-Speziale, M., Gomez-Gonzalez, J.M., Dominguez-Reyes, T., Ni, J., 2015. Crust and subduction zone structure of Southwestern Mexico. J. Geophys. Res. Solid Earth 120, 2014JB011573. doi:10.1002/2014JB011573 73

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Syracuse, E.M., van Keken, P.E., Abers, G.A., 2010. The global range of subduction zone thermal models. Phys. Earth Planet. Inter. 183, 73–90. doi:10.1016/j.pepi.2010.02.004 Tatsumi, Y., Eggins, S., 1995. Subduction zone magmatism. Blackwell Science, Cambridge London Edinburgh Carlton Paris Berlin Vienna. Tian, L., Castillo, P.R., Lonsdale, P.F., Hahm, D., Hilton, D.R., 2011. Petrology and Sr-NdPb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico. Geochem. Geophys. Geosystems 12, n/a–n/a. doi:10.1029/2010GC003319 Torres, R., Ruiz, J., Patchett, P.J., Grajales, M.J., 1999. Permo-Triassic continental arc in eastern Mexico: Tectonic implications for reconstructions of southern North America, in: Bartolini, C., Wilson, J.L., Lawton, T.F. (Eds.), Mesozoic Sedimentary and Tectonic History of North-Central Mexico. Geol. Soc. Am. Spec. Pap., Boulder, CO, pp. 191–196. Turner, S., Hawkesworth, C., Gallagher, K., Stewart, K., Peate, D., Mantovani, M., 1996. Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: Assessment of a conductive heating model and application to the Paraná. J. Geophys. Res. Solid Earth 101, 11503–11518. doi:10.1029/96JB00430 Turner, S.J., Langmuir, C.H., 2015a. The global chemical systematics of arc front stratovolcanoes: Evaluating the role of crustal processes. Earth Planet. Sci. Lett. 422, 182–193. doi:10.1016/j.epsl.2015.03.056 Turner, S.J., Langmuir, C.H., 2015b. What processes control the chemical compositions of arc front stratovolcanoes? Geochem. Geophys. Geosystems 16, 1865–1893. doi:10.1002/2014GC005633 Urrutia-Fucugauchi, J., Böhnel, H., 1987. Tectonic interpretation of the Trans-Mexican Volcanic Belt. Tectonophysics 138, 319–323. Urrutia-Fucugauchi, J., Flores-Ruiz, J.H., 1996. Bouger gravity anomalies and regional crustal structure in central Mexico. Int. Geol. Rev. 38, 176–194. Valdéz-Moreno, G., Aguirre-Díaz, G., López-Martínez, M., 1998. El Volcán La Joya, Edos. de Querétaro y Guanajuato. Un estratovolcán antiguo del cinturón volcánico mexicano. Rev. Mex. Cienc. Geológicas 15, 181–197. Valdez-Moreno, G., Schaaf, P., Macías, J.L., Kusakabe, M., 2006. New Sr-Nd-Pb-O isotope data for Colima volcano and evidence for the nature of the local basement. Geol. Soc. Am. Spec. Pap. 402, 45–63. doi:10.1130/2006.2402(02) Valencia, V.A., Righter, K., Rosas-Elguera, J., López-Martínez, M., Grove, M., 2013. The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane. Contrib. Mineral. Petrol. 166, 801–824. doi:10.1007/s00410-013-0908-z Verma, S.P., 2002. Absence of Cocos plate subduction-related basic volcanism in southern Mexico: A unique case on Earth? Geology 30, 1095–1098. Verma, S.P., 2000. Geochemistry of the subducting Cocos plate and the origin of subduction-unrelated mafic volcanism at the front of the central Mexican Volcanic Belt, in: Delgado-Granados, H., Aguirre-Díaz, G., Stock, J.M. (Eds.), Cenozoic Tectonics and Volcanism of Mexico. Geological Society of America Special Paper 334, pp. 1–28. Verma, S.P., Carrasco-Núñez, G., 2003. Reapprisal of the Geology and Geochemistry of Volcán Zamorano, Central Mexico: Implications for Discriminating the Sierra 74

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Madre Occidental and Mexican Volcanic Belt Provinces. Int. Geol. Rev. 45, 724– 752. Verma, S.P., Luhr, J.F., 2010a. Sr, Nd, and Pb isotopic evidence for the origin and evolution of the Cántaro-Colima volcanic chain, Western Mexican Volcanic Belt. J. Volcanol. Geotherm. Res. 197, 33–51. doi:10.1016/j.jvolgeores.2010.08.019 Verma, S.P., Luhr, J.F., 2010b. Sr, Nd, and Pb isotopic evidence for the origin and evolution of the Cántaro-Colima volcanic chain, Western Mexican Volcanic Belt. J. Volcanol. Geotherm. Res. 197, 33–51. doi:10.1016/j.jvolgeores.2010.08.019 Verma, S.P., Nelson, S.A., 1989. Isotopic and trace element constraints on the origin and evolution of alkaline and calc-alkaline magmas in the northwestern Mexican volcanic belt. J. Geophys. Res. 94, 4531–4544. Vigouroux, N., Wallace, P.J., Kent, A.J.R., 2008. Volatiles in High-K Magmas from the Western Trans-Mexican Volcanic Belt: Evidence for Fluid Fluxing and Extreme Enrichment of the Mantle Wedge by Subduction Processes. J. Petrol. 49, 1589– 1618. doi:10.1093/petrology/egn039 Wallace, P.J., Carmichael, I.S.E., 1999. Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions. Contrib. Mineral. Petrol. 135, 291– 314. Wallace, P.J., Carmichael, I.S.E., 1994. Petrology of Volcán Tequila, Jalisco, México: disequilibrim phenocryst assemblages and evolution of the subvolcanic magma system. Contrib. Mineral. Petrol. 117, 345–361. Wallace, P.J., Carmichael, I.S.E., 1989. Minette lavas and associated leucitites from the western front of the Mexican Volcanic Belt: petrology, chemistry and origin. Contrib. Mineral. Petrol. 103, 470–492. Wang, Z., Gaetani, G., 2008. Partitioning of Ni between olivine and siliceous eclogite partial melt: experimental constraints on the mantle source of Hawaiian basalts. Contrib. Mineral. Petrol. 156, 661–678. doi:10.1007/s00410-008-0308-y Weber, B., Scherer, E.E., Martens, U.K., Mezger, K., 2012. Where did the lower Paleozoic rocks of Yucatan come from? A U–Pb, Lu–Hf, and Sm–Nd isotope study. Chem. Geol. 312–313, 1–17. doi:10.1016/j.chemgeo.2012.04.010 Weber, R., Wallace, P., Dana Johnston, A., 2012. Experimental insights into the formation of high-Mg basaltic andesites in the trans-Mexican volcanic belt. Contrib. Mineral. Petrol. 163, 825–840. doi:10.1007/s00410-011-0701-9 White, W.M., 1985. Sources of oceanic basalts: Radiogenic isotopic evidence. Geology 13, 115–118. doi:10.1130/0091-7613(1985)13<115:SOOBRI>2.0.CO;2 Wilcox, R.E., 1954. Petrology of Paricutin Volcano, Mexico. U. S. Geol. Surv. Bull. 965–C, 281–354. Yang, T., Grand, S.P., Wilson, D., Guzman-Speziale, M., Gomez-Gonzalez, J.M., Dominguez-Reyes, T., Ni, J., 2009. Seismic structure beneath the Rivera subduction zone from finite-frequency seismic tomography. J. Geophys. Res. Solid Earth 114. doi:10.1029/2008JB005830 Zack, T., Kronz, A., Foley, S., Rivers, T., 2002. Trace elementabundances in rutiles from eclogites and associated garnet mica schists. Chem. Geol. 184, 97–122. Zanetti, A., Mazzucchelli, M., Rivalenti, G., Vannucci, R., 1999. The Finero phlogopiteperidotite massif: an example of subduction-related metasomatism. Contrib. Mineral. Petrol. 134, 107–122. 75

ACCEPTED MANUSCRIPT Zindler, A., Hart, S., 1986. Chemical geodynamics. Ann Rev Earth Planet Sci 14, 493–571.

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Figure captions 1. Tectonic map of the Mexican Subduction Zone and the Transmexican Volcanic Belt.

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Slab depth contours (km) as outlined by Ferrari et al. (2012) are based on hypocenters of local and teleseismic earthquakes (Pardo and Suárez, 1993, 1995),

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on a receiver function analysis of teleseismic waves and on seismic tomographies

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(Melgar and Pérez-Campos, 2011; Pérez-Campos et al., 2008; Yang et al., 2009). Slab ages (Ma) and convergence rates (cm/year) based on offshore seismic and

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magnetic surveys (DeMets and Traylen, 2000; Pardo and Suárez, 1995; PeláezGaviria et al., 2013). Also shown are locations of some large andesitic stratovolcanoes; from west to east: Sangangüey (S), Ceboruco (Ce),Tequila (Tq),

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Colima (C), Tancítaro (T), Palo Huérfano (Ph), La Joya (J), Zamorano (Z), Xinantécatl (X; also known as Nevado de Toluca), Popocatépetl (P), Malinche (M),

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Cerro Grande (Cg), Citlaltépetl (Ci; also known as Pico de Orizaba). Miocene stratovolcanoes in orange, Quaternary stratovolcanoes in red. Stippled lines show the subdivision of the TMVB in western, central and eastern sectors.

2. Regional crustal thickness of southern Mexico (km) as outline by Ferrari et al. (2012) based on seismic (Suhardja et al., 2015) and gravimetric profiles (MolinaGarza and Urrutia-Fucugauchi, 1993; Urrutia-Fucugauchi and Flores-Ruiz, 1996). Inferred basement geology underneath the TMVB according to Ortega-Gutiérrez et al. (2014). Guerrero Morelos Platform (GMP). Also shown are localities with deep

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ACCEPTED MANUSCRIPT crustal xenoliths: Valle Santiago (VS; Ortega-Gutiérrez et al., 2014b), Pepechuca (Pep; Elías-Herrera and Ortega-Gutiérrez, 1997) and Chalcatzingo (Chal; Chal;

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Ortega-Gutiérrez et al., 2012). Stippled lines show the subdivision of the TMVB in

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western, central and eastern sectors. Large andesitic stratovolcanoes as in figure 1.

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3. The geologic history of the TMVB based on regional geologic maps (Ferrari et al.,

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2005a; Ferrari et al., 2012; Gómez-Tuena et al., 2007b). (A) The Miocene intermediate and mafic TMVB. Miocene stratovolcanoes in orange as in figure 1.

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Numbers in boxes refer to the age range of volcanic rocks (in Ma). Los Altos de Jalisco plateau (LAJ); Palma Sola massif (PS). Also shown is early Miocene

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volcanism in the Sierra Madre del Sur (SMS) and locations for Mexico City (M) and Guadalajara City (G). (B) The latest Miocene to Pliocene silicic and bimodal TMVB. (C) The Pliocene to Holocene TMVB. Mascota volcanic field (MAS),

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Michoacán-Guanajuato volcanic field (MGVF), Valle de Bravo-Zitácuaro volcanic

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field (VBZ), and Chichinautzin volcanic field (CVC). Large andesitic Quaternary stratovolcanoes as in figure 1.

4. Chemical compositions of rocks from the TMVB compared to a thin-crusted arc like Izu-Bonin and the thick-crusted Andean arc. The chemical compositions of volcanic rocks in the TMVB are remarkably similar to the Andes, except for the potassium-rich varieties. (A) Total alkali versus silica classification diagram (Le Bas et al., 1986). Line dividing the alkaline and subalkaline fields from McDonald and Katsura (1964). Potassic and sodic subdivision of alkaline rocks based on Na2O-2 = K2O (Le Bas et al., 1986). Basalt (B), basaltic-andesite (BA), andesite (A), 77

ACCEPTED MANUSCRIPT dacite (D) and rhyolite (R). (B) Potassium versus silica classification diagram (Ewart, 1982). (C) La/Yb versus La variation diagram. (D) Silica distribution of

the

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analyzed samples in the TMVB compared to the Andean and Izu-Bonin arcs. Data georoc

database

(http://georoc.mpch-mainz.gwdg.de/georoc/). A compilation of published data for

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the TMVB is provided in electronic Appendix 1. For simplicity, data is plotted as

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provided in the original publications without any artificial filtering or normalization.

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5. Summary of major element compositions of the entire TMVB dataset compared to selected rock series for better clarity. (A) TiO2/K2O vs. SiO2. (B) Mg# vs. SiO2.

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Calculated molar Mg#= Mg/(Mg+0.85Fe2+tot). (C) Mg# vs. FeOtot. (D) CaO vs. SiO2. Unfilled triangles, diamonds and circles represent potassic, sodic and calc-alkaline rocks following the conventional classification scheme as in Figure 4. Green

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triangles are primitive potassic rocks from the volcanic front of the western sector

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(Gómez-Tuena et al., 2011). Blue diamonds are mafic intraplate-like rocks from the rear arc of the western sector (Díaz-Bravo et al., 2014; Gómez-Tuena et al., 2014a). Orange dots are calc-alkaline rocks from major stratovolcanoes. Yellow dots are trondhjemitic rocks from the Miocene Chalcatzingo domes in the eastern sector (Gómez-Tuena et al., 2008). Black line is a simple mixing model between a calcalkaline basaltic-andesite (RTZ-11-04) and an evolved rhyolite (TQ-10-08) from the western sector (Gómez-Tuena et al., 2014a). Red line is an isobaric fractional crystallization MELTS model (Ghiorso and Sack, 1995) of the same basalticandesite at 1GPa, starting at a liquids temperature of 1300 °C, with a fixed oxygen

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6. N-MORB normalized trace element patterns of representative samples of the main rock series all across the TMVB. Plotted data are provided in Table 1. (A)

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Representative intraplate-like rocks from the rear arc of the western sector in the

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Tepic-Zacoalco rift (TZR; sample TPZ-10-23; Díaz-Bravo et al., 2014), the Michoacán-Guanajuato volcanic field of the central sector (MGFV; sample MG-05-

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14; Ortega-Gutiérrez et al., 2014), the Chichinautzin (CVF; sample CH-09-11; Straub et al., 2011) and the Palma Sola (PS; sample PS-99-25; Gómez-Tuena et al.,

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2003) volcanic fields of the eastern sector. The gray field represents the trace element variation of Socorro ocean island basalts (Bohrson and Reid, 1995). (B) Representative potassic rocks from the western volcanic front in the Mascota and

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Colima volcanic fields (samples JAL-05-33 and JAL-07-12B; Gómez-Tuena et al.,

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2011), the MGVF and the Valle de Bravo-Zitácuaro volcanic field in the central sector (samples MG-05-52 and ZIT-99-12; Cavazos-Tovar, 2006; Gómez-Tuena et al., 2007a), and CVC of the eastern sector (sample CH-07-19; Straub et al., 2015). (C) Average trace element compositions of major calc-alkaline andesitic stratovolcanoes.

7. Geographic distribution of TiO2/K2O ratios in post-Pliocene alkaline rock samples with MgO contents > 4 wt.%. Highly potassic rocks erupt exclusively at volcanic front whereas intraplate-like volcanoes are more abundant towards the rear arc.

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rocks following the conventional classification scheme as in Figure 4.

Nd/144Nd vs. 87Sr/86Sr for potassic and intraplate-like rocks and (B) calc-alkaline

and trondhjemitic rocks. (C)

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Pb/204Pb vs.

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Pb/204Pb for potassic and intraplate-

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Representative rocks for each series as in Figure 5 are shown for better clarity. (A)

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like rocks and (D) calc-alkaline and trondhjemitic rocks. Note that high Nb/La ratios in alkaline rocks correlate positively with

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Pb/204Pb, indicating that

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intraplate-like rocks with a moderately high-isotopic composition (or FOZO) could be sodic or potassic in terms of the major elements. Also shown: data field of

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the EPR-MORB, corresponding to Rivera and Cocos plates (Lehnert et al., 2000; http://www.petdb.org/); the isotopic composition of Cocos plate sediments and altered oceanic crust (AOC) drilled at DSDP Site 487 (LaGatta, 2003; Verma,

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2000); and data field intraplate-like rocks from the Mexican Pacific Islands of

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Socorro and Isabel (Bohrson and Reid, 1997; Housh et al., 2010), the Mexican Basin and Range (http://georoc.mpch-mainz.gwdg.de/georoc/) and fossil spreading centers off the western coast of Baja California (Tian et al., 2011).

9. (A) Sr/Y vs. Y and (B) La/Yb vs. Yb variation diagrams highlight the existence of adakite-like rocks in all rock series of the TMVB. The strongest garnet signatures are observed in potassium-rich alkaline magmas from the volcanic front. Unfilled triangles, diamonds and circles represent potassic, sodic and calc-alkaline rocks

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units for each series as in Figure 5 are shown for better clarity.

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10. Isotopic profiles of calc-alkaline volcanic rocks along strike of the TMVB. (A) Nd/144Nd and (B) 87Sr/86Sr vs. longitude for major andesitic stratovolcanoes. (C)

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Nd/144Nd and (D) 87Sr/86Sr vs. longitude for mafic and intermediate calc-alkaline

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monogenetic volcanism with MgO > 5 wt.%. The isotopic composition of the background mantle wedge is defined by the most primitive intraplate-like rocks and

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MORB (Mantle wedge). Grey field encloses Miocene stratovolcanoes. Data sources from west to east: San Juan (Luhr, 2000), Tepetiltic (Petrone et al., 2003; Verma

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and Nelson, 1989); Sangangüey (Gómez-Tuena et al., 2014a; Verma and Nelson, 1989), Ceboruco (Nelson, 1980; Wallace and Carmichael, 1994), Tequila (GómezTuena et al., 2014a; Wallace and Carmichael, 1994), Colima (Hochstaedter et al.,

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1996; Lassiter and Luhr, 2001; Luhr, 1997; Valdez-Moreno et al., 2006; Verma and

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Luhr, 2010b), Tancítaro (Cavazos-Tovar, 2006; Ownby et al., 2010), the Miocene volcanoes of Palo-Huérfano-La Joya-Zamorano (Mori et al., 2007; Verma and Carrasco-Núñez, 2003), Xinantécatl-Nevado de Toluca (Cai et al., 2014; MartínezSerrano et al., 2004), Iztaccíhuatl (Nixon, 1988b), Popocatépetl (Cai et al., 2014; Robin, 1982; Schaaf et al., 2005; Sosa-Ceballos et al., 2015; Straub et al., 2011, 2014), Malinche (A. Gómez-Tuena and S.M. Straub, unpublished data) and Citlaltépetl-Pico de Orizaba (Cai et al., 2014; Schaaf and Carrasco-Núñez, 2010).

11. Olivine forsterite (mole%) vs. Ni (ppm) of TMVB olivines. (A) Olivines from potassic and calc-alkaline rocks have more Ni than (B) olivines from intraplate-like 81

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MORB olivines. Most forsteritic olivines in intraplate-like rocks of the western

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TMVB have Ni contents similar to olivines from MORB. Fields labeled ―peridotite‖ and ―pyroxenite‘ are from Straub et al. (2011) and illustrate olivines in equilibrium

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with partial melts of peridotite and pyroxenite, respectively. Details of models can

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be found in Straub et al. (2008; 2011). Olivine data from Straub et al. (2008; 2011; 2013a; 2015) and Diaz-Bravo et al. (2014). Olivines from MORB and thick-

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lithosphere intraplate magmas after Sobolev et al (2007).

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12. The tectonic origin of intraplate-like magmatism as a consequence of slab rollback. Low extents of decompression melting of a relatively dry peridotitic mantle are a natural consequence of convergent geodynamics but can be enhanced during

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periods of slab rollback.

13. Processes of elemental recycling in the Mexican subduction zone. (A) Rb/Ta ratios correlate positively with Nb/Ta, (B) Gd/Yb and (b) La/Ta ratios in potassic volcanic rocks from the volcanic front, whereas andesitic stratovolcanoes usually follow the opposite trend. Potassic rocks require a high temperature subduction flux extracted from an eclogite residuum rich in garnet and rutile, while andesitic stratovolcanoes necessitate a low temperature flux extracted from an amphibole-rich lithology. Unfilled triangles, diamonds and circles represent potassic, sodic and calc-alkaline rocks respectively, following the conventional classification scheme in Figure 4. Some representative rock units are shown for better clarity: green triangles are 82

ACCEPTED MANUSCRIPT primitive potassic rocks from the volcanic front of the western sector (GómezTuena et al., 2011); blue diamonds are mafic intraplate-like rocks from the rear arc

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of the western sector (Díaz-Bravo et al., 2014; Gómez-Tuena et al., 2014a); orange

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dots are andesitic rocks from Tequila volcano (Gómez-Tuena et al., 2014a).

Nd/144Nd and (D)

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Sr/86Sr vs. SiO2 from the Chichinautzin volcanic field in the

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14. (A) 3He/4He Ra and (B) δ18Omelt determined in olivine phenocrysts; (C) bulk rock

eastern TMVB (Straub et al., 2011, 2014, 2015). δ18Omelt recalculated from

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measured δ18Ooliv [δ18Omelt = δ18Ooliv +0.088*SiO2-3.57] after Bindeman (2008). Red arrows illustrate the effects of assimilation with continental crustal rocks

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having radiogenic 3He/4He Ra and high δ18O (for modeling details see (Straub et al., 2014). 3He/4He Ra in MORB and continental crust from Farley and Neroda (1998)

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and O‘Nions and Oxburgh (1988); δ18O in mantle rocks from Bindeman (2008).

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15. The tectonic origin of subduction-related volcanism as melts of rising diapirs made of mixtures of hydrous mantle, subducted basalts, sediments and eroded crustal fragments (i.e. a subduction mélange). Calc-alkaline andesites, such as those erupting from large stratovolcanoes, form by fluid-present melting of large diapirs at low P-T, triggered by decomposition of minerals like chlorite (Marschall and Schumacher, 2012). Highly potassic volcanoes form by fluid-absent melting of smaller diapirs by decomposition of minerals like phengite, allanite and monazite at high P-T conditions (Schmidt et al., 2004). Formation of secondary pyroxenites by melt/rock reaction within the peridotitic mantle should also occur under these circumstances. 83

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16. The tectonic origin of the late Miocene mafic burst forming the Los Altos de Jalisco

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plateau as a consequence of lithospheric dripping (i.e. delamination). Foundering

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beneath the region generated enhanced magmatism by a combination of hydrousfluxing and decompression melting of the mantle flowing around the foundered

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mass. Graphic representation of dripping after Elkins-Tanton (2007).

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Figure 16

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