Cerro Grande volcano: the evolution of a Miocene stratocone in the
Transcripción
Cerro Grande volcano: the evolution of a Miocene stratocone in the
Tectonophysics 318 (2000) 249–280 www.elsevier.com/locate/tecto Cerro Grande volcano: the evolution of a Miocene stratocone in the early Trans-Mexican Volcanic Belt Arturo Gómez-Tuena a, *, Gerardo Carrasco-Núñez b a Instituto de Geologı́a, Universidad Nacional Autónoma de México, Cd. Universitaria, Coyoacán 04510, Mexico, DF b Unidad de Investigación en Ciencias de la Tierra, Instituto de Geologı́a, UNAM, Campus Juriquilla, Apdo. Postal 1-742, Querétaro, Qro. 76230, 04510, Mexico Received 4 July 1998; accepted for publication 30 August 1999 Abstract Cerro Grande volcano is a well-preserved, low-angle composite volcano that evolved between 11 and 9 Ma. It represents the beginning of volcanism in the eastern sector of the early Trans-Mexican Volcanic Belt. Extensive field, stratigraphic and petrologic work revealed a complex volcanic evolution that can be summarized in six principal stages. (I ) construction of a shield-like lava cone; (II ) generation of unusual lithic-rich ignimbrites, apparently related to powerful eruptions that caused rapid and significant vent erosion — these eruptions could have resulted from shallow magma–water interactions; (III ) eruption of peripheral fissural lava flows following a NNW–SSE-trending fissure system; (IV ) period of dome growth and explosive collapse; ( V ) short period of repose followed by open-vent explosive magmatic and hydromagmatic eruptions; ( VI ) radial eruption of lava flows, related to a ring-fissural system. Geochemistry data of late Miocene volcanics show a continental arc magmatic origin, and show that crystal fractionation contributed strongly to magma differentiation. Several lava flows were erupted with a persistent NNW– SSE-trending orientation, and the entire late Miocene geologic record is affected by highly dislocated normal faults with the same orientation; this may indicate that late Miocene volcanism was under tectonic control. Finally, late Miocene volcanics, and the inception of the early Trans-Mexican Volcanic Belt, could be related to a shallow subduction angle influenced by a significant increase in the convergence rate along the Middle American Trench at about 20 Ma. © 2000 Elsevier Science B.V. All rights reserved. Keywords: hydromagmatic eruptions; late Miocene; lithic-rich ignimbrites; Mexico; Trans-Mexican Volcanic Belt; volcanic evolution 1. Introduction The evolution of the Trans-Mexican Volcanic Belt ( TMVB) as a distinctive geologic entity has been a long-standing geological problem for the last three decades. However, in the last few years, a substantial amount of information on the space– time distribution of volcanic rocks, and the * Corresponding author. Fax: +52-5-622-4326. E-mail address: [email protected] (A. Gómez-Tuena) recognition of geometric variations in the subduction system, has provided important insights into the origin of the magmatic arc. However, it is surprising that very few systematic studies have been written about the evolution of individual volcanoes in Mexico, and even fewer about ancient volcanic centers. Miocene volcanism is particularly important and needs to be well documented because of its implications on our understanding of the inception of the TMVB. In this regard, the basic geologic investigations, such as mapping, stratigraphy and petrology, must be achieved in 0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 04 0 - 1 95 1 ( 9 9 ) 00 3 1 4- 5 250 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 order to obtain a better framework of the temporal and spatial variations of the arc and a better understanding of crustal evolution in Mexico. Cerro Grande volcano is located in La Sierra de Tlaxco mountain range, in east-central Mexico ( Fig. 1). Geologically, it is situated at the intersection of the TMVB with the Mesozoic Sedimentary Fold and Thrust Province (Ortega-Gutiérrez et al., 1992). Effusive, explosive and volcaniclastic material associated with the activity of Cerro Grande volcano occupies almost 2000 km2 ( Fig. 2). The most conspicuous geomorphologic feature is a 4.5 km in diameter and 650 m deep caldera-like structure truncated in its NE flank by a NW–SEtrending normal fault ( Fig. 2). Previous work in Cerro Grande volcano includes a regional geologic map with a general stratigraphy (Carrasco-Núñez et al., 1997), two technical reports from the local electricity company (CFE) (Garduño et al., 1985; Carrasco-Núñez et al., 1995), and a regional geological recognition (López-Hernández, 1995). Here, we present a comprehensive study on the evolution of Miocene volcanism and discuss its implications on the early development of the TMVB. An interesting aspect from the volcanological point of view was found in a distinctive stratigraphic level of Cerro Grande volcano. This horizon displays a highly indurated lithic-rich pyroclastic rocksequence that is apparently related to powerful volcanic eruptions that caused rapid and significant vent erosion. The mechanism of formation of such eruptions is still not completely understood. This is because the only well-documented similar deposit, known as the Roque Nublo Formation in the Island of Gran Canaria (Spain), has been interpreted in many different ways (Anguita, 1973; Brey and Schmincke, 1980; Perez-Torrado et al., 1995). Nevertheless, recent studies in this rock-succession showed that its lithologic character can be associated with intense hydromagmatic explosions (PerezTorrado et al., 1997). We conclude that a similar phenomenon played a significant role in the evolution of Cerro Grande volcano. 2. Geologic setting: the Trans-Mexican Volcanic Belt The TMVB is a continental magmatic arc 1000 km long and between 20 and 200 km wide that extends from the Pacific Coast to the Gulf of Mexico ( Fig. 1). The magmatic arc crosses central Mexico from east to west, delimited by the 19 and 21° N parallels, with an oblique (~16°) distribution relative to the Middle American Trench (MAT ). This particular orientation led to the definition of the arc as the Trans-Mexican Volcanic Belt because it is clearly transversally emplaced over most of the NW–SE-trending Mexican geologic provinces (Ortega-Gutiérrez et al., 1992). Several historic volcanic eruptions (Simkin and Siebert, 1994), active faulting (Suter et al., 1991, 1995a,b) and high heat flow (Polak et al., 1985; Prol-Ledezma and Juárez, 1985) have been recognized as prominent features of its activity. Over a century, several hypothesis had been proposed for the origin and the striking nonparallelism of the volcanic arc relative to the trench (Humboldt, 1808; De Cserna, 1958; Mooser, 1972; Gastil and Jensky, 1973; Johnson and Harrison, 1989). Now, it is generally accepted that the TMVB is related to geometric variations in the subducting angle of the Cocos and Rivera Plates underneath the North American Plate ( UrrutiaFucugauchi and Del Castillo, 1977; UrrutiaFucugauchi and Böhnel, 1987; Pardo and Suárez, 1993, 1995). The petrologic spectrum of the TMVB is dominated by calc-alkaline andesites and dacites (Aguilar-y-Vargas and Verma, 1987). However, several alkaline volcanic rocks have been extruded in the eastern and western limits of the province (Cantagrel and Robin, 1979; Luhr and Carmichael, 1985; Negendank et al., 1985; Besch et al., 1988; Nelson and González-Caver, 1992; Luhr, 1997). The connection between these two contrasting petrogenetic suites is still in debate. It has been suggested that: (1) they related to the same subduction process along the MAT (Negendank et al., 1985; Besch et al., 1988; Nelson and GonzálezCaver, 1992); (2) they are the product of fusion of different mantle reservoirs ( Verma and Nelson, 1989; Luhr, 1997); and (3) at least on the eastern portion of the arc, they represent two distinct petrogenetic provinces ( Thorpe, 1977; Cantagrel and Robin, 1979). Structurally, the TMVB cannot be visualized as a uniform entity. Several studies based on the Fig. 1. Regional localization of Cerro Grande Volcano, major tectonic lineaments and regional geology of the TMVB. Modified from Aguirre-Dı́az et al. (1997). A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 251 252 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 directions of faulting and stress fields divide the province in three main sectors subject to different tectonic styles: Western, Central and Eastern ( Fig. 1) (Demant, 1978; Pasquaré et al., 1988). The brittle deformation features (fractures and faults) are better organized and defined in the western and central sectors, where tectonic extension have been coeval with volcanism (Luhr, 1997 and references therein). Nevertheless, the structural features of the eastern sector are not easily recognized, either because they have been obscured by erosion or recent volcanism, or because the crust in this sector responded differently to similar stress fields. These differences can also be related in some extent to the nature of basement rocks. It has been suggested that the TMVB is emplaced over boundaries of ancient terranes that have undergone episodic reactivations after accretion (Sedlock et al., 1993; Ortega-Gutiérrez et al., 1994) and that their tectonic limits may play an important role in magma ascent and emplacement (Alaniz-Alvarez et al., 1998). Indeed, the crustal thickness increases from the coasts to the continental interior reaching nearly 50 km beneath the eastern sector of the TMVB (Molina-Garza and Urrutia-Fucugauchi, 1993; Urrutia-Fucugauchi and Flores-Ruiz, 1996). Therefore, the variation in composition, age and basement structure over which the arc is emplaced may be related to differences in crustal response to a stress field, which would significantly influence magma ascent, differentiation paths and emplacement conditions. The structural framework of the eastern sector is of relevance here because it may have an important influence in the distribution of volcanic vents (Cantagrel and Robin, 1979; Negendank et al., 1985; Höskuldsson and Robin, 1993; AlanizAlvarez et al., 1998). Fault population analyses are only documented in a few technical reports from Comisión Federal de Electricidad (Garduño 253 et al., 1985; Carrasco-Núñez et al., 1995; LópezHernández, 1995) and in some regional geologic recognitions (Negendank et al., 1985; Pasquaré et al., 1986). The volcanic rocks of this sector commonly lie over late Cretaceous–Early Tertiary, NNW–SSE-trending, laramidic folds and thrusts of the Mesozoic Sedimentary Folds and Thrusts Province (Ortega-Gutiérrez et al., 1992). Toward the northern and eastern limit of the eastern sector, greatly fragmented and dislocated NNW–SSEtrending normal faults cut late Miocene volcanic rocks, but apparently, they do not affect postPliocene rock sequences (Garduño et al., 1985; Negendank et al., 1985; Pasquaré et al., 1986; Carrasco-Núñez et al., 1995, 1997; LópezHernández, 1995). Moreover, several late Miocene volcanic vents were clearly erupted along NNW– SSE-trending fissures (i.e. Teziutlán andesite ( Yañez-Garcia, 1980) and the Terrenate group and Crestón andesite (Carrasco-Núñez et al., 1997 and this paper). These faults, and the alignment of volcanic structures in the same direction, may indicate the existence of a late Miocene tectonic extension period coeval with volcanism in the eastern TMVB. The NNW–SSE-trending structures are also present in other areas of the TMVB (e.g. the Taxco–San Miguel fault system in Fig. 1), and they have usually been related to a southern protraction of the Basin and Range style of deformation (Pasquaré et al., 1988; Henry and ArandaGómez, 1992; Suter et al., 1995a; Jansma and Lang, 1997). However, a clear definition of its temporal and spatial extent has not been achieved due to the lack of a comprehensive study on the tectonics of the eastern TMVB. East from the Taxco–San Miguel fault system (e.g. south of the Mexico City Basin, Fig. 1), a set of ENE–WSW-trending normal faults cut Pliocene–Quaternary volcanic rocks. Although this system is not well defined eastwards, the NE– Fig. 2. Geologic map of Cerro Grande volcano modified from Carrasco-Núñez et al. (1997). Rock-unit abbreviations: Qdpa= pyroclastic and alluvial deposits, undivided; Qix=Xaltipan ignimbrite; Qgt=Tlaxco group; Qatc=Tres Cerros andesite; Tacl=Cruz de Leon andesite; Tsvn=Nocayoco volcano-sedimentary sequence; Tiba=Ixtacamaxtitlán block-and-ash-flow deposit; Tac=El Crestón andesite; Tgc=Cuyoaco group; Toi=Oriental lithic rich ignimbrite; Tacg=Cerro Grande andesite; Tgtl and Tgtu=Terrenate group; Tdpc=Coyoltepec pyroclastic deposits, undivided; Tsbj=Benito Juárez pyroclastic sequence; Ti=Tertiary intrusives, undivided; J-K=Mesozoic sedimentary rock sequence, undivided. 254 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 SW alignment of cinder cones and minor fractures suggest its presence (Pasquaré et al., 1988; Carrasco-Núñez et al., 1997). Therefore, it is apparent that a second period of tectonic extension acted in the eastern TMVB in close relation with volcanism since the Pliocene age. The recognition that large stratovolcanoes in the TMVB are aligned in an approximately N–S direction led to several interpretations on the relation of volcanism with tectonic style (Cantagrel and Robin, 1979; Negendank et al., 1985; Höskuldsson and Robin, 1993; Siebe et al., 1993). Recently, Alaniz-Alvarez et al. (1998) proposed that the large stratovolcanoes are located over N– S-trending faults with slow slip rates, while monogenetic volcanism is emplaced over E–W-trending normal structures with higher slip rates. This hypothesis implies that these two almost orthogonal fault systems (NNW–SSE and ENE–WSW ) have been simultaneously active since the late Miocene. However, this interpretation has been challenged by several authors in the lack of unequivocal evidence supporting the tectonic control of volcanism (Contreras and Gómez-Tuena, 1999; Suter, 1999). Furthermore, as noted above, at least in the northeastern sector of the TMVB where the present study was performed, the NNW– SSE system only affects the late Miocene geologic record. 3. Samples and procedures Geologic mapping was performed using: a 1:100 000 scale LANDSAT satellite image, 1:50 000 scale aerial photographs and 1:50 000 scale topographic maps. Geologic contacts and stratigraphic sections were later verified and measured during several field seasons from 1995 to 1997. Up to 250 rock samples were collected and described from outcrops during field work. Petrographic thin sections of nearly 150 samples were used to characterize stratigraphic rock units. Whole-rock major-element geochemistry was obtained in the laboratories of the Instituto de Geologı́a ( UNAM ) by wavelength-dispersive X-ray fluorescence spectroscopy ( XRF ). Sr and Nd isotope determinations were obtained by mass spectrometry in the laboratories of the Instituto de Geofı́sica ( UNAM ). Trace-element analyses of whole-rock samples were obtained by ICP-MS at the Lamont-Doherty Earth Observatory of Columbia University. Whole-rock lava samples for geochemistry were mechanically crushed and pulverized in a Herzog mortar. However, rocks as heterogeneous as the Oriental lithic-rich ignimbrites ( Toi, see below for descriptions) are difficult to sample and analyze for petrological and geochemical studies due to the high induration of the deposits, their lithicrich character that blur the recognition of the juvenile material, and the high degree of alteration of the glassy-juvenile components to secondary minerals. For these reasons, an attempt to recognize the composition was carried out selecting relatively homogenous samples from different sites and stratigraphic positions. These samples were mechanically crushed in a ceramic mortar and the juvenile components separated by hand-picking using a binocular microscope. The separated juvenile components were pulverized and analyzed by XRF. Major element chemistry results showed that the extensive alteration of the glass could not be avoided (see below). Pyroclastic rock units of Cerro Grande volcano are invariable highly indurated. Thus, a conventional grain-size separation analysis is not possible because a mechanical desegregation of the rock will inevitably fragment the components. Therefore, an estimation of the grain size distribution was obtained in the field using a 64 mm grid mounted in a 1 m2 wood frame (see Fig. 4b). A volume proportion of the >64 mm components and surrounding matrix was estimated with a methodology similar to that used for point-counting in thin-section petrography. 4. Geology of Cerro Grande volcano A general description of the stratigraphic units, areas and rock volumes that crop out in the Cerro Grande area can be found in Carrasco-Núñez et al. (1997). Table 1 and Fig. 2 summarize the geology of the map area. This study focuses on the evolu- 255 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Table 1 Chronostratigraphic units of the Cerro Grande volcanic fielda Unit Absolute and relative ages (Ma) Type of deposit Composition Area (km2)Interpretation average thickness (m)volume (km3) Plio-Quaternary volcanics (Qgt, Qatc, Qix, Qdpa) 0.45±0.09b, 0.49±0.07b Cinder cones, lava flows, ignimbrites Basalt to rhyolite Undivided. See Carrasco-Núñez et al. (1997). Strombolian eruptions, lava flows and caldera collapse ignimbrites Cruz de León andesite (Tacl ) 9.0±0.4b Lava flows and minor pyroclastic flows Andesite to dacite 140-280-39.2 Ring-fissural lava flows Pyroclastic fall and surges. Fluviolacustrine sediments Dacite to rhyolite. Polymictic conglomerates, sands and clays 120-200-24 Open-vent magmatic and hydromagmatic explosive eruptions Nocayoco volcanoLate Miocene sedimentary sequence ( Tsvn) Ixtacamaxtitlán block-and-ash-flow deposit (Tiba) 9.2±0.3c Block-and-ash-flow deposit Dacite 45-30-1.35 Dome explosive collapse El Crestón andesite ( Tac) 9.7±0.5b Lava flows with auto-breccias Andesite 175-400-70 Fissural lava flows probably related to a NW–SE fault system Cuyoaco Group ( Tgc) Oriental lithic rich ignimbrite (Toi) 8.9±0.4b, 10.5±0.4d Late Miocene Andesite 120-400-48 Basaltic-andesite to dacite 40-70-2.8 Cerro Grande andesite (Tacg) Terrenate group ( Tgtl, Tgtu) 11.0±0.6b Columnar jointed, massive lava flows Pyroclastic flows, falls, surges and minor lahars Blocky to slabjointed lava flows Lava flows with auto-breccias and minor volcaniclastics Andesite to dacite 140-400-56 Fissural and central lava flows Intense magmatic and hydromagmatic explosive eruptions Shield-like volcano Basaltic-andesite to dacite 48-250-12 Volcanic cones emplaced along a NW–SE-trending lineament Lacustrinepyroclastic units ( Tsbj, Tdpc) Late Miocene? Lacustrine sediments Clay-rich sediments and minor pyroclastics Undivided. See Carrasco-Núñez et al. (1997). Ponds and lakes occupying small depressions Tertiary intrusives ( Ti) 31–14.5c Plutonic rocks Granitoids Undivided. See Carrasco-Núñez et al. (1997). Roots of the ancient TMVB volcanic arc? Basement sedimentary rocks (J–K ) Jurassic– Cretaceous Platform and basin sediments Mainly calcareous and minor terrigenous Undivided. See Carrasco-Núñez et al. (1997). Shallow marine environment Late Miocene a Cerro Grande volcano rock units are shown in bold. b Carrasco-Núñez et al. (1997). c This study. d Yañez-Garcia (1980). 256 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 tion of Cerro Grande volcano based on a detailed description of the Miocene stratigraphic units. The oldest exposed rocks are highly deformed marine calcareous and terrigenous sedimentary rocks of Mesozoic age (J–K, Fig. 2 and Table 1). In several places, these rocks are intruded by granitoids (Ti; Fig. 2 and Table 1) of unknown age, but similar rocks were dated in nearby areas between 31 and 14.5 Ma ( Yañez-Garcia, 1980). These dates suggest a long period of intrusions that may represent the roots of volcanic centers in the early TMVB. However, the lack of a comprehensive study in these rocks prevents further interpretations about their origin and their relation to surrounding volcanism. During the Cenozoic, the area was affected by two different volcanic phases that occurred in the late Miocene and Pleistocene ages. Contemporaneous with the Miocene volcanism, several lacustrine basins and fluvial drainage generated thick (e.g. 45 m in section A3, Fig. 2), stratified sedimentary rock sequences commonly interbedded with pyroclastic deposits. These units ( Tsbj and Tdpc; see Fig. 2 and Table 1) were described by Carrasco-Núñez et al. (1997), but their relation to Cerro Grande evolution is unclear. From the early Pliocene to Pleistocene, a period of volcanic repose very likely occurred in the Cerro Grande area as no volcanic rocks of these ages were identified. At about 0.5 Ma, a NE–SW-trending monogenetic volcanic field was emplaced around the Miocene volcanic structures (Fig. 2 and Table 1, Qgt, Qatc). Regionally, caldera-collapse ignimbrites from Los Humeros Caldera ( Fig. 2; Qix) filled valley depressions in the center of the map area. Several ash flows and pyroclastic falls were deposited over the Miocene sequences ( Fig. 2; Qdpa). Some of these deposits are probably related to the activity of La Malinche stratovolcano (about 30 km south from Cerro Grande, Fig. 1), 257 4.1. Late Miocene stratigraphic record 4.1.1. Terrenate group (Tgtl and Tgtu) The Miocene volcanic phase started as lava flow fields, and minor pyroclastic and volcaniclastic rocks, erupted from three different vents located along a NW–SE-trending lineament (Fig. 2; El Conejo, Pilancón and Tlacoxolo vents). The Terrenate group is divided into two members, lower Terrenate ( Tgtl ) and upper Terrenate ( Tgtu). The Tgtl member consists of stratified, gray to dark-gray lava flows and auto-breccias with a composition ranging from two-pyroxenebearing basaltic andesites to andesites with similar mineralogy (SiO =56.5–62%). The Tgtu member 2 comprises a thick sequence of gray to brown, slabjointed, porphyritic augite-bearing andesitic lava flows (SiO =61.6%) that commonly show vertical 2 jointing in the flow fronts. Although there is no isotopic date for these rocks, field evidence indicates that the Tgtl lavas erupted contemporaneously or slightly earlier than the base of Cerro Grande volcano (~11 Ma). Lava flow fronts from El Crestón andesite (~9.7 Ma) and from Cruz de León andesite (~9 Ma) were clearly deflected by the Pilancón volcanic structure, hence being older. The Tgtl rock-unit overlies lacustrine sediments of the Tsbj sequence at site S-2 (Fig. 2). 4.1.2. Cerro Grande andesite (Tacg) The initial volcanic phase of Cerro Grande volcano is composed of a ~400 m thick sequence of blocky to slab-jointed lava flows. Composition of these rocks vary from low-silica andesites (SiO =58%) to dacites (SiO =67%). The 2 2 lowermost unit is a gray, massive to blocky, porphyritic andesitic lava-flow, which contains plagioclase, augite, and magnetite phenocrysts. The middle part is composed of gray to brown, subhorizontally slab-jointed porphyritic andesites with Fig. 3. Representative stratigraphic columns of the Oriental lithic rich ignimbrite (Toi). Letters beside columns depict different units and sample labels. Note that sections are drawn at different scales due to variable thicknesses. This rock unit represents a radical change in Cerro Grande’s eruptive behavior from effusive volcanism to powerful hydromagmatic eruptions (see text and Stage II for details). 258 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 phenocrysts of plagioclase, augite and magnetite with traces of chlorite, hematite and clay minerals. The uppermost lava flow unit is composed of gray to brown, massive to blocky, porphyritic to subophitic hornblende-bearing dacites with less augite and magnetite. The geographic distribution of this rock sequence is apparently very wide, but it is usually covered by later volcanism and lacustrine sedimentation. Lava flows related to this volcanic phase crop out at nearly 30 km from the vent, but the base is not exposed ( Fig. 2, site 6), and its association with a central vent cannot be unambiguously defined. The base of this unit usually rests over highly deformed Mesozoic sedimentary rocks in the northern ( Fig. 2, site 31) and western sectors ( Fig. 2, site 34). A whole-rock andesitic sample from the base of the sequence was K–Ar-dated at 11.0±0.6 Ma (Fig. 3, site J-8) (Carrasco-Núñez et al., 1997). 4.1.3. Oriental lithic rich ignimbrite (Toi) Carrasco-Núñez et al. (1997) named Ixtacamaxtitlán Breccias a pyroclastic rock-sequence with two different members ( Tbil and Tbiu) that apparently had a close relation in their stratigraphic record and genesis. Further investigations indicated that each member must be separated into two different stratigraphic units because they were clearly generated by distinct eruptive mechanisms, different magma compositions, and are separated by a period of intense erosion and lava-flow emplacement (Gómez-Tuena and Carrasco-Núñez, 1997). Therefore, in this work, the Ixtacamaxtitlán breccias lower member ( Tbil ) of Carrasco-Núñez et al., (1997) was renamed as Oriental lithic rich ignimbrite ( Toi), and the upper member ( Tbiu) as Ixtacamaxtitlán block-and-ashflow deposit ( Tiba). Also, the stratigraphic correla- 259 tion scheme of Carrasco-Núñez et al. (1997) is modified (Fig. 2 and Table 1). The Oriental lithic rich ignimbrite ( Toi) is a major rock unit widely distributed around the volcano. It is mainly composed of highly indurated, unwelded, massive, polymictic, pyroclastic breccia-type deposits [sensu Fisher (1961) and Schmid (1981)]. The thickness of the breccia-type depositional units ranges from 1 to 4 m near the crater rim (Figs. 2 and 3, sections 2 and 8), to 2– 16 m in distal parts ( Figs. 2 and 3, section 4). They are normally massive, occasionally showing either normal or inverse grading in large lithic clasts (Fig. 3). Degassification pipes are absent or poorly preserved, although some horizons show a red thermal alteration. Breccia-type deposits always show sharp planar, non-erosive contacts with underlying units, even if they are composed of non-indurated material. Breccia units are usually separated by thinly laminated, well-sorted pumice-rich fall layers or lithic-rich surge deposits ( Fig. 4a). These horizons can be traced laterally for tens of meters, but they cannot be unambiguously verified between outcrops. In distal areas, there are some reworked, laharic horizons near the top of the stratigraphic section ( Figs. 2 and 3, section 4). Although a conventional grain-size analysis was impossible because of the highly indurated nature of the deposits, a field estimation was performed using a 64 mm grid (Fig. 4b, see above). Lithic clasts with a diameter >64 mm (i.e. blocks) constitute between 10 and 45% of the total rock volume. If the complete grain-size spectra is considered, lithic clasts comprise between 25 and 80% by volume. Juvenile fragments >64 mm across constitute less than 5% of the deposit and are typically less than 5 cm in diameter. A field estimation suggests that juvenile clasts comprise between 10 Fig. 4. Field and microscope photographs of the Toi. (a) Lithic-rich surge deposit divides two breccia-type layers in section 8 ( letters represent layers in Fig. 3). (b) Breccia-type horizon ‘f ’ in section 8. The photograph shows the 1 m2 wood frame with a 64 mm grid used in field grain-size estimations. (c) Microphotograph (10×) of juvenile material with low vesicularity of breccia-like layer ‘a’ in section 4 (see Fig. 3). A large normally zoned plagioclase (Plag), and palagonitization of the glassy material (Dv), can be observed. (d ) Microphotograph (10×) of pumice fragment from layer 4d’3 in section 4 (see Fig. 3), showing well-developed subspherical vesicles (v), resorbed plagioclase phenocrysts (plag) and altered glassy matrix. (e) Microphotograph (10×) of lithic-rich surge deposit layer h-2 in section 8 (See Fig. 3). Non-vesiculated glassy material (gl ) occupies interstitial spaces between lithic fragments (L). 260 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 and 35% of the total rock volume. These constituents are always enclosed by a criptocrystalline, devitrified glassy matrix (10–40%). Lithic clasts are often angular to subrounded, and range in size from a few millimeters to 2 m across. Although they are heterogeneous in composition, a two-pyroxene porphyritic andesite is the most abundant component. Hornblende-bearing dacites and augite-bearing andesites are also present in minor proportions. No accidental lithologies were found in the deposits (i.e. limestones or plutonic rocks). Therefore, it is apparent that lithics fragments have a cognate character, derived from the walls of the eruptive vent at a shallow depth. The principal juvenile material is composed of yellow, and sometimes banded yellow–brown, twopyroxene-bearing pumice with angular to subrounded morphologies. However, some hornblende-bearing pumice is observed at the base of sections 2 and 8 in pyroclastic-fall sequences. Also, some dark-brown, ‘bread-crusted’ to ‘cauliflower’shaped andesitic bombs were observed in breccia layers of section 8. In thin-sections, the glassy matrix from breccia layers exhibit a strong devitrification to palagonite and clay minerals ( Fig. 4c). This alteration accounts for the induration of the deposits and represents the principal factor that obscures the magma composition in the wholerock chemical analyses (see below). Juvenile fragments also display variable degrees of vesicularity ( Fig. 4c–e). Juvenile material from breccia-type layers usually does not exhibit vesicles (Fig. 4c), although some fragments with a low vesicularity are present. Juvenile fragments in fall deposits commonly exhibit well-developed subespherical vesicles ( Fig. 4d ). However, juvenile material from pyroclastic surge deposits usually does not show vesicles, and the glassy matrix commonly fills interstitial spaces between lithic fragments ( Fig. 4e). The most representative stratigraphic sections of the Toi can be found at the caldera rim (Sites 8 and 2, Figs. 2 and 3), at the center of the structure (Site 11, Figs. 2 and 3) and 30 km SE from the vent (Site 4 and 6, Figs. 2 and 3). Fig. 3 depicts a stratigraphic scheme of these sections. The initial eruptive stage emplaced a series of thinly laminated pyroclastic surge and fall horizons that incorporated a few accessory lithics (sections 2 and 8; Fig. 3). In both sites, surge deposits show the same phenocryst suite: plagioclase, orthopyroxene, clinopyroxene and traces of hornblende. After this initial phase, a series of breccia layers, and interbedded pumice-rich fall deposits or lithic-rich surge horizons, were emplaced around the volcano. The mineralogy of these rocks is very similar: plagioclase and two-pyroxenes. However, the base of the section 11, northeast from the vent, is composed of breccia-type layers with only normally zoned plagioclase phenocrysts, overlaid by lithic-rich ignimbrites with plagioclase and twopyroxenes. Therefore, it is possible that the base of section 11 was the second magma batch extruded from a different portion of the chamber. The following blasts might be regarded as the climactic phase of the eruption where the main portion the chamber was extruded as volumetric breccia layers, and associated falls and surges, with very similar juvenile material containing plagioclase and two pyroxenes. The base of the Toi commonly overlies andesitic lava flows of the Cerro Grande andesite ( Tacg; ~11 Ma; sites 8 and 6) and is covered by 50–70 m thick fluvial conglomeratic horizons that underlie the Ixtacamaxtitlán block-and-ash-flow deposit ( Tiba; ~9.2 Ma, site 38). Andesitic lava flows of the El Crestón andesite ( Tac; ~9.7 Ma; sites 4 and 6), and from the Cruz de León andesite ( Tacl; ~9 Ma; sites 8 and R-4), directly overlie the Toi. Although no isotopic dates were obtained directly from the Toi because of its alteration and lithicrich character, stratigraphic relations showed that it is directly constrained between lava flows of 11 and 9.7 Ma. However, no stratigraphic relations were verified between the Toi and the Cuyoaco group ( Tgc; 10.5–8.9 Ma). For this reason, it is either possible that old lava flows of the Cuyoaco group entirely covered the Toi or that the Toi was never emplaced toward the eastern sector of the map area. As it is apparent that the Toi had a wide and radial distribution, the former possibility seems the most favorable explanation. Therefore, the Toi may reflect a single eruptive phase of a zoned magma chamber as no interbedded lava A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 flows or recognizable paleosoils were identified within the sequence. 4.1.4. Cuyoaco group (Tgc) The Cuyoaco group is composed of gray to brown, two-pyroxene porphyritic andesitic lava flows with ophitic and microlitic textures (SiO =62–63%), and some hornblende-bearing 2 porphyritic dacites (SiO =67–70%). The lava 2 flows are commonly massive, with a persistent columnar jointing and are usually affected by a intense hydrothermal alteration. These rocks were erupted from different volcanic vents that are currently deeply eroded, but they appear aligned in the N–S direction. They were dated by YañezGarcia (1980) at 10.5±0.4 Ma ( K–Ar whole rock, site D-1) and by Carrasco-Núñez et al. (1997) at 8.9±0.4 Ma ( K–Ar whole rock, site X-4). These dates imply a long period of effusive activity for the emplacement of this group. 4.1.5. El Crestón andesite (Tac) This unit is composed of thick, massive, porphyritic andesitic lava flows, and associated autobreccias, that form a 23 km long NW–SE-trending elongated ridge. The composition varies from fairly homogeneous two-pyroxene-porphyritic andesites to a two-pyroxene-hornblende bearing dacites (SiO =56–64%). These lavas were clearly related 2 to a NW–SE fissure system, as no eruptive centers were identified. Also, the distribution of lava flow fronts implies that they were extruded from a fissural axis (Fig. 2). This rock unit directly overlies the Terrenate group ( Tgtl-Tgtu), the Cerro Grande andesite ( Tacg) and the Ixtacamaxtitlán lithic rich ignimbrites ( Toi). A whole-rock andesitic sample from Site G-4 was K–Ar dated at 9.7±0.5 Ma (Carrasco-Núñez et al., 1997). 4.1.6. Ixtacamaxtitlán block-and-ash-flow deposit (Tiba) This rock unit is exposed toward the northern and northeastern flanks of the volcano ( Fig. 2, Sites D-9, 16, 19, 20, and 38). It is composed of white to light gray and pink, highly indurated, poorly sorted, stratified block-and-ash-flow deposits with a homogenous, non-vesicular, biotite-hornblende bearing porphyritic dacite as the principal 261 clast component (SiO =65%). Hornblende and 2 biotite euhedral phenocrysts are often 0.5 cm in size. Individual strata vary in thickness between 2 and 10 m, but the layer thickness decreases as the distance from the vent increases ( Fig. 5). In general, individual block-and-ash-flow layers are poorly sorted, with a sandy matrix at distal sites, but showing a clast-supported, blocky-sized character at proximal positions. Large clasts up to 2 m in diameter are common, and are usually located at the top of the deposits forming inverse graded horizons (Fig. 6). Lithic clasts are angular at proximal sites, but at distal positions, larger clasts are often well rounded. Some blocks contain radially arranged cooling joints, which show that they were emplaced as hot blocks. Individual block-and-ashflow layers are often divided by a planar or undulated, thinly laminated, sandy-sized, ash-cloud surge deposit ( Figs. 5 and 6). Very well preserved degassing pipes are observed in all outcrops ( Fig. 6), but they are not continuous through different layers, indicating that the deposits were emplaced from several discrete pyroclastic flows. The initial eruptive stage apparently emplaced an interbedded sequence of well-sorted, pumicefall and surge deposits with a composition of the juvenile material similar to that of the block-andash-flow deposits. These layers are only observable at the base of site 38 (Fig. 5), the only outcrop where the base is exposed. These layers are overlaid by massive, 40 m thick, block-and-ash-flow deposits. This unit rests over 50–70 m thick fluvial conglomeratic horizons (Fig. 5, site 38), and is overlaid by the Nocayoco volcano-sedimentary rocksequence ( Tsvn) and by the Cruz de León andesite ( Tacl ). A pure biotite concentrate, separated from a fresh 50 kg dacitic block collected in Site 16 ( Figs. 2 and 5), was K–Ar-dated at 9.2±0.3 Ma in Geochron Laboratories. 4.1.7. Nocayoco volcano-sedimentary sequence (Tsvn) This unit has a conspicuous morphology toward the northern and northwestern sectors of the volcano forming white-colored, stratified, relatively flat plateaus. It is composed of ~650 m thick, moderately indurated, fluvial conglomeratic hori- 262 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Fig. 5. Representative stratigraphic columns of the Ixtacamaxtitlán block-and-ash-flow deposit ( Tiba). The distance from the vent increases from left to right (see Fig. 2 for site locations). The base of the sequence is only exposed in section 38. A pure biotite concentrate separated from a fresh block collected from the top of section 16 was K–Ar dated at 9.2±0.3 Ma. This rock unit represents the product of a central dome explosive collapse (see text and Stage IV for details). zons, interbedded with laminated sandy and clayey layers of lacustrine origin with sparse conglomeratic lenses. At the base of the sequence, the conglomeratic layers are considerably more abundant. They usually contain different clasts of older volcanic sequences, but calcareous and terrigenous Mesozoic rocks are also present. The sedimentary units are often interbedded with pumice-rich, pyroclastic fall and base-surge horizons that have a general tendency to become more abundant toward the upper 250 m of the sequence ( Fig. 2, sites 27 and 28). The juvenile material of the pyroclastic A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 263 4.1.8. Cruz de León andesite (Tacl) This unit is formed by blocky to slab-jointed andesitic to dacitic lava flows, associated breccias, and a very minor proportion of highly weathered pyroclastic flow-deposits. Two main lava compositions were identified: a light-gray, hornblendeaugite-bearing porphyritic dacite (SiO =65%), 2 and a gray, slightly porphyritic to microlitic and subophitic textured, two-pyroxene bearing andesite (SiO =58%). The lava-breccias are commonly 2 associated with slab-jointed lava flows, and show a poorly sorted arrangement of rounded to subrounded clasts up to 1.2 m in size ( Fig. 2, site 8). This volcanic-succession constitutes the external structure of Cerro Grande volcano as low-angle lava flows radially distributed from the crater-rim. This unit rests over the Toi ( Fig. 2, Site 8) and over the Tsvn (Fig. 2, Site 32). An andesitic wholerock lava flow sample from the top of the sequence was K–Ar-dated as 9.0±0.4 Ma (Fig. 2, Site R-1) (Carrasco-Núñez et al., 1997). 5. Geochemistry 5.1. Major-element chemistry Fig. 6. Top of section 16 of the Tiba showing degassing pipes at the base of the deposit, ash cloud layers separating each block and ash flow horizon, and a laminated hyperconcentrated-laharic deposit separating two block-and-ash-flow deposits at the top. See geologist for scale. deposits is a fairly homogeneous hornblende-bearing dacite (SiO =66–68%). Fall deposits are usu2 ally well-sorted, with abundant highly vesiculated pumice fragments and minor andesitic lithics. The base surges are usually planar-bedded and occasionally cross-bedded, with well-rounded, lowvesiculated pumice fragments and minor lithics. The base of this unit directly overlies the Tiba (~9.2 Ma; Fig. 2, site 16) and is covered by the Cruz de León andesites (Tacl; ~9 Ma). No isotopic date is available for this unit, but stratigraphic relations reflect that it was deposited during an interval of about 0.2 Ma, in which volcanic repose and fluvio-lacustrine deposition was replaced by volcanic explosive activity. Whole-rock compositions of Cerro Grande volcanic rocks and peripherally emplaced Miocene vents are shown in Table 2. Rock types found, in order of decreasing abundance, are andesite, dacite and basaltic-andesite. No basaltic or rhyolitic rocks were identified in the Miocene geologic record ( Fig. 7). The rock compositions define a subalkaline magmatic trend in a total alkalis vs. silica diagram, located below the Irvine and Baragar (1971) subdivision. When plotted on an AFM diagram (not shown), most samples define a calc-alkaline trend, and are mainly located in the medium-K field on a K O–SiO plot (not 2 2 shown). However, whole-rock juvenile samples from the Toi depart from this trend displaying a FeO1 enrichment and a low K O character. It is 2 apparent that this variation is associated with an alkali depletion due to alteration of the glassy material. Major-element variation diagrams of SiO vs. 2 TiO , Fe O1 , CaO and MgO exhibit well-defined 2 2 3 95-S5 Tgtl Lava 19° 27.3∞ 97° 53.0∞ 56.52 0.8 18.64 7.02 0.09 2.8 8.59 3.61 1.74 0.19 100 1.63 2.04 44.14 CG-2e Toi Pumice 19° 34.8∞ 97° 50.0∞ 54.62 1.04 23.65 9.16 0.05 1.65 6.16 3.29 0.31 0.06 100 – 4.5 26.25 Sample: Unit: Material Lat N Lon W SiO 2 TiO 2 Al O 2 3 Fe O1 2 3 MnO MgO CaO Na O 2 KO 2 PO 2 5 Total HO 2 LOI Mg # Sample: Unit: Material Lat N Lon W SiO 2 TiO 2 Al O 2 3 Fe O1 2 3 MnO MgO CaO Na O 2 KO 2 PO 2 5 Total HO 2 LOI Mg # Pumice 19° 33.3∞ 97° 37.9∞ 58.39 0.85 21.31 7.24 0.08 3.37 5.88 2.24 0.59 0.06 100 – 5.4 47.99 CG-8m6 Toi Lava 19° 26.8∞ 97° 53.5∞ 61.95 0.76 17.37 6.12 0.05 2.05 5.38 3.78 2.34 0.2 100 0.48 1.25 39.91 95-S3A Tgtl M Glass 19° 22.1∞ 97° 37.9∞ 58.97 0.88 19.35 6.95 0.04 3.8 6.67 2.61 0.58 0.15 100 – 7 51.97 CG-4b∞ Toi Lava 19° 30.0∞ 97° 53.2∞ 61.63 0.78 16.65 6.13 0.09 2.95 5.39 3.68 2.51 0.19 100 0.41 1.26 48.8 95-S8 Tgtu Pumice 19° 22.1∞ 97° 37.9∞ 59.19 0.94 18.87 7.83 0.06 3.67 6.73 2.11 0.45 0.15 100 – 6.1 48.17 CG-4c∞ Toi Lava 19° 33.6∞ 97° 50.6∞ 58.35 0.8 17.38 6.66 0.09 3.65 7.33 3.64 1.82 0.26 100 0.53 0.86 52.06 95-J8 Tacg Bomb 19° 33.3∞ 97° 37.9∞ 59.75 0.7 19.9 5.89 0.08 4.68 6.26 2.13 0.57 0.04 100 – 4.8 61.13 CG-8o Toi Lava 19° 22.3∞ 97° 38.6∞ 58.65 0.92 18.16 7.54 0.13 2.43 6.77 3.86 1.38 0.16 100 – 1.06 38.99 CG-6a Tacg M Glass 19° 33.3∞ 97° 37.9∞ 59.82 0.66 18.57 5.54 0.09 3.76 7.06 2.99 1.36 0.14 100 – 2.57 57.32 CG-8L Toi Lava 19° 34.8∞ 97° 50.0∞ 58.92 0.77 18.33 6.44 0.1 3.24 6.26 3.82 1.98 0.14 100 – 0.67 49.91 CG-2a Tacg Pumice 19° 34.0∞ 97° 49.5∞ 63.56 0.8 17.29 7.68 0.05 2.97 5 1.74 0.82 0.1 100 – 7.42 43.35 CG-11E3 Toi Lava 19° 22.3∞ 97° 38.6∞ 59.17 1.05 17.81 8.04 0.1 1.99 5.72 4.32 1.58 0.23 100 0.81 0.92 32.95 95-C5b Tacg Table 2 Major-element data of Cerro Grande volcanics and surrounding Miocene ventsa Lava 19° 34.1∞ 97° 40.3∞ 62.11 0.72 18.14 5.38 0.07 2.07 5.49 4.29 1.57 0.16 100 0.14 0.82 43.3 95-X7 Tgc Lava 19° 33.3∞ 97° 37.9∞ 59.2 0.75 17.72 6.37 0.07 3.58 6.39 3.61 2.14 0.2 100 – 1.42 52.66 CG-8p Tacg Lava 19° 43.0∞ 97° 46.6∞ 62.28 0.72 18.4 5.19 0.07 1.78 5.46 4.44 1.46 0.2 100 – 0.97 40.45 95-L10 Tgc Lava 19° 33.9∞ 97° 51.5∞ 59.45 0.79 18.77 6.39 0.11 2.46 5.89 4.41 1.59 0.13 100 – 1 43.21 CG-9 Tacg Lava 19° 36.2∞ 97° 42.5∞ 62.87 0.77 19.14 5.49 0.05 1.31 4.32 4.17 1.71 0.17 100 0.08 2.09 32.08 95-X4D Tgc Lava 19° 34.8∞ 97° 50.0∞ 59.83 0.72 17.6 5.92 0.09 3.44 6.32 3.61 2.27 0.2 100 – 1.08 53.47 CG-2c Tacg Lava 19° 42.7∞ 97° 38.4∞ 67.38 0.43 17.33 3.04 0.06 1.16 4.59 4.59 1.27 0.14 100 – 0.81 42.93 95-M6 Tgc Lava 19° 38.0∞ 97° 49.4∞ 62.66 0.68 18.69 4.74 0.04 1.42 5.7 4.17 1.73 0.17 100 – 0.71 37.25 CG-31 Tacg Lava 19° 29.6∞ 97° 44.0∞ 70 0.62 15.51 4.93 0.08 1.17 2.23 2.38 2.93 0.15 100 1.59 2.9 31.87 95-B2a Tgc Lava 19° 34.8∞ 97° 50.0∞ 62.85 0.74 16.09 5.7 0.07 2.59 5.85 4.05 1.88 0.18 100 – 1.07 47.37 CG-2b Tacg Lava 19° 22.3∞ 97° 39.8∞ 56.34 0.81 18.12 7.5 0.11 3.94 8.32 3.42 1.18 0.26 100 0.43 0.64 50.97 95-C3a Tac Lava 19° 34.1∞ 97° 48.4∞ 63.12 0.59 17.71 4.76 0.05 2.29 5.3 4.16 1.92 0.1 100 – 1 48.78 CG-15 Tacg Lava 19° 25.1∞ 97° 43.0∞ 57.23 0.89 18.66 6.96 0.11 3.3 7.31 3.69 1.59 0.25 100 0.09 1.01 48.39 95-G4A Tac Lava 19° 36.0∞ 97° 46.1∞ 67.11 0.39 17.36 2.99 0.05 1.46 4.8 4.83 0.89 0.11 100 0.31 0.77 49.21 95-D7 Tacg 264 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Lava 19° 34.0∞ 97° 49.5∞ 57.32 0.99 17.7 7.67 0.12 4 7.3 3.44 1.25 0.21 100 – 0.81 50.78 95-R3B Tacl Lava 19° 34.2∞ 97° 53.1∞ 58.51 0.72 18.39 6.85 0.1 3.28 6.43 4.13 1.43 0.15 100 0.49 1.05 48.69 Material Lat N Lon W SiO 2 TiO 2 Al O 2 3 Fe O1 2 3 MnO MgO CaO Na O 2 KO 2 PO 2 5 Total HO 2 LOI Mg # Sample: Unit: Material Lat N Lon W SiO 2 TiO 2 Al O 2 3 Fe O1 2 3 MnO MgO CaO Na O 2 KO 2 PO 2 5 Total HO 2 LOI Mg # Lava 19° 33.3∞ 97° 37.9∞ 59.08 0.74 18.14 6.45 0.08 3.09 6.61 3.96 1.63 0.21 100 – 1.23 48.72 CG-8a Tacl Lava 19° 22.1∞ 97° 37.9∞ 58.39 0.88 17.77 6.91 0.12 3.4 7.4 3.7 1.16 0.26 100 – 1.09 49.35 CG-4j Tac Lava 19° 33.6∞ 97° 53.6∞ 59.49 0.55 19.17 5.77 0.09 2.61 6.55 4.13 1.5 0.15 100 – 0.4 47.31 JCG44 Tacl Lava 19° 29.1∞ 97° 48.3∞ 63.38 0.62 17.91 5.22 0.08 1.62 5.5 3.96 1.53 0.18 100 0.62 1.08 38.03 95-A6 Tac Lava 19° 36.5∞ 97° 50.8∞ 59.51 0.73 18.69 6.59 0.08 2.85 6.17 3.73 1.5 0.15 100 0.29 0.69 46.13 95-R1 Tacl Lava 19° 31.0∞ 97° 48.5∞ 63.71 0.63 17.57 5.01 0.07 2.63 4.58 3.55 2.09 0.16 100 0.15 1.44 50.93 95-H5 Tac Lava 19° 34.0∞ 97° 51.0∞ 59.57 0.75 18.66 6.28 0.08 2.76 6.24 4.13 1.36 0.18 100 – 0.91 46.53 CG-10 Tacl Lava 19° 27.9∞ 97° 50.9∞ 63.77 0.62 17.44 5.02 0.09 1.77 5.49 4.02 1.58 0.19 100 0.96 1.26 41.18 95-A5a Tac Lava 19° 33.3∞ 97° 37.9∞ 59.7 0.78 18.46 6.4 0.1 2.64 5.81 4.08 1.91 0.13 100 – 1.26 44.95 CG-8c Tacl Lava 19° 34.6∞ 97° 46.5∞ 63.86 0.72 17.86 4.92 0.06 1.73 4.72 4.2 1.74 0.2 100 0.68 1.67 41 95-J5 Tac Lava 19° 31.4∞ 97° 51.7∞ 60.32 0.65 17.53 5.76 0.09 3.83 6.61 3.67 1.41 0.14 100 – 0.77 56.81 JCG36 Tacl Lava 19° 34.1∞ 97° 47.0∞ 64.42 0.61 17.52 4.96 0.05 1.54 4.66 4.08 2.02 0.14 100 0.37 1.18 38.13 95-H6A Tac Lava 19° 33.4∞ 97° 55.0∞ 60.39 0.51 18.93 5.53 0.09 2.85 6.36 3.83 1.38 0.13 100 – 1.24 50.54 JV5 Tacl Block 19° 37.5∞ 97° 48.6∞ 65.02 0.65 17.06 4.18 0.07 2.04 4.73 3.69 2.42 0.13 100 – 1.37 49.2 CG-16 Tiba Lava 19° 32.8∞ 97° 52.2∞ 60.79 0.63 17.57 5.56 0.08 3.53 6.43 3.85 1.42 0.14 100 – 0.96 55.72 JCG37 Tacl Pumice 19° 39.4∞ 97° 48.5∞ 65.6 0.58 18.4 4.35 0.07 2.18 4.34 2.16 2.15 0.17 100 – 6.59 49.81 CG-27 Tsvn Lava 19° 37.0∞ 97° 49.2∞ 62.02 0.63 17.53 5.34 0.06 2.71 6.12 3.69 1.77 0.13 100 – 1.16 50.09 JCG40 Tacl Pumice 19° 36.7∞ 97° 49.2∞ 66.1 0.49 18.31 3.66 0.07 1.54 4.18 2.97 2.54 0.14 100 1.31 4 45.42 95-001 Tsvn Lava 19° 37.7∞ 97° 52.6∞ 62.62 0.6 17.4 5.44 0.08 2.56 5.21 3.87 2.06 0.15 100 0.45 0.97 48.3 95-E5 Tacl Pumice 19° 34.2∞ 97° 48.6∞ 66.78 0.52 17.86 3.73 0.07 1.02 4.36 3.29 2.26 0.11 100 – 3.6 35.09 CG-14e Tsvn a LOI: loss of ignition; total iron as Fe O1 ; FeO1 recalculation FeO1=FeO+(Fe O × 0.89981); Mg #=100(Mg2+/(Fe2++Mg2+). 2 3 2 3 CG-11a Tac Sample: Unit: Table 2 (continued ) Lava 19° 38.3∞ 97° 52.0∞ 64.55 0.68 17.33 5.17 0.13 1.43 4.37 3.59 2.6 0.15 100 – 4.3 35.45 CG-32 Tacl Pumice 19° 39.4∞ 97° 48.5∞ 67.91 0.51 18.53 3.75 0.05 1.18 3.63 2.5 1.85 0.08 100 3.25 5.75 38.46 95-Q2E Tsvn Lava 19° 34.6∞ 97° 50.9∞ 65.36 0.64 18.01 4.86 0.05 0.57 3.24 4.59 2.53 0.13 100 – 1.66 18.93 CG-7 Tacl Lava 19° 37.5∞ 97° 52.8∞ 58.33 0.92 17.63 6.89 0.07 3.01 7.18 3.66 2.04 0.27 100 – 1.09 46.35 CG-33 Tacl A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 265 266 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Fig. 7. Total alkalis-silica diagram after Le Maitre (1989). Classification is grounded on the weight per cent of major elements recalculated to 100% anhydrous. The thick line marks the subdivision of volcanic rocks into alkaline and subalkaline fields (Irvine and Baragar, 1971). Rock-unit abbreviations as in Fig. 2. Note alkali depletion in the Toi, probably due to intense alteration of the glassy components (see text for details). Data are given in Table 2. negatively correlated linear trends (Fig. 8). Al O 2 3 and MnO values display a slight decrease with increasing SiO , but a linear trend is not clearly 2 observed. Positive correlation trends are roughly defined for Na O and K O in lavas, but low values 2 2 of alkalis in pyroclastic samples from the Toi and Tsvn rock-units are clear. Na O/K O ratios vary 2 2 from 10.6 to 2.1 in Toi samples, and from 1.0 to 1.5 in Tsvn samples. The high values in Na O/K O ratio and low alkali content in the Toi 2 2 reflect the fact that alteration is always present, obscuring the magma composition involved in this particular rock sequence. The Tsvn sequence shows a somewhat lower degree of alteration. The composition of lavas and pyroclastic units does not show a systematic variation with stratigraphy. Although pyroclastic rocks from Tiba and Tsvn have a dacitic composition, a correlation between rock chemistry and eruptive style cannot be postulated since lava flows from other units have similar compositions. Moreover, the rock chemistry from the Toi is comparable to that of lava flows from other units, albeit it exhibits an evident loss of alkalis due to alteration. 5.2. Trace-element chemistry and Sr and Nd isotopes Trace-element concentrations in whole-rock lava and pyroclastic rocks (Table 3) are used as indicators of magma differentiation in samples that span the major-element chemistry variation, and the stratigraphic record of Cerro Grande volcano. Incompatible trace-element concentrations (e.g. Nb, Hf, La, Th) show a general increase with increasing differentiation, using Rb as the differentiation index ( Fig. 9). Sr exhibits a general decrease in abundance in more evolved rocks, presumably due to plagioclase control during crystal fractionation. The correlation of these trace elements with Rb suggests that compositional trends of these elements were influenced by the same magmatic processes. Thus, it is likely that crystal fractionation of the observed mineral phases (Plag+Opx+Cpx±Hbl ) contributed strongly to magma differentiation. The chondrite-normalized trace-element data for Cerro Grande volcanics (Fig. 10a) have similar shapes and abundances, exhibiting depletions in the high field strength elements (HFSE ) Nb, Ta and Ti relative to large ion lithophile elements (LILE) Rb, Ba, K, and Sr and the light rare earth elements (LREE). This is a typical pattern of trace elements in arc magmas. REE patterns for the Cerro Grande suite ( Fig. 10b) exhibit LREE enrichment (La/Sm =3.53–2.02), and relatively flat but N enriched HREE patterns, Tb/Yb =1.54–1.22, N Yb =10.91–5.73. Negative Eu anomalies are N weak with Eu/Eu1=0.85–0.98. Low positive Eu anomalies are observed in two andesitic samples with Eu/Eu1=1.064–1.066. These values suggest that plagioclase played an important role either as a cumulate or as separation product during crystal fractionation. A moderately defined convexupward mid-REE pattern in some samples indicates that the amphibole fractionation occurred to some extent. Sr and Nd isotope determinations were made for only one sample of the Ixtacamaxtitlán blockand-ash-flow deposit (site 16; Fig. 3). Sample CG-16 has an 87Sr/86Sr initial ratio of 0.703930 and e =1.21. These data fall within the field of Nd the TMVB ( Verma, 1983; Verma and Verma, 1986). In summary, the major- and trace-element characteristics are typical of a calc-alkaline arc volcanic series. These include the relative enrichments of Fig. 8. Harker variation diagrams of Cerro Grande whole-rock samples. Data are given in Table 2. Rock-unit abbreviations as in Fig. 2. A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 267 268 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Table 3 Trace-element data of Cerro Grande volcanics Sample: Unit: 95-J8 Tacg CG-6a Tacg CG-2a Tacg CG-31 Tacg CG-8L Toi CG-11a Tac CG-4j Tac CG-16 Tiba CG-8a Tacl JCG44 Tacl CG-8a Tacl CG-7 Tacl Material Lat N Lon W Pb Sn W Mo Sb Rb Cs Ba Sr Tl Li Ta Nb Hf Zr Y Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Be Lava 19° 33.6∞ 97° 50.6∞ 6 1.11 0.71 0.71 0.07 29 0.44 427 1030 0.13 7.47 0.37 6.2 3.07 116 21 3.2 0.9 22.9 49.85 6.43 26.55 5.08 1.47 4.57 0.65 3.55 0.69 1.85 1.81 0.27 1.67 Lava 19° 22.3∞ 97° 38.6∞ 5 0.99 0.89 1.34 0.26 33 1.22 434 373 0.15 10.68 0.36 5.9 3.28 148 25 2.59 0.81 14.91 32.35 4.24 18.28 4.2 1.21 4.36 0.69 4.09 0.83 2.34 2.4 0.36 1.3 Lava 19° 34.8∞ 97° 50.0∞ 7 0.94 1 1.05 0.14 34 1.3 432 775 0.15 9.81 0.37 5.8 3.85 154 19 3.77 1.12 18.24 38.44 5.39 22.46 4.68 1.3 4.1 0.58 3.13 0.64 1.76 1.7 0.25 1.62 Lava 19° 38.0∞ 97° 49.4∞ 6 0.63 0.65 0.89 0.09 23 0.35 395 853 0.07 7.59 0.37 5.9 3.71 141 14 2.33 0.74 16.67 35.33 4.51 18.76 3.58 1.17 3.18 0.46 2.58 0.49 1.31 1.26 0.19 1.53 M Glass 19° 33.3∞ 97° 37.9∞ 6 0.9 1.85 1.27 0.17 19 0.83 300 889 0.17 7.56 0.27 4.2 3.76 129 15 2.4 0.75 13.77 29.95 4.06 17.21 3.62 1.1 3.31 0.48 2.73 0.54 1.48 1.51 0.23 1.41 Lava 19° 34.0∞ 97° 49.5∞ 4 0.97 0.66 0.97 0.15 30 0.76 324 513 0.15 10.22 0.45 7.9 3.48 162 26 2.59 0.73 17.56 39.18 5.16 22.17 4.84 1.31 4.67 0.71 4.15 0.84 2.35 2.35 0.34 1.33 Lava 19° 22.1∞ 97° 37.9∞ 5 0.68 0.57 0.74 0.07 13 0.42 348 989 0.09 5.76 0.31 5.9 2.95 123 24 1.84 0.52 16.83 39.72 5.45 24.23 5.12 1.51 4.82 0.72 4.02 0.83 2.31 2.34 0.35 1.37 Block 19° 37.5∞ 97° 48.6∞ 4 0.7 1.06 1.2 0.19 33 1.98 372 696 0.2 8.68 0.55 8.1 2.74 114 15 2.75 0.83 18.3 36.54 4.56 18.18 3.49 1.05 3.14 0.46 2.53 0.49 1.35 1.33 0.21 1.49 Lava 19° 33.3∞ 97° 37.9∞ 7 1.04 1.33 1.67 0.19 39 1.6 394 471 0.17 9.81 0.42 6.7 3.63 154 20 2.68 0.99 15.71 32.13 4.16 17.34 3.79 1.1 3.77 0.57 3.34 0.68 1.91 1.95 0.29 1.75 Lava 19° 33.6∞ 97° 53.6∞ 6 0.73 1.36 1.15 0.11 19 0.28 376 787 0.08 9.63 0.23 3.8 3.12 114 15 2.17 0.73 13.01 27.02 3.55 14.98 3.08 1.05 2.99 0.44 2.49 0.5 1.42 1.48 0.23 1.45 Lava 19° 33.3∞ 97° 37.9∞ 7 1.04 1.33 1.67 0.19 39 1.6 394 471 0.17 9.81 0.42 6.7 3.63 154 20 2.68 0.99 15.71 32.13 4.16 17.34 3.79 1.1 3.77 0.57 3.34 0.68 1.91 1.95 0.29 1.75 Lava 19° 34.6∞ 97° 50.9∞ 10 1.24 1.04 2.05 0.23 61 1.71 622 356 0.4 14.37 0.61 9 5.35 203 25 5.18 1.87 32.58 34.84 7.7 29.61 5.69 1.54 5.14 0.75 4.12 0.79 2.2 2.23 0.32 2.67 LILE and depletions of HFSE relative to the REE. Depletion of HFSE relative to LILE is considered a common geochemical characteristic of arc magmas and is generally attributed to modifications of the upper mantle source region by interaction with slab-derived fluids (Gill, 1981; Tatsumi and Eggins, 1995). Moreover, Nb and Ta depletion is commonly cited as a characteristic signature of subduction-related rocks that have suffered contamination of a crustal component (Rollinson, 1993). It is interesting to note that, under this criterion, the volcanic rocks exhibit a strong deple- tion for both elements, suggesting that the magmas might be contaminated to some extent by a crustal component. Nevertheless, the contamination signature is not clearly observed in the isotope geochemistry data; thus, in this regard, no definite conclusion can be made. 6. Evolution of Cerro Grande volcano The field, stratigraphic and geochemical data presented above are consistent with a volcanic Fig. 9. Trace-element variation vs Rb (ppm) as the differentiation index. Data are given in Table 3. Rock-unit abbreviations as in Fig. 2. Trace-element replicate analyses (n=2) of the US Geological Survey AGV-1, AGV-2, BCR-1, BHVO-1, BIR-1 were used as standards for precision and accuracy. Lamont-Doherty internal references 115In, 169Tm and 209Bi were used as monitors of instrumental drift. A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 269 270 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Fig. 10. Multi-element plots representative of Cerro Grande volcanics. (a) Chondrite-normalized trace-element diagram; (b) chondritenormalized REE diagram. Rock compositions are depicted as SiO %. Normalization values from Nakamura (1974) and Thompson 2 (1982). Filled diamond=Tacg; open square=Toi; filled circle=Tac; cross=Tiba; filled triangle=Tacl. Data are given in Table 3. evolution of Cerro Grande volcano, and peripheral late Miocene volcanic vents, divided into six principal stages ( Fig. 11). 6.1. Stage I. Construction of a shield-like volcano (~11 Ma) The landscape that prevailed before emplacement of volcanic rocks consisted of NW–SE mountain ranges made up of folds and trusts of Mesozoic sedimentary rocks. The presence of stratified lacustrine deposits in the map area ( Fig. 2, Tsbj and Tdpc) indicates that small ponds and lakes occupied intermountain depressions before the inception of volcanic activity in the late Miocene. Volcanism apparently began as andesitic lava flows were erupted from NW–SE-trending vents or fracture zones, forming the Terrenate group volcanics. Cerro Grande volcano initiated its activity at about 11 Ma as a mainly effusive volcano with very minor pyroclastic activity ( Fig. 11, Stage I ). Lava flows were radially distributed from the central sector of the map area forming a low-relief A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 271 Fig. 11. Schematic evolution of Cerro Grande volcano. Arrows indicate the approximate transverse direction of the sections. Rockunit abbreviations as in Fig. 2. See text for description of stages. 272 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 shield-like volcano. Fluvio-lacustrine sedimentation apparently had a close relation with volcanic activity as lava flows continuously modified the landscape, and new stream networks and sedimentary basins developed. The persistent alignment of the initial volcanic structures in a NW–SE direction, and the later dislocation of the volcanic rocks by normal faults with the same orientation [see Fig. 2 and CarrascoNúñez et al. (1997)], strongly suggest that volcanism and tectonic deformation was coeval in the area during late Miocene times. Furthermore, the recognition that most late Miocene, arc-related, volcanic structures in other areas of central Mexico are in close association with NNW–SSE normal faults ( Yañez-Garcia, 1980; Pasquaré et al., 1988; Suter et al., 1995b) may imply that this tectonic phase allowed the ascent and emplacement of volcanic rocks on a regional scale. 6.2. Stage II. Major phreatomagmatic and magmatic explosions (11–10.5 Ma) After the initial effusive phase, Cerro Grande drastically changed its eruptive behavior and entered an intense explosive activity that generated voluminous pyroclastic and volcaniclastic rocks around the volcano (Fig. 11, Stage II ). This rock sequence, here named Oriental lithic rich ignimbrites ( Toi), marks a long-standing eruptive phase that can be easily traced in the stratigraphic record of Cerro Grande volcano. The principal characteristics of this rock-sequence are depicted by: (1) an unusual cognate-lithic-rich, polymictic, matrixsupported, massive breccia-type deposits, with a close association with lithic-rich base-surge and pumice-rich fall deposits; (2) a persistent lowvesicularity and strong devitrification of the juvenile material in breccia-type and surge deposits; and (3) similar eruptive and emplacement mechanisms regardless of the magma composition. These features, together with a radial distribution of the deposits bordering the crater rim and a maximum run-out distance of 30 km SE from the vent, indicate that the Toi breccia-like deposits were produced by deposition from pyroclastic flows. Therefore, as the term ‘breccia’ has no genetic implication, such deposits can be better classified as non-welded, lithic-rich ignimbrites, following the term proposed by Perez-Torrado et al. (1997). The high vesicularity of the juvenile material is a consequence of volatile separation from the magma and is a common feature of plinian or subplinian eruptions in which a positive buoyant eruptive column develops ( Fisher and Schmincke, 1984; Cas and Wright, 1987). However, the low vesicularity of the juvenile material may indicate a rather brittle fragmentation of the magmatic component, implying that volatile separation is inhibited (Zimanowski et al., 1997). Low vesicularity in juvenile material of pyroclastic deposits is often cited as a common feature of hydromagmatic eruptions (Sheridan and Wohletz, 1983; Wohletz, 1983; Lorenz, 1986, 1987). Hydromagmatic eruptions result from explosive interaction of magma with external water (surface or ground water), as a response to a physical phenomena experimentally observed and often referred to as a molten–fuel– coolant interaction (Zimanowski et al., 1997). This is a common feature in geologically active environments where groundwater is usually located in hydraulically active zones of structural weakness (Lorenz, 1987) and is the most efficient volcanic mechanism for converting thermal energy into kinetic energy ( Wohletz, 1983, 1986; Lorenz, 1987; Zimanowski et al., 1997). The differences in vesicularity of the juvenile material between layers (i.e. surge, fall and flow) of the Toi indicate that a complex mixture of fragmentation and eruption mechanisms, from pure magmatic to hydromagmatic, operated during eruption. The low vesicularity in juvenile material from breccia layers and surge deposits clearly accounts for a hydromagmatic fragmentation mechanism. In contrast, highly vesiculated, pumice-rich fall deposits between breccia layers resulted from the progressive fall of positively buoyant columns in the same eruptive cycle. Thus, once a hydromagmatic reaction developed, an intense devastation of the host rock opened a new vent. Ejection of the cap rock may have promoted a pressure relaxation into system and a sudden volatile exsolution from the magma. This process influenced the development of vertical eruptive columns that gave rise to well-vesiculated, pumicerich fall deposits. Therefore, the Toi depicts a A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 repetitive eruptive cycle in which hydromagmatic explosions coexisted with buoyantly rising eruptive columns. The cognate lithic-rich character of the brecciatype deposits indicates that the magmatic thermal energy was efficiently transformed into kinetic energy to strongly devastate and erode the preexistent shield-like volcano. The absence of accidental basement rocks into the lithic-rich ignimbrites implies that the magma–water interaction took place at shallow depths into the volcanic edifice. The variation in magma composition (as depicted by mineralogy) within sections and between outcrops accounts for a compositionally zoned magma chamber. Nevertheless, it seems that magma composition had a limited influence on eruption mechanisms as no variations in eruptive style were identified among different rock compositions. The whole-rock geochemistry of the Toi is characterized by a conspicuous alkali depletion due to alteration (Figs. 7 and 8), a distinct feature of this unit even when compared with other pyroclastic units of Cerro Grande volcano. Therefore, the high induration and alteration of the deposits may mean that the hydromagmatic eruptions generated voluminous, high-density pyroclastic flows that were unable to separate the water vapor efficiently from the glassy components during transport. Hence, the overall data indicate that the Oriental Lithic-Rich Ignimbrite was mainly the consequence of powerful hydromagmatic, probably high-intensity vulcanian-type eruptions, that occurred at shallow depth into the preexistent shield-like volcano. The high explosibility was acquired by the ascent of magma-batches from a compositionally zoned magma chamber into a highly fractured and water-saturated host rock. It seems possible that the late Miocene regional stress field promoted the brittle fracture of rocks and favored the development of water-saturated zones of structural weakness into the volcanic edifice. 6.3. Stage III. Peripheral fissural lava flows (10.5– 9.7 Ma) After emplacement of the Toi, a period of intense effusive activity very likely occurred in 273 adjacent areas of Cerro Grande volcano ( Fig. 11, Stage III ). Rock units related to this volcanic phase covered a wide extension towards the southern and eastern portions of the map area; here named the Cuyoaco group ( Tgc) and El Crestón andesite ( Tac). This period is characterized by the emplacement of voluminous and relatively homogeneous andesitic lava flows with auto-brecciated surfaces that were clearly erupted from NNW– SSE-trending fissure system. 6.4. Stage IV. Dome formation and explosive collapse (~9.2 Ma) During the emplacement of the Cuyoaco group volcanics ( Tgc), and apparently after the eruption of the El Crestón andesite ( Tac), a homogenous biotite-hornblende bearing a dacitic dome grew inside Cerro Grande’s crater (Fig. 11, Stage IV-a). Although no relicts of this dome were identified in the study area (they may have been covered by subsequent volcanic units or completely destroyed by explosive activity), the voluminous, and compositionally homogeneous, block-and-ash flow deposits of the Tiba toward the northern and northeastern flanks of the Cerro Grande crater reflect that they were derived from an explosively directed collapse of a central dome ( Fig. 11, Stage IV-b). The explosive activity began with the generation of phreatomagmatic explosions and the development of small positively buoyant columns (see Fig. 5, section 38). This initial vent-opening phase probably caused a pressure instability in the magmatic system and the generation of climactic explosive bursts in the dome. The fine ash surge and fall layers between single block-and-ash flow deposits indicate that a buoyant, elutriated, fine ash cloud eventually decoupled from the main dense pyroclastic flows. These layers, together with the presence of hyperconcentrated horizons and lahars at distal positions, and the truncation of degassing pipes by subsequent flow-units, clearly indicate that the entire sequence is the product of several discrete explosive bursts in the central dome system. 274 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 6.5. Stage V. Volcanic repose and open-vent explosive eruptions (9.2–9.0 Ma) After dome formation and explosive collapse, Cerro Grande experienced a period of volcanic tranquillity in which a fluvial-lacustrine environment developed in a very extensive area. Up to 400 m of conglomerates and clay-rich horizons deposited in this period forming the base of the Tsvn sequence ( Fig. 11, Stage V-a). After this period of repose, Cerro Grande entered a new explosive phase that generated a thick, hornblendebearing, pumice-rich pyroclastic-rock sequence. The sequence is composed of an interstratification of pyroclastic fall and base-surge deposits, with no associated flows, that were preferentially distributed to the northern and western sectors of the volcano ( Fig. 11, Stage V-b). Well-sorted pumice-fall deposits were probably formed by the progressive fall of unstable positively buoyant eruptive columns. In contrast, basesurge deposits were very likely emplaced from turbulent, low concentration pyroclastic currents. The sharp contacts between different deposits in all outcrops indicate that a lateral transition between them is unlikely. Thus, the relation between these two contrasting pyroclastic emplacement conditions in the same eruptive cycle indicates that they were derived from two different fragmentation mechanisms in the volcanic conduit. It is possible that the ascent of a homogenous dacitic magma, with a high volatile content, developed intense buoyant eruptive columns that were later preferentially deflected downwind toward the northern and western flanks of the volcano. Sudden and repetitive access of external water (either surface or phreatic) into the erupting magma could have generated highly energetic hydromagmatic eruptions that produced turbulent base surges. Furthermore, a distinct lack of cognate or accessory lithic material in the deposits means that the Tsvn pyroclastic sequence was the product of shallow open-vent explosive eruptions. 6.6. Stage VI. Ring-fissural induced lava-flows (~9 Ma) Shortly after the emplacement of the Tsvn pyroclastic sequence, an intense period of effusive activ- ity recorded the last volcanic stage of Cerro Grande’s geologic history ( Fig. 11, Stage VI ). Radially distributed lava flows were emplaced around the former vent, probably influenced by a ring fissure system. Stratigraphic evidence indicates that these rocks were emplaced about ~0.2 Ma after the emplacement of the last explosive phase ( Tsvn). Hence, they could not have been affected by a central roof-collapse event as a response to the evacuation of a large volume of pyroclastic material. Therefore, a period of slumping of unstable crater walls, NW–SE-trending normal faulting and a long-standing erosion were probably responsible for the crater widening and the construction of a caldera-like structure, as pointed out by Lipman (1997). 7. Discussion: tectonic implications of late Miocene volcanism and the inception of the TMVB The relation of ancient volcanism in the TMVB with the pacific plate tectonics environment is still not well understood. Therefore, we will discuss the regional implications of late Miocene volcanism of the TMVB and provide a possible tectonic scenario for the inception of the volcanic arc. Although several hypothesis have been proposed for the space–time relations of the magmatic arc (Mooser, 1972; Demant, 1978; Cantagrel and Robin, 1979; Demant, 1981; Robin and Cantagrel, 1982; Venegas et al., 1985; Nixon et al., 1987), it has been reported recently that the TMVB acquired a geometric configuration similar to the present configuration since the middle to late Miocene (Ferrari et al., 1994a, 1999). This configuration was apparently the consequence of a counterclockwise migration of the near trench, NW–SE-trending, early Oligocene to early Miocene, silicic Sierra Madre Occidental to the E– W-trending andesitic TMVB. Even though the isotopic age database approach (Ferrari et al., 1994a, 1999) portrays the global space–time distribution of volcanic rocks, it does not provide information on the geologic evolution of individual volcanic centers. Indeed, only a few volcanoes of the TMVB have been studied in detail and even fewer late Miocene volcanic centers. Three late Miocene andesitic stratovolcanoes have been A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 studied in the northern portion of the central sector (Fig. 1): El Zamorano volcano ~10 Ma (Carrasco-Núñez et al., 1989), Palo Huérfano volcano ~11 Ma (Perez-Venzor et al., 1996) and La Joya volcano ( Valdéz-Moreno and Aguirre-Dı́az, 1996). Ferrari et al. (1994b) reported the existence of a widespread calc-alkaline basaltic plateau emplaced between 10 and 6 Ma in the central part of the TMVB. Toward the eastern sector, the oldest reported volcanic rocks are those from Cerro Grande volcano, and the so-called Alsaseca andesite with ~11 Ma ( Yañez-Garcia, 1980). Several studies along the present Middle American Trench (MAT ) have verified that mechanisms of subduction erosion (Mercier de Lépinay et al., 1997) and subduction accretion ( Karig et al., 1978; Moore and Shipley, 1988) have been taking place since the late Miocene, confirming the existence of an active subduction margin since that time. However, at least two independent lines of evidence suggest that the MAT acquired its present configuration much earlier since the late Oligocene (Schaaf et al., 1995; Morán-Zenteno et al., 1996). Thus, it seems that the migration of volcanism from the Sierra Madre Occidental to the TMVB occurred when the MAT was already intensively active. The reasons why the volcanic activity migrated from the near trench NW–SE-trending silicic arc to the intermediate rocks of the E–W-trending TMVB by the middle or late Miocene are not yet understood, nor sufficiently explored. However, if the MAT had its present configuration since the late Oligocene, a major reorganization in the geometry of convergence, subduction rate and angle must account for this migration (Morán-Zenteno et al., 1997; Ferrari et al., 1999). Major plate reorganizations in the Pacific occurred at 25 Ma, between 12.5 and 11 Ma and between 6.5 and 3.5 Ma (Mammerickx and Klitgord, 1982; Londsdale, 1991). Convergence rates between the Farallon plate and its remnants (Guadalupe, Cocos and Rivera) with the North American plate had a step-down decrease from the mid-Eocene (160 mm/yr) to the late Oligocene (80 mm/yr), but increased to 120 mm/yr by the early Miocene, at ~20 Ma ( Engebretson et al., 1985). Pardo and Suárez (1995) pointed out that the current active 275 front of the TMVB is located above an irregular slab, but volcanism is manifested at the surface when it reaches 100 km deep, regardless of the angle of subduction. A depth of ~100 km is a common feature of many volcanic arcs and is usually accounted for by the break-up of amphibole and the release of fluids from the slab into the mantle wedge (Gill, 1981; Tatsumi and Eggins, 1995). If magma generation by the late Miocene was induced when the subducted plate reached a similar depth, a very shallow angle of the subduction must be taken into consideration to generate volcanic rocks as far as ~500 km from the MAT (i.e. Cerro Grande volcano is located at 440 km from the trench). Exactly which was the angle of subduction at that time is unknown; furthermore, it may have had significant variations along its length as it is observed today. However, the major reorganization in the Pacific plates at about 25 Ma, and the high convergence rate at about 20 Ma, may be closely related to the shallowing of the subduction angle and the migration of volcanism toward its E–W location in central Mexico. A plausible tectonic scenario for this hypothesis may start at 20 Ma with a convergence rate of 120 mm/yr and a subduction angle of ~11° ( Fig. 12a). The subducted slab may have reached 100 km beneath the North American plate in 4.25 Ma (Fig. 12b). Therefore, the tectonic conditions for magma generation at a horizontal distance of ~500 km from the trench were already reached at 15.75 Ma. Certainly, there is the need to allow some time for magma generation and eruption. Halliday et al. (1989), for example, estimate that the silicic magma at Glass Mountain may have had a residence time of 0.7 Ma, whereas other models in arc magmas posit transit times from 8000 to 50 000 a for melt generation, transport, fractionation, and eruption (Reagan et al., 1987; Gill and Condomines, 1992). In any case, these may represent favorable tectonic conditions for magma to be emplaced in the TMVB by the middle to late Miocene. 8. Conclusions The Cerro Grande volcano is a major structure that marks the beginning of volcanism in the 276 A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 Fig. 12. Interpretative configuration of Mexico’s subduction zone for the Miocene. The inception of TMVB in the middle to late Miocene may be the result of a lower subduction angle, probably influenced by a faster convergence rate along the MAT. Volcanic rocks with ages ~20 were emplaced with an orientation parallel to the trench ( Ferrari et al., 1999) and extended to northwestern Oaxaca (Ferrusquı́a-Villafranca et al., 1988). By the middle to late Miocene, the volcanic arc had a predominant E–W orientation and reached nearly 500 km from the MAT (see text for examples of volcanic centers). EPR: East Pacific rise. See text for details on plate velocities and angles. eastern sector of the Trans-Mexican Volcanic Belt by the late Miocene. Extensive geologic mapping and stratigraphic studies show that Cerro Grande volcano followed a complex geologic evolution that can be summarized in six principal stages, during a 2 Ma time span (11–9 Ma). The initial volcanic phase in Cerro Grande volcano apparently constructed a mainly andesitic shield-like volcano (Stage I ). This period was followed by a radical change in Cerro Grande’s eruptive behavior, here depicted by the Toi sequence and Stage II. This period produced voluminous lithic-rich ignimbrites and associated pyroclastic fall and surge deposits around the volcano. The unusual lithic-rich character of the ignimbrites, together with significant differences in the characteristics of the juvenile components, indicates that they were related to intense, shallow depth, hydromagmatic eruptions that caused significant devastation in the preexistent shield-like edifice. This explosive stage was followed by an intense period of effusive activity in many parts of the map area (Stage III ), and by the construction of a compositionally homogenous dome inside the Cerro Grande’s former crater (Stage IV-a). This dome eventually acquired an explosive termination forming voluminous block-and-ash-flow deposits on a wide area (Stage IV-b). After this stage, Cerro Grande entered a period of volcanic repose on which a fluvio-lacustrine environment persisted (Stage V-a). Shortly thereafter, this period of volcanic repose was followed by intense open-vent magmatic and hydromagmatic eruptions (Stage V-b). The last eruptive phase in Cerro Grande’s geologic history was an intense period of lava flow emplacement, probably related to a ring-fissural system (Stage VI ). Effusive volcanism was by far the most common volcanic phenomena. Several lava flows and conspicuous volcanic vents in Cerro Grande area show a persistent alignment in a NNW–SSE direction. Furthermore, the entire late Miocene sequence, including the NE flank of Cerro Grande’s crater, was affected by highly dislocated NW–SE-trending normal faults. These features strongly suggest that the late Miocene volcanic record was related to a period tectonic extension. However, volcanism was not a continuous phenomenon since the late Miocene. From the early Pliocene to the Pleistocene, a period of volcanic tranquillity was likely to occur. Volcanism resumed in the Pleistocene with the development of NE–SWtrending cinder cones, that were peripherally emplaced from the Cerro Grande massif. Major-element geochemistry data show that Cerro Grande volcanics, and peripherally emplaced Miocene vents, have fairly homogeneous intermediate compositions. No basaltic or rhyolitic rocks were identified in the geologic record. Major- A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280 and trace-element geochemical trends indicate that they have a co-magmatic origin and that crystal fractionation played a significant role in magma differentiation. Trace-element results also show that Cerro Grande volcanics exhibit typical continental magmatic arc patterns related to modifications of a mantle source region by the interaction with slab-derived fluids. Although it seems possible that these magmas were later modified by a crustal component, no definite conclusion was provided by the isotopic data. Several lines of evidence indicate that the beginning of the TMVB as a distinctive geologic feature occurred during the middle to late Miocene ( Ferrari et al., 1994a, 1999) as the product of subduction along a trench that had its present configuration since the late Oligocene (Schaaf et al., 1995; Morán-Zenteno et al., 1996). These temporal and tectonic constraints strongly suggest that the migration of volcanism from the NW– SE-trending Sierra Madre Occidental to the E–Wtrending TMVB must be related to variations in the Pacific plate tectonics arrangement. Therefore, it is very likely that this migration can be associated with the 25 Ma plate reorganization in the Pacific (Mammerickx and Klitgord, 1982; Londsdale, 1991) that later influenced an increase in the convergence rate along the MAT ( Engebretson et al., 1985). These constraints led us to envisage a tectonic scenario in which the 120 mm/yr peak in the subduction rate at 20 Ma produced a shallower subduction angle along the MAT and thus the migration of volcanism toward an inland position with inception of the TMVB by the middle to late Miocene age. Acknowledgements Our deep thanks to Laura Lozano and Jorge Rivera who provided invaluable help in long field seasons. Rufino Lozano, Patricia Girón, Peter Schaaf and Gabriela Solı́s (LUGIS-UNAM ) are thanked for their help in major-element and isotopic determinations. We gratefully acknowledge support in trace-element determinations from Charlie Langmuir and Kerstin Lehnert (LDEOColumbia). We thank MikeAbrams (JPL) for pro- 277 viding us with the satellite image of Cerro Grande volcano. Special thanks to the people of San Francisco Ixtacamaxtitlán who kindly sheltered us in their homes. We are grateful to Fernando Ortega-Gutiérrez and Luca Ferrari for discussions and suggestions during the preparation of this article. The manuscript was greatly improved by thoughtful reviews of Martı́n Barajas and an anonymous reviewer. This research was funded by a GSA student research grant (No. 5830-6) to A. Gómez-Tuena, and partially by Gerencia de Proyectos Geotermoeléctricos (Comisión Federal de Electricidad ) and Instituto de Geologı́a, UNAM. A. 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