Sr, Nd and Pb isotope and geochemical data from the Quaternary Nevado de Toluca volcano, a source of recent adakitic magmatism, and the Tenango Volcanic Field, Mexico

Journal of Volcanology and Geothermal Research 138 (2004) 77 – 110 www.elsevier.com/locate/jvolgeores

Sr, Nd and Pb isotope and geochemical data from the Quaternary Nevado de Toluca volcano, a source of recent adakitic magmatism, and the Tenango Volcanic Field, Mexico Raymundo G. Martı´nez-Serranoa,*, Peter Schaaf a, Gabriela Solı´s-Pichardob, Ma. del Sol Herna´ndez-Bernalb, Teodoro Herna´ndez-Trevin˜oa, Juan Julio Morales-Contrerasa, Jose´ Luis Macı´asa a

Universidad Nacional Auto´noma de Me´xico, Instituto de Geofı´sica, Laboratorio Universitario de Geoquı´mica Isoto´pica (LUGIS), Ciudad Universitaria, Me´xico D.F. 04510, Mexico b Universidad Nacional Auto´noma de Me´xico, Instituto de Geologı´a, Laboratorio Universitario de Geoquı´mica Isoto´pica (LUGIS), Ciudad Universitaria, Me´xico D.F. 04510, Mexico Received 24 November 2003; accepted 22 June 2004

Abstract Volcanic activity at Nevado de Toluca (NT) volcano began 2.6 Ma ago with the emission of andesitic lavas, but over the past 40 ka, eruptions have produced mainly lava flows and pyroclastic deposits of predominantly orthopyroxene–hornblende dacitic composition. In the nearby Tenango Volcanic Field (TVF) pyroclastic products and lava flows ranging in composition from basaltic andesite to andesite were erupted at most of 40 monogenetic volcanic centers and were coeval with the last stages of NT. All volcanic rocks in the study area are characterized by a calc-alkaline affinity that is consistent with a subduction setting. Relatively high concentrations of Sr (N460 ppm) coupled with low Y (b21 ppm), along with relatively low HREE contents and Pb isotopic values similar to MORB-EPR, suggest a possible geochemical adakitic signature for the majority of the volcanic rocks of NT. The HFS- and LIL-element patterns for most rocks of the TVF suggest a depleted source in the subcontinental lithosphere modified by subduction fluids, similar to most rocks from the Trans-Mexican Volcanic Belt (TMVB). The isotopic compositions are similar for volcanic rocks of NT and TVF regions (87Sr/86Sr: 0.703853–0.704226 and 0.703713–0.704481; qNd: +4.23–+5.34 and +2.24–+6.85; 206Pb/204Pb: 18.55–18.68 and 18.58–18.69; 207Pb/204Pb: 15.54–15.62 and 15.56–15.61; 208 Pb/204Pb: 38.19–38.47 and 38.28–38.50, respectively), suggesting a MORB-like source with low crustal contamination. Metamorphic xenoliths from deeper continental crust beneath NT volcano show isotopic patterns similar to those of Grenvillian rocks of north-central Mexico (87Sr/86Sr: 0.715653–0.721984, qNd: –3.8 to –7.2, 206Pb/204Pb: 18.98–19.10, 207Pb/204Pb: 15.68– 15.69, 208Pb/204Pb: 39.16–39.26 and Nd model age (TDM) of 1.2–1.3 Ga). In spite of a thick continental crust (N45 km) that underlies the volcanoes of the study area, the geochemical and isotopic patterns of these rocks indicate low interaction with this crust. NT volcano was constructed at the intersection of three fault systems, and it seems that the Plio–Quaternary E–W system

* Corresponding author. Tel.: +52 55 56 22 40 28; fax: +52 55 55 50 24 86. E-mail address: [email protected] (R.G. Martı´nez-Serrano). 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.06.007

78

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

played an important role in the ascent and storage of magmas during the recent volcanic activity in the two regions. Chemical and textural features of orthopyroxene, amphibole and Fe–Ti oxides from NT suggest that crystallization of magmas occurred at polybaric conditions, confirming the rapid upwelling of magmas. D 2004 Elsevier B.V. All rights reserved. Keywords: Geochemistry; isotopes; volcanic rocks; Adakites; Nevado de Toluca; Mexico

1. Introduction Nevado de Toluca (NT) volcano and the Tenango Volcanic Field (TVF) belong to the Trans-Mexican Volcanic Belt (TMVB) in central Mexico (Fig. 1). This is one of the best-studied volcanic zones because of its accessibility and position next to the cities of Mexico and Toluca. The TMVB is considered to be a continental magmatic arc that transects central Mexico with an almost E–W orientation, from the Pacific Ocean to the Gulf of Mexico. Activity in this volcanic arc apparently started at about 16 Ma (Ferrari et al., 1994) and continues until today. The TMVB is 1200 km long, and can be divided into three regions on the basis of petrological, tectonic and volcanological characteristics (Pasquare´ et al., 1988 and references therein). The western region consists of alkaline and calcalkaline volcanic rocks at the Colima–Chapala–Tepic junction. The central region is composed of extensive monogenetic volcanism and by higher stratovolcanoes with predominantly calc-alkaline compositions. The eastern region is characterized by the presence of some dacitic-rhyolitic stratovolcanoes and monogenetic volcanic fields with alkaline–calc-alkaline composition. Volcanism in the TMVB has been associated with the subduction of the Cocos and Rivera plates beneath the North America plate (Fig. 1) (Robin, 1976, 1982; Demant, 1978, 1981; Pal et al., 1978; Nixon, 1982; Negendank et al., 1985; Besch et al., 1987; Nixon et al., 1987; Verma and Nelson, 1989; Ferrari et al., 1999; Wallace and Carmichael, 1999 and others). However, Mooser (1972), Cebull and Shurbet (1987), Ma´rquez et al. (1999) and Verma (1984, 1987, 1999, 2000) proposed that the TMVB is the result of a combination of several tectonic processes including crustal fracturing, a continental rifting scenario associated with an upwelling mantle, and production of ocean island basalts (OIB) by a propagating rift opening from

west to east that is related to the effects of a mantle plume. Many geological, geochemical, geophysical and volcanological studies have been carried out on the TMVB during the past 35 years. In spite of these studies, little attention has been focussed on the geochemical and isotopic variations existing in the rock sequences of stratovolcanoes or monogenetic volcanic products. Many geochemical and isotopic (Sr, Nd and Pb) data exist in all regions along the TMVB. However, most active volcanoes lack detailed geochemical and isotopic characterization of their main magmatic events. In the present study, geochemical and isotopic characterization of the main products of Nevado de Toluca and the Tenango Volcanic Field was carried out in order to improve the understanding of the chemical evolution of these volcanic structures. Geochemical data for the Nevado de Toluca and Tenango Volcanic Field products are then used to test models accounting for compositional variations in the source region, contamination and mixing of magmas. These results can be used to further study volumetric discharges, cyclicity of eruptions and tectonic models.

2. Geological setting NT is the fourth highest peak in Mexico (4680 m asl) and a thick sequence of several hundreds meters of Mesozoic and Tertiary metamorphic, carbonate and volcanic sequences from the Guerrero Block (Johnson and Harrison, 1990) underlies the volcanic structures in the study area. Stratigraphic and petrographic descriptions of these older sequences are summarized by Garcı´a-Palomo et al. (2000, 2002). In these papers and references therein, the authors propose that NT volcano was constructed at the intersection of three complex fault systems of

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

79

Fig. 1. Location of the TMVB in central Mexico (RFZ=Rivera Fracture Zone, EPR=East Pacific Rise, MAT=Middle America Trench, CP=Cocos Plate, C=Ceboruco, Co=Colima, P=Paricutı´n, NT=Nevado de Toluca, Po=Popocate´petl, Pi=Pico de Orizaba, SM=San Martı´n Tuxtla, C=Chicho´n and T=Tacana´ volcanoes) and the study area. The Tenango Volcanic Field is in the westermost part of the Sierra Chichinautzin. This was characterized by Bloomfield (1975), Martin del Pozzo (1989), Ma´rquez et al. (1999) and others. Stratovolcanoes are indicated as triangles and important volcanic cones are shown by white squares. T=Tenango, TC=Tres Cruces, TX=Texontepec, X=Xitle, TA=Tabaquillo, CH=Chichinautzin, P=Pelado, D=Dos, Iz=Iztaccı´huatl and Po=Popocate´petl.

different ages, orientations and kinematics. These fault systems are Taxco-Quere´taro with a NNW– SSE orientation, San Antonio with a NE–SW orientation and Tenango with an E–W direction (Fig. 2). This last fault system controls the position

of monogenetic volcanoes in the TVF. The fault systems have coexisted since the late Miocene (Garcı´a-Palomo et al., 2000). Fig. 2 shows a generalized geologic map and locations of samples included in this study.

80

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 2. Schematic geologic map of Nevado de Toluca and the Tenango Volcanic Field with location of samples (modified from Macı´as et al., 1997). Inset shows distribution of the main fault systems (H=horst and G=graben).

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

2.1. Nevado de Toluca volcano Cantagrel et al. (1981) proposed that volcanic activity at NT started some 1.5 Ma with the emplacement of andesitic lava flows that constructed the primitive volcano (bPaleo-NevadoQ). However, recently published K–Ar ages from some andesitic lavas suggest that the volcanic activity started at 2.6 Ma (Garcı´a-Palomo et al., 2002). The volcanic activity at NT between 1.5 Ma and 100 ka was volcaniclastic, according to Cantagrel et al. (1981). A thick volcaniclastic sequence of debris avalanches, lahars and fluvial deposits on the southern flanks of the volcano give evidence that bPaleo-NevadoQ was

81

destroyed at least twice by failure of the volcanic structure (Capra and Macı´as, 2000). Macı´as et al. (1997) and Garcı´a-Palomo et al. (2002) presented a detailed stratigraphic description of the main units emplaced during late Pleistocene and Holocene time at NT (Fig. 3): –

A debris-avalanche deposit (DAD1), up to 15 m thick, composed of blocks showing jigsaw-fit structures, embedded in an indurated coarse sandy matrix, overlies a paleosoil and a thick sequence of epiclastic deposits from bPaleo-NevadoQ. Blocks of porphyritic gray juvenile dacite and red altered dacite from the volcanic structure are

Fig. 3. Composite stratigraphic sequences of NT and, in italics, the TVF. Also indicated are the samples analyzed in this study. Ages from: 1. Macı´as et al. (1997), 2. Bloomfield (1974), 3. Bloomfield and Valastro (1977), 4. Cantagrel et al. (1981), 5. Garcı´a-Palomo et al. (2002), 6. Arce et al. (2003) and 7. Caballero et al. (2001).

82

–

–

–

–

–

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

present. DAD1 spreads south to a distance of 55 km from the volcano. The Pilcaya and Mogote debris flow deposits (PDF and MDF, Capra and Macı´as, 2000) were emplaced on a pale brown paleosoil. The PDF deposit shows blocks of gray porphyritic dacite, red altered dacite, green altered andesite, basalt and schist from the local basement, all embedded in a coarse sandy matrix. A block-and-ash flow deposit that covers the PDF and MDF deposits yielded a 14 C age of 37 ka (Macı´as et al., 1997). Thus, the age of these debris flow deposits must be N37 ka. A thick, pink pumice-rich pyroclastic flow deposit (PPF) with at least four units was emplaced around NT. A radiocarbon age obtained by Macı´as et al. (1997) from a tree trunk within the deposit indicated 42 ka. Outcrops of this unit are scarce and no samples for geochemical studies were available. The first pumice-fall deposit, dated between 36 and 39 ka, was emplaced on the northern slope of NT (5 km from the summit, Garcı´a-Palomo et al., 2002). It consists of an alternating sequence of pumice-fall, pyroclastic-surge and pyroclasticflow deposits (3.5 m thick). Two violent eruptions at NT produced large magmatic explosions that destroyed old dacitic central domes and excavated the present-day crater. The explosions produced two block-andash flow (BAF in Fig. 3) deposits at 37 and 28 ka, with a similar maximum thickness of 35 m (Macı´as et al., 1997). Garcı´a-Palomo et al. (2002) identified four BAF deposits that consist almost entirely of gray porphyritic juvenile dacitic clasts with minor amounts of pumice, glassy dacitic lithic clasts and red oxidized dacitic clasts from the volcanic structure set in ash matrix. A fallout deposit with inverse grading, denominated Lower Toluca Pumice (LTP), covers the BAF deposits. The LTP is clast-supported with 62% pumice, 27% lithic clasts and 11% crystals for the entire deposit (Bloomfield et al., 1977). In the present study, we observed abundant ochre dacitic pumice fragments with hornblende and orthopyroxene, lesser amounts of gray dense juvenile dacite, altered dacitic clasts and metamorphic fragments (xenoliths) such as gneiss, schists and phyllites from the local basement.

–

–

–

–

Isolated crystals of euhedral amphibole, pyroxene and feldspar are disseminated in this deposit. A thin ash-flow deposit and a dark-brown paleosoil dated at 24.3 ka (Bloomfield and Valastro, 1977) overlie the LTP. A younger gray BAF overlies the LTP. It differs strikingly from the older BAF because it has a more radial distribution around the volcano. This unit consists of a gray cross-bedded pyroclasticsurge deposit overlain by two massive gray blockand-ash flow units made of block-sized lithic fragments in a coarse ash matrix with a total thickness of 10 m (Garcı´a-Palomo et al., 2002). The age of this deposit is uncertain, although Caballero et al. (2001) assumed an age N14 ka. The Middle Toluca Pumice (MTP; Cervantes et al., 2004) consists of a complex sequence of three fallout layers, two pyroclastic-surge deposits, and two massive pumice-rich pyroclastic-flow deposits (7 m thick), the reason for which it was first dubbed as the White Pumice Flow (WPF). The age of the MTP was defined through 14C dates of charcoal found inside the pyroclastic-flow deposits at 12.1 ka (Garcı´a-Palomo et al., 2002). Pumice-fall deposits, a pyroclastic-flow, and pyroclastic-surge beds, with a minimum age of 11 ka (14C, Bloomfield and Valastro, 1974, 1977) (Fig. 3) were referred to by these authors as the Upper Toluca Pumice (UTP). Macı´as et al. (1997) and Arce et al. (2003) refined the stratigraphy of this deposit, that in some places reaches a total thickness of about 30 m. These authors recognized four pumice-fall layers that contain abundant pink pumice, banded pumice, gray juvenile dacite clasts and altered andesitic lithics, within an ash matrix. Pyroclastic-surge deposits with crossstratification, dunes and antidunes are intercalated in the sequence. The volcanic products of the UTP cover an area of 2000 km2 with a minimum estimated volume of 8 km3 with an age of 10.5 ka (14C) (Arce, 2003). The UTP event ended with the emplacement of a dacitic dome in the Nevado de Toluca crater. Gray cross-stratified pyroclastic-surge deposits and pyroclastic-flow deposits dated by Macı´as et al. (1997) at 3.3 ka (14C) represent the most recent volcanic event at NT, and indicate that activity has continued into Holocene time.

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

2.2. Tenango Volcanic Field The TVF represents the westernmost part of the Sierra Chichinautzin (Fig. 1) described by Bloomfield (1975), Martin del Pozzo (1989), Ma´rquez et al. (1999, 2001) and others. The Sierra Chichinautzin consists of nearly 220 Quaternary monogenetic volcanic cones with an E–W general structural orientation. This volcanic province is bracketed by NT to the west, Popocate´petl volcano to the east and by the Mexico Basin in the north (Fig. 1). The TVF (Fig. 2) is composed of more than 40 volcanic monogenetic cones and associated lava flows with ages from 8 to N38.5 ka BP (Bloomfield, 1975). Cinder cones, lava cones and effusive fissural lava flows with andesite and basaltic-andesite compositions dominate in the TVF. The cone density in the TVF is 0.5/km2 but locally reaches 1/km2; the mean cone height is 650 m. The cinder cones consist of dark gray to brick-red scoria and ash fragments with diameters of 0.5–7 cm, in layers 3–15 cm thick and dips from 208 to 268. Well-sorted scoria and a small proportion of bombs are the main ejecta products of the cinder cones. Thin lenses of lava are present in some cone sequences. Bloomfield (1975) described the presence of at least 220 separate beds of black ash in some cinder cones that indicate repeated short eruptive pulses. The lava cones are made up of angular to subangular lava blocks with sizes of 1–30 cm, that form dip layers of 258. Although scoria fragments are rare in these volcanoes, thin lenses (35 cm thick) are occasionally observed in lavas. The effusive fissurefed lava flows are the most recent volcanic events (b8.5 ka, Bloomfield, 1975) in the TVF. These lava flows have the greatest length (~8 km long) and volume of those in the area. The fissure-fed lava flows are mainly aa-type and in minor proportion appear as pahoehoe and lava blocks. A complex set of faults and fractures with a similar E–W orientation to that observed in the Sierra Chichinautzin seems to control the distribution of volcanic cones in the TVF as well (Fig. 2). Mooser and Maldonado-Koerdell (1961) and Bloomfield (1974) have presented the relationships between these structures and the monogenetic activity, and Garcı´a-Palomo et al. (2000) proposed that this fault system is a continuation of the older ChapalaTula Fault Zone that has been reactivated during

83

Pleistocene–Holocene times. Fig. 3 shows the relative ages of volcanic events for the TVF on the basis of radiocarbon age data and morphologic characterization of cones developed by Bloomfield (1975). We used the nomenclature proposed by Bloomfield (1975) to describe the volcanic cone ages: PLV1c40 ka, PLV2c30 ka, PLV3 from 18.6 to 21.9 ka and HVb8.5 ka (Fig. 3). These data suggest that volcanic events in the TVF have occurred simultaneously with the emplacement of some pyroclastic deposits at NT.

3. Analytical methods A representative suite of volcanic products (N100 samples) was obtained on the flanks of NT and around the TVF (Figs. 2 and 3). Pumice, lava and scoria fragments were sampled considering their stratigraphic position. Thin sections were studied to assess the mineralogy and petrography of the rocks and fresh samples were selected for bulk chemical analyses and Sr, Nd and Pb isotopic determinations. Metamorphic xenolith fragments from the LTP were handpicked for geochemical and isotopic studies. Major-elements and Sc abundances were determined by inductively coupled plasma-emission spectroscopy, and all other trace elements by inductively coupled plasma mass spectrometry (ICP-MS) at the analytical laboratories of the Centre de Recherches Pe´trographiques et Ge´ochimiques, Nancy, France (SARM, 2003). Sr, Sm, Nd and Pb isotopic ratios of whole rock samples were measured using a Finnigan MAT 262 thermal ion mass spectrometer at LUGIS (Laboratorio Universitario de Geoquimica Isotopica), UNAM. The spectrometer is equipped with a variable multicollector system (eight Faraday cups) and all measurements were done in static mode. Rb isotope ratios were measured with an NBS type single collector mass spectrometer (Teledyne Model SS-1290). Rb, Sr, Sm and Nd samples were loaded as chlorides on double rhenium filaments and measured as metallic ions. Lead samples were loaded with a mixture of silica gel+phosphoric acid. Sixty isotopic ratios were determined for Rb, Sr, Sm and Nd, and 100 for Pb on each sample. Elements were separated using standard ion-exchange methods. Total procedure blanks during analyses of these samples were less than: 1 ng Rb, 10 ng Sr, 1 ng

84

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

mineralogical analyses and petrography of the majority of the samples. Lava, pumice and other pyroclastic samples from NT display porphyritic textures, with a predominantly dacitic composition. However, andesites have been observed in the earliest volcanic events of bPaleo-NevadoQ and also in some pyroclastic materials such as the LTP. Lava and pyroclastic products show different proportions of crystals, 50 and 10 vol.%, respectively. Phenocrysts of plagioclase, amphibole, orthopyroxene (hypersthene) and rare biotite appear in two sizes in all samples, as macrophenocrysts (1–2.5 mm) and phenocrysts (b1 mm), although plagioclase and some pyroxene appear also as microlites in the glassy groundmass. Macrophenocrysts of hornblende, plagioclase, and biotite from dacitic lavas commonly show reaction rims. The petrography of most NT rocks is very similar. Phase abundances, without considering vesicular porosity, range as follow: plagioclase (oligoclase–andesine)

Sm, 20 ng Nd and 300 pg Pb. More than 350 analyses of pyroxene, amphibole, olivine, Fe–Ti oxides and feldspar from lava samples and juvenile fragments from pyroclastic deposits were carried on an automated CAMECA SX100 electron microprobe (University of Barcelona, Spain). An acceleration voltage of 15 kV was used. The excitation current varied from 8 to 10 nA and the counting time was 10 s. The maximum analytical error in major oxides is estimated to be less than 3%.

4. Results 4.1. Mineral studies and petrographic characteristics Sixty samples from NT and the TVF were studied petrographically, and a subset of them were analyzed by electron microprobe. Table 1 shows modal

Table 1 Modal mineral assemblages (vol.%) of selected NT and TVF lava and pyroclastic samples Sample

Phenocrysts Plag (An32–54)

Groundmass Horn

Opx

Cpx

Oliv

Qtz

Plag (Ab32–54)

Biot

Oxides

Other phases

Glass

Total

Petrography

3 4 5 5 2 4 6 5 7 5 3 4 7 3 3

2 2 2 3 2 2 1 3 4 3 3 2 4 2 3

0 0 0 0 0 0 0 0 0 1 1 0 3 4 5

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 1 1 1 1 0 0

1 1 1 20 2 2 0 5 40 25 15 15 50 41 50

0 0 0 1 0 0 1 0 1 2 1 1 0 0 0

1 1 1 2 2 2 1 1 1 1 11 7 4 5 5

0 0 Zr, Ap=T 0 0 0 0 C. mins=T 0 Zr, Ap=T Zr, Ap=T 0 0 Xen=8 0

89 85 83 48 88 83 84 75 27 38 45 50 7 7 7

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

Dacitic pumice Dacitic pumice Dacitic pumice Andesitic tuff Andesitic pumice Dacitic pumice Dacitic pumice Dacite Dacite Dacite Dacite Dacite Andesite Andesite Andesite

Tenango Volcanic Field TEO1 2 0 TEP1 0 0 RMS7 0 0 RMS8 0 0 RMS9 0 0 JAJ1 20 0

3 0 2 2 2 2

5 7 6 5 5 2

1 2 1 2 2 0

5 0 0 0 0 1

56 50 67 75 73 66

0 0 0 0 0 T

5 2 2 4 3 2

0 0 0 0 0 0

25 39 22 12 15 7

100 100 100 100 100 100

Andesite Basaltic-andesite Andesite Basaltic-andesite Basaltic-andesite Andesite

Nevado de Toluca NT9 4 NT12 7 NT13 7 NT6 21 NT8 4 NT33 7 NT14 7 NT10 11 NT11 20 NT15 24 NT17 20 NT22 20 NT24 25 NT29 30 NT30 27

Plag=Plagioclase, Horn=hornblende, Opx=orthopyroxene (hypersthene), Cpx=clinopyroxene (augite-diopside), Oliv=olivine, Qtz=quartz, Biot.=biotite, Oxides=Fe–Ti oxides, Zr=zircon, Ap=apatite, C. mins=clay minerals, T=traces, Xen=xenoliths.

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

between 4 and 50 vol.%, hypersthene between 2 and 5 vol.%, hornblende between 2 and 7 vol.%, Fe–Ti oxides b2 vol.% and biotite appears in traces (b1 vol.%). The glassy matrix is very abundant (N30 vol.%). A typical characteristic of most pumices and dacitic lavas from NT is the presence of orthopyroxene (hypersthene) and the absence of clinopyroxene, although some dacitic flows display rare phenocrysts of augite. This petrographic characteristic is rarely observed in rocks of the TMVB. Rocks from the TVF show a predominantly andesitic composition, although basaltic andesites have also been observed. All samples from this field show aphanitic textures with rare disseminated phenocrysts (~1 mm) of plagioclase, pyroxene and olivine (Table 1). Phase abundances are: plagioclase in microlites ~55 vol.%, andesitic glass between 10 and 40 vol.%, orthoand clinopyroxene ~7 vol.%, Fe–Ti oxides ~3 vol.% and olivine ~1%. Some TVF samples exhibit xenoliths of dioritic composition. Minor amounts of quartz (b1 vol.%) exist in some TVF samples and in some NT andesites. Olivine and xenocrystic quartz coexist in some TVF samples with textural evidence of disequilibrium. This is a common characteristic observed in some Chichinautzin samples. Ma´rquez and De Ignacio (2002) considered these assemblages as the result of magma mixing, whereas Siebe et al. (2004) proposed that similar mineralogy and textures are consistent with normal fractional crystallization processes and the assimilation of some wall rock material in the Sierra Chichinautzin volcanic field. Plagioclase phenocrysts and microlites in NT samples show normal and reverse zoning, twinning and sieve textures. Some phenocrysts show clear evidence of multiple periods of dissolution and growth. In the TVF, plagioclase appears mostly as microlites, but rare phenocrysts with reaction rims are also observed. Phenocrysts and microlites show similar compositions (An32–54) in NT samples, although reverse zoning leads to more An-rich rims (An55–58) in some dacitic rocks. Plagioclase in some TVF andesites displays a more rich An (An54–60) content than in NT rocks. For comparison, plagioclases from two important pumice events of Popocate´petl volcano show values more rich in An than those from our study area (An60–80, Siebe et al., 1999). Olivine is commonly present in TVF basaltic andesites as disseminated phenocrysts or as rare

85

glomerophenocrysts associated with clinopyroxene and minor plagioclase. The crystals vary from subhedral to anhedral in shape and skeletal forms are also observed. Analyses of olivine show homogeneous compositions (Fo84–86). Some crystals display evidence of disequilibrium conditions such as coronas of reaction rims of clinopyroxene. Iddingsite as secondary alteration of the olivine is observed in some rocks. In some dacitic NT samples, clinopyroxene is present as rare subhedral phenocrysts (~1 mm) with reaction rims of orthopyroxene and amphibole, and the composition ranges from salite to diopside-augite with minor proportions of endiopside (Wo41–47, En41–50, Fs5–14). Clinopyroxene is absent in most NT pumices and other pyroclasts. In TVF rocks clinopyroxene appears as subhedral to anhedral phenocrysts (~1 mm), commonly small isolated crystals in the groundmass or in minor proportions as corona reaction rims on quartz. Twinning and zoning are rarely observed, and disequilibrium conditions of crystallization can been inferred from the presence of reaction borders in some samples. Most analyses are augite to diopside, although nearly 30% of the analyses fall within the endiopside field. Clinopyroxene found in quartz corona reaction rims shows compositions of Wo38–45, En45–51, Fs8–14 and very low Al2O3 concentrations (0.29–0.77 wt.%), whereas isolated crystals disseminated in the same rock sample display similar compositions (Wo35–45, En46–53, Fs7–11) but different Al2O3 concentrations (1.44–4.0 wt.%). Clinopyroxene compositions of NT and TVF are very similar to values observed in rocks of Popocate´petl volcano (Siebe et al., 1999). Orthopyroxene is very common in NT pumice and lava samples, and a minor phase in TVF rocks. At NT, it is subhedral to euhedral with several indications of reaction with the groundmass and sometimes it is associated with amphibole. Important pumice-fall deposits such as the UTP and LTP, with wide aerial distributions in central Mexico contain only orthopyroxene as a ferromagnesian mineral. Macrocrysts and phenocrysts of hypersthene display zoned borders, and compositional ranges of En70–78 for cores and En55–76 for rims. Orthopyroxene shows relatively variable compositions in dacitic NT lavas, ranging from En84–90 to En60–67. In TVF rocks, orthopyroxene is present in microcrystals (b0.4 mm) disseminated in

86

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Table 2 Major oxide and trace element abundances of selected rocks from Nevado de Toluca (A=andesite, B-A=basaltic andesite, D=dacite, D-P=dacitic pumice, A-P=andesitic pumice, T=tuff) Sample Rock Long. W Lat. N (wt.%) SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

NT4 D 99847.41V 19813.36V 64.88 0.65 16.76 4.32 0.06 1.75 4.20 4.35 1.94 0.17 0.80 99.88

Trace elements (ppm) V 71 Cr 23 Co. 7.56 Ni 6 Cu 8 Zn 71 Rb 36.14 Sr 543 Y 14.74 Zr 149 Nb 4.23 Ba 481 La 14.41 Ce 30.51 Pr 3.84 Nd 15.55 Sm 3.33 Eu 1.08 Gd 2.78 Tb 0.44 Dy 2.57 Ho 0.51 Er 1.34 Tm 0.22 Yb 1.33 Lu 0.22 Hf 3.77 Ta 0.38 Th 3.74 U 1.47 Ta/Yb 0.28 Hf/U 2.57 Zr/U 101 U/Ta 0.39 Th/Hf 0.99 Ba/Zr 3.24 Ba/La 33.35 La/Nb 3.41 Th/Ta 9.95

NT11 D 99839.77V 19811.12V

NT15 D 99845.37V 1986.37V

NT17 D 99845.9V 1986.95V

NT22G D 99846.49V 18851.29

NT22R D 99846.49V 18851.29

NT24 D 99846.49V 18851.29

NT25 D 99846.49V 18851.29

NT29 A 99847.99V 1988.28V

NT30 A 99848.09V 1988.59V

NT6 A-P 99847.41V 19813.16V

65.98 0.64 16.44 4.29 0.05 1.72 4.10 4.41 2.01 0.17 0.04 99.85

64.87 0.62 16.57 4.31 0.05 1.79 4.15 4.37 1.94 0.18 1.02 99.87

64.25 0.63 16.51 4.50 0.05 2.67 4.71 4.26 1.98 0.20 0.13 99.89

63.83 0.70 16.76 4.72 0.06 1.99 4.51 4.27 2.02 0.18 0.82 99.86

64.05 0.69 16.66 4.66 0.06 1.98 4.43 4.19 2.05 0.18 0.91 99.86

63.41 0.62 15.75 4.85 0.08 4.00 4.56 4.05 2.16 0.17 0.75 100.40

63.13 0.70 16.05 4.97 0.06 3.29 5.06 4.25 2.02 0.25 0.15 99.93

57.41 0.90 18.47 6.77 0.09 3.23 6.26 3.92 1.76 0.23 0.86 99.90

61.93 0.75 17.08 5.46 0.08 2.65 5.03 4.36 1.87 0.18 0.47 99.86

59.95 0.70 18.56 4.77 0.07 2.01 4.64 4.28 1.28 0.18 3.44 99.88

70 23 7.52 b5 7 70 37.25 541 14.73 142 4.19 464 14.63 30.77 3.93 15.49 3.50 1.09 3.06 0.43 2.64 0.53 1.33 0.20 1.34 0.21 3.72 0.38 3.82 1.47 0.28 2.53 96 0.38 1.03 3.28 31.73 3.49 10.01

62 29 8.81 21 10 75 40.70 527 13.61 160 4.97 513 16.64 35.10 4.47 18.45 3.86 1.17 3.22 0.43 2.50 0.48 1.26 0.19 1.20 0.20 4.31 0.44 4.02 1.51 0.37 2.84 106 0.38 0.93 3.20 30.85 3.35 9.12

77 83 11.33 40 19 76 35.39 694 14.35 160 4.30 511 17.53 38.81 4.99 19.33 4.03 1.25 3.13 0.48 2.57 0.49 1.32 0.20 1.23 0.19 3.99 0.37 3.70 1.34 0.30 2.98 120 0.36 0.93 3.18 29.12 4.07 9.92

84 21 10.18 11 13 81 37.55 548 15.42 140 4.19 430 13.33 28.78 3.91 15.62 3.42 1.12 3.11 0.44 2.54 0.53 1.39 0.21 1.39 0.21 3.48 0.34 3.35 1.52 0.25 2.29 92 0.45 0.96 3.07 32.27 3.18 9.75

81 22 9.90 10 15 76 37.87 560 15.69 138 4.12 413 13.11 27.18 3.69 14.89 3.33 0.96 2.89 0.42 2.53 0.50 1.33 0.20 1.37 0.22 3.47 0.34 3.04 1.43 0.25 2.42 96 0.47 0.88 3.00 31.49 3.19 8.89

71 161 18.21 104 23 70 38.09 629 14.22 135 4.03 521 17.06 35.23 4.63 18.07 3.64 1.10 2.96 0.43 2.19 0.45 1.26 0.18 1.25 0.19 3.37 0.37 4.23 1.71 0.30 1.97 79 0.40 1.26 3.87 30.50 4.24 11.38

84 125 14.47 69 19 74 32.87 843 14.36 149 4.60 578 22.66 47.65 6.51 24.69 5.22 1.41 3.64 0.51 2.71 0.50 1.35 0.18 1.21 0.19 3.99 0.39 4.15 1.60 0.32 2.49 93 0.39 1.04 3.87 25.50 4.92 10.69

145 20 17.83 8 13 84 35.22 678 21.15 152 4.24 397 18.30 37.61 5.40 22.69 5.29 1.49 4.38 0.64 3.73 0.73 1.96 0.28 1.87 0.30 3.93 0.38 4.18 1.42 0.20 2.78 107 0.34 1.06 2.61 21.68 4.32 11.08

113 45 14.74 16 15 78 39.72 514 22.69 163 4.07 409 19.35 38.24 5.16 20.95 4.66 1.20 4.14 0.63 3.65 0.77 2.05 0.30 2.10 0.32 4.48 0.37 4.78 1.59 0.18 2.81 102 0.33 1.07 2.52 21.15 4.76 12.94

83 29 9.25 6 8 78 19.68 597 15.72 150 4.29 410 13.62 31.18 4.02 15.82 3.61 1.18 3.21 0.49 2.77 0.56 1.52 0.21 1.59 0.26 4.17 0.42 4.29 1.48 0.26 2.81 101 0.35 1.03 2.73 30.06 3.18 10.32

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

87

Sample Rock Long. W Lat. N

NT7 D 99839.77V 19811.12V

NT9 D-P 99839.77V 19811.16V

NT14 D-P 99839.05V 1982.76V

NT13 D-P 99841.37V 1986.33V

NT8 A-P 99839.77V 19811.12V

NT10 D 99839.77V 19811.12"

NT12 D-P 99841.37V 1986.33V

NT31 D 99845.9V 1986.95V

NT32 D 99845.35V 1987.17V

NT33 D 99845.66V 19813.37V

(wt.%) SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

63.26 0.58 17.39 4.10 0.05 1.78 4.28 4.29 1.69 0.21 2.25 99.88

62.35 0.65 17.03 4.34 0.05 1.79 4.28 4.21 1.75 0.18 3.24 99.87

63.30 0.58 16.52 4.16 0.06 1.83 4.19 3.96 1.89 0.16 3.22 99.87

63.76 0.62 16.54 4.13 0.05 1.69 4.18 4.35 1.90 0.17 2.46 99.85

55.59 0.68 18.61 4.82 0.05 2.08 4.44 3.31 1.28 0.21 8.79 99.86

64.69 0.64 16.66 4.32 0.05 1.79 4.32 4.29 2.06 0.15 0.89 99.86

63.46 0.61 16.53 4.21 0.05 1.75 4.22 4.32 1.87 0.19 2.65 99.86

64.90 0.61 15.96 4.16 0.07 2.44 4.41 4.31 2.00 0.17 0.62 99.65

66.48 0.64 16.25 3.95 0.07 1.65 4.12 4.47 1.98 0.15 0.04 99.72

62.02 0.64 16.93 4.01 0.06 1.70 4.05 4.14 1.78 0.20 4.21 99.74

63 29 8.21 9 10 79 37.78 557 14.31 163 5.08 524 16.57 37.56 4.61 18.58 3.86 1.28 3.25 0.52 2.54 0.46 1.26 0.19 1.31 0.19 4.34 0.46 4.05 1.49 0.35 2.91 109 0.37 0.93 3.23 31.65 3.26 8.86

70 31 8.19 13 6 69 39.44 553 14.83 123 4.14 444 12.90 28.61 3.86 15.27 3.36 1.01 3.23 0.47 2.45 0.50 1.41 0.21 1.40 0.22 3.58 0.39 3.46 1.60 0.28 2.24 77 0.46 0.97 3.61 34.40 3.12 8.81

62 28 7.98 10 6 76 39.60 547 13.47 159 4.91 528 16.62 35.25 4.47 18.92 3.91 1.23 3.21 0.46 2.30 0.46 1.21 0.16 1.19 0.17 4.27 0.45 4.02 1.65 0.38 2.58 96 0.41 0.94 3.33 31.79 3.39 8.98

89 37 9.29 12 14 75 25.23 553 16.30 173 4.93 381 15.19 29.36 4.65 19.14 4.42 1.24 3.51 0.57 3.02 0.54 1.52 0.21 1.49 0.24 4.21 0.42 4.03 1.35 0.28 3.11 128 0.34 0.96 2.20 25.06 3.08 9.67

78 27 8.38 6 6 75 37.54 559 14.67 138 3.89 433 12.50 27.77 3.58 14.77 3.31 1.02 2.71 0.44 2.43 0.46 1.31 0.21 1.35 0.21 3.43 0.35 3.55 1.42 0.30 2.40 97 0.40 1.03 3.14 34.61 3.21 10.10

64 28 8.50 10 7 84 39.43 541 13.42 160 4.89 505 16.18 33.93 4.31 17.38 3.84 1.17 3.02 0.46 2.43 0.45 1.09 0.17 1.10 0.18 3.83 0.42 3.88 1.49 0.38 2.57 107 0.38 1.01 3.16 31.20 3.31 9.25

68 162 9.49 38 21 78 39.26 608 15.12 143 4.21 543 16.93 36.16 4.29 18.85 3.87 1.20 3.59 0.51 2.74 0.50 1.46 0.21 1.38 0.20 3.35 0.30 4.20 1.38 0.21 2.43 104 0.33 1.25 3.79 32.09 4.02 14.42

64 72 6.79

55 69 7.33

13 64 41.09 482 15.25 125 4.06 457 12.41 26.12 3.13 13.58 3.09 1.01 3.12 0.48 2.69 0.51 1.52 0.22 1.44 0.22 3.02 0.30 3.76 1.43 0.21 2.11 87 0.38 1.25 3.67 36.87 3.05 12.43

32 75 36.86 470 14.80 150 4.80 531 16.42 31.43 4.11 17.51 3.75 1.17 3.64 0.51 2.77 0.49 1.35 0.19 1.25 0.18 3.50 0.40 4.19 1.42 0.29 2.47 105 0.34 1.19 3.55 32.32 3.42 11.57

Trace elements (ppm) V 66 Cr 32 Co. 7.80 Ni 7 Cu 9 Zn 72 Rb 30.15 Sr 569 Y 15.18 Zr 167 Nb 4.99 Ba 528 La 17.70 Ce 32.69 Pr 5.01 Nd 19.77 Sm 3.99 Eu 1.27 Gd 3.52 Tb 0.52 Dy 2.79 Ho 0.51 Er 1.36 Tm 0.21 Yb 1.46 Lu 0.22 Hf 4.34 Ta 0.48 Th 4.76 U 1.69 Ta/Yb 0.33 Hf/U 2.57 Zr/U 99 U/Ta 0.35 Th/Hf 1.10 Ba/Zr 3.16 Ba/La 29.81 La/Nb 3.55 Th/Ta 9.92

(continued on next page)

88

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Table 2 (continued) Major oxide and trace element abundances of selected rocks from Tenango Volcanic Field (A=andesite, B-A=basaltic andesite, D=dacite) Sample

JAJ1

TEO1

STA1

ESP1

TEP1

RMS2

RMS3

RMS4

RMS5

RMS7

Rock Long. W Lat. N

A 99833.47V 1986.91V

A 99836V 1985.76V

A 99826.97V 1989.80V

A 99826.06V 1984.23V

A 99826.29V 1983.5V

B-A 99823.88V 19813.35V

B-A 99824.26V 19811.78V

B-A 99825.26V 1988.91V

A 99829.57V 1987.95V

A 99831.11V 1987.04V

(wt.%) SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

59.52 0.79 15.99 6.08 0.09 4.42 6.06 3.73 2.03 0.25 0.95 99.91

59.78 0.72 16.42 5.77 0.07 4.50 6.06 3.91 1.71 0.19 0.76 99.89

60.46 1.01 16.55 6.42 0.10 3.93 5.51 4.14 1.90 0.31 0.02 100.31

60.60 0.75 16.41 5.73 0.08 4.38 5.90 4.12 1.67 0.25 0.02 99.91

58.74 0.78 16.58 6.06 0.09 4.92 6.41 4.05 1.58 0.26 0.46 99.93

52.71 0.89 15.21 7.24 0.11 8.21 8.15 3.59 2.17 0.62 0.60 99.50

54.76 1.32 16.02 7.81 0.11 5.80 6.97 4.34 1.63 0.49 0.43 99.68

54.62 1.20 16.33 7.69 0.11 6.24 7.05 3.96 1.22 0.41 0.90 99.73

59.42 0.91 16.21 6.01 0.08 4.36 5.73 4.39 1.71 0.31 0.60 99.73

59.47 0.75 15.66 6.05 0.09 5.24 6.12 4.06 1.62 0.27 0.40 99.73

Trace elements (ppm) V 124 Cr 170 Co. 33.22 Ni 50 Cu 11 Zn 87 Rb 33.21 Sr 611 Y 20.15 Zr 150 Nb 4.94 Ba 532 La 21.73 Ce 49.62 Pr 6.51 Nd 27.47 Sm 5.77

123 119 33.85 55 17 82 27.53 649 15.42 139 3.83 379 12.88 30.54 3.88 15.96 3.43

109 120 43.45 52 19 95 44.02 448 23.33 247 9.15 538 25.06 55.55 6.85 26.74 5.35

107 198 37.92 90 24 82 36.50 588 16.94 181 5.23 497 22.71 51.60 6.43 24.70 4.84

118 230 36.07 105 27 90 34.04 623 17.96 185 5.11 527 23.77 54.15 6.62 27.27 5.17

147 353 28.20 198 34 81 29.80 1310 26.50 192 4.27 1246 59.51 133.80 17.70 77.97 14.24

131 197 25.30 117 23 78 28.40 761 25.50 208 14.40 563 31.80 67.95 8.79 38.14 7.62

134 262 24.60 81 19 81 21.60 646 21.70 176 10.20 405 22.27 49.05 6.40 27.94 5.86

100 144 17.30 81 21 72 32.60 583 17.90 174 8.14 493 21.82 44.29 5.71 24.22 4.83

117 254 19.90 113 23 74 39.10 591 18.10 150 4.85 562 22.94 47.46 6.17 26.88 5.43

Trace elements (ppm) Eu 1.55 Gd 4.21 Tb 0.62 Dy 3.44 Ho 0.65 Er 1.70 Tm 0.25 Yb 1.72 Lu 0.30 Hf 3.55 Ta 0.51 Th 4.15 U 1.57 Ta/Yb 0.30 Hf/U 2.27 Zr/U 96 U/Ta 0.38 Th/Hf 1.17 Ba/Zr 3.54 Ba/La 24.95 La/Nb 4.40 Th/Ta 8.14

1.09 2.84 0.44 2.58 0.48 1.21 0.22 1.32 0.20 3.27 0.39 2.85 0.95 0.29 3.43 145 0.34 0.87 2.73 29.41 3.36 7.39

1.45 4.31 0.67 3.89 0.77 1.92 0.31 2.00 0.31 5.39 0.91 4.67 1.30 0.45 4.15 190 0.28 0.87 2.18 21.47 2.74 5.14

1.40 3.84 0.53 3.13 0.53 1.44 0.21 1.33 0.23 4.04 0.55 4.37 1.34 0.41 3.01 135 0.31 1.08 2.74 21.88 4.35 8.02

1.59 4.32 0.59 3.27 0.61 1.66 0.24 1.47 0.25 4.28 0.50 4.42 1.30 0.34 3.30 142 0.29 1.03 2.85 22.20 4.65 8.78

3.81 10.11 1.24 5.72 1.01 2.35 0.32 2.12 0.31

2.18 6.18 0.88 4.66 0.98 2.31 0.35 2.38 0.35

1.71 4.80 0.72 4.18 0.87 2.03 0.33 2.05 0.30

1.49 4.17 0.59 3.36 0.71 1.73 0.25 1.75 0.24

1.54 4.23 0.62 3.39 0.74 1.76 0.25 1.67 0.25

20.94

2.71 17.70 2.21

2.30 18.19 2.18

2.83 22.59 2.68

3.75 24.50 4.73

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

89

Sample

RMS8

RMS9

RMS10

MX84

MX33

MX47

MX49

MX46

MX85

MX45

Rock 99827.09V Lat. N

B-A 99823.56V 19810.31V

B-A 99823V 1989.3V

B-A 99827.19V 1989.78V

A 99826.69V 1980.96V

A 99828.66V 19813.42V

A 99831.46V 1987.76V

A 99825.91 1986.51V

A 99836.7V 1984.5V

A 99826.04V 1983.70V

A 1986.05V

(wt.%) SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

54.77 1.22 16.34 7.88 0.11 5.92 7.42 4.02 1.23 0.38 0.46 99.75

53.97 1.29 16.30 7.96 0.12 6.14 7.58 4.13 1.36 0.47 0.36 99.68

53.92 1.30 16.30 8.12 0.12 6.50 7.50 4.14 1.35 0.47 0.03 99.69

56.70 0.82 16.07 6.54 0.10 6.07 6.90 3.99 1.41 0.24 0.12 98.96

61.04 0.75 15.92 5.59 0.09 4.91 5.62 4.38 1.56 0.19 0.01 100.04

61.25 0.80 15.97 5.43 0.09 4.37 6.02 4.25 1.70 0.26 0.01 100.13

61.63 0.80 16.01 5.58 0.09 4.00 5.69 4.39 1.86 0.26 0.01 100.30

60.67 0.77 16.12 5.72 0.09 4.69 6.08 4.24 1.62 0.24 0.01 100.23

60.12 0.72 16.22 5.59 0.09 4.33 5.78 4.10 1.78 0.17 0.63 99.53

63.93 0.69 16.08 4.52 0.07 2.41 4.65 4.51 2.03 0.22 0.02 99.13

Trace elements (ppm) V 138 Cr 220 Co. 24.60 Ni 63 Cu 25 Zn 84 Rb 22.30 Sr 692 Y 23.70 Zr 173 Nb 9.73 Ba 431 La 23.93 Ce 51.99 Pr 6.86 Nd 30.04 Sm 6.34

144 221 24.80 77 25 84 24.30 679 24.80 195 12.30 491 26.89 58.26 7.78 33.64 7.20

148 250 26.10 89 27 82 23.70 685 24.70 194 12.50 494 27.30 59.25 7.85 33.38 7.34

119 262 43.90 126 35 69 34.00 545 18.00 149

101 193 48.40 106 21 63 26.00 465 17.00 137

99 167 51.50 76 21 67 51.00 661 18.00 176

105 145 56.70 67 19 71 44.00 571 18.00 174

102 200 54.90 87 22 66 35.00 573 17.00 169

115 92 43.20

20.00 633 15.00 124

79 58 70.60 18 11 62 54.00 477 16.00 185

495 21.60 50.00

389 14.90 37.00

587 28.10 61.00

528 25.10 60.00

529 24.60 58.00

398 14.20 30.00

604 24.80 51.00

24.00 4.69

18.00 3.85

31.00 5.66

27.00 5.14

28.00 4.88

14.00 3.55

25.00 4.39

2.17 5.45 0.81 4.47 0.94 2.12 0.30 2.20 0.36

2.14 5.63 0.84 4.62 0.96 2.26 0.32 2.30 0.36

1.53

1.21

1.49

1.65

1.57

1.20

1.32

0.60

0.60

0.60

0.60

0.60

0.50

0.60

1.83 0.23

1.74 0.26

1.76 0.26

1.98 0.28

1.82 0.27

1.48 0.22

1.64 0.25

0.40 3.30 1.10 0.22 3.64 135 0.33 0.83 3.32 22.92

0.50 2.70 1.10 0.29 3.09 125 0.41 0.79 2.84 26.11

1.20 4.30 1.20 0.68 4.33 147 0.28 0.83 3.34 20.89

0.60 4.00 1.70 0.30 2.59 102 0.43 0.91 3.03 21.04

0.90 3.60 1.40 0.49 3.00 121 0.39 0.86 3.13 21.50

8.25

5.40

3.58

6.67

4.00

Trace elements Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Ta/Yb Hf/U Zr/U U/Ta Th/Hf Ba/Zr Ba/La La/Nb Th/Ta

(ppm) 1.95 5.18 0.77 4.40 0.93 2.26 0.33 2.23 0.33

2.49 18.01 2.46

2.52 18.26 2.19

2.55 18.10 2.18

0.60 4.90 1.40 0.37 3.64

3.21 28.03

0.29 0.96 3.26 24.35 8.17

90

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 4. (a) Total alkalis (Na2O+K2O) vs. SiO2 diagram for volcanic rocks of NT and the TVF (after Le Maitre et al., 1989). Division between alkaline and subalkaline fields from Irvine and Baragar (1971). (b) Analyzed samples displaying a typical calc-alkaline trend in the AFM diagram, A=Na2O+K2O, B=Fe2O3T and C=MgO.

the groundmass, with compositions of En70–88, similar to that observed in orthopyroxene from Popocate´petl pumices. Amphibole is relatively abundant in NT rocks as subhedral to euhedral macrocrysts and phenocrysts, and in some samples this phase becomes resorbed by the magma. Amphiboles in dacitic NT rocks are pargasitic (nomenclature of Leake, 1978), whereas in UTP and LTP amphiboles fall within the fields of pargasitic–hornblende and edenitic–hornblende. For comparison, some amphiboles from Popocate´petl volcano show compositions that fall in the fields of pargasite and ferroan–pargasite. In the case of Popocate´petl and NT volcanoes, hornblende may be used to identify the volcanic source of regional pumice deposits. Quartz abundance is less than 1 vol.% in some andesitic and dacitic lavas from NT and the TVF. It is observed as rare phenocrysts with corona reactions of radiating augite–diopside, indicating disequilibrium. Fe–Ti oxides are present in all NT and TVF rocks as rare phenocrysts and as a groundmass

phase. In TVF samples, opaque microlites (1–6 vol.%) of titanomagnetite and ilmenite occur in the matrix as euhedral–subhedral crystals, b0.7 mm in size and randomly distributed. In NT rocks, titaniferous magnetite is present as sub- to anhedral phenocrysts (0.1–0.5 mm) and as a groundmass phase. Magnetite comprises from 1 to 2 vol.% of lavas and ilmenite is much less abundant. Sometimes, opaque oxides are present as inclusions or as reaction borders on amphibole. Glass is in most cases the most voluminous groundmass phase in NT rocks. This phase displays a dominant rhyolitic composition, regardless of bulk lava composition (Arce, 2003). In TVF samples, glass is also abundant in the matrix with an andesitic predominant composition. The LTP is a clast-supported pumice-fall deposit with 20% of metamorphic and igneous lithics derived from the local basement. Metamorphic xenoliths show diameters varying from 0.5 to 1.5 cm. Handpicking of these metamorphic fragments under a polarizing binocular microscope

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

91

Fig. 5. Variation diagrams for (a) SiO2 (wt.%), (b) Al2O3 (wt.%), (c) CaO (wt.%), (d) TiO2 (wt.%), (e) K2O (wt.%) and (f) Cr (ppm) as a function of MgO (wt.%). Samples NT24 and NT25 represent blocks from an avalanche deposit of NT (see text for discussion). Black circles represent compositions of lavas from three volcanoes oriented E–W in the TVF.

92

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 6. Trace-element diagrams for (a) NT and (b) TVF rocks. Primitive mantle normalized using the values of Sun and McDonough (1989).

allowed us to group them into two types: (1) green low-grade metaigneous schists with chlorite, epidote and plagioclase along with some phyllite fragments; (2) high-grade gneiss and some metasedimentary schists.

4.2. Major and trace element results Table 2 shows major and trace element concentrations of NT and TVF rocks analyzed in this work. Previous chemical data for these areas have been

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

obtained by Bloomfield (1974, 1975), Macı´as et al. (1997) and Verma (1999), and in some parts of this paper are cited for comparison. Chemical classification of rocks is given in the alkalis vs. SiO2 diagram in Fig. 4a (Le Maitre et al., 1989). The early products of NT are andesites (SiO2 from 58 to 62.31 wt.%), whereas the later ones are mainly dacites (SiO2 from 63 to 67 wt.%) with minor andesites (e.g. LTP). The TVF rocks have compositions ranging from basaltic andesites to andesites (53–61 wt.% SiO2) with rare dacites and basaltic trachy-andesites. All volcanic rocks in the study area belong to the subalkaline series as defined by Irvine and Baragar (1971) (Fig. 4a). They lack iron enrichment, dis-

93

playing a typical calc-alkaline trend in the AFM diagram of Fig. 4b. This is equivalent to the low- to medium-Fe suite proposed by Arculus (2003). These features are consistent with their subduction setting. Chemical data from Macı´as et al. (1997) and Verma (1999) show patterns similar to our data for rocks of NT and the western Sierra Chichinautzin. Bloomfield’s (1975) chemical data show important dispersion in values of major elements for the TVF rocks (Fig. 4a). NT rocks show a decrease in SiO2 from 67 to 57 wt.% with increasing MgO contents (1.5–4 wt.%). TVF rocks display a similar trend, but with lower SiO2 concentrations (61–54 wt.%) and higher

Fig. 7. Variation diagrams for (a) Th vs. Th/Hf, (b) U/Ta vs. Zr/U and (c) Th/Hf vs. U/Ta of the NT and TVF rocks.

94

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 8. Chondrite-normalized rare earth element data for (a) NT and (b) TVF rocks, using the values of Nakamura (1974).

MgO (4–7 wt.%) (Fig. 5a). Al2O3 contents are also characteristic in each volcanic zone. NT rocks display values from 16.2 to 20.5 wt.% with very

little variation in MgO, whereas the TVF rocks have a narrow range of Al2O3 (15.8–16.7, Fig. 5b). CaO and TiO2 concentrations in most NT and TVF

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

samples are in the range of island volcanic arcs and correlate positively with MgO (Fig. 5c,d), whereas K2O correlates negatively with MgO (Fig. 5e). Data from Macı´as et al. (1997) and Verma (1999) show similar patterns for these elements. However, TiO2

95

concentrations in some TVF samples (RMS3, RMS4, RMS8, RMS9 and RMS10; Table 2) are unusually high (1.2–1.32 wt.% TiO2) for subduction-zone magmas and more typical of intraplatetype mafic alkalic rocks. Similar concentrations

Table 3 Sr, Nd and Pb present-day isotopic compositions for NT and TVF rocks Sample

Agea

Rock type

87 86

Sr/ Sr

F1j

143

Nd/ Nd

F1j

144

qNd

206

1 S.D. (%)

207 204

1 S.D. (%)

208

204

204

1 S.D. (%)

Pb/ Pb

Pb/ Pb

Pb/ Pb

Nevado de Toluca volcano NT15 3300 dacite NT17 3300 dacite NT31 3300 dacite NT6 10,500 andes. pumice NT9 10,500 daci. pumice NT13 10,500 daci. pumice NT12 10,500 daci. Pumice NT14 12,040 daci. pumice NT8 24,000 andes. pumice NT33 24,260 andes. pumice NT4 28,000 dacite NT7 28,000 dacite NT11 37,000 dacite NT10 37,000 dacite NT22G 43,000 dacite NT22R 43,000 dacite NT24 dacite block NT25 dacite block NT29 1,500,000 andesite NT30 1,500,000 andesite NT32 dacite

0.704150 0.703868 0.703912 0.703958 0.704226 0.704205 0.704208 0.703853 0.704039 0.704211 0.703952 0.704019 0.703940 0.703965 0.703887 0.703899 0.703702 0.703776 0.703923 0.703959 0.704030

46 43 46 36 56 52 44 48 40 46 40 40 43 55 44 43 31 43 45 38 40

0.512907 0.512912 0.512887 0.512899 0.512884 0.512886 0.512876 0.512893 0.512896 0.512855 0.512870 0.512872 0.512901 0.512903 0.512888 0.512890 0.512980 0.512945 0.512862 0.512876 0.512832

17 25 16 28 23 22 21 18 19 22 24 29 20 29 17 18 21 22 38 21 24

+5.25 +5.34 +4.86 +5.09 +4.80 +4.84 +4.64 +4.97 +5.03 +4.23 +4.53 +4.56 +5.13 +5.17 +4.88 +4.92 +6.67 +5.99 +4.37 +4.64 +3.78

18.573 18.576 18.590 18.577 18.610 18.603 18.607 18.593 18.598 18.635 18.557 18.612 18.554 18.580 18.553 18.555 18.587 18.596 18.677 18.611 18.576

0.016 0.014 0.019 0.020 0.048 0.017 0.018 0.020 0.027 0.034 0.020 0.018 0.012 0.018 0.017 0.015 0.016 0.019 0.024 0.080 0.020

15.544 15.556 15.570 15.572 15.587 15.579 15.583 15.585 15.569 15.617 15.548 15.581 15.545 15.576 15.567 15.546 15.570 15.578 15.594 15.568 15.570

0.017 0.015 0.023 0.020 0.065 0.018 0.021 0.021 0.027 0.035 0.025 0.018 0.013 0.017 0.017 0.016 0.017 0.021 0.027 0.019 0.023

38.225 38.256 38.310 38.298 38.374 38.342 38.358 38.354 38.314 38.475 38.217 38.359 38.206 38.312 38.192 38.200 38.295 38.332 38.431 38.315 38.294

0.018 0.015 0.028 0.022 0.087 0.021 0.025 0.023 0.030 0.038 0.026 0.020 0.012 0.018 0.017 0.016 0.016 0.020 0.032 0.018 0.027

Tenango Volcanic Field TEO1 8500 andesite TEP1 8500 andesite RMS7 8500 andesite MX84 8500 andesite MX33 8500 andesite MX85 8500 andesite ESP1 19,530 andesite RMS8 19,530 basal. andesite RMS10 19,530 basal. andesite CUATL1 19,530 andesite SIL1 19,530 andesite STA1 30,000 andesite JAJ1 30,000 andesite RMS2 40,000 andesite

0.703728 0.704165 0.704320 0.704141 0.703994 0.703713 0.704065 0.703877 0.704040 0.704005 0.704167 0.704481 0.704028 0.704338

48 46 46 38 45 41 41 54 43 45 39 52 50 40

0.512878 0.512906 0.512948 0.512946 0.512937 0.512831 0.512901 0.512989 0.512957 0.512938 0.512954 0.512753 0.512972 0.512981

17 17 19 19 18 29 19 21 24 21 15 19 15 23

+4.68 +5.23 +6.05 +6.01 +5.83 +3.76 +5.13 +6.85 +6.22 +5.85 +6.16 +2.24 +6.52 +6.69

18.581 18.671 18.635

0.020 0.024 0.020

15.562 15.595 15.578

0.025 0.023 0.022

38.275 38.454 38.369

0.028 0.023 0.025

18.679 18.659 18.667 18.689

0.034 0.021 0.025 0.021

15.607 15.595 15.575 15.599

0.039 0.020 0.025 0.019

38.496 38.433 38.386 38.472

0.051 0.020 0.026 0.021

18.616

0.027

15.575

0.028

38.346

0.031

Isotopic composition for La Jolla Nd standard are 143Nd/144Nd=0.511880F22 (1j, n=116), for the SRM987 standard 87Sr/86Sr=0.710234F18 (1j, n=220), for NBS981 standard 206Pb/204Pb=16.89F0.04%, 207Pb/204Pb=15.42F0.06% and 208Pb/204Pb=36.50F0.09%. Values for 1j standard deviation correspond to the last two digits. a Relative age (years) proposed after data of Bloomfield (1974), Bloomfield and Valastro (1977), Cantagrel et al. (1981), Macı´as et al. (1997), Garcı´a-Palomo et al. (2002) and Arce et al. (2003). daci.=dacitic, andes.=andesitic, basal.=basaltic.

96

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

have been determined in other lava samples from the Sierra Chichinautzin volcanic field (Ma´rquez et al., 1999; Siebe et al., 2004). Samples NT24 and NT25 (black squares in Figs. 5 and 7) show a slightly different chemical composition in comparison to most NT rocks. These samples are blocks of the debris avalanche deposit DAD1 south of NT (Fig. 3). Therefore, it is very likely that the source of these blocks is not NT. On the other hand, although sample NT29 shows an andesitic composition, the presence of small xenoliths (b8 vol.%) in its matrix seems to affect its chemical patterns, shifting them away from the average trend of NT rocks (Figs. 4, 5 and 7). Samples RMS4, RMS8, RMS9 and RMS10 were obtained from lava flows produced by three E–W aligned TVF monogenetic cones with similar ages (~21 ka) (Fig. 2). Major and trace element concentrations for these samples are very similar (black

circles in Fig. 5), suggesting that the same magmatic source fed these vents. Relatively high contents of compatible elements are observed in rocks of the TVF, such as Cr 92–353 ppm, Ni 50–198 ppm and Co 17–56 ppm. These relatively high values, coupled with Mg# (MgO100/(MgO+Fe2O3)) between 34 and 53, and SiO2 compositions indicate that magmas of this region were produced by fairly primitive melts. In contrast, dacitic pumice and lava flows of NT volcano show lower concentrations of these trace elements (Cr 20–83 ppm, Ni 0–20 ppm and Co 6–11 ppm) and Mg# 28–37. The TVF rocks exhibit a positive correlation between Cr (Fig. 5f), Ni and MgO, but no similar correlation is observed in rocks of NT (Fig. 5f). These results indicate the importance of crystal fractionation processes in melts from the TVF. However, element ratios such as Th/Hf vs. Th (Fig. 7a) indicate that partial melting processes have

Table 4 Sr, Nd and Pb present-day isotopic compositions for xenoliths found in the Lower Toluca Pumice of NT volcano and for high-grade xenoliths found in the La Goleta Tertiary volcanic field by Elı´as-Herrera et al. (1998) Sample

NT35PB

NT35PV

NT35EC

NT35EM

NT35EN

NT35PN

PEP2a

PEP3a

Rock type 87 Rb/86Sr 87 Sr/86Sr F1j 147 Sm/144Nd 143 Nd/144Nd F1j qNd F1j Rb (ppm)b Sr (ppm)b Sm (ppm)b Nd (ppm)b TDM (Ga)c 206 Pb/204Pb 1 S.D. (%) 207 Pb/204Pb 1 S.D. (%) 208 Pb/204Pb 1 S.D. (%)

White phyllite 5.92 0.721098 38 0.109 0.512281 17 6.96 0.33 111.71 54.70 4.57 25.24 1.27 18.984 1.035 15.631 1.031 39.114 1.039

Green phyllite 2.94 0.716177 39 0.127 0.512351 18 5.60 0.35 115.72 113.90 3.28 15.64 1.42 19.021 0.048 15.690 0.052 39.162 0.067

Chlorite schist 0.46 0.705731 38 0.144 0.512693 18 +1.07 0.35 31.88 199.50 3.04 12.76 1.02 18.864 0.131 15.632 0.131 38.713 0.131

Gneiss 6.34 0.721887 38 0.111 0.512268 20 7.22 0.39 89.66 40.95 3.47 18.90 1.32 19.036 0.022 15.694 0.136 39.256 0.154

Schist 6.25 0.717112 37 0.109 0.512442 18 3.82 0.35 124.88 57.89 4.53 25.18 1.04 19.100 0.062 15.679 0.074 39.224 0.099

Phyllite 4.02 0.715653 32 0.114 0.512279 20 7.00 0.39 125.28 90.17 5.73 30.42 1.34 19.025 0.075 15.675 0.076 39.223 0.075

Gneiss – – – 0.130 0.512296 – 6.67 – – – 4.99 23.17 1.54 – – – – – –

Gneiss – – – 0.138 0.512263 – 7.32 – – – 4.63 20.17 1.79 – – – – – –

a

Data from Elı´as-Herrera et al. (1998). Concentrations obtained by isotope dilution. c Depleted mantle model ages calculated using 147Sm/144Sm=0.214 and 143Nd/144Nd=0.51316 (Goldstein et al., 1984). Relative uncertainties (1j) of 87Rb/86Sr=F2.0% and of 147Sm/144Nd=F2.5%. Relatively reproducibility (1j) for Rb, Sr, Sm and Nd concentrations at the laboratory are F4.5%, F1.8%, F3.2% and F2.7%, respectively. b

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

97

Fig. 9. Sr vs. qNd present-day isotopic ratios for NT and TVF rocks. Inset shows isotopic composition of metamorphic xenoliths found in the LTP and data of the Guerrero terrane from Talavera-Mendoza et al. (1995). Data of Colima volcano from Verma and Luhr (1993), data of Popocate´petl from Schaaf et al. (2002, submitted for publication), and average isotopic data of DSDP 487 and 488 Cocos plate sediments, Mexico from Verma (2000).

also played an important role in the composition of magmas from the study area. Variation diagrams for trace elements are shown in Fig. 6a,b. Trace-element patterns of NT and TVF samples are very similar, with enrichment in the large-ion lithophile elements (LILE) relative to the high-field-strength element (HFSE). This enrichment is typical of calc-alkaline volcanic arcs. All rocks show negative Nb anomalies as well as several other negative anomalies (P, Ta and Ti) that are also characteristic of subduction-related magmatism (e.g. Gill, 1981; Pearce, 1983; Walker et al., 2001). The patterns of the immobile elements (Nb, Zr, Hf, Ti, Y and Yb) on the variation diagrams and the enrichment in LILE suggest a depleted

mantle source in the subcontinental lithospheric mantle modified by subduction fluids, which have added the more mobile elements (Rb, Ba, K and maybe Pb) (Pearce, 1983; Wilson, 1989). NT rocks display little chemical variation for the major and trace elements such as Y (11–17 ppm), Hf (3.4–4.5 ppm), Zr (123–180 ppm) and Nb (3.9–5.0 ppm). In contrast, TVF rocks show a wide variation Y (15– 25 ppm), Hf (3.3–5.4 ppm), Zr (124–219 ppm) and Nb (3.8–14.4 ppm) (Table 2). These HFSE data, along with some ratios of incompatible elements such as Zr/U vs. U/Ta and U/Ta vs. Th/Hf (Fig. 7b,c), may suggest that NT products were produced by a relatively homogeneous magmatic source, whereas the volcanic rocks from the TVF were

98

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 10. Plot of 207Pb/204Pb–206Pb/204Pb for whole-rock samples of NT and the TVF. Reference lines are the two-stage terrestrial lead evolution curve (Stacey and Kramers, 1975), graduated at 250 Ma intervals (SK), and the Northern Hemisphere Reference Line (NHRL) (Hart, 1984). Pb isotopic data for the Paleozoic Acatla´n complex are from Martiny et al. (2000). Data for the Oaxaca Complex field from Solari et al. (1998). MORB-EPR data are from PETDB (2002), Pacific Ocean Sediments (POS) from Church and Tatsumoto (1975) and Hemming and McLennan (2001). Pb isotopic data of DSDP 487 and 488 Cocos plate sediments, Mexico from Verma (2000).

probably derived from a more heterogeneous source. The rare earth element (REE) abundances in the NT and TVF samples show similar trends. Chondritenormalized REE patterns display light rare earth element enrichment (LREE, La–Sm) and relatively bflatQ patterns for the heavy rare earth elements (HREE, Tb–Lu). However, NT rocks show the lowest concentrations of HREE in comparison to Popocate´petl, Sierra Chichinautzin (e.g. Siebe et al., 2004), and the TVF rocks. No Eu anomalies are observed (Fig. 8a,b) in rocks of the study area, indicating that plagioclase fractionation was not significant. This is consistent with Na2O and Al2O3 concentrations that slightly increase with SiO2 concentrations, indicating that plagioclase crystallized late and was not substantially removed during ascent to the surface. Dacites of

NT show slightly lower REE concentrations in comparison to andesites and basaltic andesites of the TVF. La/Lucn ratios range from 7 to 12 in NT dacites, from 7 to 13 in NT and TVF andesites, and from 12 to 14 in TVF basaltic andesites. In addition, NT and TVF rocks show a general decrease in incompatible elements as SiO2 increases. Verma (1999, 2000), Ma´rquez et al. (1999), and Ma´rquez and De Ignacio (2002) noticed that the suite of rocks from the Sierra Chichinautzin and other sites of the TMVB display a trend opposite of that most frequently observed at many volcanoes in continental arcs, in which abundances of incompatible trace elements (HFSE and REE) typically increase with increasing SiO2. They proposed that mixing between OIB magma types and a lower-crustal component of dacitic composition produced lavas of the Sierra Chichinautzin. However,

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Siebe et al. (2004) suggested that the trace element patterns in the Sierra Chichinautzin could be explained by polybaric assimilation and fractional crystallization (AFC; DePaolo, 1981) processes. In our case, petrographical and chemical data of NT and TVF rocks suggest that the presence of some mineral phases such as hornblende, Fe–Ti oxides, and minor clinopyroxene can explain the fractionation of these trace elements as an AFC process. 4.3. Isotopic results Isotopic analyses of Sr, Nd and Pb are given in Table 3 for volcanic samples of NT and the TVF and in Table 4 for the metamorphic xenoliths found in the LTP. Despite the contrast in the petrographic and chemical compositions between NT and TVF rocks, they show similar isotopic values. Present-day 87 Sr/86Sr ratios range from 0.70385 to 0.70423 for NT rocks and from 0.70371 to 0.70448 for TVF rocks. qNd values generally range from +3.8 to +5.3 for NT rocks and from +2.2 to +6.8 for the monogenetic field (Table 3 and Fig. 9). The narrow range of isotopic data, with generally low 87Sr/86Sr ratios and qNd values mostly above that of bulk Earth that are within the mantle array, suggest relatively low crustal contamination of the magmas. In contrast, metamorphic xenoliths representing the basement under NT show variable isotopic ratios (Table 4 and Fig. 9) and can be grouped in two lithologic types. The first type includes gneiss, mica-schist, and some phyllites with 87Sr/86Sr present-day values from 0.71565 to 0.72189 and qNd from 3.8 to 7.2. The second type, formed by green schists, is less radiogenic (87Sr/86Sr= 0.70573 and qNd=+1.1). Sr and Nd isotopic ratios, obtained by Schaaf et al. (2002, submitted for publication), for pumices and lavas from Popocate´petl show more variation than the volcanic rocks analyzed in the present study (Fig. 9), indicating a larger interaction of these magmas with the crust. Pb isotopic data of lava and pumice from the study area display a relatively narrow range, suggesting a similar source for these rocks. In Fig. 10, the ratios of volcanic rocks of NT and TVF overlap and plot below the average crust model curve of Stacey and Kramers (1975). Pb isotopic ratios for NT rocks vary as follows: 206 Pb/204Pb=18.55–18.68, 207Pb/204Pb=15.54–15.62

99

and 208Pb/204Pb=38.19–38.47, whereas for the TVF the range of isotopic ratios is 206Pb/204Pb=18.58– 18.69, 207 Pb/ 204 Pb=15.56–15.61 and 208 Pb/ 204 Pb=38.28–38.50. Volcanic rocks from the study area appear to define a steep mixing trend between a mantle component such as the MORB of the Pacific Ocean (PETDB, 2002) and a 207Pb-rich reservoir. This 207Pbrich component might correspond to the influence of fluids derived from the subduction zone in the mantle wedge or perhaps it is represented by addition of continental crust. Pb isotopic data for the metamorphic xenoliths from the basement also show two different groups (Fig. 10). Most gneiss, mica-schist and phyllite samples exhibit radiogenic values (206Pb/204Pb= 18.98–19.10, 207Pb/204Pb=15.68–15.69 and 208Pb/ 204 Pb=39.16–39.26), whereas the green schists display less radiogenic values (206Pb/204Pb=18.86, 207Pb/ 204 Pb=15.63 and 208Pb/204Pb=38.71). These isotopic data lie above and to the right of the average Pb crustal evolution curve of Stacey and Kramers (1975).

5. Discussion 5.1. Petrographic patterns Stratigraphic studies in the area are difficult because several lava flows and pyroclastic products with similar petrographic compositions were erupted and distributed over wide areas in a short period of time (between ~40 and 3 ka). Considering these complications, we used petrographic, mineralogical and geochemical data for rocks, and also 14C and K– Ar age determinations (age data of Bloomfield, 1975; Cantagrel et al., 1981; Macı´as et al., 1997; Garcı´a-Palomo et al., 2002) to understand the magmatic evolution of the NT and TVF regions. The mineral association of NT products changed from phenocrysts of plagioclase–clinopyroxene– orthopyroxene–hornblende in an andesitic glassy matrix for the events of bPaleo-NevadoQ, to dacitic pumices, lavas and domes with phenocrysts of plagioclase–hornblende–orthopyroxene. The presence of lavas with only orthopyroxene as phenocrysts is relatively uncommon in TMVB rocks. Volcanoes such as Popocate´petl, Pico de Orizaba and Iztaccı´huatl and most monogenetic volcanoes from TMVB

100

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 11. Chemical and isotopic variations over time for rocks of NT and the TVF. Measured isotopic error is indicated for each sample. Data ages are taken from Fig. 3.

show the presence of two pyroxene types (clino- and orthopyroxene) in andesitic–dacitic rocks and pumices. The distribution of most pumice and lava

erupted by NT could be evaluated by identifying the pyroxene types present in the volcanic products of central Mexico.

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Paricutı´n volcano (McBirney et al., 1987) and some of the other volcanoes in the TMVB show similar pyroxene characteristics. Activity at Paricutı´n began with the eruption of an olivine-bearing basaltic andesite (55 wt.% SiO2) and ended with a hypersthene andesite when 85% of the magma volume had been discharged (McBirney et al., 1987). These authors suggest that the absence of Ca-rich pyroxene on their liquidus could be attributed to a loss of water after differentiation by fractional crystallization and crustal contamination of the magma had taken place. The chemical compositions of Fe–Ti oxides from some UTP samples were determinated by Arce (2003) and in the present study. These chemical data were used to calculate preliminary temperatures of magma crystallization. Based on the ilmenite–titanomagnetite geothermometer of Anderson and Lindsley (1998) and the mineral reformulation model of Stormer (1983), crystallization temperatures of 815–852 8C for the magma of UTP were calculated. Only titanomagnetite crystals in equilibrium conditions with the matrix were considered for the temperature calculations.

Fig. 12. Sr isotopic variations vs. differentiation index (SiO2) for volcanic rocks of NT and the TVF. Isotopic data from Popocate´petl and Pico de Orizaba from Schaaf et al. (2002, submitted for publication).

101

Hydrothermal experiments using a UTP sample were conducted by Arce (2003). This author constructed P–T stability fields for the main mineral phases with water pressures going from 150 to 200 MPa, equivalent to depths around 4–6 km below the volcano. Most NT samples show petrographic and chemical characteristics similar to those of the dacitic rocks described by Blundy and Cashman (2001) for the Mount St. Helens volcano. Based on the compositional variations in natural and experimental glasses from this volcano, these authors proposed a model for magmatic crystallization of plagioclase, amphibole, orthopyroxene and Fe–Ti oxides at pressures between 300–400 MPa (6–8 km depth) and 11 MPa (550 m depth). Crystallization of magma at NT could follow a polybaric process over 4 km depth as was suggested by Blundy and Cashman (2001) for the Mount St. Helens volcano. Petrographic characteristics of the TVF rocks are very similar to sample descriptions reported by Ma´rquez et al. (2001), Ma´rquez and De Ignacio (2002) and Siebe et al. (2004) for the Sierra Chichinautzin rocks. The most common rocks in this region are andesites, basaltic andesites, some basalts and minor dacites with a predominant calcalkaline character. Nonetheless, some mafic rocks in the Sierra Chichinautzin bear resemblances to OIBs, as noticed by Ma´rquez et al. (1999), Wallace and Carmichael (1999) and Verma (2000). In the TVF, some lavas are characterized by the coexistence of olivine and xenocrystic quartz with evidence of disequilibrium, as mentioned above. The presence of these xenocrysts has been interpreted as some type of magma mixing (Ma´rquez et al., 2001; Ma´rquez and De Ignacio, 2002). However, as was mentioned by Blatter and Carmichael (1998) in andesites from Zita´cuaro, in the central part of the TMVB, there is no evidence in the TVF of contemporaneous silicic magmas that contain quartz phenocrysts that could explain the presence of xenocrysts in andesites. In addition, no petrographic evidences of magma mixing such as reverse zoning, dissolution structures and mottled groundmass, observed at the neighbouring Iztaccı´huatl (Nixon, 1988), were identified in NT and TVF lavas. The occurrence of partially digested sandstone and granodiorite fragments provides evidence of the assimilation of some crustal material in the TVF.

102

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

5.2. Geochemical patterns The most evident difference between NT and TVF rocks is the degree of differentiation. Volcanic rocks at NT are more silicic than in the TVF (57–67 and 54–61 wt.% SiO2, respectively). In addition, geochemical patterns of some trace elements (Fig. 7) and isotopic data (Table 3) suggest that volcanic rocks of the two regions were produced by magmas derived from similar sources. Nonetheless, Sr and Nd isotopic data are relatively more variable in rocks of the TVF than at NT, suggesting a slightly different magmatic source or crustal contamination. Fig. 11 displays SiO2, V and Y concentrations, and Sr and Nd isotopic variations over time for NT and the TVF. The time scale was constructed according to the stratigraphic column of Fig. 3. The most noticeable aspects of this figure are the sharp decreases of SiO2 and the corresponding increases of V, Y, MgO and Fe2O3t (data not shown for the oxides) for rocks of NT during at least four periods of activity. The first one occurs at 2.6 Ma (andesites and dacites were reported by Bellotti et al., 2003, but no chemical data are available), the second one at 1.5 Ma, the third one at 25 ka and the last one at 10.5 ka. These sharp chemical variations could be related to the input of new less silicic magma into the NT magma reservoir. TVF rocks only show a slight increase in SiO2 and corresponding decreases in the other elements with time (e.g. V and Y). Sr isotopic values for most rocks of NT are homogeneous; although two peaks are observed at 25 and 10.5 ka. These variations could represent mafic replenishments of the magmatic system that also led to greater crustal interactions. TVF rocks display a scattered distribution of Sr isotopes with time, indicating different amounts of magma interaction with the crust during each magmatic event. Nd isotopic values for rocks of NT seem to be less sensitive to variations over time, because all rocks show values between 0.51283 and 0.51291. TVF rocks show a slight decrease in 143Nd/144Nd over time, interpreted as progressively stronger interaction of magmas with the crust. A diagram of Sr isotope ratios vs. SiO2 differentiation index (Fig. 12) is used to characterize AFC processes. For NT rocks SiO2 concentrations are nearly similar (57–64 wt.%) with a range of Sr isotopic values from 0.70385 to 0.704230, suggesting

a slight positive correlation for them. The correlation of the data confirms the existence of assimilation and crystal fractionation processes in NT lavas. TVF rocks do not show any correlation between SiO2 wt.% and Sr isotopic compositions. A rather scattered distribution of the Sr isotopic values is observed for rocks of the monogenetic field (Fig. 12). However, assimilation and crystal fractionation can also be inferred for these rocks. The nature of continental crust in the study area is not known because a thick volcanic pile of the TMVB covers the older rocks. However, it is assumed that rocks from the Upper Cretaceous Guerrero terrane (Campa and Coney, 1983) underlie the region. This terrane is considered a major tectonic accretion of an Upper Jurassic–Lower Cretaceous intraoceanic volcanic arc complex, essentially devoid of an old sialic basement, onto the continental framework of Mexico. Geochemical and isotopic data for typical sedimentary and volcanic sequences from this terrane suggest that these rocks were formed in a complex fossil island arc-trench system similar to the present-day western Pacific island arc system (Mendoza and Suastegui, 2000). Present-day Sr and Nd isotopic values range from 0.70403 to 0.70473 and 0.51267 to 0.51282, respectively, for volcanic rocks of this Mesozoic terrane (Talavera-Mendoza et al., 1995). Nevertheless, some geological and geochemical data (Elı´as-Herrera et al., 1998; Elı´as-Herrera and Ortega-Gutie´rrez, 2000) suggest involvement of an old continental crust in the Pre-Cretaceous magmatic evolution of the Guerrero terrane. Gneiss xenoliths found by these authors in a Tertiary volcanic field (La Goleta), 50 km SW of the study area are characterized by an orogenic low-pressure granulite facies metamorphism. These xenoliths have present-day 143Nd/144Nd values of 0.51230 and 0.51226 (qNd= 6.7 and 7.3) and Nd model ages (TDM) of 1.54 and 1.79 Ga (Elı´as-Herrera et al., 1998). These data are similar to values of Grenvillian rocks in Mexico; therefore, it is very likely that in the vicinity of the study area an old continental crust covered by the Mesozoic Guerrero terrane could exist. The petrographic characteristics of gneiss and some schist xenoliths found in the LTP are similar to those described by Elı´as-Herrera et al. (1998). However, green schist xenoliths show characteristics similar to the metamorphic volcanic andesitic sequen-

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

ces described by Talavera-Mendoza et al. (1995) for the Guerrero terrane. This type of xenolith is relatively abundant in the LTP. Sr and Nd isotopic data obtained in the present work for representative xenoliths are listed in Table 4 and displayed in Fig. 9. Isotopic data are relatively variable but are mainly similar to values obtained by Elı´as-Herrera et al. (1998) for their granulitic xenoliths. Green schist xenoliths (sample NT35EC, Table 4) display values similar to those of the volcanic rocks of the Guerrero terrane (TalaveraMendoza et al., 1995). Therefore, it is very probable that the metamorphic xenoliths present in the Lower Toluca Pumice were scavenged from different crustal depths. The high Rb/Sr ratios of the gneiss, phyllites and some schists might indicate a highly evolved sedimentary crust. In addition, present-day qNd values (between 3.8 and 7.2) and Nd model ages (TDM between 1.04 and 1.42 Ga) for most of the analyzed xenoliths also show typical values of Grenvillian rocks in Mexico, supporting the existence of a recycled older continental crust. Therefore, isotopic characteristics of xenoliths from the NT deposits appear to confirm the presence of a Pre-Mesozoic sialic basement under the southern border of the TMVB. Considering the possible effect of crustal contamination on the magmas of the study area along with the geochemical and isotopic data of the continental crust, represented by the metamorphic xenoliths analyzed here, it seems plausible that any degree of assimilation would have resulted in a greater dispersion of the isotopic and trace element data in volcanic rocks. However, Sr, Nd and Pb isotopic compositions of the volcanic rocks of the Toluca area plot in a restricted range (Figs. 9 and 10), excluding an important role for crustal contamination in the generation of the chemical and isotopic compositions observed in the volcanic rocks. Therefore, fractional crystallization could represent the most important process of magma differentiation in the Toluca volcanic rocks. Coherent linear trends with little scattering of samples in the variation diagrams of major and trace elements vs. SiO2 or MgO could, thus, be explained by fractional crystallization. This does not preclude that continental crust could slightly influence the compositions of the volcanic rocks. Volcanic rocks from other volcanoes of the centraleastern TMVB, such as Pico de Orizaba and

103

Popocate´petl, show more variable isotopic compositions (e.g. qNd ranges from 2 to +2 and from +3 to +5, respectively, Schaaf et al., submitted for publication). These isotopic data suggest a more significant interaction of magmas with the continental crust than observed for NT and the TVF. The thickness (50 km average) of the crust under the central-eastern part of the TMVB is similar to that suspected beneath NT and the TVF (Urrutia-Fucugauchi and Flores-Ruiz, 1996). However, the weak interaction of magmas with continental crust beneath the study area could be explained by the presence of important fault systems (Garcı´a-Palomo et al., 2000) that facilitate magma ascent through the crust. Three complex fault systems, of different ages, orientations and kinematics, intersect at NT volcano (Fig. 2). These authors propose that at least three main deformation events affected central Mexico since the late Miocene. During the early Miocene, an extensional phase with a Basin and Range deformation style occurred in northern Mexico and produced NW–SE and NNW–SSE horsts and grabens south of NT. In the middle Miocene, a transcurrent event generated NE–SW faults with two movements: (a) left-lateral strike-slip displacement and (b) normal faulting. The latest deformation event started during the late Pliocene and involved oblique extension accommodated by E–W right-lateral faulting that changed to normal faults. It is clear that during the last tectonic event, the majority of the volcanic eruptions in the study area and also in other areas of the central TMVB were produced, indicating a narrow relationship between E–W extensional faulting and magma ascent. Recent laboratory experiments (Hall and Kincaid, 2001) have indicated that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid (b30 ka). 5.3. Relationship between tectonics and magmatism (magma source) The petrologic origin and evolution of the TMVB has been intensely debated recently. Two compositionally contrasting suites of rocks have been recognized in this volcanic region on the basis of geochemical data: (1) calc-alkaline rocks with high LILE/HFSE ratios that have been associated with a

104

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Fig. 13. Sr/Y vs. Y discrimination diagram between adakites and typical arc calc-alkaline compositions (after Drummond and Defant, 1990).

subduction environment; and (2) alkaline or transitional rocks with low LILE/HFSE ratios and compositions similar to OIB. The simultaneous existence of these two suites of rocks in the volcanic arc has been explained by means of two main divergent hypotheses. Some authors (e.g. Wallace and Carmichael, 1999; Luhr, 1997; and others) considered that the predominantly calc-alkaline character of rocks and the production of the OIB magma-types could be explained by the generation of magmas in a subduction setting along the Middle America Trench. However, Verma (1999, 2000), Sheth et al. (2000), Ma´rquez et al. (1999) and Ma´rquez and De Ignacio (2002) regard the rocks of the central part of the TMVB as produced by magma mixing between the OIBs and a lower-crustal component of dacitic composition. They deny a direct relationship between subduction processes and the genesis of the TMVB. Because these two divergent points of view have been widely discussed in several recent works such as Verma (2000), Ferrari et al. (2001), Ma´rquez and De Ignacio (2002) and Siebe et al. (2004), it is not our

aim to extend here its discussion. On the basis of our petrographic, geochemical and isotopic data of NT and the TVF, along with the present-day geodynamic facts between the North America and Cocos plates (summarized by Siebe et al., 2004), we propose that magma generation in the study area is related to a subduction environment. Geochemical and isotopic data for NT and TVF rocks, particularly LILE/HFSE ratios, suggest that these rocks were produced in a typical subduction environment where a depleted-mantle source (MORB-type) was modified by different proportions of fluids or melts from the subducted lithosphere. Rocks of NT and the TVF show narrow ranges in Pb isotopic values, suggesting similar sources. Most rocks of NT and some from the TVF fall in the DM (depleted mantle) field (206Pb/204Pb from 18.58 to 18.69 and 207Pb/204Pb from 15.54 to 15.61), represented by the EPR-MORB (Fig. 10), evoking partial fusion of the oceanic slab. Pb isotopic data from NT, the TVF, and some rocks from the Sierra Chichinautzin (Verma, 1999) define a steep mixing line with a

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

narrow range (Fig. 10), where the possible end members could be the MORB-EPR and the fluids of the Pacific Ocean Sediment type (Church and Tatsumoto, 1975; Plank and Langmuir, 1998; Hemming and McLennan, 2001; Verma, 2000). Indeed, the positive anomalies of Ba and Pb and also the high values of some element ratios (e.g. Ba/Zr and K/Nb) shown by the volcanic rocks (NT and the TVF) confirm that fluids and/or melts from the subducted slab contributed to magma genesis. Melting of subducted hydrous basaltic crust (MORB) is argued to produce magmas with a distinctive chemical signature known as adakites (Kay, 1978; Defant and Drummond, 1990; Martin, 1999; Garrison and Davidson, 2003). The major and trace element characteristics used to classify rocks as adakites are SiO2 contents between 63 and 70 wt.%, high Sr concentrations (N400 ppm) coupled with low Y (b19 ppm) and Sr/Y ratios N20. Typical adakites are phenocryst-bearing volcanic rocks with compositions of hornblende andesite to dacite, and basalts are systematically lacking. Adakites commonly show a low HREE content that is interpreted as reflecting the presence of garnetFhornblende in the residual source after partial melting. Most rocks of NT and two

105

samples (TEO1 and MX85, Fig. 2) from the westernmost part of the TVF display relatively high Sr and low Y concentrations (460–694 and 14–21 ppm, respectively) (Fig. 13). These rocks also show other characteristics such as relatively low HREE contents and Pb isotopic values similar to MORB-EPR, suggesting a possible adakitic signature for them. Typical models for the generation of adakites require the subduction of a young (b5 Ma) and warm oceanic lithosphere, where temperatures in the slab rise above the solidus of wet basalts at high pressures, producing melts (Defant and Drummond, 1990). However, some authors such as Gutscher et al. (2000) have suggested that most of the known Pliocene–Quaternary adakites are paradoxically related to subduction of N10 Ma lithosphere. They proposed an unusual mode of subduction known as flat subduction, occurring in ~10% of the world’s convergent margins, that can produce the temperature and pressure conditions necessary for the fusion of moderately old oceanic crust (e.g. Chile, Ecuador and Costa Rica subduction regions). In central Mexico, it was generally believed that the Cocos plate was subducting at a constant dip angle N258. However, seismic data show that the subducted Cocos slab is subhorizontal beneath south-

Fig. 14. Ba/Zr vs. 87Sr/86Sr diagram showing the results of a mixing model between EPR-DM (White et al., 1987) and bulk Pacific sediment (Hemming and McLennan, 2001).

106

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

central Mexico (Sua´rez et al., 1990; Singh and Pardo, 1993; Pardo and Sua´rez, 1995). A similar geometry has been observed in regions of central Peru and Chile where it was designated as a bflat-slabQ. Pardo and Sua´rez (1995) inferred that the 80–100 km depth contours of the subducted slab lie beneath the volcanic front of the TMVB in central Mexico. However, the position of the subducted slab can not be confirmed due to the paucity of earthquake hypocenters along the volcanic front. In southwestern Japan, Morris (1995) described the existence of adakitic magma in at least two Quaternary volcanoes, Daisen and Sambe, that are associated with the volcanic arc. In this area, no intermediateand deep-focus earthquakes have been identified, similar to the central part of the TMVB. However, this author proposes that melting of the Philippine Sea plate beneath southwestern Japan can explain the presence of the adakitic magmas. In addition, he argued that slab melting can account for the aseismicity in the area, because the plate behaves as a plastic body rather than a solid. In central Mexico the behavior of the subducted plate could be similar. Although adakitic signatures can be identified in rocks of NT and in some rocks from the TVF, suggesting a melting process of the subducted oceanic crust, some authors (Garrison and Davidson, 2003) proposed that similar geochemical patterns can be obtained by melting another basaltic source such as the lower continental crust. Distinction between these two sources of magma is nontrivial, and requires integrated investigations of regional geochemistry along with tectonic and geophysical data. On the basis of geochemical and isotopic data, we propose that melts of the subducted oceanic crust contributed to the magmas erupted at NT and the western part of the TVF. Melting of the lower continental crust under NT could produce magmas with higher isotopic variations, similar to values observed in metamorphic xenoliths analyzed here. However, the geochemical data attest to minor interaction of magmas with the continental crust, but interaction of adakitic magma with the mantle during its passage to the surface could produce fractionation of some elements (Y and Sr/Y ratios). This is the first time that Quaternary adakitic magmas have been identified in the central part of the TMVB. Recently, adakitic signatures have also

been determined on the eastern section of the TMVB but in Miocene volcanic rocks (Gomez-Tuena et al., 2003). The Valle de Bravo Volcanic field (VBVF) is also located in the central part of the arc front of the TMVB. It is bracketed by NT to the east and the Zita´cuaro Volcanic Complex to the west. Lava ages from this volcanic field range from 300 to b10 ka (Blatter et al., 2001) and the main rock compositions are andesite and dacite. In addition we have noticed an adakitic signature for some of the samples of the VBVF based on rock compositions and Sr and Y concentrations reported by Blatter and Carmichael (2001). In fact, this adakitic composition was also suggested by Aguirre-Diaz et al. (2003) for rocks from the same region. Therefore, it is very likely that the presence of adakites at NT is not a local phenomenon but a regional characteristic of the volcanic arc in central Mexico. However, more detailed geochemical and isotopic studies are necessary in the VBVF to further evaluate this hypothesis. On the basis of some trace element ratios and isotopic data of the studied rocks indicating participation of slab components and mantle melts, a twocomponent mixing model is proposed to explain the genesis of NT and TVF rocks. This model is characterized by the presence of a depleted mantle (DM) source and a subduction component with different fluid/melt ratios. The mixing calculation assumes that the subducted component and the DM, represented by the composition of a MORB-like composition (White et al., 1987), are the two end members of the model. The results of modeling isotopic values and trace element ratios are displayed in Fig. 14. The volcanic rocks plot above the mixing line between the DM source and the subduction component. With variable fluid/melt proportions the mixing line shifts upward. The amount of subduction component involved in the genesis of NT volcanic rocks is estimated to be around 15–20%.

6. Concluding remarks Understanding magma genesis and petrological processes in the TMVB might be a relatively difficult task because neighboring volcanic centers, such as NT and the TVF, present particular geo-

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

chemical and isotopic patterns that can not be generalized for the whole volcanic province. Detailed geochemical and isotopic studies allowed us to determine two slightly different magmatic sources for the two areas. Most rocks of NT and some from the TVF can be related to an adakite magma source that was slightly modified by its passage into the mantle wedge. It is feasible that Quaternary adakitic magmas in the central part of the TMVB are a common phenomenon associated with melting of subducting slab. On the other hand, most TVF magmas show typical calc-alkaline patterns, where the mantle wedge was modified by different fluid/melt proportions derived from the subducted slab. Melting of the Cocos plate beneath south-central Mexico could explain the distinctive chemistry of NT volcano and the aseismic nature of this region. Although a thick continental crust (~50 km) has been inferred by geophysical data in the study area, there is no strong evidence for partial melting of the lower continental crust having produced the magmas. Isotopic compositions (Sr, Nd and Pb) suggest a MORB-slab melting source for the rocks. In addition, metamorphic xenoliths in the area suggest the presence of an older continental crust (Nd model age N1.0 Ga). Mechanisms of melt generation at subduction zones and transfer to the surface still remain uncertain. However, laboratory experiments have indicated that rapid ascent (b30 ka) of magmas is possible. It is important to remember that during the Late Pleistocene (~40 ka) a distensive tectonic event produced the E–W normal fault system in the study area. Most volcanic eruptions of NT and the TVF were produced during the last 40 ka and probably were controlled by this last fault system. Therefore, this last tectonic event favored the rapid ascent of magmas to the surface and it can explain the low crustal contamination observed and the presence of typical mineral associations indicating the ascent and crystallization of magmas following polybaric processes.

Acknowledgements Financial support by the National Council of Science and Technology (CONACYT) (project

107

32330-T) in Mexico is gratefully acknowledged. The authors wish to thank Jose´ Luis Arce for assistance in the field, and Giovanni Sosa and Benjamin Domı´nguez for assistance in the field, mechanical preparation of rock samples and in the analytical aspects of the isotopic determinations. We are also grateful to Barbara Martiny for revision and comments on the English. Editorial handling by Margaret Mangan and reviews by Jim Luhr and Alvaro Ma´rquez were very helpful and are greatly appreciated.

References Aguirre-Diaz, G., Jaimes Viera, M.C., Nieto Obrego´n, J., Lozano Santacruz, R., 2003. El campo volca´nico monogene´tico de Valle de Bravo, Edo. de Me´xico. Geologı´a y Geoquı´mica. Geos 23, 202. Puerto Vallarta, Me´xico. Anderson, D.J., Lindsley, D.H., 1998. Internally consistent solution models for Fe–Mg–Mn–Ti oxides: Fe–Ti oxides. Am. Mineral. 73, 714 – 726. Arce, J.L., 2003. Condiciones pre-eruptivas y evolucio´n de la erupcio´n Pliniana Po´mez Toluca Superior, Volca´n Nevado de Toluca. PhD thesis, Posgrado en Ciencias de la Tierra, UNAM. Mexico, 136 pp. Arce, J.L., Macı´as, J.L., Va´zquez-Selem, L., 2003. The 10.5 Ka Plinian eruption of Nevado de Toluca volcano, Mexico: stratigraphy and hazard implications. Geol. Soc. Amer. Bull. 115, 230 – 248. Arculus, R.J., 2003. Use and abuse of the terms calcalkaline and calcalkalic. J. Petrol. 44, 929 – 935. Bellotti, F., Capra, L., Casartelli, M., D’Antonio, M., De Beni, E., Gigliuto, A., Groppelli, G., Lunghi, R., Merlini, A. y Norini, G. 2003. Geologı´a y geoquı´mica de los edificios volca´nicos pre-Nevado de Toluca (2.6–1.3): Resultados Preliminares. XIII Congreso de Geoquı´mica, INAGEQ, CD abstr. Besch, Th., Grauert, B., Tobschall, H.J., Negendank, J.F.W., 1987. Evidence of assimilation during the genesis of subduction related volcanics in the eastern Trans Mexican Volcanic Belt. Terra Cogn. 7, 274 – 275. Blatter, D.L., Carmichael, I.S.E., 1998. Plagioclase-free andesites from Zita´cuaro (Michoaca´n), Mexico: petrology and experimental constraints. Contrib. Mineral. Petrol. 132, 121 – 138. Blatter, D.L., Carmichael, I.S.E., 2001. Hydrous phase equilibria of a Mexican high-silica andesite: a candidate for a mantle origin? Geochim. Cosmochim. Acta 65, 4043 – 4065. Blatter, D.L., Carmichael, I.S.E., Deino, A.L., Renne, P., 2001. Neogene volcanism at the front of the central Mexican Volcanic Belt: basaltic andesites to dacites, with contemporaneous shoshonites and high-TiO2 lavas. Geol. Soc. Am. Bull. 113, 10. Bloomfield, K., 1974. The age and significance of the Tenango Basalt Central Mexico. Bull. Volcanol. 37, 585 – 595. Bloomfield, K., 1975. A late-Quaternary monogenetic volcano field in central Mexico. Geol. Rundsch. 64, 476 – 497.

108

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

Bloomfield, K., Valastro, S., 1974. Late Pleistocene eruptive history of Nevado de Toluca volcano, central Mexico. Bull. Volcanol. 85, 901 – 906. Bloomfield, K., Valastro, S., 1977. Late quaternary tephrochronology of Nevado de Toluca volcano, central Mexico. Overs. Geol. Miner. Resur. 46, 1 – 15. Bloomfield, K., Sa´nchez-Rubio, G., Wilson, L., 1977. Plinian eruptions of Nevado de Toluca volcano Mexico. Geol. Rundsch. 66, 120 – 146. Blundy, J.D., Cashman, K.V., 2001. Ascent-driven crystallisation of dacite magmas at Mount St. Helens, 1980–1986. Contrib. Mineral. Petrol. 140, 631 – 650. Caballero, M., Macı´as, J.L., Urrutia-Fucugauchi, J., LozanoGarcı´a, S., Castan˜eda, R., 2001. Volcanic stratigraphy and palaeolimnology of the Upper Lerma Basin during the late Pleistocene and Holocene: sedimentology special volume. Lacustrine Volcaniclastic-Sedimentation. 30, 57 – 71. Campa, M.F., Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resource distribution in Mexico. Can. J. Earth Sci. 20, 1040 – 1051. Cantagrel, J.M., Robin, C., Vincent, P., 1981. Les grandes e´tapes d’e´volution d’un volcan ande´sitique composite: example du Nevado de Toluca. Bull. Volcanol. 44, 177 – 188. Capra, L., Macı´as, J.L., 2000. Pleistocene cohesive debris flow at Nevado de Toluca volcano central Mexico. J. Volcanol. Geotherm. Res. 102, 149 – 168. Cebull, S.E., Shurbet, D.H., 1987. Mexican volcanic belt: an intraplate transform. Geofı´s. Int. 26, 1 – 13. Cervantes, K.E., Arce, J.L., Macı´as, J.L., Mora, J.C., 2004. The 12.1 ka Middle Toluca Pumice: a dacitic small Plinian eruption of Nevado de Toluca in central Mexico. J. Volcanol. Geotherm. Res. under revision. Church, S.E., Tatsumoto, M., 1975. Lead isotope relations in oceanic ridge basalts from the Juan de Fuca-Gorda Ridge area N.E. Pacific Ocean. Contrib. Mineral. Petrol. 53, 253 – 279. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662 – 665. Demant, A., 1978. Caracterı´sticas del Eje Neovolca´nico Transmexicano y sus problemas de interpretacio´n. Rev.-Inst. Geol., UNAM 2, 172 – 187. Demant, A., 1981. L’axe ne´ovolcanique transmexicain: etude volcanologique et pe´trographique, signification ge´odynamique. PhD thesis, Univ. De Droit, d’Economie et de Sciences d’AixMarseille, France. 259 DePaolo, D.J., 1981. Trace element and isotopic effects of combined wall-rock assimilation and fractional crystallization. Earth Planet. Sci. Lett. 53, 189 – 202. Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite–tonalite–dacite genesis and crustal growth via slab melting: Archean to modern comparisons. J. Geophys. Res. 95, 21503 – 21521. Elı´as-Herrera, M., Ortega-Gutie´rrez, F., 2000. Petrology of highgrade metapelitic xenoliths in an Oligocene rhyodacite plugPrecambrian crust beneath the southern Guerrero Terrain Mexico? Rev. Mex. Cienc. Geol. 14-1, 101 – 109.

Elı´as-Herrera, M., Ortega-Gutie´rrez, F., Lozano-Santa Cruz, R., 1998. Evidence for pre-Mesozoic sialic crust in the southern Guerrero Terrane: geochemistry of the Pepechuca high-grade gneiss xenoliths. Actas INAGEQ-Mexico 4, 169 – 181. Ferrari, L., Gardun˜o, V.H., Pasquare´, G., Tibaldi, A., 1994. Volcanic and tectonic evolution of central Mexico: Oligocene to present. Geofı´s. Int. 33, 91 – 105. Ferrari, L., Lo´pez-Martı´nez, M., Aguirre-Dı´az, G., Carrasco-Nu´n˜ez, G., 1999. Space–time patterns of Cenozoic arc volcanism in Central Mexico: from the Sierra Madre Occidental to the Mexican volcanic belt. Geology 27-4, 303 – 306. Ferrari, L., Petrone, C., Francalanci, L., 2001. Generation of oceanic-island basalt-type volcanism in the western Trans-Mexican Volcanic Belt by slab rollback, asthenosphere infiltration, and variable flux melting. Geology 20, 507 – 510. Garcı´a-Palomo, A., Macı´as, J.L., Gardun˜o, V.H., 2000. Miocene to Recent structural evolution of the Nevado de Toluca volcano region central Mexico. Tectonophysics 318, 281 – 302. Garcı´a-Palomo, A., Macı´as, J.L., Arce, J.L., Capra, L., Gardun˜o, V.H., Espindola, J.M., 2002. Geology of Nevado de Toluca Volcano and surrounding areas, central Mexico. Geol. Soc. Am. Map and Chart Series MCH089, pp. 1 – 48. Garrison, J.M., Davidson, J.P., 2003. Dubious case for slab melting in the northern volcanic zone of the Andes. Geology 31, 565 – 568. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin. 390 pp. Goldstein, S.L., O’Nions, R.K., Hamilton, P.J., 1984. A Sm–Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth Planet. Sci. Lett. 70, 221 – 236. Go´mez-Tuena, A., LaGatta, A., Langmuir, C., Goldstein, S., Ortega-Gutie´rrez, F., Carrasco-Nun˜ez, G., 2003. Temporal control of subduction magmatism in the eastern Trans-Mexican Volcanic Belt: mantle sources, slab contributions and crustal contamination. Geochem. Geophys. Geosyst. 4, doi:10.1029/ 2003GC000524. Gutscher, M.A., Maury, R., Eissen, J.P., Bourdon, E., 2000. Can slab melting be caused by flat subduction? Geology 28, 535 – 538. Hall, P.S., Kincaid, C., 2001. Diapiric flow at subduction zones: a recipe for rapid transport. Science 292, 2472 – 2475. Hart, S.R., 1984. A large-scale isotope anomaly in the Southern Hemisphere mantle. Nature 309, 753 – 757. Hemming, S.R., McLennan, S.M., 2001. Pb isotope compositions of modern deep sea turbidites. Earth Planet. Sci. Lett. 184, 489 – 503. Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci. 8, 523 – 548. Johnson, C.A., Harrison, C.G.A., 1990. Neotectonics in central Mexico. Phys. Earth Planet. Inter. 64, 187 – 210. Kay, R.W., 1978. Aleutian magnesian andesites: melts from subducted Pacific Ocean crust. J. Volcanol. Geotherm. Res. 4, 117 – 132. Leake, B.E., 1978. Nomenclature of amphiboles. Am. Mineral. 63, 1023 – 1052.

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyre Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A.R., Zanettin, B., 1989. A Classification of Igneous Rocks and Glossary of Terms. Blackwell, Oxford, 193 pp. Luhr, J., 1997. Extensional tectonics and the diverse primitive volcanic rocks in the western Mexican volcanic belt. Can. Miner 35, 473 – 500. Macı´as, J.L., Garcı´a-Palomo, A., Arce, J.L., Siebe, C., Espı´ndola, J.M., Komorowski, J.C., Scott, K.M., 1997. Late Pleistocene– Holocene cataclysmic eruptions at Nevado de Toluca and Jocotitla´n volcanoes, central Mexico. In: Kowallis, B.J. (Ed.), Proterozoic to Recent stratigraphy, tectonic and volcanology, Utah, Nevada, Southern Idaho and Central Mexico. Buffalo Y U Geology Studies, pp. 493 – 528. Ma´rquez, A., De Ignacio, C., 2002. Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin Central Mexican volcanic belt. Lithos 62, 3562. Ma´rquez, A., Oyarzum, R., Doblas, M., Verma, S.P., 1999. Alkalic (ocean island type) and calc-alkalic volcanism in the Mexican volcanic belt: a case for plume-related magmatism and propagating rifting at an active margin? Geology 27, 51 – 54. Ma´rquez, A., Oyarzun, R., De Ignacio, C., Doblas, M., 2001. Southward migration of volcanic activity in the central Mexican volcanic belt: asymmetric extension within a two-layer crustal stretching model. J. Volcanol. Geotherm. Res. 112, 175 – 187. Martin, H., 1999. Adakitic magmas: modern analogues of Archaean granitoids. Lithos 46, 411 – 429. Martin del Pozzo, A.L., 1989. Geoquı´mica y paleomagnetismo de la Sierra Chichinautzin. PhD thesis, UNAM, Mexico, 148 pp. Martiny, B., Martı´nez-Serrano, R.G., Mora´ n-Zenteno, D.J., Macı´as-Romo, C., Ayuso, R.A., 2000. Stratigraphy, geochemistry and tectonic significance of the Oligocene magmatic rocks of western Oaxaca Southern Mexico. Tectonophysics 318, 71 – 98. McBirney, A.R., Taylor, H.P., Armstrong, R.L., 1987. Paricutin reexamined: a classic example of crustal assimilation in calcalkaline magma. Contrib. Mineral. Petrol. 95, 4 – 20. Mendoza, O.T., Suastegui, M.G., 2000. Geochemistry and isotopic composition of the Guerrero terrane (western Mexico): implications for the tectono-magmatic evolution of southwestern North America during the late Mesozoic. J. South Am. Earth Sci. 13, 297 – 324. Mooser, F., 1972. The Mexican volcanic belt: structure and tectonics. Geofı´s. Int. 12-2, 55 – 70. Mooser, F., Maldonado-Koerdell, M., 1961. Mexican national report on volcanology. An. Inst. Geofı´s., UNAM VII, 46 – 53. Morris, P.A., 1995. Slab melting as an explanation of quaternary volcanism and aseismicity in southwest Japan. Geology 23, 395 – 398. Nakamura, N., 1974. Determinations of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta. 38, 757 – 775. Negendank, J.F.W., Emmermann, R., Krawczyk, R., Mooser, F., Tobschall, H., Werle, D., 1985. Geological and geochemical

109

investigations on the eastern Trans-Mexican Volcanic Belt. Geofı´s. Int. 24-4, 477 – 575. Nixon, G.T., 1982. The relationship between quaternary volcanism in central Mexico and the seismicity and structure of subducted ocean lithosphere. Geol. Soc. Am. Bull. 93, 514 – 523. Nixon, G.T., 1988. Petrology of the younger andesites and dacites of Iztaccı´huatl volcano: 2. Chemical stratigraphy magma mixing, and the composition of basaltic magma influx. J. Petrol. 29, 265 – 303. Nixon, G.T., Demant, A., Armstrong, R.L., Harakal, J.E., 1987. K– Ar and geologic data bearing on the age and evolution of the Trans-Mexican Volcanic Belt. Geofı´s. Int. 26-1, 109 – 158. Pal, S., Lo´pez, M.M., Pe´rez, J.R., Terrel, D.J., 1978. Magma characterization of the Mexican volcanic belt (Mexico). Bull. Volcanol. 41, 379 – 389. Pardo, M., Sua´rez, G., 1995. Shape of the subducted Rivera and Cocos plates in southern Mexico: seismic and tectonic implications. J. Geophys. Res. 100, 12357 – 12373. Pasquare´, G., Gardun˜o, V.H., Tibaldi, A., Ferrari, M., 1988. Stress pattern evolution in the central sector of the Mexican volcanic belt. Tectonophysics 146, 353 – 364. Pearce, J.A., 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. In: Hawkesworth, C.J., Norry, M.J. (Eds.), Continental Basalts and Mantle Xenoliths. Shiva, Nantwich, pp. 230 – 249. PETDB Database 2002. Geochemical database of the Ocean Floor. http://petdb.Ideo.columbia.edu. Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediments and its consequences for the crust and mantle. Chem. Geol. 145, 325 – 394. Robin, C., 1976. Pre´sence simultane´e de magmatismes et significations te´ctoniques oppose´es dans l’est du Mexique. Bull. Soc. Ge´ol. Fr. 7 (XVII), 1637 – 1645. Robin, C., 1982. Relations volcanologie–magmatologie–ge´odynamiques: applications au passage entre volcanismes alcalins et ande´sitiques dans le sud Mexicain (Axe Trans-mexicain et Province alcaline Orientale). Ann. Sci. Univ. Clermont, Biol. Anim. 30, 503 pp. SARM. 2003. Service d’analyse de roches et mine´raux du CNRS. http:www.crpg.cnrs-nancy.fr/SARM/index.html. Schaaf, P., Martı´nez-Serrano, R., Solı´s-Pichardo, G., Herna´ndezTrevin˜o, T., Morales-Contreras, J.J., Herna´ndez-Bernal, M.S., Valdez-Moreno, G., Castro-Govea, R., Siebe, C., CarrascoNu´n˜ez, G., 2002. Reconocimiento de la corteza continental bajo los grandes estratovolcanes de la Faja Neovolca´nica Mexicana: evidencias isoto´picas. 3 Reunio´n Nacional de Ciencias de la Tierra, Geos vol. 22, pp. 152. Schaaf, P., Stimac, J., Siebe, C., Macı´as, J.L., 2003. Magmatic processes at Popocate´petl volcano, Mexico: petrology, geochemistry and Sr–Nd–Pb isotopes. Geol. Soc. Am. Bull. submitted for publication, 20 pp. Sheth, H.C., Torres-Alvarado, I.S., Verma, S.P., 2000. Beyond subduction and plumes: a unified tectonic–petrogenetic model for the Mexican volcanic belt. Int. Geol. Rev. 42, 1116 – 1132. Siebe, C., Schaaf, P., Urrutia-Fucugauchi, J., 1999. Mammoth bones embedded in a late Pleistocene lahar from Popocate´petl volcano,

110

R.G. Martı´nez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110

near Tocuila, central Mexico. Geol. Soc. Am. Bull. 111-10, 1550 – 1562. Siebe, C., Rodrı´guez-Lara, V., Schaaf, P., Abrams, M., 2004. Geochemistry, Sr–Nd isotope composition, and tectonic setting of Holocene Pelado, Guespalapa and Chichinautzin scoria cones, south of Mexico city. J. Volcanol. Geotherm. Res. 130, 197 – 226. Singh, S.K., Pardo, M., 1993. Geometry of the Benioff zone and state of stress in the overriding plate in central Mexico. Geophys. Res. Lett. 20, 1483 – 1486. Solari, L.A., Lopez, R., Cameron, K.L., Ortega-Gutie´rrez, F., Keppie, J.D., 1998. Reconnaissance U/Pb geochronology and common Pb isotopes from the northern portion of the 1Ga Oaxacan Complex Southern Mexico. EOS Trans. AGU. 79, F931. Abstract. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207 – 221. Stormer, J.C., 1983. The effects of recalculation on estimates of temperature and oxygen fugacity from analysis of multicomponent iron–titanium oxides. Am. Mineral. 68, 586 – 594. Sua´rez, G., Monfret, T., Wittlinger, G., David, C., 1990. Geometry of subduction and depth of the seismogenic zone in the Guerrero gap Mexico. Nature 345, 336 – 338. Sun, S., McDonough, W., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle compositions and processes. In: Saunders, A., Norry, M. (Eds.), Magmatism in the Ocean Basins, vol. 42. Geol. Soc. London Spec. Publ., pp. 313 – 345. Talavera-Mendoza, O., Ramı´rez-Espinoza, J., Guerrero-Suastegui, M., 1995. Petrology and geochemistry of the Teloloapan subterrane: a lower cretaceous evolved intra-oceanic island arc. Geofı´s. Int. 34, 3 – 22. Urrutia-Fucugauchi, J., Flores-Ruiz, H., 1996. Bouger gravity anomalies and regional structure in central Mexico. Int. Geol. Rev. 38, 176 – 194.

Verma, S.P., 1984. Alkali and alkaline earth element geochemistry of Los Humeros caldera, Puebla, Mexico. J. Volcanol. Geotherm. Res. 20, 21 – 40. Verma, S.P., 1987. Mexican volcanic belt: present stage of knowledge and unsolved problems. Geofı´s. Int. 26, 309 – 340. Verma, S.P., 1999. Geochemistry of evolved magmas and their relationship to subduction-unrelated mafic volcanism at the volcanic front of the central Mexican volcanic belt. J. Volcanol. Geotherm. Res. 93, 151 – 171. Verma, S.P., 2000. Geochemistry of subducting Cocos plate and the origin of subduction-unrelated mafic volcanism and the volcanic front of Central Mexican volcanic belt. Spec. Pap. Geol. Soc. Am. 334, 195 – 222. Verma, S.P., Luhr, J.F., 1993. Sr–Nd–Pb isotope and trace element geochemistry of calc-alkaline andesites from Volca´n Colima Mexico. Geofı´s. Int. 32-4, 616 – 631. Verma, S.P., Nelson, S.A., 1989. Isotopic and trace element constraints on the origin and evolution of alkaline and calcalkaline magmas in the northwestern Mexican volcanic belt. J. Geophys. Res. 94, 4531 – 4544. Walker, J.A., Patino, L.C., Carr, M.J., Feigenson, M.D., 2001. Slab control over HFSE depletions in central Nicaragua. Earth Planet. Sci. Lett. 192, 533 – 543. Wallace, P.J., Carmichael, I.S.E., 1999. Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and effects of crustal thickness variations on primitive magma compositions. Contrib. Mineral. Petrol. 135, 291 – 314. White, W.M., Hofmann, A.W., Puchelt, H., 1987. Isotope geochemistry of Pacific Mid-Ocean Ridge basalt. J. Geophys. Res. 92 (B6), 4881 – 4983. Wilson, M., 1989. Igneous Petrogenesis: A Global Tectonic Approach. Unwin Hyman, London, 446 pp.