Journal of Petrology Advance Access originally published online on April 30, 2007
Journal of Petrology 2007 48(5):901-950; doi:10.1093/petrology/egm006
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A North–South Transect across the Central Mexican Volcanic Belt at 
100°W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism
1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA
2Department of Geological Sciences and Cires, University of Colorado, Boulder, CO 80309, USA
3Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, USA
RECEIVED OCTOBER 24, 2004; ACCEPTED FEBRUARY 6, 2007
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONWithin the Zitácuaro–Valle de Bravo (ZVB) region of the central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary: (1) high-K2O basaltic andesites and andesites; (2) medium-K2O basaltic andesites, andesites and dacites; (3) high-TiO2 basalts and basaltic andesites. The dominant feature of the first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but the dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine. The spatial distribution of these three lava series is systematic. The southern section of the ZVB transect, 280–330 km from the Middle America Trench (MAT), is characterized by high-K2O melts that are relatively enriched in fluid-mobile elements and have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesite and andesite lavas are present throughout the transect, but those closest to the MAT are MgO-rich (3·5–9·4 wt %) and have phenocryst assemblages indicative of high magmatic water contents (3·5–6·5 wt % water) and relatively low temperatures (950–1000°C). In marked contrast, the northern section of the ZVB transect (380–480 km from the MAT) has high-TiO2, high field strength element (HFSE)-enriched magmas that have comparatively dry (< 1·5 wt % magmatic water) and hot (1100–1200°C) phenocryst equilibration conditions. The central section of the ZVB transect (330–380 km from the MAT) is a transition zone and produces moderately light rare earth element (LREE) and large ion lithophile element (LILE)-enriched, medium-K2O lavas with phenocryst assemblages indicative of intermediate (1·5–3·5 wt %) water contents and temperatures. The high-K2O series compositions are the most enriched in LILE and LREE, with a narrow range of radiogenic 87Sr/86Sr from 0·704245 to 0·704507, 143Nd/144Nd values ranging from 0·512857 to 0·512927 (
Nd = 4·27–5·63), and 208Pb/204Pb values from 38·248 to 38·442, 207Pb/204Pb values from 15·563 to 15·585, and 206Pb/204Pb values from 18·598 to 18·688. The medium-K2O series compositions are only moderately enriched in the LILE and LREE, with a broader range of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pb values to those of the high-K2O series. In contrast, the high-TiO2 series compositions have little enrichment in LILE or LREE and instead are enriched in the HFSE and heavy rare earth elements (HREE). The high-TiO2 lavas are isotopically distinct in their lower and narrower range of 143Nd/144Nd. The isotopic variations are believed to reflect the upper mantle magma source regions as the low content of phenocrysts in most lavas precludes significant upper crustal assimilation or magma mixing, other than that represented by the presence of quartz xenocrysts (< 2 vol. %) with rhyolitic glass inclusions, which are found in many of these lavas. The systematic spatial variation in composition of the three lava series is a reflection of the underlying subduction-modified mantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas
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INTRODUCTION |
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Subduction-related volcanism is the result of processes that occur within the subducting slab, the mantle wedge, and the crust above the subducting slab. These processes involve the recycling of volatile and lithophile elements between the lithosphere–atmosphere and the mantle (Coats, 1962
). This element redistribution creates a heterogeneous, metasomatized mantle wedge, from which melts are generated predominantly by flux-melting (Davies & Stevenson, 1992) as well as by decompression melting of the slowly circulating mantle above the subducting plate (Conder et al., 2002). In combination, these processes yield lavas as varied as potassic-absarokites and minettes, calc-alkaline basalts and intermediate magmas, and ocean island-type basalts (OIBs) and their congeners. These varieties all occur in the Mexican Volcanic Belt (MVB), and each has distinctive geochemical, petrological, and isotopic characteristics illustrating the variety of sources and processes that are involved.
This study concerns a north–south transect across the MVB in central Mexico, called the Zitácuaro–Valle de Bravo transect (ZVB; Fig. 1), which extends north of the city of Maravatío (
20°N) and south of Valle de Bravo at 19°N, with the city of Zitácuaro at its center (Fig. 1). The front of the volcanic arc occurs 280 km from the Middle America Trench (MAT), and within this transect contemporary Quaternary volcanism is found between 280 and 480 km from the trench.
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The basic and intermediate lavas of the ZVB transect are grouped after the classification of Carmichael (2002
), in which the high silica limit of intermediate magma (andesite) is taken as 65 wt % (Fig. 2), with basaltic andesites having 52–58% SiO2, and andesites 58–65% SiO2. Variation diagrams of K2O–SiO2 (Gill, 1981) and K2O–TiO2 allow the lavas and scoria of the ZVB transect to be subdivided into three groups (Fig. 2): (1) the least voluminous group of high-K2O basaltic andesites and andesites (SiO2 52–61%; TiO2 0·8–1·2%); (2) the more abundant medium-K2O basaltic andesites and andesites (SiO2 52–66%; TiO2 0·5–1·0%); (3) the equally voluminous high-TiO2 basalts and basaltic andesites (SiO2 50–60%; TiO2 1·1–1·9%). Dacites (SiO2 > 65%) are not abundant in this transect and continue the medium-K2O trend, whereas Quaternary rhyolites (SiO2 > 72%) are found only in two Quaternary volcanic centers (Los Azufres and Amealco; Dobson & Mahood, 1985; Aguirre-Díaz & McDowell, 2000) furthest from the volcanic front (Fig. 1). Basaltic lavas and scoria with <52 wt % SiO2 are as infrequent in this transect (Fig. 2) as they are in western Mexico (Carmichael, 2002).
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In hand sample it is difficult to distinguish between the three lava series, especially the more mafic compositions, because the majority of the samples contain either sparse phenocrysts of olivine and augite, or augite and hypersthene, in a dark glassy or microcrystalline groundmass. Plagioclase phenocrysts are absent in all but the high-TiO2 lavas and the dacites, a testament to the hydrous nature of the majority of the magmas at the phenocryst equilibration stage (Blatter & Carmichael, 1998b
; Moore & Carmichael, 1998; Carmichael, 2002), and it is the absence of plagioclase that is the most significant indicator of the role of water in the evolution of these intermediate magmas.
The objectives of this study are threefold: (1) to document the geochemical, petrographic, and isotopic characteristics of the Quaternary volcanism in the ZVB transect and demonstrate how these vary in space; (2) to illustrate the variations in magmatic water concentrations and temperatures that can be inferred from their phenocryst assemblages, and how these correlate with the spatial distribution of lavas and scoria in the ZVB; (3) to compare models (e.g. Verma & Nelson, 1989; Moore et al., 1994; Luhr, 1997; Márquez et al., 1999; Wallace & Carmichael, 1999) that have been proposed to explain the generation of the various lava types throughout the MVB.
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TECTONIC AND GEOLOGICAL SETTING |
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Quaternary volcanism in the central MVB is related to the subduction of the Cocos Plate beneath the North American Plate, at a rate of
6 cm/year (DeMets et al., 1990) (Fig. 1). Under this part of Mexico, there is abundant seismic activity defining a shallow-dipping (22°) subducting slab to a depth of about 70 km (Pardo & Suárez, 1995). Based on tomographic evidence (Van der Lee & Nolet, 1997; Goes & Van der Lee, 2002), as well as geochemical observations, Ferrari (2004) inferred that 9–7·5 Myr ago the subducting slab beneath this part of central Mexico became detached below 100 km, and a tear that began beneath western Mexico around 11·5 Myr ago migrated eastwards, and allowed hot asthenospheric mantle to rise through the gap between subducting slab segments. It has been postulated (Ferrari, 2004) that the rise of the asthenosphere caused the mafic volcanism that is exposed on the northern boundary of the MVB (Fig. 1).
Crustal thickness along the front of the MVB (Fig. 1) varies from 30 km in western Mexico to 50 km in the Sierra Chichináutzin Volcanic Field (SCVF) [Wallace & Carmichael (1999), based on gravity data from Urrutia-Fucugauchi & Flores-Ruiz (1996)]. The crustal thickness beneath the central section of the ZVB transect is estimated to be between 35 and 40 km.
Two pronounced gaps in the Late Miocene to Recent volcanism of the Mexican Volcanic Belt (MVB) have been recognized (e.g. Nixon, 1982), and the two gaps roughly correspond to the position of the subducted Rivera Fracture Zone (Gap 1), and the Orozco Fracture Zone (Gap 2) (Fig. 1). The Orozco Fracture Zone (OFZ) separates older, cooler and denser oceanic crust (17·6 Ma) from younger oceanic crust and lithosphere (14·5 Ma) (Fig. 3). This density, hence buoyancy, contrast may be reflected in the different dips of the subducting plate on either side of the OFZ, with the NW side subducting at 30°, and the SE side subducting at 5–10°, within the seismogenic zone (Pardo & Suarez, 1995). The bay north of Ixtapa Zihuatanejo (IZ, Figs 1 and 3) may be associated with fore-arc deformation where the OFZ is consumed. The ZVB transect occurs directly to the east of Gap 2, one of the most prominent and least understood features of the MVB.
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The northern segment of the ZVB transect (380–480 km from the MAT, Fig. 4) is characterized by prominent east–west extensional faulting associated with the large, active faults of the Acambay graben (Suter et al., 1995
). There is no seismically identifiable slab–wedge interface beneath this part of the transect (Pardo & Suárez, 1995). Quaternary volcanism in the region is dominated by monogenetic cinder cone fields, fissure-fed flows, and small shield volcanoes (<10 km basal diameter) of high-TiO2 basalt to basaltic andesite composition (Fig. 4). Also conspicuous are the silicic (up to 77 wt % SiO2) calderas of Los Azufres (Dobson & Mahood, 1985) and Amealco (Aguirre-Díaz & McDowell, 2000), which have produced large-volume Quaternary ash flow and fall deposits, as well as resurgent domes.
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The central section of the transect (330–380 km from the MAT) contains scattered, discontinuous east–west-trending normal faults, which cut lava flows that have been dated as Pliocene and Pleistocene (Blatter et al., 2001
). The Benioff zone beneath this region is very poorly defined at 100 km depth. Quaternary volcanism in this central part of the ZVB transect includes a variety of eruptive styles and compositions. Two prominent centers in the central part of the ZVB are the Ziráhuato domes (Fig. 4, ZR; Demant et al., 1975), and the Zitácuaro Volcanic Complex (Fig. 4, ZVC; Capra et al., 1997), both of which have produced several Quaternary ash-flow and fall deposits of medium-K2O, low-MgO (<3·5 wt %) andesitic and dacitic composition (58–67 wt % SiO2). These two centers are adjacent to several small (2–3 km diameter) to medium-sized (10–15 km diameter) shield volcanoes (Fig. 4) of high-TiO2 basaltic andesite composition (Blatter et al, 2001). Interspersed among the silicic centers and shield volcanoes are clusters of unusually Mg-rich andesite cones and flows with ubiquitous and evenly distributed quartz xenocrysts (Blatter & Carmichael, 1998b).
The southern section of the transect (<330 km from the MAT) sits above a poorly defined Benioff zone at 80 km depth (Pardo & Suárez, 1995), where the initial flux of metasomatic fluids is thought to be released from the down-going slab (Schmidt & Poli, 1998; Forneris & Holloway, 2003). At the surface, this area is dominated by two fault systems, trending NE–SW and NW–SE, consistent with the orientation of two large fault systems that are known in the adjacent Toluca region (Figs 1 and 4) as the San Antonio Fault System (NE–SW) and the Taxco–Querétaro Fault System (NW–SE) (García-Palomo et al., 2000). Along these faults, Quaternary cones, domes, and flows are concentrated (Blatter & Carmichael, 1998b; Blatter et al., 2001). The compositions of the Quaternary volcanism in the southern part of the transect are mainly medium-K2O andesites (Blatter & Carmichael, 2001). However, closest to the arc front, medium-K2O and high-K2O basaltic andesites with very high MgO (9 wt %) become prevalent (Blatter et al., 2001).
The basement terranes underlying the volcanic rocks of the ZVB transect are exposed along canyon walls and valley floors. In the north and central part of the ZVB transect, near Maravatío and south to Zitácuaro (Fig. 4), MVB volcanism has been active since the middle Miocene, as observed in the extensive lava successions of the Mil Cumbres and Angangueo volcanic sequences (Pasquarè et al., 1991). In the southern part of the ZVB transect, south of Zitácuaro, there are abundant exposures of subvolcanic basement rocks. These include Oligocene deposits of the Tuzantla conglomerate and the Eocene Molasse-type conglomerates, siltstone and sandstone of the Tzitzio formation (Pasquarè et al., 1991), and voluminous ash-flow tuff deposits that have been dated at 35 Ma (Blatter et al., 2001) and can be correlated with the Oligocene Sierra Madre Occidental eruptive episode (McDowell & Clabaugh, 1979).
The pre-Tertiary basement rocks observed in the ZVB transect include Middle Jurassic to Early Cretaceous schist and flysch formations consisting of greenschist in the southern section, and the Patambaro flysch and a sequence of mica-schist, calc-schist, andesitic pillow lavas and tuff (Pasquarè et al., 1991) in the central section of the ZVB transect. Massive and thinly bedded limestones also occur in the central and southern sections of the ZVB transect, and they have been considered Early Cretaceous because they contain Tethyan fossils (Pasquarè et al., 1991). Intrusive rocks include the Zitácuaro diorite, which crops out in the central section of the ZVB transect and is considered to be Late Jurassic (Pasquarè et al., 1991), and granite of unknown age that is exposed in the southern section of the transect near El Peñon (Fig. 1).
Proterozoic rocks are not known to be exposed at the surface; however, a Late Proterozoic (683 Ma) granulite xenolith has been found in an ignimbrite deposit from the Amealco Caldera (Figs 1 and 4; Aguirre-Díaz et al., 2002) and metamorphic xenoliths from the Lower Toluca Pumice have Nd model ages between 1·04 and 1·42 Ga (Martínez-Serrano et al., 2004) indicating that Proterozoic rocks occur at depth beneath the ZVB transect.
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ANALYTICAL METHODS |
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A total of 340 samples of Quaternary lava and scoria, along with 35 samples of older (Pliocene, Miocene, and Oligocene) lavas and pre-Tertiary basement rocks, have been collected and examined in thin section. Approximately 200 of these samples have been analyzed for major and trace elements by X-ray fluorescence spectrometry (XRF). A sub-set of 40 samples was selected from the three compositional series for radiogenic isotope (Sr, Nd, and Pb) analyses and high-precision trace element analyses by inductively coupled plasma mass spectrometry (ICP-MS) (Table 1). Samples of three types of basement rocks from the region (pelitic schist and granite collected from the southern section of the ZVB transect, and massive limestone collected from a quarry in the central section of the ZVB transect) were also analyzed for major and trace elements as well as radiogenic isotopes, to estimate the composition of the sub-volcanic basement. XRF analyses of major and selected trace elements of samples that have not hitherto been published are given in Table 2, together with their locations and distance from the MAT. Point counting for modal abundances (Table 3), and electron microprobe analyses (Tables 4–11
) were also performed on representative samples. The details of the various analytical techniques are given in the Appendix.
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PETROGRAPY AND MINERALOGY |
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The phenocryst modes of the three principal lava types are given in Table 3, and the most notable characteristic of the phenocryst assemblages is the paucity of plagioclase. It occurs together with olivine in the high-TiO2 group, but otherwise it is found only in two samples of intermediate lavas, dacites excepted (Fig. 5). The phenocryst assemblages of the intermediate magmas are dominated by the ferromagnesian minerals olivine, pyroxene (both Ca-rich and Ca-poor), and hornblende. It is these ferromagnesian assemblages (without plagioclase), rarely amounting to more than 20% of the sample (Fig. 5), that characterize the intermediate volcanic rocks of the ZVB and are a reflection of the dominant role of dissolved water at the stage of phenocryst equilibration.
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High-K2O basaltic andesites and andesites
The Quaternary high-K2O basaltic andesitic lava flows and cinder cone scoriae at the arc front (Fig. 4) contain 4–9 modal % forsteritic olivine (
Fo85) phenocrysts (Table 4) with little zoning and typically Cr-spinel inclusions (Cr-numbers between 0·72 and 0·82, Table 5). In addition to olivine, most of the high-K2O basaltic andesites have 2–5 modal % augite (Wo44En48Fs08) phenocrysts (Table 6), many of which display sector zoning and have high Cr2O3 concentrations (0·21–0·30 wt %, Table 6). Plagioclase is found only as flow-aligned acicular laths in the groundmass, which typically also contains augite, olivine, titanomagnetite (Table 7), ilmenite (Table 8), and small (<10 µm) pools of glass, which are too small for satisfactory analyses.
All of the Pliocene high-K2O lavas have undergone alteration, either by burial or by surface weathering. Thus, the older high-K2O lavas that occur within the Pliocene plateau sequence near Zitácuaro are distinctive because of the ubiquitous reddish iddingsite alteration of their olivine phenocrysts (Fo89–82; Table 4). Remnants of Cr-spinel inclusions occur in the olivine phenocrysts but their compositions (Mg-numbers of 0·17 and 0·29, Fe-numbers of 0·31 and 0·23, Table 5) indicate that they have reacted with the melt and/or been altered (Clynne & Borg, 1997). Unaltered augite (Wo44En44Fs12) phenocrysts (4–28 modal %, Table 3) are also found in all of the Pliocene high-K2O samples (Table 6). One of the high-K2O samples (Z-238) contains titanomagnetite (Table 7) and ilmenite microphenocrysts (Table 8) with exsolution lamellae, and another contains sparse plagioclase phenocrysts and microphenocrysts (Z-107, Table 10). The groundmass of the Pliocene high-K2O lavas may be fine- or coarse-grained with plagioclase, augite, and Fe–Ti oxides; phlogopite and sanidine also occur in the groundmass of the most potassic samples (Z-238, Tables 9 and 10). Small pools (20 µm) of interstitial glass were found in only one of the samples (Z-105, Table 11).
Medium-K2O basaltic andesites, andesites, and dacites
The medium-K2O basaltic andesites (52–58 wt % SiO2) that are found as lava flows and cinder cone scoriae at the arc front of the ZVB transect (Fig. 4), are petrographically similar to the high-K2O basaltic andesites described above. These samples contain Ni-rich (<0·46 wt %) olivine phenocrysts (Fo91–89) (Table 4), with Cr-spinel inclusions (Cr-numbers between 0·58 and 0·75, Table 5). Augite phenocrysts (Wo45En49Fs06) also occur (2–10 modal %, Table 3) with even higher Cr2O3 concentrations (0·35–0·68 wt %, Table 6) than their high-K2O counterparts. In a few of the medium-K2O basaltic andesites (e.g. Z-520) sparse microphenocrysts of plagioclase (An67Ab32Or01) occur (Fig. 5, Table 10); however, in the other medium-K2O basaltic andesites, plagioclase is found only in the groundmass, which typically also contains augite, olivine, titanomagnetite, ilmenite, and glass.
Medium-K2O andesites with 3·5–7·8 wt % MgO (58–65 wt % SiO2) display greater petrographic and textural diversity than lavas from any other compositional group. Varieties near the arc front typically contain phenocrysts of hypersthene and augite, with or without hornblende, commonly in glomeroporphyritic clusters (Blatter & Carmichael, 2001), indicating co-precipitation of these phases or reincorporation of cumulate material. Hypersthene phenocrysts (Wo01–03En71–88Fs09–26, Blatter & Carmichael, 2001) occur in all of these arc-front andesites and are usually zoned (normal, reversed, and oscillatory), and are accompanied by augite phenocrysts (Wo40–45En45–52Fs09–13) in all but one of these andesites (Z-386). Hornblende phenocrysts, up to 1·5 cm, are abundant (8–17 modal %) in Z-385 and Z-386, but in other samples (Z-346 and Z-388) the hornblende phenocrysts are smaller (1–5 mm) with oxide rims, or are represented by relict phenocrysts (Blatter & Carmichael, 2001). Inclusions of Cr-spinel and Fe–Ni sulfides, which have not previously been reported in MVB rocks, are found in both hypersthene and hornblende phenocrysts (Z-385, Z-386). Moderately zoned microphenocrysts (<0·5 mm) of plagioclase (An61–72Ab27–48Or00–01) occur in a few of these andesites (Z-357, Z-346). The trachytic groundmass looks similar in all of the medium-K2O andesites, with acicular, flow-aligned plagioclase surrounding augite, titanomagnetite and small pools of glass (Blatter & Carmichael, 2001). Xenoliths and xenocrysts of quartzo-feldspathic material are common in many of the arc-front medium-K2O andesitic lavas, and one of these samples (Z-509) contains small, fine-grained hornblende-peridotite xenoliths from the upper mantle (Blatter & Carmichael 1998a; Mukasa et al., 2007).
Several of the medium-K2O andesites that occur in the central section of the ZVB transect are high in MgO (up to 7·3 wt %) and contain olivine phenocrysts (Fo89) with Cr-spinel inclusions together with augite, or alternatively hypersthene and augite, with a conspicuous lack of plagioclase phenocrysts, but with ubiquitous evenly distributed quartz xenocrysts (Blatter & Carmichael, 1998b). The majority of the Quaternary medium-K2O andesites that occur in the central section of the ZVB are low in MgO (<3·5 wt %), and with the accompanying dacites (MAS-907, MAS-910), commonly have numerous phenocrysts of plagioclase (up to 17 modal %, Table 3), along with hornblende, ilmenite, and titanomagnetite (±biotite), surrounded by a groundmass of plagioclase, Fe–Ti oxides, and glass.
A few Quaternary medium-K2O andesites (Z-724, Z-726, and Z-727) occur within the cinder cone field near Maravatío in the northern section of the ZVB transect at the back of the arc (Fig. 4), and have plagioclase, hypersthene, and augite phenocrysts in a groundmass of plagioclase, augite, Fe–Ti oxides, and glass. The presence of plagioclase phenocrysts in these medium-K2O andesites is in marked contrast to the lack of plagioclase phenocrysts in the compositionally equivalent lavas and scoria closer to the volcanic front.
The medium-K2O dacites that form the large centers of the Zitácuaro Volcanic Complex (Capra et al., 1997; Blatter et al., 2001), the Ziráhuato domes (Demant, 1979; Blatter et al., 2001), contain large (up to 1 cm) phenocrysts of plagioclase, and smaller (2–5 mm) phenocrysts of hornblende and/or biotite. Fe–Ti oxide inclusions are present in all the phenocryst phases and the plagioclase contains melt inclusions (Table 11). The cores of the plagioclase (An47–49) show no zoning, but they are surrounded by thin rims that are visibly zoned (An45–46) (Table 10). The hornblende phenocrysts are brown with thin resorption rims in the lavas, and are green with almost no resorption rims in the pumice (Table 8). Microphenocrysts of plagioclase, hornblende, ilmenite, and titanomagnetite are common, and these phases also form the groundmass. Quartz generally occurs as single hexagonal grains rimmed by radiating augite crystals, but also occurs as small polycrystalline (metamorphic) xenoliths.
The Pliocene andesites and dacites that occur within the older plateau sequence (near Zitácuaro) generally contain phenocrysts of hypersthene, augite, and hornblende (or relict hornblende) and quartz xenocrysts in a matrix of acicular, flow-aligned plagioclase, titanomagnetite, and glass.
High-TiO2 basalts and basaltic andesites
The scoriae of the Quaternary basaltic cinder cones (MAS-913, Table 1) typically contain abundant plagioclase (An63Ab36Or01, Table 10) and fresh olivine phenocrysts (Fo80, Table 4) with or without Cr-spinel inclusions, in a groundmass of olivine, plagioclase, augite, and titanomagnetite. Crustal xenoliths (up to 4 cm) of calc-silicate and granitic affinity, as well as quartz xenocrysts, commonly occur in the Quaternary high-TiO2 cinder cone scoriae and feeder dikes.
The Quaternary high-TiO2 basaltic andesite lavas that form the shield volcanoes and several of the surrounding cinder cones (Fig. 4), are petrographically similar and contain sparse phenocrysts (3–7 modal %, Table 3) of olivine (Fo80–84) (Table 4) with Cr-spinel inclusions that have low Mg-numbers (0·40) and high Fe-numbers (0·20; Table 5) indicating that these Cr-spinel inclusions have reacted with the melt, perhaps through cracks in the olivine (Clynne & Borg, 1997), to form compositions intermediate between Cr-spinel and titanomagnetite (a ubiquitous groundmass phase). Most high-TiO2 basaltic andesites also have phenocrysts of plagioclase (An58–67, Table 10) (4–18 modal %, Table 3), in a groundmass of acicular, flow-aligned plagioclase (An51–57, Table 10) with augite, titanomagnetite, ilmenite, and small pools (20 µm) of glass (Table 11). Additionally, the basaltic andesites from the shield volcanoes contain either olivine (e.g. Z-309, Fo64, Table 4) or pigeonite (e.g. Z-227, Table 6) in the groundmass.
The high-TiO2 lavas that occur in the Pliocene plateau sequence in the center of the transect near Zitácuaro (e.g. Z-239C) contain marginally altered olivine phenocrysts (Fo79) (Tables 3 and 4) with pristine Cr-spinel inclusions (Mg-number = 0·51 and Fe-number = 0·08, Table 5), and plagioclase phenocrysts (An53Ab45Or02, Table 10) in a coarsely crystalline groundmass of plagioclase, subophitically enclosed in augite (Table 6), olivine (Table 4), titanomagnetite (Table 7), ilmenite (Table 8), and glass (Table 11).
Many lavas in the ZVB transect contain high concentrations of MgO (>6 wt %), Cr (>200 ppm), and Ni (>100 ppm), and have phenocrysts of magnesian olivine (Fo90) and Cr-spinel that are consistent with a peridotite (lherzolitic or harzburgitic) source. Such primitive compositions are found predominantly in the medium-K2O series, as few of the high-K2O series and none of the high-TiO2 samples contain magnesian olivines (Fo88–92; Table 4). A possible mantle source, in the case of the southern section of the ZVB transect, can be approximated by the hornblende spinel-lherzolite xenoliths that are contained in an arc-front medium-K2O andesite (Z-509, Table 1; Fig. 4, denoted by a ‘P’). These xenoliths have magnesian olivine (Fo89–91) (Blatter & Carmichael, 1998a), consistent with olivine in the medium-K2O primitive lavas (Z-351, Z-520, and Z-129, Table 4).
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MAGMATIC WATER CONCENTRATIONS AND TEMPERATURES ACROSS THE ZVB TRANSECT |
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Hydrous experiments on a variety of Mexican lavas (Blatter & Carmichael, 1998b
, 2001; Moore & Carmichael, 1998; Barclay & Carmichael, 2004) have shown that both the composition and modal abundance of the phenocryst phases in glassy lavas are functions of pressure, temperature, oxygen fugacity and water dissolved in the magma. Therefore, phenocryst abundances and compositions can be compared with those produced in experiments performed on similar bulk compositions to estimate intensive variables. This approach, in combination with two-pyroxene phenocryst thermometry (QUILF, Frost & Lindsley, 1992) and the water solubility model of Moore et al. (1998), has been used to constrain pre-eruptive temperatures (±40°C, Table 12) and water contents (±0·5 wt %, Fig. 4) for several of the ZVB lavas. In those cases where direct compositional equivalents have not been investigated experimentally, estimates of temperatures and magmatic water contents have been made according to phase equilibria data on compositionally similar lavas (Grove & Juster, 1989; Pallister et al, 1996; Barclay & Carmichael, 2004; Table 13).
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High-K2O basaltic andesites
The high-K2O basaltic andesites from the southern segment of the ZVB transect are more silica rich, but otherwise broadly similar to the high-K2O basalt (Jor 46, Table 13) from Cerro La Pilita at the front of the Michoacán–Guanajuato Volcanic Field (Luhr & Carmichael, 1985
), which has been the subject of a series of water-saturated phase equilibria experiments (Barclay & Carmichael, 2004). Based on the composition of the olivine phenocrysts (Fo85–87, Table 4), Cr-spinel (Table 5) and augite (Table 6) and their modal abundance (Table 3), the phenocryst equilibration conditions for the high-K2O lavas are estimated to be 1050°C (Table 12) with water concentrations greater than 5·5 wt % (equivalent to saturation at >2·2 kbar). Lack of early crystallizing plagioclase has also proven to be a good indicator of high pre-eruptive water contents (Blatter & Carmichael, 1998b; Moore & Carmichael, 1998; Barclay & Carmichael, 2004). Therefore, the lack of plagioclase phenocrysts in the Quaternary high-K2O lavas (Pliocene Z-107 excepted, Fig. 5) indicates that the high-K2O series equilibrated at 7 km depth, saturated with water.
Medium-K2O basaltic andesites, andesites, and dacites
The medium-K2O basaltic andesite compositions from the central segment of the arc near Zitácuaro include, and are comparable with, Mas-911 (Table 13) for which water-saturated phase equilibria data are available (Blatter & Carmichael, 1998b). The phenocryst modes and compositions of olivine (with Cr-spinel inclusions) + augite, or hypersthene + augite, assemblages constrain these medium-K2O basaltic andesites to equilibration conditions of less than 4 wt % water (equivalent to saturation at 1–1·2 kbar) and temperatures around 1050°C (Table 12).
The medium-K2O, high-MgO andesites from the Valle de Bravo area in the southern segment of the ZVB transect (Fig. 4) are represented by two compositions (Z-342 and Z-348, Table 13) for which water-saturated phase equilibria have been comprehensively studied (Blatter & Carmichael, 2001). The varied phenocryst modes and assemblages in these medium-K2O, high-MgO andesites constrain their equilibration conditions to temperatures between 950 and 1000°C (Table 12), water-saturated pressures of <1·0 to >3·0 kbar, and from 3·5 to >6·5 wt % water (Blatter & Carmichael, 2001).
The medium-K2O dacites from the Ziráhuato and Zitácuaro regions in the central segment of the ZVB transect commonly contain phenocrysts of plagioclase, hypersthene, hornblende, ilmenite, and titanomagnetite with trace amounts of biotite and quartz (MAS-910, Table 1) and are similar in composition to the Mt. Pinatubo (15 June 1991) dacite (Pallister et al., 1996; Table 13). The phenocryst equilibration pressures and water contents are thus constrained to lie between 1·2 and 1·4 kbar (below the stability field of cummingtonite) and <3·5 wt % water.
High-TiO2, basalts and basaltic andesites
The high-TiO2 basaltic andesites from the central to northern segment of the ZVB transect are comparable in bulk composition with the basaltic andesite (79-38b) from Medicine Lake that was studied by Grove & Juster (1989), with the exception that the Medicine Lake basaltic andesite has 0·5 wt % less TiO2 than the ZVB high-TiO2 basaltic andesites (Table 13). Because the early crystallizing phases are plagioclase and olivine (no Fe–Ti oxides), for the purpose of this comparison, this compositional discrepancy can be ignored. The phase assemblages and compositions (plagioclase = An58–67 + olivine = Fo78–84) in the ZVB high-TiO2 lavas are duplicated by experimental conditions bracketed by experiments run at 1 atm (essentially dry), temperatures of 1144 and 1161°C at the QFM (quartz–fayalite–magnetite) buffer and 1162 and 1172°C at NNO + 2 (where NNO is the nickel–nickel oxide buffer). The experiments at higher oxygen fugacities (NNO + 2) correlate well with the ZVB lavas containing Mg-rich (Fo84) olivine phenocrysts (e.g. Z-177, Table 4), and the lower oxygen fugacity experiments (QFM) correlate with the ZVB lavas containing less Mg-rich (Fo80) olivine phenocrysts (e.g. Z-227, Table 4), and pigeonite in the groundmass (Z-227, Table 6). Therefore, it is plausible to conclude that the ZVB high-TiO2 lavas equilibrated with low concentrations of water in the temperature range of 1150–1200°C.
Based on the estimates described above, the maximum phenocryst equilibration temperatures and corresponding water contents for the lavas from the ZVB transect have been contoured in Fig. 4, showing that the lavas that equilibrated under the wettest, coolest conditions are found in the southern segment of the transect at the front of the arc (3·5–6·5 wt % H2O and 950–1050°C). The ZVB transect central segment lavas have intermediate equilibration water contents (between 1·5 and 3·5 wt % H2O) with temperatures of 1050°C. At the back of the arc, the high-TiO2 lavas (e.g. MAS-913) equilibrated with lower water concentrations and temperatures between 1150 and 1200°C.
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GEOCHEMISTRY |
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The compositions of Quaternary and Pliocene lavas and scoriae from the ZVB transect have been plotted in Fig. 6 to illustrate the subduction-related compositional signature (e.g. high Sr, Ba, K, Rb, etc.) in contrast to the composition of intraplate oceanic lavas, such as those from Hawaii. The medium- and high-K2O intermediate compositions exhibit typical characteristics of subduction-related lavas, but overlap with the high-TiO2 compositions. Focusing on the Quaternary lavas and how their compositions change with distance from the MAT, it can be seen in Fig. 7 that MgO, K2O, Cr, Ni, Sr, and Ba concentrations generally decrease with distance from the MAT, whereas the FeOT, TiO2 CaO, Al2O3, Zr, and Nb concentrations increase with distance from the MAT. The trace element concentrations normalized to N-MORB (normal mid-ocean ridge basalt; Sun & McDonough, 1989
) for the medium-K2O and high-K2O lavas have typical spiky subduction-related patterns with enrichment in large ion lithophile elements (LILE): Rb, Ba, K, Sr, and P, as well as Th, U, and Pb (Fig. 8). Distinct negative anomalies are formed by Nb, Ta, and Ti in the medium-K2O and high-K2O lavas. The high-TiO2 lavas have more enriched Nb, Ta, and Ti, yielding a pattern of spikes with smaller magnitudes (Fig. 8), with a very small spike in Zr. Eu anomalies are absent in all three of the lava series. The medium-K2O and especially the high-K2O lavas display significant enrichment in the light rare earth elements (LREE) relative to the heavy REE (HREE) and the high field strength elements (HFSE), whereas the high-TiO2 lavas display much lower degrees of LREE enrichment and HREE depletion (Fig. 8). One medium-K2O lava (Z-351, Fig. 8) is notable for its enrichment in the middle REE (MREE) and HREE compared with all other samples and may represent a higher degree of melting of metasomatic vein material (Mukasa et al., 2007).
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Chondrite-normalized REE abundances for the three lava groups (Fig. 9) illustrate that the high-K2O lavas have the highest concentrations of LREE, and the high-TiO2 lavas have the highest concentrations of HREE. The medium-K2O lavas generally have the most fractionated REE patterns, with Z-351 exhibiting a REE pattern similar to that of the other medium-K2O lavas, but with about five times more enriched concentrations, surpassing REE abundances in all other samples. Chondrite-normalized La/Yb ratios reflect LREE enrichment relative to HREE; La/Yb ratios for the high-TiO2 lavas are 5–9, the medium-K2O lavas have ratios of 7–13, and the high-K2O lavas have the highest ratios of 7–27.
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In summary, the high-K2O and medium-K2O series generally have higher concentrations of MgO, SiO2, LILE (K, Sr, Ba), LREE, and transition elements (Ni, Cr), and lower concentrations of TiO2, HFSE (Nb, Zr), and HREE than the high-TiO2 series. Higher concentrations of K2O, Ba, and LREE distinguish the high-K2O series from the medium-K2O series.
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CRUSTAL CONTAMINATION AND Sr, Nd AND Pb RESULTS |
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Petrographic evidence for crustal assimilation or magma mixing includes the occurrence of partially digested xenoliths of upper crustal material, and xenocrysts of quartz with rhyolitic glass inclusions (Blatter & Carmichael, 1998b
) in many of the lavas in the ZVB transect. Most of the xenolithic material appears to be derived from granodioritic or calc-silicate upper crustal rocks. The Proterozoic (0·68–1·42 Ga) xenoliths that have been found in an ignimbrite deposit from the Amealco Caldera (Aguirre-Díaz et al., 2002) and the Lower Toluca Pumice of neighboring Nevado de Toluca (Martínez-Serrano et al., 2004) are not observed in any of the ZVB lavas.
One of the characteristic features of the lavas of the western MVB (west of 101° W) is that there is only a modest variation in the Sr and Nd isotope composition despite a very large variation in the bulk composition of the lavas, which vary from minettes to andesites and rhyolites (Luhr, 1997). In the lavas of the ZVB, 87Sr/86Sr ratios vary between 0·70323 and 0·70451 (Table 1) and the 143Nd/144Nd ratios range from 0·51278 to 0·51295 (Nd = 2·98–3·47), showing no correlation with 87Sr/86Sr ratios (Fig. 10). The high-K2O lavas have the highest 87Sr/86Sr values with 143Nd/144Nd ratios that are similar to those for the medium-K2O lava series, and the high-TiO2 lava series forms a group with low 143Nd/144Nd ratios and a range of 87Sr/86Sr ratios that is intermediate between those of the high-K2O and medium-K2O series (Fig. 10). The El Peñon clinopyroxene (cpx) megacrysts (Figs 4 and 11), which are contained in the same arc-front medium-K2O andesitic lava that entrained peridotite xenoliths from the lithospheric mantle (Blatter & Carmichael, 1998a; Mukasa et al., 2007), have Sr–Nd isotopic ratios similar to those of the medium-K2O lavas.
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On a larger scale (Fig. 11), the ZVB lavas define an array between the field for East Pacific Rise Mid-Ocean Ridge Basalt (PETDB, 2002
) and Proterozoic (1·04–1·42 Ga) basement gneiss and schist xenoliths that occur in the Lower Toluca Pumice of neighboring Nevado de Toluca (Martínez-Serrano et al., 2004; Fig. 1). The range in Sr isotopic compositions for the ZVB is slightly larger than that described for the Sierra Chichinautzin Volcanic Field (SCVF, Fig. 1) farther to the east, for which a large dataset is available (Velasco-Tapia & Verma, 2001; Siebe et al., 2004). The Nd values for the ZVB and SCVF volcanic rocks are essentially identical (Fig. 11).
Two-component mixing lines, representing the assimilation of solid material by magma, have been calculated using the mixing equations of DePaolo & Wasserburg (1979) and the compositions of: (1) end-member East Pacific Rise (EPR) MORB sources (MORB1 and MORB2), which represent a range of compositions possible in the depleted mantle wedge source; (2) altered basalt from the Cocos Plate [Ocean Drilling Program (ODP) Leg 66, site 487, Verma, 2000], which represents the isotopic composition of the slab-contributed fluids or melts; (3) Proterozoic basement gneiss, which is the most extreme of the several basement compositions known in the region (Fig. 11). These mixing lines indicate that the isotopic compositions of the high-TiO2 samples would require mixing of low-Nd EPRMORB2 with 0·1–0·2 weight fraction of basement gneiss. Mixing of high-Nd EPRMORB1 and 0·2–0·3 weight fraction of basement gneiss (Fig. 11) would be required to generate the medium-K2O isotopic compositions, and mixing of Cocos site 487 altered basalt with 0·1 weight fraction of basement gneiss would produce the high-K2O Sr–Nd isotopic compositions. The Cocos slab sediments (Verma, 2000), the basement granite (Z-625, Table 1), and two of the basement schist xenoliths from Nevada de Toluca (Martínez-Serrano et al., 2004) lie within these mixing curves.
It is difficult to envision that such large and variable fractions (0·1–0·3 wt %) of basement gneiss or granite could be assimilated into the ZVB magmas as they ascended from their source. The primitive bulk-rock compositions of many of the lavas, low concentrations of phenocrysts, and particularly the absence of plagioclase in the ZVB lavas preclude any significant assimilation of upper crustal solids. Although fluids with the isotopic composition of the basement gneiss and granite would achieve the same isotopic shift without causing significant precipitation of phenocrysts or change in the major element compositions of the lavas, this would require a large volume of lower crustal fluid input. The simpler and more likely explanation for the systematic variation in the Sr and Nd isotopic ratios of the three lava series is that the variation largely reflects the mantle source from which the lavas are derived.
The variation in Pb isotopes in the ZVB is consistent with the variation found in other studies of central Mexican lavas (Fig. 12). The 208Pb/204Pb ratios range from 38·252 to 38·570, the 207Pb/204Pb ratios range from 15·547 to 15·607, and the 206Pb/204Pb ratios range from 18·586 to 18·791 (see inset plots in Fig. 12) and form an array between the least radiogenic end-member (Z-342, Fig. 12) and the most radiogenic end-member (Z-239C), which is a Pliocene high-TiO2 basalt. Pb isotopic ratios of the ZVB lavas are more radiogenic than the Northern Hemisphere Reference Line (NHRL, Hart, 1984); however, the El Peñon (Fig. 1) clinopyroxene (cpx) megacrysts plot on the NHRL with compositions close to MORB.
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VARIATION IN COMPOSITION OF MAGMAS ACROSS THE ZVB TRANSECT |
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In the southern segment of the ZVB transect (280–330 km from the MAT), at the arc front, the lavas and scoriae equilibrated with the lowest phenocryst temperatures (950–1050°C), with the highest water contents (3·5–6·5 wt % water), and have the highest concentrations of fluid-mobile elements (K2O, Ba, Sr, etc.), and the highest 87Sr86Sr ratios. Based on the characteristics of the most primitive of the Quaternary lavas from the southern segment of the ZVB transect, and thermal models of the mantle wedge just to the east of this region (Manea et al, 2005
), this volcanism is taken to illustrate partial melting near the hinge of the mantle wedge (Fig. 13). This hinge region is composed of 10 km of lithospheric mantle, which has been influenced by several almost continuous subduction regimes and has undergone multiple periods of metasomatism and partial melting [based on the tectonic history of the lithospheric mantle of this region of Mexico presented by Dickinson & Lawton (2001)], resulting in a depleted major element background signal (high MgO, low Al2O3, FeOT, TiO2, and CaO) similar to that noted in several studies of central Mexican volcanism (Blatter & Carmichael, 1998a, 1998b, 2001; Wallace & Carmichael, 1999; Blatter et al., 2001; Verma, 2002). This depleted major element signature has been overprinted by various degrees of metasomatic fluids migrating into the lithospheric mantle wedge from the down-going slab. These fluids have hydrated and replenished the fluid-mobile elements in the mantle wedge many times over, allowing for a subduction signal to be imparted in plutons and lavas that date from at least the Late Jurassic (Pasquarè et al., 1991) to the present. Beneath this layer of lithospheric mantle lies the corner of convecting asthenospheric mantle (Fig. 13, number 1), which is also depleted in the easily fusible elements because it has been increasingly partially melted throughout its traverse from the back to the front of the arc. Fluids liberated through dehydration reactions from the down-going slab infiltrate this spinel harzburgite and cause small increments of melting. Because of the abundance of water in this arc-front part of the mantle wedge, melting occurs at low temperatures and pressures (1000°C and 10–15 kbar), and primary magmas are high-K2O and medium-K2O, high-MgO andesites and basaltic andesites (Hirose, 1997; Blatter & Carmichael, 2001), with unusually high (up to 6·5 wt %) phenocryst equilibration water contents. These arc-front scoria cones and flows are small in volume (Blatter et al., 2001) and either are isolated, separated sometimes by several kilometers, or occur in clusters along faults. The ascent and eruption of these particular arc-front lavas and scoriae may be favored by the approximately north–south extensional tectonic regime that has resulted in large approximately east–west-trending normal faults at the surface, as was the case with the El Peñon andesite lava (Fig. 1), which transported centimeter-sized peridotite xenoliths from the lithospheric mantle to the surface (Blatter & Carmichael, 1998a).
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The most primitive Quaternary lavas from the central segment of the ZVB transect (330–380 km from the MAT) are the medium-K2O, high-MgO andesites and basaltic andesites that equilibrated with intermediate temperatures (
1050°C) and water contents (1·5–3·5 wt %) and have moderate enrichments in fluid-mobile elements, with a broad range of 87Sr/86Sr ratios. These lavas show evidence of influence by slab-derived fluids, but none are as enriched in LILE and LREE, or have 87Sr/86Sr ratios as radiogenic as those high-K2O lavas in the southern segment of the ZVB transect. Contemporaneous with the Quaternary medium-K2O volcanism in the central segment of the ZVB transect are Quaternary high-TiO2 lavas and scoria, indicating that melting beneath this mid-arc region must be tapping a heterogeneous mantle source composed of fluid-modified depleted asthenospheric peridotite and more fertile asthenospheric peridotite that is advected into the mid-arc from behind the arc (Fig. 13, number 2). Partial melting of this heterogeneous source occurs as a result of decompression and fluxing by subduction-related fluids.
In the northern segment of the ZVB transect (380–430 km from the MAT), at the back of the arc, the primitive Quaternary lavas and scoriae reflect little involvement of slab-derived fluids. Instead, the phenocrysts of the high-TiO2 volcanism in this region equilibrated under hot (1150–1200°C) and essentially dry conditions with generally low concentrations of fluid-mobile elements, and high concentrations of HFSE. Isotopic (Nd and Pb) compositions indicate that crustal assimilation has occurred in many of the lavas in this segment of the transect, and Sr and Pb isotopic ratios indicate small but variable inputs of slab-derived fluids.
The characteristics of the volcanism in the northern segment of the ZVB transect are consistent with partial melting of a fertile asthenospheric peridotite mantle source advected from behind the arc (Fig. 13, number 3), which melts as a result of extension-related decompression at various depths within the mantle wedge (Wallace & Carmichael, 1999). The weak slab-derived fluid signature present in some of this northern segment ZVB volcanism is due to small inputs of slab-derived fluid. This slab-derived fluid signature appears to increase nearest the gap (Fig. 1) in the western part of the northern segment of the ZVB transect.
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CONCLUSIONS |
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Understanding the connection between volcanic compositions and their spatial distribution within the ZVB provides constraints on tectonic models for the MVB, where several competing models have been proposed for the contemporaneous juxtaposition of diverse volcanism. In the western MVB, the high-K2O and medium-K2O compositions are considered to be inextricably linked to melting of the subduction-modified mantle wedge (Wallace & Carmichael, 1989
; Carmichael et al., 1996; Luhr, 1997); however, the high-TiO2 volcanism (referred to as ‘ocean island basalt-type volcanism’) has been explained by invoking a mantle plume (Moore et al., 1994), or by decompression melting of peridotite that has not been modified by subduction (Verma & Nelson, 1989), perhaps emplaced from the back-arc mantle regions by slab-derived corner flow (Luhr, 1997).
Models for the Sierra Chichináutzin region (to the east of the ZVB transect) are similar to the western MVB models. One model considers the medium-K2O varieties to be generated in the subduction-modified mantle wedge and the high-TiO2 compositions to be derived from asthenospheric mantle advected from behind the arc (Wallace & Carmichael, 1999). Another model (Márquez et al., 1999) proposes that a detached mantle plume head beneath central Mexico generates high-TiO2 basaltic melts, and that mixing between these melts and a dacitic crustal component accounts for the medium-K2O compositions in this region.
The comprehensive ZVB transect data presented in this study provide constraints on generation and evolution of the diverse volcanism in the central MVB. The shallow angle of the subducting Cocos slab results in a wide zone of Quaternary volcanism in the ZVB region. This wide arc provides spatial resolution so that the effects of subduction-related processes can be evaluated in the ZVB. Based on these evaluations, it is apparent that Quaternary volcanism in the northern segment of the ZVB transect (380–430 km from the MAT) has been generated from partial melting of fertile asthenospheric mantle peridotite as a result of pervasive extension-related decompression with little input of slab-derived fluids. Quaternary volcanism in the southern segment of the ZVB is due to partial melting of depleted mantle peridotite and is strongly influenced by subduction-related fluids, and Quaternary volcanism in the central segment is due to partial melting of a heterogeneous mantle peridotite source with variable quantities of slab-derived fluids and extension-related decompression, making the central segment of the ZVB a transition zone between the arc front and back-arc.
The particular pattern of distribution described for the ZVB transect also generally coincides with the pattern of distribution for the Michoacán–Guanajuato Volcanic Field (MVGF) to the west, although compositional adjustments must be made in categorizing the sample types, because of the major element differences in the western Mexican mantle (it is much less depleted and samples tend to be richer in FeO, Al2O3, TiO2, and CaO). The same is not true of the Sierra Chichináutzin Volcanic Field (SCVF) to the east, where samples of all three types (high-K2O, medium-K2O, and high-TiO2) cluster in close proximity to the arc front.
This study has determined the geochemical, mineralogical, and isotopic (Sr, Nd, and Pb) compositions of volcanism transecting the Mexican Volcanic Belt in the Zitácuaro–Valle de Bravo (ZVB) region of central Mexico. These data establish that the three distinct volcanic series (high-K2O, medium-K2O, and high-TiO2) have erupted contemporaneously in spatially distinct regions of the arc and can be attributed to systematic variations in the mantle source from which they were generated.
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APPENDIX |
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Samples were ground to a fine powder in a tungsten carbide mill for XRF analyses and ground in an iron mill for ICP-MS and isotope analyses. Powdered samples were fused with lithium tetraborate flux for XRF major element analyses and compressed powder pellets were used for XRF trace element analyses. XRF analyses were obtained using an energy dispersive SpecTrace 440 X-ray spectrometer at U.C. Berkeley for 13 samples (denoted in Table 1) with standards of several lavas, including USGS standards. Average uncertainties of one standard deviation based on replicate analyses are equal to the following percentages of the amounts present: SiO2 (0·2%), TiO2 (0·8%), Al2O3 (0·3%), FeOt (0·5%), MnO (1·7%), MgO (1·3%), CaO (1·2%), Na2O (3·9%), K2O (1·8%), and P2O5 (4·6%). Forty-four of the XRF analyses presented (Tables 1 and 2) were performed using the Phillips PW2400 wavelength dispersive X-ray spectrometer at U.C. Berkeley with standards as above. Average uncertainties based on replicate analyses are equal to the following percentages of the amounts present: SiO2 (0·08%), TiO2 (0·22%), Al2O3 (0·15%), FeOt (0·08%), MnO (0·66%), MgO (0·24%), CaO (0·18%), Na2O (0·34%), K2O (0·30%), and P2O5 (0·58%). Several samples (denoted in Table 1) were also analyzed using wet chemical techniques with the following 2
precision: SiO2 (0·08), TiO2 (0·06), Al2O3 (0·08), Fe2O3 (0·06), FeO (0·06), MnO (0·02), MgO (0·06), CaO (0·04), Na2O (0·03), K2O (0·01), and P2O5 (0·03). ICP-MS analyses were performed at Washington State University's Geoanalytical Laboratory with the following reproducibility (average of several replicate analyses/standard deviation): La (1·86%), Ce (1·20%), Pr (0·98), Nd (1·75%), Sm (2·07%), Eu (1·92%), Gd (1·13%), Tb (1·12%), Dy (1·33%), Ho (1·53%), Er (1·37%), Tm (1·23%), Yb (0·94%), Lu (1·90%), Ba (1·89%), Th (9·50%), Nb (2·16%), Y (0·77%), Hf (1·47%), Ta (2·70%), U (9·34%), Pb (3·23%), Rb (1·39%), Cs (3·06%), Sr (1·56%), Sc (3·46%).
Whole-rock radiogenic isotopic analyses were performed at University of Colorado Boulder using a six-collector Finnigan MAT solid source mass spectrometer following the techniques described by Farmer et al. (1991). Total procedural blanks for Sr, Nd, and Pb averaged 147, 40, and 1000 pg, respectively. Over the study period, 30 measurements of SRM-987 yielded mean 87Sr/86Sr = 0·71032 ± 2 (2 mean). Measured 143Nd/144Nd was normalized to 146Nd/144Nd = 0·7219. During the study period, 33 measurements of the La Jolla Nd standard yielded a mean 143Nd/144Nd = 0·511838 ± 8 (2 mean). Sixteen measurements of SRM-981 during the study period yielded 208Pb/204Pb = 36·56 ± 0·03, 207Pb/204Pb = 15·449 ± 0·008, 206Pb/204Pb = 16·905 ± 0·007 (2 mean). Measured Pb isotope ratios were corrected to SRM-981 values (208Pb/204Pb = 36·721, 207Pb/204Pb = 15·491, 206Pb/204Pb = 16·937).
Microprobe analyses were obtained using a five-spectrometer Cameca SX-50 electron microprobe at U.C. Berkeley for all analyzable phases from 25 samples, which are representative of the three lava series. Representative phase compositions were determined by analyzing 5–10 crystals of each phase throughout each polished thin section. Every composition reported (core or rim) is the average of at least five analytical points acquired across the core or rim of a representative crystal of that phase. Analytical conditions were 15 kV accelerating voltage, 20 nA sample current, and a focused beam for each phase (except plagioclase, phlogopite, hornblende, and glass, for which a defocused beam of 20 µm and a sample current of 5 nA were used to minimize alkali loss). Standards were oxides of Si, Al, Ti, Mn, V, and Cr, synthetic fayalite (Fe), diopside (Ca), nepheline (Na), orthoclase (K), strontium titanate (Sr), fluor-phlogopite (F) and chlor-apatite (Cl and P). Ten-second counting times and MAN corrections (Donovan & Tingle, 1996) were used for major elements (Si, Al, Mg, Fe, Ca, Na, and K) and 20–40 s counting times and off-peak corrections were used for trace elements (Ti, Mn, Sr, P, Cl, and F). All microprobe data are reported in oxide weight percent (Tables 4–11
).
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ACKNOWLEDGEMENTS |
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Appreciation goes to Lisa Hammersley for assistance in the field and useful discussions concerning isotope systematics, Laura Glaser and Tim Teague for sample preparation and XRF analytical support, and Emily Verplanck for assistance in the isotope laboratory. Remarkably thorough and constructive reviews by Denny Geist, Mike Clynne, Claus Siebe, and Jen Garrison helped to strengthen and clarify the manuscript and are greatly appreciated. The support of funds from NSF grants EAR 0074610 to D.L.B. and EAR 0228919 to I.S.E.C. were vital to this work.
*Corresponding author. E-mail: dawnika{at}eps.berkeley.edu
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