Neogene volcanism at the front of the central Mexican volcanic belt

Neogene volcanism at the front of the central Mexican volcanic belt:
Basaltic andesites to dacites, with contemporaneous
shoshonites and high-TiO2 lava
Dawnika L. Blatter*
Ian S.E. Carmichael
Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
Alan L. Deino
Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
Paul R. Renne
Department of Geology and Geophysics, University of California, Berkeley, California 94720, USA, and Berkeley
Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
ABSTRACT
As part of the continuing study of the
young Mexican volcanic belt designed to
document the ages and types of volcanism
from the Gulf of California to the Valley of
Mexico, a 50-km-wide segment of the central part of the belt has been mapped, and
five types of lava have been found. Pliocene
(3.77 Ma) shoshonite lava flows (K2O .2
wt%; SiO2 52–58 wt%) form eroded plateaus more than 50 km behind the present
volcanic front, but in the past 1 m.y. shoshonites have erupted closer to the volcanic
front (;300 km from the Middle America
Trench). The shoshonite lava type is the
most enriched in Ba, Sr, and Zr of the suite,
and plagioclase phenocrysts are absent,
presumably because of high contents of dissolved water (3–5 wt%).
Quaternary shield volcanoes and several
cinder cones with small-volume lava flows
are composed of high-TiO2 lavas (.1.2
wt%), which have 51–57 wt% SiO2, ,6
wt% MgO, low Ni, Cr, Ba, and Sr, and
high Nb (.9 ppm) and Zr (160–230 ppm).
Pliocene high-TiO2 lava is found within the
eroded plateaus located ;50 km behind the
present volcanic front.
Quaternary basaltic andesite (52–57 wt%
SiO2) with up to ;10 wt% MgO is found
at the volcanic front, along with more sili*Present address: CIRES, University of Colorado, Campus Box 449, Boulder, Colorado 80309,
USA; e-mail: [email protected].
ceous andesite (57–63 wt% SiO2). Representatives of the siliceous andesites (57–63
wt% SiO2) are free of plagioclase phenocrysts, low in Al2O3 (;15.7 wt%), but rich
in MgO (;5 wt%). Experiments reported
elsewhere suggest that the magmas contained 3–7 wt% dissolved water. This andesite erupted along a normal fault between
;0.3 and 0.005 Ma, and it is associated
with dacite, similarly lacking plagioclase
phenocrysts, but having comparatively
abundant pyroxene. Other dacite in the Zitácuaro area is richly porphyritic, having
plagioclase and hornblende; this dacite
forms clusters of steep-sided domes or
widespread pyroclastic deposits, and the
latest eruptions, dated by radiocarbon and
K-Ar, occurred between 0.03–0.05 Ma.
Estimates of the volume of magma erupted in the Zitácuaro–Valle de Bravo region
in the past 1 m.y. (1.8 km3 · m.y.–1 · km–1)
show that this area is somewhat less productive per 1 km of arc than those to the
west (3.5 km3 · m.y.–1 · km–1) in the Michoacán-Guanajuato
Volcanic
Field
(MGVF). A large volume of dacite has
erupted in the Zitácuaro–Valle de Bravo
region, which is rare in the MGVF. The
cone density in the Zitácuaro–Valle de Bravo region (2.1/100 km2) is only slightly lower than the 2.6 cones/100 km2 found in the
MGVF.
Keywords: andesite, basaltic andesite, dac-
ite, Mexican Volcanic Belt, shoshonite,
eruption rate.
INTRODUCTION
One of the more difficult but important
problems in the study of magmatism associated with subduction is estimating the volumes of various lava types and relating these
to a time interval of eruption. The volume of
lava erupted in 1 m.y. along a specific length
of volcanic arc is one of the least precisely
known features of volcanic arcs, and yet estimates of this type are critical to assessing the
cycling of material from the mantle or crust
to the surface. Because older lava is commonly deeply eroded or tends to be covered by
younger lava, estimating volumes becomes increasingly uncertain with age. Nevertheless, it
is important to assess the productivity of
young volcanic arcs. Because these are typically linear features hundreds of kilometers
long, it is possible to map only short segments
of a volcanic arc in the detail required to estimate the volume of material erupted in a specific amount of time, usually extrapolated to
1 m.y. This paper is another contribution to
assessing the eruptive productivity of the
young and well-exposed Mexican Volcanic
Belt. This arc lies above the Rivera and Cocos
oceanic plates, which underthrust the continental North American plate at the Middle
America Trench (Fig. 1).
In the region near Zitácuaro, State of Michoacán, and Valle de Bravo, State of Mexico,
GSA Bulletin; October 2001; v. 113; no. 10; p. 1324–1342; 10 figures; 5 tables; Data Repository item 2001109.
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For permission to copy, contact [email protected]
q 2001 Geological Society of America
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
a series of volcanic structures reflects the
properties of the magma: broad shield volcanoes, steep-faced extrusive domes, small volume lava flows and cinder cones, and flat-lying lava plateaus. This part of the active
Mexican Volcanic Belt is related to the subduction of the Cocos plate, which subducts at
a shallow angle (;228); and the volcanic front
here lies about 80 km above the underlying
Benioff zone (Fig. 1) (Pardo and Suárez,
1995). There is a distinct gap in the volcanic
front at long 1018W, coincident with the Tzitzio anticline, which is the western boundary
of the described area. Farther to the west is
the Michoacán-Guanajuato Volcanic Field of
cinder cones and shield volcanoes (Fig. 1)
(Hasenaka and Carmichael, 1985, 1987; Hasenaka, 1994) for which the first detailed estimates of eruptive volumes of various lava
types in the Mexican Volcanic Belt were
made. From these data, and a series of carbon
and K-Ar dates, the important quantity of
eruptive volume per 1 m.y. along a 200-kmlong segment of the Mexican arc was
obtained.
The lavas that form the volcanic structures
in the region described here vary from rare
basalt to dacite, but several compositional features are distinctive. Many of the lava flows
and scoriae have high-TiO2 contents and share
chemical characteristics with oceanic-island
lava; however, the majority have the subduction-related signature of enrichment in the
large-ion lithophile elements (LILE). Included
in this group is a series of Al2O3-poor, highMgO andesites that have no equivalents in
western Mexico (Carmichael et al., 1996;
Luhr and Carmichael, 1980). This paper focuses on the compositions and ages of the Pliocene through Holocene volcanic products,
the magma eruption rate in the past 1 m.y.,
and the spatial distribution of volcanism in relation to the Middle America Trench.
GEOLOGIC SETTING
At the margins of the Zitácuaro–Valle de
Bravo area are three large volcanic centers,
Quaternary Los Azufres silicic caldera to the
north (Robin and Pradal, 1993; Dobson and
Mahood, 1985; Pradal and Robin, 1994; Ferrari et al., 1991; Ferrari et al., 1993), the Zitácuaro volcanic complex (Capra et al., 1997)
directly to the east (Fig. 2), and Nevado de
Toluca (Macı́as et al., 1997) 50 km to the east.
Farther east are the myriad of volcanic cones
and centers of the Valley of Mexico (Gunn
and Mooser, 1971; Nixon, 1989; Verma and
Armienta-H., 1985; Wallace and Carmichael,
1999) (Fig. 1).
Figure 1. The Mexican volcanic belt; dark gray indicates volcanism from 0 to 6 Ma
(modified from Pasquarè et al., 1991). The tectonic features are modified from Pardo and
Suárez (1995). RFZ—Rivera Fracture Zone, EG—El Gordo graben, EPR—East Pacific
Rise, OFZ—Orozco Fracture Zone, OGFZ—O’Gorman Fracture Zone. The numbers
along the Middle America Trench indicate the age of the subducting oceanic crust in
million years (first number) and the convergence rate at that location in centimeters per
year (number in parentheses); the black contours represent the depth to the subducted
slab (Pardo and Suárez, 1995). NWVB—Northwest Volcanic Belt, MGVF—MichoacánGuanajuato Volcanic Field (Hasenaka and Carmichael, 1985), M—Maravatı́o, A—Acambay, Z—Zitácuaro, VdB—Valle de Bravo, T—Nevado de Toluca, DF—Distrito Federal,
VM—Valley of Mexico (area studied by Wallace and Carmichael, 1999), and TA—Tzitzio
anticline. Labeled boxes indicate the areas covered by Figures 2 and 3.
Within the Zitácuaro–Valle de Bravo region, rocks with ages ranging from Late Jurassic to Holocene are exposed. Mesozoic
basement schist and deformed sedimentary
rocks (McLeod, 1989; Petersen, 1989; Pasquarè et al., 1991; Garduño-Monroy et al.,
1999; Garcı́a-Palomo et al., 2000) are exposed
along canyon walls and valley floors. The
lower basement units are composed of Late
Jurassic– to Early Cretaceous low-grade, regionally metamorphosed mica and calcareous
schist, andesitic pillow lava, and tuffs. The upper sequence consists of Cretaceous nonmetamorphosed terrigenous and volcaniclastic
sedimentary rocks, which are interbedded
with bioclastic limestone (Pasquarè et al.,
1991; Garduño-Monroy et al., 1999; Garcı́aPalomo et al., 2000). The basement rocks in
the Zitácuaro region are generally considered
to be part of the Guerrero terrane (Pasquarè et
al., 1991; Garduño-Monroy et al., 1999), a
composite island-arc assemblage with volcaniclastic sediments interstratified with lime-
stone, shale, and sandstone (Campa and Coney, 1983). The Valle de Bravo region (Fig.
3) is on the eastern edge of the Guerrero terrane, which forms a thrust margin boundary
with Cretaceous shelf carbonates of the upper
Mixteca terrane (Campa et al., 1976).
The dioritic stock found directly north of El
Bosque (Fig. 2) is of Jurassic age, according
to Pasquarè et al. (1991). In the southern part
of the Valle de Bravo region (near El Peñon,
Fig. 3), there are several granitic intrusive
bodies. The extent and ages of these are not
known, but they are undeformed and therefore
younger than the late Mesozoic–earliest Cenozoic Laramide Orogeny (Armstrong, 1974).
Early Tertiary (Paleocene to Oligocene) deposits in the Zitácuaro–Valle de Bravo region
include successions of volcaniclastic rocks
(exposed in the southern part of the Zitácuaro
region near Paricuaro, (Fig. 2), lava, and ashflow tuffs (exposed to the west and northeast
of Zitácuaro Fig. 2). Several of these units
have been correlated with the Sierra Madre
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NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
Occidental eruptive episode (Pasquarè et al.,
1991; Garduño-Monroy et al., 1999). Subhorizontal ignimbrite sheets form prominent cliffs
to the south of Valle de Bravo; near El Peñon
(Fig. 3) this sequence is at least 1000 m thick.
One of these ignimbrite sheets (Z-365) has
been dated at 34.9 Ma (Table 1; Fig. 3), consistent with the date of the main pulse (34–27
Ma) of the Sierra Madre silicic volcanism in
northern Mexico (McDowell and Clabaugh,
1979; Aguirre-Diaz and McDowell, 1991).
To the west of Zitácuaro, approximately
north-south normal faults, associated with
late-Miocene Basin and Range–type extension
(Pasquarè et al., 1988), separate flat-lying,
lava-capped plateaus (Fig. 2); the intervening
valleys are river courses, such as that of the
Rio Tuxpan (Fig. 2). Several large landslides
along the walls of these steep canyons have
created hummocky landslide topography, as
well as small horizontal and slightly tilted mesas made of stratigraphically intact blocks,
which have slipped hundreds of meters vertically down the canyon walls. These landslides
may have been triggered by motion along
north-south normal faults, which separate the
mesas, or along the east-west faults that dissect them (Fig. 2). One such east-west–striking fault cuts the easternmost mesa with ;100
m of offset; another cuts younger shield lava
flows south of the city of Jungapeo and has
;60 m of offset.
The geology of the Valle de Bravo region
(Fig. 3) is dominated by two normal faults,
which bound an uplifted block of Oligocene
silicic ignimbrite. The longest fault trends
;N658E and extends ;60 km along the canyon of the Rio Tilostoc (Fig. 3). The orientation of this fault is consistent with faults characterized directly to the east as the San
Antonio fault system (Garcı́a-Palomo et al.,
2000), indicating that this fault may be part of
the regional Tenochititlán shear zone, which
runs from Zihuatanejo to the north of Mexico
City (De Cserna et al., 1988). The other fault
is truncated against the main fault and extends
20 km to the southeast with a variable strike,
ranging from ;N308W at the northwest end
to ;N808W along its southeast trace. Displacement along this fault has created an escarpment of up to 1000 m in places.
PLIOCENE TO HOLOCENE
VOLCANISM
Approximately 120 samples from the Zitácuaro–Valle de Bravo region were analyzed for
major and trace elements by X-ray fluorescence spectrometry (XRF) and/or wet chemistry (Tables 1, 2; Figs. 2, 3; more complete
analyses are in Tables DR1 and DR21). Eighteen samples were dated by 40Ar/39Ar or radiocarbon techniques (Table 3; Table DR3–see
footnote 1).
The K2O content in lavas found at arcs can
vary by two orders of magnitude, from about
;0.08 wt% in the low-potassium olivine tholeiites that are found in the Cascades (Donnelly-Nolan, 1991), to 8 wt% in the leucitites
in western Mexico (Wallace and Carmichael,
1992). The lavas of Zitácuaro–Valle de Bravo
show a much smaller range in K2O (0.87 to
4.13 wt%); nevertheless, this component, together with TiO2, can be used to define five
compositional groups (Fig. 4): (1) Pliocene
through Quaternary plateau-forming shoshonites with 52–58 wt% SiO2 and K2O .2
wt%; (2) Pliocene through Quaternary shield
volcanoes, cinder cones, and lava flows composed of high-TiO2 (.1.2 wt%) lavas with
51–57 wt% SiO2; (3) Quaternary basaltic andesites (52–57 wt% SiO2, K2O ,2 wt%, and
9–5 wt% MgO); (4) andesites (57–63 wt%
SiO2), commonly having high-MgO and low
Al2O3; and (5) Quaternary dacites with 63–66
wt% SiO2; there is no contemporary rhyolite
(72–77 wt% SiO2) in the area described, but
it is found at Los Azufres to the north.
As the lavas merge petrographically and
compositionally from one group to another,
these arbitrary boundaries are used for convenience and are in general accord with the
current terminology used for the Mexican Volcanic Belt.
Pliocene-Quaternary Shoshonite
The Pliocene shoshonite lava flows that cap
the dominant plateaus of the Zitácuaro region
(Fig. 2) have been found on three mesas; the
farthest northwest mesa, the northern middle
1
GSA Data Repository item 2001109 is available
on the Web at http://www.geosociety.org/pubs/
ft2001.htm. Requests may also be sent to
[email protected].
mesa, and the eastern mesa (Z-162, Z-161,
and Z-105, respectively; Fig. 2). One of the
group (Z-238) has been dated at 3.77 Ma (Fig.
2, Table 3), which is within error of the age
(4.2 6 0.5 Ma) determined by Petersen (1989)
by fission-track dating of a similar lava from
Carrizal (Fig. 2), directly west of the Z-238
locality. An age of 6.0 6 0.7 m.y. was obtained by K-Ar methods for another similar
sample collected from Los Escobas, just west
of the northwesternmost plateau in Figure 2
(Peterson, 1989). Quaternary shoshonite occurs as one lava flow and a small cinder cone
at the current arc front (near San Pedro Tenayac, Fig. 3).
The typical Pliocene shoshonite lava (Z105, Z-107; Table 1) is not fresh; it has small
but abundant (8%–15%) phenocrysts of olivine (Fo84–73) that have ubiquitous reddishbrown iddingsite alteration at their margins.
These phenocrysts are contained in a groundmass of plagioclase (An65–55), which shows a
progressive range in size from microphenocrysts to the tiny laths of holocrystalline
groundmass. Other groundmass phases are augite, accessory Fe-Ti oxides, and subordinate
sanidine; in some samples, a sporadic interstitial residual brown glass riddled with oxide
dust is present. Rare xenocrysts of plagioclase
and augite exhibit reaction textures with the
surrounding matrix. Unlike the type shoshonite (Iddings, 1895; Nicholls and Carmichael, 1969), the Zitácuaro–Valle de Bravo
shoshonite lacks plagioclase phenocrysts,
which, from phase equilibria on a comparable,
but not identical composition (Moore and Carmichael, 1998; Righter and Carmichael,
1996), indicate that magmatic water concentrations of 3–5 wt% may be necessary to suppress the early crystallization of plagioclase.
One lava (Z-238; Table 1), an absarokite
(the most basic member of the high-K series,
,53 wt% SiO2; Gill and Whelan, 1989), contains large, extensively altered olivine phenocrysts (Fo89), together with pale yellow-green
augite phenocrysts, many with darker green
cores, but plagioclase phenocrysts are again
absent. The phenocrysts are set in a coarse
holocrystalline groundmass of distinctly yellow-green augite, plagioclase (;An50), the
smaller crystals being enclosed by sanidine
(Ab45Or49An06), Fe-Ti oxides, and occasional
flakes of Ti-rich (6.7 wt%) fluorphlogopite.
Figure 2. Volcanic deposits and sample localities in the Zitácuaro region (for location, see Fig. 1). Isopach data for the La Dieta pumice
in the Zitácuaro volcanic complex are labeled as follows: the top number is pumice thickness, in centimeters; the bottom number is the
largest pumice fragment size, in centimeters. Contacts and unit names for the complex are based on Capra et al. (1997).
N
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NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
Representative of the petrographically related
Quaternary shoshonite is Z-353 (Table 1),
which has abundant pale yellow-green augite
phenocrysts, with subordinate fresh olivine, in
a brown, glassy groundmass dominated by
plagioclase laths commonly in crude alignment. These lavas are similar in mineralogy
but less magnesian than absarokites of western
Mexico (Carmichael et al., 1996) and are
therefore closer in composition to the nearby
basaltic andesites (Table 1).
Although these lavas contain substantial
amounts of Ni and Cr, none is as rich in either
element as the most Mg-rich basaltic andesites
(Fig. 5), and in comparison to the absarokites
and minettes of western Mexico, they are relatively poor in both. Similarly, the concentrations of Ba and Sr, which are high in this lava
type, never reach the limits of the lavas of
western Mexico (Fig. 6). Along with the highTiO2 samples, the shoshonite samples have the
highest Zr concentrations of the five lava
groups, and they generally have low (4–13
ppm) Nb content (Table 2; Fig. 7).
Pliocene Through Quaternary High-TiO2
Lava Flows, Shield Volcanoes, and Cones
Because the plateaus of high-TiO2 lava
flows just west of the Rio Tuxpan (Fig. 2) in
the Zitácuaro region directly underlie a small
remnant of 3.77 Ma absarokite, these highTiO2 lavas must be Pliocene or older. Younger
high-TiO2 lava flows and scoriae make up the
three shield volcanoes surrounding Zitácuaro
(Fig. 2), which are each ;10 km in basal diameter and about 600 m high (above the basement rocks) and have slope angles of ;78.
Most of the morphology is preserved in the
youngest shield volcano (0.49–0.60 Ma) (Table 3), near Tuxpan; it forms a gently sloping
shield topped by cinder cones with contrasting
steep sides. The original morphology of the
older two shield volcanoes, one south of Benito Juarez dated at 0.95 Ma (Table 3) and the
even older eroded shield near Ocampo (Fig.
2), has been obscured by erosion and soil development, but they have retained ;78 dip angles and eroded cinder cones near their summits. All of the shield volcanoes are
symmetrical, except where they flow into obstructions of an older eroded landscape, and
each has a volume of ;15 km3. Their morphology is similar to the ‘‘Type A’’ shield volcanoes studied by Hasenaka (1994) in the Michoacán-Guanajuato Volcanic Field.
The lack of erosion or soil between the thin
(;1–2 m) lava flows of the Tuxpan shield volcano is evidence for the eruption of a rapid
succession of low-viscosity, voluminous lava
flows, but the same type of high-TiO2 lava
(Table 1) is found in small volume lava flows
and cones near Irimbo, north of Zitácuaro
(Fig. 2). Most of the cones occur along east–
west trends, indicating that their spatial distribution may be fault controlled. This pattern of
east–west alignment is similar to local alignments of cinder cones in the Michoacán-Guanajuato Volcanic Field (Hasenaka and Carmichael, 1985). One cone has been dated at
0.26 Ma (Table 3, Fig. 2).
Most of these lavas are not fresh, and they
show either marginal or extensive replacement
of the olivine phenocrysts by red-brown iddingsite; only in MAS-913 and Z-309 (Table
1) is the alteration of olivine (cores of Fo82–80)
either absent or very slight. In the oldest lava
flow of Tuxpan volcano (Z-177, Table 3), olivine (Fo84) is replaced by carbonate, presumably a consequence of being inundated by a
lake that formed when the Rio Tuxpan was
dammed by subsequent lava flows. Typically,
this group of lavas has olivine phenocrysts
(Fo80–70) with chromite inclusions, which are
accompanied by abundant plagioclase (An64–
52) phenocrysts. The groundmass can be dark
and extremely fine grained, or coarser with subophitic augite, plagioclase, and both acicular
and equant Fe-Ti oxides; interstitial glass riddled with opaque grains is very common.
These lavas contain more than 1.2 wt%
TiO2, and this lower limit separates them from
the other lavas in the Zitácuaro–Valle de Bravo area (Fig. 4). They range in SiO2 from 51
to 57 wt% (Fig. 4; Table 1), but MgO does
not exceed 6 wt%, and in this way they differ
from not only the Mg-rich basaltic andesites
in the area, but also from the high-TiO2 alkali
basalts in the Valley of Mexico (Wallace and
Carmichael, 1999), which contain 6–10 wt%
MgO (Fig. 8). Typically, these lavas contain
normative quartz (Table 1), as their silica concentrations are higher than the alkali basalt
(50–52 wt%) from the Valley of Mexico. The
characteristic trace element assemblage of
these lavas is low Ni and Cr (Fig. 5), low Ba
and Sr (Fig. 6), high Nb (.9 ppm), and 160–
230 ppm Zr (Table 2, Fig. 7), which is the
same range found in the corresponding alkali
basalt of the Valley of Mexico (Wallace and
Carmichael, 1999). In terms of the ratios K2O/
TiO2 and Zr/Ba, the high-TiO2 lavas plot close
to those from Hawaii, but their scatter suggests that they may be transitional to the calcalkalic lavas of the area (Fig. 7).
Quaternary Basaltic-Andesite and
Andesite
Volcanism within the Cerro Gordo complex
(Fig. 3) has occurred ;75 km above the Benioff zone and closer to the Middle America
Trench than any other segment of the central
Mexican Volcanic Belt (Pardo and Suárez,
1995). Farther north, near the town of Zacazonapan (Fig. 3), lava flows and cindercomes
have erupted along the escarpment of the uplifted Cerro El Peñon block of Oligocene ashflow tuffs. Many of these lava flows are preserved as inverted valleys due to differential
erosion of the friable basement schist and the
resistant capping lava flows; the Rio Temescaltepic (Fig. 3) has generated this inverted
topography along the southern fault system.
These relatively flat, lava-capped plateaus
have well-developed soil, and they are almost
entirely cultivated. One of the andesite flows
near Zacazonapan (Z-354; Fig. 3; Table 1) has
been dated at 290 ka (Table 3). On the basis
of the flow morphology, it appears that most
of the other southern flows are of comparable
age.
Small-volume, predominantly andesite
flows and domes exhibit rough alignment
along the Rio Tilostoc and the rugged fault
escarpment of Oligocene ash-flow tuffs (Fig.
3); however, these Quaternary flows and
domes are not evidently affected by motion
along this fault. River erosion has cut through
older flows and left remnants hanging at the
top of what is now a deep, narrow canyon.
The youngest flows, however, erupted at the
bottom of the canyon to form young lava
dams, and upstream lacustrine deposits provide evidence of transitory lakes. One of these
(Z-388; Table 3) erupted at the bottom of the
canyon near Nuevo Santo Tomas de los Plátanos (Fig. 3) and represents the only Holocene activity at 5 ka. This small-volume flow
and the adjacent canyon-bottom flow to the
southeast both have blocky, sparsely vegetated
surfaces, pressure ridges, and sharply defined
flow fronts.
Another young, sparsely vegetated flow (Z342; 22 ka, Table 3) is at the northeast end of
the map area (Fig. 3). However the main pulse
of eruption along the Rio Tilostoc occurred at
about 0.31 to 0.37 Ma, when several of the
Figure 3. Volcanic deposits and sample localities in the Valle de Bravo region (for location, see Fig. 1).
N
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Z228*
9.72 13.96 8.59
3.87 8.17 15.90
9.47 8.45
3.53
3.50 3.22 2.47 2.66
1.35 0.63 0.87 0.66
45
47
50
41
2.00
2.49 3.81 2.76
0.59 0.66 0.94
46
53
47
5.45 11.08 9.00
19.48 15.26 17.80
1.61
10.81
11.92
9.95
0.96
8.21 8.26 9.21
36.43 35.50 33.56
26.50 21.48 22.37
1.10
Z309
Z371*
1.64
0.52
11.89 12.64
19.97 9.22
14.34
2.61
0.92
52
Z358†
Z374†
1.50
0.41
10.05
13.04
8.86
8.38
32.63
25.19
55.20
1.00
15.30
4.05
2.76
0.10
5.80
7.40
3.50
3.10
0.40
N.A.
N.A.
98.61
Z105*
55.60
1.00
15.50
2.28
4.25
0.10
6.50
7.00
4.30
2.50
0.30
N.A.
N.A.
99.33
Z107*
Z353
Z176A*
Z509
Z356†
Z388
Andesite
Z348
Z342
14.92 10.88 7.99
8.88 19.15 9.41
1.79
5.36
3.14
1.62 1.90 1.90 2.19 1.94 1.60 1.85
0.61 0.87 0.66 1.35 1.27 0.46 0.52
68
60
88
65
62
55
13.58 7.50 11.22
29.42 35.59 30.43
20.69 19.64 22.18
3.31 11.95
63.90
0.66
17.10
1.29
2.94
0.08
2.20
4.40
3.80
1.90
0.17
N.A.
N.A.
98.44
Z133*
12.97 16.76 15.63 19.34
1.20
10.98 9.86 11.81 11.22
33.73 25.70 32.12 32.12
18.23 23.68 22.60 20.71
62.70
0.70
16.70
N.A.
4.00
0.10
2.80
5.30
3.80
2.00
0.20
N.A.
N.A.
98.30
Z354†
Z357
Z385
1.26
0.33
1.82 1.21 0.97
1.27 1.27 1.35
0.35 0.26 0.35
64
54
59
1.33
0.44
1.26
0.37
43
Z386
1.14
0.22
0.93 1.54 2.25
1.20 1.18 1.08
0.31 0.28 0.31
54
56
58
1.14
0.37
3.90 6.27
9.96 12.37
2.74
5.65
2.02 1.93
1.20 1.22
0.33 0.33
45
45
6.71
12.99
39.22
15.98
18.54
64.50 60.67 65.40
0.60 0.63 0.64
15.40 17.23 15.87
N.A. 1.39 1.33
3.70 2.26 2.34
0.10 0.06 0.06
3.80 1.87 1.90
5.10 4.20 4.10
4.50 3.98 4.64
1.80 1.81 2.20
0.17 0.15 0.15
N.A. 4.72 0.66
N.A. 1.78 0.15
99.67 99.68 99.44
Z- MAS- MAS390† 907 910
13.84 17.58 18.03 16.81 14.30 18.32
1.45
10.04 11.27 11.27 10.57 10.63 10.68
36.35 32.71 31.11 34.74 38.04 33.64
19.33 19.22 21.60 19.62 16.50 19.85
63.50 63.93 63.61 63.74
0.60 0.63 0.62 0.57
16.00 15.48 16.04 15.89
N.A. 0.64 1.06 1.55
3.90 3.02 2.83 2.67
0.10 0.05 0.06 0.07
3.60 3.49 3.67 3.63
5.50 5.19 5.04 5.12
4.30 3.87 3.68 4.11
1.70 1.91 1.91 1.79
0.10 0.14 0.13 0.14
N.A. 0.82 0.55 0.30
N.A. 0.28 0.39 0.15
99.30 99.45 99.59 99.73
Z346†
Dacite
5.02
6.57 1.26 1.97
5.97 4.54 2.05
18.94 17.87 13.39 18.70 12.35 12.05 12.37 10.54 11.53
11.41 17.63
0.47
10.27 8.68
32.04 26.46
20.71 23.90
52.63 52.05 57.17 59.37 61.94 61.94 62.04 62.17
1.15 1.02 0.84 0.97
0.66 0.67 0.67 0.71
14.15 15.80 15.50 16.11 15.71 15.97 15.26 15.49
6.40 3.69 3.07 2.16 N.A.
1.25 0.83 0.67
0.75 3.53 3.54 3.34
5.21 3.14 3.86 3.83
0.11 0.12 0.09 0.10
0.08 0.07 0.08 0.07
5.60 6.61 5.69 4.14
5.14 5.69 4.48 5.59
8.91 8.74 6.91 6.80
5.61 5.03 5.47 5.30
3.31 3.48 4.21 3.60
3.79 3.13 3.99 3.04
4.13 2.30 1.27 1.90
1.74 1.47 1.86 1.67
0.62 0.58 0.21 0.24
0.15 0.16 0.12 0.16
N.A. 1.42 N.A.
0.30 N.A.
0.50 0.69 0.30
N.A. 0.42 N.A.
0.66 N.A.
0.05 0.22 0.08
97.76 99.76 98.50 99.69 100.04 99.07 99.57 99.08
Z238*
Shoshonite
7.44 18.30 14.76 24.38
29.92 29.59 36.35 21.06
21.86 16.88 15.61 11.55
3.75
11.71 14.12 14.01 23.48
21.27 12.85 5.89
5.55 3.62 9.89 9.31
53.15 54.44 54.85
0.86
0.79
0.85
15.93 17.12 15.20
N.A. N.A.
N.A.
7.74
7.00
7.17
0.12
0.11
0.12
8.86
7.37
9.09
8.17
7.81
7.68
3.63
3.86
3.54
1.26
1.42
1.26
0.24
0.19
0.28
N.A. N.A.
N.A.
N.A. N.A.
N.A.
99.96 100.11 100.04
Z520†
8.97 7.44
30.68 30.68
24.72 23.44
0.21
54.50 54.81 54.64
1.30 1.45
1.37
16.30 16.42 16.68
2.62 1.38
3.01
5.34 6.30
5.04
0.10 0.13
0.12
6.00 5.60
5.49
7.50 7.30
8.44
4.20 3.97
3.63
1.40 1.56
1.52
0.30 0.43
0.42
N.A. 0.32 N.A.
N.A. 0.07 N.A.
99.56 99.74 100.36
Z269*
6.85 6.85 7.67 9.44
29.92 34.91 33.81 32.12
25.64 26.90 22.13 25.41
3.24
51.43 50.95 52.50 55.20 55.78
1.84 1.69 1.30 1.40
1.31
16.48 17.91 16.10 17.30 18.31
3.76 2.43 2.28 1.74
4.09
7.11 6.46 5.75 5.43
4.37
0.16 0.14 0.10 0.10
0.12
5.72 5.74 5.70 3.80
3.77
8.60 8.22 8.40 7.60
7.01
3.54 4.13 4.00 3.80
4.31
1.16 1.16 1.30 1.60
1.39
0.62 0.29 0.40 0.30
0.27
N.A.
0.42 N.A. N.A. N.A.
N.A.
0.02 N.A. N.A. N.A.
100.42 99.56 97.83 98.27 100.73
Z227*
Basaltic andesite
TABLE 1. ANALYSES WITH CIPW NORMS FOR REPRESENTATIVE WHOLE-ROCK SAMPLES
Note: Reported oxide data are in wt%. Samples were ground to a fine powder in a tungsten carbide mill. Underlined samples were analyzed by I.S.E. Carmichael using wet chemical techniques with 2s precision of: 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). Alkali flame photometry was done by J. Hampel and D. Blatter. N.A.—not analyzed.
*XRF analyses were performed by D. Blatter and T. Teague using a SpecTrace 440 X-ray spectrometer on powdered samples fused with lithium tetraborate flux. Standards were several natural 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%).
†
XRF analyses were performed using the Phillips PW2400 wavelength dispersive X-ray Spectrometer at U.C. Berkeley on powdered samples fused with lithium tetraborate flux. Standards were as above. Average uncertainties
based on replicate analyses are equal to the following percentage 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%).
§
FeO is total iron in samples where Fe2O3 is not reported; where Fe2O3 is reported, FeO was analyzed by titration.
#
Mg#—MgO/(MgO 1 FeO).
Q
Co
Or
Ab
An
Ne
Di
Hy
Ol
Mt
Il
Ap
Mg#
SiO2
TiO2
Al2O3
Fe2O3
FeO§
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O1
H2O–
Total
CIPW
ZMAS- Z239C* 913 177*
High-TiO2 Lavas
BLATTER et al.
Note: Reported results are in parts per million (ppm). Samples were ground to a fine powder in a tungsten carbide mill. XRF—X-ray fluorescence spectrometry.
*XRF analyses were performed by D. Blatter and T. Teague using a SpecTrace 440 X-ray spectrometer on powdered samples mixed with polyvinyl alcohol. Standards were several natural lavas, including USGS standards.
Average uncertainties of one standard deviation based on duplicate analyses of approximately 50 samples (Righter and Carmichael, 1992) and are equal to the following percentages of the amounts present: V (2.8%), Cr (4.3%),
Ni (6.4%), Cu (8.1%), Zn (3.4%), Rb (5.4%), Sr (0.55%), Y (4.5%), Zr (2.6%), Ba (0.57%), La (8.6%), Ce (3.0%).
†
XRF analyses were performed using a Phillips PW2400 wavelength dispersive X-ray spectrometer on powdered samples mixed with polyvinyl alcohol. Standards were as above. Average uncertainties, based on duplicate
analyses of 30 samples are equal to the following percentages of the amounts present: V (2.80%), Cr (9.63%), Ni (1.00%), Cu (1.66%), Zn (0.60%), Rb (1.68%), Sr (0.17%), Y (0.53%), Zr (0.25%), Ba (2.32%), La (25.39%), Ce
(9.80%).
§
Nb analyzed by ICP-MS by WSU GeoAnalytical Laboratory, precision is 4.7%. Samples for ICP-MS analyis were ground to a fine powder in a steel mill.
105
143
121
108
124
131
96
99
94
90
82
102
95
90
78
255
117
22
236
144
170
257
42
111
128
154
142
144
64
29
124
60
25
162
59
57
176
19
73
104
82
89
96
43
16
36
48
5
17
19
20
22
9
6
11
19
12
16
8
6
73
80
54
56
57
66
62
66
43
57
55
60
56
79
52
15
23
31
28
30
24
31
37
22
27
31
27
33
36
20
853
1273
837
644
544
762
753
506
855
915
793
720
924
659
789
20
22
16
14
18
12
14
17
14
16
18
17
15
15
16
103
159
124
119
125
129
130
126
100
126
132
136
135
128
114
453
455
386
382
376
399
393
493
371
385
380
374
381
548
519
24
29
13
13
14
18
14
10
11
16
23
17
18
15
19
45
61
28
27
25
28
28
26
25
32
31
33
32
35
32
3.49
4.53 3.77 3.73 3.42 3.73 3.28
4.63 2.83 4.12 4.03 3.78 3.93 4.61 4.41
147
167
151
180
378
332
182
246
171
160
86
155
55
49
47
40
80
74
69
99
71
56
62
26
816
727
1322
1405
20
17
32
27
290
220
297
152
709
522
1027
998
31
19
68
52
60
52
143
106
3.97 3.88
7.05
5.52
187
166
150
469
289
491
196
146
220
50
40
41
79
72
71
19
23
13
603
620
838
17
22
15
129
98
130
342
432
489
37
16
19
42
32
50
3.39 3.55 4.28
V 177
180
197
210
181
182
211
180
Cr 162
48
186
49
10
162
154
110
Ni 38
46
84
15
17
76
74
35
Cu 23
27
25
20
27
30
26
24
Zn 94
87
82
83
84
90
80
82
Rb 22
17
21
29
24
23
26
21
Sr 457
563
772
631
584
619
614
822
Y
37
22
24
29
20
25
26
32
Zr 311
169
189
197
163
180
214
229
Ba 406
244
342
385
338
365
419
412
La 20
17
19
25
13
22
23
30
Ce 84
35
47
44
30
44
54
67
Nb§ 18.91 12.70 10.62 10.90 8.97 9.75 17.12 15.53
Z386†
Z385†
Z354†
Z342†
Z348†
Z388†
Andesite
Z356†
Z509†
Z176A*
Z353†
Z238*
Shoshonite
Z107*
Z105*
Z374†
Z358†
Z520†
Basaltic andesite
Z371*
Z309†
Z269*
Z228*
High-TiO2 lavas
Z227*
ZMASZ239C* 913* 177*
TABLE 2. XRF TRACE ELEMENT DATA FOR REPRESENTATIVE WHOLE-ROCK SAMPLES
Z133*
Z346†
Z357†
Dacite
ZMAS- MAS390† 907* 910*
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
flows (Z-346, Z-348, Z-356, and Z-386) erupted (Fig. 3; Table 3). These ;300-k.y.-old
flows are cut by the river canyon, and their
surfaces cap the tops of the cliffs on either
side of the canyon. Little of the original volcanic surface morphology is preserved on
these flows, although generally the volcanic
vent can be identified.
As these lavas were being erupted along the
Rio Tilostoc, a group of andesites, notably
plagioclase free and with abundant quartz xenocrysts, erupted southwest of Zitácuaro (Fig.
2) over the same period (0.24–0.38 Ma; Blatter and Carmichael, 1998a). Several of these
andesites form clusters of small-volume flows
(0.05 to 0.5 km3) and cones (Fig. 2).
Within the general andesite group (basaltic
andesite and andesite), there is considerable
petrographic variation; the basaltic-andesites
(SiO2 ,57 wt%) are similar to the high-TiO2
lavas, and they may have small abundant olivine phenocrysts that are partially replaced
by alteration (Z-176a), or quite fresh (Z-520;
Tables 1 and 2), with or without less abundant
augite, enclosed in a groundmass of plagioclase, augite, Fe-Ti oxides, and interstitial
glass.
Several of the more siliceous andesites have
closely similar compositions (Z-342, Z-348,
Z-356, Z-388; Table 1) and yet have very different phenocryst assemblages. These variations are: orthopyroxene (opx) alone, opx and
augite, plagioclase with augite and opx, hornblende and opx, or hornblende with opx and
augite, all contained in a brown glassy
groundmass with abundant plagioclase, commonly flow aligned. Experiments demonstrate
that these phenocryst assemblages are the result of varying water concentrations (Blatter
and Carmichael, 2001). Many of the andesites
contain quartz xenocrysts, which generally
have augite reaction rims (Blatter and Carmichael, 1998a), together with opx 6 augite,
plagioclase, and hornblende phenocrysts in a
groundmass of plagioclase, Fe-Ti oxides, pyroxene, and interstitial glass.
Because these andesites range from 52 to
63 wt% SiO2 (Fig. 4), in petrographic character they extend from basaltic-looking lava
with almost 10 wt% MgO, and correspondingly high Cr (400–500 ppm) but lower Ni
(Fig. 5), to almost dacitic lava with only 2
wt% MgO. Among this group is a distinctive
cluster of siliceous andesites that erupted
along the Rio Tilostoc (Fig. 3) with 62 wt%
SiO2 (Z-342, Z-348, Z-356, Z-388; Table 1);
they are unusually Mg rich (5.7–4.5 wt%) and
Al poor (;15.7 wt%) compared to andesites,
for example, from Volcán Colima (3.2–2.6
wt% MgO; .17.2 wt% Al2O3) (Luhr and Car-
Geological Society of America Bulletin, October 2001
1331
1332
19823.59
19825.39
19828.89
19833.09
19823.89
19833.39
19814.79
19843.09
19815.89
19810.29
19811.59
19804.79
19810.79
;19803.09
19810.09
19811.09
19843.09
19843.09
;19824.09
;19828.59
19832.09
;19825.09
;19825.09
;19825.09
Latitude
100826.79
100825.49
100829.89
100828.39
100830.99
100827.49
100826.29
100827.99
100802.39
100818.09
100811.59
100815.39
100814.09
;100809.09
100813.09
100815.39
100827.99
100827.99
100833.09
100833.09
100824.09
100820.09
100815.09
100815.09
Longitude
Location
Andesite
Andesite
Basalt
Andesite
Hi-K Basalt
Andesite
Andesite
Basalt
Andesite
Dacite
Andesite
Andesite
Andesite
Rhyolite
Dacite
Andesite
Dacite
Dacite
Andesite
Andesite
Dacite
Dacite
Dacite
Dacite
Rock type
Gms
Gms
Gms
Gms
Gms
Gms
Gms
Gms
Gms
Gms
Plag
Gms
Gms
San
Gms
Gms
Charcoal
Charcoal
Zircon
Hbl
Unknown
Unknown
Charcoal
Charcoal
Material
dated
40
Ar/39Ar
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
40
Ar/39Ar
Radiocarbon
Radiocarbon
Fission track
K-Ar
K-Ar
K-Ar
Radiocarbon
Radiocarbon
40
1.2
0.5
0.04
0.02
0.05
0.01
0.03
0.01
0.006
0.005
0.7
0.12
0.12
0.14
0.13
0.013
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
0.38 6 0.02
0.24 6 0.14
0.60 6 0.03
0.49 6 0.01
3.77 6 0.05
0.52 6 0.01
0.95 6 0.04
0.26 6 0.02
0.022 6 0.004
0.313 6 0.006
0.37 6 0.03
0.29 6 0.12
0.310 6 0.007
34.87 6 0.15
0.35 6 0.13
0.005 6 0.002
.45,650
39 290 6 460
4.2 6 0.5
6.0 6 0.7
0.05 6 0.03
0.67 6 0.10
31 350 6 1,500
30 630 6 520
3.2
3.3
0.70
0.41
3.75
0.51
0.95
0.26
0.028
0.282
0.4
0.31
0.31
34.92
0.62
–0.028
Integrated
age
Preferred
age*
TABLE 3. GEOCHRONOLOGY SUMMARY
Dating
method
0.38 6 0.02
Not detected
0.60 6 0.03
0.49 6 0.01
3.77 6 0.05
0.52 6 0.01
0.95 6 0.04
0.26 6 0.02
0.22 6 0.004
Not detected
0.37 6 0.03
0.29 6 0.12
0.310 6 0.007
34.87 6 0.15
0.35 6 0.13
–0.002 6 0.004
Plateau
age
3
7
4
5
6
12
7
10
9
6
10
4
6
10
Number
of steps
0.371
0.236
0.542
0.545
3.759
0.545
0.963
0.311
0.0211
0.3131
0.4303
0.0275
0.3150
35.076
–0.391
0.0052
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
0.04
0.14
0.09
0.07
0.05
0.01
0.05
0.03
0.0053
0.0063
0.0371
0.0090
0.0087
0.1440
0.099
0.0023
Isochron
age
0.84
0.62
0.57
0.21
0.79
0.53
1.20
0.68
0.23
3.52
0.63
1.85
0.63
1.01
1.41
0.35
MSWD†
297.0
313.3
298.0
290.4
304.8
291.9
294.7
292.1
295.9
289.9
286.4
298.8
294.1
241
302.7
292.9
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
3.1
7.1
4.1
5.7
13.0
1.2
3.1
2.2
1.9
1.5
5.8
1.6
1.8
17.7
1.6
1.1
Initial
Ar/36Ar
40
Note: Gms—groundmass, Plag—plagioclase, Hbl—hornblende, San—sanidine.
*40Ar/39Ar, K-Ar, and fission track ages are in million years; radiocarbon ages are in years before present (1950).
†
Mean square of weighted deviates.
§
Ages were obtained at the Berkeley Geochronology Center. Isotopic data are in the GSA Data Repository (Table DR3).
#
Radiocarbon ages were obtained by Beta Analytic Inc. using acid-alkali-acid pretreatment. Extended counting times were used for ZC-1 and the accelerator mass spectrometry technique was used for ZC-11.
**Dates from Peterson (1989).
††
Dates from Demant et al. (1975).
§§
Dates from Comisión Federal de Electricidad (1986).
##
Dates from Capra et al. (1997).
Z-111§
Z-129§
Z-177§
Z-227§
Z-238§
Z-269§
Z-309§
MAS-913§
Z-342§
Z-346§
Z-348§
Z-354§
Z-356§
Z-365§
Z-386§
Z-388§
ZC-1#
ZC-11#
Carrizal**
Escobas**
Zirahuato††
Candelero§§
La Dieta##
La Dieta##
Sample no.
BLATTER et al.
michael, 1980). The high MgO in these Valle
de Bravo andesites is accompanied by corresponding levels of Ni (176–57 ppm; Table 3)
and Cr (236–144 ppm; Table 3).
Quaternary Dacites
Directly east of Zitácuaro is a complex of
domes and pyroclastic deposits that extends
;20 km to the east, to the town of Villa de
Allende (Fig. 2). Capra et al. (1997) interpreted this dome complex to be the result of
Pliocene to Quaternary resurgent growth inside a middle Miocene caldera structure.
The pyroclastic flow, ignimbrite, and airfall
deposits that originated from this complex
cover an area of about 700 km2 (Capra et
al., 1997).
Several of these units have been dated by
the Comisión Federal de Electricidad (1986)
and yielded ages between 5 and 0.58 Ma.
The La Soledad unit covers nearly 175 km2
and is composed of massive pyroclastic flow
deposits, some with basal surge units (Capra
et al., 1997). These voluminous (;9 km3)
pyroclastic deposits emanate in three principal directions down slope from a central
dome area in the middle of the Zitácuaro
Volcanic Complex (Fig. 2). The age of the
La Soledad unit is constrained to be .45
650 yr B.P. by charcoal found within the deposit near Llano Redondo de Zaragoza and
,0.67 Ma, which is the age of an adjacent,
older unit that was dated by the Comisión
Federal de Electricidad (1986) (Table 3).
The overlying La Dieta pumice (Fig. 3)
blankets an area of ;100 km2. Charcoal collected from a paleosol directly below this
unit near Macho de Agua yielded an age of
39 290 yr B.P (Table 3). Radiocarbon dates
from Capra et al. (1997) on charcoal found
within the La Dieta pumice deposit are 31
350 and 30 630 yr. B.P (Table 3). From the
size of pumice clasts and the deposit thickness, the source of La Dieta pumice is probably a northeast flank vent from near the
center of the Zitácuaro volcanic belt, where
the pumice deposit thickens to .2500 cm.
The estimated volume of La Dieta pumice
is ;1.5 km3.
A smaller dacitic dome complex is found
to the north of Zitácuaro (Fig. 2). These
domes are small, most having basal diameters less than 2 km, heights between 140 and
600 m, and volumes ranging from 0.1 to 1.2
km3. Several of the domes are aligned in an
east-west direction, possibly along faults
(Fig. 2). Surrounding these domes are pyroclastic flow and fall deposits. The southernmost Zirahuato dome (Fig. 2) has been
Geological Society of America Bulletin, October 2001
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
Figure 4. Analyses for the five major lava groups in the Zitácuaro–Valle de Bravo area. The data are from Table 1 and GSA Data
Repository Table DR1 (see footnote 1).
dated by Demant et al. (1975) at 0.050 Ma
(Table 3).
The typical dacite is porphyritic with abundant large phenocrysts of zoned plagioclase,
brown or green hornblende 6 opx (En57)
(MAS-907, MAS-910, Z-133; Table 1). There
are examples with no plagioclase phenocrysts
(Z-346, Z-385, Z-386; Table 1) but with orthopyroxene and/or augite accompanying the
hornblende, which in some lavas may be com-
pletely replaced by oxides (Z-346). The
groundmass ranges from a clear vesicular
glass (MAS-907) to an assemblage of plagioclase, Fe-Ti oxides, glass, and augite in some
samples. Xenocrysts of plagioclase and quartz
Figure 5. Ni and Cr plotted against MgO for the lavas (except dacites) from Zitácuaro–Valle de Bravo (Table 2 and DR2; see footnote
1) and lavas from the Mascota area of western Mexico (Carmichael et al., 1996).
Geological Society of America Bulletin, October 2001
1333
BLATTER et al.
Figure 6. Ba plotted against Sr for the lavas (except dacites) from Zitácuaro–Valle de
Bravo (Table 2 and Table DR2; see footnote 1) and lavas from the Mascota area of western
Mexico (Carmichael et al., 1996).
exhibit reaction textures with the surrounding
matrix and are common but not ubiquitous,
and some lavas have plagioclase phenocrysts
with a narrow zone of concentric inclusions
with clear overgrowths, indicating mixing or
assimilation.
The dacites (63.2 to 65.9 wt% SiO2; Fig. 4)
of the two dome complexes to the east and
north of Zitácuaro (Fig. 2), differ from those
of the Valle de Bravo region (Fig. 3). Quaternary dacites from the Valle de Bravo (Z-346,
Z-357, Z-385, Z-386, Z-390; Table 1) are very
distinctive in their high MgO (3.5–3.8 wt%),
Cr (110–155 ppm) and Ni (70–100 ppm), in
clear contrast to those from the Zitácuaro
dome complexes (Z-133, MAS-907, MAS-
1334
910; Table 1), which have lower concentrations of Mg, Ni, and Cr, as well as about 1
wt% lower CaO (Table 1).
MAGMA ERUPTION RATE
Because older lava tends to be buried by
younger lava or fragmental deposits, or eroded
by the extensive drainage systems, we have
confined our estimates of eruptive volume to
lavas younger than 1 m.y. Although we do not
know the age of all the volcanic deposits in
the Zitácuaro Valle de Bravo region, we believe that we have made reasonable estimates
of the ages of undated deposits by comparison
of their morphologic features to deposits of
known age, and by using field relations (Table
3). We calculated the magma volumes of the
.1 Ma volcanic deposits in the Zitácuaro Valle de Bravo region following the methods of
Hasenaka (1994); those methods yield uncertainties of ;20% (the difference between
model volume calculations and volumes calculated from actual volcanic profiles). We also
used the correction factors from Hasenaka
(1994) to convert lava, cinder cones, and tephra to dense-rock (magma) equivalents (Table
4). The total magma volumes are: high-TiO2
lava ;24 km3; shoshonite ;0.85 km3; basaltic
andesite, ;6 km3; andesite, ;13 km3; and
dacite, ;45 km3. The dacite fragmental deposits have a minimum estimated volume of
;14 km3; this estimate is one of considerable
uncertainty due to ambiguous underlying topography, and we did not account for ash not
observed. The dacite volume translates to a
substantial fraction of the total volume (Table
4), which is not the case in the MichoacánGuanajuato Volcanic Field, where dacite is an
insignificant magma type (Hasenaka and Carmichael, 1985, 1987). We estimate that the total volume of magma erupted in the past 1
m.y. is ;88 km3. If this volume rate per 1 m.y.
is then divided by the length of arc (;50 km)
that the mapped region projects onto the arc
(Fig. 1), the result is ;1.8 km3 · m.y.–1 · km–1.
Because we have not yet mapped the back part
of the arc (Fig. 1) and have not estimated unobserved ash volumes from the dacite, the
eruption volume estimated for this section
should be considered a minimum value. Both
the continuity and youth of the volcanism (5
ka) suggest that more volcanism should be expected in the Zitácuaro–Valle de Bravo region.
Eruption rates for Western Mexico (Table 5)
range over almost two orders of magnitude,
from a low of 0.14 km3 · m.y.–1 · km–1 in the
Talpa-Mascota arc-front graben, where minettes are abundant, to a high at Volcán Colima (;9 km3 · m.y.–1 · km–1), which is dominated by andesitic eruptions since the
collapse of the caldera. The eruption rate at
Colima is close to that of the Mt. Adams volcano in the Cascades, but both are far below
the eruption rates of the Hawaii volcanoes or
the Columbia River basalts (Table 5).
SPATIAL DISTRIBUTION OF LAVAS
The Zitácuaro–Valle de Bravo area is a segment of the Mexican Volcanic Belt ;50 km
along the strike of the arc, and about 100 km
wide (Fig. 9). The number of cones or volcanoes erupted in the past 1 m.y. is about 107,
giving a cone density, within the area of
;5000 km2, of 2.1 · m.y.–1 · 100 km–2. This
Geological Society of America Bulletin, October 2001
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
cone density is slightly lower than the cone
density (2.6 · m.y.–1 · 100 km–2) in the Michoacán-Guanajuato Volcanic Field (Hasenaka
and Carmichael, 1985). The lower cone density in the Zitácuaro–Valle de Bravo area reflects the large volume of magma associated
with the dacite eruptions.
The distribution of centers in the two areas
is similar; in the Michoacán-Guanajuato Volcanic Field the distribution of ;1000 volcanoes is related to their distance from the Middle America Trench; their greatest
concentration is at about 250 km from the
trench, but none is closer than ;190 km (volcanic front). The number of eruptive centers
falls off with distance from the trench, and
there are only two or three cones farther than
;200 km from the volcanic front. In the Zitácuaro–Valle de Bravo region, the highest
concentration of eruptive centers is between
300 and 350 km from the trench, and none is
closer than about 280 km (Fig. 9). The volcanic front in this area is farther from the
trench than to the west in the Michoacán-Guanajuato Volcanic Field because the dip of the
Benioff zone (Fig. 1) shallows under this part
of Mexico, in contrast to the west (Pardo and
Suárez, 1995).
Trenchward migration of volcanism with
time has been reported in several regions of
the Mexican Volcanic Belt; including: Nevado
de Colima–Volcán Colima (Luhr and Carmichael, 1980) in western Mexico, Iztaccı́huatlPopocatépetl (Nixon, 1989) in central Mexico,
and within the Michoacán-Guanajuato volcanic field (Hasenaka, 1994; Delgado Granados
et al., 1995). Hasenaka (1994) stated that volcanism in the Michoacán-Guanajuato Volcanic
Field migrated trenchward around 1 Ma; however, magnetostratigraphic studies (Delgado
Granados et al., 1995) indicate that trenchward migration occurred around 0.78 Ma. Because we do not have data for the back part
of the arc, trenchward migration of volcanism
in the Zitácuaro–Valle de Bravo region cannot
be clearly shown. However, we do observe
that the most recent eruptions in that region
have occurred near the arc front (0.005 and
0.022 Ma; Fig. 9).
The pattern of compositional variation
within the Zitácuaro–Valle de Bravo region
(Fig. 9) parallels that of the Michoacán-Guanajuato Volcanic Field (Hasenaka and Carmichael, 1987). The ,1 Ma, high-MgO basaltic andesite and shoshonite within the
Zitácuaro–Valle de Bravo region correspond
compositionally to the high-MgO (.9 wt%)
calc-alkalic and alkalic lavas within the Michoacán-Guanajuato Volcanic Field. In both
arc segments, these lava types are found clos-
Figure 7. K2O/TiO2 plotted against Zr/Ba (Tables 1 and 2; Data Repository Tables DR1
and 2; see footnote 1) for the five groups of lavas from Zitácuaro–Valle de Bravo together
with examples from Hawaii taken from Clague and Dalrymple (1988) and Chen et al.
(1991). Values of Nb and Zr for Zitácuaro–Valle de Bravo lavas are in the lower plot.
TABLE 4. ESTIMATES OF VOLUMES OF VOLCANIC DEPOSITS AND MAGMA OUTPUT
Composition
High-TiO2
High-TiO2
High-TiO2
Shoshonite
Shoshonite
Shoshonite
Basaltic andesite
Basaltic andesite
Basaltic andesite
Andesite
Andesite
Andesite
Dacite
Dacite
Dacite
Total
Type of deposit
Shield volcanoes
Cinder cones
Tephra and ash§
Lava flows
Cinder cones
Tephra and ash
Lava flows
Cinder cones
Tephra and ash
Lava flows
Cinder cones
Tephra and ash
Domes
Observed pyroclastic
Observed air fall**
Number of
centers
2
9
1
1
8
0#
24
10
53
107
Volume: dense
rock equivalent*
(km3)
Total output rate†
(km3 · m.y.–1 · km–1)
21
0.3
2.4
0.5
0.04
0.31
2.0
0.4
3.3
7.5
0.6
4.7
31
12
2
;88
0.42
0.006
0.048
0.01
0.0008
0.0062
0.04
0.008
0.066
0.15
0.012
0.094
0.62
0.24
0.04
;1.8
*Total volumes were estimated according to the methods described in Hasenaka (1994). Factors of 0.7 (lava),
0.5 (cinder cone), and 0.5 (tephra) were multiplied by total volumes to convert to dense rock equivalents.
†
Total output rate was calculated by dividing the total volume by 1 m.y. and 50 km.
§
Tephra and ash volume was estimated to be 7.7 times that of a cinder cone (Hasenaka, 1994).
#
Each basaltic andesite cinder cone is associated with a flow.
**Minimum estimate.
Geological Society of America Bulletin, October 2001
1335
BLATTER et al.
er to the trench than the less-Mg-rich varieties. The ,1 Ma, high-TiO2 lavas (with low
Mg contents; Table 1) from within the Zitácuaro–Valle de Bravo region correspond compositionally and spatially (Fig. 9) to the lowMg alkalic samples (.2.0 wt% TiO2) from
the Michoacán-Guanajuato Volcanic Field
that are found farthest from the trench in both
regions.
Within the Zitácuaro–Valle de Bravo region, dacites (.63 wt% SiO2) are generally
found between 20 and 70 km from the volcanic front, and they show a curious pattern.
Those associated with the linear cluster of
andesites along the Rio Tilostoc in the Valle
de Bravo area (Fig. 3) are, like their associated andesites, without plagioclase phenocrysts and rich in MgO, Ni, and Cr. Those to
the east and north of Zitácuaro (Fig. 2) are
distinguished by their abundant plagioclase
phenocrysts and lower ferromagnesian components. The abundance of dacite in the Zitácuaro–Valle de Bravo region (Figs. 2, 3)
has no counterpart in the Michoacán-Guanajuato Volcanic Field, where dacite magma is
insignificant.
Contrasts Between Volcanism in Western
Mexico and the Zitácuaro–Valle de Bravo
Region
Figure 8. Zr and TiO2 plotted against MgO for the high-TiO2 lavas from the Zitácuaro–
Valle de Bravo area (Tables 1 and 2; Tables DR 1 and 2; see footnote 1) together with
the corresponding alkaline basalts from the Valley of Mexico (Wallace and Carmichael,
1999).
TABLE 5. MAGMA ERUPTION RATES
Location
Comment
Reference
Leucite Hills, Wyoming
Mascota-Talpa, Mexico
Zitácuaro-Valle de Bravo,
Mexico
Michoacán-Guanajuato, Mexico
Michoacán-Guanajuato, Mexico
Mount Adams, Washington
1 Ma to present
0.04 Ma to present
Total
Mount Adams, Washington
Background trickle
Volcán Colima complex, Mexico
Mid-Atlantic Ridge
Famous
Iceland
Postglacial calc. to 1
Ma
Hawaii
Mauna Loa, Kilauea
Columbia River
Grande Ronde
1336
Magma eruption rate
(km3 · m.y.–1 · km–1)
Lange et al. (2000)
Lange et al. (1999)
This paper
0.01
0.14
1.8
Hasenaka (1994)
Hasenaka (1994)
Hildreth and Lanphere
(1994)
Hildreth and Lanphere
(1994)
Luhr and Carmichael (1980)
Moore et al. (1974)
Jakobsson (1972)
3.5
6.0
6.3
Crisp (1984)
Tolan et al. (1989)
1.0
9
8.8
92
;800
;510
The western part of the Mexican Volcanic
Belt (west of the Tzitzio Anticline volcanic
gap; Fig. 1) has been extensively studied, and
two distinct types of volcanism have been
recognized there. Among the lava flows, central volcanoes, and cinder cones of the Michoacán-Guanajuato Volcanic Field (Hasenaka and Carmichael, 1985, 1987), Volcán
Colima (Luhr and Carmichael, 1980), Mascota (Lange and Carmichael, 1990; Carmichael et al., 1996), San Sebastián (Lange and
Carmichael, 1991), and Los Volcanes (Wallace and Carmichael, 1989), calc-alkalic basaltic-andesite, andesite, and dacite predominate. The subordinate alkalic varieties such
as absarokite and minette are found mainly
within small, arc-front grabens in the interior
of the Jalisco block (Fig. 1), and they are
invariably flows and cones of small volume.
There is the question of why these highly alkaline absarokites and particularly minettes,
with their great enrichment in LILEs (Fig. 6),
are confined to the Jalisco block, and are not
found in central Mexico. Only less extreme
alkaline lavas are found in the MichoacánGuanajuato volcanic field (Hasenaka and
Carmichael, 1987), the Zitácuaro–Valle de
Bravo region, and the Valley of Mexico
Geological Society of America Bulletin, October 2001
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
is considered to have been through a prior
cycle of magma genesis, because the calcalkaline lavas there are characteristically low
in CaO and Al2O3 and high in SiO2, indicative of a refractory source (Wallace and Carmichael, 1999). Similar features are found in
the siliceous andesite of the Valle de Bravo
region (Table 1), and the high MgO and low
Al2O3 could reflect the hydrous melting of a
refractory mantle (Blatter and Carmichael,
2001). Thus, the absence of such extremely
alkaline lava types as minette in the Michoacán-Guanajuato Volcanic Field, Valley of
Mexico, and Zitácuaro–Valle de Bravo regions could perhaps be attributed to the underlying refractory mantle, which, although
enriched by the subduction process, was not
endowed with those components in the abundance required to form minette magma such
as that in the Jalisco block of western
Mexico.
CONCLUSIONS
Figure 9. Distribution of ,1 Ma volcanic centers in the Zitácuaro–Valle de Bravo area
by magma type. Dashed contours show the distance from the Middle America Trench
(after Pardo and Suárez, 1995).
(Wallace and Carmichael, 1999), typically
close to the volcanic front.
There are two broad conjectures to account
for these observations; one is that the extensional environment in western Mexico, represented by the grabens to which the extremely alkaline flows are confined, provides
favorable pathways for the ascent of magmas
of small volume, without freezing en route.
The underlying assumption is that highly alkaline magmas are generated at depth
throughout the Mexican volcanic belt, but
they are unable to ascend except in tectonically favored localities. The second conjecture is that the pre-enrichment mantle is different in western Mexico from the mantle
that underlies the very thick crust (;50 km,
Wallace and Carmichael, 1999) of the Valley
of Mexico and, presumably, also underlies
the Zitácuaro–Valle de Bravo region.
Although it may be plausible to attribute
the absence of alkaline lavas, such as minettes, to the absence of the tectonic pathways
in the Zitácuaro–Valle de Bravo region, the
presence of hornblende peridotite nodules in
an andesite lava in the El Peñon area (Blatter
and Carmichael, 1998b) indicates that such
pathways do exist. The rapid ascent of this
hydrous andesite magma is clearly required
if it is not to lose the dense entrained xenoliths en route.
The mantle source region in western Mexico may be quite different from that in central
Mexico; the nearby Valley of Mexico mantle
As more segments of the young and active Mexican Volcanic Belt (Fig. 1) are examined, it becomes evident that there are
significant contrasts in the character of volcanism within different segments. Although
calc-alkaline basaltic andesite, andesite, and
dacite predominate across the entire Mexican volcanic belt, two aspects of the Zitácuaro–Valle de Bravo volcanism are unique
in comparison to the neighboring segments
of the Valley of Mexico to the east and the
Michoacán-Guanajuato Volcanic Field to
the west (Fig. 1). First, shoshonite is found
in the Zitácuaro–Valle de Bravo (Figs. 2, 3),
but not in the Valley of Mexico, and only
sporadic examples are present in the Michoacán-Guanajuato Volcanic Field (Hasenaka and Carmichael, 1987). In the Michoacán-Guanajuato Volcanic Field, the
shoshonite is concentrated 350–400 km
from the trench, far behind the volcanic
front, whereas the youngest shoshonite lava
in the Zitácuaro–Valle de Bravo region defines the volcanic front, just as the alkalic
lava (minette and absarokite) does in the
Jalisco block of western Mexico (Fig. 1).
Second, the presence of andesite lava with
small nodules of hornblende-spinel-lherzo-
Figure 10. Apparent age spectra from 40Ar/39Ar step heating data for groundmass separates from Zitácuaro–Valle de Bravo samples.
The individual heating increments are labeled with their identifying step letters and temperatures (8C), or watts in the case of Z-309
and Z-365. The Ca/K ratios for each increment are also plotted as a function of cumulative percent 39Ar released. Age uncertainties on
individual steps are 2 s and do not include uncertainty in J, the neutron fluence parameter. Quoted plateau ages and integrated ages
are also 2 s, and include analytical uncertainties in J.
M
Geological Society of America Bulletin, October 2001
1337
BLATTER et al.
Figure 10. (Caption on p. 1337.)
1338
Geological Society of America Bulletin, October 2001
NEOGENE VOLCANISM AT THE FRONT OF THE CENTRAL MEXICAN VOLCANIC BELT
Figure 10. (Continued).
Geological Society of America Bulletin, October 2001
1339
BLATTER et al.
lite (Blatter and Carmichael, 1998b) is
unique to the Zitácuaro–Valle de Bravo, and
it suggests that andesite magmas can indeed
be generated in the mantle. The most likely
candidates for this mantle origin are richer
in MgO, Ni, and Cr than their andesitic
counterparts in western Mexico (Blatter and
Carmichael, 2001).
Another contrast between the MichoacánGuanajuato Volcanic Field and the Zitácuaro–
Valle de Bravo is the somewhat smaller output
of eruptive material (cubic kilometers per 1
m.y. per 1 km) in the latter (Table 4). However, because our estimates of the Zitácuaro–
Valle de Bravo eruption volumes are minimal
values and the volcanic deposits near the back
of the arc have not been analyzed or dated,
the output rates of the Zitácuaro–Valle de Bravo and the Michoacán-Guanajuato Volcanic
Field may prove to be more similar than our
current estimation suggests. Furthermore, dacite, which is of trivial volume in the Michoacán-Guanajuato Volcanic Field, forms a
substantial fraction of the Zitácuaro–Valle de
Bravo erupted volume. This high proportion
of dacite is similar to that of the Valley of
Mexico, where Volcán Toluca is just one example of several imposing volcanoes having
significant volumes of dacite (Macı́as et al.,
1997).
The contemporaneous juxtaposition of
oceanic-island-type and subduction-related
volcanism is a characteristic feature of volcanism in Mexico, and evidence of it is
found in the intersection of the Mexican
volcanic belt with the Gulf of California
(Righter and Carmichael, 1992), in the
northwest arm of the belt to Guadalajara
(Nelson and Carmichael, 1984; Nelson and
Livieres, 1986; Righter and Carmichael,
1992; Moore et al., 1994) (Fig. 1), and from
there to the Valley of Mexico (Wallace and
Carmichael, 1999), including the Zitácuaro–
Valle de Bravo region. The high-TiO2 lavas
of the Zitácuaro–Valle de Bravo are not part
of the calc-alkaline suite, as they are less
rich in LILEs and have higher Nb (.9 ppm;
Table 2); accordingly, they are separated
from the calc-alkaline lavas in Figure 9.
They share some of the compositional characteristics of lava from Hawaii (Fig. 7);
thus, in western Mexico lava of this type is
called oceanic-island type (Nelson and Carmichael, 1984; Nelson and Livieres, 1986;
Righter and Carmichael, 1992; Moore et al.,
1994). In the Valley of Mexico, similar
high-TiO2 alkaline basalt has been called intraplate lava (Wallace and Carmichael,
1999), but we prefer the emphasis on ‘‘oceanic,’’ as it avoids confusion with very un-
1340
usual intraplate lava such as that of the Leucite Hills (Cross, 1897) on the North
American plate.
The source of the oceanic-island-type volcanism in the Mexican volcanic belt is controversial. Several workers have proposed a
plume- or rift-related setting involving the upwelling of anomalous mantle (Moore et al.,
1994; Márquez et al., 1999); others have considered advection of asthenospheric mantle
from behind the arc to the region of magma
production as the most viable mechanism for
producing the oceanic-island-type lava (Luhr,
1997; Wallace and Carmichael, 1999). Because the volume of high-TiO2 lava in the Zitácuaro–Valle de Bravo region is small and the
eruption rates low in comparison to plume or
rift-related volcanism (Table 5), we favor the
idea of advected asthenospheric mantle from
behind the arc as the source for the Zitácuaro–
Valle de Bravo oceanic-island-type lava. Another unresolved problem is whether the oceanic island-type lava of the Mexican volcanic
belt shows any systematic compositional
trends from west to east in the belt, parallel to
the increase in MgO and decrease in Al2O3
shown by the calc-alkalic basaltic andesite and
andesite.
APPENDIX 1
Ar/39Ar GEOCHRONOLOGY
40
The 40Ar/39Ar ages reported in this study are
from clean groundmass separates, with the exception of Z-365 and Z-348, which are from sanidine and plagioclase separates, respectively.
These groundmass and mineral separates were
prepared by crushing (using a jaw crusher), and
grinding (in a disk mill) each sample until we
obtained material in the size range of 10–100
mesh (;0.25 to 0.025 cm). This material was
then sieved to 40–60 mesh (;0.064 to 0.042 cm),
and groundmass and plagioclase were separated
using magnetic separation and handpicking. The
sanidine-rich sample (Z-365) was sieved to 24–
32 mesh (;0.11 to 0.08 cm), and sanidine was
separated by handpicking. At this point, samples
were ultrasonically washed for 5 min sequentially
in distilled water, 5% HCl solution, and 5% HF
solution in order to remove any adhering grains,
secondary carbonates, or other alteration products. All samples were then rinsed in distilled water and dried under a heat lamp.
As preparation for neutron irradiation, each sample was weighed, wrapped in Cu foil, and loaded
into containment vessels interspersed with neutronfluence monitors (Fish Canyon sanidine and Alder
Creek sanidine; Renne et al., 1998). These samples
were then irradiated at Oregon State University in
the Cadmium-Lined In-Core Irradiation Tube (CLICIT) facility. Samples were incrementally degassed
at the Berkeley Geochronology Center in a doublevacuum resistance furnace and analyzed with a
MAP 215–50 mass spectrometer. Two samples (Z309 and Z-365) were degassed by CO2 laser with
an integrator lens for incremental heating; the meth-
ods and facilities described by Renne et al. (1997)
were used. Apparent ages were calculated from the
isotopic data given in Table DR1 (see footnote 1)
and corrected for background, mass discrimination,
radioactive decay, and interfering nucleogenic
reactions.
Apparent age spectra (Fig. 10) show the temperature steps included in the plateau age and integrated age calculations for each sample. A plateau
age is defined by at least three contiguous steps,
whose ages do not differ at the 95% confidence level (when only internal errors are considered), and
which contain .50% of the total 39ArK released.
The integrated age contains all heating steps and is
equivalent to a K-Ar age (if no Ar is lost during
sample irradiation). The isochron ages were determined by calculation of intercepts on 36Ar/40Ar versus 39Ar/40Ar diagrams, corresponding to so-called
inverse isochrons.
Plateau ages were used as the preferred ages for
all the samples in this study, with the exceptions
of Z-129, Z-346, and Z-388 (Table 3). Discordant
apparent age spectra were produced for samples
Z-111 and Z-129, yielding spuriously old integrated ages (Fig. 10; Table 3). On the basis of field
observations of well-preserved eruptive features,
these lavas are much younger than the ;3 m.y.
integrated ages determined. The anomalous integrated ages may be a result of 39Ar loss from recoil
during irradiation of these glassy lavas, or due to
inherited 40Ar from assimilation of older material.
The Z-111 data yielded a plateau that is used as
the preferred age (the plateau age is similar to the
isochron age). Samples Z-129 and Z-346 do not
yield plateaus according to our criteria; therefore,
the isochron age is considered the preferred age
for both. Sample Z-388 released so little 39Ar that
the plateau and integrated ages produced for Z-388
are negative; therefore, the isochron age is used in
this case.
ACKNOWLEDGMENTS
We thank Brooks Bonstin, Carrie Friedman, Paul
Craig, Mauricio Escobar, Kristin Fillmore, Anthea
Carmichael, and Rachel McPhail for assistance in
the field; Warren Sharp, Tim Becker, Tim Teague,
and Peter McIntyre for lab assistance; and Gerardo
Aguirre-Dı́az, Rebecca Lange, and Joann Stock for
constructive reviews that led to numerous improvements in the final manuscript. Support for this research was provided by National Science Foundation grants EAR 94–18105, EAR 93–15844, and
EAR 97–06129 and the Ann and Gordon Getty
Foundation.
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