Ducea et al., 2004

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Geologic evolution of the Xolapa Complex, southern Mexico:
Evidence from U-Pb zircon geochronology
Mihai N. Ducea†
George E. Gehrels
Sarah Shoemaker
Joaquin Ruiz
Victor A. Valencia
University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA
ABSTRACT
The Xolapa Complex of southern Mexico
is composed of mid-crustal arc-related
gneisses of poorly resolved ages, intruded by
undeformed Cenozoic calc-alkaline plutons.
Twelve undeformed and deformed tonalitic/
granodioritic samples from three transects
across the Sierra Madre del Sur (Acapulco,
Puerto Escondido, and Puerto Angel) were
chosen for U-Pb zircon analysis. The measurements were performed on single crystals
of zircons, using a multiple-collector laserablation inductively coupled plasma–mass
spectrometer (MC-LA-ICP-MS). About
20–30 crystals were measured from each
sample. Three gneisses and migmatites from
the eastern transect (Puerto Angel), located
30–42 km from the coast, yielded Grenvilleaged zircons (970–1280 Ma), suggesting that
the samples represent Oaxacan basement,
not deformed Xolapa Complex. The central
transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons
as well as mid-Mesozoic and Permian ages
on gneisses. Most samples along the Puerto
Escondido transect contain inherited ca.
1.1 Ga xenocrystals of zircons. The western
transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on
gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons,
with no inherited Grenville ages. The older
ages and xenocrystic zircons in arc-related
Xolapa Complex mirror the crustal ages
found in neighboring terranes (Mixteca and
Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin
of Xolapa with respect to its neighboring
†
E-mail: [email protected].
north-bounding terranes. The new data and
previously published ages for Xolapa suggest
that metamorphism and migmatization of
the deformed arc rocks took place prior to
the Cenozoic. Eocene and Oligocene plutons
representing renewed arc-related magmatism
in the area are common throughout Xolapa,
and probably represent the more deeply
exposed continuation of the Sierra Madre
Occidental arc to the northwest. The available U-Pb data argue against the previously
proposed eastward migration of magmatism
between Acapulco and Puerto Angel during
the Oligocene.
Keywords: Xolapa Complex, arc magmatism,
U-Pb zircon, geochronology, deformation.
INTRODUCTION
Geometric arguments based on plate-tectonic reconstructions and inferred boundaries
between various apparently unrelated terranes
in Mexico indicate that most of Mexico is composed of crustal elements that were accreted to
North America after the Carboniferous (Dickinson and Lawton, 2001). The southern slopes
of the Sierra Madre del Sur range comprise
primarily arc-related rocks of the Xolapa Complex (Campa and Coney, 1983), also known as
the Chatino terrane (Sedlock et al., 1993). It is
a 600-km-long, relatively narrow (<70 km) strip
of mostly arc-related rocks straddling the coastal
Pacific margin of southern Mexico The local
geology of Xolapa is not known in detail, partly
because of the thick vegetation cover and scarce
access, and also because of the predominance of
high-grade basement rocks, the age and origin
of which are not resolved. Overall, Xolapa has
Gondwanan affinities, as do the neighboring terranes of Mixteca and Oaxaca (e.g., Dickinson
and Lawton, 2001). It is plausible that Xolapa
may not be a far-traveled terrane, but instead
might simply be the west-facing magmatic arc
for Pacific Mexico (Herrmann et al., 1994),
presumably formed between the Jurassic and
the Late Eocene.
Two of the key elements required in deciphering the tectonic history of Xolapa are (1) sorting out the age and origin of its basement, and
(2) resolving the timing of arc magmatism and
relationship to the surrounding arc-related products in Central America. Both of these tasks
require high-precision geochronology of basement rocks. There is evidence that the arc was
active in the Mesozoic and continued throughout
much of the Cenozoic (Ortega-Gutierrez, 1981).
Unfortunately, most of the published Xolapa
ages employed isotopic techniques (K-Ar,
Rb-Sr) that yield cooling ages but not necessarily crystallization ages (Morán-Zenteno et
al., 1999, for a compilation of ages and review).
One of the major limitations of the few previous
U-Pb zircon studies (e.g., Robinson et al., 1989;
Herrmann et al., 1994; Schaaf et al., 1995) is
that they employed multigrain fractions that
commonly yielded discordant ages; thus, the
age interpretations are commonly nonunique.
The purpose of this study was to acquire
additional geochronological data on key locations within the Sierra Madre del Sur region.
The strategy was such that we would fill gaps
that remained after the publication of the previous zircon U-Pb geochronology studies, in order
to obtain a more complete picture of the plutonic and metamorphic age distribution within
Xolapa. The Xolapa zircons are commonly very
complicated, and some record multiple ages
even at crystal scale (Herrmann et al., 1994).
We conducted our U-Pb study on small domains
within single zircon crystals, via multicollector laser-ablation ICP-MS (MC-LA-ICP-MS),
GSA Bulletin; July/August 2004; v. 116; no. 7/8; p. 1016–1025; doi: 10.1130/B25467.1; 8 figures; 1 table; Data Repository item 2004109.
1016
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© 2004 Geological Society of America
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U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO
in order to efficiently sort out crystallization
from inherited ages. MC-LA-ICP-MS U-Pb
geochronology (Kidder et al., 2003; Dickinson
and Gehrels, 2003; Ducea et al., 2003) is a new
technique that allows rapid and inexpensive age
determinations with a precision similar to secondary ion mass spectrometry. The technique
can be used for both detrital zircon (provenance)
studies and igneous geochronology.
Three key long-standing regional geologic
questions are addressed in this study using U-Pb
geochronology: (1) Is the Xolapa Complex a
traveled terrane, i.e., was it largely assembled
at a remote location relative to its neighboring
blocks in southern Mexico? (2) When was the
magmatic arc active in Xolapa? (3) Does the
time-space distribution of Cenozoic magmatism in the Xolapa constrain the reorganization
of plate boundaries in southwestern Mexico?
We show that (1) inherited zircons suggest an
autochthonous origin for the Xolapa Complex,
(2) magmatism was active in distinct episodes
with two major pulses in the Late Jurassic–Early
Cretaceous and Eocene-Oligocene, and (3) magmatism ceased abruptly in Xolapa at ca. 25 Ma
as the North American–Pacific plate boundaries
were reorganized to form the modern Acapulco
Trench. In addition, we use the regional geologic
data to address some general questions on continental arc magmatism, specifically, the causes of
episodic high-flux magmatic events.
A
Guerrero
Geology
All rocks studied here are part of the Xolapa
terrane (Campa and Coney, 1983), which
extends along the Pacific margin of southern
Mexico in the states of Guerrero and Oaxaca
and is ~600 km long and 50–70 km wide
(Fig. 1). Although thought by many authors to
represent an out-of-place terrane that may have
docked to mainland Mexico by the Late Cretaceous (Dickinson and Lawton, 2001; Campa
and Coney, 1983; Sedlock et al., 1993), some
studies have argued that the geology of Xolapa,
while somewhat distinctive from the neighboring terranes, represents a magmatic arc that
formed in place during the Mesozoic and
continued into the Cenozoic (e.g., Herrmann et
al., 1994; Schaaf et al., 1995; Morán-Zenteno
et al., 1999). Thus, the Xolapa Complex is
referred to as the Xolapa Complex by Herrmann et al. (1994), a nongenetic terminology
that we also adopt here.
The geology of the Xolapa Complex consists of high-grade orthogneisses and paragneisses as well as migmatites (Ortega-Gutierrez, 1981), intruded by generally undeformed
tonalitic to granodioritic plutons (Herrmann
et al., 1994; Meschede et al., 1997; MoránZenteno et al., 1999). Figure 2 is a geologic
Trans-Mexican
Volcanic Belt
Maya
20
Morelia
Mixteco
O
Mexico City
Toluca
Oaxaca
Xolapa
Pacific Ocean
map of the Xolapa Complex in the vicinity
of Acapulco (modified after Morán-Zenteno,
1992) and shows the overall geologic features
of the complex. The orthogneisses are ductilely
deformed and metamorphosed calc-alkaline
diorites, tonalites, and granodiorites, and predominate over the paragneisses by a factor of
at least 10. The orthogneisses represent a metamorphosed sequence of continental arc rocks,
whereas the paragneisses are rare framework
rocks of this arc (Ortega-Gutierrez, 1981). The
protolith age for the gneisses and migmatites
is ca. 1.0–1.3 Ga, based on Nd model ages and
U-Pb ages (Herrmann et al., 1994), whereas
metamorphism is thought to have occurred
sometime between the Early Cretaceous and
Eocene (Riller et al., 1992; Herrmann et al.,
1994; Meschede et al., 1997; Morán-Zenteno
et al., 1996, 1999). The undeformed plutons,
calc-alkaline tonalites, and granodiorites
are also characteristic of rocks formed in a
continental magmatic arc setting (Martiny et
al., 2000), and are Eocene-Oligocene in age:
36–23 Ma between Zihuatanejo and Puerto
Angel (Schaaf et al., 1995; Morán-Zenteno
et al., 1999; Herrmann et al., 1994). Trace
element patterns in these plutons are characteristic of subduction-related magmatism,
which suggests an enriched mantle source in
the subcontinental lithosphere, modified by
subduction fluids (Martiny et al., 2000). The
C
Mexico
City
TMVB
GEOLOGICAL BACKGROUND
Juarez
Inland Tertiary
volcanic sequences
Jalapa
Tertiary Coastal
plutonic belt
Puebla
Mesozoic andCenozoic
metamorphic rocks
100 km
18
B
U.S.A
Mexico
30oN
300Km
Puerto
Angel
o
Pacific Ocean
C
90 W
SMO
TMVB
Xolapa
0
50
Sample Locations
Puerto
Escondido
o
o
Grenvillian Oaxacan
Complex
Oaxaca
20 N
110 W
Early Mesozoic
sedimentary cover
Ordovician-Devonian
Acatlan Complex
Acapulco
0
O
Zihuatanejo
Gulf of
Mexico
Pacific
Ocean
SMO and Eocene
magmatic rocks
100km
100
O
98
O
96
O
Figure 1. (A) The tectonostratigraphic division of southern Mexico showing the main terranes as well as the location of the Xolapa Complex
(after Campa and Coney, 1983). (B) Distribution of Tertiary magmatic provinces in Mexico (Morán-Zenteno et al., 1999); SMO—Sierra
Madre Occidental; TMVB—Trans-Mexican Volcanic Belt. (C) Schematic geologic map of southern Mexico, showing sample locations (after
Ortega-Gutierrez et al., 1999).
Geological Society of America Bulletin, July/August 2004
1017
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DUCEA et al.
initial 87Sr/86Sr and εNd values of these plutons
suggest a relatively low degree of crustal contamination (Martiny et al., 2000). Herrmann et
al. (1994) proposed that the Eocene-Oligocene
magmatism has a distinct migration pattern
from older in the west to younger in the east at
a rate of 56 km/m.y.
The Xolapa Complex does not have an onland sedimentary cover proper (Were-Keeman
and Estrada-Rodarte, 1999; Castillo-Nieto and
Rodriguez-Luna, 1996), in contrast to the adjacent terranes (e.g., Campa and Coney, 1983;
Sedlock et al., 1993). The only sedimentary
cover to Xolapa is represented by a narrow
accretionary wedge of the modern Acapulco
Trench, which represents the southern boundary of the Xolapa Complex, with the oldest
sediments being Miocene in age (20 Ma, Watkins et al., 1981). The northern boundary of
the complex is poorly known. This boundary
is best studied in the Tierra Colorada region,
north of Acapulco (Fig. 2), where it is marked
by a zone of cataclastic rocks and mylonites,
indicating sinistral shear and north-south extension (Meschede et al., 1997; Ratschbacher et al.,
1991). In the Tierra Colorada region, mylonitization occurred between the Late Cretaceous and
Paleocene-Eocene (Ratschbacher et al., 1991;
Herrmann et al., 1994; Morán-Zenteno et al.,
1999), and possibly later to the east near Puerto
Angel (Morán-Zenteno et al., 1999).
100˚00'
17˚15'
M002
TIERRA
COLORADA
M01-04
EL TREINTA
M01-46
ACAPULCO
Quaternary
M01-02
Undeformed Tertiary Plutonic Rocks
Cretaceous Morelos Formation
PACIFIC
OCEAN
Mesozoic Gneisses and Migmatites
Tectonic History
Acatlan Complex
Since the middle Cretaceous, the relationship between the Farallon and North American
plates changed several times (e.g., Engebretson
et al., 1985): The plate boundary was predominantly convergent, with both periods of oblique
subduction and periods when the margin was a
continental transform boundary. By 20 Ma, the
Farallon plate had fractured into the Cocos and
Nazca plates (Meschede et al., 1997). The Cocos
plate continued on a north-northeast trajectory
with respect to the North American plate along
the current southern Mexican border. Plate-tectonic reconstructions are subject to large errors
for southern Mexico because the Farallon plate
has been entirely subducted. The timing and
patterns of migration of magmatic arcs on the
continental Mexico side of this subduction zone
are used to better understand the major changes
in relative motion between the Pacific and North
American plates in southern Mexico (Ferrari et
al., 1999) especially in late Cenozoic time, for
which numerous age data are available for volcanic and intrusive rocks.
The current southern margin of the North
American plate has experienced truncation
along the southern margin of the Xolapa
1018
10 km
16˚00'
100˚00'
99˚00'
Figure 2. Geologic map of the Acapulco transect (modified after Morán-Zenteno, 1992),
showing the distribution of gneisses/migmatites and undeformed plutons in the area. The
location of samples studied here from the Acapulco transect are also shown.
Complex (Malfait and Dinkelman, 1972;
Ross and Scotese, 1988; Ferrari et al., 1994;
Herrmann et al., 1994; Schaaf et al., 1995).
Though subduction has been occurring at most
times since the early Mesozoic, the current
margin displays none of the mature margin
characteristics of other 100 Ma margins. The
Middle America Trench today lies only ~75 km
offshore, and thus, the distance between the
modern trench and the Eocene-Oligocene magmatic arc is much less than in other arc systems.
The margin displays a very narrow upper slope
and forearc basin when compared to other areas
with long subduction histories (Karig-Cordwell
et al., 1978). Structural trends of metamorphic
rocks in the Xolapa Complex intersect the coast
at steep angles, also indicating margin truncation (Malfait and Dinkleman, 1972). The lack
of high-pressure/low-temperature rocks along
the continental margin, as well as the lack of an
accretionary prism landward from the Middle
America Trench, add additional support for
the hypothesis that truncation has occurred at
some time (Schaaf et al., 1995; Meschede et al.,
1997). The Middle America (Acapulco) trench
along the southern Mexican margin has experienced accretionary sedimentation continuously
since the Middle Miocene. The two mechanisms
possibly responsible for truncation are (1) subduction erosion (Morán-Zenteno et al., 1996)
and (2) lateral transport via transform faults,
which may have removed the older wedges, the
Geological Society of America Bulletin, July/August 2004
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U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO
TABLE 1. SAMPLE LOCATION AND PETROGRAPHY
Sample
Latitude
(N)
Puerto Angel transect
M01-11
15°56′40″
M01-14
15°58′38″
M01-16
16°02′51″
Other ages
(Ma)
Longitude
(W)
Petrography
U-Pb age summary
96°27′55″
96°29′58″
96°30′22″
Bt tonalitic gneiss
Bt + Grt tonalitic gneiss
Hbl + Bt tonalitic gneiss
1252 ± 24, 1106 ± 10 Ma
1029 ± 28 Ma
1119 ± 24 Ma
23.4
Bt tonalitic gneiss
Bt + Hbl granodiorite
Bt + Hbl diorite
leucotonalite
leucogranodiorite
272 ± 10 Ma; 1100 Ma
158 ± 8.1 Ma; 1.1–1.2 Ga
25.4 ± 2.9 Ma; 1.1 Ga
31.2 ± 1.5 Ma; 1016 ± 28 Ma
29.6 ± 4.0 Ma; 50–71 and 100–126 Ma
27.9
15.5
Hbl-syenite
Bt + Hbl tonalitic gneiss
Bt + Hbl tonalitic gneiss
Bt granodiorite
54.9 ± 2.0 Ma
140.9 ± 4.5 Ma
136.6 ± 4.0 Ma
34.5 ± 1.2 Ma
26.7
17.7
Puerto Escondido transect
M01-17
15°54′55″
97°04′47″
M01-19
15°59′46″
97°04′46″
M01-26
16°12′54″
97°08′10″
M01-27
16°17′05″
97°08′41″
M01-28
15°51′28″
97°03′33″
Acapulco transect
M01-46
16°47′28″
M01-02
16°22′37″
M002
17°13′53″
M01-04
16°46′26″
†
99°49′43″
99°26′21″
99°31′16″
99°37′35″
‡
17.6
16.2
15.7
37.6
Note: Bt—biotite, Hbl—hornblende, Grt—garnet.
†
Crystallization ages are listed first; inherited ages, when determined, are shown in italics.
‡
Apatite fission-track ages determined on the same samples by Shoemaker et al. (2002).
forearc, and even parts of the Xolapa Complex
(Schaaf et al., 1995).
SAMPLES AND ANALYTICAL METHODS
Samples were collected along three northsouth transects in the Sierra Madre del Sur:
Acapulco, Puerto Escondido, and Puerto Angel
(Fig. 1). At each sample locality, we collected
1–2 kg of fresh whole rock. Samples were prepared for analysis using standard crushing and
separation techniques, including heavy liquids
and magnetic separation, at the University of
Arizona. Inclusion-free zircons were then handpicked under a binocular microscope. At least
50 zircons from each sample were mounted in
epoxy and polished.
Single zircon crystals were analyzed in
polished sections with a Micromass Isoprobe
multicollector ICP-MS equipped with nine
Faraday collectors, an axial Daly detector,
and four ion-counting channels (Kidder et
al., 2003). The LA-ICP-MS analyses involve
ablation of zircon with a New Wave DUV193
Excimer laser (operating at a wavelength of 193
nm) using a spot diameter of 25 to 35 microns.
The ablated material is carried in argon gas into
the plasma source of a Micromass Isoprobe,
which is equipped with a flight tube of sufficient
width that U, Th, and Pb isotopes are measured
simultaneously. All measurements are made in
static mode, using Faraday detectors for 238U,
232
Th, 208–206Pb, and an ion-counting channel
for 204Pb. Ion yields are ~1 mV per ppm. Each
analysis consists of one 20-second integration
on peaks with the laser off (for backgrounds), 20
1-second integrations with the laser firing, and a
30-second delay to purge the previous sample
and prepare for the next analysis. The ablation
pit is ~20 microns in depth.
Common Pb correction is performed by
using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers
(1975) (with uncertainties of 1.0 for 206Pb/204Pb
and 0.3 for 207Pb/204Pb). Measurement of 204Pb
is unaffected by the presence of 204Hg because
backgrounds are measured on peaks (thereby
subtracting any background 204Hg and 204Pb), and
because very little Hg is present in the argon gas.
For each analysis, the errors in determining
206
Pb/238U and 206Pb/204Pb result in a measurement error of several percent (at 2-sigma level)
in the 206Pb/238U age (Table DR1).1 The errors
in measurement of 206Pb/207Pb are substantially
larger for younger grains due to low intensity of
the 207Pb signal. The 207Pb/235U and 206Pb/207Pb
ages for younger grains accordingly have large
uncertainties (Table DR1). Interelement fractionation of Pb/U is generally <20%, whereas
isotopic fractionation of Pb is generally <5%.
In-run analysis of fragments of a large zircon
crystal from a Sri Lanka pegmatite (e.g. Dickinson and Gehrels, 2003) with known age of 564
± 4 Ma (2-sigma error) is used to correct for this
fractionation (generally run every fifth measurement). The uncertainty resulting from the calibration correction is generally ~3% (2-sigma)
for both 207Pb/206Pb and 206Pb/238U ages.
The pooled crystallization ages reported in
this paper are weighted averages of individual
spot analyses. The stated errors (2-sigma)
on the assigned ages are absolute values and
include contributions from all known random
and systematic errors. The Mesozoic and
Cenozoic ages interpreted from the ICP-MS
1
GSA Data Repository item 2004109, U-Pb zircon
analytical data, is available on the Web at http://
www.geosociety.org/pubs/ft2004.htm. Requests may
also be sent to [email protected].
analyses are based on 206Pb/238U ratios because
errors of the 207Pb/235U and 206Pb/207Pb ratios
are significantly greater. This is due primarily
to the low intensity (commonly <1 mV) of the
207
Pb signal from these young, U-poor grains.
For grains older than 800 Ma, both 207Pb/206Pb
and 206Pb/238U are reported.
Twelve samples were analyzed for U-Pb geochronology in this study and are grouped based
on their location along the three studied transects,
Puerto Angel, Puerto Escondido, and Acapulco
(Fig. 1). Ages of ~20–30 zircon grains were
measured from each sample. Results and errors
are reported in Tables 1 and DR1. Each line in
Table DR1 represents a spot analysis. The samples
are either deformed metamorphic rocks of clearly
metaigneous origin (orthogneisses and migmatitic orthogneisses) or unmetamorphosed plutonic
rocks. Although we have not analyzed the geochemistry of these rocks, their study under optical
microscope and analyses of their mineralogy and
modes indicate clearly that all rocks are igneous
or metaigneous, and calc-alkaline or alkaline in
composition. Their compositions (Table 1) are
tonalitic, granodioritic, granitic, and in one case
(M01-46 hornblende-bearing quartz-syenite.
RESULTS
Zircons from the Xolapa Complex samples
were analyzed optically under a petrographic
microscope and a scanning electron microscope
(SEM) in backscattered electron (BSE) mode.
Zircons were grouped according to the morphology classification of Pupin (1983) (Fig. 3).
Almost all zircons that yielded Cenozoic ages
display igneous morphologies (e.g., euhedral
crystals) with no detectable optical zoning. The
older zircons we found are smaller rounded
grains that indicate a detrital origin and/or corrosion in magma. For these zircons, we report
the core ages, which are always older than rim
ages. Rim ages typically have large errors (probably because the new rim growths are very narrow) and are not reported in Table DR1, but the
ages are generally Cenozoic.
Puerto Angel Transect
Three samples collected from north of
Puerto Angel (M01-11, M01-14, and M01-16)
yield exclusively Precambrian ages (Fig. 4).
Sample M01-11 (n = 31 zircons) has a bimodal
distribution of U-Pb ages, with one group averaging 1252 ± 24 Ma (MSWD = 1.2), and a second group averaging 1109 ± 10 Ma (MSWD =
0.9). All zircons in M01-11 are concordant or
near concordant within error (Fig. 4). M01-14
has an average age of 1092 ± 28 Ma (n = 29)
and may also have a bimodal distribution of
Geological Society of America Bulletin, July/August 2004
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DUCEA et al.
ages, although not as pronounced as M01-14.
Peak ages are 1244 ± 10 Ma (MSWD = 0.4),
and 1163 ± 15 Ma (MSWD = 1.1). Most
determined ages in M01-14 are concordant.
Most zircons in sample M01-16 (21 out of 28)
cluster around an average age of 1119 ± 24 Ma
(MSWD = 0.4), although individual ages range
from 950 to 1250 Ma. Most ages in this sample
are also concordant (Fig. 4). There are no
Phanerozoic zircons in these samples. Overall,
these ages are identical to the ca. 1.0–1.2 Ga
Grenville ages obtained on the high-grade
basement of the Oaxaca terrane (Ruiz et al.,
1988; Herrmann et al., 1994).
A.
C.
Puerto Escondido Transect
Tonalitic gneiss M01-17 collected south of
Puerto Escondido has a 206Pb/238U age of 272 ±
10 Ma (n = 21 zircons) (Fig. 5A), MSWD = 4.5.
Three zircons contain an inherited Precambrian
component (Table DR1). These crystals are internally zoned; the old ages are discordant but suggest that the inherited component is ca. 1.1 Ga,
consistent with Oaxacan basement. Another
analyzed gneiss (M01-19) has complicated zircon systematics (Figs. 5B and 5F) due to zoning
of individual crystals at a scale smaller than that
of the individual laser pits. The crystallization
age of this metaplutonic rock is interpreted to
be Jurassic (158.3 ± 8.1 Ma, MSWD = 10.5) (n
= 15 zircons). The discordant ages displayed by
the other 12 zircons (Fig. 5) are suggestive of an
inherited 1.1–1.2 Ga component.
Three other samples along this transect have
Cenozoic ages (Figs. 5C, 5D, and 5E). A leucogranite (M01-28) yielded an age of 29.6 ±
4.0 Ma, MSWD = 38 (n = 8 zircons), although
inherited grains with middle Cretaceous (100–
126 Ma) and latest Cretaceous–early Cenozoic
(50–71 Ma) 206Pb/238U ages are found. One
zircon has a 206Pb/238U age of 1051 Ma. Tonalite
M01-27 has a 31.9 ± 1.5 Ma crystallization age
(n = 22 zircons), MSWD = 5, as well as one
Precambrian zircon (1016 ± 28 Ma). Finally, a
quartz-diorite (M01-26) yielded a 25.4 ± 2.9 Ma
crystallization age (n = 8 zircons), MSWD = 19.
However, this sample contains complexly zoned
zircons that yield discordant ages spanning
the range between crystallization ages and an
inferred inherited Grenville component, similar
to the other samples collected along the Puerto
Escondido transect.
Acapulco Transect
Undeformed syenite sample M01-46,
located in Acapulco Bay, has a crystallization age of 54.9 ± 2.0 Ma (MSWD = 3.3),
which is similar to a 50 Ma K-Ar age on a
1020
B.
D.
Figure 3. Zircon morphology microphotographs. (A) SEM-CL image of zoned zircon from
sample M01-17; scale bar = 20 microns. (B) SEM-CL image of several analyzed zircons that
display complex zoning from sample M01-19; scale bar = 50 microns; laser ablation pits are
~20 microns deep. (C) SEM-CL image of individual zircon from sample M01-14; no zoning
is evident in this sample. (D) SEM-CL image of euhedral zircon from sample M01-46; both
the core and the rim of this sample yielded the same age within errors; the age was interpreted to be the crystallization age.
nearby quartz-syenite (Morán-Zenteno, 1992).
Twenty-four zircons from this sample yielded
all crystallization ages, with no detectable
inherited component (Fig. 6). Two garnet- and
biotite-bearing tonalitic gneisses (M01-02 and
M002) yielded early Cretaceous ages (Fig. 6),
identical within errors: M01-02 has an age of
140.9 ± 4.5 Ma (n = 41 zircons, MSWD = 2.0),
whereas M002 has an age of 136.6 ± 4.0 Ma
(n = 33 zircons, MSWD = 1.1). There are no
inherited zircons in this gneiss. Finally, sample
M01-04, an undeformed hornblende- and
biotite-bearing granodiorite that was collected
along the new Highway 95, yields a 206Pb/238U
age of 34.5 ± 2.2 Ma (n = 19, MSWD = 0.4)
with no inherited grains, consistent with previously determined U-Pb ages on neighboring
undeformed plutons (Herrmann et al., 1994).
INTERPRETATIONS
The Boundaries of the Xolapa Complex
Samples of the Puerto Angel transect are
interpreted to represent basement rocks of the
Oaxaca terrane, thus limiting the extent of the
Xolapa Complex to less than 30 km from the
coast along this transect. While most rocks
investigated along this transect have a distinctive L-S tectonite fabric, they do not differ
compositionally (tonalites, granodiorites) from
most of the deformed rocks of the Xolapa
Complex. In addition, one of the samples
analyzed here (M01-16) is in fact a slightly
deformed coarse biotite- and amphibole-bearing granodiorite, identical in both composition
and fabric to a typical Xolapa postkinematic
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U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO
A. M01-11
0.23
1300
206
Pb/238 U
0.21
1200
1100
0.19
1000
0.17
900
0.15
0.13
1.3
1.5
1.7
1.9
2.1
207
2.3
2.5
2.7
2.9
Pb/235 U
0.28
B. M01-14
1500
1400
0.24
206
Pb/238 U
1300
1200
0.20
1100
1000
Eocene-Oligocene granitoid. This sample was
collected just south of the Chacalapa shear
zone, which is defined by some as representing
the boundary between Xolapa and Oaxaca. The
Xolapa Complex of Mesozoic-Cenozoic arcrelated rocks becomes much narrower along
the Puerto Angel transect than to the west, at
Puerto Escondido, where it is 100 km wide
(Campa and Coney, 1983). The narrowness
of the Xolapa Complex at Puerto Angel compared to Puerto Escondido may be explained
by the presence of a major fault orthogonal
to the strike of the Xolapa Complex between
Puerto Escondido and Puerto Angel, e.g., the
Oaxaca fault (Nieto-Samaniego et al., 1995;
Alanis-Alvarez et al., 1996). A significant
late Cenozoic dextral strike-slip component
on the Oaxaca fault could explain the change
in the width of the Xolapa Complex east of
that fault; however, field data can document
only dip-slip displacements on the Oaxaca
fault during the Cenozoic (Alanis-Alvarez et
al., 1996; D. Morán-Zenteno, 2003, written
commun.), whereas most of the strike-slip
displacement is Mesozoic. Alternatively, the
Grenville ages reported in this study may suggest that the basement of the Xolapa Complex
and the Oaxacan basement are indistinct and
that perhaps the deformed basement for the
Xolapa Complex may be in fact Oaxacan basement at many other locales within the coastal
Sierra Madre del Sur.
0.16
900
1.2
Evidence for Permian Magmatism
1.6
2.0
2.4
207
3.2
Pb/ U
C. M01-16
0.23
1300
1200
0.21
1100
0.19
206
Pb/238 U
2.8
235
1000
0.17
900
0.15
0.13
1.2
1.6
2.0
207
2.4
2.8
235
Pb/ U
Figure 4. U-Pb ages for Puerto Angel in concordia plots. A: Sample M01-11. B: Sample
M01-14. C: Sample M01-16.
Sample M01-17, a tonalitic gneiss collected
~7 km north of the Pacific Coast at Puerto
Escondido, yielded a Permian U-Pb zircon age
(272 ± 10 Ma). This is an unusual age for the
Xolapa Complex, as it predates the Mesozoic
and Cenozoic metamorphism and magmatism
that essentially define the Xolapa Complex.
Plutons of Cenozoic ages are found both north
and south of the location of M01-17. Therefore, sample M01-17 represents a framework
rock of the Xolapa Complex. Similar Permian
ages have been reported in the Acatlan basement of the Mixteca terrane (Yanez et al.,
1991; Robinson et al., 1989; Elias-Herrera and
Ortega-Gutierrez, 2002) and the Juchatengo
terrane (Grajales-Nishimura et al., 1999). If
the Mixteca terrane, which is located north of
the Xolapa Complex at the longitude of Puerto
Escondido, is in fact the basement for Xolapa,
an out-of-place origin for the Xolapa Complex
could be ruled out. Future data collection will
be aimed at basement rocks of the Xolapa
Complex (e.g., rare metasedimentary rocks) in
order to further sort out the age distribution of
the pre-arc Xolapa basement.
Geological Society of America Bulletin, July/August 2004
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DUCEA et al.
310
A.
Age = 272 ± 10 Ma
Mean = 272 ± 6 Ma
(2σ)
D.
M01-17
Age = 31.2 ± 1.5 Ma
Mean = 31.2 ± 1.2 Ma
(2σ)
37
M01-27
Age (Ma)
Age (Ma)
290
270
33
29
250
25
230
190
B.
M01-19
Age = 158.3 ± 8.1 Ma
Mean = 158.3 ± 6.4 Ma
(2σ)
E.
Age = 29.6 ± 4.0 Ma
Mean = 29.6 ± 3.8 Ma
(2σ)
Age (Ma)
Age (Ma)
36
170
28
150
M01-28
20
130
C.
32
Age = 25.4 ± 2.9 Ma
Mean = 25.4 ± 2.9 Ma
(2σ)
M01-26
1200
F.
0.20
1000
27
206 238
Pb/ U
Age (Ma)
0.16
800
0.12
600
0.08
400
22
0.04
200
17
0.00
0.0
M01-19
0.4
0.8
1.2
1.6
2.0
2.4
207 235
Pb/ U
Figure 5. Zircon U-Pb ages along the Puerto Escondido transect. (A) 206Pb/238U ages of sample M01-17. (B) 206Pb/238U ages of sample M01-19.
(C) 206Pb/238U ages of sample M01-26. (D) 206Pb/238U ages of sample M01-27. (E) 206Pb/238U ages of sample M01-28. (F) Concordia diagram for
sample M01-19, showing the presence of inherited Grenville (ca. 1.1 Ga) ages as well as individual ages that lie on a mixing line between the
inherited ages and the interpreted crystallization age of the rock.
1022
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U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO
63
A.
Age = 54.9 ± 2.0 Ma
Mean =54.9 ± 1.1 Ma
160
C.
M01-46
Age =140.9 ± 4.5a
Mean =140.9 ± 1.5 Ma
M01-02
150
Age (Ma)
Age (Ma)
59
55
140
130
51
120
47
160
B.
37
M01-04
Age = 34.5 ± 1.2 Ma
Mean =34.5 ± 0.2 Ma
M002
D.
Age (Ma)
Age (Ma)
140
34
120
31
Age = 136.6 ± 4.0 Ma
Mean =136.6 ± 1.4 Ma
100
28
Figure 6. 206Pb/238U ages for samples along the Acapulco transect. (A) Sample M01-46. (B) Sample M01-04. (C) Sample M01-02. (D) Sample M002.
Mesozoic Arc Evolution and Ductile
Deformation
Three of our deformed samples, M01-19 (Puerto
Escondido), and M002 and M01-2 (Acapulco),
yielded Late Jurassic–Early Cretaceous ages.
Based on the zircon morphology (Pupin, 1983),
we interpret these data to represent igneous crystallization ages. Field observations (this study and
Morán-Zenteno, 1992) suggest that the timing of
ductile deformation was concomitant with the
intrusion (these are synkinematic plutons).
Several of the earlier geochronologic studies
of the Xolapa Complex reported similar ages,
based on U-Pb zircon geochronology (Fries
and Rincon-Orta, 1965; Guerrero-Garcia, 1975;
Robinson et al., 1989; Herrmann et al., 1994) and
Rb-Sr whole-rock ages (Guerrero-Garcia, 1975;
Morán-Zenteno, 1992). However, Herrmann et al.
(1994) considered the pre-Cenozoic ages “speculative” and instead interpreted their U-Pb data to
show evidence for an early Cenozoic (66–46 Ma)
age of magmatism, ductile deformation, and
metamorphism of the Xolapa Complex. The new
data presented in this paper lend strong evidence
to the previously postulated Late Jurassic–Early
Cretaceous magmatic/deformational event.
The alkaline rock suite of Acapulco has a
distinctively older age of ca. 55 Ma compared
to other undeformed plutons in the region. It is
part of a composite alkaline pluton extending
~15 km across Acapulco Bay (Morán-Zenteno
et al., 1993), the eastern edges of which are only
30 km from gneissic sample M01-02, which
yielded a Jurassic age. The emplacement depth
for this suite was calculated by Morán-Zenteno
et al. (1996) to be ~20 km, using Al-in-hornblende barometry. The emplacement of the Acapulco suite is coeval with the proposed Cenozoic
metamorphic event (Herrmann et al., 1994), and
the calculated depth of emplacement requires
that these rocks would have been subject to
crystal plastic deformation in such a metamorphic regime. The lack of plastic deformation
in the Acapulco intrusive suite puts important
constraints on the timing of metamorphism and
ductile deformation of the Xolapa Complex; we
interpret these data to strongly argue for a preCenozoic age of metamorphism.
Evidence Against Along-Strike Migration
of Eocene-Oligocene Arc Magmatism
Our new data, together with previously
published U-Pb zircon age data on the EoceneOligocene undeformed magmatic rocks within
the Xolapa Complex, do not support the hypothesis of eastward migration of magmatism (Herrmann et al., 1994). Figure 7 shows that plutons
with ages of 24–37 Ma were found within the
western, central, and eastern transects described
here, representing the eastern ~360 km along
the strike of the arc. While it is possible that the
cessation of magmatism was diachronous along
the strike of the Oligocene Xolapa Complex,
as a result of a change in the relative motion
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DUCEA et al.
1024
Age (Ma)
Acapulco
A proposed synthesis of magmatic and deformation events that led to the development of
the Xolapa Complex, based on our new results
and previous data, is summarized in Figure 8.
The main intrusive and deformational events
that led to the development of gneisses in the
Xolapa Complex are latest Jurassic–Cretaceous.
The framework rocks to the Mesozoic arc may
have been basement rocks of the Mixteca and
Oaxaca terranes, suggesting, as did Herrmann et
al. (1994), that the Xolapa Complex is not out of
place with respect to these terranes. We also propose that renewed magmatism, not accompanied
by deformation, took place in the Paleogene,
This study
Herrmann et al.
0
Figure 7. Diagram showing the distribution
of zircon U-Pb crystallization ages on undeformed plutons from the Xolapa Complex
between Acapulco and Puerto Angel. Age
sources are this study and Herrmann et al.
(1994). The diagram shows that there is no
clear pattern of along-strike migration of
arc magmatism; instead, all plutons formed
within an ~10–15 m.y. window coincident with
the magmatic flare-up in the Sierra Madre
Occidental to the north (from McDowell et al.,
1978, and McDowell and Mauger, 1994).
Magmatism
Ma
0 Pc
Deformation
Truncation #2-subduction
erosion and arc inland
migration
Mc
Oc
50
Puerto
Angel
Puerto
Escondido
20
Sierra Madre Occidental
magmatic flare-up
1,2,3,4
Eocene
The Pacific margin of southern Mexico was
a convergent margin after Middle Triassic time.
Arc-related magmatism is therefore expected to
have occurred at least intermittently for most of
the last 200 m.y. However, our data and previously published data for the Xolapa Complex
indicate a rather episodic record of magmatism
consisting primarily of two volumetrically
significant pulses: one in the Early Cretaceous
(possibly extending into the Late Jurassic), and
a second one that is Late Eocene–Oligocene
(Fig. 8). Plutons of different ages, such as the
Paleogene Acapulco composite intrusion, are
present locally but appear to be volumetrically
less significant in the Xolapa Complex. Other
continental arcs in North and South America,
such as the composite Sierra Nevada (Ducea,
2001), Coast Mountains (Armstrong, 1988), and
Peruvian batholiths (Pitcher, 1993), were not
steady-state but were rather formed during short
flare-up events separated by longer magmatic
lulls. Available mapping and geochronologic data
indicate that the Early Cretaceous arc is exposed
to mid-crustal levels and represents some 40% of
the exposed Xolapa Complex (Morán-Zenteno,
1992). This magmatic pulse is correlative with
the Late Jurassic–Early Cretaceous flare-up of
the California arc (Ducea, 2001). Some 50% of
the Xolapa Complex consists of large EoceneOligocene (35–25 Ma) calc-alkaline plutons.
The match in age with the magmatic flare-up of
the Sierra Madre Occidental to the north (Ferrari et al., 1999) suggests that these plutons may
represent the southern continuation of that arc.
Compositionally, the Eocene-Oligocene plutons
of the Sierra Madre del Sur between Acapulco
and Puerto Angel (Martiny et al., 2000) are
remarkably similar to the average composition
of the Sierra Madre Occidental arc. The deeper
levels of exposure (Morán-Zenteno et al., 1996)
of the Eocene-Oligocene plutons in the Xolapa
Complex are due to relatively high erosion rates
that characterize the Sierra Madre del Sur mountain range since the Miocene (Morán-Zenteno et
al., 1996; Shoemaker et al., 2002).
Magmatism ended at ca. 25 Ma in the Sierra
Madre del Sur. The Eocene-Oligocene continental margin was later truncated because the
40
CONCLUSIONS
Truncation #1- left
lateral migration of the
Chortis block
1,5 Acapulco
intrusive
Pc
?
100
Cretaceous
Episodicity of Magmatism
modern Middle America (Acapulco) trench
is adjacent to the Eocene-Oligocene arc and
oblique to it. The leading hypothesis for the
late Cenozoic continental truncation in Pacific
southern Mexico is tectonic erosion (or subduction erosion) (Morán-Zenteno et al., 1996).
?
2
Regional ductile
deformation
ductile deformation
at mid-crustal depths
1,5,6,7
150
?
Jurassic
of the Farallon and North American plates and
a transition from a subduction to transform
margin, there is no obvious reason why the
entire arc-related magmatic pulse should have
migrated along the strike of the arc (Herrmann
et al., 1994). In conclusion, our new data and
previously published data show that the EoceneOligocene magmatic pulse was not diachronous
between Acapulco and Puerto Angel.
7
1-this study; 2-Herrmann et al., 1994; 3-Martiny et al., 2000; 4-Schaaf et al., 1995;
5- Moran-Zenteno, 1992; 6- Robinson, 1989; 7-Guerrero-Garcia, 1975
Major magmatic flare-ups
Timing poorly constrained
Figure 8. Summary diagram showing the major tectonic and magmatic events in the
Xolapa Complex.
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U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO
and then again, more volumetrically significant,
during the Eocene-Oligocene. The available data
on the Eocene and Oligocene magmatism do not
support the hypothesis of eastward migration
of magmatism, but rather point to a regionally extensive, relatively short-lived (~10 m.y.)
flare-up episode that may be related to the
development of the Sierra Madre Occidental arc.
Magmatism ceased abruptly at ca. 25 Ma in the
study area. Magmatism then migrated inland and
resulted in the formation of the Trans-Mexican
Volcanic Belt (Ferrari et al., 1999), which is
oblique to the strike of the Xolapa Complex.
ACKNOWLEDGMENTS
This research was partly supported by a research
grant from the University of Arizona to Mihai Ducea.
Fieldwork was supported by the University of Arizona
Geo-Structure Program and a grant from Chevron to
Sarah Shoemaker. Interactions with Maria-Fernanda
Campa, Dante Morán-Zenteno, Barbara Martiny,
Luca Ferrari, and Joel Ramirez-Espinosa helped stimulate this study. Journal reviews by Jim Mortensen,
James McLelland, and Associate Editor Calvin Miller
have significantly improved the quality of the manuscript. We are indebted to Rebekah Wright and Alex
Pullen for assistance in sample preparation and mass
spectrometry. Mark Baker provided maintenance support of the MC-LA-ICP-MS. Funding for this study
was supported by grant 38958-G2 from the Petroleum
Research Fund of the American Chemical Society
awarded to M. Ducea.
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MANUSCRIPT RECEIVED BY THE SOCIETY 6 AUGUST 2003
REVISED MANUSCRIPT RECEIVED 6 DECEMBER 2003
MANUSCRIPT ACCEPTED 12 JANUARY 2004
Printed in the USA
Geological Society of America Bulletin, July/August 2004
1025
Downloaded from gsabulletin.gsapubs.org on August 10, 2015
Geological Society of America Bulletin
Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb
zircon geochronology
Mihai N. Ducea, George E. Gehrels, Sarah Shoemaker, Joaquin Ruiz and Victor A. Valencia
Geological Society of America Bulletin 2004;116, no. 7-8;1016-1025
doi: 10.1130/B25467.1
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