Geologic evolution of the Xolapa Complex, southern : 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 bounding terranes. The new data and previ- and Lawton, 2001). It is plausible that Xolapa ously published ages for Xolapa suggest that may not be a far-traveled terrane, but instead The Xolapa Complex of southern Mexico metamorphism and migmatization of the might simply be the west-facing magmatic is composed of mid-crustal arc-related deformed arc rocks took place prior to the arc for Pacifi c Mexico (Herrmann et al., 1994) gneisses of poorly resolved ages, intruded by Cenozoic. Eocene and Oligocene plutons rep- presumably formed between the Jurassic and undeformed Cenozoic calc-alkaline plutons. resenting renewed arc-related magmatism the Late Eocene. Twelve undeformed and deformed tonalitic/ in the area are common throughout Xolapa, Two of the key elements required in decipher- granodioritic samples from three transects and probably represent the more deeply ing the tectonic history of Xolapa are (1) sort- across the Sierra Madre del Sur (Acapulco, exposed continuation of the Sierra Madre ing out the age and origin of its basement and Puerto Escondido, and Puerto Angel) were Occidental arc to the northwest. The avail- (2) resolving the timing of arc magmatism chosen for U-Pb zircon analysis. The mea- able U-Pb data argue against the previously and relationship to the surrounding arc-related surements were performed on single crystals proposed eastward migration of magmatism products in Central America. Both of these tasks of zircons, using a multiple-collector laser- between Acapulco and Puerto Angel during require high-precision geochronology of base- ablation inductively coupled plasma–mass the Oligocene. ment rocks. There is evidence that the arc was spectrometer (MC-LA-ICP-MS). About active in the Mesozoic and continued through- 20–30 crystals were measured from each Keywords: Xolapa Complex, arc magmatism, out much of the Cenozoic (Ortega-Gutierrez, sample. Three gneisses and migmatites from U-Pb zircon, geochronology, deformation. 1981). Unfortunately, most of the published the eastern transect (Puerto Angel), located Xolapa ages employed isotopic techniques (K- 30–42 km from the coast yielded Grenville- INTRODUCTION Ar, Rb-Sr) that yield cooling ages but not nec- aged zircons (970–1280 Ma), suggesting essarily crystallization ages (Morán-Zenteno et that the samples represent Oaxacan base- Geometric arguments based on plate-tec- al., 1999, for a compilation of ages and review). ment, not deformed Xolapa Complex. The tonic reconstructions and inferred boundaries One of the major limitations of the few previous central transect (Puerto Escondido) yielded between various apparently unrelated terranes U-Pb zircon studies (e.g., Robinson et al., 1989; Oligocene ages (25–32 Ma) on undeformed in Mexico indicate that most of Mexico is com- Herrmann et al., 1994; Schaaf et al., 1995) is plutons as well as mid-Mesozoic and Perm- posed of crustal elements that were accreted to that they employed multigrain fractions that ian ages on gneisses. Most samples along the North America after the Carboniferous (Dick- commonly yielded discordant ages; thus the age Puerto Escondido transect contain inherited inson and Lawton, 2001). The southern slopes interpretations are commonly nonunique. ca. 1.1 Ga xenocrystals of zircons. The west- of the Sierra Madre del Sur range comprise The purpose of this study was to acquire ern transect (Acapulco) yielded Late Juras- primarily arc-related rocks of the Xolapa Com- additional geochronological data on key loca- sic–Early Cretaceous ages (160–136 Ma) on plex (Campa and Coney, 1983), also known as tions within the Sierra Madre del Sur region. gneisses, and Paleocene (55 Ma) and Oligo- the Chatino terrane (Sedlock et al., 1993). It is The strategy was such that we would fi ll gaps cene (34 Ma) ages on undeformed plutons, a 600 km long, relatively narrow (<70 km) strip that remained after the publication of the previ- with no inherited Grenville ages. The older of mostly arc-related rocks straddling the coastal ous zircon U-Pb geochronology studies, in order ages and xenocrystic zircons in arc-related Pacifi c margin of southern Mexico The local to obtain a more complete picture of the plu- Xolapa Complex mirror the crustal ages geology of Xolapa is not known in detail, partly tonic and metamorphic age distribution within found in neighboring terranes (Mixteca and because of the thick vegetation cover and scarce Xolapa. The Xolapa zircons are commonly very Oaxaca) to the north of the Xolapa Com- access, and also because of the predominance of complicated, and some record multiple ages plex, suggesting an autochthonous origin of high-grade basement rocks, the age and origin even at crystal scale (Herrmann et al., 1994). Xolapa with respect to its neighboring north- of which are not resolved. Overall, Xolapa has We conducted our U-Pb study on small domains Gondwanan affi nities, as do the neighboring ter- within single zircon crystals, via multicollec- †E-mail: [email protected]. ranes of Mixteca and Oaxaca (e.g., Dickinson tor laser-ablation ICP-MS (MC-LA-ICP-MS),

GSA Bulletin; July/August 2004; v. 116; no. 7/8; p. 000–000; doi: 10.1130/B25467.1; 8 fi gures; 1 table; Data Repository item 2004xxx.

For permission to copy, contact [email protected] © 2004 Geological Society of America DUCEA et al. in order to effi ciently sort out crystallization GEOLOGICAL BACKGROUND map of the Xolapa Complex in the vicinity from inherited ages. MC-LA-ICP-MS U-Pb of Acapulco (modifi ed after Morán-Zenteno, geochronology (Kidder et al., 2003; Dickinson Geology 1992) and shows the overall geologic features and Gehrels, 2003; Ducea et al., 2003) is a new of the complex. The orthogneisses are ductilely technique that allows rapid and inexpensive age All rocks studied here are part of the Xolapa deformed and metamorphosed calc-alkaline determinations with a precision similar to sec- terrane (Campa and Coney, 1983), which diorites, tonalites, and granodiorites, and pre- ondary ion mass spectrometry. The technique extends along the Pacifi c margin of southern dominate over the paragneisses by a factor of can be used for both detrital zircon (provenance) Mexico in the states of Guerrero and Oaxaca at least 10. The orthogneisses represent a meta- studies and igneous geochronology. and is ~600 km long and 50–70 km wide morphosed sequence of continental arc rocks, Three key long-standing regional geologic (Fig. 1). Although thought by many authors to whereas the paragneisses are rare framework questions are addressed in this study using U-Pb represent an out-of-place terrane that may have rocks of this arc (Ortega-Gutierrez, 1981). The geochronology: (1) Is the Xolapa Complex a docked to mainland Mexico by the Late Cre- protolith age for the gneisses and migmatites traveled terrane, i.e., was it largely assembled taceous (Dickinson and Lawton, 2001; Campa is ca. 1.0–1.3 Ga, based on Nd model ages and at a remote location relative to its neighboring and Coney, 1983; Sedlock et al., 1993), some U-Pb ages (Herrmann et al., 1994), whereas blocks in southern Mexico? (2) When was the studies have argued that the geology of Xolapa, metamorphism is thought to have occurred magmatic arc active in Xolapa? (3) Does the while somewhat distinctive from the neighbor- sometime between the Early Cretaceous and time-space distribution of Cenozoic magma- ing terranes, represents a magmatic arc that Eocene (Riller et al., 1992; Herrmann et al., tism in the Xolapa constrain the reorganization formed in place during the Mesozoic and 1994; Meschede et al., 1997; Morán-Zenteno of plate boundaries in southwestern Mexico? continued into the Cenozoic (e.g., Herrmann et et al., 1996, 1999). The undeformed plutons, We show that (1) inherited zircons suggest an al., 1994; Schaaf et al., 1995; Morán-Zenteno calc-alkaline tonalites, and granodiorites autochthonous origin for the Xolapa Complex, et al., 1999). Thus, the Xolapa Complex are are also characteristic of rocks formed in a (2) magmatism was active in distinct episodes referred to as the Xolapa Complex by Her- continental magmatic arc setting (Martiny et with two major pulses in the Late Jurassic–Early rmann et al. (1994), a nongenetic terminology al., 2000), and are Eocene-Oligocene in age: Cretaceous and Eocene-Oligocene, and (3) mag- that we also adopt here. 36–23 Ma between Zihuatanejo and Puerto matism ceased abruptly in Xolapa at ca. 25 Ma The geology of the Xolapa Complex con- Angel (Schaaf et al., 1995; Morán-Zenteno as the North American–Pacifi c plate boundaries sists of high-grade orthogneisses and parag- et al., 1999; Herrmann et al., 1994). Trace were reorganized to form the modern Acapulco neisses as well as migmatites (Ortega-Gutier- element patterns in these plutons are char- Trench. In addition, we use the regional geo- rez, 1981), intruded by generally undeformed acteristic of subduction-related magmatism, logic data to address some general questions tonalitic to granodioritic plutons (Herrmann which suggests an enriched mantle source in on continental arc magmatism, specifi cally the et al., 1994; Meschede et al., 1997; Morán- the subcontinental lithosphere, modifi ed by causes of episodic high-fl ux magmatic events. Zenteno et al., 1999). Figure 2 is a geologic subduction fl uids (Martiny et al., 2000). The

A Mexico C Trans-Mexican TMVB City Volcanic Belt SMO and Eocene Maya 20O Morelia magmatic rocks Mixteco Guerrero Mexico City Inland Tertiary Toluca volcanic sequences Oaxaca Jalapa Tertiary Coastal plutonic belt Xolapa Juarez Puebla Pacific Ocean Mesozoic andCenozoic metamorphic rocks 100 km Early Mesozoic

O sedimentary cover 18 B Ordovician-Devonian

U.S.A o Acatlan Complex 30 N Zihuatanejo Gulf of Oaxaca Grenvillian Oaxacan Mexico Complex Mexico Acapulco Puerto Sample Locations Pacific 20o N Angel Ocean Pacific Ocean 0 300Km C

o o Puerto 110 W 90 W 0 50 100km Xolapa Escondido SMO TMVB 100O 98O 96O

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).

2 Geological Society of America Bulletin, July/August 2004 U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO

87 86 ε initial Sr/ Sr and Nd values of these plutons ary of the Xolapa Complex, with the oldest Tectonic History suggest a relatively low degree of crustal con- sediments being Miocene in age (20 Ma, tamination (Martiny et al., 2000). Herrmann et Watkins et al., 1981). The northern boundary Since the middle Cretaceous, the relation- al. (1994) proposed that the Eocene-Oligocene of the complex is poorly known. This bound- ship between the Farallon and North American magmatism has a distinct migration pattern ary is best studied in the Tierra Colorada plates changed several times (e.g., Engebretson from older in the west to younger in the east at region, north of Acapulco (Fig. 2), where et al., 1985): The plate boundary was predomi- a rate of 56 km/m.y. it is marked by a zone of cataclastic rocks nantly convergent, with both periods of oblique The Xolapa Complex does not have an on- and mylonites, indicating sinistral shear and subduction and periods when the margin was a land sedimentary cover proper (Were-Keeman north-south extension (Meschede et al., 1997; continental transform boundary. By 20 Ma, the and Estrada-Rodarte, 1999; Castillo-Nieto and Ratschbacher et al., 1991). In the Tierra Colo- Farallon plate had fractured into the Cocos and Rodriguez-Luna, 1996), in contrast to the adja- rada region, mylonitization occurred between Nazca plates (Meschede et al., 1997). The Cocos cent terranes (e.g., Campa and Coney, 1983; the Late Cretaceous and Paleocene-Eocene plate continued on a north-northeast trajectory Sedlock et al., 1993). The only sedimentary (Ratschbacher et al., 1991; Herrmann et al., with respect to the along cover to Xolapa is represented by a narrow 1994; Morán-Zenteno et al., 1999), and possi- the current southern Mexican border. Plate-tec- accretionary wedge of the modern Acapulco bly later to the east near Puerto Angel (Morán- tonic reconstructions are subject to large errors Trench, which represents the southern bound- Zenteno et al., 1999). 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 100˚00' are used to better understand the major changes 17˚15' M002 in relative motion between the Pacifi c and North American plates in southern Mexico (Ferrari et al., 1999) especially in late Cenozoic time, for TIERRA which numerous age data are available for vol- COLORADA canic and intrusive rocks. The current southern margin of the North American plate has experienced truncation along the southern margin of the Xolapa Com- plex (Malfait and Dinkelman, 1972; Ross and Scotese, 1988; Ferrari et al., 1994; Herrmann et M01-04 al., 1994; Schaaf et al., 1995). Though subduc- tion has been occurring at most times since the EL TREINTA early Mesozoic, the current margin displays M01-46 none of the mature margin characteristics of other 100 Ma margins. The 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 ACAPULCO 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 Quaternary M01-02 steep angles, also indicating margin truncation (Malfait and Dinkleman, 1972). The lack of Undeformed Tertiary Plutonic Rocks high-pressure/low-temperature rocks along the continental margin as well as the lack of an Cretaceous Morelos Formation accretionary prism landward from the Middle P A C I F I C O C E A N America Trench add additional support for Mesozoic Gneisses and Migmatites the hypothesis that truncation has occurred at some time (Schaaf et al., 1995; Meschede et al., Acatlan Complex 1997). The Middle America (Acapulco) trench 10 km along the southern Mexican margin has experi- enced accretionary sedimentation continuously 16˚00' since the Middle Miocene. The two mechanisms 100˚00' 99˚00' possibly responsible for truncation are (1) sub- Figure 2. Geologic map of the Acapulco transect (modifi ed after Morán-Zenteno, 1992), duction erosion (Morán-Zenteno et al., 1996) showing the distribution of gneisses/migmatites and undeformed plutons in the area. The and (2) lateral transport via transform faults, location of samples studied here from the Acapulco transect are also shown. which may have removed the older wedges, the

Geological Society of America Bulletin, July/August 2004 3 DUCEA et al. forearc, and even parts of the Xolapa Complex TABLE 1. SAMPLE LOCATION AND PETROGRAPHY (Schaaf et al., 1995). Sample Latitude Longitude Petrography U-Pb age summary† Other ages‡ (N) (W) (Ma) SAMPLES AND ANALYTICAL METHODS Puerto Angel transect M01-11 15°56′40″ 96°27′55″ Bt tonalitic gneiss 1252 ± 24, 1106 ± 10 Ma 23.4 M01-14 15°58′38″ 96°29′58″ Bt + Grt tonalitic gneiss 1029 ± 28 Ma Samples were collected along three north- M01-16 16°02′51″ 96°30′22″ Hbl + Bt tonalitic gneiss 1119 ± 24 Ma 17.6 south transects in the Sierra Madre del Sur: Puerto Escondido transect Acapulco, Puerto Escondido, and Puerto Angel M01-17 15°54′55″ 97°04′47″ Bt tonalitic gneiss 272 ± 10 Ma; 1100 Ma 27.9 M01-19 15°59′46″ 97°04′46″ Bt + Hbl granodiorite 158 ± 8.1 Ma; 1.1–1.2 Ga 15.5 (Fig. 1). At each sample locality, we collected M01-26 16°12′54″ 97°08′10″ Bt + Hbl diorite 25.4 ± 2.9 Ma; 1.1 Ga 1–2 kg of fresh whole rock. Samples were pre- M01-27 16°17′05″ 97°08′41″ leucotonalite 31.2 ± 1.5 Ma; 1016 ± 28 Ma 16.2 M01-28 15°51′28″ 97°03′33″ leucogranodiorite 29.6 ± 4.0 Ma; 50–71 and 100–126 Ma 15.7 pared for analysis using standard crushing and Acapulco transect separation techniques, including heavy liquids M01-46 16°47′28″ 99°49′43″ Hbl-syenite 54.9 ± 2.0 Ma 26.7 and magnetic separation, at the University of M01-02 16°22′37″ 99°26′21″ Bt + Hbl tonalitic gneiss 140.9 ± 4.5 Ma 17.7 M002 17°13′53″ 99°31′16″ Bt + Hbl tonalitic gneiss 136.6 ± 4.0 Ma Arizona. Inclusion-free zircons were then hand- M01-04 16°46′26″ 99°37′35″ Bt granodiorite 34.5 ± 1.2 Ma 37.6 picked under a binocular microscope. At least Note: Bt—biotite, Hbl—hornblende, Grt—garnet. 50 zircons from each sample were mounted in †Crystallization ages are listed fi rst; inherited ages, when determined, are shown in italics. epoxy and polished. ‡Apatite fi ssion track ages determined on the same samples by Shoemaker et al. (2002). Single zircon crystals were analyzed in polished sections with a Micromass Isoprobe multicollector ICP-MS equipped with nine the 207Pb signal. The 207Pb/235U and 206Pb/207Pb analyses of their mineralogy and modes indicate Faraday collectors, an axial Daly detector, and ages for younger grains accordingly have large clearly that all rocks are igneous or meta-igne- four ion-counting channels (Kidder et al., 2003). uncertainties (Table DR1). Interelement frac- ous, and calc-alkaline or alkaline in composi- The LA-ICP-MS analyses involve ablation of tionation of Pb/U is generally <20%, whereas tion. Their compositions (Table 1) are tonalitic, zircon with a New Wave DUV193 Excimer isotopic fractionation of Pb is generally <5%. granodioritic, granitic, and in one case (M01-46 laser (operating at a wavelength of 193 nm) In-run analysis of fragments of a large zircon hornblende-bearign quartz-syenite. using a spot diameter of 25 to 35 microns. The crystal from a Sri Lanka pegmatite (e.g. Dickin- ablated material is carried in argon gas into the son and Gehrels, 2003) with known age of 564 RESULTS plasma source of a Micromass Isoprobe, which ± 4 Ma (2-sigma error) is used to correct for this is equipped with a fl ight tube of suffi cient width fractionation (generally run every fi fth measure- Zircons from the Xolapa Complex samples that U, Th, and Pb isotopes are measured simul- ment). The uncertainty resulting from the cali- were analyzed optically under a petrographic taneously. All measurements are made in static bration correction is generally ~3% (2-sigma) microscope and a scanning electron microscope mode, using Faraday detectors for 238U, 232Th, for both 207Pb/206Pb and 206Pb/238U ages. (SEM) in backscattered electron (BSE) mode. 208–206Pb, and an ion-counting channel for 204Pb. The pooled crystallization ages reported in Zircons were grouped according to the mor- Ion yields are ~1 mV per ppm. Each analysis this paper are weighted averages of individual phology classifi cation of Pupin (1983) (Fig. 3). consists of one 20-second integration on peaks spot analyses. The stated errors (2-sigma) Almost all zircons that yielded Cenozoic ages with the laser off (for backgrounds), 20 one- on the assigned ages are absolute values and display igneous morphologies (e.g., euhedral second integrations with the laser fi ring, and a include contributions from all known random crystals) with no detectable optical zoning. The 30-second delay to purge the previous sample and systematic errors. The Mesozoic and older zircons we found are smaller rounded and prepare for the next analysis. The ablation Cenozoic ages interpreted from the ICP-MS grains that indicate a detrital origin and/or cor- pit is ~20 microns in depth. analyses are based on 206Pb/238U ratios because rosion in magma. For these zircons, we report Common Pb correction is performed by errors of the 207Pb/235U and 206Pb/207Pb ratios the core ages, which are always older than rim using the measured 204Pb and assuming an ini- are signifi cantly greater. This is due primarily ages. Rim ages typically have large errors (prob- tial Pb composition from Stacey and Kramers to the low intensity (commonly <1 mV) of the ably because the new rim growths are very nar- (1975) (with uncertainties of 1.0 for 206Pb/204Pb 207Pb signal from these young, U-poor grains. row) and are not reported in Table DR1, but the and 0.3 for 207Pb/204Pb). Measurement of 204Pb For grains older than 800 Ma, both 207Pb/206Pb ages are generally Cenozoic. is unaffected by the presence of 204Hg because and 206Pb/238U are reported. backgrounds are measured on peaks (thereby Twelve samples were analyzed for U-Pb Puerto Angel Transect subtracting any background 204Hg and 204Pb), and geochronology in this study and are grouped because very little Hg is present in the argon gas. based on their location along the three studied Three samples collected from north of Puerto For each analysis, the errors in determining transects, Puerto Angel, Puerto Escondido, and Angel (M01-11, M01-14, and M01-16) yield 206Pb/238U and 206Pb/204Pb result in a measure- Acapulco (Fig. 1). Ages of ~20–30 zircon grains exclusively Precambrian ages (Fig. 4). Sample ment error of several percent (at 2-sigma level) were measured from each sample. Results and M01-11 (n = 31 zircons) has a bimodal distri- in the 206Pb/238U age (Table DR1).1 The errors errors are reported in Tables 1 and DR1. Each bution of U-Pb ages, with one group averaging in measurement of 206Pb/207Pb are substantially line in Table DR1 represents a spot analysis. 1252 ± 24 Ma (MSWD = 1.2), and a second larger for younger grains due to low intensity of The samples are either deformed metamor- group averaging 1109 ± 10 Ma (MSWD = 0.9). phic rocks of clearly meta-igneous origin All zircons in M01-11 are concordant or near (orthogneisses and migmatitic orthogneisses) concordant within error (Fig. 4). M01-14 has an 1GSA Data Repository item 2004xxx, U-Pb zircon analytical data, is available on the Web at http:// or unmetamorphosed plutonic rocks. Although average age of 1092 ± 28 Ma (n = 29) and may www.geosociety.org/pubs/ft2004.htm. Requests may we have not analyzed the geochemistry of these also have a bimodal distribution of ages, although also be sent to [email protected]. rocks, their study under optical microscope and not as pronounced as M01-14. Peak ages are

4 Geological Society of America Bulletin, July/August 2004 U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO

(50–71 Ma) 206Pb/238U ages are found. One A. C. 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 B. D. 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 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 tonal- itic 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 granodio- Figure 3. Zircon morphology microphotographs. (A) SEM-CL image of zoned zircon from ritethat was collected along the new Highway sample M01-17; scale bar = 20 microns. (B) SEM-CL image of several analyzed zircons that 95, yields a 206Pb/238U age of 34.5 ± 2.2 Ma (n display complex zoning from sample M01-19; scale bar = 50 microns; laser ablation pits are = 19, MSWD = 0.4) with no inherited grains, ~20 microns deep. (C) SEM-CL image of individual zircon from sample M01-14; no zoning consistent with previously determined U-Pb is evident in this sample. (D) SEM-CL image of euhedral zircon from sample M01-46; both ages on neighboring undeformed plutons (Her- the core and the rim of this sample yielded the same age within errors; the age was inter- rmann et al., 1994). preted to be the crystallization age. INTERPRETATIONS

1244 ± 10 Ma (MSWD = 0.4), and 1163 ± component (Table DR1). These crystals are inter- The Boundaries of the Xolapa Complex 15 Ma (MSWD = 1.1). Most determined ages in nally zoned; the old ages are discordant but sug- M01-14 are concordant. Most zircons in sample gest that the inherited component is ca. 1.1 Ga, Samples of the Puerto Angel transect are M01-16 (21 out of 28) cluster around an average consistent with Oaxacan basement. Another interpreted to represent basement rocks of the age of 1119 ± 24 Ma (MSWD = 0.4), although analyzed gneiss (M01-19) has complicated zir- Oaxaca terrane, thus limiting the extent of the individual ages range from 950 to 1250 Ma. con systematics (Figs. 5B and 5F) due to zoning Xolapa Complex to less than 30 km from the Most ages in this sample are also concordant of individual crystals at a scale smaller than that coast along this transect. While most rocks (Fig. 4). There are no Phanerozoic zircons in of the individual laser pits. The crystallization investigated along this transect have a distinc- these samples. Overall, these ages are identical to age of this metaplutonic rock is interpreted to tive L-S tectonite fabric, they do not differ com- the ca. 1.0–1.2 Ga Grenville ages obtained on the be Jurassic (158.3 ± 8.1 Ma, MSWD = 10.5) (n positionally (tonalites, granodiorites) from most high-grade basement of the Oaxaca terrane (Ruiz = 15 zircons). The discordant ages displayed by of the deformed rocks of the Xolapa Complex. et al., 1988; Herrmann et al., 1994). the other 12 zircons (Fig. 5) are suggestive of an In addition, one of the samples analyzed here inherited 1.1–1.2 Ga component. (M01-16) is in fact a slightly deformed coarse Puerto Escondido Transect Three other samples along this transect have biotite- and amphibole-bearing granodiorite, Cenozoic ages (Figs. 5C, 5D, and 5E). A leu- identical in both composition and fabric to a Tonalitic gneiss M01-17 collected south of cogranite (M01-28) yielded an age of 29.6 ± typical Xolapa postkinematic Eocene-Oligocene Puerto Escondido has a 206Pb/238U age of 272 ± 4.0 Ma, MSWD = 38 (n = 8 zircons), although granitoid. This sample was collected just south 10 Ma (n = 21 zircons) (Fig. 5A), MSWD = 4.5. inherited grains with middle Cretaceous (100– of the Chacalapa shear zone, which is defi ned Three zircons contain an inherited Precambrian 126 Ma) and latest Cretaceous–early Cenozoic by some as representing the boundary between

Geological Society of America Bulletin, July/August 2004 5 DUCEA et al.

Xolapa and Oaxaca. The Xolapa Complex of A. M01-11 Mesozoic-Cenozoic arc-related rocks becomes 0.23 1300 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 0.21 1200 narrowness of the Xolapa Complex at Puerto Angel compared to Puerto Escondido may U 1100 0.19 be explained by the presence of a major fault 238 orthogonal to the strike of the Xolapa Complex Pb/ between Puerto Escondido and Puerto Angel, 206 0.17 1000 e.g., the Oaxaca fault (Nieto-Samaniego et al., 1995; Alanis-Alvarez et al., 1996). A signifi cant 900 late Cenozoic dextral strike-slip component on 0.15 the Oaxaca fault could explain the change in the width of the Xolapa Complex east of that fault; however, fi eld data can document only 0.13 dip-slip displacements on the Oaxaca fault dur- 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 ing the Cenozoic (Alanis-Alvarez et al., 1996; 207Pb/235U D. Morán-Zenteno, 2003, written commun.), whereas most of the strike-slip displacement 0.28 is Mesozoic. Alternatively, the Grenville ages B. M01-14 1500 reported in this study may suggest that the base- ment of the Xolapa Complex and the Oaxacan 1400 basement are indistinct and that perhaps the 0.24 deformed basement for the Xolapa Complex 1300 may be in fact Oaxacan basement at many other U

locales within the coastal Sierra Madre del Sur. 238 1200

Evidence for Permian Magmatism Pb/ 0.20 206 1100 Sample M01-17, a tonalitic gneiss collected ~7 km north of the Pacifi c Coast at Puerto 1000 Escondido, yielded a Permian U-Pb zircon age 0.16 (272 ± 10 Ma). This is an unusual age for the 900 Xolapa Complex, as it predates the Mesozoic and Cenozoic metamorphism and magmatism 1.2 1.6 2.0 2.4 2.8 3.2 that essentially defi ne the Xolapa Complex. 207Pb/235U 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 0.23 C. M01-16 1300 been reported in the Acatlan basement of the Mixteca terrane (Yanez et al., 1991; Robinson 1200 et al., 1989; Elias-Herrera and Ortega-Gutier- 0.21

rez, 2002) and the Juchatengo terrane (Grajales- U

Nishimura et al., 1999). If the Mixteca terrane, 238 1100 which is located north of the Xolapa Complex at 0.19 Pb/

the longitude of Puerto Escondido, is in fact the 206 basement for Xolapa, an out-of-place origin for 0.17 1000 the Xolapa Complex could be ruled out. Future data collection will be aimed at basement rocks 900 of the Xolapa Complex (e.g., rare metasedimen- 0.15 tary rocks) in order to further sort out the age distribution of the pre-arc Xolapa basement. 0.13 Mesozoic Arc Evolution and Ductile 1.2 1.6 2.0 2.4 2.8 Deformation 207Pb/235U

Three of our deformed samples, M01-19 (Puerto Figure 4. U-Pb ages for Puerto Angel in concordia plots. A: Sample M01-11. B: Sample Escondido), and M002 and M01-2 ( Acapulco), M01-14. C: Sample M01-16.

6 Geological Society of America Bulletin, July/August 2004 U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO

A. Age = 272 ± 10 Ma M01-17 D. Age = 31.2 ± 1.5 Ma M01-27 310 Mean = 272 ± 6 Ma 37 Mean = 31.2 ± 1.2 Ma (2σ) (2σ)

290 )

a 33 (M

ge 270 Age (Ma) A

29 250

230 25

Age = 158.3 ± 8.1 Ma M01-19 190 E. Age = 29.6 ± 4.0 Ma B. Mean = 158.3 ± 6.4 Ma σ Mean = 29.6 ± 3.8 Ma (2 ) (2σ) 36 )

a 170 (M ge Age (Ma) A 28 150

M01-28

130 20

Age = 25.4 ± 2.9 Ma M01-26 1200 0.20 F. C. Mean = 25.4 ± 2.9 Ma 32 (2σ) 1000 0.16

800

U 27 0.12

238 600

Pb/ Age (Ma) 206 0.08 22 400 0.04 200 M01-19

17 0.00 0.0 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.

Geological Society of America Bulletin, July/August 2004 7 DUCEA et al.

160 A. Age = 54.9 ± 2.0 Ma M01-46 C. M01-02 63 Mean =54.9 ± 1.1 Ma Age =140.9 ± 4.5a Mean =140.9 ± 1.5 Ma

150 59

140 55 Age (Ma) Age (Ma)

130 51

47 120 160 B. M01-04 M002 Age = 34.5 ± 1.2 Ma D. Mean =34.5 ± 0.2 Ma 37

140

34 Age (Ma) Age (Ma) 120 31 Age = 136.6 ± 4.0 Ma Mean =136.6 ± 1.4 Ma

28 100 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. yielded Late Jurassic–Early Cretaceous ages. The alkaline rock suite of Acapulco has a Evidence against Along-Strike Migration of Based on the zircon morphology (Pupin, 1983), distinctively older age of ca. 55 Ma compared Eocene-Oligocene Arc Magmatism we interpret these data to represent igneous crys- to other undeformed plutons in the region. It is tallization ages. Field observations (this study and part of a composite alkaline pluton extending Our new data, together with previously Morán-Zenteno, 1992) suggest that the timing of ~15 km across Acapulco Bay (Morán-Zenteno published U-Pb zircon age data on the Eocene- ductile deformation was concomitant with the et al., 1993), the eastern edges of which are only Oligocene undeformed magmatic rocks within intrusion (these are synkinematic plutons). 30 km from gneissic sample M01-02, which the Xolapa Complex, do not support the hypoth- Several of the earlier geochronologic studies yielded a Jurassic age. The emplacement depth esis of eastward migration of magmatism (Her- of the Xolapa Complex reported similar ages, for this suite was calculated by Morán-Zenteno rmann et al., 1994). Figure 7 shows that plutons based on U-Pb zircon geochronology (Fries et al. (1996) to be ~20 km, using Al-in-horn- with ages of 24–37 Ma were found within the and Rincon-Orta, 1965; Guerrero-Garcia, 1975; blende barometry. The emplacement of the Aca- western, central, and eastern transects described Robinson et al., 1989; Herrmann et al., 1994) and pulco suite is coeval with the proposed Cenozoic here, representing the eastern ~360 km along Rb-Sr whole-rock ages (Guerrero-Garcia, 1975; metamorphic event (Herrmann et al., 1994), and the strike of the arc. While it is possible that the Morán-Zenteno, 1992). However, Herrmann et al. the calculated depth of emplacement requires cessation of magmatism was diachronous along (1994) considered the pre-Cenozoic ages “specu- that these rocks would have been subject to the strike of the Oligocene Xolapa Complex, lative” and instead interpreted their U-Pb data to crystal plastic deformation in such a metamor- as a result of a change in the relative motion show evidence for an early Cenozoic (66–46 Ma) phic regime. The lack of plastic deformation of the Farallon and North American plates and age of magmatism, ductile deformation, and in the Acapulco intrusive suite puts important a transition from a subduction to transform metamorphism of the Xolapa Complex. The new constraints on the timing of metamorphism and margin, there is no obvious reason why the data presented in this paper lend strong evidence ductile deformation of the Xolapa Complex; we entire arc-related magmatic pulse should have to the previously postulated Late Jurassic–Early interpret these data to strongly argue for a pre- migrated along the strike of the arc (Herrmann Cretaceous magmatic/deformational event. Cenozoic age of metamorphism. et al., 1994). In conclusion, our new data and

8 Geological Society of America Bulletin, July/August 2004 U-Pb GEOCHRONOLOGY OF SOUTHERN MEXICO

40 previously published data show that the Eocene- 2001), Coast Mountains (Armstrong, 1988), and Oligocene magmatic pulse was not diachronous Peruvian batholiths (Pitcher, 1993), were not between Acapulco and Puerto Angel. steady-state but were rather formed during short fl are-up events separated by longer magmatic Acapulco Episodicity of Magmatism lulls. Available mapping and geochronologic data Puerto Puerto 20

Age (Ma) Escondido Angel indicate that the Early Cretaceous arc is exposed The Pacifi c margin of southern Mexico was to mid-crustal levels and represents some 40% of Sierra Madre Occidental a convergent margin after Middle Triassic time. the exposed Xolapa Complex (Morán-Zenteno, magmatic flare-up Arc-related magmatism is therefore expected to 1992). This magmatic pulse is correlative with This study Herrmann et al. have occurred at least intermittently for most of the Late Jurassic–Early Cretaceous fl are-up of 0 the last 200 m.y. However, our data and previ- the California arc (Ducea, 2001). Some 50% of ously published data for the Xolapa Complex the Xolapa Complex consists of large Eocene- Figure 7. Diagram showing the distribution indicate a rather episodic record of magmatism Oligocene (35–25 Ma) calc-alkaline plutons. of zircon U-Pb crystallization ages on unde- consisting primarily of two volumetrically The match in age with the magmatic fl are-up of formed plutons from the Xolapa Complex signifi cant pulses: one in the Early Cretaceous the Sierra Madre Occidental to the north (Fer- between Acapulco and Puerto Angel. Age (possibly extending into the Late Jurassic), and rari et al., 1999) suggests that these plutons may sources are this study and Herrmann et al. a second one that is Late Eocene–Oligocene represent the southern continuation of that arc. (1994). The diagram shows that there is no (Fig. 8). Plutons of different ages, such as the Compositionally, the Eocene-Oligocene plutons clear pattern of along-strike migration of arc Paleogene Acapulco composite intrusion, are of the Sierra Madre del Sur between Acapulco magmatism; instead all plutons formed within present locally but appear to be volumetrically and Puerto Angel (Martiny et al., 2000) are an ~10–15 m.y. window coincident with the less signifi cant in the Xolapa Complex. Other remarkably similar to the average composition magmatic fl are-up in the Sierra Madre Occi- continental arcs in North and South America, of the Sierra Madre Occidental arc. The deeper dental to the north from McDowell et al., 1978, such as the composite Sierra Nevada (Ducea, levels of exposure (Morán-Zenteno et al., 1996) and McDowell and Mauger, 1994). of the Eocene-Oligocene plutons in the Xolapa Complex are due to relatively high erosion rates that characterize the Sierra Madre del Sur moun- Ma Magmatism tain range since the Miocene (Morán-Zenteno et Deformation al., 1996; Shoemaker et al., 2002). 0 Pc Truncation #2-subduction Magmatism ended at ca. 25 Ma in the Sierra Madre del Sur. The Eocene-Oligocene conti- Mc erosion and arc inland migration nental margin was later truncated because the modern Middle America (Acapulco) trench Oc 1,2,3,4 is adjacent to the Eocene-Oligocene arc and Truncation #1- left oblique to it. The leading hypothesis for the lateral migration of the 50 Acapulco late Cenozoic continental truncation in Pacifi c

Eocene 1,5 Chortis block intrusive southern Mexico is tectonic erosion (or subduc- Pc tion erosion) (Morán-Zenteno et al., 1996). ? 2 ? CONCLUSIONS Regional ductile A proposed synthesis of magmatic and defor- deformation 100 mation events that led to the development of the Xolapa Complex, based on our new results

Cretaceous ductile deformation and previous data, is summarized in Figure 8. 1,5,6,7 at mid-crustal depths 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 ? 150 have been basement rocks of the Mixteca and 7 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 pro-

Jurassic 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 pose that renewed magmatism, not accompanied by deformation, took place in the Paleogene, and then again, more volumetrically signifi cant, Major magmatic flare-ups Timing poorly constrained during the Eocene-Oligocene. The available data on the Eocene and Oligocene magmatism do not support the hypothesis of eastward migration Figure 8. Summary diagram showing the major tectonic and magmatic events in the of magmatism, but rather point to a region- Xolapa Complex. ally extensive, relatively short-lived (~10 m.y.)

Geological Society of America Bulletin, July/August 2004 9 DUCEA et al.

plates in the Pacifi c basin: Boulder, Colorado, Geologi- de Oaxaca (Mexico) e infl uencia de las anisotropias fl are-up episode that may be related to the cal Society of America Special Paper 206, p. 1–64. litologicas durante su actividad Cenozoica: Univer- development of the Sierra Madre Occidental arc. Ferrari, L., Garduno, V.H., Innocenti, F., Manetti, P., sidad Nacional Autonoma de Mexico, Instituto de Magmatism ceased abruptly at ca. 25 Ma in the Pasquare, G., and Vaggelli, G., 1994, Volcanic and Geologia, Revista Mexicana de Ciencias Geologicas, tectonic evolution of Central Mexico: Geofi sica Inter- v. 12, p. 1–8. study area. Magmatism then migrated inland and national, v. 33, p. 91–105. Ortega-Gutierrez, F., 1981, Metamorphic belts of southern resulted in the formation of the Trans-Mexican Ferrari, L., Lopez-Martinez, M., Aguirre-Diaz, G., and Mexico and their tectonic signifi cance: Geofi sica Inter- Carrasco-Nunez, G., 1999, Space-time patterns of national, v. 20, p. 177–202. Volcanic Belt (Ferrari et al., 1999), which is Cenozoic arc volcanism in central Mexico: From the Ortega-Gutierrez, F., Elias-Herrera, M., Reyes-Salas, M.A., oblique to the strike of the Xolapa Complex. Sierra Madre Occidental to the Mexican Volcanic Belt: Macias-Romo, C., and Lopez, R., 1999, Late Ordovi- Geology, v. 27, p. 303–306. cian–Early Silurian continental collisional orogeny in Fries, C., and Rincon-Orta, C., 1965, Nuevas aportaciones southern Mexico and its bearing on Gondwana-Lau- ACKNOWLEDGMENTS geocronologicas y tecnicas empleadas en al labora- rentia connections: Geology, v. 27, p. 719–722. torio de geocronologia: Boletin Universidad Nacio- Pitcher, W.S., 1993, The nature and origin of granite: Lon- This research was partly supported by a nal Autonoma Mexico Instituto Geologico, v. 73, don, Blackie, Academic and Professional, 321 p. p. 57–134. Pupin, J.P., 1983, Typologie des zircons des termes satures research grant from the University of Arizona to Grajales-Nishimura, J.M., Centeno-Garcia, E., Keppie, J.D., intermediaries et differencies des series alkalines du Mihai Ducea. Fieldwork was supported by the and Dostal, J., 1999, Geochemistry of Paleozoic Mont-Dore et de la Chain de Puys (Massif Central basalts from the Juchatengo Complex of southern francais): Paris, Comptes Rendus des Seances de University of Arizona Geo-Structure Program Mexico: Tectonic implications: Journal of South l’Academiedes Sciences, v. 296, p. 761–764. and a grant from Chevron to Sarah Shoemaker. American Earth Sciences, v. 12, p. 537–544. Ratschbacher, L., Riller, U., Meschede, M., Herrmann, U., Interactions with Maria-Fernanda Campa, Dante Guerrero-Garcia, J.C., 1975, Contributions to paleomagne- and Frisch, W., 1991, Second look at suspect terranes tism and Rb-Sr geochronology [Ph.D. thesis]: Dallas, in southern Mexico: Geology, v. 19, p. 1233–1236. Morán-Zenteno, Barbara Martiny, Luca Ferrari, University of Texas, 131 p. Riller, U., Ratschbacher, L., and Frisch, W., 1992, Left-lat- and Joel Ramirez-Espinosa helped stimulate Herrmann, U., Nelson, B.K., and Ratschbacher, L., 1994, eral transtension along the Tierra Colorada deforma- this study. Journal reviews by Jim Mortensen, The origin of a terrane: U/Pb zircon geochronology tion zone, northern margin of the Xolapa magmatic arc and tectonic evolution of the Xolapa Complex (south- of southern Mexico: Journal of South American Earth James McLelland, and Associate Editor Calvin ern Mexico): Tectonics, v. 13, p. 455–474. Sciences, v. 5, p. 237–249. Miller have signifi cantly improved the quality Karig-Cordwell, D.E., Moore, G.F., and Moore, D.G., 1978, Robinson, K.L., Gastil, R.G., Campa, M.F., and Ramirez- Late Cenozoic subduction and continental margin trun- Espinosa, J., 1989, Geochronology of basement and of the manuscript. We are indebted to Rebekah cation along the Middle America Trench: Geological metasedimentary rocks in southern Mexico and their Wright and Alex Pullen for assistance in sample Society of America Bulletin, v. 89, p. 265–276. relation to metasedimentary rocks in Peninsular Cali- preparation and mass spectrometry. Mark Kidder, S., Ducea, M., Gehrels, G., Patchett, P.J., and Ver- fornia: Geological Society of America Abstracts with voort, J., 2003, Tectonic and magmatic development of Programs, v. 21, no. 5, p. 135. Baker provided maintenance support of the the Salinian Coast Ridge Belt, California: Tectonics, Ross, M., and Scotese, C.R., 1988, A hierarchical tectonic MC-LA-ICP-MS. 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MANUSCRIPT RECEIVED BY THE SOCIETY 6 AUGUST 2003 Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Rela- Nieto-Samaniego, A.F., Alanis-Alvarez, S.A., and Ortega- REVISED MANUSCRIPT RECEIVED 6 DECEMBER 2003 tive plate motions between oceanic and continental Gutierrez, F., 1995, Estructura interna de la Falla MANUSCRIPT ACCEPTED 12 JANUARY 2004

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