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Geologic Evolution of the Xolapa Complex, Southern Mexico: Evidence from U-Pb Zircon Geochronology

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Geologic Evolution of the Xolapa Complex, Southern Mexico: Evidence from U-Pb Zircon Geochronology 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 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.
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