Earth and Planetary Science Letters 369–370 (2013) 188–199

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Earth and Planetary Science Letters

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Origin and provenance of basement metasedimentary rocks from the Xolapa Complex: New constraints on the Chortis–southern Mexico connection

Oscar Talavera-Mendoza a,n, Joaquín Ruiz b, Pedro Corona-Chavez c, George E. Gehrels b, Alicia Sarmiento-Villagrana a, José Luis García-Díaz a, Sergio Adrian Salgado-Souto b a U.A. Ciencias de la Tierra, Universidad Autónoma de Guerrero, Ex-Hacienda San Juan Bautista s/n, Taxco el Viejo, Guerrero 40323, Mexico b Department of Geosciences, The University of Arizona, Gould-Simpson Building ♯77, 1040 E 4th St., Tucson, AZ 85721, USA c Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio “U” Cd. Universitaria, Morelia Michoacán 58030, Mexico article info abstract

Article history: The U–Pb (LA-MC-ICPMS) geochronology of Xolapa metasedimentary rocks from Tierra Colorada, Received 7 November 2012 Guerrero to Puerto Ángel, Oaxaca in southern Mexico reveals that their protoliths accumulated in two Received in revised form distinctive cycles of sedimentation, one of Early Jurassic age and another of Late Cretaceous age. These 17 March 2013 ages are younger than thought and demonstrate that Xolapa metasedimentary rocks are not rocks from Accepted 18 March 2013 the Acatlán or Oaxacan complexes or their Paleozoic sedimentary covers as claimed. However, detrital Editor: T.M. Harrison Available online 21 April 2013 zircon ages indicate that Xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably (para-)autochthonous origin for the Xolapa terrane. Xolapa rocks Keywords: record two major tectonothermal events of 64–59 Ma and ~34 Ma; the first event produced the high- U–Pb geochronology grade metamorphism and widespread migmatization that characterize Xolapa and the second event is provenance likely related to extended heating produced by coeval arc plutonism. tectonothermal evolution origin of Xolapa Pre-Jurassic assemblages of the Chortis block of Central America contain zircon populations that Chortis paleogeography significantly coincide with those recorded in both, the Acatlán and Oaxacan complexes and their Paleozoic sedimentary covers as well as with those recorded in Xolapa metasediments, which suggests a spatial connection among these petrotectonic assemblages during much of the Mesozoic. It is proposed that Xolapa was generated in a basin floored by Permian rocks flanked on one side by southern Mexico terranes and on the other side by the Chortis block. Contraction of the basin tied to the approach and accretion of the Guerrero terrane arc assemblages during Late Cretaceous time produced crustal thickening and high-grade metamorphism and migmatization at mid-crustal levels. Diachronic exhumation of Xolapa began during Early Paleogene time very likely promoted by the detachment and migration of the Chortis block. The migration would additionally produce slicing of Xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern Mexico. & 2013 Elsevier B.V. All rights reserved.

1. Introduction Morán et al., 1996; Ducea et al., 2004; Corona et al., 2006) and thus, the knowledge of its nature, age, provenance and tectonothermal The Xolapa terrane occupies much of the Pacific margin of evolution is required in the geodynamic reconstruction of southern southern Mexico and represents one of the most enigmatic terranes Mexico. Most paleogeographic models place the Chortis block of of southern North America (Campa and Coney, 1983). It has been Central America facing southern Mexico (e.g., Pindell and Kennan, suggested that this terrane records processes of terrane accretion, 2009 and references therein). Based on these models, some authors terrane translation, margin truncation, subduction erosion, arc have suggested that the interaction between these two continental magmatism, high-grade metamorphism and migmatization (e.g., masses produced much of the petrotectonic structures of Xolapa and the intricate geological evolution that characterizes the Pacific coast of southern Mexico (e.g. Morán et al., 1996; Corona et al., n Corresponding author. Tel./fax: þ52 762 621 34 66. 2006; Rogers et al., 2007; Ratschbacher et al., 2009; Torres de León E-mail addresses: [email protected] (O. Talavera-Mendoza), et al., 2012). Accordingly, the evolution of Xolapa may also have [email protected] (J. Ruiz), [email protected] (P. Corona-Chavez), important consequences in the understanding of the Chortis– [email protected] (G.E. Gehrels), [email protected] (A. Sarmiento-Villagrana), [email protected] (J.L. García-Díaz), southern Mexico connection and on the tectonic process that [email protected] (S.A. Salgado-Souto). assembled the Caribbean Plate.

0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.03.021 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 189

The origin of the Xolapa terrane is controversial. While some with neighboring terranes is complex involving left-lateral authors (e.g., Campa and Coney, 1983; Dickinson and Lawton, transtensional shear zones and brittle faults (e.g., Riller et al., 2001; Corona et al., 2006) propose an allochthonous origin based 1992; Tolson, 2005). on differences in lithology, style of deformation and thermal The Xolapa terrane comprises (1) a basement assemblage evolution relative to adjacent terranes, others (e.g., Riller et al., represented by the Xolapa Complex, which includes metaplutonic 1992; Herrmann et al., 1994; Morán et al., 1996; Ducea et al., 2004) rocks of Mesoproterozoic (?) and Paleozoic age (Herrmann et al., claim an autochthonous origin founded on the geochronology and 1994; Ducea et al., 2004), metasedimentary rocks of unknown deformation of metaplutonic rocks. These authors further propose origin and age and metaplutonic continental arc assemblages of that Xolapa represents a composite of rocks from adjacent terranes Jurassic–Cretaceous age (e.g., Morán et al., 1996; Solari et al., 2007; in situ transformed by high-grade metamorphism and migmatiza- Pérez et al., 2009). High-grade metamorphism, ductile deforma- tion. Workers agree, however, that Xolapa represents the root of tion and widespread migmatization affected the Xolapa Complex continental magmatic arcs of Jurassic, Cretaceous and Paleogene (Corona et al., 2006) between 130 and 46 Ma (Herrmann et al., age, experiencing metamorphism and migmatization during 1994; Pérez et al., 2009); (2) undeformed granitic to dioritic Cretaceous–Paleogene time, and rapid exhumation during Neogene plutons of Oligocene–Miocene age (Herrmann et al., 1994; Ducea time. et al., 2004); and (3) narrow doleritic dikes of unknown origin and Little or nothing is known about depositional age and prove- age. Except for Miocene to Quaternary sediments accumulated nance of Xolapa metasedimentary rocks, and the age of tecto- near the Acapulco trench, this terrane lacks a volcanic or sedi- nothermal events is only roughly constrained or even contradictory. mentary cover (Campa and Coney, 1983). It is believed that these rocks enclose important insights on the origin of the Xolapa terrane, its tectonothermal evolution and its possible connection with the Chortis block and other Mexican 3. Sampling and analytical methods terranes. To place constraints on these subjects, we performed a detailed U–Pb (LA-MC-ICPMS) geochronological study of zircons Zircons from fifteen samples of metasedimentary rocks from from metasedimentary basement rocks of Xolapa cropping out the Xolapa Complex were analyzed by MC-LA-ICPMS. Samples between Tierra Colorada, Guerrero, and Puerto Ángel, Oaxaca. The come from localities between Tierra Colorada, Guerrero, and new data provide an estimation of the age of sedimentation and Puerto Ángel, Oaxaca, facing the Mixteca, Juchatengo and Oaxaca provenance of Xolapa and precise ages for its major tectonothermal terranes (Fig. 1). Two additional samples from the Tecpan area events. The data also provide a robust database: (a) to compare (facing the Guerrero terrane) were processed but no zircons were Xolapa with adjacent terranes and the Chortis block; (b) to discuss recovered. The goal of this sampling strategy is twofold: (1) to the origin of Xolapa, (c) to evaluate the interaction of Chortis with analyze samples from the most representative regions of Xolapa; southern Mexico; and, (d) to place constraints on the paleogeography and (2) to document if detrital ages of Xolapa metasediments of Chortis before the Paleogene. mirror detrital ages of formations from neighbor terranes as proposed by Herrmann et al., 1994 and Ducea et al. (2004) in metaigneous rocks. The geographic location and analytical data of 2. Geological setting samples are reported in Table A.1. The procedures for zircon extraction and the analytical method The Xolapa terrane is a crustal block ~600 km long and for LA-MC-ICPMS, U–Pb geochronology have been comprehen- 30–65 km wide extended along the Pacific coast of southern sively described by Talavera et al. (2005), Gehrels et al. (2006, Mexico (Fig. 1). It is juxtaposed against the Mixteca, Juchatengo 2008) and Gehrels (2011). Procedure for zircon extraction included and Oaxaca terranes of Mesoproterozoic–Paleozoic, Paleozoic and jaw crushing, Wilfley table pre-concentration, heavy liquids and Mesoproterozoic age, respectively, and in part against the Guerrero magnetic separation methods. A large fraction of the recovered and Juárez terranes of Late Jurassic–Cretaceous age (Fig. 1). Where zircons was mounted in epoxy resin and polished. Both low and not obscured by Cenozoic plutons, the contact of the Xolapa high resolution CL images were obtained from all dated samples in

Guerrero Tehuacan Mixteca Acatlán Juarez Xolapa Oaxaca

Chilpancingo 04 Juchatengo

230 136 52 54 53 198 298 Oaxaca Legend 200111 Acapulco 201 204 Post-Jurassic rocks Xolapa Complex N Punta Maldonado Acatlán Complex 211 213A Puerto 73 Oaxacan Escondido Puerto Angel Complex

Fig. 1. Sketch map of the Xolapa terrane and its relationship with major terranes of southern Mexico (after Ortega-Gutiérrez et al., 1999). Inset shows terrane map of southern Mexico after Campa and Coney (1983). Leveled stars indicate location of dated samples. Black stars¼Samples defining the Lower Jurassic depositional cycle; White stars¼Samples defining the Late Cretaceous depositional cycle. 190 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 a Hitachi 3400N SEM equipped with a Gatan Chroma CL system at metamorphic sources. Rims and some bipyramidal grains are the University of Arizona and were used to select dated zircons characterized by variable but generally high (0.6–505) U/Th ratios and domains. One hundred zircons were analyzed from each rock and U contents (516–3054 ppm) compatible with overgrowth and/ and analyzed grains were selected at random from all of the or crystallization during fluid-present metamorphism and migma- zircons mounted from each sample. The exception was in samples tization events (see Fig. 5). that yielded few zircons in which the largest possible number of The U–Pb geochronology of detrital cores shows that metase- zircon grains was analyzed. Cores of grains were preferred to dimentary rocks of Xolapa form two distinctive groups: bracket detrital ages and rims to reveal metamorphic overgrowths. We ensured that selected cores and overgrowths were larger than Group I is defined by samples Xo-04, PO111, PO136, PO198, the laser spot used for ablation to avoid compromising ages as PO200, PO201, PO298, PO204, PO211 and Xo-73 (Fig. 3) with much as possible. the youngest zircon clusters between 272 and 199 Ma. Some of Analyses were performed at the Arizona LaserChron Laboratory these rocks are undoubtedly intruded by 179–158 Ma meta- from the Geosciences Department, University of Arizona, USA granitoids (Herrmann et al., 1994; Morán et al., 1996; Ducea using a Nu Plasma HR MC-ICPMS equipped with twelve faraday et al., 2004; Pérez et al. 2009), which constrains the age collectors to measure U and Th isotopes and four low-side ion of deposition between 199 and 179 Ma (Early Jurassic). Major counters to measure 208−204Pb. Plasma is connected to a new zircon populations in these samples occur at 1325–1032 Ma, Photon Machines Analyte G2 excimer laser for ablation. Analyses 984–551 Ma, 531–445 Ma, 437–365 Ma, 352–299 Ma, 298– were conducted in static mode using a laser beam diameter 203 Ma and 199 Ma (Fig. 3). The degree of overlap among ranging from 50 to 25 μm. Isotopic fractionation was monitored Group I samples is moderate to high ranging from 0.51 to 0.87 by analyzing an in-house zircon standard with a concordant TIMS (most are 40.65) whereas the degree of similarity is 0.46–0.85 age of 56474 Ma. This standard was analyzed once for every five (most are 40.65; Table A.2). This is consistent with the unknowns. The age probability plots used in this study were similarity observed in their cumulative patterns and the age constructed using the 206Pb/238U age for young (o1.0 Ga) zircons of their major zircon populations (Fig. 3) and suggests deriva- and the 206Pb/207Pb age for older (41.0 Ga) grains using Ludwig tion from similar parental sources. (2008). We only considered an age geologically meaningful if three Group 2 is defined by samples PO52, PO53, PO54, PO213A and or more grains yield overlapping 206Pb/238Uor206Pb/207Pb ages PO230 (Fig. 4) with the youngest zircon clusters between 152 defining a cluster although analyses that do not belong to a cluster and 101 Ma. These ages together with the ~59 Ma age of may also be accurate but, its significance is uncertain (for a crosscutting leucogranites (e.g. Morán, 1992) and the age of detailed explanation of these criteria see Talavera et al., 2005; metamorphism and migmatization (64–59 Ma; see below) Gehrels et al., 2006, 2011). constrain the age of deposition between 101 and 64 Ma (Late The degree of overlap and similarity of samples was calculated Cretaceous). These samples contain up to 98% of detrital grains following Gehrels (2000) as a quantitative way of comparing age with ages younger than 201 Ma except for sample PO213A, cumulative patterns and to reveal common parental sources. which has a large population of zircons between 345 and Following Gehrels (2000), the degree of overlap is here considered 214 Ma. Four grains yield older ages of 1158, 773, 313 and as a measure of whether two or more age cumulative patterns 219 Ma (Fig. 3). Major age peaks in Group 2 samples are at 188, contain the same age clusters attesting to derivation of common 178–162 Ma, 152 Ma and 138–101 Ma. Sample PO213A contains parental sources whereas the degree of similarity is a measure of major peaks at 321, 306–294, 263, 206–166, 138–130 and whether overlapping ages have similar proportions, i.e. similar 104 Ma. Despite the great similarity of their cumulative age number of grains forming a cluster or age peak. Similarity is highly patterns and their similar age peaks (Fig. 4), the degree of dependent on the number of grains analyzed in each compared overlap is rather low falling in the range 0.20–0.71 (most are sample. Correlation matrices are presented in Table A.2. o0.50). The degree of similarity is also low ranging from 0.12 to 0.87 (most are o0.50). These apparent low correlations are very likely an artifact caused by the unimodal age spectra of 4. Results most samples (Gehrels, 2000).

A detailed description of each sample including rock type, field Zircon rims in samples of both groups define two lines of Pb relationships, zircon morphology, U/Th ratios, U contents and loss evidencing the existence of at least two major tectonothermal zircon geochronology is presented in Appendix A as a supplemen- events affecting Xolapa rocks. The plot age versus U/Th of zircon tary data file. Figs. 3 and 4 show concordia diagrams and rims (Fig. 5) depicts the two tectonothermal events evidenced by cumulative patterns as well as the major zircon populations an increase of the U/Th ratios and a U enrichment, which have recorded in each sample. The full set of analytical data and sample been classically related to fluid-rich metamorphism and migma- location is shown in Table A.1. tization (Rubatto, 2002; Gehrels et al., 2009). The cumulative age With the exception of two samples from the Tecpan area (NW pattern of zircon rims (Fig. 5)defines three major age clusters Xolapa), which yielded no zircons, all processed samples had at 64 Ma (n¼31), 59 Ma (n¼33) and 34 Ma (n¼12). The two zircons 40–200 μm long and ranging in morphology from elongate former are interpreted to define a same tectonothermal event of (~4:1) to nearly round (~1:1). Some samples yielded also bipyr- 64–59 Ma. amidal zircons. Catholuminiscence images (Fig. 2a and b) show that some grains do contain a large core of obvious detrital origin and rims, generally less than 25 μm wide, inferred to be of 5. Discussion metamorphic or migmatitic origin. Some bipyramidal zircons certainly derive from migmatitic veins but others are definitely 5.1. Depositional ages detrital. Regardless of variations, cores of zircons in all samples have low (o8) U/Th ratios and low U contents (most samples Our data indicate that protoliths of Xolapa metasedimentary o500 ppm on average), which indicates that most zircons prob- rocks undoubtedly derived from two cycles of sedimentation ably derivate from ultimate magmatic sources. Cores having of the Early Jurassic and Late Cretaceous age. These cycles of high U/Th ratios and U contents may derive, however, from sedimentation are separated and constrained by two periods of arc O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 191

Fig. 2. Cathodoluminiscence images of representative zircons from Xolapa metasedimentary samples of (a) Lower Jurassic age and (b) Late Cretaceous age showing the internal structure of dated grains. Note the presence of wide metamorphic overgrowths around some detrital cores in many grains of both depositional cycles. Circles indicate the sites of ablation and the number indicates individual ages. Solid scale bar¼100 μm; dashed scale bar¼50 μm. magmatism of Early to Late Jurassic (179–158 Ma) and Early (Campa and Coney, 1983; Sedlock et al., 1993) or as a composite of Cretaceous (ca. 130 Ma) age (Ducea et al., 2004; Solari et al., in situ transformed rocks from the neighboring Guerrero, Mixteca, 2007; Pérez et al., 2009). These depositional ages represent the Juchatengo, Oaxaca and Juárez terranes (Riller et al., 1992; first reliable age estimation of sedimentation in Xolapa and refute Herrmann et al., 1994; Keppie, 2004; Ducea et al., 2004). Authors the Paleozoic or even Precambrian ages previously inferred further proposed that Xolapa is mainly composed of rocks from (Morán et al., 1996 and references therein). the Acatlán (Mesoproterozoic–Paleozoic), Juchatengo (Late Paleo- The Xolapa terrane is interpreted either as an allochthonous zoic) and Oaxaca (Mesoproterozoic) Complexes (e.g., Herrmann terrane whose geological evolution is exotic to adjacent terranes et al., 1994: Ducea et al., 2004) and/or their sedimentary covers 192 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199

Fig. 3. Concordia diagram and cumulative age patterns of Lower Jurassic metasedimentary rocks of Xolapa. Concordia inset shows the youngest ages in sample. Numbers in cumulative patterns indicate the age of major clusters. All significant zircon populations are reported in inset table. The number (n) of analyzed zircons is also indicated. Met/Migma¼Age of metamorphism and/or migmatization; MDAge¼Maximum depositional age. metamorphosed and migmatized during the Late Mesozoic–Early Formation and is composed of acidic lava flows dated at 179 Ma Cenozoic as a consequence of the interaction of Chortis with (U–Pb SHRIMP; García-Díaz, 2004), which is the minimum deposi- southern Mexico. The Early Jurassic and Late Cretaceous deposi- tional age of the Lower Jurassic metasedimentary rocks of Xolapa. tional ages reported here demonstrate that Xolapa metasediments The El Rosario Formation has been reported in Central-Northeast cannot be rocks of the Acatlán, Juchatengo or Oaxacan complexes Mexico in the Sierra Madre Oriental Province and in the Tlaxiaco or their respective Paleozoic covers. Pre-volcanic basement rocks Basin in eastern Oaxaca and Veracruz States (López-Ramos, 1981), in the Guerrero terrane are known in the Arteaga area, but they but no reports of this formation exist in the Mixteca or Oaxaca have been dated at Permian–Triassic (Centeno et al., 1993), and terranes. Moreover, this formation has been dated at Toarcian consequently, Xolapa rocks cannot be correlated with Arteaga [latest Early Jurassic (183–176 Ma); López-Ramos, 1981], which rocks either. Our data further indicate that Xolapa evolution is overlaps the minimum depositional age of 179 Ma of the Lower younger than thought placing sedimentation in the time of Jurassic cycle of Xolapa constrained by well-dated crosscutting deposition of the Mesozoic units in adjacent terranes. Sedimentary plutons (Ducea et al., 2004; Solari et al., 2007; Pérez et al., 2009). strata of Early Jurassic (199–179 Ma) age have not yet been The Early Jurassic (199–179 Ma) age of the first depositional cycle reported in the Mixteca or Oaxaca terranes of southern Mexico. of Xolapa metasedimentary rocks rather coincides with the age of The Tecocoyunca Formation is Bajocian to Callovian (Middle a major event of deformation and erosion recorded in the Mixteca, Jurassic; 172–161 Ma) in age (Erben, 1956; Alencaster, 1963). The Juchatengo and Oaxaca terranes (Campa and Coney, 1983; Sedlock underlying Cualac Formation, considered in some maps of Mexico et al., 1993), which produced denudation of their basement rocks of Early Jurassic age based on its stratigraphical position, contains and their Paleozoic covers. On the contrary, sedimentary rocks of at the base acidic lava flows dated at 177 Ma (U–Pb SHRIMP; Late Cretaceous age are well known in all adjacent terranes and García-Díaz, 2004). The Las Lluvias Formation underlies the Cualac correspond to Albian–Cenomanian platform limestone of Morelos O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 193

Fig. 4. Concordia diagram and cumulative age patterns of Upper Cretaceous metasedimentary rocks of Xolapa. Concordia inset shows the youngest ages in sample. Numbers in cumulative patterns indicate the age of major clusters. All significant zircon populations are reported in inset table. The number (n) of analyzed zircons is also indicated. Met/Migma¼Age of metamorphism and/or migmatization; MDAge¼Maximum depositional age. Formation and Turonian–Maastrichtian limestone, sandstone and Cambro-Ordovician (531–445 Ma), Devonian–Silurian (437–365 Ma), shale of Mexcala Formation and thus, these units and Upper Carboniferous (352–299 Ma), Permo-Triassic (298–203 Ma) and Cretaceous Xolapa metasediments may be correlatives. Earliest Jurassic (199 Ma) age. Paleozoic to Proterozoic rocks have been extensively reported in units from the Acatlán and Oaxacan 5.2. Provenance complexes and their Paleozoic covers (Gillis et al., 2005; Talavera et al., 2005), which represent the more plausible sources of Xolapa zircons. Cores of zircons in Lower Jurassic rocks define major populations Mesoproterozoic–Neoproterozoic detrital grains are widely distributed of Mesoproterozoic (1325–1032 Ma), Neoproterozoic (984–551 Ma), in all metasedimentary units from the Acatlán Complex (Xayacatlán, 194 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199

Fig. 5. Age versus U/Th and cumulative plots of rims and some bipyramidal zircons of Xolapa metasedimentary rocks evidencing the two major tectonothermal events recorded. The age of major clusters and the number (n) of analyses composing the cluster are indicated in the cumulative patterns.

Rodeo, Esperanza, Ixcamilpa, Cosoltepec, Chazumba) and its Paleozoic cover (Olinala Fm) and are also common as inherit cores in its granitic rocks (Talavera et al., 2005; Vega-Granillo et al., 2007). Mesoproter- ozoic zircons are the main age component of Oaxacan rocks and their Paleozoic sedimentary cover (Gillis et al., 2005) and Neoproterozoic (ca. 917 Ma) granites are known for intruding into the Oaxacan Complex (Ortega-Obregón et al., 2003). Some units from the Acatlán (Ixcamilpa, Tecomate and Chazumba) contain a large population of Cambro-Ordovician zircons (Sánchez-Zavala et al., 2004; Talavera et al., 2005). Furthermore, granites of Ordovician age are common in the Acatlán (Talavera et al., 2005)andzirconsofsuchanageare present in the Santiago and Ixtaltepec formations covering the Oaxacan Complex (Gillis et al., 2005). Devonian to Silurian granites are the most widespread plutonic rocks in Acatlán (e.g., La Noria Granite, Esperanza Granitoids). Devonian zircons are also present in some metasedimentary units from Acatlán (Cosoltepec, Tecomate, Chazumba) and in the Ixtaltepec Formation on the Oaxacan Complex (Sánchez-Zavala et al., 2004; Gillis et al., 2005; Talavera et al., 2005). Carboniferous leucogranites have been reported in the Acatlán Com- plex (Murphy et al., 2006; Vega-Granillo et al., 2007). Permo-Triassic zircons derive probably from coeval granitic rocks occurring in the Xolapa (Ducea et al., 2004), Acatlán (Yañez et al., 1991), Oaxacan (Solari et al., 2001)andJuchatengo(Grajales et al., 1999) complexes and/or from the Chiapas Massif (Weber et al., 2007). Some units from the Acatlán Complex (Tecomate, Chazumba) and its Permian cover (Olinala Fm.) also contain zircons of these ages and represent addi- tional potential sources. There are no reports of Lower Jurassic (204– 199 Ma) magmatic or sedimentary rocks in the Mixteca or Oaxaca terranes. The nearest rocks of 204–199 Ma occur in the Nazas arc of central Mexico (Dickinson and Lawton, 2001; Centeno et al., 2008)and in Chiapas (Godínez-Urban et al., 2011) representing the most prob- Fig. 6. Integrated cumulative patterns of Xolapa metasedimentary rocks, Acatlán able parental source for the Earliest Jurassic zircons in Xolapa rocks. and Oaxacan complexes and their Paleozoic sedimentary covers, Guerrero Terrane Upper Cretaceous rocks have very similar cumulative zircon arc assemblages and Chortis block. Dark gray plots are patterns of zircons older patterns (Fig. 4) indicating derivation from similar sources. Cores than 199 Ma (Early Jurassic); dotted line in the Xolapa plot represents zircon in these samples define populations of Early Jurassic (188 Ma), populations of Upper Cretaceous rocks; dotted lines in the Acatlan Pz cover, – – Guerrero and Chortis plots represent ages of Jurassic Cretaceous rocks or zircons Middle Jurassic (178 162 Ma), Late Jurassic (152 Ma) and Early in their sedimentary covers. Zircons older than 2000 Ma were not considered in the Cretaceous (138–101 Ma) age. Sample PO213A contains, in addi- comparison. For references see the Table A.2 in the supplementary files. tion, zircons of Carboniferous (321–306 Ma) and Permo-Triassic (294–206 Ma) age. As stated before, Lower Jurassic rocks are unknown in the Mixteca and Oaxaca terranes and the nearest the participation of more distant sources cannot be completely known sources are the Nazas arc of central Mexico and the Chiapas ruled out. As noted before, Carboniferous zircons may derive from Massif. Jurassic rocks of 179–152 Ma are known in the Guerrero Acatlán units whereas Permian rocks are present in the Xolapa, (Tumbiscatio), Mixteca (La Montaña) and Xolapa (La Venta) Acatlán and Oaxacan complexes as well as in the Chiapas Massif terranes (García-Díaz, 2004; Centeno et al., 2003; Pérez et al., representing the most likely sources. 2009) representing the most likely sources. Lower Cretaceous Fig. 6 shows integrated cumulative age patterns of Xolapa volcanic rocks of 138–101 Ma are reported in the Guerrero metasedimentary rocks as well as from the neighboring Mixteca, (Teloloapan, Arcelia and Zihuatanejo) and Mixteca (Taxco–Taxco Oaxaca and Guerrero terranes. Aside from obvious differences in Viejo) terranes representing potential sources (Talavera et al., the proportion of zircon grains composing age peaks, the matching 2007). Plutonic rocks of ~140–130 Ma are reported in Xolapa of major zircon clusters of Lower Jurassic rocks of Xolapa with (Solari et al., 2007) representing an additional source. Even though those of Acatlán and Oaxacan complexes is noticeable. The degree the elongate to near euhedral morphology of zircons suggests little of overlap of Lower Jurassic Xolapa rocks with Acatlán and transport and point to these rocks as the most probable sources, Oaxacan rocks is coherently high (0.89 and 0.70, respectively). O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 195

The degree of similarity is slightly lower, 0.76 with Acatlán and reconstructions of Pangea place the Chortis block in a Pacific 0.58 with Oaxacan, reflecting the obvious differences in the position and suggest continuous interaction with Mexican terrains number of grains composing major zircons populations. These during the Mesozoic and its translation to the Caribbean plate at observations ratify the Acatlán Complex and its Paleozoic sedi- the beginning of the Paleogene (e.g. Pindell and Kennan, 2001; mentary cover as the most plausible source for zircons recorded in Pindell and Kennan, 2009). Even though the argument is geody- the Lower Jurassic metasediments of Xolapa. Even though this namic, some authors have envisaged direct lithological, structural source alone can account for all zircon populations in Xolapa, and geophysical correlations. Basement rocks of Chortis consist of contribution of the Oaxacan Complex and its Paleozoic cover Paleozoic low grade metamorphic rocks and isolated inliers of cannot be ruled out. Mesoproterozoic medium to high grade gneissic rocks, which have Cretaceous volcaniclastic rocks from the Guerrero terrane also been correlated with the Acatlán and Oaxacan complexes (Rogers have the zircon populations recorded in the Lower Jurassic et al., 2007; Ratschbacher et al., 2009). Lower Jurassic and Cretac- metasediments of Xolapa (Fig. 5). These rocks are now regarded eous plutonic rocks are widespread in both Chortis and southern as having been formed in off-shore arcs fringing the Mixteca Mexico. Mills (1998) suggested that lower Cretaceous platform (Acatlán Complex) during Late Jurassic–Cretaceous and hence limestones and mylonite zones in Chortis and southern Mexico their zircon populations (Talavera et al., 2007). The degree of shared the same geological evolution and that they may be overlap between Xolapa and Guerrero is high (0.91), which correlatives. In turn, Silva (2008) suggests that the Guayape indicates that both assemblages received detritus from the same (Chortis) and Papalutla (southern Mexico) fault systems are parental sources. correlated. Rogers et al. (2007) recognize a distinctive magnetic Jurassic and Cretaceous age peaks in the Upper Cretaceous boundary between the Guerrero terrane and nuclear Mexico that metasediments of Xolapa match perfectly with ages recorded in correlates with a similar boundary between the southern and volcanics of the Guerrero and western Mixteca arc assemblages central Chortis terranes, which would evidence geological con- (Fig. 5) but coeval granitic rocks are also reported in the Xolapa tinuity between southern Mexico and Chortis during part of the (Solari et al., 2007; Talavera et al., 2007; Pérez et al., 2009). Mesozoic. Finally, the petrogenetic study by Corona et al. (2006) of southern Xolapa indicates that migmatization and the associated 5.3. Tectonothermal evolution metamorphism was produced by continental-arc shortening as a consequence of the collision of a continental block with nuclear Previous studies constrained the age of metamorphism and Mexico proposing the Chortis block as the most probable colliding migmatization in the Xolapa between 66 and 46 Ma. U–Pb ages, as block. old as 136 Ma and as young as 35 Ma, have been reported for some Based on the lack of deformation in the Upper Cretaceous to migmatites suggesting long-term metamorphism and migmatiza- Recent sediments in the Gulf of Tehuantepec, which is seen as an tion (Herrmann et al., 1994; Solari et al., 2007; Pérez et al., 2009). evidence that the Chortis block did not displace along the The new data indicate that metamorphism and migmatization in Acapulco–Motagua–Cayman fault zone, some workers (Keppie Xolapa is younger than 101 Ma, which is the age of the youngest and Morán, 2005; Morán et al. 2009) suggested that during much detrital cluster in Upper Cretaceous metamorphosed rocks. Actu- of the Cenozoic the Chortis block occupied an intra-pacific pole- ally, our data demonstrate that at least two major tectonothermal oposition and that its translation to the Caribbean plate occurred events affected Xolapa rocks, one of 64–59 Ma (Paleocene) and by counterclockwise rotation about a pole near Santiago, Chile. No another of ~34 Ma (Eocene–Oligocene). The former is interpreted data for older (Pre-Paleogene) paleogeography of Chortis is given as the metamorphic event that led to the development of the main by these authors. metamorphic–migmatitic structure of Xolapa and is consistent A recently proposed model by Keppie (2012) and Keppie and with ages reported for garnet–muscovite leucogranites and peg- Keppie (2012) places the Chortis block in a western Gulf of Mexico matites (~59 Ma; Morán, 1992; Solari et al., 2007) generated by position during much of the Mesozoic and proposes its translation the migration of melt during the latest stage of migmatization to the Caribbean through westward displacement of Chortis (Corona et al., 2006). The age of the later tectonothermal is accompanying the Maya block during the opening of the Gulf of identical within analytical uncertainty to the age of many unde- Mexico followed by Jurassic clockwise rotation along the Mojave– formed granitic plutons and would suggest Pb-loss and/or zircon Sonora and West Florida megashears and Cenozoic counterclock- overgrown by large-scale intrusions. Although there is clear wise rotation along the Sierra Madre Oriental and East Yucatán evidence that Upper Jurassic sediments were intruded by Middle megashears. According to these authors, Chortis and southern Jurassic and Lower Cretaceous plutons, no chronological evidence Mexico never shared their geological record and consequently of Jurassic or Cretaceous thermal events was recorded. Why they evolved separately. zircons recorded the 34 Ma thermal event but not the Jurassic or We used a U–Pb geochronological approach to evaluate these Cretaceous ones is unclear, but it is likely that the Jurassic– models. The statement underlying this approach is that if Chortis Cretaceous thermal events produced no zircon overgrowths or shared stratigraphical and tectonothermal evolution with south- that zircon overgrowths were too small to be recorded in CL ern Mexico during much of the Mesozoic and even before, some images and, consequently, to be analyzed. The explanation of the degree of geochronological coincidence must remain in their rocks processes promoting the formation and recrystallization of zircon beside differences produced by their subsequent, separated tec- under metamorphic conditions is beyond the scope of our paper tonic evolution. On the other hand, coincidence would be low or and the reader is referred to the work by Hoskin and Schaltegger null if Chortis and southern Mexico evolved separately. (2003) for a detailed explanation of the issue. Compared to southern Mexico terranes, the geochronological data of Chortis is scarce. We used ages compiled by Rogers et al. (2007), 5.4. Chortis–southern Mexico connection Solari et al. (2009), Ratschbacher et al. (2009) and Torres de León et al. (2012) to construct the cumulative age pattern of Chortis shown in One of the most stimulating debates on the geology of southern Fig. 6. Major peak ages in the Chortis block occur at ~1142, ~963, ~824, Mexico is the paleoposition of the Chortis block during the 440–402, 271–238, ~168 and ~131 Ma. These peak ages are very Mesozoic–Cenozoic, its role in the tectonic evolution of southern similar to those recorded in Xolapa metasedimentary rocks (Fig. 6). Mexico and the tectonic framework that led to its translation The degree of overlap (0.76) and similarity (0.66) between Xolapa and to the Caribbean plate during the Paleogene. Most accepted Chortis rocks is significant (higher than those recorded between 196 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199

Fig. 7. Proposed tectonic evolution of the Xolapa Terrane. (a) Early Jurassic (190–180 Ma) stage: deposit of Lower Jurassic sediments in the Xolapa Basin flanked on one side by the Acatlán–Oaxaca and their Paleozoic covers (southern Mexico) and on the other side by the Chortis Block assemblages. (b) Middle Jurassic to Early Cretaceous stage (180–130 Ma): widespread arc magmatism intruding into Lower Jurassic sediments of Xolapa. Coeval plutons are recorded in both Chortis and southern Mexico (Mixteca– Oaxaca terranes). (c) Late Cretaceous (110–65 Ma) stage: platform limestone (Albian–Cenomanian) and turbidite (Turonian–Maastrichtian) deposition. Strata of these ages are also present in both southern Mexico (Mixteca–Oaxaca) terranes and the Chortis Block. (d) Early Paleocene (64–59 Ma) stage: thrusting of the Chortis and Acatlán assemblages and their covers over Xolapa rocks. Deformation produced folds and thrusting in the upper (Mixteca, Oaxaca, Chortis) nappes and folding, ductile deformation, high-grade metamorphism and migmatization in Xolapa. (e) Early Paleocene (~58–45 Ma) stage partial exhumation of Xolapa assemblages entrained by the detachment and the beginning of the migration of the Chortis Block, which also produced left-lateral transtension in the southern Mexico side. (f) Eocene–Oligocene stage (42−32 Ma): installation of subduction behind the Chortis Block during its migration and translation to the Caribbean Plate.

Xolapa and Oaxaca; see Table A.2) indicating that some spatial and on the other side by the Chortis block (Fig. 7a). Following Vega- connection between these two geological provinces could have Granillo et al. (2007), by this time (~190 Ma) southern Mexico terranes existed. These data further indicate that contribution of Chortis rocks occupied a Gulf of Mexico position, north of the Maya block (Fig. 8a). as sources of Xolapa zircons must not be neglected. According to our data, Chortis must have occupied a close position. Fig. 6 shows that cumulative patterns of Chortis and other This idea is compatible with the paleogeography of Chortis proposed (Acatlán and Oaxacan complexes and their Paleozoic covers) by Keppie (2012) and Keppie and Keppie (2012) but differs from the assemblages of southern Mexico also have similar age clusters generally assumed Pacific position of Chortis (see Pindell and Kennan, reinforcing a spatial connection of Chortis with southern Mexico. 2009). Overlap and similarity between Chortis and Acatlán are 0.75 and After deposition and burial of Lower Jurassic sediments, intense 0.83, respectively, whereas between Chortis and Oaxacan correla- arc magmatism occurred during Late Jurassic (180–152 Ma) and tion is even higher (overlap¼0.88; similarity¼0.81; Table A.2). Early Cretaceous (ca. 130 Ma) time (Fig. 7b). Upper Jurassic arc magmatism is also recorded in the Mixteca, Oaxaca and Chortis as 5.5. Tectonic evolution of Xolapa well as in the Arteaga region in the Guerrero terrane (Centeno et al., 2003). Lower Cretaceous arc magmatism is coeval with Metasedimentary rocks of Xolapa deposited in two distinctive volcanism recorded in the Guerrero and western Mixteca terranes cycles of sedimentation of Early Jurassic and Late Cretaceous age. (Talavera et al., 2007). Arc magmatism of such an age is also widely We propose that Lower Jurassic sediments deposited in a basin reported in the Chortis block (Rogers et al., 2007; Ratschbacher floored, at least in part, by Permian (ca. 271 Ma) granitic orthog- et al., 2009) and suggests lengthy subduction in southern Mexico neiss, the oldest confirmed rock known in Xolapa (Ducea et al., at that time. Upper Cretaceous metasediments derive essentially 2004). Precambrian rocks reported by other authors are still from adjacent or local sources. If Upper Jurassic and Lower required to be substantiated. The basin would be flanked on one Cretaceous plutonic rocks of Xolapa shed zircons to Cretaceous side by the Acatlán–Oaxacan complexes and their Paleozoic covers sediments, exhumation of former Xolapa assemblages is required. O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 197

Fig. 8. Simplified paleogeographic reconstructions of the Xolapa Terrane and its relations with southern Mexico Terranes and the Chortis Block. (a) Early Jurassic (~190 Ma) reconstruction showing the Chortis Block attached to southern Mexico terranes in a Gulf of Mexico position. The paleogeographic framework is after Pindell and Kennan (2009), but the position of the Mixteca and Oaxaca terranes is after Vega-Granillo et al. (2007). The proposed Xolapa basin is located between Chortis and Mixteca–Oaxaca terranes. (b) Early Cretaceous (~130 Ma) reconstruction. At this time, southern Mexico terranes (including the Maya Block) reached its actual position. The Chortis Block already occupied the Pacific position that has been suggested in most paleogeographic reconstructions. The Guerrero arc subduction, which produced widespread volcanism in western Mexico, extended southward producing numerous plutons and volcanic successions in Chortis, Xolapa and Mixteca assemblages. (c) Late Cretaceous–Early Paleocene (~65−59 Ma) reconstruction. Shortening of the Xolapa basin related to crowding of Chortis with nuclear Mexico provoked by the approach and final accretion of the Guerrero arc assemblages during Late Cretaceous time produced thrusting, shearing, high grade metamorphism and migmatization of Xolapa rocks. (d) Paleocene (~58 Ma) reconstruction showing the beginning of the detachment and southeastern migration of the Chortis Block. Coa¼Coahuila Block; SM¼Sierra Madre Terrane; Ch¼Chortis Block; Mi¼Mixteca Terrane; Ox¼Oaxaca Terrane; My¼Maya Block; Chis¼Chiapas Massif; V¼Volcanics; G¼Granite (s.l.) intrusions.

According to most authors (e.g. Pindell and Kennan, 2009 and suggested that transtension faulting could begin as early as Late references therein), by this time (~130 Ma), southern Mexico (Mix- Paleocene (ca. 58 Ma). teca–Oaxaca) terranes had reached their current positions (Fig. 8b) As the Chortis displaced along the newly established Polochic– and the Xolapa basin, still trapped between southern Mexico and Cayman fault system, a convergent margin established behind produ- Chortis, had connection with the Guerrero Arc Assemblages, which cing the emplacement of voluminous calc-alkaline granodioritic to account for the abundant Cretaceous zircons recorded in Xolapa dioritic plutons (Fig. 7f). Some authors see in the decreasing age trend Upper Cretaceous sediments. During Late Cretaceous (~100–65 Ma) of arc plutons along the Pacific Coast from ca. 60 Ma in the northwest deposition of platform limestone and turbidites occurred everywhere to ca. 17 in the southeast an evidence of the translation of the Chortis in southern Mexico including the Chortis, Xolapa, Mixteca and block (Schaaf et al., 1995). This translation would additionally be the Oaxaca terranes (Fig. 7c). cause of the margin truncation that characterizes southern Mexico. The major tectonothermal event in Xolapa is dated here at 64– Large-scale plutonism at 34–32 Ma could produce an extended heat- 59 Ma and produced syn-kinematic and syn-metamorphic large- ing of country rocks involving U-rich fluids to produce partial resetting scale migmatization. Physical conditions have been constrained at of the U–Pb system and/or zircon overgrowth, which may explain the 4800 1C and about 6–9 kbar following a clockwise P–T–d path ~34 Ma tectonothermal event recorded in Xolapa metasedimentary (Corona et al., 2006). Syn-deformational migmatites are contem- rocks. Final exhumation of Xolapa occurred, at least in its northern poraneous with compressive ductile deformation, isoclinal folding segment, between 34 and 23 Ma (Ducea et al., 2005). and show feedback relations during contractional deformation (Corona et al., 2006). Shortening and crust thickening of Xolapa related to crowding of Chortis with nuclear Mexico provoked by 6. Conclusions the approach and final accretion of the Guerrero arc assemblages during Late Cretaceous time (Figs. 7d and 8c). The shortening The U–Pb geochronology of metasedimentary rocks from the would also be the cause of the thrusting of the Acatlán Complex Xolapa terrane collected between Tierra Colorada, Guerrero and Puerto over Xolapa rocks described in several places (Campa and Coney, Ángel, Oaxaca indicates that their protoliths derive from two cycles 1983; Solari et al., 2007). The rearrangement of tectonic plates and of sedimentation of Early Jurassic and Late Cretaceous age. The two the change in the kinematics of their boundaries after the accre- cycles are separated by two magmatic events of Middle Jurassic (180– tion generated an extensional regimen related to left-lateral 152 Ma) and Early Cretaceous (ca. 130 Ma) age. These depositional transtensional faulting entraining the exhumation of Xolapa to ages represent the first reliable estimation of sedimentation of Xolapa upper crustal levels. This process has classically been related to the rocks and challenge the Paleozoic or even Precambrian ages previously detachment of Chortis from southern Mexico and the beginning inferred. These depositional ages further demonstrate that Xolapa of its translation to the Caribbean Plate (Figs. 7e and 8d). Most metasedimentary rocks are not rocks from the Acatlán and Oaxacan authors (e.g. Schaaf et al., 1995) propose a Middle Eocene (ca. complexes and/or their sedimentary covers in situ transformed by 44 Ma) age for the beginning of this process but Solari et al. (2007) high-grade metamorphism and migmatization as claimed. However, 198 O. Talavera-Mendoza et al. / Earth and Planetary Science Letters 369–370 (2013) 188–199 the ages of major zircon populations in Lower Jurassic metasedimen- Campa, U.M., Coney, P., 1983. Tectonostratigraphic terranes and mineral resource tary rocks of Xolapa are consistent with derivation from the neighbor- distributions in Mexico. Can. J. Earth Sci. 20, 1040–1051. Centeno, G.E., Ruiz, J., Coney, P.J., Patchett, P.J., Ortega-Gutiérrez, F., 1993. Guerrero ing Acatlán and Oaxacan complexes and their Paleozoic sedimentary terrane of Mexico: its role in the Southern Cordillera from new geochemical covers as well as the Chortis assemblages indicating a para(auto- data. Geology 21, 419–422. chthonous) origin for the Xolapa terrane. Upper Cretaceous metase- Centeno, G.E., Corona, Ch.P., Talavera, M.O., Iriondo, A., 2003. Geology and evolution of the western Guerrero terrane—a transect from Puerto Vallarta dimentary rocks of Xolapa contain zircon grains deriving essentially to Zihuatanejo, Mexico. In: Guidebook for field trips of the 99th Annual from the nearby Guerrero and western Mixteca terranes and/or from Meeting of the Cordilleran Section of the Geological Society of America, local (Xolapa) sources. Publ. Esp. 1. Instituto de Geología, Universidad Nacional Autónoma de México, – The Xolapa metasedimentary rocks record two major tecto- pp. 201 228. Centeno, G.E., Guerrero, S.M., Talavera, M.O., 2008. The Guerrero Composite Terrane nothermal events of Paleocene (64–59 Ma) and Eocene–Oligocene of western Mexico: collision and subsequent rifting in a supra-subduction zone. (~34 Ma) age. The former is interpreted as the main thermal event Geol. Soc. Am. Spec. Pap. 436, 279–308. leading to the main metamorphic and migmatitic structure of Corona, Ch.P., Poli, S., Bigioggero, B., 2006. Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico. Xolapa and refutes the Early Cretaceous age recently proposed for J. Metamorph. Geol. 24, 169–191. this event. The later event seems to be related to the emplacement Dickinson, W.R., Lawton, T.F., 2001. Carboniferous to Cretaceous assembly and of voluminous arc plutons of identical (34–32 Ma) age, which may fragmentation of Mexico. GSA Bull. 113, 1142–1160. fl Ducea, M.N., Gehrels, G.E., Shoemaker, S., Ruiz, J., Valencia, V.A., 2004. Geologic produce extended heating and remobilization of U-rich uids to evolution of the Xolapa Complex, southern Mexico: evidence from U–Pb zircon generate zircon overgrowths. geochronology. Geol. Soc. Am. Bull. 116, 1016–1025. The major U–Pb age clusters of Chortis assemblages have Ducea, M., Valencia, V.A., Shoemaker, S., Reiners, P.W., DeCelles, P.G., Campa- fi Uranga, M.F., Morán Zenteno, D.J., Ruiz, J., 2005. Rates of sediment recycling signi cant concordance with those recorded in the Acatlán and beneath the Acapulco trench: constraints from (UTh)/He thermochronology. Oaxacan complexes and their Paleozoic sedimentary covers as well J. Geophys. Res., 109, http://dx.doi.org/10.1029/2004JB003112. as with metasedimentary rocks of Xolapa implying, on one hand, a Erben, H.K., 1956. El Jurásico Medio y el Caloviano de México. XX Congreso Geológico Internacional, México, D.F. Monografía, 140 p. spatial connection of Chortis with southern Mexico since Early García-Díaz, J.L., 2004. Étude géologique de la Sierra Madre del Sur aux environs de Jurassic and, on the other hand, that Chortis could also be a Chilpancingo et d'Olinalá, Gro: Une contribution à la connaisance de l'évolution significant source of Xolapa zircons. géodynamique de la marge pacifique du Mexique depuis le Jurassique. Ph.D. ' Xolapa evolved in a basin floored by Permian rocks flanked on one Ecole Doctorale de l Universite de Savoi, France, p. 148. Gehrels, G.E., 2000. Introduction to detrital zircon studies of Paleozoic and Triassic side by southern Mexico terranes and on the other side by the Chortis strata in western Nevada and northern California. Geol. Soc. Am. Spec. Pap. 347, block. Spatial connection with the Guerrero terrane is suggested 1–17. – because both assemblages share the Middle Jurassic and Lower Gehrels, G., 2011. Detrital zircon U Pb geochronology: current methods and new opportunities. In: Busby, C., Azor, A. (Eds.), Tectonics of Sedimentary Basins: Cretaceous magmatic pulses and both contain very similar zircon Recent Advances. John Wiley & Sons, Ltd, Chichester, UKhttp://dx.doi.org/ populations. Basin shortening tied to the approach and accretion of 10.1002/9781444347166. (Chapter 2). the Guerrero terrane assemblages during the Late Cretaceous pro- Gehrels, G., Valencia, V., Pullen, A., 2006. Detrital zircon geochronology by laser ablation multicollector ICPMS at the Arizona LaserChron Center. In: duced crustal thickening and high-grade metamorphism and migma- Olszewski, T. (Ed.), Geochronology: Emerging Opportunities, 12. Paleont. Soc., tization at mid-crustal levels. 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