Pressure-temperature-time evolution of Paleozoic high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic Oceans Ricardo Vega-Granillo† Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000 Oscar Talavera-Mendoza Unidad Académica de Ciencias de la Tierra, Universidad Autónoma de Guerrero, A.P. 197, Taxco, Guerrero, México 40200 Diana Meza-Figueroa Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000 Joaquin Ruiz George E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Margarita López-Martínez Departamento de Geología, CICESE, Km 107 Carr, Tijuana–Ensenada, Ensenada, Baja California, México Julio C. de la Cruz-Vargas Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México 83000 ABSTRACT 830–730 °C and 17–15 kb. Amphibole from INTRODUCTION eclogite yields a 430 ± 5 Ma 40Ar/39Ar age, dat- New thermobarometric and U/Pb and ing the high-pressure (HP) event. P-T paths of Owing to their tectonic setting and viability 40Ar/39Ar geochronologic data coupled with high-temperature (HT) eclogites like those of for providing quantitative thermobarometric and ages obtained from the Acatlán Complex, the the Esperanza suite have been related to the geochronological measurements, high-pressure basement of the Mixteco terrane of southern collision of continental blocks. Partial over- (HP) assemblages are frequently used to recon- Mexico, reveal the existence of three distinc- printing occurred at 690–640 °C and 14–10 kb struct the tectonic evolution of ancient orogenic tive high-pressure metamorphic events of prior to 374 ± 2 Ma (40Ar/39Ar, phengite). The chains (e.g., Ernst, 1977, 1988; England and early to middle Paleozoic age, each recorded three HP suites were tectonically juxtaposed Richardson, 1977; Godard, 2001). Moreover, in a separate lithological suite. Xayacatlán at different times before the Mississippian these rocks commonly form distinctive petrotec- suite eclogites with oceanic affi nity under- Period, resulting in the closure of the Iape- tonic assemblages that facilitate regional corre- went peak metamorphism at 609–491 °C tus Ocean. Phengite 40Ar/39Ar geochronology lations and paleogeographic reconstructions. and 13–12 kb during the Early Ordovician reveals the existence of a widespread tec- The Acatlán Complex of southern Mexico (ca. 490–477 Ma, U-Pb zircon), followed by a tonothermal event between 345 and 323 Ma, (Fig. 1) contains the largest exposures of eclog- partial overprint at 600 °C and ~9.6 kb and which may be related to the juxtaposition of ites, HP garnet-amphibolites, blueschists, and then at 500 °C and ~6.7 kb. An overprinting the HP-composed block and the Gondwa- eclogitized granitoids of Paleozoic age in Mex- event at 525–500 °C and ~9.5 kb is ascribed nan-affi nity Cosoltepec suite, causing the clo- ico. The complex is framed of tectonic slices to the Devonian. The pressure-temperature sure of the Rheic Ocean. The tectonothermal containing Laurentian and Gondwanan affi nity (P-T) path of the Xayacatlán suite indicates events in the Acatlán Complex coincide in assemblages, and hence it may be useful for a subduction-exhumation process followed time, physical conditions, and tectonic setting interpreting interactions between these paleo- by tectonically related reburial. Ixcamilpa with events in the Appalachian-Caledonian continents during the Paleozoic. In this paper, suite blueschists with oceanic affi nity under- orogen, suggesting their relation. On that we present detailed petrographic and thermo- went epidote-blueschist metamorphism (T, basis the geology of the Acatlán Complex can barometric data of HP suites as well as new 200–390 °C; P, 6–9 kb) and then epidote- lead to a more comprehensive understanding U/Pb and 40Ar/39Ar ages of key rocks to defi ne amphibolite (T, 390–580 °C; P, 9–6 kb) events of the tectonic evolution of the Appalachian the ages of protoliths and metamorphic events ascribed to the Late Ordovician–Early Silu- orogen and of the Gondwana-Laurentia inter- in the Acatlán Complex. These data reveal the rian. Esperanza suite eclogites with continen- actions preceding the Pangean assembly. existence of three distinctive events of HP meta- tal affi nity underwent peak metamorphism at morphism, each with contrasting pressure-tem- Keywords: eclogite, blueschist, Paleozoic, perature (P-T) conditions that occurred at differ- †[email protected] Appalachian, Acatlán Complex, Mexico. ent times throughout the early-middle Paleozoic GSA Bulletin; September/October 2007; v. 119; no. 9/10; p. 1249–1264; doi: 10.1130/B226031.1; 10 fi gures; 2 tables; Data Repository item 2007192. For permission to copy, contact [email protected] 1249 © 2007 Geological Society of America Vega-Granillo et al. and in different lithological suites. Furthermore, GEOLOGICAL SETTING lithological suite characterized by a distinctive this paper includes the fi rst detailed description stratigraphy (Figs. 1 and 2). of Paleozoic blueschists in Mexico, which are The Acatlán Complex is presently seen as a We briefl y describe here the main strati- uncommon around the world. pile of thrust sheets containing Mesoproterozoic graphic, geochronological, and geochemical Our data, combined with previously published to Paleozoic petrotectonic suites with Lauren- characteristics of each suite. The Xayacatlán, geochemical and geochronological data (Yañez tian or Gondwanan affi nities juxtaposed during Ixcamilpa, and Esperanza suites include the HP et al., 1991; Ortega-Gutiérrez et al., 1999; Meza- the Paleozoic (Talavera-Mendoza et al., 2005). rocks treated in the study and are emphasized. Figueroa et al., 2003; Talavera-Mendoza et al., The geology of the complex has been compre- The Tecolapa suite is the oldest assemblage 2005; Murphy et al., 2006), help to defi ne the hensively treated by Ortega-Gutiérrez (1978), recognized in the Acatlán Complex. It con- Paleozoic evolution of the Acatlán Complex and Ramírez-Espinosa (2001), and Talavera-Men- sists of megacrystic granitoids and tonalitic provide the basis for establishing correlations doza et al. (2005). At present, seven major thrust gneisses of Mesoproterozoic age (1165 ± 30 with suites of coeval orogenic belts. sheets have been recognized, each containing a to 1043 ± 50 Ma; U/Pb; Campa et al., 2002; Izúcar de ACA-57 RAC-101 Matamoros ACA-58 ACA-67 Mimilulco RAC-20 ACA-60 RAC-192 RAC-190 N El Rodeo RAC-195 RAC-138 RAC-201 RAC-188 RAC-148 RAC-41 18° 30′ RAC-42 RAC-43 Acatlán ACA-39 Oaxaca Santa Cruz Organal MIXTECO OAXACA TERRANE TERRANE Tehuitzingo Acatlán ACA-125 de Osorio ACA-8 Piaxtla ACA-7 IX-19 RAC-81 A Cuatlaxtecoma Ixcamilpa ′ Tecolapa 18° 00 0 5 25 Km IX-13 IX-12 IX-13 Chazumba suite Lower Permian? IX-25 IX-28 Patlanoaya Fm Upper Dev.- IX-29 IX-30 (Tecomate Fm) Lower Perm. MI-6 Totoltepec granite Penn.-Permian Olinalá Cosoltepec suite Post-Lower Dev. Olinalá La Noria granite Middle Devonian A' Esperanza suite Lower Silurian Teticic granite Middle Ordovician Ixcamilpa suite Post-Middle Ord. ′ 98° 30′ 98° 00 El Rodeo suite Camb?-Lower Ord. Section A-A′ SE Xayacatlán suite Camb?-Lower Ord. NW 2000 2000 Tecolapa suite Mid-Proterozoic Oaxacan Complex Mid-Proterozoic 500 500 Major thrust faults Studied localities Mesozoic folding Figure 1. Geologic map of the Acatlán Complex, showing locations of studied samples. Samples in italics were analyzed by micro- probe; samples in bold were dated by U/Pb or 40Ar/39Ar methods. Map modifi ed from Ramírez-Espinosa (2001). Depth values in cross section are meters above sea level. 1250 Geological Society of America Bulletin, September/October 2007 P-T-t evolution of Paleozoic high-pressure Acatlán Complex Talavera-Mendoza et al., 2005) that crop out in show geochemical and isotopic characteris- which is considered to have undergone a single the southwestern region of the complex (Olinalá tics of suites similar to mid-ocean-ridge basalt HP metamorphism event. and Tecolapa areas in Fig. 1). The suite is cut (MORB) (Yañez et al., 1991; Meza-Figueroa The Cosoltepec suite is the most widespread by Early Ordovician leucogranites of 478 ± 5 to et al., 2003), although a continental affi nity has unit in the Acatlán Complex, forming up to 80% 471 ± 5 Ma (U/Pb, zircon; Campa et al., 2002; also been proposed for rocks of this suite (Dos- of the complex (Fig. 1). It consists of a monoto- Talavera-Mendoza et al., 2005; Fig. 2). tal et al., 2004). Protolith age has been ascribed nous succession of quartzite and phyllite with a The El Rodeo suite is a sequence of metaba- to the Late Cambrian–Early Ordovician (Talav- greenschist facies metamorphism (Fig. 2). The sites interbedded with quartzites and schists era-Mendoza et al., 2005). unit contains meter- to kilometer-size tectonic affected by greenschist metamorphism that The Ixcamilpa suite is a recently recognized slices of pillow basalts with oceanic affi nity crop out throughout the complex (Fig. 1). It unit (Talavera-Mendoza et al., 2002, 2005) that (Ramírez-Espinosa, 2001). The Cosoltepec suite is intruded by granitoids of Ordovician (476 consists of blueschists with intercalations of has a maximum Early Devonian, ca. 410 Ma ± 8 and 461 ± 7 Ma; Talavera-Mendoza et al., pelitic-psammitic schists in the western realm of (U-Pb zircon), depositional age (Talavera-Men- 2005), Devonian (371 ± 34 Ma; Yañez et al., the complex (Figs. 1 and 2). Its maximum depo- doza et al., 2005). 1991), and Early Permian (287 ± 2 Ma; Yañez sitional age is Middle Ordovician (Talavera- Major thrust faults juxtapose suites meta- et al., 1991) age (Fig. 2) (all U/Pb zircon ages). Mendoza et al., 2006). There are no constraints morphosed under contrasting P-T conditions. Geochemical and isotopic data of El Rodeo for its minimum age. Thrusting produced overprinting of defor- metabasites (Ramírez-Espinosa, 2001) indicate The Esperanza suite is one of the most mational phases, described in detail by Vega- a continental-rift magmatic affi nity.