RESEARCH A Late Triassic tectonothermal event in the eastern Acatlán Complex, southern Mexico, synchronous with a magmatic arc hiatus: The result of flat-slab subduction? Moritz Kirsch1,*, Maria Helbig2, J. Duncan Keppie3, J. Brendan Murphy4, James K.W. Lee5, and Luigi A. Solari2 1
INSTITUT FÜR GEOLOGIE, UNIVERSITÄT HAMBURG, BUNDESSTRASSE 55, 20146 HAMBURG, GERMANY CENTRO DE GEOCIENCIAS, UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO, CAMPUS JURIQUILLA, 76230 QUERÉTARO, MEXICO 3 DEPARTAMENTO DE GEOLOGÍA REGIONAL, INSTITUTO DE GEOLOGÍA, UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO, 04510 MÉXICO D.F., MEXICO 4 DEPARTMENT OF EARTH SCIENCES, ST. FRANCIS XAVIER UNIVERSITY, ANTIGONISH, NOVA SCOTIA B2G 2W5, CANADA 5 DEPARTMENT OF GEOLOGICAL SCIENCES AND GEOLOGICAL ENGINEERING, QUEEN’S UNIVERSITY, BRUCE WING 325, MILLER HALL, KINGSTON, ONTARIO K7L 3N6, CANADA 2
ABSTRACT
Basement rocks exposed in the Acatlán Complex of the Mixteca terrane in southern Mexico record two tectonothermal events: (1) a Devonian–Mississippian (ca. 365–318 Ma) event, recording extrusion and exhumation of high-pressure rocks; and (2) an Early to Middle Permian (ca. 289–263 Ma) event, involving N-S dextral shearing, transtensional deformation, and local S-vergent thrusting in a magmatic arc environment. We document an additional, regionally significant, tectonothermal event during the Middle to Late Triassic recorded by 40Ar/39Ar step-heating laser-probe ages ranging from ca. 239 and 219 Ma (≡ cooling from ca. 525 °C to 300 °C) for amphibole, muscovite, and biotite from: (1) the Carboniferous Amarillo unit, consisting of medium-grade, metasedimentary rocks intruded by mafic dikes; and (2) the Pennsylvanian–Middle Permian, low-grade, clastic-calcareous, arc-related Tecomate Formation. U-Pb laser ablation–inductively coupled plasma– mass spectrometry (LA-ICP-MS) data yield an age of 339 ± 6 Ma for the youngest population of detrital zircon grains in the Amarillo unit. Lithogeochemical and Sm-Nd isotopic data for the Amarillo unit dikes are very similar to those of other Carboniferous meta-igneous rocks in the eastern and southwestern part of the Acatlán Complex, displaying affinities transitional between mid-ocean-ridge basalt (MORB) and continental tholeiites, and initial εNd (t = 339 Ma) values from −6.6 to +6.4, indicating both depleted and enriched mantle sources, as well as variable contamination by continental crust or by subduction-derived fluids. The 40Ar/39Ar cooling ages coincide with an apparent hiatus in magmatic activity in southern Mexico, which is inferred to record a change from steep to flat subduction.
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INTRODUCTION
Basement rocks exposed in the Acatlán Complex of the Mixteca terrane in southern Mexico record a polyphase deformational and metamorphic history and provide an excellent opportunity to investigate the Paleozoic to Mesozoic tectonic evolution of the North American Cordillera. Two dominant tectonothermal events have been recognized in rocks of the Acatlán Complex (Keppie et al., 2012, and references therein): (1) a Devonian–Mississippian (ca. 365–318 Ma) event, recording extrusion and exhumation of high-pressure rocks (Middleton et al., 2007; ElíasHerrera et al., 2007; Ramos-Arias et al., 2008, 2012; Vega-Granillo et al., 2009; Keppie et al., 2010, 2012); and (2) an Early to Middle Permian (ca. 289–263 Ma) event, involving N-S dextral shearing, transtensional deformation, and local S-vergent thrusting (Elías-Herrera et al., 2005; MoralesGámez et al., 2009a; Kirsch et al., 2013). In addition, Vega-Granillo et al. (2009) reported two early Paleozoic tectonothermal events, although their existence is controversial (Keppie, J.D., et al., 2009). Reconnaissance studies suggest a Middle to Late Triassic tectonothermal event north of Petlalcingo, bracketed between a single U/Pb detrital zircon age of 239 ± 4 Ma (Keppie et al., 2006a) and a 224 ± 2 Ma muscovite cooling age (Keppie et al., 2004a). In this paper, we provide further evidence for this tectonothermal event by presenting lithogeochemical *
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doi: 10.1130/L349.1
and Sm-Nd isotopic, and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb detrital zircon data, as well as 40Ar/39Ar step-heating laser-probe data from amphibole, muscovite, and biotite from late Paleozoic low- to medium-grade units in the northeastern part of the Acatlán Complex. We show that this tectonothermal event is regional in extent, affecting Carboniferous–Permian rocks, and we investigate its tectonic significance in the context of subduction processes at the North American Cordilleran margin. GEOLOGICAL SETTING
The Ordovician to Middle Permian Acatlán Complex forms the basement of the Mixteca terrane and is the largest inlier of Paleozoic rocks in Mexico (Ortega-Gutiérrez, 1978; Campa and Coney, 1983; Sedlock et al., 1993; Keppie, 2004). The Acatlán Complex is bounded to the east by the Permian, N-S, dextral Caltepec fault zone, which separates it from the ca. 1.0 Ga granulite-facies gneisses of the Oaxacan Complex (Figs. 1A and 1B; Elías-Herrera and Ortega-Gutiérrez, 2002) and to the south by the Cenozoic La Venta–Chacalapa fault zone (Tolson, 2007; Solari et al., 2007), which juxtaposes it against the Mesozoic–Cenozoic plutonic and high-grade metamorphic rocks of the Xolapa Complex (Pérez-Gutiérrez et al., 2009b). To the west, the Acatlán Complex is thrust over Cretaceous carbonates of the Guerrero-Morelos platform, which are exposed between the Acatlán Complex and the obducted Guerrero composite terrane (Centeno-
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García et al., 2008; Ramos-Arias and Keppie, 2011). To the north, the Acatlán Complex is unconformably overlain by continental and marine sedimentary rocks of Upper Permian to Middle Jurassic age (Piedra Hueca and Otlaltepec units: Morán-Zenteno et al., 1993; Matzitzi Formation: Centeno-García et al., 2009), and by Neogene–Holocene volcanic and volcaniclastic rocks of the Trans-Mexican volcanic belt (Ferrari et al., 1999). The Acatlán Complex records a complex Paleozoic tectonothermal history reflecting the opening and closure of one or more ocean basins and their consequent continental interactions, culminating in the amalgamation of Pangea (e.g., Keppie et al., 2008, 2012, and references therein). These events were accompanied by Carboniferous to Permian subduction beneath southern Mexico (see inset of Fig. 1A). The Middle–Late Triassic of central and southern Mexico marked the development of a passive or rifted margin along the western margin of Oaxaquia, associated with a thick succession of turbiditic siliciclastic rocks deposited in a submarine fan environment (Centeno-García, 2005; Centeno-García et al., 2008; Martini et al., 2009; Helbig et al., 2012a, 2012b), before subductionrelated igneous activity was re-established by the Early Jurassic (Fig. 1A; Bartolini et al., 2003). Study Area Amarillo Unit (New Name)
The Amarillo unit occurs on the eastern side of the Totoltepec pluton, north of the road between Santo Domingo Tianguistengo and Santiago de Chazumba (Fig. 2). This unit was previously mapped as the Cosoltepec Formation (Ortega-Gutiérrez et al., 1999), which is now considered to be a composite of both Cambrian–Ordovician and Devonian–Carboniferous units (Talavera-Mendoza et al., 2005; Keppie et al., 2006a, 2008; Morales-Gámez et al., 2008; Ortega-Obregón et al., 2009). The Amarillo unit is thrust upon low-grade metasedimentary rocks of the Tecomate Formation (Fig. 2; Kirsch et al., 2012) and tectonically juxtaposed against the Totoltepec pluton along a normal fault. To the east, the unit is unconformably overlain by Jurassic clastic rocks (Morán-Zenteno et al., 1993). The Amarillo unit is primarily composed of interbedded metapelites and metapsammites that are intruded by mafic dikes. All rocks of the Amarillo unit have undergone intense, polyphase deformation under amphibolite-facies metamorphism, which renders measurement of a type section impossible. The metasedimentary rocks consist of quartz, biotite, muscovite, garnet, and accessory apatite, zircon, and opaque minerals. Secondary minerals include chlorite and calcite. The mafic dikes are made up of amphibole (ferrotschermakite: Table DR11), plagioclase, biotite, quartz, and accessory apatite and ilmenite. Local shearing of the mafic dikes is associated with retrogression to chlorite, epidote, calcite, and apatite. Seven metasedimentary and seven amphibolite samples were collected in the southern part of the Amarillo unit for geochemical, U-Pb, and 40Ar/39Ar geochronological analyses (for locations, see Fig. 2; Tables DR3 and DR4 [see footnote 1]). Furthermore, an ~3 m thick, undeformed dike of andesitic composition that cuts the metasedimentary rocks in the eastern part of the Amarillo unit was sampled for geochemical and U-Pb geochronological analysis. Tecomate Formation
The Tecomate Formation is a mildly metamorphosed, but intensely deformed clastic unit consisting of thinly bedded pelitic and psammitic sedimentary rocks, and a few marbles, conglomerates, and volcanic rocks (Keppie et al., 2004b; Sánchez-Zavala et al., 2004; Kirsch et al., 2012).
Geochronological and geochemical data indicate that the Tecomate Formation is a composite unit of Pennsylvanian–Middle Permian age, the detritus of which was predominantly derived from a regional continental arc (Kirsch et al., 2012). Rocks of the Tecomate Formation are tectonically juxtaposed against the Carboniferous Salada unit along N-striking, dextral-normal faults and N-dipping shear zones in the western part of the study area (Fig. 2; Morales-Gámez et al., 2008). To the north, they are overlain by red beds of inferred Jurassic age (Malone et al., 2002). In the southern part of the study area, the Tecomate Formation is overthrust by the Totoltepec pluton, and in the northeast, it is overthrust by the Amarillo unit. To the south, the Tecomate Formation is inferred to structurally overlie rocks of the Cosoltepec Formation (Malone et al., 2002). Three metasedimentary rock samples from the Tecomate Formation were collected for 40Ar/39Ar geochronological analysis (for location, see Fig. 2; Table DR3 [see footnote 1]). RESULTS U-Pb Geochronology Analytical Methods
An Amarillo unit metapelite sample (lat 18°15′25.26″N, long 97°46′33.66″W) and an undeformed andesitic dike (lat 18°14′45.59″N, long 97°45′41.87″W) intruding this unit were collected for U-Pb zircon dating and analyzed by LA-ICP-MS at the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Mexico. Zircons were extracted using standard mineral separation techniques, as described by Solari et al. (2007). For details of the analytical procedures, see Solari et al. (2010) and Kirsch et al. (2012). In figures, tables, and results, 206Pb/238U ages are quoted for zircons younger than 1.0 Ga, whereas older grains are quoted using their 207 Pb/206Pb ages (e.g., Gehrels et al., 2006). The latter ages become increasingly imprecise below 1.0 Ga due to small amounts of 207Pb. Zircon analyses with 99.9% purity. For muscovite and biotite, grain sizes typically ranged from 250 to 420 µm (see Fig. 4), except for mylonitic sample TT-61a and metapelite sample TT-45b (both 177–250 µm). The amphibolites were finer grained and yielded grain sizes between 149 and 200 µm. Mineral separates and flux monitors were irradiated in the McMaster Nuclear Reactor (Hamilton, Ontario), and 40Ar/39Ar analyses were performed at the 40Ar/39Ar Geochronology Research Laboratory at Queen’s University in Kingston, Canada (for details of the analytical procedure, see Cubley et al., 2013). Complete results of the 40Ar/39Ar analyses are presented in Table DR3, with representative microprobe analyses of amphibole listed in Table DR1 (see footnote 1). All data have been corrected for blanks, mass discrimination, and neutron-induced interferences. The dates are referenced to the Hb3Gr hornblende standard at 1072 Ma (Turner et al.,
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Figure 4. Laser step-heating 40Ar/39Ar age spectra for samples from the eastern Acatlán Complex. For sample locations, see Figure 2 and Table DR-3A (see text footnote 1). Data in part I were recalculated from Keppie et al. (2004a). MSWD—mean square of weighted deviates.
1971; Roddick, 1983). Errors shown in the tables and on the age spectra and inverse isotope correlation diagrams represent the analytical precision at 2σ. This is suitable for comparing within-spectrum variation and determining which steps form a plateau (e.g., McDougall and Harrison, 1988, p. 89). For the purposes of this paper, a plateau is defined as three or more contiguous steps containing >50% of the 39Ar released, with a probability of fit >0.01 and mean square of weighted deviates (MSWD)
