Gondwana Research 21 (2012) 1050–1065

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Constraints on the subduction erosion/extrusion cycle in the Paleozoic AcatlánComplex of southern Mexico: Geochemistry and geochronology of the typePiaxtla Suite

J. Duncan Keppie a,⁎, R.D. Nance b, J. Dostal c, J.K.W. Lee d, A. Ortega-Rivera e

a Depto de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México, D.F., Mexicob Dept. of Geological Sciences, 316 Clippinger Laboratories, Ohio University, Athens, Ohio 45701, USAc Dept. of Geology, St. Mary's University, Halifax, Nova Scotia, Canada, B3H 3C3d Dept. of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario, Canada, K7L3N6e Instituto de Geología, Universidad Nacional Autónoma de México, Estación Regional del Noroeste, Apartado Postal 1039, Hermosillo, Sonora 83000, Mexico

⁎ Corresponding author. Tel.: +52 902 542 5320.E-mail address: duncan@servidor.unam.mx (J.D. Kep

1342-937X/$ – see front matter. © 2011 International Adoi:10.1016/j.gr.2011.07.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 June 2011Received in revised form 19 July 2011Accepted 25 July 2011Available online 29 July 2011

Handling Editor: M. Santosh

Keywords:Subduction erosionExtrusionHigh-pressureTectonicsAcatlán ComplexMexico

The type high-pressure (HP) Piaxtla Suite in the Acatlán Complex of southern Mexico consists of retrogressedeclogite (amphibolite), megacrystic granitoids and high-grade meta-sedimentary rocks. Exhumation of theseHP rocks has recently been interpreted as the result of extrusion into the upper plate, rather than by returnflow up the subduction zone. Geochemical analyses of the retrograde eclogites indicate that they have a rifttholeiitic-transitional alkalic composition. These are closely associated with amegacrystic meta-granitoid thathas yielded an intrusive age of 452±6 Ma (concordant U–Pb zircon analyses) with inherited zirconpopulations at ca. 800–950 Ma and 1000–1200 Ma derived from the underlying basement, probably theOaxacan Complex which borders the Acatlán Complex to the east. The bimodal nature of these igneous rocksand their spatial and temporal close association with continentally-derived sedimentary rocks is similar tomost HP rocks in the Acatlán Complex derived from a rifted passive margin. The youngest detrital zirconpopulation in a meta-psammite sample yielded a U–Pb age of 365±15 Ma with older analyses distributedalong a chord with an upper intercept of 1287±29 Ma. The ca. 365 Ma age provides a maximum age for thetime of deposition of this sample. 40Ar/39Ar ages from the retrogressed eclogites provided hornblende plateauages of 342±2 Ma and 344±2 Ma, whereas muscovite from the granitoid andmeta-psammite yielded 334±2 Ma plateau ages. These data constrain the subduction erosion–extrusion cycle to ≤35 Ma during which therocks were taken to a depth of ca. 40 km at a rate of 2.7 km/Ma and back to the surface at 2.4 km/Ma. Suchexhumation rates are slower than those in continent–continent collision zones, but similar to those in theIberia–Czech Variscan belt where tectonic interpretation also suggests extrusion into the upper plate.

© 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Understanding the origin of high pressure (HP) rocks is critical forpaleogeographic reconstructions because they are generally inferred tomark either the vestiges of an oceanic suture between two continentalplates (Ernst et al., 1997; Liou et al., 2004; Ernst, 2009) or oceanic rocksextruded into the upper plate (Stöckhert and Gerya, 2005; Keppie et al.,2010a; Butler et al., 2011). Discrimination between these two settings iscritical to palinspastic studies since the former marks a fossil plateboundarywhereas the latter doesnot. Discrimination ismainly basedoncontrasts in the geological/paleontological/paleomagnetic record oneither side of an oceanic suture, whereas such parameters are similaracross a HP belt extruded into the upper plate (Keppie et al., 2010a). A

pie).

ssociation for Gondwana Research

well-exposed example of an extrusion zone occurs in the AcatlánComplex of southern Mexico (Fig. 1a). This complex contains two HPbelts, a N–S extrusion belt locatedmedially within the Acatlán Complex(Mimilulco-Piaxtla: Keppie et al., 2008a, 2010b) and a western klipperooted in the median HP belt (Ramos-Arias and Keppie, 2011; Ramos-Arias et al., in press, and references therein)(Fig. 1c). The geochemistryand geochronology of these HP rocks constrains both the protolith andthe longevity of the subduction erosion and extrusion cycle that led totheir formation. To date, most of the HP protoliths in the AcatlánComplex are inferred to have beenpart of a Cambrian–Ordovician, riftedpassive margin sequence (Murphy et al., 2006; Keppie et al., 2008a;author's unpublished data) with some forearc rocks of unknown age(Proenza et al., 2004; Galáz et al., 2009). In this paper, we presentgeochemical and geochronological data from the type area of the HPsuite, which lies near the southern end of the median HP belt (Fig. 1c).These rockswere thought to representmid-ocean ridge basalts (MORB)and ocean-island basalts (OIB) that underwent HP metamorphism at

. Published by Elsevier B.V. All rights reserved.

Fig. 1. (a) Terrane map of Middle America showing the location of the Acatlán Complex (modified after Keppie, 2004); (b) Time and Space diagram showing the geological recordafter Ortega-Gutiérrez et al. (1999) and this paper; (c) Geological map of the Acatlán Complex (modified after Keppie et al., 2010b) showing the location of the type Piaxtla Suite.

1051J.D. Keppie et al. / Gondwana Research 21 (2012) 1050–1065

560±60 °C and 11–15 kbar during inferred Ordovician subductionfollowed by retrogression with temperatures decreasing from 500 to300 °C and pressures falling from 6 to 3.5 kbar (Meza-Figueroa et al.,2003). However, the new data we present here indicate that theretrogressed eclogites (amphibolites) of the HP suite have a rifttholeiitic/alkalic protolith and are associated with continentally-derived, rift-passive margin sediments and intruded by an Ordovicianmegacrystic granitoid. Significantly, our data also indicate that the HProcks are tectonically interleaved with Upper Devonian–lowest Car-

boniferous metasedimentary rocks, and all were subjected to HPmetamorphism during the Lower Carboniferous.

2. Geological setting

The predominantly Paleozoic Acatlán Complex is bounded by onthree sides by tectonic contacts: (i) to the east, the Permian, Caltepecdextral ductile shear zone juxtaposes the Acatlán Complex against theca. 1 Ga Oaxacan Complex (Elías-Herrera and Ortega-Gutiérrez, 2002);

Fig. 2. U–Pb zircon data: (a) concordia plot of sample PIX-18 from megacrysticgranitoid; (b) PIX-18 ages with 1σ error bars showing Tuffzirc age for the youngest agecluster; (c) concordia plot of U–Pb zircon analyses of meta-psammitic sample PIX-17:ellipses represent 2σ uncertainties (inset shows an enlargement of the 300–600 Mapart of the concordia plot); (d) combined binned frequency and probability densitydistribution plot of meta-psammite sample PIX-17, with the data presented in Table 1as “best age”.

1052 J.D. Keppie et al. / Gondwana Research 21 (2012) 1050–1065

(ii) to the south, the Cenozoic, sinistral transtensional Chacalapa–LaVenta fault places the complex against the Mesozoic–Cenozoic XolapaComplex (Tolson, 2005); and (iii) to the west, the Late CretaceousPapalutla thrust places the Acatlán Complex over the CretaceousMorelos-Guerrero Platform (De Cserna et al., 1980 and Cerca et al.,2006). The northern margin of the Acatlán Complex is unconformablyoverlain by volcanic and volcaniclastic sequences of the CenozoicTransmexican Volcanic Belt (Fig. 1a).

The Paleozoic rocks of the Acatlán Complex can be subdivided intotwo assemblages of contrasting metamorphic grade and geochronol-ogy (Fig. 1b)(Keppie et al., 2008a and b, and references therein;Ramos-Arias and Keppie, 2011). The first is a low-grade sequencecomposed of: (i) a Neoproterozoic–Ordovician clastic sequenceintruded by ca. 480–440 Ma bimodal, rift-related igneous rocks; and(ii) a latest Devonian–Permian shallow marine sequence (N906 m)consisting of metapsammites, metapelites and tholeiitic maficvolcanic rocks. The second is a HP metamorphic suite that occurs inthe N–S, Piaxtla–Mimilulco median belt as a klippe rooted in themedian belt. Rocks of the HP suite consist of: (i) a Neoproterozoic–Ordovician rift–shelf meta-psammites intruded by bimodal (retrogressedeclogite/amphibolite, granitoid) rift-related intrusions that are similar tothose of the low-grade sequence; (ii) periarc ultramafic bodies, andpossibly (iii) arc and MORB rocks. Neoproterozoic–Ordovician rocks ofboth theHPsuite andthe low-grade sequenceare intrudedbymegacrysticgranitoids of Ordovician age that contain concordant inherited zirconsranging in age from ca. 900 to 1300 Ma, suggesting a source in theOaxacan Complex.

Concordant ages of detrital zircons in both the low- and high-grade Cambro-Ordovician metasedimentary rocks indicate a prove-nance in local Ordovician plutons and/or ca. 1 Ga Oaxacan basement,together with distal northwestern Gondwana sources including aunique population of detrital zircons inferred to have been derivedfrom the 900–750 Ma Goiás magmatic arc within the Brasilianoorogen (Nance et al., 2009). The 900–750 Ma ages are particularlysignificant because they discriminate Amazonian sources from thoseof Laurentia (Keppie et al., 2008a).

The HP belts in the Acatlán Complex have generally been inferredto occur in synformal keels of a single nappe rooted to the east in theCaltepec shear zone (Ortega-Gutiérrez et al., 1999; Talavera-Mendozaet al., 2005; Vega-Granillo et al., 2007, 2009). However, themedian HPbelt is bounded by a listic normal shear zone above (Ramos-Arias etal., 2008) and a thrust below (Galáz et al., 2009). This geometry isincompatible with a simple nappe structure, but is consistent with anextrusion zone (Keppie et al., 2008a). The age of HP metamorphism isalso controversial and has been variously assigned to the Ordovician,Silurian and Carboniferous (Ortega-Gutiérrez et al., 1999; Talavera-Mendoza et al., 2005; Vega-Granillo et al., 2007, 2009) or just to theCarboniferous (Middleton et al., 2007; Elias-Herrera et al., 2007;Keppie et al., 2010b; Ramos-Arias et al., in press). But metamorphicminerals analyzed by different techniques and with a range of closuretemperatures, such as low Th/U zircon, glaucophane and phengite,only yield Carboniferous ages.

Uncertainties in the age and tectonic setting of the HP metamor-phic events as well as the geodynamic scenario responsible for theirexhumation is reflected in the highly variable models proposed withup to five oceanic tracts within the Iapetus and Rheic oceans byvarious author's (Ortega-Gutiérrez et al., 1999; Talavera-Mendoza etal., 2005; Vega-Granillo et al., 2007, 2009; Nance et al., 2010).Conversely, Keppie et al. (2008a, 2010b) suggested that the HP rockswere extruded into the Acatlán Complex during the Carboniferousalong the northwestern margin of Pangea with protoliths that formedalong the southwesternmargin of the Rheic Ocean. In order to providefurther constraints on the nature of the HP protoliths, the conflictingtectonic models for their origin and the paleogeography of the AcatlánComplex, we provide geochemical and geochronological data fromthe rocks at the southern end of the median HP belt, the type area of

the Piaxtla Metamorphic Suite. Resolution of the age of HPmetamorphism has first-order implications for Paleozoic paleoconti-nental reconstructions before and after the formation of Pangea.

3. U–Pb geochronology

3.1. Analytical methods

Two samples of the Piaxtla Suite were collected for U–Pb analysis: amegacrystic granite (PIX-18) sampled along a new roadcut just south ofthe village of Piaxtla (N18° 11.728′, W98° 14.690) and a quartz-micameta-psammite (PIX-17) from the Piaxtla river (N18° 08.767′, W98°17.467)(Fig. 1c). None of the sampled retrogressed eclogite yieldedzircon. All of the rocks record polyphase deformation and eclogite faciesmetamorphism followed by retrograde metamorphism through theamphibolite andgreenschist facies. The twoU–Pbsampleswere crushedand the zircons separated, processed and analyzed by LA-ICPMS U–Pbanalysis at the University of Arizona (Tuscon, Arizona) (Fig. 2). Theprocedures used are described in Gehrels et al. (2006), who quoted206Pb/238U ages for those b1000 Ma due to the low precision inmeasurement of 207Pb, and 207Pb/206Pb ages for those N1000 Ma(Tables 1 and 2).

3.2. Results

Zircons from sample PIX-18 yielded ages ranging from 440 to1500 Ma with the youngest cluster of 11 concordant ages yielding anage of 452±6 Ma (Fig. 2a and b). Older age populations occur at ca.800–950 Ma and 1000–1200 Ma.

The detrital zircons from PIX-17 yielded data that vary fromdiscordant to concordant with a concordant age range of ca. 320 to1300 Ma (Fig. 2c and d). The youngest population has a peak age of ca.375 Ma (Fig. 2 d), which is within error of the mean of the fiveyoungest concordant cluster at 365±15 Ma (Table 2).

Table 1LA-ICPMS U–Pb analyses of zircon from megacrystic granitoid (sample PIX-18) from the type Piaxtla Suite, Acatlán Complex, southern Mexico. Errors are 1-sigma.

Analysis Isotope ratios Apparent ages(Ma)

U 206Pb U/Th 206Pb* ± 207Pb* ± 206Pb* ± Error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± Conc

(ppm) 204Pb 207Pb* (%) 235U* (%) 238U (%) corr. 238U* (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma) (%)

PIX18-23 251 13,010 6.2 17.3483 2.1 0.5633 2.2 0.0709 0.7 0.33 441.4 3.1 453.7 8.0 516.2 45.3 441.4 3.1 85.5PIX18-47 339 32,488 10.3 17.3279 2.4 0.5676 2.7 0.0713 1.4 0.49 444.1 5.8 456.4 10.1 518.8 52.5 444.1 5.8 85.6PIX18-44 183 16,414 7.0 18.0179 3.4 0.5521 3.4 0.0722 0.5 0.15 449.1 2.2 446.4 12.2 432.4 74.7 449.1 2.2 103.9PIX18-13 482 20,916 15.8 17.5021 1.9 0.5699 2.2 0.0723 1.2 0.53 450.3 5.1 458.0 8.2 496.8 41.7 450.3 5.1 90.6PIX18-59 41 4890 1.7 18.9423 10.2 0.5276 10.3 0.0725 1.6 0.15 451.1 6.8 430.2 36.3 319.9 232.8 451.1 6.8 141.0PIX18-34 202 13,452 3.3 17.6247 2.2 0.5676 2.3 0.0726 0.5 0.22 451.5 2.2 456.4 8.3 481.4 48.5 451.5 2.2 93.8PIX18-42 354 27,456 10.1 17.6755 1.7 0.5662 1.7 0.0726 0.6 0.33 451.7 2.5 455.5 6.4 475.0 36.6 451.7 2.5 95.1PIX18-19 311 16,300 7.9 17.5797 2.2 0.5702 2.4 0.0727 0.8 0.35 452.4 3.6 458.1 8.8 487.0 49.2 452.4 3.6 92.9PIX18-26 594 38,062 28.7 17.5676 0.9 0.5711 1.0 0.0728 0.5 0.51 452.8 2.2 458.7 3.7 488.6 18.8 452.8 2.2 92.7PIX18-58 62 7538 3.2 18.3473 6.6 0.5490 6.7 0.0731 0.8 0.11 454.5 3.3 444.4 24.0 391.9 148.7 454.5 3.3 116.0PIX18-51 481 43,364 12.5 17.6898 1.1 0.5796 1.6 0.0744 1.1 0.71 462.4 5.0 464.2 5.9 473.2 24.7 462.4 5.0 97.7PIX18-4 270 16,352 6.9 17.8062 3.2 0.5764 3.3 0.0744 0.8 0.25 462.8 3.6 462.1 12.1 458.7 70.2 462.8 3.6 100.9PIX18-40A 227 20,298 7.9 17.8697 3.5 0.5870 3.6 0.0761 0.5 0.14 472.7 2.3 469.0 13.3 450.8 78.1 472.7 2.3 104.9PIX18-21 359 27,576 13.5 17.0738 1.3 0.6327 1.4 0.0784 0.7 0.49 486.3 3.3 497.8 5.7 551.1 27.4 486.3 3.3 88.2PIX18-45A 276 32,092 11.4 15.2826 2.5 0.8116 4.1 0.0900 3.3 0.80 555.3 17.3 603.4 18.5 788.3 51.7 555.3 17.3 70.4PIX18-56 358 58,986 9.4 15.6716 3.8 0.8107 4.9 0.0921 3.1 0.63 568.2 16.6 602.9 22.2 735.3 80.5 568.2 16.6 77.3PIX18-22 260 21,822 11.5 14.9104 1.1 0.9346 2.3 0.1011 2.0 0.87 620.7 11.8 670.1 11.2 839.8 23.3 620.7 11.8 73.9PIX18-36 282 31,036 8.9 15.6763 1.8 0.9584 2.0 0.1090 0.9 0.45 666.8 5.8 682.5 10.0 734.7 37.8 666.8 5.8 90.7PIX18-3 228 21,288 6.8 14.5762 1.3 1.0784 1.4 0.1140 0.5 0.36 695.9 3.3 742.8 7.4 886.9 27.0 695.9 3.3 78.5PIX18-8 282 35,016 15.3 14.6652 1.8 1.0824 2.2 0.1151 1.2 0.57 702.5 8.3 744.8 11.4 874.3 36.5 702.5 8.3 80.3PIX18-7 435 46,234 13.5 14.8895 2.6 1.1096 3.6 0.1198 2.4 0.68 729.5 16.8 758.0 19.0 842.8 54.1 729.5 16.8 86.6PIX18-27 164 17,752 3.8 13.2087 2.3 1.3779 5.1 0.1320 4.6 0.90 799.3 34.7 879.6 30.2 1087.3 45.1 799.3 34.7 73.5PIX18-50 159 27,654 5.6 14.0239 2.2 1.3273 2.3 0.1350 0.6 0.24 816.4 4.2 857.7 13.0 966.2 44.6 816.4 4.2 84.5PIX18-10A 143 15,350 3.2 15.0419 2.3 1.2400 2.7 0.1353 1.4 0.52 817.9 10.8 818.9 15.2 821.6 48.3 817.9 10.8 99.6PIX18-54 215 33,470 6.5 13.9632 1.9 1.3694 2.1 0.1387 1.0 0.45 837.2 7.5 875.9 12.5 975.1 38.7 837.2 7.5 85.9PIX18-35 250 32,632 4.3 13.4267 1.4 1.4377 2.8 0.1400 2.4 0.87 844.7 19.2 904.8 16.7 1054.5 27.5 844.7 19.2 80.1PIX18-25 479 60,034 8.5 13.6122 0.6 1.4315 1.5 0.1413 1.4 0.90 852.2 10.8 902.2 8.9 1026.8 13.0 852.2 10.8 83.0PIX18-6 200 27,364 3.4 14.2547 1.9 1.3683 2.1 0.1415 1.0 0.46 852.9 7.8 875.4 12.4 932.8 38.4 852.9 7.8 91.4PIX18-31 175 24,084 4.3 14.4053 2.5 1.3758 2.9 0.1437 1.5 0.52 865.8 12.2 878.6 17.0 911.2 51.0 865.8 12.2 95.0PIX18-12 79 11,818 1.1 13.7473 4.7 1.4643 5.7 0.1460 3.2 0.56 878.5 26.0 915.8 34.1 1006.7 95.0 878.5 26.0 87.3PIX18-32 169 25,114 3.7 14.1190 1.5 1.4635 1.7 0.1499 0.8 0.48 900.2 6.7 915.5 10.0 952.4 29.9 900.2 6.7 94.5PIX18-53 110 23,238 2.4 14.0984 2.1 1.4693 2.9 0.1502 2.0 0.69 902.3 17.0 917.9 17.8 955.4 43.6 902.3 17.0 94.4PIX18-60 108 22,976 2.3 13.9683 2.5 1.5005 2.9 0.1520 1.5 0.52 912.2 12.8 930.6 17.7 974.4 50.6 912.2 12.8 93.6PIX18-43 196 55,894 6.2 14.2820 0.7 1.4766 1.2 0.1530 0.9 0.78 917.5 7.9 920.9 7.1 928.9 15.1 917.5 7.9 98.8PIX18-29 159 19,598 1.9 13.9186 2.4 1.5331 3.2 0.1548 2.0 0.64 927.6 17.4 943.7 19.5 981.6 49.8 927.6 17.4 94.5PIX18-28 702 85,424 6.6 13.8011 2.0 1.7024 2.1 0.1704 0.5 0.24 1014.3 4.7 1009.4 13.1 998.8 40.4 998.8 40.4 101.6PIX18-20 47 12,214 1.9 13.5427 4.1 1.6201 5.3 0.1591 3.3 0.63 951.9 29.1 978.0 33.0 1037.1 82.9 1037.1 82.9 91.8PIX18-48 922 18,548 20.4 13.3102 3.0 1.6858 3.5 0.1627 1.7 0.49 972.0 15.6 1003.2 22.3 1072.0 61.2 1072.0 61.2 90.7PIX18-17 450 48,372 3.7 13.2065 2.0 1.6604 2.2 0.1590 0.9 0.42 951.4 8.2 993.5 14.0 1087.7 40.3 1087.7 40.3 87.5PIX18-57 378 52,696 6.1 13.1820 1.6 1.6734 1.7 0.1600 0.5 0.30 956.7 4.4 998.5 10.5 1091.4 31.7 1091.4 31.7 87.7PIX18-30 189 24,038 3.5 13.1681 1.8 1.5410 3.4 0.1472 2.9 0.85 885.1 24.0 946.9 20.9 1093.5 35.5 1093.5 35.5 80.9PIX18-41 106 22,820 4.3 13.1175 1.9 1.6896 3.3 0.1607 2.6 0.81 960.9 23.5 1004.6 20.8 1101.2 38.3 1101.2 38.3 87.3PIX18-37 388 56,118 7.2 13.0081 2.1 1.5194 4.2 0.1433 3.7 0.87 863.6 29.7 938.3 25.8 1118.0 41.1 1118.0 41.1 77.2PIX18-1 252 45,496 3.5 12.8985 1.6 1.9348 2.4 0.1810 1.8 0.73 1072.4 17.3 1093.2 16.0 1134.8 32.5 1134.8 32.5 94.5PIX18-49 1214 140,212 22.4 12.8675 1.5 1.9656 1.9 0.1834 1.1 0.59 1085.7 11.1 1103.8 12.6 1139.6 30.0 1139.6 30.0 95.3PIX18-15 350 41,742 3.5 12.8599 0.8 1.7922 2.1 0.1672 2.0 0.92 996.4 18.2 1042.6 14.0 1140.8 16.7 1140.8 16.7 87.3PIX18-18 208 43,752 3.5 12.8134 1.8 2.0749 1.9 0.1928 0.7 0.35 1136.7 6.8 1140.6 12.9 1148.0 35.0 1148.0 35.0 99.0PIX18-38 48 15,460 1.5 12.7766 3.3 2.0651 4.3 0.1914 2.7 0.64 1128.7 28.3 1137.3 29.4 1153.7 65.8 1153.7 65.8 97.8PIX18-9 443 75,248 3.4 12.7079 1.2 1.9181 1.6 0.1768 1.1 0.68 1049.4 ` 1087.4 10.8 1164.4 23.6 1164.4 23.6 90.1PIX18-40 67 21,300 4.4 12.5982 2.3 2.1837 2.7 0.1995 1.4 0.53 1172.8 15.3 1175.9 18.8 1181.6 45.2 1181.6 45.2 99.3PIX18-46 298 70,288 3.1 12.3150 0.5 2.3136 1.6 0.2066 1.5 0.95 1210.9 16.6 1216.5 11.2 1226.3 9.8 1226.3 9.8 98.7PIX18-52 294 53,902 33.0 12.2176 1.2 2.2059 1.4 0.1955 0.6 0.46 1150.9 6.6 1182.9 9.7 1241.9 24.1 1241.9 24.1 92.7PIX18-39 304 34,622 5.6 11.9774 3.9 1.9779 4.7 0.1718 2.7 0.57 1022.1 25.3 1108.0 31.8 1280.7 75.7 1280.7 75.7 79.8PIX18-55 228 49,296 6.1 11.9463 2.8 2.0986 3.9 0.1818 2.7 0.69 1076.9 26.5 1148.3 26.7 1285.8 54.7 1285.8 54.7 83.8PIX18-5 1265 98,396 3.2 11.8991 1.8 2.3210 3.5 0.2003 2.9 0.85 1176.9 31.6 1218.7 24.6 1293.5 35.6 1293.5 35.6 91.0PIX18-11 90 15,994 2.5 11.7747 5.6 2.4753 5.7 0.2114 1.3 0.22 1236.2 14.3 1264.9 41.5 1313.9 108.5 1313.9 108.5 94.1PIX18-33 621 109,542 29.4 11.6639 1.0 2.5178 1.3 0.2130 0.9 0.68 1244.7 10.0 1277.2 9.5 1332.2 18.6 1332.2 18.6 93.4PIX18-24 54 13,708 5.6 11.0854 6.2 2.8266 6.2 0.2273 0.7 0.11 1320.1 8.4 1362.6 46.6 1429.9 117.8 1429.9 117.8 92.3PIX18-16 126 37,442 2.7 10.7151 0.8 2.9291 1.4 0.2276 1.2 0.84 1322.1 14.3 1389.5 10.9 1494.5 14.8 1494.5 14.8 88.5

1053J.D. Keppie et al. / Gondwana Research 21 (2012) 1050–1065

4. 40Ar/39Ar geochronology

4.1. Analytical methods

Hornblende, muscovite and biotite mineral grains were separatedfrom four samples of the Piaxtla Suite, two retrogressed maficamphibolites (PIX-24: N18° 11.652′, W98° 15.065′ and PIX-25: N18°15.630, W98° 15.061′), a megacrystic granitoid (PIX-18), and a

quartz-mica meta-psammite (PIX-17), the latter two being the samesamples used in the U–Pb geochronology (Fig. 1a). These mineralswere pre-treated and concentrated by standard techniques and laterselected by handpicking under a binocular microscope from fractionsthat ranged in size from 40 to 60 mesh at the mineral separationlaboratory at UNICIT-Universidad Nacional Autónoma de México(Campus-Juriquilla, Querétaro, Qro). Mineral separates were loadedinto Al-foil packets and irradiated together with Hb3gr as a neutron-

Table 2LA-ICPMS U–Pb analyses of detrital zircon from meta-psammite (sample PIX-17) from the type Piaxtla Suite, Acatlán Complex, southern Mexico. Errors are 1-sigma.

File name Notes 207/206 1σ+/−abs

206/238

1σ+/−abs

207/235

1σ+/−abs

207*/235

t σ +/−abs

207*/235 t σ (95%) 206*/238

t σ +/−abs

206*/238 t σ (95%) 207*/206*

t σ +/−abs

207*/206* t σ (95%) Bestage

Error rsd

Age, Ma +/−(abs)

Age,Ma

+/−(abs)

Age(Ma)

+/−(abs)

Age(Ma)

+/−(abs)

DY01-3C0. B5, core 0.10964 0.00268 0.1572 0.0036 2.3760 0.061 1.635 0.157 984 ±61 0.1418 0.0091 855 ±52 0.0836 0.0069 1283 ±160 855.0 51.6 0.06DY01-3C4. A15 0.08244 0.00241 0.1372 0.0028 1.5594 0.039 1.143 0.082 774 ±39 0.1252 0.0058 761 ±33 0.0662 0.0051 812 ±160 760.6 33.3 0.04DY01-3C5. A14 0.07532 0.00097 0.1394 0.0028 1.4473 0.030 1.305 0.065 848 ±28 0.1293 0.0059 784 ±34 0.0732 0.0021 1019 ±57 784.0 33.8 0.04DY01-3C6. A13 0.08630 0.00156 0.1951 0.0039 2.3219 0.051 1.991 0.107 1113 ±36 0.1792 0.0082 1062 ±45 0.0806 0.0033 1212 ±79 1211.8 79.5 0.07DY01-3C7. A12,

inclusion?0.08939 0.00278 0.1533 0.0042 1.8898 0.060 1.353 0.142 869 ±61 0.1394 0.0100 841 ±57 0.0704 0.0066 940 ±191 841.5 56.6 0.07

DY01-3C8. A10 0.08204 0.00286 0.1751 0.0035 1.9806 0.053 1.739 0.111 1023 ±41 0.1616 0.0074 966 ±41 0.0781 0.0060 1148 ±153 965.8 41.1 0.04DY01-3C9. A9 0.08970 0.00164 0.1750 0.0040 2.1638 0.053 1.730 0.126 1020 ±47 0.1601 0.0094 958 ±52 0.0783 0.0039 1156 ±98 957.5 52.5 0.05DY01-3CA. A6 0.08133 0.00106 0.1655 0.0033 1.8555 0.039 1.631 0.082 982 ±32 0.1529 0.0069 917 ±39 0.0773 0.0022 1130 ±57 917.3 38.9 0.04DY01-3CB. A5 0.09541 0.00139 0.1558 0.0032 2.0495 0.044 1.485 0.098 924 ±40 0.1416 0.0070 854 ±40 0.0761 0.0031 1097 ±81 853.8 39.6 0.05DY01-3CC. A4 0.09633 0.00195 0.2047 0.0045 2.7185 0.066 2.021 0.152 1122 ±51 0.1851 0.0104 1095 ±56 0.0792 0.0045 1176 ±112 1176.1 112.2 0.10DY01-3CD. A3 0.05996 0.00078 0.0654 0.0013 0.5406 0.011 0.454 0.024 380 ±17 0.0602 0.0028 377 ±17 0.0548 0.0017 402 ±68 376.6 17.0 0.05DY01-3CE. A2 0.09578 0.00147 0.1932 0.0039 2.5519 0.055 1.885 0.115 1076 ±41 0.1751 0.0082 1040 ±45 0.0781 0.0031 1149 ±78 1148.6 78.5 0.07DY01-3D9. B31 0.06276 0.00095 0.0748 0.0015 0.6472 0.014 0.526 0.029 429 ±19 0.0687 0.0032 428 ±19 0.0555 0.0020 434 ±80 428.5 19.0 0.04DY01-3DA. B32 0.07713 0.00101 0.1727 0.0035 1.8366 0.039 1.619 0.082 978 ±32 0.1595 0.0073 954 ±41 0.0736 0.0021 1031 ±58 954.2 40.7 0.04DY01-3DB. B33 0.07519 0.00097 0.1401 0.0030 1.4521 0.032 1.236 0.070 817 ±32 0.1294 0.0065 784 ±37 0.0693 0.0021 906 ±63 784.3 37.0 0.05DY01-3DC. B34 0.07871 0.00100 0.1620 0.0034 1.7578 0.038 1.564 0.080 956 ±32 0.1499 0.0071 900 ±40 0.0757 0.0021 1087 ±56 900.5 39.7 0.04DY01-3DD. B35 0.06731 0.00108 0.0639 0.0014 0.5931 0.014 0.454 0.030 380 ±21 0.0584 0.0030 366 ±18 0.0565 0.0023 472 ±89 365.6 18.3 0.05DY01-3DE. B36 0.07615 0.00148 0.0629 0.0013 0.6600 0.015 0.454 0.034 380 ±23 0.0569 0.0029 357 ±18 0.0579 0.0033 525 ±123 357.0 17.7 0.05DY01-3DF. B37 0.07413 0.00095 0.1580 0.0033 1.6149 0.036 1.352 0.078 868 ±34 0.1456 0.0073 876 ±41 0.0673 0.0021 848 ±64 876.4 40.9 0.05DY01-3E0. B38, low

6/40.11321 0.00369 0.1063 0.0024 1.6591 0.046 0.791 0.104 592 ±59 0.0933 0.0054 575 ±32 0.0615 0.0083 655 ±291 575.2 31.8 0.06

DY01-3E1. B39 0.08071 0.00102 0.1651 0.0033 1.8375 0.039 1.625 0.082 980 ±32 0.1527 0.0070 916 ±39 0.0772 0.0021 1126 ±55 916.0 39.2 0.04DY01-3E2. B42, low

6/40.12187 0.00296 0.0964 0.0021 1.6204 0.041 0.867 0.091 634 ±49 0.0850 0.0047 526 ±28 0.0740 0.0066 1041 ±180 525.8 28.2 0.05

DY01-3E6. A27 0.07862 0.00103 0.1340 0.0033 1.4530 0.037 1.241 0.081 819 ±37 0.1238 0.0073 753 ±42 0.0727 0.0022 1005 ±63 752.6 41.7 0.06DY01-3E7. A26, low

6/40.15406 0.00274 0.1404 0.0034 2.9833 0.077 1.344 0.170 865 ±74 0.1207 0.0075 735 ±43 0.0807 0.0060 1215 ±147 734.6 43.3 0.06

DY01-3E8. A24 0.07914 0.00170 0.1015 0.0037 1.1080 0.042 0.812 0.104 604 ±58 0.0927 0.0091 571 ±54 0.0635 0.0042 726 ±139 571.4 53.8 0.09DY01-3E9. A23 0.09659 0.00125 0.2375 0.0048 3.1624 0.067 2.718 0.144 1333 ±39 0.2119 0.0103 1239 ±55 0.0930 0.0027 1489 ±55 1488.6 54.9 0.04DY01-3EA. A22 0.09676 0.00139 0.1887 0.0038 2.5179 0.054 1.936 0.116 1094 ±40 0.1717 0.0082 1021 ±45 0.0818 0.0030 1241 ±71 1240.9 71.3 0.06DY01-3EB. A21 0.09297 0.00137 0.1449 0.0033 1.8574 0.044 1.414 0.092 895 ±39 0.1324 0.0069 801 ±39 0.0775 0.0029 1134 ±74 801.4 39.0 0.05DY01-3EC. A20 0.08637 0.00110 0.2073 0.0042 2.4694 0.052 2.216 0.110 1186 ±35 0.1907 0.0088 1125 ±47 0.0843 0.0023 1299 ±53 1299.4 53.1 0.04DY01-3ED. A19 0.08126 0.00101 0.1822 0.0037 2.0413 0.043 1.872 0.090 1071 ±32 0.1686 0.0077 1005 ±42 0.0805 0.0021 1210 ±52 1209.7 51.8 0.04DY01-3EE. A18 0.07961 0.00106 0.1471 0.0030 1.6144 0.034 1.364 0.078 874 ±33 0.1357 0.0067 820 ±38 0.0729 0.0024 1012 ±67 820.3 38.2 0.05DY01-3EF. A17 0.08154 0.00106 0.1662 0.0033 1.8688 0.039 1.667 0.082 996 ±31 0.1538 0.0070 922 ±39 0.0786 0.0022 1162 ±56 922.1 38.9 0.04DY01-3F7. B14 0.08732 0.00125 0.1958 0.0041 2.3576 0.052 1.998 0.113 1115 ±38 0.1796 0.0089 1065 ±48 0.0807 0.0027 1214 ±66 1214.2 65.7 0.05DY01-3F8. B12 0.08843 0.00195 0.1507 0.0031 1.8369 0.043 1.381 0.090 881 ±38 0.1376 0.0065 831 ±37 0.0728 0.0041 1009 ±114 830.9 36.6 0.04DY01-3F9. B11 0.09371 0.00117 0.2148 0.0045 2.7755 0.061 2.427 0.127 1251 ±38 0.1898 0.0094 1120 ±51 0.0927 0.0024 1483 ±50 1482.6 50.0 0.03DY01-3FA. B10 0.11543 0.00190 0.0586 0.0012 0.9332 0.020 0.442 0.044 372 ±31 0.0511 0.0026 322 ±16 0.0627 0.0041 699 ±141 321.5 15.8 0.05DY01-3FB. B8 0.08426 0.00106 0.1859 0.0037 2.1597 0.046 1.946 0.096 1097 ±33 0.1717 0.0079 1021 ±43 0.0822 0.0022 1250 ±53 1250.0 53.1 0.04DY01-3FC. B3 0.07421 0.00124 0.1434 0.0029 1.4673 0.032 1.253 0.067 825 ±30 0.1325 0.0060 802 ±34 0.0686 0.0026 886 ±78 802.0 34.3 0.04DY01-3FD. B2 0.07884 0.00126 0.1503 0.0030 1.6338 0.035 1.402 0.074 890 ±31 0.1388 0.0063 838 ±36 0.0733 0.0026 1021 ±73 838.0 35.9 0.04DY01-3FE. B1 0.07060 0.00110 0.0744 0.0015 0.7243 0.016 0.521 0.033 426 ±22 0.0677 0.0032 422 ±19 0.0558 0.0023 443 ±92 422.4 19.3 0.05DY01-3FF. A31

(firsthalf)0.11802 0.00199 0.2439 0.0051 3.9681 0.090 2.917 0.203 1386 ±53 0.2145 0.0116 1253 ±61 0.0986 0.0045 1598 ±85 1598.2 85.4 0.05

DY01-3FF. A31(lasthalf)

0.13506 0.00246 0.2503 0.0050 4.6612 0.103 3.022 0.232 1413 ±59 0.2174 0.0113 1268 ±60 0.1008 0.0056 1639 ±103 1639.1 102.6 0.06

DY01-3FF. A31(alltwenty)

0.12654 0.00274 0.2502 0.0050 4.3648 0.099 3.102 0.208 1433 ±51 0.2197 0.0105 1280 ±55 0.1024 0.0057 1668 ±103 1667.9 103.5 0.06

DY01-400. A28 0.06124 0.00091 0.0773 0.0016 0.6527 0.014 0.554 0.030 448 ±20 0.0713 0.0034 444 ±20 0.0564 0.0020 469 ±77 443.7 20.3 0.05DY01-404. C26 0.06959 0.00184 0.0713 0.0015 0.6841 0.017 0.484 0.037 401 ±25 0.0648 0.0032 405 ±20 0.0542 0.0041 379 ±171 404.9 19.6 0.05

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File name Notes 207/206 1σ+/−abs

206/238

1σ+/−abs

207/235

1σ+/−abs

207*/235

t σ +/−abs

207*/235 t σ (95%) 206*/238

t σ +/−abs

206*/238 t σ (95%) 207*/206*

t σ +/−abs

207*/206* t σ (95%) Bestage

Error rsd

Age, Ma +/−(abs)

Age,Ma

+/−(abs)

Age(Ma)

+/−(abs)

Age(Ma)

+/−(abs)

DY01-405. c25 0.07322 0.00105 0.0983 0.0020 0.9923 0.021 0.866 0.045 633 ±24 0.0909 0.0042 561 ±25 0.0691 0.0022 901 ±66 560.9 24.8 0.04DY01-406. C24 0.06128 0.00091 0.0653 0.0013 0.5514 0.012 0.453 0.025 380 ±17 0.0599 0.0027 375 ±17 0.0549 0.0019 407 ±78 375.2 16.7 0.04DY01-407. C27 0.08106 0.00102 0.1801 0.0037 2.0133 0.043 1.809 0.092 1049 ±33 0.1665 0.0078 993 ±43 0.0788 0.0022 1167 ±55 992.7 43.3 0.04DY01-408. C28 0.07390 0.00116 0.0748 0.0015 0.7620 0.017 0.533 0.037 434 ±24 0.0680 0.0034 424 ±20 0.0569 0.0025 488 ±98 423.8 20.4 0.05DY01-40A. C29 0.06529 0.00099 0.0801 0.0018 0.7209 0.017 0.568 0.040 457 ±26 0.0735 0.0042 457 ±25 0.0561 0.0023 457 ±90 457.0 25.5 0.06DY01-40B. C30 0.09261 0.00178 0.1047 0.0022 1.3371 0.031 0.927 0.067 666 ±35 0.0950 0.0048 585 ±28 0.0708 0.0039 951 ±112 584.8 28.2 0.05DY01-40C. C32

(firsthalf)0.09543 0.00137 0.1892 0.0039 2.4896 0.054 1.830 0.122 1056 ±44 0.1715 0.0087 1020 ±48 0.0774 0.0031 1131 ±80 1131.1 79.6 0.07

DY01-40C. C32(lasthalf)

0.08872 0.00138 0.1872 0.0039 2.2899 0.051 1.811 0.116 1050 ±42 0.1709 0.0089 1017 ±49 0.0769 0.0031 1118 ±81 1117.8 80.9 0.07

DY01-40C. C32(alltwenty)

0.09208 0.00147 0.1894 0.0038 2.4049 0.052 1.834 0.109 1058 ±39 0.1723 0.0080 1025 ±44 0.0772 0.0031 1127 ±80 1126.9 79.6 0.07

DY01-40D. C33 0.09071 0.00149 0.1936 0.0040 2.4212 0.054 1.932 0.112 1092 ±39 0.1766 0.0084 1049 ±46 0.0793 0.0031 1180 ±78 1179.9 77.9 0.07DY01-40E. D4 0.06446 0.00117 0.0710 0.0014 0.6309 0.014 0.504 0.032 414 ±21 0.0651 0.0032 407 ±20 0.0561 0.0026 458 ±105 406.5 19.6 0.05DY01-40F. D12 0.09603 0.00126 0.1623 0.0034 2.1485 0.047 1.868 0.098 1070 ±35 0.1498 0.0071 900 ±40 0.0905 0.0026 1436 ±56 899.6 39.6 0.04DY01-410. D13 0.06120 0.00093 0.0607 0.0012 0.5124 0.011 0.420 0.024 356 ±17 0.0557 0.0027 350 ±16 0.0546 0.0020 397 ±82 349.5 16.2 0.05DY01-411. D15 0.10766 0.00148 0.2527 0.0051 3.7513 0.080 3.056 0.173 1422 ±43 0.2259 0.0110 1313 ±58 0.0981 0.0032 1589 ±61 1588.8 60.7 0.04DY01-412. D16 0.08390 0.00124 0.1863 0.0037 2.1545 0.046 1.843 0.097 1061 ±35 0.1713 0.0079 1019 ±43 0.0781 0.0026 1148 ±67 1148.5 66.5 0.06DY01-413. D19 0.09633 0.00122 0.2293 0.0046 3.0452 0.064 2.646 0.135 1313 ±37 0.2039 0.0097 1196 ±52 0.0941 0.0026 1510 ±51 1509.9 51.2 0.03DY01-414. D20 0.08309 0.00105 0.1716 0.0035 1.9656 0.042 1.682 0.088 1002 ±33 0.1581 0.0073 946 ±41 0.0772 0.0022 1125 ±57 946.1 40.8 0.04DY01-415. D22 0.08364 0.00104 0.1923 0.0039 2.2177 0.047 2.022 0.099 1123 ±33 0.1776 0.0082 1054 ±45 0.0826 0.0022 1259 ±52 1258.7 52.1 0.04DY01-416. D23 0.08135 0.00109 0.1713 0.0034 1.9218 0.041 1.727 0.086 1019 ±32 0.1585 0.0073 949 ±40 0.0790 0.0023 1173 ±58 948.7 40.3 0.04DY01-417. D25 0.09230 0.00218 0.0725 0.0015 0.9224 0.022 0.508 0.046 417 ±31 0.0645 0.0031 403 ±19 0.0571 0.0046 495 ±178 403.1 18.9 0.05DY01-418. D36 0.07505 0.00106 0.0889 0.0018 0.9194 0.020 0.634 0.042 498 ±26 0.0808 0.0039 501 ±23 0.0569 0.0023 488 ±88 500.8 23.1 0.05DY01-419. D37 0.09447 0.00120 0.2255 0.0046 2.9375 0.062 2.570 0.131 1292 ±37 0.2004 0.0096 1177 ±52 0.0930 0.0025 1489 ±51 1488.6 51.5 0.03DY01-422. E3 0.08534 0.00176 0.0798 0.0016 0.9384 0.022 0.567 0.048 456 ±31 0.0716 0.0037 446 ±22 0.0574 0.0039 506 ±150 446.0 22.0 0.05DY01-423. E8 0.05964 0.00083 0.0690 0.0014 0.5670 0.012 0.477 0.025 396 ±17 0.0635 0.0029 397 ±18 0.0545 0.0017 394 ±72 396.7 17.6 0.04DY01-424. E9 0.08048 0.00102 0.1808 0.0036 2.0058 0.042 1.814 0.089 1050 ±32 0.1671 0.0076 996 ±42 0.0787 0.0021 1165 ±54 996.4 42.1 0.04DY01-425. E10 0.09509 0.00122 0.1978 0.0040 2.5927 0.055 2.296 0.114 1211 ±35 0.1819 0.0083 1078 ±45 0.0915 0.0025 1458 ±53 1457.9 52.9 0.04DY01-426. E12 0.08816 0.00172 0.1516 0.0047 1.8423 0.060 1.404 0.131 890 ±55 0.1385 0.0103 836 ±58 0.0735 0.0037 1028 ±103 836.3 58.3 0.07DY01-427. E21 0.09855 0.00162 0.2135 0.0044 2.9017 0.065 2.411 0.135 1246 ±40 0.1876 0.0093 1108 ±50 0.0932 0.0034 1492 ±69 1492.4 69.0 0.05DY01-428. E22 0.09801 0.00130 0.2363 0.0048 3.1937 0.068 2.781 0.142 1351 ±38 0.2110 0.0100 1234 ±53 0.0956 0.0027 1540 ±54 1539.8 53.8 0.03DY01-429. E28 0.09399 0.00142 0.1714 0.0034 2.2209 0.048 1.719 0.101 1015 ±38 0.1564 0.0073 937 ±41 0.0797 0.0030 1189 ±74 936.7 40.7 0.04DY01-42A. E32 0.11403 0.00540 0.0765 0.0016 1.2029 0.038 0.665 0.081 517 ±49 0.0676 0.0034 422 ±21 0.0713 0.0114 965 ±326 422.0 20.6 0.05DY01-42B. E37 0.08476 0.00124 0.1491 0.0043 1.7418 0.051 1.457 0.118 913 ±49 0.1373 0.0098 830 ±56 0.0770 0.0029 1120 ±74 829.6 55.6 0.07DY01-42C. E38 0.09269 0.00118 0.2274 0.0046 2.9057 0.061 2.514 0.129 1276 ±37 0.2020 0.0096 1186 ±52 0.0903 0.0025 1431 ±52 1431.2 52.4 0.04DY01-42D. E40 0.07767 0.00103 0.0832 0.0017 0.8907 0.019 0.689 0.041 532 ±24 0.0761 0.0036 473 ±22 0.0657 0.0022 795 ±70 472.9 21.6 0.05DY01-4BF. F31a

(core?)0.18564 0.00203 0.0885 0.0020 2.2647 0.052 0.791 0.122 592 ±69 0.0705 0.0046 439 ±28 0.0814 0.0048 1231 ±116 439.0 28.0 0.06

DY01-4BF. F31a(rim?)

0.15855 0.00250 0.0589 0.0060 1.2870 0.131 0.519 0.338 425 ±226 0.0490 0.0154 308 ±95 0.0769 0.0064 1118 ±167 308.3 94.7 0.31

DY01-4C0. F31b(core?)

0.20999 0.00172 0.0844 0.0025 2.4449 0.073 0.814 0.179 605 ±100 0.0663 0.0061 414 ±37 0.0890 0.0042 1405 ±90 414.0 37.0 0.09

DY01-4C1. F30 0.08210 0.00084 0.1973 0.0042 2.2335 0.049 1.675 0.103 999 ±39 0.1595 0.0088 954 ±49 0.0761 0.0018 1099 ±47 954.1 49.2 0.05DY01-4C2. F29 0.08343 0.00061 0.1000 0.0020 1.1505 0.024 0.870 0.053 636 ±29 0.0871 0.0046 538 ±27 0.0725 0.0014 999 ±39 538.3 27.0 0.05DY01-4C3. F28 0.08149 0.00064 0.1861 0.0037 2.0908 0.043 1.654 0.090 991 ±34 0.1525 0.0078 915 ±44 0.0786 0.0013 1163 ±34 915.1 43.9 0.05DY01-4C4. F27 0.06986 0.00062 0.0915 0.0019 0.8812 0.019 0.696 0.041 537 ±24 0.0805 0.0041 499 ±25 0.0627 0.0013 699 ±46 499.0 24.6 0.05DY01-4C7. F25 0.08247 0.00062 0.1742 0.0036 1.9810 0.042 1.596 0.088 969 ±34 0.1444 0.0076 869 ±43 0.0802 0.0013 1201 ±32 869.4 42.7 0.05DY01-4C8. F24 0.09222 0.00073 0.2354 0.0047 2.9934 0.061 2.237 0.129 1193 ±40 0.1804 0.0099 1069 ±54 0.0899 0.0015 1424 ±32 1423.7 32.5 0.02DY01-4C9. F23 0.08693 0.00110 0.0851 0.0017 1.0200 0.022 0.607 0.045 482 ±29 0.0736 0.0036 458 ±22 0.0599 0.0023 598 ±83 457.6 21.8 0.05DY01-4CA. F22a 0.08491 0.00085 0.1962 0.0039 2.2971 0.048 1.726 0.104 1018 ±39 0.1588 0.0086 950 ±48 0.0788 0.0018 1168 ±46 950.0 47.8 0.05DY01-4CB. F22b 0.07815 0.00064 0.1717 0.0034 1.8498 0.038 1.495 0.080 929 ±32 0.1426 0.0072 859 ±41 0.0760 0.0014 1096 ±36 859.4 40.8 0.05DY01-4CF. F43 0.08473 0.00074 0.1666 0.0034 1.9459 0.040 1.447 0.085 909 ±35 0.1379 0.0071 833 ±40 0.0761 0.0016 1097 ±41 832.9 40.3 0.05DY01-4D0. F42 0.08635 0.00066 0.3421 0.0074 4.0732 0.089 2.785 0.189 1352 ±51 0.2474 0.0156 1425 ±81 0.0817 0.0014 1237 ±34 1237.2 33.6 0.03DY01-4D1. F41 0.08014 0.00065 0.1248 0.0029 1.3792 0.032 1.118 0.068 762 ±33 0.1074 0.0061 658 ±35 0.0755 0.0014 1081 ±37 657.8 35.3 0.05DY01-4D2. F40 0.10476 0.00133 0.1858 0.0039 2.6838 0.059 1.693 0.144 1006 ±54 0.1490 0.0096 895 ±54 0.0824 0.0033 1255 ±77 895.4 53.6 0.06DY01-4D3. F39 0.08982 0.00074 0.3510 0.0073 4.3472 0.092 2.999 0.194 1407 ±49 0.2530 0.0153 1454 ±79 0.0860 0.0016 1338 ±35 1337.6 35.2 0.03DY01-4D4. F38 0.11815 0.00178 0.3171 0.0073 5.1656 0.125 2.966 0.272 1399 ±70 0.2274 0.0159 1321 ±83 0.0946 0.0039 1520 ±77 1520.1 77.1 0.05DY01-4D5. F37 0.24821 0.00350 0.2894 0.0061 9.9058 0.219 3.185 0.466 1453 ±113 0.1890 0.0129 1116 ±70 0.1222 0.0075 1989 ±108 1988.7 108.4 0.05

(continued on next page)

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Table 2 (continued)

File name Notes 207/206 1σ+/−abs

206/238

1σ+/−abs

207/235

1σ+/−abs

207*/235

t σ +/−abs

207*/235 t σ (95%) 206*/238

t σ +/−abs

206*/238 t σ (95%) 207*/206*

t σ +/−abs

207*/206* t σ (95%) Bestage

Error rsd

Age, Ma +/−(abs)

Age,Ma

+/−(abs)

Age(Ma)

+/−(abs)

Age(Ma)

+/−(abs)

DY01-4D6. F36 0.08344 0.00072 0.2049 0.0048 2.3576 0.056 1.835 0.118 1058 ±42 0.1651 0.0101 985 ±56 0.0806 0.0015 1211 ±37 985.3 56.0 0.06DY01-4D7. F35 0.08228 0.00059 0.2283 0.0046 2.5901 0.053 2.001 0.111 1116 ±38 0.1802 0.0096 1068 ±53 0.0805 0.0012 1210 ±30 1210.3 30.3 0.03DY01-4D8. F34 0.08533 0.00086 0.2450 0.0054 2.8829 0.065 2.141 0.148 1162 ±48 0.1875 0.0122 1108 ±66 0.0828 0.0019 1265 ±46 1264.6 45.9 0.04DY01-4D9. F33 0.08572 0.00145 0.1161 0.0024 1.3726 0.031 1.009 0.064 708 ±32 0.0998 0.0050 613 ±29 0.0734 0.0030 1024 ±84 612.9 29.4 0.05DY01-4DA. F34 0.10046 0.00148 0.1902 0.0078 2.6343 0.110 1.699 0.282 1008 ±106 0.1524 0.0201 914 ±112 0.0809 0.0038 1219 ±93 914.4 112.3 0.12DY01-4E1. F11 0.08172 0.00063 0.1950 0.0039 2.1973 0.045 1.755 0.094 1029 ±35 0.1588 0.0082 950 ±46 0.0802 0.0013 1201 ±32 950.2 45.6 0.05DY01-4E2. F10 0.08104 0.00060 0.1845 0.0037 2.0617 0.042 1.674 0.089 999 ±34 0.1518 0.0079 911 ±44 0.0800 0.0013 1197 ±31 910.9 43.9 0.05DY01-4E3. F9 0.07810 0.00056 0.1607 0.0032 1.7307 0.035 1.440 0.075 906 ±31 0.1350 0.0068 816 ±39 0.0774 0.0012 1131 ±31 816.3 38.7 0.05DY01-4E4. F8 0.08178 0.00072 0.1808 0.0036 2.0387 0.042 1.614 0.088 976 ±34 0.1488 0.0077 894 ±43 0.0786 0.0015 1163 ±38 894.5 43.0 0.05DY01-4E5. F7 0.08091 0.00064 0.1908 0.0038 2.1289 0.043 1.696 0.091 1007 ±34 0.1559 0.0080 934 ±45 0.0789 0.0013 1169 ±34 934.0 44.6 0.05DY01-4E6. F6 0.06465 0.00104 0.0688 0.0014 0.6130 0.013 0.467 0.028 389 ±19 0.0613 0.0029 384 ±18 0.0553 0.0022 424 ±88 383.7 17.6 0.05DY01-4E7. F5 0.08854 0.00081 0.1993 0.0041 2.4330 0.051 1.830 0.112 1056 ±40 0.1609 0.0089 962 ±50 0.0825 0.0018 1258 ±42 961.5 49.6 0.05DY01-4E8. F4 0.10788 0.00142 0.2768 0.0058 4.1168 0.090 2.746 0.208 1341 ±56 0.2054 0.0133 1204 ±71 0.0970 0.0033 1567 ±63 1566.7 63.5 0.04DY01-4E9. F3 0.08419 0.00071 0.2247 0.0045 2.6082 0.054 1.976 0.113 1107 ±38 0.1776 0.0095 1054 ±52 0.0807 0.0015 1214 ±37 1213.9 36.6 0.03DY01-4EA. F2a 0.11454 0.00227 0.2049 0.0041 3.2363 0.072 1.871 0.161 1071 ±57 0.1602 0.0092 958 ±51 0.0847 0.0051 1308 ±116 958.0 50.9 0.05DY01-4EB. F2b 0.10413 0.00087 0.1808 0.0037 2.5956 0.054 1.654 0.117 991 ±45 0.1457 0.0080 877 ±45 0.0823 0.0019 1253 ±45 876.9 44.9 0.05DY01-4EC. F1 0.15342 0.00583 0.0783 0.0017 1.6569 0.047 0.593 0.099 473 ±63 0.0640 0.0035 400 ±21 0.0673 0.0122 846 ±379 399.9 21.1 0.05DY01-4F1. F21 0.10027 0.00081 0.2627 0.0053 3.6319 0.075 2.607 0.157 1303 ±44 0.1999 0.0111 1175 ±60 0.0946 0.0017 1520 ±34 1519.7 34.1 0.02DY01-4F2. F20 0.11901 0.00103 0.4349 0.0089 7.1357 0.149 4.751 0.320 1776 ±56 0.2999 0.0191 1691 ±95 0.1149 0.0022 1879 ±35 1878.5 34.6 0.02DY01-4F3. F19 0.08384 0.00061 0.2992 0.0062 3.4588 0.073 2.513 0.154 1276 ±44 0.2219 0.0131 1292 ±69 0.0821 0.0013 1249 ±31 1248.6 30.9 0.02DY01-4F4. F18 0.10009 0.00095 0.1111 0.0024 1.5337 0.034 0.953 0.073 680 ±38 0.0943 0.0052 581 ±30 0.0733 0.0020 1022 ±56 580.8 30.4 0.05DY01-4F5. F17 0.10844 0.00108 0.2040 0.0050 3.0508 0.076 1.768 0.173 1034 ±63 0.1599 0.0113 956 ±63 0.0802 0.0024 1201 ±60 956.4 63.0 0.07DY01-4F6. F16 0.06460 0.00049 0.0736 0.0015 0.6554 0.013 0.520 0.032 425 ±21 0.0656 0.0035 409 ±21 0.0575 0.0011 511 ±44 409.4 21.1 0.05DY01-4F7. F15 0.09512 0.00068 0.2331 0.0047 3.0569 0.062 2.283 0.131 1207 ±41 0.1786 0.0098 1060 ±54 0.0927 0.0014 1481 ±29 1481.4 29.3 0.02DY01-4F8. F14 0.07877 0.00069 0.1590 0.0032 1.7265 0.035 1.350 0.074 868 ±32 0.1330 0.0067 805 ±38 0.0736 0.0014 1031 ±40 805.0 38.1 0.05DY01-4F9. F13 0.08357 0.00064 0.2133 0.0043 2.4572 0.051 1.896 0.106 1080 ±37 0.1705 0.0091 1015 ±50 0.0806 0.0013 1213 ±33 Ma +/− rsdDY01-4FA. F12 0.08856 0.00111 0.1847 0.0037 2.2549 0.048 1.702 0.106 1009 ±40 0.1509 0.0083 906 ±47 0.0818 0.0025 1241 ±59 #DIV/0!

Corrected for Mass Fractionation, Elemental (i.e., Pb/U) Bias, 207 bias, laser system blank and common Pb.+/−(abs)=1 s propagated uncertainties (absolute); includes errors on alpha and on Pb/U bias, rho=0.8.Corrected radiogenic isotopic ratios.Best age=206Pb/238U for ages b1000 Ma, and 207Pb/206Pb for ages N1000 Ma.

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Table 340Ar/39Ar analyses of hornblende, muscovite and biotite from the type Piaxtla Suite in the Acatlán Complex of southern Mexico.

AOR-564: PIX-18 Ms 40/60

# Steps in theplateau

Laser power Isotope volumes %40Ar

(Watts) 40Ar 39Ar 38Ar 37Ar 36Ar Ca/K Cl/K atm f 39Ar 40Ar*/39ArK Age

1 0.75 0.848±0.010 0.034±0.001 0.002±0.001 0.002±0.001 0.001±0.000 0.191 0.007 17.13 0.61 20.172±4.001 246.59±45.722 1.50 2.204 0.011 0.079 0.002 0.003 0.001 0.002 0.001 0.001 0.000 0.053 0.002 8.03 1.43 25.676 1.457 308.41 16.093* 2.00 3.218 0.010 0.114 0.002 0.002 0.001 0.001 0.001 0.001 0.000 0.012 0.001 0.25 2.08 28.188 1.121 335.93 12.194* 3.00 12.624 0.021 0.446 0.003 0.007 0.001 0.001 0.001 0.001 0.000 0.006 0 0.88 8.22 28.143 0.316 335.45 3.445* 4.00 30.951 0.040 1.101 0.007 0.016 0.001 0.001 0.001 0.001 0.000 0.003 0 0.45 20.32 28.091 0.206 334.88 2.256* 5.00 26.012 0.030 0.932 0.006 0.013 0.001 0.001 0.001 0.001 0.000 0.002 0 0.18 17.19 27.967 0.225 333.53 2.457* 6.00 10.365 0.024 0.373 0.004 0.005 0.001 0.001 0.001 0.001 0.000 0.005 0 0.25 6.85 27.840 0.454 332.14 4.958* 7.00 65.300 0.064 2.344 0.013 0.035 0.002 0.005 0.002 0.001 0.001 0.007 0 0 43.3 27.960 0.179 333.01±0.80Total/Average 151.108±0.089 5.384±0.017 0.078±0.003 0.021±0.002 0.002±0.001 0.007 0.001 100 28.000±0.057 333.45 1.94

Isotope correlation data Footnotes: Isotope production ratios

36Ar/40Ar 39Ar/40Ar r J=0.007261±0.000024 (40Ar/39Ar)K=0.03020.000580±0.000545 0.041073±0.001471 0.01 Volume 39ArK=53.84000000 (37Ar/39Ar)Ca=1416.43060.000272 0.000162 0.035818 0.000787 0 Integrated Date=333.01±1.59 (36Ar/39Ar)Ca=0.39520.000008 0.000121 0.035388 0.000604 0 Plateau Date=333.89±1.57 Ca/K=1.83 x (37ArCa/39ArK)0.000030 0.000028 0.035221 0.000263 0 %39ArK for PA=97.960000000.000015 0.000013 0.035438 0.000216 0 Isotope Correlation Date=333.26±6.60 N-Sigma=20.000006 0.000015 0.035691 0.000240 0 Initial 40Ar/36Ar Ratio=582.40±1853.510.000009 0.000040 0.035829 0.000396 0 MSWD=0.09000000

%39ArK for CA=97.96000000

AOR-588: PIX24 Amph 40/60

# Steps in theplateau

Laser power Isotope volumes %40Ar

(Watts) 40Ar 39Ar 38Ar 37Ar 36Ar Ca/K Cl/K atm f 39Ar 40Ar*/39ArK Age

1 2.00 7.911±0.088 0.231±0.004 0.013±0.001 0.127±0.002 0.010±0.000 2.32 0.007 35.88 6.4 21.990±0.719 267.35±8.122 4.00 9.916 0.071 0.387 0.005 0.020 0.001 0.163 0.002 0.003 0.000 1.781 0.008 6.12 10.69 24.169 0.425 291.81 4.743 5.00 10.564 0.049 0.414 0.005 0.054 0.001 0.598 0.007 0.002 0.000 6.177 0.026 3.52 11.44 24.783 0.374 298.64 4.154 6.00 15.096 0.041 0.543 0.004 0.115 0.002 1.656 0.015 0.004 0.000 13.083 0.045 4.77 15.01 26.730 0.295 320.14 3.235 6.50 12.095 0.026 0.422 0.005 0.093 0.002 1.459 0.014 0.004 0.000 14.848 0.047 5 11.66 27.498 0.372 328.54 4.076* 7.00 3.016 0.009 0.103 0.002 0.026 0.001 0.384 0.004 0.001 0.000 16.152 0.052 4.56 2.82 28.261 0.934 336.86 10.167* 7.50 45.294 0.049 1.515 0.008 0.444 0.005 6.358 0.056 0.012 0.000 18.023 0.064 4.77 41.98 28.775 0.182 342.44 1.98Total/Average 103.653±0.141 3.589±0.013 0.751±0.006 24.879±0.060 0.032±0.001 12.748 0.036 100 28.743±0.066 322.18±0.89

Isotope Correlation Data Footnotes: Isotope Production Ratios

36Ar/40Ar 39Ar/40Ar r J=0.007264±0.000026 (40Ar/39Ar)K=0.03020.001215±0.000049 0.029146±0.000600 0.155 Volume 39ArK=35.89000000 (37Ar/39Ar)Ca=1416.43060.000207 0.000033 0.038840 0.000545 0.026 Integrated Date=322.18±1.79 (36Ar/39Ar)Ca=0.39520.000120 0.000030 0.038929 0.000476 0.027 Plateau Date=342.09±2.25 Ca/K=1.83 x (37ArCa/39ArK)0.000162 0.000030 0.035625 0.000293 0.107 %39ArK for PA=44.800000000.000170 0.000033 0.034545 0.000388 0.097 Isotope Correlation Date=235.45±73.73 N-Sigma=20.000155 0.000092 0.033771 0.000637 0.034 Initial 40Ar/36Ar Ratio=2091.01±2267.88

MSWD=0.57000000

(continued on next page)

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Table 3 (continued)

Isotope Correlation Data Footnotes: Isotope Production Ratios

AOR-591: PIX-25 Amph 40/60

# Steps in theplateau

Laser Power Isotope Volumes %40Ar

(Watts) 40Ar 39Ar 38Ar 37Ar 36Ar Ca/K Cl/K atm f 39Ar 40Ar*/39ArK Age

1 2.50 2.459±0.022 0.064±0.002 0.005±0.001 0.168±0.003 0.003±0.000 12.61 0.01 34.75 4.23 25.509±1.554 306.60±17.172 4.00 2.499 0.016 0.095 0.002 0.004 0.001 0.079 0.002 0.001 0.000 3.91 0 8.3 6.34 24.245 0.986 292.58 10.993 5.00 3.235 0.020 0.125 0.002 0.007 0.001 0.162 0.003 0.001 0.000 6.09 0.01 2.68 8.39 25.280 0.883 304.07 9.784 6.00 5.943 0.041 0.210 0.003 0.016 0.001 0.521 0.006 0.002 0.000 11.8 0.01 6.66 14.08 26.712 0.642 319.86 7.045 6.50 1.863 0.022 0.066 0.002 0.006 0.001 0.178 0.003 0.001 0.000 12.82 0.02 1.96 4.41 27.735 1.771 331.05 19.316* 7.00 10.105 0.249 0.341 0.006 0.022 0.001 0.951 0.015 0.002 0.000 13.21 0.01 1.72 23.02 29.350 0.952 348.57 10.287* 7.50 17.190 0.068 0.585 0.006 0.045 0.001 2.177 0.028 0.004 0.000 17.67 0.01 3.1 39.53 28.757 0.409 342.15 4.43Total/Average 43.043±0.264 1.471±0.010 0.098±0.003 10.824±0.033 0.011±0.001 13.54 0.01 100 28.894±0.159 332.22± 1.80

Isotope correlation data

36Ar/40Ar 39Ar/40Ar r Footnotes: Isotope production ratios

0.001177±0.000121 0.025570±0.000698 0.06 J=0.007262±0.000024 (40Ar/39Ar)K=0.03020.000281 0.000107 0.037819 0.000810 0.01 Volume 39ArK=14.71000000 (37Ar/39Ar)Ca=1416.43060.000091 0.000101 0.038496 0.000668 0.01 Integrated Date=332.22±3.61 (36Ar/39Ar)Ca=0.39520.000226 0.000060 0.034939 0.000545 0.04 Plateau Date=343.64±4.69 Ca/K=1.83 x (37ArCa/39ArK)0.000067 0.000188 0.035346 0.001079 0.02 %39ArK for PA=66.960000000.000058 0.000047 0.033485 0.001022 0.08 Isotope Correlation Date=296.71±65.33 N-Sigma=20.000105 0.000045 0.033697 0.000392 0.19 Initial 40Ar/36Ar Ratio=648.07±1256.05

AOR-592: PIX17 Ms 40/60

# Steps in theplateau

Laser power Isotope volumes %40Ar

(Watts) 40Ar 39Ar 38Ar 37Ar 36Ar Ca/K Cl/K atm f 39Ar 40Ar*/39ArK Age

1 2.50 14.425±0.029 0.511±0.004 0.009±0.001 0.004±0.001 0.004±0.000 0.03 0 7.72 4.38 26.116±0.338 313.38±3.722* 2.75 8.592 0.017 0.306 0.003 0.004 0.001 0.001 0.001 0.001 0.000 0 0 1.16 2.61 27.891 0.436 332.83 4.763* 3.00 23.894 0.033 0.846 0.006 0.011 0.001 0.001 0.001 0.001 0.000 0 0 0.8 7.25 28.115 0.246 335.27 2.684* 4.00 116.107 0.136 4.138 0.023 0.054 0.004 0.003 0.002 0.002 0.001 0 0 0.32 35.52 28.069 0.176 334.77 1.915* 5.00 45.669 0.059 1.629 0.009 0.021 0.002 0.003 0.001 0.002 0.000 0.01 0 0.7 13.97 27.945 0.179 333.41 1.956* 6.00 34.673 0.043 1.242 0.008 0.016 0.001 0.002 0.001 0.001 0.000 0 0 0.81 10.65 27.791 0.203 331.74 2.227* 7.00 83.351 0.102 2.984 0.017 0.037 0.003 0.012 0.002 0.001 0.001 0.02 0 0.32 25.61 27.949 0.194 333.46 2.11Total/Average 326.455±0.190 11.592±0.032 0.146±0.005 0.047±0.003 0.009±0.002 0.01 0 100 27.987±0.046 332.98±0.74

Isotope correlation data

36Ar/40Ar 39Ar/40Ar r Footnotes: Isotope production ratios

0.001177±0.000121 0.025570±0.000698 J=0.007264±0.000026 (40Ar/39Ar)K=0.03020.000281 0.000107 0.037819 0.000810 0.06 Volume 39ArK=115.92000000 (37Ar/39Ar)Ca=1416.43060.000091 0.000101 0.038496 0.000668 0.01 Integrated Date=332.98±1.48 (36Ar/39Ar)Ca=0.39520.000226 0.000060 0.034939 0.000545 0.01 Plateau Date=333.87±1.51 Ca/K=1.83 x (37ArCa/39ArK)0.000067 0.000188 0.035346 0.001079 0.04 %39ArK for PA=95.620000000.000058 0.000047 0.033485 0.001022 0.02 Isotope Correlation Date=320.25±17.540.000105 0.000045 0.033697 0.000392 0.08 Initial 40Ar/36Ar Ratio=2214.44±2602.98 N-Sigma=2

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fluence monitor at the McMaster Nuclear Reactor (Hamilton, Ontario,Canada). 40Ar/39Ar analyses were performed by standard laser step-heating techniques described in detail by Clark et al. (1998) at theGeochronology Research Laboratory of Queen's University (Kingston,Ontario, Canada). The data are given in Table 3 and plotted in Fig. 3. Alldata have been corrected for blanks, mass discrimination, andneutron-induced interferences. For the purposes of this paper, aplateau age is obtained when the apparent ages of at least threeconsecutive steps, comprising aminimum of 45% of the 39Ark released,agree within 2σ error with the integrated age of the plateau segment.Errors shown in Table 2 and on the age spectra and isotope-correlation diagrams represent the analytical precision at±2σ.

4.2. Results

Hornblende from both Piaxtla retrogressed eclogite samples(PIX-24 and PIX-25) produced 40Ar/39Ar age spectra with datesmonotonically increasing to plateau ages of 342±2 Ma (PIX-24: 45%of the 39Ar released,MSWD=1.2) and 344±5 Ma (PIX-25: 67% of the39Ar released, MSWD=1.4) as defined by the higher-temperaturesteps (Fig. 3a and b).

Muscovite from both the megacrystic granitoid (PIX-18) and themeta-psammite (PIX-24) yielded identical plateau ages of 334±2 Ma(98% of the 39Ar released, MSWD=0.5; and 96% of the 39Ar released,MSWD=1.2, respectively)(Fig. 3c and d). In contrast, the coexistingbiotite in sample PIX-18 yielded a plateau age of 395±9 Ma (not shownbecause geologically meaningless — see below).

5. Interpretation of the geochronology

U–Pb zircon ages are inferred to indicate cooling through ca.800 °C (Heaman and Parrish, 1991), whereas peak metamorphicpressures and temperatures in the Piaxtla area have been estimated tohave been 11–15 kbar and 560±60 °C (Meza-Figueroa et al., 2003).Thus, the concordant U–Pb zircon age of ca. 452±6 Ma from theyoungest zircons in the megacrystic granitoid is interpreted to date itstime of intrusion. The 800–950 Ma and 1000–1200 Ma ages from thesample are considered to be inherited from the crustal source, whichis consistent with the geochemistry. Similarly, the ca. 365 Ma age ofthe youngest concordant zircon population in the meta-psammite isinferred to date the youngest detrital zircons from the source region,and so provides an older age constraint on the timing of its deposition.The concordant Cambro-Ordovician ages are inferred to recordprovenance from plutons in the Acatlán Complex, such as themegacrystic granite dated herein (PIX-18) and others dated else-where in the Acatlán Complex (Miller et al., 2007; Talavera-Mendozaet al., 2005; Murphy et al., 2006). The Middle Proterozoic ages are alsointerpreted as provenance ages, either in the Amazon craton or in thebasement source of the granitoids.

Hornblende and muscovite 40Ar/39Ar plateau ages are inferred toreflect cooling through ca. 530 °C and 370 °C, respectively, assuming acooling rate of 5 °C/Ma (Purdy and Jäger, 1976; Harrison et al., 1985).Thus, the ca. 343 Ma amphibole ages provide a close younger limit onthe age of eclogite facies metamorphism. This age is consistent withthe muscovite plateau ages of ca. 334 Ma, which are inferred to recordfurther rapid cooling through ~370 °C. The 395±9 Ma biotite plateauage from the granitoid sample PIX-18 is older than these muscoviteages and suggests the presence of excess argon, which has been well-documented in eclogitized rocks elsewhere (e.g. Boundy et al., 1997).

6. Geochemistry

6.1. Analytical methods

15 mafic amphibolite samples, 6 granitoid samples, and 1 quartz-mica paragneiss were collected for geochemical analyses and were

analyzed by X-ray fluorescence spectrometer for major and severaltrace (Rb, Sr, Ba, Zr, Nb, Y, Zn, V, Cr and Ni) elements in the Nova ScotiaRegional Geochemical Centre at SaintMary's University (Halifax, NovaScotia, Canada)(Table 3). From this set, 10 mafic rocks, 4 granitoidsand the paragneiss sample were selected for additional trace elementsanalysis (rare earth elements, Hf, Zr, Nb, Ta and Th) by inductively-coupled plasma-mass spectrometer (ICP-MS) in the geochemicallaboratory of the Ontario Geological Survey in Sudbury (Table 4). Theprecision and accuracy of the ICP-MS data were reported by Ayer andDavis (1997). Briefly, analytical errors for trace elements are generallyb5 rel.%.

The use of chemical data in metamorphosed rocks assumes thatthe elements under consideration remained relatively immobileduring secondary processes. There are several lines of evidence tosuggest that most major elements, such as Si, Al, Mg, Fe and Ti, as wellas many trace elements, including high-field strength elements (Zr,Hf, Nb, Y, Ta, and Th), rare earth elements (REE) and the transitionelements (Cr, Ni and V), were not redistributed in the analyzed rocks.In particular, their consistent trends and similarities to modernvolcanic rocks suggest that they essentially retained their originalcharacteristics.

6.2. Results

The igneous rocks are bimodal with respect to SiO2, which rangesfrom 45 to 54 wt.% and from 69 to 74 wt.% (on a volatile-free basis)indicating that the rocks are either basaltic or granitic in composition(Fig. 4a). The basaltic amphibolites have a wide range of Fe2O3 (10 to16 wt.%), and Mg# (=100xMg/Mg+Fetot: 37 to 58), and strongcorrelations of FeOtot/MgO with TiO2, P2O5, V and Zr indicatingenrichment of these elements during fractionation (e.g. Fig. 5a),trends typical of tholeiitic suites. The high Ti and Cr contents arecharacteristic of within-plate tholeiites rather than arc types (Fig. 5b).In comparison with typical arc basalts, the Piaxtla basaltic amphib-olites have wider ranges in Cr (69–338 ppm), Ni (11–80 ppm) andalso high Ti/V (33–50) and Zr/Y (3.2 to 9.3) values. However, the rocksshow a spread of Nb/Y ratios (0.2 to 1.1: Table 3) implying that someof them are transitional to alkaline types. This conclusion is alsosupported by the shapes of the chondrite-normalized rare earthelement (REE) patterns (Fig. 6) that are of two types. The first type isrelatively flat with (La/Yb)n~1.3–1.7, intermediate between N-typeand E-type MORB (Fig. 6a). The second is distinctly enriched in lightREE with (La/Yb)n~4.1, resembling those of oceanic island basalts(Fig. 6b). The differences between the two types are shown on themantle-normalized trace element diagrams (Fig. 7). The first typeshows no depletion of high-field strength elements, resemblingMORBandwithin-plate basalts derived from asthenospheric source (Fig. 7a).The second type peaks at Nb–Ta and is similar in shape to basalts suchas BHVO-1 (Hawaiian basalt-USGS standard rock) likewise derivedfrom an asthenospheric source but without obvious influence of asubduction component (Fig. 7b). The low Th/La (0.08 to 0.1) andTh/Ta (1.3 to 1.6) ratios also suggest that the rocks were notsignificantly affected by crustal contamination.

The granitoid rocks have Al2O3N13 wt.% and significantly lowerFe2O3, MgO, CaO, TiO2, Ni, V and Cr compared to the basaltic rocks. Theircontents of (Na2O+K2O) vary from 6 to 8 wt.%, whereas Na2ONK2O isusuallyb1. Compositionally, the rocks resemble calc-alkaline rhyolitesshowing nearly constant TiO2 with FeOtot/MgO (Fig. 5a). Their REEpatterns are enriched in light REE but have variable heavy REE (Fig. 6)with (La/Yb)n between 5 and 13. The patterns display variable butdistinct negative Eu anomalies (Fig. 6c). Their mantle-normalizedincompatible trace element patterns (Fig. 7c) are marked by negativeNb, Ta and Ti anomalies and resemblemodern rhyolites including thoseassociated with continental rifting and arc environments. The granitoidrocks do not appear to be directly related to the basaltic amphibolites bydifferentiation.

Fig. 3. 40Ar/39Ar incremental release spectra from: (a) hornblende in amphibolite sample PIX-24; (b) hornblende in amphibolite sample PIX-25; (c) muscovite in megacrysticgranitoid sample PIX-18; and (d) muscovite in meta-psammite sample PIX-17.

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Compositionally rather similar Neoproterozoic–Ordovician basal-tic rocks occur at Olinalá and Asis (western and northern AcatlánComplex: Murphy et al., 2006; Ortega-Obregón et al., 2009)(Fig. 4).The latter display a distinct tholeitiic trend (e.g. Fig. 5a) and havechemical characteristics of within-plate basalts (Fig. 6b).

Table 4Major, selected trace element and rare earth element analyses of rocks from the Piaxtla SuitRay Fluorescence at St. Mary's University, Halifax, Nova Scotia: the precision and accuracy ofis generally better that 10%. Rare earth elements were determined by instrument neutron aprecision generally between 5 and 10%.

Sample PIX-11 PIX-13 PIX-15 PIX-16 PIX-19 PIX-20 PIX-21

SiO2 (%) 52.88 49.47 50.98 43.49 66.55 69.76 68.15TiO2 1.50 1.85 1.59 1.93 0.71 0.66 0.67Al2O3 14.46 15.16 13.16 17.30 15.02 14.45 14.80Fe2O3 10.83 12.10 14.06 12.45 4.54 3.74 4.24MnO 0.19 0.20 0.18 0.21 0.06 0.03 0.06MgO 6.03 6.89 7.12 6.32 1.27 0.59 0.87CaO 7.48 10.08 6.99 12.93 0.57 0.48 1.27Na2O 4.38 2.70 3.62 1.88 2.46 2.33 2.66K2O 0.69 0.25 0.49 0.79 5.31 4.26 3.93P2O5 0.16 0.21 0.16 0.19 0.17 0.16 0.16LOI 1.87 1.21 1.81 3.17 2.24 2.12 2.97Total 100.47 100.11 100.16 100.66 98.89 98.58 99.78Mg# 52.44 53.00 50.07 50.13 35.65 23.80 28.89Cr (ppm) 141 117 89 295 30 26 20Ni 22 24 17 77 1 1 1Co 44 46 57 49 14 11 13V 212 279 285 293 83 77 77Cu 99 35 87 165 20 14 16Zn 77 119 120 66 58 55 59Rb 30 15 19 24 175 172 160Ba 70 91 154 125 1105 582 491Sr 200 159 58 537 96 82 117Ga 16 17 17 23 17 17 19TaNb 8 5 5 16 16 18 17HfZr 135 156 85 160 253 264 251Y 23 31 26 26 30 30 36

7. Tectonic implications

The tholeiitic–alkalic nature of the mafic amphibolites in thePiaxtla area suggests that they form part of the Cambro-Ordovicianrifted passive margin suite exposed elsewhere in the Acatlán Complex

e, Acatlán Complex, southern Mexico. Major and trace elements were determined by X-these determinations is generally better than 5%, with the exception of Nb and Cr whichctivation by X-Ray Assay Laboratories, McMaster University, Hamilton, Ontario, with a

PIX-22 PIX-25 PIX-26 PIX-10 PIX-12 PIX-17 PIX-18 PIX-24

49.76 48.19 47.55 47.07 47.93 72.31 72.74 48.252.65 2.93 3.75 1.92 1.32 0.29 0.52 2.7813.99 13.67 13.05 14.09 13.79 14.49 13.43 13.8214.11 15.46 17.13 14.05 12.83 2.75 3.35 14.530.19 0.22 0.25 0.28 0.22 0.15 0.03 0.225.80 6.22 5.16 6.68 9.06 0.78 0.83 6.259.24 9.02 9.88 10.98 11.25 0.33 0.51 9.982.80 3.35 3.02 2.49 1.62 3.65 2.52 2.500.55 0.37 0.18 0.48 0.59 3.09 3.72 0.450.26 0.24 0.38 0.18 0.11 0.05 0.15 0.281.05 1.11 0.00 1.96 2.37 1.62 1.74 1.06100.40 100.79 100.35 100.17 101.09 99.51 99.54 100.1244.88 44.34 37.36 48.50 58.31 35.97 32.92 46.0068 82 108 158 329 7 14 16011 22 18 35 62 1 1 4051 57 55 55 60 5 9 51330 397 447 302 238 35 61 35475 116 141 177 144 2 4 126104 194 137 89 83 67 47 9719 18 12 20 25 129 148 20105 56 10 110 45 1082 570 168227 182 207 134 160 47 93 26822 21 21 17 16 21 16 20

0.51 0.28 2.20 0.86 1.1417 17 27 8.7 4.7 30.7 12.9 19.4

2.45 1.70 7.90 4.20 4.16167 156 241 108 75 328 212 18323 22 26 30 23 74 25 31

Table 4 (continued)

Sample PIX-11 PIX-13 PIX-15 PIX-16 PIX-19 PIX-20 PIX-21 PIX-22 PIX-25 PIX-26 PIX-10 PIX-12 PIX-17 PIX-18 PIX-24

Th 0.83 0.37 27.40 17.00 1.74U 0.24 0.11 5.21 2.52 0.48La 38 18 20 18 53 44 55 33 16 16 8.51 4.93 69.60 47.30 17.80Ce 21.10 12.80 154.00 102.00 43.20Pr 3.09 1.95 17.80 12.00 5.91Nd 30 20 22 21 36 33 37 29 17 22 15.20 9.66 66.90 45.50 27.30Sm 4.31 3.02 13.80 8.99 6.65Eu 1.52 1.08 0.88 1.36 2.16Gd 5.19 3.76 12.00 6.95 6.74Tb 0.88 0.65 2.07 1.00 1.06Dy 5.58 4.12 13.00 5.36 6.14Ho 1.16 0.89 2.82 0.96 1.19Er 3.34 2.59 8.89 2.56 3.26Tm 0.47 0.36 1.38 0.35 0.43Yb 2.96 2.29 8.85 2.26 2.64Lu 0.44 0.34 1.26 0.33 0.38

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(Murphy et al., 2006; Keppie et al., 2008b, author's unpublished data).This is consistent with the inference that the Piaxtla amphiboliteshave a Late Ordovician–Early Silurian protolith age because amphib-olites with a similar geochemistry have yielded ages of: (a) the442±2 Ma the age of a zircon core from an amphibolitized eclogite inthe Asis area (Elías-Herrera et al., 2004); and (b) the Xayacatlán dikes(eastern Acatlán Complex) cutting low-grade metasedimentary rocksthat have an intrusive age of 442±1 Ma (Keppie et al., 2008b). Acontinental rift interpretation for the Upper Ordovician (ca. 452 Ma)granitoid rocks in the Piaxtla area (rather than arc magmatism) isbased on the bimodality of the coeval suite, a characteristic that issimilar most other 440–480 Ma megacrystic granitoids in the AcatlánComplex, some of which have undergone high-grade metamorphismand polyphase deformation, whereas others are undeformed tomildly

Fig. 4. Rocks of the type Piaxtla Suite plotted on: (a) Zr/TiO2 versus SiO2 (wt.%) diagramof Winchester and Floyd (1977), and (b) Zr/TiO2 versus Nb/Y diagram of Winchesterand Floyd (1977). Abbreviations: Tr = trachyte; TrAn = trachyandesite; Bas =basanite; Trach= trachyte; Neph=nephelinite. Symbols: Basaltic rocks (circles), felsicrocks (crosses).

deformed and exhibit only sub-greenschist facies metamorphism(Miller et al., 2007, and references therein). The similar ages suggestthat the datedmegacrystic granitoid belongs to the same 440–480 Mamagmatic episode and are broadly coeval with the mafic rocks, thetwo representing a bimodal rift-related suite.

The detrital zircon ages in the meta-psammite suggests derivationfrom various sources. The 440–500 Ma zircons are most probablyderived from the numerous megacrystic granitoids in the AcatlánComplex. Similarly, the 950–1300 Ma detrital zircons match datedevents in the adjacent Oaxacan Complex (Keppie et al., 2003; Ortega-Obregon et al., 2003; Solari et al., 2003; Keppie et al., 2008a). TheMesoproterozoic ages derived for themegacrystic granitoid suggests asource either in the underlying Oaxacan basement or in sedimentaryrocks derived from this basement. On the other hand, the mean ca.

Fig. 5. Rocks of the Piaxtla Suite plotted on: (a) FeO*/MgO versus TiO2 (wt.%) diagram(vectors for tholeiitic and calc-alkaline trends are after Miyashiro, 1974), and (b) Cr(ppm) versus Ti (ppm) diagram of Pearce (1975). Fields for Olinalá (western AcatlánComplex are from Ortega-Obregón et al., 2009). Symbols: Basaltic rocks (filled circles),felsic rocks (crosses).

Fig. 6. Chondrite-normalized REE abundances for: (a) light REE enriched Piaxtlaamphibolites; and (b) depleted REE amphibolites: oceanic island basalt (OIB) of Sunand McDonough (1989) and BHVO-1 (USGS standard rock— Hawaiian tholeiitic basalt:Govindaraju, 1994) are shown for a comparison; and (c) Piaxtla felsic rocks.Normalizing values are after Sun and McDonough (1989).

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365 Ma age of the youngest detrital zircons in the meta-psammitesample places an older limit on the timing of its deposition. A youngerlimit on the time of deposition is provided by the age of the eclogite

Fig. 7. Primitive mantle-normalized trace element patterns of: (a) light REE enrichedPiaxtla amphibolites; (b) depleted REE Piaxtla amphibolites: oceanic island basalt (OIB)of Sun and McDonough (1989) and BHVO-1 (USGS standard rock— Hawaiian tholeiiticbasalt: Govindaraju, 1994) are shown for a comparison; and (c) Piaxtla granitoid andgneiss. Normalizing values are after Sun and McDonough (1989).

facies metamorphism in the median HP belt as constrained by:(i) 346±3 Ma and 353±1 Ma metamorphic zircon ages (Middletonet al., 2007; Elías-Herrera et al., 2007); (ii) the ca. 343 Ma amphiboleages reported here; (iii) a 344±5 Ma glaucophane age (Keppie et al.,2010b); and (iv) 338±3 Ma and 334 Ma phengite ages (Keppie et al.,2010b, this paper). The low Th/U ratio of the analyzed zircons andtheir similar age to the amphibole ages are consistent withmetamorphic growth. Together these ages indicate an UpperDevonian–lowest Carboniferous depositional age for the datedmeta-psammite sample. The data also indicate that, followingdeposition, subduction erosion took the rocks to a depth of ca.40 km (Meza-Figueroa et al., 2003) at ca. 350 Ma, a burial rate of~2.7 km/Ma (Figs. 8 and 9A). Subsequently, extrusion brought themthrough 340–300 °C at ca. 8 kmdepth (Meza-Figueroa et al., 2003) byca. 334 Ma at a rate of ~2.4 km/Ma. Assuming exhumation at a rate of~2.4 km/Ma continued until the rocks reached the surface, fullexhumation would have taken 20 Ma (Figs. 8 and 9B). Thus, thecomplete subduction erosion/extrusion cycle in the Piaxtla rockstook place within a relatively short (≤35 Ma) period.

The ≤35 Ma subduction erosion/exhumation cycle in the Piaxtlarocks may be compared with other natural and numerical models.Natural exhumation rates for ultra-high pressure (UHP) rocks incontinent–continent collisional zone are 7.5–58 km/Ma and ca.4 km/Ma, respectively (Warren et al., 2008; Butler et al., 2011), and≥4 km/Ma for the HP rocks in Crete (Thomson et al., 1998). Theserates are 2–24 times faster than those of the Piaxtla Suite. However,the 2.4 km/Ma exhumation rate in the Piaxtla Suite is similar to the2 km/Ma exhumation rate of the HP Maksyutov Complex in thesouthern Urals of Russia (Hetzel and Romer, 2001) and the1–5 km/Ma rate in the Iberia–Czech Variscan belt, which has beenrecently interpreted as an extrusion into the upper plate (Keppie et al.,2010b). Such extrusion would be facilitated by extension duringtrench rollback during steepening of the subduction zone followingsubduction erosion during flat-slab subduction (c.f. D.F. Keppie et al.,2009). Numericalmodels predict exhumation rates of 5–8 km/Ma forsubduction channel extrusion of lower plate material (Warren et al.,2008), and ca. 6 km/Ma where material is removed from, andextruded into, the upper plate (Stöckhert and Gerya, 2005), i.e. 2–3times faster than in the Piaxtla Suite. However, the exhumation rateis partly dependant on the convergence rate, which can varyconsiderably from 20 to 150 km/Ma, which may have been anadditional factor in the slower rate of exhumation of the PiaxtlaSuite.

The burial rate of material removed by subduction erosion fromthe upper plate can proceed at or below the convergence ratedepending on the absence or presence, respectively, of internal shearzones. In the Stöckhert and Gerya (2005) model, where steady-statesubduction erosion takes place, the predicted burial rate of 12 km/Mawould take the subducted material to 40 km in 3.3 Ma, 5 times fasterthan in the Piaxtla Suite. Alternatively, unsteady-state subductionerosion capable of removing strips of continental margin up to 200 kmwide, proceeds under a wide range of convergence rates(25–175 km/Ma) and takes 4–16 Ma to suduct material to depths of40–100 km (D.F. Keppie et al., 2009; see also Stern, 2011). Thisduration spans the 15 Ma time frame for subduction erosion of thePiaxtla Suite. Thus, whereas exhumation of the Piaxtla Suite appearsto have been slower than that recorded in UHP rocks and steady-statenumerical models, its burial is similar to that predicted in unsteady-state subduction erosion. Based on the model of D.F. Keppie et al.(2009), the HP Piaxtla rocks would have been removed during flat-slab subduction some 25 Ma after such subduction was initiated. Thiscalculation would suggest that flat-slab subduction beneaththe Acatlán Complex started at or before 385 Ma (i.e. close to theMiddle–Late Devonian boundary). This is consistent with thepresence in Carboniferous rocks in the Acatlán Complex andIxtaltepec Formation (part of the Paleozoic sequence lying

Fig. 8. Geochronological constraints on the subduction erosion/extrusion cycle in the Acatlán Complex plotted on a Temperature versus Time graph.

1063J.D. Keppie et al. / Gondwana Research 21 (2012) 1050–1065

unconformably on the Oaxacan Complex) of 380–405 Ma detritalzircons that may have been derived from the arc (Keppie et al., 2008a,2010b, and references therein). That extrusion took place into theAcatlán Complex is indicated by the similarity of the rocks on eitherside of the median HP belt (Fig. 9B)(Keppie et al., 2008a; Ortega-Obregón et al., 2009).

The data also indicate the presence of both lower and upperPaleozoic continentally-derived clastic sedimentary rocks, an obser-vation consistent with tectonic interleaving. Such extrusional inter-

Fig. 9. Tectonic evolution of the Piaxtla Suite: (A) subduction erosion of part of the arc compl≥40 km producing eclogites during the Upper Devonian–Lower Carboniferous over a periodover a period of 20 Ma.

leaving would also account for the presence of periarc serpentinites(Proenza et al., 2004) and forearc igneous rocks (Galáz et al., 2009)near the base of the extruded median HP belt, and may indicate thatthat an upper Paleozoic arc originally overlay the lower Paleozoicrifted passive margin of Oaxaquia/Amazonia (Fig. 10a). If so, most ofthis arc appears to have been removed by subduction erosion as onlya remnant occurs in the low-grade rocks of the Acatlán Complex,near Patlanoaya (Keppie et al., 2010b). This is consistent with theinference that the eclogite facies metamorphism in the type Piaxtla

ex and underlying rift-passive margin from the upper plate involving burial to depths ofof 15 Ma; and (B) extrusion of the eclogites into the upper plate during theMississippian

Fig. 10. Paleogeographic reconstructions for: (A) Ordovician showing the rift-passive margin deposition of the Acatlán Complex on the northern Gondwana margin (modified afterKeppie, 2004); and (B) Devono-Carboniferous showing subduction erosion of Ordovician rift-passive margin and Devonian arc complex (Piaxtla Suite) along the western margin ofPangea (modified after Keppie, 2004).

1064 J.D. Keppie et al. / Gondwana Research 21 (2012) 1050–1065

Suite took place during a subduction erosion/extrusion cyclealong the paleo-Pacific margin of Pangea (Keppie et al., 2008a)(Fig. 10b).

Acknowledgments

We would like to acknowledge Papiit grant IN103003 to JDK, a NSFgrantEAR0308105 toRDN, andNSERCDiscovery grants to JD, and JKWL.We would also like to thank Drs. J.B. Murphy and L.A. Solari for theirconstructive reviews, and Fabian Duran for assistancewith drawing thefigures.

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