Detrital Zircon Data from the Eastern Mixteca Terrane, Southern Mexico: Evidence for an Ordovician—Mississippian Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia

International Geology Review, Vol. 48, 2006, p. 97–111.Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.

Detrital Zircon Data from the Eastern Mixteca Terrane,Southern Mexico: Evidence for an Ordovician–Mississippian

Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia

J. DUNCAN KEPPIE,1

Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., México

R. D. NANCE,Department of Geological Sciences, Ohio University, Athens, Ohio 45701

JAVIER FERNÁNDEZ-SUÁREZ,Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain

CRAIG D. STOREY,Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom

TERESA E. JEFFRIES, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

AND J. BRENDAN MURPHY Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada, B2G 2W5

Abstract

The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group(part of the Acatlán Complex), and has been interpreted as either a Lower Paleozoic passive margin,or a trench/forearc sequence deposited in either the Iapetus or Rheic oceans. The group, frombottom to top, consists of: (1) the Magdalena Migmatite protolith (metapsammites, metapelites,calsilicates, and marbles), which grades up into (2) the meta-psammitic Chazumba Formation; over-thrust by (3) the Cosoltepec Formation (phyllites and quartzites with minor mafic meta-volcanichorizons). The group is unconformably overlain by the Pennsylvanian–Middle Permian TecomateFormation, which is overthrust by the ~288 Ma Totoltepec pluton and unconformably overlain byMiddle Jurassic rocks. In contrast to previous inferences that the protoliths of the units (1) to (3)were early Paleozoic in age, detrital zircon LA-ICPMS ages combined with published data constraindepositional ages as follows: (i) Magdalena Migmatite protolith: post-303 Ma–pre-171 Ma (Permian–Early Jurassic); (ii) Chazumba Formation: post-239 Ma–pre-174 Ma (Middle Triassic–Early Juras-sic); and (iii) Cosoltepec Formation: post-455 Ma–pre-310 Ma (uppermost Ordovician–Mississip-pian). Given the different ages and depositional environments of the Cosoltepec Formation versusthe Chazumba Formation and Magdalena protolith, we recommend redefining the Chazumba andMagdalena as lithodemes grouped in the Petlalcingo Suite and excluding the Cosoltepec Formation.Detrital zircons in all three units show a population peak at ~850–1200 Ma, suggesting derivationfrom the adjacent ~1 Ga Oaxacan Complex. A ~470–640 Ma peak is limited to the CosoltepecFormation whose source may be found in ~470 Ma plutons in the Acatlan Complex, beneath theYucatan Peninsula, and in the Brasiliano orogens of South America. The inferred turbiditic protolithof the Chazumba Formation and Magdalena protolith suggests that it represents a clastic wedgedeposited in front of S-verging Permo-Triassic thrusts on the western margin of Pangea. The mainlyoceanic affinity of the basalts in the Cosoltepec Formation suggests deposition of sedimentaryprotoliths in a continental rise fringing Oaxaquia. These data are more consistent with deposition ofthe Cosoltepec Formation in the Rheic Ocean than in the Iapetus Ocean.

1Corresponding author; email: [email protected]

970020-6814/06/856/97-15 $25.00

98 KEPPIE ET AL.

Introduction

THE ACATLÁN COMPLEX forms the basement of theMixteca terrane of southern Mexico (Fig. 1A). Thecomplex is juxtaposed on its eastern side againstthe ~1 Ga Oaxacan Complex of the Oaxaquia ter-rane along a Permian dextral flower structure,where syntectonic migmatites have yielded an ageof 276 ± 1 Ma (Elías-Herrera and Ortega-Gutiérrez,2002). The ~1 Ga rocks of southern and centralMexico belong to the Middle American microconti-nent, which was fringed by Paleozoic passive mar-gin and oceanic sequences, one of which is theAcatlán Complex (Fig. 1B). Ortega-Gutiérrez et al.(1999) claimed that the Acatlán Complex consistsof two major thrust sequences: (1) a lower Petla-lcingo Group made up of the Magdalena Migmatite,Chazumba and Cosoltepec formations of inferredearly Paleozoic age (Fig. 2), and (2) an upper Piax-tla Group. Both groups are unconformably overlainby the Pennsylvanian–Lower Permian TecomateFormation: ~305–270 Ma (Keppie et al., 2004c)(Fig. 2), and the upper Fammenian–Middle Per-mian Patlanoaya Formation (~370–260 Ma: UpperDevonian) is reported to rest unconformably onthe Cosoltepec Formation in the northern Mixtecaterrane (Vachard and Flores de Dios, 2002). ThePetlalcingo Group is interpreted to be either apassive margin sequence (Ramirez-Espinoza,2001) or a trench and forearc deposit (Ortega-Gutiérrez et al., 1999), and the complex as a wholeis inferred to be the vestige of either the IapetusOcean (Ortega-Gutiérrez et al., 1999) or the Gond-wanan margin of the Rheic Ocean (Keppie andRamos, 1999; Keppie, 2004).

Constraining the protolith ages of the sedimen-tary successions such as the Petlalcingo Group is akey to resolving this controversy. In the absence offossils in the Petlalcingo Group, this paper presentsdetrital zircon ages from the Magdalena Migmatitepaleosome, a Chazumba Formation metapsammite,and a semipelite of the Cosoltepec Formation,which provide an older limit on the time of deposi-tion of the units. An upper limit is provided by pre-viously published Jurassic ages for igneous andmetamorphic events (Keppie et al., 2004b). Theseages, together with geochemical data, providethe basis for deducing the depositional environ-ment of the units, and permit evaluation of theirpaleogeography.

Geological Setting

Petlalcingo Group

The Petlalcingo Group consists of the MagdalenaMigmatite, and the Chazumba and Cosoltepec for-mations (Figs. 1 and 2), the most complete section ofwhich is exposed in an antiform in the eastern partof the Mixteca terrane. The Chazumba Formationconsists of a thick, polydeformed sequence ofmetapsammites and metapelites that were metamor-phosed under amphibolite-facies conditions duringthe Jurassic and contain several tectonic lenses ofJurassic, mafic-ultramafic rocks (Keppie et al.,2004b). The formation appears to grade structurallydownwards into a similar lithological unit that addi-tionally includes calcsilicate and marble lenses andbands. This unit was pervasively migmatized andrepeatedly deformed during the Jurassic (~175–170Ma; Keppie et al., 2004b) producing a mapable unitcalled the Magdalena Migmatite (Keppie et al.,2004b).

The Cosoltepec Formation structurally overliesthe Chazumba Formation and comprises extensivephyllites and quartzites and minor mafic metavolca-nic units (Figs. 1B and 1C). These rocks have beenpenetratively deformed three times, generally atgreenschist facies; however, a mafic unit at the baseof the formation was metamorphosed in the amphib-olite facies. This latter unit has yielded 40Ar/39Arplateau ages of 218 ± 11 Ma (hornblende) and 224 ±2 Ma (muscovite; Keppie et al., 2004b). A minimumdepositional age for the Cosoltepec Formation isprovided by the fault-modified unconformity withthe overlying Pennsylvanian–Middle Permian Teco-mate Formation (Figs. 1B and 1C; Keppie et al.,2004c).

Piaxtla Group

The Petlalcingo Group is structurally juxtaposedagainst locally eclogitic mafic and ultramafic rocks,high-grade metasedimentary units, granitoid rocks,and migmatites of the Piaxtla Group that are inferredto have been thrust over the Petlalcingo Group(Ortega-Gutiérrez et al., 1999; Fig. 1). A mafic eclog-ite from the northern part of the Mixteca terrane hasyielded a concordant U-Pb zircon age of 346 ± 3 Mathat is inferred to date the eclogite-facies metamor-phism followed by migmatization that has yieldedSHRIMP ages of ~350–330 Ma (Keppie et al.,2004a). The rocks in sheared contact with the PiaxtlaGroup are generally of lowgrade, suggesting that thePiaxtla Group had cooled before the two groups were

DETRITAL ZIRCON DATA 99

tectonically juxtaposed. The Piaxtla Group may com-prise several thrust slices and is inferred to representobducted oceanic and/or continental lithosphere(Ortega-Gutiérrez et al., 1999). As with the Pet-

lalcingo Group, a minimum depositional age for thePiaxtla Group is provided by the unconformablyoverlying Pennsylvanian–Middle Permian TecomateFormation (Ortega-Gutiérrez et al., 1999).

FIG. 1. A. Map showing the location of the Mixteca terrane (modified after Keppie et al., 2003b). B. Cross-sectionA–B located on map below. C. Geological map of the Mixteca terrane (modified after Ortega-Gutiérrez et al., 1999).

100 KEPPIE ET AL.

Tecomate FormationThe Tecomate Formation consists of conglomer-

ate, sandstone, slate, within-plate mafic and felsicvolcanic rocks, and limestones that contain latestPennsylvanian–Middle Permian conodonts (Keppieet al., 2004c). Zircons from granite pebbles in aconglomerate horizon have yielded SHRIMP ages ofca. 264–320 Ma and were likely sourced in theTotoltepec pluton (287 ± 2 Ma, Yañez et al, 1991;289 ± 1 Ma, Keppie et al., 2004b), which over-thrusts the Tecomate and Cosoltepec formations(Figs. 1B and 1C). The Tecomate Formation hasbeen penetratively deformed by two sets of struc-tures under greenschist-facies conditions: (1) isocli-nal folding associated with N-S dextral shearing andsouth-vergent thrusting during which the syntec-tonic Totoltepec pluton was emplaced; and (2) NW-through N- to NE-trending upright open folding witha axial planar crenulation cleavage (Malone et al.,2002). K-Ar data on muscovite from the TecomateFormation has yielded an age of 288 ± 14 Ma(Weber et al., 1997). In the antiform in the eastern

part of the Mixteca terrane, the Acatlán Complex inthe antiform is unconformably overlain MiddleJurassic rocks in the north, and Eocene–Oligocenerocks in the south (Keppie et al., 2004b).

Orogenic eventsTaking all these data together, several tectono-

thermal events appear to have affected the Pet-lalcingo Group, including: (1) a Jurassic eventlocalized in the southeastern part of the Mixtecoterrane (Keppie et al., 2004b); (2) a Permian eventthat started in the Early Permian synchronous withintrusion of the ~288 Ma Totoltepec pluton anddeforms rocks as young as lower Middle Permianthat are unconformably overlain by Middle Juras-sic rocks (Keppie et al., 2004c); and (3) an eventyet to be dated that produced the first phase ofdeformation in the Cosoltepec Formation—thismay have been synchronous with polyphase defor-mation under eclogite-facies metamorphic condi-tions in the Piaxtla Group, which yielded an ageof 346 ± 3 Ma, i.e. Mississippian (Keppie et al.,2004a).

FIG. 2. Stratigraphic columns for the eastern part of the Mixteca terrane: Petlalcingo Group, Tecomate Formation,and Totoltepec pluton; Acatlán Complex after Ortega-Gutiérrez et al. (1999) and using data in this paper and Keppie etal. (2004c). Abbreviations: C = Cambrian, O = Ordovician, S = Silurian, D = Devonian, C = Carboniferous, P = Permian,Tr = Triassic.

DETRITAL ZIRCON DATA 101

Analytical Techniques

Zircons were separated from three samples ofclastic rocks, one from each of the MagdalenaMigmatite protolith, the Chazumba Formation, andthe Cosoltepec Formation. Details of the widelyused separation methodology can be found inFernández-Suárez et al. (2002) and Jeffries et al.(2003). Separated zircons were examined opticallyunder a binocular microscope, and representativegood-quality grains were picked and mounted inepoxy resin. The mounts were then ground down sothat ~50% of the grains were exposed, and thenpolished to high quality. Grains were then imagedby cathodoluminescence (CL) in a JEOL 5900LVscanning electron microscope (SEM) at the NaturalHistory Museum, London (NHM). Finally, themounts were cleaned thoroughly by immersion in anultrasonic bath containing a dilute HNO3 acid, anddried before being introduced to the laser ablationchamber. Analytical instrumentation, analyticalprotocol and methodology, data reduction, agealculation and common Pb correction followedthose described in Fernández-Suárez et al. (2002)and Jeffries et al. (2003).

In this study, nominal laser beam diameter was30 µm for >75% of the analyses, but where the areato be analyzed was deemed to be large enough a45µm beam was used to ensure the analysis wascollected with the optimal signal strength that theanalyte volume allowed.

Data were collected in discrete runs of 20 analy-ses, comprising 12 unknowns bracketed before andafter by 4 analyses of the standard zircon 91500(Wiedenbeck et al., 1995). Concordia age calcula-tions, and concordia and frequency histograms/probability density distribution plots wereperformed using Isoplot v.3.00 (Ludwig, 2003).

Results

Two hundred eighty-eight (288) analyses, nearlyall representing one analysis per grain, were per-formed on zircons from samples MM-10 (MagdalenaMigmati te paleosome: 96 analyses) , A-12(Chazumba metapsammite: 96 analyses), and COS-100 (Cosoltepec semipelite: 96 analyses). Of those,15 were rejected (4 in MM-10, 3 in A-12, and 8 inCOS-100) based on the presence of features such asdiscordance >20%; high common Pb detected in theU-Pb, Th-Pb, and Pb-Pb isotope ratio plots; and/orelemental U-Pb fractionation or inconsistent behav-

ior of U-Pb and Th-Pb ratios in the course ofablation (see Fernández-Suárez et al., 2002; Jeffrieset al., 2003). Figure 3 shows concordia plots andcombined binned frequency and probability densitydistribution plots for the three samples, with thedata presented in Table 1: 2σ errors are quotedthroughout. Where the analyses overlap concordia,we assign a U-Pb Concordia age (sensu Ludwig,2003) as the best age estimate (see bold type inTable 1). Where analyses are normally discordant(i.e., they plot below concordia), we assign the207Pb/206Pb age inasmuch as we are confident thatany discordance is not a result of excess common Pbin the analysis or analytically induced problemssuch as laser-induced elemental fractionation (seeFernández-Suárez et al., 2002 and Jeffries et al.,2003 for details). Consequently, these ages willapproximate the “correct” age, assuming a zero-agePb-loss event, and there is a small danger that a nonzero-age thermal event could result in these agesrepresenting minimum ages. However, the amountof discordance within these zircons is minor (seeFig. 3 and Table 1), and therefore this phenomenonis unlikely to affect any of the conclusions reachedregarding this dataset.

Magdalena Migmatite

The sample of the paleosome of the MagdalenaMigmatite (MM-10) was collected south ofMagdalena (17°59.26', 97°48.42': Fig. 1B) andconsists of quartz, plagioclase (An37-26), biotite,hornblende, and accessory zircon, titanite, apatite,rutile, tourmaline, and opaques: chlorite and musco-vite are common alteration products. Most of the zir-cons yielded concordant to nearly concordant dataranging from ~850 to 1250 Ma with one concordantpoint at ~1575 Ma (Fig. 3, Table 1). The youngestdetrital zircon yielded a U-Pb Concordia age of 303± 6 Ma. And another grain yielded a Concordant ageof 521 ± 8 Ma.

Chazumba Formation

The metapsammite sample of the ChazumbaFormation (A-12) was collected at the village of Tul-titlan (18°04.65', 97°02.93') and consists of quartz,plagioclase (An35-20), biotite, muscovite, garnet, andaccessory zircon, tourmaline, and opaques. Most ofthe zircons are concordant to nearly concordant,with ages ranging from ~920 to 1150 Ma (Fig. 3,Table 1). The youngest grain has a Concordia age of239 ± 4 Ma.

102 KEPPIE ET AL.

TABLE 1. U-Pb LA-ICPMS Data from samples MM-10, A-12, and COS-100

Isotopic ratios (2σ errors) Age, Ma

Best age estimate, Ma

Sample/analysis

206Pb/238U

Pct.error

207Pb/235U

Pct. error

207Pb/206Pb

Pct. error

206Pb/238U (2σ)

207Pb/235U (2σ)

207Pb/206Pb (2σ)

MM-10au11a05 0.1955 1.30 2.1988 2.92 0.0815 2.56 1151 15 1181 34 1234 36 1234 36au11a06 0.0483 2.32 0.3456 2.78 0.0518 4.28 304 7 301 8 278 8 303 6au11a07 0.1431 2.08 1.4144 2.18 0.0717 3.04 862 18 895 20 976 21 976 21au11a08 0.1415 1.88 1.3169 5.12 0.0675 5.92 853 16 853 44 852 44 853 15au11a09 0.2022 1.02 2.2317 2.14 0.0801 2.54 1187 12 1191 25 1198 26 1188 10au11a10 0.1838 2.00 1.9862 2.36 0.0784 2.86 1088 22 1111 26 1156 27 1102 13au11a11 0.1811 2.62 1.9301 3.58 0.0773 3.12 1073 28 1092 39 1128 40 1084 22au11a12 0.2141 1.40 2.5144 1.46 0.0852 1.44 1251 18 1276 19 1319 19 1319 19au11a13 0.1487 1.72 1.4219 4.46 0.0693 3.78 894 15 898 40 909 41 894 14au11a14 0.1474 1.38 1.4045 3.36 0.0691 2.84 886 12 891 30 902 30 886 11au11a15 0.1853 1.04 2.0219 1.96 0.0791 2.12 1096 11 1123 22 1175 23 1175 23au11a16 0.1917 2.02 2.1471 4.10 0.0812 3.22 1131 23 1164 48 1227 50 1227 50au11b06 0.1724 1.22 1.8072 1.88 0.0760 1.86 1025 13 1048 20 1096 21 1096 21au11b09 0.2061 1.48 2.3503 2.66 0.0827 3.22 1208 18 1228 33 1261 34 1216 13au11b10 0.1817 2.82 1.9477 2.64 0.0777 4.28 1076 30 1098 29 1139 30 1093 16au11b11 0.1476 0.98 1.4023 1.84 0.0689 1.72 887 9 890 16 895 16 888 8au11b13 0.1801 1.02 1.9571 2.52 0.0788 2.58 1068 11 1101 28 1167 29 1167 29au11b14 0.1916 1.78 2.2176 2.46 0.0839 3.14 1130 20 1187 29 1291 32 1291 32au11b15 0.1770 1.12 1.8959 1.10 0.0777 1.68 1051 12 1080 12 1139 13 1139 13au11b16 0.1808 1.18 1.9702 2.48 0.0790 2.76 1071 13 1105 27 1173 29 1173 29au12a05 0.1526 1.80 1.5184 2.60 0.0721 3.02 916 16 938 24 990 26 990 26au12a06 0.1792 1.20 1.9294 1.84 0.0781 1.96 1063 13 1091 20 1149 21 1149 21au12a07 0.2159 1.76 2.5188 1.66 0.0846 0.64 1260 22 1277 21 1306 22 1306 22au12a08 0.1956 1.66 2.1297 2.34 0.0790 2.90 1152 19 1158 27 1171 27 1155 12au12a09 0.1877 4.08 2.0493 2.36 0.0792 3.80 1109 45 1132 27 1176 28 1132 16au12a10 0.1868 1.58 1.9826 1.50 0.0770 1.98 1104 17 1110 17 1120 17 1108 9au12a11 0.1956 1.14 2.1210 2.06 0.0786 2.38 1152 13 1156 24 1163 24 1153 9au12a12 0.1832 1.56 2.0010 2.36 0.0792 2.30 1084 17 1116 26 1178 28 1178 28au12a13 0.1875 2.34 2.0015 2.12 0.0774 1.56 1108 26 1116 24 1132 24 1118 14au12a14 0.1902 3.66 2.0907 2.36 0.0797 3.88 1122 41 1146 27 1190 28 1144 16au12a15 0.1512 2.08 1.5077 2.08 0.0723 1.56 908 19 934 19 994 21 994 21au12a16 0.1588 2.84 1.6378 4.20 0.0748 3.88 950 27 985 41 1063 45 1063 45au12b05 0.1843 2.68 1.9864 2.58 0.0782 1.62 1090 29 1111 29 1151 30 1151 30au12b06 0.1834 1.08 1.9229 1.48 0.0760 1.44 1086 12 1089 16 1096 16 1088 9au12b07 0.1844 2.10 2.0044 1.94 0.0788 1.32 1091 23 1117 22 1168 23 1168 23au12b08 0.2011 2.14 2.2302 1.92 0.0804 3.34 1181 25 1191 23 1208 23 1189 13au12b09 0.1602 1.08 1.5958 1.28 0.0722 1.00 958 10 969 12 992 13 992 13au12b10 0.2069 1.32 2.3392 2.64 0.0820 1.94 1212 16 1224 32 1245 33 1213 15au12b11 0.0490 0.94 0.3533 3.30 0.0523 3.52 308 3 307 10 298 10 308 3au12b12 0.1899 2.30 1.9789 4.24 0.0756 5.48 1121 26 1108 47 1084 46 1116 21au12b13 0.1321 1.22 1.3093 1.16 0.0719 0.62 800 10 850 10 982 11 982 11au12b14 0.1803 1.42 1.9533 2.16 0.0786 2.24 1069 15 1100 24 1161 25 1161 25au12b15 0.1908 1.00 2.0652 1.52 0.0785 1.06 1126 11 1137 17 1159 18 1159 18au12b16 0.2316 1.60 2.8960 2.80 0.0907 2.32 1343 21 1381 39 1440 40 1440 40au13a05 0.1976 1.52 2.1983 1.64 0.0807 1.94 1162 18 1181 19 1214 20 1214 20au13a06 0.1897 1.92 2.0508 2.94 0.0784 2.46 1120 21 1133 33 1157 34 1126 18au13a07 0.1854 1.38 1.9937 1.22 0.0780 1.50 1096 15 1113 14 1146 14 1146 14au13a08 0.1873 1.28 2.0032 2.10 0.0775 2.06 1107 14 1117 23 1135 24 1111 11au13a09 0.2108 1.08 2.3357 2.12 0.0804 2.22 1233 13 1223 26 1206 26 1229 10au13a10 0.1856 2.36 1.9589 1.92 0.0765 1.60 1097 26 1102 21 1109 21 1102 12au13a11 0.1914 2.38 2.0061 4.60 0.0760 5.04 1129 27 1118 51 1096 50 1125 20au13a12 0.1924 0.94 2.1208 1.44 0.0799 1.66 1134 11 1156 17 1195 17 1195 17

Table continues

DETRITAL ZIRCON DATA 103

TABLE 1. (Continued)

Isotopic ratios (2σ errors) Age, Ma

Best age estimate, Ma

Sample/analysis

206Pb/238U

Pct.error

207Pb/235U

Pct. error

207Pb/206Pb

Pct. error

206Pb/238U (2σ)

207Pb/235U (2σ)

207Pb/206Pb (2σ)

MM-10 (continued)au13a13 0.1920 2.66 2.1097 3.02 0.0797 1.60 1132 30 1152 35 1189 36 1189 36au13a14 0.0836 2.40 0.6729 2.18 0.0584 2.80 518 12 522 11 543 12 521 8au13a15 0.2765 2.56 3.7296 2.34 0.0978 1.50 1574 40 1578 37 1583 37 1579 17au13a16 0.1729 1.28 1.8574 1.96 0.0779 1.48 1028 13 1066 21 1144 22 1144 22au13b05 0.2033 1.66 2.2292 2.24 0.0795 2.56 1193 20 1190 27 1185 27 1191 13au13b06 0.1865 0.74 2.0125 1.04 0.0783 0.98 1102 8 1120 12 1153 12 1153 12au13b07 0.1831 0.90 1.9686 1.30 0.0780 1.14 1084 10 1105 14 1146 15 1146 15au13b08 0.1838 2.36 1.9682 2.44 0.0776 2.20 1088 26 1105 27 1138 28 1103 16au13b09 0.1947 2.40 2.0375 3.28 0.0759 3.14 1147 28 1128 37 1092 36 1135 20au13b10 0.1921 1.84 2.1578 2.56 0.0815 1.56 1133 21 1168 30 1233 32 1233 32au13b11 0.1944 1.64 2.0869 1.72 0.0778 1.40 1145 19 1145 20 1143 20 1145 12au13b12 0.1922 1.76 2.1143 2.48 0.0798 2.02 1133 20 1153 29 1191 30 1191 30au13b13 0.2590 0.98 3.5396 1.36 0.0991 1.14 1485 15 1536 21 1607 22 1607 22au13b14 0.1744 1.74 1.8355 2.06 0.0763 1.68 1036 18 1058 22 1103 23 1103 23au13b15 0.1740 1.28 1.8247 1.24 0.0761 1.34 1034 13 1054 13 1097 14 1097 14au13b16 0.1836 1.50 1.9408 2.20 0.0766 3.12 1087 16 1095 24 1112 24 1091 12

A12au18a05 0.1437 2.48 1.3984 4.80 0.0706 4.28 866 21 888 43 945 40 870 19au18a06 0.1554 1.06 1.6125 1.46 0.0753 0.72 931 10 975 14 1075 8 1075 8au18a08 0.1540 1.02 1.5166 1.88 0.0714 1.76 923 9 937 18 969 17 969 17au18a09 0.1573 1.28 1.6055 2.46 0.0740 2.92 942 12 972 24 1041 30 1041 30au18a10 0.1623 1.60 1.6155 1.68 0.0722 1.86 970 16 976 16 991 18 974 10au18a11 0.1956 1.16 2.0937 1.84 0.0776 2.10 1152 13 1147 21 1137 24 1149 9au18a12 0.2017 2.24 2.1546 2.00 0.0775 0.66 1184 27 1167 23 1133 7 1133 7au18a13 0.1679 1.52 1.6452 2.02 0.0711 1.36 1001 15 988 20 959 13 959 13au18a14 0.1494 1.00 1.4708 1.94 0.0714 1.68 898 9 918 18 969 16 969 16au18a15 0.1744 1.66 1.8429 2.20 0.0766 1.56 1036 17 1061 23 1112 17 1112 17au18a16 0.1658 1.36 1.6448 1.86 0.0719 2.04 989 13 988 18 984 20 988 10au18b05 0.1575 2.18 1.5344 4.66 0.0706 5.14 943 21 944 44 946 49 943 16au18b06 0.1526 2.96 1.5253 3.48 0.0725 3.90 916 27 941 33 999 39 931 18au18b07 0.1828 1.24 1.9914 1.98 0.0790 2.16 1082 13 1113 22 1172 25 1172 25au18b08 0.1463 2.32 1.4357 3.60 0.0712 2.78 880 20 904 33 962 27 962 27au18b09 0.1777 1.94 1.8694 3.74 0.0763 4.00 1054 20 1070 40 1102 44 1060 16au18b10 0.1545 1.62 1.5702 1.34 0.0737 1.02 926 15 959 13 1033 11 1033 11au18b11 0.1585 2.00 1.6255 1.94 0.0744 0.90 948 19 980 19 1052 9 1052 9au18b12 0.1812 1.44 1.9051 1.90 0.0762 1.96 1074 15 1083 21 1101 22 1079 11au18b13 0.1538 0.82 1.5243 1.22 0.0719 1.06 922 8 940 11 982 10 982 10au18b14 0.1558 1.40 1.5350 2.12 0.0714 2.12 933 13 945 20 970 21 938 10au18b15 0.1354 2.36 1.3384 2.52 0.0717 0.96 819 19 863 22 977 9 977 9au18b16 0.1516 0.96 1.5102 1.82 0.0722 1.38 910 9 935 17 992 14 992 14au18c05 0.1563 1.88 1.5432 2.98 0.0716 2.92 936 18 948 28 974 28 941 14au18c06 0.1601 1.38 1.5617 1.66 0.0707 2.10 957 13 955 16 950 20 956 8au18c07 0.1582 1.02 1.4932 2.26 0.0684 2.40 947 10 928 21 882 21 882 21au18c08 0.1533 2.02 1.4822 1.56 0.0701 2.56 919 19 923 14 932 24 922 8au18c09 0.1635 1.74 1.6680 1.84 0.0740 2.14 976 17 996 18 1041 22 1041 22au18c10 0.1560 2.34 1.5829 3.38 0.0736 3.30 935 22 964 33 1030 34 1030 34au18c11 0.1786 1.62 1.8958 2.06 0.0770 1.26 1059 17 1080 22 1120 14 1120 14au18c12 0.1577 1.34 1.5562 2.22 0.0716 2.10 944 13 953 21 974 20 947 11au18c13 0.1562 1.08 1.5600 1.48 0.0724 1.88 936 10 954 14 998 19 998 19au18c14 0.1545 2.36 1.5268 2.76 0.0717 2.54 926 22 941 26 976 25 936 16au18c15 0.1837 0.98 2.0096 1.72 0.0793 1.74 1087 11 1119 19 1180 21 1180 21au18c16 0.1766 1.54 1.8701 2.20 0.0768 1.74 1048 16 1071 24 1116 19 1116 19

Table continues

104 KEPPIE ET AL.

TABLE 1. (Continued)

Isotopic ratios (2σ errors) Age, Ma

Best age estimate, Ma

Sample/analysis

206Pb/238U

Pct.error

207Pb/235U

Pct. error

207Pb/206Pb

Pct. error

206Pb/238U (2σ)

207Pb/235U (2σ)

207Pb/206Pb (2σ)

A12 (continued)au19a05 0.1906 1.20 2.0713 1.50 0.0788 1.24 1125 13 1139 17 1167 14 1167 14au19a06 0.1610 1.30 1.6133 2.54 0.0727 2.04 962 13 975 25 1004 20 1004 20au19a07 0.1925 1.58 2.0870 3.00 0.0786 2.64 1135 18 1145 34 1163 31 1138 15au19a08 0.1816 1.60 1.9229 1.58 0.0768 1.08 1076 17 1089 17 1116 12 1116 12au19a09 0.1856 1.16 1.9722 2.22 0.0771 2.28 1097 13 1106 25 1123 26 1100 10au19a10 0.1576 1.10 1.5449 2.22 0.0711 2.54 943 10 948 21 959 24 945 8au19a11 0.1617 1.44 1.6174 1.68 0.0726 1.58 966 14 977 16 1001 16 974 10au19a12 0.1552 1.20 1.5298 2.48 0.0715 2.88 930 11 942 23 971 28 934 9au19a13 0.1391 2.48 1.3531 2.54 0.0706 1.04 840 21 869 22 944 10 944 10au19a14 0.1632 1.32 1.6269 1.86 0.0723 1.54 975 13 981 18 994 15 978 11au19a15 0.1813 1.46 1.9410 3.24 0.0776 2.22 1074 16 1095 35 1138 25 1138 25au19a16 0.1615 3.38 1.6593 5.22 0.0745 5.34 965 33 993 52 1055 56 977 25au19b05 0.1818 1.10 1.9239 1.50 0.0768 1.50 1077 12 1089 16 1115 17 1115 17au19b06 0.1632 2.90 1.6347 3.92 0.0726 2.38 975 28 984 39 1004 24 980 24au19b07 0.1684 3.46 1.6569 6.72 0.0714 5.38 1003 35 992 67 967 52 1001 32au19b08 0.1610 1.16 1.6448 2.68 0.0741 2.30 962 11 988 26 1043 24 1043 24au19b09 0.1623 1.74 1.6401 2.06 0.0733 2.22 970 17 986 20 1021 23 980 12au19b10 0.1709 1.24 1.7162 1.26 0.0728 0.98 1017 13 1015 13 1009 10 1015 8au19b11 0.1946 1.14 2.1276 1.32 0.0793 1.10 1146 13 1158 15 1179 13 1179 13au19b12 0.1575 1.08 1.5681 1.30 0.0722 1.58 943 10 958 12 991 16 991 16au19b13 0.1595 1.78 1.5632 1.92 0.0711 1.20 954 17 956 18 959 12 956 12au19b14 0.1575 1.58 1.5977 2.82 0.0736 2.80 943 15 969 27 1029 29 1029 29au19b15 0.1835 2.32 1.9820 2.90 0.0783 2.06 1086 25 1109 32 1155 24 1155 24au19b16 0.1570 1.94 1.6286 2.86 0.0752 3.22 940 18 981 28 1074 35 1074 35au19c05 0.1678 1.98 1.7591 3.90 0.0760 3.74 1000 20 1031 40 1096 41 1096 41au19c06 0.1574 0.76 1.5812 1.30 0.0729 1.36 942 7 963 13 1010 14 1010 14au19c07 0.1594 1.00 1.5998 1.76 0.0728 1.54 953 10 970 17 1008 16 1008 16au19c08 0.1539 0.74 1.5250 1.78 0.0719 1.52 923 7 940 17 982 15 982 15au19c09 0.1546 1.72 1.5663 3.98 0.0735 3.78 927 16 957 38 1027 39 1027 39au19c10 0.1546 2.24 1.4563 1.54 0.0683 2.50 927 21 912 14 878 22 915 9au19c13 0.1630 2.52 1.6428 3.04 0.0731 3.08 973 25 987 30 1016 31 982 17au19c14 0.1704 0.98 1.7271 2.26 0.0735 2.10 1014 10 1019 23 1027 22 1015 9au19c15 0.0378 1.76 0.2605 6.26 0.0500 6.82 239 4 235 15 196 13 239 4au19c16 0.1579 1.58 1.5666 2.16 0.0720 2.50 945 15 957 21 985 25 951 10

COS-100au02a05 0.0832 1.58 0.6462 3.50 0.0563 3.68 515 8 506 18 465 17 513 7au02a06 0.0915 1.60 0.7367 2.12 0.0584 1.80 564 9 560 12 544 10 563 8au02a07 0.1669 1.58 1.7406 2.12 0.0756 2.22 995 16 1024 22 1085 24 1085 24au02a08 0.0915 1.98 0.7491 2.86 0.0594 2.52 564 11 568 16 581 15 566 10au02a09 0.0785 2.46 0.6345 2.52 0.0586 1.92 487 12 499 13 554 11 554 11au02a10 0.1918 1.56 2.0850 1.58 0.0788 1.84 1131 18 1144 18 1168 21 1141 10au02a11 0.2126 0.84 2.5192 1.50 0.0859 1.52 1243 10 1278 19 1336 20 1336 20au02a12 0.1922 3.36 2.0551 3.24 0.0775 1.90 1133 38 1134 37 1135 22 1134 21au02a13 0.0879 2.02 0.7378 1.58 0.0608 1.92 543 11 561 9 634 12 634 12au02a14 0.0914 1.66 0.7585 1.90 0.0602 2.96 564 9 573 11 609 18 569 7au02a15 0.0974 1.94 0.8165 1.40 0.0608 1.80 599 12 606 8 631 11 606 6au02a16 0.0937 1.66 0.7718 2.86 0.0597 1.78 577 10 581 17 593 11 577 9au03a05 0.0989 1.14 0.8254 1.74 0.0605 1.56 608 7 611 11 622 10 609 6au03a06 0.1315 1.64 1.2361 0.80 0.0682 1.46 796 13 817 7 873 13 873 13au03a08 0.0868 1.72 0.7292 3.04 0.0609 3.66 537 9 556 17 636 23 636 23au03a09 0.1020 1.24 0.8459 2.50 0.0602 2.66 626 8 622 16 609 16 625 7

Table continues

DETRITAL ZIRCON DATA 105

TABLE 1. (Continued)

Isotopic ratios (2σ errors) Age, Ma

Best age estimate, Ma

Sample/analysis

206Pb/238U

Pct.error

207Pb/235U

Pct. error

207Pb/206Pb

Pct. error

206Pb/238U (2σ)

207Pb/235U (2σ)

207Pb/206Pb (2σ)

COS-100 (continued)au03a10 0.1333 1.08 1.2841 1.96 0.0699 1.06 807 9 839 16 924 10 924 10au03a11 0.0850 0.96 0.6731 3.30 0.0574 3.10 526 5 523 17 507 16 526 5au03a12 0.0746 1.04 0.5889 1.28 0.0573 1.14 464 5 470 6 502 6 502 6au03a13 0.1026 0.94 0.8531 1.50 0.0603 1.44 630 6 626 9 613 9 629 5au03a14 0.1741 1.50 1.7751 2.12 0.0739 2.32 1035 16 1036 22 1040 24 1036 11au03a16 0.1281 3.42 1.1796 1.94 0.0668 2.62 777 27 791 15 831 22 793 10au03b05 0.2029 2.32 2.2772 1.40 0.0814 2.76 1191 28 1205 17 1230 34 1204 10au03b07 0.0761 0.98 0.5952 1.26 0.0567 1.34 473 5 474 6 479 6 473 4au03b08 0.1607 1.92 1.6495 2.56 0.0744 0.72 961 18 989 25 1053 8 1053 8au03b10 0.2782 1.10 4.0681 1.16 0.1061 1.08 1582 17 1648 19 1733 19 1733 19au03b11 0.2047 0.48 2.2888 0.84 0.0811 0.82 1201 6 1209 10 1223 10 1223 10au03b12 0.1513 3.72 1.6345 3.60 0.0783 2.34 908 34 984 35 1155 27 1155 27au03b13 0.1046 1.14 0.8920 1.92 0.0618 1.40 641 7 647 12 669 9 642 7au03b14 0.0823 1.38 0.6554 2.20 0.0578 2.00 510 7 512 11 521 10 510 6au03b15 0.1836 1.02 1.9537 0.88 0.0771 1.08 1087 11 1100 10 1125 12 1125 12au03b16 0.0864 1.60 0.6995 2.96 0.0587 2.36 534 9 538 16 557 13 534 8au04a05 0.0834 2.04 0.6830 2.78 0.0594 2.48 516 11 529 15 581 14 581 14au04a06 0.1682 1.40 1.7148 0.80 0.0739 1.30 1002 14 1014 8 1039 14 1039 14au04a07 0.0904 0.98 0.7548 1.84 0.0605 1.98 558 5 571 11 623 12 623 12au04a08 0.2820 0.94 3.9943 0.84 0.1027 0.72 1601 15 1633 14 1673 12 1673 12au04a09 0.0842 0.92 0.6694 1.26 0.0577 1.18 521 5 520 7 517 6 521 4au04a10 0.0972 1.14 0.7945 2.30 0.0593 2.68 598 7 594 14 578 15 597 6au04a11 0.0817 0.86 0.6484 1.74 0.0576 1.36 506 4 507 9 513 7 506 4au04a12 0.1850 1.76 1.8590 2.40 0.0729 3.40 1094 19 1067 26 1010 34 1010 34au04a13 0.1371 2.06 1.3380 3.74 0.0708 2.94 828 17 862 32 950 28 950 28au04a14 0.2079 1.70 2.3279 1.82 0.0812 1.32 1218 21 1221 22 1226 16 1221 13au04a16 0.2027 0.94 2.2733 1.04 0.0813 0.98 1190 11 1204 13 1229 12 1229 12au05a05 0.0815 1.24 0.6597 1.04 0.0587 1.18 505 6 514 5 557 7 557 7au05a06 0.3195 0.74 5.0701 0.90 0.1151 0.70 1787 13 1831 16 1881 13 1881 13au05a07 0.1909 1.08 2.0856 1.72 0.0792 1.64 1126 12 1144 20 1178 19 1178 19au05a08 0.0821 1.30 0.6670 2.38 0.0589 2.08 509 7 519 12 565 12 565 12au05a10 0.0732 0.90 0.5633 1.88 0.0558 1.72 455 4 454 9 444 8 455 4au05a11 0.1568 1.40 1.5494 2.12 0.0716 1.20 939 13 950 20 976 12 976 12au05a12 0.0866 1.38 0.6967 2.54 0.0583 2.46 535 7 537 14 542 13 536 7au05a13 0.3823 1.36 7.0795 1.20 0.1343 0.98 2087 28 2121 25 2154 21 2154 21au05a15 0.0819 0.98 0.6524 1.44 0.0578 1.50 507 5 510 7 520 8 508 4au05a16 0.1571 1.18 1.5737 1.86 0.0726 1.42 941 11 960 18 1004 14 1004 14au05b05 0.1270 0.90 1.1504 2.12 0.0657 1.94 771 7 777 16 796 15 771 6au05b06 0.2413 2.02 3.4451 1.80 0.1036 0.78 1393 28 1515 27 1689 13 1689 13au05b07 0.0918 1.56 0.7814 3.18 0.0617 3.60 566 9 586 19 664 24 664 24au05b08 0.3587 1.22 6.0778 1.34 0.1229 0.68 1976 24 1987 27 1998 14 1992 10au05b09 0.0931 0.82 0.7586 1.28 0.0591 1.48 574 5 573 7 571 8 574 4au05b10 0.0733 0.68 0.5921 2.30 0.0586 2.36 456 3 472 11 551 13 551 13au05b11 0.0851 1.80 0.6896 2.10 0.0588 2.34 526 9 533 11 558 13 530 7au05b12 0.3319 1.34 5.6099 0.90 0.1226 0.62 1848 25 1918 17 1994 12 1994 12au05b13 0.1324 1.54 1.2365 1.96 0.0677 1.08 802 12 817 16 859 9 859 9au05b15 0.4747 1.06 12.2978 1.70 0.1878 1.08 2504 27 2627 45 2723 29 2723 29au05b16 0.0872 1.32 0.7016 1.24 0.0584 1.24 539 7 540 7 543 7 540 5

106 KEPPIE ET AL.

FIG. 3. U-Pb detrital zircon data plotted on concordia diagrams from samples of Magdalena Lithogene (A), ChazumbaLithogene (B), and Cosoltepec Formation (C). Error ellipses are shown with 2σ errors on the Concordia diagrams andinput values for the histograms and probability density plots are 2σ age errors from the “best age estimate” discussed inthe text and included in Table 1.

DETRITAL ZIRCON DATA 107

Cosoltepec FormationThe low-grade semipelitic sample of the Cosolte-

pec Formation (COS-100) was collected on theAca t lán-To to l tepec road near La Huer ta(18°15.244', 98°00.132') and consists of quartz,biotite, phengite, chlorite, and opaques. Analyzedzircons are generally concordant to nearly concor-dant with two main populations at ~455–630 Maand ~770–1200 Ma (Fig. 3, Table 1). There are sev-eral other discordant points with 207Pb/206Pb agesranging from ~1700 to 2700 Ma. The youngest zir-con gives a concordant age of 455 ± 4 Ma.

Depositional Ages of Units

Magdalena Migmatite protolithThe depositional age of the Magdalena Migma-

tite protolith is constrained between the youngestdetrital zircon (303 ± 6 Ma) and the age of migmati-zation (171 ± 1 Ma; Keppie et al., 2004b)—that is,between the Permo-Carboniferous boundary and theMiddle Jurassic (Gradstein et al., 2004). However,because migmatization took place at depths of 19 ±2 km (Keppie et al., 2004b), the younger time limitis probably somewhat older to allow time for sedi-mentary+tectonic burial (Fig. 2).

Chazumba FormationThe youngest detrital zircon has a concordant

age of 239 ± 4 Ma (Middle Triassic; Gradstein et al.,2004) and provides an older limit on the time of for-mation deposition. Emplacement of the Tultitlanmafic lens into the Chazumba Formation at 174 ± 1Ma provides a younger limit near the base of theMiddle Jurassic (Keppie et al., 2004b). Inasmuch asintrusion was synchronous with the growth of garnetin the country rocks at a minimum depth of ~15 km(Keppie et al., 2004b), the younger constraint ondeposition is also somewhat older to allow time forsedimentary + tectonic burial. The 224 ± 2 Ma 40Ar/39Ar plateau age on metamorphic muscovite fromthe adjacent Cosoltepec Formation (Keppie et al.,2004b) probably provides a tighter constraint of thetime of deposition of the Chazumba Formation—that is, post-Anisian–pre-Norian (Gradstein et al.,2004). This younger time constraint also applies tothe Magdalena Migmatite protolith. But because zir-cons from only one sample of each of the Magdalenaprotolith and Chazumba Formation have beenanalyzed, deposition of these two units may haveextended throughout the Permian and into the EarlyTriassic (Fig. 2). Clearly more sampling is required;

however, the polydeformed nature of the two unitsmakes it difficult to determine stratigraphic top andbottom.

Cosoltepec Formation

Deposition of the Cosoltepec Formation is con-strained between the youngest concordant detritalzircon age, which has a 206Pb/238U age of 455 ± 4 Ma(Mid-Caradoc, Upper Ordovician; Gradstein et al.,2004), and the Pennsylvanian unconformity beneaththe Tecomate Formation: ~305 Ma (Keppie et al.,2004c), although an older constraint is provided bythe upper Fammenian unconformity beneath thePatlanoaya Formation: 370 Ma (Vachard and Floresde Dios, 2002). However, the youngest detritalzircons in the Pennsylvanian–Middle PermianTecomate Formation (Sanchez-Zavala et al., 2004)are ~480–450 Ma (Early–Late Ordovician; Grad-stein et al., 2004), suggesting caution should beexercised in assigning a more precise age to theCosoltepec Formation (Fig. 2).

Redefinition of units

The significantly different ages of the CosoltepecFormation versus the Chazumba Formation andMagdalena protolith suggests that the Cosoltepec beremoved from the Petlalcingo Group. Furthermore,the complex structure and composite metasedimen-tary and meta-igneous nature of the Chazumba andMagdalena units makes it impossible to determinethe stratigraphy, suggesting that they be designatedlithodemes grouped in the Petlalcingo Suite. Untilsuch time as further work is carried out on theCosoltepec Formation, its name and formationalstatus are retained.

Provenance

Magdalena and Chazumba lithodemes

The ~239 Ma zircon in the Chazumba Lithodememay have been derived from the Permo-Triassic arcthat extended throughout eastern and central Mex-ico (Fig. 4; Centeno-Garcia and Silva-Romo, 1997;Torres et al., 1999; Dickinson and Lawton, 2001).The ~303–308 Ma zircon in the protolith of theMagdalena Lithodeme is similar to those fromgranitic pebbles in the Tecomate Formation,which were inferred to have been derived from theTotoltepec pluton that is also part of the Permo-Triassic arc (Figs. 1C and 4; Keppie et al., 2004b).A source for the ~920–1250 Ma detrital zircons inthe Magdalena and Chazumba lithodeme samples

108 KEPPIE ET AL.

may by found either in the adjacent OaxacanComplex (Figs. 1C and 4; Keppie et al., 2001,2003a; Solari et al., 2003; Ortega-Obregon et al.,2003) or recycled from various units of the AcatlánComplex (e.g., Cosoltepec and Tecomate formations;this paper and Sanchez-Zavala et al., 2004). Possi-ble provenances for the ~850-920 Ma detritalzircons include the basement of Avalonia (e.g.,Murphy et al., 2004) and the Goiás magmatic arc ofeastern Amazonian (Pimental et al., 2000)(Fig. 4).

Cosoltepec FormationThe ~473 Ma detrital zircon in the Cosoltepec

Formation may have come directly from plutons ofthis age in the Acatlán Complex (such as the Esper-anza Granitoids), which have yielded a concordantage of 471 ± 6 Ma (U-Pb zircon, Sánchez-Zavala etal., 2004), and a leucogranite in the western part ofthe complex that has yielded a concordant age of478 ± 5 Ma (U-Pb zircon; Campa-Uranga et al.,2002). On the other hand, the Neoproterozoic–Cam-brian detrital zircons could have come from eitherthe basement beneath the Yucatan Peninsula (Figs.

1C and 5; Krogh et al., 1993a, 1993b), or the Brasil-iano orogens of South America (e.g., Pimental et al.,2000). Most of the older zircons probably had asource in the Oaxacan Complex.

Tectonic and Stratigraphic Implications

The Permian–Late Triassic depositional age ofthe Magdalena and Chazumba lithodemes overlapsthe time of deposition of the Tecomate Formationand the intrusion of the syntectonic Totoltepecpluton (Keppie et al., 2004c). Kinematic studiesindicate that this deformation involved S-vergentthrusting associated with dextral, N-S vertical shearzones (Malone et al., 1999; Elías-Herrera andOrtega-Gutiérrez, 2002). Such overthusting wouldhave led to depression of the lithosphere anddeposition of a clastic wedge (Magdalena-Chazumbalithodemes) in front of the advancing thrust sheets.In particular, overthrusting of the Totoltepec plutonand Tecomate Formation would have provided asource for the ~304 Ma detrital zircon in theprotolith of the Magdalena Lithodeme. The absence

FIG. 4. Silurian reconstructions showing the location of the Cosoltepec Formation as a continental rise deposit adja-cent to Oaxaquia (modified after Keppie, 2004).

DETRITAL ZIRCON DATA 109

of 460–630 Ma detrital zircons in the Magdalenaand Chazumba samples may have a number ofexplanations: (1) that the Cosoltepec Formation wasalso was being depressed beneath the overlyingallochthons and so was not exposed to erosion; (2)that Cosoltepec detritus may be present elsewherein the Chazumba and Magdalena lithodemes eitherexposed or buried beneath Mesozoic and Teriaryunits; or (3) that units containing Cosoltepec detri-tus have been eroded away. Clearly further samplingis in order. However, progressive southward advanceof the thrust front eventually placed the CosoltepecFormation above the Chazumba Lithodeme in amanner consistent with a clastic wedge setting.Support for the deposition of the Chazumba andMagdalena lithodemes in a clastic wedge is also pro-vided by the turbiditic nature of the metasedimentsin these two units. The synchroneity of their deposi-tion with arc magmatism farther east and with dex-tral N-S shear zones suggests oblique subduction ofthe paleo-Pacific plate beneath the western Mexicanpart of Pangea at this time (Fig. 4A). Such obliquesubduction has also been postulated for the Middle

Permian–earliest Triassic Sonoma orogeny in theU.S. Cordillera (Saleeby and Busby-Spera, 1992).

The Latest Ordovician—Middle Devonian depo-sitional age of the Cosoltepec Formation, its distalturbiditic nature with continental-derived detritus(this paper), and the presence within it of inter-leaved oceanic tholeiitic basalts (authors’ unpubl.data) suggests that it represents the part of a conti-nental rise deposited on oceanic lithosphere. Itsage range is more consistent with deposition inthe Rheic Ocean (Ordovician–Carboniferous; Fig.4B) than in the Iapetus Ocean (latest Precambrian–Early Paleozoic), which had closed by latestOrdovician.

Acknowledgments

We would like to acknowledge a Papiit grantIN103003 to JDK that facilitated the field work; anNSF grant (EAR 0308105) and an Ohio University1804 Award to RDN; and an NSERC Discoverygrant to JBM. We also thank Miguel Morales forassistance with drawing the figures. This paper

FIG. 5. 300–230 Ma reconstruction showing the location of the Petlalcingo Suite (Chazumba and Magdalena litho-genes) as a clastic wedge (modified after Keppie, 2004).

110 KEPPIE ET AL.

represents a contribution to IGCP Project 453 (Mod-ern and Ancient Orogens) and IGCP Project 498(The Rheic Ocean).

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