Ordovician and Late Paleozoic–Early Mesozoic tectonothermal history of the La Noria area, northern...

Tectonophysics 461 (2008) 324–342

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Ordovician and Late Paleozoic–Early Mesozoic tectonothermal history of the La Noriaarea, northern Acatlán Complex, southern Mexico: Record of convergence in theRheic and paleo-Pacific Oceans

H.R. Hinojosa-Prieto a,1, R.D. Nance a, J.D. Keppie b,⁎, J.V. Dostal c, A. Ortega-Rivera d, J.K.W. Lee e

a Department of Geological Sciences, Ohio University, Athens, Ohio, 45701 USAb Depto. de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexicoc Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada B3H 3C3d Instituto de Geología, Universidad Nacional Autónoma de México, Estación Regional del Noroeste, Apartado Postal 1039, Hermosillo, Sonora 83000 Mexicoe Department of Geology, Queens University, Kingston, Ontario, Canada K7L3NG

⁎ Corresponding author.E-mail address: [email protected] (J.D. Kep

1 Now at Miami University, Oxford, Ohio, USA.

0040-1951/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tecto.2008.06.002

A B S T R A C T
A R T I C L E I N F O
Article history:
The La Noria area lies in t Received 4 June 2007Received in revised form 5 February 2008Accepted 3 June 2008Available online 11 June 2008
Keywords:Acatlán ComplexMexicoStructureGeochronologyGeochemistry

he northern part of the polydeformed Acatlán Complex, (southern MexicanMixteca terrane), and the rocks record the following sequence of events: (i) Early–Middle Ordoviciandeposition of the volcaniclastic El Epazote and Las Calaveras units; (ii) late Middle Ordovician intrusion ofthe 467±16 Ma megacrystic, peraluminous, rift-related granitoids; (iii) late Devonian, D1, greenschistfacies deformation; (iv) intrusion of the Los Malpasos leucogranite and associated minor intrusions;(v) Middle Mississippian, D2, dextral N–S deformation also under greenschist facies metamorphic con-ditions; and (vi) undated D3 kink band development. U–Pb LA-ICPMS detrital zircon ages: (a) the ElEpazote unit yielded: a mean 206Pb/238U age from the youngest five concordant 206Pb/238Pb age of 488±10 Ma, with other age peaks at ca. 506, ca. 1077, and ca. 1779 Ma and a few concordant Neoproterozoicages: and (b) the Las Calaveras unit yielded a mean 206Pb/238U age from the sixteen youngest detritalzircons of 466±10 Ma with other population age peaks ca. 1111, and ca. 1753 Ma. These data imply thatgranitoid intrusion was roughly synchronous with deposition of some of the host rocks. Whereas sourcesfor most of the detrital zircons may be found in either the Acatlán and Oaxacan complexes, Laurentia orGondwana, a Neoproterozoic source is most likely in Amazonia. The rocks record three low-gradedeformational episodes: (i) D1 produced a weak compositional banding and/or schistosity (S1) undergreenschist facies conditions; (ii) D2, also occurred under greenschist facies conditions, and developedtight to isoclinal folds (F2) in S1 and an axial planar spaced-cleavage (S2) that is co-planar with S1; and(iii) D3 produced reverse and conjugate kink bands (F3) that deform the S1/S2 composite foliation. Theleucogranite and related dikes that intrude the complex record only the latter two deformational events.Ca. 330 Ma 40Ar/39Ar muscovite plateau ages probably closely post-date the D2 event. D1 may be correlatedwith early Carboniferous deformation elsewhere in the Acatlán Complex. On the other hand, the initial40Ar/39Ar steps at ca. 300, 220 and 172 Ma probably indicate thermal disturbances below 300 °C during thePermo-Carboniferous, Triassic and Jurassic, respectively. Whereas the Ordovician history of the plutonsand volcano-sedimentary units coincides with the lifespan of both the Iapetus and Rheic oceans, the LatePaleozoic–Early Mesozoic deformation better reflects closure of the Rheic Ocean and convergencetectonics on the paleo-Pacific margin following the amalgamation of Pangea.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The Acatlán Complex, the crystalline basement of the southernMexican Mixteca terrane (Fig. 1), comprises repeatedly deformedand metamorphosed sedimentary, igneous, and mafic–ultramafic

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rocks. The eclogite and greenschist facies tectonothermal eventswere initially respectively ascribed to the Late Ordovician–EarlySilurian Acatecan orogeny (Ortega-Gutiérrez et al., 1999) and theLate Devonian Mixtecan orogeny (Sánchez-Zavala et al., 2000),which were correlated with the Taconian and Acadian orogenies inthe Appalachians of eastern Laurentia, respectively (Ortega-Gutiér-rez et al., 1999). However, recent data suggests that the greenschistevent is Permian (Keppie et al., 2004b), whereas the eclogite eventhas been dated as early Carboniferous (Middleton et al., 2007).These differences have led to alternative paleogeographic models

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Fig. 1. Tectonic map of the southern Mexican Mixteca terrane. Index map: Mixteca (Mx) terrane; Oaxaquia terrane (Oax), Xolapa terrane (Xo); Trans-Mexican Volcanic Belt (TMVB)(after Ortega-Gutiérrez et al., 1999; Keppie, 2004). Inset of paleogeographic reconstruction of the Rheic Ocean realm in the Late Carboniferous (after Nance et al., 2007): Avalonia (A),Carolina (C), AC-OX (Acatlán–Oaxaca), Ossa–Morena (OM), Massif Centrale (MC), Bohemian Massif (BM), Northwestern Iberia (NW-I), Florida (F), Armorica (ARM).

325H.R. Hinojosa-Prieto et al. / Tectonophysics 461 (2008) 324–342

for the Acatlán Complex. Thus, Ortega-Gutiérrez et al. (1999) pro-posed that it records the Ordovician–Silurian closure of the IapetusOcean, whereas Keppie and Ramos (1999), Nance et al. (2006, 2007)and Keppie et al. (2008a) suggested that it formed in the Rheic andpaleo-Pacific oceans. On the other hand, Talavera-Mendoza et al.(2005) and Vega-Granillo et al. (2007) infer that the Acatlán Com-plex formed in several oceanic tracts including the Iapetus andRheic oceans.

The Mixteca terrane lies to the west of ca. 1 Ga granulites of theOaxacan Complex that forms the basement of the Oaxaquia terrane,the two being separated byan ENE-dipping dextral transpressive shearzone of Early Permian age (Elías-Herrera and Ortega-Gutiérrez, 2002).To thewest, the Acatlán Complex is juxtaposed againstMesozoic rocksalong the E-dipping Papalutla thrust (Ortega-Gutiérrez et al., 1999). Tothe south, the Acatlán Complex is truncated by the E–W, sinistralstrike-slip Chacalapa fault (Tolson et al., 2005), and yet farther southliesMesozoic and Tertiarymetamorphic and plutonic arc-related rocksof the Xolapa terrane (Ortega-Gutiérrez et al.,1999; Ducea et al., 2004a,

b). To the north, Mesozoic–Cenozoic rocks of the Miocene–PresentTrans-Mexican Volcanic Belt obscure the Acatlán Complex (Yañezet al., 1991; Ortega-Gutiérrez et al., 1999).

Sánchez-Zavala et al. (2000) first proposed the existence of theLate Devonian Mixtecan orogeny in rocks of the La Noria area, north-ern Acatlán Complex, the topic of this paper. Evidence for this orog-eny was based on a lower intercept U–Pb TIMS zircon age of 371±34 Ma (Yañez et al., 1991) on a megacrystic, K-feldspar La Noriagranite that was inferred to have been emplaced syntectonically(Sánchez-Zavala et al., 2000). However, this granite was redatedby Miller et al. (2007) and yielded a 467±16 Ma crystallization age(U–Pb SHRIMP zircon data). Additionally, this granite is reported tointrude volcano-sedimentary rocks ascribed to the TecomateFormation of inferred Silurian–Devonian age (Ortega-Gutiérrez,1975; Yañez et al., 1991; Sánchez-Zavala et al., 2000; Talavera-Mendoza et al, 2005). However, this age assignment is precluded bythe recent discovery of latest Pennsylvanian–Middle Permian fossilsin limestones of the Tecomate Formation (Keppie et al., 2004b).

Fig. 2. Structural geologic map of the La Noria area, northern Acatlán Complex, southern Mexico, with schematic cross-section (along A–A′). Inset of equal-area stereographic projection of the ENE-dipping La Escalerilla fault: structural data isconsistent with a south-vergent dextral strike-slip sense of movement. Mapped area covers ca. 50 km2.

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Hence, these inconsistencies demand a re-examination of the LaNoria area, and in turn, a re-evaluation of the evidence for the LateDevonian Mixtecan orogeny. For this reason, remapping of the LaNoria area has been undertaken followed by the acquisition of U–Pbdetrital zircon ages and igneous geochemistry to reassess thegeological history of this tectonically critical area.

2. The Acatlán Complex

In light of new geochronological and faunal data, the geologicalhistory of the Acatlán Complex has undergone major revisions in thepast few years. These changes have been reviewed by Nance et al.(2006, 2007) and Keppie et al. (2008a,b) and a brief summary of thecurrent understanding is presented below:

(i) (Cambrian–)Ordovician deposition of rift-passive margin sedi-mentary rocks accompanied by intrusion by Ordovicianbimodal tholeiites and megacrystic granitoids;

(ii) latest Devonian–Permian deposition of periarc, shallow mar-ine–continental rocks (Vachard et al., 2000) locally associatedwith tholeiites that was synchronous with Mississippianexhumation of high pressure, subduction-related rocks (Mid-dleton et al., 2007), Permo-Triassic S-vergent thrusting anddextral shearing on N–S shear zones (Malone et al., 2002), andPermian arc magmatism (Keppie et al., 2004a);

(iii) Jurassic polyphase deformation and high grade metamorphismaccompanied by migmatization possibly related to a mantleplume (Keppie et al., 2004a)

(iv) Mesozoic deposition of continental-shelf sedimentary rocksoverlain by Cenozoic arc-related rocks.

3. Geology of the La Noria area

The La Noria areawasmapped at a 1:50,000 scale and exposes five,N–S trending, mappable units (Fig. 2, Table 1): (i) a clastic-to-volcaniclastic unit, here termed the El Epazote unit; (ii) a metasedi-mentary unit, here termed the Las Calaveras unit, previously mappedas part of the Tecomate Formation by Ortega-Gutiérrez (1975), (iii) themegacrystic, K-feldspar La Noria granite dated at ca. 467 Ma (Milleret al., 2007); (iv) the El Zapote Negro granite, which is cut by maficdikes, (v) the cross-cutting Los Malpasos leucogranite and relateddikes. A post-Triassic, undeformed and unmetamorphosed package ofvolcanic ash beds and andesitic lava flows unconformably overlies

Table 1Summary of mineralogy of the metavolcano-sedimentary and metaplutonic rocks of the La

Unit Rock types Mineralogy

El Epazote Metapelites: chl–plag–musc schist and musc–plag schist RecrystallizeFine-grained metavolcaniclastics: epidote–chlorite schistand garnet–clinozoisite muscovite–chlorite schist

Chloritized band zr.

Very fine-to-fine grained metapsammites RecrystallizeLas Calaveras Very fine-to-medium grained quartzites Recrystallize

Metagreywackes Musc, bt, chlZr, sph, pyr, hmicrostructu

La Noria granite Undeformed granites, mylonites and ultramylonites Megacrysticbrownish-to-secondary mwith zr and F

El Zapote Negrogranite

Mylonitic granite and augen granite garnet–actinolite–muscovite schist

Quartz, alb aand microscoand other op

Los Malpasosleucogranite(and dikes)

Gently foliated and weakly deformed leucogranite andrelated dikes

Qtz, orth, prtplag) occur aThin veins offractures in c

Abbreviations: chorite = chl; muscovite = musc; biotite = bt; quartz = qtz; plagioclase = plag; gclinozoisite = clz; pyrite = pyr; zircon = zr; microcline = mcr; stilpnomelane = stp; perthite

sheared rocks of the La Noria granite and associated ductile shearzones. These beds dip moderately west (Fig. 2).

The El Epazote unit comprises metapelites, metavolcaniclasticsand metapsammites (Table 1), and it is intruded by the La Noriagranite, which varies from undeformed granites to mylonites andultramylonites. The El Zapote Negro granite varies from undeformedaugen granite to mylonitic granite (Table 1). The contact between theEl Epazote unit and the El Zapote Negro granite is a dextral shear zone(La Escalerilla fault), in which sigmoidal quartz and plagioclaseporphyroclasts reveal the kinematics. The Las Calaveras unit, com-prises very fine-to-medium grained quartzites and metagreywackes(Table 1) and is cut by the Los Malpasos leucogranite. The coarse-grained Los Malpasos leucogranite is cut by gently foliated leuco-granitic dikes that also intrude the Las Calaveras unit, and both the LaNoria and the El Zapote Negro granites. No dikes were observed tointrude the El Epazote unit. The leucogranite and related dikes areweakly deformed.

4. U–Pb detrital zircon geochronology

4.1. LA-ICPMS analyses

A fine-grained, epidote–chlorite, metavolcaniclastic schist fromthe El Epazote unit and a clastic metagreywake from the Las Calaverasunit were collected for laser ablation-inductively coupled plasmamass spectrometry (LA-ICPMS) U–Pb dating of detrital zircons at theArizona LaserChron Center. Detrital zircons were extracted usingstandard procedures for mineral separation (Gehrels, 2000). Around300–500 randomly selected zircons were mounted in epoxy, sandedand polished to expose the interiors of most zircon grains. U–Pbisotopic analyses were performed with a Micromass Isoprobe multi-collector ICPMS linked to a New Wave DUV193 Excimer laser ablationsystem (Gehrels et al., 2006). The collectors are configured forsimultaneous measurement of 204Pb, 206Pb, 207Pb, 208Pb, 232Th, and238U. All isotope masses are measured in a static mode Faradaydetectors except for 204Pb, which is measured using an ion-countingchannel. All analyses were conducted with a laser beam diameter of35–50 µm operated with an output energy of ca. 32 and pulse rate of9 Hz to yield ablation pits with a depth of ca. 15–20 µm. Each analysisconsisted of one 20-second on-peak blank integration with no laserfiring (background) and twenty 1-second integrations on peaks withthe laser firing. Any Hg contribution to the 204Pb mass position wasremoved by subtracting the on-peak background values. The total

Noria area, northern Acatlán Complex, southern Mexico

d qtz, musc, chl, plag, orth, cal, and opaques as accessory grains.t, musc, epd, plag, recrystallized qtz, cal, orth, snowball gnt, clz, and accessory pyr

d qtz, plag, mcr and orth, stp, epidote, biotite, chlorite, and accessory zr and opaques.d, elongate polygonal qtz aggregates, few orth and prth, plag, musc, chl, and epd., epd, stp, plag, mcr, and recrystallized equant-to-irregular polygonal qtz aggregates.em, calc, and cross-cutting qtz microveins occur as accessory minerals andres, respectively.(ca. 10–15 cm) pink K-feldspar crystals (±mcr, prth) set in a matrix of qtz, plag,greenish bt, mus and euhedral and anhedral epd. Chl and src (after plag) are commoninerals,e–Ti oxide and sph as accessory minerals.nd mcr, orth, and prth augens set in a matrix of epd, brownish-to-greenish bt, acti,pic gnt. Common secondary minerals are musc, chl, stp, src, and calcite veins. Zr, pyr,aques occur as accessory minerals.h, alb, prc, musc, stp, epd, and extremely atypical microscopic gnt. Chl and src (afters secondary minerals. Apt, zr, and Fe–Ti oxide minerals occur as accessory minerals.calc, src, and recrystallized equant-to-irregular polygonal aggregates of qtz sealrystals of qtz and K-feldspar.

arnet = gnt; actinolite = acti; orthoclase = orth; albite = alb; calcite = calc; epidote = epd;= prth; sphene = sph; hematite = hem; sericite = src; pericline = prc; apatite = apt.

https://www.researchgate.net/publication/233312094_Terranes_of_Mexico_Revisited_A_13_Billion_year_Odyssey?el=1_x_8&enrichId=rgreq-6abc5056-fae6-428e-a279-8ce3ff185dee&enrichSource=Y292ZXJQYWdlOzIyMzI4MTQ1MDtBUzoxMDM0OTU5OTQ3MDc5ODBAMTQwMTY4Njc3ODE0Mw==

328 H.R. Hinojosa-Prieto et al. / Tectonophysics 461 (2008) 324–342

analysis time per zircon grainwas ca. 90 s. Inter-element fractionationwas monitored by analyzing fragments of a large concordant zir-con crystal from Sri Lanka with a known (ID-TIMS) age of 564±4 Ma(2-sigma error). This reference zirconwas analyzed once for every fiveunknowns. The isotopic ratios were corrected for common Pb, using themeasured 204Pb, assuming an initial Pb composition according to Staceyand Kramer (1975) and respective uncertainties of 1.0, 0.3, and 2.0 for206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. The systematic error (age ofstandard, calibration correction from standard, composition of commonPb, decay constant uncertainty) in the session ranged from 1.15–1.20%for 206Pb/238U and from 1.07–1.33% for 206Pb/207Pb. Uranium andthorium concentrations were monitored by analyzing a NIST 610 glass.Grains for analysis were selected randomly from all sizes andmorphologies present in the sample mounts, except for avoidance ofgrainswith visible fractures, inclusions, or compositional zoning. Grainsb35 µm in diameter were also avoided to ensure that each ablation pitwas entirely located within a single zircon grain.

The age probability plots in this paper were constructed from the206Pb/238U age for young (b1.3 Ga) zircon grains and the 206Pb/207Pbage for older (N1.3 Ga) zircon grains. In old zircons, analyses with N20%discordance or N10% reverse discordance are considered unreliableandwere rejected (Gehrels et al., 2006). Analyses were not rejected fordiscordance or reverse discordance for grains that have 206Pb/238Uages b800Ma because of the low precision of 207Pbmeasurement, andhence the large errors on the 207Pb/238U age for young grains, making

Fig. 3. Geochronologic analyses for the metavolcaniclastic El Epazote and the clastic Las Calavzircons, and (c–d) age probability curves for each lithodemes, respectively.

the geological significance of concordance and discordance difficult orimpossible to assess. Thus, the 206Pb/238U age is robust if it belongs to acluster of three ormore zircons with similar ages (Gehrels et al., 2006).The resulting U–Pb data are shown on concordia diagrams (Fig. 3aand b) and as relative age probability curves (Fig. 3c and d), which addprobability distributions from all analyses from a given sample into anunique composite probability distribution. Age probability plots wereconstructed using IsoPlot 3.00 of Ludwig (2003). Table 2 displays acomplete set of analytical U–Pb data and sample locations.

4.2. Results: U–Pb detrital zircon ages

The analyzed sample of the El Epazote unit (sample D7) is a fine-grained epidote–chlorite schist (metavolcaniclastic horizon), consist-ing of chloritized biotite, muscovite, epidote, volcanic plagioclase,recrystallized quartz, calcite, K-feldspar, occasional detrital snowballgarnet, clinozoisite, and accessory pyrite and zircon. Itwas collected ca.5.5 km east of San Juan Las Calaveras village along the Las Calaverasgorge (Fig. 2). This horizon is intercalated with polydeformedmetapelites and metapsammites. Detrital zircons ages range from349±8 to 2440±32 Ma (Fig. 3a, Table 2). The youngest concordantzircon has an age of 349±8Ma, however, it was not reproducible, so, asindicated above, its reliability is questionable, a conclusion in accordwith the fact that the El Epazote unit is intruded by the 467±16 Ma LaNoria granite. A similar conclusion applies to two other single zircon

eras units (or lithodemes): (a–b) concordia diagrams showing U–Pb age data of detrital

Table 2LA-ICPMS U–Pb isotopic data of detrital zircons from the El Epazote and Las Calaveras units in the La Noria–Las Calaveras area, northern Acatlán complex, southern Mexico

U U/Th Isotopic ratios Apparent ages (Ma)

206Pb 207Pb ±(%) 206Pb ±(%) Errorcorrection

206Pb ±(Ma) 207Pb ±(Ma) 206Pb ±(Ma) Best

(ppm) 204Pb 235U 238U 235U 207Pb Age (Ma)± (Ma)

El Epazote Formation: metavolcaniclastic (D7: N18° 18.614′ W098° 08.707′)38 0.5 3024 2.42473 12 0.21624 4 0.31 1262 41 1250 84 1229 219 1262 41144 1.0 5614 0.88775 8 0.10541 1 0.15 646 8 645 39 642 172 646 8318 0.8 5254 0.61676 6 0.08234 3 0.48 510 14 488 23 385 118 510 14126 0.9 2392 0.67796 12 0.07903 3 0.25 490 14 526 49 682 249 490 14193 1.6 7962 1.61255 4 0.16533 1 0.32 986 13 975 27 950 84 986 13219 3.2 7311 1.91109 5 0.18810 4 0.83 1111 43 1085 34 1033 58 1111 43255 3.0 7944 0.78213 6 0.09329 2 0.33 575 12 587 28 632 129 575 12311 1.2 8581 0.69115 7 0.08750 1 0.16 541 6 533 30 503 159 541 6236 2.3 7318 2.02215 2 0.18975 1 0.37 1120 7 1123 13 1129 35 1120 7424 3.4 2314 2.15547 4 0.20340 3 0.74 1194 36 1167 31 1118 60 1194 36227 1.9 9341 2.32044 4 0.21305 3 0.66 1245 33 1219 31 1172 65 1245 33245 1.4 7216 2.77853 5 0.23703 4 0.85 1371 48 1350 34 1316 47 1316 47191 1.7 1539 1.20928 36 0.13250 6 0.16 802 44 805 204 812 773 802 44127 5.5 4036 1.48489 7 0.15729 2 0.31 942 18 924 40 883 128 942 18216 1.7 12,057 3.68483 3 0.25218 3 0.81 1450 35 1568 27 1731 36 1731 36132 1.0 3139 0.60429 21 0.08209 3 0.13 509 14 480 82 345 487 509 14291 4.9 9686 2.75228 10 0.23100 5 0.50 1340 61 1343 76 1347 170 1347 17038 2.0 2021 1.73480 19 0.17161 4 0.19 1021 33 1022 121 1023 374 1021 33528 12.1 29,134 1.80653 1 0.17345 1 0.57 1031 7 1048 8 1083 20 1031 7233 1.2 8240 0.49098 9 0.06947 3 0.30 433 11 406 30 253 194 433 11118 1.8 12,439 1.92467 4 0.18819 2 0.34 1112 15 1090 29 1046 83 1112 15156 1.7 8624 1.71301 6 0.16802 2 0.29 1001 15 1013 36 1040 108 1001 1585 1.0 10,653 7.11855 2 0.39587 2 0.70 2150 31 2126 22 2104 30 2104 30137 1.4 29,678 3.22479 4 0.25113 2 0.48 1444 22 1463 27 1491 59 1491 59484 6.0 4915 0.79181 7 0.09024 1 0.14 557 5 592 32 730 149 557 5551 13.9 34,612 2.52460 8 0.21096 5 0.53 1234 51 1279 62 1356 138 1234 51213 1.7 15,561 1.84250 4 0.18025 3 0.61 1068 27 1061 29 1045 71 1068 27199 1.7 9320 1.29842 9 0.13179 6 0.65 798 43 845 50 970 135 798 43474 4.7 37,448 2.11631 3 0.18885 2 0.78 1115 25 1154 21 1228 37 1115 25103 3.9 10,850 1.15361 6 0.11803 2 0.26 719 11 779 34 954 124 719 11100 1.1 4775 0.78305 10 0.08845 2 0.20 546 11 587 45 748 209 546 11223 1.2 16,487 4.47799 4 0.30000 3 0.83 1691 48 1727 32 1770 39 1770 39154 0.9 5009 0.73968 8 0.09509 1 0.18 586 8 562 35 469 176 586 8483 1.8 4035 2.27380 6 0.19296 6 0.94 1137 58 1204 42 1326 40 1137 58116 0.6 5555 0.88912 10 0.10264 4 0.34 630 21 646 50 702 209 630 21243 3.9 11,066 1.61801 5 0.16197 2 0.35 968 14 977 28 999 86 968 14102 1.3 4208 1.76682 8 0.17556 4 0.55 1043 41 1033 51 1014 134 1043 41237 1.8 4798 1.85369 3 0.18095 1 0.34 1072 10 1065 20 1050 57 1072 10291 0.9 7491 0.68184 9 0.08338 3 0.31 516 14 528 38 579 192 516 14125 0.8 3020 0.65728 10 0.08335 2 0.22 516 10 513 38 499 205 516 10164 0.8 4890 1.77669 8 0.17576 6 0.70 1044 54 1037 52 1023 115 1044 54280 1.8 7849 3.08793 3 0.24041 2 0.79 1389 30 1430 23 1491 35 1491 35245 2.0 16,208 1.70222 5 0.17139 2 0.41 1020 20 1009 33 987 96 1020 20582 2.5 6204 0.87597 7 0.09986 4 0.52 614 21 639 33 729 124 614 21261 3.7 17,936 1.18554 4 0.12969 3 0.65 786 19 794 22 816 64 786 19535 9.0 51,900 2.47785 6 0.21287 6 0.98 1244 71 1266 46 1302 23 1244 71299 1.4 5562 0.39637 8 0.05563 2 0.28 349 8 339 23 271 179 349 8153 0.6 4421 0.67051 7 0.07879 2 0.29 489 9 521 27 665 137 489 9184 1.1 1439 0.52101 11 0.07150 5 0.43 445 19 426 37 322 218 445 19238 1.0 5592 1.93342 8 0.18357 7 0.92 1086 74 1093 54 1105 64 1086 7421 1.1 837 1.81618 32 0.17801 4 0.14 1056 44 1051 215 1041 662 1056 44137 1.9 7702 2.33151 3 0.21454 2 0.50 1253 17 1222 21 1168 51 1253 17622 1.1 9816 3.35989 4 0.24119 4 0.96 1393 46 1495 30 1643 20 1643 20428 2.1 28,692 4.65372 3 0.30698 3 0.97 1726 45 1759 26 1799 15 1799 15413 0.5 8530 0.80105 4 0.09698 2 0.43 597 9 597 17 600 73 597 9207 0.9 2968 0.68227 12 0.08775 2 0.19 542 11 528 48 468 254 542 11228 1.8 9488 1.79942 4 0.17933 3 0.70 1063 26 1045 25 1008 56 1063 26108 2.9 9021 2.14261 7 0.19406 2 0.21 1143 16 1163 50 1199 139 1143 16305 2.3 2901 1.80767 5 0.17976 4 0.82 1066 42 1048 34 1012 62 1066 4291 0.8 4536 2.05988 10 0.19378 8 0.77 1142 82 1136 70 1124 130 1142 82302 4.7 3819 4.89575 5 0.28887 5 0.89 1636 65 1802 43 1999 42 1999 42430 5.5 7633 0.59015 5 0.07783 1 0.22 483 5 471 17 412 100 483 5162 3.1 17,572 2.01828 9 0.19497 8 0.85 1148 83 1122 63 1071 97 1148 83339 3.3 8437 1.94320 6 0.19145 5 0.75 1129 47 1096 40 1031 80 1129 471475 7.5 20,347 0.87502 2 0.10201 1 0.44 626 5 638 9 681 35 626 5323 1.7 8025 0.62803 7 0.07838 3 0.46 486 15 495 27 534 133 486 15440 1.3 6304 0.91421 5 0.10510 5 0.85 644 28 659 26 711 60 644 28242 3.0 7343 0.71184 10 0.08976 2 0.24 554 12 546 41 512 206 554 12

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Las Calaveras Formation: metapsammite (E6: N18° 18.295′ W098° 09.831′)45 0.5 1010 1.34357 11 0.15693 2 0.18 940 18 865 66 677 239 940 18407 0.3 8005 1.77030 1 0.17464 1 0.52 1038 7 1035 9 1028 23 1038 7275 0.7 5969 1.72014 3 0.17579 1 0.38 1044 13 1016 22 957 65 1044 13440 2.7 9862 2.13684 2 0.19730 1 0.72 1161 13 1161 12 1161 23 1161 13497 1.3 7900 2.17491 1 0.19937 1 0.67 1172 11 1173 10 1175 21 1172 11896 2.9 19,336 2.17081 4 0.19330 4 0.96 1139 42 1172 29 1232 22 1139 42118 0.7 1067 1.93433 8 0.18201 1 0.15 1078 12 1093 53 1123 156 1078 12220 1.1 1038 1.59275 11 0.16228 4 0.33 969 33 967 70 963 217 969 33963 3.6 8948 0.57477 2 0.07517 1 0.46 467 5 461 9 431 47 467 5411 2.0 8532 1.73400 2 0.17102 1 0.43 1018 7 1021 10 1029 30 1018 7182 1.9 4986 2.09761 3 0.19486 2 0.55 1148 20 1148 23 1149 56 1148 20560 2.4 14,033 1.68038 3 0.16998 2 0.70 1012 17 1001 16 977 37 1012 17529 2.0 11,997 3.07180 3 0.23399 2 0.80 1355 25 1426 19 1532 28 1532 28507 0.8 1277 1.18665 7 0.12823 3 0.37 778 18 794 38 841 134 778 18203 1.0 5140 5.23403 2 0.33964 2 0.85 1885 28 1858 17 1828 19 1828 19335 1.0 10,330 2.70193 2 0.22673 1 0.71 1317 15 1329 13 1348 24 1348 24103 0.8 1367 0.99729 10 0.12098 6 0.59 736 39 702 49 596 168 736 39250 0.7 6097 1.79056 2 0.18303 1 0.76 1084 15 1042 13 956 26 1084 1561 1.3 769 0.60955 21 0.07635 2 0.10 474 10 483 80 526 460 474 10371 1.5 2388 2.31343 3 0.21000 1 0.55 1229 16 1216 19 1194 44 1229 16150 0.6 4690 0.57891 13 0.07586 1 0.09 471 5 464 48 426 285 471 588 0.8 1535 0.70161 16 0.07533 1 0.09 468 6 540 66 855 329 468 6134 0.9 4072 1.86894 6 0.18535 3 0.58 1096 33 1070 37 1018 93 1096 3380 2.1 2826 1.33219 14 0.12423 8 0.56 755 58 860 84 1141 237 755 5824 0.6 1161 1.39897 23 0.17421 2 0.09 1035 21 889 138 539 510 1035 21238 1.3 16,501 1.74862 3 0.17419 2 0.53 1035 15 1027 19 1009 50 1035 15114 1.0 10,833 2.31665 4 0.20865 3 0.62 1222 30 1217 31 1210 68 1222 30104 1.0 9418 2.43510 5 0.21734 2 0.35 1268 21 1253 38 1228 97 1268 21103 0.8 8693 2.42390 5 0.21505 2 0.35 1256 21 1250 38 1239 97 1256 21191 1.1 4915 1.59896 3 0.16251 1 0.38 971 9 970 16 968 48 971 9255 1.0 7835 0.60200 9 0.07598 3 0.34 472 14 479 36 509 194 472 14174 2.0 11,096 1.77412 3 0.17369 2 0.47 1032 15 1036 22 1044 61 1032 1519 0.3 699 1.61490 42 0.16243 2 0.05 970 19 976 268 989 889 970 19836 6.9 19,761 0.57634 3 0.07195 1 0.28 448 3 462 10 533 56 448 361 0.5 12,499 2.06967 8 0.19999 2 0.19 1175 18 1139 58 1070 167 1175 181017 2.9 2633 1.35151 5 0.13431 3 0.55 812 21 868 29 1013 83 812 21248 0.9 2120 2.25781 3 0.21115 1 0.38 1235 14 1199 24 1135 62 1235 14354 0.8 7784 2.50405 2 0.22002 1 0.56 1282 12 1273 13 1258 29 1282 12351 0.9 12,140 3.93617 1 0.27238 1 0.86 1553 17 1621 12 1711 14 1711 14875 5.1 20,013 1.90501 2 0.17976 1 0.78 1066 12 1083 10 1118 20 1066 12601 3.2 12,552 1.91518 1 0.18269 1 0.48 1082 7 1086 10 1096 26 1082 7325 0.8 8879 2.51778 2 0.21898 1 0.39 1277 8 1277 13 1278 33 1277 8301 1.4 14,106 3.20268 3 0.23585 2 0.92 1365 29 1458 20 1596 18 1596 1894 0.8 2154 1.86856 6 0.19574 3 0.50 1152 30 1070 37 906 100 1152 30202 0.8 4077 2.18290 4 0.20209 3 0.69 1187 32 1176 29 1156 61 1187 3284 0.6 2161 1.44354 9 0.14974 1 0.16 900 12 907 55 926 187 900 12422 1.0 3341 0.61570 4 0.07583 2 0.39 471 7 487 15 563 80 471 7100 1.1 2996 2.29737 7 0.20591 5 0.79 1207 59 1211 48 1219 82 1207 59241 0.5 3456 1.71059 5 0.17765 2 0.29 1054 15 1013 35 924 106 1054 15116 0.9 2433 1.89449 8 0.18311 4 0.56 1084 43 1079 51 1069 128 1084 4328 0.5 270 0.53964 30 0.13355 3 0.10 808 23 438 109 NA NA87 0.7 3969 4.50285 3 0.30235 2 0.56 1703 29 1731 28 1766 52 1766 52278 1.3 11,646 2.32975 2 0.20799 1 0.50 1218 13 1221 16 1227 39 1218 1341 0.9 1150 1.84187 11 0.19951 4 0.35 1173 41 1061 72 836 214 1173 41197 0.6 11,106 4.33741 2 0.29636 1 0.64 1673 21 1701 19 1734 32 1734 32185 1.2 5950 2.33341 3 0.20949 2 0.48 1226 18 1223 24 1216 58 1226 1849 0.7 990 1.86720 14 0.17439 3 0.22 1036 28 1070 91 1138 267 1036 28430 1.0 5497 0.58818 6 0.08022 2 0.30 497 9 470 24 336 140 497 927 0.5 857 1.56393 28 0.17589 5 0.18 1044 50 956 178 758 600 1044 50182 1.6 12,136 4.30621 3 0.30403 3 0.90 1711 44 1695 27 1674 26 1674 2670 0.7 2745 2.67113 5 0.24025 3 0.51 1388 35 1321 41 1213 93 1213 93137 1.1 1655 2.18908 7 0.20498 3 0.47 1202 38 1178 51 1133 130 1202 38161 1.6 4773 2.28521 4 0.20201 1 0.34 1186 14 1208 27 1247 72 1186 14200 1.8 9348 3.21275 3 0.26039 2 0.83 1492 32 1460 23 1415 32 1415 32672 2.3 7309 0.61307 5 0.07454 4 0.86 463 18 485 18 591 53 463 18221 0.6 8349 2.24785 3 0.20495 3 0.90 1202 29 1196 21 1186 26 1202 29201 0.4 3036 0.58928 9 0.07769 2 0.19 482 8 470 34 413 196 482 8258 1.4 17,440 2.41856 3 0.21904 2 0.67 1277 23 1248 22 1199 45 1277 2340 0.5 3389 1.78038 19 0.17296 3 0.14 1028 24 1038 122 1059 375 1028 24208 3.3 11,866 1.66249 4 0.16581 2 0.59 989 22 994 26 1006 68 989 2261 0.3 4400 2.55937 6 0.22181 3 0.52 1291 40 1289 47 1285 108 1291 40

Table 2 (continued )


86 2.6 1543 2.30951 9 0.20869 4 0.44 1222 46 1215 67 1203 166 1222 46112 1.8 5978 2.24222 5 0.20847 1 0.24 1221 13 1194 36 1147 99 1221 13469 1.4 1301 1.84335 4 0.18387 2 0.47 1088 16 1061 23 1006 63 1088 16393 1.5 1672 1.82140 6 0.18217 3 0.49 1079 28 1053 38 1000 102 1079 2893 0.8 1668 2.12211 6 0.20161 2 0.29 1184 19 1156 43 1104 120 1184 19357 2.4 4160 0.57416 4 0.07923 3 0.74 492 14 461 15 310 62 492 14293 1.0 9042 1.75905 4 0.17577 3 0.73 1044 26 1030 24 1002 51 1044 26207 0.8 13,821 4.49728 5 0.30480 4 0.92 1715 65 1730 39 1749 33 1749 33551 0.8 13,783 4.03246 3 0.27128 3 0.97 1547 41 1641 25 1763 13 1763 13139 0.9 1129 0.57675 19 0.07764 2 0.13 482 11 462 71 366 432 482 11252 0.8 4281 0.55860 8 0.07257 2 0.31 452 11 451 29 445 166 452 11301 0.6 8908 0.56501 12 0.07544 3 0.22 469 12 455 44 384 266 469 12157 0.9 10,628 2.37958 4 0.20967 1 0.33 1227 14 1236 27 1253 70 1227 14157 1.6 8322 2.29639 5 0.20838 1 0.22 1220 11 1211 33 1195 89 1220 11234 0.6 13,915 4.48703 3 0.30076 2 0.86 1695 34 1729 22 1769 25 1769 25659 1.2 8885 0.62794 2 0.07839 1 0.61 487 6 495 9 533 39 487 6170 0.6 10,927 4.19412 3 0.28052 2 0.60 1594 24 1673 23 1773 41 1773 41319 1.2 22,562 1.93586 4 0.17836 1 0.23 1058 10 1094 29 1165 85 1058 10207 0.7 10,081 5.29987 3 0.32064 2 0.64 1793 27 1869 23 1954 37 1954 3776 0.5 3638 2.43982 9 0.19660 1 0.14 1157 13 1254 65 1426 172 1157 12.9908All errors 2 sigmaSystematic errors E6 206/238 1.15 206/207 1.33

D7 206/238 1.2 206/207 1.07

U concentration and U/Th are calibrated by comparison with NBS SRM 610 and have uncertainty of ~25%.Decay constants: 235U=9.8485×10–10. 238U=1.55125×10–10, 238U/235U=137.88.Isotope ratios are corrected for Pb/U fractionation by comparison with standard zircon with an age of 564±4 Ma (2-sigma).Initial Pb composition interpreted from Stacey and Kramer (1975), with uncertainty of 1.0 for 206Pb/204Pb, 0.3 for 207Pb/204Pb and 2.0 for 208Pb/204Pb.Discordance is calculated from comparison of 206Pb/238U and 206Pb/207Pb ages.

Table 2 (continued)





Las Calaveras Formation: metapsammite (E6: N18° 18.295′ W098° 09.831′)


206Pb/238U ages of 433±11 Ma and 445±19 Ma (Table 2). On the otherhand, a cluster of five 206Pb/238U ages between ca. 483 and ca. 491 Mahas a mean age of 488±10 Ma, and provides a maximum depositionalage for the unit. This, together with the ca. 467 age of the La Noriagranite, bracket deposition of the unit in the Early–Middle Ordovician.Whereas these data preclude correlation with the Carboniferous–Permian Tecomate Formation (c.f. Sánchez-Zavala et al., 2000), they dosuggest a correlation with the Las Minas unit in the Patlanoaya area(Ramos-Arias et al., 2008; Keppie et al., 2008b). The cumulative agegraphs show detrital zircon age peaks in the Cambrian, Mesoproter-ozoic and Paleoproterozoic, with respective peaks at ca. 506, ca. 1077,ca. 1779 Ma (Fig. 3c). There are also a few concordant detrital zirconsbetween ca. 620 and 700 Ma and one at 807±49 Ma.

The analyzed sample (sample E6) of the Las Calaveras unit is ametagreywake containing recrystallized quartz, muscovite, chlor-itized biotite, epidote, stilpnomelane, plagioclase, K-feldspar, andaccessory zircon, sphene, pyrite, and hematite. It was collected ca.3.5 km east of San Juan de Las Calaveras village along the LasCalaveras gorge (Fig. 2). Detrital zircons ages range from 448±3 to1954±37 Ma (Fig. 3b, Table 2). The youngest sixteen zircons have206Pb/238U ages ranging from ca. 448 to ca. 497 Ma (Table 2, Fig. 3b),and a population peak at ca. 466±10 Ma (Fig. 3d): this establishes apost-Tremadocian Ordovician depositional age for the unit. This,combined with the 49Ar/39Ar data for the cross-cutting Los Malpasosleucogranite described below, only poorly constrains deposition ofthe Las Calaveras unit to the Ordovician–Devonian. However, it islithologically similar to the Huerta unit in the Xayacatlán area,which was deposited in the mid-late Ordovician (Morales-Gámezet al., 2008; Keppie et al., 2008b). The sample's cumulative agepattern also shows prominent older detrital zircon populations inthe Mesoproterozoic and Paleoproterozoic, with respective peaks atca. 1111, and ca. 1753 Ma (Fig. 3d).

The closest source for the Ordovician and ca.1 Ga detrital zircons inthese two units is in the adjacent Acatlán and Oaxacan complexes

(Keppie et al., 2003), respectively, although a more distal source ineither the Grenville or Sunsas orogens cannot be eliminated. Similarlyprovenance of the 1750–1780 Ma zircons could be in either Laurentiaor Amazonia. However, the few 620–700 Ma and 807 Ma detritalzircons in the El Epazote unit are most likely derived from Amazonia(Keppie et al., 2008a).

5. Geochemistry of intrusions

5.1. Analytical methods

Sixteen plutonic rock samples were collected for geochemicalanalyses (Fig. 2): four from the La Noria megacrystic granite, six fromthe El Zapote Negro granite, and six from the Los Malpasosleucogranite. The samples were analyzed by X-ray fluorescencespectroscopy for major oxides and several trace elements at theNova Scotia Regional Geochemical Centre at Saint Mary's University,Halifax, Canada. The major oxides and trace element analyses ofrepresentative samples and their geographic locations are given inTable 3.

5.2. Results: major oxides and trace elements

The rocks of the La Noria and the El Zapote Negro granites, and thecross-cutting Los Malpasos leucogranite are primarily granodioritic–granitic with subordinate tonalitic composition (Fig. 4a). All ana-lyzed plutonic rocks are felsic and peraluminous [molar Al2O3N (CaO+Na2O+K2O)]. They all show similar patterns in their mantle-normal-ized spider diagrams with overall pattern of incompatible elementenrichment and marked depletions in Ba, Nb, Sr, and Ti, signatures,which are typical of subduction-zone magmas (Fig. 4b). In general, allsamples straddle the volcanic arc granite—within plate graniteboundary in the Y–Nb tectonic discrimination diagram (Fig. 4c) ofPearce et al. (1984).

https://www.researchgate.net/publication/222690559_Geochronology_and_geochemistry_of_Grenvillian_igneous_suites_in_the_northern_Oaxacan_Complex_southern_Mexico_Tectonic_implications?el=1_x_8&enrichId=rgreq-6abc5056-fae6-428e-a279-8ce3ff185dee&enrichSource=Y292ZXJQYWdlOzIyMzI4MTQ1MDtBUzoxMDM0OTU5OTQ3MDc5ODBAMTQwMTY4Njc3ODE0Mw==

https://www.researchgate.net/publication/229417019_Extensional_Late_Paleozoic_deformation_on_the_western_margin_of_Pangea_Patlanoaya_area_Acatlan_Complex_southern_Mexico?el=1_x_8&enrichId=rgreq-6abc5056-fae6-428e-a279-8ce3ff185dee&enrichSource=Y292ZXJQYWdlOzIyMzI4MTQ1MDtBUzoxMDM0OTU5OTQ3MDc5ODBAMTQwMTY4Njc3ODE0Mw==

https://www.researchgate.net/publication/223834300_Synthesis_and_tectonic_interpretation_of_the_westernmost_Paleozoic_Variscan_orogen_in_southern_Mexico_From_rifted_Rheic_margin_to_active_Pacific_margin?el=1_x_8&enrichId=rgreq-6abc5056-fae6-428e-a279-8ce3ff185dee&enrichSource=Y292ZXJQYWdlOzIyMzI4MTQ1MDtBUzoxMDM0OTU5OTQ3MDc5ODBAMTQwMTY4Njc3ODE0Mw==

Table 3Major oxide and some trace element compositions of plutonic rocks in the La Noria–Las Calaveras area, northern Acatlán Complex, southern Mexico

La Noria granite El Zapote Negro quartz augen granite Los Malpasos leucogranite

A1 A2 A3 A4 B1 B2 B3 B4 B5 B6 C1 C2 C3 C4 C5 C6

Oxides (wt.%)L.O.I. 2.34 2.43 2.04 2.51 2.54 2.94 2.82 2.6 1.81 1.64 0.85 1.8 2.19 1.9 1.83 1.84SiO2 67.91 66.70 63.23 67.70 63.95 61.91 67.89 66.66 63.83 69.68 63.65 72.62 62.77 68.94 69.02 67.76TiO2 0.653 0.751 0.931 0.625 0.933 1.378 0.647 0.67 0.632 0.537 0.114 0.344 0.729 0.441 0.451 0.458Al2O3 14.11 14.11 16.57 14.06 15.78 15.79 14.1 14.54 15.75 13.58 19.66 12.23 15.81 14.71 14.23 15.12Fe2O3 4.54 5.36 5.43 4.33 6.74 8.44 4.23 4.46 6.21 3.78 1.23 1.86 5.54 3.31 3.14 3.62MnO 0.048 0.064 0.089 0.053 0.171 0.144 0.076 0.066 0.091 0.063 0.003 0.022 0.099 0.046 0.033 0.058MgO 1.56 1.71 1.49 1.32 1.93 2.17 1.07 1.24 2.44 0.83 0.18 0.31 1.77 0.92 1.56 1.31CaO 0.76 1.55 3.36 1.28 1.11 1.08 1.59 1.75 2.35 1.57 0.37 1.95 3.56 1.6 2.18 2.84Na2O 2.48 2.25 3.61 2.39 1.73 1.56 2.97 2.39 5 2.58 5.31 3.6 2.62 2.13 3.25 3.07K2O 4.33 4.15 2.41 4.7 4.03 3.4 3.64 4.9 0.79 4.45 7.94 3.84 3.4 5.56 3.51 3.02P2O5 0.176 0.153 0.173 0.136 0.161 0.163 0.152 0.155 0.129 0.144 0.205 0.116 0.167 0.109 0.11 0.102Total 98.91 32.53 99.33 31.40 99.08 98.98 99.19 99.43 99.03 98.85 99.51 98.69 98.66 99.67 99.31 99.20

Trace elements (ppm)V 30 43 51 38 55 77 37 39 35 33 7 20 43 29 27 29Cr 10 16 22 13 44 43 7 12 24 15 b4 b4 8 b4 16 4Co 12 15 16 13 25 28 12 12 28 8 b5 b5 17 7 8 7Zr 245 285 392 234 271 285 270 268 185 265 72 167 298 270 208 189Ba 201 255 208 339 245 254 324 388 77 475 282 213 304 389 281 287La 43 47 75 33 73 59 49 70 33 46 32 47 56 50 41 30Nd 29 34 55 26 51 43 38 51 24 37 22 30 44 39 33 24Ni b3 b3 b3 b3 55 13 23 b3 b3 b3 b3 b3 b3 b3 b3 b3Cu 10 10 b4 b4 22 59 7 17 b4 7 28 9 11 b4 b4 b4Zn 50 64 58 56 92 103 45 66 44 56 12 18 80 38 27 48Ga 20 19 23 19 23 22 19 22 18 18 20 16 22 18 20 18Rb 159 144 86 160 142 109 117 142 41 142 224 116 106 167 107 97Sr 45 73 396 57 120 109 78 84 234 88 81 68 229 137 209 279Y 38 39 47 37 36 34 41 40 30 47 35 49 32 31 36 26Nb 19 19 21 16 19 21 17 17 13 17 17 16 16 9 17 12Pb 9 6 6 10 11 16 19 39 6 30 133 935 16 19 26 16Th 11 11 18 10 14 11 12 10 12 16 5 9 9 8 13 3U 5 4 2 4 4 3 3 4 1 4 7 4 3 4 3 2K 35,900 34,500 20,000 39,000 33,500 28,200 30,200 40,700 6600 36,900 65,900 31,900 28,200 46,200 29,100 25,100Ti 3900 4500 5600 3700 5600 8300 3900 4000 3800 3200 700 2100 4400 2600 2700 2700Location N18°

20.554′W098°09.575′

N18°17.966′W098°10.077′

N18°19.320′W098°11.911′

N18°18.622′W098°10.458′

N18°18.466′W098°08.879′

N18°18.528′W098°09.047′

N18°18.534′W098°09.392′

N18°20.556′W098°09.708′

N18°20.312′W098°09.341′

N18°19.550′W098°09.600′

N18°17.837′W098°09.579′

N18°18.209′W098°09.853′

N18°20.037′W098°10.152′

N18°20.500′W098°10.810′

N18°19.512′W098°10.843′

N18°18.676′W098°10.458′


The peraluminous affinity, high-K calc-alkaline character, andmixed arc-within plate signature of the La Noria granite are alsocommon in other megacrystic granitoids in the Acatlán Complex.Indeed, available U–Pb igneous zircon ages (Ortega-Gutiérrez et al.,1999; Sánchez-Zavala et al., 2004; Talavera-Mendoza et al., 2005;Murphy et al., 2006; Miller et al., 2007), whole-rock igneousgeochemistry, trace element data (Ramirez-Espinoza, 2001; Murphyet al., 2006; Miller et al., 2007), neodymium isotopic studies (Yañezet al., 1991; Murphy et al., 2006) show that all the megacrysticgranitoids of the Acatlán Complex likely originated in the same, orsimilar, Ordovician–Early Silurian (ca.480–440 Ma) geodynamicsetting. Recent data indicate that these ca. 480–440 Ma granitoidsare synchronous with intrusion of rift tholeiites during extension,with the arc-within plate, geochemical signature being inherited fromthe source region (Keppie et al., 2008b).

6. Deformational history of the La Noria area

6.1. Structural geometry

Detailed macro- and micro-structural/kinematic and petro-graphic (Tables 1 and 4) analyses demonstrate that the metaplutonicand metavolcano-sedimentary units in the area were subjected tobrittle and ductile deformation under greenschist facies conditions.NNW–SSE trending, vertical, ductile shear zones also occur inlocalized areas. The structural geometry of the El Epazote and theLas Calaveras units and the La Noria and the El Zapote Negro

megacrystic granites reveals two phases of ductile deformation,only one of which is recorded by the cross-cutting Los Malpasosleucogranite and related dikes. A later phase of ductile–brittle kinkband deformation is recorded in all units. Based on structural styleand overprinting relations, these phases are assigned to D1, D2, andD3. Table 4 provides a detailed explanation for the deformationalfabrics in all units. Figs. 5 and 6 summarize the deformationalpatterns for all units. Associated with D2 are NNW trending ductileshear zones that are parallel to the dextral strike-slip La Escalerillafault. Other mylonitic shear zones occur in the villages of La Noriaand Las Calaveras, along the NNW–SSE trending intrusive contactsbetween the La Noria granite and the El Epazote unit, and along thecontact between the Los Malpasos leucogranite and the LasCalaveras unit. The greenschist facies planar fabrics were deformedby D3 kink bands during the exhumation of these metavolcano-sedimentary and metaplutonic rocks from depths of ca. 5–10 km.

7. 40Ar/39Ar geochronology

Muscovite was separated from four samples (Fig. 2, Table 5):(i) mica schist of the El Epazote unit; (ii) ultramylonitic granite ofthe La Noria granite; (iii) mylonized granite of the El Zapote Negrogranite; and (iv) leucogranite from the Los Malpasos leucogranite.The muscovite was pre-treated and concentrated by standardtechniques and later selected by handpicking under a binocularmicroscope from fractions that ranged in size from 40–60 mesh atthe mineral separation laboratory at UNICIT-Universidad Nacional

Fig. 4. Summaryof geochemical analyses for the felsic rocks of the LaNoria and theEl ZapoteNegro granites, and the LosMalpasos leucogranite plottedon: (a)Na2O+K2Oversus SiO2 (wt.%)diagram (Middlemost, 1985); (b) primitive mantle-normalized spider diagrams of trace element abundances; and (c) Y–Nb tectonic discrimination diagrams of Pearce et al. (1984).


Autónoma de México, Campus-Juriquilla, Querétaro, Qro. Themuscovite separates were loaded into Al-foil packets and irradiatedtogether with Hb3gr (1072 Ma) as a neutron-fluence monitor at theMcMaster Nuclear Reactor (Hamilton, Ontario). 40Ar/39Ar analyseswere performed by laser step-heating techniques described byClark et al. (1998) at the Geochronology Research Laboratory ofQueen's University, Kingston, Ontario, Canada. The data are given inTable 5 and plotted in Fig. 7. All data have been corrected forblanks, mass discrimination, and neutron-induced interferences.For the purposes of this paper, a plateau age is obtained when theapparent ages of at least three consecutive steps, comprising aminimum of 40% of the 39Ark released, agree within 2 sigma errorwith the integrated age of the plateau segment, and plateau arehere interpreted to represent the best estimate for the time ofcooling through a mineral's closure temperature (e.g., 300–400 °Cfor the muscovite; Hames and Bowring 1994). Errors shown inTable 5 and on the age spectrum represent the analytical precisionat ±2 sigma.

7.1. Results

Muscovite from the El Epazote mica schist yielded a 40Ar/39Arplateau age of 328±3 Ma for 86% of the gas released (Fig. 7A,Table 5). The lowest power step yielded an age of 300±2 Ma. Incontrast, the granitoid rocks yielded variable discordant spectra. Themildly leucogranite yielded discordant data that step downwardsfrom 355±8 Ma to 244±6 Ma (Fig. 7B, Table 5). A lithologicallysimilar leucogranite dike in the Orgonal area (10 km to the north of

La Noria) has yielded a 372±8 Ma U–Pb zircon age (Vega-Granilloet al., 2007), and may be part of the same magmatic event. Themylonitic granite of the El Zapote granite yielded a plateau age of333±3 Ma in the higher four power increments stepping down to172±3 Ma in the lowest power increment (Fig. 7C, Table 5). Theultramylonite of the La Noria granite yielded discordant data thatdecrease from an age of 297±2 Ma to 220±2 Ma (Fig. 7D, Table 5).

8. Structural correlation and timing of deformation

The D2 structures developed under greenschist facies meta-morphic conditions and dextral shearing are remarkably similaracross the area, which suggests correlation. Thus the orientation ofthe S2 foliation (steeply WSW and ENE ENE dipping) and L2 minerallineation (moderately-steeply, NW and SE plunging), the geometryand orientation of F2 folding (steeply NW-to-S-plunging tight-isoclinal folds), and the orientation of the shear zones are almostidentical (Figs. 2, 5 and 6). During D1, the El Epazote and LasCalaveras units and the La Noria and El Zapote Negro megacrysticgranites developed their S1 foliation also under greenschist faciesconditions. The geometries and orientation of D3 kink bands andtheir spatial distribution are likewise similar in all units, and alloverprint those associated with D2. Whereas D2 and D3 affected allof the units in the area, D1 did not affect the Los Malpasosleucogranite and associated dikes. Thus, D1 is bracketed betweenthe youngest detrital zircons in the Las Calaveras unit (ca. 466 Ma),and the 355±8 Ma age in the Los Malpasos leucogranite. Withinerror limits, D1 is synchronous with the earliest deformation (ca.

Table 4Summary of deformational patters produced by D1, D2 and D3 events for the metavolcano-sedimentary and metaplutonic rocks of the La Noria area, northern Acatlán complex,southern Mexico


347 Ma) in the Patlanoaya area (Ramos-Arias et al., 2008) thatappears to be related to the ca. 346 Ma exhumation of eclogite faciesrocks in the Asis lithodeme (Middleton et al., 2007; Keppie et al.,2008a).

The 328±3 Ma and 333±3 Ma, Middle Mississippian plateau agesin the mica schist and mylonized granite come from muscovite thatgrew during the D1 and D2 events, the latter at temperatures ca.400 °C. Thus, the ca. 330 Ma 40Ar/39Ar muscovite plateau ages areinferred to closely post-date the D2 event. This deformation and thedextral N–S shearing associated with it may have continued into theEarly Permian, during which the Acatlán and Oaxacan complexeswere tectonically juxtaposed (Elías-Herrera and Ortega-Gutiérrez,2002).

The younger 40Ar/39Ar ages suggest one or more thermal events attemperatures below ca. 300 °C, possibly during the Triassic andJurassic, one of which may be associated with the D3 event. Similarstyles of deformation have been documented elsewhere in the AcatlánComplex where they are thought to be associated with Permo-Triassicconvergent tectonics on the paleo-Pacific margin of Pangea (Keppieet al., 2006, 2008a; Nance et al., 2006) and a Jurassic tectonothermalevent associated with a mantle plume (Keppie et al., 2004a). Aproposed model for the deformational history of the La Noria area isshown in Fig. 8.

9. Conclusions

This paper documents the following events in the La Noria area ofnorthern Acatlán Complex in southern Mexico: (i) Early–MiddleOrdovician deposition of the volcaniclastic El Epazote and LasCalaveras units after ca. 488 Ma in the El Epazote unit, and after ca.466±10 Ma in the Las Calaveras unit; (ii) late Middle Ordovicianintrusion of the 467±16 Ma La Noria megacrystic granite (Miller et al.,2007), which is likely to have been co-magmatic with the megacrysticEl Zapote Negro granite, because they have similar geochemicalcharacteristics; (iii) late Devonian D1 greenschist facies deformation;(iv) intrusion of the Los Malpasos leucogranite and associated minorintrusions; (v) Middle Mississippian, D2, dextral N–S deformation alsounder greenschist facies metamorphic conditions; and (vi) subse-quent D3 kink band development. Whereas a local provenance for theyoungest Ordovician and ca. 1 Ga detrital zircons may be found in theAcatlán and Oaxacan complexes, the source of the older 1.75 Gadetrital zircons may be found in either Laurentia or Gondwana. On theother hand, provenance of the Neoproterozoic detrital zircons is mostlikely in Amazonia. The ca. 480–440 Ma, peraluminous plutons andtheir association with rift tholeiites suggest that they formed duringrifting on the northern margin of Gondwana (Keppie et al., 2008a,b),andwere produced bymelting the underlying 1 Ga Oaxacan basement

Fig. 5. Summary of structural and kinematic data of the El Epazote and the Las Calaveras units: equal-area stereographic projections of (a) D2 structures (composite foliation S2 and mineral lineation L2), (b) poles to F2 axial planes and F2 axes,and (d) poles to F3 kink band axial planes and related axes for the El Epazote unit. (c) poles to S1/S2 composite foliation, F2 axial planes, F2 fold axes and mineral lineation L2, and (e) D3 ductile–brittle structures (kink band fold axis and axialplanes) for the Las Calaveras unit.

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Fig. 6. Summary of structural and kinematic data of the granitoids in the La Noria area, Acatlán Complex: La Noria granite: equal-area stereographic projection of (a) poles to F2, F2 axial planes, and fold axes; (b) poles to S1/S2 composite foliationand mineral lineation L2; and (e) pole to F3 conjugate, (f) reverse and normal kink band axial plane and fold axis (F3) deforming S1/S2 composite foliation in highly deformed areas of the La Noria granite. El Zapote Negro granite: equal-areastereographic projection of (c) poles to F2, F2 axial planes, and fold axes; (d) poles to S1/S2 composite foliation and mineral lineation L2, poles to reverse (g) and (h) conjugate kink band axial plane and fold axis (F3). Los Malpasos leucogranite:equal-area stereographic projection of (i) poles to S1 and L1, (j) orientation of related dikes and associated S1 and L1 along its sheared intrusive contact with the Las Calaveras unit, and (l) pole to F2 fold axis and axial plane of kink band withrespect to S1 planes.

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Table 540Ar/39Ar geochronological analyses of samples from the La Noria area, northern Acatlán Complex, southern Mexico: (a) mica schist of the El Epazote unit (LN-D5, AOR 1020: 18°18.614′ 098° 08.706); (b) ultramylonitic granite of the La Noria granite (LN.A7, AOR-1023: 18° 18.170′ 098° 11.064′); (c) mylonized granite of the El Zapote Negro granite (LN-B11, AOR-1024: 18° 18.532′ 098° 09.062); and (d) leucogranite from the Los Malpasos leucogranite (LN-G3, AOR-1022; 18° 18.203′ 098° 10.001′)

AOR-1020: LN-D5-HH-04 Ms 40/60

Steps Laser power(W)

Isotope volumes

40Ar 39Ar 38Ar 37Ar 36Ar Ca/K Cl/K %40Ar atm f 39Ar 40Ar⁎/39ArK Age

1 0.50 12.652±0.021 0.439±0.002 0.007±0.000 0.001±0.000 0.005±0.000 0.015 0.000 10.52 12.53 25.85±0.20 299.7±2.22⁎⁎ 0.75 77.189±0.112 2.384±0.014 0.036±0.002 0.001±0.001 0.034±0.001 0.000 −0.000 12.42 68.05 28.49±0.24 327.6±2.53⁎⁎ 1.00 18.589±0.023 0.628±0.004 0.008±0.001 0.001±0.000 0.002±0.000 0.000 −0.000 3.00 17.92 28.82±0.22 331.1±2.34 1.25 1.600±0.004 0.053±0.001 0.001±0.000 0.001±0.000 0.000±0.000 0.000 −0.000 0.25 1.50 29.85±0.79 341.9±8.2

Total/average 109.905±0.116 3.482±0.015 0.051±0.002 0.004±0.001 0.039±0.001 0.002 −0.000 100.00 28.56±0.09 325.0±1.5

Isotope correlation data

36Ar/40Ar 39Ar/40Ar r

1 0.50 0.000356±0.000016 0.034606±0.000198 0.0252⁎⁎ 0.75 0.000421±0.000017 0.030742±0.000191 0.0273⁎⁎ 1.00 0.000102±0.000014 0.033656±0.000217 0.0274 1.25 0.000008±0.000088 0.033417±0.000479 0.125

Footnotes: Isotope production ratios

(40Ar/39Ar)K=0.0302(37Ar/39Ar)Ca=1416.4306(36Ar/39Ar)Ca=0.3952Ca/K=1.83×(37ArCa/39ArK)J=0.006990±0.000054Volume 39ArK=34.82

Integrated date=324.98±2.92Plateau date=328.33±3.10**%39ArK for PA=85.97Isotope correlation date=358.25±317.68Initial 40Ar/36Ar ratio=−552.92±−8821.06MSWD=1393.88%39ArK for CA=100.00

AOR-1022: LN-G3-HH-04 Ms 80/100

Steps Laser power(W)

Isotope volumes


1 0.20 2.349±0.007 0.095±0.001 0.002±0.000 0.001±0.000 0.002±0.000 0.283 0.002 15.06 16.30 20.74±0.50 244.1±5.52⁎⁎ 0.40 7.213±0.050 0.257±0.002 0.004±0.000 0.002±0.000 0.001±0.000 0.157 0.000 1.75 43.98 27.57±0.31 317.8±3.23⁎⁎ 0.50 3.452±0.025 0.123±0.001 0.002±0.000 0.002±0.000 0.000±0.000 0.333 0.000 1.25 21.04 27.60±0.47 318.1±5.04 0.70 1.721±0.004 0.056±0.001 0.001±0.000 0.002±0.000 0.000±0.000 0.559 0.000 1.42 9.56 29.95±0.70 342.8±7.35 5.00 1.699±0.004 0.053±0.001 0.001±0.000 0.001±0.000 0.000±0.000 0.149 0.000 0.94 9.12 31.11±0.75 354.9±7.8

Total/average 16.274±0.057 0.581±0.003 0.010±0.001 0.080±0.001 0.002±0.000 0.252 0.001 100.00 27.58±0.10 311.9±1.6


36Ar/40Ar 39Ar/40Ar r 36Ar/39Ar

1 0.20 0.000510±0.000065 0.040949±0.000427 0.046 0.016±0.0922⁎⁎ 0.40 0.000059±0.000019 0.035636±0.000351 0.052 0.003±0.1603⁎⁎ 0.50 0.000042±0.000042 0.035778±0.000447 0.041 0.004±0.2944 0.70 0.000048±0.000071 0.032916±0.000438 0.073 0.007±0.2655 5.00 0.000032±0.000072 0.031841±0.000456 0.071 0.008±0.264



Integrated date=311.91±3.19Plateau date=317.91±3.58**%39ArK for PA=65.02Isotope correlation date=−149.94±2092.12Initial 40Ar/36Ar ratio=31,506.98±483,115.80MSWD=68.94%39ArK for CA=100.00

AOR-1023: LN-A7-HH-04 Ms-Wr 80/100

Steps Laserpower (W)

Isotope volumes


1 0.20 11.572±0.016 0.501±0.003 0.010±0.000 0.002±0.000 0.008±0.000 0.093 0.001 19.79 9.55 18.55±0.18 219.7±2.02 0.30 31.730±0.036 1.232±0.006 0.017±0.001 0.002±0.001 0.009±0.000 0.041 −0.000 7.87 23.47 23.82±0.16 277.5±1.8

(continued on next page)(continued on next page)


Table 5 (continued)

AOR-1023: LN-A7-HH-04 Ms-Wr 80/100

Steps Laserpower (W)

Isotope volumes


1 Total/average

11.572±0.016 0.501±0.003 0.010±0.000 0.002±0.000 0.008±0.000 0.093 0.001 −0.000 9.55 18.55±0.18 219.7±2.0

8 0.75 0.017±0.001 0.000±0.000 0.000±0.000 0.001±0.000 0.000±0.000 0.000 0.1050 −0.34 100.00 14.59±276.56 175.0±3161.6

3 0.40 36.894±0.036 1.441±0.007 0.019±0.001 0.001±0.001 0.004±0.000 0.017 −0.000 2.76 27.46 24.99±0.15 290.1±1.64 0.50 19.886±0.020 0.777±0.004 0.010±0.001 0.001±0.000 0.002±0.000 0.023 −0.000 2.01 14.81 25.16±0.16 291.9±1.75⁎⁎ 0.60 19.461±0.019 0.752±0.004 0.010±0.001 0.002±0.000 0.002±0.000 0.046 −0.000 2.00 14.33 25.43±0.16 294.9±1.76⁎⁎ 0.65 12.340±0.011 0.472±0.002 0.006±0.000 0.002±0.000 0.001±0.000 0.079 −0.000 2.27 8.98 25.64±0.17 297.1±1.87 0.70 0.487±0.002 0.000±0.000 0.000±0.000 0.001±0.000 0.002±0.000 0.000 0.068 99.77 0.00 5.76±242.77 71.1±2939.98 0.75 0.017±0.001 0.000±0.000 0.000±0.000 0.001±0.000 0.000±0.000 0.000 0.105 −0.34 0.00 14.59±276.56 175.0±3161.69 3.00 0.477±0.003 0.018±0.000 0.000±0.000 0.001±0.000 0.000±0.000 0.000 0.000 0.57 0.34 26.21±1.45 303.2±15.410 6.00 1.533±0.012 0.056±0.001 0.001±0.000 0.001±0.000 0.000±0.000 0.003 0.001 2.51 1.06 26.80±0.56 309.4±5.9

Total/average

134.127±0.062 5.217±0.011 0.072±0.002 0.115±0.001 0.025±0.001 0.040 0.017 100.00 25.51±0.04 282.4±1.1



1 0.20 0.000671±0.000021 0.043233±0.000227 0.0192 0.30 0.000267±0.000015 0.038675±0.000193 0.0233 0.40 0.000093±0.000011 0.038911±0.000192 0.0154 0.50 0.000068±0.000011 0.038952±0.000210 0.0175⁎⁎ 0.60 0.000068±0.000010 0.038529±0.000208 0.0236⁎⁎ 0.65 0.000077±0.000014 0.038112±0.000205 0.0327 0.70 0.003376±0.009916 0.000398±0.025074 1.0008 0.75 0.000000±0.064176 0.068783±0.100674 0.2239 3.00 0.000020±0.000193 0.037930±0.000882 0.12710 6.00 0.000085±0.000056 0.036378±0.000530 0.066




AOR-1024: LN-B11-HH-04 Ms 40/60

Laser power(W)

Isotope volumes


1 0.75 2.508±0.009 0.138±0.001 0.003±0.000 0.001±0.000 0.002±0.000 0.068 0.001 19.67 1.88 14.53±0.30 171.7±3.42 1.25 4.874±0.016 0.213±0.002 0.003±0.000 0.001±0.000 0.001±0.000 0.030 −0.000 7.00 2.89 21.34±0.23 247.0±2.53 1.50 5.908±0.028 0.220±0.0020 0.003±0.00 0.001±0.000 0.001±0.000 0.039 −0.000 3.77 2.99 25.90±0.27 295.7±2.84 2.00 31.110±0.099 1.064±0.006 0.014±0.001 0.001±0.001 0.002±0.000 0.030 −0.000 1.88 14.47 28.81±0.21 326.0±2.25 2.50 31.929±0.117 1.119±0.007 0.015±0.001 0.001±0.001 0.001±0.000 0.025 −0.000 0.96 15.22 28.38±0.22 321.5±2.36 3.00 26.478±0.076 0.942±0.005 0.012±0.001 0.001±0.000 0.001±0.000 0.001 −0.000 1.00 12.82 27.93±0.19 316.9±2.07 3.50 16.428±0.044 0.577±0.003 0.008±0.000 0.002±0.000 0.001±0.000 0.088 −0.000 1.21 7.85 28.23±0.20 320.0±2.18⁎⁎ 4.00 20.241±0.064 0.688±0.004 0.009±0.001 0.001±0.000 0.001±0.000 0.027 −0.000 0.95 9.36 29.24±0.20 330.5±2.09⁎⁎ 5.00 23.316±0.078 0.788±0.004 0.010±0.001 0.001±0.000 0.001±0.000 0.041 −0.000 0.79 10.72 29.48±0.21 333.0±2.210⁎⁎ 6.00 14.746±0.043 0.499±0.002 0.007±0.000 0.001±0.000 0.001±0.000 0.065 −0.000 0.79 6.79 29.42±0.18 332.3±1.811⁎⁎ 7.00 32.892±0.066 1.104±0.006 0.014±0.001 0.002±0.001 0.001±0.000 0.060 −0.000 0.61 15.01 29.76±0.18 335.8±1.9

Total/average 210.247±0.221 7.309±0.014 0.095±0.002 0.155±0.002 0.011±0.001 0.039 −0.000 100.00 29.52±0.04 320.8±1.3



1 0.75 0.000667±0.000051 0.055279±0.000527 0.0362 1.25 0.000237±0.000023 0.043574±0.000351 0.0413 1.50 0.000128±0.000019 0.037148±0.000326 0.0504 2.00 0.000064±0.000010 0.034059±0.000235 0.0465 2.50 0.000032±0.000011 0.034903±0.000254 0.0486 3.00 0.000034±0.000009 0.035441±0.000224 0.0327 3.50 0.000041±0.000011 0.034990±0.000226 0.0568⁎⁎ 4.00 0.000032±0.000010 0.033870±0.000210 0.0379⁎⁎ 5.00 0.000027±0.000010 0.033650±0.000222 0.04010⁎⁎ 6.00 0.000027±0.000011 0.033720±0.000187 0.07611⁎⁎ 7.00 0.000021±0.000011 0.033401±0.000185 0.056


corr


(Keppie et al., 2008a) that resulted in their mixed arc-within platesignature. Deposition of the host rock (El Epazote and Las Calaverasunits) may have been synchronous with the emplacement of the ca.

Table 5 (continued)

Isotope

Footnotes: Isotope production ratios(40Ar/39Ar)K=0.0302(37Ar/39Ar)Ca=1416.4306(36Ar/39Ar)Ca=0.3952Ca/K=1.83×(37ArCa/39ArK)J=0.006875±0.000056Volume 39ArK=73.09


36Ar/40Ar

467 Ma La Noria and El Zapote Negro granites. The Ordoviciangeological units in the La Noria area overlap both the Late Ordovician–Early Silurian closure of Iapetus Ocean at ca. 450–430 Ma and theOrdovician to Carboniferous lifespan of the Rheic Ocean (Fig. 9). Onthe other hand, the Late Paleozoic–Mesozoic deformational eventsmay be only correlated with closure of the Rheic Ocean and tectonicevents on the paleo-Pacific margin of Pangea (Keppie et al., 2008a).

Acknowledgements

Special thanks to David Schneider, Greg Nadon, and Dina Lopez forcomments; to Brent Barley for field assistance, and to George Gehrelsand Victor A. Valencia for assistance during U–Pb geochronologiclaboratory analysis. Reviews by B.V. Miller, J. B. Murphy, G.E Gehrels,and an anonymous person improved the content of the manuscript.This project was supported by the following Ohio University awards:Department of Geological Sciences Alumni Grant, Council of Research,Scholarship, and Creative Activity Undergraduate, Graduate andMedical Student Enhancement Award, the Graduate Student SenateGrant for Original Work award. The work was also supported by aPapiit grant (IN103003) and a CONACyT grant (CB-2005-1: 24894) toJDK that facilitated the fieldwork, an NSF grant (EAR 0308105) and anOhio University 1804 Award to RDN, and NSERC Discovery grant to JD.I dedicate this paper to Héctor J. Hinojosa-Prieto.

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Fig. 7. 40Ar/39Ar incremental release spectrum from muscovite in: (a) El Epazote micaschist; (b) Los Malpasos leucogranite; (c) mylonized El Zapote Negro megacrysticgranite; and (d) ultramylonized La Noria granite.


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Fig. 8. Proposed model for the tectonothermal history of the La Noria area, northern Acatlán Complex, based on detailed field mapping, structural/kinematic analysis, igneousgeochemistry, and U–Pb SHRIMP (published) and LA-ICPMS geochronology of igneous and detrital zircons, respectively. (a) deposition of the El Epazote unit (and possibly the LasCalaveras unit) in the Early–Middle Ordovician; (b) intrusion of Middle Ordovician megacrystic granitoids. (c) deformation (D1) under the upper-to-middle greenschist faciesconditions in the Late Devono-Mississippian. (d) intrusion of the co-magmatic Los Malpasos leucogranite and its related dikes. (e) deformation (D2) and related N–S dextral shearingunder greenschist facies conditions at ca. 330 Ma. (f) low temperature, brittle deformation (D3).


Fig. 9. Geologic time-lime comparing the geological record of the La Noria areawith thelifespan of both the Iapetus and Rheic Oceans (after Keppie and Ramos, 1999; andOrtega-Gutiérrez et al., 1999). Radiometric ages for the granitoids are from Ortega-Gutiérrez et al. (1999) and Miller et al. (2007).


Ordovician and Late Paleozoic–Early Mesozoic tectonothermal history of the La Noria area, northern...

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