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Geochemistry of paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications
Journal of South American Earth Sciences 12 (1999) 537±544
Geochemistry of Paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications J.M. Grajales-Nishimura a,*, E. Centeno-Garcia a, J.D. Keppie a, J. Dostal b a
Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, 04510, MeÂxico D.F., Mexico b Department of Geology, St. Mary's University, Halifax, Nova Scotia, Canada, B3H 3C3
Abstract Analyses of Lower Permian or older basalts and associated dykes of the Juchatengo sequence indicate that they are rift tholeiites that formed in a continental rift or back-arc tectonic setting. Age constraints include a Middle Permian fossil recovered from the tectonically overlying sediments and a cross-cutting, post-tectonic pluton dated by K/Ar on hornblende at 2822 6 Ma. A location adjacent to the Oaxacan Complex or other old continental crust is suggested by (1) an eNdi isotopic value of ÿ8.95 and a TDM age of 1487 Ma in the overlying sediments, which are similar to the Oaxacan Complex; (2) enrichment of incompatible elements in the lavas, suggesting old crustal contamination; and (3) the presence of Permian±Triassic calc-alkaline plutons that stitch the Juchatengo±Oaxaca boundary. The possible tectonic models depend on the age of the Juchatengo basalts, which requires future geochronological work. If the Juchatengo basalts are Permo-Carboniferous, they could have formed near the eastern edge of a back-arc basin: the contemporaneous arc would have rifted away to the west. Eastward migration of the arc magmatism indicated by the Permian±Triassic calc-alkaline plutonism may re¯ect shallowing of the dip of the subduction zone, which probably also produced the deformation of the Juchatengo sequence. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The tectonic evolution of the North American Cordillera in Middle to Late Devonian was characterized by marginal arc magmatism with development of back-arc rifting. This was followed by Early Mississippian shortening and accretion of an arc. During Pennsylvanian time, arc magmatism developed in the southern parts of the Cordillera. To the west of this arc sequence lay ophiolitic rocks that contain plagiogranite blocks and ma®c dike complexes with radiometric ages around 300 Ma (Miller et al., 1992). In this scenario, the evolution and chemical composition of the Paleozoic basalts and associated deep marine sedimentary rocks of the Juchatengo complex of southern Mexico are a critical test of this tectonic
model at the southern end of the North American Cordillera. The Juchatengo complex was accreted to the Oaxaca Complex prior to intrusions of granitoids that yield Early Permian K/Ar radiometric ages. The Permian in the Americas represents a transitional period following the amalgamation of Pangea and preceding its breakup in the Late Triassic. Most authors infer that there was subduction beneath the western margin of the Americas (e.g. Golonka and Ross, 1995) expressed as arc magmatism that extends from southwestern US, through the backbone of Mexico as isolated plutons, and into the northern Andes (TorresVargas et al., 1993). Early Permian granitoids of the Juchatengo area may correspond to this arc magmatism. 2. Geological setting
* Corresponding author. E-mail address: [email protected] Nishimura).
(J.M.
Grajales-
Mexico has been divided into several tectonic regimes (Fig. 1). The southern edge of cratonic North
0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 9 9 ) 0 0 0 3 7 - 1
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Fig. 1. Tectonic subdivision of Mexico and location of the Juchatengo terrane.
America, lying near the US±Mexican border, is bounded by the Ouachita Orogen (Stewart et al., 1993), followed southward by a variety of terranes (from east to west) (Fig. 1): (1) the Neoproterozoic
Maya Terrane; (2) the Early Mesozoic Juarez Terrane; (3) Oaxaquia (ca 1 Ga); (4) the Upper Paleozoic or older Juchatengo Terrane; (5) the Lower Paleozoic Mixteco Terrane; (6) the Mesozoic Guerrero terrane; and (7) the Mesozoic-Tertiary Xolapa Terrane (Campa-Uranga and Coney, 1983; Sedlock et al., 1993; Ortega-GutieÂrrez et al., 1995; Keppie and OrtegaGutieÂrrez, 1995). Some authors have correlated the Juchatengo and Mixteco terranes but their age constraints are poorly known. The Mixteco Terrane consists of Lower Paleozoic ophiolites that have been metamorphosed up to eclogite facies and thrust over trench-®ll and ¯ysch deposits before intrusion of a Devonian syntectonic granitoid (Ortega-GutieÂrrez, 1993). The Mixteco Terrane was accreted to Oaxaquia during the Devonian. Subsequently, the MixtecoOaxaquia-Maya collided with Laurentia in the Late Paleozoic during the amalgamation of Pangea. The boundary between Oaxaquia and the Juchatengo terrane is not exposed; however, the Juchatengo Terrane is inferred to have accreted to the southern margin of Oaxaquia by the Permian because both terranes are
Fig. 2. Geological map of the Juchatengo area.
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
539
Table 1 Major element analyses of basalt and associated basic dykes samples from the Juchatengo Complexa Sample
I-73
I-74
I-75
I-76
I-77
I-78
I-79
I-80
I-81
I-82
QUI-6A
QUI-15
QUI-17A
Rock type SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total LOI Mg]
pbas 53.44 1.98 14.67 9.89 0.15 4.26 7.69 5.49 0.14 0.21 97.92 1.5 0.43
pbas 53.3 1.91 14.18 10.05 0.14 3.59 9.1 4.98 0.19 0.19 97.63 2.7 0.38
pbas 50.21 2.16 14.68 11.41 0.15 3.48 11.65 3.5 0.09 0.2 97.53 2.1 0.35
pbas 48.55 2.41 15.13 12.67 0.16 4.37 7.4 4.87 0.13 0.22 95.91 2.5 0.38
pbas 51.27 1.95 13.82 10.46 0.14 3.17 9.27 4.37 0.1 0.18 94.72 4.6 0.35
mdyke 48.35 2.15 14.74 10.23 0.23 5.65 9.1 3.6 0.4 0.56 95.01 4.3 0.5
pbas 45.7 3.08 14.61 12.89 0.23 5.53 8.55 3.19 0.19 0.38 94.36 4.7 0.43
mlava 46.95 1.25 15.43 9.17 0.14 8.48 8.83 3.29 0.55 0.12 94.21 5.3 0.62
mlava 48.06 1.37 15.44 8.71 0.12 8.27 8.41 3.61 0.6 0.14 94.73 4.8 0.63
mlava 44.78 1.14 14.6 9.07 0.16 7.16 10.08 3.77 0.46 0.13 91.35 7.9 0.58
mdyke 48.47 1 18.15 8.71 0.16 5.87 9.96 2.13 0.72 0.14 95.31 ± 0.54
mdyke 49.78 1.34 13.1 9.74 0.2 6.73 12.01 2.77 0.11 0.2 95.98 ± 0.55
mdyke 54.76 0.68 15.17 7.91 0.16 4.52 12.63 0.92 0.01 0.11 96.88 ± 0.5
a
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
intruded by a suite of Permian±Triassic calc-alkaline plutons (Grajales-Nishimura, 1988). The WNW-trending Xolapa Terrane truncates the N-trending Juchatengo Terrane along a major sinistral Late Mesozoic-Tertiary fault zone (Ratschbacher et al., 1991; Tolson et al., 1993). The Juchatengo Terrane consists of a sequence of rocks (Fig. 2) (from west to east: inferred base to top) (Grajales-Nishimura, 1988): (1) gabbroic and plagiogranitic dykes and stocks which have yielded K±Ar hornblende ages of 289 2 6 Ma, 2342 5 Ma, 225 25 Ma, and 219 2 5 Ma, i.e., Early Permian±Late Triassic (Time Scale of Okulitch, 1995). The younger ages were recorded in altered hornblende separates involving replacement by chlorite, epidote and actinolite; (2) basalts and pyroclastic rocks; and (3) interbedded
black shale, chert, siltstone, sandstone and minor limestone from which a cephalopod (ammonite?) of possible Middle Permian(?) age was recovered (Mullerried, 1945). These rocks are locally intensely deformed, striking N±S to NW±SE with moderately to steep dips, and most boundaries are tectonic. This sequence and the Oaxacan Complex were subsequently intruded by calc-alkaline stocks and a batholith composed of quartz diorite and tonalite dykes that have yielded K±Ar hornblende ages of 282 2 6 Ma, 251 2 6 Ma, and 238 25 Ma, and a K±Ar biotite age of 2782 6 Ma (Grajales-Nishimura, 1988): the younger ages are from localities closer to Tertiary plutons. It is perhaps signi®cant that these ages are similar to those recorded in the basal unit of the Juchatengo Terrane suggesting thermal re-equilibration
Table 1a Trace element analyses of basalt and associated basic dykes samples from the Juchatengo Complexa Sample
I-73
I-74
I-75
I-76
I-77
I-78
I-79
I-80
I-81
I-82
QUI-6A
QUI-15
QUI-17A
Rock type Cr Ni Co V Pb Zn Rb Ba Sr Ga Ta Nb Hf Zr Y Th
pbas 41 33 24 317 15 51 4 45 172 ± 0.28 6.2 2.95 125 27 1.1
pbas 56 37 20 316 ± 43 3 50 280 ±
pbas 61 23 18 422 ± 36 3 29 411 ± 0.25 5.1 2.1 89 24 0.8
pbas 57 37 25 406 ± 66 4 23 174 ±
pbas 50 33 32 369 ± 49 5 38 179 ± 0.22 4.7 4.17 79 20 0.65
mdyke 93 49 23 250 7 69 8 293 255 ± 0.43 7.6 1.84 140 37 1.42
pbas 63 40 33 403 ± 110 6 115 251 ± 0.83 14.6 1.73 200 32 1.76
mlava 334 119 50 269 30 59 16 145 184 ±
mlava 324 105 48 282 23 55 12 197 230 ± 0.16 3.3 2.16 69 20 0.42
mlava 328 117 43 255 18 75 13 137 140 ± 0.18 3.5 1.73 70 21 0.76
mdyke 89 32 22 258 ± 61 15 450 450 16 0.21 4.2 2.07 78 17 3.67
mdyke 248 79 24 298 ± 73 ± ± 266 17 0.22 4.2 2.16 98 23 0.71
mdyke 74 24 23 230 ± 56 ± ± 375 16 ± ±
a
5 2.09 103 26
4 3.27 80 23
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
1 2.07 52 16
54 16
540
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
Table 1b Rare Earth element analyses of basalt and associated ma®c dykes samples from the Juchatengo Complexa Sample
I-73
I-75
I-77
I-78
I-79
I-81
I-82
QUI-6A
QUI-15
Rock type La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
pbas 7.19 18.73 2.86 14.22 4.07 1.55 4.92 0.82 5.34 1.14 3.29 0.45 3.12 0.46
pbas 5.96 15.63 2.36 11.9 3.42 1.41 4.32 0.7 4.54 0.98 2.82 0.38 2.34 0.35
pbas 5.1 13.49 2.1 10.18 3.04 1.19 3.84 0.61 3.94 0.86 2.51 0.35 2.2 0.33
mdyke 10.27 27.21 4.05 21.18 5.95 2.24 7.44 1.15 7.06 1.45 3.99 0.55 3.58 0.53
pbas 14.43 36.03 4.9 22.65 6.08 1.94 6.71 1.06 6.43 1.32 3.7 0.52 3.47 0.47
mlava 4.45 11.74 1.78 9.4 2.94 1.05 3.7 0.6 3.98 0.81 2.26 0.33 2.32 0.33
mlava 8.5 21.19 2.41 10.18 2.68 1 3.33 0.57 3.72 0.82 2.37 0.34 2.23 0.31
mdyke 12.83 28.1 3.33 14.02 3.42 1.17 3.65 0.52 3.37 0.67 2 0.26 1.77 0.26
mdyke 6.81 16.49 2.32 11.65 3.42 1.11 4.12 0.67 4.41 0.89 2.59 0.37 2.39 0.36
a
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
of the K±Ar ages in this basal unit. Based on these radiometric ages we consider the deposition and deformation of the Juchatengo complex to be Early Permian or older. On the other hand, we suggest that the discrepancy between the oldest K±Ar hornblende age in the calc-alkaline stock intruding the sedimentary unit and the Middle Permian fossil determination is due to misidenti®cation of the fossil. Massive and pillow basalts from the middle unit of Juchatengo complex were sampled for analysis. They display subvariolitic texture, with amygdules and consist of plagioclase microlites partially altered to actinolite, epidote, chlorite and opaque minerals, set in a matrix of sodic plagioclase, actinolite, chlorite, epidote, magnetite and leucoxene.
formed following procedures described in Patchett and Ruiz (1987). Blanks were Nd < 300 pg. Duplicates are less than 20.2eNd units.
3. Analytical techniques Major and some trace (Rb, Sr, Ba, Zr, Nb, Y, Cr, Ni, Sc, V, Cu and Zn) elements from eighteen samples were analyzed by X-ray ¯uorescence at the Regional Geochemical Center, Saint Mary's University, using a Phillips PW-2400 spectrometer (Table 1). The reproducibility of these data is about 1±5%. In addition, ten samples were analyzed for rare earth elements (REE), Th, Ta, and Hf, by inductively coupled plasma-mass spectrometry (ICP-MS) at the Department of Earth Sciences, Memorial University of Newfoundland (Table 1a and b). Reproducibility is estimated to be better than 5%. A representative basalt sample, along with a siltstone sample from the overlying sediments, were selected for Sm±Nd isotopic analyses (Table 2). These analyses were carried out at the Department of Geosciences of the University of Arizona in Tucson. Whole-rock Sm±Nd isotopic analysis were performed on a VG-354 mass spectrometer. Analysis were per-
Fig. 3. Variations of TiO2(%), Cr (ppm) and Ni (ppm) vs Mg]=(100 Mg/Mg+Fe tot) in mol% for the ma®c metavolcanic rocks of the Juchatengo complex.
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
541
Fig. 6. Ti±Zr±Y diagram of Pearce and Cann (1971) for the ma®c metavolcanic rocks of the Juchatengo complex. Fig. 4. Cr vs Ti diagram of Pearce (1975) for the ma®c metavolcanic rocks of the Juchatengo complex.
4. Geochemistry The rocks were aected to varying degrees by secondary processes including low-grade metamorphism which modi®ed the chemical composition of these rocks. For example, most samples yielded elevated LOI values (Table 1). The concentrations of most major elements, high-®eld-strength elements (HFSE), rare earth elements and transition elements are, however, thought to re¯ect the primary magmatic distribution (Winchester and Floyd, 1977) and are used for consideration of the petrogenesis and tectonic setting of the metavolcanic rocks. The Juchatengo volcanics are ma®c rocks with <55% SiO2 (normalized to volatile-free) and a wide spread of Mg]=100 Mg/Mg+Fetot) in mole% values ranging between 0.63 and 0.35. The ma®c rocks display tholeiitic Fe- and Ti-enrichment trends with increasing dierentiation (Fig. 3) that preclude exten-
Fig. 5. Variations of V (ppm) vs Ti/1000 (ppm) for the ma®c meta volcanic rocks of the Juchatengo complex. The dividing line between Arc and MORB ®elds after Shervais (1982).
Fig. 7. Chondrite-normalized REE abundances of (a) pillow basalts and (b) ma®c dykes. Normalizing values after Sun (1982).
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J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
Fig. 8. MORB-normalized abundances in the (a) pillow basalts and (b) ma®c dykes of the Juchatengo complex. Normalizing values after Pearce (1983).
sive early fractionation of Fe±Ti oxides. Their TiO2 contents range between 1 and 3% (Fig. 3). The abundances of Cr and Ni decrease steeply with the decrease of the Mg] up to a value of 0.45 and then remain approximately constant (Fig. 3), consistent with fractionation of pyroxenes. The metavolcanic rocks have higher abundances of Ti for a given Cr content than oceanic island arc tholeiites (Fig. 4), and higher Ti/V ratios (20±50; Fig. 5). The major element composition of these rocks resembles that of rift-related basalts. On various tectonic discrimination diagrams such as that of Pearce and Cann (1971), the ma®c rocks plot into the within-plate basalts and MORB ®elds (Fig. 6). Transition elements in the rocks are highly variable, indicating that these rocks underwent fractional crystallization. The rocks have two slightly dierent types of the chondrite-normalized REE patterns although they are both relatively ¯at (Fig. 7). The predominant type is pillow basalts and is characterized by ¯at light REE (LREE) accompanied by slightly sloping heavy REE (HREE) with (La/Yb)n between 1.1 and 1.8 and (La/ Sm)n between 0.8 and 1.0. The second type is ma®c dykes that display a negative slope with (La/Yb)n around 2.3±2.5 and (La/Sm)n around 1.5. These dier-
ences suggest a slightly heterogeneous source, with a more evolved magma source for the dykes. The relatively shallow slope of HREE might imply that the magma was derived from a predominantly spinel-bearing source. The magma subsequently underwent extensive fractional crystallization. The MORB-normalized trace element pro®les of the rocks (Fig. 8) usually show an enrichment of incompatible elements compared to N-MORB and primitive oceanic island arc basalts. Although the Juchatengo volcanics are clearly tholeiitic, their trace elements suggest that they most likely are rift-related basalts, slightly contaminated by continental crust. A basalt sample yielded an eNdi value of +8.16 recalculated for 296 Ma, and a TDM age of 477 Ma. This is consistent with derivation of the basaltic magma by melting of a depleted mantle source. A consistently low Th/La ratio (<0.15) and high eNd value indicate that crustal contamination did not play a signi®cant role in its genesis. This eNd value is much higher than those obtained from igneous rocks of the Acatlan Complex (Fig. 9, values recalculated for 296 Ma) by YanÄez et al. (1991) suggesting that the Juchatengo unit did not form as part of the Acatlan Complex.The geochemistry of the basalts suggests a rift where magmas were generated in a man-
Table 2 Whole-rock Sm/Nd analysis from the Juchatengo Complex Rock type
Sample
Sm
Nd
147
Sm/144Nd
143
Nd/144Nd
TDM
eNd0
eNdi (296 Ma)
Basalt Shale
I-73 I-67
4.06 4.84
13.99 24.43
0.1755 0.1198
0.512936 0.512125
477 1487
5.63 ÿ10.01
8.16 ÿ8.95
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
543
Fig. 9. Initial eNd vs model age for the Juchatengo basalts and sediments. eNd and TDM for the Acatlan and Oaxaca complexes were taken from YanÄez et al. (1991), Patchett and Ruiz (1987), and Ruiz et al. (1988). All eNdi were recalculated for 296 Ma.
tle that was previously aected by subduction processes. Most likely this could be either in intra-continental or back-arc environments. A siltstone sample of the upper unit of the Juchatengo complex yielded an eNd value of ÿ8.95 recalculated for 296 Ma, and a TDM age of 1487 Ma (Table 2). These data suggest derivation from an ancient cratonic source similar to the Oaxacan Complex (Ruiz et al., 1988), and there is no indication of material derived from a contemporaneous volcanic source. 5. Tectonic implications The tholeiitic within-plate/MORB nature of the Juchatengo basalts suggests that they formed in a rift setting that may be either a continental rift or a backarc basin (Fig. 10). A location adjacent to the Oaxacan Complex is suggested by the Nd isotopic values of the overlying sediments, and the minor old crustal contamination shown by the basalts. This is consistent with the presence of Permian±Triassic calcalkaline plutons that stitch the Juchatengo-Oaxaca boundary (Grajales-Nishimura, 1988). Regional considerations indicate that subduction was occurring along the western margin of the Americas during Devonian and Carboniferous times. Therefore, backarc rifting is favored over intracontinental rifting. Given these constraints, the Juchatengo basalts could have formed near the eastern edge of a back-arc basin that developed over thinned continental crust: the contemporaneous arc would have lain to the west of this back-arc basin (Fig. 10). The age of this arc-back arc system is not well constrained, a problem requiring further geochronological work. However the regional tectonic scenario requires that the Juchatengo basalts
Fig. 10. Paleogeographic location of the Juchatengo terrane. (a) inferred subduction zone and back-arc rifting for pre-Early Permian time. (b) Pangea reconstruction for southern US, Middle America and northwestern South America showing the Permo-Triassic arc that developed after telescoping of the Juchatengo basin. A=Acatlan complex, O=Oaxaquia, J=Juchatengo terrane.
be older than the Permo-Triassic calc-alkaline plutonism. Eastward migration of the Permian±Triassic arc magmatism may re¯ect shallowing of the dip of the subduction zone, this could also have produced the deformation of the Juchatengo complex that preceded the 280 Ma plutonism (Fig. 10). Torres-Vargas et al. (1993) have proposed that the calc-alkaline plutonism belongs to a series of Permian±Triassic plutons intruded along the backbone of Mexico. They are a continuation of the arc magmatism in northern Mexico and adjacent US that was produced by subduction along the Paci®c margin of the Americas (Fig. 10). We conclude that during mid- to Late Paleozoic time in southern Mexico, an arc rifted away from the Oaxaquia margin forming a back-arc basin (Juchatengo complex). This was followed by telescoping of the basin and eastward migration of the arc, which produced a Permo-Triassic continental arc on the Juchatengo terrane and Oaxaquia (Fig. 10). Although the evolution of the Juchatengo complex seems to be similar to the evolution of the western
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J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
margin of US, further studies are needed to constrain its relationship with the geology of the North American Cordillera.
Acknowledgements The authors wish to acknowledge funding from CONACYT Project 0255P-T9506, Universidad Nacional Autonoma de Mexico (Project IN10195), the Instituto de Geologia, and the Natural Sciences and Engineering Council (NSERC) to JD. JMGN also thanks the Instituto Mexicano del PetroÂleo and the Department of Geosciences at the University of Arizona for economic support. Special thanks to J. Patchett and C. Isachsen (University of Arizona) for allowing and helping us to carry out the Nd analysis in their Isotope Laboratory. Thanks to Pablo PenÄa¯or for his help in the preparation of samples. Special thanks to J. W. Shervais and an anonymous reviewer for their constructive comments that helped to improve the manuscript. This is a contribution to IGCP project 376: Laurentia-Gondwana Connections Before Pangea.
References Campa-Uranga, M.F., Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resource distribution of Mexico. Can. J. Earth Sci. 20, 1040±1051. Grajales-Nishimura, J.M., 1988. Geology, geochronology, geochemistry and tectonic implications of the Juchatengo green rock sequence, State of Oaxaca, southern Mexico. M.Sc thesis, University of Arizona, Tucson, Arizona. U.S.A. Golonka, J., Ross, M.I., 1995. Phanerozoic paleogeographic and paleoclimatic modeling maps. In: Beauchamp, B., Embry, A., Glass, D. (Eds.), Pangea 17, Can. Soc. Petr. Geol. Mem 1±47. Keppie, J.D., Ortega-GutieÂrrez, F., 1995. Provenance of Mexican terranes: isotopic constraints. Int. Geol. Rev. 37, 813±824. Miller, E.L., Miller, M.M., Stevens, C.H., Wright, J.E., Madrid, R., 1992. Late Paleozoic paleogeographic and tectonic evolution of western U.S. Cordillera. In: Burch®el, B.C., Lipman, P.W., Zoback, M.L. (Eds.), The Cordilleran Orogen: conterminous U.S. The Geology of North America, G-3, 57±106. Geol. Soc. Am., USA. Mullerried, F.K.G., 1945. ContribucioÂn a la geologia de Mexico y noroeste de la America Central. Universidad nacional Autonoma de Mexico, Imprenta Universitaria, MeÂxico. Okulitch, A.V., 1995. Geological Time Chart. Geological Survey of Canada Open File 3040. Ortega-GutieÂrrez, F., 1993. Tectonostratigraphic analysis and signi®cance of the Paleozoic Acatlan Complex of southern Mexico. In: Ortega-GutieÂrrez, F., Centeno-Garcia, E., Moran-Zenteno, D.J., Gomez-Caballero, A. (Eds.), Terrane Geology of southern
Mexico. Universidad Nacional Autonoma de Mexico, Instituto de Geologia, MeÂxico, pp. 61±65 Guidebook of Field Trip B. Ortega-GutieÂrrez, F., Ruiz, J., Centeno-Garcia, E., 1995. Oaxaquia Ð a Phanerozoic microcontinent accreted to North America during the Late Paleozoic. Geology 23, 1127±1130. Patchett, P.J., Ruiz, J., 1987. Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico. Contrib. Mineral. Petrol. 96, 523±528. Pearce, J.A., 1975. Basalt geochemistry used to investigate past tectonic settings on Cyprus. Tectonophysics 25, 41±67. Pearce, J.A., 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. In: Hawkesworth, C.J., Norry, M.J. (Eds.), Continental basalts and mantle xenoliths. Shiva, Nantwich, pp. 230±249. Pearce, J.A., Cann, J.R., 1971. Ophiolite origin investigated by discriminant analysis using Ti, Zr and Y. Earth Planet. Sci. Lett. 12, 339±349. Ratschbacher, L., Riller, U., Meschede, M., Herrmann, U., Frisch, W., 1991. Second look at suspect terranes in southern Mexico. Geology 19, 1233±1236. Ruiz, J., Patchett, P.J., Ortega-GutieÂrrez, F., 1988. Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies. Geol. Soc. Am. Bull. 100, 274±281. Sedlock, R.L., Ortega-GutieÂrrez, F., Speed, R.C., 1993. Tectonostratigraphic terranes and tectonic evolution of Mexico. Geol. Soc. Am. Sp. Pap. 278, 153. Shervais, J.W., 1982. T-V plots and the petrogenesis of modern and ophiolitic lavas. Earth Planet. Sci. Lett. 59, 101±118. Stewart, J.H., Blodget, R.B., Boucot, A.J., Carter, J.L., 1993. Middle Paleozoic exotic terrane near Ciudad Victoria, northeastern Mexico, and the southern margin of Paleozoic North America. In: Ortega-GutieÂrrez, F., Centeno-Garcia, E., MoranZenteno, D.J., Gomez-Caballero, A. (Eds.), Proceedings of the 1st Circum-Paci®c and Circum-Atlantic Terrane Conference. Universidad Nacional Autonoma de Mexico, Instituto de Geologia, MeÂxico, pp. 147±149. Sun, S.S., 1982. Chemical composition and origin of the Earth's primitive mantle. Geochim. Cosmochim. Acta 46, 179±192. Tolson, G., Solis-Pichardo, G., Moran-Zenteno, D.J., VictoriaMorales, A., Hernandez-Trevino, T., 1993. Naturaleza petrogra®ca y estructural de las rocas cristalinas en la zona de contacto entre los terrenos Xolapa y Oaxaca, region de Santa Naria Huatulco, Oaxaca. In: Delgado-Argote, L.A., Martin-Barajas, A. (Eds.), Contribuciones a la tectonica del occidente de Mexico. Monogra®as de la Union Geo®sica Mexicana, MeÂxico. Torres-Vargas, R., Ruiz, J., Murillo-Muneton, G., GrajalesNishimura, J.M., 1993. The Paleozoic magmatism in Mexico: evidences for the shift from circum-Atlantic to circum-Paci®c tectonism. In: Ortega-GutieÂrrez, F., Centeno-Garcia, E., MoranZenteno, D.J., Gomez-Caballero, A. (Eds.), Proceedings of the 1st Circum-Paci®c and Circum-Atlantic Terrane Conference. Universidad Nacional Autonoma de Mexico, Instituto de Geologia, MeÂxico, pp. 154±155. Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of dierent magma series and their dierentiation products using immobile elements. Chem. Geol. 20, 325±343. YanÄez, P., Ruiz, J., Patchett, P.J., Ortega-GutieÂrrez, F., Gehrels, G., 1991. Isotopic studies of the Acatlan Complex, southern Mexico: Implications for Paleozoic North American Tectonics. Geol. Soc. Am. Bull. 103 (6), 817±828.
Geochemistry of Paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications J.M. Grajales-Nishimura a,*, E. Centeno-Garcia a, J.D. Keppie a, J. Dostal b a
Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, 04510, MeÂxico D.F., Mexico b Department of Geology, St. Mary's University, Halifax, Nova Scotia, Canada, B3H 3C3
Abstract Analyses of Lower Permian or older basalts and associated dykes of the Juchatengo sequence indicate that they are rift tholeiites that formed in a continental rift or back-arc tectonic setting. Age constraints include a Middle Permian fossil recovered from the tectonically overlying sediments and a cross-cutting, post-tectonic pluton dated by K/Ar on hornblende at 2822 6 Ma. A location adjacent to the Oaxacan Complex or other old continental crust is suggested by (1) an eNdi isotopic value of ÿ8.95 and a TDM age of 1487 Ma in the overlying sediments, which are similar to the Oaxacan Complex; (2) enrichment of incompatible elements in the lavas, suggesting old crustal contamination; and (3) the presence of Permian±Triassic calc-alkaline plutons that stitch the Juchatengo±Oaxaca boundary. The possible tectonic models depend on the age of the Juchatengo basalts, which requires future geochronological work. If the Juchatengo basalts are Permo-Carboniferous, they could have formed near the eastern edge of a back-arc basin: the contemporaneous arc would have rifted away to the west. Eastward migration of the arc magmatism indicated by the Permian±Triassic calc-alkaline plutonism may re¯ect shallowing of the dip of the subduction zone, which probably also produced the deformation of the Juchatengo sequence. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The tectonic evolution of the North American Cordillera in Middle to Late Devonian was characterized by marginal arc magmatism with development of back-arc rifting. This was followed by Early Mississippian shortening and accretion of an arc. During Pennsylvanian time, arc magmatism developed in the southern parts of the Cordillera. To the west of this arc sequence lay ophiolitic rocks that contain plagiogranite blocks and ma®c dike complexes with radiometric ages around 300 Ma (Miller et al., 1992). In this scenario, the evolution and chemical composition of the Paleozoic basalts and associated deep marine sedimentary rocks of the Juchatengo complex of southern Mexico are a critical test of this tectonic
model at the southern end of the North American Cordillera. The Juchatengo complex was accreted to the Oaxaca Complex prior to intrusions of granitoids that yield Early Permian K/Ar radiometric ages. The Permian in the Americas represents a transitional period following the amalgamation of Pangea and preceding its breakup in the Late Triassic. Most authors infer that there was subduction beneath the western margin of the Americas (e.g. Golonka and Ross, 1995) expressed as arc magmatism that extends from southwestern US, through the backbone of Mexico as isolated plutons, and into the northern Andes (TorresVargas et al., 1993). Early Permian granitoids of the Juchatengo area may correspond to this arc magmatism. 2. Geological setting
* Corresponding author. E-mail address: [email protected] Nishimura).
(J.M.
Grajales-
Mexico has been divided into several tectonic regimes (Fig. 1). The southern edge of cratonic North
0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 9 9 ) 0 0 0 3 7 - 1
538
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
Fig. 1. Tectonic subdivision of Mexico and location of the Juchatengo terrane.
America, lying near the US±Mexican border, is bounded by the Ouachita Orogen (Stewart et al., 1993), followed southward by a variety of terranes (from east to west) (Fig. 1): (1) the Neoproterozoic
Maya Terrane; (2) the Early Mesozoic Juarez Terrane; (3) Oaxaquia (ca 1 Ga); (4) the Upper Paleozoic or older Juchatengo Terrane; (5) the Lower Paleozoic Mixteco Terrane; (6) the Mesozoic Guerrero terrane; and (7) the Mesozoic-Tertiary Xolapa Terrane (Campa-Uranga and Coney, 1983; Sedlock et al., 1993; Ortega-GutieÂrrez et al., 1995; Keppie and OrtegaGutieÂrrez, 1995). Some authors have correlated the Juchatengo and Mixteco terranes but their age constraints are poorly known. The Mixteco Terrane consists of Lower Paleozoic ophiolites that have been metamorphosed up to eclogite facies and thrust over trench-®ll and ¯ysch deposits before intrusion of a Devonian syntectonic granitoid (Ortega-GutieÂrrez, 1993). The Mixteco Terrane was accreted to Oaxaquia during the Devonian. Subsequently, the MixtecoOaxaquia-Maya collided with Laurentia in the Late Paleozoic during the amalgamation of Pangea. The boundary between Oaxaquia and the Juchatengo terrane is not exposed; however, the Juchatengo Terrane is inferred to have accreted to the southern margin of Oaxaquia by the Permian because both terranes are
Fig. 2. Geological map of the Juchatengo area.
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
539
Table 1 Major element analyses of basalt and associated basic dykes samples from the Juchatengo Complexa Sample
I-73
I-74
I-75
I-76
I-77
I-78
I-79
I-80
I-81
I-82
QUI-6A
QUI-15
QUI-17A
Rock type SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total LOI Mg]
pbas 53.44 1.98 14.67 9.89 0.15 4.26 7.69 5.49 0.14 0.21 97.92 1.5 0.43
pbas 53.3 1.91 14.18 10.05 0.14 3.59 9.1 4.98 0.19 0.19 97.63 2.7 0.38
pbas 50.21 2.16 14.68 11.41 0.15 3.48 11.65 3.5 0.09 0.2 97.53 2.1 0.35
pbas 48.55 2.41 15.13 12.67 0.16 4.37 7.4 4.87 0.13 0.22 95.91 2.5 0.38
pbas 51.27 1.95 13.82 10.46 0.14 3.17 9.27 4.37 0.1 0.18 94.72 4.6 0.35
mdyke 48.35 2.15 14.74 10.23 0.23 5.65 9.1 3.6 0.4 0.56 95.01 4.3 0.5
pbas 45.7 3.08 14.61 12.89 0.23 5.53 8.55 3.19 0.19 0.38 94.36 4.7 0.43
mlava 46.95 1.25 15.43 9.17 0.14 8.48 8.83 3.29 0.55 0.12 94.21 5.3 0.62
mlava 48.06 1.37 15.44 8.71 0.12 8.27 8.41 3.61 0.6 0.14 94.73 4.8 0.63
mlava 44.78 1.14 14.6 9.07 0.16 7.16 10.08 3.77 0.46 0.13 91.35 7.9 0.58
mdyke 48.47 1 18.15 8.71 0.16 5.87 9.96 2.13 0.72 0.14 95.31 ± 0.54
mdyke 49.78 1.34 13.1 9.74 0.2 6.73 12.01 2.77 0.11 0.2 95.98 ± 0.55
mdyke 54.76 0.68 15.17 7.91 0.16 4.52 12.63 0.92 0.01 0.11 96.88 ± 0.5
a
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
intruded by a suite of Permian±Triassic calc-alkaline plutons (Grajales-Nishimura, 1988). The WNW-trending Xolapa Terrane truncates the N-trending Juchatengo Terrane along a major sinistral Late Mesozoic-Tertiary fault zone (Ratschbacher et al., 1991; Tolson et al., 1993). The Juchatengo Terrane consists of a sequence of rocks (Fig. 2) (from west to east: inferred base to top) (Grajales-Nishimura, 1988): (1) gabbroic and plagiogranitic dykes and stocks which have yielded K±Ar hornblende ages of 289 2 6 Ma, 2342 5 Ma, 225 25 Ma, and 219 2 5 Ma, i.e., Early Permian±Late Triassic (Time Scale of Okulitch, 1995). The younger ages were recorded in altered hornblende separates involving replacement by chlorite, epidote and actinolite; (2) basalts and pyroclastic rocks; and (3) interbedded
black shale, chert, siltstone, sandstone and minor limestone from which a cephalopod (ammonite?) of possible Middle Permian(?) age was recovered (Mullerried, 1945). These rocks are locally intensely deformed, striking N±S to NW±SE with moderately to steep dips, and most boundaries are tectonic. This sequence and the Oaxacan Complex were subsequently intruded by calc-alkaline stocks and a batholith composed of quartz diorite and tonalite dykes that have yielded K±Ar hornblende ages of 282 2 6 Ma, 251 2 6 Ma, and 238 25 Ma, and a K±Ar biotite age of 2782 6 Ma (Grajales-Nishimura, 1988): the younger ages are from localities closer to Tertiary plutons. It is perhaps signi®cant that these ages are similar to those recorded in the basal unit of the Juchatengo Terrane suggesting thermal re-equilibration
Table 1a Trace element analyses of basalt and associated basic dykes samples from the Juchatengo Complexa Sample
I-73
I-74
I-75
I-76
I-77
I-78
I-79
I-80
I-81
I-82
QUI-6A
QUI-15
QUI-17A
Rock type Cr Ni Co V Pb Zn Rb Ba Sr Ga Ta Nb Hf Zr Y Th
pbas 41 33 24 317 15 51 4 45 172 ± 0.28 6.2 2.95 125 27 1.1
pbas 56 37 20 316 ± 43 3 50 280 ±
pbas 61 23 18 422 ± 36 3 29 411 ± 0.25 5.1 2.1 89 24 0.8
pbas 57 37 25 406 ± 66 4 23 174 ±
pbas 50 33 32 369 ± 49 5 38 179 ± 0.22 4.7 4.17 79 20 0.65
mdyke 93 49 23 250 7 69 8 293 255 ± 0.43 7.6 1.84 140 37 1.42
pbas 63 40 33 403 ± 110 6 115 251 ± 0.83 14.6 1.73 200 32 1.76
mlava 334 119 50 269 30 59 16 145 184 ±
mlava 324 105 48 282 23 55 12 197 230 ± 0.16 3.3 2.16 69 20 0.42
mlava 328 117 43 255 18 75 13 137 140 ± 0.18 3.5 1.73 70 21 0.76
mdyke 89 32 22 258 ± 61 15 450 450 16 0.21 4.2 2.07 78 17 3.67
mdyke 248 79 24 298 ± 73 ± ± 266 17 0.22 4.2 2.16 98 23 0.71
mdyke 74 24 23 230 ± 56 ± ± 375 16 ± ±
a
5 2.09 103 26
4 3.27 80 23
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
1 2.07 52 16
54 16
540
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
Table 1b Rare Earth element analyses of basalt and associated ma®c dykes samples from the Juchatengo Complexa Sample
I-73
I-75
I-77
I-78
I-79
I-81
I-82
QUI-6A
QUI-15
Rock type La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
pbas 7.19 18.73 2.86 14.22 4.07 1.55 4.92 0.82 5.34 1.14 3.29 0.45 3.12 0.46
pbas 5.96 15.63 2.36 11.9 3.42 1.41 4.32 0.7 4.54 0.98 2.82 0.38 2.34 0.35
pbas 5.1 13.49 2.1 10.18 3.04 1.19 3.84 0.61 3.94 0.86 2.51 0.35 2.2 0.33
mdyke 10.27 27.21 4.05 21.18 5.95 2.24 7.44 1.15 7.06 1.45 3.99 0.55 3.58 0.53
pbas 14.43 36.03 4.9 22.65 6.08 1.94 6.71 1.06 6.43 1.32 3.7 0.52 3.47 0.47
mlava 4.45 11.74 1.78 9.4 2.94 1.05 3.7 0.6 3.98 0.81 2.26 0.33 2.32 0.33
mlava 8.5 21.19 2.41 10.18 2.68 1 3.33 0.57 3.72 0.82 2.37 0.34 2.23 0.31
mdyke 12.83 28.1 3.33 14.02 3.42 1.17 3.65 0.52 3.37 0.67 2 0.26 1.77 0.26
mdyke 6.81 16.49 2.32 11.65 3.42 1.11 4.12 0.67 4.41 0.89 2.59 0.37 2.39 0.36
a
Rock type: pbas=pillowed basalts, mlava=massive lavas, mdyke=ma®c dyke.
of the K±Ar ages in this basal unit. Based on these radiometric ages we consider the deposition and deformation of the Juchatengo complex to be Early Permian or older. On the other hand, we suggest that the discrepancy between the oldest K±Ar hornblende age in the calc-alkaline stock intruding the sedimentary unit and the Middle Permian fossil determination is due to misidenti®cation of the fossil. Massive and pillow basalts from the middle unit of Juchatengo complex were sampled for analysis. They display subvariolitic texture, with amygdules and consist of plagioclase microlites partially altered to actinolite, epidote, chlorite and opaque minerals, set in a matrix of sodic plagioclase, actinolite, chlorite, epidote, magnetite and leucoxene.
formed following procedures described in Patchett and Ruiz (1987). Blanks were Nd < 300 pg. Duplicates are less than 20.2eNd units.
3. Analytical techniques Major and some trace (Rb, Sr, Ba, Zr, Nb, Y, Cr, Ni, Sc, V, Cu and Zn) elements from eighteen samples were analyzed by X-ray ¯uorescence at the Regional Geochemical Center, Saint Mary's University, using a Phillips PW-2400 spectrometer (Table 1). The reproducibility of these data is about 1±5%. In addition, ten samples were analyzed for rare earth elements (REE), Th, Ta, and Hf, by inductively coupled plasma-mass spectrometry (ICP-MS) at the Department of Earth Sciences, Memorial University of Newfoundland (Table 1a and b). Reproducibility is estimated to be better than 5%. A representative basalt sample, along with a siltstone sample from the overlying sediments, were selected for Sm±Nd isotopic analyses (Table 2). These analyses were carried out at the Department of Geosciences of the University of Arizona in Tucson. Whole-rock Sm±Nd isotopic analysis were performed on a VG-354 mass spectrometer. Analysis were per-
Fig. 3. Variations of TiO2(%), Cr (ppm) and Ni (ppm) vs Mg]=(100 Mg/Mg+Fe tot) in mol% for the ma®c metavolcanic rocks of the Juchatengo complex.
J.M. Grajales-Nishimura et al. / Journal of South American Earth Sciences 12 (1999) 537±544
541
Fig. 6. Ti±Zr±Y diagram of Pearce and Cann (1971) for the ma®c metavolcanic rocks of the Juchatengo complex. Fig. 4. Cr vs Ti diagram of Pearce (1975) for the ma®c metavolcanic rocks of the Juchatengo complex.
4. Geochemistry The rocks were aected to varying degrees by secondary processes including low-grade metamorphism which modi®ed the chemical composition of these rocks. For example, most samples yielded elevated LOI values (Table 1). The concentrations of most major elements, high-®eld-strength elements (HFSE), rare earth elements and transition elements are, however, thought to re¯ect the primary magmatic distribution (Winchester and Floyd, 1977) and are used for consideration of the petrogenesis and tectonic setting of the metavolcanic rocks. The Juchatengo volcanics are ma®c rocks with <55% SiO2 (normalized to volatile-free) and a wide spread of Mg]=100 Mg/Mg+Fetot) in mole% values ranging between 0.63 and 0.35. The ma®c rocks display tholeiitic Fe- and Ti-enrichment trends with increasing dierentiation (Fig. 3) that preclude exten-
Fig. 5. Variations of V (ppm) vs Ti/1000 (ppm) for the ma®c meta volcanic rocks of the Juchatengo complex. The dividing line between Arc and MORB ®elds after Shervais (1982).
Fig. 7. Chondrite-normalized REE abundances of (a) pillow basalts and (b) ma®c dykes. Normalizing values after Sun (1982).
542
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Fig. 8. MORB-normalized abundances in the (a) pillow basalts and (b) ma®c dykes of the Juchatengo complex. Normalizing values after Pearce (1983).
sive early fractionation of Fe±Ti oxides. Their TiO2 contents range between 1 and 3% (Fig. 3). The abundances of Cr and Ni decrease steeply with the decrease of the Mg] up to a value of 0.45 and then remain approximately constant (Fig. 3), consistent with fractionation of pyroxenes. The metavolcanic rocks have higher abundances of Ti for a given Cr content than oceanic island arc tholeiites (Fig. 4), and higher Ti/V ratios (20±50; Fig. 5). The major element composition of these rocks resembles that of rift-related basalts. On various tectonic discrimination diagrams such as that of Pearce and Cann (1971), the ma®c rocks plot into the within-plate basalts and MORB ®elds (Fig. 6). Transition elements in the rocks are highly variable, indicating that these rocks underwent fractional crystallization. The rocks have two slightly dierent types of the chondrite-normalized REE patterns although they are both relatively ¯at (Fig. 7). The predominant type is pillow basalts and is characterized by ¯at light REE (LREE) accompanied by slightly sloping heavy REE (HREE) with (La/Yb)n between 1.1 and 1.8 and (La/ Sm)n between 0.8 and 1.0. The second type is ma®c dykes that display a negative slope with (La/Yb)n around 2.3±2.5 and (La/Sm)n around 1.5. These dier-
ences suggest a slightly heterogeneous source, with a more evolved magma source for the dykes. The relatively shallow slope of HREE might imply that the magma was derived from a predominantly spinel-bearing source. The magma subsequently underwent extensive fractional crystallization. The MORB-normalized trace element pro®les of the rocks (Fig. 8) usually show an enrichment of incompatible elements compared to N-MORB and primitive oceanic island arc basalts. Although the Juchatengo volcanics are clearly tholeiitic, their trace elements suggest that they most likely are rift-related basalts, slightly contaminated by continental crust. A basalt sample yielded an eNdi value of +8.16 recalculated for 296 Ma, and a TDM age of 477 Ma. This is consistent with derivation of the basaltic magma by melting of a depleted mantle source. A consistently low Th/La ratio (<0.15) and high eNd value indicate that crustal contamination did not play a signi®cant role in its genesis. This eNd value is much higher than those obtained from igneous rocks of the Acatlan Complex (Fig. 9, values recalculated for 296 Ma) by YanÄez et al. (1991) suggesting that the Juchatengo unit did not form as part of the Acatlan Complex.The geochemistry of the basalts suggests a rift where magmas were generated in a man-
Table 2 Whole-rock Sm/Nd analysis from the Juchatengo Complex Rock type
Sample
Sm
Nd
147
Sm/144Nd
143
Nd/144Nd
TDM
eNd0
eNdi (296 Ma)
Basalt Shale
I-73 I-67
4.06 4.84
13.99 24.43
0.1755 0.1198
0.512936 0.512125
477 1487
5.63 ÿ10.01
8.16 ÿ8.95
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543
Fig. 9. Initial eNd vs model age for the Juchatengo basalts and sediments. eNd and TDM for the Acatlan and Oaxaca complexes were taken from YanÄez et al. (1991), Patchett and Ruiz (1987), and Ruiz et al. (1988). All eNdi were recalculated for 296 Ma.
tle that was previously aected by subduction processes. Most likely this could be either in intra-continental or back-arc environments. A siltstone sample of the upper unit of the Juchatengo complex yielded an eNd value of ÿ8.95 recalculated for 296 Ma, and a TDM age of 1487 Ma (Table 2). These data suggest derivation from an ancient cratonic source similar to the Oaxacan Complex (Ruiz et al., 1988), and there is no indication of material derived from a contemporaneous volcanic source. 5. Tectonic implications The tholeiitic within-plate/MORB nature of the Juchatengo basalts suggests that they formed in a rift setting that may be either a continental rift or a backarc basin (Fig. 10). A location adjacent to the Oaxacan Complex is suggested by the Nd isotopic values of the overlying sediments, and the minor old crustal contamination shown by the basalts. This is consistent with the presence of Permian±Triassic calcalkaline plutons that stitch the Juchatengo-Oaxaca boundary (Grajales-Nishimura, 1988). Regional considerations indicate that subduction was occurring along the western margin of the Americas during Devonian and Carboniferous times. Therefore, backarc rifting is favored over intracontinental rifting. Given these constraints, the Juchatengo basalts could have formed near the eastern edge of a back-arc basin that developed over thinned continental crust: the contemporaneous arc would have lain to the west of this back-arc basin (Fig. 10). The age of this arc-back arc system is not well constrained, a problem requiring further geochronological work. However the regional tectonic scenario requires that the Juchatengo basalts
Fig. 10. Paleogeographic location of the Juchatengo terrane. (a) inferred subduction zone and back-arc rifting for pre-Early Permian time. (b) Pangea reconstruction for southern US, Middle America and northwestern South America showing the Permo-Triassic arc that developed after telescoping of the Juchatengo basin. A=Acatlan complex, O=Oaxaquia, J=Juchatengo terrane.
be older than the Permo-Triassic calc-alkaline plutonism. Eastward migration of the Permian±Triassic arc magmatism may re¯ect shallowing of the dip of the subduction zone, this could also have produced the deformation of the Juchatengo complex that preceded the 280 Ma plutonism (Fig. 10). Torres-Vargas et al. (1993) have proposed that the calc-alkaline plutonism belongs to a series of Permian±Triassic plutons intruded along the backbone of Mexico. They are a continuation of the arc magmatism in northern Mexico and adjacent US that was produced by subduction along the Paci®c margin of the Americas (Fig. 10). We conclude that during mid- to Late Paleozoic time in southern Mexico, an arc rifted away from the Oaxaquia margin forming a back-arc basin (Juchatengo complex). This was followed by telescoping of the basin and eastward migration of the arc, which produced a Permo-Triassic continental arc on the Juchatengo terrane and Oaxaquia (Fig. 10). Although the evolution of the Juchatengo complex seems to be similar to the evolution of the western
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margin of US, further studies are needed to constrain its relationship with the geology of the North American Cordillera.
Acknowledgements The authors wish to acknowledge funding from CONACYT Project 0255P-T9506, Universidad Nacional Autonoma de Mexico (Project IN10195), the Instituto de Geologia, and the Natural Sciences and Engineering Council (NSERC) to JD. JMGN also thanks the Instituto Mexicano del PetroÂleo and the Department of Geosciences at the University of Arizona for economic support. Special thanks to J. Patchett and C. Isachsen (University of Arizona) for allowing and helping us to carry out the Nd analysis in their Isotope Laboratory. Thanks to Pablo PenÄa¯or for his help in the preparation of samples. Special thanks to J. W. Shervais and an anonymous reviewer for their constructive comments that helped to improve the manuscript. This is a contribution to IGCP project 376: Laurentia-Gondwana Connections Before Pangea.
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