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Revista mexicana de ciencias geológicas

versión On-line ISSN 2007-2902

Rev. mex. cienc. geol vol.27 no.2 México ago. 2010

 

Geochemistry of Carboniferous low metamorphic grade sedimentary and tholeiitic igneous rocks in the western Acatlán complex, southern Mexico: deposition along the active western margin of Pangea

 

Geoquímica de rocas carboníferas sedimentarias e ígneas tholeíticas de bajo grado metamórfico de la parte occidental del complejo Acatlán, sur de México: depósito a lo largo del margen occidental activo de Pangea

 

Carlos Ortega–Obregón1, J. Duncan Keppie1,*, J. Brendan Murphy2

 

1 Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico. * duncan@servidor.unam.mx

2 Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5a, Canada.

 

Manuscript received: November 6, 2009.
Corrected manuscript received: March 11, 2010.
Manuscript accepted: March 23, 2010.

 

ABSTRACT

Low grade metasedimentary rocks in the western Acatlán complex (Olinalá area) of southern Mexico occur in two units termed here the Progreso and Zumpango units of Lower and middle Carboniferous ages, respectively. The Zumpango Unit is dominated by phyllite and quartzites, but the Progreso Unit is more highly deformed and is dominated by quartzite that is locally interbedded with pillow basalt, and intruded by a suite of mafic dikes. Geochemical and Sm–Nd isotopic data indicate that the basalts and the mafic dikes are tholeiitic, have N–MORB affinities and are moderately fractionated. Deposition of the Progreso Unit was coeval with Carboniferous extrusion of high metamorphic grade rocks of the Acatlán complex above of an active subduction zone, and deposition of Mississippian rocks in the adjacent Oaxaquia terrane. The tholeiitic affinity of mafic rocks, suggests extension on this active margin. The Mid–Continent (USA) affinities of the fauna in the latter rocks suggest that Pangea had already amalgamated and that these events took place on the western margin of Pangea.

Key words: geochemistry, active margin, Carboniferous, Pangea, Acatlán complex, Mexico.

 

RESUMEN

Las rocas metasedimentarias de bajo grado en el complejo Acatlán occidental (área de Olinalá) del Sur de México, ocurren en dos unidades diferentes de cuarcitas y filitas denominadas aquí como Progreso y Zumpango de edad Carbonífero Inferior y medio, respectivamente. La Unidad Zumpango está caracterizada porfilitas y cuarcitas, mientras que la Unidad Progreso, está formada principalmente por cuarcitas, localmente interestratificadas con basaltos almohadillados e intrusionadas por diques máficos, toleiíticos con afinidades N–MORB y fraccionamiento moderado. El depósito de la unidad Progreso fue contemporáneo con la extrusión de las rocas metamórficas de alto grado del complejo Acatlán en una zona de subducción activa y con el depósito de rocas misisípicas en el margen del terreno Oaxaqueño adyacente. La afinidad toleiítica de las rocas máficas sugiere extensión en esta margen activa. La afinidad meso–continental (EE.UU.) de la fauna en estas últimas sugiere que Pangea estaba ya amalgamada y que estos eventos tuvieron lugar en la margen occidental de Pangea.

Palabras clave: geoquímica, margen activo, Carbonífero, Pangea, Acatlán complex, México.

 

INTRODUCTION

The Acatlán complex, synonymous with the Mixteca terrane, underlies much of southern Mexico and its evolution provides important constraints for Late Paleozoic continental reconstructions along the western edge of Pangea (e.g., Ortega–Gutiérrez et al., 1999; Keppie and Ramos, 1999; Talavera–Mendoza et al., 2005; Nance et al., 2006, 2007; Vega–Granillo et al., 2007; Keppie et al., 2008a). Determination of the depositional setting of low–grade psammitic–pelitic and mafic rocks (originally assigned to the Cosoltepec Formation; Ortega–Gutiérrez et al., 1999), which underlie ca. 60% of the Acatlán complex of southern Mexico is critical to the understanding of this evolution. Recent dating of detrital zircons in several samples from rocks assigned to this formation has shown it to be composite with both Ordovician and Devonian–Carboniferous units being present (Talavera–Mendoza et al., 2005; Keppie et al., 2006, 2008a; Ortega–Obregón et al., 2009).

Although a rift–passive margin setting has been determined for the Ordovician units (Keppie et al., 2006, 2008b; Ortega–Obregón et al., 2009; Morales–Gámez et al., 2009; Dostal and Keppie, 2009), the depositional setting of these rocks is still unclear and various models have been proposed. Talavera–Mendoza et al. (2005) have suggested that the Devonian–Carboniferous rocks were deposited along the passive margin of Gondwana (Amazonia–Oaxaquia). However, Keppie et al. (2008a) and Morales–Gámez et al. (2009) infer deposition in an active margin along the western side of Pangea. In an attempt to provide additional constraints, we present geochemical data for Carboniferous rocks in the western part of the Acatlán complex associated with clastic detritus derived mainly from the adjacent Oaxacan complex, Maya block and high grade metamorphic units of Acatlán complex (Ortega–Obregón et al., 2009). Penetrative deformation synchronous with deposition suggests that these Carboniferous rocks were deposited along an active margin (Keppie et al., 2008a, 2008b; Ortega–Obregón et al., 2009). Given that the Rheic Ocean had all but closed by the beginning of the Carboniferous (Matte, 2001; Winchester et al., 2002; Keppie et al., 2008b), the active margin is inferred to have been located along the western margin of Pangea.

 

GEOLOGICAL SETTING

The Acatlán complex is tectonically juxtaposed on its eastern, southern and western sides against the ca. 1 Ga Oaxacan and Mesozoic Xolapa complexes, and Cretaceous carbonates, respectively (Figure 1a). To the north, the complex is unconformably overlain by uppermost Devonian–Lower Permian rocks of the Patlanoya Group (Vachard et al., 2000; Vachard and Flores–de Dios, 2002) and by Cenozoic rocks of the Trans–Mexican Volcanic Belt. As the geology (Keppie et al., 2008a and references therein) and detailed mapping supported by geochronology (Ortega–Obregón et al., 2009) of the Olinalá area in the western part of the Acatlán complex has been recently published, we present only a summary of the western Acatlán complex geological record here.

1. Ediacaran to Lower–Middle Ordovician deposition of the Zacango Unit clastic rocks.

2. Ordovician intrusion into the Zacango Unit of ca. 465 Ma bimodal granitoids and amphibolites.

3. Lower–Middle Mississippian deposition of clastic rocks associated with mafic lavas and minor intrusions (Progreso Unit) that partially overlaps with latest Devonian–Middle Mississippian metamorphism, deformation and uplift/cooling through ca. 400 °C.

4. Upper Mississippian (–Lower Permian) deposition of clastic rocks and felsic ashes (Zumpango Unit).

5. Lower Permian low–grade deformation.

6. Middle–Upper Permian deposition of the Olinalá Formation, which consists of clastic units at the base that grade upward into calcareous clastic rocks.

7. Intrusion of a ca. 61 Ma diorite (lower Paleocene).

This paper focuses on the Carboniferous Progreso and Zumpango units, previously mapped as part of the Cosoltopec Formation and Tecomate Formation (Ortega–Gutiérrez et al., 1999, Ramírez–Espinoza, 2001, respectively). The Carboniferous Progreso Unit occurs in the Southern and Naranjo fault blocks (Figure 2). Deposition of this unit is bracketed by 403 ± 7 Ma mean of three zircons and by a 335 ± 2 Ma 40Ar/39Ar age on metamorphic muscovite (Ortega–Obregón et al., 2009). The Progreso Unit consists of pelitic and psammitic rocks interbedded with pillow lavas (Naranjo block) and intruded by mafic dikes (Southern block) that underwent greenschist facies metamorphism and polyphase deformation during the Carboniferous and Permian (Ortega–Obregón et al., 2009). Despite the different age populations of zircons from these blocks (Naranjo and Progreso; Ortega–Obregón, et al., 2009), the similar lithology, style of deformation and grade of metamorphism suggest contemporaneous deposition and tectonic evolution. In thin section, the psammitic–pelitic rocks exhibit a spaced foliation defined by microlithons of stretched quartz and fine grained white mica. The pillow basalts have hyalopilitic textures with very fine grained plagioclase laths, accessory opaque minerals, and chlorite microphenocrysts inferred to represent pseudomorphs after pyroxene, set in a greenish, extensively altered glassy matrix. The mineralogy indicates lower greenschist facies metamorphism.

The Progreso Unit is very similar in lithology, structure, and metamorphic grade to the Coatlaco Unit, which crops out just to the southeast of Olinalá and consists of low–grade psammites, pelites and pillow basalts (Grodzicki et al., 2008). The age of the the Coatlaco Unit post–dates its youngest population of detrital zircons, dated at 357 ± 35 Ma (Grodzicki et al., 2008). The close lithological similarity between the Progreso and Coatlaco units suggests that they are correlatives, implying that their deposition probably post–dates the Devonian.

The Zumpango Unit occurs in two fault blocks and consists of sub–greenschist facies clastic rocks associated with felsic ash and minor mafic volcanic rocks (Figure 2). A volcanic epiclastic rock (felsic ash) within the Zumpango Unit yielded a single concordant zircon with an age of 327 ± 2 Ma (upper Mississippian: Ortega–Obregón et al., 2009). A maximum age for the clastic rocks deposited prior to this ash is partly constrained by the youngest detrital zircon in a psammite (348 ± 3 Ma) or the ca. 350–385 Ma of four crystals and the 338 ± 2 Ma plateau age yielded by muscovite inferred as detrital (Ortega–Obregón et al., 2009). No mafic rocks were found interbedded or intruding these metasedimentary rocks. The Zumpango Unit was deformed prior to deposition of the unconformably overlying Middle–Upper Permian Olinalá Formation (Vachard et al., 2000).

 

ANALYTICAL METHODS

In order to determine the tectonic setting of the Progreso Unit, 15 samples of the pillow basalts from the Naranjo block and one sample of a mafic dike from the Southern block (Figure 2) were analyzed for major and trace elements including rare earth elements (REE) along with four samples of the psammites and pelites. In addition, ten samples of metavolcanic felsic ashes and quartzites from the Zumpango Unit were also analyzed. The data are shown in Table 1.

Major and selected trace elements (Rb, Sr, Ga, Co, Cu, Pb, Zn, V, Cr and Ni) were determined by X–ray fluorescence spectrometry at the Nova Scotia Regional Geochemical Centre at Saint Mary's University, Nova Scotia using a Philips PW2400 X–ray spectrometer. Analytical procedures, precision and accuracy for the X–ray data are described by Dostal et al. (1994). Rare earth and other trace elements (Y, Zr, Nb, Cs, Ba, Hf, Th, U and Ta) and Sm–Nd isotopic analyses were determined at Memorial University, Newfoundland by Inductively Coupled Plasma–Mass Spectrometry (ICP–MS) and Isotope Dilution–Thermal Ionization Mass Spectrometry (ID–TIMS), respectively. Analytical methods are described in Jenner et al. (1990). Details of all geochemical analyses and analytical method are given in Table 1 (see footnote).

The Sm–Nd isotopic data from six samples of pillow basalts from the Naranjo block, as well as one sample of metasedimentary quartzite from the Progreso Unit and four samples of the Zumpango Unit are shown in Table 2. Details of the analytical procedures for the Sm–Nd isotopic analyses are described by Kerr et al. (1995). La Jolla Nd standard gave an average value of 0.511860. εNd parameters were calculated relative to 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.196593 for present day CHUR (Jacobsen and Wasserburg, 1980) and a decay constant 1 147Sm = 6.54 x 10–12/year. Nd Model ages [T(DM)] were calculated using the model of DePaolo (1981, 1988). εNd values are calculated for t = 330 Ma, which represents the probable extrusive age of mafic magmatic rocks.

 

GEOCHEMICAL AND ISOTOPIC RESULTS

Metasedimentary rocks

Results from the low grade metasedimentary rocks of the Progreso (including those from El Naranjo block; Figure 2) and Zumpango units were plotted together in order to assess their respective provenance. Both units show considerable variation in chemical composition, exemplified by the range in SiO2 from ca. 57.3 to 82.6 wt. % and ca. 70.2 to 92.1 wt.%, respectively (on a volatile free basis). Sample N2 from the Naranjo block, which has the lowest SiO2 value (ca. 57.3 wt%), was not plotted on geochemical diagrams due to its high lost of ignition (LOI) value (ca. 14.1 wt%; Table 1). MgO, TiO2 and Cr contents in Zumpango samples display negative correlations with SiO2 (Figure 3a, 3c and 3d). Although some samples from Zumpango Unit are detrital sedimentary rocks, this negative correlation could be inherited from a fractionated igneous felsic rock source. Positive correlations between some trace element ratios (e.g., Ti/Y and Ti/V vs. Zr/Y and Zr/V; Figures 3e and 3f) suggest a contribution of minor phases (e.g., zircon, magnetite and titanite) to the detritus. This suggests an important contribution from calcalkaline granitoid rocks (e.g., Murphy et al., 2005).

Chondrite–normalized REE patterns for both metasedimentary units display moderate LREE enrichment relative to HREE with chondrite normalized (La/Sm)N varying from 4.8 and 6.3, (La/Yb)N varying from 11.2 to 14.1, and flat to slightly depleted HREE patterns (Figure 4a). These samples also display a minor negative Eu anomaly (EuN/Eu* ranges from 0.59 to 0.78). These features reflect the average of detrital contributions from the source area (e.g., Murphy et al., 1996). With the exception of low Sr, Nb and Ta, the REE and trace element abundances are similar to typical upper continental crust (Figure 4b). Negative anomalies in Nb and Ta may reflect source rock depletion (i. e., derivation from arc–related rocks) whereas Sr negative anomalies may reflect weathering of feldspar.

Several geochemical features indicate that the clastic rocks were predominantly derived from felsic to intermediate source rocks: i) the enrichment in incompatible (e.g., Th) relative to compatible trace elements (e.g., Cr) is indicated by low Cr/Th (ranging from 0.3 to 5.3), values typical of a dominant felsic mixed with mafic source (Cullers, 1994); ii) the La/Th versus Hf diagram, where most rocks plot close to the field of the acid arc source (Figure 5; after Floyd and Leveridge, 1987); iii) the high Al2O3/TiO2 values of six of the ten Zumpango Unit samples (>20) also suggest derivation from predominantly felsic sources (e.g., Girty et al. , 1996); and; iv) the relatively low contents of Fe2O3and MgO in some samples (Table 1). The negative Nb and Ta anomalies evident in Figure 3b, suggest that these felsic rock sources were produced in a magmatic arc.

All metasedimentary samples have relatively homogenous Sm–Nd isotopic characteristics, with εNd (t= 330 Ma) of –6 in Progreso quartzite and a range from –7.5 to –8.4 in the Zumpango Unit samples. Ratios of 147Sm/144Nd of all samples range from 0.1156 to 0.1242. Depleted mantle model age (TDM, DePaolo, 1981) of the Progreso Unit quartzite is 1.7 Ga and range from 1.66 to 1.82 Ga in the Zumpango Unit samples (Table 2).

Progreso Unit mafic rocks

Despite the well preserved shapes of the Progreso pillow basalts, secondary processes (such as alteration and metamorphism) have affected the primary concentrations of many major and some trace elements. The effects of alteration are indicated by the high loss on ignition (LOI) in several samples (e.g., OL36–1, OL36–2 and OL500–M1), and by the scatter of data points on diagrams containing alkali and alkali–earth elements, indicating that many of the primary characteristics of the basalts have been modified. We therefore describe the major element abundances only in very general terms, and focus on the abundances of selected trace elements, such as high field strength (HFSE) and rare earth (REE) elements, which are both considered to be "relatively" immobile during hydrous alteration (e.g., Winchester and Floyd, 1977; Pearce, 1996).

The pillow basalts range in SiO2 content between 47.6 to 52.6 wt% (on a volatile free basis). Pillow basalts have a relatively homogeneous geochemical composition and on most diagrams generally plot in a cluster. Mg# (= molar {MgO/[MgO+Fe2O3t]}) ranges between 32.8 and 58.4 (Table 1). In general, the variation observed in elements such as TiO2 and V is typical of a tholeiitic trend (Figure 6a and e). Ni content ranges from 90 to 156 ppm and Cr content ranges from 250 to 374 ppm. Ni and Cr display a slight negative correlation with Mg#. Fe2O3t/MgO ranges from 1.2 to 3.6 with a high Fe2O3t (between 9.7 and 13.6 wt %) and a moderately high TiO2 content (between 1.0 and 1.7 wt %) contents. These features are also typical of tholeiitic mafic rocks as exemplified by the tholeiitic Skaergaard intrusion (Boudreau and McBirney, 1997; Figure 6f).

The mafic rocks are generally characterized by low Nb/Y (between 0.11 and 0.34), and are therefore subalkalic in composition (Figure 7a).. These ratios are typical of either volcanic arc or mid–ocean ridge basalts (MORB). However, on a V vs. Ti/1000 discrimination diagram (Figure 7b), the samples clearly exhibit MORB–type characteristics.

On the Th–Hf–Ta diagram (Figure 8), the Olinalá samples plot in the N–MORB field. Relative abundances of the less mobile trace elements suggest a within plate to ocean floor tectonic setting.

On the Ta/Yb vs. Ce/Yb and diagram, most samples cluster very close to the mantle array (Figure 9) suggesting negligible contamination from either crustal or arc sources contamination. A few samples plot significantly above the mantle array indicating either a subduction component in the source or crustal contamination. Cr–Y abundances (Figure 10) suggest that the mantle source for the mafic rocks underwent 5–25% partial melting followed by minor fractionation of mafic phases.

 

Most, but not all, pillow basalts display no LREE enrichment (Figure 11a), with La/SmN varying between 0.93 and 1.8. The low La/YbN values (1.3 to 2.8) together with the low Nb/Y suggest derivation from a spinel lherzolite mantle. The lack of a significant Eu anomaly indicates that plagioclase fractionation was relatively minor.. Spider diagrams normalized to N–MORB indicate a slight enrichment of large ion lithophile elements (LILE) and absence of Nb or Ta negative anomalies (Figure 11b). This is consistent with other evidence indicating minimal contamination by either crustal or subduction components. The pillow basalts also display positive and negative anomalies in Sr, probably related to plagioclase alteration.

In general, the pillow basalts from Olinalá area have similar trends to those pillow basalts of the Coatlaco Unit south of Olinalá (Grodzicki et al., 2008) and to the Carboniferous mafic dikes in the Salada Unit in the eastern Acatlán complex (Morales–Gámez et al., 2009).

Sm–Nd analyses of the pillow basalts of the Naranjo block display a wide range of εNd values (from +0.4 to +9.5; t = 330 Ma; Figure 12a, Table 2). Two samples (OL36–1 and OL36–2) have a higher εNd (+8.9 and +9.5, respectively) than the depleted mantle model (De Paolo, 1981) for the same age. Samples have high 147Sm/144Nd ratios ranging from 0.1860 to 0.2442 (Figure 12b; Table 2) and thus their model age calculations are too imprecise to be relevant to this study (see Stern, 2002). Only sample OL500M1 (a highly deformed pillow basalt) has a relatively low ratio (147Sm/144Nd= 0.1545) and a model age of 1.5 Ga (Table 2), suggesting that the source rocks was old sub–continental lithospheric mantle (e.g., Murphy and Dostal, 2007).

 

DISCUSSION AND CONCLUSIONS

The geochemistry of Carboniferous mafic rocks of the El Naranjo block, west of Olinalá indicates that they are tholeiitic with N–MORB characteristics and were derived from asthenospheric mantle (e.g., Figures 3 to 7) (4, 5, 6). They lack evidence of significant fractionation or crustal contamination. These basalts show close petrological and geochemical similarities with coeval basalts from the Coatlaco area (Grodzicki et al., 2008) located ~3 km to the east of Olinalá, and also with sparse mafic dikes that intrude the Carboniferous Salada Unit (Morales–Gámez et al., 2009).

The high εNd values are also characteristic for several Devonian and Carboniferous ophiolites in the Variscan orogen, including the Morais complex (Pin et al., 2006), the Lizard complex (Davies et al., 1984), Aracena (Castro et al., 1996) and Massif Central (Pin and Paquette, 2002), which are all characterized by εNd values that are equivalent to or slightly higher than the isotopic composition of the contemporary depleted mantle (DePaolo, 1981, 1988) suggesting derivation from a depleted mantle source (Murphy et al., 2010).

The fact that these mafic rocks are interbedded with a thick sequence of continental–derived clastic rocks suggests likely that they formed on the margin of a continent rather than in an open oceanic environment (cf. Keppie et al., 2006). This scenario is consistent with the occurrence of a crustally–derived Ordovician granitoid pluton in the Naranjo block (near Olinalá), which contains ca. 1 Ga zircons derived from a source similar to the Oaxacan complex that most probably underlies most of the Acatlán complex (e.g., Keppie et al., 2008a, 2008b; Ortega–Obregon et al., 2009). Based on youngest detrital zircon age (403 ± 7 Ma; Ortega–Obregón et al., 2009), deposition of the Progreso Unit (in the Naranjo block) occurs within the Mississippian.

The age of one horizon in the Zumpango Unit is best constrained by the 327 ± 3 Ma U–Pb age of the youngest concordant zircon in a felsic tuff interbedded with the clastic rocks. The angular unconformity of the overlying Olinalá Formation, suggests a pre–Middle Permian minimum depositional age. The age of the felsic source is indicated by the predominant detrital zircons ages: Oaxacan complex for the Progreso Unit, Neoproterozoic Maya block with a lesser contribution from the Oaxacan complex for the Naranjo and Zumpango units.

Deposition of the Progreso and Zumpango units is regarded by Keppie et al. (2008a, 2008b) as coeval with deformation associated with extrusion of high–pressure rocks into the Acatlán complex in an active tectonic margin (Ramos–Arias et al., 2009). Extrusion of high pressure rocks is a process that occurs only in an active margin setting. Thus the rift setting suggested by the geochemistry probably indicates extension during extrusion. Furthermore, Keppie et al. (2008b) have proposed that the Devonian–Carboniferous arc was removed by subduction erosion: remnants occur as Devonian–Carboniferous igneous detrital zircons, and in high–pressure extruded rocks (e.g., Meza–Figueroa et al., 2005; Galaz et al., 2009). Paleogeographic reconstructions consistent with paleomagnetic data (Figure 13; Keppie et al., 2008a, 2008b and references therein; Morales–Gámez et al., 2009) indicate that this active margin is probably the western margin of Pangea, as suggested by the Mid–continent (USA) affinities of Mississippian fauna found above the Oaxacan complex (Navarro–Santillán et al., 2002), which presumably underlies the Acatlán complex (Keppie et al., 2008b, Ortega–Obregón et al., 2009, 2010).

 

ACKNOWLEDGEMENTS

We thank CONACyT (Project CB–2005–1: 24894), PAPIIT (IN 100108–3) and NSERC for grants to support the work reported in this paper. COO was supported during his PhD by a CONACYT scholarship. JBM acknowledges the continuing support of NSERC (Canada) via Discovery and Research Capacity Development grants. We thank Drs. Peter Schaaf and Ricardo Vega–Granillo for their constructive comments.

 

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