The Early Mesozoic volcanic arc of western North America in
Transcription
The Early Mesozoic volcanic arc of western North America in
Journal of South American Earth Sciences 25 (2008) 49–63 www.elsevier.com/locate/jsames The Early Mesozoic volcanic arc of western North America in northeastern Mexico José Rafael Barboza-Gudiño a,*, Marı́a Teresa Orozco-Esquivel b, Martı́n Gómez-Anguiano c, Aurora Zavala-Monsiváis d a Instituto de Geologı́a, Universidad Autónoma de San Luis Potosı́, Manuel Nava No. 5. Zona Universitaria, 78240 San Luis Potosı́, S.L.P., Mexico b Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, Qro., Mexico c Universidad Tecnológica de La Mixteca, Carretera a Acatuma Km. 2.5, 69000 Huajuapan de León, Oaxaca, Mexico d Posgrado en Geologı́a Aplicada, Universidad Autónoma de San Luis Potosı́, Manuel Nava No. 5, Zona Universitaria, 78240 San Luis Potosı́, S.L.P., Mexico Abstract Volcanic successions underlying clastic and carbonate marine rocks of the Oxfordian–Kimmeridgian Zuloaga Group in northeastern Mexico have been attributed to magmatic arcs of Permo–Triassic and Early Jurassic ages. This work provides stratigraphic, petrographic geochronological, and geochemical data to characterize pre-Oxfordian volcanic rocks outcropping in seven localities in northeastern Mexico. Field observations show that the volcanic units overlie Paleozoic metamorphic rocks (Granjeno schist) or Triassic marine strata (Zacatecas Formation) and intrude Triassic redbeds or are partly interbedded with Lower Jurassic redbeds (Huizachal Group). The volcanic rocks include rhyolitic and rhyodacitic domes and dikes, basaltic to andesitic lava flows and breccias, and andesitic to rhyolitic pyroclastic rocks, including breccias, lapilli, and ashflow tuffs that range from welded to unwelded. Lower–Middle Jurassic ages (U/ Pb in zircon) have been reported from only two studied localities (Huizachal Valley, Sierra de Catorce), and other reported ages (Ar/ Ar and K–Ar in whole-rock or feldspar) are often reset. This work reports a new U/Pb age in zircon that confirms a Lower Jurassic (193 Ma) age for volcanic rocks exposed in the Aramberri area. The major and trace element contents of samples from the seven localities are typical of calc-alkaline, subduction-related rocks. The new geochronological and geochemical data, coupled with the lithological features and stratigraphic positions, indicate volcanic rocks are part of a continental arc, similar to that represented by the Lower–Middle Jurassic Nazas Formation of Durango and northern Zacatecas. On that basis, the studied volcanic sequences are assigned to the Early Jurassic volcanic arc of western North America. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Stratigraphy; Volcanic rocks; Arc; Jurassic; Mexico 1. Introduction Volcanic successions underlie clastic and carbonate marine sequences of the Oxfordian–Kimmeridgian Zuloaga Group in northeastern Mexico. Studies of the successions (e.g., Pantoja-Alor, 1972; Blickwede, 2001; López-Infanzón, 1986; Jones et al., 1990, 1995; Bartolini, 1998; Barto- * Corresponding author. Fax: +52 444 8111741. E-mail address: rbarboza@uaslp.mx (J.R. Barboza-Gudiño). 0895-9811/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2007.08.003 lini et al., 2003; Barboza-Gudiño et al., 1998, 1999, 2004) reveal diverse lithologies and stratigraphic position below Oxfordian limestones. In northern Durango and Zacatecas, the volcanic pre-Oxfordian rocks have been assigned to the Jurassic continental volcanic arc, related to the active continental margin of southwestern North America (Grajales-Nishimura et al., 1992; Jones et al., 1995; Bartolini, 1998; Bartolini et al., 2003), whereas in areas of San Luis Potosı́, Nuevo León, and Tamaulipas, they have been considered products of a Permo–Triassic volcanic arc (Meiburg et al., 1987; Bartolini et al., 1999). 50 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 However, the assignments are uncertain because reliable isotopic data are lacking, and petrographic and geochemical information is scarce. In this article, we report new geochemical, petrographic, and stratigraphic data for pre-Oxfordian rocks exposed in northeastern Mexico in the Sierra de Salinas, Sierra de Charcas, and Sierra de Catorce in San Luis Potosı́; the Aramberri and San Marcos areas in Nuevo León; and the Huizachal Valley in Tamaulipas (Fig. 1). The main purpose of the geochemical analyses is to characterize the rocks with regard to the tectonic setting in which they could have originated, rather than providing information leading to a detailed evolutionary model of the magmas. The new data, supported by information from the literature, help establish the tectonic setting and the correlations among pre-Oxfordian volcanic rocks in northeastern Mexico. 2. The exposed sequences Localities described in this section are shown in Fig. 1. Some outcrop aspects and textural or microstructural details of the pre-Oxfordian volcanic rocks studied in localities from northeastern Mexico are illustrated in Fig. 2. In northern Durango, intermediate to felsic volcanic rocks are exposed in the Villa Juárez area. Pantoja-Alor (1972) defines this unit as the Nazas Formation, with its type locality at Cerritos Colorados and a reported Pbage of 230 ± 20 Ma from a rhyolitic flow. In the Villa Juárez area, Bartolini and Spell (1997) obtain a 40Ar/39Ar age of 195 ± 55 Ma from plagioclase in rhyolitic rocks, probably comparable to those dated by Pantoja-Alor (1972). The Nazas Formation is the oldest exposed unit in the Villa Juárez region, but 200 km northwest of this locality, in Santa Marı́a del Oro, northern Durango, the Fig. 1. Geologic map of northeastern Mexico, showing outcrops of pre-Oxfordian volcanic and sedimentary rocks and location of samples. J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 51 Fig. 2. Details of the textures and microstructures observed in outcrops, hand samples, and thin sections of the volcanic rocks. (a) Volcanic breccia in the pyroclastic deposits of the Aramberri area, Nuevo León. Knife is 11 cm long. (b) General aspect of a rhyolitic dike (R) in the Sierra de Catorce; Juj, Upper Jurassic La Joya Formation; volc., intermediate pre-Oxfordian volcanic rocks. Note person for scale. (c) Lithophyse contained in pyroclastic deposits outcropping west of Charcas, San Luis Potosı́ (long edge = 10 cm). (d) Fiamme structures in ignimbrites from Aramberri, Nuevo León; hand-lens diameter is 2.5 cm. (e) Fragments of partially collapsed pumice in an ignimbrite of Charcas, San Luis Potosı́. (f) Trachytic or pylotaxitic texture in a basaltic andesite of La Ballena, Zacatecas, San Luis Potosı́. unit overlies Paleozoic metamorphic rocks (Bartolini, 1998) and unconformably underlies Upper Jurassic sandy limestone of La Gloria Formation (Imlay, 1936). In northern Zacatecas, lava flows, airfall and ashflow tuffs, and lahars correlated with the Nazas Formation have been described in the Caopas–Rodeo area, including Sierra de Teyra to the west and Sierra de San Julián to the east (Blickwede, 1981, 2001). Some units (Caopas schist and Rodeo Formation) initially were considered pre-Jurassic because of their very deformed and metamorphosed aspects (de Cserna, 1956; Córdoba-Méndez, 1964). Subsequent studies indicated that the deformed units are coeval with the Nazas Formation and belong to the same Jurassic continental volcanic arc sequence (López-Infanzón, 1986; Jones et al., 1990, 1995). This hypothesis is supported by a K–Ar age of 183 Ma determined in hornblende from the so-called Rodeo Formation (López-Infanzón, 1986) and an apparent U–Pb age of 158 ± 4 Ma in zircon grains from the Caopas schist (Jones et al., 1995). In the Sierra de Teyra, the Nazas Formation overlies the marine siliciclastic Taray Formation (Córdoba-Méndez, 1964) of Triassic age (Silva-Romo et al., 2000) and underlies Upper Jurassic continental deposits of La Joya Formation (Mixon et al., 1959) and shallow marine limestones of the Zuloaga Formation (Imlay, 1938). In western San Luis Potosı́, volcanic sequences comparable to those of Durango and Zacatecas rest on Triassic thin-bedded strata interpreted as part of a turbiditic sequence known as the Zacatecas Formation (Martı́nezPérez, 1972) or the La Ballena Formation (Silva-Romo, 0.717 0.592 0.690 193.6 193.7 199.4 Pb/206Pb age (Ma) 207 193.1 191.2 183.0 Pb/235U age (Ma) 207 0.04997 0.04997 0.05010 0.16 0.22 0.17 0.20943 0.20720 0.19744 0.11 0.12 0.11 0.030397 0.030071 0.028584 %Err Pb Pb/ Rad. %Err U Pb/ Rad. %Err U Pb/ 2276.1 2019.8 2234.7 0.6 0.6 0.6 12.4 4.4 6.3 1.7 4.4 3.5 Z1 Z2 Z3 402 142 211 / Pb (corr.) / 0.133 0.155 0.169 Pb (corr.) Rad. 0.11 0.18 0.12 193.0 191.0 181.7 Pb/238U age (Ma) 206 206 207 235 207 238 206 208 206 Corrected atomic ratios 206 204 Com. Pb (pg) Pb (ppm) U (ppm) Weight ( g) Sample 1993; Centeno-Garcı́a and Silva-Romo, 1997). The volcanic units are unconformably overlain by Upper Jurassic redbeds of La Joya Formation and shallow marine limestones of the Zuloaga Formation. The volcanic sequences crop out in the Sierra de Salinas, at the state border between Zacatecas and San Luis Potosı́ (Silva-Romo, 1993; Barboza-Gudiño et al., 1998, 1999; Zavala-Monsiváis, 2000; Gómez-Anguiano, 2001), in the Sierra de Charcas, and in the region of Tepozan in western San Luis Potosı́ state (Tristán-González and Torres-Hernández, 1992, 1994; Tristán-González et al., 1995; Zavala-Monsiváis, 2000), as well as in the Sierra de Catorce (LópezInfanzón, 1986; Barboza-Gudiño et al., 1998, 1999; Zavala-Monsiváis, 2000; Hoppe, 2000; Gómez-Anguiano, 2001). Barboza-Gudiño et al. (2004) report U–Pb isotopic analyses of zircon for rhyolite of the Sierra de Catorce. The fraction recording the least inheritance yields an age of 174.7 ± 1.3 Ma, a maximum age of the rock. In Nuevo León state, volcanic rocks comparable in their lithology and stratigraphic position with those described previously have been observed in a sequence exposed in Aramberri (Jones et al., 1995). The sequence consists predominantly of ignimbrites, volcanic breccias, and tuffs of intermediate to felsic composition, which overlie Paleozoic schist and unconformably underlie the transgressive Upper Jurassic strata. Some authors consider these volcanic units related to a Permo–Triassic volcanic arc (Meiburg et al., 1987; Bartolini et al., 1999). We analyze three single zircon grains from a rhyodacitic ignimbrite at the Mezquital section, north of Aramberri. One grain yields essentially concordant Pb/U ratios that indicate an age of 193.1 ± 0.3 Ma for the rock (Table 1); an upper intercept at 193.3 ± 1.5 Ma is also shown in a discordia plot (Fig. 3). In the redbed sequence exposed near San Marcos, south of Galeana, Nuevo León, the volume of volcanic and subvolcanic rocks is small, and the stratigraphic relations are uncertain. At this locality, some dikes and sills of trachytic composition intrude Upper Triassic redbeds of the Lower Huizachal Group and are truncated by the unconformity beneath Upper Jurassic breccias of La Joya Formation and Oxfordian–Kimmeridgian evaporites of the Minas Viejas Formation (Gómez-Anguiano, 2001). Pre-Oxfordian volcanic rocks are also present in the State of Tamaulipas, at Huizachal Valley (Jones et al., 1995; Fastowsky et al., 2005), the Miquihuana area (Bartolini et al., 2003), and La Boca Canyon (Fig. 1). Metarhyolite exposed in the nearby Caballeros Canyon are apparently older and possibly of Paleozoic age (Gursky and Ramı́rez-Ramı́rez, 1986; Stewart et al., 1999). The best exposures of pre-Oxfordian volcanic and subvolcanic rocks in Tamaulipas are found in the Huizachal Valley, where they represent the basal part of the exposed Mesozoic sequence, underlying and partially intruding Lower Jurassic redbeds; no older units crop out in this area. Volcanic rocks in this locality consist of ignimbrite, finegrained tuff, breccia, and lava of intermediate to felsic Corr. coef. J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 Table 1 U–Pb data for single zircon crystals from a rhyodacitic ashflow tuff, Aramberri area (sample MZQTG1) 52 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 53 Fig. 3. Concordia diagram for U–Pb isotope ratios of zircons from a rhyodacitic ashflow tuff in the Aramberri area (see Table 1). Error ellipses of individual spots are 2 . composition. In the Huizachal Valley, zircon grains from a pyroclastic flow are dated by U–Pb at 189 ± 0.2 Ma (Fastowsky et al., 2005). At this locality, Fastowsky et al. (2005) recognize an older volcanic sequence beneath an angular unconformity that underlies the dated pyroclastic flow at the base of the Lower Jurassic redbed sequence of La Boca Formation. All features described by Fastowsky et al. (2005) and those we observe in the same outcrops of the ‘‘older’’ sequence are typical textures and structures of rhyolitic domes: spherulitic structures, steeply dipping flow-like bands, lithophysae, peripheral lava flows or lobes, and associated ash flow tuffs. In addition, we observe intrusive relationships to Lower Jurassic redbeds of La Boca Formation. We interpret the steeply dipping features in this sequence as subvertical flow bands resulting from magma injection into a volcanic dome, whose emplacement age is not necessary much older than the dated Lower Jurassic pyroclastic flow, and thus, we consider plausible the inclusion of this lower sequence as part of the same Lower Jurassic volcanic arc. Isotopic ages reported to date for pre-Oxfordian volcanic rocks in northeastern Mexico are summarized in Table 2. Three Lower–Middle Jurassic ages were obtained by the U–Pb method in zircons, whereas other included K–Ar and Ar/Ar ages (whole-rock or feldspar) indicate Cretaceous– Paleogene ages and probably reflect age resetting during the Laramide event. 3. Petrography The petrography and petrology of the Lower Jurassic volcanic rocks in the Nazas Formation and comparable rocks from northeastern Mexico have been described from different localities by various authors. Blickwede (1981, 2001) provides petrographic and petrologic data on rocks from Sierra de San Julián; López-Infanzón (1986) offers data on rocks from the Caopas–Rodeo and Sierra de Teyra areas and compares them with rocks of the Sierra de Catorce; Jones et al. (1995) describe volcanic rocks of the Caopas–Pico de Teyra region and compare them with rocks Table 2 Summary of isotopic ages of pre-Oxfordian volcanic rocks from northeastern Mexico, including several Ar/Ar and K–Ar ages determined in whole-rock or feldspar that are considered reset ages Location name State Rock type Method Material dated Age (Ma) Error (Ma) Source Huizachal Valley Sierra de Salinas Aramberri Huizachal Valley Miquihuana San Marcos Sierra de Salinas Sierra de Catorce Sierra de Catorce Aramberri Tamaulipas S. Luis Potosı́ Nuevo León Tamaulipas Tamaulipas Nuevo León S. Luis Potosı́ S. Luis Potosı́ S. Luis Potosı́ Nuevo León Pyroclastic flow Basalt Rhyolite Rhyolite Rhyolite Andesitic dike Andesitic basalt Rhyolite Rhyolite Pyroclastic rock U–Pb 40 Ar/39Ar K–Ar K–Ar K–Ar K–Ar K–Ar K–Ar U–Pb U–Pb Zircon Whole rock Whole rock Whole rock Whole rock Whole rock Whole rock Feldspar Zircon Zircon 189.0 82.9 70.7 52.1 57.8 104 81.9 110.0 174.7 193.1 0.2 0.6 1.8 1.4 1.5 3 4.1 1.9 1.3 0.3 Fastowsky et al. (2005) Bartolini (1998) Fastowsky et al. (2005) Bartolini (1998) Bartolini (1998) Bartolini (1998) Barboza-Gudiño et al. (1999) This work Barboza-Gudiño et al. (2004) This work 54 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 from Torreón, Sierra de Catorce, Charcas, Aramberri, Miquihuana, and Huizachal Valley; and finally, ZavalaMonsiváis (2000) describes the petrography of pre-Oxfordian volcanic rocks included in this study from western San Luis Potosı́ State at Sierra de Salinas, Charcas, Tepozán, and Sierra de Catorce. We compile relevant petrographic data and add new petrographic descriptions of the main lithologic features, which we summarize in Table 3. 3.1. Rhyolite Porphyritic rhyolite with quartz and sanidine phenocrysts is abundant in the ‘‘Caopas schist,’’ exposed near the town of same name, and in the Nazas Formation outcrops of Villa Juárez and Sierra de San Julián. In the Sierra de Catorce, a rhyolitic dike (Fig. 2b) exhibits porphyritic texture, with phenocrysts of hypidiomorphic quartz in a groundmass of feldspar and quartz. The rhyolitic dike is strongly altered, as shown by totally kaolinitized feldspar phenocrysts and subsequent silicification of the rock. Locally, foliation with lepidoblastic texture results from the presence of oriented sericite associated with dynamic metamorphism. In the Huizachal Valley, rhyolitic domes are characterized by steeply dipping flow bands and the development of abundant spherulites, ranging from one to several centimeters in diameter. 3.2. Basalts and basaltic andesites In the Caopas–Rodeo area, Jones et al. (1995) describe sparse basaltic lavas in the Nazas Formation, whereas andesitic lava flows are more common in the unit known as the Rodeo Formation. In the Sierra de Salinas, north of the town La Ballena, basaltic–andesitic lava is the dominant rock type; some lavas display fluidal porphyritic texture with highly altered, probable hornblende phenocrysts, scarce pyroxene, olivine, and plagioclase in a fine groundmass composed of acicular plagioclase, ferromagnesian, and opaque grains. At the base of the sequence, the lavas are brecciated. Similar basaltic–andesitic lavas crop out at Sierra de Catorce and Charcas. In the latter area, andesitic lava flows contain a brecciated zone that, given the presence of autoclasts arranged in a ‘‘puzzle structure,’’ is interpreted as an autoclastic breccia and probable peperite, associated with a flow front or basal breccia that was apparently engulfed by igneous material of the same composition, and in part mixing with wet sediment. Finally, volcanic products in the Huizachal Valley include layers of cinders, scoreaceous material, and andesitic lava. These rocks are dark to reddish brown due to oxidation and have a very compact, deformed aspect, with vestiges of vesicles. 3.3. Pyroclastic deposits The most common rocks are andesitic to dacitic and rhyolitic pyroclastic products that show clear flow indicators. The textures are diverse, including breccia, lapilli tuff, and fine volcanic ash. Some deposits display welding and devitrification, whereas others appear to be unwelded rocks, compacted by deformational processes, that record intense foliation and jointing. In the Sierra de San Julián, Blickwede (1981, 2001) describes a 1000 m thick volcanic sequence consisting of 65% pyroclastic rocks, 25% sedimentary and volcaniclastic rocks, and only 10% lavas. Volcanic breccias or tuff-breccias are the predominant rocks in the various small outcrops exposed in the Arroyo El Tepozán, northern Sierra de Charcas. A volcanic breccia that crops out in Aramberri, Nuevo León (Fig. 1), consists of angular fragments of rhyodacitic to rhyolitic welded tuffs in a sandy matrix (Fig. 2a). These deposits are directly overlain by shallow marine sandstone with calcareous cement of La Joya Formation that represents the base of the Late Jurassic marine transgression (Mixon et al., 1959). Typical ignimbrites with notable development of fiamme structures are identified in outcrops at Charcas, Aramberri, and Huizachal Valley (Fig. 2d and e). In La Boca Canyon, the pyroclastic deposits are represented by fine-grained tuffs that partly contain deformed accretionary lapilli. 4. Geochemistry Ten samples were collected for geochemical analysis from six exposures of pre-Oxfordian volcanic units in the states of San Luis Potosı́, Nuevo León, and Tamaulipas (Fig. 1). Sampling of the diverse lithologies in the studied areas was strongly restricted by the availability of acceptably fresh samples. Major and trace-element analysis of whole-rock samples was performed by ICP-MS at the Centre des Recherches Pétrographiques et Geóchimiques (CNRS) of Nancy, France (samples: PBLG1 from Sierra de Salinas, CHRG1 from Sierra de Charcas, RCG1 from Sierra de Catorce; MZQTG1 collected in the Mezquital area, north of Aramberri; SM1 from the San Marcos area south of Galeana, Nuevo León; HZCHG1 and HZCHG2 from Huizachal Valley in Tamaulipas), and Activation Laboratories Ltd. (samples: TPZ1 from Tepozán outcrops, northern Sierra de Charcas; RCG3 from north of Real de Catorce, Sierra de Catorce; SM3 from San Marcos area in Nuevo León). The composition of analyzed samples is summarized in Table 4. The analyzed volcanic rocks are classified as intermediate to felsic after the silica content limits proposed by Peccerillo and Taylor (1976). Following the total alkalis vs. silica (TAS) classification of Le Bas et al. (1986), the samples are classified as trachyandesite, andesite, dacite, and rhyolite. The use of the TAS diagram to classify volcanic rocks is restricted to fresh rocks because of the high mobility of alkaline elements during secondary processes. The analyzed samples have high volatile contents (LOI) and show evidence of oxidation, which basically makes them unsuitable for TAS classification. Nevertheless, the obtained chemical classification is supported by its agree- Table 3 Petrographic features observed in the studied pre-Oxfordian volcanic rocks of northeastern Mexico Feature RhyoliteRhyodacite Macrostructure La Ballena Charcas Texture Components Alteration minerals Basalt– Andesite Macrostructure Texture Components Alteration minerals Pyroclastic rocks Lava flow, basal flow breccia Airfall deposits, laminated ash, porphyritic, trachytic, pilotaxitic Plagioclase, olivine, pyroxene, hornblende Chlorite, epidote, oxide Lava flow, basal flow breccia Porphyritic, trachytic, pilotaxitic Plagioclase, olivine, Pyroxene, Chlorite, epidote, oxide Sierra de Catorce Aramberri Huizachal Valley Lava dome, dikes, flow banding Porphyritic Quartz, feldspar Kaolinite, sericite opaque minerals Lava dome, lava flow flow banding Glassy Quartz, feldspar Kaolinite, opaque minerals (oxide) Lava flow, basal flow breccia Porphyritic, trachytic, pilotaxitic Lava flow basal flow breccia Vesicular Plagioclase, olivine, pyroxene, biotite, hornblende Chlorite, sericite, epidote, oxide Plagioclase La Boca Canyon Opaque minerals (oxide) Macrostructure Airfall deposits, laminated ash Ash flow tuff (ignimbrites), airfall deposits, welded basal vitrophyre, spherulites vapour zone, massive welded zone, eutaxitic structure, fiammen, volcaniclastic breccia Airfall deposits, laminated ash Ash flow tuff (ignimbrites), airfall deposits, laminated ash, massive welded zone, eutaxitic structure, fiammen, volcaniclastic breccia Texture Unwelded Unwelded Components Quartz, lithic fragments, pumice fragments Porphyritic, glassy, welded, unwelded Quartz, lithic fragments, pumice fragments Quartz, lithic fragments, pumice fragments Porphyritic, glassy, welded, unwelded Quartz, lithic fragments, pumice fragments Alteration minerals Kaolinite, epidote, opaque minerals (oxide) Kaolinite, sericite, epidote, opaque minerals (oxide) Sericite, opaque minerals (oxide) Kaolinite, opaque minerals (oxide) Ash flow tuff (ignimbrites), airfall deposits, spherulites vapor zone, massive welded zone, eutaxitic structure, fiammen, rheomorphic folding, ash flows, flow banding Glassy welded, unwelded Quartz, lithic fragments, pumice fragments Kaolinite, opaque minerals (oxide) Airfall deposits, laminated ash Unwelded Quartz plagioclase, lithic fragments, pumice fragments, distorted lapillus Kaolinite,opaque minerals (oxide) J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 Rock 55 56 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 Table 4 Major- and trace-element composition of pre-Oxfordian volcanic rocks, northeastern Mexico CHRG1a SCH Dacite TPZ1b SCH Andesite RCG1a SC Dacite RCG3b SC Rhyolite MZQTG1a A Rhyolite SM1a SM Trachyandesite SM3 b SM Trachyandesite HZCHG1a HV Rhyolite HZCHG2a HV Rhyolite Major elements (wt%) SiO2 53.16 TiO2 1.25 Al2O3 18.79 4.54 Fe2O3 FeO 1.92 MgO 5.21 MnO 0.11 CaO 2.81 Na2O 6.2 K2O 1.82 P2O5 0.32 LOI 3.55 Total 99.68 65.07 0.48 15.23 5.6 0.79 1.53 Traces 0.67 2.21 3.72 0.16 3.02 98.48 54.50 0.954 16.19 6.37 1.12 6.06 0.081 5.38 3.83 0.76 0.19 4.17 99.60 62.19 2.11 11.85 11.97 0.66 0.75 Traces 2.19 0.08 4.24 1.34 2.35 99.73 78.75 0.235 14.10 0.69 0.13 0.09 0.005 Traces 0.24 3.57 0.05 2.19 100.05 71.97 0.15 13.25 2.06 0.55 1.09 0.02 1.74 0.53 4.59 0.04 3.83 99.82 52.53 0.78 18.34 2.94 4.59 2.23 0.12 4.58 5.52 1.64 0.3 6.42 99.99 54.68 0.638 18.0 6.33 1.12 1.86 0.098 4.03 5.52 2.07 0.32 4.78 99.44 77.34 0.24 12.51 3.16 0.06 0.26 Traces 0.31 0.25 3.17 Traces 2.49 99.79 76.89 0.27 14.24 0.57 0.22 0.36 Traces 0.11 0.21 3.88 0.06 2.74 99.51 Trace elements (ppm) Rb 57.82 Sr 387 Y 28.6 Zr 212 Nb 12.93 Cs 5.1 Ba 538 La 20.41 Ce 47.53 Pr 5.93 Nd 23.5 Sm 5.12 Eu 1.57 Gd 5.16 Tb 0.76 Dy 4.52 Ho 1.02 Er 2.56 Tm 0.4 Yb 2.42 Lu 0.4 Hf 4.96 Ta 0.943 Th 4.58 U 1.26 154.0 156 36.1 338 17.51 12.96 1987 38.73 81.14 9.85 38.77 7.92 2.12 6.75 1.04 6.18 1.25 3.46 0.53 3.42 0.54 8.62 1.33 10.07 1.82 14 480 18.8 129 5.7 6.6 501 15.1 33.3 4.10 17.9 4.20 1.33 4.04 0.65 3.78 0.77 2.26 0.331 2.03 0.325 3.2 0.35 2.10 0.63 139.3 107 32.9 804 27.74 10.65 1069 128.8 331.3 35.48 129.4 20.68 8.04 13.27 1.51 6.95 1.06 2.73 0.34 2.09 0.29 20.4 1.5 28.55 5.05 90 56 14.4 113 8.9 8.4 318 34.8 68.3 7.43 27.8 5.58 1.59 4.65 0.62 2.94 0.54 1.63 0.245 1.62 0.253 3.4 1.42 8.44 1.15 166.6 30 16.9 142 7.27 7.38 709 25.55 46.74 4.92 16.9 3.17 0.72 2.71 0.41 2.48 0.542 1.54 0.249 1.66 0.27 3.94 0.749 12.54 2.97 51.09 320 20.7 126 5.78 2.16 455 38.68 76.64 8.82 34.83 6.93 2.18 5.25 0.76 3.99 0.66 1.97 0.29 1.98 0.3 3.32 0.424 9.25 2.28 54 251 27.9 120 4.2 4.0 227 39.0 79.9 8.94 35.5 7.0 1.65 5.83 0.88 4.91 1.03 3.18 0.475 3.06 0.502 3.0 0.18 13.8 3.69 94.91 29.1 31.4 230 9.87 2.99 799 27.81 60.28 7.05 27.8 5.69 1.08 4.52 0.77 4.86 1.01 3.04 0.51 3.3 0.52 5.93 0.872 10.44 1.91 94.21 30.5 23.7 219 11.01 3.22 607 41.55 82.97 9.43 35.11 5.9 0.95 4.02 0.65 4.29 0.82 2.51 0.4 2.5 0.43 6.02 0.974 11.56 3.75 11.8 – 16.3 – 43.2 6.6 6.5 – 65.0 10.1 0.1 – 48.3 4.5 3.0 – 1.4 – 8.9 – 3.6 0.1 8.1 – 64.3 8.3 1.5 – 62.5 10.0 0.6 – Sample Locality Type PBLG1a SS Trachyandesite CIPW normative minerals (wt.%) qz – 35.9 c 2.4 7.1 hy 9.9 6.7 ol 2.3 – Notes: SS, Sierra de Salinas; SCH, Sierra de Charcas; SC, Sierra de Catorce; A, Aramberri; SM, San Marcos; HV, Huizachal Valley. For sample locations, see Fig. 1. a Analyses performed by ICP-MS at Centre des Recherches Pétrographiques et Geóchimiques (CNRS) of Nancy, France. b Analysis performed by ICP-MS at Activation Laboratories, Ltd., Ancaster, Ontario, Canada. ment with the petrographic classification based on the mineralogical composition of the samples. Sample PBLG1 from Sierra de Salinas is a silica undersaturated trachyandesite with 2.3% normative olivine and 9.9% normative hypersthene (hy). Samples SM1 and SM3 from the San Marcos area in Nuevo León are slightly oversaturated trachyandesite with low normative quartz (qz) content (1.4 and 3.6%, respectively) and 8.1–8.9% hy. A sample collected in the northern Sierra de Charcas (TPZ1) is a silica saturated andesite with 11.8% qz and 16.3% hy. Samples classified as dacites were collected at Sierra de Charcas (CHRG1) and Sierra de Catorce (RCG1); they have qz contents of 35.9% and 43.2% and hy contents of 6.7% and 6.5%, respectively. Samples RC3 (Sierra de Catorce), MZQTG1 (Aramberri area), HZCHG1 and HZCHG2 (Huizachal Valley) are rhyolite, J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 with variable qz contents between 48.3% and 65.0% and hy contents of 0.1–3.0%; these rocks are peraluminous with normative corundum contents varying from 4.5% to 10.1%. The analyzed samples trend toward lower contents of most oxides (e.g., CaO, Al2O3, MgO, NaO, TiO2, and P2O5) as silica increases, whereas K2O tends to be enriched as silica increases and then diminish at higher silica contents, as is characteristic of calc-alkaline magmas evolving through the fractional crystallization of plagioclase and ferromagnesian minerals. The late potassium depletion could indicate K-feldspar crystallization in the most evolved rocks. The samples represent a broad region, so they cannot be treated as comagmatic, though the observed tendencies prompt us to interpret them as originating in a common tectonic setting. Trace-element abundances are shown in a multi-element diagram (Fig. 4), normalized against the primordial mantle composition of Sun and McDonough (1989). Although the 57 samples have different trace-element enrichments, their patterns are similar and show common features. The most relevant feature observed in this diagram is a well-developed negative anomaly in the elements Nb and Ta, considered one of the most distinctive characteristics of rocks generated by subduction processes (e.g., Hawkesworth et al., 1993), and generally related to the enrichment of largeion lithophile elements (e.g., Rb, Ba, K) and light rare earth elements (e.g., La, Ce) in fluids and melts released from the subducting plate to the overlying mantle. The negative anomalies in Sr, P, and Ti are most likely controlled by the fractionation of individual minerals, in that they become more pronounced in the most differentiated rocks. Because Sr is a fluid-mobile element, the Sr anomalies could result partly from element mobilization during alteration, though the development of similar anomalies for relatively immobile elements such as P and Ti as differentiation proceeds indicate that mineral fractionation is the Fig. 4. Normalized multielement diagrams for samples collected from six different outcrops of pre-Oxfordian volcanic rocks in northeastern Mexico (for sample location, see Fig. 1). Sample elemental abundances are normalized to the primitive mantle values of Sun and McDonough (1989). A, andesite; TA, trachyandesite; D, dacite; R, rhyolite. 58 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 dominant process of depletion. In the analyzed samples, concentrations of P could have been controlled by apatite fractionation; Sr by plagioclase, and Ti by ilmenite, rutile, or sphene (Rollinson, 1993). The patterns in this diagram do not show significant deviations caused by element mobilization, despite of the observed alteration in the rocks. The trace-element patterns in the normalized multi-element diagram are strong evidence of an origin of pre-Oxfordian rocks in a continental arc setting. Rare earth element (REE) abundances normalized against chondrite values from Anders and Grevesse (1989) are shown in Fig. 5. The REE can be used reliably in this type of rock because they are considered to remain immobile during alteration. All samples are enriched in light REE relative to heavy REE. With the exception of sample RCG1 from Sierra de Catorce, all samples have a relatively steep slopes for the LREE (La-Eu) and a flat pattern for the HREE (Gd-Lu). The most evolved samples (northeast area: MZQTG1, HZCHG1, and HZCHG2) show weak negative Eu-anomalies, indicating plagioclase fractionation. The REE pattern of sample RCG1 is characterized by a stronger enrichment in LREE relative to HREE and a continuous decrease of element abundances between La and Lu. The REE diagrams show that the pre-Oxfordian volcanic rocks from broadly distributed localities have similar compositions and probably originated in similar conditions. The differences observed in sample RCG1 could be attributed to differences in source composition or melting process, but the available data do not allow a more definitive interpretation. Discrimination among tectonic settings on the basis of geochemical data has been proposed in several works (e.g., Pearce and Cann, 1971, 1973; Wood, 1980; Pearce, Fig. 6. Tectonomagmatic discrimination diagrams showing the composition of analyzed samples. (a) Th–Ta–Hf/3 ternary diagram of Wood (1980). (b) Rb vs. Y + Nb diagram after Pearce et al. (1984). Symbols as in Fig. 5. Fig. 5. Rare earth element diagrams of the pre-Oxfordian volcanic units from northeastern Mexico, normalized to chondrites values of Anders and Grevesse (1989). A, andesite; TA, trachyandesite; D, dacite; R, rhyolite. J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 1982; Shervais, 1982; Meschede, 1986). The diagram Hf/3– Th–Ta of Wood (1980) is particularly useful for discriminating altered and metamorphosed rocks because of the relative immobility of these elements during secondary processes. This diagram was originally proposed for basic rocks but can be satisfactorily applied to rocks of intermediate to felsic composition (Wood, 1980). The pre-Oxfordian volcanic rocks analyzed in this work plot within the field of calc-alkaline continental volcanic arcs (Fig. 6A). To confirm the tectonomagmatic discrimination, the data were plotted in the Rb vs. Y + Nb diagram (Fig. 6B) proposed by Pearce et al. (1984), which applies to rocks of granitic composition and is thus more indicative for the evolved rocks. Again, all pre-Oxfordian volcanic rocks plot in the field of volcanic arc granites (VAG), independently of their composition. It is noteworthy that the samples plot in the same field in both diagrams, though Rb is considered a mobile element during secondary processes. The general features of the exposed volcanic sequences, the petrography of the diverse materials, and the geochemical data support the conclusion that all the studied preOxfordian volcanic rocks originated in a continental arc. Our analyses provide a general idea of the compositional variations among the sequences or localities, which, however, are common in volcanic arcs composed of different volcanic centers. These variations also document the changes in composition of all volcanic products during magmatic evolution in space and time. The following observations suggest an origin of these rocks in a continental volcanic arc: 59 1. Pyroclastic volcanic products, in the form of ashflows and ignimbrites, airfall tuffs, breccias, and probable lahars and avalanches predominate at all localities, whereas lava flows, rhyolitic domes, and dikes are present in lesser proportion. 2. The observed volcanism is of eminently subaerial character. The existence of large volcanoes is suggested by the presence of lahars and possible avalanche deposists. Furthermore, there is evidence of separate volcanic centers or volcanic fields in the region. 3. The rocks in this study have intermediate to felsic compositions and calc-alkaline characters, as found in volcanic arcs worldwide. The abundance of silica-rich magmas, partly K-rich, is also typical for continental arcs. Moreover, the patterns observed in normalized multi-element diagrams are characteristic of subduction-related rocks. 4. Two different discrimination diagrams that show the behavior of some trace elements for different rock compositions support an origin of these rocks in a continental volcanic arc. 5. Correlation The stratigraphic correlation of pre-Oxfordian units in northeastern Mexico is shown in Fig. 7. In most outcrops of Zacatecas and San Luis Potosı́ (southwestern part of the studied region), the volcanic rock sequences overlie Upper Triassic siliciclastic marine sequences of the Zacate- Fig. 7. Stratigraphic correlation of the pre-Oxfordian units of northeastern Mexico. The studied volcanic sequences belong to the Nazas Formation, which overlies Triassic units of marine origin in central Mexico and sequences of continental origin in northeastern Mexico, and underlies or is partly interlayered with the basal parts of the Middle–Upper Jurassic redbed sequences (La Boca Formation). 60 J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 Fig. 8. Tectonic setting of pre-Oxfordian volcanic rocks in a model of continental volcanic arc associated with the development of the active continental margin of southwestern North America during Late Triassic–Middle Jurassic time. J.R. Barboza-Gudiño et al. / Journal of South American Earth Sciences 25 (2008) 49–63 cas and Taray formations (Cantú-Chapa, 1969; GalloPadilla et al., 1993; Gómez-Luna et al., 1998), whereas in the northeastern part of the region, the underlying rocks are continental redbed sequences of Late Triassic age (Weber, 1997; Silva-Pineda and Buitrón-Sánchez, 1999). Triassic rocks are absent in localities where the volcanic rocks directly overlie older sedimentary or metamorphic rocks. In turn, the volcanic rocks underlie or are interbedded in the basal part of redbeds of either La Boca Formation (Mixon et al., 1959) – dated as Sinemurian by RuedaGaxiola et al. (1993) and as Early–Middle Jurassic by Fastowsky et al. (2005) – or the Upper Jurassic La Joya Formation (Mixon et al., 1959). Both Jurassic redbed sequences frequently contain clasts of pre-Oxfordian volcanic rocks and, in some localities, are almost solely constituted by these rocks. La Joya Formation, in turn, is overlain by the transgressive carbonate sequences of the Oxfordian–Kimmeridgian Zuloaga Group. The lithological similarities, stratigraphic position, and available absolute ages of the localities studied in this work lead us to conclude that all the volcanic sequences described here are part of a Jurassic volcanic arc, related to the active margin of southwestern North America. The available ages (Tables 1 and 2) indicate that the volcanic arc was probably active for a period of 40 Ma during the Jurassic. The volcanic rocks of the described localities correlate with the Nazas Formation of northern Durango and Zacatecas and therefore represent a key unit for the stratigraphic subdivision, as well as paleogeographic and paleotectonic interpretations of north and northeastern Mexico. 6. Conclusions The evidence presented herein indicates that the preOxfordian volcanic rocks of Tamaulipas and Nuevo León are similar in composition, age, and tectonic setting to those of Durango and Zacatecas and that these sequences continue toward the western part of San Luis Potosı́. The tectonic setting, defined on the basis of petrographic and geochemical studies of the volcanic rocks, is a continental arc. Fig. 8 summarizes the tectonic evolution of pre-Oxfordian units from northeastern Mexico according to their stratigraphic positions, geochemical–petrological character, and available paleontological and isotopic age determinations. The Upper Triassic and Lower Jurassic rocks from northeastern México may be interpreted as related to the evolution of the Cordilleran system (Jones et al., 1995; Bartolini et al., 2003). During the Middle–Late Triassic, turbiditic sequences (Zacatecas Formation) related to submarine fans formed at the western margin of North America; subsequent eastward subduction of the Kula plate caused the first deformation of the turbiditic sequences. In the Early Jurassic, the development of a continental volcanic arc (recorded by the Nazas Formation) began as part of the Cordilleran magmatic arc. Finally, the volcanic arc was eroded and buried beneath the Upper Huizachal 61 Group and La Joya Formation, followed by the eastward displacement of Mexico along the Mojave–Sonora megashear (Silver and Anderson, 1974; Anderson and Silver, 2005) and/or other Late Jurassic sinistral strike-slip faults. Roughly coeval to sinistral transcurrent faulting, eastward subduction of a North American segment of oceanic lithosphere took place under the Pacific plate (Barboza-Gudiño et al., 1998; Dickinson and Lawton, 2001), resulting in the development of an intraoceanic volcanic arc complex (Guerrero Terrane). Since Late Jurassic time, the evolution of northeastern Mexico has been associated with the evolution of the Gulf of Mexico Basin, and the ‘‘Cordilleran terranes’’ in this region were covered by the Upper Jurassic–Cretaceous Gulf sequences (e.g., Wilson and Ward, 1993). The stratigraphic position and existing absolute ages of the studied volcanic sequences indicate they belong to a Jurassic arc, related to the active Pacific continental margin of southwestern North America, rather than to the Permo– Triassic arc. This older arc has been inferred from isolated localities east and north of the studied area under the Gulf of Mexico coastal plain in Tamaulipas and near Coahuila and Chihuahua (Bartolini et al., 1999; Torres et al., 1999). The assignment of the studied volcanic sequences to the Jurassic arc provides an excellent guide for discriminating between Triassic and Jurassic redbeds, which commonly have been considered a single unit (La Boca Formation [Mixon et al., 1959]; Huizachal Formation [Carrillo-Bravo, 1961]; or Los San Pedros Alogroup [Rueda-Gaxiola et al., 1993), which blurs stratigraphic details and leads to a misunderstanding of the tectonic evolution and main processes acting in these periods. Acknowledgments We acknowledge support from SEP/CONACYT (project 485100-5-25400T and 2002-CO2-41239) and FAI/ UASLP (project C02-FAI-11-27.88) and thank J. Blickwede and C. Bartolini for their revisions and suggestions. Detailed reviews by the JSAES-designed reviewers, T.H. Anderson and T.F. Lawton, greatly helped improve this work. References Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197–214. Anderson, T.H., Silver, L.T., 2005. The Mojave-Sonora Megashear – field and analytical studies leading to the conception and evolution of the hypothesis. In: Anderson, T.H., Nourse, J.A., McKee, J.W., Steiner, M.B. (Eds.), The Mojave-Sonora Megashear Hypothesis: Development, Assessment and Alternatives. Geological Society of America Special Paper, vol. 393, pp. 1–50. Barboza-Gudiño, J.R., Tristán-González, M., Torres-Hernández, J.R., 1998. 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