Revista mexicana de ciencias geológicas

 

Provenance ages of late Paleozoic sandstones (Santa Rosa Formation) from the Maya block, SE Mexico. Implications on the tectonic evolution of western Pangea

 

Bodo Weber¹, Peter Schaaf², Victor A. Valencia³, Alexander Iriondo⁴, and Fernando Ortega–Gutiérrez²

 

1 División Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Km. 107 carretera Tijuana–Ensenada, 22860 Ensenada BC, Mexico.
E–mail: bweber@cicese.mx

2 Instituto de Geofísica, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., Mexico.

3 Department of Geosciences, University of Arizona, 1040 East Fourth St., Tucson AZ, 85721–0077 U.S.A.

4 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, Mexico. and Department of Geological Sciences, University of Colorado at Boulder, Boulder CO, 80309 U.S.A.

 

Manuscript received: March 14, 2006
Corrected manuscript received: July 3, 2006
Manuscript accepted: July 7, 2006

 

ABSTRACT

The Santa Rosa Formation in the State of Chiapas is a sequence of flysch–type sediments of Mississippian to Pennsylvanian age. These sedimentary rocks correlate with the Santa Rosa Group of Guatemala and Belize and crop out along the southern limit of the Maya block north of the Motagua fault, which is currently considered the border between the North American and the Caribbean plates. Ages of individual zircon grains from sandstones of the Upper Santa Rosa Formation in southern Mexico were analyzed by Laser Ablation Multicollector ICPMS and by SHRIMP. The youngest zircon population is of Silurian age (˜420 Ma), but most grains have ages that correspond to the Pan–African–Brasiliano orogenic cycle (500–700 Ma). Other minor populations have ˜820 Ma, Grenville (1.0–1.3 Ga), Mesoproterozoic (1.4–1.6 Ga), Paleoproterozoic (1.8–2.2 Ga), and Archean (2.7–3.1 Ga) ages. Most of the sediments came from either present–day West Africa or NE South America, where both Pan–African–Brasiliano orogens and cratonic landmasses are present. In our model, southwestward progressive collision of Gondwana with Laurentia during the Alleghanian orogeny resulted in erosion and deposition of flysch–type sediments to the west, followed by westward movement of the Maya block and adjacent lithosphere.

Key words: provenance ages, zircon, sediments, SE Mexico, Gondwana, Pan–African–Brasiliano.

 

RESUMEN

La formación Santa Rosa en el estado de Chiapas es una secuencia de sedimentos tipo flysch de edad Misisípica a Pensilvánica. Estas rocas sedimentarias correlacionan con el Grupo Santa Rosa de Guatemala y de Belice y afloran a lo largo del límite sur del bloque Maya al norte de la falla Motagua, la cual se considera actualmente como el límite entre las placas de Norteamérica y del Caribe. Se analizaron edades de zircones individuales de areniscas de la Formación Santa Rosa Superior en el Sur de México por ICPMS multicolector con ablación con láser y con SHRIMP. La población más joven de zircones es de edad silúrica (˜420 Ma), pero la mayoría de los zircones tiene edades que corresponden con el ciclo orogénico Pan–Africano–Brasiliano (500–700 Ma). Otras poblaciones menores tienen edades de ˜820 Ma, del Grenvilleano (1.0–1.3 Ga), del Mesoproterozoico (1.4–1.6 Ga), del Paleoproterozoico (1.8–2.2 Ga) y del Arqueano (2.7–3.1 Ga). La mayoria de los sedimentos proviene del oeste de Africa o del este de Sudamérica, donde se encuentran tanto orógenos con edades del ciclo Pan–Africano–Brasiliano como cratones precámbricos. En nuestro modelo, la colisión progresiva entre Gondwana y Laurentia durante la orogenia Alleghaniana resultó en erosión y deposición de los sedimentos flyschoides hacia el oeste, seguido por un movimiento del bloque Maya y la litosfera adyacente en dirección poniente.

Palabras clave: edades de proveniencia, zircón, sedimentos, SE México, Gondwana, Pan–Africano–Brasiliano.

 

INTRODUCTION

Since the first reconstruction of Pangea by Bullard et al. (1965) with overlapping areas between South America and parts of southern Mexico, the paleogeographic position of pre–Mesozoic crustal blocks in Mexico, Central America, and the Caribbean region have been of special interest in solving the spatial problems posed by the Pangea reconstruction (Ross and Scotese, 1988; Pindell and Barrett, 1990; Pindell et al., 2000). Southern Mexico and Central America consist of several blocks with different crustal evolution that are separated by major fault zones, and hence these crustal blocks were defined as tectono–stratigraphic terranes whose origin and relation to each other is uncertain (Campa and Coney, 1983; Sedlock et al., 1993; Ortega–Gutiérrez et al., 1994). Large scale sinistral strike–slip movements along the hypothetical Mojave–Sonora Megashear (e.g., Anderson and Schmidt, 1983), the Trans–Mexican Volcanic Belt (e.g., Shurbet and Cebull, 1984), and along the Motagua–Polochic fault system (e.g., Anderson and Schmidt, 1983; Burkart and Serf, 1985) have been assumed to accommodate the Mexican terranes and the Chords block of Central America prior to the opening of the Gulf of Mexico in Triassic to Jurassic times. Plate reconstructions on the basis of geological and geophysical constraints have shown that most of the Mexican terranes reached their present position with respect to Laurentia after Carboniferous times (Dickinson and Lawton, 2001). On the other hand, Precambrian (Grenville) granulite basement, the Oaxaquia micro–continent, which is thought to underlie most of central and southern Mexico (Ortega–Gutiérrez et al., 1995) and a Permian magmatic arc which occur along the entire length of Mexico (Torres et al., 1999), indicate that these crustal blocks were in contact at least since Permian times. The pre–Mesozoic positions of all these land masses either as peri–Gondwanan blocks between Gondwana and Laurentia or as outboard terranes in the Pacific margin, are important for the understanding of the late Paleozoic assembly of western Pangea.

Provenance studies of zircons from late Paleozoic sedimentary rocks in southeastern Mexico provide important arguments (1) to define the paleo–positions of the crustal blocks prior to the assemblage of Pangea and (2) to test geologic relations proposed between adjacent geologic units and crustal blocks. In this paper, we present U–Pb ages of individual zircons from sandstones of the late Paleozoic Santa Rosa Superior Formation from southern Mexico, which were obtained both by SHRIMP (sensitive high–resolution ion microprobe) and LA–MC–ICPMS (laser ablation– multi–collector–inductively coupled plasma mass–spectrometer) analysis.

 

GEOLOGICAL BACKGROUND

Regional geology

The pre–Mesozoic basement rocks of Central America extent from the state of Chiapas in Mexico to Guatemala, Belize, and Honduras (Figure 1). The sinistral Motagua–Polochic fault system is the most important tectonic structure that crosses the entire region. This Neogene fault system is the plate boundary between the North American and Caribbean plates, continuing from the Cayman trough in the Caribbean Sea to the east into the Gulf of Tehuantepec to the west (e.g. Muehlberger and Ritchie, 1975; Burkart, 1983).

The Maya block (Dengo, 1985), Mayaterrane (Sedlock et al., 1993), or Yucatan–Chiapas block (Dickinson and Lawton, 2001) is exposed north of the Motagua fault as the southeasternmost of the Mexican terranes and includes the Yucatan peninsula, parts of the coastal plain of the Gulf of México, and southeastern México to the Tehuantepec isthmus. Pre–Mesozoic rocks crop out only in the southern part of the Maya block. East of the Tehuantepec isthmus, the Chiapas Massif extends over an area of more than 20,000 km² parallel to the Pacific coast (Figure 1b). It is the most voluminous of the Permian crystalline complexes in México and is composed mainly of deformed plutonic and metamorphic rocks (Damon et al, 1981; Schaaf et al, 2002; Weber et al., 2005). Most of the metamorphic rocks in the Chiapas massif are orthogneisses; however, metasedimentary rocks crop out in several areas of the massif (Weber et al., 2002). The protoliths of these metasedimentary rocks are pelitic andpsammitic clastic sediments, graywackes, calcsilicates, and limestones. These sequences reached medium– to high–grade metamorphic conditions and anatexis during a late Permian orogenic event, and they were synchronously intruded by igneous rocks of the Chiapas batholith (Weber et al., 2006; Hillere et al., 2004). Late Paleozoic (Mississipian to Leonardian) sediments of the Santa Rosa Group crop out east of the Chiapas Massif, in the central cordillera of Guatemala, and in the Maya Mountains of Belize (Figure 1b). At least in the Maya Mountains, older sediments intruded by Silurian granites (Steiner and Walker, 1996) underlie the Santa Rosa Group. The Chuacús group (Dengo, 1985) or Chuacús complex (Ortega–Gutiérrez etal., 2004) is situated between the Polochic and the Motagua faults in central Guatemala (Figure 1b) and contains Paleozoic medium– to high–grade metamorphic rocks witheclogitic relicts. Dengo (1985) correlated the Chuacús complex with metasediments from the Chiapas Massif, and thus the Chuacús complex was considered as belonging to the Maya block. However, recent studies suggested that the Chuacús complex is possibly an independent fault–bounded terrane between the Maya and the Chortis blocks (Ortega–Gutierrez et al, 2004).

The Chortis block is located south of the Motagua fault and includes southern Guatemala, El Salvador, Honduras, and most of Nicaragua (e.g., Dengo, 1985 and references therein). The Chortis block does not contain unmetamorphosed Paleozoic rocks like the Maya block. Immediately south of the Motagua fault zone, amphibolite–facies rocks and migmatites of the Las Ovejas complex represent the basement of the Chortis block in southern Guatemala. The most extensive sequence in the Chortis block of possible Paleozoic age is composed of low–grade metasedimentary rocks that were tentatively correlated with the Santa Rosa Group by Clemons (1966). However, a correlation of the different basement units throughout the block seems to be problematic (Donelly et al, 1990). The metamorphic basement of the Chortí s block is overlain by a thick sequence of Mesozoic sedimentary rocks, somewhat different from the Mesozoic sequence in the Maya block (e.g., Dengo, 1985, and references therein).

 

The Santa Rosa Group

The name "Santa Rosa" was first applied by Dollfus and Montserrat (1868) to clastic sedimentary rocks with interbedded limestones exposed in western Guatemala. Sapper (1937) applied the name "Santa Rosa" to a sequence of Carboniferous shales with intercalated limestone in central and western Guatemala. The original type locality (village of Santa Rosa, Baja Verapaz, Guatemala) was later demonstrated as being correlative with the Todos Santos Formation of Jurassic age (Vinson, 1962). In order to avoid confusion because of its absence at the type locality of Santa Rosa, the most abundant Upper Paleozoic shale sequence was renamed as Tactic Formation (Warper, 1960). This shale sequence is transitional into overlying massive–bedded dolomites and limestones of the Permian Chochal Formation with highly fossiliferous zones of fusulinids, corals, and brachypods of Leonardian age (Roberts and Irving, 1957; Clemons and Burkart, 1971). Clemons and Burkart (1971) proposed the use of the name Santa Rosa Group, which, in western Guatemala, includes three formations (Figure 2): (1) the lower Chicol Formation exposed near Huehuetenango is a sequence of conglomerates and breccias between 800 and 1,200 m thick; (2) the middle Tactic Formation of Pennsylvanian–Permian age is the most widespread unit of Paleozoic rocks in northwestern Guatemala, mainly composed of brown to black shales, mudstone, minor siltstone and fine sandstone beds with a maximum thickness of 1,000 m; (3) the Esperanza Formation is the uppermost unit of the Santa Rosa Group in Guatemala. Fusulinids of the genus Schwagerina cf. S. campenis indicate Wolfcampanian age for these limestones with interbedded fossiliferous shales, sandstones and dolomites (Clemons and Burkart, 1971).

In Chiapas, Paleozoic sedimentary rocks correlative with the Santa Rosa Group were described from northwest and around the village of Chicomuselo (Figure 3). They constitute a thick sequence of flysch–type rocks with an estimated overall thickness of ˜5,800 m (López–Ramos, 1979) that was subdivided into two major sequences, the Lower and the Upper Santa Rosa Formation (Figure 2; Hernández–García, 1973). The Lower Santa Rosa Formation is exposed east of Angel Albino Corzo (Figure 3), and is composed of metamorphosed (partly garnet–bearing) schists andphyllites with intercalated horizons of metaquartzites and a 10 m thick conglomerate (Hernández–García, 1973). Based on a fossiliferous horizon with Paleozoic crinoids and pelecypods (lamellibranchia), Hernández–García (1973) interpreted this sequence as being at least of late Mississippian age. Its lower limit is not exposed, and its upper limit is discordant with respect to the overlying Upper Santa Rosa Formation of late Pennsylvanian age.

The Upper Santa Rosa Formation in Chiapas is best exposed in the area around Chicomuselo (Figure 3); it is a sequence of shale, slightly calcareous siltstone, and rarely sandy siltstone, which occasionally alternates with 0.5 to 1.2 m thick sandstones of yellowish brown and grayish green colors. These sediments do not contain identifiable fossils, but on the basis of their lithological similarity and stratigraphic relations they were correlated with the Tactic Formation of Guatemala (Hernández–García, 1973). North of Chicomuselo, the Upper Santa Rosa Formation is unconformably covered by siliceous shales and limestones of the Grupera Formation which contains fusulinids (Schwagerina) of Early Permian (Wolfcampanian) age (Hernández–García, 1973; López–Ramos, 1979). Therefore, the Grupera Formation correlates with the Esperanza Formation (Figure 2), which forms the uppermost part of the Santa Rosa Group in Guatemala. Similar to the Chochal Formation in Guatemala, fossiliferous gray limestones of Leonardian age concordantly cover the Grupera Formation. These limestones crop out extensively south and southeast of Chicomuselo and are named Paso Hondo Formation (Figures 2 and 3; Hernández–García, 1973).

The Santa Rosa Group was also correlated with the Paleozoic sediments and low–grade metasediments that form the largest part of the Maya Mountains in Belize (Figure lb; Dixon, 1956; Bateson and Hall, 1977). Dixon (1956) defined a lower Maya Series with metamorphosed and isoclinally folded metasediments, which is discordantly overlain by the Macal series, consisting of a sequence of open–folded clastic sediments with distinctive limestone layers. (Figure 2). Bateson and Hall (1977) discarded Dixon's dichotomy and combined the Maya and the Macal series into the Santa Rosa Group with continuous sedimentation from the Pennsylvanian to the middle Permian. However, since Steiner and Walker (1996) reported Silurian crystallization ages of granites intruded into the sediments of the Maya Mountains, it seems improbable that all these sediments belong to the Santa Rosa Group. If this intrusive relation is true, Dixon's Maya Series, which do not contain fossils, might actually be of pre–Silurian age.

 

ANALYTICAL TECHNIQUES

Zircons were separated by standard procedures at the Geology Department at Centro de Investigación Científica y de Educación Superior de Ensenada Baja California (CICESE), and at the Instituto de Geología at Universidad Nacional Autónoma de México (UNAM), by using a Wilfley® table, a Frantz® isodynamic separator, heavy liquids, and handpicking techniques.

 

SHRIMP–RG

The SHRIMP procedures used in this study are similar to those reported inNourse et al. (2005). Zircons handpicked from a total sample population and chips of zircon standard R33 were mounted in epoxy, ground to nearly half their thickness, and polished with 6– and l–µm–grit diamond suspension abrasive. The mounts were cleaned in IN HC1 to avoid surface related common lead introduced to mount during polishing, and gold coated for maximum surface conductivity.

The U–Th–Pb analyses were made on 49 individual zircon grains from sample SR01 using the SHRIMP–RG (Sensitive High Resolution Ion Microprobe – Reverse Geometry) housed at Stanford University, California. The primary oxygen ion beam, operated at about 2–4 nA, excavated an area of about 25–30 µm in diameter to a depth of about 1 µm; sensitivity ranged from 5 to 30 cps per ppm Pb. Data for each spot were collected in sets of five scans through the mass range. Isotope ratios were corrected for common Pb using the measured ²⁰⁴Pb. The reduced ²⁰⁶Pb/ ²³⁸U ratios were normalized to the zircon standard R33 which has a concordant TIMS age of 418.9 ± 0.4 Ma (2σ) (Black et al., 2004). For the closest control of Pb/U ratios, one standard was analyzed after every four unknown samples. Uranium concentrations were monitored by analyzing a standard (CZ3) with ˜550 ppm U. U and Pb concentrations are accurate to about 10–20%. SHRIMP isotopic data were reduced and plotted using the Squid and IsoplotEx programs of Ludwig (2001, 2003).

 

Laser ablation multicollector ICP–MS

LA–MC–ICPMS analyses were conducted following the method described by Dickinson and Gehrels (2003). Briefly, several hundred zircon crystals from CB–55 and fragments of a standard zircon were mounted in the inner half of the mount area. One hundred zircons were analyzed from polished section. The grains analyzed were selected at random from all of the zircons mounted. Cores of grains were preferred to avoid possible metamorphic overgrowths.

Zircon crystals were analyzed with a Micromass Isoprobe multicollector ICPMS equipped with nine Faraday collectors, an axial Daly collector, and four ion–counting channels. The Isoprobe is linked to a New Wave ArF Excimer laser ablation system, which has an emission wavelength of 193 nm. The collector configuration allows measurement of ²⁰⁴Pb with an ion–counting channel whereas ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th and ²³⁸U measured simultaneously with Faraday detectors. All analyses were conducted in static mode with a laser beam diameter of 35 µm, operated with an output energy of ˜32 mJ (at 23 kV) and a pulse rate of 9 Hz. Each analysis consisted of one 20–second integration on peaks with no laser firing for backgrounds and twenty 1–second integrations on peaks with the laser firing. Hg contribution to the ²⁰⁴Pb mass position was removed by subtracting on–peak background values. Inter–element fractionation was monitored by analyzing an in–house zircon standard which has a concordant ID–TIMS age of 564 ± 4 Ma (2σ) (Dickinson and Gehrels, 2003). This standard was analyzed once for every five unknown zircon grains. Uranium and Thorium concentrations were monitored by analyzing a standard (NIST 610 Glass) with ˜500 ppm Th and U. The lead isotopic ratios were corrected for common Pb, using the measured ²⁰⁴Pb, assuming an initial Pb composition according to Stacey and Kramers (1975) and respective uncertainties of 1.0, 0.3 and 2.0 for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb.

Systematic errors are propagated separately, and include the age of the standard, calibration correction from standard analyses, composition of commonPb, and U decay constant uncertainties. For these samples the systematic errors were 0.9 % for ²⁰⁶Pb/²³⁸U and 1.3 for ²⁰⁷Pb/²⁰⁶Pb. The age probability plots (Ludwig, 2003) used in this study were constructed using the ²⁰⁶Pb/²³⁸U age for young (<0.9 Ga) zircons and the ²⁰⁷Pb/²⁰⁶Pb age for older (>0.9 Ga) grains. Interpreted ages are based on ²⁰⁶Pb/ ²³⁸U for <900 Ma grains and on ²⁰⁷Pb/²⁰⁶Pb for >900 Ma grains. This division at 900 Ma results from the increasing uncertainty of ²⁰⁶Pb/²³⁸U ages and the decreasing uncertainty of ²⁰⁷Pb/²⁰⁶Pb ages as a function of age. The resulting interpreted ages are shown on relative age–probability diagrams (from Ludwig, 2003). These diagrams show each age and its uncertainty (for measurement error only) as a normal distribution, and sum all ages from a sample into a single curve. All errors of both techniques are reported at the 1–σ level in Tables 1 and 2.

For a better comparison of the reliabilities between both methods, the individual errors are depicted in the diagrams (Figures 4, 5, 6, 7) at the 2–σ level as it is common practice for LA–MC–ICPMS (e.g., Dickinson and Gehrels, 2003).

Both analytical techniques (ion probe and LA–MC–ICPMS) are widely used for the determination of provenance ages of individual zircons (e.g., DeGraaff–Surpless et al, 2002; Dickinson and Gehrels, 2003; Weislogel et al., 2006). The results and errors of both methods are similar for zircons that are not complex. The main difference, in practice, is the cost and the number of grains analyzed given that each ion probe analysis takes ˜15 minutes whereas an analysis by LA–MC–ICPMS takes only ˜90 seconds. For complex zircons, however, an ion probe is a more powerful tool because the excavation depth is <1 micron in comparison with 10–15 microns for laser ablation.

 

RESULTS

We analyzed zircons from two samples of fine–grained sandstone layers from the Upper Santa Rosa Formation close to Chicomuselo (Figure 3). Both samples have similar mineralogical compositions with 40–50% quartz, 20–30% altered feldspar, 10–15% altered oxides, 5–10% white mica, ˜5% fresh plagioclase, and another 5% composed of chlorite, biotite, and heavy minerals like green tourmaline, zircon, titanite, and apatite. Matrix composed of sericite is rare. Clastic grains are principally quartz, plagioclase and altered feldspar. Occasionally, lithic fragments of very fine–grained quartzite could be observed (CB55). The clastic grains are little rounded, indicating, together with abundant detrital phyllosilicates, a nearby provenance of most of the detritus.

Zircons from sample CB55 were analyzed by LA–MC–ICPMS, and the data are listed in Table 1. Figure 4 shows a Concordia plot of all data (a) together with a relative probability plot (b) and a histogram plot (c). The oldest zircon analyzed has an apparent minimum ²⁰⁷Pb/²⁰⁶Pb age of 3.1 Ga, although its isotope ratios yield discordant ages. This grain and another almost concordant at 2.8 Ga indicate the presence of Archean zircons in the sample. A small group of four zircons represent a Paleoproterozoic population (²⁰⁷Pb/²⁰⁶Pb ages = 2.0–2.16 Ga). One concordant grain is about 1.88 Ga old. The next small group of three zircons has ages of 1.4 to 1.6 Ga. Several fairly concordant zircons (eight grains) have ²⁰⁷Pb/²⁰⁶Pb apparent ages between 1.3 and 0.95 Ga, which can be grouped together with the Grenville–type zircons.

The great majority of zircons (about 75 %) are younger than Mesoproterozoic. A detailed concordia plot of these mostly concordant zircons together with weighted mean age calculations of prominent age groups is given in Figure 5. A small group of five early Neoproterozoic grains (group 5, Figure 5) yielded a mean ²⁰⁶Pb/²³⁸U age of 816 ± 47 Ma (2σ,). Isotopic ratios of 36 grains have apparent ²⁰⁶Pb/²³⁸U ages between 500 and 700 Ma, of which 28 are between 500 and 600 Ma (Figure 4c). From these zircons, three age groups were distinguished. A group of six grains yielded a mean ²⁰⁶Pb/²³⁸U age of 629.0 ± 9.6 Ma (2σ, group 4, Figure 5), a group of nine grains yielded a mean ²⁰⁶Pb/²³⁸U age of 544.7 ± 6.0 Ma (2σ, group 3, Figure 5), and another group of nine grains yielded a mean ²⁰⁶Pb/²³⁸U age of 521.0 ± 7.0 Ma (2σ, group 2, Figure 5). The youngest zircons (group 1, Figure 5) yielded a late Silurian mean ²⁰⁶Pb/²³⁸U age of 422.0 ± 12.4 Ma (2σ).

Zircons from sample SR01 were analyzed by SHRIMP, and the data are shown in Table 2. Figure 6 shows a concordia plot of all data (a) together with a relative probability plot (b) and a histogram plot (c). Three grains, of which two are nearly concordant and one discordant (Figure 6a), are of late Archean age. A group of five grains have apparent ²⁰⁷Pb/²⁰⁶Pb ages between 1.9 and 2.1 Ga, and another measurement yielded a ²⁰⁷Pb/²⁰⁶Pb age of 1.76 Ga. Three concordant grains are about 1.4 Ga old, and another discordant zircon has a similar ²⁰⁷Pb/²⁰⁶Pb age. Each of the 1.2 to 1.3 Ga and the 0.95 to 1.0 Ga Grenville–type ages are represented by two concordant grains. About half of the analyzed spots yielded Pan–African–Brasiliano ages between 500 and 700 Ma. Two groups of concordant zircons yielded mean ²⁰⁶Pb/²³⁸U ages of 629 ± 21 Ma (2σ, group 3, Figure 6) and 558 ± 7 Ma (2σ, group 2, Figure 7). Another group of seven zircons (group 1, Figure 7) yielded a mean ²⁰⁶Pb/²³⁸U ageof416±19Ma(2σ).

 

DISCUSSION

The U–Pb zircon data presented here clearly show that the main source area of detrital components for the sandstone samples from the Upper Santa Rosa Formation at Chicomuselo is dominated by the Pan–African–Brasiliano orogenic cycle. This source includes rocks with ages of ˜630 Ma, 540–560 Ma, ˜520 Ma and, in a broader sense, also a less pronounced population of ˜820 Ma. There are no outcrops of igneous and metamorphic rocks of any of those ages known from Mexico and Central America that may be considered as the local source of sediments for the Upper Santa Rosa Formation. However, Krogh et al. (1993) reported an average age of 545 ± 5 Ma for shocked zircons from ejecta of the Chicxulub impact structure of northwestern Yucatán, and Lopez et al. (2001) obtained a 580 ± 4 Ma concordant zircon age from granitic boulders within sediments of the Mexican state of Coahuila. Therefore, detrital zircons from the Pan–African–Brasiliano orogenic cycle are not uncommon in the Maya block and probably occur in similar Paleozoic sediments of eastern and northeastern Mexico.

It is widely accepted that, after fragmentation of Rodinia in the early Neoproterozoic, Western Gondwana was assembled by diachronous convergence and collision of several cratonic landmasses which lasted from 650 to 500 Ma (e.g., Veevers, 2003). Circum–cratonic convergence and collision of the West African craton started in the Bassarides–Mauritanides (BA, Figure 8) of present–day West Africa at 665–655 Ma, progressing clockwise around the craton, arriving at the Brasiliano belt (BR, Figure 8) at 600–550 Ma, and finally at the Rokelides (R, Figure 8) with ages from 547–500 Ma in West Africa (e.g., Doblas et al., 2002) and Florida (e.g., Hatcher, 2002). The Brazilide Ocean was closed at 650–600 Ma, followed by intracontinental convergence that culminanted at ˜550 Ma during the final assemblage of Western Gondwanaland (e.g., Alkmin et al., 2001). Taking into consideration these models, the most probable source regions for the Pan–African–Brasiliano zircons of the Upper Santa Rosa Formation are West Africa and northeastern South America. Archean (2.6–3.1 Ga) zircons and more abundant Paleoproterozoic (1.8–2.2 Ga) populations probably came either from the northern Amazonian or West African cratons, indicating similar source regions as for the Pan–African–Brasiliano zircons. In Florida, detrital zircons from a subsurface sandstone sample have main age populations of (1) 515 to 637 Ma and (2) 1.9 to 2.3 Ga (Mueller et al, 1994), indistinguishable from our present zircon age data and indicating a similar provenance for the Florida sedimentary basement and the Upper Santa Rosa Formation.

Zircons of Mesoproterozoic (1.4–1.6) and Grenville (0.95–1.3 Ga) ages must be from a different source, as there is little or no record of those ages from eastern South America and West Africa. These zircons, although the populations are of minor importance, may either come from one of the terranes with Grenville affinity, namely Oxaquia and the Colombian or Venezuelan terranes where such ages are common (Restrepo–Pace et al, 1997; Alemán and Ramos, 2000), or they come from eastern Laurentia. The youngest zircon population from the Upper Santa Rosa Formation is of Silurian age (˜420 Ma). Granite intrusions of this age are exposed in the southern Maya block in the Maya Mountains of Belize (Figure 1; Steiner and Walker, 1996). These intrusive rocks were either already exposed to erosion in the late Carboniferous, or the zircons came from contemporaneous felsic volcanic rocks which can be observed as boulders in some of the conglomerate layers of the Santa Rosa Formation.

The provenance ages of the Upper Santa Rosa Formation support a model in which the Maya block, together with other terranes with similar late Paleozoic flysch–type sedimentary rocks, like the Delicias Basin in Coahuila, Mexico (McKee et al, 1999), Florida, and the Mérida terrane of Venezuela (Alemán and Ramos, 2000), were located at the northwestern Gondwana margin during the late Carboniferous (Figure 8). By closing the Theic Ocean, Gondwana collided with Laurentia during the Alleghanian orogeny. This orogeny lasted from 320 to 280 Ma, starting in the northern Appalachians and ending in the Marathon–Ouachita belt. Hatcher (2002) explained the diachronous closure of the former Theic ocean and the Alleghanian orogeny by so–called "zipper tectonics" which means rotational (clockwise) transpressive continent–continent collision. Zipper closing started in the present northwest, inducing dextral srike–slip deformation in the southern and central Appalachians.

On the basis of this hypothesis, our new zircon data, and previous paleogeographic reconstructions (Rowley and Pindell, 1989; Dickinson and Lawton, 2001; Elias–Herrera and Ortega–Gutiérrez, 2002) we suggest the following model (Figure 8): 1) During the late Carboniferous (deposition of the Upper Santa Rosa Formation), the Maya block together with Florida and other supracrustal blocks was located close to West Africa or northeastern South America, defined as Gondwana margin or Perigondwanan terranes; 2) most of the flysch–type sedimentation occurred southwestward by erosion of the newly formed mountain chains of the early Alleghanian orogen; (3) ongoing zipper tectonics towards the southwest caused dextral strike–slip movement south of the future Ouachita–suture and westward movement of the Maya block, probably together with other similar crustal blocks, and attached oceanic lithosphere. The accommodation of these blocks and Grenville–type basement terranes, especially Oaxaquia, at the western margin of the newly formed Pangea supercontinent, together with initiating subduction and arc–magmatism along the western margin of former Gondwana, stopped sedimentation and westward movement of the Maya block. This is documented by deformation, metamorphism, and magmatism during the Permian in the southern Maya block, namely in the Chiapas Massif (Weber ei a/., 2005, 2006).

 

ACKNOWLEDGEMENTS

This work was supported by CONACYT project D41083–F and DFG/BMZ collaboration project HE2893/4–1. Many thanks to Susana Rosas–Montoya, Victor Pérez–Arroyoz, and Gabriel Rendón–Márquez (CICESE) for their help with preparing the zircon separates. Many thanks go to Teodoro Hernández–Treviño (UNAM) for sample preparation. We are grateful to Joe Wooden and Wayne Premo (both USGS) for assistance with running the SHRIMP–RG at Stanford University and to George Gehlers and Joaquin Ruiz for their assistance with running the LA–MC–ICPMS and data reduction at University of Arizona, Tucson (NSF EAR–0443387). We thank Alfred Kroner (University of Mainz) and Luigi Solari (UNAM) for their helpful reviews.

 

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