Sección Especial

 

U–Pb geochronology of granitoids in the north–western boundary of the Xolapa Terrane

 

Geocronología U–Pb de granitoides del límite noroeste del terreno Xolapa

 

Victor A. Valencia^{1, *}, Mihai Ducea¹, Oscar Talavera–Mendoza², George Gehrels¹, Joaquin Ruiz¹, and Sarah Shoemaker¹

 

¹ University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA. ^*victorv@email.arizona.edu

² Unidad Académica Ciencias de la Tierra, Universidad Autónoma de Guerrero, A.P. 197, 40200 Taxco, Guerrero, Mexico.

 

Manuscript received: December 12, 2007
Corrected manuscript received: August 28, 2008
Manuscript accepted: August 26, 2008

 

ABSTRACT

The Sierra Madre del Sur, a Mesozoic–Cenozoic magmatic arc in southern Mexico, was studied using U–Pb zircon geochronology. Undeformed to slightly deformed plutons from two transects were sampled at the limit between the Guerrero and Xolapa terranes, in order to constrain the magmatic history, nature of the basement and terrane boundaries. Four samples from the Zihuatanejo, Guerrero, transect within the Guerrero terrane, yielded crystallization ages of 41.8 ± 1.4, 43.4 ± 1.6, 40.8 ± 1.4 and, 41.8 ± 4.6 Ma. No inherited zircons were detected in these plutons indicating that pre–existing zircons from continental basement or sediments are not a significant component in these rocks. Five samples from the Atoyac, Guerrero transect within the Xolapa terrane, yielded crystallization ages of 53.5 ± 1.9, 52.7 ± 1.9, 57.3 ± 2.2, 54.4 ± 1.7, and 57.0 ± 2.1 Ma, analogous to the ages reported for the Acapulco intrusive. One sample of this transect yielded an age of 40.2 Ma with an inherited component of 58–64 Ma, similar to the ages determined for the first five samples. Several clusters of Mesozoic inherited zircons with ages of 72–74 Ma, 83–87 Ma, 90–92 Ma, 105–111 Ma and, 143–153 Ma, indicate that the magmatism in the Xolapa terrane was active since the Jurassic, and that multiple episodes of magmatism occurred during the Cretaceous. Inherited zircons also indicate that processes of assimilation and recycling of previous intrusive bodies have played an important role in the evolution of the Xolapa Complex. Older Paleozoic (~320 Ma; ~360 Ma) and Grenvillian (~960–1085 Ma) inherited zircons ages suggest an affinity of the Xolapa Complex with the Acatlán and Oaxaca Complexes, even though the metasedimentary basement of the Xolapa complex (of unknown age) may be the source of these Paleozoic and Grenvillian zircons. The presence of inherited zircons in the Atoyac transect suggests that the limit between the Xolapa and Guerrero terranes is located between these two transects.

Key words: U–Pb, zircon, arc magmatism, Xolapa, Mexico.

 

RESUMEN

La Sierra Madre del Sur, un arco magmático del Mesozoico–Cenozoico localizado en el sur de México, fue estudiado usando geocronología de U–Pb en circones. Plutones no deformados a ligeramente deformados de dos transectos localizados en el límite entre los terrenos Guerrero y Xolapa fueron muestreados, buscando precisar la historia magmática, naturaleza del basamento y límite de terrenos. Cuatro muestras del transecto Zihuatanejo, Guerrero, dentro del terreno Guerrero, produjeron edades de cristalización de 41.8 ± 1.4, 43.4 ± 1.6, 40.8 ± 1.4 y, 41.8 ± 4.6 Ma. No se detectaron circones heredados en estos cuerpos plutónicos, lo cual indica que el basamento continental o sedimentos no son un componente significantivo en estas rocas. Cinco muestras del transecto Atoyac, Guerrero, dentro del terreno Xolapa, produjeron edades de cristalización de 53.5 ± 1.9, 52.7 ± 1.9, 57.3 ± 2.2, 54.4 ± 1.7 y 57.0 ± 2.1 Ma, análogas a las reportadas para el intrusivo de Acapulco. Una de las muestras de este transecto produjo una edad de 40.2 Ma con un componente heredado de 58–64 Ma, similar a las edades determinadas para las otras cinco muestras. Las diferentes poblaciones mesozoicas de circones heredados con edades de 72–74 Ma, 83–87 Ma, 90–92 Ma, 105–111 Ma y 143–153 Ma, indican que el magmatismo en el terreno Xolapa estaba activo desde el Jurásico, y que múltiples episodios de magmatismo ocurrieron durante el Cretácico. Circones heredados también indican que el proceso de asimilación y reciclaje de los cuerpos intrusivos previos han tenido un papel importante en la evolución del complejo Xolapa. Las edades más antiguas de circones heredados: Paleozoico (~320 Ma; ~360 Ma) y Grenviliano (~960–1085 Ma), sugieren una afinidad del complejo Xolapa con los complejos Acatlán y Oaxaca, aunque el basamento metasedimentario del complejo Xolapa podría ser la fuente de estos circones. La presencia de circones heredados en el transecto de Atoyac sugiere que el límite entre los terrenos Xolapa y Guerrero está localizado entre los dos transectos estudiados.

Palabras clave: U–Pb, circón, magmatismo de arco, Xolapa, México.

 

INTRODUCTION

The Xolapa Terrane (Campa and Coney, 1983), also known as the Chatino Terrane (Sedlock et al., 1993) is a long belt of high grade metamorphic and plutonic rocks of Proterozoic to Cenozoic age facing the Pacific coast in the states of Guerrero and Oaxaca in the Sierra Madre del Sur, Mexico (Figure 1). The origin of this terrane is in debate and has been interpreted as an allochtonous magmatic arc accreted during Late Cretaceous–Early Tertiary time (Campa and Coney, 1983; Coney, 1983), whereas an autochtonous magmatic arc origin has been proposed by others (i.e., Ratschbacher et al.,1991; Morán Zenteno, 1992; Hermann et al. 1994; and Ducea et al. 2004).

Due to widespread Cenozoic volcanic cover, intrusive rocks and abundant vegetation, the precise location of the boundary between the Xolapa and the Guerrero terranes is also unclear. Campa and Coney (1983) and Coney and Campa (1987) placed this boundary between Zihuatanejo and Petatlán. Meschede et al. (1997) characterized this limit as a normal fault with the hanging wall moving to the northwest as registered by mylonites and ultramylonites. On the other hand, Sedlock et al. (1993) placed the boundary between the Guerrero and Xolapa terranes following the trace of the Papalutla fault, which according to these authors is the limit between the Guerrero and Mixteco terranes (Figure 1). They argue that this limit is obscured by Tertiary granites emplaced in the contact between both terranes in the area east of Petatlán (Sedlock et al., 1993) or between Petatlán and Atoyac (Tolson et al., 1993).

At present, most geochronological data obtained in the Xolapa terrane has been obtained from Rb–Sr and K–Ar mineral data (e.g., Guerrero–García, 1975; Morán–Zenteno, 1992; Schaaf et al., 1995) that may not necessarily reflect the crystallization age of plutons but rather their cooling history defined by through their respective closure temperatures. Some valuable U–Pb zircon multifraction data has been obtained in different areas (Robinson et al., 1989; Herrmann et al., 1994; Schaaf et al., 1995). Ducea et al. (2004) obtained U–Pb single zircon ages of plutonic and metaplutonic rocks from three transects south of Acapulco using LA–MC–ICPMS. Recently, Solari et al. (2007) reported ID–TIMS U–Pb ages of metaplutonic rocks from the Tierra Colorada–Acapulco sector.

The aim of this study is to present new LA–MC–ICPMS U–Pb data for rocks from Atoyac de Álvarez and Zihuatanejo–Altamirano transects in order to document the crystallization ages of plutonic rocks, to constrain the nature of the basement, and to precisely locate the boundary between the Guerrero and Xolapa terranes.

 

GEOLOGICAL SETTING

The Xolapa Terrane (Campa and Coney, 1983) is a fault–bounded crustal block located in the Pacific margin of southern Mexico (Figure 1). It is bounded by the late Mesozoic Guerrero arc terrane and by the Proterozoic to Mesozoic Mixteco and Oaxaca terranes (Figure 2a). The Xolapa terrane mainly includes orthogneiss, paragneiss and rare marble of Proterozoic to Mesozoic age, which experienced regional deformation, amphibolite facies metamorphism and migmatization, and intrusion by undeformed, mid–Tertiary calc–alkaline granites (De Cserna, 1965; Herrmann et al., 1994; Morán–Zenteno et al., 1996; Ducea et al., 2004; Corona–Chávez et al., 2006; Solari et al., 2007). The high–grade metamorphism and migmatization is early Tertiary (65–46 Ma; Herrmann et al., 1994). Extensional deformation and uplift of southern Mexico occurred during the mid–Tertiary (30–25 Ma; Morán–Zenteno et al., 1996; Meschede et al., 1997). The northern boundary of the Xolapa terrane was mapped as a belt of mylonites with a normal–fault geometry (Ratschbacher et al., 1991; Riller et al., 1992, Herrmann, 1994). The other limits are still poorly understood.

 

ANALYTICAL METHOD

Around 10 kg sample of igneous rocks were collected at sites shown in Figures 1 and 2, and crushed and milled. Heavy mineral concentrates of the <350 microns fraction were separated magnetically. Inclusion–free zircons from the non–magnetic fraction were then handpicked under a binocular microscope. When possible at least fifty zircons from each sample were mounted in epoxy and polished for laser ablation analyses. Single zircon crystals were analyzed in a VG isoprobe multi–collector ICPMS equipped with nine Faraday collectors, an axial Daly detector, and four ion–counting channels (Gehrels et al., 2008). The isoprobe is equipped with an ArF Excimer laser, which has an emission wavelength of 193 nm. The analyses were conducted on 35 or 50 micron spots with an output energy of ~32 mJ and a repetition rate of 8 Hz. Each analysis consisted of one 20–second integration of background on peaks with no laser firing and twenty 1–second integrations on peaks with the laser firing. The depth of each ablation pit was ~15 microns. The collectors were configured to simultaneously measure ²⁰⁴Pb in an ion–counting channel, while ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U are measured with Faraday collectors. All analyses were conducted in static mode. Inter–element fractionation was monitored by analyzing fragments of SL–1, a large concordant zircon crystal from Sri Lanka (SL–1) with a known (ID–TIMS) age of 563.5 ± 3.2 Ma (2σ) (Gehrels et al., 2008). The reported ages for zircon grains are based on ²⁰⁶Pb/²⁰⁸U ratios because errors of the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²⁰⁷Pb ratios are significantly greater. This is due primarily to the low intensity (commonly <1 mV) of the ²⁰⁷Pb signal from these young, low–U grains. The ²⁰⁶Pb/²³⁸U ratios are corrected for common Pb by using the measured ²⁰⁶Pb/²⁰⁴Pb, and the common Pb composition (Stacey and Kramers, 1975) with an uncertainty of 1.0 unit on the assigned common ²⁰⁶Pb/²⁰⁴Pb (Gehrels et al., 2008).Zircons were studied optically under SEM in back–scattered electron (BSE) mode and cathodoluminescence (CL) images. Almost all zircons that yielded Cenozoic ages display igneous morphologies (e.g., euhedral crystals). Older zircons are generally smaller, rounded grains with overgrowths, which possibly indicates an inherited origin (Figure 3).

Ages from 5–25 zircon grains were measured from each sample. Results are reported in Table 1 where each line represents a spot analysis. The weighted mean of individual analyses were calculated according to Ludwig (2003). The mean age (Mean) considered only the measurement or random errors (errors in ²⁰⁶Pb/²³⁸U and ²⁰⁶Pb/²⁰⁴Pb of each unknown). For these samples the random error are 0.7–1.7 Ma (2σ), and represents ~1–2.9% of the age.

Age of standard, calibration correction from standard, composition of common Pb, decay constant uncertainty are the other sources that contributed to the error in the final age determination. These uncertainties are grouped and are known as the systematic error. For these samples the systematic error is ~1.2–2.0%. The error of the age for the sample is calculated adding quadratically the two components (random or measurment error and systematic error). All age uncertainties are reported at the 2–sigma level (2σ).

 

RESULTS

The plutonic rocks studied here are characterized by medium– to coarse–grained hypidiomorphic granular textures. The samples range in composition from quartz–monzodiorite to granite (Figure 4) and can be divided into two groups which correlate with the two studied transects. Rocks from the Atoyac transect consist exclusively of granites, whereas rocks from the Zihuatanejo transect consist of quartz monzodiorites, granodiorites and granites (Figure 4). Both groups are dominated by quartz, plagioclase, biotite, hornblende and magnetite in different proportions. Biotite is the principal mafic phase in the leucocratic rocks (Atoyac transect), whereas hornblende and biotite are abundant in the less silicic rocks of the Zihuatanejo transect.

Zihuatanejo transect

Four samples were analyzed from the Zihuatanejo transect (Table 2 and Figures 1 and 2). The rocks contain euhedral zircons that have typical igneous morphologies: prominent sharp pyramidal terminations, clear, transparent, and no detectable optical zoning. Their low U/Th ratios (<3) are consistent with a magmatic origin (Rubatto, 2002). The resulting ²⁰⁶Pb/²³⁸U ages are 41.8 ± 1.4 Ma (n=5, MSWD=1.6) for MO136; 43.4 ± 1.6 Ma (n= 17, MSWD =1.4) for MO137; 40.8 ± 1.4 Ma (n=20, MSWD =1.4) for MO138; and a concordant age of 41.8 ± 4.6 Ma (individual zircons are 39.1 ± 3.8 Ma and 45.1 ± 3.4 Ma) for MO139 (Figure 5). There are no inherited zircons in any of the analyzed samples. Overall, these ages are slightly older than the 37.4 to 40.5 Ma K–Ar ages in biotite–chlorite separates obtained by Stein et al. (1994) from similar units.

Atoyac transect

Six samples from the Atoyac transect were analyzed (Figure 1 and Table 2). Sample MO140 yielded a ²⁰⁶Pb/²³⁸U age of 53.5 ± 1.9 Ma (n=22 zircons, MSWD=2.7) with no inherited grains (Figure 6). Sample MO141 has a ²⁰⁶Pb/²³⁸U age of 52.7 ± 1.9 Ma (n= 14 zircons, MSWD=4.0). One zircon contains an inherited core of Carboniferous age (326 Ma). Sample MO142 has a ²⁰⁶Pb/²³⁸U age of 57.3 ± 2.2 Ma (n=13 zircons, MSWD=2.0); inherited cores yielded ages of ~70 Ma, ~90 Ma, 385 Ma and 1085 Ma. Sample MO143 has a ²⁰⁶Pb/²³⁸U age of 54.4 ± 1.7 Ma (n=17 zircons, MSWD=1.2), with inherited cores of ~100 Ma, (n=3), ~150 Ma, (n=3), 960 Ma and 1848 Ma. Sample MO144 has a ²⁰⁶Pb/²³⁸U age of 57.0 ± 2.1 Ma (n=13 zircons, MSWD=2.4) with a Late Cretaceous (~85 Ma, n=4) inherited component. Sample MO145 has a ²⁰⁶Pb/²³⁸U age of 40.2 ± 2.1 Ma (n = 14 zircons, MSWD=2.4); inherited components in this sample are ~60 (n=6), ~110 Ma (n=2) and 1024 Ma (n=1).

 

DISCUSSION

Granitic rocks from Zihuatanejo have mid–Tertiary (43–40 Ma) crystallization ages with no inherited components. Although it is well known that the region is underlain by modified oceanic crust containing old (Archean to Paleozoic) zircons of the Arteaga Complex (Centeno–Garcia et al., 1993; Talavera et al., 2007), the absence of inherited old grains indicates that crustal contamination was not a significant process in the genesis of mid–Tertiary granitic magmas near Zihuatanejo.

On the other hand, most granitic plutons at Atoyac, Guerrero yielded 52.7–58.1 Ma crystallization ages. Only one granitic pluton in this transect has a younger, 40 Ma age. Younger Rb–Sr biotite–whole rocks ages have been obtained from the Atoyac transect (e.g., 28.3 ± 0.6 Ma, Schaaf et al., 1995, but there are also comparable K–Ar ages from the Instituto Mexicano del Petróleo, IMP), indicative for a prolonged thermal event in that area (Schaaf, written communication).

The former are identical in age to the Acapulco granite dated at 55 Ma (Ducea et al., 2004) whereas the later is within the range recorded in plutons at Zihuatanejo. Regardles of their age, granitic plutons at Atoyac also contain abundant inherited zircon grains with ages ranging from 58 to 1085 Ma. Major clusters of inherited zircon are early Tertiary (58–64 Ma), mid– to Late Cretaceous (72–111 Ma), Jurassic (143–153 Ma), mid–Paleozoic (320–360 Ma) and Mesoproterozoic (960–1085 Ma). Rocks with zircons of such age are widespread in the Acatlán and Xolapa complexes (Keppie et al., 2004; Talavera et al., 2005), but they are not found in the Guerrero Terrane (Talavera et al., 2007). It is not possible to discern which of these two complexes is the source of inherited zircons recorded in granitic plutons at Atoyac given the limited available data.

Two tectonostratigraphic terrane configurations of south Mexico are widely used (Campa and Coney, 1983; Sedlock et al., 1993). Names are different and some of the limits are slightly different, but both are essentially similar. Nevertheless, in the case of the Campa and Coney (1983) configuration, the Xolapa terrane is juxtaposed against the Guerrero Terrane as well as the Mixteca and Oaxaca terranes. In contrast, the Sedlock et al. (1993) configuration shows the Xolapa Terrane juxtaposed only against the Mixteca and Oaxaca terranes. Location of the true limits of Xolapa is not only of geometric significance, but it has important implications for the origin and geologic evolution of Xolapa and southern Mexico. Our data indicate that along the Atoyac transect, granitic magmas had significant interactions with older continental rocks similar to those of the Xolapa or Acatlán complexes. This fact suggests that the Atoyac region most probably forms part of the Xolapa terrane and not of the Guerrero terrane as suggested by the configuration of Sedlock et al. (1993); this is supported by Nd model ages around 0.8 Ga (Schaaf et al., 1995). Based on our data, we believe that the boundary between the Guerrero and Xolapa terranes is located between these studied transects, which does not support a boundary between Chatino (Xolapa) and Nahuatl (Guerrero) terranes located closely west of Acapulco as proposed by Sedlock et al. (1993).

These new age data provide additional information regarding coastal arc magmatism along the Pacific margin of Mexico during the Cenozoic. The ages presented herein provide strong evidence for a protracted episode of Cordilleran style arc magmatism within the Xolapa complex for much of the Cenozoic, prior to the early Miocene.

The Eocene–Oligocene post–kinematic (i.e., largely undeformed) arc–related intrusives within the Sierra Madre del Sur formed during a ~20 Ma high flux magmatic event, that is typical for Cordilleran arcs (e.g., Ducea and Barton, 2007). If about half of the width of the Xolapa Complex is occupied by these plutons, as indicated by available geologic mapping, apparent intrusive fluxes for this flare–up event are ~800–1000 km²/m.y., comparable to the largest flare–up events in the Cordillera, such as the Late Cretaceous event that built much of the Sierra Nevada (Ducea, 2001), or the Eocene flare–up of the Coast Mountains batholith (Gehrels et al., 2007).

 

ACKNOWLEDGEMENTS

Arizona LaserChron Center is partially supported by NSF Instrumentation and Facilities Program grant (NSF–EAR 0443387). We would like to thank Peter Schaaf, Fernando Barra, Luigi Solari and an anonymous reviewer for their constructive comments and suggestions on the manuscript.

 

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