A chronological and chemical zircon study of some pegmatite dikes and lenses from the central part (Ayoquezco-Ejutla) of the Oaxacan Complex, southern Mexico

Un estudio cronológico y químico de zircones de algunos diques y lentes de pegmatita de la zona central (Ayoquezco-Ejutla) del Complejo Oaxaqueño, sur de México

Valentina Shchepetilnikova^1,*, Jesús Solé², Luigi Solari³, and Fanis Abdullin¹

¹ Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Coyoacán, México D.F., Mexico. * shepetilnikova@gmail.com

³ Centro de Geociencias, Universidad Nacional Autónoma de México, Campus UNAM Juriquilla, Blvd. Juriquilla No. 3001, Querétaro, 76230, Mexico.

Manuscript received: November 20, 2013
Corrected manuscript received: January 23, 2015
Manuscript accepted: February 1, 2015

ABSTRACT

We carried out a geochronological and geochemical study of zircons from seven pegmatite intrusions collected from the central part (Zimatlán-Ayoquezco-Ejutla villages) of the Oaxacan Complex, southern Mexico. U-Pb ages and trace element chemistry were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The objective of this work is to determine the time of pegmatite emplacement and its high grade metamorphism, if present, by U-Pb dating and identify its possible source and crystallization environment, using trace element concentrations in zircons. The geochronological study allowed to distinguish three main groups of pegmatites: post-tectonic, syntectonic, and pre-tectonic with respect to the granulite facies metamorphism event, which have ages in the ranges of 963 ± 7 to 977 ± 5 Ma, 980 ± 5 to 981 ± 7 Ma, and 1190 ± 7 to 1201 ± 5 Ma, respectively. The REE geochemistry in pegmatite zircons shows that the mechanism of pegmatite formation was in some cases magmatic, in others metamorphic or in between. It has been suggested before that all pegmatites of this region are "granitic" and are the result of a classical evolution of a felsic melt formed in situ during the anatexis of the Oaxacan Complex rocks. The interpretation of our chemical data indicates that the composition of the initial melt, from which each class of pegmatite was formed, can be ultramafic, alkaline or carbonatitic, and only one sample shows a granitic-like initial composition. This means that the pegmatites of the Oaxacan Complex are of diverse origin and only those of quartz-feldspar mineralogy are actually granitic in origin.

Key words: Oaxacan Complex; pegmatite; zircon; rare earth elements: U-Pb geochronology.

RESUMEN

Hemos realizado un estudio geocronológico y geoquímico de los circones de siete pegmatitas colectadas en la zona central (Zimatlán-Ayoquezco-Ejutla) del Complejo Oaxaqueño, Sur de México. Las edades U-Pb y los elementos traza se obtuvieron mediante LA-ICP-MS. El objetivo de este trabajo es determinar la edad de emplazamiento de cada pegmatita y su metamorfismo de alto grado, si está presente, mediante datación U-Pb, así como identificar su posible roca fuente y ambiente de cristalización, usando los elementos traza en circón. Este estudio geocronológico ha permitido identificar tres grupos de pegmatitas: sintectónicas, post-tectónicas y pretectónicas, con respecto del evento metamórfico granulítico, que presentan edades en los rangos de 963 ± 7 a 977 ± 5 Ma, 980 ± 5 a 981 ± 7 Ma, y 1190 ± 7 a 1201 ± 5 Ma, respectivamente. La geoquímica de REE en circones muestra que el mecanismo de formación de las pegmatitas en algunos casos fue magmático, en otros – metamórfico, o una combinación de los dos. Se ha sugerido previamente que todas las pegmatitas de esta región son "graníticas" y son el resultado de la evolución clásica de un magma félsico formado in situ durante los procesos de anatexis de las rocas del Complejo Oaxaqueño. La interpretación de los análisis químicos indica que la composición inicial del fundido del cual derivan los cuerpos pegmatíticos puede ser ultramáfico, alcalino o carbonatítico, y sólo una muestra presenta una composición granítica original. Esto significa que las pegmatitas del Complejo Oaxaqueño son de origen diverso y sólo las de mineralogía cuarzo-feldespática son de origen granítico.

Palabras clave: Complejo Oaxaqueño; pegmatita; circón; elementos de las tierras raras; geocronología U-Pb.

INTRODUCTION

The Oaxacan Complex constitutes the largest (10000 km2) outcrop of ~1 Ga rocks in Mexico. From the regional point of view, the Oaxacan Complex is the largest exposure of the NW-SE extending Oaxaquia microcontinent (e.g., Ortega-Gutiérrez et al., 1995), underlying most of the Mexican territory. Oaxaquia is a portion of the Grenville-aged belt in the North American continent, extending from northeastern Canada to Southern Mexico (Ruiz et al., 1999) (Figure 1a).

Till now, the pegmatites of the Oaxacan Complex have not received much attention, in spite of their importance for the understanding of the Mexican Grenville basement. Pegmatite formation processes in this region are closely related to the tectonic history of the Oaxacan Complex (e.g., Solari et al., 2003; Prol-Ledesma et al., 2012). The first published ages of the Oaxacan Complex rocks were obtained from pegmatite zircons with the Pb-α method, yielding ages in the range of 960 – 1110 Ma (Fries et al., 1962; Fries and Rincón-Orta, 1965).

Most of the pegmatites in the world contain zircon in trace quantities only, but its abundance increases as pegmatite compositions become more alkaline (London and Černy, 2008). Some well-known pegmatites of the Oaxacan Complex contain abundant zircon megacrysts (e.g., Fries et al., 1962; Fries and Rincón-Orta, 1965; Haghenbeck-Correa, 1993; Arenas-Hernández, 1999; Prol-Ledesma et al., 2012).

Zircon is considered to be one of the first phases to crystallize in most igneous rocks (Nagasawa, 1970) and is enriched in REE, Y, Th, U and Hf. The concentrations of these elements in zircons may provide information concerning the nature of the primary melt (e.g., Nagasawa, 1970; Pupin, 2000; Belousova et al., 2002; Hoskin and Schaltegger, 2003; Lesnov, 2012). Moreover, the morphology of zircon crystals can give information about formation conditions (Pupin, 1980). Due to its chemical resistance and ability to survive weathering and transport processes, as well as high temperature metamorphism and anatexis, zircon is able to record information about conditions and time of its crystallization and recrystallization (Belousova et al., 2002).

GEOLOGICAL FRAMEWORK

The Oaxacan Complex consists of a diversity of rock types that were metamorphosed up to the granulite facies and were derived from either a sedimentary protholith (marbles, calcsilicates, quartzo-feldspathic and graphitic gneiss) or from igneous rocks (granite, tonalite, syenite, gabbro, anorthosite, charnockite). Other igneous rocks are present in the form of pegmatite intrusions (Fries et al., 1974; Bloomfield and Ortega-Gutiérrez, 1975; Ortega-Gutiérrez et al., 1977; Keppie et al., 2003). According to Solari et al. (2003), these rocks were involved in two tectonothermal events during the Grenville orogeny: the Olmecan event (1106 ± 6 Ma), and the Zapotecan event (1004 ± 3 to 979 ± 3 Ma). The existence of the former event was recently questioned by Weber et al. (2010), who did not find its evidence in the southernmost edge of the Oaxacan Complex, near Pluma Hidalgo (Oaxaca). The temperature and pressure conditions of the Zapotecan granulite facies event were 700 – 825 °C at 7.2 – 8.2 kb (Mora et al., 1986) in the northern part and 800 – 900 °C at 8 kb in the southern portion of the Oaxacan Complex (Schulze-Schreiber, 2011).

There is a large age gap between the end of the Grenville orogeny and the deposition of the oldest discordant unit above the Oaxacan Complex, the Early Ordovician sedimentary marine rocks from Tiñú Formation (Pantoja-Alor and Robinson, 1967). This formation is only found in the northern Oaxacan Complex, whereas in the southern part the stratigraphic constraints are <500 m thick Jurassic red beds (?) and Cretaceous platform carbonate rocks and flysch (Schulze-Schreiber et al., 2004).

The Oaxacan Complex contains multiple pegmatite dikes that are either concordant or discordant, with respect to the host rock foliation. Some of these pegmatites are undeformed and others show features of syntectonic deformation (Schaaf and Schulze-Schreiber, 1998; Solari et al., 1998). In the northern Oaxacan Complex, Solari et al. (2003) reported the presence of at least three types of pegmatites, which, based on their relationships with the Grenvillian deformation, were classified as pre-tectonic, syntectonic, and post-tectonic. The central part of the Oaxacan Complex, south of Oaxaca City, between Zimatlán, Ejutla, and Ayoquezco, is also characterized by multiple pegmatitic intrusions, but to this day there is only little information published on this topic (Elías-Herrera and Obregón-Ramos, 1983). In general, all the pegmatites belonging to this sector and mentioned in literature, are dikes or lenses with very coarse-grained textures and were identified as late-tectonic granitic pegmatites, formed by partial melting of the host rock gneisses (Haghenbeck-Correa, 1993; Arenas-Hernández, 1999). Only few of these pegmatites were previously dated by K-Ar, Rb-Sr or Pb-α, with ages in the range of 670 – 980 Ma (Fries and Rincón-Orta, 1965; Fries et al., 1962; Anderson and Silver, 1971; Ortega-Gutiérrez et al., 1977).

For this study seven pegmatites in the central part of the Oaxacan Complex were chosen. Three of them have already been mentioned in literature (La Ofelia, La Panchita, and OC22-4AB) (e.g., Fries and Rincón-Orta, 1965; Anderson and Silver, 1971; Arenas-Hernández, 1999). The remaining four were discovered during the field work (183-2, 176-1, 213-1 and AYQ25-7). All pegmatites and pegmatites-migmatites (leucosome) are characterized by the presence of zircon and, in some cases, show a very unusual mineralogy for a "granitic" pegmatite (London and Černy, 2008). Among the most remarkable minerals, they contain scapolite, clinopyroxene and primary calcite.

La Ofelia, 213-1, 183-2, La Panchita, and AYQ25-7 are located in the western part, OC22-04 and 176-1 are located in the eastern part of the study area. The pegmatite bodies OC22-04 and 213-1 have lenticular forms following the foliation of the host rocks (Figure 1c). The La Ofelia, 176-1, La Panchita, 183-2, and AYQ25-7 pegmatites are dikes that cut the host rock foliation (Figure 1c). Pegmatite 213-1 is composed of several lenses which are concordant with the host rock foliation and show large grain size (crystals are several cm in length). This pegmatite, in particular, has geological features typical of a migmatite (Mehnert, 1968): the limits between the pegmatite body and the host rock are gradual and no zonation is observed within the pegmatite lenses.

ANALYTICAL METHODS

Samples were dated by the U-Pb method using a Resonetics M50 workstation coupled to a Thermo X series II quadrupole ICP-MS at the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, UNAM, following the methodology reported in Solari et al. (2010).

The analytical data were filtered by discordance: results which yielded more than 10% or less than -5% of discordance were considered unreliable and were eliminated. Raw data were reduced using Iolite (Paton et al., 2010) and the Vizual Age data reduction scheme (Petrus and Kamber, 2012), whereas all the Concordia plots were obtained, using Isoplot v. 3.70 software (Ludwig, 2008).

Trace elements (REE, Y and Hf) were collected from all zircons during U-Pb isotopic analysis. REE patterns were normalized to the chondrite values of McDonough and Sun (1995).

The size of Cerium and Europium anomalies was calculated from measured REE concentrations, using the following formulas:

Ce_anomaly=Ce/Ce*, where Ce is the chondrite-normalized Ce concentration and Ce* is the average of the chondrite-normalized La and Pr; Eu_anomaly=Eu/Eu*, where Eu is the chondrite-normalized Eu concentration and Eu* is the average of the chondrite-normalized Sm and Gd concentrations.

The ages of each sample and the concentrations of Y, REE, Hf, Th, and U are shown in Table 1 and 2.

RESULTS

A detailed description of the seven pegmatite bodies sampled and the obtained analytical data (age and chemistry) is given in this section. The coordinates (latitude/longitude) of the pegmatite locations are shown in Table 3.

La Ofelia pegmatite

This is a discordant pegmatite body approximately 150 m wide and 300 m long, showing gradual grain size decrease towards the host quartzo-feldspathic gneiss. It is composed of three main zones: a pure phlogopite core, a phlogopite-clinopyroxene intermediate zone, and an external quartz-feldspar zone. The intermediate zone is represented by diopside highly altered to epidote and by phlogopite altered to chlorite. The external zone is made up of perthitic crystals of microcline with andesine-oligoclase highly altered to sericite and quartz. This zone has the largest abundance of accessory phases, such as apatite, zircon, titanite, and monazite. The zircon sample was taken from the external zone and, in spite of the metamorphic overprint it seems that the zircon crystals grew in the early stage of the pegmatite formation. During this work 500 µm fragments of the fractured pink to purple colored subhedral zircon megacrysts were analyzed. Most of them have inherited cores and metamorphic rims in CL images. Overall 17 spots were analyzed from the core zones and 11 spots were analyzed from rims. Two distinct isotopic populations emerge from the data (Figure 2a). Cores yielded ages ranging from 1091 to 1367 Ma, whereas rims yield ages ranging from 969 to 1045 Ma. Most U-Pb ages are concordant. A mean age of 1190 ± 7 Ma (MSWD = 0.79) was calculated for core spots and a mean age of 991 ± 12 Ma (MSWD = 1.8) was obtained for rims (Figures 2a and 2c). Trace element concentrations in zircon cores are similar to zircon rims. La Ofelia zircons display moderate Hf (avg = 13123 ppm) and dispersed Y (496 to 2194 ppm; avg = 825) concentrations. The Th/U ratios range from 0.32 to 0.99, and this variation is independent of age. The analyzed zircons have uniform REE characteristics (Figure 2b), with a significant enrichment in HREE (Lu_N/La_N avg = 4325), well-defined positive Ce/Ce* (avg = 21.3) and negative Eu/Eu* (avg = 0.20) anomalies. HREE enrichment measured by the ratio Lu_N/Gd_N gives an average of 23.9 and for LREE the Sm_N/La_N ratio averages 54.

La Panchita pegmatite

La Panchita pegmatite consists of several pegmatite lenses located inside of an extensional pyroxenitic dike. During the present work only one pegmatite lens was studied. This pegmatite body measures 2 – 4 m across and up to 10 m long and intrudes into a large (more than 200 m across and some kilometers long) pyroxenite dike that in turn is intruding the quartzo-feldspathic gneisses of the Oaxacan Complex. The pyroxenite, as well as the pegmatite body, do not show any sign of deformation. The pegmatite is composed of diopside megacrysts (up to 40 cm long) in contact with scapolite, or symplectitic intergrowth of these two minerals, phlogopite and a calcite core. It also presents dispersed megacrysts (up to 10 cm) of zircon, titanite and apatite; these minerals are mainly located in the contact between scapolite, diopside crystals and the calcite core. The crystals of zircon and titanite show petrographic evidence of crystallizing at equilibrium with diopside, which means that they have crystallized during the early stage of the pegmatite formation. During the field work no presence was found of the classical quartz core or any other manifestation of quartz in this pegmatite body. Dated zircon fragments were taken from euhedral megacrysts of ~2 cm and smaller in length, elongated, prismatic with bipyramidal terminations and with well-defined facets, and colored pink to purple. The obtained U-Pb ages of 20 spots vary from 955 to 1005 Ma, with a mean age of 981.4 ± 7.1 Ma (MSWD = 2.2) (Figure 2d). The analyzed zircons display relatively uniform low Y (avg = 263) and moderate Hf (avg = 7045) concentrations. Th/U ratios range between 0.38 and 0.68. These zircons show also similar REE patterns (Figure 2e), steeply increasing from La to Lu (Lu_N/La_N avg = 6556), a strong positive Ce/Ce* (avg = 43.3) and a very slight negative Eu/Eu* anomalies (avg = 0.45). The HREE enrichment expressed by the Lu_N/Gd_N value averages 37.6 and the average Sm_N/La_N value is 47.8.

Pegmatite sample AYQ25-7

Pegmatite sample 176-1

This sample is part of a moderately deformed 0.5 – 1 m wide pegmatite dike that intrudes the amphibolite gneiss host rock. The pegmatite dike does not show much zonation from borders to center. It is mainly composed of altered amphibole megacrysts (>15 cm), quartz, K-feldspar, mesoperthite, magnetite-titanomagnetite, and accessory zircons. The zircon crystals are always in association with hornblende, and their morphological character gives evidence that they crystallized during the early stage of the pegmatite formation. All analyzed zircon crystals are smaller than 200 µm in length, euhedral, prismatic elongated with bipyramidal terminations, and slightly pink-colored. We analyzed 2 spots from small cores and 23 spots from rims (Figure 2k). The obtained U-Pb ages of the cores are concordant at 1156 ± 24 and 1134 ± 25 Ma and the rim ages vary from 953 to 993 Ma with a mean age of 977 ± 4.6 Ma (MSWD = 0.89) (Figure 2i). The Y concentrations vary from 162 to 993 ppm (avg = 416) whereas Hf ranges from 9200 to 13984 ppm (avg = 11451). Th/U ratios are uniformly high (0.29 – 0.71), with the exception of one spot (Th/U = 0.08) located in the core. Other geochemical signatures of these zircons are homogeneous REE patterns, steeply increasing from La to Lu (Lu_N/La_N avg = 4567), with well-defined positive Ce/Ce* (avg = 10.4) and negative Eu/Eu* (avg = 0.14) anomalies (Figure 2j). The cores have REE patterns slightly different from the rims, and show the strongest (Eu/Eu* = 0.03), as well as the weakest (Eu/Eu* = 0.22) Eu anomaly. HREE enrichment is characterized by Lu_N/Gd_N avg = 13.1 and the average value of Sm_N/La_N is 126.1.

Pegmatite sample 183-2

This sample was taken from a 0.5 – 1 m sized pegmatite dike cutting the host rock (quartzo-feldspathic gneiss). It is composed of large (1 – 2 cm) crystals of ilmenite with secondary titanite, calcite megacrysts (up to 5 cm), quartz, altered calcic feldspars and abundant crystals of zircon. Zircon crystals are euhedral and less than 200 µm in length, prismatic elongated with bipyramidal terminations, pink-colored and always show spatial relation with Fe-oxids. The obtained ages of 32 spots vary from 938 to 995 Ma, with a mean age of 969.8 ± 4.6 Ma (MSWD = 1.3) (Figures 2l and 2n). Hf concentrations in the zircons belonging to this sample range from 9989 to 13664 ppm (avg = 11693), whereas Y ranges from 208 to 1043 ppm (avg = 508), and Th/U ratios are uniformly high between 0.46 and 0.95. These zircons have homogeneous REE patterns, gradually rising from La to Lu (Lu_N/La_N avg = 2522), with well-defined positive Ce/Ce* (avg = 25.9) and negative Eu/Eu* (avg = 0.21) anomalies (Figure 2m). The HREE enrichment is characterized by Lu_N/Gd_N avg = 12.2 and the average value of Sm_N/La_N is 76.1.

Pegmatite-migmatite (leucosome) sample 213-1

A series of pegmatite-migmatite (leucosome) lenses (1 – 5 m wide and some meters long) are located within a mafic host rock (amphibolite gneiss). According to Mehnert (1968) and London and Černy (2008), this group of pegmatite bodies could be named pegmatite-migmatite: the lenses are concordant with the foliation of the host rock, the limits between pegmatite bodies and the host rock are not well defined, and there is no internal zonation of the pegmatite body. The series of pegmatite leucosome lenses consist of megacrysts (up to 5 cm and more) of chloritized amphiboles, sericitized andesine-labradorite, perthitic microcline, opaque Fe-minerals (magnetite-titanomagnetite-ilmenite), apatite, abundant zircon, and small amounts of quartz (<5%). The analyzed zircon crystals have irregular subhedral form, are up to 200 µm in length, prismatic, elongated with bipyramidal terminations, pink-colored and are in association with hornblende and Fe-oxides. The obtained U-Pb ages of 27 spots vary from 927 to 1035 Ma, with a mean age of 980.1 ± 4.7 Ma (MSWD = 0.98) (Figures 2o and 2q). The zircons separated from this sample have uniform and moderate Hf concentrations (9521 – 12439 ppm; avg = 11020) and relatively low concentrations of Y (204 – 632 ppm; avg = 362). Th/U ratios are uniformly high between 0.47 and 0.86. They also have homogeneous REE patterns, steeply increasing from La to Lu (Lu_N/La_N avg = 4272), with well-defined positive Ce/Ce* (avg = 7.8) and negative Eu/Eu* (avg = 0.23) anomalies (Figure 2p). The HREE enrichment is characterized by Lu_N/Gd_N avg = 15.5 and the average value of Sm_N/La_N is 91.7.

Pegmatite sample OC22-4AB

This pegmatite group is located 5 km to the south of Ejutla, along the road Ejutla-Miahuatlán. This is a group of 2 – 8 m wide and 12 – 35 m long pegmatite lenses, which follow the foliation of the biotite-garnet gneissic host rock. These pegmatite lenses are represented by a thin (some cm long) biotite-andesine border zone and a central zone composed of deformed quartz, sericitized andesine, pyroxene (spodumene?), muscovite, primary calcite, abundant zircon and titanite central zone. Zircon crystals extracted from the central zone are euhedral, up to 500 µm in length, and range in shape from moderately elongated to prismatic, with bipyramidal endings to ovoid or sub-spherical ("soccerball") forms with well-defined faces, colored dark-red to purple. The zircon grains from this sample are almost non-luminescente, and their internal structure is hard to see, but we found some zircon crystals with dark cores and slightly light rims (Figure 2t). U-Pb ages obtained from 39 zircon cores range from 988 to 1267 Ma, with a mean age of 1201.2 ± 4.8 Ma (MSWD = 1.4), whereas zircon rims yielded 6 U-Pb ages, ranging from 965 to 1069 Ma. These 6 ages are dispersed and two of them are slightly discordant. The mean age for this younger age group was not calculated (Figure 2r). The Hf content of the analyzed zircons varies ranging from 7090 to 20265 ppm (avg = 13182), whereas Y ranges between 514 and 3958ppm (avg = 1931). Th/U has variable ratios (0.07 – 0.23) and do not show much dependence from age. The REE patterns steeply increase from La to Lu (Lu_N/La_N avg = 7703) and are somewhat variable on the LREE side, with some spots showing relative LREE enrichments (Figure 2s). The Nd concentrations vary from 0.55 to 5.9 ppm. Grains with LREE enrichment have weaker Ce (e.g., Ce/Ce* = 2) and Eu anomalies (e.g., Eu/Eu* = 0.28). The LREE-depleted zircons have stronger Ce (e.g., Ce/Ce* = 7.9) and Eu anomalies (e.g., Eu/Eu* = 0.04).

DISCUSSION

Geochemistry

In general, the chondrite-normalized patterns of all zircon samples can be characterized by a steeply-rising slope from LREE to HREE, which can be defined by Lu_N/La_N values ranging from 1504 to 7703, and a different intensity of positive Ce and negative Eu-anomalies typical for igneous zircons (Hoskin and Schaltegger, 2003). Compared to classic granitic magmatic zircons, samples from our work are characterized by a similar LREE behavior, but a relatively higher HREE depletion (Figure 3). It should be noted that, in general, magmatic zircons have a trend of increasing REE content from ultramafic through mafic to granitic rocks (Belousova et al., 2002). The zircon patterns of the Oaxacan Complex show similarity with zircon patterns of Phalaborwa Complex carbonatites in South Africa (Hoskin and Ireland, 2000). There are two types of REE patterns in the studied zircons: those with normalized Lu values that do not exceed 10³ (183-2, 213-1, AYQ25-7, 176-1, La Panchita) and those with Lu values up to 10⁴ (OC22-4AB, La Ofelia). Samples 176-1, 183-2 and 213-1 show patterns which are similar to each other.

Using the Ce/Ce* vs. Eu/Eu* genetic diagram of Belousova et al. (2002), the intensity of Ce and Eu anomalies in chondrite-normalized patterns can be evaluated. Pegmatites 213-1, 183-2, AYQ25-7 and La Ofelia have the same Eu anomaly size (Figure 4b), and the weakest Eu anomaly (Eu/Eu* avg = 0.45) of this series is from La Panchita. All of the above zircon samples have very similar Ce anomalies. Sample OC22-04AB shows variable magnitudes of Ce (Ce/Ce* vary from 2.0 to 15) and Eu (Eu/Eu* vary from 0.015 to 0.7) anomalies. However, it should be noted that OC22-04AB has some spots with the weakest Ce (Ce/Ce* = 2) and the most significant Eu (Eu/Eu* = 0.04) anomaly among other samples. There is no age dependence of rims and cores in regard to the REE and other trace element concentrations in all analyzed zircon grains. Likewise, there is not any significant relation between the Th/U ratios and the rim-core age difference. Most of the analyzed zircon samples (182-2, 213-1, La Ofelia and AYQ25-7) have high Th/U values, between 0.24 and 0.88. Only one sample (OC22-4AB) shows a relatively low Th/U ratio (from 0.06 to 0.2), that can be caused by its possible metamorphic genesis (e.g., Hoskin and Schaltegger, 2003; Rubatto, 2002). The interpretation of Th/U values is still in dispute, because it is not exactly known which are the values of the Th/U ratios in zircon for magmatic and metamorphic rocks and the distribution coefficients between zircon and melt at diverse P and T (e.g., Harley et al., 2007).

Genetic diagrams

Shnukov et al. (1997), Pupin (2000) and Belousova et al. (2002) elaborated some diagrams which display concentrations of Hf, Y, U, Ce and Eu in zircons, with the aim to estimate the composition of the melt where zircons were crystallized (Figure 4). These diagrams are based on the theory that zircon grows in the early crystallization stages of igneous rocks and may strongly affect the behavior of many trace elements during magma crystallization (e.g., Nagasawa, 1970; Watson, 1979; Hoskin et al., 2000; Belousova et al., 2002).

According to Shnukov et al. (1997) and using a Hf (wt%) vs. Y (ppm) diagram, at least three groups of pegmatites can be discriminated (Figure 4a). (1) The first group (183-2, 213-1, 176-1) lies within the fields I, II and VI of "kimberlites", "ultramafic, mafic and intermediate rocks" and "alkaline rocks and alkaline metasomatites of alkaline complexes". It shows a relatively large range of Y (160 – 990 ppm) and Hf (9000 – 14000 ppm) concentrations. (2) The second group (OC22-04AB; La Ofelia) plots in the fields II and III of "ultramafic, mafic and intermediate rocks", and "quartz-bearing intermediate and felsic rocks". It has relatively moderate to high Y (450 – 3960 ppm) and high Hf contents (10000 – 20000 ppm). (3) The third group (La Panchita; AYQ25-7) straddle fields VI and VII of "alkaline rocks and alkaline metasomatites of alkaline complexes" and "carbonatites". It can be characterized by moderate to low Y (120 – 770 ppm) and low Hf concentrations (5120 – 10700 ppm).

On the Ce/Ce* versus Eu/Eu* graph of Belousova et al. (2002) (Figure 4b) most analyses lie in the area of syenitic rocks; the exception is sample OC22-4AB, which displays scattered points. The Y (ppm) versus U (ppm) diagram (Figure 4c) shows that most of the analyses plot within the area of mafic and carbonatitic rocks (La Ofelia, AYQ25-7, 183-2, 176-1 and 213-1). Sample OC22-4AB points lie in the area of granitic rocks and La Panchita lies outside of all fields because of its high U concentrations (avg = 231 ppm) and moderate Y concentrations (avg = 262 ppm). The zircons which are most depleted in these elements are from sample AYQ25-7 (Y <400 ppm and U_avg = 63 ppm). These diagrams of Belousova et al. (2002) should be interpreted together with other genetic diagrams (e.g., diagrams of Shnukov et al. (1997), Pupin (2000), and comparative REE pattern diagrams), as suggested by Hoskin and Schaltegger (2003). On the other hand, the fields for zircon-bearing rock types of Belousova et al. (2002) overlap each other at different degrees in most plots. A comparison of several plots or together with the Hf vs. Y diagram of Shnukov et al. (1997) and Y₂O₃ vs. HfO₂ graph of Pupin (2000) can help to identify zircons from different rock types.

Pupin (2000) elaborated the Y₂O₃ (ppm) versus HfO₂ (ppm) diagram basically for granitic rocks, but some special areas for basic to intermediate rocks are also included (Figure 4d). The group of samples 183-2, 213-1 and 176-1 lies in the field of "basic to intermediate calc-alkaline rocks" and the La Panchita sample is positioned in the "peralkaline syenites" field. It is difficult to make a petrogenetic conclusion, based only on these diagrams, but at least it is shown that the geochemical nature of some of the studied pegmatites must have been related to the presence of ultramafic-alkaline precursors, probably unrelated to granitic sensu lato magmas.

Geochronology

There are three types of pegmatites according to their crystallization ages (Figure 5). The first group is represented by samples AYQ25-7, 183-2 and 176-1. Their mean ages are in the range of 963 ± 7 to 977 ± 4.6 Ma. This age range can be described as post-tectonic (Anderson and Silver, 1971), with respect to the last episode of the Grenville orogeny (Zapotecan orogeny: 1004 to 978 ± 3 Ma) in the Oaxacan Complex (Solari et al., 2003) (Figure 5). Sample 176-1 has also two older ages of 1156 ± 24 and 1134 ± 25 Ma, which are interpreted as xenocrystic cores.

The second group is represented by samples taken from pegmatite bodies of La Panchita and 213-1. The mean ages of these samples are almost identical, 981.4 ± 7.1 and 980.1 ± 4.7 Ma (Figure 5), respectively. La Panchita pegmatite did not suffer any type of metamorphism but, as it was mentioned before, the 213-1 pegmatite lenses show migmatitic features (Mehnert, 1968), so they must have been formed during a high grade metamorphic event. La Panchita and 213-1 pegmatite bodies were formed during the same time period, corresponding to the last stage of the Zapotecan orogeny (Solari et al., 2003). The distance between La Panchita and 213-1 is ~10 km and it is noteworthy that they have different metamorphic grades. One of the several possibilities is that the uncertainty of the LA-ICP-MS U-Pb method is underestimated. If we look at the age dispersion for La Panchita and ignore the statistical calculations, we can conclude that its age is post-tectonic. This is coherent with the rest of the available data and suggests an age of 978 – 980 Ma for the last episode of granulite facies metamorphism. More data are necessary to resolve this issue.

The third group is represented by pre-tectonic pegmatite bodies such, as La Ofelia and OC22-4AB. Zircon cores of OC22-4AB have a mean crystallization age of 1201.2 ± 4.8 Ma (Figure 5) and dispersed ages of recrystallized rims in the range of 965 ± 17 to 1069 ± 20 Ma. The dispersion and discordance of these ages could be the result of Pb-loss during the granulite facies metamorphism. Metamorphic overprints are also indicated by some distinctive features, like the deformation of the pegmatite body, the "soccerball" zircon morphology and relative low Th/U ratios. Zircons from La Ofelia pegmatite also show these two age groups. The oldest ages are represented by cores with a mean age of 1190.1 ± 7.3 Ma and the younger ages are represented by rims with a mean age of 991 ± 12 Ma. There are also some spots with intermediate concordant ages (1045 ± 23; 1091 ± 25; 1114 ± 28 and 1119 ± 25 Ma) which we interpret as mixed ages, probably a result of analyzing the limit between cores and rims. There is a clear relation between metamorphic rims, older cores and the irregular zircon morphology.

General discussion

The U-Pb LA-ICP-MS ages obtained from zircons confirmed the pre-tectonic ("old"), syn-tectonic and post-tectonic division of all pegmatites (Solari et al., 1998) (Figure 5). The ages of post-tectonic pegmatites obtained in this work vary from 963 ± 7 to 977 ± 5 Ma. The syn-tectonic pegmatites, formed during the last phase of the Oaxacan Complex Orogeny (Zapotecan) have an age of 981 ± 7 Ma. The "old" pegmatites formed during the period between 1190 ± 7and 1201 ± 5 Ma. These "old" pegmatite bodies do not show any ages that can be interpreted as being part of the Olmecan orogeny (1106 ± 6 Ma). The youngest ages of OC22-4AB define a range of ages between 965 ± 17 and 1069 ± 20 Ma, which is the result of different degrees of Pb loss in zircons during the Zapotecan orogeny (from 1004 to 978 ± 3 Ma, Solari et al., 2003).

The zircon geochemistry shows that all studied pegmatite intrusions, with the exception of OC22-4AB, are not "granitic" pegmatites at all, which is in contrast with previous works (Haghenbeck-Correa, 1993; Arenas-Hernández, 1999). This means that pegmatites from the central portion of the Oaxacan Complex reflect the composition of non-granitic rocks or melts. The zircon trace element geochemistry shows that La Panchita pegmatite has been formed during the evolution of an alkaline and SiO2-depleted melt, like carbonatite or syenitic rocks. The group of pegmatites 183-2, 176-1, 213-1 and La Ofelia shows similar chemical behavior and a possible mafic composition of the source rock type. AYQ25-7 has transitional trace element patterns between La Panchita pegmatite and the group of 183-2, 176-1 and 213-1. Only one zircon sample, that from OC22-4AB, shows a granitic type initial composition of this pegmatite body.

With respect to pegmatite generation, it should be noted that the pegmatites studied in this work could have three different ways of formation: pure magmatic, pure metamorphic, and a mixture of magmatic and metamorphic processes. The first is the classical process of formation during the last stage of magmatic intrusions (London and Černy, 2008). The second way implies a pegmatite formation by partial melting or anatexis of the Oaxacan Complex rocks during high-grade metamorphism, without involving magma melt from another source (Mehnert, 1968). All previous authors (Haghenbeck-Correa, 1993; Schaaf and Schulze-Schreiber, 1998; Arenas-Hernández, 1999) claim the anatectic way for the Oaxacan Complex pegmatite formation. The third and last possible mode of pegmatite formation is a mixture of the two aforementioned processes: partial melting of the Oaxacan Complex rocks during high grade metamorphism involving foreign magma melt from another source (Mehnert, 1968).

It seems that the studied zircons have magmatic REE compositions rather than metamorphic (Figure 3), which means that they underwent a magmatic mechanism of formation; but according to Hoskin and Schaltegger (2003), the composition of metamorphic zircon in equilibrium with an anatectic melt does not differ greatly from igneous zircon. So it is difficult to give a final opinion about the magmatic, metamorphic or hybrid process of pegmatite formation using only trace element chemistry of zircons.

CONCLUSIONS

From the geochemical and geochronological study of zircons from seven pegmatites located in the Oaxacan Complex, Oaxaca, México, we can conclude the following:

a) The numerous pegmatites in this complex can be roughly divided into granitic, displaying a simple quartzo-feldspathic mineralogy and non-granitic, showing a less-common mineralogy with pyroxenes, scapolite, large zircon or calcite.

b) The obtained ages confirm the division in pre-tectonic (~1200 Ma), syntectonic (~980 Ma) and post-tectonic (<980 Ma) time of the pegmatite formation.

c) The Olmecan orogeny cannot be identified in the central part of the Oaxacan Complex with the available data.

d) The selected pegmatites for this study, mainly non-granitic in nature, indicate the presence of mafic and alkaline parental magmas for these intrusives.

e) The application of geochemical discrimination diagrams for zircons is useful, but lacks a systematic approach. More work is needed on these diagrams to obtain a reliable solution for magmas source identification.

]]> f) Age and chemistry correlation of the studied zircons corroborate that the tectonic setting has shifted from compressive (collision) to extensive (rifting?) during the last stages of the Zapotecan orogeny.

ACKNOWLEDGMENTS

This work is part of the PhD thesis of the first author, who acknowledges a CONACYT scholarship. This paper has benefited from funds granted to J. Solé by PAPIIT-DGAPA, UNAM, number IN112711-2. We thank Carlos Ortega-Obregón for assistance during LA-ICP-MS analytical work, Jaime Díaz Ortega and María del Consuelo Macías Romo for support during the sample preparation for U-Pb analysis. We are also thankful to Peter Schaaf for his editorial handling, and to Thomas Sperling and one anonymous reviewer, for their critical comments which improved the manuscript.

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