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Revista mexicana de ciencias geológicas

versión On-line ISSN 2007-2902versión impresa ISSN 1026-8774

Rev. mex. cienc. geol vol.26 no.1 México abr. 2009

 

Sección Especial

 

Mesozoic geologic evolution of the Xolapa migmatitic complex north of Acapulco, southern Mexico: implications for paleogeographic reconstructions

 

Evolución geológica mesozoica del Complejo Xolapa al norte de Acapulco, sur de México: implicaciones para las reconstrucciones paleogeográficas

 

Rosalva Pérez–Gutiérrez1, Luigi A. Solari2, *, Arturo Gómez–Tuena2, and Uwe Martens3

 

1 Instituto de Geología, Universidad Nacional Autónoma de México, Cd. Universitaria, Del. Coyoacán, 04510 México D.F., Mexico.

2 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Blvd. Juriquilla 3001, 76230 Querétaro, Qro., Mexico. *solari@servidor.unam.mx

3 Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA 94304, United States.

 

Manuscript received:January 22, 2008
Corrected manuscript received: August 13, 2008
Manuscript accepted: September 9, 2008

 

ABSTRACT

The Xolapa Complex in the Acapulco–Tierra Colorada area, southern Mexico, is made up of orthogneisses and paragneisses, both affected by a variable degrees of migmatization. These rocks are intruded by several episodes of Jurassic–Oligocene, calc–alkaline granitic magmatism. Three phases of ductile deformation affected the gneisses and migmatites. D1 produced an amphibolite–facies metamorphic banding (M1) and a penetrative S1 foliation, axial planar to isoclinal, recumbent F1 folds with axes parallel to a NE to NW, gently plunging, L1 stretching lineation. D2 consists of a S2 foliation defined by hornblende, biotite and garnet, synchronous with M2 migmatization that generated leucosomes, which are generally parallel to S1. D3 is made up of asymmetric, chevron F3 folds that deform the composite S1/S2 foliation during greenschist to lower amphibolite metamorphism (M3). U–Pb SHRIMP (Sensitive High–Resolution Ion Micropobe) geochronology carried out on zircon separated from two orthogneisses yielded an Early Jurassic magmatic event (178.7 ± 1.1 Ma) and the age of migmatization (133.6 ± 0.9 Ma). Two episodes of Pb loss were also recognized, the first at 129.2 ± 0.4 Ma, and the second during the earliest Paleocene (61.4 ± 1.5 Ma); they are probably associated with two episodes of magmatism. The Early Jurassic magmatic arc may be correlated with a magmatic arc in the eastern Guerrero terrane. The Early Cretaceous migmatization is inferred to have resulted from shortening, possibly due to the accretion of an exotic block, such as the Chortís block along whose northern margin contemporaneous, high–pressure metamorphism has been recorded.

Key words: migmatization, U–Pb, Mesozoic, Xolapa Complex, southern México.

 

RESUMEN

El Complejo Xolapa es el terreno metamórfico más extenso en el sur de México. En el segmento que va de Acapulco a Tierra Colorada está constituido por ortogneises y paragneises, afectados por un grado variable de migmatización. El basamento del Complejo Xolapa está también afectado por diversos episodios de magmatismo granítico de afinidad geoquímica calcialcalina y edades que van del Jurásico al Oligoceno. Tres fases de deformación dúctil afectan los gneises y migmatitas. Un bandeamiento metamórfico se desarrolla durante D1, y está asociado con una foliación penetrativa S1, en facies de anfibolita, que actúa como plano axial de pliegues de recumbentes a isoclinales F1. Una lineación de estiramiento L1 está presente en toda el área de estudio, con buzamiento tanto al NE como NW. La orientación de las estructuras D1 controla el emplazamiento de los lentes de leucosoma D2, generados durante el proceso de migmatización. Afuera de los dominios leucosomáticos, la foliación S2 está caracterizada por la asociación hornblenda + biotita + granate. Pliegues asimétricos de tipo chevron relacionados a la fase F3 deforman la foliación compuesta S1/S2, en condiciones de facies de esquistos verdes hasta anfibolita. La geocronología de U–Pb por SHRIMP (microsonda iónica de alta resolución) realizada en zircones separados de dos muestras de ortogneises permitió reconocer un evento magmático jurásico temprano (178.7 ± 1.1 Ma), así como la edad de migmatización en esta porción del complejo de Xolapa calculada en 133.6 ± 0.9 Ma. Dos episodios de pérdida de Pb se pueden reconocer en algunos de los zircones de las muestras fechadas. El primero es posterior al pico de migmatización (129.2 ± 0.4 Ma), y el segundo ocurrió durante el Paleoceno temprano (61.4 ± 1.5 Ma), y probablemente ambos son un efecto térmico asociado a dos de los episodios magmáticos posteriores. Estos datos sugieren que un arco magmático del Jurásico temprano habría constituido el basamento del Complejo Xolapa; se sugiere que este mismo arco corresponda a la porción este del terreno Guerrero, en donde algunas edades magmáticas similares han sido reportadas. La migmatización ocurrió durante el Cretácico temprano como consecuencia del acortamiento producido por la accreción de un bloque exótico. El bloque de Chortís es un candidato posible para tal colisión, ya que registra un evento metamórfico de alta presión contemporáneo con el evento de migmatización en el Complejo Xolapa.

Palabras clave: migmatización, U–Pb, Mesozoico, Complejo Xolapa, sur de México.

 

INTRODUCTION

High–grade metamorphic terrains form a large portion of the continental crust exposed in Phanerozoic and Precambrian orogens. Inasmuch as these kinds of metamorphic terrains represent exhumed portions of the middle and lower continental crust, understanding their geologic evolution sheds light on the deep crustal processes.

The geology of southern Mexico is composed of a mosaic of tectonostratigraphic terranes, (Figure 1) (Campa and Coney, 1983; Sedlock et al., 1993; Keppie, 2004). Along the Pacific coast, the Xolapa (or Chatino) terrane, ~600 km long and ~50–80 km wide, is a composite plutonic and metamorphic terrane. The Xolapa terrane truncates the terranes exposed to the north, which consist (from W to E) of: (a) the Mesozoic Guerrero terrane (Talavera–Mendoza et al., 2007; Centeno–García et al., 2008), (b) the polymetamorphic, Paleozoic–bearing Mixteca Terrane (Ortega–Gutiérrez et al., 1999), and (c) the Mesoproterozoic, granulite–bearing Zapoteco terrane (Keppie et al., 2001; Keppie et al., 2003; Solari et al., 2003; Solari et al., 2004). The Mixteca and Zapoteco terranes were amalgamated during the Permian, whereas the Guerrero terrane was accreted in the Mesozoic (Centeno–García et al., 1993; Elias–Herrera and Ortega–Gutiérrez, 2002; Keppie, 2004; Centeno–García et al., 2008). The allochthonous or autochthonous nature of the Xolapa Complex is still debated (e.g., Keppie, 2004; Ducea et al., 2004; Corona–Chávez et al., 2006). One view suggests that the Xolapa Complex is autochthonous, originated in situ by thinning and reheating of the continental crust during Tertiary (e.g., Herrmann et al., 1994; Schaaf et al., 1995; Morán–Zenteno et al., 1996; Ducea et al., 2004). A contrasting idea suggests that the Xolapa is allochthonous, and was accreted to the continental margin of southern Mexico during Mesozoic–Tertiary (e.g., Corona–Chávez et al., 2006).

In an attempt to resolve this debate, we undertook a combination of fieldwork, petrologic and geochronologic analyses to better document the sequence of tectonothermal events in the Xolapa Complex. A key question to be addressed is the timing of migmatization and high–grade metamorphism of the Xolapa Complex. Various time ranges have been proposed: Late Jurassic to earliest Cretaceous (e.g., Ducea et al., 2004; Solari et al., 2007), Late Cretaceous to earliest Paleocene (~60–110 Ma, Corona–Chávez et al., 2006), and Early Tertiary (46–66 Ma; Herrmann et al., 1994). In this paper, we present detailed structural description of the Xolapa Complex north of Acapulco (Figures 1 and 2), the geochemical characteristic of some of the high–grade metamorphic rocks, and U–Pb zircon geochronology. These data are then applied to Tertiary reconstructions (e.g., Solari et al., 2007).

 

REGIONAL GEOLOGY

The Xolapa Complex is made up by a sequence of high–grade metasedimentary and metaigneous rocks that are frequently intruded by both deformed and undeformed plutonic rocks. Previous work was generally limited to either field descriptions, (i.e., De Cserna, 1965), combined geochemical and geochronological analyses (Morán–Zenteno, 1992; Herrmann et al., 1994; Ducea et al., 2004), or combined field and petrological work without geochronology (Corona–Chávez et al., 2006). As there is some confusion about what constitutes the Xolapa Complex, we define the Xolapa Complex as those rocks that are affected by migmatization and ductile deformation, which predate the intrusion and further solid–state shearing of the ~130 Ma deformed granites (e.g., Solari et al., 2007, and see below). Previous definitions of the Xolapa Complex included all the metamorphic and igneous pre–Oligocene rocks exposed in the area (e.g., De Cserna, 1965; Alaniz–Álvarez and Ortega–Gutiérrez, 1997).

The northern boundary of the Xolapa terrane (Figure 1) is marked by normal and left–lateral ductile shear zones, which were active from the Eocene on the west (Tierra Colorada shear zone, Riller et al., 1992; Solari et al., 2007), to the Oligocene in the east (Chacalapa shear zone, Tolson, 2005). While protolith ages of the metasediments have yet to be determined, the oldest magmatic arc intruding the Xolapa Complex is documented as Jurassic to Early Cretaceous (e.g., Guerrero–García et al., 1978; Morán–Zenteno, 1992; Herrmann et al., 1994; Ducea et al., 2004). The youngest undeformed plutons intruding the Xolapa Complex range from ~34 Ma in the Acapulco – Tierra Colorada area (Hermann et al. 1994; Ducea et al., 2004), and become younger toward SE (~25–29 Ma in Puerto Escondido – Huatulco; Hermann et al. 1994; Ducea et al., 2004). At least two other intrusive events occurred between the oldest and youngest magmatic events. Thus, north of Acapulco, Solari et al. (2007) presented U–Pb concordant ages of ~129 Ma on undeformed to slightly deformed granites, and ~55 Ma ages for other granites locally affected by ductile shearing. The ~129 Ma ages are in the range of the deformed granites of Morán–Zenteno (1992), whereas the ~55 Ma ages are similar to those previously determined by Ducea et al. (2004) on the Acapulco syeno–granite. Geochronology of deformed samples from the Xolapa Complex indicates the presence of protoliths similar to the neighboring metamorphic complexes. For instance, north of Puerto Ángel (Figure 1) rocks have both igneous and metamorphic Grenvillian ages, similar to those in the Oaxacan Complex (e.g., Keppie et al., 2001; Ducea et al., 2004). North of Puerto Escondido, Ducea et al. (2004) reported Permian ages in a biotite–bearing gneiss that are similar to ages reported in granite and granodiorite belonging to the Acatlán Complex (Yañez et al., 1991; Keppie et al., 2004, 2008), the Juchatengo complex (Grajales–Nishimura, 1988; Grajales–Nishimura et al., 1999), and the stitching granites along the Caltepec fault zone (Elias–Herrera and Ortega–Gutiérrez, 2002). In the Acapulco transect, ages reported so far are no older than Middle Jurassic, and are similar to those in the Guerrero Terrane (Talavera–Mendoza et al., 2007; Centeno–García et al., 2008). These correlations can be interpreted as the prime evidence for the autochthonous nature of the Xolapa terrane.

Corona–Chávez et al. (2006) recognized a single high–grade metamorphic event (830–900 °C and 6.3–9.5 kbar) with a clockwise pressure–temperature (P–T) path in the Xolapa Complex of the Puerto Escondido – Puerto Ángel area (Figure 1). Fluids released by the breakdown of biotite and amphibole produced intense migmatization. The vast majority of plutonic rocks of the Xolapa Complex are calc–alkaline granites and granodiorites (Herrmann, 1994). The Tertiary (Paleocene to Oligocene) granites and granodiorites have similar compositions (Morán–Zenteno, 1992; Schaaf et al., 1995; Morán–Zenteno et al., 1996; Morán–Zenteno et al., 1999; Morán–Zenteno et al., 2005).

 

ROCK TYPES

The study area, located north of Acapulco, covers about 700 km2. Previous studies in this area were focused on general field descriptions of the northern part of the Xolapa Complex in the Barranca de Xolapa (Figure 2), where the Xolapa Complex was first defined (De Cserna, 1965). Subsequently, Alaniz–Álvarez and Ortega–Gutiérrez (1997) described medium to high–grade metasediments at this locality (quartz–K–feldspar–biotite–muscovite–sillimanite–garnet, and rare cordierite and corundum) that are intruded by different generations of variably deformed pegmatite, mafic dikes and granites. Other studies dealt with regional geological mapping (e.g., Sabanero–Sosa, 1990), or combined geologic mapping with isotopic determinations (Morán–Zenteno, 1992). For instance Morán–Zenteno (1992) produced a regional map, and geochemical, Rb–Sr and Sm–Nd data on the deformed granitoids, which yielded an age of ~130 Ma, interpreted as the age of intrusion. Other studies north of Acapulco provided geochronological data on gneisses (~134–140 Ma, Ducea et al., 2004) and undeformed, Oligocene plutons (e.g., Herrmann et al., 1994; Schaaf et al., 1995; Ducea et al., 2004), but the lack of detailed mapping makes it difficult to establish a complete geological record. Northeast of the study area, Torres–de León (2005) and Solari et al. (2007) documented the allochthonous nature of the Tierra Colorada area (Figure 1). Torres–de León (2005) and Solari et al. (2007) established the structural and magmatic evolution of the Tierra Colorada area as a series of alternating magmatic (~130, ~55, and ~34 Ma) and deformation pulses (>130, ~45, and between 45 and 34 Ma).

Our geologic mapping (Figure 2) revealed the existence of several lithologies exposed north of Acapulco that belong to the Xolapa Complex. The recognized pre–migmatitic rocks were grouped into para– and ortho–gneisses, which are affected by variable degrees of migmatization. Their petrographic characteristics and field relationships are described in the following section.

Metasediments

Metasediments, including psammitic–pelitic paragneisses as well as marbles, are mainly distributed to the south and southeast of the study area and constitute ~30% of the area (Figure 2). They generally consists of 5–30 cm thick bands of grey to ochre–brown colored quartzofeldsphatic micaschists, whose main constituents are granoblastic quartz, plagioclase, biotite, muscovite, cordierite, garnet, ± sillimanite, ± staurolite. Staurolite and muscovite are not present in the migmatitic rocks. The association biotite + sillimanite + cordierite + corundum + hercynite is quite common in the metasedimentary restites. Zircon and apatite are common accessory minerals. Tourmaline is an abundant accessory mineral in some samples. Biotite, muscovite and sillimanite (often present as fibrolite) are generally intergrown, and aligned along foliation planes. The mineralogy suggests a greywacke/minor pelites protolith. Up to 1 km wide marble bodies within the sequence are made up of ≤85% calcite, minor dolomite, and accessory quartz, plagioclase, diopside and detrital zircons.

Orthogneisses

Orthogneisses, the most abundant lithology in the study area, are mainly leucocratic to mesocratic, pervasively foliated granites and granodiorites (not distinguished on Figure 2). The orthogneisses are banded and often characterized by up to 20 cm thick dark bands mainly composed of biotite, hornblende, plagioclase and opaque minerals. Leucocratic bands are composed of quartz, plagioclase, potassic feldspar, biotite, ± hornblende. Common accessory minerals are zircon and apatite. Chlorite develops as a secondary mineral phase at the expense of biotite. Orthoclase porphyroclasts are present, sometimes developing core and mantle textures, with sodium–rich mantles around potassium–rich cores. The foliation is defined by biotite and feldspar porphyroclasts in the leucocratic bands, and hornblende and biotite in the melanosomes (e.g., Figure 3a). The sharp contacts between paragneisses and orthogneisses, as well as some preserved cross–cutting relationships, indicate of original intrusive relationships.

Ortho– and para–migmatites

Migmatization is superimposed on both ortho– and para–gneisses (Figure 2). The main recognizable types are (following Mehnert, 1968, and modifications in Milord and Sawyer, 2003, and Brown, 2004): (1) stromatic, i.e. banded migmatites, where up to 10 cm thick melanosomatic bands, made up of hornblende, biotite, plagioclase, and opaque minerals, alternate with leucosomes made up of quartz, plagioclase, ± feldspar, ± garnet, ± muscovite (Figure 3b); (2) Agmatic to diktyonitic migmatites, in which a breccia–like paleosome is injected by a network of neosome veins (Figure 3c) of trondhjeimitic to tonalitic compositions (the latter often shows green poikiloblasts of hornblende, ± garnet ± biotite); (3) Raft migmatites (Figure 3d), in which small pieces of paleosomatic gneisses float in a heterogeneous leucosome; (4) nebulitic migmatites (Figure 3e), in which the advanced degree of migmatization almost completely erases the original structures of the former gneisses. Pods of granitic leucosome are common, and they generally show centimetric porphyroblasts of biotite and hornblende (Figures 3e and 3f). Based on the degree of migmatization, four different units were mapped (Figure 2): (1) migmatitic paragneisses, i.e. rocks with well–preserved gneissosity, <30% of partial melting, and sedimentary (peraluminous) protolith; (2) migmatitic orthogneiss, same as (1) but with igneous protolith; (3) para–migmatite, where the leucosomes constitute >30% and the rock lack of clear, traceable gneiss bands; the abundant Al–rich phases, such as sillimanite and garnet, are indicative of a sedimentary origin; and (4) ortho–migmatite, same as (3), but with a clear igneous protolith suggested by the absence of Al–rich phases, together with the presence of biotite and hornblende as principal mafic phases.

Post–migmatization units

Three groups of post–migmatization rock types were recognized in the study area (Figure 2).

Deformed plutons

The deformed plutons have compositions ranging from granite to granodiorite and locally tonalite. Plutonic rocks cropping out towards the SE of the study area usually display a magmatic foliation that was later overprinted by solid–state deformation (e.g., Morán–Zenteno, 1992). Based on field relations and geochronologic data, Morán–Zenteno (1992) argued that the emplacement of deformed granites was syntectonic with the main tectonothermal event that affected the protoliths of the Xolapa Complex. However, in the N–NNW–vergent, La Venta shear zone, the 129 Ma El Pozuelo granite (Figure 2) was deformed under greenschist facies conditions at ~45 Ma (Torres–de León, 2005; Solari et al., 2007). As the El Pozuelo granite is not sheared outside the ~100 m wide La Venta shear zone, Solari et al. (2007) argued that the crystallization of El Pozuelo granite postdates migmatization in the Xolapa Complex. Ducea et al. (2004, 2006) also reported ~134 to ~140 Ma U–Pb zircon ages on "syntectonic" hornblende–biotite gneisses in the study area.

Undeformed, post–Laramide plutons

A second group of post–migmatitic granites typically have ~55 Ma ages. They slightly postdate Laramide deformation in southern Mexico (Cerca et al., 2007; Solari et al., 2007). Such bodies lack of solid–state deformation, although they are locally affected by low–grade ductile shearing or brittle deformation. For instance, the Acapulco granite, intruding the Xolapa migmatites and gneisses just SW of the mapped area, was dated by Ducea et al. (2004) at 54.9 ± 2 Ma. The Salitre peraluminous and Las Piñas granites were dated at 55.3 ± 3.3 and 54.2 ± 5.8 Ma, respectively (Solari et al., 2007), and crop out northeast of the study area, just west of the La Venta shear zone. The plutons are either foliated (El Salitre) or unfoliated (Las Piñas), and locally, along La Venta shear zone, they display S–C fabrics associated with the shearing.

Eocene–Oligocene undeformed granitoids

A third group of undeformed, Eocene–Oligocene granitoids crop out in the studied area (Herrmann et al., 1994; Schaaf et al., 1995; Ducea et al., 2004; Hernández–Pineda, 2006). These plutonic bodies lack ductile, solid–state deformation, but they generally show magmatic banding as well as brittle faults and joints.

Several generations of dioritic–quartzodioritic dikes, pegmatites, and minor aplites, intrude both the Xolapa basement, and the Eocene–Oligocene granitoids. Some pegmatites cropping out in the Barranca de Xolapa were dated by Morán–Zenteno (1992) at ~59 Ma, and by Solé (2004) at 60–62 Ma. Although no isotopic ages are available for the dioritic dikes, the youngest generation post–dates the 34 Ma Tierra Colorada granite.

 

STRUCTURAL EVOLUTION

Three phases of ductile deformation were recognized in the gneisses and migmatites of the study area that are absent in the post–migmatization granitoids. Post–migmatization deformation was previously described by Solari et al. (2007) as ~45 Ma greenschist facies extension, 45–34 Ma brittle–semibrittle SW–vergent thrusting and open folding.

D1: metamorphic banding and initial stages of migmatization

S1 metamorphic banding consists of alternating leucocratic and melanocratic horizons, which range from 0.5 to 10 cm in the orthogneisses but are as thin as few millimeters in paragneisses. Penetrative ductile deformation is associated with development of a gently to moderately, NE to ENE dipping S1 foliation in the west and southwest of the study area, whereas it is moderately W to WNW dipping in the east (stereonets of Figures 4a and 4b). Pre–D1 granitic dikes intruding the paragneisses (e.g., Figure 3c) were deformed by recumbent to isoclinal, rarely rootless, F1 folds, with axial planes subparallel to the S1 foliation. S1 is defined by polygonal quartz, micas and sillimanite in the paragneisses, and by biotite, hornblende and quartz in orthogneisses. The grain size of the quartz varies from band to band. Stretched quartz and elongated feldspar porphyroclasts define the gently NE– to NW–plunging L1 lineation (Figures 4a and 4b).

D2 /M2: ductile deformation, migmatization and leucosome emplacement

In the para–gneisses outside the migmatitic zone, a foliation, S2, is made up of aligned sillimanite + biotite ± muscovite ± staurolite. Garnet is often present in the foliation planes. In the orthogneisses, aligned brown hornblende + biotite + titanite ± garnet constitute S2 (e.g., Figure 3a). S1 and S2 appear to form a composite foliation, S1/2, formed at the highest metamorphic grade as part of one continuous tectonothermal event. Undeformed, 1 to 7 cm thick leucosomatic lenses injected into the paleosome parallel to S1 (Figure 3g) have randomly oriented minerals suggesting that the migmatization postdates the D1 deformation. However, it is possible that the heat associated with D1 friction was responsible for partial melting, and acted as the driving mechanism for neosome generation during the high–grade metamorphism (cf. Brown, 2005). The absence of a S2 foliation in the migmatites and migmatitic gneisses is inferred to be due to the high ductility of molten leucosome during D2, which precluded the recrystallizing minerals from becoming oriented (e.g.), Solari et al., 2003).

D3: ductile deformation of migmatitic structures

The S1/2 foliation and banding are locally deformed by chevron, gently to moderately NW to SE plunging, subvertical to steeply NNE dipping, slightly asymmetric F3 folds (Figures 4c and 5a). These folds are common in both the paragneisses and migmatitic gneisses, but are less common in orthogneisses. In paragneisses, the S3 axial planes are characterized by alignment of biotite and muscovite; in the orthogneisses S3 is defined by aligned hornblende and biotite (Figure 5b). This suggests that the metamorphic grade associated with D3 was in the upper greenschist to amphibolite facies. A L3 mineral lineation varies in orientation from north to south in the study area (Figure 4d). This variation is probably due to the later D4 shearing and D5 refolding.

D4: greenschist facies extension in the post–migmatization units

Top–to–the NNW D4 shear zones, developed under greenschist facies conditions, affected both Xolapa Complex and post–Xolapa Complex units in the northeastern part of the study area (Figure 2). For example, F3 fold limbs are deflected in the shear zones (e.g., Figure 5c). Whereas S4 shear zones are mappable in supracrustal rocks of the Chapolapa Formation, and in both post–migmatization ~130 and ~55 Ma granites (cf. Solari et al., 2007), in the Xolapa gneisses, S4 is generally parallel to S2, constituting a composite S2/S4 foliation, associated with a composite L2/4 stretching lineation (Figures 4e and 4f, respectively).

D5: folding of ~55 Ma pegmatites

Garnet and muscovite–bearing pegmatites, up to 60 cm thick, were closely folded during D5. The S5 axial planar cleavage is not present in folded pegmatites, but it is visible in the host paragneisses. F5 fold axes (e.g., Figure 3h) are roughly subhorizontal, and EW trending, whereas axial plane are subvertical to NNW steeply plunging, and ESE–WNW trending.

 

SUMMARY OF STRUCTURAL EVENTS IN THE XOLAPA COMPLEX AND ITS COVER

Five phases of ductile deformation have been recognized in the Tierra Colorada area of the Xolapa Complex:

(a) D1 and D2 are associated with high–grade metamorphism and migmatization;

(b) D3 is characterized by chevron folds;

(c) D4 is present in the northwestern part of the study area, where it is congruent with the ~45 Ma, normal to left–lateral ductile shearing in the La Venta shear zone (D2 of Solari et al., 2007);

(d) D5 produced folds in some pegmatites cropping out in the northwestern portion, and it can be correlated with the adjacent SSW–vergent thrusting of the Morelos Fm. over the Chapolapa Fm. (D3 of Solari et al., 2007).

 

ANALYTICAL METHODS

Geochemical analyses were performed at Actlabs Labortories (Texas, EE.UU). Major elements were determined by Instrumental Neutron Activation (INA), whereas trace elements were measured by Inductively Coupled Plasma–Mass Spectrometry (ICP–MS) employing lithium metaborate/tetraborate fusions. Common Pb analyses were performed at Laboratorio Universitario de Geoquímica Isotópica, UNAM on feldspars separated from the crushed rocks using methods described in Solari et al. (2004).

Zircons were separated by means of standard crushing and concentration techniques, such as Wilfley shaking table, Frantz isodynamic magnetic separator, and heavy liquids (e.g., Solari et al., 2007). About 10 kg of sample were processed to ensure that every morphological type of zircon was adequately represented in the final concentrate. Representative zircons were then handpicked in ethanol, mounted in epoxy resin together with chips of standard zircon R33 (Black et al., 2004). The mount was then ground to expose zircons to approximately half their thickness, and polished using diamond compound. Polished mounts were cleaned in 1N HCl, to avoid surficial common Pb contamination, and gold coated. Cathodoluminescence imaging (CL) was performed on the polished zircons to gain knowledge on their internal morphology, guide the analysis and help later with the age interpretations. CL imaging was performed in a JEOL 5600 LV SEM, equipped with a Hamamatsu CL Detector, in Stanford–USGS facility. A total of 36 analyses, 20 on zircons belonging to sample Xo0301, and 16 on zircons belonging to sample Xo0303 were performed for U–Th–Pb geochronology, following the methodology reported in Nourse et al. (2005) and Weber et al., (2006), and utilizing the Stanford–USGS SHRIMP–RG (Sensitive High Resolution Ion Microprobe – Reverse Geometry) facilities. Oxygen was used as primary ion beam, and operated at about 2–4 nA, excavating 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 analysis was performed 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 using SQUID (Ludwig, 2001) and plotted using IsoplotEx (Ludwig, 2004).

 

GEOCHEMISTRY

After extensive petrographic screening to eliminate altered samples, six orthogneiss and two paragneiss were chosen for geochemical analysis (Table 1). These samples include unmigmatized samples (RB71 and RB76), gneisses (Xo0303, Xo0230, Xo0214), and migmatites (Xo0301, RB105, RB109). As most of the analyzed samples are high–grade metamorphic rocks, classification and interpretation using diagrams designed for unmetamorphosed rocks should be viewed with caution. Nevertheless, comparisons between the compositions of the analyzed samples and pristine igneous rocks can provide some useful insights into the sources and processes that lead to their formation.

The rocks can be classified as metaluminous I–type diorites to granites, displaying a variable range in SiO2 (58.8 to 74.7 wt%) that correlates negatively with TiO2, MgO and Fe2O3 contents. On the AFM diagram, the rocks plot in the calc–alkaline field (Figure 6a). MgO contents (3.6–0.2 wt%) and Mg# (51–26) of the studied rocks indicate that they are significantly more differentiated than any magma in equilibrium with the upper mantle (Langmuir and Hanson, 1980), and therefore cannot be considered as primary mantle melts.

Chondrite–normalized rare earth patterns (Figure 6e) are enriched in light–rare earth elements (LREE; La/SmN 2.3–6.2), and unfractionated in the heavy rare earths (HREE; Dy/Ybn 1.0–1.4). The Xolapa orthogneisses also display strong negative Eu anomalies (Eu/Eu*=0.44–1) that are coupled with high Rb/Sr ratios (Figure 6b). These geochemical features indicate that magmatic differentiation occurred in the stability field of plagioclase.

Incompatible trace element patterns show enrichments of large ion lithophile elements (LILE) over high–field strength elements (HFSE) typical of arc magmas (Figure 6d). The rocks also display low Nb/La ratios (Figure 6b) and incompatible element relationships that invariably plot within the field of volcanic–arc granites in the tectonic discrimination diagrams of Pearce et al. (1984) (Figure 6f). Nonetheless, the Xolapa rocks also tend to have much lower U/Th and Ce/Th ratios (Figure 6c) than is typical for oceanic island arcs, which tend to preserve the chemical signature of fluids released from the downgoing slab. Thus, the relative enrichment in the fluid–immobile element Th likely indicates strong continental contributions to the original magmas, either through subducted sediments or by means of crustal contamination.

Crustal contributions are also supported by the overall radiogenic Pb isotopic compositions of feldspars from the orthogneisses, which form a restricted positive correlation that is bracketed between an evolved upper–crustal–like component similar to the isotopic composition of the analyzed paragneiss, and an unradiogenic lower–crustal–like component with higher 207Pb/204Pb and 208Pb/204Pb ratios that is typically observed in magmas derived from the partial fusion of the upper mantle (Figures 6g and 6h). In summary, the chemical compositions of the Xolapa rock suites likely reflect the differentiation stages of a magmatic arc that either grew in close proximity to a continent, or was directly constructed over a mature continental crust.

 

GEOCHRONOLOGY

Two samples of migmatitic orthogneiss were chosen for SHRIMP–RG U–Pb analysis. Sample Xo0301 is a migmatite leucosome made up of quartz, plagioclase (Ab 30–50%), poikilitic garnet, brown biotite, and rare hornblende porphyroclasts. Large allanite crystals, abundant zircon and apatite are common accessory minerals. Sample Xo0303 is a banded migmatitic gneiss, in which leucocratic portions are made up of quartz, plagioclase and subordinate K–feldspar and zircon, whereas mesocratic bands are almost entirely composed of biotite, magnetite, tiny relict garnet, apatite and subordinate zircons as inclusions within biotite. Samples locations are in Figure 2, whereas samples coordinates can be found in Table 1.

Sample Xo0301: migmatitic leucosome

Zircons from sample Xo0301 are pale yellow to honey–colored, range from 60 µm to >250 µm, and show oscillatory zoning in cathodoluminescence (CL) images (Figures 7a to 7l) , which is generally interpreted as igneous zoning (Connelly, 2001; Corfu et al., 2003). Some of the zircons show the presence of xenocrystic cores, whose structures are discordant with igneous oscillatory zoning, indicating that igneous zircon grew around preexisting, un–dissolved, older zircons. SHRIMP analyses were focused on the igneous zoning and late–stage recrystallization rather than the inherited cores. Analyses yielded two age groups (Figure 8a): (i) seven analyses (discontinuous ellipses in Figure 8b) are concordant to nearly concordant, and have 206Pb/238U ages between 135.5 ± 1.0 and 131.3 ±0.7 Ma (1 sigma); and (ii) five analyses (thick line ellipses in Figure 8b) are concordant within the analytical error, and have 206Pb/238U ages between 129.8 ± 1.0 and 126.2 ± 1.3 Ma (1 sigma). Comparing the analyzed spots with the CL images (e.g., Figures 7a to 7l), it is evident that the two age groups are related to different events. Whereas, some of the analyzed zircons show continuous oscillatory zonation with similar ages in both core and rim (e.g., Figures 7b, 7c, and 7e), others show a clear discontinuity between core and rim. In one case (Figure 7d), a 129.5 Ma zircon rim grew over an oscillatory–zoned core with a 206Pb/238U age of 135.5 Ma. The Early Cretaceous age group, clustering at 133.6 ± 0.9 Ma (Figure 8b) is interpreted as the age of migmatization. The second age group clustering at 129.2 ± 0.4 Ma, is interpreted as either an episode of Pb loss and/or partial recrystallization.

Another group of analyzed spots yielded more discordant isotope ratios, generally with larger errors (Figure 8a). However, the apparent 206Pb/238U ages of such zircons define a mean of 61.4 ± 1.5 Ma (Figure 8c), which is interpreted as Early Paleocene Pb loss or zircon growth. CL imaging of such domains (e.g., Figures 7a, 7h, 7i, 7l) reveal that they cannot be texturally distinguished from the first episode of Pb loss or recrystallization at ~129 Ma.

Sample Xo0303: banded migmatitic orthogneiss

Zircons separated from sample Xo0303 are yellow to reddish colored, <280 µm in size, and range from prismatic to stubby with or without bypyramidal terminations. Imaged under CL, most of them show well–developed oscillatory zoning. Inherited cores are visible in some crystals, but they do not show any significant age difference (e.g., Figure 6m) when compared to the oscillatory overgrowths (e.g., Figures 7n to 7r). The isotopic analyses are concordant within analytical error (Figure 9a and Table 2). Together, they define a cluster yielding an average concordia age of 178.7 ± 1.1 Ma interpreted as the crystallization age of the orthogneiss (Figure 9b). Some zircon crystals have brighter rims around oscillatory zones (e.g., Figures 7s to 7z), and analyses of these rims yielded discordant ages (Figure 9a) with 206Pb/238U ages ranging between 168.7 ± 1.2 Ma and 116.2 ± 1.0 Ma, suggesting variable degrees of Pb loss. Two of the analyzed crystals, although discordant (Figures 7v and 7z, and Table 2), have 206Pb/238U ages that are similar to the migmatization age of 133.6 ± 0.9 Ma determined on sample Xo0301. The youngest, discordant analysis (Figure 9a) is probably the result of a younger Pb loss as the youngest zircons analyzed in sample Xo0301.

 

DISCUSSION AND CONCLUSIONS

Early Jurassic magmatism

The 178 ± 1.1 Ma igneous age calculated from the migmatitic orthogneiss Xo0303 is the first report of a Lower Jurassic magmatic event in the Xolapa Complex (time scale of Gradstein et al., 2004). Because of its calc–alkaline geochemical signature, we argue that the igneous activity is related to a magmatic arc. This is similar to: (a) a Middle Jurassic age of 165 Ma reported by Guerrero–García et al. (1978) from the Acapulco – Tierra Colorada transect of the Xolapa Complex; (b) a ~158 Ma age reported by Ducea et al. (2004) from north of Puerto Escondido; and (c) an Early Jurassic magmatic age reported by Elías–Herrera et al. (2000) in the Tizapa metagranite (186.5 ± 7.4 Ma, lower intercept of discordant data) in the eastern Guerrero Terrane, Teloloapan subterrane, 200 km north of the studied area (Figure 2). The Lower Jurassic age has several paleogeographic implications. First, it indicates the presence of a pre–Toarcian sedimentary sequence, which was intruded by the Early Jurassic igneous protolith and then metamorphosed to form the Xolapa paragneiss. Ortega–Gutiérrez and Elías–Herrera (2003) argued that ammonite–bearing sediments of the mid–JurassicTecocoyunca Group (Burckhardt, 1927; Corona–Esquivel, 1981), which are extensively exposed north of the Tierra Colorada–Chacalapa shear zone, grade into the metasedimentary protolith of the Xolapa Complex. However, this suggestion is not supported by the data presented in this paper because the Tecocoyunca Group is younger than pre–178 Ma paragneisses of the Xolapa Complex. Another possibility is that the rocks in the study area are correlatives of the easternmost part of the Guerrero Terrane, i.e., the Teloloapan subterrane (Elías–Herrera et al., 2000; Centeno–García et al., 2008). The Teloloapan terrane was accreted to the backbone of Mexico (Mixteca and Zapoteco terranes) either during the Middle–Late Jurassic (Elías–Herrera et al., 2000), during the Early Cretaceous (Dickinson and Lawton, 2001), or during the Late Cretaceous (Talavera–Mendoza et al., 2007). Thus, the 178 Ma magmatism in the high–grade Xolapa Complex could have been part of an Early Jurassic magmatic arc fringing the Mixteca Terrane of southern Mexico, which was subsequently accreted and deformed.

Early Cretaceous Migmatization

Our zircon ages indicate that high–grade metamorphism and migmatization of the Xolapa Complex in the study area occurred ~133 Ma ago. This interpretation is consistent with field observations reported by Solari et al. (2007), who concluded that migmatization occurred prior tothe ~129 Ma crystallization age of undeformed granites intruding migmatites. Our structural data and petrographic studies of Xolapa high–grade rocks are also consistent with those of Corona–Chávez et al. (2006) in the Puerto Escondido area (Figure 1), where only one high–grade metamorphic event in the lower granulite facies is discernible in migmatites and gneisses. Similar lithologies, metamorphic grade and intrusive relationships (cf. Robinson et al., 1989; Herrmann et al., 1994; Corona–Chávez et al., 2006) throughout the Xolapa Complex suggest that the Early Cretaceous age of migmatization can be extrapolated across the whole complex. Support for this is provided by the 131.8 ± 2.2 Ma age calculated by Herrmann et al. (1994) on a migmatite cropping out north of Puerto Escondido.

Late Mesozoic to Early Cenozoic evolution

High–grade gneisses and migmatites in the study area underwent limited re–heating during the Late Cretaceous–Paleocene as indicated by Pb loss in zircons of sample Xo0301. Four tectonothermal events were previously reported for the early Cenozoic in the Xolapa Complex: (1) magmatism at ~55 Ma (Acapulco, El Salitre and Las Piñas granites, Morán–Zenteno, 1992; Ducea et al., 2004; Solari et al., 2007); (2) NNW–vergent, normal and left–lateral ductile shearing at ~45 Ma (Riller et al., 1992; Solari et al., 2007); (3) SW–vergent brittle thrusting between ~45 and ~34 Ma, and (4) magmatism at ~30–34 Ma (Tierra Colorada and Xaltianguis undeformed granites, Herrmann et al., 1994; Schaaf et al., 1995; Ducea et al., 2004; Hernández–Pineda, 2006). Such events are probably responsible for the reorientation of previous structures (e.g., D4, see above), Pb loss in zircons, recrystallization or resetting of isotopic chronometers in micas due to intrusion or hydrothermal fluid circulation, specially along the boundaries of major intrusions (e.g., Solé, 2004).

Tectonic implications

Previous studies of the geological record of the Xolapa Complex have been interpreted in terms of extension and exhumation during the Late Cretaceous – Early Tertiary causing uplift of the isotherms and partial melting (e.g., Robinson et al., 1989; Herrmann et al., 1994; Meschede et al., 1997). Such models were partly based on a 66–46 Ma age for the migmatization (Herrmann et al., 1994). These authors suggested that exhumation of the Xolapa Complex was related to the southeastward displacement of the Chortís block from a location off Puerto Vallarta along the southern Mexican coast at ~100 Ma, towards a location off Huatulco at ~29 Ma.

Our present data suggest two alternative models for the Lower Cretaceous migmatization, both involving accretion of outboard terranes: (1) accretion of the Guerrero terrane to the backbone of southern Mexico could possibly have occurred during the Early Cretaceous (but see discussion above); or (2) accretion of the Chortís block to southern Mexico. The absence of an exposed suture east of the Guerrero terrane argues against hypothesis #1, which suggests that the Guerrero terrane may have been a continental margin arc (cf. Elías–Herrera and Ortega–Gutiérrez, 1998). Moreover, the accretion of the Guerrero terrane cannot explain the synchronous migmatization as far as 500 km SE of the study area. Several lines of evidence indicate a possible correlation between the Xolapa Complex and the northern Chortís block: (i) a similar metamorphic grade occurs in the exposed basement of Las Ovejas Complex in southern Guatemala and northwestern Honduras (Horne et al., 1976; Schwartz, 1977; Ortega–Gutiérrez et al., 2007; Martens et al., 2007), (ii) a ~170 Ma magmatic arc is present in the Chortís block (Martens et al., 2007), and (iii) the presence of eclogites in the Motagua mélange (central Guatemala), which have Sm–Nd mineral isochron ages spanning ~125 to ~136 Ma (Brueckner et al., 2005; Martens et al., 2007). These latter ages are interpreted in terms of collision between the Chortís block and southern Mexico (cf. Harlow et al., 2004; Martens et al., 2007). The similar ~133 Ma migmatization age in the Xolapa Complex could represent collision between the Xolapa Complex and the (current) northern margin of the Chortís block marked by the HP metamorphism in the Motagua melange. Exhumation of such eclogites in blueschist metamorphic conditions at ~70–75 Ma (e.g., Harlow et al., 2004; Martens et al., 2007) could mark the separation of the Chortís block from southern Mexico. This is consistent with the conclusions of Corona–Chávez et al. (2006), who suggested that the clockwise P–T path followed by the Xolapa Complex represent a compressional regime in the roots of a continental magmatic arc. Other processes, such as shortening due to accretion of nappes and/or terranes (e.g., Whitney et al., 1999), or magma–loading processes (e.g., Brown, 1996; Warren and Ellis, 1996) are normally invoked in such a tectonic scenario. Removal of the Chortís block during the mid–Late Cretaceous would have allowed reorganization of plates, and onset of subduction along the Middle America trench to generate the ~55 and ~30 Ma arc plutons that intrude the Xolapa Complex (Ducea et al., 2004; Solari et al., 2007) and the Guerrero terrane (e.g., Levresse et al., 2004).

 

ACKNOWLEDGEMENTS

Several people are thanked for their support during field work (e.g., Rafael Torres de León, Guillermo Hernández Pineda, Teodoro Hernández Treviño, Bernardo Villacura), and for fruitful discussions on the geology of the Xolapa Complex in various stages of this work, such as Pedro Corona–Chávez, Fernando Ortega–Gutiérrez, Mariano Elías–Herrera, J. Duncan Keppie. The authors acknowledge a CONACyT scholarship to RPG, CONACyT grant 54559 to LAS, and PAPIIT–DGAPA grant IN101407 to LAS, which funded field and analytical work. Diego Aparicio made a lot of thin sections we studied during different stages of this work, and Consuelo Macías helped with mineral separations. We also thank Joe Wooden, USGS at Stanford University, for assistance during SHRIMP analyses. Thorough reviews by Bodo Weber, Pedro Corona–Chávez and Dante Morán–Zenteno improved the readability of the concepts expressed in this work.

 

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