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BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA http://dx.doi.org/10.18268/BSGM2009v61n3a9 |
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Petrology and geochemistry of Tehuitzingo serpentinites (Acatlán Complex, SW Mexico)
Petrología y geoquímica de las serpentinitas de Tehuitzingo (Complejo Acatlán, SW México): implicaciones tectónicas
Guillermina González–Mancera*1, Fernando Ortega–Gutiérrez2, Joaquín A. Proenza3 y Viorel Atudorei4
1 Facultad de Química, Universidad Nacional Autónoma de México, 04510, México, D.F.
2 Instituto de Geología, Universidad Nacional Autónoma de México, 04510, México, D.F.
3 Departament de Cristallografa, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona, C/Martí i Franquès s/n 08028 Barcelona, Spain.
4 Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, NM 87131, USA.
* This email address is being protected from spambots. You need JavaScript enabled to view it..
Abstract
Petrographic and geochemical studies of the serpentinites from the Tehuitzingo body, the main ophiolitic outcrop of the Paleozoic Acatlán Complex of southern Mexico, provide new petrogenetic evidence and preliminary data on the nature of the fuids that interacted with an original mantle peridotite. Textures of the studied serpentinites show the principal events of recrystallization and metasomatism, but the diagnostic phases associated with the high pressure events related to subduction were erased. Preliminary H and O isotope studies in serpentinite and chorite suggest the involvement of marine water, probably under oceanic conditions during the frst serpentinization event.
Accessory chromite in the serpentinites is characterized by #Cr ~0.6 and serpentinites display low abundances of Ti, Na, Nd, Sm, Lu and Hf, which suggest that Tehuitzingo serpentinites represent relicts of a depleted mantle formed in a suprasubduction zone, probably in a back–arc setting that experienced partial melting in excess of 18%. Normalized REE patterns of the studied serpentinite samples are characteristic of peridotites from both suprasubduction (SSZ) and mid–ocean ridge (MOR) zones. This preliminary, but important, result may be related to the probable presence of lithospheric mantle slivers tectonically juxtaposed on the Acatlán Complex.
Key words: Serpentinites, Cr–spinel, "Xayacatlán" ophiolites, trace elements, Tehuitzingo, Mexico.
Resumen
El estudio petrológico y geoquímico de las serpentinitas de la región de Tehuitzingo proporciona nuevas evidencias petrogenéticas y datos preliminares de la naturaleza de los fluidos que interactuaron con el manto peridotítico que fue el protolito del cuerpo ultramáfico de Tehuitzingo, el mayor cuerpo ofolítico Paleozoico expuesto del Complejo Acatlán al sur de México. Las texturas de las serpentinitas estudiadas muestran los principales eventos de recristalización y metasomatismo, pero las fases diagnósticas asociadas con eventos de alta presión relacionados con subducción fueron borradas. Estudios preliminares de isótopos de H y O en serpentina y clorita sugieren que agua marina fue involucrada probablemente durante el primer evento de serpentinización bajo condiciones oceánicas.
La cromita accesoria en las serpentinitas tiene un #Cr ~0.6 y las serpentinitas presentan bajos contenidos de Ti, Na, Nd, Sm, Lu y Hf, lo cual sugiere que las serpentinitas de Tehuitzingo representan relictos de un manto empobrecido formado en una zona de suprasubducción, probablemente en un ambiente de trasarco que experimentó una tasa de fusión parcial mayor al 18%. Los patrones normalizados de REEs de las muestras de serpentinita estudiadas son característicos de peridotitas representativas de zonas de suprasubducción (peridotitas tipo SSZ) y de dorsales oceánicas (peridotitas tipo MOR). Este resultado preeliminar podría estar relacionado con la presencia de fragmentos de la cuña del manto litosférico y de la placa subducida, yuxtapuestos tectónicamente en el Complejo Acatlán.
Palabras claves: Serpentinitas, Cr espinela, Ofolita "Xayacatlán", elementos traza, Tehuitzingo, México.
1. Introduction
Serpentinite bodies, up to 500 meters thick and 8 km long, are present in the Tehuitzingo area of the Acatlán Complex, in Puebla State, SW Mexico (Figure 1a). The Acatlán Complex comprises a deformed and polymetamorphic assemblage of Paleozoic metasedimentary and metavolcanic rocks, granitoids and serpentinized ultramafic bodies (Figure 1b). The Tehuitzingo serpentinite body (TUB) is by far the largest ultramafic body within the Acatlán Complex. The sequence extends from Tlachinola to Atolpotitlan, about 10 km south of Tehuitzingo (Figure 2). The serpentinites are juxtaposed along a regional thrust fault above low–grade metasedimentary rocks of the Cosoltepec lithodeme (Ortega–Gutiérrez, 1993).
Previous studies of the Tehuitzingo serpentinites mainly focused on geology (Ortega–Gutiérrez, 1978; Carballido–Sánchez and Delgado–Argote, 1989), mineralogy (Solís–Muñoz, 1978; González–Mancera, 2001), and associated chromitite bodies (Proenza et al., 2004; Zaccarini et al., 2005). However, the signifcance of Tehuitzingo serpentinites is still a matter of debate. These serpentinites were frst interpreted as part of an ophiolite originated at a mid–ocean ridge (Ortega–Gutiérrez, 1978). In contrast, Proenza et al. (2004) suggested that Tehuitzingo serpentinites were formed at a convergent plate boundary as part of an island arc structure, and were interpreted as a fragment of oceanic lithosphere (Xayacatlán Ophiolite) formed in an arc/back–arc environment. In a more recent paper (e. g. Nance et al., 2006 and references therein), the ultramafic bodies of the Acatlán Complex have been interpreted as being associated with the early stages (Ordovician) of the Rheic Ocean opening in the western margin of Gondwana.
In this paper, we provide detailed petrographic and mineral assemblages together with composition of bulk–rock (major and trace elements), minerals, and stable isotopes (D, O), which are used to assess the petrogenesis and tectonic setting of the Tehuitzingo serpentinites.
2. Geological setting
The Acatlán Complex is a polymetamorphic unit of Paleozoic age composed of metasediments, granitoids and mafic–ultramafic rocks metamorphosed at high pressure/ low temperature (subduction event) and high temperature/oderate pressure (collision event) (Ortega–Gutiérrez, 1978, 1993; Yañez et al., 1991; Weber et al., 1997; Ortega Gutiérrez et al., 1999; Malone et al., 2002; Elías–Herrera and Ortega–Gutiérrrez, 2002; Meza–Figueroa et al., 2003). According to Ortega–Gutiérrez (1978, 1993) the Acatlán Complex formed essentially as a collisional orogen in Cambro–Devonian times associated with closure of the Iapetus Ocean. However, recent views (e. g. Murphy et al., 2006; Nance et al., 2006) relate it to the opening and closure of the younger Rheic Ocean, and to Permo–Triassic convergence of Pacifc plates on the western margin of Pangea.
These serpentinite bodies are part of the Xayacatlán Formation (Ortega–Gutiérrez, 1978) and comprise greenschists, pelitic schists, gneisses, quartzites, amphibolites, metagabbros, mafic eclogitic and pelitic rocks, and the serpentinites studied in this work.
The Tehuitzingo serpentinite bodies, containing chromitite lenses, are in close relationship with eclogitic rocks embedded within a metasedimentary matrix rich in garnet, rutile and phengite, suggesting that the entire assemblage underwent a common high–pressure metamorphic history. The serpentinites elsewhere in the Acatlán Complex also form lenses associated with tabular units of ecloglitic metabasite and high pressure metapelitic rocks (Figure 2). They are frequently mylonitic and commonly display foliation defined by the preferred dimensional orientation of antigorite blades and spinels (Proenza et al., 2004).
The Tehuitzingo ultramafic body can be interpreted as a completely serpentinized harzburgite, where the dominant occurrence of antigorite and interpenetrative and interlocking (non–pseudomorphic) textures are in agreement with the geological history suggested for the Acatlán Complex (Ortega–Gutiérrez, 1978). It forms the base of a dismembered ophiolite consisting of eclogitized mafic metabasites and metapelites thrust over low grade phyllites and quartzites of the Cosoltepec Formation (Figure 1b). Talc rock, chloritites and other metasomatic monomineralic rocks such as tremolitites and epidotites commonly occur in the sole of the thrust. However, because of the wide pressure stability of serpentine (antigorite) and protracted retrogression of the studied rocks, the high pressure regime that affected the ultramafic rocks was only partially preserved in the accompanying metabasites and metasediments.
3. Sampling and analytical methods
The serpentinites studied here were collected at Los Venaditos Canyon and El Tigre Canyon at the Tehuitzingo ultramafic body in the Xayacatlán Formation of the Acatlán Complex in southern Mexico (Figure 1, Table 1). Petrographic, mineralogical and textural analyses of serpentinitic rocks were determined frst on six thin sections and then electron microprobe analyses were performed on selected samples.
Mineral compositions were obtained in rock thin sections by electron microprobe using a CAMECA SX 50 instrument at the Serveis Cientifcotècnics of the Universitat de Barcelona (Spain). Excitation voltage was 20 kV and beam current 15 nA, except for analyses of Cr–spinel for which a current of 20 nA was preferred. Most elements were measured with a counting time of 10 s, except for Ni, V and Zn (30 s). Calibrations were performed using natural and synthetic reference materials: chromite (Cr, Al, Fe), periclase (Mg), rhodonite (Mn), rutile (Ti), NiO (Ni) and metallic V. The chemical data for Cr–spinels were stoichiometrically recalculated in order to distinguish FeO from Fe2O3 according to the procedure described by Carmichael (1967).
The serpentine polymorphs in twelve serpentinite samples were analyzed using X–ray diffractometry at the Institute of Materials, and electronic transmission microscopy JEOL/2010 at the Faculty of Chemistry, both at the National Autonomous University of Mexico, at 200 Kv operating conditions.
Bulk rock major and minor elements were measured by X–ray fuorescence (XRF) spectrometry at the University Isotope Geochemistry Laboratory (LUGIS) in the Instituto de Geología, UNAM, using a SIEMENS SRS 3000 spectrometer. Rare earth elements (REE), Sc, As, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, Th, U, Pb, Nb, Ta and Hf were analyzed by a commercial laboratory (ACT–LABS) using lithium metaborate/tetraborate fusion with inductively coupled plasma mass spectrometry (ICP–MS), except Sc and As that were analyzed by Instrumental Neutron Activation Analysis (INAA).
Three serpentine and three chlorite separates from the serpentinites were used for oxygen and hydrogen isotope analyses. Samples were grounded and disaggregated in distilled water using an ultrasonic cleaner. The size fraction to mesh <80 was separated in order to remove contaminants. Then a magnet was used to separate magnetite from this fraction. These processes were repeated until no magnetic fraction remained in the separate. Purity of separates was checked by X–ray diffraction and is estimated to be better than 95%. Isotope stable analyses were carried out at the University of New Mexico Department of Earth and Planetary Sciences. Oxygen isotope analyses were carried out using the Sharp (1990) laser fuorination technique. Silicate samples were reacted with BrF5 and heated with a 25–W Merchantek CO2 laser. Liberated O2 gas was purifed and collected on a 13X molecular sieve. The δ18O values were performed with 1–2 mg of sample and measured on a Finnigan MAT Delta XL mass spectrometer. Hydrogen isotopic analyses were performed with 2–4 mg of sample using the Sharp et al. (2001) method. The δD was measured on a Finnigan MAT DeltaPlus XL mass spectrometer.
4. Results
4.1. Petrography
The studied samples (Table 1) consist essentially of 100% serpentinized ultramafic bodies of antigorite serpentinites (Ve02) and occasional chloritites (Ve38) or tremolitites (Ve11), indicating pervasive events of retrogression and metasomatism. The only preserved primary phase is Cr–spinel (Figure 3a). Some samples (Te03, Te04) show a penetrative fabric defined by the preferred dimensional and crystallographic orientation of antigorite blades with grain size between 100–500 μm, but many other lack any foliation and the antigorite blades in some samples tend to form radial aggregates. Clinochlore, rarely accompanied by secondary tremolite and clinopyroxene (Ve06), is a common accessory phase in calcium–rich metasomatized serpentinites. Opaque phases consist of minor Ni–Fe–Cu sulphides, Cr–spinel, and magnetite (Figure 3b), the last usually altered to hematite and goethite. Magnetite and its alteration products occur dispersed regularly or remobilized along the foliated or blastomylonitic serpentinite matrix, although magnetite is more commonly associated with secondary veins of carbonates (Figure 3c). Occasionally, the largest magnetite crystals (up to 1.5 mm) preserved cores of red Cr–spinel, indicating that many grains may be the oxidation end product of primary Cr–spinel. In other samples, it defines symplectitic patterns that suggest a complex metamorphic origin and evolution.
Pseudomorphic textures include bastite (Ve26) in grains about 3–6 mm in size, mainly after orthopyroxene (Figure 3d), and in some samples (Ve02) many crystals of antigorite contain lamellar rods (Figure 3e) of a translucent phase, probably hematites or ilmenite, exsolved along two intersecting planes at 60° from an original iron–bearing pyroxene. Carbonate pseudomorphs after olivine (Ve38) replaced the mesh texture (Figure 3f). In some cases (Ve23), carbonates define pseudomorphic structures up to 1 mm in size (Figure 3g), apparently after clinopyroxene cut by veins of deformed antigorite. Serpentinized olivine pseudomorphs that escaped late deformation were occasionally distinguished forming grains up to 850 μm long (Te18).
Chloritites consist of two generations of chlorite and very fine–grained opaque oxides. Chlorite 1 (older and magnesian) is pleochroic from green to pale yellow (probably Cr–spinel rich), aligned in the foliation and kinked, whereas chlorite 2 (younger and ferrian) flls spaces between Mg–chlorite grains and is not pleochroic. Fe–chlorite appears deformed and foliated inside deformed veins indicating syntectonic emplacement. Moreover, the oxide dust appears to have been formed by pressure–solution phenomena during deformation because it is concentrated in folded stylolitic surfaces in the chloritite.
Irregular, discontinuous veins cutting or following the foliated serpentine groundmass consist of carbonate and magnetite (Figure 3c), indicating late introduction or remobilization of the oxide phase during vein formation. Some of the secondary carbonates replace the cores of many serpentine crystals (Figure 3f), and numerous opaque grains show rims of clinochlore about 200 μm wide cut by the carbonate veins, further suggesting that they represent high–Al primary Cr–spinel altered before carbonate metasomatism. These secondary carbonates (calcite and dolomite) may form up to 20 % of the surface of the thin section area and they are usually very fne grained (10–20 μm), dirty, and associated with very fne grained chrysotile, altering from the antigorite blades.
Deformation fabrics include textures with three penetrative phases of deformation, D1 represented by intrafolial isoclinal folds (Te01), D2 by a mylonitic–phyllonitic (Te04) superposed fabric, and D3 by crenulation of these early folds. S1 is defined by coarse antigorite blades and large (up to 2 cm; Te01) elongate porphyroclasts (Figure 3h), whereas fner grained (0.5 mm) antigorite (blastomylonitic) is associated with the S2 and oriented subparallel to S1. S3 is associated to a late crenulation cleavage and to the injection of carbonate–magnetite veins, which show an irregular distribution, arrangements sub–parallel to S3, or simple patches replacing the serpentine matrix. Only one carbonate vein traversed the entire thin section, showing in this case a more complex mineralogy that includes a phase optically identifed as prehnite, probably indicating incipient calcium metasomatism that ended locally with rodingite formation, where garnet and clinopyroxene replaced small patches of the serpentinite. Stylolitic folded surfaces defined by concentrations of opaque oxides, phrenite and probably stilpnomelane appear to be related to D2.
4.2. Mineral chemistry
4.2.1. Serpentine
The chemical compositions of serpentines from Tehuitzingo are presented in Table 2. These results illustrate that SiO2 exhibits contents from 40.2 to 43.4 wt.%, MgO from 36.3 to 43.1 wt.%, FeO from 1.0 to 6.9 wt.%, Al2O3 from 0.01 to 2.5 wt.% and Cr2O3 from 0.03 to 1.9 wt.%. The binary diagram Mg# vs. Si (Figure 4) shows the wide chemical diversity of the Tehuitzingo serpentines, which also matches the feld representing the compositional range of serpentines described in the literature (D'Antonio and Kristensen, 2004). The Si content of some serpentines shows values slightly higher (>4 a.p.f.u) than commonly reported (Figure 4), which could be explained by the occurrence of antigorite, as it has slightly higher SiO2 (D'Antonio and Kristensen, 2004).
The chemical composition related to different textures of serpentine analyzed here show signifcant overlap (Table 2, Figures 4–5). Nevertheless, Figure 5 shows that bastite textures are different from mesh textures, the former having signifcant amounts of Al and Cr. Serpentine after olivine generally has an Al content below 0.04 and a Cr content below 0.01 a.p.f.u (Table 2). Serpentine minerals flling veins are similar in terms of Al and Cr content, to those replacing olivine (mesh texture).
Serpentines showing interpenetrative–blade texture have Si contents that are between 3.8 and 4.0 (a.p.f.u.), which overlaps with the other studied textures (Figure 4). Some of their Mg# values (Figure 4) are similar to those textures but are generally lower (0.9). Al can enter in serpentines by tschermak substitution (VIAlIVAlVIMg–1IVSi–1), which is characterized by increasing Al (and Fe) and decreasing Mg and Si (Li et al., 2004; Hajialioghli et al., 2007).
4.2.2. Accesory Cr–spinel
Ch–spinel (chromite) occurs in irregular to amoeboid shapes. In the serpentine matrix, spinel grains display three different zones: the core (deep red color in thin section when viewed under plane polarized light) represents unaltered (primary) chromite, the intermediate zone (black when viewed under plane polarized light) corresponds to chromite altered to "ferritchromite" and the outer zone is evolved magnetite. Chlorite also occurs as thin rims surrounding altered chromite. Only analyses performed in unaltered cores have been considered in the interpretation of the primary chromite and used for petrogenetic purposes.
The chemical composition of chromite cores in four samples is listed in Table 3. The plots of Cr# (Cr/Cr+Al) vs. Mg# (Mg/Mg+Fe2+), and Cr# vs. TiO2 show that primary chromite compositions are uniform, and plot close to the intermediate part of the SSZ feld, near the depleted end of the abyssal peridotite spinels (Figure 6a).
The Cr# ranges between 0.54–0.63 (Table 3), Mg# ranges from 0.33 to 0.51, and Fe+3# = [Fe+3/ (Fe+3+Cr+Al)] is lower than 0.03, corresponding to Fe2O3 contents between 0.2 and 2.6 wt. %; TiO2 (<0.16 wt.%), MnO (<1 wt.%), ZnO (<1 wt.%), V2O3 (<0.28 wt.%), and NiO (<0.12 wt.%) concentrations are low.
In a Cr# vs. TiO2 diagram (Figure 6b) chromite compositions show an increase in TiO2 at practically constant Cr#. This trend is probably the result of melt–mantle interactions (e. g. Kelemen et al., 1995; Dupuis et al., 2005; Choi et al., 2008). On the other hand, chromite compositions from Tehuitzingo serpentinites have Al2O3 and TiO2 contents similar to those from subduction related mantle peridotites and highly depleted MOR peridotites (Kame–netsky et al., 2001).
4.2.3. Chlorite
Chlorite in the serpentinites is found associated with serpentine, around altered chromite and in massive form. According to the nomenclature proposed by Hey (1954), the first two types were classified as clinochlore with smooth variations to pennine, whilst the last was classifed as chamosite. The FeOtotal content in chamosite (6.7–7.2 wt.%) is higher than in clinochlore (3.2–4.6 wt.%), whereas its Cr2O3 content is lower (< 1 wt.%) than in the latter.
4.3. Whole rock geochemistry
4.3.1. Major elements
The whole rock major elements (Table 4) plotted in a CaO–Al2O3–MgO ternary diagram (Figure 7) show that the ultramafic serpentinized rocks of Tehuitzingo correspond to a harzburgitic protolith. Because of the Tehuitzingo serpentinite alteration and its lack of primary mineral phases, it was impossible to calculate the normative values.
4.3.2. Minor and trace elements
Whole rock trace element data of six serpentinite samples from the Tehuitzingo ultramafic body are listed in Table 5.
Figure 8 shows the distribution of lithophile trace elements normalized to primitive upper mantle (McDonough and Sun, 1995) for the Tehuitzingo serpentinites. They are depleted in terms of lithophile trace elements. Nevertheless, they show variable relative enrichment in the most of incompatible trace elements (Cs, U, and Nb), and exhibit a positive Sr spike. The REE patterns of Tehuitzingo serpentinites show two groups: group 1 (Ve11 and Ve20) has lower HREE abundances and display relatively fat REE patterns (Ve20) (Figure 8), and group 2 (Ve03, Ve06, Ve23) is characterized by LREE–HREE profles with positive slopes (Figure 8). A characteristic feature of all REE patterns is the negative Ce anomaly, which probably results from the mobility of the trivalent LREE during secondary alteration (e. g. Gruau et al., 1998), such as, seafoor weathering or serpentinization (Niu, 2004).
The compositions of Tehuitzingo serpentinites indicate the enrichment of fuid mobile elements (As, Sb, Pb, Sr; Figure 9). In this fgure, fuid–mobile elements refer to those with high solubilities in aqueous fluids, whereas fuid–immobile elements are listed in order of compatibility with mantle minerals during partial melting (see Hattori and Guillot, 2007).
4.4. D, O isotopes
Most analyzed serpentines and chlorites from TUB exhibit similar isotopic compositions (δ18O between +7.04
and +6.29 and δD values from –47.4 to –66.0) (Table 6). Such values are typical for serpentine formed in the presence of fuids dominated by oceanic water (Figure 10). It should be noted that the preliminary nature of these data does not exclude other possible sources in the crust or mantle. The chrysotile vein separate, however, has the lowest δD value (–112; Table 6), which indicates that it formed at a lower temperature than the interpenetrative serpentine.
5. Discussion
5.1. Implications of serpentinite textures
The composition, mineralogy, and textures of studied serpentinites indicate different processes of serpentinization. The common presence of pseudomorphic textures, representing former orthopyroxene (bastite) and olivine (hour–glass textures), supports the harzburgitic nature of the original mantle rock. However, occasional tremolite–rich rock associated with the serpentinites indicates that clinopyroxene may also have been altered from an original fertile lherzolite by reactions such as Atg + 2 Di + 2 SiO2 (in the fuid) = Tr + H2O, or Di + H+ = Srp + SiO2 + Ca++ + H2O, rather than by Cametasomatism of the associated metabasites (cf. Frost and Beard, 2007). Bastite pseudomorphs, with their higher content of chromium and aluminum and probably derived from the breakdown of orthopyroxene as previously noticed by Wicks and Plant (1979), represent the frst event of peridotite serpentinization. The pseudomorphic textures probably resulted from ocean foor hydrothermal metamorphism, with some domains of bastite or mesh texture surviving subsequent prograde metamorphism and deformation. Mylonitic fabrics and folded bladed texture are dominated by anti–gorite (determined by XRD), which may have developed during subduction of the oceanic lithosphere or collisional emplacement. Interpenetrative and interlocking textures, which are interpreted as having formed by retrograde metamorphism during exhumation of a previously existing non–pseudomorphic texture, are composed of prograde antigorite. Veins of chrysotile and blades of serpentine accompanied by abundant magnetite were produced in a late–stage hydrothermal activity. Some of these veins crosscut earlier carbonate generations.
Cr–spinel is the only residue of the original mantle peridotite and, except for a few pseudomorphic textures (bastite and mesh), the majority of the Tehuitzingo serpentinites exhibit interpenetrating textures, where most serpentine is antigorite with minor lizardite. Experimental studies have confrmed that antigorite is the most stable mineral at high pressures and moderate temperatures in subduction zones (Chernosky et al., 1988; Ulmer and Trommsdorff, 1995).
5.2. P–T conditions of serpentinization
Geothermometry in completely serpentinized ultra–mafic bodies, devoid of higher temperature metamorphic silicates such as anthophyllite, olivine and pyroxenes, can only be based on the stability of the high–temperature serpentine polymorph antigorite. Temperatures above 650° C at any pressure would have originated olivine and pyroxenes for which textural evidence (bastite and hourglass pseudomorphs) is scant, but in this work these textures are ascribed to the original mantle peridotite olivine and pyroxene before its frst serpentinization.
The residual pseudomorphic textures could represent the oceanic stage alteration within the stability of lizardite and chrysotile relative to antigorite. However, the neocrystallization of olivine and pyroxene (deserpentinization produced by high–grade metamorphism) as the source of pseudomorphic textures of TUB is discarded on the basis of two criteria: (a) the large size (up to 0.5 cm; Figure 3d) of pseudomorphs, which is similar to most mantle peridotite primary textures, and (b) because it has been shown that bastite may be resistant to later changes within the antigorite feld (Dungan, 1979). Moreover, it must be kept in mind that the vast majority of analyzed serpentines (Table 3) have relatively high concentrations of Al2O3 and SiO2, which would considerably increase the stability of the lizardite in pseudomorphic textures (Dungan, 1979). Strongly sheared and folded serpentines, composed of antigorite, may correspond to the different orogenic stages leading to tectonic collisions, while the complex textures formed by replacement of antigorite by chrysotile and late emplacement of veins of hydroxides, carbonates and oxides were formed during the fnal stages of exhumation and under a stress regime in the brittle–ductile transition.
The Mg/Si rates between 1.30 and 1.47 obtained for most of Tehuitzingo serpentines are different from the theoretical stoichiometric value (1.5) for certain serpentine minerals, which implies an excess of silica more common in antigorite compared to lizardite and chrysotile. This, in turn, would indicate silica metasomatism during formation of antigorite through mobilization of subduction fluids towards the mantle wedge and before its incorporation to the continental crust during the collision process. Antigorite could also have been generated by the orogenic interaction of the ultramafic mass with continental crust. However, the frst explanation is more reasonable because the silicifcation process would have permeated the entire ultramafic body, whereas interaction of the ultramafic body with crustal rock usually only results in local steatization, where serpentinite is altered across metric sized aureoles in its contact with country rocks.
Most probably the serpentinization events represented by replacement of antigorite by chrysotile–lizardite, occurred between 250 °C and 350 °C, at pressures of about two kbar, corresponding to the late Paleozoic exhumation of the orogen, associated with early stages of the Rheic Ocean closure (e. g., Nance et al., 2006; Keppie et al., 2008).
On the other hand, it is clear that the Tehuitzingo ultramafic body, whatever its ultimate origin, has undergone a complex petrological evolution associated with its birth in the mantle to incorporation in the core of a collisional orogen. Clearly distinguishing each of these stages in the present mineralogy and textural relations was not possible because the integrated processes lead to total serpentinization of the original peridotite accompanied by intense shearing, hydrothermal alteration, and metasomatism. Nonetheless, pseudomorphic textures and local preservation of igneous chromite permitted some important inferences about the nature of the precursor mantle rock and the physicochemical conditions that assisted the main events. Whole rock chemistry indicates, assuming an isochemical process except for the massive access of water to the system, that the Tehuitzingo ultramafic body massif evolved from an original suprasubduction zone harzburgite.
5.3. Accessory chromite composition of TUB serpentinites and tectonic implications
The composition of accessory chromite shows that the TUB was formed in a suprasubduction zone in arc/back–arc environment (Figure 6a). Two arguments for this interpretation are: (i) the chromite composition in Tehuitzingo serpentinite has predominantly Cr#<0.6, whereas fore–arc peridotites usually have Cr#>0.65 and up to 0.85 (Dick and Bullen, 1984; Niu et al., 2003), and (ii) the presence of Al–rich chromitites associated with the Tehuitzingo serpentinites. Al–rich chromitites tend to form in nascent spreading centers, such as back–arc basins. By contrast, Al–rich chromitites have not been reported in forearc environments. In addition, no ophiolitic chromitites are thought to form in mature spreading centers, such as mid–ocean ridges (e. g. Arai and Yurimoto, 1995; Zhou and Robinson, 1997; Proenza et al., 1999).
The accessory chromite in Tehuitzingo serpentinites have Cr# that plot outside the felds defined by boninites of primitive oceanic arcs and mid–ocean ridge basalts (MORB) (Figure 6b). Instead, the compositions fall between these felds, which may be explained by a magma of transitional provenance such as a young back–arc, where the chromite would originate from a depleted mantle affected by high degrees of partial melting (~20%; Kamenetsky et al., 2001). The variation in the Mg# depends on the Cr–spinel/olivine ratio since it is the result of subsolidus Mg–Fe exchange between olivine and Cr spinel on cooling.
5.4. Parental melts
Further insights into the chemistry of the parental melts for the studied chromites can be gained using the equation of Maurel and Maurel (1982), namely, (Al2O3 wt%)Sp = 0.035(Al2O3 wt%)Liquid2.42.
The results show that melts had an average Al2O3 content of 13.74% (Table 7). This value is similar to the Al2O3 contents of mid–ocean ridge and backarc basin basalts (Wilson, 1989; Fryer et al., 1990).
Probably this Al2O3 content represents transitional compositions associated with non–evolved back arc basin basalt. We calculated the degree of partial melting (F) of the TUB precursor peridotite, based on the empirical equation proposed by Hellebrand et al. (2001), F = 10 ln (Cr#) + 24, suggesting partial melting up to 18 % (Table 8), which is within the range of peridotites from suprasubduction zones (Table 8).
5.5. Geochemistry
Preliminary REE geochemistry of TUB serpentinites distinguished two groups of samples, as observed in primitive–normalized REE patterns (Figure 8). One group (Ve11, Ve20) displays morphological REE patterns with fat to negative slope trends (Figure 8). This preferential fractionation of LREE in relation to the rest of the REEs cannot be explained exclusively in terms of partial melting with extraction of melt. In contrast, it can be interpreted as a result of fractionation of the most incompatible REE associated with percolation of small fractions of volatile–rich melt through porous channels (Van der Wal and Bodinier, 1996; Proenza et al., 1999; Melcher et al., 2002). The LREE enrichment with respect to the HREE is probably not related to the addition of seawater (serpentinization), as seawater is depleted in REE (e. g., Li and Lee, 2006). These patterns are more characteristic of suprasubduction zones (SSZ) and may be due to secondary metasomatism during subduction or metamorphic interaction with the continental crust. However, because the TUB in Acatlán Complex underwent a very complex orogenic history, these effects could not be evaluated in this paper.
The other group (Ve03, Ve06, Ve23) shows higher REE values, and in general developed patterns with positive slopes (Figure 8). These REE patterns are characteristic of abyssal peridotites (MOR–type), but such geochemical signatures in mantle peridotites (SSZ and MOR) are common in many ophiolites. MOR–type signatures may be naturally preserved because melt/peridotite interaction in SSZ does not affect equally the entire wedge.
On the other hand, all samples are enriched in LILE (Figures 8–9) (As, Sb, Cs, U, Sr), which could be associated with seawater alteration or fuids produced by dehydration of subducted slab (Stolper and Newman, 1994; Keppler, 1996; Stalder et al., 1998). Thus, this enrichment of Tehuitzingo serpentinites can be explained by infltration of hydrothermal fuids derived from seawater and/or fuids related to dehydration of the subducted slab. Fluids related to the subducted slab are hotter than those related to seawater infltration; in the former case, LREE enrichment can be expected relative to HFSE.
The extremely low content of tantalum in the TUB serpentinites and low content of thorium (Table 5), compared to ytterbium also indicates a suprasubduction environment (Gorton and Schandl, 2000), probably associated with an oceanic arc.
5.6. Preliminary study on the isotopic composition of 18O and D
The isotopic compositions of oxygen and hydrogen have been used (Wenner and Taylor, 1973) to identify the source of water that produced the serpentinization. Sheppard (1986) mentions that the D/H ratio rather than the 18O/16O ratio of water is often the most defnitive parameter to determine the source of water because oxygen isotope composition of water may not retain the "label" of its source. In this study, the calculated 18O fuid compositions of fuid in equilibrium with serpentine (antigorite) are consistent with marine water interaction (Table 6). Figure 10 shows that chlorites and serpentines fall near the range of "oceanic serpentine" (δ18O= +0.8 to +6.7, δD= –68 to – 35) as defined by Wenner and Taylor (1973), whereas the D/H values exhibit values that could suggest moderate temperatures (350 ± 50°C). If continental serpentinization had occurred, we would expect minerals with relatively high 18O values (around 12–15 or higher).
Analyses of hydrogen and oxygen isotopes in serpentine minerals indicate that lizardite and chrysotile can be distinguished from antigorite by their isotopic signature (Wicks and O'Hanley, 1988).
Chrysotile veins in TUB have lower δD (–112) than matrix antigorite or chlorite (Figure 10, Table 6), probably indicating that chrysotile experienced exchange with water at low temperatures. Moreover, the relatively higher δ18O values of chrysotile (7.02) suggest that the fuid activity occurred at temperatures lower than antigorite crystallization.
Data for serpentine and other phyllosilicates show that chrysotile and lizardite readily exchange hydrogen with ambient fuids at low temperatures, leading to very low δD (Kyser and Kerrich, 1991; Kyser e/ ah, 1999). In contrast, coarse grained antigorite exchanges hydrogen much more slowly and retains its original δD.
The oxygen and hydrogen isotopic compositions of the analyzed serpentine and accessory chlorite mainly fall within the "oceanic serpentine" (Figure 10) or forearc sea–mount felds defined by Wenner and Taylor (1973, 1974) and Sakai et al. (1990) respectively, which suggests that chloritization and serpentinization took place in an oceanic arc setting process at moderate temperatures (~300°C) by seawater–derived fuids. Wenner and Taylor (1973) concluded that antigorite serpentinization apparently occurs at higher temperatures (220° to 460°C) than lizardite–chrysotile serpentinization of alpine ultramafic rocks. Chlorite geothermometry (González–Mancera et al., 2006) suggests that serpentinization–chloritization processes occurred at 250° to 400°C.
6. Conclusions
The chemical composition and petrographic observations of textures in TUB serpentinites show different stages of alteration from original mantle harzburgites. Residual pseudomorphic coarse grained textures (0.5–1 cm) could represent the oceanic stage developed under anorogenic conditions, within the stability of lizardite and chrysotile. In this phase, a process of neocrystallization of olivine and pyroxene (deserpentinization) as the source of pseudomorphic textures was discarded.
The dominant serpentinization process found is characterized by interpenetrative textures composed of antigorite, which indicate prograde metamorphism conditions and P(H2O)=Ptotal (O'Hanley, 1996). The estimated temperature during TUB peak metamorphism was below 600°C and probably occurred during a collisional orogeny that closed the ocean tract where the original peridotites were emplaced. Subsequent serpentinization events represented by replacement of antigorite for chrysotile–lizardite occurred between 300 °C to 500 °C, at unknown pressures. These events should correspond to the orogenic process that exhumed the Acatlán Complex by Devonian–Mississippian times.
The composition of accessory chromite suggests that Tehuitzingo serpentinites represent residual mantle that interacted with some melt in a back arc setting.
Our main conclusion is that Tehuitzingo serpentinites represent the relicts of depleted mantle peridotite formed in a suprasubduction zone (probably in a arc/back–arc environment), which experienced a high grade of partial melting (>18%). The enrichment of Mn and Zn in the "ferritcromite" aureoles of Tehuitzingo accessory chromites may be associated with different stages of hydrothermal alteration related to a polymetamorphic history as proposed by Ortega–Gutiérrez (1981).
The REE patterns obtained from TUB serpentinites are characteristic of suprasubduction peridotites. Two geochemical signatures were found: MOR and SSZ, which could suggest the presence of two tectonic domains: the mantle wedge and the subducted slab. The enrichment in incompatible LILE (Sr, As, Sb, Pb) suggest addition of a fuid component rich in these elements, presumably transferred from the subducted slab in to the mantle wedge within the suprasubduction zone.
Preliminary results from δ18O and δD compositions determined in serpentine and chlorite minerals from TUB suggest interaction with marine waters.
Acknowledgements
The EPMA analyses were carried out in the Serveis Cientifcotècnics of the Universitat of Barcelona (UB). We also acknowledge the assistance of R. Lozano Santa Cruz for his help with XRF analyses and of Antoni Camprubí for his constructive comments to the analytical work. In addition, the stay to carry out most of analytical part of this work at the Universitat de Barcelona, was possible due to fnancial support from the student mobility program DGEP–UNAM. This paper was supported by a COSUA of the Chemistry Faculty and an UNAM DGAPA project to Fernando Ortega Gutiérrez. Detailed and critical reviews by F. Zaccarini, I. Uysal and S. Guillot signifcantly improved the manuscript.
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Manuscript received: March 30, 2009.
Corrected manuscript received: July 12, 2009.
Manuscript accepted: August 20, 2009.
