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Andean geology

versión On-line ISSN 0718-7106

AndGeo vol.38 no.2 Santiago jul. 2011

 

Andean Geology 38 (2): 268-283. July, 2011
formerly Revista Geológica de Chile
www.andeangeology.cl

 

Polyphase white mica growth in low-grade metapelites from La Cébila Metamorphic Complex (Famatinian Belt, Argentina): evidence from microstructural and XRD investigations

Crecimiento polifásico de mica blanca en metapelitas de bajo grado del Complejo Metamórfico La Cébila (Faja Famatiniana, Argentina): evidencias a partir de investigaciones microestructurales y de DRX

 

Sebastián O. Verdecchia1, Gilda Collo1, Edgardo G. Baldo1

CICTERRA-Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)-Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales, Av. Vélez Sarsfields 1611, CP. X5016GCA Córdoba, Argentina. sverdecchia@efn.uncor.edu; gcollo@efn.uncor.edu; ebaldo@efn.uncor.edu


ABSTRACT. Two tectono-thermal metamorphic events, M1-D1 (S1, with associated white mica and chlorite: WM1-Chl1) and M2-D2 (S2, with development of WM2-Chl2), are established from polyphase white mica growth for low-grade units from the Ordovician metasedimentary successions of La Cébila Metamorphic Complex in the Famatinian belt (western-central Argentina). The thermobarometric characterization of the M1 main event was carried out by means of clay-mineral analysis and crystallo-chemical parameter measurements. Epizonal (temperatures between 300 and 400°C) and low-pressure conditions are suggested for M1 event, based in Kübler index values ranging from 0.23 to 0.17 A°20, white mica b parameter values between 9.004 and 9.022 Á (mean of 9.014 Á, n=16) and Si contents between 3.13-3.29 a.p.f.u. Temperatures of ~180-270°C are estimated for the M2 event, with Kübler index values ranging from 0.31 to 0.46 A°20. The M1-D1 event of La Cébila could be linked to high-strain heating tectono-metamorphic Ordovician regime recorded in others complexes from Famatinian foreland region of Sierras Pampeanas.

Key words: Sierras Pampeanas, Low pressure, Kübler index, White mica b parameter.


RESUMEN. Dos eventos tectono-metamórficos fueron establecidos a partir de la blastesis superpuesta de mica blanca en sucesiones metasedimentarias ordovícicas de bajo grado del Complejo Metamórfico La Cébila, cinturón Famati-niano (centro oeste de Argentina): M1-D1 (S1, con blastesis asociada de mica blanca y clorita: WM1-Chl1) y M2-D2 (S2, con desarrollo de WM2-Chl2). La caracterización termobarométrica del evento principal M1 fue llevada a cabo a través del análisis de minerales de arcilla y de la medición de parámetros cristaloquímicos. Las condiciones de epizona con temperaturas entre 300 y 400°C, y baja presión fueron estimadas para el evento M1 sobre la base de valores de índice de Kübler de 0,23 a 0,17 A°20, parámetro b de la mica blanca entre 9,004 y 9,022 Á (valor medio de 9,014 Á, n=16) y contenidos de Si entre 3,13-3,29 a.p.f.u. Se estimaron temperaturas de ~180-270°C para el evento M2, con valores de índice de Kübler entre 0,31 y 0,46 A°20. El evento M1-D1 registrado en La Cébila podría ser vinculado al evento tectono-metamórfico ordovícico con calentamiento bajo un régimen de alta deformación ocurrido en otros complejos de la región del antepaís famatiniano de las Sierras Pampenas.

Palabras clave: Sierras Pampeanas, Baja presión, Índice de Kübler, Parámetro b de la mica blanca.


 

1. Introduction

The Early Cambrian to Late Ordovician record of South American Central Andes comprises widespread igneous, metamorphic and sedimentary complexes forming the southwestern margin of Gondwana in the Early Paleozoic (e.g., Pankhurst and Rapela, 1998; Chew et al., 2007; Ramos, 2008). In the central and northwestern regions of Argentina (between 22°S and 33°S), the Ordovician record is represented by the Famatinian orogenic belt (Ace-ñolaza and Toselli, 1976), with the main outcrops situated within the Puna, Cordillera Oriental and the eastern Sierras Pampeanas (Fig. 1a). Although different geotectonic contexts have been proposed for the Famatinian orogeny evolution (e.g., subduction processes followed by collision of para-autochthonous or exotic terranes; Thomas and Astini, 1996; Rapela et al., 1998; Aceñolaza et al., 2002; Ramos, 2008 and references therein; crustal shortening and extension in a mobile belt setting; Lucassen et al., 2000), most of the authors agree with the development of a complex back-arc basin with a high-thermal regime (e.g., Rapela et al., 1998; Astini and Dávila, 2004; Büttner, 2009; Coira et al., 2009). The subduction of oceanic lithosphere along the continental margin allowed the development of a widespread continental magmatic arc (Famatinian magmatism, ca. 460-480 Ma; Rapela et al., 2001; Viramonte et al., 2007 and references therein; Dahlquist et al., 2008), and of low-to medium-pressure metamorphic complexes from sedimentary protoliths and pre-Ordovician metamorphic and igneous basement (Büttner et al., 2005; Murra and Baldo, 2006; Steenken et al.,, 2006; Collo et al., 2008; Otamendi et al., 2008). The evolution of the arc overlapped in time with the deposition of marine and volcaniclastic successions within the associated foreland region (Late Cambrian to Middle Ordovician; Bahlburg, 1991; Astini, 2003 and references therein; Verdecchia et al., 2007).

Cébila Metamorphic Complex (previously named La Cébila Formation, González Bonorino, 1951), located within the Famatinian belt is composed of a variety of metamorphic and igneous rocks (Espi-zúa and Caminos, 1979; Fig. 1b), including low- to high-grade metasedimentary rocks metamorphosed under low-pressure conditions (Verdecchia, 2009). An Early Ordovician depositional age has been recently constrained for this complex (Verdecchia et al., 2007), which makes its previously assumed association with the Neoproterozoic to Early Cambrian Pampean Orogeny (e.g., Zimmermann, 2005) unlikely.

Its location, immediately east of the Famatinian volcanic arc, its well-constrained depositional age and the relative lack of data related to the tectono-thermal evolution at this latitude makes the La Cébila Metamorphic Complex an interesting area for the understanding of the Famatinian foreland evolution. In this work, we present X-ray diffraction analyses of clay minerals and crystallo-chemical parameters such as the Kübler index and white mica b parameter carried out in the very low- to low-grade rocks within the La Cébila. The tectono-metamorphic events with their associated thermobarometric conditions, as well as some retrograde diagenetic processes, have been established in order to understand the post-depositional history of this complex.

2. Geological setting

The Early to Middle Ordovician metasedimen-tary rocks of La Cébila Metamorphic Complex are located in the eastern-central part of the Famatinian belt, outcropping discontinuously along the eastern edge of the sierra de Velasco (Fig. 1b). This range is mainly composed of Ordovician peraluminous to metaluminous granitic rocks (e.g., ortogneiss Antinaco complex and Mazán granite; Pankhurst et al., 2000; Toselli et al., 2007) and locally intruded by undeformed Carboniferous granitic plutons such as the San Blas, Huaco, and Sanagasta granites (Dahl-quist et al., 2006; Grosse et al., 2008).

The outcrops of La Cébila Metamorphic Complex are located in: 1. quebrada de La Cébila, 2. quebrada de Cantadero, 3. quebrada de La Rioja, and 4. La puerta de Arauco (Fig. 1b). The low-grade metamorphic successions are found in areas 1 and 2. This complex is composed of phyllites, metapsammites, quartzites, micaceous and quartz-micaceous schists, gneisses and migmatites (e.g., Espizúa and Caminos, 1979; Verdecchia, 2009). The metasedimentary rocks are intruded by Ordovician granitic plutons (e.g., Antinaco complex, 476.4±1.5 and 461±2.2 Ma U-Pb ID-TIMS monazite intrusion ages in quebrada de La Rioja area; De los Hoyos et al., 2008; see Fig. 1b) and discordant pegmatitic dikes (Verdecchia, 2009), are partially covered by the Carboniferous continental successions of the Trampeadero Formation (Gutiérrez and Barreda, 2006), and by Cenozoic sediments.


The La Cébila Metamorphic Complex was traditionally correlated with the older Puncoviscana Formation and equivalents (e.g., Zimmermann, 2005; Fig. 1 a). However, the Early Ordovician primary age (Floian: 472-479 Ma; ICS Stratigraphic Chart, Ogg, 2009) of this complex has been recently established biostratigraphically in quartzites of quebrada La Cébila area (Fig. 1b; Verdecchia et al., 2007). U-Pb isotope data obtained from detrital zircon from the same li-thologic unit, show Mesoproterozoic, Neoproterozoic and Cambrian ages but no Ordovician populations (cf. Rapela et al., 2007), with a minimum peak age of ~530 Ma. Based on these data, the La Cébila Metamorphic Complex was interpreted as part of a shallow-water marine succession within a siliciclas-tic platform in the Ordovician foreland region, and coeval with the deposition of volcano-sedimentary successions of the Suri Formation associated with the volcanic arc to the west (e.g., Astini et al., 2004; Verdecchia et al., 2007). However, the absence of Ordovician zircon ages in this complex indicates sedimentation without influence of Famatinian volcanic sources (Verdecchia and Baldo, 2010). Furthermore, pre-Ordovician age populations together with geochemical analysis, compatible with acidic arc sources, obtained from this complex have been related with provenance from recycling of older units from the eastern Pampean belt and the Río de La Plata craton (Verdecchia and Baldo, 2010).

The metamorphic grade in the La Cébila Me-tamorphic Complex increases from very low- and low-grade in the east, to medium to high-grade toward the west in quebrada de La Cébila and quebrada de Cantadero areas (Verdecchia, 2009; Fig. 2). The very low- and low-grade metasedimentary rocks of La Cébila Metamorphic Complex comprise a succession of kilometer-thick phyllites and metap-sammitic layers, with some subordinate quartzitic layers (Figs. 2, 3a, b), with a general NNE-SSW strike and NW high dip. In the quebrada de La Cébila area, the low-grade units (~2.5 km wide; Fig. 2) are in tectonic contact along a ductile shear belt with medium-grade andalusite-cordierite schists. To the SE, the low-grade succession is faulted and tectonically overlaid by the high-grade migmatites and granites of sierra de Ambato complex. In the quebrada de Cantadero area, 45 km southeast from quebrada de La Cébila area, the low-grade succession (~7 km width; Fig. 2) grades into micaceous and quartz-micaceous schists with a biotite-cordierite parageneses.


3. Sampling and clay mineral analysis

Clay-size fraction (<2 ^m) samples for X-ray diffraction analysis were prepared following the recommendations of Moore and Reynolds (1997). The <2 nm fraction, assumed to be representative of the neoformed and transformed phases and conventionally used for main crystallographic index measurements, was separated from 14 samples of metapelites and 3 samples of fine-grained metapsam-mites. Clay-mineral composition was established by the comparison of orientated aggregates that were air-dried (AD), ethylene-glycol solvated (EG; 24 hours), and heated at 500°C (H500; 4 hours). X-ray analyses were determined with Philips PW1050 (INGEIS) and X-Pert Pro (Departamento de Físico Química-UNC) diffractometers, employing Cu radiation from 4 to 30°20 with a step size of 0.03°20 and a count time of 0.5 s per step. Clay-mineral phases were semi-quantified using MIF factors and the recommendations of Moore and Reynolds (1997).

The Kübler index (KI; Kübler, 1968; Kisch, 1991) was measured in the white mica (001) reflection in both AD and EG oriented clay mineral aggregates and KIcis values (CIS: Crystallinity Index Standard, Warr and Rice, 1994) were established from the regression equations for the diffractometers employed. The b parameter of white mica (Sassi, 1972; Sassi and Scolari, 1974; Guidotti and Sassi, 1986) was measured in rock slices oriented perpendicular to the main foliation (S1; see below); the (211) reflection of quartz, positioned at ~1.541Á, was used as internal standard. The chemical composition of white micas was established through SEM-EDX (Energy Dispersive X-ray spectrometry) analysis on carbon-coated polished thin sections of the studied samples. We used a Zeiss DSM950 SEM and a Variable Pressure SEM at the Centro de Instrumentación Científica (Universidad de Granada). The structural formulae of dioctahedral micas were calculated considering 22 negative charges and 0.45 Fe37(Fe2++Fe3+) ratio (cf. Guidotti et al., 1994).

4. Low-grade units in La Cébila Metamorphic Complex

4.1. Macrostructural characterization

Original sedimentary bedding (S0) is preserved in the low-grade successions and represented by a decimeter to meter thick alternating phyllite and metapsammite layers (Figs. 3a, b). These metasedi-mentary rocks also show two penetrative secondary foliations (S1 and S2, Figs. 3c, d, e). The S1 foliation, predominantly subparallel to S0 and most prominently developed, is oriented parallel to the axial surface of F1 folds associated with a ductile deformation episode (D1). The F1 folds are characterized by tight to close inter-limb angle, some meters in wavelength, and they are well developed in the metapelitic levels. This main foliation is overprinted by a weak second folding phase (F2) with an associated crenulation foliation (S2), subparallel to S1.

4.2. Petrography

The phyllites are characterized by >50% phyllo-silicate content, well-developed cleavage, and green, grey and black colour. Their mineral assemblage is composed of fine-grained white mica, quartz, chlorite, plagioclase, K-feldspar, and calcite. Rocks with <50% phyllosilicates were defined as metapsammites and rocks with >80% quartz were classified as quartzites. Metapsammites and quartzites have coarser grain size (>0.05 mm), less-defined cleavage and an overall grey colour. Their mineral assemblage is composed of quartz, white mica, chlorite, plagioclase and scarce calcite. The low-grade rocks have detrital accessory phases such as ilmenite, zircon, apatite, tourmaline and opaque minerals. No biotite neoblasts were identified in the studied sequence.

Fine-grained white mica (WM) was characterized as the most abundant phyllosilicate phase through X-ray diffraction (see section below: Clay-size fraction mineral assemblages) and microscopic observations. Two textural varieties of white mica and chlorite were recognized: WM1+Chl1 and WM2+Chl2 (mineral abbreviations follow Kretz, 1983, except WM for white mica), associated to S1 and S2 respectively. Physical characteristics are similar for both varieties, with grains <0.03-0.1 mm long and weak-to-moderate undulose extinction. Scarce larger (<0.4 mm) and irregular shaped muscovite was also identified, with moderate to intense undulose extinction, and is interpreted as detrital. Quartz is anhedral, with grains <0.1 mm long in phyllites and <0.7 mm long in metapsammites and quartzites. All quartz grains show undulose extinction and irregular grain boundaries related to grain boundary migration (cf. Passchier and Trouw, 2005). Plagioclase (<0.15 mm long) displays anhedral grain shapes and polysynthetic twinning; it frequently exhibits weak alteration to fine-grained white mica and kaolinite. K-feldspar appears as scarce thick clasts (>0.2 mm) surrounded by the fine-grained matrix.

4.3. Microstructural analysis

The primary layering (S0) (<1-5 mm thick in phyllites and <3 -20 mm in metapsammites) is defined by grain size variations, variable thickness of layers and compositional variations, specifically variable proportions of quartz, feldspar, and phyllosilicates (Figs. 3f, g). In coarse-grained metapsammitic layers the quartz grains show undulose extinction, subgrains and concave-convex boundaries; they could either be associated with inherited features from the protolith, pressure-solution during pre-metamorphic diagenesis, or with the syn-metamorphic development of foliation (Fig. 4a).

Secondary foliations (S1 and S2) are well developed in phyllites and weakly developed in metapsammites and quartzites. In phyllites, S1 is defined by oriented WM1 and Chl1 in phyllosilicate layers (<1-5 mm thick) and preferred orientation of quartz and feldspar grain-shapes in quartz-feldspatic layers (1 -7 mm thick) (Figs. 3f, g). In metapsammites and quartzites S1 is defined as a continuous foliation with weak to moderate grain-shape orientation of quartz and feldspars with a minor percentage of oriented phyllosilicate minerals (Figs. 4a, b, c).


S2 is less well-developed than S1 and appears as a discrete axial plane foliation due to symmetric to asymmetric crenulation of S1 (Figs 3f, g, i, j). S2 is defined by oriented blastesis WM2 and Chl2 in thin cleavage domains (<0.5 mm thick) alternating with microlithons. However, in the quebrada de La Cébila area S2 is defined by preferential concentration of opaque phases in very thin layers (<0.1 mm), consistent with dissolution creep processes, and less intense development of blastic white mica (Figs. 3e, g, j). In the quebrada de Cantadero area S2 constitutes a well-developed discontinuous foliation in the eastern sector and is frequently highlighted by iron oxide concentrations (Figs. 3d, f, h, i).

Another feature observed in the metasedimentary rocks is the presence of veinlets (<1 mm thick) which were classified into five types according to their mineral composition: A. quartz veinlet; B. quartz-calcite vein-let; C. calcite veinlet; D. chlorite veinlet (radial habit), and E. veinlets rich in opaque minerals. These veinlets are generally discordant to S0/S1 and affected by the D2 event associated with S2 development, as is shown by deformed chlorite veinlets (Fig. 3f) and quartz and quartz-calcite veinlets (with deformed twins in calcite, Figs. 4d, e, f). However, veinlets rich in opaque minerals are discordant to concordant to S2, suggesting that they could be contemporaneous with or younger than D2. These opaque veins were observed in macroscopic and microscopic scale (see iron oxide veins in Fig. 3d). Deformed grains of calcite in quartz-calcite veinlets from quebrada de La Cébila show widened twins, bent twins, lens-like twin shape and irregular twin boundaries (Figs. 4e, f). These twin geometries are transitional between types II and III according to Burkhard (1993) and Ferril et al. (2004).


4.4. Clay-size fraction mineral assemblages

The mineralogical composition of the <2 um fraction was determined by X-ray diffraction, with white mica (38-83%, see Table 1) and chlorite (11-44%) as the main phyllosilicate phases, plus quartz, albite (Ab)±K-feldspar±goethite (Goe) as subordinate phases (Table 2). White mica was identified in all samples (n=17) by the presence of the 10.1 Á (001), 5.0 Á (002) and 3.38 Á (003) reflections in AD diagrams, that show no modifications in the EG and H500 diagrams (Figs. 5a, b). Chlorite was identified in most of the samples by the 14.2 Á (001), 7.1 Á (002), 4.74 Á (003) and 3.55 Á (004) reflections in the AD diagrams (Figs. 5a, b), that do not show significant modifications in the EG and H500 diagrams. The 4.27 and 3.34 Á reflections were assigned to quartz, and the 3.24 and 3.19 Á reflections were assigned to K-feldspar and albite, respectively.



The appearance of a reflection at ~17 Á in the EG diagrams from three of the samples, together with a lower intensity of the 14.2 Á reflection, indicate the presence of minor amounts of expandable phases such as smectite (Sm, 12-24%, Figs. 5d, e).

Weak reflections at ~12.4 and ~8.8 Á were identified in H500 diagram from two samples. This suggests the presence of high-charge corrensite (interstratified chlorite/vermiculite; Co, Fig. 5a).


Four samples from the quebrada de Cantadero area show strong reflections at ~7.1 and ~3.57 Á that disappear or lose intensity in the H500 diagrams, which suggests the presence of kaolinite (Kln, 18-58%). In most cases this phase is associated with chlorite, although in some samples it appears as the only phase, with a spacing of ~ 7 Á (Fig. 5c).

In most of the samples a reflection was identified at ~4.2 Á in AD diagrams, which disappears in the H500 diagrams, suggesting the presence of goethite (Fig. 5b). The occurrence of this mineral species is consistent with macroscopic and microscopic observations of significant quantities of Fe-oxides coatings.

4.5. Kübler Index

Kübler indices (KICIS values) were determined in metapelites and some fine-grained metapsammites. KI values were measured both in air-dried and ethylene glycol-solvated aggregates and similar KIcis values were obtained (Fig. 6a).


Two different IKcis data sets could be distinguished in the quebrada de Cantadero area, broadly corresponding to the eastern and western areas. Those samples showing D1 (S1) but not D2 (S2) structures exhibit KIcis values ranging between 0.23 and 0.17 A°29 (mean value of 0.20 and standard deviation of 0.03; 95%, n=5), indicating epizone conditions (Table 2; Fig. 6a). Samples recording D2 structures (S2), mainly from the eastern area, show values ranging between 0.43 and 0.26 A°29 (mean value of 0.36 and standard deviation of 0.06; 95%, n=7), characteristic of the late diagene-sis-low anchizone transition (Table 2; Fig. 6a). In the quebrada de La Cébila area, where all samples recorded D2 structures (S2), similar values KIcis ranging between 0.46 and 0.32 A°26 (mean value of 0.39 A°26 and standard deviation of 0.06; 95%, n=5) were recorded (Table 2; Fig. 6a).


4.6. White mica b parameter

The white mica b parameter was measured in most of the samples, and values from 9.004 to 9.022 Á with a mean of 9.014 Á (0.005 Á standard deviation, confidence level of 95% and n=16, Table 2) were obtained. According to the classification of Guidotti and Sassi (1986), these values are within the range established for intermediate pressure conditions (9.0009.040 Á); the minimum value of 9.004 Á would be close to the boundary between intermediate and low pressures. The cumulative frequency curve of the La Cébila Metamorphic Complex (Fig. 6b) is similar to those obtained for sequences with post-depositional histories developed under low-pressure conditions (see Ryoke Belt in Sassi and Scolari, 1974).

5. Discussion: Tectono-thermal history and ther-mobarometric conditions for the low-grade metamorphism of the La Cébila Metamorphic Complex

Petrographical and mineralogical analyses carried out on the low-grade units of La Cébila Metamorphic Complex show at least two white mica growing episodes interpreted as the result of two tectono-thermal events: M1-D1 (F1-S1) and M2-D2 (F2-S2). WM1 and Chl1 are associated with the main M1 event while WM2 and Chl2 are related to the less developed M2 event.

The age of intrusion of granitic bodies in the medium- to high-grade metasedimentary sequence (e.g., granites from the Antinaco complex; De los Hoyos et al., 2008), together with the Floian sedimentation age (Verdecchia et al., 2007) suggest a Middle Ordovician maximum age for the M1-D1 event. The discordance between undeformed Ordovician granites and pegmatitic dykes, and the secondary foliation (S1) in quebrada de La Cébila (Verdecchia, 2009), allow interpreting the magmatism as post-cinematic to D1, and the thermal imput related with M1 would not be linked with the intrusion of these igneous bodies.

Epizonal conditions established for some samples in the quebrada de Cantadero area are consistent with petrographical observations pointing to a strong development of a spaced cleavage (S1), with blastesis of white mica grains larger than ~0.1 mm (Figs. 3h, j). This type of foliation is generally associated with conditions above the high anchizone (cf. Merriman and Peacor, 1999) and is typical of the epizone. Although the S1 foliation is well developed in all analyzed samples, petrographically inconsistent late-diagenesis to low-anchizone values were obtained within the samples from the quebrada de La Cébila and quebrada de Cantadero areas which also show S2 foliation (Table 2). Compositional variations established through petrography and XRD analysis (e.g., Kfs content) are insufficient to explain the different development of phyllosilicate phases during a single metamorphic event (M1), as relationships between the compositional variations and KIcis values were not observed. Consequently, variations of KIcis values within the two data sets would not be attributable to compositional variations. Moreover, although the expandable phases identified in the analyzed low-grade rocks could be associated with retrograde diagenesis (cf. Nieto et al., 2005), typical sequences affected by this process do not record alterations in the measured KIcis values (Personal communication, F. Nieto, 2010) and consequently we cannot interpret the inconsistency as a product of retrograde diagenesis. Therefore, high KIcis values are probably related to (a) the existence of a less-developed white mica population (probably associated with the petrographically identified WM2 linked to M2 event) or (b) composite (001) reflections as consequence of the coexistence of WM1 and WM2 (associated with the main M1 and M2 events, respectively). In either case, values obtained from those samples that were strongly affected by both events (e.g., presence of both D1 and D2 structures) are inadequate to estimate the thermal conditions linked to the M1-D1 event. Based on these interpretations, thermal conditions for the M1-D1 event were estimated from the samples that were not significantly affected by the M2-D2 event (see Table 2). The KIcis values of these samples range between 0.17 and 0.23 A°29, belonging to the epizone field, suggesting temperatures higher than 300°C (cf. Merriman and Frey, 1999) but, as is pointed by the absence of biotite neoblasts, lower than 400°C (Spear and Cheney, 1989).

As the WM1 is the dominant phase, we considered the measured b parameter values as representative of the conditions reached by the sequence during the M1 event. Moreover, no difference was found between values obtained in samples that record D2 structures (S2) and those only showing D1 structures (S1), as well as between samples with and without Kfs (Table 2). Values established for the low-grade rocks of La Cébila Metamorphic Complex (9.0049.022 Á) correspond to the intermediate pressure facies series (9.000-9.040 Á; cf. Guidotti and Sassi, 1986), although it is clear that most of the values are below the average established for this field (9.020 Á). Comparison of the results obtained in this work with the cumulative frequency curves published by Sassi and Scolari (1974) for typical low-, medium- and high- pressure metamorphic environments, suggests that the low-grade rocks within La Cébila Metamorphic Complex were metamorphosed at least in the low-to intermediate-pressure transition. EDX analyses of WM1 (Table 3) show Si contents between 3.13 to 3.29 a.p.f.u. (atom per formula unit) that suggest minimum pressure conditions between <100 and 300 IMPa (Massone and Szpurka, 1997) assuming an average temperature of 350°C. The estimated conditions are also supported by the similarity between La Cébila Metamorphic Complex and Ryoke Belt (Japan) white mica b parameters (Fig. 6b), the latter having been interpreted as a classical low-pressure regional metamorphic complex (cf. Brown, 1998). Furthermore, pressure and temperature conditions estimated in this work for the M1-D1 event in low-grade rocks from La Cébila Metamorphic Complex are consistent with the conditions established in higher-grade sequences located immediately to the west. The metamorphic zonation in medium- to high-grade metamorphic rocks from this complex showed andalusite-cordierite, sillimanite-K-feldspar, cordierite-K-feldspar parageneses, interpreted as product of regional low-pressure metamorphism (Verdecchia, 2009).

Consequently, the M1-D1 event could be interpreted as part of an Ordovician regional low-pressure meta-morphic event associated with a high thermal gradient. The thermobarometric conditions here estimated for low-grade rocks from La Cébila Metamorphic Complex are similar to those from most of the Ordovician metamorphic complexes belonging to the Argentinian Famatinian foreland area within the Sierras Pampeanas (high-temperature and low-pressure, e.g., sierra de Pa-ganzo, Los Llanos-Chepes-Ulapes, Pringles Complex in sierra de San Luis, sierra de Quilmes; see Fig. 1a; Saal et al, 1996; Pascua, 1998; Dahlquist et al., 2005; Steenken et al., 2006; Delpino et al., 2007; Büttner et al., 2005). In the metamorphic complexes of sierras de San Luis (Pringles Complex) and sierras de Chepes-Ulapes-Los Llanos, main Early Paleozoic metamorphic events were interpreted as consequence of high-strain heating in a compressive context, that would have been subsequent to the back-arc basin extension (e.g., Dahlquist and Galindo, 2004; Dahlquist et al., 2005; Steenken et al., 2006).


The M2-D2 represents a tectono-metamorphic event subsequent and subordinate to the main M1-D1. It is characterized by a weak compressional episode that develops folds and a weak secondary foliation with similar main stress direction than the D1 structures. Temperatures of ~180-270°C estimated from KIcis values ranging between 0.31 and 0.46 A°29 in samples with white mica blastesis associated to S2, could be considered as maximum temperatures linked with the M2 event. In addition, the calcite twins described in quartz-calcite veins from quebrada de La Cébila area (II-III type) suggest a deformation temperature (D2) of ~200-300°C.

Temperatures estimated for the M1 and M2 events would imply that the small amounts of smectite, co-rrensite and kaolinite identified in some samples could be related to subsequent retrograde diagenesis (Nieto et al., 2005) given the instability of some of these phases above ~180°C.

6. Conclusions

Petrographical analysis of the low-grade metamorphic successions from La Cébila Metamorphic Complex allows establishing at least two with mica growth episodes associated with the development of secondary foliations (S1 and S2) and related to two tectono-thermal events (M1-D1 and M2-D2). Clay minerals analysis, Kübler indices, white mica b parameter values and Si contents enable the estimation of temperatures between 300 and 400°C and low-pressure conditions for the M1 metamorphic event in the low-grade metasedimen-tary rocks from La Cébila Metamorphic Complex. Temperatures of ~180-270°C are estimated for the M2 event. A subsequent retrograde diagenetic episode is recorded, with formation of associated expandable clay phases. The tectono-thermal M1-D1 event recorded in this complex could be linked with the low-pressure metamorphism recorded in others complexes from Sierras Pampeanas where the metamorphic event was associated to an Ordovician high-strain heating stage.

Acknowledgements

We thank the Centro Regional de Investigaciones Científicas y Transferencia Tecnológica (CRILAR) for providing logistic support, and SPECTRAU laboratory (University of Johannesburg), Instituto de Geocronología y Geología Isotópica (INGEIS, Universidad de Buenos Aires) and Facultad de Ciencias Químicas (Universidad Nacional de Córdoba) for given us access to their facilities. Financial support for this paper was provided by Argentine public grants FONCyT PICT-1009 (Fondo para la Investigación Científica y Tecnológica) and CONICET PIP-1940 (Consejo Nacional de Investigaciones Científicas y Técnicas). We are grateful to Drs. F. Nieto, M. Do Campo, R. Pankhurst and F. Colombo for suggestions on several aspects of the manuscript. Dra. Brime and one anonymous reviewer are thanked for their constructive comments that enabled us to improve the manuscript.

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Manuscript received: May 28, 2010; revised/accepted: November 5, 2010; available online: March 23, 2011.

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