ARTÍCULOS

La Aguadita Trachyte: new exponent of triassic alkaline magmatism in Sierra de Valle Fértil, western sierras Pampeanas

 

Brígida Castro De Machuca^1,2, María Gimena López^1,2, Diego Morata³, Alicia Conte-Grand¹, Sandra Pontoriero¹

¹ Instituto de Geología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, San Juan. E-mail: bcastro@unsj-cuim.edu.ar
² CIGEOBIO-CONICET
³ Departamento de Geología y Centro de Excelencia en Geotermia de los Andes (CEGA, FONDAP-CONICYT), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile.

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ABSTRACT

La Aguadita Trachyte, in Sierra de Valle Fértil, is a new outcrop referring to the Triassic alkaline magmatism. The trachyte body was emplaced along a NNW-trending fault zone overlying the crystalline basement. The trachyte consists mainly of anorthoclase and sanidine as pheno- and microphenocrysts, with lesser amounts of arfvedsonite and ferro-eckermannite in the groundmass. It is moderately evolved in composition with 66.5-69.1 wt.% SiO2 and exhibits a limited compositional range in most of the major elements. The rare earth elements (REE) pattern is somewhat uniform and highly fractionated, being enriched in light REE over heavy REE and showing a prominent negative Eu anomaly. The trachyte is enriched in high field strength elements Nb, Zr, and Y, consistent with typical intraplate alkaline magmatism associated with continental rifting. It is proposed to be genetically linked to other alkaline volcanic rocks outcropping in the area, and generated from the same parental source (i.e., asthenospheric mantle source) through open-system fractional crystallization. ɛNd values of +2.6 are consistent with slightly crustal contamination during trachyte generation. Thus, La Aguadita Trachyte could be the result of a combination of fractional crystallization of mantle-derived basaltic magma and crustal assimilation. Radiometric age dating of 218 ± 7 Ma on the trachyte (K-Ar age, whole rock) expands the range of the alkaline volcanism known in the area to the Upper Triassic, and confirms a regional volcanic event related to uplift in an extensional tectonic environment.

Palabras clave: Trachyte; Geochemistry; Petrogenesis; Upper Triassic; Intraplate continental rifting.

RESUMEN

Traquita La Aguadita: Nuevo exponente del magmatismo alcalino triásico en la Sierra de Valle Fértil, Sierras Pampeanas Occidentales

La Traquita La Aguadita, en la Sierra de Valle Fértil, es un nuevo afloramiento del magmatismo alcalino triásico. Se dispone sobre el basamento cristalino y su emplazamiento estuvo controlado por una falla de dirección NNO. La traquita está compuesta principalmente por feno y microfenocristales de anortoclasa y sanidina, con cantidades subordinadas de arfvedsonita y ferro-eckermannita en la mesostasis. Es moderadamente evolucionada en composición con 66,5-69,1 % de SiO2 y exhibe un rango composicional restringido para la mayoría de los elementos mayoritarios. El patrón de distribución de los elementos de tierras raras (REE) es uniforme y altamente fraccionado, con enriquecimiento de las LREE respecto de las HREE, y prominente anomalía negativa de Eu. La traquita está enriquecida en elementos de alto potencial iónico como Nb, Zr e Y, congruente con magmatismo alcalino de intraplaca asociado a rifting continental. Se propone su vinculación genética con otras rocas volcánicas alcalinas aflorantes en el área, y su generación a partir de la misma fuente primaria (manto astenosférico) mediante cristalización fraccionada. Los valores de ɛNd = +2.6 son consistentes con un cierto componente de contaminación cortical. De este modo, la Traquita La Aguadita sería resultado de la cristalización fraccionada de magma basáltico derivado del manto combinado con asimilación cortical. La edad radimétrica de 218 ± 7 Ma (K-Ar en roca total) expande el alcance del magmatismo alcalino conocido en el área hasta el Triásico superior, y confirma el carácter regional de este evento volcánico relacionado con la tectónica extensional imperante.

Keywords: Traquita; Geoquímica; Petrogénesis; Triásico Superior; Rift intraplaca continental.

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INTRODUCTION

In Sierra de Valle Fértil, Western Sierras Pampeanas, a wide variety of alkaline volcanic rocks ranging from olivine alkali-basalt, basaltic trachyandesite, phonolite, trachyte to alkali rhyolite, intrude through and erupt over the crystalline basement of Lower Palaeozoic age (Mirré 1976). The alkaline volcanic suite was mostly emplaced along fractures or pre-existing lithospheric weakness zones (Uliana and Biddle 1988, Ramos and Kay 1991). Most outcrops are located within and on both margins of the NNW-trending Río del Potrero depression extending from Estancia Quiroga in the north to Los Bretes-Los Molles localities in the south (Fig. 1a).

Figure 1: a) Satellite image (from Google Earth 2013) showing the location of La Aguadita Trachyte in Sierra de Valle Fértil, Western Sierras Pampeanas; b) Schematic geologic map of the trachyte outcrop with sample locations.

Exposures are poor and occur scattered over the basement within an area of less than 500 km², showing that the igneous activity was volumetrically a subsidiary event though significant in the geotectonic evolution of the region. The geology of these volcanic rocks is not well known; the only study dealing with the volcanism of the area is due to Mirré (1976), who grouped these lithologies as part of a non-orogenic continental alkaline association of uncertain Triassic age. However, detailed petrological investigations on these volcanic rocks are still absent.
Based on petrographic and geochemical similarities with continental alkaline basalts and related rocks from the nearby triassic Ischigualasto-Villa Union basin (San Juan and La Rioja provinces), the alkaline rocks from Sierra de Valle Fértil were included in the Baldecitos Formation and referred to the Middle Triassic by Limarino et al. (2008 and references therein). Radiometric age data (K/Ar and Ar/Ar methods) from basalts interlayered and/or overlying the sedimentary sequence that infilled the Ischigualasto-Villa Union Basin, are constrained between 231 ± 4 Ma and 215 ± 5 Ma (Limarino et al. 2008, Curriea et al. 2009, Martinez et al. 2011, and references therein). These ages indicate a significant magmatic activity concentrated at the base of the Upper Triassic.
The petrology of basaltic rocks from the Baldecitos Formation and lavas of similar composition outcropping in the surroundings has been studied by Page et al. (1997). According to these authors the magmatic series extends from basalt to trachybasalt to trachyte, and was originated under high pressure and temperature conditions involving the fractionation of olivine, ulvöspinel, ilmenite, clinopyroxene, An-rich plagioclase and apatite. The alkaline volcanic rocks seem to be derived from a common source, but basaltic melts were originated separately representing different mantle-melting degrees from source. The resulting rocks are interpreted as being the product of fast magmatic ascent and subsequent intrusion or flow. Temporary storage in smaller chambers at intermediate depth could have been the mechanism for the differentiation process (Page et al. 1997).
The present study outlines the petrological, mineralogical, geochemical and isotopic characteristics of an outcrop of volcanic rock named La Aguadita Trachyte (nov. nom.), first recognized by Castro de Machuca et al. (2012), and discusses its petrogenetic implications. Data obtained will allow to reconstruct the evolution of this lithology and the tectonic setting of emplacement.

GEOLOGIC SETTING

The Valle Fértil Complex (Cuerda et al. 1984) that make up Sierra de Valle Fértil, is composed of medium-pressure upper amphibolite to granulite facies metasedimentary rocks (0.6 ± 0.1 GPa, 800 ± 40 °C), predominantly sillimanite-garnet (±cordierite) migmatitic gneisses of Middle to Late Cambrian age, and minor metacarbonates and calc-silicate rocks of unknown age (Casquet et al. 2012 and references therein). During the Famatinian orogeny (Ordovician) voluminous bodies of gabbro, diorite, tonalite, granodiorite and granite, largely concordant with foliation of the host rocks, intrude the metamorphic basement. Field relations suggest that metagabbros are the oldest intrusive rocks with metatonalites, granodiorites and granites being slightly younger (Otamendi et al. 2009 and references therein). Magmatism and metamorphism here took place within a narrow timespan at ca. 470 Ma (Casquet et al. 2012 and references therein).
Since the Lower Triassic, the already uplifted Sierra de Valle Fértil (Mirré 1976, Otamendi et al. 2009) experienced an extensive system of north-northwest striking, and continental-rift basins were developed to the north (Ischigualasto-Villa Unión basin) and to the south (Marayes-El Carrizal basin). Transtension was responsible for the main NNW to NW-trending fractures and conjugated NNE to NE-fractures -whose orientation is possibly controlled by the basement structure-, and the subsequent formation of the sedimentary basins (Page et al. 1997 and references therein). During the Mesozoic and Palaeogene the whole area has been exposed to strong erosion (Limarino et al. 2008).
The structure of Sierra de Valle Fértil is the result of several overimposed tectonic deformational events. It is made up of two main components: a complex fold and fault system that deformed the Valle Fértil Complex during Lower Palaeozoic times, and a Tertiary fracture system responsible for the present morphology (Mirré 1976).

ANALYTICAL TECHNIQUES

Electron microprobe analyses of alkali feldspar and amphibole on a selected sample of trachyte (sample LA12-2) (Tables 1-2), were carried out using a JEOL JXA-8230 SuperProbe electron microprobe equipped with two wavelength dispersive X-ray spectrometers (WDS) and an energy dispersive X-ray spectrometer (EDS), hosted at LAMARX (Laboratorio de Microscopía Electrónica y Análisis por Rayos X), University of Córdoba, Argentina. Microprobe running conditions were 15.0 kV high voltage, 20 nA beam current and 3 μm electron beam. Detection limits were approximately 0.03 % for light elements (expressed as oxides) and 0.08 % for heavy elements (expressed as oxides). Standard materials used in alkali feldspar determination were: anorthoclase (Na, Al, Si), benitoite (Ba), fayalite (Fe2O3), orthoclase (K), wollastonite (Ca) and celestine (Sr). Standard reference materials for amphibole were: topaz (F), anorthoclase (Na, Al), diopside (Mg, Si, Ca), fayalite (FeO), orthoclase (K), sphene (Ti), Bushveld chromite (Cr), sodalite (Cl) and rodonite (Mn). Ferrous and ferric Fe were calculated using the equation of Droop (1987). Data processing was made using the Excel spreadsheets GabbroSoft 2013 (http://www.gabbrosoft.org/spreadsheets.html).

Table 1: Representative electron microprobe analyses of feldspars from La Aguadita Trachyte (sample LA12-2).

Table 2: Representative electron microprobe analyses of amphiboles from La Aguadita Trachyte (sample LA12-2).*

Twenty seven representative fresh samples of mafic and felsic volcanic rocks from Sierra de Valle Fértil -amongst which four samples belong to La Aguadita Trachyte-, were selected and analysed for whole rock geochemistry. Major and minor oxides were determined on fused lithium borate glass pellets by X-ray fluorescence spectrometry (XRF), and trace elements and rare earth element (REE) contents were determined by inductively coupled plasma-mass spectrometry (ICP-MS) at ALS Minerals Laboratories, Canada. Analytical precision is 0.01 % for major element oxides Al2O3, CaO, Fe2O3, MgO and SiO2, and 0.001 % for BaO, Cr2O3, K2O, MnO, Na2O, P2O5 and SrO. For trace elements and REE precision is 5 ppm (V), 2 ppm (Mo, Zr), 1 ppm (W), 0.5 ppm (Ba, Ce, Co, La, Tl, Y), 0.2 ppm (Hf, Nb, Rb), 0.1 ppm (Ga, Nd, Sr, Ta), 0.05 ppm (Dy, Gd, Th, U), 0.03 ppm (Er, Eu, Pr, Sm, Yb) and 0.01 ppm (Cs, Ho, Lu, Tb, Tm). Whole rock and REE analyses for the trachyte samples are given in table 3.

Table 3: Whole rock chemical composition and some geochemical parameters of La Aguadita Trachyte.

The K/Ar dating on a selected sample of the La Aguadita Trachyte (sample LA12- 2) was performed in the Laboratory of Geochronology, Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile. Sample was degassed by high-frequency induction heating. The argon was cleaned and purified in Pyrex extraction lines and the radiogenic 40Ar volumes were determined using isotopic dilution techniques in an MS10S Mass Spectrometer, with a total accuracy of 1-2%. The K analyses were done in triplicate by atomic absorption techniques with an accuracy of 0.7-1.3%, depending on the K content and using a Li-internal standard. The interlaboratory standard GL-0 was used for controlling and for the calibration of Ar and K determination. Errors in K-Ar ages are quoted at the two sigma (2σ) level and the decay constants are those suggested by Steiger and Jager (1977). These include an estimated error for the standards and an error for the Ar isotope ratios. Data are reported in table 4. Rb/Sr and Sm/ Nd isotope ratios from a representative sample (sample LA7) were obtained by whole rock isotope dilution technique at the Centro de Pesquisas Geocronologicas (CPGeo), Universidade de Sao Paulo, Brazil. Data and procedures for the analysis are specified in tables 5 and 6.

Table 4: K/Ar age dating on La Aguadita Trachyte.

Table 5: Rb-Sr isotope analyses of La Aguadita Trachyte.

Table 6: Sm-Nd isotope analyses of La Aguadita Trachyte.

LA AGUADITA TRACHYTE: FIELD OCCURRENCE AND PETROGRAPHY

La Aguadita Trachyte lies on the eastern side of the Río del Potrero graben (30°46'24.26" S - 67°26'16.02" W), along the main NNW-trending Portezuelo Lineament (Furque et al. 1999) that separates the main body of Sierra de Valle Fértil from its eastern portion (Fig. 1b). Uplift and strong erosion have resulted in incomplete exposure of this lithology, supposed to be a remnant of a lava dome.
La Aguadita Trachyte consists of a 15 - 20 m thick whitish-grey microporphyritic to aphanitic lava body that overlies metatonalites, granodiorites and granites of the crystalline basement covering an area of approximately 120 m × 200 m (Figs. 2a-b). Outcrops of Los Molles - Los Bretes Basaltic Trachyandesite (López 2011) are also exposed in the surroundings; however, no contact relationships are seen between them.

Figure 2: a) View from the top of the trachyte outcrop to the west showing the main body of Sierra de Valle Fértil at the bottom; b) Massive trachyte outcrop; c) Flow-banded structure; d) Vesicles in the upper section of the trachyte body.

The trachytic rock exhibits a distinct but thinly flow structure in the field (Fig. 2c), clearly observed on weathered surfaces of the outcrop with the appearance of light grey and ochre colour bands or streaks. Moreover, small ellipsoidal and rounded vesicles up to 0.7 cm in diameter are occasionally seen in the upper part of the body (Fig. 2d).
The trachyte consists of alkali feldspar ± sodium-rich amphibole. Iron oxides (magnetite and hematite), and tiny zircon grains with pleochroic halos included in the feldspars, are the main accessories. The feldspar, ranging from sanidine to anorthoclase, is completely dominant (≈ 90 % total volume). It is present both as twinned and zoned euhedral phenocrysts and microphenocrysts (up to 1 mm in size) as in the orthophyric to slightly trachytic groundmass (0.07 - 0.6 mm in size), (Figs. 3a-b). Feldspar crystals have micro- to cryptoperthitic textures, anomalous extinction, and show polysynthetic or combined Carlsbad-polysynthetic twinning (Figs. 3c-e). Feldspars are generally fresh or slightly altered to kaolinite. Most feldspar phenocrysts are oscillatory zoned (Figs. 4a-c), revealing regular variations in the Or molecule between cores and rims as seen in figure 4b, where the feldspar crystal shows a core of Ab72 mantled by Ab45.5 and again by Ab72 in the outer zone of the crystal. Na content in feldspars varies from Ab 43 to 83 % averaging about An0.32-Ab67.33-Or32.35 (Table 1). On the ternary Ab-An-Or diagram of Deer et al. (1966), the feldspars display a compositional array from sodic sanidine to anorthoclase (Fig. 4d, Table 1). The average mineral formula is Na0.66K0.32Fe2+0.01Al0.98 Si3.01O8.

Figure 3: Photomicrographs of La Aguadita Trachyte: a) Orthophyric groundmass characterized by closely packed feldspar crystals with quadratic or short and stumpy rectangular cross sections (NX); b) Trachytic texture due to the alignment of feldspar laths (NX); c) Euhedral anorthoclase microphenocryst exhibiting zoning and tartan twinning; d) Zoning and anomalous extinction in anorthoclase microphenocryst (NX); e) Normal-zoned feldspar microphenocryst (NX); f) Zoned and strongly pleochroic alkali amphibole crystals (N//). N//: plane polarized light; NX: crossed polarized light. Mineral abbreviations after Siivola and Schmid (2007).

Figure 4: La Aguadita Trachyte, sample LA12-2: a) False-color backscattered electron (BSE) image showing the orthophyric texture and strong zoning in anorthoclase crystals; b) BSE image of alkali feldspar microphenocryst showing the position of the EDS analysed points; c) BSE image of zoned anorthoclase crystals; d) Chemical classification of feldspars in the Ab-An-Or diagram (after Deer et al. 1966). Filled squares: phenocrysts/microphenocrysts, filled circles: groundmass feldspars; e) BSE image of interstitial amphibole (dots); f) Classification diagram of amphiboles (dots) from La Aguadita Trachyte (after Leake et al. 2003).

Subsidiary deep-blue sodium-rich amphibole, zoned and strongly pleochroic in shades of deep blue, indigo to yellow-brown, is present as interstitial crystals in the groundmass. The fine-grained alkali amphibole is recorded as rare groundmasses phase in trachyte elsewhere (Nono et al. 1994). The amphibole occurs as small (< 0.09 mm) anhedral to subhedral crystals and as spongy aggregates, filling the interstices between the feldspars (Fig. 3f). It is often rimmed by iron oxides. All the analysed amphibole crystals occurring in the trachyte are of sodic amphibole in the sense of Leake et al. (1997), where NaB ≥ 1.5 and (Na+K)A ≥ 0.5. They have a maximum Mg/(Mg+Fe2+) ratio of 0.0029 indicating Fe-rich types (Table 2). The high fluorine content (≈ 2.33 wt.% in average) should be emphasized. According to the amphibole nomenclature of Leake et al. (2003), the composition fluctuates between arfvedsonite to ferro-eckermannite (Fig. 4f). Compositional variations of feldspar and amphibole are shown in figures 4d and 4f, respectively.
Even though La Aguadita Trachyte contains nearly 70 wt.% SiO2 and up to 10.7 % normative quartz, modal quartz is absent or rarely seen and confined to the groundmass.

GEOCHEMISTRY

The trachyte has SiO2 content which ranges from 66.5 % to 69.1 and exhibits a limited compositional range in most of the major elements: 15.5 - 16.1 % Al2O3, 2.56 - 2.76 % Fe2O3, 0.03 - 0.09 % MgO, < 0.01 - 0.02 % TiO2, 0.01 - 0.14 % P2O5, 0.36 - 1.22 % CaO, 7.2 - 7.35 % Na2O and 4.3 - 4.5 % K2O (Table 3). The average alkali content is of 11.7 %, with average Na2O content of 7.27 %. In contrast, the trachyte shows a wide compositional range in Zr (982 - 1580 ppm) and Sr (7 - 82.5 ppm). On the total alkalis vs. silica diagram (after Le Maitre 1989, Fig. 5a), the trachyte lies within the alkaline field. Consistently, its composition occurs on the comenditic trachyte field of the binary Nb/Y-SiO2 diagram (after Winchester and Floyd 1977, Fig. 5b). The trachyte samples have significant amount of normative quartz (6.5 to 10.7 wt.%) and also have acmite (up to 4.4 wt.%) in the norm.

Figure 5: a) Total alkali vs. SiO2 diagram for the entire analysed alkaline rocks of Sierra de Valle Fértil (after Le Maitre 1989, normalized to 100%, water free). The line after Irvine and Baragar (1971) illustrates the alkaline nature of the rocks; b) Nb/Y-SiO2 diagram (after Winchester and Floyd 1977). Oxide contents in wt.%; trace elements in ppm. Symbols: filled rhombus: San Agustín Basalt, open circle: Potrerillos Basalt, filled circle: Usno Basalt, unfilled inverted triangle: Tala - Turbante Basalt, unfilled rhombus: Los Molles - Los Bretes Basaltic Trachyandesite, unfilled square: Jaboncillo Trachyphonolite Phonolite, filled square: Monte Chumao Phonolite, filled triangle: Loma Negra Trachyte, unfilled triangle: La Aguadita Trachyte.

La Aguadita Trachyte displays remarkable Zr enrichment when compared with other incompatible trace elements (e.g. Zr/Rb = 5.73, Zr/Th = 22.72, Zr/La = 10.64 and Zr/Ce = 5.45, in average). On the primitive mantle normalized trace element plot (Fig. 6a) using normalization values of Sun and McDonough (1989), the La Aguadita Trachyte shows significant negative anomalies in Ba, Sr, P and Ti, which indicates plagioclase, apatite and Fe-Ti oxide fractionation. Low concentrations for Ba (≤ 28.8 ppm) are also attributed to alkali feldspar fractionation. On the other hand, Rb reaches levels of 193-259 ppm equal to 400 times primitive mantle levels, Zr is at up 1580 ppm (1235 ppm average) and Nb is at up 231 ppm (207 ppm average) with Zr/ Nb = 5.97. The abundance of high field strength elements (HFSE) such as Y, Yb, Lu (20-40 times primitive mantle values), is also remarkable.

Figure 6: a) Primitive mantle-normalized incompatible trace element pattern for La Aguadita Trachyte (normalization values after Sun and McDonough 1989); b) Chondrite-normalized REE pattern for La Aguadita Trachyte (normalization values after Nakamura 1974); c) In the Al2O3 vs. FeOtotal diagram (after Macdonald 1974 modified by Kozlovsky et al. 2007) the analysed trachyte samples are grouped into the comenditic field; d-e) La/10-Y/15-Nb/8 (Cabanis and Lecolle 1989) and Th-Hf/3-Ta (Wood 1980) discrimination diagrams showing that the entire alkaline rocks from Sierra de Valle Fértil -including La Aguadita Trachyte-, plot in the within-plate setting; f) Y vs. Nb tectonic discriminant diagram of Pearce et al. (1984). WPG: within-plate granites, COLG: collisional granites, VAG: volcanic arc granites, ORG: ocean-ridge granites. Oxide contents in wt.%; trace elements and REE in ppm. Symbols as in Fig. 5.

The chondrite-normalized REE pattern of the La Aguadita Trachyte (chondrite values after Nakamura 1974) has steep light rare earth elements (LREE) and almost flat heavy rare earth elements (HREE) profile (Fig. 6b). The REE pattern is enriched in LREE [(La/Sm)N = 5.08] relative to HREE [(Gd/Lu)N = 0.93] and shows a fractionated ratio between (La/Yb)N = 5.64, with negative Eu anomaly (Eu/Eu* ≈ 0.185 on average).

K-AR AGE DATING AND ISOTOPY

The radiometric age of 218 ± 7 Ma obtained by the K/Ar method on whole rock (Table 4) provides evidence that igneous activity, which is related to extensional tectonics and could have begun in the Lower Triassic (Page et al. 1997), certainly continued until at least the uppermost Triassic.
Initial (87Sr/86Sr)0 ratios were calculated for an age of 218 Ma and also for the extreme values from the error interval 211 - 225 Ma (Table 5). An anomalous low Sr0 isotopic ratio was obtained (Sr0 = 0.65993). Because alteration observed in La Aguadita Trachyte is insignificant, the most probable explanation for the anomalously low calculated initial value would be in relation to the very low Sr content (5.5 ppm). Due this very low Sr content, small variations produced by slightly alteration could produce perturbation on the Rb-Sr system during or after trachyte emplacement. A similar explanation was proposed by Dahlquist et al. (2010) in Carboniferous A-type granitoids from Sierras Pampeanas. These authors indicate that anomalous samples also show lower Sr contents, probably reflecting perturbation of the Rb-Sr systematics after crystallization by an alteration event. In our case, originally samples would have very low Sr contents, and under this condition slightly modifications of Rb-Sr contents for incipient alteration could affect the (87Sr/86Sr) ratio.
Sm-Nd isotopes are quoted in Table 6. (143Nd/144Nd)0 calculated for an age of 218 Ma yields a value of 0.512357, with an ɛNd = +2.6, ɛNdCHUR = 0.3 and a TDM of 0.65 Ga. Positive but relatively low ɛNd values (+2.6) suggest a parental mantle-derived magma that could incorporate some crustal component during its ascent throughout continental crust (Nohda and Wasserburg 1981, Davidson 1987).

DISCUSSION

The tectonic setting of La Aguadita Trachyte is considered by taking into account its geochemistry in order to understand its geological history and the nature of any genetic process during its evolution. The largest volumes of peralkaline silicic rocks are found in areas of rift formation on the continents, but they may also form in several other tectonic settings such as in areas of extensional tectonics (MacDonald 1974b). The peralkaline character of La Aguadita Trachyte is supported by the presence of arfvedsonite and ferro-eckermannite as minor phase. Furthermore, in the FeOT (total iron) vs. Al2O3 diagram (MacDonald 1974a, Fig. 6c), the studied trachyte samples are grouped together in the field of the peralkaline trachytes (comenditic trachytes) which are typical of continental rift-valley systems. The Rb, U, Th, Nb, Ta, Y, Zr, and REE contents are relatively abundant suggesting geochemical features of intraplate magmatism (White and Urbanczyk 2001). Enrichment in alkalis, as well as in LREE relative to HREE [(La/Yb)N = 5.64 chondrite normalized according to Nakamura 1974], are features of intraplate alkaline magmas worldwide. In the appropriate discrimination diagrams (Figs. 6d-e) the geochemistry of La Aguadita Trachyte is consistent with a continental rift setting. The high Nb contents are also typical of intraplate magmas (Wilson 1989). The tectonic setting is corroborated by the Nb-Y ratios, plotting the analysed trachyte in the within-plate granitoids domain defined by Pearce et al. (1984) (Fig. 6f).
As with most continental alkaline rocks, the whole volcanic rocks from Sierra de Valle Fértil are associated with extensional block faulting resulting from distant and/or local stress fields related to the Triassic rifting, and were emplaced along lines of lithospheric weakness. Rift-triggered magmatism with Middle- to Upper Triassic ages ranging from 231 to 215 Ma is common in the Ischigualasto-Villa Union basin. Moreover, extensional fault-controlled A-type granitoids have been also defined by Dahlquist et al. (2010) during Carboniferous time in the Eastern Sierras Pampeanas, indicating that this relation between alkaline acid magmatism and extensional setting is recurrent in the time and evolution of the region.
In general, and as proposed by Gharib et al. (2012 and references therein), the various sources of trachytic rocks are summarized as follows: (1) fractional crystallization of mantle-derived magma source, either during major lithospheric extension or when a mantle plume impinges the base of the lithosphere; (2) interaction of mantle-derived magmas with continental crustal materials to produce trachytic melts; and (3) mixing of independent mafic and felsic magmas.
Field, petrographic and geochemical characteristics of the La Aguadita Trachyte such as the absence of any acidic or basic microgranular xenoliths, its homogeneous petrography, and the limited major and trace element variations, contradicts the magma mixing model as the process responsible for the magmatic evolution. Thus, the two remaining possible sources are (1) and (2). Moreover, most of feldspar phenocrysts are oscillatory zoned clearly indicating the changing chemical environment the crystals were subjected to during crystallization.
Many alkaline rocks representing fractionates of dominant mantle-derived magma are characterized by low Y/Nb ratios (O’Halloran 1985) and low initial 87Sr/86Sr values (≈ 0.704; Faure and Powell 1972) and consequent high ɛNd values. La Aguadita Trachyte displays fractionated REE pattern, and shows narrow variation range of Zr/Nb (4.65 - 7.46) and low Y/Nb ratio (< 0.8) close to oceanic island basalts (OIB) derived from mantle source. This variation range is due to the fact that Zr and Nb behave as very incompatible elements in alkaline magmas (Macdonald et al. 1987) (Fig. 7a). In addition, the positive but relatively low obtained ɛNd value of +2.6 would suggest a dominant primary mantellic source but with a slightly degree of crustal participation. This geochemical signature is also consistent with the projection in the Eby (1992) discriminant diagram proposed for distinguishing between mantle derived vs crustal-derived source in A-type magmatism. Relatively low Nb contents, compared with Y and Ga, plot our samples in the A1 group (Fig. 7b), suggesting a distinct mantle-source for La Aguadita Trachyte genesis.

Figure 7: a) Y/Nb vs. Zr/Nb plot. The CRZ/OIB and CFB fields represent continental rift zone, oceanic island alkali basalts and continental flood basalts according to Wilson (1989); b) Y-Nb-Ga diagram (Eby 1992). A1 represents differentiates of magmas derived from mantle sources like those of oceanic-island basalts but emplaced in continental rifts or during intra-plate magmatism, A2 represents magmas derived from continental crust or under-plated continental crust that had been through a cycle of continent-continent collision or island-arc magmatism; c) Nb/U vs. Ce/U variation diagram. Oceanic island basalt (OIB), mid-oceanic ridge basalt (MORB), and average crust data are from Hofmann et al. (1986) in Gharib et al. (2012). Trace elements in ppm. Unfilled triangle: La Aguadita Trachyte.

The deep and nature of the mantle source are the dominant factors controlling the mantle-derived melts. According to Smith et al. (1999), HFS elements such as Nb are depleted in the lithospheric mantle relative to the LREE (such as La) with Nb/La ratios < 0.5. The high Nb/La ratio (≈ 1.85) and low La/Yb ratio (≈ 8.43) of La Aguadita Trachyte would suggest a depth mantle source.
The evolved geochemical signature of La Aguadita Trachyte, reflected by the negative Ba, Sr, and Eu anomalies, the low contents of CaO, Fe2O3T, MgO, TiO2 and P2O5 and the decrease in LREE from La to Sm, indicate that the trachyte has experienced a significant fractionation process. The Ba, Sr, Ti and P anomalies can be attributed to the fractionation of feldspars (Ba and Sr), titanomagnetite (Fe, Ti), and apatite (P). The negative Eu anomaly, together the very low-Sr content, reflect that Ca-rich plagioclases were major fractionation phases during the magmatic evolution. Dahlquist et al. (2010) find similar REE patterns for the Early Carboniferous A-type granites of Sierras Pampeanas. These authors conclude that progressive fractional crystallization with dominant fractionation of feldspar could explain the distinctive negative Eu anomalies observed in these A-type granites. Also, the gradual increase in Nb and Y contents with the increase of Zr content supports the fractional crystallization model of La Aguadita Trachyte. The high fluorine content (about 2.33 wt.%) in the alkali amphibole could reflects late-stage crystallization of volatile-rich alkaline liquids, or even an originally F enrichment in the parental magma as suggested by Dahlquist et al. (2010, 2014) for A-type granitoids.
The assimilation of crust by mantle derived melts is often invoked to explain the genesis of silica-oversaturated trachytic magmas (Panter et al. 1997 and references therein). Trace element Nb/U and Ce/U ratios in mafic and intermediate magmas are useful in distinguishing between mantle and crustal components and could help in the identification of magmatic sources (McDonough 1990). The studied trachyte has high Nb/U and Ce/U average ratios of 28.7 and 31.7, respectively. In the Nb/U vs. Ce/U diagram (Fig. 7c) the samples fall outside but close to the OIB field, slightly shifted toward the average continental crust. The Th/Nb ratio is also an index of the contribution of the continental crust material during the magmatic evolution. The Th/Nb ratios of La Aguadita Trachyte ranging from 0.22 to 0.35 are very high compared with the corresponding mantle value (≈ 0.07, Pin et al. 1992), suggesting possible crustal contamination. This crustal contamination is consistent with positive K, Rb and REE (La, Ce, Nd) anomalies, which in concordance with the positive but relatively low ɛNd value (+2.6) could indicate that the trachyte was derived from a mantle source with some degree of crustal contamination.

CONCLUSIONS

La Aguadita Trachyte is a new exponent of the Triassic volcanic activity in Sierra de  Valle Fértil, Western Sierras Pampeanas. The discovery of this trachyte expands the number of outcrops attributed to the Triassic alkaline magmatism known in the area, and confirms the regional extent of the volcanic event.
The geochemical signature showed by La Aguadita Trachyte is typical of intraplate magmatism originated in an extensional tectonic regime. Moreover, the fractionated REE pattern of the trachyte is representative of a melt derived from a dominant mantle source (ɛNd = +2.6). The negative Ba, Sr, TiO2, P2O5 and Eu anomalies, and the gradual decrease in LREE, confirm the important role of fractional crystallization during the magmatic evolution of this rock. It has a narrow variation range of Zr/Nb, low Y/Nb, and high Nb/La ratios resembling OIB signature, which evolve from asthenospheric mantle source. Such mantle source would have been subjected to fractional crystallization processes to yield La Aguadita Trachyte. On the other hand, the Th/ Nb ratio (0.22 - 0.35), low but positive ɛNd values, coupled with trace element data, would support the participation of a crustal component during ascent and crustal emplacement.
Alkaline volcanic rocks from Sierra de Valle Fértil are closely associated in space and time. They belong to the olivine alkali- basalt suite which represents an important crystal fractionation series. Consequently, La Aguadita Trachyte would be derived from a mantellic source that evolved by feldspar fractionation coupled with slightly crustal component.
Moreover, the obtained age of (218 ± 7 Ma), suggests that igneous activity related to extensional tectonics could have begun in the Lower Triassic and continued until at least the Upper Triassic. The radiometric age and the petrographic, geochemical and tectonic features of this lithology, are consistent with alkaline magmatism generated in an intraplate continental environment during the process of rifting that affected the Western Sierras Pampeanas region.

ACKNOWLEDGEMENTS

This work was funded by CONICET-Argentina (Projects PIP 09-0878 and 112-00294) and CICITCA-UNSJ (Project E935). We are particularly grateful to J. Dahlquist for his helpful comments which significantly improved the manuscript. Review by P. Sruoga as well as the editorial work of V. Litvak is also appreciated. Professor L. Ureta (UNSJ) helped us with the English translation.

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Recibido: 14 de octubre, 2014
Aceptado: 09 de septiembre, 2015