The Antilla magmatic complex (26°10′S, 64°40′W, NW Argentina) attests to magma eruption at ca. 4.7 Ma in the Central Andes backarc region, 300 km E of the active arc. The Antilla lavas have an alkaline, predominantly mafic composition and record the most primitive isotopic ratios (87Sr/86Sr = 0.704360 and 143Nd/144Nd = 0.512764) of Central Andes Neogene backarc volcanism between 24°S and 27°S. They show trace-element patterns recalling backarc Pliocene-Quaternary intraplate mafic rocks, but they show lower silica and higher alkali contents, and are interpreted to derive from the depleted subcontinental mantle.
A revision of the structural and volcanological characteristics of the Central Andes between 24°S and 27°S shows that this region was, during the Miocene-Pliocene, the site of lithospheric processes that account for partial melting in the mantle wedge, in the subcontinental mantle, and in the continental crust. The existing geophysical and petrological data agree with a model in which magma production was related to a process of lithospheric delamination. The Antilla rocks are the easternmost volcanic products with intraplate characteristics, located beside a large zone of partial melting of the continental crust, at the intersection of the NW-trending Archibarca and NE-trending Tucumán transversal lineaments. Their age of 4.7 Ma corresponds to the acme of mafic monogenetic and silicic ignimbrite volcanism in the backarc at the same latitude. This provides new constraints for the spatial and temporal reconstruction of deformation events in the crust and of the lithospheric delamination process and its bearing on magmatic activity.
In the Central Andes, long-lived Jurassic to present subduction has led to a complex magmatic and tectonic evolution and to substantial modifications of the lithospheric mantle and crust. The Miocene to present magmatic arc is linked to the subduction of the Nazca plate under the South America continent, and it forms a continuous N-S–striking belt of calc-alkaline volcanoes and calderas representing the Western Cordillera (Fig. 1). Behind the arc, between 24°S and 27°S, magmatic belts have developed eastward for ∼300 km from the active arc across the 4000-m-high Puna Plateau. These magmatic belts are connected to NW-SE–trending strike-slip fault systems (Lipez, Calama–Olacapato–El Toro, Archibarca, Culampaja, and Ojos del Salado fault systems; Fig. 1) (Coira et al., 1993; Riller et al., 2001; Matteini et al., 2002; Chernicoff et al., 2002; Acocella et al., 2007). Volcanic centers in backarc position (i.e., Farallón Negro [Sasso and Clark, 1998]; Portezuelo de Las Animas [González et al., 2005]; Antilla [Viramonte et al., 1994a]) are also present along the NE-trending Tucumán transfer zone (Fig. 1; de Urreiztieta et al., 1996). The whole history of volcanic activity in the backarc region is an important indicator of the time-space evolution of the Andean orogens, and, therefore, it plays a fundamental role in the reconstruction of the thermal and compositional state of the upper mantle and lithosphere and of the tectonic history of the upper crust.
The backarc magmatic products between 24°S and 27°S show different geochemical characteristics, in terms of both isotopic ratios and trace-element composition, indicating different sources involved in magma genesis and complex processes responsible for their origin (e.g., Kay et al., 1994; Matteini et al., 2002; Petrinovic et al., 2005; Mazzuoli et al., 2008). Moving from the active arc (Western Cordillera) to the Eastern Cordillera (Fig. 1), the geochemical magma signature changes from typical calc-alkaline to crustal to intraplate (Kay et al., 1994; Mazzuoli et al., 2008). Recently, different isotopic compositions determined in backarc lithospheric magmas have suggested the existence of different chemical domains in the subcontinental lithospheric mantle (Mazzuoli et al., 2008; Drew et al., 2009). Most reconstructions based on geophysical and petrological data refer to a piecemeal lithosphere delamination process leading to thinning of lithospheric mantle and consequent ascent and melting of asthenosphere, which in turn induces partial melting of lower and upper crust and lithosphere mantle itself (Kay et al., 1994; Mazzuoli et al., 2008; Drew et al., 2009). Although models invoking lithospheric loss can reconcile the genesis of magmas from different sources, the crustal deformation, and the uplift of the Puna Plateau, our understanding of the geodynamic forces that drive these regional-scale events remains incomplete. Some of the major topics to address in order to understand the general and specific processes of magma genesis and evolution in the Central Andes backarc magmatism are: (1) the nature and structure of the subcontinental lithospheric mantle and its role in the generation of magmas, particularly in the easternmost sector of the backarc, and (2) the role of deformation of the crust in magma transport, shallow storage, and ascent to the surface. The present research was conducted in an attempt to provide a solution to these questions by studying backarc products very far from the trench.
Our study is focused on a small magmatic complex, dispersed over a 150 km2 area near the village of Antilla (26°10′S, 64°40′W, Salta Province, NW Argentina), ∼650 km from the Pacific trench (Figs. 1 and 2). The Pliocene Antilla magmatic complex, mainly formed of mafic rocks (Viramonte et al., 1994a), is located in a key area at the boundary of the morphotectonic units of the Eastern Cordillera, Sierras Pampeanas, and Santa Barbara System and at the intersection of the NW-trending Archibarca fault zone with the NE-trending Tucumán lineament (Figs. 1 and 2). This work presents a new geologic field survey, K/Ar chronology, and geochemical, petrological, and isotopic data on the Antilla rocks with the purpose of discussing the petrogenesis and tectonic setting of this volcanism in a regional geologic context, by comparing the Antilla rocks with those of Miocene-Pliocene age set in the southern Puna Plateau. Although these new data focus on a small volcanic center far from the active arc, they are of great importance for characterizing the lithospheric mantle in the backarc, and they contribute to define the limits, in space and time, of lithospheric delamination, one of the most important geological events controlling the evolution of Central Andes in the past 15 Ma.
CENTRAL ANDES AT 26°S
Regional Geologic Setting
The Central Andes at ∼26°S represent the southern part of the transition zone developing between the 30°-dipping (N of 24°S) and the subhorizontal (S of 27°30′S) sections of the Nazca plate slab (Cahill and Isacks, 1992). In recent years, the application of high-precision geophysical techniques (see Prezzi et al., 2009, and references therein) has significantly influenced the new tectonic models for this sector. At present, the weight of evidence clearly supports the architecture of the transition zone as oblique (SW-NE), approximately coincident with the direction of the Tucumán transfer zone. This configuration of the subducting Nazca plate is interpreted to be responsible for the major structural differences in the style of foreland deformation, in conjunction with inherited paleogeographic (Grier et al., 1991; Marquillas et al., 2005) and structural (Allmendinger et al., 1983) pre-Andean features.
The Eastern Cordillera is a basement-involved fold-and-thrust system with bivergent, N-S– to NE-striking thrust faults (Kley et al., 1996; Mon, 1999), and it is characterized by a low-grade metamorphosed Proterozoic–early Paleozoic basement (Puncoviscana Formation; Omarini et al., 1999) that is unconformably covered by a thick Paleozoic sedimentary sequence. In the southern Eastern Cordillera, Paleozoic rocks are absent, and the well-developed Cretaceous–Paleogene Salta Group strata were deposited directly on the basement (Fig. 2). In this region, the NNE-SSW–striking and W-dipping frontal thrust-fault system of the Eastern Cordillera represents a main border fault of the Cretaceous rift that was reactivated by inversion during the Cenozoic (Carrera et al., 2006; Carrera and Muñoz, 2008), and it poses a basement slice (Sierra de Metán) over the Cenozoic sediments of the Metán Basin with a thrust offset of ∼10–15 km (Fig. 2) (Mon et al., 2005). East of this frontal thrust, a structural high belonging to the Santa Barbara System (Cerro Colorado; Fig. 2) shows exposures of Paleozoic units and an eastward thickness reduction of the Salta Group strata (Mon et al., 2005).
The Santa Barbara System (Kley and Monaldi, 1999) is a fold-and-thrust belt harmonically involving Proterozoic, lower Paleozoic (Ordovician-Devonian), Cretaceous-Paleogene (Salta Group), and Cenozoic (Orán Group) units. In the area of interest, the Santa Barbara System coincides with a NE-SW–trending branch of the Cretaceous continental rift, with a depocenter in the Metán Basin (González and Mon, 1996; Kley and Monaldi, 1999; Mon et al., 2005). The pre-Cretaceous basement, composed of Proterozoic–early Paleozoic highly microfolded metapelites, is exposed at the core of asymmetric anticlines forming laterally discontinuous, subparallel ranges of moderate topography with frequent and abrupt changes of strike from N-S to NE-SW (Fig. 2) (Allmendinger et al., 1983; González and Mon, 1996; Kley et al., 1999). The anticline-bounding faults were extensional normal faults bordering the Cretaceous rift basin that were inverted during the Andean orogeny (Cristallini et al., 1997; Kley and Monaldi, 2002). They dip both to SE and NW following the original geometry of rift-related faults. Across these faults, pre-Neogene sediments record an abrupt change of thickness and lithofacies (Mon et al., 2005). Several NE-striking reverse faults show a dextral, strike-slip component (Fig. 2). The Cenozoic sequence (Orán Group) is well developed and covers the Salta Group with a low-angle unconformity (Mingramm et al., 1979).
The Sierras Pampeanas consist of spatially isolated, N-trending crystalline basement blocks that are uplifted along NNE-striking reverse faults and that overthrust Neogene and Quaternary sediments of intermontane basins (Mon, 1976, 1979; Jordan and Allmendinger, 1986; González and Mon, 1996). In the northernmost Sierras Pampeanas ranges (Sierra de Quilmes, Sierra del Aconquija, and Cumbres Calchaquies; Fig. 2), the crystalline basement consists of Upper Proterozoic and/or lower Paleozoic rocks metamorphosed from upper greenschist to amphibolite facies, sometimes migmatized, and intruded by lower Paleozoic granitoids. The upper Paleozoic and Mesozoic sequences are lacking. The Cretaceous units related to the Salta rift thin westward and disappear on the eastern flank of the Cumbres Calchaquies. The Neogene–Quaternary strata unconformably overlie a basement peneplain surface that formed in the Late Cretaceous due to the denudation of the rift shoulders (Allmendinger et al., 1983; Coughlin et al., 1998; Sobel and Strecker, 2003). The Neogene–Quaternary sediments filled the intermontane basins (Coutand et al., 2001, 2006; Mortimer et al., 2007) and are well represented by the stratigraphic sequence of the Santa Maria Valley (Bossi et al., 2001).
The southern Puna (Allmendinger, 1986; Allmendinger et al., 1989; Marrett et al., 1994) has two distinct basement sequences (Fig. 2). The western part of the Puna Plateau is characterized by elongated fault-bounded ranges of Paleozoic sedimentary strata (Ordovician) folded along N-S axes and intervening, internally drained basins (salars). In the eastern part, the crystalline basement consists of amphibole-grade metasedimentary strata of Proterozoic or early Paleozoic age with granitoid intrusions similar to the basement of the Sierras Pampeanas (Siebel et al., 2001). Neogene–Quaternary andesitic to dacitic volcanic sequences unconformably mantle older deposits and are poorly deformed (Fig. 2). At the southeastern border of the Puna, transitional with the Sierras Pampeanas, the Neogene sedimentary and volcanic sequence spans from ca. 11 Ma to 2.3 Ma, without evidence of internal unconformities, and rests directly on the crystalline basement (Strecker et al., 1987; Allmendinger et al., 1989). The Miocene rocks are transitional to or unconformably overlain by a boulder conglomerate (Punaschotter Formation; Allmendinger, 1986) of probable Pliocene age (3.8 Ma; Carrapa et al., 2008).
Elements and Timing of Upper-Crust Deformation
The Puna Plateau and the eastern Andean foreland region at ∼26°S show three prominent regional structural trends running about N-S, NE-SW, and NW-SE, parallel and transversal, respectively, to the main direction of the orogenic structures.
The major N- to NNE-trending set corresponds to the regional-scale thrust faults of the Eastern Cordillera and northern Sierras Pampeanas and to the fold axes of the Paleozoic ranges in the Puna Plateau (Fig. 2). These thrusts were reactivated during Neogene as sinistral transtensional strike-slip faults, determining the major NNE-elongated salar depressions (i.e., Salar de Antofalla; Fig. 2; Kraemer et al., 1999; Coutand et al., 2001).
The NW- and NE-trending transverse systems cut across the Puna Plateau and the eastern foreland provinces of the Eastern Cordillera and Sierras Pampeanas (Figs. 1 and 2). During the Neogene, the NW- and NE-trending faults formed conjugate systems (Allmendinger, 1984; Strecker et al., 1989; Assumpção and Araujo, 1993) and had sinistral and dextral motion, respectively, consistent with dominant regional E-W shortening (Allmendinger et al., 1983). Many of these transverse lineaments were active prior to the Cenozoic and are interpreted to represent crustal structures, compositional anisotropies in the lithosphere, and/or pre-Andean paleogeographic boundaries, exerting an important influence on regional tectonic style.
At ∼26°S, the NW-striking Archibarca lineament develops (Fig. 1). On the Puna Plateau, NW-striking faults can be traced discontinuously and are obscured by Neogene–Quaternary volcanic cover, whereas they are more evident on the southeastern Puna border, Sierras Pampeanas, and Santa Barbara System (Fig. 2) (Allmendinger, 1984, 1986).
The NE-striking Tucumán lineament (Mon, 1976; Allmendinger et al., 1983) was defined by de Urreiztieta et al. (1996) as a dextral transpressional transfer zone accommodating the lateral decrease in amounts of bulk horizontal shortening and crustal thickening occurring at ∼27°S. Along this lineament, a horizontal offset of ∼350 km was produced between the main deformation front of the Subandean foreland provinces to the north and the Precordillera to the south (Fig. 1). It also coincides with structural and stratigraphic regional boundaries between the Puna Plateau and Eastern Cordillera and Santa Barbara System (Figs. 1 and 2). The Tucumán lineament is 100 km wide, can be followed for at least 350 km (Fig. 1), and appears as a broad diffuse zone of distributed right-lateral shearing rather than strike-slip faulting along discrete faults (Allmendinger, 1984; Allmendinger et al., 1989; de Urreiztieta et al., 1996). Notwithstanding these factors, several NE-striking faults of the Sierras Pampeanas and Santa Barbara System show a dextral strike-slip component (de Urreiztieta et al., 1996) (Fig. 2).
The onset and evolution of Cenozoic deformation along the transect at ∼26°S have been recently constrained by apatite fission-track thermochronology on uplifted blocks of the Eastern Cordillera and Sierras Pampeanas (Coutand et al., 2001, 2006; Sobel and Strecker, 2003; Deeken et al., 2006; Mortimer et al., 2007; Carrapa et al., 2008). Along this transect, timing of the upper-crustal deformation becomes more recent eastward. In the southern Puna Plateau, the last significant compressive deformation occurred during the middle to late Miocene (<15 Ma; Vandervoort et al., 1995). Uplift of the Sierras Pampeanas ranges is interpreted to have started at ca. 6 Ma (Strecker et al., 1989; Sobel and Strecker, 2003; Carrapa et al., 2008). In the Santa Maria Basin, the sediments do not record any evidence for tectonic deformation until ca. 7 Ma (Kleinert and Strecker, 2001). Two phases of deformation are recorded by the Neogene–Quaternary sequence of this basin, both with WNW-ESE shortening and subvertical extension along thrust faults. Similarly, at the southeastern border of the Puna, at the transition with the Sierras Pampeanas, two phases of late Cenozoic deformation are stratigraphically separated by the deposition of the boulder conglomerate of the Punaschotter Formation (Allmendinger, 1986; Allmendinger et al., 1989; Carrapa et al., 2008). The first phase started between 10 and 5 Ma and is older than 3.53 Ma (Allmendinger, 1984, 1986; Trumbull et al., 2006). The younger phase produced normal, reverse, NE-striking right-lateral, and NW-striking left-lateral strike-slip faults. On the Santa Barbara System (Carrera and Muñoz, 2008), a first deformation event is recorded by the decrease in thickness of the Guanaco Formation (10–5 Ma) across the crest of anticlinal folds (Reynolds et al., 1994; Gomez Omil and Albarino, 1996), and the main phase of deformation developed during the deposition of the Piquete Formation since ca. 5 Ma.
Time-Space Distribution and Characteristics of Backarc Magmatism
In the Central Andes between 24°S and 27°S, late Miocene–Quaternary backarc magmatism was focused in the southern Puna Plateau and Eastern Cordillera along NW-trending lineaments (Calama–Olacapato–El Toro, Archibarca, and Culumpajá) and in the northern Sierras Pampeanas along the NE-trending Tucumán lineament (Figs. 1 and 2).
The backarc volcanism in the southern Puna, along the Calama–Olacapato–El Toro and Archibarca fault systems, was characterized by three different volcanic kinds: stratovolcanoes, ignimbrites, and small mafic centers (Coira et al., 1982, 1993; Allmendinger et al., 1997), and sporadically by intrusive massifs. Along the Calama–Olacapato–El Toro, the products of some Miocene stratovolcanoes have been interpreted as deriving by partial melting of lower crust (Matteini et al., 2002). Along the easternmost portion of the same lineaments, Miocene intrusive and volcanic rocks with intraplate signature were emplaced (La Burras–Almagro–El Toro magmatic complex; Mazzuoli et al., 2008). Along the Archibarca lineament, the middle Miocene–Pliocene andesitic to dacitic stratovolcanoes exhibit a calc-alkaline signature with a higher degree of crustal contamination than the andesite-dacite lavas of the frontal arc (Western Cordillera) (Coira and Pezzutti, 1976; Coira et al., 1993; Kay et al., 1999; Kraemer et al., 1999; Richards et al., 2006). It is worth noting that these stratovolcanoes have an extraordinary long-lived history, spanning 6–9 Ma (Risse et al., 2008), compared to the normal evolution of such kind of volcanoes. Small-volume, rhyolitic ignimbrites, middle Miocene–Pliocene in age (11–4 Ma; Siebel et al., 2001), show an isotopic composition similar to that of the stratovolcanoes of the frontal arc. They are interpreted as products of crystal fractionation of magmas similar to the contemporaneous subduction-related andesites, with little intervention of crustal contamination (Coira et al., 1993; Siebel et al., 2001; Schnurr et al., 2007). Contemporaneously, the Cerro Galan caldera outpoured dacitic, crystal-rich, large-volume ignimbrites (Fig. 2) with geochemical and isotopic signatures (Francis et al., 1980, 1989; Sparks et al., 1985) that provide evidence for large-scale intracrustal melting with a minor mantle component, similar to the ignimbrite flare-ups of the Altiplano-Puna volcanic complex (de Silva, 1989). Mafic volcanism (mainly basaltic andesite and andesite) formed small, scattered, monogenetic cinder and spatter cones and fissures with associated low-volume lava flows (Fig. 2) during the late Miocene–Pleistocene (Coira et al., 1982, 1993; Knox et al., 1989; Kay et al., 1994, 1997, 1999; Kraemer et al., 1999; Risse et al., 2008). Based on their geochemical characteristics, the products of the mafic backarc centers have been distinguished into K-rich calc-alkaline, shoshonite, and intraplate, reflecting their different mantle sources (Kay et al., 1994).
The onset of the backarc mafic volcanism in the southern Puna occurred at 7.3 Ma and reached a maximum of activity at 5.3–3.6 Ma (Kay et al., 1999; Kraemer et al., 1999; Risse et al., 2008; Drew et al., 2009). Volcanic activity continued with some fluctuations through the Pleistocene. The main phases of mafic backarc volcanism coincided with the waning activity of backarc stratovolcanoes (Risse et al., 2008). Moreover, there is a correspondence in time between the peak of mafic volcanism and the major silicic ignimbrite eruptions of the Cerro Galan caldera (Sparks et al., 1985; Francis et al., 1983).
Miocene magmatism in the northern Sierras Pampeanas is represented by the Farallón Negro and Agua Rica volcanic centers, and the andesitic breccias and lavas of the Portezuelo de Las Animas on top of Sierra del Aconquija (Figs. 1 and 2). At Farallón Negro, basalts and basaltic andesites were erupted between 12.5 and 8.5 Ma, and an andesitic stratovolcano was active between 8.5 and 7.5 Ma (Sasso and Clark, 1998). The youngest phase of magmatic activity in the Farallón Negro was the intrusion of a NW-trending dacite and rhyolite dike swarm between 7 and 5.1 Ma (Caelles et al., 1971; Sasso and Clark, 1998). The Agua Rica center includes a monzonitic intrusive complex emplaced at 8.6 Ma and younger andesite porphyry mineralized at 5.4 Ma (Sasso and Clark, 1998). The Portezuelo de Las Animas volcanism is inferred to have been active between 11.6 and 7.7 Ma (González et al., 2005).
The only volcanic activity in the Santa Barbara System is the Antilla magmatic complex, the study of which is presented in this paper.
System at Depth: Crust, Lithosphere, and Upper Mantle
The thickness of the crust and lithospheric mantle and the depth of the subducted slab are critical constraints for geodynamic models on Andean structural evolution and genesis of backarc magmatism. Currently, the continental crust thickness estimated beneath southern Puna and Eastern Cordillera at ∼25°S is 42–59 km, with an irregular distribution and a southward decreasing trend (Yuan et al., 2002; McGlashan et al., 2008, and references therein). Across the transect of the Central Andes at 26°S, lithosphere thickness is modeled at 60 km beneath the southern Puna Plateau and 90–100 km eastward beneath the transitional area with the Sierras Pampeanas and Santa Barbara System (Whitman et al., 1992, 1996). Across the same transect at 26°S, the subducting Nazca slab is 90–100 km deep beneath the Western Cordillera and 300 km deep beneath the Antilla magmatic complex (Cahill and Isacks, 1992).
Geophysical data in the southern Puna, with strong seismic attenuation, low P- and S-wave velocities, and high electrical conductivity (Whitman et al., 1992; Yuan et al., 2000; Schurr et al., 2003; Koulakov et al., 2006; Asch et al., 2006), indicate anomalously high temperatures and the presence of partial melts in the midcrust at ∼20–25 km depth beneath the Galan caldera region, similar to the Altiplano-Puna volcanic complex.
The crustal thinning, high elevation of ∼4000 m, a felsic composition of the crust (Zandt et al., 1994; Beck and Zandt, 2002), the absence of a thick mantle lid (Whitman et al., 1996), the extensive crustal melting, and the mafic backarc magmatism (Kay et al., 1994; Risse et al., 2008) are features that have been interpreted to indicate the foundering of lower portions of the lithosphere (lithospheric delamination), replaced by an upflow of hot asthenosphere (Kay and Kay, 1993; Kay et al., 1994; Schurr et al., 2006; Kay and Coira, 2009).
ANTILLA MAGMATIC COMPLEX
Our work is focused on the Neogene magmatic rocks exposed near the village of Antilla (Salta Province, 26°10′S, 64°40′W, 550–750 m above sea level [asl]) in the southern part of the Subandean foreland province called the Santa Barbara System (Figs. 1, 2, and 3). This is the only and most eastern occurrence of Neogene backarc magmatism documented in this sector of the Andean foreland. Insight into the stratigraphy and structure of the Antilla rocks from surface exposure is limited because the area is covered by extensive scrub vegetation, shows scarce erosion, and was subjected to intensive ground leveling for agricultural purposes. The Antilla magmatic complex was first recognized by Viramonte et al. (1994a). Presently, it appears as a volcanic field of different, monogenetic lava vents rather than a central volcanic edifice, extending over an area of ∼150 km2 around the Cerro Negro, on the eastern side of the Sierra de la Candelaria. These volcanic rocks crop out as lava flows and feeder dikes interlayered and intruded within Neogene sedimentary deposits (Fig. 3; Table 1).
In the study area, the crystalline basement forms the core of the anticline of Sierra de la Candelaria (Figs. 2 and 3) and is composed of medium-grade metamorphic rocks of the Medina Formation (Bossi, 1969). These microfolded schists probably represent the Proterozoic–early Paleozoic Puncoviscana Formation, but pervasive shearing that developed during the Early Cambrian deformational event has obscured the original lithofacies (Willner, 1990). The overlying Paleozoic sedimentary rocks show variable distribution and thickness. The Middle Cambrian Mesón Group is composed of fine-grained whitish and pinkish quartzite overlying the crystalline basement with angular unconformity in the Sierra de la Candelaria range. The conformable Ordovician Santa Victoria Group includes shallow-marine fossiliferous sandstone, mudstone, and shale (Aceñolaza, 1992). The overlying Silurian-Devonian strata are unconformable.
The Paleozoic sequence is unconformably overlain by the Cretaceous–Paleogene Salta Group (Figs. 2 and 3), coinciding in the study area with the Metán Basin, which includes two sedimentary stages. The first stage represents the infilling of a synrift basin and is represented by the Cretaceous–Campanian Pirgua Subgroup, including reddish conglomerates, sandstone, shale, and alkaline volcanic rocks (Marquillas and Salfity, 1988; Viramonte et al., 1999). Conglomerates of the basal Pirgua Subgroup overlie the Devonian quartzite at the core of the Cerro Colorado and Cerro Remate anticlines (Fig. 2). The conglomerate has a vertical and lateral transition to red and pink, massive quartz sandstone with interlayers of mudstone, as in the Cerro Negro anticline. The thickness of the Pirgua Subgroup is 1100 m in the Cerro Colorado and 3500 m in the central part of the Metán Basin. The second stage (Maastrichtian–Eocene) is related to the basin postrift infilling and includes the Balbuena and Santa Barbara Subgroups, both intruded by basaltic lavas and dikes (Viramonte et al., 1999). The Balbuena Subgroup is composed of basal massive calcareous sandstone (Lecho Formation) and white and yellow oolitic limestone (Yacoraite Formation), distributed uniformly over the entire area with maximum thickness of 200 m in the Cerro Colorado. The Santa Barbara Subgroup is represented only by the Mealla Formation, cropping out around the Cerro Colorado, which is composed of yellow and red sandstone with interlayers of calcareous sandstone and calcrete paleosols. The exposed sequence lacks the younger units of the subgroup.
The stratigraphy of the Cenozoic units in the Antilla area is poorly known. According to Reynolds et al. (2000), the reference sequence of the Neogene sediments is represented by the 5000-m-thick Orán Group in the Arroyo Gonzalez, ∼150 km to the north of the study area. This sequence includes the basal Metán Subgroup and the unconformably overlying Jujuy Subgroup (Gebhard et al., 1974; Galli and Hernández, 1999; Reynolds et al., 2000). The Metán Subgroup is formed, from base to top, by the Río Seco, Anta, and Jesús María Formations. In the study area, around the Cerro Colorado and Cerro Negro (Figs. 2 and 3), the Anta Formation directly overlies the Mealla Formation of the Salta Group. The Anta Formation is composed of well-stratified, reddish, fine-grained sandstone, siltstone, and mudstone interbedded with laminated greenish mudstone, whitish oolitic limestone, and abundant beds and lenses of gypsum. Geochronology analyses from tuffs collected on the Anta Formation have produced ages of 13.2−14.5 Ma (Reynolds et al., 2000). Lithofacies indicate that the Anta Formation corresponds to lacustrine and shallow-marine environments developed at the margins of the Miocene transgression of the Paraná Sea (Ramos and Aleman, 2000; Aceñolaza and Sprechmann, 2002; Vezzoli et al., 2009). The Jujuy Subgroup shows a basal unconformity and consists of the Guanaco and Piquete Formations. The Guanaco Formation is dominated by red and gray sandstones, siltstones, and conglomerates. Interbedded tuffs yielded U-Pb ages on zircon of 10.9 and 6.9 Ma (Viramonte et al., 1994b); the base of this unit is interpreted as younger than 9 Ma (Reynolds et al., 2000). The Guanaco Formation is in turn unconformably overlain by the Piquete Formation, which has an estimated age of 5–2 Ma (Reynolds et al., 2000) and is composed of dark-red polymictic cobble and boulder conglomerate, subordinate sandstone, and rare siltstone.
Volcanic and Structural Features
The Antilla volcanic rocks crop out in distinct lava bodies interpreted as lava flows and feeder dikes. Each exposure shows individual or superposed lavas with distinctive petrographic texture and composition (Table 1). On the northern side of the Cerro Negro, two dikes have N-S and NW-SE strike, respectively. They intruded a sedimentary sequence composed, from the base, of: (1) reddish and pinkish, laminated, fine-grained sandstone and siltstone interbedded with laminated greenish mudstone, and (2) stratified, whitish oolitic limestone, in 5–20-cm-thick beds. Lithofacies suggest the correlation of these sediments with the Anta Formation. On the southern side of the Cerro Negro (Río Quevitas), almost six different lava flows, 3–20 m thick, are interbedded within red and gray sandstone and laminated reddish and yellow siltstone. Several textures and structures at the lava-sediment interface suggest a synsedimentary emplacement of lava flows. The attribution of these sediments to a distinct Cenozoic lithostratigraphic unit is not obvious. Lithofacies may suggest a correlation with the Guanaco Formation. On the eastern side of the Cerro Negro, lavas are exposed as isolated relict hills representing WNW- to NW-striking dikes that are surrounded by yellowish and grayish, fine-grained sandstone and siltstone with lenses of gypsum that can be correlated to lithofacies of the Anta Formation.
The present regional structural setting of the Antilla area is represented by the N-S–striking Sierra de la Candelaria anticline, showing crystalline basement and Paleozoic rocks at its core (Fig. 3). On the eastern flank of the fold, a N-S–striking and W-dipping high-angle fault thrusts the basement rocks over the Cenozoic sedimentary strata of the Orán Group, which generally gently (5°–1°) dip toward the east. In the footwall of this fault, the hill of the Cerro Negro is a small dome-shaped anticline plunging northward and southward and involving pink and red, medium-grained sandstones of the Pirgua Subgroup (Cretaceous Salta Group) at its core and the Cenozoic sediments of the Orán Group (Fig. 3). Due to the scarce erosion and abundant vegetation cover, no structures (fracture or fault) were observed throughout the studied area, except for a N-S–striking compressional dip-slip fault that was measured along the Río Quevitas on the southern side of the Cerro Negro. The fault plane dips 40° toward the east and offsets a lava flow (locality number 6 in Fig. 3 and Table 1) and the underlying Cenozoic sediments. All these deformations postdate the emplacement of the Antilla volcanics. Therefore, the only possible tectonic indicators of local or regional synvolcanic deformation are represented by the attitude of the Antilla dikes (Fig. 3), which coincides with the N-S– and NW-SE–trending directions of the regional structures.
Three samples of the Antilla lavas (Table 2) were selected for geochronological study (see analytical methods in Appendix) after macroscopic and petrographic inspections to exclude alteration. Sample ANT16 is a lava flow interbedded with red sandstone and siltstone along the Rio Quevitas, sample ANT24 represents a WNW-striking dike intruding the Anta Formation on the eastern side of the Cerro Negro, and sample ANT8 forms an isolated exposure near San Lorenzo (Table 1).
The K/Ar determinations are reported in Table 2. They give consistent ages of 4.83–4.67 Ma (Table 2). Viramonte et al. (2008) proposed an age of 7.7 Ma from Ar/Ar experiments on hornblende separates. This age is significantly older than the very precise and self-consistent K/Ar ones obtained in this work. Because of the lack of detailed data about the Ar/Ar experiments and of clear geological information about the Ar/Ar dated sample, it is not possible for us to comment on this age difference.
Petrography, Rock Classification, and Mineral Chemistry
The Antilla rocks have a SiO2 content ranging from 49 to 61 wt%, LOI (loss on ignition)–free, and moderately high and variable alkali content (Table 3). The analyzed rocks fall in the alkaline field (Fig. 4), and in the K2O versus Na2O diagram, they plot in the field of the K-series (inset of Fig. 4). Most of the rocks are mildly nepheline-normative and range from trachybasalts to trachytes of the moderately alkaline series (Middlemost, 1980).
The Antilla rocks show a porphyritic texture, and the majority have high phenocryst content. The mafic samples are dominated by phenocrysts of calcic amphibole and clinopyroxene in variable mutual proportions (Table 1). The calcic amphibole has Mg# (molar Mg/Mg + Fe) ranging from 68 to 42 and commonly shows reverse zoning and disequilibrium rims. The clinopyroxene phenocrysts are represented by augite, with Mg# ranging from 88 to 67 and normal zoning (Table 4). Olivine is present as scarce, completely iddingsitized microcrystals. Apatite occurs as a common accessory. Labradorite plagioclase joins the phenocryst assemblage in part of the analyzed rocks (Table 1). The groundmass grain size is highly variable, from coarse in dikes to mainly medium-fine-grained in lava samples; it is composed of plagioclase, clinopyroxene, K-feldspar, Fe-Ti oxides, biotite, and olivine. The more silicic rocks (MgO < 3 wt%) contain phenocrysts of normally zoned plagioclase (andesine from An49Ab47 to An26Ab64, unzoned amphibole [Mg# around 50], aegirine-augite clinopyroxene [Mg# from 67 to 59 with rare Mg-rich cores], Na2O up to 1.2 wt%; Table 4), large titanite and apatite crystals, Fe-Ti oxides, and minor biotite. The groundmass is made of the same minerals plus K-feldspar. Fe- and Cu-sulfides occur in some samples. Many rocks are altered to a variable extent, presenting phyllosilicates (probably chlorite minerals) substituting mafic grains in the groundmass, zeolites, and secondary calcite in the groundmass and filling vacuoles, sericitization on plagioclase, and, locally, albite and epidote.
The chemical analyses of selected Antilla samples are presented in Table 3. The alkali content is rather variable, and two groups of samples, with lower and higher alkali content, may be recognized (Fig. 4). Some of the analyzed rocks (belonging mainly, but not exclusively, to the group with higher alkalies) show a relatively high value of LOI (Table 3). Although this high LOI value is related to a moderate alteration, we have preferred to report all the analyzed rocks because of the paucity of geochemical data on volcanic rocks related to the geologic context of the Antilla magmatic complex. Necessarily, we have taken into account the possible effects of the alteration process on the interpretation and discussion of the geochemical data, checking the eventuality of correlations between geochemical parameters and LOI.
The well-defined trends of the major elements, and in particular those of K2O (very mobile element) and Al2O3, suggest that the alteration processes did not deeply modify the content of these elements (Fig. 5). When plotted with MgO taken as a differentiation index, alkali and Al2O3 increase regularly with decreasing MgO, whereas TiO2, FeO, and CaO exhibit a negative trend (Fig. 5), indicating fractionation of mafic minerals such as those in the phenocrysts assemblage of the mafic and intermediate rocks, i.e., clinopyroxene, amphibole, and titanite. Regarding the trace elements, the Antilla samples describe regular and well-defined trends in variation diagrams of all the compatible or the incompatible elements (Fig. 5). The two groups observed in the alkali-silica diagram are evident in Na2O, Fe2O3, and P2O5 variation diagrams, but are not particularly highlighted by the trace element (Fig. 5), except for a very moderate dispersion of Zr and Th contents in the mafic analyzed samples (Table 3).
In the spider diagrams of trace elements normalized to the primitive mantle (Fig. 6A), the Antilla mafic samples display positive anomalies of Rb, Th, K, and Pb and moderate negative troughs of Nb, Ta, and Ti. The rare earth element (REE) patterns of the mafic Antilla samples are moderately fractionated and show no Eu anomaly (Fig. 6B). Also, the most differentiated samples, which bear plagioclase as main phenocrysts, do not show Eu anomalies, indicating that crystallization occurred at high-oxygen-fugacity conditions. This evidence, joined to the petrographic features suggesting a large presence of hydrous minerals, may be taken as an indication of the volatile-rich nature of Antilla magmas.
The Antilla samples show Sr isotopic ratios ranging from 0.704360 to 0.706406 and Nd from 0.512764 to 0.512625 (Table 5; Fig. 7). In the 87Sr/86Sr versus 143Nd/144Nd diagram, the Sr and Nd isotopic ratios of Antilla rocks are correlated to each other, following the Sr and Nd Central Andes main trend, but neither the Sr nor the Nd ratio shows any correlation with the degree of evolution represented by SiO2.
Magma Source Characteristics
The Antilla volcanics are moderately alkaline and involve mafic rocks with a Mg-value around 70 and a maximum Cr and Ni content of 147 and 49 ppm, respectively. Moderately differentiated rocks with SiO2 content close to 60 wt% (Table 3) are present as well. The scarcity of olivine, the moderate Ni and Cr content, and the chemical composition of phenocrysts (Tables 3 and 4) suggest that even the most primitive rocks underwent some differentiation processes.
The low values of discriminant trace-element ratios such as La/Ta and Zr/Y indicate an intraplate character (Fig. 6A). The low Ba/Nb ratio excludes a contribution from subduction-related fluids during the partial melting of the mantle source (Fig. 8).
Compared to the backarc mafic Miocene to Quaternary products between 24°S and 27°S, the Antilla samples show some trace-element characteristics similar to intraplate mafic rocks (i.e., Las Burras, Farallón Negro, and Puna monogenetic centers) in the positive anomalies of Rb and K and the negative anomaly of Ti (Fig. 6A), the moderately fractionated REE patterns (Fig. 6B), and the weak heavy (H) REE depletion. The latter suggests the absence of residual garnet in the mantle source and a melt equilibration shallower than 60–70 km (Fig. 9). However, the Antilla mafic rocks differ from the Puna intraplate rocks, even the most primitive basalt yet reported by Drew et al. (2009), for their alkaline nature, the lower content of silica, Cr, and Ni with a comparable MgO content, and the lower Sr and higher Nd isotope ratios. The high porphyricity and the abundance of volatile-bearing minerals are comparable to the rocks emplaced at the eastern margins of the backarc region (i.e., Farallón Negro, Las Burras, and Almagro). The remarkably primitive Sr and Nd isotope ratios of Antilla rocks plot not far from the 20 Ma Chiar Kkollu basalt (Fig. 7), which is considered to be a near-primary intraplate magma in the Central Andes (Davidson and de Silva, 1995). Moreover, the Antilla rocks overlap the composition of Las Burras, the most primitive rocks along the Calama–Olacapato–El Toro transversal structure at 24°S (Mazzuoli et al., 2008), and approach the composition of the Mesozoic peridotite xenoliths reported by Lucassen et al. (2002, 2005).
Because of the Sr and Nd isotopic compositions, the moderately alkaline character, and the shallow depth of the source, we interpreted that Antilla primitive magmas originated by partial melting of the depleted subcontinental mantle. The variable alkali enrichment of these mafic rocks, uncorrelated with the isotopic composition, may suggest variable degrees of partial melting.
The differentiation of the Antilla products was dominated by fractional crystallization processes. The isotopic composition is not correlated with differentiation, indicating a minor role for assimilation of crustal material. The Antilla volcanics have a low Ce/Pb ratio and a low Pb content (Fig. 10), similar to the Farallón Negro and Las Burras–Almagro rocks, close to the field representing the lower-crust xenoliths of Lucassen et al. (1999). Therefore, a minor involvement of lower crust, characterized by low Ce/Pb ratios and low Sr and high Nd isotopic ratios, may have had a role in determining the small range of isotopic composition shown by the mafic Antilla magmas (Fig. 7).
Antilla Volcanism in the Frame of the Evolution of the Southern Central Andes
In the backarc sector of the southern Central Andes, between 24°S and 27°S, volcanic rocks with distinct affinities have been erupting since the middle Miocene (Fig. 11A). When considering the most primitive of these rocks, three main groups can be distinguished: (1) high-K calc-alkaline rocks, associated with Miocene stratovolcanoes and monogenetic centers, mainly located in the northern (e.g., Negra Muerta) and southern (e.g., Pasto Ventura) parts of the region; (2) shoshonitic rocks, erupted in the past 2 Ma by small monogenetic centers clustered in the northeastern part of the region (i.e., San Geronimo and Chorrillos); and (3) products with intraplate affinity (low La/Ta and Zr/Y), erupted since Miocene times up to Quaternary times by monogenetic centers, emplaced in a large part of the Puna, and by lava domes and intrusions in the Eastern Cordillera and Santa Barbara System (i.e., Las Burras, Almagro, Tuzgle, and Farallón Negro).
The geochemical features of the Antilla mafic lavas relate these rocks to the mafic intraplate volcanism at the margin of the backarc region, and suggest that these primitive magmas originated by partial melting of the depleted subcontinental mantle.
A question arises: why did the mantle undergo melting so far from the active volcanic arc? The lack of a subduction imprint in the Antilla rocks and the depth of the slab beneath Antilla exclude mantle melting due to subduction-derived fluids or melts. Other scenarios must be examined that take into account the geodynamic setting of Antilla. The generation of mantle-derived magmas in an intraplate setting may occur in an extensional regime, producing large magma volumes, but both these conditions are lacking at Antilla. On the other hand, geophysical data at 25°S indicate a relatively thin lithosphere (Asch et al., 2006), which has been related to the occurrence of processes of foundering of the lower portions of the lithosphere itself, invoked in the southern Puna Plateau since the middle Miocene (Fig. 11B; Kay et al., 1994; Asch et al., 2006; Mazzuoli et al., 2008; Kay and Coira, 2009). These processes were a consequence of crustal thickening and shortening during the main Andean compressional tectonic phase (Kay and Coira, 2009). Replacement of portions of the lower lithosphere by hot asthenosphere has been interpreted as the cause of the mantle melting responsible for the Pliocene pulse of mafic magmas in the Puna, suggesting a spatial correlation between their intraplate-like geochemical signature and location of lithospheric foundering beneath the plateau (Kay et al., 1994). The new thermal regime due to the rise of asthenosphere led to the partial melting of the continental crust (de Silva et al., 2006), too, originating the large volumes of felsic magmas erupted in this Andean sector (i.e., Cerro Galan; Fig. 11B). Antilla is the easternmost volcano with intraplate-like signature, east of the Galan area, and is coeval with the acme of Pliocene magmatism in the Puna. Therefore, we favor the hypothesis that the generation of Antilla primary magmas was related to a scenario of foundering of limited portions of the lower lithosphere, inducing asthenospheric upflow and consequent melting of lower portions of the subcontinental mantle.
The results of the study of the Antilla volcanic complex enable us to provide insights into the lithospheric mantle composition in the Andean foreland. The Antilla rocks, which have low Sr and high Nd isotopic ratios, and which were emplaced out of the zone where the crust was largely affected by partial melting, represent the least contaminated products of the Central Andes backarc magmatism during the Pliocene. The comparison of the petrological characteristics of Antilla with those of the most primitive magmas of monogenetic mafic centers of the Puna Plateau indicate that, whereas under the Puna part of the mafic magmatism requires the presence of an aged, metasomatized, subcontinental lithospheric mantle portion (Drew et al., 2009), in the eastern backarc the subcontinental mantle has a depleted isotopic composition. Similar indications have been found at 24°S in the Las Burras older rocks (Mazzuoli et al., 2008).
At the margin of the backarc region, the crust did not obliterate the mantle imprint of magmas, and isotopically primitive compositions were repeatedly erupted (Figs. 11A and 12). As a whole, the mafic volcanic products emitted from the middle Miocene to Quaternary at the margins of the backarc region (Fig. 11A) show variable geochemical features, i.e., Farallón Negro, Almagro, and the Puna shoshonitic volcanism show higher Ba/Nb and La/Ta, and Almagro mafic lavas show notably higher Sr and lower Nd isotopic ratios than Antilla (Fig. 12). This picture suggests that these magmas were derived from a lithospheric mantle containing regions modified by inputs of subduction-related metasomatizing fluids that underwent different degrees of partial melting in the frame of the lithospheric delamination process.
Eastward, beyond these magmatic complexes, Miocene-Quaternary backarc volcanic activity was apparently absent (Fig. 1). The distribution and timing of the backarc magmas have some important structural implications. The absence of magmatic centers east of Antilla in the Andean foreland may be explained by the presence of pure compressive structures that inhibited the ascent of magma despite the possible presence of partially molten rocks at depth, as was proposed for the eastern termination of the magmatic belt along the Calama–Olacapato–El Toro (Acocella et al., 2007). The inferred extension of the western edge of the Brazilian craton beneath the Andes to ∼65°–66°W at the latitude of 18°–24°S (Beck and Zandt, 2002; Babeyko and Sobolev, 2005) coincides with volcanism termination along the Calama–Olacapato–El Toro lineament (Mazzuoli et al., 2008) and may be considered as a concurrent cause of backarc volcanism absence east of 64°W also along the southern lineaments.
The age of the Antilla lavas corresponds to the acme of mafic and ignimbrite volcanic activity around 25°S (Risse et al., 2008). The diagram in Figure 13 shows the relationships between the ages and latitude of the backarc magmatic centers in the interval between 24°S and 27°S. This diagram can give some general indications, despite being limited by either incomplete sampling or the lack of age determinations. At around 24°S, in coincidence with an ∼150 km segment of the Calama–Olacapato–El Toro lineament, volcanic activity lasted from ca. 14 Ma until 6 Ma and then became scarce and dominated by monogenetic centers (Kay et al., 1994). Furthermore, volcanism concentrates in a narrow belt around 24°S. Southward, between 25°S and 26°S, volcanism started at ca. 7.3 Ma and had an acme at 4–5 Ma (Risse et al., 2008); the activity, which is recorded by many age data, was widespread in a wider latitude interval than the Calama–Olacapato–El Toro belt. In this area, the NNE-SSW–striking faults, parallel to the trend of Antofalla salar (Marrett and Strecker, 2000) and Tucumán transfer zone, could have played an important role in magma ascent (Risse et al., 2008). South of 26.5°S, the only age determinations are related to the long-lived volcano Farallón Negro, aligned with Antilla along the important Tucumán lineament.
The time distribution of the backarc magmatism, and in particular, the onset of the mafic intraplate backarc volcanism, which is related to the delamination process, can be read in the frame of the tectonic history of this sector of the Central Andes. Two distinct tectonic phases have been recognized: an older NW-SE compressional event that produced thrust faults since ca. 10 Ma, and a younger NE-SW or E-W shortening event with a component of N-S extension that reactivated previous thrust faults and transverse lineaments with extensional and strike-slip components from the Pliocene to the Quaternary (5–2 Ma) (Allmendinger, 1986; Allmendinger et al., 1989; Cladouhos et al., 1994; Marrett et al., 1994; Marrett and Strecker, 2000). The ages of the oldest volcanoes with intraplate affinity at 7 Ma could record, as suggested by some authors (Kay et al., 1999; Kraemer et al., 1999; Risse et al., 2008), the beginning of the change in regional stress conditions in the southern Puna Plateau (Marrett and Emerman, 1992), and the ages of 5.3–3.6 Ma could represent the acme of volcanic activity linked to the change in stress regime. This process is recorded by the volcanic activity at 25–26°S, whereas at ∼24°S, a compressional regime was predominant due to the underthrusting of the Brazilian Shield under the Andes structure (Acocella et al., 2007; Mazzuoli et al., 2008), partially preventing the storage and ascent of magmas.
In this frame, the Antilla volcanic complex takes on a particular interest because it defines the southeastern limit of the magmatic effects of the lithospheric delamination process and of the thinned lithosphere. Furthermore, its geological position at the intersection between the Archibarca-Galan fault system and the Tucumán transfer zone could suggest that the latter lineament, practically matching the dipping direction of the southward shallowing of the subducting Nazca plate (Fig. 12), played an important role in the ascent of magmas (i.e., Farallón Negro, Portezuelo de Las Animas, and Antilla), extending backarc magmatism to 64°W.
The study of the Pliocene Antilla rocks, the easternmost magmatic outcrops in the Central Andes between 24°S and 27°S, was the starting point for a revision of our understanding of the backarc volcanism in this sector. The Antilla rocks are prevalently mafic, and have geochemical characteristics similar to those of mafic intraplate volcanism in the backarc, and their Sr and Nd isotopic characteristics are among the most primitive recorded in the backarc lavas. The Antilla rocks, with an age of 4.7 Ma, were emplaced coeval with the acme of activity of the mafic monogenetic intraplate centers and the felsic volcanoes erupting crust-derived ignimbrites in the southern Puna Plateau. This intense magmatic activity corresponds, in agreement with geophysical data in this region, to a process of lithospheric delamination, which allows the uprising of asthenosphere and is responsible for partial melting in the lithospheric mantle and continental crust. The primitive isotopic characteristics of Antilla rocks suggest that Antilla lavas were erupted at the margins of this zone and did not interact with crustal melts. In this scenario, the Antilla volcanism could represent the surface expression of the easternmost limit of the area affected by foundering of deep lithospheric portions.
A revision of backarc magmatic activity from the Miocene to Quaternary between 24°S and 27°S indicates that mafic magma production and eruption are most marked at around 25°S–26°S, with a high density of volcanic centers and an acme of activity at 4–5 Ma. This could be put in relation with the tectonic regime. At ∼24°S, after 10 Ma, the regime was mainly compressive due to the underthrusting of the Brazilian Shield under the Andes, whereas at 25°S–26°S, after 7 Ma, the tectonic regime was mainly extensional due to a change in the tectonic stress conditions, favoring magma ascent on a relatively wider area. The location of the Antilla volcanics at the intersection of the Archibarca-Galan lineament and the Tucúman transfer zone highlights the importance of the interaction between these two large regional structures.
The geochronological study was carried out preparing the groundmass from selected fresh samples following the methods detailed in Guillou et al. (1998). The groundmass is assumed to have formed shortly after eruption and should not contain any significant excess argon. All the samples were crushed and sieved to 0.250–0.125-mm-size fractions and ultrasonically washed in acetic acid (1 N) for 45 min at a temperature of 60 °C to remove any secondary mineral phases that might be present in minute amounts. Phenocrysts and xenocrysts, which are potential carriers of extraneous 40Ar (including excess and inherited components), were filtered out using magnetic, gravimetric, and visual handpicking separation. All the samples were dated using the unspiked K-Ar technique described by Charbit et al. (1998). When compared to the 40Ar/39Ar method, this alternative method of conventional K-Ar dating has proven successful in dating subaerial volcanic rocks (Guillou et al., 2004; Ackert et al., 2003; Singer et al., 2004; Guillou et al., 2010). This technique differs from the conventional isotope-dilution method in that argon extracted from the sample is measured in sequence with purified aliquots of atmospheric argon at the same working gas pressure in the mass spectrometer. This allows mass discrimination effects between the atmospheric reference and the unknown to be suppressed, and allows amounts of radiogenic 40Ar* as small as 0.14% to be detected on a single-run basis (Scaillet and Guillou, 2004). The mass spectrometer sensitivity is 5.7 × 10−3 mol/A at m/e = 40 with amplifier backgrounds of 75 × 10−12 A at m/e = 40 (109 ohm resistor), and 5.75 10−14 A at m/e = 36 (1011 ohm resistor). The determination of K was carried out by atomic absorption with a relative precision of 1%. Argon was extracted by radio frequency heating of a 0.5–1.0 g sample in an ultrahigh-vacuum glass line and purified with Ti sponge and Zr-Al getters. Isotopic analyses were performed on total 40Ar contents ranging between 2.0 × 10−11 and 2.0 × 10−10 moles using a 180°, 6-cm-radius mass spectrometer with an accelerating potential of 620 V. The spectrometer was operated in static mode, but its volume was varied to give equal 40Ar signals for the air aliquots and the samples. Beam sizes were measured simultaneously on a double Faraday collector in sets of 100 online acquisitions with a 1 s integration time. The atmospheric correction was monitored via separate measurements of atmospheric argon for each sample. Periodic cross-calibration of zero-age standards precisely constrained the mass discrimination to within ±0.5‰ on the 40Ar/36Ar ratios (Scaillet and Guillou, 2004). A manometrically calibrated dose of atmospheric argon was used to convert beam intensities into atomic abundances. This manometric calibration is based on periodic, replicate determinations of international dating standards of known K-Ar age using the same procedure for the unknowns as described in Charbit et al. (1998). This allows the total 40Ar content of the sample to be determined with a precision of about ±0.2% (2σ). Standards used include LP-6 (127.8 ± 0.7 Ma; Odin, 1982, 1995) and HD-B1 (24.21 ± 0.32 Ma; Fuhrmann et al., 1987; Hess and Lippolt, 1994; Hautmann and Lippolt, 2000). At the 95% confidence level, the values adopted here are consistent with those obtained for several 40Ar/39Ar standards through the intercalibration against biotite GA-1550 by Renne et al. (1998) and Spell and McDougall (2003). Uncertainties for the K and Ar data are 1σ analytical only, and consist of propagated and quadratically averaged experimental uncertainties arising from the K, 40Ar (total), and 40Ar* determinations. Uncertainties on the ages are given at 2σ.
In principle, the most critical uncertainty in the K-Ar method derives from the fact that it is not possible to verify the isotopic composition of the initial argon in the sample. That is, we cannot check the assumption that, at the time of its formation, its 40Ar/36Ar ratio was the modern atmospheric value (295.5). As a result, the analytical errors given in Table 2 may in principle be smaller than the true error.
This work was carried out in the framework of the scientific convention between Pisa (Italy) and Salta (Argentina) Universities. The research was supported by the Consejo Nacional de Investigaciones Cientificas y Técnicas (CONICET, Argentina, grant 5341) and Consejo de Investigaciones de la Universidad Nacional de Salta (CIUNSA, Argentina, grant 20/D-249). The authors thank two anonymous reviewers for their insightful comments and J. Pelletier for the editorial handling.