Origin and provenance of igneous clasts from late Palaeozoic conglomerate formations (Del Raton and El Planchon) in the Andean Precordillera of San Juan, Argentina.
1. Introduction and aims of this studyIn this work we investigate the age and origin (provenance) of late Palaeozoic conglomerate formations of central-western Argentina. Conglomerates are sediments closely related to source areas and frequently linked with tectonically active basin margins. The investigation of such rocks, when formed by igneous clasts, permits direct geochronology and petrology studies that help improve the stratigraphic knowledge and location of possible source areas by comparing igneous clasts with known igneous rocks nearby.
The late Palaeozoic sequences of central-western Argentina are the most complete stratigraphic record for this time in South America (Limarino and Spalletti, 2006; Astini et al., 2011). This is one of the few regions of Gondwana with a continuous fossil record from early Carboniferous to Permian, including abundant plant remains, palynomorphs and invertebrates, widely represented in the Rio Blanco, Calingasta-Uspallata, San Rafael and Paganzo basins (Cesari et al., 2011 and references therein). However, the age of these sequences established by palynofloras, macrofloras, and marine faunas remains under discussion in some areas due to the absence of species having worldwide biochronological value and the scarcity of radiometric ages (Cesari et al., 2011).
Thick sequences of early Carboniferous rocks (Mississippian) crop out along the Western Argentine Precordillera (Fig. 1a, b). This stratigraphic interval is often poorly represented in West Gondwana (Limarino and Spalletti, 2006; Limarino et al., 2006). Within these sequences the Angualasto Group represents the remains of much larger deposits that once occupied most of the Rio Blanco (to the north of study area) and Calingasta-Uspallata basins (Fig. 1b) (Limarino and Cesari, 1993). In these arc-related basins, the early Carboniferous deposits are considered synorogenic sequences of the Chanic orogeny (Limarino and Spalletti, 2006; Limarino et al, 2006; Heredia et al., 2012; Limarino et al., 2012). This Late Devonian-early Carboniferous orogeny has been ascribed to the docking of the Chilenia terrane to southwestern Gondwana margin, formed by the previously accreted Cuyania terrane (Fig. 1a) (Ramos, 1988).
The Angualasto Group, defined by Limarino and Cesari (1993), includes the Maliman and Cortaderas formations in the Rio Blanco basin, north of the Rio Jachal, and the Del Raton Formation in the Calingasta-Uspallata basin, outcropping in the vicinity of the Rio San Juan within the study area (Fig. 1b). The Del Raton Formation is stratigraphically equivalent (Sessarego and Cesari, 1986) or partially equivalent (Azcuy et al., 2000) to the Maliman Formation. The age attributed to the Maliman Formation and equivalent units, from invertebrates and palaeoflora, is late Tournaisian-early Visean; the Cortaderas Formation is late Visean, based on palynomorphs (references in Cesari et al., 2011).
Despite the importance of these deposits, the age and stratigraphic relationships between the Del Raton Formation and other late Palaeozoic units, El Planchon and Del Salto formations, is very controversial. In this work we provide a U-Pb zircon age from an igneous clast in the conglomeratic lower section of the Del Raton Formation, which establishes the maximum age of deposition, thus confirming the fossil-based age previously assigned to this formation and the start of synorogenic deposits of the Chanic orogeny in this region. We also undertake a detailed geochemical study of igneous clasts from conglomerates of the Del Raton and El Planchon formations and compare them with known igneous complexes. This provenance study indicates substantial source differences between these formations.
2. Geological framework
2.1. General overview
The Argentine Precordillera forms the northern part of Cuyania, one of the larger terranes accreted to the southwestern Gondwana margin during the Palaeozoic (Ramos et al., 1986) (Fig. 1a). This terrane has been the focus of intense research to unravel its palaeogeographic links usually interpreted in terms of its accretion to southwestern margin of Gondwana (1, in Fig. 1a) in early Palaeozoic times (Ramos et al., 1986; Ramos, 1988; Astini et al., 1995; Dalziel, 1997; Thomas et al., 2002; Thomas and Astini, 2003; Finney, 2007) and the later accretion of the Chilenia terrane against it (2, in Fig. 1a) in Late Devonian-early Carboniferous times (Ramos et al., 1984, 1986).
The Argentine Precordillera is a fold-and-thrust belt (Baldis and Chebli, 1969; Limarino et al., 2006; Limarino and Spalletti, 2006; Alonso et al., 2008; Ramos and Folguera, 2009; among others) about 80 km wide (Fig. 1b) formed by Palaeozoic and Tertiary sediments (Bracaccini, 1946; Heim, 1952), that according to stratigraphic and structural features, has been divided into Western, Central and Eastern domains (Fig. 1b). The Eastern and Central Precordillera represent a stable carbonate platform during Cambrian and Early Ordovician (Bordonaro, 1999). The Western Precordillera is characterised by Cambrian-Ordovician olistostrome or melange deposits related to extensional tectonics in a continent-ocean transition (Astini, 1997; Keller, 1999), and ocean floor-like sediments with pillow basalts in the westernmost part (Kay et al., 1984), indicating the existence of an ancient continental margin (e.g. Spalletti et al., 1989; Astini, 1997; Keller, 1999). This early Palaeozoic continental margin (2, in Fig. 1a) was affected by extension during Ordovician and remained stable until the Late Devonian (Alonso et al., 2008). Subsequently, the accretion of the Chilenia terrane against the western Cuyania margin generated the Late Devonian-early Carboniferous Chanic tectonic phase of the Famatinian orogenic cycle (Ramos et al., 1984, 1986) or Chanic orogeny (Heredia et al., 2012). This collision resulted in a complex deformation and low-grade metamorphism that affected mainly pre-Carboniferous rocks (Furque, 1979) of the Western Precordillera (Keller et al, 1993; Gosen, 1997; Davis et al., 2000). Chanic synorogenic deposits of the early Carboniferous Angualasto Group (Limarino and Cesari, 1993) in Western Precordillera overlie folded and cleaved rocks of Devonian age with a strong angular unconformity (Azcuy et al., 1981; Limarino and Cesari, 1993; Lopez Gamundi and Rossello, 1993; Alonso et al., 2008; Amenabar and di Pasquo, 2008; Colombo et al., 2012; among others).
2.2. Stratigraphic relations of the late Palaeozoic formations in the studied area.
South of the Rio San Juan (Fig. 2), Devonian rocks consist of a sequence of sandstones and shales denominated Codo Formation (Guerstein et al., 1965; Sessarego, 1988) unconformably underlying the Del Raton Formation (Azcuy et al., 1981; Lopez Gamundi and Rossello, 1993) of Angualasto Group (Limarino and Cesari, 1993). The Codo Formation has been tentatively dated as Givetian-Frasnian according its palynological assemblage (Amenabar and di Pascuo, 2008). The Del Raton Formation (Guerstein et al., 1965; Quartino et al., 1971) is a conglomeratic unit with subordinate sandstones and shales, divided in two cycles (De Rosa, 1983), three members (Sessarego and Cesari, 1988), and recently in two sections (Colombo et al., 2012). A fact highlighted by different authors is the presence of up to 60 % of igneous clasts in the conglomerates of this formation (Quartino et al., 1971; De Rosa, 1983; Tofalo et al., 1985; Sessarego et al., 1990), including granites, quartz monzonites, quartz syenites, syenites, rhyolites, rhyodacites and basaltic rocks (Sessarego et al., 1990). According to Colombo et al. (2012), the erosive surface over the Codo Formation is marked by a pavement of disordered and heterometric (30-50 cm) granitic clasts. Over this pavement, within the lower section, there are various metric conglomeratic layers with rounded or subrounded clasts of 3-5 cm in size, of whitish granites (70%), metamorphic rocks (20%) and sedimentary rocks (10%). Alternating with these conglomerates appear layers with clasts sizes of 30-40 cm. In the upper section, and above an erosive discordance, matrix-supported reddish conglomerates include clasts of sandstones and greywacke (65%), pinkish granites and rhyolites (30%), quartz and metamorphic rocks (5%). In this upper section there are also disordered and poorly sorted conglomerates with very coarse clasts (40-50 cm) of reddish granites and rhyolites. The age assigned to the Del Raton Formation is Tournaisian-Visean, based on fossiliferous assemblages (Scalabrini Ortiz, 1973; Sessarego and Cesari, 1988; Cesari and Gutierrez, 2001), or early Visean from palynological data (Amenabar and di Pascuo, 2008).
Other late Palaeozoic deposits cropping out south of the Rio San Juan are the El Planchon and Del Salto formations (Fig. 2). The El Planchon Formation (Quartino et al., 1971; Sessarego, 1983, 1988) consists of shales and sandstones which grade laterally into conglomerates (Colombo et al., 2012). Its stratigraphic relationships with the Del Raton and Del Salto formations are very controversial and the age remains undetermined because the El Planchon Formation is palynologically barren (Amenabar and Di Pascuo, 2008). Some authors proposed a Devonian age based on marine fossils (Kerllenevich, 1967), suggesting that it would be stratigraphically below the Del Raton Formation (Sessarego, 1983) or in fault contact with it (Amenabar and Di Pascuo, 2008). Others consider that the El Planchon Formation rests unconformably over the Del Raton Formation and constitutes the lower part of the Del Salto Formation (Quartino et al., 1971; Alonso et al., 2005). However according to Colombo et al. (2012), the El Planchon Formation is overlain unconformably by the Del Salto Formation (Fig. 2). The age proposed for the Del Salto Formation is late Carboniferous (Pennsylvanian)-early Permian based on marine fossils (Azcuy et al., 2007). The Del Salto Formation represents the synorogenic sequences of late Carboniferous-early Permian Gondwanan orogeny (Colombo et al., 2012). Pre-orogenic late Carboniferous (Pennsylvanian) deposits related to this orogeny are absent in this area, and in most of the Western Precordillera. The absence of most late Carboniferous sediment record is explained because the pre-Precordillera (Proto-Precordillera) probably formed a horst-like topographic high inherited from the Chanic cordillera (Limarino and Spalletti, 2006; Heredia et al., 2012). Therefore, the El Planchon conglomerate Formation could belong to the early Carboniferous (Mississippian) deposits of Angualasto Group synorogenic with the Late Devonian-early Carboniferous Chanic orogeny.
3. Samples and analytical techniques
For this study a set of 36 samples were collected south of the Rio San Juan, between the 114 and 118 km markers on the RN 20 road, near of Calingasta (Fig. 2). Most of the samples (31) correspond to igneous clasts from the conglomerates of the Del Raton Formation cropping out at the Quebrada Km 117 valley. In this formation we have studied two different conglomerate layers. One of these is a disordered and poorly sorted boulder conglomerate formed by very coarse clasts (up to 30-50 cm) of reddish or pinkish acid-intermediate igneous rocks, and smaller dark-coloured clasts of basic igneous rocks (Fig. 3a). The other conglomerate layer studied in this formation, and located above, is a poorly sorted cobble-pebble conglomerate formed by clasts smaller than 15 cm within a micro-conglomeratic matrix. Clasts are of whitish acid-intermediate igneous rocks, basic igneous rocks, and in smaller proportion, of sedimentary and metamorphic rocks (Fig. 3b). For comparative purposes, we also collected 5 representative samples in a conglomerate layer from the upper part of the El Planchon Formation, at the Quebrada del Salto valley (Fig. 2). This conglomeratic layer is 2 metres thick, poorly sorted, formed by clasts up to 15 cm within a sand-mudstone matrix. All the clasts are dark-coloured mafic igneous rocks.
3.1. Major and trace element analyses
From the total sample set, 17 representative igneous clasts (boulders) were selected for major and trace element analyses. Major and some trace elements (V to Pb) were analysed by X-ray fluorescence (XRF) in the Technical-Scientific Services of Oviedo University (Spain) using a WD-XRF spectrometer (model 2404; PANalytical) coupled with a Rh tube. Major element analyses were performed using glass beads of powdered rocks after fusion with lithium tetraborate. Precision of the XRF technique was better than [+ or -] 1% relative. Trace elements were determined on pressed pellets with Elvacite. Raw data were processed using Pro-Trace-XRF PANalytical software. Other trace elements (U, Th, Hf, Ta) and rare earth elements (REE) were analysed by inductively coupled plasma mass spectrometry (ICP-MS) following sample decomposition with lithium metaborate at the Geochronology and Geochemistry-SGIker facility of El Pais Vasco University/EHU (Spain) (see Garcia de Madinabeitia et al., 2008 for additional details).
3.2. U-Pb ICP-MS isotopic analyses
An igneous clast of the lower conglomerates layers from the Del Raton Formation (sample AN47) was processed for zircon separation and U-Pb geochronology. Rock pulverization and mineral separation using a Wilfley table, heavy liquids, and a Frantz isodynamic separator were performed at University of Oviedo (Spain). The selected zircon fractions were hand picked under a binocular microscope. The zircon mount was prepared using double-sided tape, a plexiglass ring, and Buehler Epoxicure resin. BSE and CL images of the individual grains were obtained with the Cameca SX100 electron microprobe of Oviedo University to assess the internal morphology before carrying out the U-Pb laser work.
Zircon U-Pb analyses were carried out at Johann Wolfgang Goethe-University Frankfurt/JWG (Germany) using a Thermo-Finnigan Element II SF-ICP-MS coupled to a New Wave UP213 ultraviolet laser system. Laser spot-sizes varied from 20 to 40 pm for zircon. The typical depth of the ablation crater was ~20 pm. Data were acquired in peak-jumping mode over 900 mass scans during 20 s background measurement followed by 32 second sample ablation. A teardrop-shaped, low volume laser cell was used to enable the precise detection of heterogeneous material (e.g., inclusions or different growth zones) during time resolved data acquisition (see Janousek et al, 2006).
Laser-induced elemental fractionation and instrumental mass discrimination were corrected by normalization to the reference zircon GJ-1 (Jackson et al., 2004). Prior to this normalization, the change of elemental fractionation (e.g., the Pb/Th and Pb/U ratios as a function of ablation time and thus crater depth) was corrected for each set of isotope ratios (c. 40) collected during the time of each single spot analysis. The correction was done by applying a linear regression through all measured ratios. The total offset of the measured drift-corrected [sup.206]Pb/[sup.238]U ratio from the "true" ID-TIMS value of the analysed GJ-1 grain was about 3-4%. Reported uncertainties (2[sigma]) were propagated by quadratic addition of the external reproducibility (2 s.d.) obtained from the standard zircon GJ-1 (n = 20; 1.3% and 1.2% for the [sup.207]Pb/[sup.206]Pb and 206Pb/238U, respectively) during the analytical session and the within-run precision of each analysis (2 s.e.). For further details on analytical protocol and data processing for the U-Pb method see Gerdes and Zeh (2006, 2009).
4. Petrography
A set of 36 thin sections of igneous clasts from the conglomerate layers of the Del Raton and El Planchon formations were studied. The petrographic classification of the igneous clasts is just an approximation because it is impossible to know if they were part of plutonic, subvolcanic or volcanic igneous complexes. The main petrographic features of individual clasts are summarised in Table 1.
4.1. Boulder conglomerate from the Del Raton Formation.
In this conglomerate (Fig. 3a), the studied clasts are plutonic and volcanic rocks, ranging in composition from acid to basic, although acid-intermediate compositions are prevalent. Samples were divided in two main groups: i) plutonic--volcanic rocks and ii) pyroclastic rocks. In most of the cases, but especially in the rocks of intermediate to acid composition, the rocks show moderate to severe hydrothermal alteration, with the development of potassic minerals (sericite, K-feldspar), prehnite, and carbonates accompanied by other secondary minerals (epidote, quartz, chlorite, titanite).
i) Plutonic-volcanic rocks. This group includes a wide variety of rocks ranging from gabbros and basalts to granites and rhyolites.
Gabbros. These are the least abundant rocks. Their texture is coarse- to medium-grained, porphyritic to ophitic and subophitic, with a doleritic matrix. The mineral assemblage includes clinopyroxene, plagioclase, opaque minerals, and smectites (probably pseudomorphs after olivine). Clinopyroxene is the most abundant phase (Fig. 4a), and constitutes phenocrysts in the porphyritic rocks, with sizes up to 4 mm. All these rocks show a moderate hydrothermal alteration that produced chlorite, smectites (mordenite), talc, titanite, actinolite, albite, quartz, and carbonates (Fig. 4b).
Basalts and basalt andesites. These are equigranular rocks, mainly with doleritic textures but in some cases with microlithic or fluidal microlithic textures. Their mineralogy is formed by plagioclase, amphibole, and biotite. The samples show a moderate hydrothermal alteration, with nearly complete replacement of mafic minerals by a secondary paragenesis dominated by chlorite and epidote, with hematite, quartz, and carbonates. One sample of basaltic andesite presents vugs larger than 3 mm filled by mordenite, chlorite, quartz, and carbonates.
Andesites. These are porphyritic and hypocrystalline rocks that occasionally host basic enclaves. The mineral assemblage is formed by plagioclase, biotite, quartz, and hematite. Plagioclase phenocrysts are larger than 8 mm and form > 40% of the rock (Fig. 4c). The groundmass is altered and replaced by a granoblastic mixture of quartz, plagioclase, and hematite, with grain sizes below 0.1 mm. The hydrothermal paragenesis of these rocks includes-chlorite, mordenite, albite, hematite, quartz, and carbonates, with minor epidote and titanite. Some of these minerals represent pseudomorphs after previous mafic minerals (Fig. 4c).
Granites. There are several samples of granitic clasts whose composition varies from quartz-syenites (alkali feldspar rich) to biotite-amphibole granodiorites and biotitic monzogranites. These rocks have pinkish to reddish colours, indicating hydrothermal alteration. The mineral assemblage includes K-feldspar, quartz, plagioclase, altered biotite, [+ or -] amphibole (Fig. 4d). The texture is medium to coarse-grained hypidiomorphic or allotriomorphic (2-10 mm crystal size). An important potassic alteration generated K-feldspar overgrowths and a decrease of quartz content. Other secondary minerals are carbonate, sericite, prehnite, titanite, chlorite, and minor epidote and hematite. In some samples is possible to recognize a sequence of alteration events. Initially, the rock developed pervasive potassic alteration where plagioclase was partially replaced by K-feldspar. This potassic alteration stage also produced quartz leaching, biotite replacement by chlorite ([+ or -] prehnite, [+ or -] titanite), and concentration of accessory minerals such as zircon and monazite. In a second stage, the rock underwent a process of light to moderate silicification (Fig. 4e); this produced re-precipitation of euhedral quartz filling voids. There is also evidence of infiltration and precipitation of feldspar in cracks and crystallization of chlorite and mordenite, covering the interior of the previous voids. Finally, a stage of carbonatization affected the rock, with the partial replacement of feldspar by calcite, and a complete fill of previous voids.
ii) Pyroclastic rocks. This group is formed by rocks whose composition ranges from andesite to rhyolite. Different textures are observed, from crystal-rich tuffs to vitriclastic tuffs.
Andesitic tuffs. This group includes a great diversity of textures and mineral assemblages. It varies from crystal-rich tuffs (20-70% of crystals) to vitreous-rich tuffs (>30% of vitriclasts) of andesitic composition. Crystals are mainly plagioclase, quartz, [+ or -] biotite. In some samples K-feldspar, amphibole or pseudomorphs of mafic minerals (probably olivine) were observed. All these rocks were affected by low to severe hydrothermal alteration that generated albite, sericite and quartz, with minor titanite, hematite, chlorite, and epidote [+ or -] carbonates.
Dacite-Rhyolite tuffs. These rocks are mainly crystal-rich tuffs, with K-feldspar, quartz, and minor plagioclase, biotite or amphibole. Lithic clasts are less abundant and are similar in mineralogy to the host rock. The groundmass was replaced by a granoblastic aggregate of quartz, sericite, and opaque minerals, but it is still possible to recognize ghosts of devitrification textures, such as perlitic, spherulitic and patchy structures or sintaxial growths over the crystals (Fig. 4f). Scarce samples of vitreous-tuffs occur, including microcrystalline sericite-quartz cineritic clasts.
4.2. Cobble-pebble conglomerate from the Del Raton Formation.
This conglomerate layer (Fig. 3b) is composed by clasts very similar in texture and composition to those in the conglomeratic unit described above. The samples selected correspond mainly to the micro-conglomeratic matrix between large clasts (< 15 cm). The same two groups of rocks described above also occur in this unit (plutonic-volcanic and volcaniclastic). The most abundant clasts are crystal-rich and vitreous-rich tuffs of light brown colour (Fig. 5a, b), with variable phenocrysts content, and evidence of glass hydration (devitrification) textures (perlitic, spherulitic, patchy textures, etc.). Less abundant, granitic and basaltic clasts (Fig. 5c, d) have textures, composition, and hydrothermal alteration similar to those described in the previous unit.
4.3. Cobble conglomerate from the El Planchon Formation.
In this conglomeratic layer one type of igneous clasts was found. These are gabbros/basalts, with different proportions of clinopyroxene, plagioclase, [+ or -] ilmenite, affected by hydrothermal alteration. All the clasts are dark-coloured and coarse-grained equigranular to porphyritic, although doleritic, ophitic and sometimes subophitic textures are observed (Fig. 5e, f). The phenocrysts of plagioclase or clinopyroxene are larger than 4 mm of long, while the groundmass is fine-grained and less than 0.8 mm. The studied samples record light to moderate hydrothermal alteration that generated amphibole, sericite, epidote, hematite, titanite, and in some cases talc, quartz, and carbonates.
5. U-Pb zircon age of the Del Raton Formation
A representative igneous clast sample ([approximately equal to] 50 cm in size) was selected for U-Pb isotopic analyses. This is a pinkish coloured medium- to coarse-grained holocrystalline rock of granitic composition (sample AN47), with a moderate hydrothermal alteration (Fig. 4e). Its mineralogy is composed by K-feldspar, plagioclase, and quartz, with minor biotite (replaced by chlorite [+ or -] prehnite [+ or -] titanite). Its textural features are dominated by secondary processes, with pervasive potassic alteration and less important carbonatization that are superimposed on the previous granitic texture (see Table 1).
In this study 29 isotopic analyses were obtained from 28 magmatic zircons (Table 2). The backscattered electron (BSE) images taken with electron microprobe show that the zircons are euhedral to subhedral short-prismatic crystals, with rhythmic concentric growth zoning parallel to crystal outlines (Fig. 6). Prior to isotopic analyses, zircons were classified following the method of Pupin and Turco (1972). These zircons fall into the S8 to L5 morphologies, mainly S4-S5, characteristic of rocks crystallized at low temperature (650-700 [degrees]C). Of the 29 analyses, 27 provide a Concordia age of 348 [+ or -] 2 Ma that was interpreted as the crystallization age of the granite clast (Fig. 6). This places the granite crystallization very close to the Tournaisian-Visean boundary, established at 346.3 Ma in a global Carboniferous chronostratigraphic time scale (Davydov et al., 2010) or 347 Ma in the Geological Time Scale (Walker et al., 2012). This age represents the maximum possible for deposition of the Del Raton Formation that is bound to be just at the end of the Tournaisian or more likely in the Visean, in agreement with palynological data (Amenabar and di Pascuo, 2008). This age also suggests that some of the granitic clasts incorporated into the Del Raton Formation conglomerates come from the erosion of early Carboniferous igneous rocks related to Chanic magmatism.
5.1. Comparison with ages from known igneous complexes
Within the Precordillera, the existence of igneous rocks of early Carboniferous age is restricted to dykes recognised in the Devonian Codo Formation, to the north of study area (Sessarego et al, 1990). These authors describe dykes of granodiorites, diorites, quartz monzonites, trachytes, basalts, andesites, and rhyolites, with a Rb-Sr age of 337 [+ or -] 10 Ma, related to Chanic magmatic activity.
To the E and NE of the Precordillera, minor but widespread Devonian to early Carboniferous igneous rocks are present in the Sierras Pampeanas (Fig. 1b) (Dahlquist et al., 2006; Grosse et al, 2009; among many others). This early Carboniferous magmatism is represented by A-type granites and syenogranites, alkaline S-type granodiorites to granites, and alkaline I-type tonalites to granites (Dahlquist et al., 2010; Alasino et al., 2012 and references in both) generated during crustal extension (Grosse et al., 2009), with U-Pb ages of 350-323 Ma (references in Alasino et al., 2012). Further north, Martina et al. (2011) describe an important early Carboniferous volcanic event related to coeval A-type granites of the Sierras Pampeanas and also generated in an extensional environment. This volcanism consists of calc-alkaline/A-type rhyolites similar in age (348-342 Ma) to the granitic clast of the Del Raton conglomerate.
To the W of the Precordillera, in the Andean Frontal Cordillera (Fig. 1b) there are no known early Carboniferous igneous rocks at this latitude. However, further south of the study area in the Frontal Cordillera of Mendoza, there are outcrops of calc-alkaline igneous rocks of Early Devonian-early Carboniferous age (Caminos et al., 1979; Gregori et al., 1996; Tickyj et al., 2009; Tickyj, 2011), for example the Pampa de las Avestruces granodiorite of Early Devonian age (Tickyj et al., 2009) and the Carrizalito Tonalite dated at 334 [+ or -] 17 Ma (K-Ar whole rock; Dessanti and Caminos, 1967). Also in this south sector (Cordon del Portillo) there is a plutonic association of gabbros and tonalites to granodiorites, and a volcanic sequence of andesites and dacites to rhyodacites and rhyolites (Polanski, 1972). One of these calc-alkaline rocks (Cerro Punta Blanca granodiorite) has been dated at 348 [+ or -] 35 Ma (Rb-Sr) and 337 [+ or -] 15 Ma (K-Ar) (Caminos et al., 1979). This calc-alkaline magmatism in the Frontal Cordillera of Mendoza has been interpreted as a magmatic arc (Tickyj, 2011), that could be associated with west-dipping subduction (Davis et al., 2000) prior to the accretion of the Chilenia terrane to the Precordillera (Cuyania terrane) and with crustal thickening during Chanic collision (Heredia et al., 2012).
6. Geochemistry
The geochemistry study of the igneous clasts in these conglomerates (Del Raton and El Planchon) might test the possibility of a genetic relationship among the different clasts. In this way we could judge whether the clasts were derived from a single igneous complex or from unrelated intrusives. Clast geochemistry is also an important tool to identify possible sources by comparison with known igneous complexes.
6.1. Rock classification
The studied conglomerates are formed by different igneous clasts, as previously shown in the field and petrography sections. The different composition of these clasts is also observed in their geochemistry (Table 3). The rock classification using immobile trace elements (Nb, Y) combined with major elements (Winchester and Floyd, 1977) defines these rocks as subalkaline basalts/gabbros (basic clasts), andesites, dacites/rhyodacites (intermediate clasts), and rhyolites/granites, comendites/pantellerites (acid clasts) (Fig. 7a). Regarding the two different conglomerates, the El Planchon conglomerate includes only basic clasts whereas the Del Raton conglomerate contains basic-intermediate-acid clasts.
In the Del Raton clasts, the Daly gap (Dickin et al., 1984) is observed in the lower abundance of intermediate clasts compared to the acid and basic ones. Some of the intermediate clasts from the Del Raton conglomerate had a slight alkaline (A-type) signature defined by their higher content in Zr+Nb+Ce+Y (Fig. 7b), but caution is necessary since many S-type peraluminous granitoids can also have such relatively high contents in these elements. Other geochemical parameters also indicate this alkaline signature for these samples (AN27, AN51): relatively high [Na.sub.2]O+[K.sub.2]O-CaO ([approximately equal to] 7-8) and [Fe.sub.2][O.sub.3]t/[Fe.sub.2][O.sub.3]t+MgO ([approximately equal to] 0.8) together with high Zr-saturation temperatures (T[degrees]C [approximately equal to] 850-900).
Regarding peraluminosity, the basic clasts are metaluminous but two groups can be established (Fig. 7c): the El Planchon basic clasts have the lowest values of ASI ([Al.sub.2][O.sub.3]/ [Na.sub.2]O+[K.sub.2]O+CaO in molar proportions [approximately equal to] 0.4-0.6) whereas the Del Raton basic clasts display higher and more variable ASI values ([approximately equal to] 0.55-0.88-0.95). Such high values (0.88-0.95) for these basic compositions are probably related to contamination with peraluminous crustal lithologies or/and caused by higher sub-surface/surface alteration.
Intermediate and acid clasts from the Del Raton conglomerate show variable ASI values, some of them are peraluminous to strongly peraluminous (AN52) while other clasts are metaluminous and metaluminous with elevated values of the agpaitic index ([Na.sub.2]O+[K.sub.2]O/[Al.sub.2][O.sub.3] in molar proportions) and felsic compositions. The reason for these high values is the high content of [Na.sub.2]O relative to [K.sub.2]O and [Al.sub.2][O.sub.3] (AN47-50-51 with [Na.sub.2]O wt.% of [approximately equal to] 5-7.8). These high [Na.sub.2]O contents produce high normative Ab values relative to normative Or and An giving a trondhjemitic signature to these rocks (Fig. 7d).
In order to compare the Del Raton and El Plachon basic clasts with known compositions from mafic igneous rocks located nearby we have included in some of the plots the composition of Late Ordovician basalts and gabbros from the Western Precordillera (Sierra del Tigre basalts and gabbros; data from Gonzalez-Menendez et al., 2013). As observed in the figures 7 b and c, the El Planchon basic clasts are similar to the mafic compositions of the Western Precordillera basalts and gabbros in Zr+Nb+Ce+Y contents, as well as in ASI and [Na.sub.2]O+[K.sub.2]O/CaO values. On the other hand, the basic clasts from the Del Raton conglomerate show higher ASI and [Na.sub.2]O+[K.sub.2]O/CaO values.
6.2. Geochemical variation trends
When all the samples are plotted in Harker diagrams some correlations can be observed: Si[O.sub.2] correlates well with Ti[O.sub.2] (Fig. 8a) and [Fe.sub.2][O.sub.3], but less with MnO, MgO, and CaO. Other major elements such as [Al.sub.2][O.sub.3], [Na.sub.2]O, [K.sub.2]O, and [P.sub.2][O.sub.5] show no correlation with Si[O.sub.2] (Fig. 8b, c). The elements [Na.sub.2]O and [K.sub.2]O are prone to alteration and hence mobile, which could explain the absence of correlations. On the other hand, when only intermediate and acid clasts are considered (Fig. 8b), decreasing [Al.sub.2][O.sub.3] correlates well with increasing Si[O.sub.2]. Correlations are also observed for some trace elements such as V, Sr (decrease with increasing Si[O.sub.2]), and trace element ratios such as Nb/La that increase slightly with increasing Si[O.sub.2] (Fig. 8d, f). Other trace elements show considerable scatter (Ba, Nb, Y, Zr, REE, Th) except for Rb and U, which have similar trends to Sr and [Al.sub.2][O.sub.3], decreasing with increase Si[O.sub.2]. All these trends could suggest an absence of petrogenetic relationship between the basic clasts and the intermediate-acid ones. The basic clasts fall away from the trends defined by the acid-intermediate ones for [Al.sub.2][O.sub.3], V, Sr, Rb, and U. There are also some differences in trace element ratios such as Nb/ La, or La/Sm. Fractionation vectors were generated by linear mixing calculations (Ragland, 1989) for the intermediate-acid clasts. A combination of Pl+Bt+Amp fractionation ([approximately equal to] 30%Amp + 40%Pl; 30%Bt) could explain part of the data such as the variations of [Al.sub.2][O.sub.3] and Ti[O.sub.2] with the Si[O.sub.2] (Fig. 8a, b) but other element variations such as [Na.sub.2]O and [K.sub.2]O (Fig. 8c) cannot be reproduced with these calculations (this could also be valid for some of the mentioned scattered trace elements). This lack of adjustment to simple differentiation processes could be due to the effect of alteration or to a lack of direct petrogenetic relationship.
6.3. Outstanding trace element features and REE data
Some trace element ratios and the REE contents can help to identify the existence or absence of petrogenetic links among the different clasts of these conglomerates. The La/ Nb ratio can be used to investigate the volcanic arc vs. non arc-derived sources for basic to intermediate igneous rocks (Gill, 1981). The studied rock clasts show some scatter in the La vs. Nb diagram but two groups can be established (Fig. 9a). Many of the Del Raton clasts plot in the volcanic arc settings with La/Nb > 2 values. The El Planchon basic clasts have La/Nb values < 2 and plot in the MORB field close to the Western Precordillera Late Ordovician basalts and gabbros (OIB/Within plate field). The Nb/La ratios compared with the Sr/Nd ones (Hawkesworth and Kemp, 2006) also show these differences: the basic clasts from the El Planchon conglomerate have Nb/La-Sr/Nd compositions close to primitive mantle and MORB while the ones from the Del Raton conglomerate plot close to upper and bulk crust values and also close to the field of continental arcs (Fig. 9b). Regarding REE, it is noticeable that the basic clasts from the Del Raton conglomerate have high La/Yb (> 5) and La/ Sm (> 2.5) ratios compared to those from the El Planchon basic clasts, which have similar REE patterns to those of the Late Ordovician Precordillera basalts and gabbros. The La/Sm vs. B (Mg+Ti+Fe) diagram (Fig. 9c) shows a correlation trend, defined by the Del Raton intermediate-acid clasts, of increasing La/Sm (LREE-MREE fractionation), with decreasing B (or with Si[O.sub.2] increase) possibly related to increasing concentration of LREE rich accessories typical of felsic melts (Bea, 1996). The basic clasts from the Del Raton conglomerate fall away from this trend suggesting an absence of petrogenetic relation to the intermediate-acid clasts.
The REE normalized patterns of the different clasts (Fig. 9d) show also the differences between the El Planchon basic clasts (smooth, low fractionated REE patterns, similar to those of tholeiites/enriched tholeiites, essentially without Eu negative anomalies) and the Del Raton basic clasts. The latter show contrasting patterns, some samples have fractionated patterns with relatively high La/Yb (6-8) but other samples (AN24) have much higher fractionation (La/Yb = 25) and similar La/Sm values to some of the Del Raton intermediate clasts. The Del Raton intermediate and acid clasts have enriched LREE, marked Eu negative anomalies, and nearly flat HREE. The difference between these two groups (intermediate and acid clasts) lies in the absolute lower REE contents, higher LREE fractionation, and decreasing middle-heavy REE in the acid clasts. The comparison with other early Carboniferous igneous complexes such as A-type granitoids from the Sierras Pampeanas (Dahlquist et al., 2010; Alasino et al., 2012), located to the E and NE, and also to calc-alkaline granitoids from the Frontal Cordillera (Gregori et al., 1996), located to the SW, shows differences in both REE contents and normalized patterns (Fig. 10): A-type granitoids have higher total REE contents (some with significantly higher HREE), lower La/Sm ratios (flatter REE normalized patterns) and stronger negative Eu anomalies. Calc-alkaline granitoids show lower total contents of REE, lower La/Sm (but more similar to the Del Raton clasts than A-type granitoids) and lower Eu anomalies.
6.4. Normalized trace elements and further comparisons with other magmatic units
Multi-element diagrams normalized to a primordial mantle composition (Sun and McDonough, 1989) were used for comparison between the studied conglomerates and possible igneous rock sources. As shown previously, the El Planchon basic clast compositions are different from the Del Raton basic clasts. Their mantle-normalized pattern is smooth showing low fractionation between large ion lithophile elements (LILE) and high field strength elements (HFSE). Only some negative anomalies in K and P and slightly high Rb contents break this nearly flat pattern (Fig. 10a). This geochemistry is quite similar to that of the Late Ordovician basalts and gabbros from the Western Precordillera (data from Gonzalez-Menendez et al., 2013). Some differences are the higher positive Ba and negative K anomalies and lower Rb contents of some of the Precordilleran basalts and gabbros (Fig. 10a).
The Del Raton basic clasts have much higher contents in Th, U, LREE, and negative anomalies in Nb-Ta, P, and Ti. Some of these basic clasts have a normalized pattern similar to the intermediate clasts (Fig. 10b). The Del Raton intermediate clasts have spiked mantle-normalized patterns with marked Nb-Ta, P, and Ti negative anomalies and Zr positive ones (Fig. 10b). This pattern is similar to that of the upper continental crust (but somewhat higher in REE contents). The Del Raton acid clasts (Fig. 10c) have normalized patterns similar to those of the intermediate clasts and display negative anomalies in Nb-Ta, P and Ti and positive ones in Zr, but also show negative anomalies in Sr and K in some samples. The acid clasts normalized abundances are very similar to those of the upper continental crust.
For comparison with known possible early Carboniferous igneous sources, A-type igneous rocks (Dahlquist et al., 2010; Alasino et al, 2012) from the Sierras Pampeanas (located to the E and NE) and representative calc-alkaline granitoids (Gregori et al, 1996) from the Frontal Cordillera (located to SW) were plotted on the multi-element diagrams (Fig. 10b, c). The selected A-type granitoids have marked negative anomalies in Ba, positive ones in Rb-Th-U, and very strong Nb-Ta troughs. These are the main differences with the clasts from the Del Raton conglomerate. In a more detailed comparison with the intermediate Del Raton clasts, A-type granitoids also have stronger negative Eu, Sr and Ti anomalies, and higher REE contents. The Frontal Cordillera calc-alkaline granitoids show a very similar pattern to the Del Raton clasts with only slightly higher Sr and lower HREE than the intermediate clasts, and higher Sr and deeper Nb-Ta trough than the acid clasts.
7. Discussion
7.1. Geochemical relationships among the different clasts
The petrography and geochemistry reveals that the basic clasts from the El Planchon conglomerate are different from the rest of the studied rocks. The geochemistry also indicates that the most probable source of the El Planchon basic clasts is the Late Ordovician basalts and gabbros of the Western Precordillera mafic belt (Haller and Ramos, 1984; Kay et al., 1984; Davis et al., 2000; Ramos et al., 2000; Gonzalez Menendez et al., 2013). Both have tholeiitic to transitional geochemistry probably related to extensional continental or continental-oceanic transitional settings (OIB/Withinplate/ MORB).
The basic clasts from the Del Raton conglomerate have a subduction-related geochemistry (Nb-Ta negative anomalies, elevated LILE/HFSE and La/Yb ratios, La/Nb >2) suggesting a provenance from a mantle arc source, or/and, from mantle-derived basalts contaminated with continental crust materials.
Intermediate clasts from the Del Raton conglomerate also have similar arc-related features and could have been derived by partial melting of mafic arc rocks. An alternative model could be that these intermediate rocks resulted from the magmatic differentiation of mafic arc magmas (crystal fractionation, crustal contamination). These supposed mafic precursors could be the basalt clasts mentioned above. The observed magmatic trends of the basic and intermediate rocks are substantially different for some elements (Al, Sr) but not for others (Ti, V, Mg), which seems to negate simple fractional crystallization or binary mixing.
Intermediate and acid clasts from the Del Raton conglomerate could be related by straight differentiation (fractional crystallization) processes. The observed trends in the Harker diagrams (Fig. 8) are continuous between both groups of rocks (intermediate and acid clasts). The fractionation vectors calculated by linear mixing show that coupled fractionation of Pl+Amp+Bt could explain the actual trends for most of the major elements (Al, Ti, Mg, Mn, Ca, K, Na). The amphibole fractionation could also explain the middle-heavy REE decreasing in the acid clasts (Fig. 9d).
7.2. Implications for the provenance of the clasts
The composition of the clasts from the Del Raton conglomerate and their comparison with the Frontal Cordillera calc-alkaline igneous rocks (Fig. 10) indicates a source area probably located along this Frontal Cordillera (Fig. 11a). In this N-S orientated range, the igneous calc-alkaline granitoids occur presently to the southwest of the Del Raton outcrops. These calc-alkaline complexes are Devonian to early Carboniferous in age and mostly consists of igneous rocks including gabbros, tonalites, granodiorites, granites, andesites, dacites, rhyodacites and rhyolites (Polanski, 1972; Caminos et al., 1979; Gregori et al., 1996; Tickyj, 2011). Such a provenance is in agreement with the observed similar geochemistry of the Del Raton clasts (Fig. 10b, c) and also with the U-Pb (~348 Ma) age obtained in one of these clasts. The geochemistry also indicates that the acid and intermediate clasts probably come from a single igneous batholithic complex. The basic clasts, given their calc-alkaline signature, could be either mafic intrusives forming part of the same batholithic complex as the intermediate and acid clasts or derived from different intrusive units in other domains of the Frontal Cordillera. Recent palaeocurrent research (Colombo et al., 2012) indicates that the main provenance source for the Del Raton clasts is from the Frontal Cordillera, from the west and northwest from its present outcrops. This contrasts with the absence of early carboniferous igneous rocks at this latitude. A possible solution is that such early Carboniferous calc-alkaline granitoids originally cropping out farther north in the Frontal Cordillera (Fig. 11a, b) have been oblitered by subsequent erosion and profuse Permo-Triassic plutonism (Colanguil batholith; Llambias and Sato, 1990, 1995; Sato et al., 1990) and volcanism (Choiyoi Group; Sato and Llambias, 1993; Llambias et al., 2003).
Another possible early Carboniferouos source for the Del Raton conglomerate could be calc-alkaline/A-type rhyolites, A-type granites and syenogranites, alkaline S-type granodiorites to granites, and alkaline I-type tonalites to granites, located to the N and NE in the Sierras Pampeanas (Dahlquist et al., 2010; Martina et al., 2011; Alasino et al., 2012) (Fig. 11). However, their compositions (Fig. 10b, c) differ from those of the Del Raton clats. The stratigraphic constraints of provenance from the W and NW (Colombo et al., 2012) also preclude such rocks as the source.
The petrography and geochemistry indicates that the sources of the El Planchon basic clasts are the Late Ordovician sedimentary formations (Alcaparrosa-Yerba Loca Formation) that according to Haller and Ramos (1984), Kay et al. (1984), Davis et al. (2000), and Ramos et al. (2000) host a significant volume of interlayered mafic volcanics and sills. This would indicate a provenance either from the north or south (in present coordinates). Possible source areas with the suitable basaltic compositions occur within 50 km (to both north and south). Northern provenance of these clasts (Fig. 11b) is the preferred hypothesis because it agrees with recent palaeocurrent studies (Colombo et al., 2012). Nevertheless, tholeiitic rocks of early Carboniferous age occur in southern locations of the Frontal Cordillera of Mendoza (Gregori et al., 1996). These rocks have similarities in composition to the El Planchon basic clasts and also to the Late Ordovician Precordillera mafic rocks and therefore cannot be discarded as possible sources once located along the Frontal Cordillera (Fig. 11b).
The fact that the El Planchon conglomerate contains much older clasts (probably Late Ordovician) compared to the ones from the Del Raton (early Carboniferous) has further significance since the Del Raton Formation underlies the El Planchon Formation (Quartino et al., 1971; Alonso et al., 2005; Colombo et al., 2012). The fact that the older conglomerate (Del Raton) includes younger clasts than the younger conglomerate (El Planchon) suggests that the first reliefs uplifted and eroded in the early Carboniferous (~348 Ma) where those of the Frontal Cordillera (Fig. 11a) as also indicated by Colombo et al. (2012). These clasts were transported eastwards and were deposited in the locations where the Del Raton Formation presently outcrops (i.e., western provenance). Afterwards, this sedimentary flux from the Frontal Cordillera was partially shut off, orogenic deformation was transferred towards the east (in the northern domains) and the Precordillera was probably uplifted (Proto-Precordillera block; Fig. 11b). The partial erosion of its early Palaeozoic formations produced the main clastic component delivery towards the south that formed the El Planchon conglomerate (northern provenance). This scenario suggests that the main uplift of the Frontal Cordillera domain was followed by uplifts farther east during the early to late Carboniferous period.
8. Conclusions
The Del Raton conglomerate igneous clasts are basic, intermediate and acid rocks with calc-alkaline geochemical signatures. Laser ablation has yielded a U-Pb zircon age of 348 [+ or -] 2 Ma (late Tournaisian), interpreted as the maximum deposition age of this conglomerate formation. The Del Raton clasts are similar in petrography and geochemistry to some early Carboniferous calc-alkaline complexes of the Frontal Cordillera suggesting provenance from the west or northwest. Frontal cordillera sources could have had one source location (a single igneous batholithic complex accounting for basic-intermediate-acid clasts) or varied ones (accounting for the basic clasts and for the intermediate-acid clasts).
The basic igneous clasts from the El Planchon conglomerate are different from those of the Del Raton. Their petrology and geochemistry is that of tholeiites from extensional continent-oceanic transition or intraplate settings without any arc signature. The similarities with Late Ordovician mafic igneous rocks of the western Precordillera suggest a provenance from the erosion of early Palaeozoic formations that outcrop in the Argentine Precordillera (north provenance). These formations usually contain abundant mafic volcanic and subvolcanic rocks (i.e. Late Ordovician Alcaparrosa-Yerba Loca Formation). Other possible sources along the Frontal Cordillera cannot be discarded but their clastic input was probably minor.
The erosional events that delivered the clasts of the Del Raton and El Planchon formations were probably related to mountain range uplift episodes during the early (~348 Ma) Carboniferous period. Uplift seems to have migrated from the Frontal Cordillera domain (producing clasts transported eastwards to generate the Del Raton conglomerate) eastwards to the Precordillera (shifting to clasts transported mainly from the north, generating the El Planchon conglomerates).
http://dx.doi.org/10.5209/rev_JIGE.2014.v40.n2.45298
Acknowledgments
We greatly appreciate the review of R.J. Pankhurst for their constructive comments, and English corrections, which have improved the original manuscript. We thank C.O. Limarino, S.N. Cesari, F. Colombo, N. Heredia and an anonymous reviewer for their suggestions. Catalina Suarez is thanked for producing maps figures. Financial support was provided by CGL2006-12415-C03 and CGL2009-13706-C03 projects (I+D+i Spanish Programmes) and FEDER Funds of the EU.
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G. Gallastegui (1) *, L. Gonzalez-Menendez (2), A. Rubio-Ordonez (3), A. Cuesta (3), A. Gerdes (4)
(1) Instituto Geologico y Minero de Espana (IGME). C/Matematico Pedrayes 25, 33005 Oviedo, Spain.
(2) Instituto Geologico y Minero de Espana (IGME). C/Real 1, 24006 Leon, Spain.
(3) Departamento de Geologia, Universidad de Oviedo. C/ Jesus Arias de Velasco, s/n, 33005 Oviedo, Spain.
(4) Institut fur Geowissenschaften Mineralogie, Abt. Geochemie & Petrologie, Altenhoferallee 1, D-60438 Frankfurt am Main, Germany.
e-mail addresses: [email protected] (G.G., *corresponding author); [email protected] (L.M.); [email protected] (A.R.); [email protected] (A.C.); [email protected] (A.G.)
Received: 2 October 2013 / Accepted: 5 May 2014 / Available online: 25 June 2014
Table 1.-Main petrographic features of the igneous clasts from the
Del Raton (RaFm) and El Planchon (PlaFm) conglomerate formations.
Sample Fm Rock type Texture
ANC1 RaFm Dacitic vitric-tuff
(+C+Lc)
ANC10 RaFm Granite/ Qz-syenite Allotriomorphic
ANC3 RaFm Greywacke
ANC4 RaFm Gabbro Subophitic
ANC5 RaFm Gabbro Ophitic
ANC6 RaFm Dacitic vitric-tuff
(+C+Lc)
ANC7 RaFm Rhyolitic ash-tuff
ANC8 RaFm Microgabbro Ophitic
ANC9 RaFm Bt-Granite Hypidiomorphic
AN47 RaFm Bt-Granite Hypidiomorphic
AN48 RaFm Basalt Doleritic
AN49 RaFm Dacitic crystal-rich
tuff (+Vc)
AN50 RaFm Dacitic/Rhyolitic
crystal-rich tuff (+Vc)
AN51 RaFm Andesite Porphyritic
[AN51.sub.enc] RaFm Basaltic andesite Doleritic
AN52A RaFm Dacitic vitric-tuff
(-15% C)
AN52B RaFm Dacitic vitric-tuff
(-15% C)
AN20 RaFm Andesite Doleritic
AN21A RaFm Andesite Doleritic
AN21B RaFm Bt-Granite Hypidiomorphic
AN22 RaFm Monzogranite Allotriomorphic
AN23 RaFm Granodiorite Hypidiomorphic
AN24 RaFm Basaltic andesite Trachytic
AN25 RaFm Andesitic
crystal-rich tuff
AN26 RaFm Dacitic crystal-rich tuff
AN27 RaFm Andesitic/Dacitic
crystal-rich tuff
AN20A PlaFm Gabbro/basalt Doleritic
AN20B PlaFm Gabbro/basalt Ophitic
AN20C PlaFm Gabbro/basalt Ophitic
AN20D PlaFm Gabbro/basalt Doleritic
AN20E PlaFm Gabbro/basalt Subophitic
Sample Main mineralogy
ANC1 Pl+Qz+Afs+(Bt)
ANC10 Afs+Qz+(Bt)
ANC3 Qz+Pl+Ms/Chl+(Opq)
ANC4 Pl+Cpx+Ilm
ANC5 Cpx+Pl+Ilm+(Ol?)
ANC6 Qz+Pl+Afs+(Bt+Amp)
ANC7
ANC8 Cpx+Pl+Opq
ANC9 Qz+Afs+Pl+Bt+Opq
AN47 Afs+Pl+Qz+(Bt)
AN48 Pl+(Amp+Opq)
AN49 Pl+Qz+(Bt)
AN50 Pl+Afs+Qz+(Bt)
AN51 Pl+Bt+Qz+Hem
[AN51.sub.enc] Pl+(Amp/Bt)+Vesc
AN52A Pl+Qz+(Bt)
AN52B Pl+Qz+(Bt+Amp?)
AN20 Pl+Qz+(Amp?+Bt?)
AN21A Pl+Afs+Qz+(Bt+Amp)
AN21B Afs+Pl+Qz+(Bt+Amp)
AN22 Afs+Qz+Pl+Bt
AN23 Afs+Qz+Pl+Bt+Amp
AN24 Pl+(Bt)
AN25 Pl+Afs+(Bt+Amp+Ol?)
AN26 Afs+Qz+Pl+(Bt)
AN27 Pl+Bt+Qz
AN20A Pl+Cpx+Ilm+(Ol?)
AN20B Cpx+Ilm+(Pl)
AN20C Cpx+Pl+Ilm
AN20D Cpx+Pl+Ilm
AN20E Cpx+Pl+Ilm
Sample Dominant secondary phases
ANC1 Ab+Ser+Qz+(Chl,Hem,Ttn).*Cb+Qz
ANC10 kfs+Qz+Prh+Ttn>< Cb
ANC3 Cb
ANC4 Ab+Amp+Chl+Sme+Tlc+(Ttn,Hem,Qz).*Cb
ANC5 Amp+Ab+Ep+Tlc+Sme+Hem+Ttn+(Chl)
ANC6 Hem+Qz+Chl+(Ttn)
ANC7 Ser+Qz
ANC8 Ep+Hem+(Chl,Amp,Ttn)
ANC9 Qz+Ab+Ser+(Ep,Kfs,Chl)><Cb+(Qz)
AN47 Ser+Kfs+Qz+Chl+Cb
AN48 Hem+Chl+Ep+Qz+Cb
AN49 Qz+Chl+Ser+(Hem). *Cb+Chl+Qz
AN50 Ser+Qz +Hem+(Chl,Ep,Ttn). *Cb+Qz
AN51 Ab+Chl+Hem+Qz+(Ep,Ttn). *Cb
[AN51.sub.enc] Ab+Chl+Ep. *Mor+Chl+Qz+Cb
AN52A Ser+(Ttn)+Qz
AN52B Ser+(Ttn)+Qz
AN20 Chl+Hem+Ttn+Sme+Qz. *Cb
AN21A Ser+Qz+Chl+Ep+Ttn. *Cb
AN21B Ser+Kfs+Qz+(Hem,Ttn). *Cb
AN22 Qz+Ab+Ser+(Ep,Kfs,Chl). *Cb+(Qz)
AN23 Qz+Ab+Ser+(Ep,Kfs,Chl)+Ttn. *Cb+(Qz)
AN24 Ep+Chl+Ttn+Ab+Qz. *Cb
AN25 Chl+Hem+Ttn+Ep+Qz+(Amp,Ap,Afs)
AN26 Chl+Qz+Ser+Ttn+(Ep,Hem)
AN27 Chl+Qz+Ser+Ttn+(Ep,Hem)
AN20A Amp+Ser+Ep+Ttn+(Chl)
AN20B Amp+Ser+Ep+Ttn+(Chl)
AN20C Amp+Ser+Ep+Ttn+(Chl,Tlc)
AN20D Amp+Chl+Ep+Qz+Ttn[+ or -]Hem+Cb
AN20E Amp+Chl+Ser+Ep+(Qz)
C: Crystals. Lc: Lithic clasts. Vc: Vitriclasts. Enc: Enclave. In
parentheses: minerals in very low %. (*): Minerals related with a
later alteration
Table 2.-Results of U-Pb (LA-ICP-MS) zircon isotopic analyses of a
medium-to coarse-grained Bt-granite clast (sample AN47) from the
lower unit conglomerates of the Del Raton Formation.
[sup.207]Pb (a) U (b) Pb (b) Th (b) [sup.206]Pb
grain L-No. (cps) (ppm) (ppm) U [sup.204]Pb
2 a22 5287 282 19 1,81 6674
3 a23 943 53 3,3 0,83 4796
4 a24 2576 145 9,1 0,68 19678
8 a25 5418 301 19 1,41 3778
9-1 a26 4256 244 15 0,80 6397
9-2 a27 8486 465 30 0,92 10478
11 a28 2443 136 8,7 1,19 2543
12 a29 1405 77 4,9 1,52 10659
13 a31 5989 315 20 1,84 6043
17 a32 6545 382 24 1,11 51365
18 a33 3089 169 11 1,49 23416
20 a1 1719 100 6,3 0,96 6654
21 a2 2703 157 9,8 1,03 7837
23 a3 3086 182 11 0,95 9045
28 a5 8483 494 31 1,09 22129
30 a6 2326 135 8,5 1,49 8913
31 a7 3838 205 13 1,30 2977
32 a8 2321 134 8,5 1,39 1484
35 a9 6205 360 23 1,01 15170
38 a10 4757 281 18 1,06 1223
40 a11 7978 434 29 1,11 10637
43 a12 1094 67 4,1 0,79 4219
45 a13 3615 187 12 0,54 2780
48 a14 1484 89 5,7 1,19 5874
49 a15 5583 322 21 1,03 3131
52 a16 1581 96 6,0 1,26 6110
53 a17 2183 128 8,2 0,93 8320
54 a18 1362 84 5,3 1,05 5311
55 a19 6870 413 26 0,91 7615
[sup.206]Pb (c) [+ or -] 2[sigma] [sup.207]Pb (c)
grain [sup.238]U (%) [sup.235]U
2 0,05806 1,8 0,4265
3 0,05530 2,1 0,4042
4 0,05503 1,8 0,4061
8 0,05493 2,1 0,4097
9-1 0,05539 1,8 0,3979
9-2 0,05619 1,9 0,4160
11 0,05584 2,3 0,4102
12 0,05608 1,9 0,4168
13 0,05647 2,6 0,4085
17 0,05445 2,2 0,3911
18 0,05612 1,8 0,4173
20 0,05534 2,2 0,4027
21 0,05470 2,1 0,4045
23 0,05498 2,1 0,4013
28 0,05511 2,1 0,4071
30 0,05556 2,1 0,4083
31 0,05529 2,6 0,4039
32 0,05538 2,3 0,4135
35 0,05652 2,2 0,4125
38 0,05483 2,3 0,4054
40 0,05917 2,5 0,4410
43 0,05396 2,8 0,3940
45 0,05524 2,3 0,4109
48 0,05674 2,4 0,4037
49 0,05618 2,3 0,4199
52 0,05470 2,1 0,3987
53 0,05634 2,3 0,4164
54 0,05493 2,3 0,3969
55 0,05501 2,1 0,4069
[+ or -] 2[sigma] Rho (d) [sup.208]Pb (c)
grain (%) [sup.232]Th
2 3,0 0,59 0,01765
3 8,5 0,25 0,01685
4 3,4 0,53 0,01713
8 4,3 0,49 0,01709
9-1 3,4 0,54 0,01752
9-2 2,8 0,69 0,01752
11 4,9 0,47 0,01761
12 4,4 0,42 0,01726
13 8,7 0,30 0,01646
17 3,6 0,60 0,01687
18 3,9 0,47 0,01691
20 4,2 0,52 0,01719
21 4,4 0,47 0,01721
23 4,6 0,46 0,01782
28 3,0 0,69 0,01714
30 4,5 0,47 0,01706
31 5,3 0,49 0,01732
32 4,2 0,54 0,01740
35 3,7 0,59 0,01780
38 8,6 0,26 0,01717
40 4,0 0,63 0,01890
43 7,1 0,39 0,01724
45 7,3 0,32 0,01716
48 4,9 0,48 0,01735
49 4,0 0,56 0,01696
52 4,7 0,44 0,01699
53 4,3 0,52 0,01724
54 5,0 0,47 0,01723
55 3,4 0,61 0,01734
[+ or -] 2[sigma] [sup.207]Pb (c) [+ or -] 2[sigma]
grain (%) [sup.206]Pb (%)
2 1,8 0,05328 2,4
3 5,0 0,05301 8,3
4 2,6 0,05352 2,9
8 2,7 0,05410 3,7
9-1 2,2 0,05210 2,8
9-2 2,0 0,05369 2,0
11 2,6 0,05328 4,3
12 2,8 0,05390 4,0
13 7,0 0,05247 8,3
17 7,8 0,05210 2,9
18 2,3 0,05393 3,4
20 2,7 0,05278 3,6
21 2,8 0,05364 3,9
23 2,8 0,05294 4,1
28 2,4 0,05358 2,2
30 2,2 0,05330 4,0
31 3,0 0,05299 4,6
32 3,0 0,05415 3,6
35 2,9 0,05293 3,0
38 3,3 0,05363 8,3
40 3,4 0,05406 3,1
43 3,1 0,05296 6,6
45 3,5 0,05395 6,9
48 2,5 0,05160 4,3
49 3,1 0,05420 3,3
52 2,8 0,05286 4,3
53 2,6 0,05360 3,7
54 2,9 0,05240 4,4
55 2,4 0,05364 2,7
Age (Ma)
[sup.206]Pb [+ or -] 2[sigma] [sup.207]Pb
grain [sup.238]U (Ma) [sup.235]U
2 364 6 361
3 347 7 345
4 345 6 346
8 345 7 349
9-1 348 6 340
9-2 352 7 353
11 350 8 349
12 352 6 354
13 354 9 348
17 342 7 335
18 352 6 354
20 347 7 344
21 343 7 345
23 345 7 343
28 346 7 347
30 349 7 348
31 347 9 345
32 347 8 351
35 354 7 351
38 344 8 346
40 371 9 371
43 339 9 337
45 347 8 350
48 356 8 344
49 352 8 356
52 343 7 341
53 353 8 353
54 345 8 339
55 345 7 347
[+ or -] 2[sigma] [sup.208]Pb [+ or -] 2[sigma]
grain (Ma) [sup.232]Th (Ma)
2 9 354 6
3 25 338 17
4 10 343 9
8 13 343 9
9-1 10 351 8
9-2 8 351 7
11 15 353 9
12 13 346 9
13 26 352 9
17 10 330 23
18 12 338 26
20 12 344 9
21 13 345 10
23 13 357 10
28 9 344 8
30 13 342 8
31 16 347 10
32 13 349 10
35 11 357 10
38 25 344 11
40 13 378 13
43 21 345 11
45 22 344 12
48 14 348 9
49 12 340 10
52 14 341 9
53 13 346 9
54 14 345 10
55 10 347 8
[sup.207]Pb (c) [+ or -] 2[sigma]
grain [sup.206]Pb (Ma)
2 341 55
3 329 187
4 351 66
8 375 84
9-1 290 65
9-2 358 45
11 341 98
12 367 90
13 325 104
17 306 189
18 290 66
20 319 82
21 356 87
23 326 93
28 354 50
30 341 90
31 328 105
32 377 80
35 326 68
38 355 187
40 374 71
43 327 149
45 369 155
48 268 99
49 380 75
52 323 97
53 354 84
54 303 100
55 356 61
Diameter of laser spot = 30[micro]m; depth of crater ~15-20 [micro]m.
(a) Within run background-corrected mean [sup.207]Pb signal in counts
per second. (b) U and Pb content and Th/U ratio were calculated
relative to GJ-1 reference (LA-ICP-MS values, Gerdes, unpublished).
(c) corrected for background, common Pb and within-run Pb/U
fractionation and subsequently normalised to GJ-1 (ID-TIMS value/
measured value). [sup.207]Pb/[sup.235]U calculated using
[sup.207]Pb/[sup.206]Pb/([sup.238]U/[sup.206P]bx1/137.88).
Uncertainties propagated following Gerdes & Zeh (2006, 2009).
(d) Rho is the error correlation defined as err[sup.206]Pb/
[sup.238]U/err[sup.207]Pb/235U
Table 3.--Whole-rock analyses of major and trace elements of the
igneous clasts from the Del Raton and El Planchon conglomerate
formations.
Formation Del Raton igneous clasts
R. Type Basic rocks Intermediate rocks
Sample AN20 AN48 AN24 AN25 AN22
Major elements (wt %)
Si[O.sub.2] 43.86 45.86 46.99 57.73 60.05
Ti[O.sub.2] 1.98 2.08 1.95 0.89 0.63
[Al.sub.2][O.sub.3] 16.65 16.15 14.74 18.09 16.54
[Fe.sub.2][O.sub.3] 13.86 13.73 8.09 5.95 6.62
MnO 0.38 0.30 0.26 0.10 0.12
MgO 5.12 5.37 2.92 2.31 2.86
CaO 6.45 7.47 10.24 4.42 5.36
[Na.sub.2]O 3.10 2.54 4.40 4.53 2.66
[K.sub.2]O 0.70 0.42 0.75 3.19 2.43
[P.sub.2][O.sub.5] 0.19 0.21 2.14 0.25 0.18
LOI 6.71 5.19 6.91 1.89 2.05
Total 99.29 99.28 99.40 99.35 99.50
Formation
R. Type Acid rocks
Sample AN51 AN-27 AN26 AN52 AN49
Major elements (wt %)
Si[O.sub.2] 65.31 65.84 69.82 70.43 72.41
Ti[O.sub.2] 0.83 0.81 0.43 0.45 0.39
[Al.sub.2][O.sub.3] 15.48 15.16 12.94 14.59 12.54
[Fe.sub.2][O.sub.3] 3.78 5.14 4.04 3.20 3.73
MnO 0.10 0.09 0.09 0.06 0.07
MgO 1.26 1.17 1.24 1.61 1.05
CaO 1.95 1.05 1.26 0.38 0.50
[Na.sub.2]O 7.86 4.59 3.61 1.14 3.90
[K.sub.2]O 0.55 3.01 2.59 4.04 3.16
[P.sub.2][O.sub.5] 0.42 0.02 0.10 0.14 0.06
LOI 1.82 2.34 2.68 3.14 1.58
Total 99.37 99.22 98.79 99.17 99.40
Formation El Planchon igneous clasts
R. Type Basic rocks
Sample AN50 AN47 AN20b AN20c AN20e
Major elements (wt %)
Si[O.sub.2] 73.02 75.72 45.43 45.85 46.63
Ti[O.sub.2] 0.27 0.19 1.67 2.18 1.60
[Al.sub.2][O.sub.3] 11.75 12.11 10.15 14.46 14.80
[Fe.sub.2][O.sub.3] 1.79 0.99 13.87 12.95 11.34
MnO 0.06 0.04 0.21 0.19 0.17
MgO 0.47 0.23 11.53 7.45 8.22
CaO 2.62 1.22 12.69 11.04 12.10
[Na.sub.2]O 5.02 6.9 1.16 2.05 1.79
[K.sub.2]O 1.43 0.21 0.31 0.33 0.19
[P.sub.2][O.sub.5] 0.11 0.03 0.14 0.21 0.13
LOI 2.81 1.55 2.25 2.59 2.51
Total 99.35 99.18 99.42 99.31 99.57
Formation
R. Type
Sample AN20d AN20a
Major elements (wt %)
Si[O.sub.2] 47.02 47.07
Ti[O.sub.2] 1.72 1.79
[Al.sub.2][O.sub.3] 14.86 16.17
[Fe.sub.2][O.sub.3] 11.74 11.24
MnO 0.17 0.18
MgO 7.14 6.54
CaO 11.50 11.58
[Na.sub.2]O 2.19 2.47
[K.sub.2]O 0.34 0.45
[P.sub.2][O.sub.5] 0.15 0.17
LOI 2.51 2.28
Total 99.35 99.94
Trace elements (ppm)
V 380 308 164 107 183
Cr 351 199 48 25 79
Co 51 58 32 32 43
Ni 101 96 18 6 12
Cu 393 42 10 18 12
Zn 167 175 102 82 73
Ba 236 130 118 1073 501
Nb 7.8 9.9 15.9 9.2 7.4
Rb 16 12 20 137 96
Sr 336 360 575 494 332
Y 34 30 51 26 46
Zr 124 146 282 360 131
Pb 20 20.3 10.0 12.5 15.7
U 1.9 1.0 3.5 4.9 1.7
Th 1.1 4.7 9.7 9.0
Hf 3.90 3.68 5.10 8.30 4.00
Ta 0.62 0.49 1.10 0.75 0.73
La 20.08 25.27 94.60 30.48 23.18
Ce 45.39 48.48 172.90 67.31 54.46
Pr 6.35 6.26 23.94 9.21 7.45
Nd 25.88 26.54 89.22 34.69 30.06
Sm 6.38 6.18 15.95 6.68 7.35
Eu 1.88 1.91 4.13 1.78 1.34
Gd 7.03 6.60 15.62 6.25 7.66
Tb 1.12 1.04 2.00 0.90 1.32
Dy 6.62 6.21 10.07 4.94 8.37
Ho 1.31 1.20 1.88 0.97 1.78
Er 3.54 3.26 5.13 2.84 5.44
Tm 0.49 0.45 0.66 0.42 0.83
Yb 2.92 2.85 3.86 2.68 5.11
Lu 0.41 0.39 0.58 0.40 0.74
Trace elements (ppm)
V 56 86 44 44 60
Cr 3 30 21 6 6
Co 39 42 40 25 39
Ni 2 22 11 14 4
Cu 11 35 12 12
Zn 76 73 55 55 48
Ba 228 850 495 292 932
Nb 37.2 33.1 22.6 24.6 20.3
Rb 10 69 62 129 74
Sr 292 262 131 48 206
Y 47 62 14 23 14
Zr 587 548 212 294 207
Pb 9.6 12.7 6.1 11.0 8.8
U 2.4 2.0 2.0 1.7 1.6
Th 8.1 12.3 11.2 8.4 9.9
Hf 13.74 12.70 4.20 8.46 5.37
Ta 2.24 2.23 1.97 2.15 1.82
La 108.40 78.71 54.37 49.89 14.02
Ce 221.90 154.60 86.11 100.90 27.90
Pr 24.57 19.45 11.49 11.58 3.32
Nd 86.40 63.34 37.61 38.40 11.49
Sm 13.52 11.31 5.56 5.94 2.22
Eu 3.04 2.04 1.09 0.95 0.60
Gd 12.08 12.43 5.13 5.37 2.21
Tb 1.58 1.84 0.59 0.70 0.34
Dy 8.79 11.26 2.84 4.12 2.20
Ho 1.70 2.29 0.54 0.85 0.46
Er 5.06 7.02 1.67 2.74 1.42
Tm 0.72 1.08 0.27 0.44 2.23
Yb 4.73 6.75 1.76 3.02 1.55
Lu 0.71 1.02 0.27 0.47 0.25
Trace elements (ppm)
V 26 9 376 323 289
Cr 3 4 422 324 614
Co 45 140 66 52 55
Ni 225 137 152
Cu 10 95 162 119
Zn 16 9 95 95 80
Ba 299 502 120 137 67
Nb 18.5 22.9 6.6 8.1 5.8
Rb 32 5 10 11 9
Sr 123 115 180 266 215
Y 19 22 22 27 20
Zr 139 156 90 118 78
Pb 8.2 10.3 4.3 4.9 3.7
U 3.3 1.8 2.2 1.6 1.6
Th 13.7 13.3
Hf 4.70 5.27 2.33 3.23 2.37
Ta 2.02 1.70 0.46 0.69 0.49
La 42.75 40.94 7.02 10.38 7.12
Ce 81.08 76.55 16.97 25.47 17.56
Pr 8.34 7.35 2.38 3.75 2.45
Nd 27.57 21.81 12.14 18.04 12.21
Sm 4.53 3.39 3.48 4.93 3.47
Eu 0.76 0.44 1.18 1.61 1.26
Gd 4.14 3.39 3.89 5.30 3.78
Tb 0.55 0.48 0.69 0.89 0.66
Dy 3.21 3.08 4.14 5.28 3.97
Ho 0.66 0.67 0.82 1.05 0.78
Er 2.09 2.25 2.23 2.81 2.11
Tm 0.34 0.39 0.32 0.41 0.31
Yb 2.38 2.83 1.96 2.49 1.84
Lu 0.36 0.43 0.28 0.36 0.27
Trace elements (ppm)
V 309 285
Cr 312 212
Co 50 53
Ni 103 77
Cu 118 106
Zn 85 77
Ba 51 121
Nb 7.1 7.5
Rb 11 13
Sr 238 242
Y 21 22
Zr 90 105
Pb 4.8 3.4
U 2.0
Th
Hf 2.48 2.62
Ta 0.54 0.56
La 7.56 8.25
Ce 18.53 20.31
Pr 2.59 2.86
Nd 12.92 13.92
Sm 3.62 3.89
Eu 1.31 1.35
Gd 4.01 4.16
Tb 0.68 0.71
Dy 4.13 4.26
Ho 0.81 0.84
Er 2.19 2.31
Tm 0.32 0.33
Yb 1.92 2.08
Lu 0.28 0.30
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| Title Annotation: | articulo en ingles |
|---|---|
| Author: | Gallastegui, G.; Gonzalez-Menendez, L.; Rubio-Ordonez, A.; Cuesta, A.; Gerdes, A. |
| Publication: | Journal of Iberian Geology |
| Date: | Jul 1, 2014 |
| Words: | 14561 |
| Previous Article: | Stratigraphy and structure of the Punta Negra Anticline. Implications on the structural evolution of the Argentine Precordillera. |
| Next Article: | The Palaeozoic basement of the Andean Frontal Cordillera at 34[degrees] S (Cordon del Carrizalito, Mendoza Province, Argentina): geotectonic... |