Timing of the magmatism of the paleo-Pacific border of Gondwana:
U-Pb geochronology of Late Paleozoic to Early Mesozoic igneous
rocks of the north Chilean Andes between 20° and 31°S

Víctor Maksaev1, Francisco Munizaga1, 2, Colombo Tassinari3

1 Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile.
vmaksaev@ing.uchile.cl; fmunizag@ing.uchile.cl

2 Geología, Facultad de Ingeniería, Universidad Andrés Bello, Salvador Sanfuentes 2375, Santiago, Chile.

3 Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Avda. Vélez Sársfield 299, X500JJC, Córdoba, Argentina.

4 Instituto de Geociencias, Universidade de Sao Paulo, Rua do Lago, 562-Cidade Universitária CEP 05508-080, Sao Paulo, Brasil.
ccgtassi@usp.br

Fig. 1. Late Paleozoic to Early Jurassic stratigraphic units of northern Chile from 20° to 26°S, ordered according to the new geochronological data. The different units are ordered geographically from western (left) to eastern exposures (right) and face each other according to their approximate latitude of occurrence. The reader is referred to the work of Charrier et al. (2007 and references therein) for the units that are not mentioned in text.

 

 

Fig. 2. Late Paleozoic to Early Jurassic stratigraphic units of northern Chile from 26° to 31°S, ordered according to the new geochronological data. The different units are ordered geographically from western (left) to eastern exposures (right) and face each other according to their approximate latitude of occurrence. The reader is referred to the work of Charrier et al. (2007 and references therein) for the units that are not mentioned in text.

 

The exposed Late Paleozoic to Mid-Triassic volcanic succession along the Andes in northern Chile has uneven thickness, visible successions typically range between 750 and 2,600 m, but exceeding 3,200 m in the Collahuasi area of the Precordillera (21°S latitude; Vergara and Thomas, 1984). These successions are mostly formed of felsic volcanic rocks that include extensive rhyolitic and dacitic ash flow tuffs (ignimbrites), and banded rhyolite and dacitic domes and lava-flows. A lesser proportion is composed of basaltic and andesitic lavas, and pyroclastic deposits of the same petrographic composition. Sedimentary intercalations of volcanic sandstones, minor conglomerates, and local limestones (i.e., Collahuasi area; Vergara and Thomas, 1984), are also locally present. The volcanic successions unconformably overlie Devonian to Early Carboniferous shallow-marine, sedimentary successions, but its base is only locally exposed (Mercado, 1982; Niemeyer et al., 1985, 1997; Mpodozis and Cornejo, 1988; Nasi et al., 1990; Marinovic et al., 1995). In addition, calcareous and terrigenous, shallow-marine sedimentary successions, containing Early Permian fossils are exposed in the western foothills of the High Andes and in the Central Depression. These sedimentary units represent Early Permian carbonate sedimentation within a shallow-marine platform, but some of them are either overlying or interstratified with rhyolitic rocks (Sepúlveda and Naranjo, 1982; Marinovic et al., 1995; Iriarte et al., 1996; Cortés, 2000; Díaz-Martínez et al., 2000).

Middle to Late Triassic successions are sedimentary-volcanic in nature and are separated from older rocks by unconformities. A bimodal suite composed of basalts and rhyolites, locally with pillow lavas, and associated rhyolitic and dacitic sills, domes and ash-flow deposits occur within the Mid-Late Triassic to Early Jurassic successions (Naranjo and Puig, 1984; Suárez and Bell, 1992; Tomlinson et al., 1999; Cornejo and Mpodozis, 1996, 1997; Cornejo et al., 2009). The Late Triassic to Early Jurassic magmatism is fairly common, but it has been poorly investigated (e.g., Franz et al., 2006), and has not always been recognized as a distinct unit from older Carboniferous-Triassic rocks.

Carboniferous to Triassic granitic, granodioritic, and tonalitic batholiths and lesser dioritic or gabbroic plutons were emplaced in different segments along the Precordillera (21°-24°S), Domeyko Cordillera (24°-27°S) and an extensive sector of the Chilean Frontal Cordillera between 28° and 31°S (Figs. 3 and 4) is formed of granitic rocks of Late Paleozoic to Triassic age (Parada et al., 1981, 2007; Nasi et al., 1985; Mpodozis and Kay, 1990, 1992; Mpodozis et al., 1993; Cornejo et al., 1998; Tomlinson et al., 1999; Tomlinson et al., 2001; Tomlinson and Blanco, 2008; Hervé et al., 2014). Scattered Late Carboniferous and Triassic granitic plutons also extend from 21°S latitude southwards along the Coastal Cordillera, and then forming an almost continuous intrusive belt between 33° to 38°S (Berg et al., 1983; Shibata et al., 1984; Maksaev and Marinovic, 1980; Skarmeta and Marinovic, 1981; Berg and Baumann, 1985; Mpodozis and Kay, 1992; Parada et al., 2007; Deckart et al., 2014). Some of the Carboniferous and Early Permian plutons show foliation with preferred orientation of their tabular minerals, and local bands of mylonites with high-temperature, ductile, shearing deformation (Nasi et al., 1985; Ribba et al., 1988; Marinovic et al., 1995; Murillo et al., 2012; Hervé et al., 2014). Close associations of granitic plutons and ignimbrites in the Andes have been interpreted as large relict calderas (Davidson et al., 1985; Breitkreuz, 1995; Tomlinson et al., 2001). In addition, a belt of porphyry copper prospects and hydrothermal alteration zones are associated with the Permian and Triassic intrusions in Chile and Argentina (Sillitoe, 1977; Camus, 2003; Cornejo et al., 2006; Tomlinson and Blanco, 2008).

 

Fig. 3. Map showing the distribution of the Late Paleozoic to Late Triassic rocks of northern Chile between latitudes 20° and 26°S and location of U-Pb zircon ages (Ma±2σ). New SHRIMP U-Pb ages in bold; previous U-Pb dates compiled from previous works in italics (references in Tables 1 and 2).

 

Fig. 4. Map showing the distribution of the Late Paleozoic to Late Triassic rocks of northern Chile between latitudes 26° and 31°S and location of U-Pb zircon ages (Ma±2σ). New SHRIMP U-Pb ages in bold; previous U-Pb dates compiled from previous works in italics (references in Tables 1 and 2).

 

The Carboniferous to Mid-Triassic igneous rocks constitute a calc-alkaline to potassium-rich calc-alkaline suite, are metaluminous to peraluminous in composition, and have overall geochemical characteristics consistent with subduction-related, magmatic arc origin, but with evolution into extensional magmatism and significant involvement of crustal melts, evidencing significant crustal recycling (Nasi et al., 1985; Kay et al., 1989; Breitkreuz et al., 1989; Mpodozis and Kay, 1990, 1992; Mpodozis et al., 1993; Breitkreuz and Zeil, 1994; Marinovic et al., 1995; Lucassen et al., 1999; Llambías et al., 2003; Parada et al., 2007; Urzúa, 2009; Vásquez et al., 2011; Parada, 2013). Brown (1990) compared the major and trace elements geochemistry of the Permian to Middle Triassic Coastal granitic plutons at latitude 26°S with those exposed some 120 km to the east in the High Andes, showing their similar compositions with subduction-related affinity, and concluded that they were part of the same magmatic arc split during Mesozoic extension.

The Middle to Late Triassic igneous rocks show a distinct bimodal composition with a within plate tendency, consistent with an origin in an extensional, rift-related setting (Vergara et al., 1991; Parada et al., 1991; Morata et al., 2000; Vásquez et al., 2011; Parada, 2013).

 

3. Previous U-Pb geochronological data

U-Pb geochronological data for Late Paleozoic to Early Mesozoic rocks compiled from previous studies are presented in Tables 1 and 2 and figures 3 and 4.

The oldest zircon U-Pb ages compiled for volcanic rocks range from 309.9±2.2 to 273.5±11.0 Ma (Table 1; Fig. 3); these rocks are exposed from 20°30’ to 25°30’S in a ~50 km wide, longitudinal belt along ~69°W longitude along the Precordillera and the Domeyko Cordillera (Fig. 3), and according to the U-Pb data represent a Late Carboniferous to Early Permian, felsic volcanism and subordinate intermediate and basic volcanic rocks, and minor sedimentary intercalations (Collahuasi, Agua Dulce and La Tabla Formations, and the Sierra del Jardín Rhyolites; Vergara and Thomas, 1984; Mpodozis et al., 1993; Marinovic and García, 1999; Masterman, 2003; Cornejo et al., 2006; Marinovic, 2007; Munizaga et al., 2008; Tomlinson and Blanco, 2008; Urzúa, 2009; Jara et al., 2009; Hervé et al., 2012). The Late Carboniferous to Early Permian volcanism is less known southward of 25°30’S, but a U-Pb zircon age of 300.8±4.6 Ma has been reported by Salazar et al. (2013) for rhyolites outcropping in the El Tránsito river valley in the High Andes, 65 km west of the divide (28°46’S; Fig. 4).

Rhyolitic volcanic rocks exposed along the Domeyko Cordillera and Frontal Cordillera south of 25°30’S latitude have yielded U-Pb ages of 262.9±2.0 and 265.0±5.6 Ma (Table 1; Fig. 4), thus essentially representing a Mid-Permian felsic volcanism, which has been previously ascribed to the to the Permian-Triassic (Martin et al., 1999) or to the Carboniferous-Permian (Cornejo et al., 2009).

A U-Pb zircon U-Pb age of 248±3 Ma, resulting from combining analyses of four separate samples, was reported by Breitkreuz and van Schmus (1996) for felsic volcanic rocks of the Peine Group, exposed in on the eastern border of the Salar de Atacama salt flat at about 23°30’S (Fig. 3). Thus, these easternmost outcrops of northern Chile represent an Early to Middle Triassic volcanism, which originally was assigned to the Late Permian by Breitkreuz and van Schmus (1996), as befitted to the geologic time scale in that year.

Middle Triassic to Early Jurassic volcanism has yielded U-Pb dates from 249.7±3.8 to 194.0±9.2 Ma (Table 1). During this time span andesitic to basaltic volcanism coexisted with rhyolitic volcanism (bimodal volcanism), and was synchronous with rift-related, continental and/or marine sedimentary deposits widely distributed in northern Chile (Naranjo and Puig, 1984; Charrier, 1979; Suárez and Bell, 1992; Martin et al., 1999; Tomlinson et al., 1999; Cornejo and Mpodozis, 1996, 1997; Cornejo et al., 2009).

The zircon U-Pb ages for intrusive rocks exposed along the High Andes and Domeyko Cordillera range from 328.1±2.8 to 205.9±0.5 Ma (Table 2), overlapping with the corresponding interval of U-Pb dates of volcanic rocks (ca. 310 to 194 Ma; Table 1). The intrusions include coarse-grained granitoids and porphyritic felsic plutons; the late are closely related to the volcanic rocks. Foliated plutons (ductile deformation) show U-Pb ages in the range 328 to 288 Ma (Table 2).

There are scarce and inaccurate previous U-Pb dates for granitic plutons scattered along the Coastal Cordillera of northern Chile from 21° to 31°S, which range from 291.8±14.3 to 217±12 Ma (Berg et al., 1983; Berg and Baumann, 1985). Yet, a number of whole rock Rb-Sr dates that range from 296±5.4 to 222±3 Ma have been published (Berg et al., 1983; Shibata et al., 1984; Berg and Baumann, 1985), and biotite K-Ar dates of 318±6 and 322±5 Ma (Maksaev and Marinovic, 1980; Skarmeta and Marinovic, 1981). Besides, eight U-Pb ages ranging from 319.6±2.4 to 308.9±2.4 Ma have been recently obtained for the granitoids that are exposed from 33° to 38°S along the Coastal Cordillera of central-southern Chile (Deckart et al., 2014).

 

4. Analytical methods

SHRIMP II U-Pb analyses of zircon grains were carried out at the Geochronological Research Center (Centro de Pesquisas Geocronológicas) of the Sao Paulo University, Brazil. Zircon grains were obtained from rock samples following normal mineral separation procedures in Universidad de Chile, which included crushing, washing, and heavy liquid and magnetic separation. Hand-picked zircon grains were mounted in epoxy, together with chips of the Temora reference zircons, sectioned approximately in one half, and polished in Brazil. Cathodoluminescence (CL) Scanning Electron Microscope (SEM) images were prepared for all zircons. The CL images were used to decipher the internal structures of the sectioned grains and to ensure that the ~20 μm SHRIMP spot was wholly within a single age component within the sectioned grains. Uranium-Th-Pb analyses were made using a sensitive high resolution ion microprobe (SHRIMP II) for determining the respective zircon Concordia U-Pb ages following procedures given in Williams (1998, and references therein). Each analysis consisted of six scans through the mass range, with a Temora U-Pb reference grain analyzed for every three unknown analyses. The data have been reduced using the SQUID Excel Macro of Ludwig (2001); U-Pb ratios have been normalized relative to a value of 0.0668 for the Temora reference zircon, equivalent to an age of 417 Ma (Black et al., 2003). Uncertainties given for individual analyses (ratios and ages) are at the one-sigma level (Appendix 2). Tera and Wasserburg (1972) concordia plots, and mean U-Pb age calculations were carried out using ISOPLOT/EX (Ludwig, 2003). The data tabulations for all 43 samples are presented in Appendix 1 and 2.

5. Results

The new SHRIMP U-Pb zircon concordia ages are summarized in Tables 3 and 4 for volcanic and intrusive rocks, respectively and sample locations are shown in figures 3 and 4; Tera-Wasserburg concordia plots and SHRIMP zircon U-Pb analytical data are included in the Appendix 1 and 2, respectively.

5.1. Volcanic rocks

The new zircon U-Pb ages for volcanic rocks range from 324.7±4.0 to 221.0±2.6 Ma (Table 3), which is consistent with the ca. 310 to 194 Ma interval showed by the compiled U-Pb dates (Table 1). Nevertheless, we obtained some of the oldest age determinations for Late Paleozoic volcanic rocks in Chile to date, particularly for volcanic deposits exposed in the current Intermediate Depression and the western foothills of the High Andes. These include U-Pb ages of 324.7±4.0 and 323.1±5.8 for rhyolites outcropping in the El Tránsito river valley (at 28°46’S; Fig. 4), which were previously mapped as the Permian to Triassic Pastos Blancos Formation (Moscoso et al., 2010), but were recognized as an older separate unit named Cerro Bayo Formation by Salazar et al. (2013). Besides, a U-Pb age of 320.1±9.0 Ma was obtained for an ignimbrite outcropping in the Intermediate Depression of the Antofagasta Region, which is part of a rhyolitic volcanic succession that underlies and is partly interfingered with carbonate strata that contains Early Permian marine fossils in its upper section (Cerro del Árbol Formation; Marinovic et al., 1995; Marinovic, 2007). In addition, U-Pb ages of 296.6±4.2 and 292.2±4.2 Ma were obtained for rhyolites that are part of a succession exposed in the Paipote valley which was assigned to the Pantanoso Formation (Sepúlveda and Naranjo, 1982; Iriarte et al., 1996) and unconformably underlies Late Triassic conglomerates of La Ternera Formation in the Atacama Region (Fig. 4). The same rhyolitic unit underlies limestone beds with Early Permian marine fossils in the region, though the contact between them has been interpreted as a low-angle, normal fault (Quebrada de Las Represas Beds; Mpodozis and Allmendinger, 1993; Iriarte et al., 1996).

In the High Andes a U-Pb age of 291.5±4.0 Ma was obtained for an andesitic tuff intercalated in the upper section of El Mono Beds (sensu Mercado, 1982; Mpodozis et al., 2012), which is a mostly alluvial and lacustrine sedimentary succession, ca. 1,000 m thick, formed of sandstone and conglomerate strata that unconformably overlie Devonian to Early Carboniferous sandstones of the Chinches Formation in the High Andes at ~27°S (Fig. 4) and underlies in apparent conformity Jurassic marine strata (Mercado, 1982; Mpodozis et al., 2012). This sedimentary-volcanic succession was previously attributed to the Late Triassic based on fossil flora and conchostracans (Davidson et al., 1978; Mercado, 1982; Suárez and Bell, 1991; Blanco, 1994; Cornejo et al., 1998; Gallego and Covacevich, 1998; Mpodozis et al., 2012).

U-Pb dates ranging from 282.0±11.4 to 270.4±4.6 Ma were obtained for 4 samples of banded rhyolites and rhyolitic tuff from the type locality of La Tabla Formation, (sensu García, 1967), in the eastern part of the Domeyko Cordillera and southwest of the Pedernales Salar in the Atacama Region (26°30’S; Fig. 4). One sample (CHY-14; Table 3) returned a bimodal age distribution with an older population at 311.3±8.2 Ma and a younger one at 282.0±11.4; the analytic spots on the cathode luminescence images of the zircons indicate that the older age population corresponds to inherited zircon cores, while the younger ages to zircon rims, thus the younger population corresponding to the actual crystallization age of the dated rock. The La Tabla Formation is an 800 m thick succession formed of rhyolites, rhyolitic tuffs, breccias, and basaltic intercalations (without exposed base) which originally was thought to be Early Triassic in age (García, 1967), but later assigned to the Late Paleozoic by Tomlinson et al. (1999), based on K-Ar ages of intrusions that crosscut the unit. The volcanic rocks of La Tabla Formation are crosscut by granitoids of the Pedernales Batholith and andesitic dikes that yielded U-Pb ages from 287.0±4.2 to 264.6±7.0 Ma (Table 4). According to the new U-Pb dates it represent Permian volcanism, which is a bit younger but overlapping with the U-Pb ages from 309.9±2.2 to 282±2 Ma published for rocks assigned to the La Tabla Formation along the Domeyko Cordillera north of 25°30’S latitude (Table 1). In addition, a U-Pb age of 262.0±2.8 was obtained for a sample of fluidal rhyolite farther south (26°49’S; Table 3), mapped as La Tabla Formation by Cornejo et al. (1998) that is part of a succession crosscut by granite of the Caballo Muerto Pluton, which yielded a U-Pb age of 263.5±3.4 Ma (Table 4).

A U-Pb age of 270.3±4.2 Ma was obtained for a basal dacite horizon and additional U-Pb ages of 269.9±4.0 and 266.1±4.4 Ma were obtained for dacitic tuff and rhyolitic ignimbrite of the Pantanoso Formation (Mercado, 1982; Mpodozis et al., 2012), at its type locality (Table 3; Fig. 4). The Pantanoso Formation is crosscut by granite of the El Hielo batholith that yielded a U-Pb age of 267.9±3.8 Ma (Table 4). The Pantanoso Formation is a 1,000 thick succession of rhyolitic and dacitic volcanic rocks and minor conglomerate and volcanic sandstone intercalations exposed in the High Andes at 27°28’S latitude, and according to the new U-Pb data it rep-resent Mid-Permian volcanism. It unconformably overlies mainly Devonian to Early Carboniferous sandstones of the Chinches Formation (Mercado, 1982; Bell, 1985), but also a local lens of folded sandstone and conglomerate beds up to 200 m thick of the Elba Beds, which were ascribed to the Carboniferous-Permian by Mercado (1982).

U-Pb zircon ages of 255.9±6.2 and 242.7±5.8 Ma (Table 3) were obtained for samples of ignimbrite and rhyolite from the Cas Formation (sensu Ramírez and Gardeweg, 1982), which is a succession of about 300 m thick (without exposed base), outcropping immediately east of the Peine town, in the eastern part of the Antofagasta Region (Fig. 3). The new U-Pb ages are consistent with the mean weighted U-Pb age of 248±3 based on 4 separate determinations published by Breitkreuz and van Schmus (1996) and indicate that the rhyolitic rocks of the Cas Formation represent a Late Permian to Early Triassic felsic volcanism.

A sample of basaltic tuff from the basal meters of the mostly basaltic-andesitic La Totora Formation in the homonymous creek in the High Andes of the Vallenar region (28º42’; Fig. 4) yielded a U-Pb age of 249.7±3.8 Ma. The dated sample (CHY-56) actually returned a bimodal age distribution with an older population at 323.6±5.8 Ma; the analytic spots on the cathode luminescence images of the zircons indicate that the older age population corresponds to inherited zircon cores, while the younger ages to zircon rims, thus the younger population corresponding to the actual crystallization age of the dated rock. Another sample of basalt of the same succession in the Totora creek yielded an U-Pb age of 221.0±2.6 Ma; these ages are coherent with the U-Pb ages from 217.9±1.4 to 210.4±2.9 Ma published by Salazar et al. (2013) for the same formation. This volcanic unit unconformably overlies a mylonitized tonalite of the Quebrada Pintado intrusive unit (sensu Salazar et al., 2013) for which we obtained a U-Pb age of 258.9±5.0 Ma (Table 4), and conformably underlies fossiliferous, Mid-Late Jurassic (Sinemurian) marine limestones (Reutter, 1974; Nasi et al., 1990; Salazar et al., 2013); stratigraphic position that is consistent with the new U-Pb data. In addition, strata of the western belt of La Totora Formation are in part conformably overlying the 4,000 m thick, mostly clastic-sedimentary Mid-Late Triassic San Felix Formation to the southwest (Salazar et al., 2013).

U-Pb ages of 232.1±4.6, 224.0±3.4, and 221.6±3.4 were obtained for samples of banded rhyolites and rhyolitic tuffs from the Los Tilos sequence of the Pastos Blancos Group (sensu Martin et al., 1999), exposed in the Potrerillos river valley in the High Andes, 20 km west of the divide (29°30’S; Fig. 4); this new U-Pb data confirms the Late Triassic age of the Los Tilos volcanic sequence of the Pastos Blancos Group, which unconformably overlies the Permian silicic volcanics of the Guanaco Sonso sequence as defined by Martin et al. (1999).

A U-Pb age of 154.3±3.8 Ma was obtained for andesite that is part of a succession outcropping in the west flank of the El Carmen river valley (29°17’S; Fig. 4). This intermediate volcanic unit was originally mapped as part of the Late Paleozoic-Triassic Pastos Blancos Formation (with a question mark) by Nasi et al. (1990). Actually, the new U-Pb age shows that these andesitic rocks are Late Jurassic in age, being equivalent in age and lithology to the Picudo Formation (Reutter, 1974; Moscoso et al., 2010) exposed in the El Tránsito river valley and to the Lagunillas Formation, which has been recognized farther north between 27°30’ and 28°30´S in the High Andes of the Atacama Region and these represent Late Jurassic back-arc volcanism (Iriarte et al., 1996; Oliveros et al., 2011).

5.2. Intrusive rocks

The new zircon U-Pb ages for Late Paleozoic intrusive rocks range from 315.7±4.6 to 229.6±5.2 Ma (Table 4), well within the 328 to 206 Ma interval of the compiled U-Pb dates (Table 2).

Granitoids exposed along the Domeyko Cordillera and its southern continuation in the Frontal Cordillera yielded U-Pb ages from 287 to 263 Ma (Table 4; Fig. 4), representing Permian granitic plutonism, these include: a U-Pb age of 285.3±6.4 Ma for a foliated tonalite that is part of the Sierra Castillo batholith (sensu Tomlinson et al., 1999; Fig. 4); a U-Pb age of 287.0±4.2 Ma for a hornblende-biotite monzodiorite and U-Pb ages of 280.6±4.2, and 264.9±7.0 Ma for biotite-hornblende, monzogranite plutons of the Pedernales batholiths (sensu Tomlinson et al., 1999; Fig. 4); a U-Pb age of 263.5±3.4 Ma for monzogranite of the Caballo Muerto pluton (sensu Cornejo et al., 1998); and a U-Pb age of 267.9±3.8 Ma for leucocratic, monzogranite of the El Hielo batholith (sensu Mercado, 1982; Bell, 1985; Mpodozis et al., 2012) in the High Andes of the Atacama Region (Fig. 4).

A U-Pb age of 244.0±2.8 Ma was obtained for a coarse-grained monzogranite of the Chollay Batholith (Fig. 4), which is consistent with U-Pb ages from 247.7±3.4 to 240.3±1.7 Ma that have recently been reported for the same batholith (Coloma et al., 2012; Salazar et al., 2013; Álvarez et al., 2013; Hervé et al., 2014). These U-Pb ages confirm the Mid-Triassic age ascribed to the granitic Chollay batholith by Nasi et al. (1985) and Martin et al. (1999).

The dating of plutons of the northern part of the composite Elqui-Limarí Batholith in the Frontal Cordillera of the Atacama Region (Fig. 4) have yield U-Pb ages from 315.7±4.6 to 229.6±5.2 Ma (Table 4). These are consistent with the 330 to 215 U-Pb interval of recently published U-Pb ages for the same batholith (e.g., Pineda and Calderón, 2008; Salazar et al., 2009, 2013; Coloma et al., 2012, Álvarez et al., 2013; Hervé et al., 2014). We have obtained a U-Pb age of 315.7±4.6 Ma for a coarse-grained, biotite monzogranite from the Guachicay pluton (sensu Nasi et al., 1985) exposed in the Del Carmen river valley and 311.9±6.0 Ma for a coarse-grained, biotite syenogranite exposed in the Potrerillos river valley. The latter was included in the extensive tonalitic Guanta pluton by Nasi et al. (1985, 1990). In addition, we obtained a zircon U-Pb age of 307.1±4.8 Ma for a foliated, coarse-grained, biotite-hornblende tonalite of the Guanta pluton (sensu Nasi et al., 1985, 1990) exposed in the Del Carmen river valley, and a U-Pb zircon age of 286.3±6.2 Ma was obtained for a coarse-grained, muscovite-biotite granite of the Burro Muerto pluton (sensu Nasi et al., 1985), which is emplaced within the foliated tonalite of the Guanta pluton in the El Carmen river valley, and corresponds to the Cochiguaz intrusive unit of Nasi et al. (1985, 1990); and a U-Pb zircon age of 229.6±5.2 Ma was obtained for a brick-red, fine-grained, syenogranite of the La Coneja pluton (sensu Nasi et al., 1985) that corresponds to the Colorado intrusive unit of Nasi et al. (1985, 1990).

A U-Pb zircon age of 258.9±5.0 Ma (Late Permian) was obtained for mylonitized tonalite of the Quebrada Pintado tonalitic unit of Salazar et al. (2013) in the La Totora creek (28°40’S; Fig 4). The mylonites at La Totora creek show a N35°E/75°E foliation and centimetric to decimetric white, felsic bands alternating with dark gray, biotitic bands. This mylonitized intrusion unconformably underlies the Mid-Late Triassic volcanic rocks of La Totora Formation.

Along the Pacific coast north of Chañaral, about 120 km west of the High Andes, U-Pb ages of 276.6±3.6 and 264.6±7.0 Ma were obtained for biotite-muscovite, monzogranite of the Pan de Azúcar pluton (sensu Mercado, 1980; Godoy and Lara, 1998), while two U-Pb ages of 246.5±4.0 and 237.0±4.0 were obtained for coarse-grained, muscovite-biotite monzogranite of the Cerros del Vetado pluton of the Coastal Cordillera immediately east of Chañaral (Fig. 4).

6. Discussion

The U-Pb data provide evidence for the onset of volcanism at ~325 Ma (by the end of the Missisippian), which is approximately coeval with the onset of granitic plutonism at ~328 Ma (Tables 2 and 3), confirming a previous conclusion of Hervé et al. (2014). This marks reestablishment of magmatism during the Carboniferous after a gap in igneous activity during the Devonian in northern Chile and Perú (e.g., Charrier et al., 2007; Boekhout et al., 2013).

6.1. Volcanic rocks

The oldest rhyolitic rocks (325 to 292 Ma; Table 1 and 3) are present within the current Intermediate Depression and the western foothills of the High Andes of northern Chile (Figs. 3 and 4). Part of this initial volcanism developed within a shallow marine platform represented by carbonate strata with Late Permian marine fossils that are overlying and partly interfingered with the dated rhyolitic rocks (Juan de Morales Formation in the Tarapacá Region; Galli, 1956, 1968; Díaz-Martínez et al., 2000; Cerro del Árbol Formation in Antofagasta Region; Marinovic et al., 1995; Marinovic, 2007; and Quebrada de Las Represas Beds in the Atacama Region; Iriarte et al., 1996). While, the rhyolitic succession exposed in the El Tránsito river valley includes conglomerates, sandstones and andesites (Cerro Bayo Formation; Salazar, 2012; Salazar et al., 2013).

The mainly felsic volcanic rocks widely exposed along the Domeyko Cordillera and Precordillera of northern Chile north of 25°30’S latitude have yielded U-Pb ages from 310 to 282 Ma Tables 1 and 3), thus represent a Late Carboniferous to Early Permian volcanism that is older than the Choiyoi succession of the San Rafael Block in western Argentina for which U-Pb ages from 281 to 251 Ma have been published (Rocha-Campos et al., 2011). Actually, the Chilean Carboniferous to Early Permian volcanic rocks are chronologically equivalent to sedimentary successions with submarine volcanic deposits that are present farther east in Argentina (Azcuy et al., 1999; López-Gamundí, 2006; Koukharsky et al., 2009; Gulbranson et al., 2010). Therefore, when the Carboniferous to Early Permian magmatism was active along the current Domeyko Cordillera and Precordillera northwards, in Argentina farther east developed contemporary marine basins; this situation has been interpreted as a magmatic arc and back-arc basins by Gulbranson et al. (2010).

The U-Pb age of 291.5±4.0 Ma that we obtained for an andesitic tuff intercalated in the upper part of El Mono Beds at its type locality (Mpodozis et al., 2012), in the High Andes at ~27°S, indicates an Early Permian age for the upper part of the succession. Even though it underlies in apparent conformity Jurassic marine strata, implying a hiatus in what was considered a continuous sedimentary sequence (e.g., Blanco, 1994). The mostly clastic, alluvial and lacustrine sedimentary El Mono Beds have previously been assigned to the Late Triassic, based mainly on fossil flora attributed to Dicroidium sp. (Mercado, 1982; Suárez and Bell, 1991; Blanco, 1994; Cornejo et al., 1998; Gallego and Covacevich, 1998; Mpodozis et al., 2012). We cannot exclude the possibility that Triassic sedimentary rocks exists in the same region, but the new U-Pb age of 291.5±4.0 Ma is consistent with the stratigraphic position of the El Mono Beds unconformably overlying Devonian to Early Carboniferous sandstones of the Chinches Formation (Bell, 1985; Mpodozis et al., 2012), and evidences that this unit represents a Late Carboniferous to Early Permian sedimentation and coetaneous pyroclastic volcanism, at least where sampled for dating. Therefore, El Mono Beds could be equivalent to a sedimentary unit with Late Carboniferous flora and marine fossils exposed near the watershed of the Andes at ~29°S latitude (Las Placetas Formation; Reutter, 1974; Nasi et al., 1990); these sedimentary deposits probably represent the westernmost part of sedimentary basins developed mainly farther east in Argentina during the Carboniferous to Early Permian (Azcuy et al., 1999; López-Gamundí, 2006; Koukharsky et al., 2009; Gulbranson et al., 2010).

Rhyolitic volcanism with andesitic and basaltic intercalations persisted during the Permian along the current Domeyko Cordillera and the Chilean Frontal Cordillera (High Andes) as the volcanic rocks exposed south of 25°30’S latitude have yielded U-Pb ages from 282 to 262 Ma (Figs. 3 and 4). These are represented by the La Tabla, and Pantanoso Formations, and the Guanaco Sonso Sequence of the Pastos Blancos Group (Mercado, 1982; Cornejo et al., 1993, 1998, 2009; Martin et al., 1999; Tomlinson et al., 1999; Mpodozis et al., 2012) and their U-Pb ages fall well within the 281 to 251 Ma time span of U-Pb ages published by Rocha-Campos et al. (2011) for the Choiyoi Succession of the San Rafael Block in western Argentina.

The U-Pb age of 270.3±4.6 Ma for the basal unit of the Pantanoso Formation (Table 3) indicates the onset of volcanic accumulation in the current High Andes at ~27°S latitude unconformably on top of deformed Devonian to Early Carboniferous sedimentary rocks (Chinches Formation; Mercado, 1982; Bell, 1985), but also over folded sandstone and conglomerate beds ascribed to the Carboniferous-Permian (Elba Beds; Mercado, 1982). Thus, according to the new U-Pb data the accumulation of the volcanic rocks of the Pantanoso Formation at its type locality occurred at least 20 My later than the rhyolitic succession that is exposed 47 km to the west in the Paipote valley, which in previous works has also been assigned to the Pantanoso Formation (Iriarte et al., 1996).

Late Permian to Early Triassic rhyolitic volcanism is represented by the Cas Formation that yielded U-Pb ages from 256 to 243 Ma (Fig. 3; Tables 1 and 3) and is exposed in the easternmost part of Chile (68°W), along the eastern side of the Salar de Atacama salt flat in the Antofagasta Region (Ramírez and Gardeweg, 1982; Breitkreuz and van Schmus, 1996). The rhyolitic rocks of the Cas Formation at its type locality are significantly younger than the Early Permian rhyolitic rocks exposed some 90 km to the west along the Domeyko Cordillera (e.g., Sierra del Jardín Rhyolites; Marinovic, 2007), even though the latter were mapped as the same formation by Mpodozis et al. (1993).

The occurrence of Mid-Permian to Early Triassic volcanic rocks eastward from the Carboniferous to Early Permian volcanic rocks in Chile is interpreted as a Mid-Permian to Early Triassic expansion and/or eastward migration of the volcanic front, which is consistent with evidences of expansion of the volcanic activity farther east in Argentina at that time (e.g., Ramos and Folguera, 2009; Ramos, 2009).

Middle Triassic to Early Jurassic volcanic rocks yielded zircon U-Pb ages from 249.7±3.8 to 194.0±9.2 Ma, their outcrops are more discontinuous (patchy) and dispersed compared to the older volcanic rocks, and are separated from them by erosional surfaces. During this period andesitic to basaltic volcanism coexisted with felsic volcanism (bimodal volcanism), and was synchronous with rift-related, continental and/or marine sedimentary deposits in northern Chile (e.g., Suárez and Bell, 1992; Bell and Suarez, 1994). Middle to Upper Triassic strata show strong facies changes along the longitudinal axis of the High Andes, for example according to the U-Pb data the mostly basaltic-andesitic La Totora Formation (250-210 Ma) overlaps in age with the upper part of the mainly clastic, sedimentary San Félix Formation (223-212 Ma), but also with rhyolites of Los Tilos Sequence (232-221 Ma), within less than 100 km in the Atacama Region. These striking north-south facies changes probably reflect the main northwest orientation of extensional basins that developed in the Mid-Late Triassic in Chile and Argentina (Charrier, 1979; Chong and Von Hillebrandt, 1985; Uliana et al., 1989; Ramos and Kay, 1991; Jenchen and Rosenfeld, 2002; Charrier et al., 2007; Barredo et al., 2012).

A probability density plot and stacked histogram of all U-Pb ages for volcanic rocks exposed along the segment of the Chilean Andes from 20° to 31°S (Fig. 5) evidences a protracted volcanic activity that extended for about 130 million years, from about 325 to 194 Ma (Late Mississippian to Early Jurassic). The histogram (Fig. 5) shows two largest concentrations of U-Pb dates from 310 to 250 Ma and from 240 to 210 Ma, and other less prominent concentrations of U-Pb ages from 330 to 320 Ma and at about 190 Ma. Even though the histogram probably is biased to zircon-bearing rocks and easily accessible outcrops for sampling, the statistical analysis of the U-Pb data may signify that major volcanic episodes extended from the Late Carboniferous to Late Permian and then from Mid- to Late Triassic, but were preceded by a mid-Carboniferous volcanic episode and followed by an Early Jurassic one.

 

Fig. 5. Probability density plot and stacked histogram of zircon U-Pb ages for volcanic rocks.

 

6.2 Intrusive rocks

U-Pb data for plutonic rocks show that earliest intrusions crystallized from 328.1±2.8 to 311.9±6.0 Ma, including coarse-grained, biotite-muscovite granite to granodiorite (S-type) and biotite monzogranite and biotite-hornblende granodiorite plutons (I-type). From 308.5±2.2 to 290±2 Ma rhyolitic and granitic porphyries and equigranular monzogranite plutons crystallized and biotite-muscovite granitic and granodioritic plutons, and hornblende-diorite tonalitic to dioritic plutons were emplaced. Furthermore, 33 compiled and 9 new U-Pb zircon ages, which altogether range from 298.9±2.6 to 252.5±1.8 Ma attest that plutonism was active during the whole Permian in northern Chile, when there was a strong predominance of coarse-grained monzogranite plutons, both equigranular and porphyritic, followed by coarse-grained biotite-hornblende tonalites, and only minor monzonite, diorite and gabbro intrusions; two-mica (S-type) granites, granodiorites, and tonalites also intruded during this time span. Most of the Permian plutons are exposed along the High Andes and Domeyko Cordillera, but Permian granites with U-Pb ages of 277-270 Ma are also present some 120 km west in the Pacific coast (Pan de Azúcar pluton, Fig 4).

Triassic plutonism is represented by granitic porphyries, biotite- and two-mica granites, granodiorites, and brick-red syenogranite intrusions with U-Pb ages that range from 247.0±3.1 to 215.6±1.9 Ma. Some of these intrusions are of large volume, as most of the Chollay and Montosa granitic batholiths are formed of Triassic medium to coarse-grained monzogranites for which U-Pb ages of 245.0±3.1 and 240.3±1.7 Ma have been reported (Coloma et al., 2012; Hervé et al., 2014) and we obtained a U-Pb age of 244.0±2.8 Ma. Triassic granitic plutons are also present in the current Coastal Cordillera as Cerros del Vetado pluton yielded U-Pb ages of 247-237 Ma (Fig. 4).

The U-Pb data now available evidence that muscovite-bearing intrusive, with S-type granite characteristics, were not confined to a single discrete Permian magmatic episode as previously thought, but crystallized within the entire time span from Late Carboniferous to Late Triassic and synchronically with biotite- and hornblende-bearing intrusions with petrographic characteristics of I-type granitoids; this is in agreement with a similar conclusion based on U-Pb data for the Elqui-Limarí Batholith (Hervé et al., 2014). Actually, the synchronic emplacement of I- and S-Type granitoids was already noted and sustained by chemical data by Marinovic et al. (1995) in the Domeyko Cordillera, though it was attributed to a local phenomenon related to a hypothetical longitudinal segmentation of the Late Paleozoic to Triassic plutonism, rather than to a regional characteristic.

The new U-Pb data from 277 to 237 Ma (Table 4) confirm that Permian and Mid-Triassic granitic magmatism is also exposed in the current coastal area, which combined with previously published geochronological data suggests a protracted plutonism extending from 322 to 217 Ma along the Coastal Cordillera of northern Chile from 21° to 33°S (Maksaev and Marinovic, 1980; Skarmeta and Marinovic, 1981; Berg et al., 1983; Shibata et al., 1984; Berg and Baumann, 1985). However, there appears to be a difference with the Coastal Batholith exposed to the south between 33° to 38°S, which has yielded eight 320 to 300 Ma (Pennsylvanian) U-Pb zircon ages (Deckart et al., 2014), but to date there are no Permian and Early Triassic granitoids intruded in the main body of this Coastal Batholith of central-southern Chile, and only some small Late Triassic-Early Jurassic stocks of calc-alkaline to transitional A-type affinities, with U-Pb zircon ages from 225 to 197 Ma that are emplaced into its metamorphic envelope (Vásquez et al., 2011; Hervé et al., 2014).

A probability density plot and stacked histogram of all U-Pb ages for intrusive rocks (Fig. 6) shows that plutonic activity extended for at least 120 My from about 328 to 206 Ma (Late Missisipian to Late Triassic) with a largest concentration of U-Pb dates between 310 and 280 Ma, followed by another significant concentration of U-Pb dates from 255 to 205 Ma, and other less prominent U-Pb age concentrations from 330 to 310 Ma and 280 to 260 Ma. This statistical analysis of the U-Pb data may signify that major intrusive episodes took place from the Late Carboniferous to Early Permian and from the Late Permian to Late Triassic, but also there were mid-Carboniferous and mid-Permian intrusive episodes. The inferred plutonic episodes are fundamentally coincident with the major episodes of volcanism inferred from the statistical analysis of U-Pb data (Figs. 5 and 6). A sequential plot of the U-Pb ages (Fig. 7) shows that there are no apparent periods when the volcanism or plutonism prevailed, but both developed simultaneously from the Carboniferous to the Early Jurassic. A probability density plot and stacked histogram of all U-Pb ages for igneous rocks (Fig. 8) show the largest concentration of dates from 310 to 260 Ma and from 255 to 205 Ma, which are interpreted as the major magmatic episodes developed during from the Late Carboniferous to Mid-Permian and from Late Permian to Late Triassic. Other less prominent U-Pb age concentrations reveal a mid-Carboniferous episode (330 to 320 Ma) and an Early Jurassic episode (200-190 Ma).

 

Fig. 6. Probability density plot and stacked histogram of zircon U-Pb ages for intrusive rocks.

 

Fig. 7. Plot of all zircon U-Pb ages for Late Paleozoic to Early Jurassic intrusive and volcanic rocks ordered from older to younger with 2σ error bars; black bars=intrusive rocks and gray bars=volcanic rocks.

 

 

Fig. 8. Probability density plot and stacked histogram of all zircon U-Pb ages for igneous rocks.

 

The magmatic episodes of the Chilean Andes fall well within the 336 to 230 Ma interval of published U-Pb geochronological data for igneous rocks of Argentina (Pankhurst, 2006; Gulbranson et al., 2010; Rocha-Campos et al., 2011; Barredo et al., 2012; Suárez et al., 2012; Gregori and Benedini, 2013), but also with the interval from 342 to 190 of U-Pb data from the Andes of Perú, Ecuador and Colombia (Restrepo et al., 1991; Noble et al., 1997; Ordoñez and Pimentel, 2002; Vinasco et al., 2006; Chew et al., 2007; Ibáñez-Mejía et al., 2008; Cardona et al., 2008, 2009; Mišković et al., 2009, Mišković and Shaltegger, 2009; Witt et al., 2013; Boekhout et al., 2013). Therefore, despite local particularities, there was a rather consistent evolution in time of the magmatism along the southwestern, paleo-Pacific border of Gondwana.

7. Conclusions

U-Pb zircon geochronological data provide record of about 130 Ma of igneous activity in the Andes of northern Chile that extended episodically from the latest Early Carboniferous to Early Jurassic (328-194 Ma). The overall U-Pb data show that volcanism and plutonism were essentially synchronous and inferred major episodes of igneous activity developed during the Late Carboniferous to Permian (310 to 260 Ma) and from Late Permian to Late Triassic (255-205 Ma), with less prominent episodes in the mid-Carboniferous (330 to 320 Ma), and Early Jurassic (200-190 Ma). Thus, from the Carboniferous to the Early Triassic dominantly silicic magmatism developed along the Chilean segment of the southwestern border of Gondwana supercontinent. Further magmatism developed during the Mid-Late Triassic (250-194 Ma) was bimodal and synchronous with rift-related, continental and/or marine sedimentary strata related to the early stages of break-up of Gondwana.

Most of the siliceous volcanic rocks of the Precordillera and Domeyko Cordillera of northern Chile (21°30’ to 25°30’S) are older than the silicic rocks assigned to the Choiyoi succession in Argentina, being instead equivalent in age to Carboniferous to Early Permian marine sedimentary sequences present in the eastern Argentinean foreland. On the other hand, silicic volcanic successions exposed in the easternmost part of northern Chile are equivalent in age to the Choiyoi succession of the San Rafael Block of Argentina. An eastward expansion or migration during the Mid-Permian to Early Triassic is inferred by the age difference between older western rhyolitic deposits and younger eastern rhyolitic volcanic that originally were supposed to be of the same age; interpretation that is consistent with expansion of the volcanism at that time in Argentina.

The timing of the magmatism is coincident with that of the Andes of Perú and of Argentina according to the available U-Pb data, revealing a rather consistent evolution in time of the magmatism along the southwestern, paleo-Pacific border of Gondwana.

The correct allocation of lithostratigraphic units to the geological time results in changes in the stratigraphy modifying the tectonic interpretation and evolution of the studied volcano-plutonic event. The current U-Pb data allow a better understanding of the temporal evolution of the Late Paleozoic to Early Jurassic volcano-plutonic event in northern Chile, but certainly further work is required to solve particularly stratigraphic problems still remaining and some outcrops that have not been sampled by logistic reasons.

 

Acknowledgements
This study was supported by CONICYT, Chile, through Fondecyt 1110093 grant to V. Maksaev and F. Munizaga (Universidad de Chile). We are obliged to F. Hervé and his Fondecyt project 1095099 for sharing data from the Elqui-Limarí batholith and provided some samples for dating during this study. We thank K. Sato in charge of the SHRIMP and the staff of the Geochronological Research Center (Centro de Pesquisas Geocronológicas) of the University of Sao Paulo, Brazil, for their cooperation during the U-Pb dating on the SHRIMP. Reviews by V. Oliveros and C. Mpodozis contributed to improve this paper.

 

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Appendix 1

Tera-Wasserburg concordia plots of SHRIMP U-Pb zircon ages for samples dated during this study (ordered according Tables 3 and 4). Ellipses and error in ages in the inset are at 1σ; decay constant errors included.

Appendix 2

Shimp zircon U-Pb analytical data.