Ordovician Volcanic Activity in the Puna, Argentina

Beatriz COIRA1 and Magdalena KOUKHARSKY2

1 CONICET – Instituto de Geología y Minería. Universidad Nacional de Jujuy, C. Correo 258, (4600) S.S. de Jujuy. E-mail: bcoira@idgym.unju.edu.ar

2 CONICET – Universidad Nacional de Buenos Aires. Ciudad Univeritaria, Pabellon 2, Piso 1, (1428) Buenos Aires. E-mail: mkou@gl.fcen.uba.ar

Abstract: ORDOVICIAN VOLCANIC ACTIVITY IN THE PUNA, ARGENTINA. The Ordovician volcanics in the Puna are integrated in two belts of submeridional disposition known as Faja Eruptiva de la Puna Occidental and Faja Eruptiva de la Puna Oriental. On the first one, which extends in the Northwest of the Puna and North of Chile, are found the most ancient outcroppings. Two magmatic cycles are represented there. The first cycle began with basalticandesitic manifestations in a shallow marine environment that were followed by dacitic-rhyolitic pyroclastic sequences. These ended in subaerial conditions. In general, they have arc like signature, with low K content denoting an evolution on an oceanic crust or a thin continental crust, though there exists basic volcanic rocks with transitional features to backarc type. This first cycle ended with the intrusion of plutonic rocks during the Upper Tremadocian. The second cycle is represented by andesistic-basaltic to andesitic volcanoes of reduced development, members of an arc to backarc setting, which evolved during the Arenig in a shallow marine environment. Episodes of an intermittent acid explosive volcanism are registered later on, meanwhile erosive stages are developed on the arc placed westwards. Towards the Middle to Upper Arenig, the lack of primary volcanic records, added to the existence of important turbiditic volcanoclastic sequences, mark the cease of arc volcanism and the beginning of the erosive phase.

The Faja Eruptiva de la Puna Oriental developed in a basin of trasarc. It represents a submarine lavic-subvolcanic magmatism, dominantly dacitic and subordinately basaltic, which began in the Tremadocian-Arenig boundary, extending during the Middle Arenig. Its manifestations are lavas and an important swarm of synsedimentary sills emplaced on an extentional environment. The siliceous volcanic members, of peraluminous character and weak arc signature, denote a sedimentary cortical component in the source. The alkaline to subalkaline mafic volcanic rocks are plotted in the within-plate field and can be roughly divided into low-Ti and high Ti groups, being the result of different fusion degrees of a garnet bearing mantle source. The Arenig magmatic episode ended with an Upper Arenig/Llanvirn syntectonic plutonism record along the Eastern Puna. During the Ashgill a collisional plutonism in the North of Chile and an important generalized contemporaneous deformation (Ocloyic Orogenic Phase) took place, resulting on the closure of the basin.

The most consistent model with the geological framework and the geochemical features for the Ordovician volcanism of the Puna considers the development of a volcanic arc during the Tremadoc with its axis in Chilean territory, which culminates with a plutonic episode. During the Arenig a new arc developed in that region, evolving in the North Eastern Puna a trasarc basin with associated bimodal volcanism, in a transpressive geodynamic regime with oblique strike-slip fault system, related to oblique arc convergence. That transtentional regime would have controlled the origin and location of the magmatism. Meanwhile, under that geotectonic configuration, there would have taken place an arc magmatism in the southern Puna.

Resumen: ACTIVIDAD VOLCÁNICA ORDOVÍCICA EN LA PUNA, ARGENTINA:. Las volcanitas ordovícicas en la Puna se integran en dos fajas de disposición submeridiana conocidas como Faja Eruptiva de la Puna Occidental y Faja Eruptiva de la Puna Oriental. En la primera de ellas, que se extiende en el noroeste de la Puna y norte de Chile se encuentran los afloramientos más antiguos. Allí están representados dos ciclos magmáticos. El primer ciclo se inició con manifestaciones basáltico-andesíticas en un ambiente marino poco profundo, que fueron sucedidas por secuencias piroclásticas dacítico-riolíticas, que culminaron en condiciones subaéreas. En general tienen características de arco con bajo contenido en K denotando una evolución sobre corteza oceánica o continental fina, aunque existen representantes básicos con características transicionales a retroarco. Este primer ciclo finalizó con la intrusión de rocas plutónicas durante el Tremadociano superior. El segundo ciclo está representado por volcanes andesítico-basálticos a andesíticos de reducido desarrollo integrantes de un arco-retroarco, los cuales evolucionaron durante el Arenigiano en un ambiente marino de poca profundidad. Episodios de un volcanismo explosivo ácido intermitentes son registrados posteriormente junto a etapas erosivas de un arco ubicado al oeste. Hacia el Arenigiano medio a superior, la falta de registros volcánicos primarios sumada a las importantes secuencias turbidíticas volcaniclásticas presentes, marcan el cese del volcanismo de arco y la erosión del mismo.

La Faja Eruptiva de la Puna Oriental se desarrolló en una cuenca de transarco. Representa un magmatismo submarino lávico-subvolcánico bimodal, dominantemente dacítico y subordinadamente basáltico, que se inició en el límite Tremadociano-Arenigiano extendiéndose durante el Arenigiano Medio. Sus manifestaciones son lavas y un importante cortejo de cuerpos filonianos sin-sedimentarios emplazados en un ambiente extensional. Los representantes volcánicos silíceos, de carácter peraluminoso y débil signatura de arco, denotan una componente cortical sedimentaria en su fuente. Las volcanitas máficas alcalinas a subalcalinas se ubican en el campo de intraplaca en dos grupos con diferentes contenidos de Ti, resultantes de distintos grados de fusión de una fuente mantélica portadora de granate. El episodio magmático Arengiano finalizó con un plutonismo sintectónico Arenigiano superior/Llanvirniano registrado a lo largo de la Puna Oriental. Durante el Ashgiliano tuvo lugar un plutonismo colisional en el Norte de Chile y una importante deformación contemporánea generalizada (fase orogénica Oclóyica), produciéndose el cierre de la cuenca.

El modelo más consistente con el marco geológico y las características geoquímicas del volcanismo Ordovícico de la Puna considera el desarrollo de un arco volcánico durante el Tremadociano con su eje en territorio chileno, el que culmina con un episodio plutónico. Durante el Arenigiano un nuevo arco se desarrolló en esa región, evolucionando una cuenca de transarco con volcanismo bimodal asociado, en la Puna nororiental, en donde tuvo lugar un régimen geodinámico transpresivo con un sistema de fallas oblicuas de desplazamiento tangencial vinculada a una convergencia de arco oblicuos. Dicho régimen transtensional habría controlado tanto el origen, como el emplazamiento del magmatismo. Mientras tanto bajo esa configuración geotectónica habría tenido lugar un magmatismo de arco en la Puna Sur.

Keywords: Ordovician volcanism. Arc. Back-arc. Trasarc basin. Puna.

Palabras clave: Volcanismo Ordovícico. Cuenca de arco y transarco. Puna


The Ordovician volcanism became important for the interpretation of the geological evolution of the Puna during the Lower Palaeozoic considering its extensive geographical distribution, the different types of extrusions and its geochemical features. Their records begin in the Early Tremadoc and reach their maximun expression in the Arenig. In general, they form two belts of submeridian disposition known as Faja Eruptiva de la Puna Occidental (Palma et al., 1986) and Faja Eruptiva de la Puna Oriental (Mendez et al., 1972). The former, the one that extends towards the west in Chile (Fig. 1), includes volcano-sedimentary sequences with trilobite faunas of Lower Tremadocian age, as well as others with graptofaunas Arenig s.l. to Middle late Arenig age and with less precision assignable to early Llanvirn. They correspond mainly to dacite-rhyolite pyroclastic and lavic rocks and volumetrically more limited basaltic and/or andesitic lavas , associated to volcaniclastic turbiditic sequences. Granitic-granodioritic plutons participate in this belt. Their ages K/Ar, Ar/Ar and 207Pb/206Pb are not well constrained and probably they associate to different magmatic-tectonic events that embrace the intervals 502-482 Ma, 450-440 Ma and 420-425 Ma (Palma et al., 1983, Mpodozis et al., 1986, Damm et al., 1990 and Koukharsky et al., 2002).

The Faja Eruptiva de la Puna Oriental registers, in the Argentina northern sector (22°–24° S), a dominant lavic to subvolcanic bimodal volcanism associated to clastic sequences with a fossiliferous content assignable to the early-Middle Arenig and U/Pb age of 467 Ma. Granitoids with ages U/Pb and K/Ar of 476 Ma and 428 Ma, respectively are also found in this sector. To the south of the 24° S it extends through a group of granite-granodiorites of U/Pb and K/Ar ages of 472-462 Ma, 450- 440 Ma and 420 Ma (Linares and Gonzalez 1990, Lork et al. 1991 and Lork and Bahlburg 1993), intruded in a metamorphic basement of medium to high grade with Sm/Nd ages of 515-502 Ma (Beccio et al. 1990).

Such magmatic belts have been defined and interpreted under different approaches, assigning them diverses geotectonic settings, which contemplate in their evolution the collision of allochthonous terranes with the southwest margin of Gondwana or assume parauthochthonous or authochthonous origins (Coira et al. 1982, Allmendinguer et al. 1982, Ramos et al. 1984, Dalziel and Forsythe 1985, Ramos 1986, Bahlburg 1990, Bahlburg and Breitkrenz 1991 and Conti et al. 1996). In that way such belts have been interpreted, as associated with two magmatic arcs related to subduction (Dalziel and Forsythe 1985, Ramos 1988 and Rapela et al. 1992), genesis proposed in some cases only to the occidental belt (Balhburg 1990, 1993). In opposition Davidson and Mpodozis (in Coira et al. 1982) and Aceñolaza and Toselli (1984) assign the magmatism of the Faja Oriental to an extensional ensialic region developed between the massif of Arequipa to the west and the Brazilian craton to the east and Damm et al. (1990) recognize an intrusive syntectonic arc in the Faja Magmática Oriental, which is associated with the Ocloyic collision. Coira et al. (1999), on the other hand, propose an evolutive model with initial development of an arc, in coincidence with the Faja Magmática Occidental (Upper Cambrian-Lower Arenigian), stage that is succeeded by an oblique convergent regime to which a strike-slip system associates in the north, and along which the emplacement of mantelic magmas take place in association with important continental crust fusion processes, whereas in the south an arc magmatism occurs. Towards the last stage, compressional (Ocloyic Orogeny), indicates the closure of the basin, deformation of the sequences and establishment of a plutonism of collisional type identified in the west.

Face to the various proposals of magmatic-tectonic evolutionary models for the Puna during the Lower Paleozoic, it is expected that the present work, through a critical revision and an updated synthesis of the existing data about the Ordovician volcanism, traces the geotectonic models cirscumscribing the problematic on petrological (including compositional aspects, styles and eruptive mechanisms, origin of the magmas and emplacement conditions, etc.), as well as biostratigraphical, geochronological and tectonic bases. In the treatment of the data a chronostratigraphical order is followed adopting even here the British subdivision for the Ordovician stages.

Tremadocian volcanism of the Puna

Among the most ancient representatives of the Ordovician volcanism in the Puna, the outcroppings of Vega Lari, in the Western sector are found (Fig. 1). There, covered in discordance by marine sediments with a Llandoverian age fauna, softly folded (Isaacson et al. 1976), outcrops a deformed sequence of about 360 m whose base is not visible, with hyaloclastic, pyroclastic and of volcanic sandstone levels from dacitic to rhyolitic compositions, besides siltstones, coquinas and cherts, with trilobites and sponges, intercalated between dark graptolite shales, all of them members of the Las Vicuñas Formation (Moya et al. 1993). Separated by a fracture are added rhyolitic and dacitic lavas, breccias and pumice tuffs belonging to the same association. The cuspated shards in low welded tuffs (Koukharsky 1997), as well as the presence of ripple marks in reworked tuffs and conglomerate lenses with diagonal stratification indicate a marine environment of very low depth, to which the lavic-explosive associated volcanism was partly subaerial.

The outcrops of the basal shales of Vega Lari extends towards the west reappearing above them acid volcanic rocks in Vega Pinato (Fig. 1). Between both localities, the rocks are transformed in dark cordierite hornfels by effects of intrusive granitoids which outcrop 5 km to the south. Among those hornfels there are metabasalts of several meters thickness intercalated, whose upper levels crop out in Vega Pinato associated to dark limolites and shales, thin coquines with trilobites and acid hyaloclastites (Koukharsky et al. 1996). In Vega Pinato the thickness of the Tremadocian column is estimated in more than 400 m. It culminates with a dark conglomerate, 1,5 m thick covered by 100 m of dacitic lavas, with peperites in the base and hyaloclastites facies in the roof, that reflect its emplacement in a submarine environment.

In the lower sections of Vega Lari and Vega Pinato, basalts correspond to arc tholeites with FeO*/MgO values from 1,40 to 2,10, SiO2= 50,3 to 51,7 % and low contents of K2O (0,2 to 0,5%) and TiO2 (0,8 to 1,1%), high La/Ta (40,6-49,4) and La/Th between 4,1 and 1,3 (Fig. 5) and flat REE patterns (La/Yb=0,6 and 1,9; Fig. 4), which make them comparable to the equivalent rocks outcropping in Cordon de Lila, in Chile (Coira et al. 1999). The abundant acid rocks are dominantly dacitic and rarely rhyolitic. Their geochemical features correspond to that of a calc-alkaline arc association, presenting the dacites (SiO2 between 66,0 and 67,8 %), contents of K2O between 2.2 and 3.5 %, high relations Ba/La=16,2 to 56,5 and La/Ta=31,3 to 48,6, low La/Th=2,8 to 4,2 and moderate REE patterns (La/Yb 8,5 to 14,5 – Fig. 4), with greater enrichment in LREE (La/Sm 4,0 to 6,2).

To the south of Vega Pinato, in Sierra de Macon (Fig. 1), granitoids of epizone, containing megaenclaves of volcanic and sedimentary hornfels, outcrop. Such metavolcanic rocks have compositions comprised between basaltic andesites and dacites, predominating the latter. Because of its texture and chemical composition they can be correlated with the volcanic rocks of Vega Pinato (Koukharsky et al. 2002). The granitoids are granodiorites, tonalites and granites with arc signature and an Ar/Ar hornblende age of 482,7±7,8 Ma (Koukharsky et al. 2002), which indicates its cooling during the Upper Tremadoc- Arenig. Diorites and granite porphyries of the Pocitos Complex, to the south of the Salar de Pocitos (Fig. 1), dated by the K/Ar method in amphibole and amphibolebiotite in 494±20 Ma and 470±17 Ma respectively (Blasco et al. 1996) have been correlated with those granitoids. Such plutonic event would be indicating that the magmatic arc activity would have extended up to the Upper Tremadocian, moment in which those plutons would have intruded the Lower Tremadocian volcanic arc sequences in the north of the Sierra de Macon.

In Quebrada Honda, in southern Puna (Fig. 1), mafic massive flows, sills and partially discordant bodies intercalated in a sequence of leptometamorphic wackes, siltstones and shales with chert and limestone levels, the whole section affected by an intense Pre-Devonian deformation. They are represented by basalts, microgabbros and in some cases by fine meladiorites and basaltic andesites.

They have transitional geochemical features among tholeitic basalts of volcanic arc and MORB (Fig. 6), just as they used to assemble in the tectonic environments of marginal basins (Coira et al. 2002).

These rocks show geochemical similarities with metabasalts of the Vega Pinato Tremadocian lower section.

To the south of the Salar de Pocitos, as well as in the Sierra de Calalaste, there have been recognized mafic-ultramafic complexes, originally assigned to ophiolitic successions (Allmendinger et al. 1982, Blasco et al. 1996), which present tectonic contacts with Early Ordovician sedimentary sequences. They have been considered by Zimmerman et al. (1999) as remnants of a Pre-Ordovician tectono-magmatic event with arc-backarc features, although rocks with intraplate signature (see Fig. 6) are recognized together with arc-N-type MORB rocks.

The most western representatives assignable to this initial magmatic stage belong to the Cordón de Lila Complex (North Chile). This Complex is composed by turbiditic sequences with lavic and explosive bimodal volcanism records (Niemeyer 1989). In its lower section tholeitic basaltic-andesites lava flows are registered cut by dykes and sills of similar composition and small microdiorite and gabbro stocks. Towards the medium high to upper section calc-alkaline dacitic to rhyolitic lavas and volcaniclastic rocks dominate. They correspond to shallow water marine sequences, which towards to the roof reached subaerial conditions, as their volcanic products point out. Paleocurrents determined in turbiditic sequences indicate southeast transport directions.

Chemical analysis for the Cordon de Lila volcanic sequence are given by Niemeyer (1989), Breitkreuz et al. (1989) and Damm et al. (1986, 1990, 1991). The lavas range from mafic to silicic in composition (42-68%) and have arc-like characteristics in that they plot in the low K arc field (Fig. 3), as I-type metaluminous rocks in Fig. 2 and in the arc field on the Hf-Th-Ta diagram in Fig. 7. Their extended normalized trace element patterns show little light REE enrichment and flat heavy REE, leading to low La/Yb ratio < 4 (Fig. 4) arc-like Th and U enrichments and arc-like Nb and Ta depletions (La/ Ta=25-36; Nb/Ta=17, Fig. 5).

Arenig magmatism in the Puna

Volcanism during the Arenig reachs large diffusion in the Puna. Its volcano-sedimentary successions are disconnected of the Lower Tremadocian deposits.

In the Western Puna are known volcanic-volcaniclastic and clastic sequences of Arenig age in Aguada de la Perdiz, Huaitiquina, Vega Pinato, Salina de Jama and Guayaos (Fig. 1) that are represented by pyroclastic flows, lapillites and crystal-lithic to vitric dacitic to rhyolitic tuffs intercalated in volcaniclastic turbiditic sequences, generally preceeded by basaltic to andesitic, massive, pillow or hyaloclastic lava flows. This group of rocks records events of a cal-alkaline volcanism with explosive siliceous stages, partially subaerial, and mafic lavic episodes (basaltic-andesitic pillow and massives lavas). Emergency and instability periods in the volcanic structures are represented in the thick epiclastic upper levels of volcanic sources (volcaniclastic turbidites and debris flows), which became thinner and present a less primary volcanic participation towards the tops.

The fossiliferous associations of graptolites identified in such sequences allow themselves to be placed in the Arenig, without having being proved up to now its Llanvirn age.

The volcanic-sedimentary successions of Aguada de la Perdiz (Breitkreuz et al. 1989), that totalize 2,700 m, present a coarse-grained turbiditic volcaniclastic lower section with volcanogenic debrisflows of basaltic and rhyolitic composition and an upper section, also epiclastic of volcanic origin, but fine-grained which shows intercalated tuff layers. Basaltic levels are distinguished in the lower as well as in the upper section, being clear its lavic nature only in the last case. This sequence, bearer of graptolite fauna, assignable to the bifidus Zone, is assigned on that basis to the Middle Arenig (Erdtmann, in Breitkreuz 1986).

In the Huaitiquina section (2,400 m thick) immediately to the southeast of Aguada de la Perdiz, it has been distinguished (Coira and Barber 1989) a basal unit A, represented by massive to brecciated basaltic andesites to andesites lava flows, and breccias of pillow fragments to which algal levels are associated. The next unit B is constituted by dacitic to rhyolitic crystal-vitric tuffs with frequent granulometric gradations and with scarcely intercalated epiclastic volcanic levels. The following unit C is integrated by andesitic-basaltic and dacitic massive lava flows, pillow lava breccias, and debris-flow deposits with reduced epiclastic psamo-pelitic intercalations. The suprajacent unit D corresponds to a turbidite volcaniclastic sequence in which scarcely dacite-rhyolite pyroclastic levels are recognized.

The section culminates with a succession E of volcaniclastic turbidites with exiguous fine pyroclastic intercalations.

In the upper sections D and E have been described graptofauna that include: Azygograptus eivionicus, Didymograptus (Expansograptus) sp aff D (E.) extensus, Pseudotrigonograptus minor, Dichograptus octobrachiatus, Tetragraptus sp., Xiphograptus sp., Xiphograptus svalbardensis, assigned to late Middle to Upper Aarenig (Monteros et al. 1996), wich marks out the end of the volcanism in the area.

The basaltic and andesitic units could have been gathered, from the geochemical point of view, in three groups that reflect the arc evolution throughout the time with features ranging from oceanic arc backarc to intraplate setting (Coira et al. 1999). The ones from the first group, (unit A-53 to 63 % SiO2) present arc characteristics, they are cal-alkaline with medium content in K, high Al2O3 (16-20 %), relatively low TiO2 (< 1.3 %), relations FeO/MgO<2, high La/Ta ratio (38 to 55; Fig.9), low Th/La ratio, and relations Hf-Ta-Th of arc characteristics (Fig. 7). The second group of mafic andesites (59 % SiO2), unit C, presents weak signs of arc signature as is shown by their low relations La/Ta (21), (Fig. 5) and high ratios La/Th (2.5), as well as its location in the diagram Hf-Th-Ta (Fig. 7). The third group is representated by basanitic dykes (~ 46 % SiO2) which have alkaline features and plot in the of intraplate field (Fig.7).

The dacitic and rhyolitic rocks of the units B and C have arc features as their concentrations of TiO2 < 0.5 %, Na2O> 4.0 % and K2O < 2 % indicate, as well as the high relations La/Ta (40-60) and Ba/La (20, Fig.5).

In the Salina de Jama, in sections that overcome the 1,200 m thick, levels up 400 m thick, of vitric and crystal-vitric tuffs intercalated in a turbiditic volcaniclastic sequence (Coira and Nullo 1989) are comparable to the tuffs unit B of Huaitiquina sections.

In Vega Pinato, located to the southwest of Huaitiquina, an Arenig volcano-sedimentary sequence crops out (Koukharsky et al. 1996). The presence of volcanic debris-flows, dacitic tuffs and calcareous levels ,bearing articulated trilobites and brachiopod , would allow to correlate them with the Huaitiquina sequence.

In the Sierra de Guayaos, sequences of 900 m thick were described (Coira et al. 1987, Koukharsky et al. 1989) with rhythmical intercalations of dacitic and rhyolitic pyroclastic flows and tuffs in a volcanic turbiditic sequence, with granulometric decrease towards the upper levels, in which scarcely andesitic to basaltic hyaloclastites participate.

Chemical analysis of the siliceous tuffs indicate contents in SiO2<79 %, low Al2O3 (<11 %) and Na2O that reach 0,2 %, concentrations that do not represent magmatic liquids. Considering on the other hand their high relations Na2 O/K2O (Fig. 2), high relations La/Ta (27-35; Fig. 5) and La/Th (2.5-2.9) and moderate slope of REE patterns (La/Yb=9 to 14; Fig. 4) would indicate arc/backarc conditions on a thin continental crust or oceanic crust (Coira et al. 1999). The mafic lavas (54 % SiO2), present high K2O contents (~3 %), high content in light REE (Ce ~ 105 ppm), Ba (1239), Th (15.8 ppm), and Rb (80 ppm) and enriched patterns in LREE in accordance with a shoshonitic classification (Coira et al., op cit.), showing arc signature as their relations Ba/La (24), La/Ta (58) and La/Th (3.3), high content in Al2O3 (18.3 %) and low in TiO2 (1.14 %) point out.

To the southeast of Salar de Pocitos Zappettini et al. (1994) describe shales, siltstones and subordinated sandstones with submarine acid volcanic intercalations of up to 1 m of thickness. The graptolites findings by Zimmermann et al. (1998) assign an Arenig age to these turbidites and to volcaniclastic conglomerates of the central portion of the Sierra de Calalaste . The volcanic manifestations extend up to the south of the Sierra de Calalaste (26° 30’S), where Allmendinger et al. (1982) mention the presence of pumicites fragments in rocks of Ordovician turbiditic sequences.

In the Eastern Puna the magmatic Arenig records are represented by volcanic and subvolcanic dacitic and spilitic syn-depositional rocks with thick arenaceous-pelitic sequences bearer of graptolitic faunas of Arenig age, all of which have been grouped by Coira et al. (1998) in the Complejo Magmático-Sedimentario Cochinoca-Escaya, in the homonymous ranges and also recognized in the Sierra de Taique (Pérez and Coira 1998) and in the Cerro Huancar (Coira and Koukharsky 1994) Lavas, autoclastic breccias, hyaloclastites, sills, extrusive domes, cryptodomes and volcaniclastic rocks of dacitic composition, constitute the siliceous representatives of the Complex, as well as its dominant magmatic facies (Coira 1996, Coira and Pérez 1998, Coira et al. 1999).

The lava-dome flows form layers of 20 to 150 m of thickness, which in some cases come to constitute units of multiple flows of up to 500 m thick. They are composed by dacite porphyric rocks with abundant phenocrystals of alkaline feldespar up to 5 cm long, oligoclase, quartz and biotite, in a groundmass that reflects its originally vitric nature. They generally present in their tops and marginal portions hyaloclastic facies and in some cases they are totally hyaloclastic. In some of their basal contacts it is possible to distinguish the interaction with the sedimentary substratum.

Sills and synvolcanic bodies, of similar composition, constitute layers of 10 to 30 m of thickness, concordant with the clastic sequence with which they show interaction through the peperitic borders.

(Coira and Pérez 2001).

The cryptodomes constitute discordant bodies of porfiric dacites of composition similar to the lavas, showing towards the roof the passage to dacite “in situ“ breccias succeeded by megapeperites of up to 20 m of thickness. Over them are distinguished volcaniclastic levels that correspond to redeposited hyaloclastites.

From the geochemical point of view the magmatic siliceous members, independantly from their eruptive style, show compositional uniformity; are in their majority of dacitic composition with 68 to 71 % SiO2, 2.2 to 3,2 % Na2O and > 4 % K2O (Fig. 2). They are peraluminous rocks with A/ CNK=1.1-1.4 consistent with melts derived from sedimentary sources, which gather in diagrams of granitic rocks clasification in the field of the granites S.

The dominant magmatic siliceous sequences in the Complex point out, in their relation with the hosting substratum, processes of magma-water saturated sediments interaction, with development of peperitic structures that corroborate the syn-sedimenty nature of the magmatism. The subaqueous conditions in this non explosive magmatism are revealed through the autobreccias and hyaloclastites in lava margins. On the other hand the presence of discrete levels of resedimented hyaloclastites on the tops of the dome bodies, indicate short periods of emergency and denudation of volcanic bodies with smooth relief (Coira and Perez 1998). The usual scarce participation of clastic volcanic material in the contemporary sedimentary sequences indicate a low rate for the extrusive volcanism, as well as a significant participation of it as sills and dykes emplaced in superficial levels, at the same time that an important rate of clastic sedimentation from cratonic areas.

The siliceous volcanic associations of the Complex represent facies of a mainly non explosive submarine volcanism, near to the emision zones, to which it can be estimated depths that overcome the 1,000 m (Mc Birney 1963, Cas and Wright 1987).

The linearity of the outcropping of volcanic sequences along kilometers, prove the tectonic control of the volcanism, which would have been canalized possibly along a deep cortical fracture zone.

The associations of the contemporary sedimentary facies with the volcanism and its features, reflect a sedimentation environment of the marine platform influenced by storms in which turbidites and tempestites took place, the latter under conditions of external and middle platform (Martinez and Perez 1998). The palaeocurrents and the compositional data indicate that the detritus derived from the east, recording indicators of an axial transport with north-northwest and west directions.

Among the basic volcanic representatives spilitic massive and in pillow lavas are distinguished, as well as alkaline gabbro-basalt sills and dykes.

The spilitic lavas (Coira and Koukharsky 1991) constitute layers of 6-25 m, massives, of microporphyric to fine granular texture that towards the roof are amygdaloidal. The pillow facies, of similar thickness, usually present ellipsoidal bodies of 3x1,5 m to 1x0,40 m sizes in which the cooled border highlights with abundant amygdulas and an intermediate zone, with concentric arrangement and contrasting vesiculation, poorly porphyric, with phenocrysts of albite and mafic minerals.

The basic and ultrabasic sills and dykes are represented by:

a) Stratified olivine gabbros (eg.: Santa Ana) from 5 to 500 m of thickness, located concordantly in a sandy-pelitic deformed sequence.

b) Basaltic to hornblende andesitic-basaltic dykes slightly discordant, of 1 to 3 m of thickness, deformed together with the rest of the Ordovician succession.

c) Basic and ultrabasic alkaline sills, from 10 to 1 m thickness. The first ones represented by alkaline basalts and basanites, aphyric to poorly porphyric and vesiculated (e.g.: Huancar, Sierra de Taique). The second ones belonging to the alkaline lamprophyre family (Coira and Jones 2002), are characterized by brecciation as a result of multiple intrusions and show clear peperitic contacts with the sedimentary host-rock (Sierra de Aguilar).

Chemical analysis of these mafic units indicate subalkaline to alkaline features and relations La/ Ta (8-13) that do not indicate arc environment (Fig. 5). Most of them plot in the intraplate field in the Hf-Ta-Th diagram (Fig. 7) and can be divided into two groups according to the Ti content. To the low Ti group belong the spilite lavas of the Sierras de Queta, Cordon de Escaya and Taique. This group generally has ~48 to 53 % SiO2 (anhydrous base), 1,2 % to 2 % TiO2 and < 0,6 % K2O (Fig. 3) and it locates in the normal or low in K subalkaline field and in the limit between the intraplate fields and E-MORB in the diagram Hf-Ta-Th (Fig.7). They show enrichment in the light rare earths (LREE) and relations La/Yb=10 to 12 (Fig.4), little anomalies of Eu and not of Sr, being their relations La/Th=8. The group with high Ti includes mafic and ultramafic alkaline dykes rich in Ti and K as for example the ones from Huancar, Sierra de Cobres, Cochinoca, Escaya, Taique and El Aguilar. These units are characterized by low contents in SiO2 (42 to 50 %), high TiO2 (2,8 to 4 %) and generally high K2O (0,36 to 2,3 %, Fig.3). They present normalized REE patterns with deeper slopes (La/Yb=19 to 55, Fig. 4) than the group of low Ti and a bit higher concentrations of incompatible elements. Even though there are differences between the two groups, the similarities in the relations of immobile and incompatible elements are consistent with the same garnet bearing mantelic source, representing magmas with low Ti and higher fusion percentage. Gabbro dykes of the Sierra de Taique, Rio de Las Burras and Santa Ana have aproximately 50 % SiO2, 1,7 to 2,7 TiO2 and <1,5 % K2O. Their chemical features are transitional between the spilites of low K and the alkaline intrusives with high Ti, which could be consistent with the derivation from the same source for all these mafic rocks.

The Eastern Puna volcanism previously described would have taken place in the lapse Upper Tremadoc/Lower Arenig to Middle Arenig. Data comes from the fossils records obtained in different volcano-sedimentary sections. In the Cerro Huancar the siliceous volcanic rocks are intercalated in an epiclastic sequence that towards the basal levels present a graptolite fauna (Hunnegraptus copiosus, Tetragraptus sp., Paradelograptus sp.), typical associations of the late Tremadocianearly Arenig (Benedetto et al., 2002). In the southeast of the Sierra de Cobres (Taique region) have been recognized partly hyaloclastic dacitic lavas and sills with petrographic and geochemical features similar to volcanic rocks assignable to the Upper Tremadocian-Lower Arenig outcropping in the Cerro Huancar, which intercalate in a sandy shaly sequence placed appearently continuous above a succession with trilobites: Parabolina (Neoparabolina) sp. and Leiostegium douglasi, assignable to the early Tremadocian (Vaccari et al. 1999). In the northern sector (Sierra de Quichagua) the sediments above the volcanic rocks contain graptofaunas assignable to the Middle Arenig, zone of D. bifidus (Martínez et al. 1999).

Concluding remarks

- The most ancient records of the Ordovician volcanism are found in the Western Puna – North of Chile. It encompasses the exposed sections in Cordon de Lila, Vega Pinato and Vega Lari, representative of a magmatic cycle that ended with the plutonic event represented by Choschas and Macon intrusives whose ages indicate the extension of the magmatic activity up to the Upper Tremadocian. Such submarine syn-sedimentary volcanism began with basaltic-andesitic manifestations in a shallow marine environment, that were succeeded by pyroclastic dacitic-rhyolitic sequences that ended in subaerial conditions. The succession shows arc features with low content in K. In the southwestern Puna the basic representatives denote, on the other hand, transitional features towards backarc conditions, which suggests that the main arc axis could be found in the present Chilean territory, at the Sierra de Almeida longitude.

- A new volcanic episode is registered in the Western Puna through lavic andesitic-basaltic to andesitic edifices of reduced development, representative of an arc-backarc setting which evolved during the Arenig in a shallow marine environment. Episodes of an explosive acid intermitent volcanism are later registered near erosive periods of an arc located to the west, revealed in the important volcaniclastic contribution in sandy levels as well as in debris-flows. Towards the Late Middle to Upper Arenig, the lack of primary volcanic records, added to the important turbiditic volcaniclastic sequences , mark the end of the arc volcanism and the erosion of its products. This change has been interpreted by Coira et al. (1999) as the answer to a more oblique convergence regime (Fig. 8).

- In the backarc basin, located in the Eastern Puna, a submarine bimodal volcanism basically lavic-subvolcanic, dominantly dacitic and subordinantly basaltic, starts in the Tremadocian-Arenig boundary expanding during the Middle Arenig (Fig. 8). Its manifestations, non explosive, represent facies near to the emission zone. They are lavas and an important court of syn-sedimentary sills and dykes that mark the recording of successive injections inside a sedimentary pile not yet consolidated, in an extentional environment. The latter reflects a sedimentary environment of external to medium marine platform, in which turbidites and tempestites took place, with detritic derivation from the east cratonic area, associated to a minimum volcaniclastic participation, which is indicative of a non explosive volcanism with scarce capacity for relief creation. The siliceous volcanic representatives, of peraluminous character and weak arc signature, denote a cortical sedimentary component in their magma sources. The mafic representatives, of alkaline to subalkaline features are located in the intraplate field, distinguishing among them two groups according to the Ti content, which would represent different fusion degrees of a mantelic source bearing garnet. The described features are consistent with a siliceous volcanism as a result of an important cortical fusion, to which a mafic volcanism would have associated, representative of mantle melts, that by mantle decompression ascended along the zones under extensional regime (Fig. 8). Such features agreed with the changing model for those times to an oblique convergence regime, to which it would have associated in the north Puna, in the backarc basin, a conspicuous fault system along kilometers, associated to a transtensional regime that would have controlled the origin as well as the emplacement of such magmatism. Meanwhile, under that configuration it would have taken place an arc magmatism in the southern Puna.

- A syntectonic Upper Arenig/Llanvirn plutonism, distinguished in the Eastern Puna, would mark the end of the Arenig magmatic episode. Records of a collisional Ashgill plutonism in the North of Chile and an important contemporany deformation (Ocloyic phase) state the closing of the basin


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Recibido: 21 de Octubre de 2002

Aceptado: 3 de Diciembre de 2002