Volcanic Activity in the Puna, Argentina
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: email@example.com
2 CONICET – Universidad Nacional de Buenos Aires. Ciudad Univeritaria, Pabellon 2, Piso 1, (1428) Buenos Aires. E-mail: firstname.lastname@example.org
ORDOVICIAN VOLCANIC ACTIVITY IN 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.
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.
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
EN LA PUNA,
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
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
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.
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.
Ordovician volcanism. Arc. Back-arc. Trasarc
Ordovícico. Cuenca de arco y transarco. Puna
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).
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.
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.
volcanism of the Puna
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.
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).
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.
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
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).
rocks show geochemical similarities with metabasalts of the Vega Pinato
Tremadocian lower section.
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
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.
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).
magmatism in the Puna
during the Arenig reachs large diffusion in the Puna. Its volcano-sedimentary
successions are disconnected of the Lower Tremadocian deposits.
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.
fossiliferous associations of graptolites identified in such sequences allow
themselves to be placed in the Arenig, without having being proved up to now its
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).
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.
section culminates with a succession E of volcaniclastic turbidites with
exiguous fine pyroclastic intercalations.
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.
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).
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).
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.
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
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.
analysis of the siliceous tuffs indicate contents in SiO2<79
%, low Al2O3
%) 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
%) point out.
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.
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).
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.
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.
and Pérez 2001).
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
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.
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.
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
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.
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.
the basic volcanic representatives spilitic massive and in pillow lavas are
distinguished, as well as alkaline gabbro-basalt sills and dykes.
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.
basic and ultrabasic sills and dykes are represented by:
Stratified olivine gabbros (eg.: Santa Ana) from 5 to 500 m of thickness,
located concordantly in a sandy-pelitic deformed sequence.
Basaltic to hornblende andesitic-basaltic dykes slightly discordant, of 1 to 3 m
of thickness, deformed together with the rest of the Ordovician succession.
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).
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
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.
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.
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).
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.
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21 de Octubre de
Aceptado: 3 de Diciembre de 2002