Earth-Science Reviews', 18 (1982) 303-332 Elsevier Scientific Publishing Company, Amsterdam--Printed in The Netherlands

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Tectonic and Magmatic Evolution of the Andes of Northern Argentina and Chile

~ Beatriz Coira, 2john Davidson, 2Constantino Mpodozis and ~Victor Ramos

] Secretaria de Mineria, Buenos Aires (Argentina) e Departamento de Geologia, Universidad de Chile, Santiago (Chile)

ABSTRACT

Coira, B., Davidson, J., Mpodozis, C. and Ramos, V., 1982. Tectonic and magmatic evolution of the Andes of northern Argentina and Chile. Earth-Sci. Rev., 18: 303-332.

Two orogenic cycles, both with different evolution, are developed in the western margin of the South American continent in northern Argentina and Chile: the Paleozoic "Hercynic" cycle and the Meso-Cenozoic "Andean" cycle.

The Hercynic cycle. A wide marine basin extending westward of the Cordillera Oriental which developed in Cambrian-Ordovician times marks the beginning of this cycle. In contrast to Peru and Bolivia where this basin developed between two Precambrian blocks, the western margin of this basin in northern Argentina and Chile is still unknown. The Ordovician sedimentation and accompanied volcanism ends with the Ocloyic deformation phase and its synkinematic granitic plutonism. Two basins were developed in the Silurian- Devonian separated by the Puna arc, uplifted during this ocloyic phase. The shallow-water marine terrigeneous sediments which were deposited in them were deformed by a new tectono-magmatic associated phase (Chanic phase).

Carboniferous to Lower Permian marine carbonates were deposited west of the Puna arc and red beds east of it. Later on, during the Permian to Triassic, a magmatic belt was developed along the Cordillera Occidental. The rhyolites, ignimbrites and the granitic to granodioritic related intrusives are well represented in Chile. Although the overall geologic history of this period is known, many problems concerning its origin and relations to plate tectonics are still unsolved.

The Andean cycle. During this cycle, a series of magmatic-arc systems related to the subduction of the Pacific crust was built up along the western margin of South America. Huge volumes of calc-alkaline lavas and related plutons were emplaced since the Jurassic, along belts parallel to the present coastline, showing a general eastward migration trend. Up to the Lower Cretaceous, an ensialic back-arc basin was formed east of this magmatic arc. Thousands of meters of marine and continental sediments were deposited in it. This basin disappeared during the Middle Cretaceous, probably as a result of the final opening of the Atlantic and the active westward movement of the South American Plate.

Since Middle Cretaceous times, the magmatic-arc has been the fundamental paleogeo-

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graphic element. The arc progressively migrated stepwise eastward, each step marked by a tectonic phase. Subduction-related crustal erosion could explain the lack of fore-arc petro- tectonic assemblages. The eastward magmatic polarity which characterizes this section of the Andes, could also be explained by such a mechanism.

INTRODUCTION

The geological knowledge of the Andes between 22 ° and 26°S has recently considerably increased by exploration programs like the NOA I and Plan Cordillera Norte in the Argentine side, as well as by the Instituto de Investigaciones Geol6gicas (IIG) and the Departamentos de Geologia of the Universidad de Chile (Santiago) and del Norte (Antofagasta) field work in northern Chile.

A great amount of information achieved by these programs remains unpublished and therefore has not yet been incorporated in the recent tectonic syntheses of this region. The purpose of this work is to up-date geologic and geochronologic interpretation based on the new evidence, as well as to suggest some hypotheses on the tectomagmatic evolution of this segment of the Andes. The present analysis has been restricted to this part of the Cordillera because it has a peculiar and homogeneous evolution, distinc- tive from adjacent segments. It encompasses part of the Chilean Coastal Range and the Central Valley, the Puna, a flat surface at 4000 m above sea level surrounded by the Cordillera Occidental to the west and the Cordillera Oriental to the east of the Puna, and finally the Sierras Subandinas, the eastern foothills of the Andean Range.

This synthesis has been prepared as part of tl~e Program 120 "Magmatic Evolution of the Andes" of the I.G.C.P. and the authors want to express their acknowledgement to Drs. E. Linares and U. Cordani, who encouraged this compilation, and to the Servicio de Desarrollo Cientifico, Universidad de Chile (Grant E886-801), for supporting the Chilean authors' research. The authors of the present paper disagree in regard to the meaning of some Paleozoic events. Due to this fact, two alternative interpretations are put forward. We leave it to the reader to decide which hypothesis is the most plausible one.

Two major orogenic cycles are clearly present in this section of the Andes. The older has been compared by various authors to the "Hercynic" Cycle (vide Vicente, 1975) which, as the Caledonian event, cannot be clearly separated (Aubouin and Borrello, 1970) and apparently includes the whole Paleozoic. This tectomagmatic phase developed, at least in part, in an Upper Precambrian basement assigned to the Pampean or Brazilian Cycle (600 m.y.). The second major orogenic cycle corresponds to the well-known

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Andean Cycle which developed from the Triassic onward along the Pacific margin of the South American continent.

THE PRECAMBRIAN BASEMENT (PAMPEAN CYCLE)

In the Puna of Argentina the Precambrian basement is composed of a low-grade metamorphic sequence, which bounded this morphological unit by its eastern side. The western limit of the basement outcrops goes through the west side of the Sierra de Santa Victoria to Quebrada de Huamahuaca where it is displaced by the right-lateral Salinas Grandes Lineament (Mon, 1976). This outcrop boundary continues further south in the Cord6n de San Antonio de Los Cobres and Nevados de Palermo.

Slates, phyllites and schists of greenschist facies are the dominant rock types in the northernmost exposures. Southwards, the metamorphic grade increases up to medium high-grades. Their geochronology and metamor- phism seem to characterize a mobile belt bounded on the east by belts of higher grade and older metamorphism of the Brazilian Platform. This mobile belt is intruded by post-kinematic granitic to granodioritic intrusives like the La Quesera, Tastil, Santa Victoria, Cerro Fundici6n and Molinos plutons. These are typical epizonal intrusives of calc-alkaline composition (Kilmurray and Igarzabal, 1971; Ramos, 1973; Amengual and Zanettini, 1974). Whole- rock dating by Rb /S r method of the Tastil Granite yielded ages of 601 -+ 65 m.y. and 586 -+ 70 m.y., while further south, at Molinos, ages of 604 -+ 65 to 580-+ 45 m.y. have been obtained (Halpern and Latorre, 1973).

This mobile belt is assigned to the Pampean Cycle, as defined by Acefiolaza and Toselli (1973), which is partially equivalent to the Brazilian Cycle recognized further east, although it is the authors' opinion that the main diastrophic phase took place before 570 m.y., as proved by the age of the postkinematic plutons.

At the Chilean side, the only Precambrian age known has been obtained at the Puna of Arica, north of the analysed segment (Bel6n: Rb /Sr isochrone in schists of 1000 m.y., Pacci et al., 1980). Metamorphic rocks of uncertain age outcrop in several localities of the Tarapac/l and Antofagasta Cordilleras, as in Quehuita, Sierra Moreno, Lim6n Verde, Quimal (Harrington, 1961; Vergara, 1978; Maksaev, 1978) which are partially intruded by granitoids of Lower Paleozoic age (Huete et al., 1977).

It may be concluded that indisputed Precambrian basement rocks are represented only in the eastern side of the Puna. On the Pacific side, the presence of Precambrian rocks is still uncertain at this segment of the Cordillera.

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THE PALEOZOIC "HERCYNIC" CYCLE

It has been already mentioned that the Paleozoic evolution of this section of the Andes may be described in terms of a single orogenic cycle: the "Hercynic cycle" (regardless of its meaning or semantics) was characterized by alternating sedimentary a n d / o r volcanic events separated by short defor- mative phases, frequently associated to syn-kinematic plutonism. A Paleo- zoic orogen was thus constructed on the western margin of the South American Precambrian craton. The "Hercynic" cycle may be subdivided in two major periods, separated by the Upper Devonian-Lower Carboniferous unconformity the Early Paleozoic (Cambrian-Devonian) Famatinian and the Late Paleozoic (Carboniferous-Permian) Variscan structural stages.

Famatinian structural stage

This stage starts, in the Cambro-Ordovician with an important marine sedimentary event and related synchronous volcanism. This event is followed by deformation at the Ordovician-Silurian boundary (Ocloyic phase) and new less extensive Silurian-Devonian marine sedimentation before the Late Devonian-Early Carboniferous deformation (Chanic phase).

The Cambro-Ordovician sedimentation The Paleozoic sedimentation began in the Argentinian Puna and Cordillera

Oriental with the shallow-marine sandstones of the Mesrn Group (see Fig. 1; Turner, 1960) and in the Sierras Subandinas with those of the Candelaria Formation (Ricci and Villanueva, 1969). The Ordovician, probably unconfor- mably over the Cambrian, is well developed in the Puna (Turner, 1972;

Arequmpo Massif Ocean Turbddites BtaZlilan Shletd i

/. B~'e 7 .~,..,. + ,

So toca s h a l e s , / '

Acequipo Mpcroplote P~C'Tostil Gr

P a s s i v e m a r g i n s

Late Combriort-Early Ordovicion

Fig. l. Schematic section at 25°S during Late Cambrian-Early Ordovician times. C-- Cambrian elastic platform deposits of the Mes6n Group; 1 ----Tremadoc to Arenigian deposits of near-shore facies; 2----Arenigian distal platform facies; 3~-turbiditic flysch deposits of Arenigian to Llanvirnian age (Modified after Ramos, 1973, and Vicente, 1976).

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Schwab, 1972; Ramos, 1973), the Cordillera Oriental (Ramos et al., 1967) arenaceous-pelitic sediments of the Santa Victoria Group (Turner, 1972) and in the Sierras Subandinas (Mingramm et al., 1979). In the Ordovician, until the Arenigian an east to west facies zonation is observed, as it is presented in Fig. 1 (Ramos, 1972). From the Sierra de Santa B/lrbara south to Quebrada de Huamahuaca neritic platform facies are developed (Harrington and Leanza, 1957) which, towards the west, grade into distal platform facies (Ramos, 1972; Vicente, 1975). Westwards and up to 69°W a typical flysch facies is reported (Schwab, 1971) to which the isolated outcrops in Chile may be assigned: for example Aguada de la Perdiz (Garcia et al., 1962) and Puquios (Marinovic, 1979), where similar lithologies are present. The Ordo- vician facies distribution suggests that they represent the eastern border of a deep marine sedimentary "basin" whose western boundary is not yet well established (see, for example, Schwab, 1971).

The Ordovician synsedimentary volcanism An intense volcanism, partially synchronous with and /o r interbedded

with Ordovician sediments developed during Arenigian-Llanvirnian times along the Argentine Puna (see Fig. 2). It begins with spilitic lavas as observed in the Abra Pampa region. They are characterized by KzO contents 0.32- 0.96% against 47-52% of silica, and K 2 0 / N a 2 0 ratios of 0.2-0.05. Further south, Schwab (1973) indicates similar values for spilitic rocks, which per- mits us to extend the longitudinal development of these volcanic rocks. Ordovician diabases and basalts are also mentioned in Bolivia (Suarez- Soruco, 1976). The low potassium and silica contents (47-52%) permit us to define these basic rocks as low-K arc tholeiites according to Condie (1976) and Carmichael et al. (1974).

The volcanic sequence transitionally changes upward to a calc-alkaline series as described in Abra Pampa by Coira (1973, 1975). These volcanic rocks reached a more extensive longitudinal development which exceeds the analysed segment. Andesitic to rhyolitic rocks are known in the Lower

island Arc

Spi l i t ic f lows

Crustal COnsumpflon-mo~lmotlc ore construction

Middle Ordovi¢ion

Fig. 2. Deve lopment of a magmat ic arc in a volcanic-is land system. Volcanism was active since Middle to Late Ordovician times.

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Paleozoic from 17°S in Bolivia (Suarez-Soruco, 1976, p. 117) until 29°S in the Famatina area (Lavandaio, 1973; Alba, 1979). This longitudinal develop- ment permits us to reconstruct a volcanic arc through 1300 km, composed by andesites, dacites and rhyolites in lavic, piroclastic and ignimbritic facies, as described by Lavandaio (1973), Coira (1973, 1975) and Koukharsky and Mirr6 (1974). Some chemical data from Abra Pampa indicate a potassium trend in the calc-alkaline rocks as evidenced by K 2 0 / N a 2 0 between 1 and 1.8. This ratio, together with the K20/S i20 one, seems to identify a shoshonitic calc-alkaline association.

Few geochronological data are available for these volcanic rocks. Some K / A r minimum ages on whole rock yielded 420---5 and 416 + 20 m.y. (Linares, 1977, 1979). The age must be assigned based on the field relation- ships with the interbedded Ordovician sediments.

The Ocloyic tectonic phase and two possible interpretations of Early Pa/eo- zoic paleogeography

At the Ordovician-Silurian boundary, an important deformation event took place (Ocloyic phase, Turner, 1979). It folded the Ordovician sediments and metamorphosed them under prehnite-pumpellite facies conditions (Schwab, 1971). Contemporaneously a series of syn-kinematic granitoids were emplaced and dynamically metamorphosed. This dynamic metamor- phism also affected the Lower Paleozoic lavas. These plutonic a n d / o r volcanic deformed rocks crop out along an axis named "Faja Eruptiva de la Puna" in the Argentinian geologic literature (Fig. 3; Turner and M6ndez, 1975).

An outstanding characteristic of this belt is its tectonic contact with the Pampean platform along hundreds of kilometers, as described by Salfity et al. (1975) and Mon (1979). The intrusion of syn-kinematic and late-kine- matic plutons occurred also east of the Faja Eruptiva, in the Pampean

OCLOYIC OROGEN

Fojo Eruptiva de Io Puno

Arc* Platform tecton,c contact /

z

Crustal co lhs~on-p la te convergence ceased

La te O rdov i c i a n

Fig. 3. The ocloyic orogen characterized by the collision of the Arequipa microplate against the Southamerican Plate. This collision produced the tectonic contact of the volcanic-arc over the Precambrian platform.

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platform near Cafayate. Ages from 490 to 445 m.y. were registered by Rapela (1976). This magmatic event is synchronic with a thermal event superimposed on Precambrian metamorphic rocks in the eastern border of the Puna (485 -+- 15 and 450---15 m.y. by whole-rock K / A r ; Toselli and Acefiolaza, 1978). It is also registered in the Chilean side, where granites of 431 --- 10 m.y. (K/Ar) in the Quehuita area (Huete et al., 1977) and an alkali granite of 468 --- 100 m.y. by R b / S r in the Sierra de Almeida (Halpern, 1978) are known. The Precambrian rocks of Belrn, north of the studied segment, also show a thermal event dated at 440 m.y. by K / A r in muscovite (Pacci et al., 1980) which indicates the broad areal extent of the ocloyic phase.

According to two of the authors (B.C. and V.R.) the Faja Eruptiva de la Puna could represent a calc-alkaline arc related to subduction of an oceanic crust. The underlying crust of the Peruvian-Bolivian intracratonic basin thinned towards the south, to develop in northern Argentina a quasi-oceanic crust where the volcanic arc was emplaced. There are four reasons for this interpretation: (1) the Ordovician sediments may have deposited on the slope of an oceanic-floored intracratonic basin; (2) mafic lavas interbedded in some places with the sediments could represent oceanic crust; (3) no dated Precambrian outcrops are known in this segment of the Puna; and (4) finally that the Faja Eruptiva lavas belong to low-K arc tholeiites and to a calc-alkaline series. As a consequence of this hypothesis the Ocloyic phase would represent a closure of the oceanic basin and a collision between a Lower Paleozoic oceanic magmatic arc and the South American craton along a tectonic boundary which would run along the western margin of the Faja Eruptiva against the Arequipa Microplate.

The other two authors (J.D. and C.M.), by comparison with the Early Paleozoic paleogeographic setting of Peru (Dalmayrac et al., 1980), assume that an intracratonic sedimentary basin with a deep ensialic, thinned crust exists west of the Pampean platform with a hypothetical southern extension of the Precambrian Arequipa Massif. The discontinuous outcrops of meta- morphic rocks of probably Precambrian age in Chile would favor this proposition. The Early Paleozoic magmatism would thus represent an event which is not necessarily linked to subduction (Godoy, 1979) but to crustal extension in a hercyno-type basin, such as that put forward for the Early Paleozoic in Peru (Pichler, 1979; Dalmayrac et al., 1980). In contrast to northern Chile and Argentina, Early Paleozoic magmatism in Peru and northern Bolivia, is of minor importance.

The Siluro-Devonian sedimentation The distribution of the Silurian and Devonian is different from that of the

Ordovician. During this time, most of the Argentine Puna was a positive area ("Arco Punefio", Salfity, 1980). More than 2000 m of Silurian (Zapla

310

diamictite, Mecoyita and Lipebn Formations; Turner, 1960; Turner and M6ndez, 1979) and Lower Devonian (Baritk Formation, Padula et al., 1967) shallow-marine sandstones and shales were deposited to the east of it in the Cordillera Oriental and Sierras Subandinas. The eastern boundary of the Silurian-Dev0nian sea is not well known. Formations assigned to this age have been drilled in the Chaco-Pampean Plain up to near the Paraguayan boundary (Russo et al., 1979).

West of the Arco Punefio, the Lower Devonian is present only as shallow marine facies (Salar del Rincbn Formation, Acefiolaza et al., 1972) in Argentina and has its epimetamorphic equivalent in the Chilean Antofagasta (El Toco Formation; Wetzel, 1927; Bobenrieth, 1980) and the Atacama (Miller, 1970; Mercado, 1980)Coastal Range.

The Chanic tectonic phase and the end of the Famatinian structural stage Towards the end of the Famitinian cycle, the Upper Devonian-Lower

Carboniferous orogenesis occurred in the whole Central-Southern Andes as illustrated in Fig. 4 (Eohercynic phase of Dalmayrac et al., 1980; Chanic phase, Turner and M6ndez, 1975). Two tectonic domains are distinguishable in Chile and Argentina at that time. Along the Chilean coast, south of 29°S (Southern Coastal Domain of Herv6 et al., 1980), an important metamorphic event took place in rocks belonging to a subduction complex accreted to the South American continental margin. Radiometric dating of this complex of glaucophane schists, slates, metacherts and abyssal tholeiitic metabasalts (Gonzhlez-Bonorino and Aguirre, 1970; Herv6, 1977; Godoy, 1979) range from 329--+22 m.y. K / A r to 316 m.y. (Herv6 et al., 1970). Contempora-

Pa rnpean Mass i f

. , . • , . , • /

- - ~ . . . ; 4. + . ~ . +4+

323m y

No r t h Ch i l e - - No r t hwes te rn A rgen t i ne

Ea r l y La te Devon ian Carboniferous

Fig. 4. Schematic reconstruction after the Chanic compressive phase in early Late Devonian times. Calc-alkaline intrusives may represent a postorogenic reactivation of the subduction during Early Carboniferous.

311

neously with this metamorphic event a huge batholith, which may represent the roots of an arc, was emplaced along central-south coastal Chile.

The second domain corresponds to a wide zone east of the magmatic arc where generalized tectonism and plutonism, together with mesozonal low- P / T metamorphism in some areas of the Argentinian Cordillera Frontal takes place.

Although no subduction complex petrotectonic assemblages are known in the coastal region of northern Chile, it is possible that both northern Chile and Argentina represent an extension of the second domain. According to some of the authors (J.D. and C.M.) if a subduction zone was present here at that time it must have been located west of the Arequipa Massif or its hypothetical southern extension, because of the model of Dalmayrac et al. (1980).

At this time, a major folding and plutonism took place along the whole extent of the Famatinian orogen. Plutonism is represented by granitoids such as the calc-alkaline Taca-Taca, Chuculaqui, La Casualidad and Arita plutons in Argentina, whose ages range between 360-+ 1 and 323-+ 5 m.y. K / A r (Turner and M6ndez, 1979). Also the Chilean El Toco (21°30'S), E1 Abra- Quebrada Blanca (21°-22°S) and Lim6n Verde (23°45'S) plutons point to intrusive activity during the Lower Carboniferous (320 m.y. K / A t ; Huete et al., 1977). Numerous lead-alpha ages (350 + 40 to 260 --- 30 m.y., Inst. Invest. Geol., 1972) may also be assigned to the same intrusive event. A Rb /Sr isochron in the Faja Eruptiva de la Puna (374 -+ 7 m.y., Omarini et al., 1979) testifies to its important reactivation during the Late Devonian. This major deformation and plutonism event, linked to a profound change in the Paleozoic paleogeography has also been dated Late Devonian-Early Carboniferous in Peru and Bolivia, marking the end of the Famatinian or Eohercynian structural stage (Vicente, 1975; Salfity et al., 1975; Dalmayrac et al., 1977; 1980).

The Variscan structural stage

Two periods may be distinguished in the Late Paleozoic to Early Triassic (Variscan stage). First, in the Carboniferous-Early Permian interval, sporadic deposition of shallow marine and /or continental sediments occurred from the Chilean coast to the Sierras Subandinas. This was followed by the Middle Permian to Early Triassic interval, during which a magmatic arc was active in the Cordillera Occidental. Siliceous lavas, subvolcanic intrusives and consanguineous synchronous plutons characterized this arc. A tectonic event (Saalian phase) separates these two-Late Paleozoic periods.

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The Carboniferous-Early Permian marine sedimentation Carboniferous to Lower Permian calcareous and detritic terrigenous

marine sequences were deposited in Chile and the western Argentinian Puna unconformably on top of the Famatinian deposits (Fig. 5). Such is the case of the cerro 1584 (Davidson et al., 1980), the Antofagasta Precordillera, and the Sierra Fraga limestones, Atacama (Von Hillebrandt and Davidson, 1979). In this last locality, the calcareous sediments overly Famatinian or older pink granites. The quartz sandstones and limestones near Salar del Rinc6n, in the Argentine Puna (Cerro Oscuro Formation, the Carboniferous; Arizaro Formation, Permian, Acefiolaza et al., 1972) and discontinuous outcrops of probably continental sandstones and diamictites of the Cordillera Oriental (Macharetti Group, Mississippian, Mather, 1922) are also included. The rocks of this last unit, together with those of the Mandiyuti group are well developed in the Sierras Subandinas (Mingramm et al., 1979). This facies distribution is similar to that of the Devonian, when the Sierras Subandinas also were the main area of sedimentation.

The Upper Paleozoic magmatic belt The marine sedimentation in the Paleozoic orogen was terminated by

another folding episode (Saalian phase, Vicente, 1975). After this event an intrusive and effusive magmatic belt, active at least until the Middle Triassic, developed along 69°W. (see Fig. 6). The plutonic volcanic complex is made up mainly of rhyolites, dacites (both partly ignimbritic) and consanguineous granitoids (leucocratic granites and granodiorites) interbedded with scarce

CHILE ~ ARGENTINA

A r c o PuneSo I

Cord Oriental Sierra Froga Solar de Ar,zaro I (Cuenco Torilo) Crotdn S t,

't 1 [ Marl . . . . d cont,nent(~l I Corbong e Platform I I depos ts

- - - = ~ - L - I - L - = - , ~ ~ " ~ . ~ ' ~ ~ ~ .~ - -~ . ~ Y / ~

" z ~ ' / + + + " " i ~ * * t v ~ / - - -

C a r b o n i f e r o u s - L o w e r P e r m i a n

Fig. 5. Development of a carbonate platform west of the Arco Punefio during Late Carbonif- erous-Early Permian times. Marine-continental basins were present in the eastern areas. No magmatism is registered.

313

CHILE @ ARGENTINA

acidic ignimbrites and ondesitic- basaltic Lacustrine L~vo flows deposits

o o _ - ~ . . . . ~ " m . - J ' . -

U p p e r P e r m i a n - L o w e r T r i a s s i c

Fig. 6. Intensive and extensive calc-alkaline magmatism developed during Late Permian-Early Triassic times. East of the magmatic-arc, continental taphrogenic basins had some basic alkaline magmatism.

continental red beds. It is the major Paleozoic event registered in Chile. In the Cordillera Occidental of the northern Chile numerous radiometric dates range from 246 to 267 m.y. K / A r ages (Quirt, 1972; Zentilli, 1974; Farrar et al., 1970; Mc Bride et al., 1976; Huete et al., 1977). A thermal event (250-260 m.y.) probably linked to this intrusive cycle has been found in the Argentinian Precambrian basement. Some isolated Permian granitic plutons are known west of the main magmatic belt in the Coast Range at Taltal with ages Ar39/Ar 38 of 259 +-- 8 m.y. (Ulriksen, 1979). Eastwards of the main belt near Salta, K / A r dated subvolcanic olivine basalts and nephelinite basanites yielded an age of 254-+ 10 m.y. (Cortelezzi and Tasi, 1976). The granitoids and siliceous volcanic rocks of the main belt belong to a huge magmatic belt extending continuously for nearby 4000 km from Neuquen (Choiyoi- Mahuida, Groeber, 1946) along the Cordillera Frontal of Mendoza-San Juan (Choiyoi Group of Rolleri and Criado Roque, 1969) and to northern Chile. The relationship of this calc-alkaline belt with the Peruvian Cordillera Oriental (Mitu Group and related intrusives, Megard, 1973) is still unknown. Although this belt is geographically much better defined than older plutonism, its tectonic meaning is still uncertain. The Mitu Group in Peru included alkaline rocks such as comendites (Noble et al., 1978) and nephe- line syenites (Macusani; Laubacher, 1978) which are not recognized in the Argentine belt. According to Noble et al. (1978) the chemical composition of this Permo-Triassic belt in Peru is more related to "back-arc spreading" magmatism than to subduction magmatism. Chemical analyses of Permo- Triassic plutonism in the Cordillera Frontal of Mendoza, Argentina (Coira

314

and Koukharsky, 1976), pointed out that the Choiyoi Group at that latitude is calc-alkaline and probably more directly related to subduction. No studies or analyses are available in northern Chile and Argentina with which to characterize the chemical nature of this magmatic episode.

Remarks about Paleozoic evolution

The Paleozoic paleogeography of this section of the Andes is still far from being understood. Recent paleogeographic reconstructions in Peru and Bolivia (Dalmayrac et al., 1980) show a broad intracratonic ensialic sedimen- tary basin: an "hercyno-type basin" following Pitcher (1979). This basin developed between the Amazonic Precambrian craton and an hypothetical "Southeast Pacific Continent" of which the Arequipa Massif represents a remnant. No clear evidence of subduction is present in Paleozoic rocks of that area. In Argentina and central Chile that evidence does exist. Most of the Coastal Metamorphic Belt represents a subduction complex accreted to the South American continental margin during the Late Paleozoic (Herv6 et al., 1980). Thus, broadly speaking, the northern Chile-Argentina section of the Andes may be located in a complex transitional zone between an intracratonic regime (Peru-Bolivia) and a pericratonic one (central Argentina and Chile) which at least during certain periods of the Upper Paleozoic was an active continental margin. The main question is how the transition between these regimes took place. How far south did the Arequipa Massif extend; was the Peru-Bolivian intracratonic basin directly connected to the proto-Pacific? Did its crust progressively thin out to the south in Ordovician times, to become ocean-floored? On the other hand; how is the apparent contrasting chemical character of the Permo-Triassic magmatic belt to be understood? Once we know if a magmatic belt may give way longitudinally from a pericratonic setting with active subduction to that of an intracratonic one, we will be better able to answer these unsolved questions.

ANDEAN OROGENIC CYCLE

In contrast to the previous orogenic period, the Meso-Cenozoic (Andean) evolution seems to be more directly controlled by oceanic (Pacific) crust subducting under the South American continent. Superposed magmatic arc systems showing a general migration towards the east were thus originated. Two principal periods are distinguished within this wide time span.

An early period (Jurassic-Early Cretaceous), characterized by the develop- ment of a well defined magmatic-arc-back-arc basin pair (Fig. 7).

A late period (Late Cretaceous-Recent) during which only an eastward

315

migrating magmatic arc was present. No related back-arc basin existed at this time (Fig. 8).

The Triassic represents a transitional period between the Hercynian and Andean cycles. None of the paleogeographic elements that characterize either the Hercynian or Andean may be recognized. During this period marine sediments were deposited along the actual Chilean coast while in the western side of the Cordillera Occidental huge amounts of continental sediments and andesitic to basaltic lavas were deposited, related to a possible tensional tectonic setting (Charrier, 1979). Marine Triassic deposits in the coast south of Taltal unconformably overlie metasedimentary rocks and granitoids of Late Paleozoic age. They give way to Hettangian-Sinemurian marine sediments (Biese, 1956; Zeil, 1960; Garcia, 1967). In the Domeyko Range, the western boundary of the Puna, the Triassic sequences overlie siliceous volcanic rocks and granitoids of the Permo-Triassic magmatic belt. They are characterized by thick continental and lacustrine deposits with Unio sp. (Mercado, 1980) Podozamites and Equisetites (Chong, 1977) and vertebrate remains (Chong and Gasparini, 1975). These sediments are inter- bedded with andesite, basalts and their pyroclastic equivalents. This as- semblage has been reported in the western side of the Sierra de Almeida (24°15'S, Davidson et al., 1980) in numerous Domeyko Range localities (Davidson and Godoy, 1976; Chong, 1977) and in the Copiap6 Precordillera (Segerstrom, 1968; Jensen and Vicente, 1977; Mpodozis and Davidson, 1979; Muzzio, 1980).

The Jurassic-Early Cretaceous stage

The rnagmatic arc A thick volcaniclastic sequence developed along the coastal region of

Chile after Pliensbachian times, overlying marine sediments of Early Liassic age. This sequence is known as the La Negra Formation in northern Chile (Garcia, 1967), and it is composed of an association of subsaturated ande- sites, andesites, and olivine basalts, with increasing potassium toward the east and an initial tholeiitic trend (Losert, 1973). Nc Nutt et al. (1975) have suggested that Jurassic rocks represent magmas produced at an early subduc- tion stage, when the melted oceanic subducted plate was still at shallow depths below the South-American Plate. The hybrid 87Sr/86Sr ratios for these rocks (0.7043-0.7059) may be explained by a relatively high rate of convergence which might have produced some contamination with the continental margin.

Diorites, granodiorites, monzodiorites and tonalites closely associated with the volcaniclastic sequence of the La Negra Formation, are located along north-south belts, with eastward younging ages. The ages of the

316

Essenhol Poleogeogrophlc Peir

Mogmotic Arc - - Beck Arc Basin

Ourgssic- Eorly Cfetoceoul

Fig. 7. The magmatic-arc as developed since the Early Jurassic. Extension in the back-arc area produced crustal attenuation and marine basins. This paleogeographic setting prevailed until the end of the Early Cretaceous.

coastal batholiths range between 191 -+ 5.8 m.y. and 177 -+ 5.4 m.y. by K / A r (Quirt, 1972; Zentilli, 1974) and 186+6 m.y. by K / A r (Ulriksen, 1979). Towards the east, younger ages such as 148-+ 7 to 137-+ 4.4 m.y. by K / A t (Quirt, 1972; Zentilli, 1974) and from 150--+5 to 130-+4 m.y. by K / A r (Ulriksen, 1979) are present. The eastern flank of the Coastal Range has yielded ages of 125---4 to 112-+ 3.4 m.y. by K / A r (Quirt, 1972; Zentilli, 1974) and from 129 -+ 4 to 92 -+ 2 m.y. by K / A r (Ulriksen, 1979). The Upper Jurassic calc-alkaline volcanics, represented by the uppermost section of the La Negra Formation, underlie a younger cycle of volcaniclastics, well exposed in the Taltal area (Aeropuerto Formation of Ulriksen, 1979). This is characterized by 2000m of andesites, tuffaceous sandstones and sub- ordinated limestones with Exogyra sp. intruded by plutons of Hauterivian- Albian age.

The back-arc basin deposits Eastward of the magmatic arc, in the Cordillera de Domeyko and in the

Chilean Precordillera, sedimentation of thick calcareous sequences occurred during the Jurassic. They characterize a broad marine basin located between the magmatic arc and the Paleozoic foreland (Lautaro Formation, Segerstrom, 1960; Asientos and Montandon Formations, Harrington, 1961; Caracoles, Harrington, 1961; Profeta Formation, Chong, 1973). After Oxfordian time, their western outcrops are interfingered with andesitic flows derived from the arc (Cerro Islote, Chong, 1973; Sierra Candeleros, Biese, 1956).

The back-arc area has been characterized since the Late Jurassic by red-bed sequences interbedded with tidalites and sporadic marine Lower Cretaceous deposits such as in the Cordillera de Copiap6 (Von Hillebrandt, 1973), Potrerillos (Garcia, 1967) or Vaquillas Altas (Naranjo and Co- vacevich, 1979). These red beds include vertebrate remains of possibly Late Jurassic-Early Cretaceous age (Chong and Gasparini, 1975), as well as

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evaporitic facies (Tonel Formation, Dingmann, 1963; Cerritos Bayos, Biese, 1956; Rio Frio, Chong, 1973). At the top of the sequence interbeds of mesosilicic flows and breccias are observed in Agua Verde and Arca Forma- tions (Lahsen, 1969; Masaev, 1978) in the E1 Tatio-Ayquina and the upper Rio Loa areas. A K / A r age from this least area yielded a minimum Tithonian age (Thomas, in Maksaev, 1978). The easternmost outcrops of this broad back-arc zone are the bituminous shales of the Salar de Siglia (Briaggen, 1950).

The foreland magmatism East of the back-arc basin in the central Puna of Argentina, alkaline to

calc-alkaline magmatic activity is registered during the uppermost Jurassic and Early Cretaceous. This is represented by the E1 Aguilar and Tusaquillas granitic plutons with ages by Rb /Sr ranging from 147 -+ 10 to 96 ± 5 m.y. (Halpern and Latorre, 1973) and the Cobres Stock of syenitic to alkaline granitic composition with ages between 123-+ 6 and 129 + 8 m.y. The high 87Sr/86Sr ratios (0.9501-0.7437) may indicate crustal anatexis related to their genesis.

The Mid Cretaceous orogenesis and the end of the arc-back-arc pair

At the end of the Early Cretaceous the paleogeography was fundamentally changed by a tectogenetic event along the whole Andean domain (Vicente et al., 1973). After this event the arc-back-arc pair was replaced by a single eastward migrating magmatic arc which has remained active up to recent times. The boundary between these tectonic periods corresponds to the Middle Cretaceous Peruvian or "Subhercynian" phase and is contempora- neous with a pulse of increased spreading between South America and Africa at about 100 m.y. (Larson and Pitman, 1972; Ramos and Ramos, 1979). In Peru, these two periods have been interpreted as a general distension during the first period followed by generalized and persistent compression during the second (Marocco, 1979).

The various hypotheses put forward to explain tectonic asymmetry around the Pacific margins (Wilson and Burke, 1972; Packham and Falvey, 1971) may be used to understand the contrasting tectonic regime of the two main Andean evolution periods. According to Uyeda and Kanamori (1979) and Chase (1978) active marginal basins characterize the Western Pacific because the Asiatic Plate remains stationary or is slowly retreating with respect to the trench. In contrast, compressive horizontal stress related to active advance of the American Plate over its trench since the Middle Cretaceous precludes marginal basin formation in the East Pacific. If we consider that the

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American Plate was part of Gondwana up to the Middle Cretaceous, the tectonic regime in the southern Andes during Jurassic-Early Cretaceous time may be compared well with the actual West-Pacific framework. An ocean-floored marginal basin was developed at this time only in the southern tip of South America (Dalziel et al., 1974). The back-arc basin further north, although of similar tectonic significance, remained ensialic. The Middle Cretaceous "subhercynic" tectonic phase would thus represent the beginning of the American Plate as active westward advance that introduced a stress regime incompatible with the survival of back-arc basins.

Late Cretaceous to Present

The eastward migrating arcs of the Late Cretaceous--Early Tertiary Active volcanism and plutonism related to a subduction zone char-

acterized the Late Cretaceous. The magmatic arc was shifted eastward to the western foothills of the Cordillera Occidental (68°30'-70°W). Rhyolitic and dacitic flows, intruded by rhyolitic porphyries are representative of this arc. They are dated from 109---2 to 114-+2 m.y. by K / A r (Pefia Morada Formation, Maksaev, 1978), between 21-22°S latitude. Exposures are sporadic south of 22°S latitude (Quebrada Justo Formation, Lahsen, 1969; Augusta Victoria Formation, Garcia, 1967; Cerro Islote Formation, Chang, 1973).

Several intracratonic basins, partially interconnected, also developed dur- ing the late Cretaceous on the Argentine side. These are characterized by red-bed continental deposits of the Salta Group (Turner, 1960), which are the southern extension of the Puca and Tacuru basins of Peru and Bolivia.

eastward shiffing of mogmatic

arc axis (Upper Cret)

(Jurassic) _ _~I ~ , n t rG o~c bas,ns I Poleo~o,c * Early Mesolaic Basement

I i / Grupo SoI~o , ~ .... ~\

" ~ ' . colcolkohne mogmoltsm

L a t e C r e f o c e o u | - T e r t i a r y

Fig. 8. Eastward shifting of the magmatic-arc since Late Cretaceous to Miocene times. Continental intra-arc basins and taphrogenic troughs in the sub-Andean area associated with basic alkaline magmatism in the eastern region.

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These sedimentary rocks are associated with a basic alkaline volcanism (Coira, 1979), composed of trachy andesite and alkali basalts dated from 114 to 77 m.y. by K / A r (Reyes et al., 1976; Valencio et al., 1976). Total thickness reaches 10000 m for the Salta Group, which has a thin marine level at the top of the Cretaceous. This is represented by marls and limestones of Campanian to Maastrichtian age which covered a broad extension of the foreland region.

The magmatic-arc rocks of the Late Cretaceous are characterized by initial 87Sr/86Sr ratios of 0.70-0.7035 (Mc Nutt et al., 1975), which are lower than the Jurassic-Early Cretaceous ratios. A possible but not unique explanation of the anomalous ratios may be related to the fact that the Upper Cretaceous arc has been emplaced along the previous back-arc basin axis, which was developed on attenuated continental crust. This attenuation may have pro- duced a smaller crustal contamination and therefore lower initial 87Sr/86Sr ratios for the Upper Cretaceous arc.

Late Cretaceous subduction continue into Early Tertiary times with similar characteristics. A magmatic arc which produced a calc-alkaline volcanic pile, interfingered with continental deposits but was displaced eastward of the Upper Cretaceous arc.

On the upper reaches of the Loa River (21°-22°S), Paleocene (?) red-beds and evaporites are covered by up to 4000 m of andesites (Icanche Formation) which have yielded K / A r ages ranging from 42.7-+ 1 to 50.6 -+ 1 m.y. (Maksaev, 1978). Similar Eocene ages are found in the El Tat io-Ayquina area, 22 ° 15'S (Lahsen, 1969), as well as in the Precordillera area of Copiap6 (27°S). In this last area six K / A r ages from lavas yielded ages from 62.6 + 4 to 51.7 + 3 m.y., while the age of associated intrusives vary between 61.8 -+ 2 to 42 + 1.5 m.y. (Zentilli, 1974). Andesite and dacite flows, up to 200m thick, of the Cerro La Peineta Formation, also yielded Eocene ages of 52.5 + 2 to 55-+2 m.y. by K / A r (Sillitoe et al., 1968; Mortimer, 1974; Zentilli, 1974).

The 400 m thick andesites and rhyolites at the E1 Salvador mine, which unconformably overly Cretaceous "andesites", are cut and covered by an andesitic complex and rhyolitic domes of 50.3 -+ 3.2 and 50.4 -+ 2.8 m.y. All these rocks are crossed by quartz porphyries of 45.4 -+ 1.4 m.y. by R b / S r and 45.6---3 m.y. by K / A r (Gustafson and Hunt, 1975). Undated volcanic sequences, which may be similar in age, crop out west of Domeyko Range at 25°S (Chile-Alemania Formation of Chong, 1973, 1977).

These Eocene volcanic and intrusive rocks have yielded initial 878r/86Sr values of 0.7043-0.7057, which are clearly higher than those of the Upper Cretaceous (Mc Nutt et al., 1975). The higher initial ratios may be interpre- ted as resulting from contamination related to progressive thickening of continental crust.

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The Oligocene - - a period of relative magmatic quiescence A strong Middle Eocene folding event synchronous with the Incaic

orogenesis in Peru (Steinmann, 1929) ended the Late Cretaceous-Early Tertiary volcanism. No major volcanism or plutonism is recorded during the Oligocene, a magmatic "quiescent" period during which extensive continen- tal red-beds were deposited in Chile. This event is represented by the Sichal Formation, upper Loa River area, where welded dacitic tuffs have yielded a K / A r age of 34.7 + 0.8 m.y. (Maksaev, 1978); the San Pedro and Tambores Formations, San Pedro de Atacama area, made up by 2000 m of sandstones (Ramirez, 1979) where Travisany (1978) reports a K / A r date of 28 --- 6 m.y. in a tuffaceous bed. These last two formations are similar to the Oligo- Miocene Chojfias and Cajchimayo Formations in the Zapaleri area, along the boundary between Chile, Argentina and Bolivia (Marinovic, 1979). Argentinian Puna representatives of this event are: the Log Log Formation (Schwab and Lippolt, 1974) and the Pastos Grandes Group (Turner, 1961) which extends eastward as the Chaco Group up to the Cordillera oriental (Schlaginweit, 1938). Still further eastwards, in the Sierras Subandinas, 10000 m of Eocene-Pliocene red continental detritic deposits accumulated. Oligocene to Mio-Pliocene ages are registered in tuffs cropping out in nearby Bolivia. These sedimentary rocks probably represent the filling up of in- tramontane basins with detritus eroded from mountain ranges uplifted during the Incaic phase.

Late Oligocene-Middle Miocene volcanism During the Late Oligocene to Early Miocene, and after Oligocene con-

tinental deposits were strongly folded (Lahsen, 1982), renewed magmatic activity occurred still further eastward (Cordillera Occidental and the Argentine Puna). It represents the initial phases of a later, extensive Miocene-Pliocene volcanism. In Chile this event is recorded in the old andesitic strato-volcanoes of Jotabeche, Copiap6, Ojo de Maricunga, Dofia In6s, E1 Bols6n and Chaco. Zentilli (1974) reports K / A r dates of 24.8 + 1.0, 21.3 --+0.8, 15.8 -+0.8 and 15.0-+0.6 m.y. for these volcanoes located at the western border of the Puna. In the Copiap6 Precordillera (Cerro La Coipa, 26°45'S) subvolcanic dacitic bodies have yielded K / A r ages of 23.1 -+ 0.8 and 23.3 -+ 0.8 m.y. (Mc Nutt et al., 1975).

The magmatic activity in Argentina presents similar characteristics. A small monzonitic body (Nevado de Acay stock) has been dated as 26 -+ 1 m.y. (Mirr6, 1974; Linares, 1979) and subvolcanic andesitic bodies dated 20-+ 2 m.y. are present in Catamarca (Laguna de los Patos) and Jujuy (Cerrillos). This subvolcanic activity and associated sedimentation continues between 15.4 and 11.8 m.y. (Schwab and Lippolt, 1974; Mendez, 1974; Sillitoe, 1977; Coira, 1979), with important ignimbrites and ash flows of

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dacitic to rhyolitic composition, dated by K / A r between 10.8 and 8.9 m.y. (Schwab and Lippolt, 1974). At this time the main volcanic centers were erupting dacitic to andesitic rocks with associated subvolcanism, dated between 11 and 8.8 m.y., as observed in the Antofalla, Agua de la Falda and Aguas Calientes volcanoes (Coira and Pezzutti, 1976). In more restricted areas, ignimbrites of 15 to 13.5 m.y. were erupted (Cerro Agua de la Falda) (Coira and Pezzutti, 1976).

The available radiometric data suggest that a continuous overlapping between ignimbrites, ash flows and andesite volcanism of the same age also occurred in Chile south of 20 ° latitude. Acidic and related flows are represented by the Puchuldiza Formation at 20°S (12 m.y. K /Ar , Lahsen, 1980), the lower part of the Altos de Pica at the same latitude ( K / A r 15.5 and 15.1 m.y., Baker and Francis, 1978), the Ujina and surrounding ignimbrites of Co. Alconcha with 9.3 --+ 0.4 m.y. and 9.3 -+ 0.20 m.y., at 21°S, the Rio San Pedro ignimbrite at 22°S of 10.0 + 0.3 m.y. K / A r and the Rio Salado ignimbrite at 22°15'S of 10.6 +0 .4 m.y. K / A r (biot.). On the other hand, although the bulk of the andesitic material appears to date from the Pliocene and Quaternary, andesitic lavas from central vents are known at 21030 ' nearby Salar de Ascotan with 12.2 -+ 0.3 m.y. K / A r (biot.) and at Co. Carcamal (22°S), which has given a narrow range of 10 to 11 m.y. ages (Baker, 1977). There are other examples of Upper Miocene lavas such as the Millumi volcanic complex of Vergara (1978) and the Co. Irruputuncu lavas at 20°45'S. The latter has given a 10.8 + 0.6 m.y. whole-rock K / A r age (Baker and Francis, 1978). An older example of Miocene andesitic volcanism constitutes the Machuca volcano lavas in the Cordillera east of Antofagasta (17-+ 2 m.y., Ramirez, 1979). Scarce subvolcanic activity with shoshonitic affinities was also dated at 12.88 + 0.46 m.y. ( K / A r biot.) in the Poquis- Zapaleri area (Marinovic, 1979).

Upper Miocene (Quechua) diastrophism During the Miocene a major reorganization of spreading activity in the

East Pacific area took place. The active sea-floor spreading along the old Galapagos Rise ended during the Late Miocene, between 10 m.y. and 6 m.y. (Herron, 1972; Anderson and Sclater, 1972). At the same time spreading starts westward of the former, on the present East Pacific Rise (Pautot, 1975; Mammerickx et al., 1980). This tectonic event was accompanied at the continental margin by an important deformative episode, block uplifting and interruption of volcanic activity (Schwab and Lippolt, 1974; Coira, 1978; Lahsen and Munizaga, 1979; Lahsen, 1982). This diastrophism (Quechua phase of Charrier and Vicente, 1970) was dated in the Argentine Puna between 9.5 and 8.0 m.y. (Schwab and Lippolt, 1974; Coira, 1978). The same event was bracketed between 12-4 m.y. in the Puchuldiza area (Chile, 20°S,

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Lahsen and Munizaga, 1979) and dated as 7.4 m.y. in the upper Rio Loa area (Lahsen, 1982). These movements may be related to the event described further south in the Taltal region (25°S) by Godoy and Davidson (1976). Both at 25°S and further north this event involved extensive thrust faulting and block movements that represent a major uplift stage along this trans- verse of the Andes.

The uppermost Cenozoic volcanism Intermittent ejection of ignimbrites and lavas began soon after the dated

Quechua phase, both in the Chilean and Argentine sectors. Chile. Field and radiometric data obtained in Chile (Rutland et al., 1965;

Guest, 1969; Baker, 1977; Baker and Francis, 1978; Ramirez, 1979; Lahsen, 1982) show that while the ignimbrite eruptions were dominant from the Upper Miocene to the Lower Pliocene, the occurrence of andesitic lavas is especially significant during the Late Miocene and Quaternary. Examples of Miocene-Pliocene ignimbrites are the Ichuno ignimbrite ( K / A r 7.37 --- 0.37, 8.2 -+- 1.2 and 8.5 --+ 1.4 m.y., Maksaev, 1978) on the northern reaches of the Rio Loa area; the Toconce Formation, Sif6n ignimbrite and the Rio Salado volcanic Group of Lahsen (1982), cropping out in the Rio Salado and San Pedro areas (10.6+0.4 to 6.2-+0.15 m.y.), the San Bartolo Formation (10.0---0.4 to 7.0 ± 1.0 m.y., Rutland et al., 1965; Ramirez, 1979) and some other ignimbrite flows ranging from 8.5 ± 0.25 to 5.6 ± 0.20 m.y. dated by Baker (1977).

The Puripicar ignimbrite (4.2 m.y., Rutland et al., 1965) covers the above mentioned volcanic series in the E1 Tat io-San Bartolo area (Lahsen, 1982). Prior to its deposition, dacitic subvolcanic domes such as Copacoya and Piedras Grandes dated at 7.3 m.y. (Rutland et al., 1965) were emplaced.

Further south, at latitudes 23 ° and 26°-27°S, Dingmann (1963), Sillitoe et al. (1968) and Clark et al. (1967) report 12.6, 9.2, 4.7, 7.6 and 3 m.y. old ignimbrites. It has been already mentioned that intense eruptions of ande- sitic and dacitic lavas and the building up of cones and domes took place mainly from the Late Miocene to Recent. The following are examples of such volcanism: most of the volcanic chain east of the upper reaches of the Rio Loa (5.8+0.18, 5 .7±0.2, 3.6-+0.6 m.y. in Salar de Ascot/m, 21°30'S and 3.3±0.15 m.y. west of Ollague, 21°15'S; Baker, 1977); the Millunu Volcanic Group (7 .2±0.4 to 3 .4±0 .4 m.y., Baker and Francis, 1978); dacites from the Tucle volcanic group (0.8 m.y., Lahsen, 1969) in El Tario area; andesite lavas east of Toconce, at 22 ° 15'S (1.1 ---0.1 m.y., Baker and Francis, 1978); a basal lava flow from the Saire-Cabur volcano (Guatin andesite, 1 m.y., Ramirez, 1979); and andesitic lavas of the eastern volcanic chain in the Copiap6 Andes (0.885 m.y. and 1 m.y., McNutt et al., 1975).

Some of these Pleistocene lavas such as those near the Zapaleri volcano

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have shoshonitic affinities (Munizaga and Marinovic, 1979). Similar rocks crop out along a belt running from southern Peru to southwestern Bolivia down to northwestern Argentina (Deruelle, 1978; Aquater, 1979). The only few occurrences of Recent ignimbrites are known in Chile. One of them. the Pastillitos Ignimbrite (Vergara, 1979) exposed south of the Coposa basin has yielded 0 . 7 5 - 0.20 and 0.79-0.46 m.y. ages (Baker, 1977). An ash-flow dacitic tuff in the E1 Tatio-Caspana area (22°20'S) is also less than 1 m.y. old (Lahsen and Munizaga, 1979).

Argentina. The reactivation of volcanism in the Argentine side on the latest Miocene (8.0 - 6.7 m.y.) built up huge stratovolcanoes (Coira and Pezzutti, 1976; Coira, 1978; Aquater, 1979) such as the Queva, Chipas, Rachaite-Casabindo, Alto Nevado de San Pedro-Pairique, Coranzuli, Socompa, and Antofalla (Koukharsky, 1969; Vilela, 1969; Ramos, 1973; Sillitoe, 1975; Coira, 1978; Turner, 1978). Voluminous rhyodacitic-dacitic ejections were active during 8.9 to 8.0 m.y. building a broad ignimbritic plateau, between 22 ° 15' and 23°30'S (Coira, 1978; Aquater, 1979).

During the Eate Miocene, but mainly in the Pliocene, and up to 2.03 m.y., most of the effusive centers were reactivated. Rhyodacitic, dacitic, and andesitic to basaltic lava ejections were volumetrically more restricted than in the previous period. Examples of this activity are the Socompa, Chivinar, Rinc6n, Aracar, Cajero, Aguas Blanca, Granadas, Tinte, Campanario, Vicufia Huasi, Bayo, Coyamboy and Supusami stratovolcanoes (Schwab, 1973; Koukharsky, 1969; Turner, 1978; Aquater, 1979).

Dacitic-rhyolitic ignimbritic ejections have been dated between 4.8 and 3 m.y. (Schwab and Lippolt, 1974; Coira and Pezzutti, 1976). They are widely exposed on the plateau that extends from Cerro Zapaleri (22°45'S) to Salinas de Jama and E1 Rinc6n (24°15'S) and on the plateau south of Salar Aguas Calientes (23°45'-23°55°S).

The reduced Pleistocene ejections frequently show a basification trend from dacites to olivine andesites and basalts such as in the Cerro Morado, San Ger6nimo, Chascha, Paycuqui, La Poma, Aguas Calientes, and others (Acefiolaza and Toselli, 1973; Schwab and Eippolt, 1974; M6ndez, 1974; Coira and Pezzutti, 1976; H6rmann et al., 1973). They also are of shoshonitic composition as in the Cerro Negro de Chorrillos (Aquater, 1979).

In addition, rhyolitic-dacitic ignimbrites and lavas evolve to olivine ande- sites as observed in the Cerro Tuzgle, where ignimbrites range from 0.65 to 0.5 m.y. and the lavas from 0.3 to 0.1 m.y. (Schwab and Lippolt, 1974; Aquater, 1979).

The Holocene volcanic activity is even more restricted and is represented by limited dacitic-rhyolitic ignimbrites and andesitic-basaltic lavas ejected during historical times.

In earlier works, a simple bimodal stratigraphy for the Upper Tertiary

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volcanics has been suggested. The available field studies and radiometric data here presented show that this stratigraphy is only partly correct. As was previously stated, since the Miocene there has been a generalized overlap in time and space between andesitic lavas and ignimbrite flows. Further recon- aissance mapping and K / A r dates in the area will help to define different regional episodes, both in intensity and nature of volcanic activity along the length of the Andean chain. Seismic evidence for a segmentation of the subduction zone in this part of the Andes (Swift and Carr, 1974) may provide an explanation for the large variations in lateral correlations of lhe volcanic pile, if we accept that eruptions can be directly related to the complexities of the subduction process.

Both the Upper Cenozoic volcanic rocks in Chile and Argentina along this profile have 878r/86Sr ratios which are high compared to those of similar rocks in the southern Andes. While the ratios obtained in southern Andes volcanic rocks are consistent with an upper-mantle or oceanic-plate origin, those of this transect (0.7051-0.7071, Pichler and Zeil, 1972; 0.7060-0.7077, Mc Nutt et al., 1975; 0.70575-+ 6-0.70717-+ 5, Francis et al., 1977; 0.7051 0.7105, Klerkx et al., 1977) are much higher than those of volcanic island-arc rocks with a similar SiO 2 content.

The above mentioned ratios and the high K, Rb, Sr, Ba, REE, and Ni contents have been interpreted as the result of: (1) ensialic contamination of a mantle-derived magma with initially low 87Sr/86Sr (Klerkx et al., 1977), or (2) melting of continental lithospheric mantle previously enriched through melting of graywacke residual phases with low partition coefficients for the above mentioned elements (James, 1978). This model may explain the progressive increase through time of 87Sr/~6Sr ratios in Andean eruptives as well as intrusives from the Late Cretaceous to present, as due to recycling of "aged" sialic material.

REMARKS ABOUT THE EVOLUTION OF THE ANDES

In contrast to the Paleozoic, the Andean evolution seems to have a simple development as a series of eastward migratory magmatic arc systems directly related to subduction located on the western margin of the South American continent. During a first stage (Jurassic-Lower Cretaceous), an ensialic back-arc basin developed behind the magmatic arc which disappeared in the following stage (Cretaceous to Tertiary). We relate that change in tectonic regime to the final opening to the Atlantic Ocean and active westward movement of the South American Plate.

The following points remain unexplained in this rather simple model. The magmatism and paleogeographical development of the Andean Cycle is clearly subduction-controlled. In contrast, the Paleozoic geological history,

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magmatism, subsidence and deformation are hardly related to simple models of plate interactions. The origin of the Late Paleozoic-Early Mesozoic tectonic rearrangement remains unknown.

A second set of points that is still poorly understood is the eastward migration of magmatic loci during the Meso-Cenozoic, and the lack of Meso-Cenozoic fore-arc petrotectonic assemblages (subduction complexes, fore-arc basin sediments), such as those which characterize the western Pacific Island arcs. In this regard, it is worthwhile noting that the Jurassic magmatic arc lies only 50-80 km eastward of the present Peru-Chile trench, a distance which is probably too short to correspond to the original Jurassic geography. We favor Rutland's (1971) hypothesis regarding subduction erosion of the continental margin. This hypothesis would explain the re- moval of the missing Jurassic arc-trench-gap section if it existed. It would also explain the eastward migration of magmatic loci, as well as the abnormal thickness of the continental crust under the Andes in this area. This last phenomena may have been due to subcrustal accretion of "recycled" continental crust.

REFERENCES

Acefiolaza, G. and Toselli, A.C., 1973. Consideraciones estratigrfificas y tect6nicas sobre el Paleozoico inferior del Noroeste Argentino. Mem. Segr. Congr. Latinoam. Geol., 2: 755-764.

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PP.

[Accepted for publication April 8, 1981]