Journal of South American Earth Sciences

This paper presents geochemical, petrographic, and geochronological data on the Uatumã magmatism in the Pitinga Province, where it is represented by volcanic rocks from the Iricoumé Group and granitic rocks from the Mapuera Suite. The Iricoumé Group (1.89–1.88 Ga) is constituted of the Divisor Formation (intermediate volcanic rocks), Ouro Preto Formation (acid effusive rocks), and Paraiso Formation (acid crystal-rich ignimbrites, surge deposits, and basic rocks). The volcanic sequence is intruded by granitoids from the Mapuera Suite (1.88 Ga), mainly represented by monzogranites and syenogranites. Structural and field relations suggest that caldera complex collapse controlled the emplacement of volcanics and granitoids of the Mapuera Suite. Subsequent structure reactivations allowed the younger Madeira Suite (1.82–1.81 Ga) to be emplaced in the central portion of the caldera complex. The felsic Iricoumé magmatism is mainly composed of rhyolites, trachydacites and latites, with SiO2 contents between 64 wt% and 80 wt%. The plutonic rocks from the Mapuera Suite present SiO2 between 65 wt% and 77 wt%. Volcanic and granitic rocks present identical geochemical characteristics and that is attributed to their co-magmatic character. The felsic volcanic rocks and granites are metaluminous to slightly peraluminous and show affinity with silica-saturated alkaline series or with A-type magmas. They have Na2O + K2O between 6.6% and 10.4%, FeOt/(FeOt + MgO) varying between 0.76 and 0.99, Ga/Al ratios between 1.5 and 4.9, like typical A-type rocks; and plot in the within-plate or post-collisional fields in the (Nb + Y) vs. Rb diagram. The Nb/Y ratios indicate that these rocks are comparable to A2-type granites. This magmatism can be related to the (i) potassic alkaline series, with low Sr content in the felsic rocks explained by plagioclase fractionation at low pressure and high temperature or, alternatively, (ii) a bimodal association where magma had high crustal influence. The similarity of the Iricoumé felsic magmatism with A2-type granitoids and their high ETRL/Nb ratios suggest its relation with mantle sources previously modified by subduction, probably in a post-collision environment. Alternatively, this can be interpreted as bimodal within-plate magmatism with contamination by crustal melts. In this context, the extreme F, Nb and Zr enrichment of Madeira Suite could be explained by the presence of a thin crust which favored the presence and continuity of convective systems in the upper mantle. Resumo: Este trabalho apresenta dados geoquímicos, petrográficos e geocronológicos do magmatismo Uatumã na Província Pitinga, onde é representado pelas rochas vulcânicas do Grupo Iricoumé e granitóides da Suíte Mapuera. O Grupo Iricoumé (1.89–1.88 Ga) é constituído pela Formação Divisor (vulcanitos intermediários), Formação Ouro Preto (efusivas ácidas) e Formação Paraíso (ignimbritos ácidos ricos em cristais, depósitos tipo surge e raras rochas básicas associadas). A sequência vulcânica é intrudida por granitóides da Suíte Mapuera (1.88 Ga), representados principalmente por monzogranitos e sienogranitos. Relações estruturais e de campo sugerem ambientes associados a abatimento de caldeiras, controlando o posicionamento das rochas vulcânicas e plutons graníticos da Suíte Intrusiva Mapuera. Subsequentes reativações das estruturas tectônicas permitiram o posicionamento da Suíte Madeira (1.82–1.81 Ga) na porção central do complexo de caldeiras. O magmatismo Iricoumé é predominantemente composto por riolitos e traquidacitos, latitos com conteúdos de SiO2 entre 64% e 80%. Os termos plutônicos da Suíte Mapuera apresentam teores de SiO2 entre 65% e 77%. Rochas vulcânicas e graníticas apresentam características geoquímicas que se sobrepõem, o que é atribuído ao seu caráter co-magmático. Ambas são metaluminosas a levemente peraluminosas e mostram características geoquímicas consistentes com afinidade potássica alcalina e com magmas do tipo-A. Apresentam conteúdos de Na2O + K2O entre 6.6% e 10.4%, razões FeOT/(FeOT + MgO) variando de 0.76 a 0.99, razões Ga/Al entre 1.5 e 4.9, similares às de rochas tipo-A, e se posicionam no campo dos granitóides intra-placa ou pós-colisionais no diagrama (Nb + Y) versus Rb. A razão Nb/Y indica que são comparáveis a rochas tipo A2. Sugere-se que o magmatismo pode ser relacionado (i) à série alcalina potássica, com baixo conteúdo de Sr explicado pelo expressivo fracionamento do plagioclásio sob baixa pressão e alta temperatura ou, alternativamente, (ii) a uma associação bimodal onde magmas ácidos tiveram forte influência crustal. A similaridade do magmatismo Iricoumé com granitóides do tipo A2 e a alta razão ETRL/Nb sugerem sua relação com fontes mantélicas previamente modificadas por subducção, provavelmente em ambiente pós-colisional. Alternativamente, pode ser interpretado como magmatismo bimodal intra-placa com maior participação de fusões crustais. Neste contexto, o extremo enriquecimento em flúor, Nb e Zr da Suíte Madeira poderia ser explicado pela presença de uma crosta fina, favorecendo a presença e a continuidade de sistemas convectivos no manto superior.

Age and origin of the charnockitic rocks of the central part of the Guyana Shield have been a matter of discussion. These rocks have been interpreted either as Transamazonian granulites metamorphosed around 2.02 Ga or as 1.56 Ga old igneous charnockites. Recently, most of the Roraima charnockitic rocks have been recognized as igneous rocks and included into the Serra da Prata Suite (SPS). Five Pb–Pb single-zircon evaporation ages were obtained for samples representative of different facies of the SPS and these constrained the age of the charnockitic magmatism between 1943 ± 5 Ma and 1933 ± 2 Ma. This charnockitic magmatism may be related to a post-collisional setting after the evolution of the Cauarane-Coeroeni Belt (∼2.00 Ga), or may represent a post-collisional (or intracontinental?) magmatism related to orogenic activities along the plate margins around 1.95–1.94 Ga.

Combined strontium and neodymium isotope data on the Miocene-Pliocene Cordillera Blanca batholith (9–11 °S) are compared with acid plutonic and volcanic rocks of similar age and composition from the central Andes (ca. 16–23 °S). Although intruded through 50–60 km of continental crust the batholith rocks, which range in composition from quartz diorite to high silica leucogranodiorite, show little sign of contamination by mature continental basement. Initial ratios define a range of 0.7041 to 0.7057, with average ϵNd values close to bulk earth (−0.5), over a relatively large range in SiO2. It is difficult to reconcile the isotopic composition of the batholith rocks with simple AFC models involving fractionation of either clinopyroxene (deep-level) or plagioclase (high-level) dominated assemblages, and the isotopic variation in these rocks is instead considered to be inherited from a primary subcontinental lithosphere source through a two-stage process of crustal underplating and subsequent partial melting. Estimated (mantle) magma production rates during underplating are 0.1–0.3 km3 yr−1.

The opening of the Equatorial Atlantic Gateway (EAG) during the Cretaceous was accompanied by the disruption of the sedimentary basins that had developed on the conjugate margins of Africa and South America. Drilling along the Côte d'Ivoire–Ghana Transform Margin (ODP Leg 159) provided a transect across the northern rim of this gateway. The interplay of tectonic and oceanic processes along the gateway created a complex continental margin that evolved in three stages interrupted by dramatic changes in sedimentary facies, waterdepths, and subsidence rates. The earliest stage records the formation of small basins with restricted connection to the world ocean and rapid infill with siliciclastic deposits in an Early Cretaceous intracratonic rift or wrench tectonic setting. This stage ended with an uplift event and the formation of a regional unconformity. During the late Albian to middle Coniacian, the oceanward side of the margin subsided below the calcite compensation depth (CCD) and a deepwater connection between Central and South Atlantic became established. Deepening of the basement ridge and its landward slope, in contrast, were delayed and detrital limestones intercalated with carbonaceous shales accumulated at shelf to slope depths. During the ensuing, latest Cretaceous to present stage, passive margin subsidence led to continuous deepening of the basement ridge and on its landward slope. Condensation and gradually decreasing organic contents point to an intensified exposure to deepwater circulation. The replacement of the zonal circulation system through the Mesozoic Tethys and Central Atlantic with a modern, oxidizing meridional circulation system through the EAG appears to be intimately related to the changing depositional conditions over large parts of the Cretaceous Atlantic.

Paleozoic volcanic rocks of northern Chile (21-27°S) can be subdivided into Early Ordovician, Early Carboniferous, and Late Carboniferous to Triassic episodes. At the western and eastern margin of the Alta Cordillera and in the Chilean Precordillera, isolated outcrops of Early Ordovician submarine basic lavas and siliceous volcanic apron deposits occur (submarine deposition level <500 m). The basaltic-andesitic lavas and the calc-alkaline dacitic-rhyolitic volcaniclastic rocks probably formed in an active continental margin setting. South of 26°S in the Cordillera de la Costa, submarine (ultra-)basic lavas and subordinate siliceous volcaniclastic rocks are locally intercalated with Devonian/Carboniferous flysch. The alkaline and tholeiitic (picro-)basalts are assumed to have been formed in a continental tensional regime originating from partial melts of a garnet-bearing mantle source. The magmas of the associated siliceous volcaniclastic deposits possibly evolved from the tholeiitic melts. Extensive outcrops of thick Late Carboniferous to Triassic volcanosedimentary units are known from the Pre- and Alta Cordillera. The terrestrial volcanic rocks consist predominantly of different types of siliceous tuffs with subordinate basic lavas. The calc-alkaline spectrum of basaltic andesites to rhyolites displays geochemical features of an evolved active continental margin magmatism with a pronounced within-plate tendency. The Early Ordovician volcanic deposits are assumed to form part of the active continental margin of Gondwana. The Early Carboniferous volcanic rocks are seen as having formed during a tensional stage of the flysch basin development. An active continental margin setting, which is well established as a geotectonic model for the Southern Andes in Late Carboniferous to Triassic time, might have extended northward into the southern Central Andes.

The Late Precambrian–Early Paleozoic metamorphic basement forms a volumetrically important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26°S into crustal domains by means of petrological and age data (Sm–Nd isochrons, K–Ar). The metamorphic crystallization ages and tDM ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exotic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26°S are similar and reached the upper amphibolite facies conditions indicated by mineral paragensis and the occurrence of migmatite. Sm–Nd mineral isochrons yielded 525±10, 505±6 and 509±1 Ma for the Chilean Coast Range, the Chilean Precordillera and the Argentine Puna, and 442±9 and 412±18 Ma for the Sierras Pampeanas. Conventional K–Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Paleozoic orogen is confirmed by Devonian erosional unconformities. Sm–Nd depleted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65±0.16 Ga (1σ; n=12), average tDM of both gneiss and metabasite in NW Argentina is 1.76±0.4 Ga (1σ; n=22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic metamorphic and magmatic cycle. These new data indicate a largely similar development of the metamorphic basement south of the Arequipa Massif at 18°S and north of the Argentine Precordillera at 28°S. Variations of metamorphic grade and of ages of peak metamorphism are of local importance. The protolith was derived from Early to Middle Proterozoic cratonic areas, similar to the Proterozoic rocks from the Arequipa Massif, which had undergone Grenvillian metamorphism at ca. 1.0 Ga.

Using species of the genus Sclerocalyptus Ameghino, 1891, found in Argentina and considered valid, the authors recognize: (1) four species for the Ensenadan stage (Late Pliocene-Middle Pleistocene): S. pseudornatus (Ameghino), restricted to Buenos Aires province; S. ornatus (Owen), recorded at Buenos Aires, Cordoba, and Santa Fe provinces; S. perfectus (Gervais and Ameghino) in Buenos Aires and Santiago del Estero provinces; and S. corduhensis (Ameghino), endemic to west-central Cordoba province; (2) a single species for the Bonaerian stage (Middle-Late Pleistocene), S. migoyanus, restricted to the Buenos Aires province; and (3) Lujanian taxa (Late Pleistocene-Early Holocene) represented by Sclerocalyptus cf. S. heusseri (Ameghino), distributed in Buenos Aires, Cordoba, Tucuman, Corrientes, and Santa Fe provinces, and S. evidens (Ameghino) in Salta province. From a paleoenvironmental standpoint, the Sclerocalyptus species show adaptations to arid-semiarid and cold environments, such as strong development of the fronto-nasal sinuses, a characteristic that probably appeared during the Sanadresian-Ensenadan. Sclerocalyptus is not frequent in those areas in which relatively more humid and warm climates than those inferred for the Pampean region and central northern Argentina (e.g. Mesopotamia, west of Uruguay, south of Brazil) prevailed during the Quaternary. (c) 2005 Elsevier Ltd. All rights reserved.

This article reports on new records of the bivalve Pachydon hettneri (Anderson, 1928) from Miocene deposits in Venezuela, Colombia, and Peru. In particular, we focus on findings in the Chaguaramas Formation of northern Venezuela. The stratigraphic resolution of these records was recently improved, narrowing the age of these deposits to late Early–early Middle Miocene (Burdigalian–Langhian). These new Pachydon records imply that during the Burdigalian–Langhian, a lowland aquatic biogeographic connection existed between the Amazon region and Venezuela through the East Andean foreland basins. The species Pachydon hettneri may have given rise to evolutionary radiations in the Middle Miocene Pebas ‘long-lived’ lake-wetland system in Amazonia.

The upper Paleozoic miospore genus Spelaeotriletes Neves and Owens, 1966 is reviewed as a morpho-taxonomic entity and vis-à-vis other similarly constructed (pseudosaccate) genera - Geminospora Balme, 1962, Grandispora Hoffmeister, Staplin, and Malloy, 1955, Rhabdosporites Richardson, 1960, and Retispora Staplin, 1960. Detailed studies of numerous, mainly topotype specimens of Spelaeotriletes yhertii (Marques-Toigo, 1970) Playford and Powis, 1979 from the Lower Permian of Uruguay result in its re-diagnosis, in conjunction with a survey of its exclusively Gondwanan occurrences, particularly in South American strata extending from the Upper Carboniferous (West-phalian) into the Lower Permian, and also in Australian strata of approximately equivalent age. The characteristics of other species of Spelaeotriletes reported from upper Paleozoic deposits of Gondwana are discussed, as are their temporal representations in various broad regions of the supercontinent (South America, North Africa, Australia). These species include two, perhaps three, that, like Spelaeotriletes triangulus/arenaceus, are known also from Euramerica - S. balteatus (Playford, 1963) Higgs, 1996, S. pretiosus (Playford, 1964) Utting, 1987, and possibly S. owensii Loboziak and Alpern, 1978. Other species, such as S. benghaziensis Loboziak and Clayton, 1988, S. giganteus Loboziak and Clayton, 1988, and S. vibrissus Playford and Satterthwait, 1988, have, on present knowledge, exclusively Gondwanan occurrences, S. queenslandensis Jones and Truswell, 1992, known only from Upper Carboniferous strata of northeastern Australia, is formally reassigned on sculptural grounds to Grandispora. Not unexpectedly in a paleogeographic perspective, North Africa and South America are more closely allied with each other than with Australia in terms of shared species of Spelaeotriletes.

The Serrinha magmatic suite (Mineiro belt) crops out in the southern edge of the São Francisco craton, comprising the Brito quartz-diorite, Brumado de Cima and Brumado de Baixo granodiorites, granophyres and felsic sub-volcanic and volcanic rocks, part of which intruded into the Nazareno greenstone belt. The suite rocks have petrographic features that are consistent with magma supercooling due to the low water content combined with volatile loss, leading to crystallization of quartz and alkaline feldspar at the rims of plagioclase phenocrysts (granophyric intergrowth). The investigated rocks are sub-alkaline, calc-alkaline and show low content in rare earth elements. The U–Pb zircon crystallization ages for the Brumado de Cima granodiorite [2227 ± 22 (23) Ma] and a coeval granophyre [2211 ± 22 (23) Ma], coupled with available single-zircon Pb evaporation ages for the Brito and Brumado de Baixo plutons, are significantly older than the “Minas orogeny” (ca. 2100–2050 Ga) of Quadrilátero Ferrífero area, eastward from the Serrinha suite. Our data establish an early Rhyacian event tectonically linked with the evolution of the Mineiro belt. The bulk Nd isotopic signature [low negative to positive εNd(t) values] of the Serrinha samples are consistent with the important role of Paleoproterozoic mantle components in the magma genesis. The integrated geologic, geochemical and isotopic information suggests that Paleoproterozoic evolution of the Mineiro belt initiated in a passive continental margin basin with deposition of the Minas Supergroup at ca. 2500 Ma. This stage was succeeded by outboard rupture of the oceanic lithosphere with development and coalescence of progressively younger magmatic arcs during Rhyacian time. One of the earliest arcs formed the Serrinha suite. The tectonic collage of the Serrinha and Ritápolis (2190–2120 Ma) arcs produced the NE–SW Lenheiro shear zone, resulting in mylonitization and recrystallization of both the granitoid intrusions and host rocks. As a matter of fact juxtaposition of distinct magmatic units in age and origin took place along the Lenheiros structure in this sector of the Mineiro belt.

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene.

The July 24, 2001, Mw = 6.3 earthquake in Aroma, Chile, is one of the few moderately shallow earthquakes to occur recently in northern Chile. This study uses different seismological data (short-period, broadband, strong-motion) to locate the event and its corresponding aftershocks. In addition, it carefully constrains the focal depth using SP phase and the focal mechanism of the main-shock. Finally, a model of the strong-motion waveforms discriminates the activated fault plane among the two nodal planes. The main-shock fault plane solution obtained from the strong-motion analysis is (strike, dip, rake) = (14° ± 10°, 53° ± 15°, −163° ± 15°), which indicates a right-lateral motion on an inclined fault, in agreement with the aftershock distribution, which also indicates a fault striking N14°E and dipping about 50°E.

Based on numerous geoscientific data a section through the Central Andean active continental margin at 21°S has been compiled which shows the structure of the South American upper plate and the downgoing Nazca Plate.

A set of seismological stations was deployed in the Central Andes region along a ∼600 km long profile at 21°S between Chile and Bolivia and operated for a period of almost two years, from March 2002 to January 2004. Here we present the results of the tomographic inversion for P-wave velocity anomalies, based on teleseismic data recorded at the stations. The reliability of the results has been checked by a series of synthetic tests. The tomographic images show high-velocities on the west of the profile that are indicative of cold material from the fore-arc. A low-velocity anomaly is detected at the border between the fore- and the volcanic arc where the Quebrada Blanca seismic anomaly was previously described. This anomaly might be related to the presence of fluids that originate at the cluster of earthquakes at a depth of ∼100 km in the subducted plate. A strong low-velocity anomaly is detected beneath the entire Altiplano plateau and part of the Eastern Cordillera, in agreement with previous receiver function results. The Brazilian Shield is thought to be responsible for the strong high-velocity anomaly underneath the Interandean and Subandean regions.

Early Cretaceous plutonic rocks exposed south of Copiapó form part of the Coastal Batholith of northern Chile. These rocks intrude arc-derived volcanic and volcaniclastic rocks and marine limestones that were deposited in the Early Cretaceous Atacama backarc basin. The Copiapó plutonic complex consists mainly of calc-alkaline, medium- to coarse-grained diorite, granodiorite, tonalite, monzodiorite, and quartz monzonite. The plutonic rocks are subalkaline to alkaline, metaluminous, magnetite-series, volcanic arc, I-type granitoids. Batholithic magmas are a heat, potential fluid, metal, and sulphur source for the hydrothermal iron oxide-rich Cu–Au mineralization in the Candelaria-Punta del Cobre district. Ore-related hydrothermal alteration affected large portions of the Copiapó complex. The least altered batholithic rocks have initial 87Sr/86Sr of 0.703070–0.703231; initial 143Nd/144Nd of 0.512733–0.512781; and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 18.428–18.772, 15.550–15.603, and 38.127–38.401, respectively. The δ18O values for these rocks range from +6.9 to +8.6‰. Isotope signatures and trace element distributions suggest that the magmas are mantle derived. A subduction fluid-modified mantle source may explain the geochemical characteristics of the Copiapó complex. The ascent of magmas occurred along deep-rooted structures without significant crustal contamination, though minor contamination by relatively young (e.g. Jurassic) igneous rocks during ascent is possible. Intrusive rocks with high-K to shoshonitic characteristics probably represent residual liquids of less evolved magmas. The regional geologic context suggests that the plutons of the Copiapó complex were emplaced at a relatively shallow crustal level of 2–3 km.

The distribution and character of Tertiary magmatism in the non-volcanic region of shallow subduction between 28°S and 33°S in the Andes correlates with other geological and geophysical evidence suggesting that the subduction zone in this region has shallowed over the last 18 Ma with a major change occurring between 11 and 8 Ma. Geologic and geochemical data suggest that the magmatic source region under the Chilean Cordillera (29.0-30.5°S) changed at 16-18 Ma from garnet-poor to garnet-rich coincident with cooling due to shallowing of the subduction zone and thickening of the crust. Magmas older than 16-18 Ma appear to have fractionated at shallower crustal levels and interacted less with the pre-existing crust than have younger magmas. Except for small silicic ignimbrites, volcanism ended in the Cordillera at about 11 Ma. Magmatic activity spread to east to the Calingasta Valley and Precordillera in Argentina between 18 and 7 Ma (exact ages uncertain) as the subduction zone shallowed. The chemistry of the andesitic to rhyolitic composition rocks in this region are consistent with derivation from a thickened crust over a shallowing subduction zone. After 8 Ma, the principal magmatic activity migrated far to the east to the Pocho (7.8-4.9 Ma) and San Luis regions in the Sierras Pampeanas. The volcanic rocks (54-70% SiO2) in this area are geochemically distinct from recent volcanic rocks in other parts of the Andes, but are consistent with eruption over a deep segment of the seismic zone. Present-day volcanism in the "flat slab" region is lacking in the west due to insufficient heat for melting in the mantle above the slab, and in the east because the slab has already lost the components to flux melting in the mantle above the slab by the time temperatures are high enough to produce melting.

Magmatic activity in the El Indio-Pascua Au-Ag-Cu belt, situated in the Cordillera Principal at latitudes 29-30°S at the center of the southern Central Andean flat subduction regime, did not, as previously assumed, cease at 5-6 Ma but continued locally until the Late Pliocene. New and essentially identical 40Ar-39Ar laser step-heating ages of 2.1 ± 0.5 Ma (biotite) and 2.0 ± 0.2 Ma (glass) are recorded for a rhyolitic dome, the Cerro de Vidrio, in the northern Valle del Cura region near the Veladero Au (-Ag) property. The rhyolite is geochemically distinct from local Upper Miocene volcanic rocks; it is slightly but unequivocally peraluminous and does not exhibit significant REE fractionation apart from a pronounced negative Eu anomaly, a feature also shown by the Upper Paleozoic-Lower Mesozoic basement units of the area. This suggests that magma generation occurred in a garnet-free environment, which implies anatexis at shallower levels than for the rhyolites of the Upper Miocene Vallecito Formation.

The name Farellones Formation is currently used to designate continental Miocene volcanic and volcaniclastic deposits that form a 400 km long and 26-65 km wide, N/S-oriented belt of outcrops located along the central Chilean Andes. These deposits are mainly located east of an Oligocene-Miocene volcanic belt and west of the present volcanic arc. The lava flows and pyroclastics of the Farellones Formation vary in composition from andesitic to rhyolitic and, in lesser proportion, from dacitic to basaltic. Major element geochemistry and some trace elements confirm a typical calc-alkaline nature of the continental margin. The Miocene volcanic activity that formed the Farellones Formation occurred between 19.3 and 7.4 Ma, according to available K/Ar data, but this activity was not totally synchronous along the belt. It is estimated that 15.000 km3 of effusive material was extruded during this volcanic episode. The main Miocene volcanic activity can be related to an increase in the normal convergence rate between the Nazca and South American plates, which occurred between 26 and 9.6 Ma. The Farellones volcanic belt is not related to the present segmentation of the Nazca plate and thus represents an older Andean segment with a continuous and compositionally homogeneous volcanism possibly related to segmentation of the paleo-subduction zone.

The Miocene Albarracı́n Formation is a 1400-m-thick fluvial and alluvial succession interfingered with significant volcaniclastic deposits. Six fission track ages on zircons from intercalated ignimbrites constrain the age of the succession and allow rates of sediment accumulation and vertical movements in the basin to be estimated. The age of the Albarracı́n infill ranges from 18.0±2.6 Ma to an extrapolated 7.1 Ma at the top of the preserved sequence. The rates of sediment accumulation increase upwards in the succession varying from less than 0.04 mm/a (18–13.2 Ma) to 0.09 mm/a (13.2–9.1 Ma) and finally to 0.44 mm/a (9.1–8.3 Ma). The onset of the Central Cordillera thrust system is assumed to be at about 8.4 Ma as evidenced by the sudden increase of Precordillera clasts in the conglomerates. A minimum incision rate of 0.14 mm/a is determined for the San Juan River at the front of the Precordillera since 7 Ma to present.

Chronoenvironmental and tectonic charts are presented for Mesozoic basins located along the Andean foothills of the South American plate. On the basis of the main tectonic events, pre-Andean basins, break-up-related basins, extensional back-arc basins, and Andean foreland basins are recognized. The pre-Andean basins were formed by continental extension and strike-slip movement before the development of the Mesozoic–Cenozoic Andean magmatic arc. Upper Permian to Middle Triassic extension along Palaeozoic terrane sutures resulted in rifting, bimodal magmatism (Choiyoi group), and continental deposition (Cuyo basin). From the Late Triassic to the Early Jurassic, continental extension related to the collapse of the Gondwana orogen initiated a series of long, narrow half-grabens that filled with continental volcaniclastic deposits. These depocenters were later integrated into the Neuquén basin. Coeval development of the shallow marine Pampa de Agnia basin (42–44°S) is related to short-lived extension, probably driven by dextral displacement along major strike-slip faults (e.g. the Gastre fault system).

Recent studies carried out in the High Andes of central-western Argentina in the provinces of San Juan and Mendoza have established its stratigraphic and structural evolution. This paper presents new data on the Triassic–Early Jurassic rift system, the depositional sequences, and a synthesis of the tectonic evolution of the region, along with a correlation with the Chilean continental margin.The paleogeographic evolution of the Cordillera Principal at these latitudes is controlled by the development of the Mercedario rift system. This rift began with the sedimentation of synrift deposits of the Rancho de Lata Formation, during the Rhetian (about 190 Ma). Subsidence was driven by normal faults, locally preserved in spite of the severe tectonic inversion of the Andes during the Cenozoic. Different authors have emphasized that an important extension dominated the transition between the Triassic and Jurassic periods along the magmatic arc in the Coastal Cordillera of Chile on the western side of the Andes. Extension was related to the bimodal magmatism that characterized the evolution of this segment (30°–33° SL). The granitic plutonism and the associated mafic volcanism indicate that they were controlled by extension during 220–200 Ma. The first subduction related granitoids at these latitudes are 170 Ma old (Bathonian).The geometry of the Mercedario rift system may be reconstructed by the pattern of the normal faults. Rifting was followed by a thermal subsidence that expanded the original area of sedimentation and controlled the paleogeography of the Los Patillos Formation during Pliensbachian to early Callovian times. This period of cooling and thermal subsidence is correlated with magmatic quiescence in the continental margin. The evolution of the basin closely matches the magmatic history of the Chilean continental margin. Subduction at the continental margin began in the Bathonian, together with deposition of the upper section of Los Patillos Formation.Arc magmatism shifted to the Cordillera Principal during the Kimmeridgian, where it is represented by the volcanic and volcaniclastic deposits of Tordillo Formation.Early Mesozoic evolution of the Andean system at these latitudes is, thus, reconstructed by a comparative analysis of these two adjacent regions, driven by a common tectonic regime, but through different geological processes.ResumenEstudios recientes llevados a cabo en los Andes Principales del centro-oeste de Argentina, en las provincias de San Juan y Mendoza han establecido la evolución estratigráfica y tectónica de esta área. En este trabajo se presentan nuevos datos sobre el sistema de rift del Triásico–Jurásico Temprano, las secuencias deposicionales y una sı́ntesis de la evolución tectónica de la región.La evolución paleogeográfica de la Cordillera Principal a estas latitudes estuvo controlada por el desarrollo del sistema de rift Mercedario. Este rift comenzó con la sedimentación de depósitos de sinrift de la Formación Rancho de Lata, durante el Rético (aproximadamente 190 Ma). La subsidencia fue conducida por fallas normales, las cuales se encuentran localmente preservadas a causa de la fuerte inversión tectónica ocurrida en los Andes durante el Cenozoico. Diferentes autores han enfatizado que una extensión importante dominó la transición entre el Triásico y el Jurásico a lo largo del arco magmático en la Cordillera de la Costa de Chile, en el sector occidental de los Andes. El magmatismo bimodal es asociado a un perı́odo de extensión que caracteriza la evolución de este segmento (30°–33° SL). El plutonismo granı́tico y el volcanismo máfico asociado indica que ellos fueron controlados por extensión durante los 220–200 Ma. Los primeros granitoides asociados a subducción a estas latitudes datan de aproximadamente 170 Ma (Bathoniano).La geometrı́a del sistema de rift Mercedario pudo ser reconstruida por los patrones de fallas normales. El perı́odo de rifting fue seguido por subsidencia térmica que produjo la expansión del área original de sedimentación y controló la paleogeografı́a de la Formación Los Patillos durante el intervalo Pliensbachiano–Caloviano. Este perı́odo de enfriamiento y subsidencia termal es correlacionado con un episodio de quietud magmática en el margen continental chileno. La subducción en el margen continental comenzó en el Bathoniano, junto con la sedimentación de la parte superior de la Formación Los Patillos.El arco magmático migró en los Andes Principales durante el Kimmeridgiano, donde está representado por depósitos volcánicos y volcaniclásticos de la Formación Tordillo.La evolución del sistema andino durante el Mesozoico temprano a estas latitudes es ası́ reconstruida mediante el análisis comparativo de estas dos regiones adyacentes, pero con diferentes procesos geológicos, en un régimen tectónico común.

Mesozoic and Cenozoic granitoids of this segment of the Andes occur in three N/S-trending belts: western (WB), central (CB), and eastern (EB). The WB is formed by the Mincha and Illapel superunits; the CB includes the Cogotí superunit and the San Lorenzo unit; and the EB comprises the Río Grande and Río Chicharra superunits. KAr and RbSr ages show discrete ranges for each of the belts, with a pronounced eastward migration of magmatism with time: WB, Early Jurassic to Late Cretaceous; CB, early Tertiary; and EB, late Tertiary. The jumps in the sites of the magmatic belts correspond to essentially non-magmatic intervals (86-70 Ma and 39-26 Ma) and may relate to periods of subduction-erosion or changes in the dip angle of the subducted lithosphere. Periods of rapid migration correspond to specific changes in the Pacific Ocean spreading history. The superunits show relatively uniform major element oxide variation. The exceptions are the Limahuida granitoids which have characteristics of a confirm derivation of the parent magmas from the upper mantle with virtually no continental crustal involvment. This distinguishes the Mesozoic-Cenozoic granitoids from those of the Paleozoic belts of Chile, which have values systematically higher than 0.705.

The Alto Tunuyán basin is a Neogene foreland basin located between Cordillera Principal and Cordillera Frontal, from 33°30′ to 34°00′ south latitude. At this latitude, the feature that characterizes the subduction geometry beneath the Andean Cordillera is a transition in the slab dip from nearly horizontal, north of 33°S, to normal dip, south of 34°S. This particular tectonic setting apparently controlled the Neogene tectonic history of the area. The Neogene sedimentary infill of the basin is represented by the Tunuyán Conglomerate and the Palomares, Butaló, and Papal formations. Thrusting and uplift of the Cordillera Principal began during the early Miocene. Deformation and uplift of the volcanic arc, located on the western part of the thrust belt, produced the sediment source for the lower 200 m of the Tunuyán Conglomerate. As deformation migrated progressively eastward during middle Miocene times, it involved the underlying Mesozoic sequences, the erosion of which provided the material accumulated in the rest of the Tunuyán Conglomerate. The Palomares Formation unconformably overlying the former unit reflects the uplift of Cordillera Frontal. Deposition of the Butaló and Papal formations over the partially deformed broken foreland basin reflects accumulation during a period of tectonic quiescence and low rate of erosion in the eastern part of Cordillera Principal and the western part of Cordillera Frontal. The basement uplift of Cordillera Frontal generated a sticking point that prevented the propagation of the thrust belt toward the foreland. Consequently, out-of-sequence thrusts developed in the Cordillera Principal and the basin was partially cannibalized.

New petrographic and chemical data from Upper Palaeozoic-Triassic granitoids from the Frontal Cordillera of Argentina (33 degrees 10 to 33 degrees 45) are presented. Five stocks with ages from Early Carboniferous to Late Permian were sampled. The rocks are all calc alkaline with the mineralogy of each stock comprising essential plagioclase, alkali-feldspar and quartz with monor biotite and hornblende. Chemically, the rocks are similar to granitoids from the Frontal Cordillera of Chile, although the Argentinian stocks have generally higher K/Rb ratios. REE chondrite-normalized patterns and overall abundances, along with other petrological and geochemical similarities, suggest a common mode of origin for the granitoid stocks. The absence of systematic and wide variations in abundance of compatible trace elements suggest that the stocks represent magmas largely unaffected by extensive high-level fractional crystallization, and that their compositions are controlled primarily at source. Their age, tectonic setting and geochemical trends are consistent with magma pulses generated during crustal extension. The magma source region is considered to be relatively mafic lower crust. Copyright (C) 1996 Elsevier Science Ltd & Earth Sciences & Resources Institute

The mainly volcanic Cenozoic deposits that make up much of the western part of the Principal Cordillera in Central Chile are generally subdivided into two major units: an older Abanico or Coya-Machalı́ Formation and a younger Farellones Formation. Difficulty in differentiating these units has led to considerable debate. On the basis of the wide distribution, great thickness, and presence of sedimentary intercalations, it has been postulated that these arc volcanics were deposited in an intermontane basin; more recently, it has been proposed that this basin developed under extensional conditions and underwent subsequent tectonic inversion. We present field, geochronologic, geochemical, and thermal maturity data that support the latter interpretation. Collectively, this new information clarifies the stratigraphic, tectonic, and paleogeographic evolution of these deposits.