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.
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.
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.
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 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.
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.
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.
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 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.
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.
The late Tertiary tectonic and geological evolution of the Southern Andes at 33–34°S has been strongly controlled by the Challenger–Juan Fernández–Maipo (CH–JF–M) structural system. The present configuration of a flat slab between 28–33°S may be explained by a series of favorable conditions evolving with time since the breakup of the Farallon Plate at 25 Ma. The dramatic shift of the pole of rotation and the rapid eastward propagating rift along the Challenger Fracture Zone induced an unbalanced slab pull force, south of the CH–JF–M, that may have triggered the detachment of the subducting slab. The upwelling of a warmer asthenospheric material and the partial melts of the slab are likely consequences that are consistent with the anhydrous tholeiitic late Oligocene volcanism and the anomalous adakite-type magmatism of the early Miocene, respectively. The present seismogenic zone across the CH–JF–M tectonic boundary shows a continuity for more than 600 km along the flat slab segment, in contrast with the much shorter slab southward. Such a tectonic configuration is probably a quasi-steady condition since 25 Ma. Gravity modeling along the JF chain and the broadly located focal mechanisms at the locus of the already subducted JF chain indicate a thick (>25 km), wide (100 km), and continuous belt of lighter oceanic crust, which is the major contribution to the positive buoyancy of the slab. The decoupling of the subducted slab at intermediate depths further contributes to the flattening of the slab, focusing the buoyancy forces associated with the thickened oceanic crust along the already subducted JF chain. The absolute plate reconstruction during the Miocene shows that the JF collision against the margin migrates southward, in agreement with geological and tectonic observations that further support the causative relationship between the flattening of the slab and the subduction of the JF chain. Preliminary deformation models for the indentation of the JF ridge against the continental lithosphere is consistent with the particular east–west trend of the Maipo deformation zone, connecting in this way the CH–JF–M major tectonic boundary in the ocean-continent lithosphere system.
New geologic, paleontologic and isotopic geochronometric results from the Termas del Flaco region in the upper Tinguiririca River valley in central Chile demand considerable revision of the accepted geotectonic history of the Andean Main Range in this region. A diverse, transitional Eocene-Oligocene aged, land-mammal fauna was recovered from several sites in volcaniclastic sediments of the Coya-Machalí (=Abanico) Formation. Major results of our study include: 1) The 1000 + m thick studied deposits, previously attributed to the Cretaceous Colimapu Formation, belong to the Coya-Machalí (=Abanico) Formation. Radioisotopic data from levels immediately above (31.5 Ma) and below (37.S Ma) the fossiliferous horizon indicate a latest Eocene to early Oligocene age for the basal part of the formation and the fauna contained in it. 2) The fossiliferous unit rests with slight angular offset on different Mesozoic units: “Brownish-red Clastic Unit” (BRCU) and Baños del Flaco Formation; in a limited area it also overlies a white tuff dated at 104 Ma. 3) The contacts just discussed (none of which is attributable to faulting), demonstrate the existence of two, or possibly three, unconformities in the region. 4) Sedimentological criteria argue against reference of the BRCU to the Colimapu Formation, and imply correlation of the former unit to basal levels with in the late Cretaceous Neuquén Group of western Argentina. 5) The Coya-Machalí Formation, previously viewed as representing the western volcanic equivalent of Riogrd́ndico Supercycle deposits of western Argentino, is likely coeval to much younger units in that region such as the Agua de la Piedra Formation. 6) Paleomagnetic results from the fossil producing horizon indicate about 20 ° of post-early Oligocene, counterclockwise rotation. 7) Fossil mammals from the Coya-Machalí Formation near Termas del Flaco represent a distinct biochronologic interval not heretofore clearly recognized from elsewhere on the continent. This new fauna helps fill the long recognized post-?middle Eocene, pre-late Oligocene faunal hiatus between the Mustersan and Deseadan South American Land Mammal Ages (SALMA). In addition, it records the earliest known presence of rodents in South America and otherwise differs strongly from the enigmatic Divisaderan SALMA.ResumenEstudios geológicos, paleontológicos y radioisotópicos recientes efectuados en la región de Termas del Flaco, v alle superior del no Tinguiririca, Chile central, permiten modificar considerablemente la evolución lectánica de la Cordillera Principal en esta region. Los fósiles corresponden a una variada fauna de mamíferos colectados en varias localidades y niveles de edad eocena — oligocena de la Formatión Coya-Machalí (=Abanico). Los principales resultados obtenidos son: 1) Los depósitos de esta region previamente atribuidos a la Formatión Colimapu, cretácica, pertenecen a la Formatión Coya-Machalí (=Abanico). Las dataciones radioisotópicas y indican una edad Eoceno más alto (37,5 Ma) a Oligoceno inferior (31,5 Ma) para la parte inferior de la formatión y la fauna fósil contenida en ella. 2) Los depósitos terciarios fosilíferos se apoyan con discordancia de erosión y levé angularidad sobre diferentes unidades mesozoicas: “Unidad Clástica Pardo-rojiza” (BRCU) y Formation Baños del Flaco. En un reducido sector recubren a una toba blanca datada en 104 Ma. 3) Los contactos discordantes mencionados, ninguno de los cuales es atribuible a una falla, ponen en evidencia la existencia de dos, posiblemente tres, discordancias en esta region. 4) Se excluye la pertenencia de la BRCU a la Formatión Colimapu y se la correlaciona con el Grupo Neuquén, del Cretácico Superior, conocido en la vertiente cordillerana argentina. 5) La Formación Coya-Machalí, anteriormente considerada un equivalente volcánico del Superciclo Riográndico, sería coetánea con unidades más jóvenes de Argentina como la Formación Agua de la Piedra. 6) Los estudios de paleomagnetismo en muestras de los horizontes fosilíferos indican una rotatión antihoraria post-Oligoceno inferior de unos 20 °. 7) Los restos de mamíferos fósiles encontrados representan un intervalo biocronológico bien definido, no reconocido claramente hasta la fecha en América del Sur. Esta fauna completa el hiatus post?-Eoceno medio a pre-Oligoceno superior cuya existencia se acepta entre las Edades Mamífero Mustersense y Deseadense, registra la presencia más antigua de roedores en América del Sur y se diferencia claramente de otra Edad Mamífero, el enigmático Divisaderense.
In the segment between 38°S and 39°S of the southern South American continental margin volcanic belt, the volcanoes of the Quaternary “orogenic arc” may be divided into two groups: the first forming the volcanic front, which is located within the Cordillera Principal along a trend oriented 15° east of north; the second east of the volcanic front, occurring upon an uplifted precordilleran block that is east of the Bío-Bío/Aluminé fault system, trends west of north, and merges with the Cordillera Principal near 38°S. Quaternary “back-arc” plateau lavas, consisting of basalts erupted from small cones, occur in the valley east of this precordilleran block. Quaternary intra-arc and back-arc extension appear to be occurring in this section (38–39°S) of the southern South American volcanic belt and perhaps as far north as 34°S. South of 39°S, intra-arc extension is absent and the orogenic arc is restricted to the Cordillera Principal. Differences in the structural characteristics of the arc north and south of 39°S may be due to the change in age from older to younger oceanic lithosphere being subducted beneath, respectively, the two regions. Basalts erupted from both Quaternary arc and back-arc centers located east of the volcanic front between 38°S and 39°S have higher K20, Rb, Ba, and LREE contents, and higher La/Yb but lower Ba/La ratios, than basalts erupted along the volcanic front. 87Sr/86Sr ratios for arc lavas from volcanic centers both within and east of the volcanic front range from 0.7038 to 0.7041 independent of their Si02 content, suggesting that more silicic compositions developed from basalts by crystal-liquid fractionation without significant crustal assimilation. Back-arc plateau basalts from the region 38–39°S have Sr isotopic compositions similar to those of the arc volcanoes. Basalts from both the arc and the back-arc centers between 38°S and 39°S may form by partial melting of a large-ion-lithophile-element-enriched mantle source similar to the source of mantle-xenolith-bearing Quaternary alkali plateau basalts from southern Patagonia (south of 39°S), further modified by the addition of alkali elements derived from subducted oceanic lithosphere.ResumenEn el sector entre los 38°S y 39°S del margen continental Sudamericano, los centros del arco volcánico orogénico Cuaternario puede ser divididos en dos grupos: el primero comprende el frente volcánico, que con una orientación nor-noreste se localiza en la Cordillera Principal; el segundo incluye centros ubicados al este del frente volcánico sobre un bloque precordillerano, individualizado al este del sistema de falla Bío-Bío/Aluminé, que con orientación nor-noroeste converge con la Cordillera Principal aproximadamente a los 38°S. En este sector de los Andes el volcanismo Cuaternario de tras-arco se manifiesta en erupciones basálticas relacionadas a conos pequeños que ocupan un área topograficamente deprimida al este del bloque precordillerano. Posiblemente durante el Cuaternario ha tenido lugar extensión intra-arco y tras-arco en este sector de los Andes y, también entre los 34°S y 38°S. Al sur de los 39°S no existe evidencia de extensión intra-arco y el arco volcánico orogénico está restringido a la Cordillera Principal. Las diferencias en las características estructurales del arco volcánico Cuaternario al norte y al sur de los 39°S pueden ser el resultado de variaciones de edad, desde más antigua a más jóven respectivamente, de la litósfera oceánica subductada bajo estas dos regiones. Los basaltos extruídos entre los 38° y 39°S por los centros cuaternarios del arco y tras-arco localizados al este del frente volcánico tienen mayores contenidos de K20, Rb, Ba, de elementos de las tierras raras livianas, y mayores razones La/Yb, pero menores razones Ba/La que los basaltos asociados a los centros del frente volcánico. Las razones 87Sr/86Sr para las lavas del arco volcánico, tanto para los centros volcánicos que forman el frente volcánico como para los centros del arco al este del frente volcánico, varían entre 0.7038 y 0.7041 independientemente del contenido de SiO2, sugiriendo que las composiciones mas silícicas se producen por cristalización fraccionada de magma basáltico, sin una asimilación cortical significante. Los basaltos cuaternarios de trasarco de la región 38–39° tienen composiciones isotópicas de Sr similares a los basaltos del arco. Los basaltos tanto del arco volcánico como de los centros de tras-arco, ubicados entre los 38°S y 39°S, pueden haberse fomado por fusión partial de tins fuente en el manto enriquecida en elementos litófilos de gran radio iónico, similar a la fuente de los basaltos cuaternarios alcalinos de tras-arco portadores de inclusiones ultramáficas del manto del sector sur de la Patagonia (al sur de los 39°S), pero modificada por la adición de elementos alcalinos derivados de una litósfera oceánica subductada.
The Cordillera de Carabaya region of the southern Peruvian Andes preserves a collage of Tertiary igneous suites, including extrusive and hypabyssal units, that are herein assigned to the Crucero Supergroup and Crucero Intrusive Supersuite, respectively. The supergroup consists of two petrologically and temporally distinct assemblages, the Picotani and the Quenamari Groups, both with silicic hypabyssal representatives. Stratigraphic, petrologic and Ar-40-(3)9Ar geochronologic studies of these rocks demonstrate that they are exposed in four localities, the Quenamari, Antauta, Cayconi and Picotani volcanic fields. The Picotani Group incorporates a diverse assemblage of lavas and pyroclastic rocks, including lamprophyres (minette), medium-to high-K calc-alkaline basalts, shoshonites, and S-type rhyodacites and rhyolites, as well as commingled and mixed associations of these, constituting ten mappable formations and twelve delimited hypabyssal units. These were emplaced over a brief interval from ca. 22-to-26 Ma. The Quenamari Group, in contrast, comprises entirely silicic, strongly peraluminous suites including, biotite + sillimanite +/- muscovite +/- andalusite +/- tourmaline-bearing pyroclastic units and cogenetic epizonal intrusions. These constitute three distinct volcanic formations and at least 8 intrusive bodies. Rocks of the Quenamari Group span a broad temporal interval, from ca. 6.5 to 17 Ma. The rock associations of the Crucero Supergroup are clearly distinct from time-equivalent igneous units of the calc alkaline, Main Andean are in southern Peru, thereby implying differing geodynamic conditions during their genesis. (C) 1997 Elsevier Science Ltd.
Carboniferous, igneous, and metamorphic rocks, followed by Jurassic, Cretaceous, and particularly extensive Miocene granitoid plutons, crop out in the Andes of the Lake Region, as determined by new K-Ar and Rb-Sr whole-rock age determinations. Their spatial distribution appears to define the following: the westernmost and easternmost Paleozoic belts, an oblique belt of Jurassic age, a NNW belt of Cretaceous age developed mainly in Argentina but entering Chile at 39°30′S, and a N-S belt of Miocene batholiths and stocks. This distribution of plutons is unlike the west to east younging belts that have been described from the Andes between 28°S and 32°S. This difference could be related to the presence in the Lake Region of old lineaments oblique to the direction of the Andes and to the influence of the Liquiñe-Ofqui fault zone as a pathway for Miocene magmas. The narrowness of the zone of magmatism from late Paleozoic to Miocene times, compared to the wide outcrops of the Paleozoic accretionary wedge, could be explained by the lack of tectonic erosion during Mesozoic-Cenozoic subduction and the constant subduction geometry.
Hornblende step-heating 40Ar/39Ar dating for granitic plutons along an E–W transect of central Sonora was carried out to constrain the Late Cretaceous–early Tertiary migration of the cordilleran magmatic arc across northwestern Mexico. Geochronological data from previous studies offer a good estimate of the overall process, but because they come from different dating schemes performed on a variety of rocks and/or minerals with a wide range of closure temperatures, the ages largely overlap when plotted on a map. Previous data suggest that the Cordilleran magmatic arc was nearly static in the western portion of the Peninsular Ranges batholith in Baja California (140–105 Ma), then the axis of magmatism migrated east at approximately 10 km/Ma and reached coastal Sonora approximately 90 Ma ago. The locus of the plutonic emplacement continued to migrate inland during the Laramide magmatic pulse (80–40 Ma), penetrating up to central Chihuahua. New argon data indicate that granitic plutons intruded the region northeast of Bahía Kino, in coastal Sonora, approximately 77 Ma ago. Magmatism subsequently moved east to the area surrounding the city of Hermosillo approximately 69 Ma ago and continued its easterly migration, reaching the Sonora–Chihuahua state boundary 59 Ma ago. However, the granitic rocks of east-central Sonora yield ages in a relatively wide range of 62–56 Ma. Synchronic plutons reported farther east in central Chihuahua suggest an unusually broad magmatic arc, which appears difficult to explain on the basis of the traditional subduction model assumed for southwestern North America during this time and may reflect particular – and little understood – tectonic conditions derived from the relatively flat subduction regime prior to the extinction of the Laramide magmatic arc. Moreover, volcanic rocks exposed in east-central Sonora yield fairly old U–Pb zircon dates of 90–70 Ma, which have no known contemporaneous plutons, and complicate the scenario for the Laramide event in Sonora, perhaps requiring the existence of a second volcanic arc. Considering solely the evidence from granitic plutons, the data provide a systematic way to evaluate the shift of magmatic activity across Sonora. It needs a proper restitution for the conspicuous Cenozoic extension affecting the region. After restoring the cumulative extension of 90% estimated for east-central Sonora, a rate of approximately 8.5 km/Ma of eastward migration can be roughly estimated for the Laramide arc across Sonora.
Surtseyan tuff cones of the Baño Nuevo Volcanic Complex erupted in shallow marine water during the waning stage of the Aisén Basin, the northern part of the Austral Basin of the Central Patagonian Cordillera. This volcanic complex was emplaced within the Hauterivian to early Aptian Apeleg Formation, the youngest sedimentary unit of this basin, whilst the sediments were still wet. Three new Ar/Ar dates (amphibole) and one U–Pb SHRIMP date (zircon) from this complex yielded concordant early Aptian ages of 122–121 Ma. These data agree with the contact relationships between the Apeleg Formation and the tuff cones. These dates also agree with those of early Aptian ammonites collected from the Apeleg Formation 100 km south of the study area, which represent the youngest fossils identified in this unit. Aptian to Albian subaerial volcanic rocks of the Divisadero Group, paraconformably overlie the Apeleg Formation. The emplacement of the Baño Nuevo Volcanic Complex pre-dates the disappearance of the Aisén Basin.
New K-Ar and 40Ar/39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb)N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb)N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate-K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64-59 Ma for felsic dikes. Because Jurassic-Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.
Fe–Au skarn deposits related to intrusive centers, mostly of granodioritic composition, are widespread in southern Mexico's Guerrero state. These intrusive rocks are largely associated with the NW–SE-oriented Laramide magmatic belt that extends across most of western Mexico. The geochemical composition and ages of representative rocks from the Mezcala mining district in central Guerrero are studied to evaluate the petrogenetic aspects of the ore-related magmas. Some major and trace elements display nearly linear silica variation trends, which suggest a possible comagmatic origin. However, other elements have scattered distributions, possibly due to irregular mantle-to-crust magma mixing ratios, heterogeneities in the composition of the assimilated crustal material, or modifications during the emplacement or postemplacement processes. Major element chemistry indicates calc-alkalic metaluminous compositions, whereas trace element data suggest a volcanic arc tectonic setting, confirming that these rocks evolved from magmas generated above a subduction zone. Compared with the Laramide granites from the northern part of the belt in northwestern Mexico, which intruded a crust underlain by Proterozoic North American rocks, the studied samples are similar but relatively low in Nb and high in Sr, the middle rare earth elements (REE), P, and Zr. They also display minor Ti enrichments and a moderate depletion in the heavy REE. These characteristics may indicate a source of basaltic composition. New 40Ar/39Ar dating of granodiorites and dacite porphyries shows a north-to-south age progression from 66.2±0.8 Ma in the northern part of the belt to 62.2±0.7 Ma in the south. Moreover, the argon dates identify a younger postorogenic igneous event 35–30 Ma ago. This event is poorly documented and may have occurred after the extinction of the Laramide arc and prior to the mid-Tertiary Sierra Madre Occidental ignimbrite flare-up. On the basis of limited geochemical data, these rocks appear to be depleted in P2O5 and Sr and enriched in U relative to the studied Laramide granites. A Fe skarn deposit located in Buena Vista de Cuéllar, in the north central part of Guerrero, suggests that this magmatic pulse took place after the ore development of the Mezcala district.
We present 40Ar/39Ar age determinations and chemical and Sr–Nd isotopic data for the tholeiitic dyke swarm cropping out at Arraial do Cabo peninsula at the eastern end of the Rio de Janeiro coastline. The age determinations indicate that the swarm emplaced approximately 55 Ma ago and thus is similar in age to the K-rich alkaline dykes and plugs also found in the peninsula. The dykes are basalts and basaltic andesites that belong to low Ti tholeiitic series. One dyke group is basaltic in composition, has relatively low Zr/Nb (8.9±1.8), roughly flat mantle-normalized incompatible element patterns, relatively low initial 87Sr/86Sr (0.70434–0.70426), and high εNd(55) (+2.3 to +3.8). These dykes chemically resemble enriched midocean ridge basalts (MORB). A second dyke group is formed by more evolved basaltic andesites with higher Zr/Nb ratios (12.7±2.1), negative Nb anomalies in the mantle-normalized patterns, higher initial 87Sr/86Sr (0.70548–0.70613), and lower εNd(55) (−1.8 to −2.2). Chemical and isotopic data exclude the possibility that the two dyke types are comagmatic or related to each other through crustal contamination processes. The genesis of the Arraial do Cabo tholeiites was likely caused by extension-related melting of a largely incompatible element-depleted mantle, with no trace of the enriched component in the roughly coeval, K-rich mafic alkaline magmas of the Serra do Mar province.
The Nevados del Famatina mining district (NFMD) is located in La Rioja province, Argentina. This district contains porphyry-style mineralization (Nevados del Famatina) and high sulfidation veins (La Mejicana). The stratigraphic column in the NFMD begins with Cambrian siltstones which were metamorphosed during the Late Ordovician - Early Silurian and intruded by Late Ordovician-Silurian granitic rocks. These units were covered by Upper Paleozoic and Tertiary continental sedimentary rocks which are intercalated with and overlain by dacitic-rhyodacitic porphyritic rocks (Mogote Formation) emplaced during the Pliocene. All these units are covered by Pleistocene sediments and Quaternary alluvial and colluvial deposits. Magmatic activity and related mineralization in the NFMD have been dated by the Ar-40/Ar-39 technique. Step heating studies of orthoclase and biotite phenocrysts from the Mogote Formation in the NFMD suggest that the igneous rocks were emplaced around 5.0+/-0.3 Ma ago. However, plateau ages of biotite from the outer carapace of the subjacent granodioritic magma chamber and of muscovite from quartz-sericite alteration at both Nevados del Famatina and La Mejicana are around 3.8+/-0.2 Ma. Emplacement of the shallow stocks is separated from cooling of the outer carapace of the subjacent granodioritic magma chamber to temperatures below 350-degrees-C by a time span of approximately 1 Ma. During this interval, a convective hydrothermal system was established proximal to the granodioritic magma chamber, which resulted in porphyry molybdenum-copper-gold mineralization adjacent to the igneous rocks and more distal high sulfidation veins located in fault zones.
The structure of the southern sector of the central Andes in the North Patagonian Andes of the Argentinean slope (41°-42°S) is characterized by a Tertiary fold-and-thrust belt formed by an E-vergent imbricate thrust structure and a retrovergent thrust system. The paleogeographic distribution of Jurassic rocks suggests that some of the W-vergent thrusts may have been part of a Mesozoic extensional fault system inverted during Andean compression. On the basis of the structures and rocks involved in the deformation, we distinguish a western and an eastern sector. The western sector developed on pre-Tertiary basement rocks with E-vergent thrusts and an associated backthrust system that forms a triangle zone that exposes Mesozoic and pre-Mesozoic rocks. In the eastern sector, thrusting that formed the Ñirihuau foldbelt involved Cenozoic volcanic and sedimentary rocks and affected a subsurface synorogenic wedge. The boundary between the two sectors is probably a normal fault that was active during the middle Mesozoic. The regional cartography and microtectonic observations suggest predominant dip-slip movements of the thrust sheets. However, there is no evidence of major N-S strike-slip movements as has been proposed for the forearc region (Chilean Andes) and northwestern Patagonia on the basis of fault slip data analysis. A Tertiary sedimentary basin was developed in relation to the eastward migration of the orogenic front toward the foreland.
The IGCP 449 fieldtrip in June 2003 drew attention to the Late Cenozoic fluvial sequences of western Amazonia. In Acre state in western Brazil, underlain by relatively mobile crust, rivers have incised up to 70 minto the stacked latest Miocene (?)/Early Pliocene (?) sediments of the Solimões Group, creating staircases of fluvial terraces and indicating regional uplift on this time scale. In contrast, in western Rondonia state, the Madeira River flows through the Early Proterozoic western part of the Amazon Craton, where Late Cenozoic vertical crustal motions seem minimal. The evidence in Acre suggests that the Solimões Group was deposited by an ancestral river system associated with the incipient development of the modern eastward Amazon drainage.
A succession of quartz-rich fluvial sandstones and siltstones derived from a mainly rhyolitic source and minor metamorphic rocks, located to the west, represent the first Upper Paleocene-Early Eocene deposits described in Chilean eastern central Patagonian Cordillera (46°45′S). This unit, exposed 25 km south of Chile Chico, south of lago General Carrera, is here defined as the Ligorio Marquez Formation. It overlies with an angular unconformity Lower Cretaceous shallow marine sedimentary rocks (Cerro Colorado Formation) and subaerial tuffs that have yielded K-Ar dates of 128, 125 and 123 Ma (Flamencos Tuffs, of the Divisadero Group). The Ligorio Marquez Formation includes flora indicative of a tropical/subtropical climate, and its deposition took place during the initial part of the Late Paleocene-Early Eocene Cenozoic optimum. The underlying Lower Cretaceous units exhibit folding and faulting, implying a pre-Paleocene-Lower Eocene contractional tectonism. Overlying Oligocene-Miocene marine and continental facies in the same area exhibit thrusts and normal faults indicative of post-Lower Miocene contractional tectonism.
A maximum shortening of 600 km and a minimum of 300 km was calculated for the southernmost extreme of the Patagonian Andes orocline. A regional balanced cross-section was restored in four stages that represent the main events of compressive deformation. The mid-Cretaceous event produced a maximum shortening of 430 km associated with high grade regional metamorphism in Cordillera Darwin and a period of fast subsidence in the Magallanes foreland basin during the Late Cretaceous. The Late Cretaceous-Cenozoic events produced a maximum shortening of 170 km, of which 40 km correspond to Late Cretaceous, 50 km to Paleogene, and 80 km to Neogene events. A set of evolutive profiles at lithospheric scale from Jurassic to Present time shows that, assuming a maximum shortening of 600 km, the mass balance is attained only if the back arc oceanic lithosphere created during the early rifting stage is partially consumed by a short episode of reverse subduction during the mid-Cretaceous compressive event. A set of six maps showing the palinspastic restoration of the Patagonian Andes orocline indicates that a maximum shortening of 600 km at the southern extreme is compatible with the available geologic data and geometrically compatible with paleomagnetic arc rotations of nearly 90°.
87Sr/86Sr analysis on calcitic shells of pectinids (Chlamys actinodes and Chesapecten crassus) and oysters (Ostrea sp.) shows that the ‘Entrerriense’ sequence belonging to the Puerto Madryn Formation, Valdés Penı́nsula (Chubut), was deposited at about Thus, a middle Tortonian age is assigned to the sequence. Within the sequence, the pectinids better retain their original 87Sr/86Sr ratio, whilst the oysters are altered and yield older-than-real ages. The latter is the result of greater susceptibility to interaction with diagenetic fluids bearing Sr derived from the alteration of tuffaceous materials and volcanic rocks of intermediate and basic composition.ResumenAnálisis de 87Sr/86Sr en conchillas calcı́ticas de pectı́nidos (Chlamys actinodes y Chesapecten crassus) y ostras (Ostrea sp.) demuestran que la secuencia ‘Entrerriense’ correspondiente a la Formación Puerto Madryn, de Penı́nsula Valdés (Chubut), fue depositada alrededor de los Por lo tanto esta secuencia es de edad tortoniana media. Los pectı́nidos retienen mejor la relación original de 87Sr/86Sr mientras que las ostras se alteran más fácilmente e indican edades más antiguas que las reales. Esto se debe a que son más susceptibles a interactuar con fluidos diagenéticos portadores de Sr derivado de la alteración de materiales tobáceos y rocas volcánicas de composición intermedia y básica.
The Abanico del Quindío (AQ) fan, a volcaniclastic deposit from the Ruiz–Tolima volcanic complex (RTVC), Colombia, provides insight into recent deformation in the Central Andes. The use of geological observations, geophysical measurements, and estimates of fault-scarp ages constrain timing of recent tectonic activity. Gravity and magnetic analyses, along with geomorphologic cartography, allow the detection of lateral variations in basement distribution and at least three structural trends that cut the AQ: the Armenia fault (NNE), El Danubio fault (NNW), and Hojas Anchas fault (E–W). Recent deformation in the zone results from slip on the Armenia and El Danubio faults and suggests a maximum interval magnitude of 5.1 < Mw < 6.3, with ages ranging between 2560 ± 480 yr B.P. and 4120 ± 780 yr B.P. Although no surface ruptures are associated with historical events on the fault segments in this zone, blind structures may have influenced the hypocentral distribution of events recorded after the Armenia Earthquake (Mw 6.2, 25-01-1999). Further geophysical studies are needed to understand the Romeral Fault System and assess the earthquake hazard for the city of Armenia.
The serpentinites and associated chromitite bodies in Tehuitzingo (Acatlán Complex, southern Mexico) are in close relationship with eclogitic rocks enclosed within a metasedimentary sequence, suggesting that the serpentinites, chromitites and eclogitic rocks underwent a common metamorphic history.Primary chromites from the chromitite bodies at Tehuitzingo are of refractory-grade (Al-rich) and have a chemical composition similar to that expected to be found in an ophiolitic environment. The chromite grains in chromitites and serpentinites are systematically altered to ‘ferritchromite’. The alteration trend is usually characterized by a decrease in the Al, Mg and Cr contents coupled by an increase in Fe3+ and Fe2+.The Tehutizingo chromitites have low Platinum Group Elements (PGE) contents, ranging from 102 to 303 ppb. The chondrite-normalized PGE patterns are characterized by an enrichment in the Ir-subgroup elements (IPGE=Os, Ir, Ru) relative to the Pd-subgroup elements (PPGE=Rh, Pt, Pd). In addition, all chromitite samples display a negative slope from Ru to Pd [(Os+Ir+Ru)/(Pt+Pd)=4.78−14.13]. These patterns, coupled with absolute PGE abundances, are typical of ophiolitic chromitites elsewhere. Moreover, all the analyzed samples exhibit chondrite-normalized PGE patterns similar to those found for non-metamorphosed ophiolitic chromitites. Thus, the PGE distribution patterns found in the Tehuitzingo chromitites have not been significantly affected by any subsequent Paleozoic high-pressure (eclogite facies) metamorphic event.The chondrite-normalized PGE patterns of the enclosing serpentinites also indicate that the PGE distribution in the residual mantle peridotites exposed in Tehuitzingo was unaffected by high-pressure metamorphism, or subsequent hydrothermal alteration since the serpentinites show a similar pattern to that of partially serpentinized peridotites present in mantle sequences of non-metamorphosed ophiolites.Our main conclusion is that the chromitites and serpentinites from Tehuizingo experienced no significant redistribution (or concentration) of PGE during the serpentinization process or the high-pressure metamorphic path, or during subsequent alteration processes. If any PGE mobilization occurred, it was restricted to individual chromitite bodies without changing the bulk-rock PGE composition.Our data suggest that the Tehuitzingo serpentinites and associated chromitites are a fragment of oceanic lithosphere formed in an arc/back-arc environment, and represent an ophiolitic mantle sequence from a supra-subduction zone, the chemical composition of which remained essentially unchanged during the alteration and metamorphic events that affected the Acatlán Complex.
The Paleozoic Acatlán Complex of southern Mexico comprises polydeformed metasedimentary, granitoid, and mafic–ultramafic rocks variously interpreted as recording the closure of the Iapetus, Rheic, and Ouachitan Oceans. The complex is tectonically juxtaposed on its eastern margin against Grenville-age gneisses (Oaxacan Complex) that are unconformably overlain by Lower Paleozoic strata containing fossils of Gondwanan affinity. A thick siliciclastic unit (Chazumba and Cosoltepec Formations) at the base of the complex is considered part of a Lower Paleozoic accretionary prism with a provenance that isotopically resembles the Oaxacan Complex. This unit is tectonically overridden by a locally eclogitic mafic–ultramafic unit interpreted as a westward-obducted ophiolite, the emplacement of which was synchronous with mylonitic granitoid intrusion at ca. 440 Ma. Both units are unconformably overlain by a deformed volcano-sedimentary sequence (Tecomate Formation) attributed to a volcanic arc of presumed Devonian age. Deformed granitoids in contact with this sequence have been dated at ca. 371 (La Noria granite) and 287 Ma (Totoltepec pluton).
East and Southeast Asia is a complex assembly of allochthonous continental terranes, island arcs, accretionary complexes and small ocean basins. The boundaries between continental terranes are marked by major fault zones or by sutures recognized by the presence of ophiolites, mélanges and accretionary complexes. Stratigraphical, sedimentological, paleobiogeographical and paleomagnetic data suggest that all of the East and Southeast Asian continental terranes were derived directly or indirectly from the Iran-Himalaya-Australia margin of Gondwanaland. The evolution of the terranes is one of rifting from Gondwanaland, northwards drift and amalgamation/accretion to form present day East Asia. Three continental silvers were rifted from the northeast margin of Gondwanaland in the Silurian-Early Devonian (North China, South China, Indochina/East Malaya, Qamdo-Simao and Tarim terranes), Early-Middle Permian (Sibumasu, Lhasa and Qiangtang terranes) and Late Jurassic (West Burma terrane, Woyla terranes). The northwards drift of these terranes was effected by the opening and closing of three successive Tethys oceans, the Paleo-Tethys, Meso-Tethys and Ceno-Tethys. Terrane assembly took place between the Late Paleozoic and Cenozoic, but the precise timings of amalgamation and accretion are still contentious. Amalgamation of South China and Indochina/East Malaya occurred during the Early Carboniferous along the Song Ma Suture to form “Cathaysialand”. Cathaysialand, together with North China, formed a large continental region within the Paleotethys during the Late Carboniferous and Permian. Paleomagnetic data indicate that this continental region was in equatorial to low northern paleolatitudes which is consistent with the tropical Cathaysian flora developed on these terranes. The Tarim terrane (together with the Kunlun, Qaidam and Ala Shan terranes) accreted to Kazakhstan/Siberia in the Permian. This was followed by the suturing of Sibumasu and Qiangtang to Cathaysialand in the Late Permian-Early Triassic, largely closing the Paleo-Tethys. North and South China were amalgamated in the Late Triassic-Early Jurassic and finally welded to Laurasia around the same time. The Lhasa terrane accreted to the Sibumasu-Qiangtang terrane in the Late Jurassic and the Kurosegawa terrane of Japan, interpreted to be derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The West Burma and Woyla terranes drifted northwards during the Late Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia and were accreted to proto-Southeast Asia in the Early to Late Cretaceous. The Southwest Borneo and Semitau terranes amalgamated to each other and accreted to Indochina/East Malaya in the Late Cretaceous and the Hainanese terranes probably accreted to South China sometime in the Cretaceous.RésuméLa zona Oriental y Sur-Oriental de Asia está compuesta de una asambléa de terrenos continentales alóctonos, arcos de isla, complejos acrecionarios y pequeñas cuencas pceánicas. El límite de cada terreno continental se distingue por la presencia de zonas de falla mayores, o por suturas que se pueden reconocer por la presencia de ofiolitas, complejos acrecionarios y “mélanges”. La información estratigráfica, sedimentológica, paleogeográfica y paleomagnética sugiere que todos los terrenos continentales del Oriente y Sur-Oriente de Asia, tienen su origen directo o indirecto del margen Irán-Himalaya-Australiana del continente de Gondwana. La evolución de los terrenos continentales se inicia con la separación o “rifting” del terreno de Gondwana y su desplasamiento hacía el norte, y se culmina con la amalgamación/acreción de estos terrenos para formar lo que hoy en dia compone el Oriente de Asia. Tres trozos de bloque continental fueron separados por “rifting” del margen Nor-Oriental de Gondwana durante el Silúrico-Devónico tempranío (Norte de China, Sur de China, Indochina/Malaya Oriental, Qamdo-Simao y terrenos de Tarím), Pérmico-tempranío a medio (Sibumasu, Lhasa y terrenos de Qiangtang) y el Jurásico tardío (terreno de Burma Occidental, terreno de Woyla). El traslado de estos terrenos hacía el norte fue afectado por la apertura y cierre de tres oceanos Tethys succesivos, el Paleo-Tethys, Meso-Tethys y Ceno-Tethys. La asambléa de estos terrenos ocurrió entre el Paleozóico tardío y el Cenozóico, pero las edades precisas de la amalgamación todavía están en contención del Sur de China e Indochina/Malaya Oriental ocurrío durante el Carbonífero tempranío a lo largo de la sutura de Song Ma, para formar “Cathaysialand”. Cathaysialand, en conjunto con el Norte de China, formaron una gran regíon continental dentro de Paleo-Tethys durante el Carbonífero tempranío hasta el Pémico. La información paleomagnética indica que esta región continental fue ubicada a latitudes cerca-hasta un poco norte del paleoecuador, lo cual es consistente con la flora tropical Cathaysiana desarollado dentro estos terrenos. El terreno de Tarím (en conjunto con los de Kunlum, Qaidím y Ala Shan) acretaron a Kazakhstan/Siberia durante el Pérmico. Esto fué seguido por la suturación de Sibumasu y Qiangtang a Cathaysialand durante el Pérmico tardío-Triásico tempranío, resultando en el cierre casi completo del Paleo-Tethys. El Norte y Sur de China fueron amalgamados durante el Triásico tardío-Jurásico tempranío, y finalmente fueron soldados a Laurasia alrededor del mismo tiempo. El terreno Lhasa acretó al terreno Sibumasu-Qiangtang durante el Jurásico tardío, y el terreno Kurosegawa de Japón, lo cual se interpreta con origen en la parte Australiana de Gondwana, acretó a Eurasia Japonés también durante el Jurásico tardío. Los terrenos de Burma Occidental y Woyla se trasladaron hacía el norte durante el Jurásico tardío-Cretaceo tempranío, cuando se habría el Ceno-Tethys y se destruía el Meso-Tethys por subducción bajo la placa de Eurasia. Estos fueron acretados a Proto-Asia-Sur Oriental durante el Cretaceo tempranío hasta el Cretaceo tardío. Los terrenos de Borneo y Semitau se amalgamaron uno al otro, y fueron acretados a Indochina/Malaya Oriental durante el Cretaceo tardío. Los terrenos de Hainanese probablemente fueron acretados al sur de China durante el Cretaceo.
The Precordillera terrane of Argentina was rifted as a lithospheric block from the Ouachita embayment of southeastern Laurentia and accreted to western Gondwana. In various interpretations, the time of rifting of the Precordillera terrane from Laurentia ranges from Early Cambrian to Late Ordovician, and the time of accretion to Gondwana ranges from Middle–Late Ordovician to Silurian–Devonian. This review of available data and previous interpretations, leads to the conclusion that rifting from the Ouachita embayment of Laurentia occurred in the Early Cambrian, and collision with the Famatina arc on the western margin of Gondwana occurred in the Middle–Late Ordovician. In that context, Silurian–Devonian deformation reflects the accretion of the Chilenia terrane on the outboard side of the Precordillera terrane.RésuméEl terrano de la Precordillera Argentina se separó como un bloque litosférico del engolfamiento de Ouachita en el sureste Laurentia y se acrecionó a Gondwana occidental. En diferentes interpretaciones el momento de separación de la Precordillera desde Laurentia varı́a entre el Cámbrico Temprano y el Ordovı́cico Tardı́o y el momento de acreción a Gondwana varı́a entre el Ordovı́cico Medio–Tardı́o y el Silúrico–Devónico. Esta revisión sobre la información disponible e interpretaciones previas concluye que la separación del engolfamiento de Ouachita en Laurentia ocurrió en el Cámbrico Temprano y la colisión con el arco de Famatina en el margen occidental de Gondwana ocurrió en el Orodvı́cico Medio–Tardı́o. En ese contexto la deformación silúrica y devónica refleja la acreción del terreno de Chilenia sobre el margen libre de la Precordillera.
Volcanic and interbedded volcaniclastic rocks of the lower Cretaceous island-arc series of the Teloloapan Terrane in southern Mexico contain metamorphic assemblages characteristic of the zeolite, prehnite–pumpellyite and lower greenschist facies produced by burial metamorphism prior to its accretion to nuclear Mexico. Distribution of secondary assemblages throughout the stratigraphic succession, together with the chemical evolution of metamorphic minerals, reveals a depth-controlled metamorphic zonation characterized by the presence of the diagnostic assemblages laumontite+pumpellyite+epidote and laumontite+celadonite+pumpellyite±epidote (zeolite facies) followed downward by assemblages containing prehnite+pumpellyite±white mica (prehnite–pumpellyite facies) and finally by the presence of the assemblages pumpellyite+actinolite+epidote and epidote+actinolite (greenschist facies). Analysis of assemblages in the Al–Fe3+–FM–K system, reveals that facies boundaries are discontinuous, involving the disappearance of at least one phase and the appearance and/or extension of the field of equilibrium of other diagnostic minerals and assemblages. Application of empirically based thermobarometers, phase equilibria, mineral chemistry, and petrogenetic grids indicates that the P–T conditions of metamorphism ranged from 175 to 342°C and The data further indicate high geothermal gradients of about ∼55°C km−1. Seawater-derived fluids were characterized by high aK, high fO2 and low XCO2.
The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in the region, and probably represent remnants of island and continental arcs. The Três Palmeiras succession, emplaced between 2.36 and 2.34 Ga, hosts gold deposits and represents the first record of Siderian supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of the Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of Archean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile material between 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume of granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed from reworked crust predominate in the south. The present-day configuration of the Bacajá domain results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle.
New U–Pb and Sm–Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiás magmatic arc in western Goiás constrain the geological evolution of this juvenile crust in the western Brasília belt. Orthogneiss rock samples have U–Pb crystallization ages of 804±6, 669±3, 662±12, 634±8, 630±5, and 637±20 Ma and show εNd(T) values between +2.8 and −15.1. Rock units with negative εNd(T) are more frequent in the eastern part of the studied area to the south of Anicuns, which indicates the presence of older continental crust in that part of the arc. Metagranitoids have ages of 821±10, 810±10, 792±5, 790±12, 782±14, 748±4, and 614±5 Ma and εNd(T) values between +5.1 and −3.7. The data presented here, combined with those in the literature, suggest that igneous activity in the Goiás magmatic arc took place in two episodes: between ca. 0.89 and 0.8 Ga, probably in intraoceanic settings, and between ca. 0.66 and 0.60 Ga, likely in an active continental margin at the end of the Brasiliano orogeny.
The Sinu and San Jacinto fold belts in northern Colombia from a wedge of sediments up to 12 km thick which has been accreted to the South American margin throughout the Cenozoic. The wedge is characterized by a very low topographic slope ( <-2°) and abundant mud volcanism. Many folds have a core of mobile overpressured mud. Active shortening at a rate of about 1 to 2 cm yr-1 is the result of convergence between the Caribbean plate and the northwest margin of South America. Aeromagnetic and gravity anomalies suggest a buttress of rigid basement rock dipping toward the toe of the wedge. A seismic reflector dipping gently (2°) toward the back of the wedge is interpreted as the basal décollement. According to the theory of critical taper for a noncohesive wedge of Coulomb material, fluid pressure ratios should approach lithostatic values (0.97) within the wedge and on the basal décollement. The predicted high fluid pressures within the wedge are consistent with the mud diapirism. The low slip rate and subduction of large quantities of young, high-porosity sediments explain the absence of large interplate thrust earthquakes.