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comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Río de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic-ultramafic rocks of 1,244-1,027 Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846-570 Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640-580 Ma) and Cambro-Ordovician (~550-470 Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480-450 Ma), late phase (440-420 Ma), and Chanic phase (400-360 Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt. [ABSTRACT FROM AUTHOR]

The Patagonian batholith was formed through continuous subduction in the southwestern Gondwana margin, from the Early–Middle Jurassic to present times. This study presents a petrological analysis of the La Hoya pluton, an intrusive body located in the North Patagonian Andes near Esquel that is part of the Patagonian batholith. The La Hoya pluton consists of porphyritic biotite–amphibole granodiorites that grade into porphyritic biotite monzogranites, and both facies are associated with minor occurrences of gabbroic-to-tonalitic stocks and doleritic dikes. Amphibole dating yielded a new 40Ar–39Ar stepwise age of 161.5 ± 0.5 Ma for this shallow emplaced pluton. Field, petrographic, whole rock, and mineral geochemical data suggest and indicate that the petrogenetic evolution of the La Hoya pluton involved fractional crystallization, mafic and felsic magma interaction, and possibly mixing of multiple magma sources. Early formed brown resorbed amphibole and labradoritic plagioclase antecrysts crystallized in alkaline-to-transitional (alkaline-to-subalkaline) magmas with temperatures of 834–962 °C and pressures of ~ 2 kbar (up to ~ 7 km depth). Green amphiboles, plagioclases of andesine–oligoclase composition, and biotites crystallized in equilibrium with subalkaline magmas at lower temperatures and pressures (750–806 °C and of ~ 1 kbar; ~ 3.5 km depth). Actinolitic amphibole and albitic plagioclase formed during the hydrothermal alteration associated with the overprinting of post-emplacement solid-state deformation. The La Hoya pluton involved early formed alkaline-to-transitional magmas that progressively became more calc-alkaline with ongoing differentiation. [ABSTRACT FROM AUTHOR]

The sedimentary infill of the Tandilia Basin is characterised by at least four different stages of deposition during the Neoproterozoic (< 1160 to ~ 550 Ma), two of them recognisable in the Sierras Bayas Group and two in the La Providencia Group. The Villa Mónica Formation, representing the first stage of basin infill (< 1160 to < 720 Ma), consists of a lower siliciclastic section that passes transitionally into an upper dolostone section. This dolostone platform, rich in standard Cryogenian stromatolites, was interpreted as deposited under typical cold, suboxic to oxic seawater conditions. A detailed analysis of the dolostone section allowed us to distinguish two karst events imprinted in the formation. The hypogene karst genesis is interpreted as hydrothermal with the development of Mn-oxide dykes associated with intense silicification and brecciation of the host dolostones. The younger epigene karst, which developed over the post uplifted unit, is related to intense weathering and subaerial exposure with the generation of an irregular surface in its upper contact. This unconformity, known as the Piedra Amarilla Surface, is represented regionally in the Tandilia System. It may have implied a post-uplift period of erosion, dissolution and non-deposition that predates the accumulation of the ~ 400-m-thick overlying succession (the Sierras Bayas and La Providencia groups). Accordingly, the Piedra Amarilla Surface may represent a large spatio-temporal interval bounding two contrasting histories in the basin infill, which would justify separating the Villa Mónica Formation from the Sierras Bayas Group. This karst-related surface may be considered a fingerprint of the transition from the break-up of Rodinia to the configuration of southwestern Gondwana. [ABSTRACT FROM AUTHOR]

The Oligocene–early Miocene Tranquitas Formation in the southern Chaco foreland basin records the onset of Central Andean shortening. The aims of this contribution are to: (1) provide original data of the depositional conditions of the Tranquitas Formation, (2) to determine the role of tectonic and climate influence on the depositional conditions and on the composition of these deposits and (3) propose an evolutionary model of accumulation based on a sequence stratigraphic approach. The detailed analysis of these deposits resulted in the definition of six facies associations, including: complex sandy narrow sheets (FA1), sandy narrow sheets with lateral accretion surfaces (FA2), sandstone ribbons (FA3), tabular muddy-sandstone deposits (FA4), sandstone lobes (FA5), and fine-grained deposits (FA6). Based on the spatial distribution and the vertical stacking of these associations, the Tranquitas Formation was divided into two major intervals. Interval 1 (lower section) is characterized by braided fluvial deposits and represents low accommodation conditions with a high proportion of channel deposits. Interval 2 (upper section) tends to an increase in the accommodation space characterized by the better preservation of floodplain deposits in moderate to high sinuosity meandering fluvial systems. The integration of paleocurrent, petrography, and clay mineralogy analyses indicates that detrital components come from the orogenic region located to the west, in an overfilled stage of the foreland basin. Clay mineral assemblage analysis suggests a long-term climate change from drier to more humid conditions and reflects the climate effect on the depositional paleoenvironments developed in the distal position of the basin. In addition, the resultant vertical and lateral stacking of sedimentary sequences representing different paleoenvironments of the Tranquitas Formation could be correlated with the Oligocene–early Miocene Petaca and lower-Yecua formations located in the center and north of the Chaco foreland basin. [ABSTRACT FROM AUTHOR]

The Suri-Las Planchadas Volcanic-Sedimentary Complex (SPVSC), northwest of the Sierras Pampeanas in Argentina, comprises an Early to Middle Ordovician (Floian–Dapingian) submarine and subaerial succession with a variety of mafic to felsic volcanic lithofacies. These units include mafic to intermediate submarine massive-hyaloclastic lava flows, dacitic lavas, pyroclast-rich volcaniclastic deposits, intrusions of domes/sills and rhyodacitic—rhyolitic lavas. Along Las Planchadas hill a section of ~ 3 km thickness is exposed showing vertical and lateral variations. The lower part is dominated by massive, pillowed and basaltic to intermediate lavas, autobreccia and hyaloclastic facies, representing the beginning of the volcanic arc activity. The upper almost 2 km of the succession is dominated by subaqueous-resedimented volcaniclastic siltstones, sandstones and breccias with abundant pyroclastic components and rhyodacitic to rhyolitic lava-domes, sills and dykes. Tuff and ignimbrite lithoclasts, abundant shards and pumice fragments were determined in the resedimented deposits. Volcanism in the SPVSC started with submarine eruptions and progressively grew above the wave base, accompanied by a change from mafic to intermediate and felsic members. The eruptive pattern shifted from effusive to explosive, producing a large apron of syn-and post-eruptive volcaniclastic deposits of debris flows and turbidity currents. They are the evidence of variations in composition and volcanic styles and their development contributed to generate an apron surrounding the volcanoes. The apron prograded and aggraded with the evolution of the volcanism culminating in shallow water volcanogenic beds. The final phase was characterized by felsic subaerial or shallow subaqueous explosive volcanism. [ABSTRACT FROM AUTHOR]

The Huayra Huasi Volcanic Complex of Miocene age (11.88 ± 0.15 Ma U–Pb zircon) was emplaced in the Northern Puna plateau of Argentina, spatially associated with ignimbrites erupted from Altiplano–Puna Volcanic Complex calderas. The complex comprises biotite-bearing dacites and low-SiO2 rhyolites in the northern area and high-SiO2 rhyolites in the south, all with peraluminous compositions (A/CNK > 1.0–1.22). The units have broadly similar initial Sr and Nd isotopic ratios (87Sr/86Sr ∼ 0.71013–0.71225 and εNd ∼ −5.4 to −7.0) and are composed of plagioclase, quartz, sanidine and biotite as the main phenocryst phases. All units host macroscopic microgranular enclaves and xenoliths of sillimanite–biotite schists, sillimanite- and sillimanite–garnet gneisses, as well as fibrous alumina-rich microxenoliths, the latter being especially abundant in the southern rhyolites. Petrographic, mineral and whole-rock geochemistry, geothermometric and isotopic data indicate that all units of the complex originated by contamination of andesite magmas through assimilation of upper crustal lithologies in early stages of magma evolution. The fibrous alumina-rich microxenoliths are composed almost entirely of refractory minerals (sill + Kfsp ± Pl ± Bt) and interpreted as peritectic or restitic products of partial melting of assimilated metasedimentary rocks similar to the unmodified metamorphic xenoliths in the complex. Geochemical modeling indicates that, after early-stage contamination, each magmatic unit evolved separately. Whereas the northern dacites and low-SiO2 rhyolites underwent assimilation and fractional crystallization throughout their history, the southern rhyolites mainly evolved via fractional crystallization of felsic phases alone. This study shows that the peraluminous nature of felsic magmas do not necessarily originate by partial melting of crustal material but can be acquired by metaluminous magmas during later evolution. The processes shown here of assimilation and fractional crystallization and pure fractional crystallization has relevance for other igneous bodies of similar compositions in the Puna backarc and worldwide. [ABSTRACT FROM AUTHOR]

The pressure–temperature (P–T) evolution of a deformed garnet-bearing granite of La Virgen hill, located 25 km south of the town of Azul, Argentina, was studied as an example of the slightly peraluminous and deformed granites widespread in the Paleoproterozoic Tandilia Belt of the Rio de la Plata Craton. An early isobaric magmatic–submagmatic cooling path followed by exhumation was derived by the appraisal of microstructures, chemical compositions of minerals and P–T pseudosection modeling in the system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H (1 wt.% of H2O) using the software PERPLE_X. Slightly zoned magmatic garnet with a core composition of pyr4(grs + adr)11sps10alm75 equilibrated with plagioclase (XAn = 0.37) at 6.0 kbar and 655 °C in a crystal mush with < 7 vol.% of melt. The garnet rim composition of pyr3(grs + adr)12sps14alm71, texturally equilibrated with biotite (Mg/(Mg + Fe) = 0.21) at the granitic solidus of 5.9 kbar and 645 °C. The subsequent cooling path is constrained by isopleths of low-Ti muscovite with Si contents of 3.17 per formula unit commonly located around K-feldspar and recrystallized plagioclase (XAn = 0.35). Ductile deformation at low amphibolite- to greenschist-facies metamorphic conditions produced the subvertical main foliation. Forty-two chemical analyses in seven monazite grains of the granite yielded two groups of U–Th–Pb ages at 2126 ± 10 (2σ) Ma and 2089 ± 3 Ma, interpreted to be inherited from widespread magmatic events of the Tandilia Belt and resulted from the granite crystallization, respectively. The exhumation of the deformed granite from middle (~ 22 km) to upper and cold crustal levels was probably a long-lasting process after 1.9 Ga. [ABSTRACT FROM AUTHOR]

In the Comechingones pegmatitic field, central Argentina, leucogranite and pegmatite bodies crop out in a relatively narrow (25 × 10 km) belt, and were emplaced synkinematically with the main deformational event of the crustal-scale Guacha Corral shear zone during the Early Ordovician (~ 475 Ma). These leucogranites and pegmatites are geochemically evolved rocks with high silica and alkalis, low Fe2O3, MgO, TiO2 and CaO, and high ASI values. The leucogranites display quite variable Sr and Nd isotope compositions (initial 87Sr/86Sr ratios from 0.7048 to 0.7170, and εNd values from + 2.0 to − 3.1), some of which do not overlap with almost any other pre-Famatinian rock from the Sierras de Córdoba. The major and trace element geochemistry and the particular Sr and Nd isotope compositions of the leucogranites are here explained by the following processes: (1) water-fluxed partial melting of amphibolites at relatively low P–T conditions generating currently unexposed granodioritic melts with unradiogenic 87Sr/86Sr ratios and radiogenic εNd values; (2) fractionation of mostly plagioclase and monazite leading to compositions close to the leucogranite melts; and (3) assimilation of metasedimentary rocks with crustal isotopic signatures, modelled by assimilation and fractional crystallization processes. The major, trace and isotope compositions of the pegmatites suggest a derivation from partial melting of the same metasedimentary protoliths of the Sierras de Córdoba that were assimilated by leucogranite melts. We propose a feedback relationship among deformation, anatexis, magma evolution and mass transfer in the context of such a crustal-scale shear zone in the foreland of the Famatinian orogen. [ABSTRACT FROM AUTHOR]

The interaction of tectonics and climate is considered the main determining factor in the development of different depositional systems (from aeolian to alluvial to lacustrine environments) in intermontane basins. The role of these allogenic controls forces the paleoenvironmental evolution of intermontane basins. In this work, we analyse a Miocene (15–11.5 Ma) synorogenic and volcaniclastic succession of an intermontane basin of the Northpatagonian Andean foreland region. The evolution of the infill was synchronously developed with the Andean uplift and, consequently, with the rain shadow effect generation. A detailed sedimentological and stratigraphic analysis allowed to define eight facies associations, which are lateral and vertical stacked into three different depositional systems. Alluvial depositional systems occurred between 15 and 14.6 Ma, deltaic–lacustrine systems took place between 14.6 and 12.75 Ma, and a reinstatement of alluvial depositional systems occurred between 12.75 and 11.5 Ma. These variations in the depositional systems were contrasted with synchronous tectonic and climatic changes and some considerations about the role of these allogenic controls were discussed to unravel their influence on the paleoenvironmental evolution. The onset of the deltaic–lacustrine depositional system was related to a tectonic reconfiguration whereas the reinstatement of the alluvial depositional system was associated with a regional aridization. Additionally, since the composition of the succession is mainly volcaniclastic, the explosive volcanism is considered and discussed as an extra allogenic control on the evolution of the basin. [ABSTRACT FROM AUTHOR]

We carried out a sedimentary provenance analysis of the Devonian Gualilán Group, Central Precordillera of western Argentina, focusing on the siliciclastic record of the Talacasto (Lower Devonian) and the Punta Negra (Lower-Middle Devonian) formations. Provenance is determined based on petrography, heavy minerals, whole-rock geochemistry, Sm–Nd isotopes, and U–Pb detrital zircon dating. Sandstones are composed of quartz, feldspar, and metamorphic and plutonic lithoclasts. The Gualilán Group underwent moderate weathering indicated by CIA values and Th/U ratios below 76 and 4.9, respectively. Th/Sc, Zr/Sc, Cr/V and La/Th ratios, negative Eu-anomalies and REE patterns point to felsic source compositions. The heavy minerals and zircon morphologies indicate that high-grade metamorphic and igneous rocks provided the bulk of detritus. TDM ages between 1.40 and 1.46 Ga and εNd(t) values ranging from − 9.21 to − 11.13 constrain the signature of the sources. U–Pb detrital zircon ages for the Talacasto Formation are equally distributed between the Famatinian (Late Cambrian–Late Devonian), Pampean–Brazilian (Neoproterozoic–Early Cambrian) and Grenvillian-Sunsas (Mesoproterozoic) orogenic cycles. Detrital zircon ages of the Punta Negra Formation were dominantly supplied from Mesoproterozoic rocks, linked to the Grenvillian-Sunsas orogeny. The data indicate a provenance from rocks located eastwards of the basin represented by the basement of the Pampean Ranges. The peripheral foreland basin developed during the post-collisional regime that followed the accretion of Cuyania against Gondwana. Whereas eastern and western Pampean Ranges provided detritus to the Talacasto Formation, the Punta Negra Formation was sourced by the western Pampean Ranges, implying exhumation and erosion of such basement during the Devonian. [ABSTRACT FROM AUTHOR]

Large masses of turbidite systems have been metamorphosed in orogenic systems during Earth's history. Under granulite-facies conditions, the transformation of turbidite sedimentary successions into metasedimentary sequences drives intracrustal differentiation by anatexis and melt–residuum separation. We report on a migmatite terrane developing from turbidite successions that were buried into the deep crust during the Neoproterozoic to Early Cambrian Pampean orogeny, in central Argentina. At the exposed middle crustal paleodepths, migmatites occur on the regional scale but lithological zones are characterized by (1) bedded migmatites, (2) metatexite, and (3) diatexite. Bedded migmatites are the low-temperature part of the migmatite terrane where the alternating metapelite and metagreywacke layers are traceable among migmatites of different protoliths. The temperature (> 790 °C) was sufficient high for melting the metapelite, but not for melting the metagreywacke. The majority of the migmatite terrane consists of stromatic metatexites in which the limit among different progenitors is either faint or erased by migmatization. Metatexite zone acts as melt transfer in the migmatite terrane. In the stromatic migmatites, the major oxide composition of leucosomes and tabular bodies of leucogranite resembles those of the glasses of experimental petrology. However, leucosomes and leucogranites are crystallized melts that have low Zr, Th and LREE contents and positive Eu anomalies resulting from accessory mineral retention in melting residues. The transformation of metatexite into diatexite is gradational over tens of metres, and related to an accumulated melt fraction that dismembers the stromatic fabric. The most abundant diatexite is mesocratic, and has little or lacks K-feldspar. A subordinate proportion of diatexites is leucocratic, contains K-feldspar phenocrysts, and shows igneous-like textures. Leucogranites and leucocratic diatexites are the potential carriers of an anatectic melt-rich component from granulite-facies migmatite sequences toward shallow crustal levels. Turbidite successions are fertile protoliths that undergo widespread melting under low granulite-facies temperature (< 850 °C), and the development of large diatexite massifs makes them suitable sources of granitic magmatism. [ABSTRACT FROM AUTHOR]

We introduce a method for the detailed interpretation of K-Ar illite fine-fraction ages of fault gouges from non-sedimentary host rocks. Ages are cross-evaluated with several independent parameters, e.g. illite crystallinity, illite polytype quantification, grain size, mineralogical observations, K-Ar muscovite and biotite host-rock cooling ages as well as low-temperature thermochronological data (AFT, AHe, ZHe). This interpretation approach is applied to a regional study in order to constrain the 'deformation path' of the Eastern Sierras Pampeanas in NW Argentina. In the course of this study, a large number of gouge-bearing fault zones were systematically sampled and analysed. Obtained K-Ar illite fine-fraction ages range from Devonian to Cretaceous times, documenting a long-lasting brittle fault activity in this region. Ages >320 Ma are synchronous with a period of intra-Carboniferous compressional tectonism, whereas Permo-Triassic ages are contemporaneous to a flat-slab subduction episode of the Farallon plate beneath the South American plate. Middle to Late Permian and Early Triassic ages as well as Early Jurassic to Middle Cretaceous ages correlate with extensional tectonics in this region. Additionally, K-Ar illite ages reveal a propagation of brittle deformation from north to south in the Sierras de Córdoba and San Luis. Data integrity and consistency with other chronometers and geological evidence show that the here suggested interpretation is valid and can provide a powerful tool to evaluate cooling and deformation histories. Despite of that, we could show that the reliability of fault gouge data strongly depends on the regional cooling. [ABSTRACT FROM AUTHOR]

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings. [ABSTRACT FROM AUTHOR]

The geometry and related geomorphological features of the right-lateral strike-slip El Tigre Fault, one of the main morphostructural discontinuities in the Central-Western Precordillera of Argentina, were investigated. Achievements of this survey include: recognition of structural and geometrical discontinuities along the fault trace, identification and classification of landforms associated with local transpressional and transtensional sectors, observation of significant changes in the fault strike and detection of right and left bends of different wavelength. In the Central Segment of the El Tigre Fault, 2D electrical resistivity tomography surveys were carried out across the fault zone. The resistivity imaging permitted to infer the orientation of the main fault surface, the presence of blind fault branches along the fault zone, tectonic tilting of the Quaternary sedimentary cover, subsurface structure of pressure ridges and depth to the water table. Based on this information, it is possible to characterize the El Tigre Fault also as an important hydro-geological barrier. Our survey shows that the main fault surface changes along different segments from a high-angle to a subvertical setting whilst the vertical-slip component is either reverse or normal, depending on the local transpressive or transtensive regime induced by major bends along the trace. These local variations are expressed as sections of a few kilometres in length with relatively homogeneous behaviour and frequently separated by oblique or transversal structures. [ABSTRACT FROM AUTHOR]

Early Paleozoic paleomagnetic data from NW Argentina and Northern Chile have shown large systematic rotations within two domains: one composed of the Western Puna that yields very large (up to 80°) counter-clockwise rotations, and the other formed by the Famatina Ranges and the Eastern Puna that shows (~40°) clockwise rotations around vertical axes. In several locations, lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the Paleozoic. Previous tectonic models have explained these rotations as indicative of rigid-body rotations of large para-autochthonous crustal blocks or terranes. A different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of Precordillera in the Late Ordovician. This collision should have generated dextral shear zones in the back arc region of the convergent SW Gondwana margin, where systematic domino-like clockwise rotations of small crustal blocks accommodate crustal shortening. The Western Puna block, bordering the Precordillera terrane to the north, might have rotated counterclockwise as an independent microplate due to tectonic escape processes, in a fashion similar to the present-day relationship between the Anatolia block and the Arabian microplate. [ABSTRACT FROM AUTHOR]

The Cryogenian to Uppermost Ediacaran successions of the Tandilia System (Sierras Bayas Group and Cerro Negro Formation) in central-eastern Argentina is exceptional because of its unmetamorphosed and nearly undeformed character, its sediment provenance trend and the absence of any identified glacial deposit and the deposition of warm water carbonates. We decipher a dramatic change in the basin evolution from small-scale depositional areas during the Neoproterozoic to a larger basin related to an active continental margin throughout the Uppermost Ediacaran. The base of the succession is represented by immature detritus of alkaline composition (Villa Mónica Formation), but towards the top of this formation, the material is sorted and reworked, nonetheless still reflecting in its detritus the local rocks. The clastic deposition is interrupted by diagenetic overprinted dolomites. The unconformable overlying quartz-arenitic Ediacaran Cerro Largo Formation reworked the Cryogenian Villa Mónica Formation and contains mainly felsic granitic and metamorphic basement material of slightly wider variety, while the dominant alkaline geochemical signature in rocks of the Villa Mónica Formation disappears. Based on diagenetic, petrographic and sedimentological data, we can interpret the unconformity representing a longer time of erosion. The Cerro Largo Formation shows a transition to mudstones and the heterolithic facies of the Olavarría Formation. The top of the Sierras Bayas Group is represented by limestones (Loma Negra Formation), which are discordantly overlain by the Uppermost Ediacaran Cerro Negro Formation. The latter displays detrital material derived from a continental arc, mafic and felsic sources. Several arc-related geochemical proxies (Th/Sc < 0.8; Zr/Sc < 10; La/Sc < 2; Ti/Zr > 20) are recorded in the sediment detritus, as are syn-depositional pyroclastites. The absence of volcanic material in the underlying rocks allows proposing that the Cerro Negro Formation is related to an active continental margin fringing Gondwana ('Terra Australis Orogen') as a retro-arc or retro-arc foreland basin. [ABSTRACT FROM AUTHOR]

review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm-Nd T model ages between 2.6 and 2.2 Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1 Ga for the metamorphic protoliths and 2.1-2.0 Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm-Nd T model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0-2.6 and 2.3-1.6 Ga). The crystallization ages on zircon ranges between 3.1 and 0.57 Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton. [ABSTRACT FROM AUTHOR]

The Silurian-Devonian siliciclastic sedimentary units known as Sierra Grande Formation and the upper part of the Ventana Group crop out in the eastern area of the North Patagonian Massif and in the Ventania system, toward the Atlantic border of Argentina. Both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment. Previous contributions have provided evidence of an allochthonous Patagonia terrane that amalgamate to Gondwana during the Permian-Triassic. However, other lines of research support a crustal continuity southward, where the Pampean and Famatinian events extend into the northern Patagonia. In either case, the detrital input to the Eo-Mesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources. In this contribution, new age data on the sedimentary provenance of these units is provided by U-Pb and Lu-Hf isotopic studies on detrital zircons, using LA-ICP-MS and SHRIMP methodologies. The main sedimentary sources of detrital zircons for both regions are of Cambrian-Ordovician and Neoproterozoic age, while a secondary mode is Mesoproterozoic. Zircons from older cratonic sources (Mesoarchean-Paleoproterozoic ages) are scarcely recorded. The sample from the upper section of the Devonian Lolén Formation (Ventana Group) shows an important change in the sedimentary provenance, with a main mode of Mesoproterozoic detrital zircons. Detrital source areas considering the orogenic cycles known for southwest South America (Famatinian, Pampean-Brasiliano, Mesoproterozoic-'Grenvillian' and Paleoproterozoic-'Transamazonian') are proposed. [ABSTRACT FROM AUTHOR]

Whole-rock geochemical analyses using major and trace elements in combination with the Sm-Nd and Pb-Pb isotope systems, together with SHRIMP age dating on metasedimentary rocks from the Sierras de Chepes, the Sierras de Córdoba, the Sierra Norte and the San Luis Formation in the Sierra de San Luis, have been carried out to unravel the provenance and the geodynamic history of the Eastern Sierras Pampeanas, Central Argentina. The geochemical and the Sm-Nd data point to a slightly stronger mafic and less-fractionated material in the provenance area of the Sierras de Córdoba when compared to the other units. The T model ages from the Sierras de Chepes (~1.82 Ga) and the Sierra Norte (~1.79 Ga) are significantly older than the data from the Sierras de Córdoba (1.67 Ga). The Pb data are homogeneous for the different units. Only the Pb/Pb ratios of some samples from the Sierras de Córdoba are higher. A late Pampean detrital zircon peak around 520 Ma from the Sierras de Chepes is in accordance with the new data from the San Luis Formation. This is similar to the literature data from the Famatina Belt located to the northwest of the Sierras de Chepes and also fits the detrital zircon peaks in the Mesón group. These maximum depositional ages were also reported from some locations in the Puncoviscana Formation but are absent in the Sierras de Córdoba. An improved model for the development of the Eastern Sierras Pampeanas in the area between the Sierras de Córdoba and the Puncoviscana Formation is provided. This gives new insights into the late Pampean development of the Sierra de San Luis and the complex development of the Eastern Sierras Pampeanas. This new model explains the younger detrital ages in the Puncoviscana Formation compared with the older ages of the Sierras de Córdoba. Another model of the Sierra de San Luis explains the younger depositional ages of the Pringles Metamorphic Complex and the San Luis Formation when compared to the Nogolí Metamorphic Complex and the Conlara Metamorphic Complex. Additionally, the rather fast change of the high-grade metamorphic conditions in the Pringles Metamorphic Complex and the low-grade metamorphic conditions in the San Luis Formation is explained by extension, the ascent of (ultra) mafic material and later folding and erosion. [ABSTRACT FROM AUTHOR]

The Sierras Pampeanas in central and north-western Argentina constitute a distinct morphotectonic feature between 27°S and 33°S. The last stage of uplift and deformation in this area are interpreted to be closely related to the Andean flat-slab subduction of the Nazca plate beneath the South American plate. K-Ar fault gouge dating and low-temperature thermochronology along two transects within the Sierra de Comechingones reveal a minimum age for the onset of brittle deformation about 340 Ma, very low exhumation rates since Late Paleozoic time, as well as a total exhumation of about 2.3 km since the Late Cretaceous. New Ar-Ar ages (7.54-1.91 Ma) of volcanic rocks from the San Luis volcanic belt support the eastward propagation of the flat-slab magmatic front, confirming the onset of flat-slab related deformation in this region at 11.2 Ma. Although low-temperature thermochronology does not clearly constrain the signal of the Andean uplift, it is understood that the current structural relief related to the Comechingones range has been achieved after the exhumation of both fault walls (circa 80-70 Ma). [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editor discusses reports published within the issue including one by Varela and colleagues on the Mesoproterozoic terranes attached at the western margin of the Rio de Plata Craton, Saalman and colleagues on the Encantadas block or microcontent, and Zambrano and colleagues on the calculation of sedimentation rates in the broken forleand of west Argentina.

The southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in eastern Argentina. The geological evolution comprises mainly an igneous-metamorphic Paleoproterozoic basement named Buenos Aires Complex, which is covered by Neoproterozoic to Early Paleozoic sedimentary units which display subhorizontal bedding. The basement of calc-alkaline signature consists mainly of granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks, and granitoid plutons. Subordinate rock-types include schists, marbles, and dykes of acid and mafic composition. Tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids. The tectonic scenario seems related to juvenile accretion event (2.25-2.12 Ga) along an active continental margin, followed by continental collision (2.1-2.08 Ga) after U-Pb zircon data. The collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks. The tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution. The basement was preserved from younger orogenies such as those of the Brasiliano cycle. After a long paleoweathering process, the Sierras Bayas Group (c. 185 m thick) represents a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), superposed by Cerro Negro Formation (c. 150-400 m thick, siliciclastics) assigned to Upper Neoproterozoic age. The final sedimentary transgression during Early Paleozoic was the Balcarce Formation (c. 90-450 m thick) deposited over all the mentioned Precambrian units. Based on all the geological background, a tectonic evolution is offered. [ABSTRACT FROM AUTHOR]

The Sierra de San Luis forms the southern tip of the Eastern Sierras Pampeanas in central Argentina. Two narrow belts of low-grade phyllites and quartz arenites, i.e. the San Luis Formation, have accommodated part of the strain-related differential exhumation of the medium- to high-grade metamorphic domains that constitute to the basement complex of the sierra. Eleven phyllite samples were subjected to the K/Ar fine-fraction dating technique. Results are interpreted in relation to the Kübler index of the illites, which indicate epimetamorphic conditions for the majority of the samples. Obtained ages between 330 and 290 Ma cover a period of compressional tectonics in the late Mississippian (Visean/Serpukhovian boundary) followed by the subsidence during the formation of the Paganzo Basin in the provinces of La Rioja and San Luis. These tectonic movements are coincident with the Toco orogeny in northern Chile and southern Bolivia. This suggests that the older K/Ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement. [ABSTRACT FROM AUTHOR]

In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5 Ga) and late Palaeoproterozoic (1.8-2.0 Ga) ones. No Siluro-Devonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Río de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous. [ABSTRACT FROM AUTHOR]

The application of the SHRIMP U/Pb dating technique to zircon and monazite of different rock types of the Sierras de Córdoba provides an important insight into the metamorphic history of the basement domains. Additional constraints on the Pampean metamorphic episode were gained by Pb/Pb stepwise leaching (PbSL) experiments on two titanite and garnet separates. Results indicate that the metamorphic history recorded by Crd-free gneisses (M2) started in the latest Neoproterozoic/earliest Cambrian (553 and 543 Ma) followed by the M4 metamorphism at ~530 Ma that is documented in the diatexites. Zircon ages of 492 Ma in the San Carlos Massif correlate partly with rather low Th/U ratios (<0.1) suggesting their growth by metamorphic fluids. This age is even younger than the PbSL titanite ages of 506 Ma. It is suggested that the fluid alteration relates to the beginning of the Famatinien metamorphic cycle in the neighbouring Sierra de San Luis and has not affected the titanite ages. The PTt evolution can be correlated with the plate tectonic processes responsible for the formation of the Pampean orogene, i.e., the accretion of the Pampean basement to the Río de La Plata craton (M2) and the later collision of the Western Pampean basement with the Pampean basement. [ABSTRACT FROM AUTHOR]