Puncoviscana Research Articles

Hf isotopic ratios in zircon reveal processes of anatexis and pluton construction

Heterogenous isotopic ratios in magmatic rocks have been variously interpreted to be a result of magma mixing or inheritance from the source rock. Isotopic heterogeneities in zircons in the source may be homogenised in the melt by magma flow and chemical diffusion. This implies that the degree of homogenisation of the inherited Hf isotopic signal can be indicative of the degree of magma homogenisation, which depends on the nature and duration of magma flow before crystallisation. In order to understand the systematics of Hf isotope transfer from zircons in the source to those crystallised from the melt, we measured Hf isotopic ratios of magmatic and inherited detrital zircons in migmatites from turbidites of the Puncoviscana Formation in NW Argentina, as well as a granite domain within the sequence. Detrital zircons show ∼30 Eps units of variation in Hf isotope values. Anatectic zircons also show a large range of isotopic values but it is narrower than, and lies within, the range of the detrital zircons. Using analysis of variance (ANOVA), we found that the Hf isotope ratios of zircon rims are partly controlled by their cores and partly by the mean value of the magma. This demonstrates the effect of a boundary layer around dissolving detrital zircon grains that contains a high concentration of the core's isotopic signature (the “core effect”). Anatectic systems can be divided into four end-members on the basis of the extent of the core effect, the degree of magma homogenisation, and nature of the magma prior to zircon crystallisation. Recognising these end-members allows the nature of processes in anatectic rocks that homogenise and modify Hf isotopic ratios to be constrained.

Extending the Pampean orogen in western Argentina: New evidence of Cambrian magmatism and metamorphism within the Ordovician Famatinian belt revealed by new SHRIMP U–Pb ages

New SHRIMP U–Pb zircon and monazite ages from the Sierra de Fiambalá (Sierras Pampeanas, NW Argentina) record coeval granitoid peraluminous magmatism and high-grade metamorphism at ca. 525 Ma (Lower Cambrian) that resulted from the collisional Pampean orogeny. The Sierra de Fiambalá, however, is well within the Ordovician accretionary Famatinian orogenic belt and west of what is commonly regarded as the Pampean orogenic belt in the Eastern Sierras Pampeanas (ESP). It is thus an inlier that can be correlated chronologically and stratigraphically with the Pampean high-grade metamorphic domain in the Sierras de Córdoba (ESP). Moreover, in the Fiambalá inlier, Ediacaran to early Cambrian meta-sedimentary rocks of the Ancaján Series, of an alleged Laurentian source, predominate relative to the Puncoviscana Formation of Gondwanan provenance. Two tectonic interpretations are proposed for the origin of the Fiambalá inlier: (1) it is a Pampean basement thrusted westward during the Famatinian orogeny or (2) it was rifted away from the main Pampean orogenic belt sometime between the end of the Pampean orogeny and the start of the Famatinian orogeny and later reworked by the latter in coincidence with the proposal by Weinberg et al. (2018). The evidence given here confirms that rocks of the Pampean orogen extend far westward of the main Pampean belt (≥300 km).

Paleogeographic and tectonic evolution of the Pampia Terrane in the Cambrian: New paleomagnetic constraints

Paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the late Middle to early Late Cambrian Campanario Formation exposed in NW Argentina. The study also presents preliminary results from the Early Cambrian metasediments of the Puncoviscana Formation and from Mojotoro intrusive. A new paleomagnetic pole, C2, was computed for the Campanario Formation (23.6°N, 346.5°E, A95, 7.0°, N: 11 sites) from the localities of Tilcara (T), Terma de Reyes (R) and El Perchel (EP), including previous data from Santa Victoria Oeste (M), in northernmost Argentina. A positive fold test was obtained for Tilcara locality while a positive regional tilt test was obtained for mean site directions for the four localities. This pole can be considered as the representative pole position for the Pampia terrane for the late Middle to Late Cambrian. Previous paleomagnetic data for the same formation from the Iruya-Matancillas locality show an in situ clockwise rotation of about 30° around a vertical axis when compared with the mean Pampia pole. The same is observed when comparing the early Ordovician pole for the Santa Rosita Formation at those localities with the recently obtained coeval pole of the La Pedrera Formation, indicating that local tectonic rotations, affected that area. These results suggest that previous paleomagnetic interpretations for these Late Cambrian and Early Ordovician rocks in the region as recording displacement of the Pampia terrane in Cambrian times are probably incorrect. Additionally, two virtual geomagnetic poles were computed for the Puncoviscana Formation (TP1: 34.5°N, 52.3°E, A95: 5.0°, n: 22 specimens and TP2: 38.3°N, 33.6°E, A95: 5.7°, n: 14 specimens) and one for the Mojotoro Intrusive (MO: 9.1°N, 345.6°E, A95: 16.4°, n: 11 specimens). These and previous Early Cambrian paleomagnetic data from the Pampia terrane differ from Gondwana coeval reference directions in inclination and/or declination, lacking a simple pattern.

Foreland Basin Record of Uplift and Exhumation of the Eastern Cordillera, Northwest Argentina

AbstractThe style, tempo, and timing of orogenic plateaus such as the Altiplano‐Puna in South America are actively debated. Foreland basin strata in northwestern Argentina preserve a record of the eastward propagation, uplift, and erosion of the eastern margin of the high topography of the Puna plateau and Eastern Cordillera. Herein, we present new sedimentologic and geochronologic data from the Neogene foreland basin strata and modern fluvial sediments at 23°S to reveal the timing of growth and erosion of the modern plateau margin. Traditional sedimentologic approaches, including conglomeratic clast counts and sandstone point‐counts, combined with detrital zircon U‐Pb age distributions preserve robust signatures that track changes in sediment provenance over the past 12 Ma. Growth of the modern plateau edge commenced between 12 and 7 Ma and erected a topographic barrier that cut off the supply of sediment from more westerly sources. Erosion of this rising block has progressively dissected a cover of early Paleozoic sediments of the Mesón and Santa Victoria Groups, leading to deep incision into the pre‐Cambrian metasediments of the Puncoviscana Formation that continues today. Detrital apatite and zircon fission track data from the both the Neogene strata and modern streams document erosion of the frontal block at a rate of about 0.6 mm/a.

Hybridization between I-type and S-type granites in the Ordovician Famatinian magmatic arc, Tafí del Valle, Tucumán, NW Argentina.

In the Tafi del Valle region, in northwestern Argentina, several intrusive bodies of lower Paleozoic age were emplaced in the metasedimentary Puncoviscana Formation, belonging to the Sierras Pampeanas. Four intrusive bodies outcrop in the study area: La Ovejeria, El Infiernillo, Loma Pelada and Los Cuartos. La Ovejeria and El Infiernillo intrusive bodies represent the I-type magmatism according to their major element contents and show cotectic evolutions similar to those described by Famatinian I-type batholiths. These moderately peraluminous adakitic trondhjemites have characteristic high Sr/Y ratios and low Fe 2 O 3 +MgO+MnO+TiO 2 contents. They are related to high-pressure conditions at the source, where dehydration melting of basaltic rocks may be involved and garnet is retained in the residue leading to generation of HREE depleted melts. The Loma Pelada granites show characteristics of S-type magmatism (low CaO and MgO, and high SiO 2 and K 2 O contents) typical of granites segregated in the last stages of magmatic differentiation, or anatectic granites. They show an increasing peraluminosity due to garnet entrainment and they are related to the anatectic melts generated in the Puncoviscana Formation. Both the Loma Pelada and Los Cuartos granitoids include samples with intermediate geochemical characteristics that range between those of the El Infiernillo and La Ovejeria and the regional metasedimentary rocks. These characteristics could be explained by assimilation processes involving the I-type intrusive magmas and the metasedimentary host rocks or by hybridization processes between trondhjemitic I-type magmas as La Ovejeria and El Infiernillo and anatectic S-type melts.

Low‐P melting of metapelitic rocks and the role of H2O: Insights from phase equilibria modelling

AbstractWater‐fluxed melting has long been thought to have a minor influence on the thermal and chemical structure of the crust. We report here on amphibolite facies metasedimentary rocks from the 490–450 Ma Famatinian Orogen, in northwest Argentina, that have undergone water‐fluxed incongruent biotite melting at relatively low temperature, which have produced and lost a significant volume of melt. The protoliths consist of the turbiditic Puncoviscana Formation (Neoproterozoic to Early Cambrian). The field area exhibits a condensed metamorphic field gradient, from greenschist to amphibolite facies suprasolidus conditions, recording a low pressure almost isobaric path, reaching peak conditions estimated at 700°C at 4 kbar. Thermodynamic modelling in the MnNCKFMASHTO system is applied to investigate melting at such low pressure as a function of water content. Calculations using a typical turbidite composition show how small amounts of added free H2O may increase significantly the melt fraction with little or no change in either the melt or residual phase compositions. They indicate negligible difference in normative An–Ab–Or proportions and ferromagnesian contents between melts derived by dehydration and water‐fluxed melts. The same is true for the content of H2O dissolved in melts, which remains constant and the melt produced is granitic whether or not aqueous fluids are present. Thus, neither the residue nor the melt composition are indicators of the presence of aqueous fluids during anatexis. Recognizing the impact of small additions of H2O to an anatectic terrane may therefore be difficult. The most significant change related to water‐fluxing is the relative proportions of minerals and melt fraction, rather than the actual mineral assemblage. The modal proportion of feldspar decreased while those of cordierite and biotite increased in the residual assemblages, as <5 mol.% of free H2O was added. The impact of this addition is to more than double the proportion of water‐undersaturated melt to 25–30 mol.%. We have also developed a simple way to estimate how much melt a residual rock has lost, if the compositional trends of the protoliths are known. In summary, we find that even though the addition of small amounts of free H2O impacts significantly on rock fertility, there is little obvious record in the field. The combined application of careful petrological investigation and thermodynamic modelling is the key to identify the influence of aqueous fluids, and exploit systems that became open not only to fluid influx but also to the extraction of melt.

Isotope (Sr, C) and U–Pb SHRIMP zircon geochronology of marble-bearing sedimentary series in the Eastern Sierras Pampeanas, Argentina. Constraining the SW Gondwana margin in Ediacaran to early Cambrian times

The Sierras de Córdoba Metasedimentary Series consists of marbles and metasiliciclastic rocks of Ediacaran to early Cambrian age (ca. 630 and 540Ma) that underwent high-grade metamorphism during the collisional Pampean orogeny in the early Cambrian. The ages of the marbles were determined from the Sr-isotope composition (blind dating) of screened samples of almost pure calcite marble and were further constrained with C- and O-isotope data and U–Pb SHRIMP detrital zircon ages of an interbedded paragneiss. Two groups of samples are recognised with Sr-isotope composition ca. 0.7075 and 0.7085 that are considered stratigraphically significant. The first is inferred early Ediacaran, the second late Ediacaran to early Cambrian. The Sierras de Córdoba Metasedimentary Series is correlated for the first time with marble-bearing metasedimentary series in several sierras to the west and north of Sierras de Córdoba (e.g., the Difunta Correa Sedimentary Sequence and the Ancaján Series), implying that all were probably parts of an originally extensive sedimentary cover. These series bear evidence of sedimentary sources in the Mesoproterozoic (and Paleoproterozoic) basement of the Western Sierras Pampeanas (part of the large MARA continental block) and farther west (Laurentia?). In terms of the age of limestones/marbles and detrital zircon patterns, the Sierras de Córdoba Metasedimentary Series differs strongly from the older section of late Ediacaran to early Cambrian Puncoviscana Formation of northwestern Argentina, which mostly outcrops in northern Sierra Chica and Sierra Norte, with sedimentary input from western Gondwana sources.The Sierras de Córdoba Metasedimentary Series and the Puncoviscana Formation were probably juxtaposed during the Pampean orogeny along a complex suture zone that was further folded and/or imbricated at mid-crustal depths. The peak of metamorphism was attained at 527±2Ma. According to the evidence found here most of the Sierras Pampeanas to the west of the Sierras de Córdoba were part of the lower colliding plate during the final amalgamation of SW Gondwana.

Pre-Pampean metasedimentary rocks from the Argentinian Puna: Evidence for the Ediacaran margin of Gondwana or the Arequipa–Antofalla–Western Pampeanas block

Quartzites exposed in the southern Puna (northwest Argentina) form faulted contacts with Late Ordovician volcaniclastic rocks and are discordantly overlain by Permo-Carboniferous red-beds. These strongly recycled rocks lack sedimentary structures and fossils. Therefore, a geochemical approach was preferred for this study. U–Pb isotope analysis of detrital zircon yielded an unusual age population. Ten grains lie within error of 570 and 590Ma—with the youngest detrital zircon at 571±14Ma—and are therefore interpreted as defining the maximum sedimentation age range.There is no evidence of Pampean orogeny event detritus (c. 550–520Ma). Therefore, we argue that these rocks are unlikely to be syn-tectonic with the event, or even younger. Alternatively, it can be argued that this component is absent due to differences in paleocurrent direction, sorting during transportation, or a hypothetical structural high; all of which could have blocked the Pampean provenance signal from reaching the depositional basin. Sorting related to the depositional environment can be excluded as the sample is a combination of all occurring grain sizes. There is no preserved geological evidence of an obstruction, which allows us to envisage different scenarios for deposition of these rocks.The most likely interpretation is that these successions predate the Pampean orogeny and deposition of the Cambrian Puncoviscana Formation. Consequently, the rocks could either have been deposited within the (passive) margin of the Arequipa–Antofalla–Western Pampeanas terrane (western Gondwana) or MARA block; or are unusual, reworked sedimentary rocks related to the active continental margin of the Pampean orogen.

Paleomagnetic studies on the late Ediacaran – Early Cambrian Puncoviscana and the late Cambrian Campanario formations, NW Argentina: New paleogeographic constraints for the Pampia terrane

A magnetofabric and paleomagnetic study was carried out in the Late Ediacaran – Early Cambrian Puncoviscana and the early Late Cambrian Campanario formations, exposed in Santa Victoria Oeste, in northwestern Argentina. Ten sites (93 samples) were located in tuffs and volcanic sills interbedded in the Puncoviscana Formation, one of which had been dated at 537 ± 0.9 Ma. On the other hand, 42 samples were collected at three sites from red to purple sandstones of the Campanario Formation. The analysis of anisotropy of magnetic susceptibility (AMS) allowed to recognize a pre-Andean fabric in both formations indicating that it is previous to the Andean cycle. In the paleomagnetic study reliable magnetic components were isolated in only two sites of the Puncoviscana formation whose virtual geomagnetic poles (VGPs) are close to but do not coincide with the apparent polar wander path of Río de la Plata craton – Gondwana for the late Neoproterozoic – Cambrian. A new paleomagnetic pole (18.2°S, 358.8°W, K: 27.9; A95: 3.9) was computed for the Campanario formation which is close to but does not coincide with those obtained in other locations for this unit and were considered anomalous respect to the expected position in the Gondwana path. The pole presented here is closer to the apparent polar wander path of Gondwana for the Late Cambrian-Early Ordovician suggesting two possible interpretations; the presence of different amounts of Andean tectonic rotations between different sampling locations of the Campanario Formation or the recording of a rapid Pampia dextral displacement along the Rio de la Plata craton margin in Cambrian times.

The ages and tectonic setting of the Faja Eruptiva de la Puna Oriental, Ordovician, NW Argentina

The Ordovician Faja Eruptiva de la Puna Oriental is a magmatic, predominantly intrusive belt in the Puna of northwestern Argentina with a N-S extension of ca. 400km. Scarce isotope geochemical ages and biostratigraphic data on some of the folded Faja Eruptiva country rocks assign the magmatism either to the Lower and lower Middle Ordovician, or to the latest Ordovician. Interpretations of origin and tectonic framework of the Faja Eruptiva are controversial and vary between arc, back-arc and collisional-orogenic settings.We present high-resolution La-ICP-MS U–Pb age and Hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200km in the northern Argentinian Puna. The xenocrystic and magmatic zircon age data have a wide spread between 2700Ma and 440Ma. Concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460Ma, and between 453 and 444Ma, and constrain the time of the last intrusions at 444±3Ma and at 445±2Ma thus defining this last and main phase of intrusion at 444Ma.εHf(t) values define a main vertical trend centered at 500Ma with εHf(t) values between +3 and −16 indicating significant mixing of juvenile early Paleozoic melts with Paleoproterozoic crustal components. A second trend is formed by zircons with ages between 1.1Ga and c. 500Ma and predominantly positive εHf values between +8 and −3 and originates in juvenile mantle compositions between 1.6 and 1.1Ga. The spread of the zircon and Hf data document that the Faja Eruptiva intrusives have experienced large-scale contamination by the hosting crustal basement. It follows that the basement of the Puna is formed either by the upper Proterozoic-lower Cambrian Puncoviscana Formation as an erosional product of the Proterozoic orogenic belts of SW Amazonia or that the Puna including its Puncoviscana basement is underlain by a crust shaped by these orogenies.The main intrusive event at 444Ma has been linked to the Oclóyic orogeny in the Late Ordovician. The plutons intruded very likely in a sinistral strike-slip regime after the main folding phase of the Oclóyic orogeny had deformed the Ordovician sedimentary country rocks.

Provenance from zircon U–Pb age distributions in crustally contaminated granitoids

The basement of sedimentary basins is often entirely covered by a potentially multi-stage basin fill and therefore removed from direct observation and sampling. Melts intruding through the basin stratigraphy at a subsequent stage in the geological evolution of a region may assimilate significant volumes of country rocks. This component may be preserved in the intrusive body either as xenoliths or it may be reflected only by the age spectrum of incorporated zircons. Here we present the case of an Ordovician calc-alkaline intrusive belt in NW Argentina named the “Faja Eruptiva de la Puna Oriental” (Faja Eruptiva), which in the course of intrusion sampled the unexposed and unknown basement of the Ordovician basin in this region, and parts of the basin stratigraphy. We present new LA–ICP-MS U–Pb ages on zircons from 9 granodiorites and granites of the Faja Eruptiva.The main part of the Faja Eruptiva intruded c. 445Ma in the Late Ordovician. The zircon ages obtained from the intrusive rocks have a large spread between 2683.5±21.6 and 440.0±4.9Ma and reflect the underlying crust and may be interpreted in several ways. The inherited zircons may have been derived from the oldest known unit in the region, the thick siliciclastic turbidite successions of the upper Neoproterozoic–lower Cambrian Puncoviscana Formation, which is inferred to represent the basement of the NW Argentina. The basement to the Puncoviscana Formation is not known. Alternatively, the inherited zircons may reflect the geochronological structure of the entire unexposed Early Paleozoic crust underlying this region of which the Puncoviscana Formation was only one component. This crust likely contained rocks pertaining to and detritus derived from earlier orogenic cycles of the southwestern Amazonia craton, including sources of Early Meso- and Paleoproterozoic age.Detritus derived, in turn, from the Faja Eruptiva intrusive belt reflects the origin of the granitoids as well as the inherited geochronological and isotope geochemical structure of either the basement and/or distant sources having supplied material to the basement rocks. If unrecognized, sediment formed from such granitoid sources may erroneously be used to infer the exposure of, and direct detrital contributions from, a variety of older source rocks in fact not directly involved in the studied source–sink system.

Geochronology of igneous rocks in the Sierra Norte de Córdoba (Argentina): Implications for the Pampean evolution at the western Gondwana margin

U-Pb zircon data (secondary ion mass spectrometry [SIMS] and thermal ionization mass spectrometry [TIMS] analyses) from igneous rocks with differing structural fabrics in the Sierra Norte de Cordoba, western Argentina, suggest that the sedimentary, tectonic, and magmatic history in this part of the Eastern Sierras Pampeanas spans the late Neoproterozoic–Early Cambrian. A deformed metarhyolite layer in metaclastic sedimentary rocks gives a crystallization age of 535 ± 5 Ma, providing a limit on the timing of the onset of D 1 deformation and metamorphism. The new data coupled with published Neoproterozoic zircon dates from a rhyolite beneath the metaclastic section and detrital zircon ages from the section indicate a late Neoproterozoic to Early Cambrian depositional age, making this section time equivalent with the Puncoviscana Formation (sensu lato) of northwest Argentina. A synkinematic granite porphyry gives a crystallization age of 534 ± 5 Ma, providing a limit on the age of dextral mylonitization in the Sierra Norte area (D 2 event). The new age is consistent with ages of 533 ± 4 Ma from a mylonitic granite with dextral sense-of-shear fabrics and 531 ± 4 Ma from a late-synkinematic dacitic porphyry, which broadly indicates the final age of dextral deformation. A crystallization age of zircons from the postkinematic, high-level El Tio granite (530 ± 4 Ma) suggests that both stages of Pampean deformation and regional metamorphism, accompanied by synkinematic intrusions, were followed by uplift and took place during a very short time span in the Early Cambrian. This is supported by zircon dates of 523 ± 5 Ma from a rhyolite to dacite in the western part of the Rodeito area and dates from the undeformed El Escondido rhyolite and granite of 519 ± 4 Ma and 521 ± 4 Ma, respectively. These three crystallization ages also indicate that ductile dextral shearing and mylonitization associated with the Pampean D 2 event terminated in the Early Cambrian. Both stages of Pampean deformation in this segment of the western pre-Andean Gondwana margin seem to represent a continuous event that can be related to oblique dextral convergence between the overriding plate in the east and the subducting and finally colliding plate in the west. The postkinematic intrusions and extrusions are related to the late stage of the Pampean magmatic history, which terminated before Early Ordovician (Famatinian) time.

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have eNd t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. 147Sm/144Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe3+ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.

Geothermobarometry of very low-grade metamorphic pelites of the Vendian–Early Cambrian Puncoviscana Formation (NW Argentina)

The Vendian–Early Cambrian Puncoviscana Formation is a pelite-greywacke turbidite sequence affected by polyphase deformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analytical high-resolution TEM studies on southern locations found anchizonal grade and proposed medium–high pressure metamorphism followed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on small uncontaminated areas selected using SEM in backscattered electron mode. The application of mica-chlorite geothermobarometry to mineral grains defining two different foliations has produced consistent pressure/temperature results based on the intersection of six reactions: 9 kbar/250 °C peak conditions were followed by an isothermal decompression to 3 kbar and a successive increase of temperature to nearly 350 °C at 1.5 kbar. The shales from the overlying Meson and Santa Victoria Cambro–Ordovician groups do not contain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KICIS obtained (0.41–0.68Δ°2θ) and their mineralogical assemblage (illite ± kaolinite ± corrensite + Qtz + Pl ± Kfs), these rocks have not surpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100 °C, consistent with the sharp angular unconformity between the two units.

Petrogenesis of migmatites and leucogranites from Sierra de Molinos, Salta, Northwest Argentina: A petrologic and geochemical study

In Sierra de Molinos, Eastern Cordillera, NW Argentina the relationship between different components of an anatectic system developed in high temperature and low pressure conditions (<800°C and 4–6kbar) can be observed. This allows the direct insight on the origin of migmatites and granites by anatexis of psammo–pelitic metasedimentary rocks. Four principal rock types have been recognized in Sierra de Molinos 1) metasedimentary rocks, assigned to the Puncoviscana Formation, 2) metatexite migmatites, 3) diatexite migmatites and 4) anatectic granitoids including: leucogranites, trondhjemites and pegmatite/aplitic dykes. The metamorphic grade increases from phyllites and schists at sub-greenschist and greenschist facies in the west, to upper amphibolite facies migmatites in the east. The mineral assemblages indicate a progressive increase in temperature without a significant change in pressure. The ages obtained for granitoids and migmatites are identical within analytical errors and indicate that the metamorphic peak and anatectic granite generation occurred at ca. 470Ma.We use petrological and geochemical variations to investigate the formation of granite magma from migmatites that derived from metasedimentary protoliths. This provides evidence of the processes that occur during the evolution (melting, segregation and crystallization stages) of partially molten terranes, for which particular components: protolith, residuum and melt fractions are identified. Mass balance calculations are used to estimate the degree of partial melting and the degree of melt extraction in migmatites. Petrological and compositional data suggest that migmatites from this region are the result of open system processes and that fluid-absent melting played a major role in the formation of migmatites and leucogranites. The geochemical evidence suggests that the compositional variation within the anatectic rocks from Molinos can be understood as a result of 1) variable degrees of melt–residuum separation and 2) fractional crystallization of the anatectic melt. Compared to the protolith, most of the migmatites have melt-depleted compositions (up to 0.25 of melt extraction). In the same fashion most of the leucosomes represent cumulate products of fractional crystallization. This means that they have lost some liquid of evolved composition and represent evidence of the melt extraction pathways and linkage to the magma ascent conduits.

Provenance and tectonic setting of Neoproterozoic to Early Cambrian metasedimentary rocks from the Cordillera Oriental and Eastern Sierras Pampeanas, NW Argentina

Major, trace, and rare-earth element data for the Neoproterozoic to early Cambrian Puncoviscana Formation and related highergrade metasedimentary rocks in Eastern Sierras Pampeanas, NW Argentina, were analyzed to evaluate their provenance, weathering, and tectonic setting during the upper Neoproterozoic-lower Paleozoic. The studied rocks are metapelites and metapsammites that were mainly affected by the lower Cambrian Pampean orogeny. Geochemical analyses indicate that the Puncoviscana Fm. and related rocks are characterized by moderate Chemical Index of Alteration (CIA) values. REE element distributions, Eu/Eu* values, and high Zr/Sc and Th/Sc ratios indicate that the samples were likely derived from predominantly upper-crust felsic sources. Tectonic discrimination diagrams based on immobile trace elements (e.g. Th-La-Sc, Sc-Th-Zr/10), multi-element patterns, REE characteristics, and diagnostic trace element ratios (i.e. Th/Sc, Zr/Hf, La/Th, La/Sc) imply that the tectonic setting of the source area was a continental island arc and/ or an active continental margin. Geochemical signatures of low-grade metapsammitic samples (high Hf and Zr content, and high Zr/ Sc ratios) indicate crustal recycling. The geometry and sequence of structures and microstructures preserved in all samples are pervasive and suggest a common and uniform deformational history during the Pampean orogeny. Geochemical signatures and structures of the studied samples are consistent with protolithic sediments of the Puncoviscana Fm. and related rocks initially deposited in an arc-related basin that was subsequently deformed during the Pampean orogeny.

Major Miocene exhumation by fault‐propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina

The central Andean retroarc thrust belt is characterized by a southward transition at ∼22°S in structural style (thin‐skinned in Bolivia, thick‐skinned in Argentina) and apparent magnitude of Cenozoic shortening (&gt;100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo‐thermochronological study of the Cachi Range (∼24–25°S), which is a prominent topographic feature at this latitude. Our U‐Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro‐Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U‐Th)/He zircon results require 8–10 km of Miocene exhumation that was likely associated with fault‐propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike‐slip faults demonstrates a change in stress state to a non‐vertically orientedσ3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin‐skinned Bolivian fold‐thrust belt. Our results also underscore the importance of the pre‐existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation.

Basin evolution of the margin of Gondwana at the Neoproterozoic/Cambrian transition: the Puncoviscana Formation of Northwest Argentina

The Puncoviscana Formation is an important stratigraphic sequence of sandstones and shales, with intercalated conglomerates, limestones and volcanic rocks. It was deposited at the Neo- proterozoic-Cambrian boundary in a shallow basin on the western margin of Gondwana. Trace fos- sils of the Puncoviscana Formation s.l. are represented by the Oldhamia and Nereites ichnoassocia- tions and the geographical distribution of assemblages are aligned as parallel belts, with a shallower eastern Nereites association and a deeper western Oldhamia association. They represent different temporal levels on the evolution of the basin. Detrital zircon spectra display a wide range of ages, which indicate their provenance from Neoproterozoic to Cambrian source areas as well as Meso- to Paleoproterozoic basement units, exhibiting typical Gondwanan ages. Thick limestone banks are interbedded in the Puncoviscana Formation s.l. that included the siliciclastic sequences, with very low- to medium-metamorphic grade. δ13C values for the limestones vary from -1.57 to +3.40/00 VPDB, while in the Sierras Pampeanas, δ13C values vary from +2.6 to +80/00 VPDB. Also, reported 87Sr/86Sr values allow a clear differentiation of two rock sets. The limestones interbedded with the Puncoviscana Formation yielded values typical for the Neoproterozoic-Cambrian transition, between 0.70868 and 0.70896; while the limestones of the Sierras Pampeanas display values between 0.70748 and 0.70756, common in the middle to late Ediacaran. Deformation and uplift of the Puncoviscana Formation is clearly represented in northern Argentina by the Tilcarian unconformity, which is over- lain by the Meson Group.

Detrital zircon U–Pb ages of metasedimentary rocks from Sierra de Valle Fértil: Entrapment of Middle and Late Cambrian marine successions in the deep roots of the Early Ordovician Famatinian arc

We obtained detrital zircon ages from three metasedimentary rocks with siliciclastic protoliths that were buried to paleodepths of 20–27 km during Early Ordovician Famatinian arc magmatism. In all three samples, U–Pb zircon age distributions are polymodal but the dominant peaks make well-defined clusters, matching the ages of orogenic systems that characterize West Gondwana. The most prominent peaks of constituent age populations at 520 Ma, 600 Ma, and 1050 Ma reflect derivation from sources that are characteristic of the West Gondwana margin. Moreover, grains with Meso- and Paleo-Proterozoic ages appear as small fractions (<5%). The siliciclastic protoliths to metasedimentary rocks from the deep Famatinian crust are dominantly composed of detritus derived from two sources: 1) a nearby magmatic arc or a rapidly exhuming orogen, and 2) sedimentary recycling of Neoproterozoic–Lower Cambrian sedimentary successions broadly grouped as Puncoviscana Formation. Our findings reveal that granulite–facies metasedimentary rocks in the deep crust of the Famatinian arc have detrital zircon patterns closely resembling those found in nearby Middle to Upper Cambrian quartz-rich turbidite sequences from central Famatina, the Cordillera Oriental and the Puna. At the regional scale, these sedimentary successions yield maximum depositional ages corresponding to the Middle Cambrian (<520 Ma). This indicates that metasedimentary rocks in the Sierra de Valle Fértil were rapidly buried to deep levels after deposition. Our results are also consistent with the existence of a crustal scale paleo-suture located at the western margin of the Sierras Valle Fértil and La Huerta that explains the differences of detrital zircon age patterns between metasedimentary rocks in the Valle Fértil and age-equivalent sedimentary units in the nearby Cuyania Terrane.

El Alisal : a new locality with trace fossils of the Puncoviscana Formation (late Precambrian-early Cambrian) in Salta Province. Argentina

The Puncoviscana, Suncho and Las Aguaditas/Negro Peinado formations represent a thick siliciclastic folded succession that crops out on a strip about 800 km long and 150 km wide in northwestern Argentina (Jujuy, Salta, Tucuman, Catamarca and La Rioja provinces). These sequences are lithologically characterized by very lowgrade metamorphic sediments grading from slates to schists; turbidites, pelagic clays, limestones and volcanic rocks. On the basis of paleoichnological data, these units have been assigned to the late Precambrian-early Cambrian. A new locality bearing well preserved trace fossils from the Puncoviscana Formation is described herein. The outcrop is located about 45 km west of Salta city (Salta Province), displaying an alternation of colored differentiated grayish-bluish slates and fine sandstones. Ichnofossils include Cochlichnus anguineus HITCHCOCK, Helminthoidichnites tenuis FITCH, Helminthoida isp., Monomorphichnus lineatus CRIMES, LEGG and MARCOS ARBOLEYA, Planolites isp. and Torrowangea? isp., as well as some limb marks and a few slightly bended smooth trails (indet.). An Early Cambrian age for the succession at El Alisal as well as some environmental considerations are allowed by the ichnological association. In addition, the ichnogenus Planolites is mentioned for the first time from the “red shales and conglomerate type” facies at the neighbor locality of Chorrillos.