Pampean Orogen Research Articles

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).

Constraining the timing and evolution of a long-lived tectonic boundary: an example from the Early Paleozoic, Argentina

The Guacha Corral shear zone (GCSZ) is one of the most prominent crustal-scale shear zones in the Eastern Sierras Pampeanas of Argentina, juxtaposing two different metamorphic terranes: the eastern hanging wall, which was metamorphosed in the 550-510 Ma Pampean Orogen, and the western footwall, metamorphosed in the 500-440 Ma Famatinian Orogen. In this study, the tectono-thermal evolution of the footwall was constrained using U–Pb dating of zircon, monazite and apatite, and results were compared to the broader evolution of the shear zone. The main episode of migmatization was recorded in the footwall at ~475 Ma by zircon and monazite, indicating anatexis during the Famatinian Orogeny at amphibolite facies conditions (~700 °C – Ti-in-zircon thermometer). It was at this time that the major thrusting event was taking place due to the docking of the Precordillera/Cuyania terrane to the proto-Andean margin. Anatexis in the footwall was followed by cooling reaching upper greenschist facies conditions recorded by apatite U/Pb ages at ~400 Ma. The cooling rates during retrogression of footwall migmatites from ~690 °C to ~500 °C were of ~3–4 °C/Ma. The cooling processes after peak metamorphism were temporally related to the waning of the Famatinian Orogeny during the Early Devonian. After crossing apatite closure temperature, rocks entered greenschist facies conditions and continued motion on the shear zone favored the influx of fluids to retrogress the footwall. This late motion is temporally related to the collision of the Chilenia terrane to the proto-Andean margin at ~390 Ma, marking the start of the Chanic/Achalian Orogeny.

The geodynamic history of the Famatinian arc, Argentina: A record of exposed geology over the type section (latitudes 27°- 33° south)

The Famatinian arc, over central-western Argentina, is one of the few examples of exposed crustal arc cross-section in the world. This Paleozoic magmatic arc and orogenic system on the Earth's surface offers unique insights into the nature of whole-arc processes, continental crust generation, and preservation. A detailed account of birth, growth, and closure is integrated throughout the region, and the entire geodynamic history of the Famatinian arc is presented. The Famatinian magmatic arc grew during a single-cycle episode that spanned a few tens of million years. Upon waning of the Cambrian Pampean orogen, an Upper Cambrian – Lower Ordovician marginal and open sea basin developed mostly on the recently stabilized Cambrian crystalline crust and was flooded by a turbidite wedge. Typical subduction zone magmatism resumed outboard of the Pampean orogen in the Lower Ordovician. After about 20 My of magmatism driven by subduction zone dynamics at plate scale, magmatism waned and stopped with the entry of a Laurentia-rifted continental microplate in the subduction zone. A full phase of a mountain-building process accompanied the continent-arc collision. The orogenic collapse occurred during the Devonian, ending the Famatinian system after nearly 150 My of magmatic arc, plate convergence, and continent-arc collision development on the proto-Andean Gondwana margin.

Depositional age and provenance in the San Luis Formation, Sierras Pampeanas, Argentina: Evidence from detrital zircon studies

The San Luis Formation is one of the lithostratigraphic units constituting the early Paleozoic crystalline basement of the Sierras de San Luis, which are part of the Sierras Pampeanas of central Argentina. The low-grade metamorphic overprint of the San Luis Formation makes it a key unit for the modelling of its sedimentary environment, which belongs to a deep-marine environment. This formation crops out in two belts named as Eastern and Western, whose stratigraphic correlation has not been clarified. Five samples from both belts of the San Luis Formation show U-Pb age distributions of detrital zircons with dominant peaks that are characteristic of orogenic systems of the West Gondwanan landmasses. However, detrital zircon age spectra and maximum depositional ages differ substantially in each belt of the San Luis Formation. While the Western belt displays polymodal age patterns with its most prominent age peaks at ∼635 Ma, a second most important population appears at 535–575 Ma with subordinate groups at 720–775 Ma and 1.7–2.2 Ga; the Eastern belt shows an almost unimodal age pattern dominated by an age peak at ∼535 Ma, with very subordinate peaks at ∼620 Ma, ∼780 Ma and ∼1,017 Ma. Regarding the maximum depositional age, two different ages have been recognized in the Western belt, ∼555 Ma at its bottom and ∼530 Ma at its top; whereas in the Eastern belt the maximum depositional age is ∼515 Ma. The age spectra of detrital zircons from the San Luis Formation record the evolution of the nearby Pampean orogen. At an early stage of exhumation, the Pampean orogenic system chiefly liberated recycled sedimentary detritus containing zircons with Neoproterozoic (∼635 Ma), Mesoproterozoic (∼1,000 Ma) and subordinate amounts of Paleoproterozoic (∼2,200 Ma) ages. Upon advanced unroofing and active erosion, the Pampean sequences sourced first-cycle detritus with latest Neoproterozoic to early Cambrian ages (555–525 Ma). A major conclusion is that the San Luis Formation accumulated in at least two stages, filling successive deep-marine basins that were marginal to the Pampean orogen during and after its latest Neoproterozoic to Cambrian development.

Syn-deformational anatexis along the Santa Rosa river section, Argentina: Feedback relations between deformation, metamorphism and melt extraction

Mass transference from lower to upper crustal levels requires an efficient network of pathways that act as channels for draining melts from the source and transferring magmas through the crust. We report on a mid-crustal section in the Santa Rosa river from the Eastern Pampean Ranges that underwent partial melting under active deformation. The study focuses on the feedback relations between structural features and the position and shape of leucosomes that record the remnant magma network in both metasedimentary and meta-igneous rocks. A complete range of migmatites encompassing from patch to stromatic metatexite to schollen-schlieren and nebulitic diatexite formed during a single high-grade metamorphic event that postdated a previous sub-solid evolution. The diversity in migmatites structures suggests that the layered sequence behaved heterogeneously under a non-coaxial deformational regime of progressive deformation. The relationships between migmatitic structures and the progressive production of melts resulted in feedback relations that persisted as a time-marker during the Pampean event. U-Pb dating of leucogranitic zircons allows constraining the time of melt generation and segregation to the lower Cambrian (534-520 Ma). Migmatites in the lower crust of the Pampean orogen acted as channels of melt migration towards upper crustal levels.

Petrological and geochemical variations of a turbidite-like metasedimentary sequence over the metatexite to diatexite transition within the Pampean Orogen, Argentina

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.

A Cambrian mixed carbonate–siliciclastic platform in SW Gondwana: evidence from the Western Sierras Pampeanas (Argentina) and implications for the early Paleozoic paleogeography of the proto-Andean margin

The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to the early Paleozoic tectonic evolution of SW Gondwana. Two well-known orogenies took place at the proto-Andean margin of Gondwana in the Cambrian and the Ordovician, i.e., the Pampean (545–520 Ma) and Famatinian (490–440 Ma) orogenies, respectively. Between them, an extensive continental platform was developed, where mixed carbonate–siliciclastic sedimentation occurred. This platform was later involved in the Famatinian orogeny when it underwent penetrative deformation and metamorphism. The platform apparently extended from Patagonia to northwestern Argentina and the Eastern Sierras Pampeanas, and has probable equivalents in SW Africa, Peru, and Bolivia. The WSP record the outer (deepest) part of the platform, where carbonates were deposited in addition to siliciclastic sediments. Detrital zircon U–Pb SHRIMP ages from clastic metasedimentary successions and Sr-isotope compositions of marbles from the WSP suggest depositional ages between ca. 525 and 490 Ma. The detrital zircon age patterns further suggest that clastic sedimentation took place in two stages. The first was sourced mainly from re-working of the underlying Neoproterozoic metasedimentary rocks and the uplifted core of the early Cambrian Pampean orogen, without input from the Paleoproterozoic Rio de la Plata craton. Sediments of the second stage resulted from the erosion of the still emerged Pampean belt and the Neoproterozoic Brasiliano orogen in the NE with some contribution from the Rio de la Plata craton. An important conclusion is that the WSP basement was already part of SW Gondwana in the early Cambrian, and not part of the exotic Precordillera/Cuyania terrane, as was previously thought.

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.

A provenance study of the Paleozoic Ventania System (Argentina): Transient complex sources from Western and Eastern Gondwana

The U–Pb and Lu–Hf isotopic analyses of the different sedimentary sequences of the Ventania System, an old Paleozoic orogenic belt exposed in the southern region of the Río de la Plata Craton in the province of Buenos Aires, Argentina, provide new evidence for the understanding of the tectonic evolution of the western sector of the Gondwanides mountain belt. These ranges formed as result of the late Paleozoic collision of the Patagonia terrane against the continental margin of Gondwana. The provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the Tandilia System, a Paleoproterozoic mountain belt formed during the amalgamation of the Río de la Plata Craton at about 1800–2200Ma, and incorporated to Western Gondwana during the Brasiliano Orogeny at 550–530Ma. The local dominant source at the base of the early Paleozoic changed to more distant supplies toward the top of the sequences, when an increasing participation of detritus from first, Cambrian (560–520Ma) zircons from the Pampean Orogen, and later on Ordovician (480–460Ma) zircons from the Famatinian Orogen is recorded. The detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early Paleozoic stratigraphy. The Balcarce Formation, an early Paleozoic sedimentary cover of the Tandilia metamorphic and igneous basement, shows striking differences when compared with the new data from the Ventania System. The two data-sets reveal different sources for the two regions. The late Paleozoic foreland basin deposits mark an abrupt change of 180° in the paleocurrent directions, in the petrographic composition of the sediments, and in the provenance of detrital zircons. These data indicate a southern provenance with the first evidence of Carboniferous and Permian magmatic zircons. The oldest Archean zircons together with the finding of clasts with archeocyathids support the provenance from Patagonia, which was derived from Eastern Gondwana. The U–Pb ages of the ash-fall tuffs in the Tunas Formation confirm the Early Permian age of the Eurydesma Fauna in the Ventania System. The U–Pb data together with the Lu–Hf isotopic data enhance the comprehension of the tectonic evolution of the Ventania System as part of the larger Gondwanides Belt that amalgamated to Western Gondwana during Late Permian times with some independent pieces derived from Eastern Gondwana.

Was there an Ediacaran Clymene Ocean in central South America?

Previous studies have proposed that a major suture resulted from the collision between the Amazonian and Sao Francisco-Congo cratons during the Cambrian, following the closure of a supposed Clymene Ocean. The proposal tentatively located this ocean along the Araguaia and Paraguay belts at the eastern margin of the Amazonian Craton, and its southern extension reached the Pampean belt in Argentina. In the present study we will argue that the existence of Ediacaran-Cambrian oceanic lithosphere in central South America is highly unlikely. West Gondwana was assembled during the convergence between the Amazonian, West African, Sao Francisco-Congo and Rio de La Plata cratons as well as the Saharan Metacraton, leading to the closure of the Goias-Pharusian Ocean during the Neoproterozoic. Final closure and continental collision resulted in the development of the Transbrasiliano-Kandi mega-shear zone that cuts through several mobile belts, but leaves the cratonic areas totally untouched. Consistent results of radiometric dating along the Transbrasiliano (TB) mega-shear in South America and of metamorphic rocks of the Brasilia Belt have indicated that the Neoproterozoic collision finished at ca. 620 Ma. After isostatic uplift, cooling, and denudation, between 590 and 500 Ma, emplacement of undeformed K-rich postorogenic granites represented the main tectonic event. At this time or afterwards, a series of small extensional sedimentary basins formed in graben troughs, most of which are within the TB tectonic corridor. They all were of extensional character, contrasting clearly with the convergent tectonics occurring within the coeval Pampean Orogen in Argentina. The main arguments showing that an Ediacaran to Cambrian oceanic closure in central Brazil is untenable include: (i) the assembly of West Gondwana was completed by ca. 600 Ma, when the convergence between the Amazonian, Sao Francisco and Rio de La Plata cratons had already ended. After this, there is no geological evidence of an oceanic lithosphere (for example, ophiolites, magmatic arcs, et cetera), ruling out the possible existence of an Ediacaran or Cambrian Clymene Ocean in Central Brazil; (ii) the Gurupi and Araguaia belts in Brazil, as well as the Bassaride and Rokelide belts in West Africa, are regarded as aulacogenic-type systems formed within an intraplate tectonic setting. Their tectonic history precedes the collision between the Amazonian and Sao Francisco-Congo cratons, as demonstrated by the linear structures of the Transbrasiliano megashear which truncate the N-S structural trends of the Araguaia Belt; (iii) there is a close correlation between the Corumba Group of the Paraguay Belt in Brazil and the Arroyo del Soldado Group in Uruguay. These sedimentary sequences belonged to the same Ediacaran continental shelf and this is a powerful indicator for an Ediacaran connection between the Amazonian and Rio de La Plata cratons, which precludes the existence of a wide ocean (for example, the Clymene) between them. On the other hand, the tentative correlation between the Sierras de Cordoba and the Paraguay Belt cannot be accepted, because these are far apart and there is no similarity in lithology, metamorphism, or structural trends; (iv) the Puga paleopole is the most important evidence for the hypothesis of the Cambrian Clymene Ocean, however the age of about 600 Ma for this paleopole, taken on the basis of Sr and C isotopes, is loosely constrained. In addition this is located at low latitude, not far from the present pole, and therefore could be related to a younger remagnetization; (v) the Pampean Orogen is made up of medium- to high-grade metamorphic rocks constrained between 560 Ma and 520 Ma and therefore was tectonically active during most of the Cambrian. However, at this time, an oceanic lithosphere is not evident in the vicinity of the Paraguay belt, and in central Brazil extensional rather than convergent tectonic processes have been observed.

The Sierra Norte-Ambargasta batholith: Late Ediacaran–Early Cambrian magmatism associated with Pampean transpressional tectonics

The Sierra Norte-Ambargasta batholith is one of the largest plutonic expressions of the Pampean orogeny in western Argentina. A thorough petrographic, geochemical, isotopic (Sr and Nd) and geochronological (UePb SHRIMP) study is reported. The batholith comprises granitoid rocks that may be subdivided into those affected by Pampean D2 dextral shearing and mylonization and those emplaced after deformation had ceased; representative samples gave UePb zircon ages of 537 ±4 Ma and 530 ±4 Ma respectively. The earlier, dominant, groupwere derived largely frommetaluminous calc-alkaline subduction-related magmas, whereas the late granites are peraluminous. However, all have relatively high initial 87Sr/86Sr ratios (0.706 to at least 0.710, strongly negative eNdt values (-1.7 to -5.9) and, in some cases inherited 600 Ma and 970 Ma zircon, similar to the isotopic and zircon provenance seen in the metamorphic host rocks. A high degree of contamination of the magmas, possibly anatexis in the case of the post-mylonite granite, is related to emplacement during the latestage transpressional docking of the Pampean terrane against the Rio de la Plata craton. The absence of detrital zircon derived from the craton in either the Pampean metasedimentary host rocks or the batholith supports this collisional model for the Pampean orogen.

Early uplift and orogenic deformation in the Neuquén Basin: Constraints on the Andean uplift from U–Pb and Hf isotopic data of detrital zircons

Provenance of detrital sediments of the Neuquén Basin in Central Argentina was investigated using U–Pb and Lu–Hf isotopic composition of zircon grains in order to evaluate the timing of uplift of the southern Andes at these latitudes (36°–39°S). Samples of fluvial synorogenic deposits of the Candeleros Formation, at the base of the Neuquén Group (Upper Cretaceous), as well as from older deposits of the Rayoso and Agrio Formations (Lower Cretaceous) were investigated. A regional angular unconformity separates the Upper from the Lower Cretaceous units and is generally attributed to the onset of the foreland basin stage. The evident differences between these suites (above and below the unconformity) are clearly shown by their distinct provenance patterns, which correspond to different bedrock sources and patterns of dispersal. Detrital zircons of the Rayoso and Agrio Formations beneath the unconformity indicate provenance from the eastern foreland basement, presently exposed along the Sierras Pampeanas of central Argentina. Detrital zircon ages correspond to the Permian and Triassic Choiyoi province, the Ordovician Famatinian orogen, the Lower Cambrian–Neoproterozoic Pampean orogen, the Grenvillian age Mesoproterozoic basement and some other older sources. In contrast, detrital zircons above the unconformity, from the Candeleros Formation, have striking different prominent peaks. The pattern indicates direct derivation from the Early Cretaceous magmatic arc, probably from plutons exhumed during the Late Cretaceous deformation along the western Chilean slope of the Andean cordillera. The contrasting dispersal patterns are interpreted as a result of the uplift of the southern Central Andes at these latitudes. The youngest detrital zircons constrain the timing of uplift as being younger than 99 Ma, supporting the assumed Cenomanian age for the synorogenic deposits. This age is further constrained by fission-track data from zircons from an ash-tuff layer of the lowest parts of the Neuquén Group, which yielded ages of about 88 Ma. The Hf isotopic data indicate derivation from a juvenile mantle for the Grenvillian age zircons, which corroborates the previously assumed island arc setting. The intermediate Hf isotopic values for the Pampean age zircons indicate a transition to the extensive recycling of the continental crust for the oldest Paleoproterozoic zircons with negative values that characterize the foreland basement.

Suya Taco and Sol de Mayo mafic complexes from eastern Sierras Pampeanas, Argentina: Evidence for the emplacement of primitive OIB-like magmas into deep crustal levels at a late stage of the Pampean orogeny

The age and petrogenetic features of the Suya Taco and Sol de Mayo mafic complexes reported here provide important new insights into the tectonic setting and evolution of the Pampean orogen. Specifically, the results of this study support models that are capable of explaining the introduction of non-arc mantle-derived mafic magmas into the developing orogen at or very near the time of its metamorphic peak. Although a continental collision model for the Pampean orogen cannot be categorically ruled out, an additional slab break-off or lower crustal delamination event immediately following collision would be required in order to account for the observations presented here. Alternatively, the recently proposed hypothesis of subduction of a mid-ocean ridge beneath an accretionary prism as the cause for Pampean orogeny can account for rise of non-arc, OIB-like mafic magmas through an already thickened supracrustal assemblage, as well as the close spatial and temporal association of this magmatism with the development of high-grade granulitic metamorphic and migmatitic assemblages in the Pampean supracrustal rocks.

Timing and Duration of the Calc‐Alkaline Arc of the Pampean Orogeny: Implications for the Late Neoproterozoic to Cambrian Evolution of Western Gondwana

The Pampean Orogen in the Eastern Sierras Pampeanas contains two paired magmatic belts, an eastern calc‐alkaline magmatic belt and a western peraluminous granite/high‐grade metasedimentary belt. The relationship between the two belts and their relative timing are constrained through new U‐Pb zircon ages on granodiorites, monzogranites, and associated volcanic rocks from Sierra Norte and the easternmost Sierras de Córdoba. These ages indicate that calc‐alkaline arc magmatism was active over at least a 30‐m.yr. period from 555 to 525 Ma, terminating at the same time that peraluminous magmatism and associated high‐grade metamorphism began in the adjoining metasedimentary belt (525–515 Ma). These temporal relationships and the metamorphic characteristics of the two belts appear to be in conflict with previously proposed models for the Pampean Orogeny as a continental‐collision event, but they are consistent with models that propose eastward‐dipping subduction of oceanic crust initiated at ca. 555 Ma, followed by ridge‐trench collision at ca. 525 Ma. Similar‐aged belts of arc‐related and peraluminous magmatism occur elsewhere along the paleo‐Pacific margin of Gondwana, suggesting that similar processes of subduction and noncollisional peraluminous magmatism occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.

Neoproterozoic to Early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina)

The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c . 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c . 496 Ma El Peñón pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208 Pb/ 232 Th monazite age of 478 Ma for a migmatite from the Nogolí Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c . 500 Ma and c . 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

Crustal Provenance and Cooling of the Basement Complexes of the Sierra de San Luis: An Insight Into the Tectonic History of the Pro to-Andean Margin of Gondwana

The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian. The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.

Cambrian to Devonian Geologic Evolution of the Sierra de Comechingones, Eastern Sierras Pampeanas, Argentina: Evidence for the Development and Exhumation of Continental Crust on the Proto-Pacific Margin of Gondwana

Crystalline rocks from the Sierra de Comechingones, eastern Sierras Pampeanas, evolved through three distinct orogenic cycles during the Eopalaeozoic: (1) the first tectono-thermal event named Pampean orogeny (550 to 505 Ma), which peaked in the Early Cambrian, was responsible for extensive metamorphism, partial melting, juvenile magmatism, rapid decompression, and persistent tectonic activity. Large part of the crustal section that was residing at middle levels (c. 27 km) was heated above 800 °C during the thermal peak stage of the Pampean orogeny; decompression of the Pampean orogen's core took place at this high temperature. The exhumation mechanism that assisted rapid uplifting combined the effects of ongoing tectonic forces with a buoyant instability created by a large amount of anatectic magmas in the middle to lower crust. (2) Beginning at the Early Ordovician, the Famatinian orogeny produced an overall shortening, causing pervasive textural reworking of the Cambrian metamorphic sequences under a high-strain regime. By being adjacent to the Famatinian magmatic arc, the western border of the Cambrian crystalline package absorbed imposed deformation along a crustal scale ductile shear zone. Within this zone, the high-grade metamorphic rocks were reworked and re-hydrated to lower temperature assemblages (<600°C and 3–6 kbar). Early Ordovician subduction-related igneous activity, even though manifested as small plutons, intruded Cambrian crystalline sequences, and experienced textural reworking during Late Famatinian tectonic exhumation. Late Famatinian convergence resulted in west-vergent ductile shear zones that placed Cambrian onto Ordovician crystalline sequences. (3) During post-Famatinian times (360–400 Ma) enduring crustal perturbation produced intra-crustal-derived granitic magmatism. West- to northwest-directed thrusting was concentrated in belts nucleated along crustal-scale tectonic boundaries formed between older tectono-stratigraphic units. As a result, Devonian anatectic granites were formed and tectonically extruded among Pampean and Famatinian crystalline sequences. The post-Famatinian event is also characterised by the intrusion of batholith-scale monzogranites into Pampean and Famatinian crystalline sequences residing in the upper crust. Crystalline rocks currently exposed in the Sierra de Comechingones show that they crystallised and were exhumed in a setting where tectono-thermal activity lasted, even though it might have waned, until the Middle Palaeozoic. From the latest Neoproterozoic (c. 550 Ma) until the Late Devonian (c. 360 Ma) tectonic activity was intermittently acting, indicating continuous convergence along the proto-Pacific margin of Gondwana.

Significado geotectónico de las suturas de Sierras Pampeanas orientales en la faja orogenica cordoba: posible correlacion con orogenos brasilianos

We propose to correlate the Pampean orogen with the Brasiliano belts, based on work started in 1994. The Precambrian-Cambrian tectonic evolution of the Pampean Ranges of Cordoba includes the Neoproterozoic development of a passive margin on the western side of the Rio de La Plata Craton. Subsequently, this passive margin evolved into an eastward dipping subduction zone as shown by the relative position of the rocks interpreted to have formed in an accretionary prism, a magmatic arc and a retroarc basin. The latest Precambrian event was the collision between the Rio the La Plata Craton and the Pampia Terrane (Grenville age craton?) which followed the closure of the ocean basin to the west of this arc.The suture between the Rio de La Plata Craton and the Pampia Terrane is located in the western ultramafic belt of the Sierras de Cordoba, it coincides with the Sierras Grandes Ranges. This suture separates the two distinctive tectonic domains which form the basis for the above tectonic interpretation. 1) The external, western domain is characterized by a peripheral fold and thrust belt developed in a dominantly clastic sedimentary prism, and a post collisional foreland basin. Evidence for the post-collisional basin comes from small outcrops of coarse clastics in the Sierras de San Luis to the South and the Puncoviscana basin to the north. 2) The internal, eastern domain is characterized by magmatic and high grade metamorphic rocks (migmatites and granulites) associated with ductile shear zones. These features are superimposed on a carbonate-clastic prism which appears to have formed on a passive margin. The polarity of the internal and external domains, the ultramafic belts, the age and evolution of the Cordoba Orogen are consistent with a correlation of the belt with the Paraguay-Araguaia belt in Brazil.

Calc-alkaline arc granitoid activity in the Sierra Norte-Ambargasta Ranges, central Argentina

A petrological and geochemical study of an area of approximately 750 km 2 has been carried out on representative lithological units of the extensive Sierra Norte-Ambargasta batholith (SNAB), in the Sierra Norte-Ambargasta range of the easternmost Sierras Pampeanas system of central Argentina. This batholith has intruded a dominantly metasedimentary basement of Precambrian age, which is mainly composed of quartzofeldsphatic-biotite or sericite-chlorite schists, and minor quartz-arenites of local distribution. The most primitive rocks of the magmatic sequence are diorite to tonalite microgranular enclaves (ME), which are included in the granodiorite-monzogranite (GM) basement. A large differentiated dacite-rhyolite (DR) stock intruded the earlier granitoids. All of these units were later intruded by highly evolved subalkaline granitoids (HESG) composed of miarolitic monzogranites, granite porphyries and large aplite dykes, which emplacement was controlled by old regional structural corridors. Geochemical parameters show that these granitoids series were derived from crystal fractionation of calc-alkaline magmas of metaluminous to weakly peraluminous composition. Discriminatory diagrams for tectonomagmatic settings, and the petrological and geochemical signature of the studied lithologies indicate that most granitoids of the intrusive sequence are the result of I-Type calc-alkaline magmatism. These granitoids are typical representatives of a precollisional volcanic arc environment, spatially related to an active continental margin. A significant chemical gap along the fractional crystallization trend separates the most evolved silica-rich granitoids from the less evolved units. This gap is attributed to the absence of representative magmatism of the syncollisional stage within the orogenic cycle. Silica and K-feldspar rich-rocks of the HESG-group are the result of intrusions that occured after the main magmatic activity, in a late-to post-orogenic tectonic setting. The occurrence of molybdenum, copper and other base-metal sulfides, associated with alteration assemblages that are common in porphyry systems, are in line with I-Type dacitic-rhyolitic magmatism in a volcanic arc geotectonic regime. Barren quartz-seriate greisen alteration is characteristic of the late-stage evolution of miarolitic monzogranites of the HESG-group. Radiometric ages for the SNAB granitoids suggest Upper Precambrian to Lower Paleozoic magmatic activity. Calc-alkaline arc magmatic activity of this age in the Sierra Norte of Córdoba, the easternmost block of the Oriental Sierras Pampeanas, raises the important question whether to assign the main magmatic activity of this region to the Pampean or to the Famatinian orogenic cycles. Our data suggest that the main igneous basement of the Sierra Norte-Ambargasta batholith consist of precollisional granitoids related to an orogenic cycle that most likely overlaps the G1 magmatic cycle of the Pampean orogen.