Famatinian Arc Research Articles

Geology and geochronology of the El Peñón granitic pluton: A contribution to the famatinian magmatic evolution in the Sierra de San Luis, Argentina

The El Peñón pluton is a peraluminous granite located in the northeastern sector of the Sierra de San Luis. According to its textural and mineral variations two endmember facies have been distinguished along it: a coarse-grained muscovite-bearing facies, and a fine-grained biotite-bearing facies. Field relationships aligned with mineralogical and geochemical compositions, allow us to establish an anatectic origin for this pluton. Regarding the inherited zircon ages, the provenance area of the metasedimentary source could be linked to the orogenic systems of Western Gondwana. The analysis of the internal structure of the intrusive body related with the structural arrangement observed in the country rocks indicates a syn-kinematic emplacement of the igneous body with respect to the main deformational event described in the Sierra de San Luis, which has been assigned to the closure of the Famatinian arc. New U-Pb (LA-ICP-MS) zircon ages constrain the emplacement/crystallization age of the El Peñón granite within the time lapse between ∼470 and 460 Ma. The geochronological data presented here, together with geochemical and structural analysis, enables us to correlate the origin of the El Peñón pluton and the deformation that occurred during its emplacement with the Famatinian Orogeny. Thus, these results allow us to rule out the presence of Pampean magmatism in the Sierra de San Luis but also establish a new absolute age for the main deformational event of the range.

Elastic thermobarometry on metapelites across the crustal section of the Famatinian Arc, Argentina

We applied elastic thermobarometry on garnet-bearing migmatites along two transects through the crustal section at Sierra Valle Fértil-La Huerta, Argentina. We performed quartz-in-garnet barometry and zircon-in-garnet thermometry on metapelites from different paleo-depths across the crustal section. Our work recovers entrapment pressures ranging from 240 to 1330 MPa and entrapment temperatures between 691 and 1574 °C. The entrapment conditions are broadly consistent with anticipated pressures and temperatures along the crustal section derived previously using conventional, thermodynamic thermobarometers. The quartz-in-garnet barometer reproduces those conventionally established entrapment conditions when samples only experienced conditions within the alpha-quartz stability field. Raman-derived pressures for samples that experienced beta-quartz reference conditions are commonly much higher than those established by conventional barometry. Samples that preserve compressive (positive) residual pressures best reproduce reference entrapment pressures. Entrapment temperatures show high variability and overestimation of temperature conditions compared to conventional results. These results indicate elastic thermobarometry over- or under-estimates crystallization conditions in rocks crystallized at high temperatures, as is common in the Famatinian Arc deep-crust. We suggest that modeling quartz behavior across the alpha–beta transition may present challenges, as does shape maturation, viscous deformation, and radiation damage in zircon.

Petrological and chemical evidence for polybaric differentiation across the preserved middle crustal plutonic column of the Famatinian arc, Argentina

Petrological and chemical evidence for polybaric differentiation across the preserved middle crustal plutonic column of the Famatinian arc, Argentina

Interrelated pressure-temperature-time-paths of medium to high pressure metamorphic rocks in the Sierra Pie de Palo (W-Argentina): Evolution of a “hard” collisional wedge during an Ordovician microcontinent-arc collision

The Sierra Pie de Palo (W-Argentina) is part of an Ordovician collisional wedge between the Cuyania microcontinent and the Ordovician Famatinian arc along the west Gondwana protomargin. The flatly E-dipping collisional wedge formed by polyphase shearing. The lower plate consists of Neoproterozoic-Cambrian metasedimentary rocks and Mesoproterozoic mafic-ultramafic basement of eastern Cuyania. The Famatinian forearc represents the upper plate composed of Mesoproterozoic metasedimentary rocks intruded by Mesoproterozoic and Ordovician plutons and overlying Neoproterozoic metasedimentary rocks. Pseudosection modeling on 20 samples distributed over the entire wedge yields a range of recorded maximum PT conditions of 7–14 kbar/480–590 °C along metamorphic gradients of 10–20 °C/km. Clockwise PT-paths are common starting from a midcrustal level with gradients of 20–30 °C to maximum depth followed by thermal relaxation. Local anticlockwise PT-paths are observed (1) in high-grade basement klippen of the overriding Famatinian arc (8–10 kbar/700–765 °C) caused by late emplacement onto the nappe stack and (2) within late upper crustal ductile zones with normal shear sense.Isotopic ages of (re)crystallisation of metamorphic minerals and assemblages comprise: 429 ± 2 to 434 ± 7 Ma (Lu-Hf; garnet), 460 ± 6 Ma (UPb monazite in garnet), 404 ± 7 to 422 ± 8 Ma (Rb-Sr; white mica) and 403 ± 4 to 440 ± 8 Ma (40Ar/39Ar white mica). Including previous data, ages cluster around major peaks at 464 ± 1 Ma, 436 ± 3 Ma, 427 ± 3 Ma, 415 ± 2 Ma and 405 ± 1 Ma related to deformation during three orogenic stages: (1) subduction of the lower plate (eastern Cuyania) to the basis of the wedge at 470–455 Ma and later burial of the upper plate (Famatinian forearc) to similar depths at 440–415 Ma, a characteristic of a “hard” collision; (2) exhumation mainly by erosion and (3) late dissection of the wedge by upper crustal thrusts and some normal faulting at the top of the wedge during arrival of the colliding microcontinent Chilenia (405–360 Ma) at the new Devonian leading margin.

Sediment dispersal and basin evolution during contrasting tectonic regimes along the western Gondwanan margin in the central Andes

A >15-20 km-thick succession in southern Bolivia forms the most-complete stratigraphic record in western Gondwana. Upper Neoproterozoic–Carboniferous clastic rocks record ∼300 Myr of marine, nonmarine, and glacially influenced sedimentation in diverse basin systems generated by variable tectonic regimes along the western edge of Gondwana during active and passive-margin conditions. New provenance results help resolve key uncertainties regarding source regions and sediment dispersal patterns. The findings are integrated with spatial variations in stratigraphic thicknesses to evaluate regional patterns of basin subsidence, magmatism, and deformation during long-term evolution of the western Gondwanan margin in the central Andes.Detrital zircon U–Pb geochronological data for 17 sandstone samples reveal sedimentary input from Precambrian cratonic basement provinces and pre-Andean basement, magmatic arc, and fold-thrust belt source regions. The basement age signatures indicate derivation from the flanking Brasiliano (900-560 Ma) and Pampean (650-500 Ma) provinces to the south and east, and the distal Rio de la Plata craton (2400-2000 Ma) along the eastern South American margin ∼1000–1500 km to the southeast. Although the greater Amazonian craton was not a major contributor, subordinate Amazonian signatures from the Sunsás (1300-950 Ma) province to the east and northeast selectively fed the northern basin regions of the central Andes. Despite the lack of Paleozoic igneous rocks in Bolivia, detrital zircons of Ordovician age attest to the pre-Andean influence of the subduction-related Famatinian magmatic arc. Limited Devonian-Carboniferous igneous material was contributed locally from western pre-Andean highlands or regionally by axial northward transport from selected igneous sources in Argentina and Chile. Episodic recycling of Neoproterozoic–Paleozoic basin fill, including a sharp reappearance of Famatinian-age detritus, can be linked to periods of Paleozoic crustal shortening and foreland sedimentation ascribed to Famatinian, Ocloyic, Chañic, or Gondwanide phases of deformation.The spatial distribution of sediment sources along with temporal shifts in sediment routing highlight several stages in the paleogeographic evolution of the western Gondwanan margin preserved in the central Andes. Initial regional subsidence spanned a multiphase Neoproterozoic to early Paleozoic history of Rodinia breakup, Brasiliano-Pampean orogenesis, and post-orogenic back-arc extension prior to final late Paleozoic amalgamation of Gondwana. The early Paleozoic onset of subduction and Famatinian arc magmatism led to high-magnitude subsidence (>10–15 km) likely driven by Ordovician slab rollback in an extensional back-arc basin. Thereafter, intermittent Paleozoic contraction in a poorly understood pre-Andean system (best expressed in the Eastern Cordillera of Bolivia and neighboring segments of northern Argentina and southern Peru) generated transient topographic loads that produced superimposed flexural foreland and successor basin systems.

Generation of crystal-poor rhyolites from a shallow plutonic reservoir in the Famatinian arc (Argentina)

The Ordovician rocks of Sierra Las Planchadas are an exceptionally preserved natural example of a plutonic-volcanic connection. Located in northwestern Argentina, the Las Planchadas Formation consists of a monzogranitic batholith that displays a window into the source of large subvolcanic rhyolitic rocks, cryptodomes and volcanic feeder-dykes. Mafic intrusions and felsic dykes occur cutting the plutonic rocks. Grain-scale relationships, whole-rock compositions and mineral chemistry indicate that monzogranites and rhyolites are two comagmatic members which differentiated within upper crust magma reservoirs. Monzogranites accumulated some plagioclase and trapped a large volume of melt, denoted by a high modal percentage (~64–71%) of near-solidus minerals, comprised of alkali feldspar, albite-rich plagioclase (An<30), and late crystallized quartz. The formation of felsic dykes and rhyolitic cryptodomes is the result of a late melt extraction stage facilitated by the injection of hot mafic magmas from deeper in the system. The mafic-induced thermal reactivation increased the amount of melt-filled pore stored within a highly-crystallized monzogranite reservoir. The breakdown of the low-temperature phases overpressurized the mush and opened a network of microfractures into which residual melts, depleted in feldspar-compatible trace elements (Ba, Sr, and Eu), infiltrated. The draining system involved thin interconnected vein-like channels that merged into wider sheet-like conduits. Our results suggest that channelized segregation of late-stage melts in the subvolcanic environment is a mechanism for triggering rhyolitic eruptions.

El Plutón sintectónico ordovícico El León, extremo suroccidental del Cordón de Lila, región de Antofagasta, Chile

The El León pluton, located in the Cordón de Lila in the Precordillera of the Antofagasta region, Chile, is an intrusive body emplaced in the famatinian magmatic arc in the southwestern margin of Gondwana during the Darriwilian (Middle Ordovician). It has a strong magmatic foliation and it is has a granodioritic composition that is coarse grained in its central part and fine grained in its borders. It contains metric and decametric fringes of mylonitic rocks. Aditionally it develops a contact metamorphic aureole in its country rock and it was intruded in the mesozone. There was collected ca. 400 structural data from magmatic and mylonitic foliations inside the pluton. There was established that the general trend of the magmatic foliation is N77E and the shear sense from S-C foliations and mica-fish in the mylonites is normal-sinistral. By means of the analysis of microstructures and their associated deformation mechanisms it has established a continuous deformation sequence that started with magmatic deformation followed by submagmatic and solid-state deformations. The latter begun at high temperatures and was followed by low temperatures. An important magmatic stoping occurred during the plutonic emplacement. Besides, the development of high temperature mylonites, the sinmagmatic character of the deformation and a shear subvertical-sinistral component is compatible with a descending piston mechanism model. Its deformation is asigned to the compressive Guandacol tectonic phase in the Cordón de Lila.

The ≥6-km Cuesta de Randolfo mylonite zone in Ordovician Famatinian peraluminous granites, NW Argentina: Strain-localization as a function of protolith composition

The Cuesta de Randolfo mylonite zone (CRMZ) in the southern Puna of Argentina comprises ≥6-km of moderately to steeply dipping proto-to ultramylonites developed in peraluminous intrusive rocks of the Famatinian arc. From youngest to oldest, these include a tourmaline-bearing granite, K-feldspar-biotite granite, and biotite-plagioclase granodiorite. Based on whole rock chemistry, the former is identified as a distinct, more evolved and particularly silica-rich intrusive phase. LA-ICP-MS U–Pb zircon ages indicate that the intrusive units crystallized during two periods of Famatinian arc magmatism (∼484 and ∼471 Ma). Distinctive solid-state deformation microstructures and temperatures characterize four structural zones, revealing strain localization governed by protolith mineralogy during cooling. Early deformation by quartz subgrain-rotation recrystallization and minor K-feldspar bulging at ∼500-450 °C resulted in moderate strain distributed across the width of the youngest, quartz-rich intrusive unit (the tourmaline-bearing granite), forming the Western Distributed Zone. With cooling, strain localized into reverse-sense shear zones active at 450–400 °C, forming the Western and Eastern Shear Zones along the contacts between the youngest and the older intrusive units. Lastly, narrow ultramylonites developed along the contacts between all intrusive units and along country-rock rafts, in particular in the Eastern Domain. Deformation at ∼400–280 °C was driven by plagioclase-reaction weakening in the feldspar-rich older intrusive units. Progressive CRMZ deformation therefore transitioned from broadly- and homogenously-distributed at moderate temperatures—enabled by the abundance and weakness of quartz of the youngest units—to localized at low temperatures, driven by reaction softening of plagioclase in the older units. We conclude that the composition of intrusive units (silicic and peraluminous) has a first order control on strain localization. The CRMZ is part of a larger N–S striking network of wide ductile shear zones deforming peraluminous Famatinian–aged igneous rocks. Their similar composition likely caused the development of wide shear zones in the eastern part of the Famatinian orogen.

Neoproterozoic to Jurassic tectono-metamorphic events in the Sierra Nevada de Santa Marta Massif, Colombia: insights from zircon U-Pb geochronology and trace element geochemistry

ABSTRACT The Sierra Nevada de Santa Marta Massif (SNSM) in the Northern Andes affords an insight into lower crustal section structural relations. This polymetamorphic basement comprises high-grade gneisses, which alternate with imbricate slivers of medium-to high-grade schists. New field observations coupled with LA-ICP-MS U-Pb zircon geochronology and trace element geochemistry reveal some hallmarks on the evolution of this strongly remobilized crustal section. The Proterozoic Rodinia-type basement (1100 − 880 Ma) recorded at least two separated tectono-metamorphic events (M0 and M1) and was subsequently affected by a magmatic episode during the Ordovician (~450 Ma; M2). During the Permian, subduction of Panthalassian oceanic crust beneath Pangaea led to the installation of a magmatic-arc and probably coeval metamorphism (~288-266 Ma; M3). Middle-Late Triassic roll-back extension of the upper plate led to anatexis and metamorphism (M4) reaching peak conditions at ca. 239 Ma with concurrent crystallization of garnet and sillimanite, followed by a decreasing geothermal gradient at ca. 202 Ma. Early Jurassic arc build-up under a rejuvenated convergence was contemporaneous to metamorphism on an accretionary wedge (M5) at ca. 185 Ma. The SNSM Massif basement units yield metamorphic ages that compare to similar tectono-metamorphic provinces in NW-Gondwana, including the Central Cordillera of Colombia, the Guajira Peninsula, the Mérida Andes, the Perijá Range, the Santander Massif, and the Central American and Mexican terranes to the south of the Ouachita-Marathon Suture (Oaxaquia, Mixteca, and Maya blocks). Age equivalence of these events is best explained by the existence of a conjugate margin with Laurentia until the Late Triassic ~216 Ma, which preceded the inception of a Colombian marginal seaway during the break-up of western Pangaea. Furthermore, the presence of Ordovician metamorphic rocks in the SNSM Massif confirms the extent of a Famatinian arc along NW-Gondwana during the Early Palaeozoic.

Geoquímica de las rocas plutónicas del batolito cambro-ordovícico del Cordón de Lila y de la Sierra de Almeida, Región de Antofagasta, Chile

An Upper Cambrian-Ordovician (490-460 Ma) batholith crop out along the Cordón de Lila and Sierra de Almeida, in the Antofagasta region, northern Chile, which belongs to the Famatinian arc. Two plutonic rock types can be distinguished: granodiorites and monzogranites. The first lithology is positioned along an occidental fringe in as much the second is located along an oriental part of the batholith. The main geochemical difference between the two lithologies is the enrichment in silica and K of the second with respect of the first one. Most of the granodiorites are metaluminous or are located between the metaluminous and peraluminous fields. The monzogranites are mostly peraluminous. The chemical composition of the plutons (mayor and trace elements and rare earth elements) allow to infer a common origin for both groups, from a mafic magma that evolved by fractional crystallization. The difference in composition between both lithologies, specially the meta or peraluminous character, can be explained by means a different degree of interaction of the original magma with the felsic crustal rocks.

Upper-crustal architecture and record of Famatinian arc activity in the Sierra de Narváez and Sierra de Las Planchadas, NW Argentina

The 495 to 450 Ma Famatinian orogen, exposed throughout central and northwestern Argentina, formed from east-directed subduction under the Gondwanan margin. The Sierra de Narváez and Sierra de Las Planchadas preserve a rare upper-crustal section of the Famatinian arc. New mapping, structural analysis, detrital U–Pb zircon geochronology, as well as major and trace element geochemistry in the Sierra de Narváez – Las Planchadas are presented to give a comprehensive geodynamic portrait of the volcano-sedimentary, igneous, and deformational processes acting within the top of the Famatinian arc in the Ordovician.Field observations and bulk rock geochemistry agree with previous work indicating that the top of the Famatinian arc consisted of volcanic centers, mafic and felsic feeders, and plutons built into continental crust in a shallow marine arc setting, characterized by fossil-bearing, fine-grained marine sediments interbedded with coarse-grained volcanic-clastic material. Trace element chemistry is consistent with the Sierra de Narváez – Las Planchadas region being a continuation along the main arc axis from the more southerly Sierra de Famatina, not a back arc setting as previously interpreted. Detrital zircon geochronology in Permian and Carboniferous sedimentary units unconformably overlying Ordovician units adds further constraints to the duration of Famatinian arc activity and the source of sedimentary material. Two peaks in detrital zircon ages within Carboniferous and Permian strata at 481 Ma and from 474 to 469 Ma, record periods of enhanced magma addition during Famatinian arc activity. Structural analysis establishes both Famatinian and post-Famatinian (largely Andean) deformation; contractional deformation in the Ordovician, although small relative to middle- to lower-crustal levels of the Famatinian orogen, caused crustal thickening and likely initiated surface uplift. Unlike the Famatinian middle to lower crust, however, where widespread ductile deformation is ubiquitous, shortening here is accommodated by open folding, pressure solution, and likely localized brittle faulting. We briefly speculate on the implications of variable shortening recorded at different crustal levels.

The Late Neoproterozoic-Early Paleozoic basin of the western Argentine Precordillera: Insights from zircon U-Pb geochronology

In central-western Argentina, a belt including marine metasedimentary rocks and mafic-ultramafic bodies occurs throughout the western margin of the Precordillera. The belt is considered as the suture zone between the poorly known Chilenia terrane and the Cuyania terrane, part of the composite West Gondwana margin. It is assigned to the Late Neoproterozoic-Early Devonian based on fossil fauna and radiometric ages. In the southern sector of this belt, in the Peñasco area, two units crop out. The Peñasco Formation comprises metasandstone and metapelite spatially associated with mafic metavolcanic and metahyaloclastic rocks. Metagabbro bodies intrude the succession. The Garganta del León Formation consists of metasandstone and scarce metapelite where tractive and deformational sedimentary structures are preserved. Both units are affected by low-grade metamorphism, but the main foliation S1 and crenulation cleavage S2 are better developed in the Peñasco Formation rocks. U-Pb data on detrital zircon of two metasandstone samples from these units show a dominant detrital input from sources with 1.0–1.3 and 0.65–0.53 Ga ages. Detritus may come from reworked sedimentary units or from igneous/metamorphic complexes from the Cuyania terrane basement that was possibly exhumed in the Ocloyic orogen. A Gondwanan provenance for the Late Neoproterozoic-Cambrian population would also be plausible. A ca. 460 Ma zircon population in the Garganta del León Formation is interpreted to be derived from the Famatinian Arc. This would imply that the deposition of the sediment occurred after the collision of the Cuyania terrane against West Gondwana, and that the Ocloyic orogen acted as a barrier for detritus from the Famatinian Arc and other rocks further east.

O–H–Sr–Nd isotope constraints on the origin of the Famatinian magmatic arc, NW Argentina

AbstractWe report a study of whole-rock O–H–Sr–Nd isotopes of Ordovician igneous and metamorphic rocks exposed at different crustal palaeodepths along c. 750 km in the Sierras Pampeanas, NW Argentina. The isotope compositions preserved in the intermediate rocks (mostly tonalite) (average δ18O = +8.7 ± 0.5‰, δD = −73 ± 14‰, 87Sr/86Srt = 0.7088 ± 0.0001 and εNdt = −4.5 ± 0.6) show no major difference from those of most of the mafic rocks (average δ18O = +8 ± 0.8‰, δD = −84 ± 18‰, 87Sr/86Srt = 0.7082 ± 0.0016 and εNdt = −4 ± 1.1), suggesting that most of their magmas acquired their crustal characteristics in the mantle. The estimate of assimilation of crustal material (δ18O = +12.2 ± 1.7‰, δD = −89 ± 21‰, 87Sr/86Srt = 0.7146 ± 0.0034 and εNdt = −6.9 ± 0.7) by the tonalite is in most samples within the range 10–20%. Felsic magmas that reached upper crustal levels had isotope values (δ18O = +9.9 ± 1.5‰, δD= −76 ± 5‰, 87Sr/86Srt = 0.7067 ± 0.0010, εNdt = −3.5 ± 1.4) suggesting that they were not derived by fractionation of the contaminated intermediate magmas, but evolved from different magma batches. Some rocks of the arc, both igneous (mostly gabbro and tonalite) and metamorphic, underwent restricted interaction with meteoric fluids. Reported values of δ18O of magmatic zircons from the Famatinian arc rocks (+6 to +9‰) are comparable to our δ18O whole-rock data, indicating that pervasive oxygen isotope exchange in the lower crust was not a major process after zircon crystallization.

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.

Geology, petrology and geochronology of sierra Valle Fértil - La Huerta batholith: Implications for the construction of a middle-crust magmatic-arc section

The Valle Fértil-La Huerta batholith is a differentially tilted, compositionally stacked, calc-alkaline plutonic sequence in the central Famatinian arc of Argentina. It consists of two major lithologic units: these are (1) an intermediate unit largely dominated by amphibole- and biotite-bearing tonalites but encompassing from gabbro to leucogranite; and (2) an overlying silicic unit that shows the same petrological diversity as the intermediate unit, but granodiorite and monzogranite are its prevailing lithologies. The silicic unit is separated into four subunits using lithological changes at regional mapping scale. All the boundaries are gradational among lithologic units and subunits. The internal fabric along the entire batholith is either magmatic or submagmatic and it was developed during the arc formation. The original magmatic foliation is north–south striking and steeply-dipping. The magmatic foliation crosses over and transposes the gradational contacts among lithologic units. Petrological architecture, structural geology, geobarometric estimates and geochronology show that the entire batholith is a comagmatic system exposing more than 13 km of a middle arc crust. Crystallization-fractionation models computed using whole rock and mineral chemistry account for the mineralogy of plutonic rocks and coincide with the results of experimental petrology. Mass-balanced modeling predicts that the unexposed cumulate roots of the Famatinian arc should have been about 1.3 times larger than the intermediate and silicic plutonic batholith. However, the mass of calculated cumulate would diminish significantly if a portion of the host metasedimentary material were consumed in constructing the batholith. The batholith grew progressing upward from the lowest levels and acquired a stratified compositional organization. The driving mechanism was continuous influxes of mantle-derived mafic magmas that made up the lower crust, caused crustal-level melting and magma mixing, and formed intermediate – silicic rocks at increasingly shallower depths.

Evidence for an Ordovician continental arc in the pre-Mesozoic basement of the Huizachal–Peregrina Anticlinorium, Sierra Madre Oriental, Mexico: Peregrina Tonalite

The Huizachal–Peregrina Anticlinorium basement of the Sierra Madre Oriental in north-eastern (NE) Mexico comprises a wide variety of Precambrian and Paleozoic units. A granitic unit, described in this work as the Peregrina Tonalite (PTo), intruded the Neoproterozoic Novillo Gneiss, which is the northernmost expression of the Oaxaquia microcontinent. In contrast, the PTo’s contact with the Carboniferous Granjeno Schist is tectonic along vertical faults with a strike-slip component. This Paleozoic lithodeme is part of the Granjeno–Acatlan Belt. The PTo has been described as an orphaned block associated with an enigmatic Carboniferous magmatic arc located along the north-western (NW) margin of Gondwana. In this study, new U–Pb LA–ICP–MS data from PTo zircons included a youngest Late Ordovician (Katian stage) population at 448.8 ± 2.9 Ma, interpreted as the crystallisation age, and an oldest Grenvillian population interpreted as the potential age of its protolith. Additionally, major and trace element concentrations and ratios indicate a link to a continental arc that developed along the NW margin of Gondwana. Given this proposed arc’s age and position, a comprehensive model for the development of the NE Mexican basement without the involvement of exotic or orphaned terranes is now proposed. The PTo outcropping near Ciudad Victoria, Tamaulipas, is interpreted to be part of a previously unreported magmatic arc in NE Mexico established during the Late Ordovician, herein described as the Peregrina–Mochonian Orogeny. It represents an extension of the South American Famatinian arc into Mexico.

Ordovician to Silurian igneous rocks in southern Mexico and Central America: geochronologic and isotopic constraints on paleogeographic models

Exposures of Ordovician to Silurian magmatic rock in southern Mexico and Central America constitute dismembered parts of an early Paleozoic belt related to the interaction of terranes of northwestern Gondwana with southern Laurentia. Convergent and extensional tectonic scenarios have been proposed to explain the origin of the magmatic activity along this belt. New U–Pb geochronological data, as well as Hf and Nd isotopic analyses were performed on five representative granitoids from southern Mexico and Central America to evaluate the timing and environment of emplacement. Samples from the Acatlán Complex in southern Mexico, as well as from the Altos Cuchumatanes, Rabinal and Chuacús areas along the southern Maya block yield Ordovician (470.9 Ma) to Silurian (427.1 Ma) crystallization ages. These samples have ɛHf values ranging from −6.4 to +1.0 and extraction ages (TDM) between 1.15 and 1.53 Ga, which match with ɛNd whole rock values ranging from −2.4 to −5.3 and TDM between 1.20 and 1.53 Ga. The Hf and Nd isotope compositions suggest incorporation of an evolved crustal reservoir, probably an Oaxaquia-type basement. Our data would better support a convergent tectonic scenario for the origin of the lower Paleozoic granitoids, possibly comprising an extension of the Famatinian arc of northwestern south America.

Geochronology and facies analysis of subaqueous volcanism of lower ordovician, Famatinian arc, Argentina

The Chuschín formation is a sedimentary-volcanic succession of the Famatina System subduction-related arc magmatism. This formation developed along the western margin of Gondwana during the early and middle Ordovician. The interpretation of the Chuschín formation is significant for the understanding of the Famatinian arc. This study provides a detailed facies analysis, which allows characterizing the deposits as well as defining some of the characteristics of the volcanism and sedimentation. Coherent, autoclastic, peperitic, pyroclastic and volcaniclastic facies were recognized between the different volcanic facies. The characteristics and relationships between these facies indicate events of effusive subaqueous volcanism as well as explosive volcanism showing predominantly high temperature features. Both types of volcanism are interbedded with volcanogenic deposits and epiclastic sediment that are associated mainly with turbidity currents. The magmatic and volcanic rocks are dominantly subalkaline, weakly peraluminous and silicic. Two new U-Pb zircon crystallization ages on volcanic rocks from the Chuschín Formation show that this section of the Famatinian arc formed between 463 and 469 Ma (Middle Ordovician).

U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U–Th–Pb and Lu–Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472–465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (−0.3 to −8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc–alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

The crustal evolution of South America from a zircon Hf‐isotope perspective

AbstractHf‐isotope data of &gt;1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.