Famatinian Arc Research Articles

Granites of the intracontinental termination of a magmatic arc: an example from the Ediacaran Araçuaí orogen, southeastern Brazil

The Araçuaí orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Araçuaí–West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the São Francisco–Congo Craton, this portion of the Adamastor was only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630Ma and 580Ma. The Rio Doce magmatic arc terminates in the northern portion of the Araçuaí orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to study magmatism at arc terminations, and to understand the interplay between calc-alkaline magma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb–Ta, Sr and Ti anomalies, typically associated with subduction-related magmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. Zircon U–Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590Ma, experienced a migmatization event at ca. 585Ma. The younger granitoids, emplaced between 570 and 590Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydration-melting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done.

Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru

Research Article| June 01, 2015 Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru Angela M. Hessler; Angela M. Hessler 1The Deep Time Institute, 8711 Bluegrass Drive, Austin, Texas 78759, U.S.A. e-mail: ahessler@deeptimeinstitute.org Search for other works by this author on: GSW Google Scholar Andrea Fildani Andrea Fildani 2Statoil Research, Development and Innovation Center, 6300 Bridge Point Parkway, Suite 2–500 Austin, Texas 78730, U.S.A. Search for other works by this author on: GSW Google Scholar Journal of Sedimentary Research (2015) 85 (6): 646–659. https://doi.org/10.2110/jsr.2015.45 Article history received: 07 Oct 2014 accepted: 24 Mar 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Angela M. Hessler, Andrea Fildani; Andean Forearc Dynamics, As Recorded By Detrital Zircon From the Eocene Talara Basin, Northwest Peru. Journal of Sedimentary Research 2015;; 85 (6): 646–659. doi: https://doi.org/10.2110/jsr.2015.45 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract: Eocene strata of the Talara forearc basin of northwest Peru contain detrital-zircon (DZ) populations that, when integrated with other provenance indicators, record paleogeographic shifts in the Andean forearc that can be linked to larger-scale tectonic drivers such as subduction erosion, plate convergence rate, and regional accretion events. Because forearc basins along the western margin of South America are particularly difficult to preserve, the new DZ results provide rare insight into the trench–arc dynamics associated with the Andean orogenic cycle. The Talara basin tapped into both the Andean volcanic arc and older basement, and the DZ populations subsequently reflect long-lived plate interactions and progressive crustal growth along the northwestern edge of Amazonia, Gondwana, and South America. Dominant U–Pb zircon age populations include: 36–110 Ma (Andes convergence); 215–285 Ma (western Gondwana extension); 460–625 Ma (Rodinia extension and western Gondwana convergence); 950–1250 Ma (Grenville orogeny); and > 1250 Ma (Amazonia assembly). The data corroborate previous geochronology on pre-Andean basement and support the existence of a Famatinian arc (∼ 480 Ma) and Carboniferous–Triassic magmatism along northwestern Gondwana. Pre-Andean and Amazonian DZ likely was recycled from Paleozoic metasedimentary rocks of the Andean Cordillera and Amotape terrane, although the data do not preclude delivery of Amazonian detritus directly into the forearc via a large-scale, westward drainage. The Andean DZ population constrains the timing of arc activity and quiescence, with two peaks at ∼ 75 Ma and ∼ 50 Ma, and a gap at ∼ 70 Ma, linked to accretion of the Caribbean Oceanic Plateau against northwest South America. Arc volcanism was concurrent with Eocene forearc deposition, and syndepositional DZ is used to revise maximum depositional age for the upper Eocene Helico Member and Verdun Formation. Progressive subsidence and arc shut-down in the late Eocene coincide with arc collision to the north, uplift of the Amotape block, subduction erosion, and slab shallowing. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

0.3 byr of drainage stability along the Palaeozoic palaeo-Pacific Gondwana margin; a detrital zircon study

The palaeo-Pacific margin of Gondwana in the present-day south–central Andes is marked by tectonic activity related to subduction and terrane accretion. We present detrital zircon U–Pb data encompassing the Palaeozoic era in northern Chile and northwestern Argentina. Cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase. The main age clusters for these zircon grains are Ediacaran to Palaeozoic with an additional peak at 1.3–0.9 Ga and they can be correlated with ‘Grenvillian’ age, and the Brasiliano, Pampean, and Famatinian orogenies. The zircon data reveal main transport from the nearby Ordovician Famatinian arc and related rocks. The Silurian sandstone units are more comparable with Cambrian units, with Brasiliano and Transamazonian ages (2.2–1.9 Ga) being more common, because the Silurian deposits were situated within or east of the (extinct) Famatinian arc. Hence, the arc acted as a transport barrier throughout Palaeozoic time. The complete suite of zircon ages does not record the accretions of exotic terranes or the Palaeozoic glacial periods. We conclude that the transport system along the palaeo-Pacific margin of Gondwana remained stable for c . 0.3 byr and that provenance data do not necessarily reflect the interior of a continent. Hence, inherited geomorphological features must be taken into account when detrital mineral ages are interpreted. Supplementary material: U–Pb data, CL images, and detailed geological maps are available at www.geolsoc.org.uk/SUP18796 .

A Middle Paleozoic shear zone in the Sierra de Valle Fértil, Argentina: Records of a continent-arc collision in the Famatinian margin of Gondwana

Geological, petrological and structural observations were obtained along a 30-km-long traverse across a segment of the Valle Fértil shear zone, central-western Argentina. On a regional scale, the shear zone appears as numerous discontinues belts over 25 km in width and is approximately 140 km in length, extended on the western section of the Sierras Valle Fértil – La Huerta mountain range. The steeply dipping shear zone with a vertical mylonitic lineation is composed of amphibolite facies ribbon mylonites and amphibolite to greenschist facies ultramylonites derived from Early Ordovician plutonic and metasedimentary parent rocks. Locally, syn-kinematic retrogression of mylonites formed greenschist facies phyllonites. During the later stages of deformation, unstrained parent rocks, mylonites, ultramylonites and phyllonites were affected by pervasive cataclasis under low greenschist facies conditions associated with localized faulting. One new 40Ar/39Ar age on biotite and published 40Ar/39Ar ages on amphibole in the shear zone yield an average cooling rate of 6.2 °C/Ma for a time period that crosses the Silurian–Devonian boundary. Since in metasedimentary rocks the youngest zircon's rims dated at 465 Ma marks the beginning of cooling, nearly continuous uplift of rocks within the shear zone occurred over a minimum time span of 55 Ma. During the period of active deformation, dip-slip movement can explain uplift of several kilometers of the Early Ordovician arc crust. The Valle Fértil shear zone, which was formed near above the inferred suture zone between the Famatinian arc and Cuyania microcontinent, is a major structural boundary nucleated within the Early Ordovician crust. The simplest geodynamic model to explain the evolution of the Valle Fértil shear zone involves the collision of the composite Cuyania/Precodillera microcontinent against the Famatinian arc.

Multi-pulse cotectic evolution and in-situ fractionation of calc-alkaline tonalite–granodiorite rocks, Sierra de Velasco batholith, Famatinian belt, Argentina

The study of a magmatic series composed of Qtz-diorites to granites of the south part of the Sierra de Velasco batholith (Famatinian arc) in Argentina reveals that most rocks follow a coherent cotectic trend, which is comparable to a cotectic line of liquids (CLL) traced from experimental liquid compositions of calc-alkaline (andesitic) systems. The identification of cotectic relationships is crucial to interpret the meaning of geochemical variation trends in terms of phase equilibria. We show the contrast between typical CLL trends defined by rocks of the Palanche pluton and rocks departing from CLL array, which are interpreted as resulting from local processes of bulk assimilation of a pelitic host. Zircon U–Pb age determinations with SHRIMP support a coeval zircon crystallization record from of about 480 to 450Ma of samples plotting on the CLL. The absence of intrusive contacts between these coeval cotectic granites implies that they were fractionated in situ at the level of emplacement. Other younger pulses (442±5Ma), also fall on the CLL, denoting that uniform processes of melting and fractionation were operative for at least 40Ma at the same locus at the active continental margin of Gondwana. During this protracted magmatic activity, sporadic pulses of mafic magma were injected (e.g., at 456±7Ma), revealing a complex process of pulse amalgamation in the building up of the batholith. These mafic magma pulses may have contributed to basification of the host granitoid by enclave dissolution at the low pressure of emplacement. The reported geochemical trends, as true CLL, together with observed textures and crystallization sequences, point to a parental intermediate system of andesite composition (SiO2>55wt.%) with a water content of about 2.5wt.% H2O, similar to those formed by sediment–basalt subducted mélanges. A sublithospheric origin for the mafic precursor is proposed in agreement with prediction by recently published thermo-mechanical modeling and experimental phase equilibria.

Reconstruction of the Early Ordovician Famatinian arc through thermobarometry in lower and middle crustal exposures, Sierra de Valle Fértil, Argentina

The crustal structure of the Famatinian paleoarc is reconstructed by determining the metamorphic crystallization P–T conditions from metasedimentary rocks at various structural levels in the Valle Fértil section. The bulk section exhibits a 15-km-thick arc crustal section. Thermobarometry shows that nested tonalitic and granodioritic plutons constructed the arc crust at depths <20km. Dioritic and tonalitic bodies dominated between 20 and 25km. The deepest exposed paleodepths are formed by cumulate and non-cumulate gabbroic rocks with interlayered quartz diorites that crystallized below 25km. The boundary between the crust and the lithospheric mantle is not observed and would have been underneath a depth of 27km. P–T estimates throughout the section reveals high geothermal gradients ranging from 25°C/km to 35°C/km. The thermal regime inferred for middle crustal levels is observed in active and ancient magmatic arcs. Thermophysical models predicting the array of retrieved P–T estimates require heat advection conducted by mafic magmas which either emplaced in the lower crust and/or intruded into middle crustal levels. Calculated seismic wave velocities of plutonic rocks dominating progressively deeper paleodepths are used to deduce the internal architecture of the Valle Fértil section. This result indicates that the Famatinian arc had a middle crustal structure very similar to that of the ancient Talkeetna arc; however a rapid increase of seismic wave velocities from ~6.3km/s to >6.6km/s is located at deeper depths in the Famatinian arc than in Talkeetna arc. The thickness of a crustal layer dominated by plutonic rocks with low seismic wave velocities (<6.2km/s) is 10km thinner than the crustal layer with similar physical properties in the Sierra Nevada batholith. A putative model for the whole Famatinian arc suggests a total crustal thickness between 30 and 35km with three distinct layers.

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.

Geological, Petrological and Geochemical Evidence for Progressive Construction of an Arc Crustal Section, Sierra de Valle Fertil, Famatinian Arc, Argentina

The petrogenesis of calc-alkaline magmatism in the Famatinian arc is investigated in the central Sierra Valle Fe¤ rtil, a major, lower to middle crustal section of the Early Ordovician active margin of West Gondwana. Large-scale field relationships show a gradual and continuous compositional variation of the plutonic sequence, ranging from olivine-bearing gabbronorites to hornblendeand biotite-bearing granodiorites. Distinctive lithostratigraphic units are, however, discernible as one compositional type of plutonic rock dominates over mappable areas.These results allow us to identify a continuous plutonic arc stratigraphy that progressively exposes shallower paleo-depths towards the east. At all the exposed levels, calc-alkaline plutonic rocks are volumetrically dominant, interrupted only by granulite-facies migmatites and leucogranites. The migmatites are interpreted to be refractory remnants of supracrustal sedimentary successions, whereas the peraluminous leucogranites have field relationships and chemical and isotopic compositions suggesting that they were produced via anatexis of metasedimentary packages. Massbalance calculations predict that a parental gabbroic magma after progressive closed-system fractionation would crystallize about 80% of the original mass to yield a granodioritic daughter. Because the crystallizing mineral assemblage comprises hornblende and plagioclase, mass balance suggests a volume of residual amphibole-rich gabbroic rocks much larger than that observed, suggesting that differentiation is significantly driven by open-system processes. Indeed, the combination of field and petrographic observations with bulk-rock geochemistry and petrogenetic modeling demonstrates that most dioritic and tonalitic rocks are hybrids formed by either (1) bulk assimilation of metasedimentary materials into gabbroic magmas, or (2) multi-stage and complex interactions between gabbroic rocks and metasedimentary-derived leucogranitic melts. The source region of the granodioritic magmas is located at the transition zone between a tonalite-dominated intermediate unit and a granodiorite-dominated silicic unit. Typical granodiorites have a hornblende-bearing mineralogy, metaluminous chemical signature and isotopic compositions [Sr/Sr(T)1⁄4 0·7075^0·7100 and eNd(T) 5·0] broadly overlapping those of the tonalites of the intermediate rock unit. These major compositional features of the granodiorites can be best explained if three end-member components contribute to their generation. As field observational data suggest, primitive gabbroic rocks, metaluminous intermediate magmas and anatectic leucogranitic melts mixed to produce the calc-alkaline granodiorites; however, the exact petrological process generating the granodioritic magmas is unclear because the mafic end-member may have been incorporated as mafic inclusions in the intermediate magmas or as syn-magmatic dikes, or both.The polygenetic nature of the intermediate to silicic plutonic rocks, along with the preponderance of parental gabbroic rocks at the inferred base of the plutonic column, suggests an upward growth of the intermediate to silicic crust that involved the complete reconstitution of the pre-existing crustal configuration. The main implication of this study is that intermediate and silicic plutonic rocks in the Valle Fe¤ rtil section formed within a crustal column in which the mass transfer and heat

Age and magmatic evolution of the Famatinian granitic rocks of Sierra de Ancasti, Sierras Pampeanas, NW Argentina

The granitic rocks of the Sierra de Ancasti represent one of the easternmost outcrops of the Famatinian arc of the Sierras Pampeanas. We report here new U–Pb SHRIMP and LA-ICP-MS Ordovician ages and a petrological and geochemical study of the Las Canadas I-type granitic complex in the central sector of Sierra de Ancasti. Numerical modelling indicates that calc-alkaline and metaluminous monzogranites (SiO2 = 69.8–73.0 wt.%) were produced by fractional crystallization of plagioclase-rich tonalite (∼60% plagioclase and SiO2 = 60.9–62.5 wt.%) from an inferred parental magma of granodioritic composition (SiO2 = 67.4 wt.%). Sm–Nd isotopic data on the dated rocks indicate a dominant continental crustal input, as reported for other Ordovician granitic rocks (Famatinian magmatism) farther west in the Sierras Pampeanas. Combined ages and isotopic data are consistent with previous studies indicating that Famatinian magmatism was short-lived (∼20 Ma) and lacked a significant asthenospheric contribution. The occurrence of inherited zircon and the petrological and geochemical data from the Las Canadas complex indicate that the parental magma was derived from mixed igneous and sedimentary material in the source or by partial melting of igneous rocks of different ages. Systematic dating of inherited zircon cores is required to clarify this issue. Zircon saturation geothermometry combined with geochemical data indicates that the onset of zircon crystallization, together with variable amounts of other minerals such as plagioclase, quartz, biotite, oxides and apatite, occurred when the parental magma reached 67 wt.% SiO2 with ∼440 ppm of Zr at ∼878 °C.

Crustal segments in the North Patagonian Massif, Patagonia: An integrated perspective based on Sm–Nd isotope systematics

Abstract New insights on the Paleozoic evolution of the continental crust in the North Patagonian Massif are presented based on the analysis of Sm–Nd systematics. New evidence is presented to constrain tectonic models for the origin of Patagonia and its relations with the South American crustal blocks. Geologic, isotopic and tectonic characterization of the North Patagonian Massif and comparison of the Nd parameters lead us to conclude that: (1) The North Patagonian Massif is a crustal block with bulk crustal average ages between 2.1 and 1.6 Ga T DM (Nd) and (2) At least three metamorphic episodes could be identified in the Paleozoic rocks of the North Patagonian Massif. In the northeastern corner, Famatinian metamorphism is widely identified. However field and petrographic evidence indicate a Middle to Late Cambrian metamorphism pre-dating the emplacement of the ca. 475 Ma granitoids. In the southwestern area, are apparent 425–420 Ma (?) and 380–360 Ma metamorphic peaks. The latter episode might have resulted from the collision of the Antonia terrane; and (3) Early Paleozoic magmatism in the northeastern area is coeval with the Famatinian arc. Nd isotopic compositions reveal that Ordovician magmatism was associated with attenuated crust. On the southwestern border, the first magmatic recycling record is Devonian. Nd data shows a step by step melting of different levels of the continental crust in the Late Palaeozoic. Between 330 and 295 Ma magmatism was likely the product of a crustal source with an average 1.5 Ga T DM (Nd). Widespread magmatism represented by the 295–260 Ma granitoids involved a lower crustal mafic source, and continued with massive shallower-acid plutono volcanic complexes which might have recycled an upper crustal segment of the Proterozoic continental basement, resulting in a more felsic crust until the Triassic. (4) Sm–Nd parameters and detrital zircon age patterns of Early Paleozoic (meta)-sedimentary rocks from the North Patagonian Massif and those from the neighboring blocks, suggest crustal continuity between Eastern Sierras Pampeanas, southern Arequipa-Antofalla and the northeastern sector of the North Patagonian Massif by the Early Paleozoic. This evidence suggests that, at least, this corner of the North Patagonian Massif is not allochthonous to Gondwana. A Late Paleozoic frontal collision with the southwestern margin of Gondwana can be reconcilied in a para-autochthonous model including a rifting event from a similar or neighbouring position to its post-collision location. Possible Proterozoic or Early Paleozoic connections of the NPM with the Kalahari craton or the western Antartic blocks should be investigated.

Timing constraints on building an intermediate plutonic arc crustal section: U- Pb zircon geochronology of the Sierra Valle Fértil-La Huerta, Famatinian arc, Argentina

[1] The Sierra Valle Fertil Range in northwestern Argentina exposes a tilted crustal section through the Ordovician Famatinian arc, from >25 km to shallow crustal paleodepths. Fourteen new U-Pb zircon crystallization ages of magmatic rocks from Sierra Valle Fertil area show that this section of the arc was built over a short time interval during the Ordovician, between 485 and 465 Ma. Zircon rim ages demonstrate that high-grade metamorphism and migmatization were synchronous with magmatic emplacement. Inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the Famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto-South American continent in the Ordovician, which represents a part of Gondwana. Docking of the Precordilleran terrane outboard of proto-South America led to the cessation of arc magmatism in the Valle Fertil area and preservation of the arc in its early stages after <60 Myr of subduction during the Pampean and Famatinian magmatic stages. Our age data from the Sierra Valle Fertil indicate that wet mafic magmas were emplaced into the section at a rate typical for modern island arcs, ∼30 km3 km−1 Myr−1. The production of intermediate to felsic magmas in the upper plate was a result of partial melting of the metasedimentary framework and hybridization with mantle-derived melts and resulted in the generation of tonalites, granodiorites, and granites in the upper crust at a rate of ∼100 km3 km−1 Myr−1, similar to major magmatic flare-ups in the mature arcs of North American Cordillera. The Sierra Valle Fertil arc section is a type example of an immature Cordilleran arc, before major crustal thickening and crustal overturn takes place in the upper plate. The Famatinian arc section may also be an equivalent to the crustal architecture of more mature modern island arcs, such as parts of the Aleutians, Caribbean, and Japan.

Tectonothermal ordovician evolution of the western margin of the Famatinian magmatic arc: metamorphism of the mafic and ultramafic rocks of Sierra de la Huerta -Las Imanas (Pampean Ranges, Argentina)

An important magmatic and tectonometamorphic activity of Early and Middle Ordovician age is registered in the pre-Andean basement of the Sierras Pampeanas of Argentina. These were linked to the development of a continental magmatic arc during the Famatinian Orogeny, resulting from the approach and attachment of an alleged exotic terrane (the Precordillera Terrane), to the south western Gondwana's margin (present coordinates). A suit of meta-mafic and ultramafic rocks are exposed in the Sierras de La Huerta and Las Imanas, at the western limit of the famatinian orogen. Metaperidotites (Ol-Opx-Cpx-Am-Spl), coronitic metapyroxenites (Opx-Cpx-Spl-Am-Pl), metaquartz-norites (Opx-Pl-Am-Qtz-Bt±Grt) and metadiorites (Pl-Am-Qtz-Bt-Ep) are associated with metasedimentary rocks (marbles, gneisses and migmatites with Sil+Kfs+Grt) that reached the peak and post-peak conditions of metamorphism at middle Ordovician time. The meta-mafic rocks record a first high-grade metamorphic event (M1-730ºC and 8.4±0.5 kbar) and a second lower pressure event (M2, 720ºC and 4.5 kbar) with Cum+Hbl+Mag in a coronitic assemblage. The meta-ultramafic rocks also record the two metamorphic events, but only for the second one it was possible to calculate the P-T conditions. At latitude 32º30'S, the Famatinian magmatic arc shows a systematic compositional variation normal to its trend, i.e. in an east-west direction, which could be related to present erosion levels. In this context, the mafic and ultramafic units of Sierras de La Huerta and Las Imanas, probably represent the deepest levels of the magmatic arc which is consistent with the position that they show marginal to the orogenic belt, i.e., where the uplift and erosion rates were larger.

The southern segment of the Famatinian magmatic arc, La Pampa Province, Argentina

The present study led to the identification of a largely hidden southern segment of the Famatinian magmatic arc in the province of La Pampa, south-central Argentina. The arc is represented by scarce outcrops of metaigneous rocks (mostly meta-quartz-diorites and metagabbros) but stands out in the aeromagnetic data as a conspicuous and continuous, NNW-trending magnetic anomaly. The geochemical signature of this magmatic suite points to a pre-collisional continental magmatic arc. Its crystallization age (U–Pb SHRIMP dating on magmatic zircon) ranges from ca. 476 to 466 Ma, being comparable, therefore, to the central and northern segments of the Famatinian arc. All dated zircon of the metagabbro have similar 176Hf/ 177Hf ratios, negative e Hf (from − 5.02 to − 3.62) and average Lu–Hf model ages of ca. 1.7 Ga, indicating a crustal contamination of a mantle-derived mafic magma (of ca. 466 Ma) with much older rocks, probably older than 2 Ga, hence suggesting that part of the underlying basement of the southernmost Pampia terrane is at least this old. U–Pb SHRIMP dating on metamorphic zircon rims/areas yielded ca 454 Ma, that is within the 465–450 Ma age range of Famatinian metamorphism previously recorded in La Pampa. The amphibolite-facies metamorphism that affects the magmatic arc in La Pampa is attributed to the collision of the Cuyania (greater Precordillera) terrane against the western margin of Gondwana.

Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley, Sierra de Valle Fértil, Argentina: Implications for the evolution of magmas in the lower crust of the Famatinian arc

Abstract This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fertil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ∼ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO2 ∼ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO2) of the parental magma estimated between +0.8 and −0.6 log fO2 units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.

Tectonomagmatic evolution of Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic constraints from the Peruvian Eastern Cordilleran granitoids

The results of a coupled, in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb study on zircon and geochemical characterization of the Eastern Cordilleran intrusives of Peru reveal 1.15 Ga of intermittent magmatism along central Western Amazonia, the Earth's oldest active open continental margin. The eastern Peruvian batholiths are volumetrically dominated by plutonism related to the assembly and breakup of Pangea during the Paleozoic-Mesozoic transition. A Carboniferous-Permian (340-285 Ma) continental arc is identified along the regional orogenic strike from the Ecuadorian border (6 degrees S) to the inferred inboard extension of the Arequipa-Antofalla terrane in southern Peru (14 degrees S). Widespread crustal extension and thinning, which affected western Gondwana throughout the Permian and Triassic resulted in the intrusion of the late- to post-tectonic La Merced-San Ramon-type anatectites dated between 275 and 220 Ma, while the emplacement of the southern Cordillera de Carabaya peraluminous granitoids in the Late Triassic to Early Jurassic (220-190 Ma) represents, temporally and regionally, a separate tectonomagmatic event likely related to resuturing of the Arequipa-Antofalla block. Volcano-plutonic complexes and stocks associated with the onset of the present Andean cycle define a compositionally bimodal alkaline suite and cluster between 180 and 170 Ma. A volumetrically minor intrusive pulse of Oligocene age (ca. 30 Ma) is detected near the southwestern Cordilleran border with the Altiplano. Both post-Gondwanide (30-170 Ma), and Precambrian plutonism (691-1123 Ma) are restricted to isolated occurrences spatially comprising less than 15% of the Eastern Cordillera intrusives. Only one remnant of a Late Ordovician intrusive belt is recognized in the Cuzco batholith (446.5 +/- 9.7 Ma) indicating that the Famatinian arc system previously identified in Peru along the north-central Eastern Cordillera and the coastal Arequipa-Antofalla terrane also existed inboard of this parautochthonous crustal fragment. Hitherto unknown occurrences of late Mesoproterozoic and middle Neoproterozoic granitoids from the south-central cordilleran segment define magmatic events at 691 +/- 13 Ma, 751 +/- 8 Ma, 985 +/- 14 Ma, and 1071-1123 +/- 23 Ma that are broadly coeval with the Braziliano and Grenville-Sunsas orogenies, respectively. Our data suggest the existence of a continuous orogenic belt in excess of 3500 km along Western Amazonia during the formation of Rodinia, its "early" fragmentation prior to 690 Ma, and support a model of reaccretion of the Paracas-Arequipa-Antofalla terrane to western Gondwana in the Early Ordovician with subsequent detachment of the Paracas segment in form of the Mexican Oaxaquia microcontinent in Middle Ordovician. A tectonomagmatic model involving slab detachment, followed by underplating of cratonic margin by asthenospheric mantle is proposed for the genesis of the volumetrically dominant Late Paleozoic to early Mesozoic Peruvian Cordilleran batholiths

Generation of Tonalitic and Dioritic Magmas by Coupled Partial Melting of Gabbroic and Metasedimentary Rocks within the Deep Crust of the Famatinian Magmatic Arc, Argentina

The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate igneous rocks are located at the transition between a gabbro-dominated mafic unit and a tonalite-dominated intermediate unit. In the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall-rocks, crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments. In turn, the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks. The gabbroic rocks became mafic migmatites comprising amphibole-rich pyroxene-bearing mesosomes and leucotonalitic veins. Significantly, most of the mafic migmatites have isotopic compositions [87Sr/86Sr(T) < 0·7063 and εNd(T) = –0·94 to +2·24] similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins [87Sr/86Sr(T) = 0·7075–0·7126 and εNd(T) < –2·65], providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts [87Sr/86Sr(T) = 0·715 and εNd(T) = –6·21]. Mass-balance calculations show that the model reaction plagioclase + amphibole + leucogranitic melt → leucotonalitic melt + clinopyroxene ± orthopyroxene can better explain the partial melting of the gabbroic rocks. Based on field observations, we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We envisage that once detached from their source region the leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process gave rise to melts that became tonalitic and dioritic magmas. This study concludes that the generation of intermediate magmas is a multistage process with three critical steps: (1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; (2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies; (3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal-scale volumes of intermediate igneous rocks. However, the Famatinian paleo-arc crust would expose only those magma source zones that were still active during the tectonic closure of the arc. Ultimately, a time-integrated perspective indicates that early active source zones were cannibalized during the downward expansion of the plutonic bodies already dominated by intermediate plutonic rocks.

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number ( ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al 2O 3 contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al 2O 3 content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.

Metamorphic evolution of migmatites from the deep Famatinian arc crust exposed in Sierras Valle Fértil–La Huerta, San Juan, Argentina

Meta-supracrustal migmatitic packages in the Sierras Valle Fértil–La Huerta of northwestern Argentina occur as wedge- or strip-shaped septa interlayered among mafic to intermediate igneous plutonic rocks. Meta-supracrustal rocks were metamorphosed under granulite-facies conditions during the development of the Famatinian magmatic arc, and are among the structurally deepest rocks exposed within the belt dominated by Ordovician plutonism. Petrographic analysis, mineral chemistry and whole rock geochemistry of granulite-facies migmatites are used to argue that the meta-supracrustal packages comprise a sequence of pelitic to quartzo-feldspathic sedimentary rocks that achieved peak metamorphic P– T conditions of 5.2–7.1 kbar and 770–840 °C. There are no resolvable differences in peak P– T conditions for migmatites separated 70 km along strike of the Sierras Valle Fértil and La Huerta, suggesting that similar levels of the Famatinian paleo-arc crust are currently exposed in these ranges. Idioblastic poikilitic garnets displaying weak to absent chemical zoning profiles developed at or close to the peak metamorphic stage are used in conjunction with petrogenetic grid constraints to interpret the prograde evolution. At the time the supracrustal rocks experienced maximum thermal conditions, they underwent dehydration partial melting. Microtextural features show that felsic melt (leucosome) back reacted with the adjacent coexisting mineral assemblage (mesosome). These observations are interpreted as evidence that the migmatites evolved through a continuous heating–cooling cycle with minor pressure change. This is consistent with the general lack of reaction textures denoting decompression at high temperatures, and with the possibility that in some migmatites retrograde reactions formed staurolite. Collectively, these features indicate that when the migmatites attained peak thermal conditions, the deepest exposed arc crust was about 20–25 km beneath the Ordovician surface. Comparing these results with metamorphic studies elsewhere in the Famatinian arc between 31° and 32°S. indicates that much of the main-arc records primarily prograde P– T trajectories associated with a regional contact metamorphism, but that specific locations in the back-arc, main-arc and accretionary wedge also record post-peak retrogression during crustal exhumation. These differences are attributed to the fact that a collisional orogeny closely followed the cessation of arc magmatism, a collision we infer to be associated with the accretion of a Laurentian terrane to the Gondwana margin. The results of this study therefore provide important insights into the geodynamic context of the formation and closure of the central segment of the Famatinian magmatic arc.

P– T– t evolution of an Early Silurian medium-grade shear zone on the west side of the Famatinian magmatic arc, Argentina: Implications for the assembly of the Western Gondwana margin

The geodynamic evolution of the proto-Andean margin of Gondwana during the Paleozoic was characterized by repeated subduction processes associated with the docking of several terranes, including the Cuyania–Precordillera terrane and the Chilenia terrane, and the development of the calc–alkaline Famatinian continental magmatic arc. In the Sierra de La Huerta (30°56′–31°29′ S and 67°17′–67°32′ W), at the southwestern end of the Western Sierras Pampeanas, some mafic to ultramafic igneous bodies belonging to the Famatinian arc were affected by regional metamorphism of the medium-pressure granulite facies (7–7.5 kbar, ≈ 850 °C). After this regional metamorphism, local mylonitization under amphibolite facies along discrete NW–SE to NNW–SSE striking ductile shear zones occurred. A 40Ar/ 39Ar plateau age of 432 ± 4 Ma was obtained on rather homogeneous hornblende porphyroclasts from a metagabbro mylonite. The textures and mineral chemistry of the mylonitized metagabbro allow its P– T– t evolution from magmatic crystallization to mylonitization to be constrained. Geochronological data obtained from mylonite provide evidence that orogenesis was active at least until the Early Silurian. This deformational event would have been related to uplift and decompression during the later stages of the orogenesis, probably associated with the accretion of the Precordillera terrane to the southwestern Gondwana margin.

Foreland basin deposits associated with Cuyania accretion in La Pampa Province, Argentina

In the province of La Pampa, south-central Argentina, adjacent to the Cuyania–Pampia terrane boundary, a Late Ordovician–Devonian marine foreland basin generated after the Middle Ordovician (Darriwilian) collision of Cuyania has been recognised, and is herein referred to as the Curacó basin. This basin was formed on crust of the Cuyania terrane, immediately to the west of the Pampia terrane. Its fill primarily comprises sandstones and shales of the La Horqueta Formation, unconformably overlain by Permian continental sediments (Carapacha Formation). The extent of the Curacó basin is investigated using aeromagnetic data; its depocentre is conspicuously delineated by a narrow NNW-orientated magnetic low. In addition, scarce outcrops assigned to the same sequence are found on the Pampian continental crust, about 50 km to the east, in the proximity of the Valle Daza estate (Valle Daza sub-basin). The studied marine sequence of La Pampa is equivalent to other Lower to Middle Paleozoic units of the eastern margin of the Cuyania terrane and is clearly distinguished from the overlying Permian continental sedimentary sequence. U–Pb SHRIMP dating of detrital zircons from Valle Daza indicates a maximum deposition age of ca. 466 Ma; the minimum age is given by the intrusion of granites at ca. 405 Ma (i.e. Early Devonian). Geochemical data indicate that the Curacó basin was primarily sourced by an active continental margin, possibly the Ordovician Famatinian arc.