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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
Neoproterozoic crystalline basement rocks are exposed as fault-bounded blocks over the high Andes of Catamarca. The crystalline basement is stratigraphically grouped into the unique Laguna Amarga Metamorphic Complex and represents the northern extension of the Cuyania/Precordillera terrane. Tabular bodies of meta-mafic rocks are widespread in the basement interspersed within a thick sequence of meta-sedimentary rocks derived from siliciclastic, calc-silicate, and limestone protoliths. Overall, the geochemical characteristics of meta-mafic rocks are in the compositional range of Normal-Mid Ocean Ridge Basalt (Normal-MORB), reflecting a common depleted-mantle source with varying degrees of partial melting. While preserving the typical bulk chemistry of MORB magmatism, some mafic magma underwent differentiation at emplacement, leading to the development of high-Ti mafic rocks. New U-Pb zircon geochronology reveals three distinct age peaks, with two coinciding with ages identified in the metasedimentary host rocks. The dominant Mesoproterozoic age cluster is linked to inherited zircon crystals assimilated within a single meta-mafic rock. In contrast, zircon ages from the late Ordovician to early Devonian are attributed to metamorphic overgrowths. Notably, the third age cluster, delineates a Late Neoproterozoic magmatic event, indicating the temporal span of mafic magmatism. The finding agrees with the best available age (576 ± 17 Ma) for mafic magmatism on the Precordillera Mafic-Ultramafic Belt. Stratigraphic relationships and geochemical fingerprints enable correlation among the meta-mafic rocks from Laguna Amarga, tracing a belt of mafic magmatism with an oceanic affinity that extends southward. Building upon previous works, this study reaffirms that the rift-drift transition of the Cuyania/Precordillera terrane, linked to the Ouachita rift opening from southeastern Grenville, evolved during the latest Neoproterozoic.
The Famatinian orogeny along proto-Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, was of the accretional type during the Paleozoic (Ordovician to early Devonian). Information gathered from the Sierras Pampeanas (Argentina) reveals magmatism (plutonism and volcanism) between 490–465 Ma, with a peak at 473–467 Ma, similar to other regions along the margin. Metamorphic events in the middle crust occurred between 484 and 465 Ma, and at ca. 440 Ma. Recent reviews propose a lithospheric extension phase (>480 Ma) before contraction episodes (ca. 470 and 440 Ma). Magmatism transitioned from peraluminous to metaluminous intrusions, with prolonged mafic magmatism throughout. Mafic intrusions show a broad spectrum of isotope compositions (87Sr/86Srt = 0.705 to 0.711 and ƐNdt = +5 to –6) but with general trends toward isotopically enriched magmas. The main arc involved tonalite-granodiorite magmas (ca. 480–470 Ma) with <25% crustal contamination but crustal-like isotopic signatures (87Sr/86Srt = 0.707 to 0.711 and ƐNdt = –4 to –6). The wide isotopic variations in granodiorite-monzogranite (and volcanic equivalents) (87Sr/86Srt = 0.705 to 0.711 and ƐNdt = +3 to –6) suggest two origins: differentiation and contamination of tonalite-granodiorite magmas (AFC model) for the most evolved, and differentiation of juvenile mafic additions for the isotopically juvenile ones. Regardless of depth (~1 to 6 kbar) and position within the magmatic arc, analyzed areas display north-south striking and steep magmatic fabrics. These fabrics may have rotated by no more than 20-28°. The petrogenetic model involves isotopically diverse sub-arc mantle sources. Famatinian magmatism ceased abruptly around 465 Ma (magmatic lull), correlating with arcs along the SW Gondwana margin, indicating significant geodynamic changes in the subduction regime.
The southwestern paleo-margin of Gondwana is interpreted as an accretionary margin that was active from the late Neoproterozoic to the late Paleozoic. The basement of this paleo-margin is widely exposed in the central-western part of Argentina (Sierras Pampeanas area), where a protracted evolution from the Mesoproterozoic to the late Paleozoic is recorded. Part of this evolution is preserved in the El Gigante Metamorphic Complex (Sierra de El Gigante; Western Sierras Pampeanas), a small rotated block within the Valle Fértil Lineament fault zone that separates the Western and Eastern Sierras Pampeanas. The complex is composed of medium-grade meta-siliciclastic and meta-carbonate rocks and medium- to high-grade meta-igneous rocks, affected by tight to isoclinal folds and a pervasive east–west foliation resulting from the Famatinian orogeny (broadly late Cambrian to early Devonian). Later events include localized ductile shear zones. The isotopic and geochronological data from El Gigante Metamorphic Complex reveal at least three distinct lithological assemblages: (1) metamorphosed felsic igneous rocks of ca. 1.11 Ga, i.e., late Mesoproterozoic, (2) a Neoproterozoic metasedimentary succession composed of quartzites and mica-schists, (3) possibly mid- to late-Cambrian marble and graphite-schist. The Grenvillian assemblage (1) was part of a large reworked Paleoproterozoic continental block called MARA (acronym of Maz-Arequipa-Río Apa) and was the basement over which the Neoproterozoic and
Cambrian sedimentary successions were deposited. Based on U-Pb zircon ages, lithological similarities and Sr-isotope data, the three lithological assemblages of the El Gigante Metamorphic Complex can be correlated with similar ones in the nearby geologically better-known Sierra de Pie de Palo. The Neoproterozoic and the
Cambrian metasedimentary successions are respectively equivalent to the Difunta Correa Metasedimentary Sequence and the Nikizanga-Caucete Groups, which are recognized throughout the Sierras Pampeanas, east and west of the Valle Fértil Lineament.
In this contribution, we present new early middle Devonian igneous and metaigneous units with a major juvenile magmatic source input in the North Patagonian Massif, which were discovered through U‐Pb and Lu‐Hf zircon analyses. Afterward, we assessed their tectonic implications for northwestern Patagonia and then for southern South America, combining our results with available database information consisting of igneous crystallization ages and isotopic data of the Devonian to early Carboniferous magmatic units, tectonic‐metamorphic analyses, and thermochronologic record. This study allows for distinguishing retreating and advancing subduction switching in northwestern Patagonia (38°30′ to 44°S) and a contrasting coetaneous evolution for basement outcrops exposed further north (27°30′ and 37°30′S). The early middle Devonian (400–380 Ma) northwestern Patagonian magmatism is characterized by widespread magmatism and positive εHf–εNd linked to forearc and backarc magmatism that evolved within a retreating subduction stage. A tectonic switching toward advancing orogeny stage began in the late Devonian, evidenced by a lull in magmatic activity with a negative εHf–εNd trend, possibly contemporaneous with the first tectonic‐metamorphic event in western Patagonia. An early Carboniferous magmatic gap, followed by the subsequent development of the main foliation in the basement during the Carboniferous‐Permian period, denotes the acme of this contractional stage. In contrast, the Devonian period in the northern segment is characterized by mostly negative εHf–εNd values, reverse shear zone activity in the foreland, and an inboard magmatism migration, evidencing a compressive tectonic setting that changed to an extensional configuration in the early Carboniferous with widespread arc magmatism development.
The tectonic and magmatic characteristics of the Alps and Pyrenees during convergence are quite distinct from characteristics associated with classic Benioff-type oceanic subduction. From the initiation of subduction at passive margins until the onset of continental collision, the closure of the Western Tethys never produced a long-lived magmatic arc. This is a consequence of the 3-D architecture of the Western Tethys (a series of hyper-thinned basins and continental blocks) and its narrow width (<500–700 km) prior to convergence. Subduction primarily involved the slow and amagmatic subduction of a narrow domain of dry lithospheric mantle. This type of congested Ampferer subduction led to the sequential and coherent accretion of inherited rifted domains which today form the Alpine and Pyrenean orogens.
The International Mineralogical Association’s approved amphibole nomenclature has been revised in order to simplify it,
make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely, and include new species
of amphibole discovered and named since 1978, when the previous scheme was approved. The same reference axes form the
basis of the new scheme, and most names are little changed, but compound species names like tremolitic hornblende (now
magnesiohornblende) are abolished, as are crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite),
tirodite (now manganocummingtonite) and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain
tremolite and actinolite as in the 1978 scheme; the sodic–calcic amphibole range has therefore been expanded. Alkali amphiboles
are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite,
kornite, ungarettiite, sadanagaite and cannilloite. All abandoned names are listed. The formulae and source of the amphibole
end-member names are listed, and procedures outlined to calculate Fe3+ and Fe2+ where not determined by analysis.
The Sierra de Pie de Palo (SPP, Western Sierras Pampeanas) shows evidence of two regional metamorphisms: one Mesoproterozoic attributed to the Grenvillian orogeny and other of Ordovician age related to the Famatinian orogeny. The Neoproterozoic-to-Cambrian sedimentary successions that cover the Grenvillian basement only record the Ordovician event. One staurolite-schist from the Ediacaran Difunta Correa Metasedimentary Sequence collected in the southeastern side of the SPP allows to constrain, by means of pseudosections, a prograde evolution from ca. 3 kbar and 515 ºC up to ca. 9 kbar and 640 ºC corresponding to a high P/T gradient. The SPP and the immediately east Loma de Las Chacras outcrop were part of the famatinian forearc which shows a progressive decrease of P (from ca. 13 kbar to 6 kbar), T (from ca. 900 ºC to 450 ºC), and P/T gradient (from ca. 85 ºC/kbar to 35 ºC/kbar) towards the active continental margin on the west. The Caucete Group, in the western side of the SPP, represents the westernmost part of the forearc, near to the active continental margin. Metamorphism was apparently coeval with the Famatinian magmatism and with ductile underthrusting at ca. 470-465 Ma, which led to burial of the forearc beneath the magmatic arc.
The proto-Andean margin of Argentina consists of several suspect terranes, the origins of which are disputed. The Cuyania (greater Precordillera) suspect terrane was originally interpreted to be of southeast Laurentian affinity, but more recently a southwestern Gondwanan provenance has been argued. Both potential source regions comprise Mesoproterozoic rocks, but we show they are isotopically distinct, using previously published zircon Lu-Hf data. Detrital zircon εHf data from southwestern Gondwana (Namaqua-Natal belt) show no correlation with new zircon U-Pb and Lu-Hf data from Cuyania, suggesting that Gondwana was not the source of these sediments. Rather, detrital zircons from Cambrian strata in Cuyania yield Mesoproterozoic zircons with depleted εHf that correlate to the Grenville margin of Laurentia, and a ca. 535 Ma zircon population sourced directly from rift-related rocks of the Ouachita Embayment, thus recording rifting and drifting of Cuyania from Laurentia. By contrast, zircons from Middle to Late Ordovician strata of Cuyania record a larger range of εHf values, correlated with Western Sierras Pampeanas Mesoproterozoic basement inliers of Argentina. These synorogenic clastic deposits record the Ordovician arrival of Cuyania at the proto-Andean margin of Gondwana. The new data require the terrane boundaries of Cuyania to be redefined, thereby excluding Western Sierras Pampeanas basement inliers. The results verify the Laurentian microcontinent model for the origin of Cuyania.
The basement of the Central Andes located in central-western Argentina (31º20′S - 69º22′W) is composed by the Cuyania and Chilenia terranes which were amalgamated to Gondwana in the Early-Mid Paleozoic. Between the Precordillera (Cuyania) and Frontal Cordillera (Chilenia) there are exposures of marine metasedimentary rocks associated with mafic rocks with an E-MORB chemical signature that represent the remnants of an extensional basin developed between both terranes. The stratigraphic features and the distribution of the Early-Mid Paleozoic units along the Western Precordillera were constrained by remote sensing techniques. This allowed us to identify two stages in the evolution of the sedimentary in-fill of the marine basin: an initial stage (Mid-Late Ordovician) marked by widespread extensional tectonics and a finning-upwards sequence interbedded with volcanic-plutonic mafic rocks; and a Late Ordovician?-Devonian where the sedimentation was characterized by the development of coarsening-upwards sequences with low participation of mafic rocks. Flattened parallel folds associated with pre-Andean thrusts have locally a top-to-the SW vergence. These pre-Andean (Late Devonian) structures are the relics of the Chanic orogen whose double vergence is the result of the control exerted by previous structures related to the ordovician rifting. This is constrained by the residual and regional magnetic anomalies which reflect an important correlation between deep and surface structures. We propose the inception of a subduction zone with an eastward polarity on the proto-Pacific margin of Gondwana as the responsible for the compressive geotectonic framework that led to the closure of the Western Precordillera basin during the Late Devonian and the development of the Chanic thick-skinned-dominated orogen.
Hf-isotope data of > 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 zircons were mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc. This article is protected by copyright. All rights reserved.
New geologic mapping, structural, and geochronologic analyses of the pre-Carboniferous ultramafic,mafic, and metasedimentary rocks exposed on the southwest margin of the Precordillera terrane, western Argentina, show that the ultramafic and mafic rocks belong to at least four distinct tectonic units, not one ophiolite pseudostratigraphy, as previously thought. One unit comprises gabbro, microgabbro, diabase, and minor plagiogranite that are interpreted as the mafic crustal section of an ophiolite pseudostratigraphy. Another unit consists of serpentinized peridotite, ultramafic cumulate, layered gabbro, and quartzofeldspathic gneiss that experienced granulite facies metamorphism in a deep continental crust environment. Other mafic rocks include highly altered basaltic flows interlayered with low-grade clastic metasedimentary rocks and diabase and microgabbro sills that intrude the metasedimentary rocks. The pre-Carboniferous rock units were juxtaposed along synmetamorphic, ductile, top-to-the-east shear zones in the Early to Middle Devonian. The intense deformational fabrics formed during the juxtaposition event were overprinted by strong Permian folding and top-to-the-west brittle thrust faulting, Late Permian to middle Mesozoic extension, and Late Tertiary west-vergent folding and top-to-the-west brittle thrust faulting. A speculative tectonic scenario for the juxtaposition of the pre-Carboniferous in the Early to Middle Devonian places the deep continental crust at the base of the upper plate (eastern Chilenia) of a west-dipping subduction zone, the mafic ophiolite crustal section in an ocean basin between the western Precordillera terrane and eastern Chilenia, and the mafic sills and flows on the extended western margin of the Precordillera terrane. Early to Middle Devonian closure of the ocean basin resulted in juxtaposition of the pre-Carboniferous rock units.
The Sierra de Pie de Palo (Western Sierras Pampeanas, Argentina) in the Andean foreland is mainly formed by a Mesoproterozoic basement and an Ediacaran metasedimentary cover referred to as the Difunta Correa metasedimentary sequence. The latter is key to understanding the characteristics of this region prior to the early Cambrian assembly of SW Gondwana. It is composed of low- to medium grade metamorphic rocks (metasandstones, mica-schists, Ca-pelitic schists, metaconglomerates, marbles and less abundant amphibolites) that can be grouped into four informal lithostratigraphic units. The chemical composition of these rocks allows to classify the siliciclastic protoliths as shales, Fe-shales and immature sandstones (wackes, sub-litharenites, litharenites and Fe-sandstones). The sediments were derived from an evolved felsic to intermediate continental source and were deposited on a continental passive margin overlaying a Mesoproterozoic basement that crops out at several places of the Western Sierras Pampeanas. Thick marine carbonate beds with seawater isotope composition, phosphatic clasts and the lack of contemporaneous, arc related igneous rocks, also support a passive margin sedimentation. Phosphatic clasts within metaconglomerates are described for the first time in the Sierras Pampeanas and were probably formed after an important Neoproterozoic glaciation (Marinoan). We further suggest, based on our data and previous works, that the passive margin probably belonged to the Paleoproterozoic MARA (acronym of Maz, Arequipa, Río Apa) continental block. MARA, which remained juxtaposed to Laurentia since the middle to late Mesoproterozoic orogenies until its eventual drifting in the late Neoproterozoic, finally accreted to SW Gondwana in early Cambrian times during the Pampean orogeny.
The International Mineralogical Association's approved amphibole nomenclature has been revised to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely, and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed, but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished, as are crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite), and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme; the sodic-calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyböite, leakeite, kornite, ungarettiite, sadanagaite, and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end-member names are listed and procedures outlined to calculate Fe3+ and Fe2+ where not determined by analysis.
The provenance of Neoproterozoic to Early Paleozoic sedimentary rocks in the Sierras Pampeanas has been established using U–Pb SHRIMP age determination of detrital zircons in twelve metasedimentary samples, with supplementary Hf and O isotope analyses of selected samples. The detrital zircon age patterns show that the western and eastern sectors of the Sierras Pampeanas are derived from different sources, and were juxtaposed during the Early Cambrian ‘Pampean’ collision orogeny, thus defining initiation of the supercontinent stage of southwestern Gondwana. The Western Sierras Pampeanas (WSP), which extend northwards to the southern Puna (Antofalla) and the Arequipa Massif (Peru), constitute a single large continental basement of Paleoproterozoic age – the MARA block – that was reworked during the Grenvillian orogeny. The MARA block probably extends eastwards to include the Río Apa block (southern Brazil), but in this case without a Mesoproterozoic overprint. Detrital zircons from the WSP and Antofalla yield age peaks between 1330 and 1030 Ma, remarkably similar to the range of ages in the Grenville province of eastern Laurentia. The WSP Neoproterozoic sedimentary cover to this basement shows the same 1330–1030 component, but also includes important 1430–1380 Ma zircons whose juvenile Hf and O isotopic signatures strongly suggest derivation from the Grenville and the Southern Granite-Rhyolite provinces of eastern Laurentia. In contrast the Eastern Sierras Pampeanas metasedimentary rocks have a typically bimodal detrital zircon pattern with peaks at ca. 1000 and 600 Ma, which respectively indicate sources in the Natal–Namaqua belt and the East African orogen and/or the Dom Feliciano belt of SE Brazil and Uruguay. Sedimentary rocks in the Eastern Sierras Pampeanas and Patagonia deposited during the Late Early Cambrian–Early Ordovician interval, after the Pampean orogeny, have detrital patterns common to many sectors along the Terra Australis orogen, reflecting increasingly dominant input to the Paleozoic basins from the Neoproterozoic to Early Cambrian orogenic belts of the Gondwana margin.
Despite the recongnized importance of monazite in geochronology and petrology, a range of fundamental analytical and preparational problems remains. For example, chemical Th-U-Pb dating of monazite requires special lead-free sample preparation. This is achieved efficiently and at high quality with specially developped grooved ND-PE polyethylene polishing disks. Techniques useful in locating and characterizing monazite are evaluated. Back scattered electron imaging is an effective way to determine zonation patterns, particularly with respect to thorium. Quantitative analysis of monazite by EMP is delicate and time consuming. A whole series of X-ray peak interferences has been ignored in published work. For example, for monazite containing 12% Th, the commonly disregarded interference of Th Mz on Pb Ma causes an overestimation of 11% (relative) in Pb. This propagates to an age overestimation of ~50 Ma for a sample of 400 to 500 Ma in age. A judicious choice of X-ray peaks used in quantitative EMP analysis avoids or minimises peak overlap for all elements, including REE. Only for U a correction factor is required: U wt% corrected = U wt% measured -(0.0052 * Th wt% measured) based on the analytical lines U Mb and Th Ma.
This paper is included in the Special Publication entitled 'The proto- Andean margin of Gondwana', edited by R.J. Pankhurst and C.W. Rapela. 40Ar/ 39Ar incremental-release ages have been determined for muscovite and hornblende concentrates prepared from basement rocks of the Sierra de Pie de Palo, in the western Sierras Pampeanas, east of the Early Palaeozoic Precordillera. The basement is mainly represented by variably metamorphosed units, including ophiolites, orthogneisses and schists. Previous U-Pb, Rb-Sr, K-Ar, and 40Ar/ 39Ar dating of magmatic and metamorphic zircons from the basement has indicated a Middle Proterozoic age. The 40Ar/ 39Ar plateau ages constrain a series of ductile deformational events that are correlated with development of structural discontinuities in adjacent foreland basins. The following deformational events are postulated: (1) initiation of collision and drowning of the Precordillera platform against Gondwana (470-460 Ma); (2) flexural extension associated with normal faulting due to tectonic loading of the Gondwana margin; and (3) development of a foreland basin (450-430 Ma). These Mid-Ordovician-Silurian events were related to collision of several exotic Early Palaeozoic terranes along the Gondwana margin. The Punta Negra foreland basin developed further west during Early to Mid-Devonian (410-380 Ma) times, and was linked to the beginning of the docking of the Chilenia terrane along the western margin of the Precordillera.
A paradox in dating metamorphic events in low-grade, polymetamorphic terranes is exemplified by the eastern Lower Austroalpine nappes of the Eastern Alps. Here, the last metamorphic event, best recorded in post-Variscan cover rocks, is dated as Late Cretaceous in age (c. 80–85 Ma) by white mica Rb/Sr and ⁴⁰ Ar/ ³⁹ Ar systems. Within the underlying polymetamorphic basement, ⁴⁰ Ar/ ³⁹ Ar and Rb/Sr ages of phengitic white mica record only Early and/or Late Variscan ages (375–270 Ma), indicating that the Alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of Variscan mineral ages.
Based on these results, the timing of a penetrative, ductile, top–to-WNW simple shear deformation recorded within both basement and cover rocks was contradictory. Deformation within the post-Variscan cover rocks had to be Alpine in age, whereas phengite ⁴⁰ Ar/ ³⁹ Ar ages from basement mylonites yield Variscan ages. To date this deformation directly, we isolated different mineral size fractions (63–30 and 30–10 μm) from highly strained shearbands within the Wechsel basement nappe. A resulting Rb/Sr errorchron pointed to an Upper Cretaceous age (c. 85 Ma) for the deformation, consistent with the timing of Alpine metamorphism in the cover rocks. Coarser-grained white mica (150–300 μim) from similar basement mylonites do not reflect any Alpine overprint of either K/Ar and/or Rb/Sr systems. It follows that dynamic re-and/or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the Rb/Sr system locally virtually re-equilibrated. This is valid even for overprinting metamorphic conditions below the temperatures required for Ar diffusional loss in phengitic white mica (c. <350°C). The data suggest that mineral ages that date low-grade mylonitization (e.g., white mica ⁴⁰ Ar/ ³⁹ Ar) should be considered with caution.
We argue there is no distinction between accretion and collision as a process, except when accretion is used in the sense of incorporating small bodies of sedimentary and/or volcanic rocks into an accretionary wedge by off-scraping or underplating. There is also a distinction when these terms are used in classifying mountain belts into accretionary and collisional orogens, although such classifications are commonly based on a qualitative assessment of the scale and nature of the accreted terranes and continents involved in formation of mountain belts. Soft collisions occur when contractional deformation and associated metamorphism are principally concentrated in rocks of the leading edge of the partially pulled-down buoyant plate and the upper plate forearc terrane. Several young arc-continent collisions show evidence for partial or wholesale subduction of the forearc such that the arc is structurally juxtaposed directly against lower plate rocks. This process may explain the poor preservation of forearcs in the geological record. Soft collisions generally change into hard collisions over time, except if the collision is rapidly followed by formation of a new subduction zone due to step-back or polarity reversal. Thickening and metamorphism of the arc's suprastructure and retro-arc part of upper plate due to contractional deformation and burial are the characteristics of a hard collision or an advancing Andean-type margin. Strong rheological coupling of the converging plates and lower and upper crust in the down-going continental margin promotes a hard collision. Application of the soft–hard terminology supports a structural juxtaposition of the Taconic soft collision recorded in the Humber margin of western Newfoundland with a hard collision recorded in the adjacent Dashwoods block. It is postulated that Dashwoods was translated dextrally along the Cabot-Baie Verte fault system from a position to the north of Newfoundland where the Notre Dame arc collided ca. 10 m.y. earlier with a wide promontory in a hyperextended segment of the Laurentian margin.
New analytical and field techniques, as well as increased international communication and collaboration, have resulted in significant new geological discoveries within the Appalachian-Caledonian-Variscan orogen. Cross-Atlantic correlations are more tightly constrained and the database that helps us understand the origins of Gondwanan terranes continues to grow. Special Paper 554 provides a comprehensive overview of our current understanding of the evolution of this orogen. It takes the reader along a clockwise path around the North Atlantic Ocean from the U.S. and Canadian Appalachians, to the Caledonides of Spitsbergen, Scandinavia, Scotland and Ireland, and thence south to the Variscides of Morocco.
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.
New structural data from a mid-crustal segment in the Eastern Sierras Pampeanas, coupled with geochronological methods and P – T estimates, reveal polyphase contractional deformation and metamorphism during the Famatinian Orogeny over a long period of time. Peaks of metamorphic monazite and zircon ages are recorded at c. 500 Ma, between 484 and 465 Ma and at c. 440 Ma. Between 484 and 465 Ma the region attained high-temperature (HT) low-pressure (LP) conditions that resulted in widespread partial melting (peak at c. 470 Ma). A contractional phase occurred during this event, as suggested by syn-anatectic structures, as well as folding at subsolidus conditions. Renewed contraction under subsolidus conditions is evidenced by reverse ductile shearing and folding at c. 440 Ma. Thrusting along the La Chilca Shear Zone caused metamorphic inversion. The consistent orientation of folds in the Quebrada del Molle Metamorphic Complex, El Portezuelo Metamorphic–Igneous Complex and the La Chilca Shear Zone as well as WSW-directed thrusting at the later shear zone indicate uniform WSW–ENE-directed shortening. This late deformation records the Ocloyic tectonic phase and brought the Famatinian Orogeny to an end in the Late Ordovician to Early Silurian.
Supplementary material: Analytical methods, methods and description of the calculated P – T pseudosection diagrams, bulk compositions, and electron microprobe, U–Pb and ⁴⁰ Ar– ³⁹ Ar data are available at https://doi.org/10.6084/m9.figshare.c.4697189
Along the proto-Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, the Early–Middle Ordovician Famatinian orogeny was the first orogenic event following assembly of the supercontinent. Previous isotope studies of the igneous and (meta-)sedimentary rocks of southwestern Gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin. To interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern South America, including the 700 × 600 km type sector of the orogen in the Sierras Pampeanas (27°–33°S), the Precordillera, and northeastern Patagonia. Previous geological, geochemical and geochronological results are reviewed together with new U—Pb SHRIMP crystallization ages, 177Hf/176Hf and 18O/16O data for dated zircon, and whole-rock Sr and Nd isotope compositions.
Four geological domains are recognized in the Sierras Pampeanas (Western, Central, Eastern and Foreland Famatinian domains). Magmatism is mostly restricted to the interval 463 ± 4 to 486 ± 7 Ma, with the most intense period of emplacement between 468 and 472 Ma constituting a magmatic flare-up. Granitoid emplacement in both northeastern Patagonia and the Cordon de Lila (Puna Altiplano, Chile) was effectively synchronous with that in the Sierras Pampeanas, defining a continuous belt. Combined geochemical and isotopic data (whole-rock Sr, Nd; Hf, O in zircon) indicate that the source of calcic metaluminous suites is the subcontinental lithosphere – both mantle and mafic lower crust – with variable contamination by the Early Paleozoic metasedimentary country rocks. The lithospheric mantle involved is assumed to underlie the outcropping 1330–1030 Ma age basement of the Western Domain, which exhibits tectonic characteristics of active continental margin in the north and oceanic arc-back arc in the south. The latter sector is the potential source of some minor Famatinian igneous rocks with less evolved isotopic compositions, although a restricted asthenospheric addition cannot be discarded in this case. Minor peraluminous granites are spatially associated with the metaluminous sequence, but major highly-peraluminous batholiths occur on the eastern flank of the Central Domain. Field relations and geochemical/isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post-early Cambrian metasedimentary sequence comprising the host country rocks.
Episodic tectono-magmatic evolution of the Famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector:
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ca. 474–486? Ma, roll-back stage. This is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle; full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly-peraluminous batholiths in the Central and Eastern Famatinian domains. Trondhjemite plutons with an adakitic signature were emplaced in the Foreland Domain
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ca. 468–472 Ma, slab break-off stage. Steepening of the oceanic slab and arc migration to the southwest ended with slab break-off due to subduction of continental crust during continental collision with the Precordillera terrane. This stage produced voluminous metaluminous magmatism at the western edge of the Central Domain (the flare-up episode), K-bentonites in the Precordillera, leucogranites in the Western Domain and scattered metaluminous and peraluminous plutons in all Famatinian domains.
Both slab roll-back and break-off stages developed during a high-T regime typical of hot orogens.
Although asthenospheric mantle was a necessary heat source for lithospheric melting, its material contribution to the growth of Early Paleozoic crust was apparently very minor. Recycling of Mesoproterozoic lithosphere, including the subcontinental mantle, coupled with crustal melting of Early Paleozoic metasedimentary sequences, accounts for most Famatinian magmatism. Comparable results from the Central Andes and East Antarctica confirm that the early stages of the Terra Australis orogen in SW Gondwana were dominated by lithospheric reworking processes.
Shear zones play a major role in the deformation of the crust at a variety of scales, as expressions of strain localization during orogeny and rifting, and also as reactivated structures. They influence the geometry and evolution of orogenic belts and rifts, crustal rheology, magma ascent and emplacement, and fluid flow. Consequently, assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere. The interpretation of thermochronologic data from shear zones is, however, not straightforward. In the first place, closure temperatures depend on a number of factors (grain size, cooling rate, mineral composition and pressure, among others). On the other hand, deformation-related processes such as dynamic recrystallization, neocrystallization and fluid circulation seem to be crucial for isotopic systems and, thus, the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks. For this reason, geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo- or recrystallization, fluid-assisted deformation and inheritance of the protolith age(s). In order to robustly reconstruct P-T-ε-t paths of long-term crustal-scale shear zones, structural, microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems. In addition, geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications. One of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion, particularly the coupling with deformation processes, based on natural, theoretical and experimental data. Future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks.
The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at ~452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at ~471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles).
Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2–7 kbar, 230–450 °C during increase of the metamorphic field gradient from ~12 °C/km to ~32 °C/km over time. Multiple metamorphic crystallisation stages were dated by white mica ⁴⁰Ar/³⁹Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 ± 4 Ma and 356 ± 16 Ma postdate the closure of Iapetus.
Results differ along two transects: The oldest ages of 443–421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0–7.0 kbar, 230–340 °C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically.
In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408–390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (~250–450 °C, 2.5–4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393–340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL.
The ³⁹Ar-⁴⁰Ar technique is often used to date the metamorphic evolution of basement rocks. The present review article examines systematic aspects of the K-Ar decay system in different mineral chronometers frequently found in mono- and polymetamorphic basements (amphibole, biotite, muscovite/phengite, K-feldspar). A key observation is that the measured dissolution rate of silicates in aqueous fluids is many orders of magnitude faster, and has a much lower activation energy, than the rate of Fickian diffusion of Ar. The effects of this inequality are patchy age zonations, very much like those observed in many U-Pb chronometers, unaccompanied by intra-crystalline bell- shaped Ar loss profiles. Recognizing the importance of the respective rates in field situations leads to re-evaluating the ages and the interpretive paradigms in classic examples such as the Central Alpine "Lepontine" amphibolite facies event and the Western Alpine eclogitic event.
Systematic 40Ar/39Ar feldspar data obtained from the Sierras Pampeanas are presented, filling the gap between available high- (>~300 °C) and low-temperature (<~150 °C) thermochronological data. Results show Silurian–Devonian exhumation related to the late stages of the Famatinian/Ocloyic Orogeny for the Sierra de Pocho and the Sierra de Pie de Palo regions, whereas the Sierras de San Luis and the Sierra de Comechingones regions record exhumation during the Carboniferous. Comparison between new and available data points to a Carboniferous tectonic event in the Sierras Pampeanas, which represents a key period to constrain the early evolution of the proto-Andean margin of Gondwana. This event was probably transtensional and played a major role during the evolution of the Paganzo Basin as well as during the emplacement of alkaline magmatism in the retroarc.
In central-western Argentina, an Early Paleozoic belt including mafic-ultramafic bodies, marine metasedimentary rocks and high-pressure rocks occur along the western margin of the Precordillera and in the Frontal Cordillera. First pressure-temperature estimates are presented here for low-grade rocks of the southern sector of this belt based on two metasedimentary and one metabasaltic sample from the Peñasco Formation. Peak metamorphic conditions resulted within the range of 345–395 °C and 7.0–9.3 kbar within the high-pressure greenschist facies. The corresponding low metamorphic gradient of 13 °C/km is comparable with subduction related geothermal gradients. Comparison between these results and data from other localities of the same collision zone (Guarguaraz and Colohuincul complexes) confirms a collision between Chilenia and the composite margin of western Gondwana and suggests a stronger crustal thickening in the south of the belt, causing exhumation of more deeply buried sequences. During the Early Paleozoic a long-lived marine sedimentation coupled with the intrusion of MORB-like basalts occurred along a stable margin before the collision event. This contrasts with the almost contemporaneous sedimentation registered during accretion in accretionary prism settings and additionally proves the development of a collision zone along western Precordillera and the eastern Frontal Cordillera as well as the existence of Chilenia as a separate microcontinent.
The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle-Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge. The dominant mode (similar to 1350-1000 Ma) of the detrital-zircon ages corresponds to the ages of basement rocks in the Western Sierras Pampeanas along the eastern side of the Precordillera. A secondary mode (1500-1350 Ma) corresponds in age to the Granite-Rhyolite province of Laurentia, an age range which is not known in ages of basement rocks of the Western Sierras Pampeanas; however, detritus from Granite-Rhyolite-age rocks in the basement of the Precordillera was available through recycling of synrift and passive-margin cover strata. Igneous clasts in the conglomerates have ages (647-614 Ma) that correspond to the ages of minor synrift igneous rocks in the nearby basement massifs; the same ages are represented in a minor mode (similar to 750-570 Ma) of detrital-zircon ages. A quartzite clast in a conglomerate, as well as parts of the population of detrital zircons, indicates the importance of a source in the metasedimentary cover of the leading edge of the Precordillera. The Famatina continental-margin magmatic arc reflects pre-collision subduction of Precordillera lithosphere beneath the western Gondwana margin; however, no detrital zircons have ages that correspond to Famatina arc magmatism, indicating that sedimentary detritus from the arc may have been trapped in a forearc basin and did not reach the foreland. The indicators of sedimentary provenance for the foreland deposits are consistent with subduction of the Precordillera beneath western Gondwana, imbrication of basement rocks from either the Precordillera or Gondwana into an accretionary complex, and recycling of deformed Precordillera cover rocks.
The topographic data combined with information on structure, magmatism, seismicity, and paleomagnetism support a simple kinematic model for the late Cenozoic evolution of the central Andes. The model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning. The model interrelates Andean uplift, a changing geometry of the subducted Nazca plate, and a changing outline (in map view) of the leading edge of the S American plate.-from Author
P-T pseudosections were calculated in the system SiO2-TiO2-Al2O3-MgO-MnO-FeO-O-2-CaO-Na2O-K2O-H2O-CO2 with the PERPLE_X software package for the pressure-temperature range 1-25 kbar and 150-450 degrees C to gain a better understanding of the phase relations of metamorphosed calcareous sediments at low temperature including their dehydration behaviour during prograde metamorphism. For this purpose the applied thermodynamic data set of Holland and Powell, augmented by data of Massonne and Willner, was enlarged by end-member data for Mn-stilpnomelane. In addition, a three-component solid-solution model for stilpnomelane and a four-component model for Ca-Mg-Mn-Fe carbonate with calcite structure were introduced. For geotherms of 10-15 degrees C/km, which are typical for the metamorphism of rocks involved in accretionary wedge systems, a major dehydration event occurs at temperatures between 270 to 330 degrees C in both carbonate-free and calcareous greywackes. For an investigated marly limestone this event takes place at about 100 degrees C higher temperatures. The H2O-CO2 fluid formed is characterized by very low CO2 contents. The major dehydration event is made responsible for the detachment of sediments on top of a subducting slab.
The IUPAC-IUGS joint Task Group “Isotopes in Geosciences” recommends a value of (49.61 ± 0.16) Ga for the half life of 87Rb, corresponding to a decay constant λ87 = (1.3972 ± 0.0045) × 10-11 a-1.
The reigning paradigm for the formation and exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction
of crust to mantle depths and the return of crustal slices within the subduction channel—all at plate tectonic rates. Additional
processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study
of UHP terranes—for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism,
and exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into
the formation and exhumation of UHP terranes.
We investigated Lu-Hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c. 500 °C to biotite dehydration melting at c. 800 °C in the Sikkim Himalaya, India. Exceptionally precise Lu-Hf ages obtained for the Barrovian metasedimentary sequence in the Lesser Himalaya (LH) correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth. The youngest age is reported for the garnet zone ( ) and then the ages become progressively older in the staurolite ( ), kyanite ( ) and sillimanite ( ) zones. The oldest age of was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting, directly below the Main Central Thrust (MCT). These ages provide a tight constraint on the timing and duration of the Barrovian sequence formation which lasted about 6 Ma. The age pattern is clearly inverted with respect to structural depth but shows “normal” correlation with the metamorphic grade, i.e. earlier garnet growth in higher grade rocks.
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.
The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross-section through the ~80-meter thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures show that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved.
Abstract A-type orthogneisses of mid Neoproterozoic age (774 ± 6 Ma, U-Pb SHRIMP zircon age), are reported for the first time from the Grenvillian basement of the Western Sierras Pampeanas in Argentina. These anorogenic meta-igneous rocks represent the latest event of Rodinia break-up so far recognized in Grenvillian basement exposures across Andean South America. Moreover, they compare well with A-type granitoids and volcanic rocks along the Appalachian margin of Laurentia (Blue Ridge), thus adding to former evidence that the Western Sierras Pampeanas Grenvillian basement was left on the conjugate rifted margin of eastern Laurentia during Rodinia break-up and the consequent opening of the Iapetus ocean. Terra Nova, 18, 388–394, 2006
[1] The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ~12 kbar and 868 °C at 461 ± 1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional mid-crustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20-30 million years following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of syn-convergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ~436 Ma. The second migmatite event was regionally localized and occurred at ~700 °C and 12 kbar between 411-407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.
Tectonometamorphic zones were defined within the lower Paleozoic
basement of the NW Argentine Andes in a transitional zone between two
Andean segments of different geotectonic evolution. In the Cambrian, the
Pacific edge of Gondwana changed from a passive to an active continental
margin. This event began with folding of a Vendian/Eocambrian sediment
wedge (Puncoviscana Formation and equivalents). The effects can be
traced progressively over all structural levels with exposed depth
increasing from north to south. Phenomena of a second deformation are of
different nature and age but mostly characterized by shear belts causing
large-scale crustal imbrication. In the lower tectonic levels this phase
coincides with subduction-related magmatism of Ordovician age. A flat
subduction slab is supposed, somewhat steeper in the northern than in
the southern segment. The following anatectic-granitic magmatism and
weak deformation in the Devonian may have marked a new change to passive
margin conditions.
The Caucete Group and structurally overlying Pie de Palo Complex in western Argentina are characterised by two generations of west-verging folds and thrust-related shear zones, which formed under amphibolite facies conditions. The Caucete Group is separated from the Pie de Palo Complex by the Pirquitas thrust. These structures are interpreted to have formed as a result of a progressive deformation, generated during Middle Ordovician, underthrusting of the Laurentian-derived Cuyania microcontinent beneath the active Famatina margin. Geometrical relationships are most simply explained if the Pie de Palo Complex was basement to the Caucete Group prior to Ordovician orogenesis. We propose that this basement-cover relationship was established during Cambrian rifting of the Cuyania microcontinent from Laurentia. The Pirquitas fault may have been initiated during this extension prior to its long-lived remobilization as a thrust. We cannot rule out the possibility that the Pie de Palo Complex was exotic with respect to the Caucete Group, but for this to be possible we have to introduce an extra generation of structures, for which no evidence is preserved.The deformation was characterised by early strain localization followed by a more homogeneously distributed non-coaxial flow during F2. Thermal softening probably dominated over fabric softening during this stage.
In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone – 21 analyses: Th/U ≥0.023 and 337.0±2.7 Ma (2, combined ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U age); (2) diamond-bearing intermediate zone – 23 analyses: Th/U ≥0.037 and 336.8±2.8 Ma; and (3) rim zone – 12 analyses: Th/U = 0.015−0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥0.8) yielded (∼concordant) ages of ∼400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2) Ma was acquired from 113 analyses.
By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (∼1.8 GPa, 650°C) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050°C) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.
Electron-probe microanalysis (EPMA) is applicable to rare-earth elements (REE) in minerals with relatively high REE concentrations (e.g. hundreds of parts per million). However, given that each of the 14 REE has at least 12 X-ray lines in the L spectrum, finding peak-free regions for background measurement can be problematical. Also, measured peak intensities are liable to require correction for interferences. Hitherto, little attention has been paid to the optimisation of background offsets and the implications of the wide variation in REE distribution patterns in different minerals. The 'Virtual WDS' program, which enables complex multi-element spectra to be synthesised, has been used to refine the conditions used for different REE distributions. Choices include whether to use the L1 rather than the Lα1 line, background offsets, and counting times for comparable relative precision. Correction factors for interferences affecting peak and background measurements have also been derived.
This paper is included in the Special Publication entitled 'The proto- Andean margin of Gondwana', edited by R.J. Pankhurst and C.W. Rapela. A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Cordoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513±31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490±5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470-450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87Sr/ 86Sr (0.7075-0.7105) and low εNd 1 (-4.6 to -6.3), inherited from lower crust. Sm-Nd model ages of 1600-1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.