Ordovician Arc Research Articles

The deep crustal magnetic structure of Britain

The deep crustal magnetic structure of Britain has not previously been described in a uniform manner. We provide a new assessment of the deep crustal magnetic bodies responsible for the long wavelength magnetic features. The study area contains deep crustal relics of the destruction of early Palaeozoic oceanic lithosphere along the Thor-Tornquist Suture and primarily the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Spectral decomposition is applied to a merged onshore and offshore magnetic anomaly data set. Thirty idealised basement bodies are compared with a representation of the subsurface obtained by a coarse 3D inversion of the data. The central area separating Laurentia and Avalonia, is largely characterised by an absence of high susceptibilities throughout the whole crustal volume. We find that the idealised basement bodies are largely consistent with relatively high susceptibility zones at depths in excess of 10km. The zones of higher relative susceptibility are referenced to the tectonic-terrane framework of the area and possible geological explanations for the contrasts are reviewed. In the north, the Laurentian terranes are diverse, comprising crust first created in the Archaean (Hebridean Terrane), Palaeoproterozoic (Rhinns Terrane), Mesoproterozoic? (Midland Valley Terrane), Neoproterozoic (sub-Southern Upland rocks) and Ordovician. Magnetic anomalies further record the assembly of the Gondwanan (Eastern Avalonian) part of the country through Neoproterozoic and Ordovician (Tornquist) arc magmatism and accretion. The convergence zones between Laurentia, Avalonia and Baltica have all left a magnetic imprint, as has Variscan convergence to the south.

New zircon ages on the Cambrian–Ordovician volcanism of the Southern Gemericum basement (Western Carpathians, Slovakia): SHRIMP dating, geochemistry and provenance

The Southern Gemericum basement in the Inner Western Carpathians, composed of low-grade volcano-sedimentary rock complexes, constitutes a record of the polyphase Cambrian–Ordovician continental volcanic arc volcanism. These metavolcanic rocks are characterized by the enrichment in K, Rb, Ba, Th and Ce and Sm relative to Ta, Nb, Hf, Zr, Y and Yb that are the characteristic features for volcanic arc magmatites. The new SHRIMP U–Pb zircon data and compilation of previously published and re-evaluated zircon ages, contribute to a new constrain of the timing of the Cambrian–Ordovician volcanism that occurred between 496 and 447 Ma. The following peaks of the volcanic activity of the Southern Gemericum basement have been recognized: (a) mid-late Furongian at 492 Ma; (b) Tremadocian at 481 Ma; (c) Darriwilian at 464 Ma prolonged to 453 Ma within the early Upper Ordovician. The metavolcanic rocks are characterized by a high zircon inheritance, composed of Ediacaran (650–550 Ma), Tonian–Stenian (1.1–0.9 Ma), and, to a lesser extent, Mesoproterozoic (1.3 Ga), Paleoproterozoic (1.9 Ga) and Archaean assemblages (2.6 Ga). Based on the acquired zircon populations, it could be deduced that Cambrian–Ordovician arc crust was generated by a partial melting of Ediacaran basement in the subduction-related setting, into which old crustal fragments were incorporated. The ascertained zircon inheritances with Meso-, Paleoproterozoic and Archaean cores indicate the similarities with the Saharan Metacraton provenance.

Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints

In the early Cambrian Bithia Formation in the Variscan foreland of Sardinia, a Middle Ordovician granitic intrusion (478–457Ma) is hosted by marly metasedimentary rocks that were affected by high-temperature (HT) metamorphism. A detailed structural–petrographical transect was conducted through the granitic intrusion and its host rocks. Field data and relationships between HT/low-pressure (LP) mineral assemblages in the metasedimentary rocks (Grt+Wo+Ves in carbonate lenses and And in pelite) demonstrate that the study area was affected by a polyphase HT overprint (I: T=520–620°C at XCO2=0.1, P: 0.2–0.4GPa; and II: T=600–670°C at XCO2=0.1, P=0.2–0.4GPa) that pre-dates the Variscan tectonic, metamorphic, and igneous phases.In the Canigò or Canigou Massif (Eastern Pyrenees), the Somail Massif (Montagne Noire), and the Ruitor Massif (Internal Massifs, NW Alps), Middle Ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by Variscan amphibolite-to-granulite-facies metamorphism and subsequent Alpine low-grade metamorphism. Comparisons of associated igneous and metasedimentary rocks in the Sardinia foreland with the High-Grade Metamorphic Complex in the Variscan Axial Zone and the Canigou Massif indicate a convergent Middle Ordovician evolution that was overprinted by HT Variscan metamorphism.

Middle to Late Ordovician arc system in the Kyrgyz Middle Tianshan: From arc-continent collision to subsequent evolution of a Palaeozoic continental margin

New geological, geochronological and isotopic data reveal a previously unknown arc system that evolved south of the Kyrgyz Middle Tianshan (MTS) microcontinent during the Middle and Late Ordovician, 467–444Ma ago. The two fragments of this magmatic arc are located within the Bozbutau Mountains and the northern Atbashi Range, and a marginal part of the arc, with mixed volcanic and sedimentary rocks, extends north to the Semizsai metamorphic unit of the southern Chatkal Range. A continental basement of the arc, indicated by predominantly felsic volcanic rocks in Bozbutau and Atbashi, is supported by whole-rock Nd- and Hf-in-zircon isotopic data. εNd(t) of +0.9 to −2.6 and εHf(t) of +1.8 to −6.0 imply melting of Neo- to Mesoproterozoic continental sources with Nd model ages of ca. 0.9 to 1.2Ga and Hf crustal model ages of ca. 1.2 to 1.7Ga. In the north, the arc was separated from the MTS microcontinent by an oceanic back-arc basin, represented by the Karaterek ophiolite belt. Our inference of a long-lived Early Palaeozoic arc in the southwestern MTS suggests an oceanic domain between the MTS microcontinent and the Tarim craton in the Middle Ordovician.The time of arc-continent collision is constrained as Late Ordovician at ca. 450Ma, based on cessation of sedimentation on the MTS microcontinent, the age of an angular unconformity within the Karaterek suture zone, and the age of syncollisional metamorphism and magmatism in the Kassan Metamorphic Complex of the southern Chatkal Range. High-grade amphibolite-facies metamorphism and associated crustal melting in the Kassan Metamorphic Complex restricts the main tectonic activity in the collisional belt to ca. 450Ma. This interpretation is based on the age of a synkinematic amphibolite-facies granite, intruded into paragneiss during peak metamorphism. A second episode of greenschist- to kyanite–staurolite-facies metamorphism is dated between 450 and 420Ma, based on the ages of granitoid rocks, subsequently affected or not affected by this metamorphism. The latest episode is recorded by greenschist-facies metamorphism in Silurian sandstones and granodiorites and by retrogression of the older, higher-grade rocks. This may have occurred at the Silurian to Devonian transition and reflects reorganization of a Middle Palaeozoic convergent margin.Late Ordovician collision was followed by initiation of a new continental arc in the southern MTS. This arc was active in the Early Silurian, latest Silurian to Middle Devonian, and Late Carboniferous, whereas during the Givetian through Mississippian (ca. 385–325Ma) this area was a passive continental margin. These arcs, previously well constrained west of the Talas-Ferghana Fault, continued eastwards into the Naryn and Atbashi areas and probably extended into the Chinese Central Tianshan. The disappearance of a major crustal block with transitional facies on the continental margin and too short a distance between the arc and accretionary complex suggest that plate convergence in the Atbashi sector of the MTS was accompanied by subduction erosion in the Devonian or Early Pennsylvanian. This led to a minimum of 50–70km of crustal loss and removal of the Ordovician arc as well as the Silurian and Devonian forearcs in the areas east of the Talas-Ferghana Fault.

Laurentian and Amazonian sediment sources to Neoproterozoic–lower Paleozoic Maryland Piedmont rocks

Several terranes of variable tectonic affinity and history underlie the central Appalachian Piedmont Province (eastern United States). These terranes mostly consist of widespread metasedimentary and lesser metavolcanic rocks. Intense and pervasive deformation and metamorphism have made the depositional ages and provenance of sediment in these rocks difficult to determine. The lack of tight constraints on such basic information led to a century-long debate about the tectonic significance of these rocks, particularly how they correlate to similar rocks along and across strike in the Appalachian orogen. We address these issues using U/Pb isotopic ages from single spots in 2433 zircon grains from 18 metasedimentary rock samples distributed across the Maryland Piedmont. The resulting age signatures indicate that the Marburg Formation and Prettyboy Schist, heretofore assigned to the Westminster terrane, actually belong to the Potomac terrane, making the Hyattstown thrust the contact between the two terranes. Ediacaran Laurentia could have supplied all Potomac terrane sediment except for the detritus in one sample from the northern part of the terrane that likely came from Amazonia. This is one of the first recognitions of a Gondwana-derived terrane between Carolinia to the south and Ganderia to the north. Maximum depositional ages for Potomac terrane suprasubduction zone sedimentary rocks are latest Neoproterozoic or early Cambrian, and some may have been deposited ca. 510 Ma. Continental rifting ended ca. 560 Ma at the longitude of our study, so the transition from rifting to subduction at this location in eastern Laurentia may have lasted only 50 M.y. Lower Ordovician arc intrusions into these rocks demonstrate that the transition lasted no longer than 90 M.y. The Iapetan margin of central-eastern Laurentia was one of the shortest lived passive margins that formed in Neoproterozoic time.

Evolution processes of Ordovician–Devonian arc system in the South‐Kitakami Massif and its relevance to the Ordovician ophiolite pulse

AbstractThe South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.

From Large Zones to Small Terranes to Detailed Reconstruction of an Early to Middle Ordovician Arc–Backarc System Preserved Along the Iapetus Suture Zone: A Legacy of Hank Williams

The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during the Middle to Late Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of the composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites (481 to 477 Ma) with organized ridges indicated by the presence of well-developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This Laurentian crust fragment then formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc (473 to 468 Ma) underwent extensive rifting indicated by organized spreading in the Lloyds River backarc basin, which was floored by juvenile backarc ophiolitic crust (472 Ma). The establishment of the Darriwilian arc (467 to 462 Ma) was in part coeval with yet another stage of rifting. Darriwilian magmatism is characterised by significant along-strike variability, ranging from continental to primitive calc-alkaline arc to tholeiitic backarc-like magmatism. The diversity of Darriwilian magmatism can be attributed to fragmentation and magmatic reworking of Laurenian-derived basement along strike in the same arc undergoing disorganized spreading. The development of the AAT is interpreted to be similar to that of the modern Izu – Bonin – Mariana arc in the western Pacific.SOMMAIRELa bande d’accretion d’Annieopsquotch (AAT) est constituee d’un empilement de chevauchements de l’Ordovicien precoce a moyen, et de terranes d’arc et d’arriere-arc qui se sont accretes a la marge composite laurentienne japetienne a l’Ordovicien moyen a tardif. Les faits geologiques releves portent a penser que les terranes constitutifs de l’AAT se sont constitues a l’exterieur de la marge laurentienne dans un arc d’extension qui a subi de multiples episodes de rifting avant son accretion. L’initiation du magmatisme de l’AAT a mene au developpement de zones d’ophiolites de supra-subduction du Tremadocien au Floien (481 Ma a 477 Ma), avec des cretes ordonnees mises en evidence par la presence de complexes de tapis de dikes bien developpes. Ce centre d’extension s’est propage a travers un fragment de la croute laurentienne, et l’a ultimement separe de la marge composite laurentienne. Et, du Floien au Darriwilien, ce fragment de croute laurentienne a servi de substratum au magmatisme d’arc de l’AAT. Au Floien (473 Ma a 468 Ma), cette zone d’arc a subi un important rifting, comme l’indique la distension ordonnee du bassin d’arriere-arc de Lloyds River, lequel a servi de semelle a une croute ophiolitique d’arriere-arc (472 Ma). La mise en place de l’arc au Darriwilien (467 Ma a 462 Ma) a coexiste pour un temps avec un autre episode de rifting. Le magmatisme darriwilien est caracterise par une variabilite de composition importante parallelement a sa direction, passant d’une composition d’arc continental a celle d’arriere-arc primitif calco-alcalin jusqu’a une composition de magmatisme de type tholeiitique d’arriere-arc. La diversite du magmatisme darriwilien peut etre attribuee a la fragmentation et au remaniement magmatique de la croute d’origine laurentienne parallelement a la direction d’un meme arc subissant une distension desordonnee. Nous proposons que le developpement de l’AAT a ete similaire a celui de l’arc moderne Izy–Bonin–Marianne du Pacifique occidental.

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.

Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Perú (Tarma province)

High-pressure conditions of 11–13kbar/500–540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm–Nd mineral-whole rock isochron at 465 ± 24Ma dates fluid influx at peak temperatures of ∼600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12–13 °C/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Marañon Complex. Here, low PT conditions at 2.4–2.6 kbar, 300–330 °C were estimated for a phyllite–greenschist assemblage representing a contrasting metamorphic geotherm of 32–40 °C/km characteristic for a magmatic arc environment.

Subaqueous eruption-fed mass-flow deposits: Records of the Ordovician arc volcanism in the northern Famatina Belt; Northwestern Argentina

This study is focused on the analyses of a Chaschuil section (27° 49′ S–68° 04′ W), north of the Argentina Famatina Belt, where Ordovician explosive-effusive arc volcanism took place under subaerial to subaqueous marine conditions. In analyzing the profile, we have recognized an Arenigian succession composed by dominant volcaniclastic lithofacies represented by volcaniclastic debris flow, turbidity current and minor resedimented syn-eruptive pyroclastic depositsand lavas. The upper portions of succession are represented by volcanogenic sedimentary lithofacies with fossiliferous levels. Great volumes of the volcaniclastic deposits are strongly controlled in their transport by mass flow processes. These representative deposits provide significant data in relation to the coeval volcanic events for recognizing a continuous explosive volcanism together a minor effusive activity and the degradation of volcanic edifices. Likewise mass flow deposits give indications of the high rate of sedimentation, strong slope control and instability episodes in the basin, typical of those volcanic environments. That substantial information was the key to understand the features and evolution of the Arenigian basin in the north of the Famatina System.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc- alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U-Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic-volcanoclastic succession belong- ing to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in con- junction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean- type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle-Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.

Low-grade metamorphism of Cambro-Ordovician successions in the Famatina belt, Southern-Central Andes: Burial-inversion history linked to the evolution of the proto-Andean Gondwana margin

The metamorphic P-T conditions of low-grade units from the Famatina belt, Central Andes of Ar¬gentina, were estimated through petrography, X-ray diffraction, and electron microscopy. For the Middle-Upper Cambrian Negro Peinado Formation a tectono-metamorphic event associated with intense intrafoliar folding, with estimated temperatures between 290 and 400°C (KICIS: 0.16-0.27Δº2θ, biotite blastesis and compositional homo¬geneity in dioctahedral micas) and intermediate pressure conditions (white mica b parameter: 9.010Å-9.035Å), was recognized. The Achavil Formation (Middle-Upper Cambrian) presents a main metamorphic event associated with temperatures between 200 and 290°C (KICIS: 0.26-0.41Δº2θ) and intermediate- to low-pressure conditions (white mica b parameter values: 8.972Å-9.017Å). Some illitic substitution in dioctahedral micas also indicates lower metamorphic grade than the Negro Peinado Formation. For Upper Cambrian to Middle Ordovician sequences a burial metamorphic pattern, with a progressive decrease in metamorphic grade from Volcancito Formation to Cerro Morado Group (ca. 490-465 Ma; KICIS: 0.31-0.69Δº2θ) and absence of tendency changes linked to strati¬graphic discontinuities was proposed. Mica and chlorite are the main phyllosilicates in the oldest units, while Ilt/Sme (R3) mixed-layer is almost the only one in the youngest. White mica b parameter indicates intermediate- to low-pressure conditions for all these sequences. This burial metamorphic pattern presents a marked break as the youngest Ordovician unit (La Aguadita Formation, after ca. 452 Ma) records higher metamorphic conditions (IKCIS: 0.28-0.19Δº2θ) than units from the Ordovician arc, with estimated temperatures between 270 and 330ºC and intermediate-pressure conditions. Our results indicate that basin contraction and inversion processes related to the Ordovician Ocloyic Orogeny involved at least two well-discriminated and not superposed metamorphic episodes in this region.

Collisional accretion of a Late Ordovician oceanic island arc, northern Tasman Orogenic Zone, Australia

A distinctive Late Ordovician volcano-sedimentary terrane, embracing the Carriers Well Formation and Everetts Creek Volcanics and dismembered slivers now structurally intercalated in the adjoining Wairuna Formation, is located within the Broken River Province of the northern Tasmanides. It abuts a basement of mafic–ultramafic rocks (Gray Creek Complex) and overlying Early Ordovician deep marine sedimentary and volcanic strata (Judea Formation) which host tonalitic plutons. The terrane lies at the western, inboard-margin of the Camel Creek Subprovince, a broad tract of multiply deformed mid-Paleozoic turbidites with minor basalt and chert variously interpreted as the infill of a backarc basin or an accretionary wedge. U–Pb dates from detrital zircon indicate a maximum Late Silurian age for siliciclastic rocks from the previously undated Wairuna Formation. Geochemistry of volcanic rocks from the volcano-sedimentary terrane show them to be largely of mafic to intermediate compositions of calc-alkaline affinity, comparable with broadly coeval Macquarie Arc volcanic suites of the southern Tasmanides. Trace-element systematics identify a subduction relationship for the volcanic suite and V/Ti employed as a discrimination tool identifies the terrane as representing an oceanic island arc, consistent with its sedimentary facies which include volcaniclastic mass flow deposits, limestone, and radiolarian chert. Continent-derived sandstone in the sedimentary assemblage, confirmed by the ages of detrital zircon from a sandstone sample from the Carriers Well Formation, indicates that the oceanic island arc developed proximal to the Late Ordovician continental margin of East Gondwana. Its nature and location bear on the tectonic setting of the entire Camel Creek Subprovince, for which interpretation as an Early Silurian–Early Devonian accretionary wedge is favoured. Collision of the island arc with the continental margin, and associated deformation of part the intervening oceanic crustal tract, now represented by the Gray Creek Complex and its sedimentary cover (Judea Formation) registers the initiation of subduction accretion in late Early Silurian (Llandoverian) time. It marks early-stage orogenesis in the Broken River Province, accurately timed by stratigraphic relationships in the basinal succession developed in the Graveyard Creek Subprovince immediately to the west of the arc assemblage. Tectonism was regionally developed in north Queensland at this time, coeval with the Benambran Orogeny of the Lachlan Orogen in which the Macquarie Arc was likewise accreted to the East Gondwana margin. Benambran orogenesis marks a general phase of shortening, and removal by subduction, of oceanic crust and inversion of continent-derived overlying sedimentary cover along the East Gondwana margin.

Dynamics of accretion of arc and backarc crust to continental margins: Inferences from the Annieopsquotch accretionary tract, Newfoundland Appalachians

The Annieopsquotch accretionary tract comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the Laurentian margin of Iapetus during Middle to Upper Ordovician. Geological relationships suggest that the constituent terranes of the Annieopsquotch accretionary tract initially formed outboard of a peri-Laurentian Dashwoods microcontinent in an extensional arc, but occupied a lower plate setting with respect to Dashwoods during accretion. Metamorphic mineral assemblages indicate that the terranes were underplated beneath the composite Laurentian margin at depths ranging from ~ 3 km up to > 18 km. We infer the accretion of the terranes to be controlled by the brittle–ductile transition in the hydrated crust. The decoupling of brittle from ductile crust resulted in very high aspect ratios of the terranes, which comprise thin (< 5 km) but very large (up to 25 × 250 km) slabs of supracrustal arc rocks and ophiolite crust. Arc basement and ophiolitic mantle are not preserved and were either underplated at a greater depth or subducted and recycled back in the mantle. The accreted crust forms a reasonable approximation to bulk continental crust requiring little post-accretionary modification; hence, the accretion of arc–backarc complexes which occupy a lower plate setting can form an important mechanism for creation of new continental crust required to balance crustal loss at convergent margins.

Isotopic Age Constraints and Metamorphic History of the Talladega Belt: New Evidence for Timing of Arc Magmatism and Terrane Emplacement along the Southern Laurentian Margin

Abstract U‐Pb and 40Ar/39Ar dates from the Talladega belt, southernmost Appalachians, provide insight into the timing and nature of pre‐Alleghanian tectonism in this region. Low‐grade metasedimentary rocks of the Talladega belt represent the outermost preserved portions of the southern Laurentian margin, thus recording the earliest orogenic events that affected the margin, in addition to later overprinting events. These rocks are structurally overlain by the Hillabee Greenstone metavolcanic sequence, from which metadacite yields an age of 470±4 Ma (ion microprobe U‐Pb zircon age). Hillabee geochemistry indicates formation in an arc or back‐arc setting. We suggest correlation of the Hillabee Greenstone with 460–470‐Ma arc‐related rocks in the Dahlonega gold belt, which extends along strike with the Hillabee through Georgia and into North Carolina, and interpret the Hillabee as the southernmost volcanic expression of an Early‐Middle Ordovician arc that formed outboard of Laurentia. Rocks of the Talladega be...

Late Paleozoic tectonic history of the Ertix Fault in the Chinese Altai and its implications for the development of the Central Asian Orogenic System

The Central Asian Orogenic System (CAOS) is one of the largest Phanerozoic accretionary orogens in the world and may represent a signifi cant site of continental growth. Its origin has been explained by two competing models: syn-subduction strikeslip duplication of a single (>1000 km) long-lived arc (ca. 630‐360 Ma) or collision of multiple arcs and micro-continents. Central to the debate are the relative roles of syn-subduction strike-slip faulting versus thrusting. In both models, the Ertix fault fi gures prominently, either as a roof fault of a large strike-slip duplex system developed during oceanic subduction or as a suture of arc-continent or continent-continent collision. In order to differentiate between the above models, we conducted fi eld mapping, detailed kinematic analysis, and geochronological dating of the Ertix fault zone in the Chinese Altai. Our work indicates that the fault is a crustal-scale thrust that was active in the Permian. Its hanging wall records two pulses of magmatism ca. 450 Ma and ca. 280 Ma and experienced peak pressure and temperature of 6.2‐7.7 kbar and 560‐ 670 °C. Our geologic observations, together with the existing geologic information, favor a tectonic model that involves two episodes of subduction below the Altai arc: fi rst, in the Ordovician, along a south-dipping subduction zone; and second, in the late Carboniferous and early Permian along northdipping subduction of the Junggar ocean. It was during the latter event that a melange complex was underplated below the older Ordovician arc, metamorphosed at lower crustal depths, and then exhumed to the upper crust along the south-directed Ertix thrust zone.

Junee – Narromine Volcanic Belt, Macquarie Arc, Lachlan Orogen, New South Wales: components and structure

The Junee – Narromine Volcanic Belt of Ordovician volcanic, volcaniclastic and intrusive rocks in central New South Wales is the most westerly structural belt of the now disrupted Macquarie Arc. Although more than 200 km long (far longer than other belts of Ordovician arc volcanics), most of this belt is concealed by younger Palaeozoic to Holocene cover. Most of our knowledge of this belt thus comes from scattered outcrops and drillhole information, augmented by the interpretation of aeromagnetic and gravity data and, to a lesser extent, information from deep seismic-reflection profiling. This combined geological and geophysical approach suggests that the Junee – Narromine Volcanic Belt consists of major separate igneous complexes, each consisting of lavas and volcaniclastic sediments intruded by various mafic to felsic intrusive porphyries. Variations in age and geochemical affinities are used to suggest that the Junee – Narromine Volcanic Belt approximates most closely the magmatic core of the Macquarie Arc. The present geometry of the Junee – Narromine Volcanic Belt reflects the interplay of two major ductile – brittle fault systems: the north-northwest-trending Gilmore Fault System mainly in the south and the north-trending Tullamore Fault System, mainly in the north and centre, together with local west-northwest cross faults. These systems were generated during deformation in the Early Silurian, Early Devonian and Carboniferous.

Resolution of regional fluid flow related to successive orogenic events on the Laurentian margin

Seven localities distributed along a 180-km-long outcrop within the UK sector of the Laurentian margin were sampled, and all showed two phases of authigenic K-feldspar. The first phase was precipitated from a cool fluid of moderate salinity, and the second phase was precipitated from a hotter, more saline fluid. Ar-Ar ages for the two episodes of authigenesis are ca. 470 Ma and ca. 432 Ma. The data can be interpreted to represent two phases of regional-scale fluid migration related to Ordovician arc accretion and Silurian continent-continent convergence, corresponding to the Grampian and Scandian episodes of the Caledonian orogeny, respectively.

Shrimp U-Pb evidence for a Late Silurian age of metasedimentary rocks in the Merrimack and Putnam-Nashoba terranes, eastern New England

U-Pb ages of detrital, metamorphic, and magmatic zircon and metamorphic monazite and titanite provide evidence for the ages of deposition and metamorphism of metasedimentary rocks from the Merrimack and Putnam-Nashoba terranes of eastern New England. Rocks from these terranes are interpreted here as having been deposited in the middle Paleozoic above Neoproterozoic basement of the Gander terrane and juxtaposed by Late Paleozoic thrusting in thin, fault-bounded slices. The correlative Hebron and Berwick formations (Merrimack terrane) and Tatnic Hill Formation (Putnam-Nashoba terrane), contain detrital zircons with Mesoproterozoic, Ordovician, and Silurian age populations. On the basis of the age of the youngest detrital zircon population (∼425 Ma), the Hebron, Berwick and Tatnic Hill formations are no older than Late Silurian (Wenlockian). The minimum deposition ages of the Hebron and Berwick are constrained by ages of cross-cutting plutons (414 ± 3 and 418 ± 2 Ma, respectively). The Tatnic Hill Formation must be older than the oldest metamorphic monazite and zircon (∼407 Ma). Thus, all three of these units were deposited between ∼425 and 418 Ma, probably in the Ludlovian. Age populations of detrital zircons suggest Laurentian and Ordovician arc provenance to the west. High grade metamorphism of the Tatnic Hill Formation soon after deposition probably requires that sedimentation and burial occurred in a fore-arc environment, whereas time-equivalent calcareous sediments of the Hebron and Berwick formations probably originated in a back-arc setting. In contrast to age data from the Berwick Formation, the Kittery Formation contains primarily Mesoproterozoic detrital zircons; only 2 younger grains were identified. The absence of a significant Ordovician population, in addition to paleo-current directions from the east and structural data indicating thrusting, suggest that the Kittery was derived from peri-Gondwanan sources and deposited in the Fredericton Sea. Thus, the Kittery should not be considered part of the Laurentian-derived Merrimack terrane; it more likely correlates with the early Silurian Fredericton terrane of northeastern New England and Maritime Canada.

Evidence of Early to Middle Ordovician arc volcanism in the Cordillera Oriental and Altiplano of southern Peru, Ollantaytambo Formation and Umachiri beds

Volcanic rocks of intermediate to predominantly mafic composition are exposed in the Cordillera Oriental and Altiplano of southern Peru in two localities: (1) the Ollantaytambo Formation, which crops out near Cusco, and (2) the Umachiri beds in the Departamento de Puno. The most notable unit in the Ollantaytambo Formation is an approximately 100 m thick succession of lapilli tuffs that originated by hydroclastic fragmentation. These volcanic rocks are of calc-alkaline basaltic to andesitic composition. The tuffs are overlain by fine-grained volcaniclastic sandstones and shales, which grade into quartz-rich turbidites. The Umachiri beds consist of monomict lapilli tuffs with varying lapilli abundance and size, formed by hydroclastic fragmentation, and volcaniclastic sandstones. The rocks have a uniform basaltic andesite composition and tholeiitic affinity. Chondrite-normalized element patterns of both volcanigenic units show a moderate enrichment of LREE and a weak negative Eu anomaly. Pronounced negative Nb and Ta anomalies indicate magma evolution in an arc environment. In contrast, the overlying epiclastic sandstones and shales in the Ollantaytambo Formation reflect upper crustal sources. Our data indicate that the deposition of the lapilli tuffs of the Ollantaytambo Formation and Umachiri beds likely was connected to the westward-lying Ordovician magmatic arc on the Arequipa Massif. If the Ordovician active margin in southern Peru had the same east-dipping subduction polarity as the active Gondwana margin in Argentina and Chile, the sedimentary basin in which the Ollantaytambo Formation and Umachiri beds were deposited was in a backarc position.