Paleozoic Evolution Research Articles

Zircon U–Pb geochronology and Hf isotopic composition of the Hongqiyingzi Complex, northern Hebei Province: New evidence for Paleoproterozoic and late Paleozoic evolution of the northern margin of the North China Craton

The Hongqiyingzi Complex, located at the northern margin of the North China Craton, was previously defined as a Paleoproterozoic unit. It consists of volcanic and sedimentary rocks, which underwent greenschist- to amphibolite-facies metamorphism. This study presents zircon U–Pb ages and Hf isotopic composition and whole-rock Nd isotopic composition of metamorphosed igneous and sedimentary rocks from the Hongqiyingzi Complex. The age dating results show that different gneisses of this complex contain mainly late Paleoproterozoic (~ 1800 Ma) and late Paleozoic (~ 275 to ~ 300 Ma) zircons and minor late Archean zircons. Late Paleozoic zircons either occur as magmatic grains or as metamorphic overgrowth and both types of zircon can be distinguished by their Th/U ratios and internal structures. Most zircon grains of a Paleozoic magmatic rock (sample Hq14) have high ε Hf(t) values of 1.5 to 10.8, suggesting significant contribution of mantle or juvenile crustal material to the magma source(s). The results imply that the Hongqiyingzi Complex contains Precambrian supracrustal sequences and Paleozoic rocks of magmatic origin that underwent strong metamorphism and deformation during the late Paleozoic related to the closure of the Paleo-Asian Ocean between Siberia and the North China Craton.

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

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

Silurian clastic sediments in the North Qilian Shan, NW China: Chemical and isotopic constraints on their forearc provenance with implications for the Paleozoic evolution of the Tibetan Plateau

The North Qilian Shan is an accretion–collision orogenic belt in the northeastern margin of the Tibetan Plateau that connects the North China plate to the north with the Tethyan orogenic assemblage to the south. Its Early Paleozoic temporal–spatial tectonic evolution has been extensively studied, but the nature and tectonic setting of its Silurian deposition is poorly understood. Our new petrological analysis suggests that the clast composition of fan conglomerates is complex. In the Danbanshan and Zhongdabanshan sections, abundant lenticular conglomerate beds contain mainly subrounded–rounded metamorphic clasts; whereas the Tongziba and Haichaoba sections contain volcanic, chert, limestone, ultramafic and granitoid clasts. However, in the Sunan area, clast types are relatively simple, dominated by basalt and granitoids, and at Tongziba gabbro and basalt clasts are subrounded and angular, indicating two different types of provenance. Ultramafic, limestone and abundant chert clasts in the Haichaoba and Tonghe conglomerates indicate an ophiolitic source, whereas subrounded–rounded quartzite and granitoid gneiss clasts were probably derived from the Central Qilian basement. Geochemical data from the volcanic and granitoid clasts suggest an arc-related source and their zircon U–Pb ages of 515–429 Ma suggest that the arc magmatism did not end in the Mid-Silurian. Sandstone petrology and detrital modes suggest that the Silurian sediments were deposited in a forearc basin. Paleogeographic reconstructions inferred from NNW–NNE paleocurrent data indicate that an island arc was present off the southern margin of the Silurian basin, and that southward subduction of oceanic crust between North China and the Qilian Shan was active or continued in the Silurian.

Paleozoic and early mesozoic magmatism manifestations in the early Precambrian structure of the South Yenisei Ridge

This study characterizes some issues of the Paleozoic and Mesozoic tectonomagmatic evolution of Precambrian structures from the southwestern margin of the Siberian craton. The relationship between the Devonian and Triassic magmatic events is demonstrated from the example of the Severnaya rift-related structure, South Yenisei Ridge. U-Pb SHRIMP dating yielded ages of 387 ± 5 Ma for leucogranites and 240 ± 3 Ma for the overlying alkaline trachytes. These ages show good agreement with Ar-Ar geochronological data (392–387 Ma) obtained for micas from paragneisses and leucogranite dykes of the Yenisei suture zone, the extension of which is superimposed by the studied rift-related structure. The previous geological evidence and the Devonian age estimate first obtained for magmatic rocks of the Yenisei Ridge allow us to interpret the studied leucogranites as products of Devonian continental rifting, similar to volcanic and intrusive rocks of the North Minusa depression and Agul graben. Like other localities within the western margin of Siberian craton, the formation of Triassic alkaline rocks may be related to the Siberian superplume activity.

Paleosols in Late Moscovian (Carboniferous) Marine Carbonates of the East European Craton Revealing "Great Calcimagnesian Plain" Paleolandscapes

Abstract Late Paleozoic strata commonly include lithologic cycles (“cyclothems”), driven by sea-level fluctuations that repeatedly flooded and exposed broad expanses of epeiric shelves. Such successions, commonly with multiple unconformities, are well documented in Permo-Pennsylvanian sedimentary successions, but remain less understood in the East European Craton (EEC), part of the Euramerican continental shelf rim. This study documents unconformities and paleosols from the Podolskian to Myachkovian (middle Pennsylvanian) epeiric-carbonate succession of the EEC and explores the non-actualistic soil types, landscapes, and the character and density of vegetation in this Pennsylvanian calcimagnesian dryland environment. Thin (mostly < 1 m) paleosols developed at seven successive unconformities consist of an upper terrestrial clay layer or “topclay”, a crust of beta calcrete beneath the topclay, and a weakly karsted substrate limestone with rare rhizocretions. Solution pits and small (< 3 m deep) karst sinkholes are rare. Nontruncated topclays commonly consist of two layers: in situ clayey epipedons and resedimented terrestrial clays. The latter are interpreted as playa sediments. The Myachkovian unconformities of the northernmost localities studied, near Arkhangelsk, lack beta calcrete and root traces, and the limestone beneath these unconformities is sepiolitized. In Podolskian paleosols of the southern Moscow Basin, the topclays are predominantly palygorskitic. The Myachkovian paleosol clays are smectitic–illitic with occasional chlorite admixture. The majority of late Moscovian paleosols surveyed formally conform to lithic (rendollic) haplocalcids within the aridisol class or rendzic calcic leptosols. A detailed study of a representative Podolskian paleo-pedon reveals development of shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter in the form of covalently bound organomineral complexes, and is fulvate-dominated resembling organic matter from extant dryland soils. The palygorskitic composition of Podolskian topclays and shallow (< 30 cm) pedogenic carbonates is interpreted to reflect hot, well-drained semidesert conditions with precipitation less than 300 mm/yr. The transition from palygorskitic to smectitic soil clays across the Podolskian–Myachkovian boundary, apparently deeper soil carbonate, and an increase in the degree of karstification record a general increase in humidity such that annual precipitation may have exceeded 300–400 mm. The available data reconstruct the “great calcimagnesian plain” landscapes formed during low sea level, supposedly glacial phases, across tens of thousands of square kilometers of the EEC. This non-actualistic system apparently lacked a fluvial network and was covered by uniform soils and playas. The primary productivity of this early (310–305 Ma old) dryland ecosystem inferred from high TOC and rhizogenic beta calcretes was not markedly different from present-day steppe or savanna grasslands and semideserts. In the northernmost outcrops surveyed, near Arkhangelsk, drier conditions probably limited colonization by higher plants. The available data open a “hidden chapter” in the Paleozoic evolution of terrestrial ecosystems which is not recorded by paleobotany and urges modifications in the concept of low-productivity character of pre-Miocene dry landscapes.

The origin of the Baydaric microcontinent, Mongolia: Constraints from paleomagnetism and geochronology

Existing views on the tectonic evolution of the Central Asian orogenic belt (CAOB) are highly controversial and the Neoproterozoic to Cambrian stages of this evolution remain the most enigmatic. However, the views on the Paleozoic evolution of the CAOB crucially depend on these early stages, as different choices of the starting point lead to very dissimilar Paleozoic reconstructions. In this context numerous microcontinents with the Precambrian basement that are included in the mosaic structure of Kazakhstan, Tien Shan, Altai and Mongolia are of particular interest. We undertook a paleomagnetic, geochemical and geochronological study of the Neoproterozoic volcanics from one of these units — the Baydaric microcontinent in Central Mongolia. According to U–Pb (laser ablation) dating the age of the studied Dzabkhan Volcanics is about 770–805 Ma. Thermal demagnetization revealed that most of the studied samples retained a pre-tilting component, whose primary origin is supported by a conglomerate test. These new data, together with available geological information allow us to conclude that about 770–800 Ma ago the Baydaric domain was located at a latitude of 47 − 12 + 16 ° N and belonged to one of the following plates: India, South China, Tarim or Australia.

Age constraints on Late Paleozoic evolution of continental crust from electron microprobe dating of monazite in the Peloritani Mountains (southern Italy): another example of resetting of monazite ages in high-grade rocks

In situ monazite microprobe dating has been performed, for the first time, on trondhjemite and amphibolite facies metasediments from the Peloritani Mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the Hercynian Belt. All samples show single-stage monazite growth of Hercynian age. One migmatite and one biotitic paragneiss yielded monazite ages of 311 ± 4 and 298 ± 6 Ma, respectively. These ages fit with previous age determinations in similar rocks from southern Calabria, indicating a thermal metamorphic peak at about 300 Ma, at the same time as widespread granitoid magmatism. The older of the two ages might represent a slightly earlier event, possibly associated with the emplacement of an adjacent trondhjemite pluton, previously dated by SHRIMP at 314 Ma. No evidence for pre-Hercynian events and only a little indication for some monazite crystallization starting from ca. 360 Ma were obtained from monazite dating of the metasediments, suggesting either a single-stage metamorphic evolution or a significant resetting of the monazite isotope system during the main Hercynian event (ca. 300 Ma). Rare monazite from a trondhjemite sample yields evidence for a late-Hercynian age of about 275 Ma. This age is interpreted as representing a post-magmatic stage of metasomatic monazite crystallization, which significantly postdates the emplacement of the original magmatic body.

Geochemistry and tectonic significance of the Stony Mountain gabbro, North Carolina: Implications for the Early Paleozoic evolution of Carolinia

Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia. The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO 2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas. Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~ 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.

Late Proterozoic–Paleozoic evolution of the Arctic Alaska–Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska–Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska–Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700–540 Ma, with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440–400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic–Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska–Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska–Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska–Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

Paleozoic evolution of the External Crystalline Massifs of the Western Alps

The External Crystalline Massifs (ECMs) of the Alps record, during the Paleozoic, the progressive closure of oceanic domains between Gondwana, Armorica and Avalonia in three contrasting tectonic domains. The eastern one shows the Early Devonian closure of the Central-European Ocean between Armorica and Gondwana along a northwest dipping subduction zone. The western domain is marked by Lower Ordovician rifting followed by Mid-Devonian obduction of the back-arc Chamrousse ophiolite. The central domain underwent Late Devonian to Dinantian extension in a back arc setting associated with southeast dipping subduction of the Saxo-Thuringian Ocean. Based on tectonostratigraphic correlations, we propose that the western domain shows an affinity to the Barrandian domain while the eastern and central domains correspond to the north-eastward extension of the Moldanubian zone, to the south of the present-day Bohemian Massif. From Mid-Carboniferous to Permian, the eastern and central domains of the ECMs, including the internal parts of the Maures Massif, Sardinia and Corsica were stretched towards the south-west along the ca. 1500 km long dextral ECMs shear zone preceding the opening of the Palaeo-Tethys ocean.

The Cambrian pre-Uralide-Timanide orogen: Structural evidence for its collisional origin

At present, there are two alternative standpoints on the northeastern fringing of the Late Precambrian– Early Paleozoic evolution of the East European Platform: accretionary and collisional. According to the accretionary concept [1], the Timan margin of the Baltia continent (Precambrian framework of the East European Platform) successively amalgamated island arcs, microcontinents, relicts of basins underlain by the oceanic crust, and other structures. Contrary to this concept, other researchers, myself included, share the collisional hypothesis, according to which the Precambrian–Cambrian transition was marked by collision between the Late Precambrian Timan passive margin of the Baltia continent with the Bolshezemel’skaya active margin of the Arctida continent [2]. The Pechora– Ilych–Chiksha zone of faults traceable in the basement of the Pechora Plate and its continuations in the Barents Sea shelf and Kvarkush Anticlinorium in the Urals represent a relict of the collisional suture (Fig. 1). The main structural consequence of collision was the formation of the Cambrian collisional divergent orogen of pre-Uralides–Timanides, the axial part of which is marked by the suture. The southwestern limb of the orogen is largely composed of dislocated sedimentary complexes of the Late Precambrian passive margin of the Baltia continent, which were thrust over its adjacent areas, while the northeastern limb is mostly represented by complexes of the Bol’shezemel’skaya active margin of Arctida and its rear zones that were involved into large-amplitude overthrust displacements toward the latter. This communication is dedicated to structural analysis of relict complexes formerly constituting the pre-Uralides–Timanides of the Timan– Pechora–Barents Sea region and neighboring areas (Fig. 2). Relicts of the southwestern limb of the orogen. The structural paragenesis of pre-Uralide–Timanide complexes constituting the southwestern limb of the orogen has been studied in the Varanger, Srednii, and Rybachii peninsulas. Some information on the structure of the pre-Uralides–Timanides in the southwestern limb of the orogen can be derived from the structural analysis of the Kanin Kamen Range and the near-Timan part of the Pechora Plate basement based on seismic materials. In all these areas, the pre-Uralide–Timanide complexes are characterized by a distinct southwestern vergence of fold–thrust dislocations and the approximately NW– SE strike of mesostructural linear elements (hinges of different-scale asymmetrical folds, mineral and deformation-related linearity).

Paleozoic tectonic evolution of the Yili Block, western Chinese Tianshan

Abstract The Yili block is a triangular area bordered by sutures and fault zones in the western Chinese Tianshan belt. It is often considered as a part of the Central Tianshan micro-continent with Proterozoic basement extending westward into Kazakhstan and Kirgizstan, but this interpretation is questionable. This paper aims to synthesize the available data, discusses the meaning of the tectonic boundaries and proposes a model for the Paleozoic evolution of the Yili block. Alike the entire Tianshan belt, the Yili block underwent a polyphase evolution including subduction of oceanic crust and collision with micro-continents and volcanic arcs. The southern boundary of the Yili block is formed of Proterozoic basement and Early Paleozoic platform sediments, tectonically overlain by oceanic high-pressure metamorphic rocks and ophiolite. It has been involved in a south-dipping subduction associated with the closure of the Tianshan Ocean and the subsequent collision with a micro-continent correlated with Central Tianshan. This tectonic event resulted in top-to-the-north ductile thrusting observed in oceanic HP metamorphic rocks and Proterozoic basement as well. During the Late Paleozoic, the northern boundary of the Yili block was an active continental margin related to the southward subduction of the North Tianshan oceanic basin, this boundary is represented by Late Carboniferous turbidite and ophiolitic mélange. The southern and northern boundaries have been both reworked by Permian strike-slip faults.

Vendian stage in formation of the Early Caledonian superterrane in Central Asia

Granitoids and metamorphic rocks of the Baidarik basement block of the Dzabkhan microcontinent are studied in terms of geology, geochronology (U-Pb dating of zircon microfractions and individual grains) and Nd isotopic-geochemical systematics. As is established, the formation history of metamorphic belt (disthene-sillimanite facies) in junction zone of the Baidarik block and Bayankhongor zone of the Late Riphean (∼665 Ma) ophiolite association characterizes development of the Vendian (∼560–570 Ma) active continental margin. The high-P metamorphic rocks of that time span evidence formation of structures with the Earth’s crust of considerable thickness. In Central Asia, events of the Vendian low-gradient metamorphism are established also in the Tuva-Mongolian massif, Kan block of the East Sayan Mountains, and South Chuya inlier of the Caledonides in the Altai Mountains. Based on these data, it is possible to distinguish the Late Baikalian stage in development of the Early Caledonian superterrane of Central Asia, which antedated the subsequent evolution of this structure during the Late Cambrian-Ordovician. The high-gradient metamorphism that affected most intensively the southeastern part of the Baidarik block can be correlated with the Early Paleozoic (525–540 Ma) evolution of active continental margin and associated development of the Vendian oceanic basins and island arcs of the Ozernaya zone.

The Late Paleozoic structural evolution of the Magnitogorsk Megazone in the southern Urals

Various models of the evolution of the southern Urals at the stage of the Late Paleozoic collision of the East European and Kazakhstan continents have been proposed [3‐5, 7, and others]. The diversity of geodynamic models is caused by poor knowledge of regional tectonics in terms of structural geology. The results of regional structural and tectonophysical studies in the Magnitogorsk Megazone allowed us to recognize and characterize two stages of the Late Paleozoic (postDevonian) tectonic deformations: (1) thrusting in the Middle Carboniferous and (2) strike-slip faulting in the left-lateral transpression regime in the Middle Carboniferous‐Permian. Middle Carboniferous thrusting. The most intense tectonic deformations were confined to the flanks of the megazone within the Main Ural and East Magnitogorsk boundary fault zones, where the high-amplitude counter-dipping listric thrust faults were formed at a depth of 25‐30 km (Figs. 1, 2). The Late Paleozoic thrust structure of the Main Ural Fault Zone is characterized by nappes of higher orders deformed into NNEstriking antiforms and synforms. The westward verging reverse and thrust faults of the second generation are conjugated with the folds. The identification of these structural elements is difficult because of the distortion of the East Magnitogorsk Fault Zone by late strike-slip faults. They are represented by fragments of bivergent imbricate reverse and thrust faults retained in shear slices. The inner sectors of the Magnitogorsk Megazone are characterized by thin submeridional thrust and reverse faults with an amplitude of a few kilometers, which are accompanied by subordinate transfer faults and folds of mainly the open type (Figs. 1‐3a). The transfer faults bound the thrust and reverse fault zones, as well as imbricate packets therein, and serve as

New porcellioidean gastropods from early Devonian of Royal Creek area, Yukon Territory, Canada, with notes on their early phylogeny

This paper presents a description of new gastropods belonging to the superfamily Porcellioidea (Vetigastropoda) from the richly diverse Lower Devonian gastropod fauna of the Road River Formation in the Royal Creek area, Yukon Territory. This fauna belongs to Western Canada Province of the Old World Realm. The Pragian speciesPorcellia(Porcellia)yukonensisn. sp. andPorcellia(Paraporcellia) sp. represent the oldest presently known members of subgeneraPorcellia(Porcellia) andPorcellia(Paraporcellia). Their simple shell ornamentation fits well with an earlier described evolutionary trend in shell morphology of the Porcellinae. Late Pragian to early EmsianPerryconcha pulchran. gen. and n. sp. is the first member of the Porcellioidea bearing a row of tremata on adult teleoconch whorls. The occurrence of this shell feature in the Porcellioidea is additional evidence that the evolution of the apertural slit was much more complicated than has been proposed in classical models of Paleozoic gastropod evolution.

High-pressure rocks in the serpentinite melange of the Chara zone, Eastern Kazakhstan: Geochemistry, petrology, and age

The nature of the protolith of high-pressure rocks occurring as exotic blocks in serpentinite melange in the Chara ophiolite belt, Eastern Kazakhstan, is identified based on petrological and geochemical evidence. The distribution of incompatible HFSE in these rocks suggests that their protoliths were similar to E- and N-MORB tholeiites and, more rarely, to OIB alkaline basalts. This testifies to the activity of plume magmatism in the Early Paleozoic evolution of the Paleoasian Ocean. The Ar-Ar ages of phengite and Na-amphibole from the rocks date the subduction/exhumation of high-pressure rocks in the Chara ophiolite belt at the Late Ordovician (450 ± 5 Ma).

The Olivos Olistostrome: Remnant of a Late Permian Oceanic Basin along the Southwestern Margin of Laurentia, Chihuahua, Mexico

We report on the finding of an olistostrome containing Late Permian basalts (257 ± 7 Ma K-Ar, whole-rock) with MORB affinity embedded within deep-water Jurassic sediments (Upper Bajocian—Middle Callovian) in south-central Chihuahua, northern Mexico, at the southwestern margin of the North American craton (Laurentia). The largest Paleozoic outcrops in northern Mexico occur in the Chihuahua, Caborca, and Cortez terranes; the Los Olivos area is a small outlier with no discernible relation with either of those outcrops. Study of the Sierra de Olivos, in south-central Chihuahua, offers key elements to decipher the Late Paleozoic and Mesozoic evolution of the southern North American craton. Elements of oceanic crust in the olistostrome of Los Olivos are massive and pillowed basalts. Their chemistry shows that they are K-poor (K2O ~0.1%) and TiO2 rich (~0.8%); they are extremely depleted in elements such as Rb, Sr, and Zr, and trace-element compositions place them in the N-MORB and IAT fields of tectonic discrimination diagrams. The basalts, together with associated granitic tectonic blocks exhibiting a well-developed mylonitic texture, lie at the southern margin of Laurentia, and south of the inferred trace of the Mojave-Sonora megashear. They also lie along one of the inferred traces of the Ouachita suture. The Permian basaltic slabs were tectonically emplaced within the Middle Jurassic sedimentary sequence, but we infer that they were deposited as gravity slides. Their contacts with less competent rocks were reactivated, facilitating tectonic displacement during the Hidalgoan orogeny. The deep-marine Middle Jurassic sequence is informally assigned to the Pelayo Formation, which is covered by deep-water Cretaceous sedimentary facies of the Mezcalera Formation and by Tertiary volcanic rocks of the Sierra Madre Occidental magmatic arc. If the radiometric age determination of the oceanic basalts correctly dates the emplacement of these lavas, our study provides evidence on the existence of a Late Paleozoic—Mesozoic oceanic plate, subducted collision of either Gondwanic or Pacific elements with Laurentia, or during accretion of the Guerrero terrane (e.g., the Mezcalera plate). Characteristics of the markedly epiclastic Middle Jurassic units that host the olistostrome supports the assumption of an active margin setting for Jurassic deposition at the southern extreme of the Laurentia craton.

Re-deciphering the tectonic jigsaw puzzle of northern Eurasia

Northern Eurasia consists of the East European, Siberian, North China, and Alai-Tarim cratons, fragments of the supercontinent Rodinia, and the orogens of the Baikalides, Timanides, Uralides, Altaids, and Mongolides. These can be collectively classified as the Central Asian supercollage. The Baikalides and Timanides host Meso- and Neoproterozoic magmatic arc terranes that were sutured with the adjacent East European and Siberian cratons in the end of the Neoproterozoic. The Paleozoic part of the supercollage consists of three almost synchronous and subparallel Neoproterosoic to Paleozoic magmatic arc and turbidite superterranes, as well as overlap assemblages, bent into the world’s largest oroclines. Analysis of their structural pattern, supported by paleontological, lithological, and paleomagnetic data, indicates that these superterranes might have been produced via formation of arc–backarc systems at the margin of combined North China, East European and Siberian cratons and then deformed during Paleozoic westward-directed strike-slip translation between the clockwise rotating Siberian and eastward moving North China cratons. It is proposed that this development took place against the respective breakup of the above-mentioned cratons from the northern and southern margins of Eastern Europe in the Neoproterozoic, initially as a group of cratons called Nena, which reassembled in late Paleozoic to early Mesozoic times into Laurasia, part of the new supercontinent Pangea. In Mesozoic–Cenozoic times, the subduction-related continental growth of northern Eurasia continued in the Nipponide, Kamchatka and Kolyma–Alaska orogenic collages of the northern Circum-Pacific, which consist of Paleozoic to Cenozoic turbidite to island arc superterranes and overlap assemblages, generally younging towards the Pacific oceanic plate and also severely oroclinally bent. It is proposed that terranes of the Kolyma–Alaska and Kamchatka collages were translated westward, dextrally relative to Siberia, whereas Nipponides were translated northward, relative to North China, similarly to the better constrained Mesozoic–Cenozoic reconstructions of southeastern Asia. The two groups of collages started to collide along the Mongol-Okhotsk suture zone in the south of the Siberian craton in the end of the Mesozoic and then continued to collide along the presently active plate boundary at the island of Sakhalin. The proposed scenario suggests similarities in Paleozoic evolution of the Central Asian and Mesozoic–Cenozoic evolution of the northern Circum-Pacific supercollages, both possibly formed in response to westward subduction and related strike-slip translation of the (Paleo)-Pacific oceanic plates. The individual superterranes might have been consequently translated for as much as 4000–6000 km and oroclinally bent during such translation or/and rotation of the adjacent cratons.

Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their implications on the geodynamic evolution of Chinese Tianshan Belt

In order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NW of China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples showing a weak and unstable magnetic remanence, the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite. In the study area, though positive fold test has been observed on the Early Carboniferous rocks, a general re-magnetization of these rocks has been identified and attributed to the Late Carboniferous magmatism. Moreover, all Early and Late Carboniferous samples from the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity. The Late Carboniferous (C 2) is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to ∼ 310 Ma. The C 2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with A 95 = 6.4° and n = 15. The Late Carboniferous rocks located close to a deformation zone present a consistent magnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P 2) red beds in the interior of the Yili Block with also a solo reverse magnetic polarity, the P 2 paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with A 95 = 11.3° and n = 5. Keeping important uncertainties in mind, comparisons of the C 2 and P 2 paleomagnetic poles of the Yili Block with available coeval poles of Junggar, Tarim and Siberia indicate (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2° ± 15.1° and the 31.6° ± 15.1° counterclockwise rotations of the Yili–Junggar blocks with respect to Tarim and Siberia took place during C 2 to P 2. These rotations are accommodated by the Permian dextral strike–slip faults along the northern and southern sides of Tianshan Belt and sinistral strike–slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively.

U-Pb geochronologic evidence for the evolution of the Gondwanan margin of the north-central Andes

We investigated the Neoproterozoic–early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon analysis and dating of syn- and post-tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45–0.65 Ga and 0.9–1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474–442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician.