Time scales and mechanisms of growth of active margins of Gondwana: A model based on detrital zircon ages from the Neoproterozoic to Cambrian Blovice accretionary complex,Bohemian Massif

New U–Pb detrital zircon ages from (meta-)graywackes of the Blovice accretionary complex, Bohemian Massif, provide an intriguing record of expansion of the northern active margin of Gondwana during late Neoproterozoic and Cambrian. The late Neoproterozoic (meta-)graywackes typically contain a smaller proportion of Archean and Paleoproterozoic zircons and show a 1.6–1.0 Ga age gap and a prominent late Cryogenian to early Ediacaran age peak. The respective zircon age spectra match those described from other correlative Cadomian terranes with a West African provenance. On the other hand, some samples were dominated by Cambrian zircons with concordia ages as young as 499 Ma. The age spectra obtained from these samples mostly reflect input from juvenile volcanic arcs whereas the late Cambrian samples are interpreted as representing relics of forearc basins that overlay the accretionary wedge.The new U–Pb zircon ages suggest that the Cadomian orogeny, at least in the Bohemian Massif, was not restricted to the Neoproterozoic but should be rather viewed as a continuum of multiple accretion, deformation, magmatic and basin development events governed by oceanic subduction until late Cambrian times. Our new U–Pb ages also indicate that the Cadomian margin was largely non-accretionary since its initiation at ~ 650–635 Ma and that most of the material accreted during a short time span at around 527 Ma, closely followed by a major pulse of pluton emplacement. Based on the new detrital zircon ages, we argue for an unsteady, cyclic evolution of the Cadomian active margin which had much in common with modern Andean and Cordilleran continental-margin arc systems. The newly recognized episodic magmatic arc activity is interpreted as linked to increased erosion–deposition–accretion events, perhaps driven by feedbacks among the changing subducted slab angle, overriding plate deformation, surface erosion, and gravitational foundering of arc roots. These Cadomian active-margin processes were terminated by slab break-off and/or slab rollback and by a switch from convergent to divergent plate motions related to opening of the Rheic Ocean at around 490–480 Ma.The proposed tectonic evolution of the Teplá–Barrandian unit is rather similar to that of the Ossa Morena Zone in Iberia but shows significant differences to that of the North Armorican Massif and Saxothuringian unit in Western and Central Europe. This suggests that the Cadomian orogenic zoning was complexly disrupted during early Ordovician opening of the Rheic Ocean and Late Paleozoic Variscan orogeny so that the originally outboard tectonic elements are now in the Variscan orogen's interior and vice versa.     

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif,Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.     

Mineral chemical and geochronological constraints on the age and provenance of the eastern Circum-Rhodope Belt low-grade metasedimentary rocks, NE Greece

In north-eastern Greece the mid-greenschist facies Makri Unit and the anchizonal Melia Formation belong to the eastern Circum-Rhodope Belt that forms the uppermost tectonostratigraphic unit of the Rhodope metamorphic nappe pile. The two metasedimentary successions had different source areas, although they now lie in close proximity in the Rhodope Massif. The U-Pb isotopic ages of detrital zircons from a metasandstone of the Makri Unit analysed using LA-SF-ICP-MS and SHRIMP-II gave age clusters at ca. 310-290 Ma and at ca. 240 Ma for magmatic zircons, which may have been derived from Carboniferous-Permian basement rocks of the Thracia Terrane (Lower Tectonic Unit of the Rhodope Massif) that subsequently underwent Triassic rifting. The youngest detrital zircon grains found so far indicate that the metasedimentary succession of the Makri Unit, or at least parts of it, cannot be older than Late Triassic. By contrast, clastic sedimentary rocks of the Melia Formation contain the primary detrital mineral assemblage of epidote, zoisite, garnet, and phengitic mica, which is absent in the Makri Unit, and clearly points to metamorphic rocks being the major source for these sediments. U-Pb analyses of detrital zircons gave a prominent age cluster at ca. 315-285 Ma for magmatic zircons. Inherited cores indicate the involvement of Pan-African and Late Ordovician-Early Silurian crustal sources during Late Carboniferous-Early Permian igneous event(s). Moreover, U-Pb detrital zircon geochronology indicates that the Melia Formation cannot be older than latest Middle Jurassic. We suggest that the Melia Formation was deposited in front of a metamorphic nappe pile with Rhodopean affinities in Tithonian or Cretaceous times. Both the Makri Unit and the Melia Formation have been tectonically juxtaposed from different sources to their present location during Balkan and Alpine orogenic processes.     

http://www.sciencedirect.com/science/article/pii/S1674987113001333

The basement of the Romanian Carpathians is made of Neoproterozoic to early Paleozoic periGondwanan terranes variably involved in the Variscan orogeny,similarly to other basement terrains of Europe.They were hardly dismembered during the Alpine orogeny and traditionally have their own names in the three Carpathian areas.The Danubian domain of the South Carpathians comprises the Dragsan and Lainici-Paius peri-Amazonian terranes.The Dragsan terrane originated within the ocean surrounding Rodinia and docked with Rodinia at ~800 Ma.It does not contain Cadomian magmatism and consequently it is classified as an Avalonian extra-Cadomian terrane.The Lainici-Paius terrane is a Ganderian fragment strongly modified by Cadomian subduction-related magmatism.It is attached to the Moesia platform.The Tisovita terrane is an ophiolite that marks the boundary between Dragsan and Lainici-Paius terranes.The other basement terranes of the Romanian Carpathians originated close to the Ordovician NorthAfrican orogen,as a result of the eastern Rheic Ocean opening and closure.Except for the Sebes-Lotru terrane that includes a lower metamorphic unit of Cadomian age,all the other terranes(Bretila,Tulghes,Negrisoara and Rebra in the East Carpathians,Somes,Biharia and Baia de Aries in the Apuseni mountains,Fagaras,Leaota,Caras and Pades in the South Carpathians) represent late Cambrian—Ordovician rock assemblages.Their provenance,is probably within paleo-northeast Africa,close to the Arabian-Nubian shield.The late Cambrian-Ordovician terranes are defined here as Carpathian-type terranes.According to their lithostratigraphy and origin,some are of continental margin magmatic arc setting,whereas others formed in rift and back-arc environment and closed to passive continental margin settings.In a paleogeographic reconstruction,the continental margin magmatic arc terranes were first that drifted out,followed by the passive continental margin terranes with the back-arc terranes in their front.They accreted to Laurussia during the Variscan orogeny.Some of them(Sebes-Lotru in South Carpathians and Baia de Aries in Apuseni mountains) underwent eclogite-grade metamorphism.The Danubian terranes,the Bretila terrane and the Somes terrane were intruded by Variscan granitoids.     

U–Pb LA-SF-ICP-MS zircon geochronology of the Serbo-Macedonian Massif, Greece: palaeotectonic constraints for Gondwana-derived terranes in the Eastern Mediterranean

The Pirgadikia Terrane in northern Greece forms tectonic inliers within the Vardar suture zone bordering the Serbo-Macedonian Massif to the southwest. It comprises Cadomian basement rocks of volcanic-arc origin and very mature quartz-rich metasedimentary rocks. U–Pb laser ablation sector-field inductively-coupled plasma mass spectrometry analyses of detrital zircons from the latter reveal a marked input from a Cadomian–Pan-African source with minor contribution from Mesoproterozoic, Palaeoproterozoic and Archaean sources. The metasedimentary rocks are correlated with Ordovician overlap sequences at the northern margin of Gondwana on the basis of their maturity and zircon age spectra. The Pirgadikia Terrane can be best interpreted as a peri-Gondwana terrane of Avalonian origin, which was situated close to the Cadomian terranes in the Late Neoproterozoic–Early Palaeozoic, very much like the Istanbul Terrane. The second unit investigated is the Vertiskos Terrane, which constitutes the major part of the Serbo-Macedonian Massif in Greece. It comprises predominantly igneous rocks of Silurian age and minor metasedimentary rocks of unknown age and provenance. U–Pb analyses of detrital zircons from a garnetiferous mica schist of the Vertiskos Terrane indicate derivation from 550 to 1,150 Ma-old source rocks with a major Cadomian peak. This, combined with minor input of >1,950 Ma-old zircons and the absence of ages between ca. 1.2 and 1.7 Ga suggests a NW Africa source. The protolith age of the garnetiferous mica schist is presumably Early Ordovician. One sample of garnet-bearing biotite gneiss, interpreted as meta-igneous rock, comprises predominantly subhedral zircons of igneous origin with late Middle Ordovician to Silurian ages. We suggest that the rock association of the Vertiskos Terrane is part of an ancient active-margin succession of the Hun superterrane, comparable to successions of the Austro- and Intra-Alpine Terranes. The new data of this study provide evidence of occurrences of Avalonia- and Armorica-derived terranes in the Eastern Mediterranean and moreover help to clarify palaeogeographic reconstructions for the peri-Gondwana realm in the Early Palaeozoic.     

Palaeozoic to Early Jurassic history of the northwestern corner of Gondwana,and implications for the evolution of the Iapetus,Rheic and Pacific Oceans

The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic.     

Refining the age of magmatism in the Altos Cuchumatanes,western Guatemala,by LA–ICPMS,and tectonic implications

The Altos Cuchumatanes Range is made up of a core of igneous and metamorphic rocks, surrounded by lower Palaeozoic and Mesozoic sedimentary strata. These units constitute the westernmost exposure of basement rocks in Guatemala and represent some of the most important crustal units in the Maya Block. New laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon geochronology allows better definition of their igneous ages, inheritance and petrologic evolution. The Altos Cuchumatanes magmatism occurred during the Middle Ordovician (461 Ma) and lower Pennsylvanian (312–317 Ma), replicating similar age trends present in southern Mexico (Acatlán Complex) and the Maya Block, from Chiapas to central Guatemala (Rabinal-Salamá area) and Belize (Maya Mountains). The U-Pb inheritance from cores of the studied zircons makes it possible to decipher the pre-magmatic history of the area. During the Late Ordovician to Permo-Carboniferous, the Altos Cuchumatanes and Maya Block were located adjacent to northeastern Mexico, near the Mixteco terrane, where Ordovician megacrystic granites intruded a passive-margin sedimentary sequence. The Ordovician granites present at the southern limit of the Maya Block, in the Altos Cuchumatanes, in central Guatemala and in Belize, are the result of partial crustal melting during the initial opening of the Rheic Ocean, when both Maya and Mixteco terranes would have lain close to NW Gondwana until the closure of that ocean. The crystallization of the early Pennsylvanian granites seems to be the result of an E-dipping subduction zone that accommodated convergence between Laurentia and Gondwana.     

Pan-African magmatism in the Menderes Massif: geochronological data from leucocratic tourmaline orthogneisses in western Turkey

The Menderes Massif, exposed in western Anatolia, is a metamorphic complex cropping out in the Alpine orogenic belt. The metamorphic rock succession of the Massif is made up of a Precambrian basement and overlying Paleozoic-early Tertiary cover series. The Pan-African basement is composed of late Proterozoic metasedimentary rocks consisting of partially migmatized paragneisses and conformably overlying medium- to high-grade mica schists, intruded by orthogneisses and metagabbros. Along the southern flank of the southern submassif, we recognized well-preserved primary contact relationship between biotite and leucocratic tourmaline orthogneisses and country rocks as the orthogneisses represent numerous large plutons, stocks and vein rocks intruded into a basement of garnet mica schists. Based on the radiometric data, the primary deposition age of the precursors of the country rocks, garnet mica schist, can be constrained between 600 and 550?Ma (latest Neoproterozoic). The North Africa–Arabian-Nubian Shield in the Mozambique Belt can be suggested as the possible provenance of these metaclastics. The intrusion ages of the leucocratic tourmaline orthogneisses and biotite orthogneisses were dated at 550–540?Ma (latest Neoproterozoic–earliest Cambrian) by zircon U/Pb and Pb/Pb geochronology. These granitoids represent the products of the widespread Pan-African acidic magmatic activity, which can be attributed to the closure of the Mozambique Ocean during the final collision of East and West Gondwana. Detrital zircon ages at about 550?Ma in the Paleozoic muscovite-quartz schists show that these Pan-African granitoids in the basement form the source rocks of the cover series of the Menderes Massif.     

Nature and evolution of the lower crust under central Spain: Inferences from granulite xenoliths (Calatrava Volcanic Field-Spanish central system)

So far, the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System (SCS). In recent years, ultramafic volcanics from the Calatrava Volcanic Field (CVF) have also provided deep-seated crustal xenoliths which have not been studied in detail. Our data, combining mineral, whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths, highlight the felsic composition of the lower crust under central Iberia. A number of the Calatrava xenoliths represents Variscan igneous protoliths, which are a minor population in the SCS, and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny (leucodiorite protolith age of 314 ± 3 Ma and leucogranite protolith age of 308 ± 2.5 Ma). U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas. These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue, which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions. The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana. These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component, ultimately extracted from an Eburnean crust. Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.     

West African provenance for Saxo-Thuringia (Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana? – U/Pb-SHRIMP zircon evidence and the Nd-isotopic record

Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data.     

Re-interpreting the Devonian ophiolites involved in the Variscan suture: U–Pb and Lu–Hf zircon data of the Moeche Ophiolite (Cabo Ortegal Complex,NW Iberia)

New U–Pb zircon data of a mylonitic greenschist from the Moeche Ophiolite, one of the mafic units involved in the Variscan suture in the Cabo Ortegal Complex (NW of the Iberian Massif), yielded an age of 400 ± 3 Ma. Consequently, this unit can be considered one of the Devonian ophiolites, the most extended group of oceanic units in the Variscan belt. The mafic rocks show transitional compositions between N-MORB and island-arc tholeiites, although Lu–Hf isotope signatures of its zircons clearly indicate contribution from an old continental source. εHf values in the analysed zircons are negative (generally below εHf = ?5), and thence, they are not compatible with their generation from a juvenile mantle source. Accordingly, the igneous protoliths were generated in a setting where juvenile mafic magmas interacted with an old continental crust. The Devonian ophiolites from the Variscan suture have been repeatedly interpreted as remnants of the Rheic Ocean. However, the presence of a continental source in the origin of the mafic rocks of the Moeche Ophiolite allows discarding an intraoceanic setting for their generation, at least for the NW Iberian counterparts. The tectonic setting for the Devonian ophiolites of NW Iberia is very likely represented by an ephemeral oceanic basin opened within a continental realm. Herein, the real Rheic Ocean suture could only be located west of the terrane represented by the upper units of the allochthonous complexes. Apparently that suture is not represented in NW Iberia.     

Cambro-Silurian magmatisms at the northern Gondwana margin (Penninic basement of the Ligurian Alps)

The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in the Variscan blocks incorporated in the Ligurian Alps because they experienced low temperature Alpine metamorphism. During the Paleozoic, these blocks, together with the other Alpine basements, were placed between the Corsica-Sardinia and the Bohemian Massif along the northern margin of Gondwana. In this framework, they host several a variegated lithostratigraphy forming two main complexes(Complexs I and II) that can be distinguished by both the protoliths and their crosscutting relationships, which indicate that the acidic and mafic intrusives of Complex II cut an already folded sequence made of sediments, basalts and granitoids of Complex I. Both complexes were involved in the Variscan orogenic phases as highlighted by the pervasive eclogite-amphibolite facies schistosity(foliation II). However, rare relicts of a metamorphic foliation at amphibolite facies conditions(foliation I)is locally preserved only in the rocks of Complex I. It is debatable if this schistosity was produced during the early folding event e occurred between the emplacement of Complex I and II e rather than during an early stage of the Variscan metamorphic cycle.New SHRIMP and LA ICP-MS Ue Pb zircon dating integrated with literature data, provide emplacement ages of the several volcanic or intrusive bodies of both complexes. The igneous activity of Complex I is dated between 507 ± 15 Ma and 494 ± 5 Ma, while Complex II between 467 ± 12 Ma and 445.5 ± 12 Ma.The folding event recorded only by the Complex I should therefore have occurred between 494 ± 5 Ma and 467 ± 12 Ma. The Variscan eclogite-amphibolite facies metamorphism is instead constrained between ~420 Ma and ~300 Ma. These ages and the geochemical signature of these rocks allow constraining the Early Paleozoic tectono-magmatic evolution of the Ligurian blocks, from a middleeupper Cambrian rifting stage, through the formation of an Early Ordovician volcanic arc during the Rheic Ocean subduction, until a Late Ordovician extension related to the arc collapse and subsequent rifting of the PaleoThetys. Furthermore, the ~420-350 Ma ages from zircon rims testify to thermal perturbations that may be associated with the Silurian rifting-related magmatism, followed by the subduction-collisional phases of the Variscan orogeny.     

Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)

The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.     

内蒙古西乌旗罕乌拉地区下二叠统寿山沟组碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

中亚造山带南缘二叠纪的构造背景一直存在争议。以内蒙古西乌旗罕乌拉地区发育的寿山沟组为研究对象,开展了野外地质、岩石学、碎屑锆石LA-ICP-MS U-Pb年龄研究。2个样品锆石阴极发光图像和Th/U值指示,锆石为岩浆成因锆石。103颗碎屑锆石年龄测试结果显示,年龄信息可划分为5组:285~328Ma,峰期年龄302Ma;338~361Ma;455~490Ma;757Ma;1278Ma、2380Ma。最年轻的年龄为285Ma,结合侵入其中的花岗岩同位素年龄,指示西乌旗罕乌拉地区寿山沟组沉积时限介于285~280Ma之间,主体沉积时代应为早二叠世Sakmarian期—Artinskian期。寿山沟组碎屑锆石反映出近源、快速沉积的特点,沉积物源中含有较多的火山碎屑物,可能代表弧后盆地沉积,为早二叠世古亚洲洋闭合前洋壳俯冲消减作用的沉积响应。结合区域资料,寿山沟组碎屑锆石的年龄对应于东北地区的变质基底及其后的构造岩浆事件,物源区物质主要来自于苏尼特左旗—锡林浩特—西乌旗一带早石炭世末—晚石炭世岩浆弧及贺根山—东乌旗一带,并进一步限制了华北与西伯利亚两大板块的缝合线应位于寿山沟组发育地区的南部,即索伦缝合带,拼合时代最可能为晚二叠世—早三叠世。     

How far did the Cadomian ʽterranesʼ travel from Gondwana during early Palaeozoic? A critical reappraisal based on detrital zircon geochronology

In this paper, laser ablation ICP-MS U–Pb detrital zircon ages are used to discuss provenance and early Palaeozoic palaeogeography of continental fragments that originated in the Cadomian–Avalonian active margin of Gondwana at the end of Precambrian, were subsequently extended during late Cambrian to Early Ordovician opening of the Rheic Ocean, and finally were incorporated into and reworked within the European Variscan belt. The U–Pb detrital zircon age spectra in the analysed samples, taken across a late Neproterozoic (Ediacaran) to Early/Middle Devonian metasedimentary succession of the southeastern Teplá–Barrandian unit, Bohemian Massif, are almost identical and exhibit a bimodal age distribution with significant peaks at about 2.1–1.9 Ga and 650–550 Ma. We interpret the source area as an active margin comprising a cratonic (Eburnean) hinterland rimmed by Cadomian volcanic arcs and we suggest that this source was available at all times during deposition. The new detrital zircon ages also corroborate the West African provenance of the Teplá–Barrandian and correlative Saxothuringian and Moldanubian units, questioned in some palaeogeographic reconstructions. Finally, at variance with the still popular concept of the Cadomian basement units as far-travelled terranes, we propose that early Palaeozoic basins, developed upon the Cadomian active margin, were always part of a wide Gondwana shelf and drifted northwards together before involvement in the Variscan collisional belt.     

中国东北麻山杂岩晚泛非期变质的锆石SHRIMP年龄证据及全球大陆再造意义

中国东北地区佳木斯地块南部麻山杂岩正、副片麻岩 7个样品的锆石 SHRIMP年龄数据首次明确地表明 ,东北地区存在 500 Ma的晚泛非期高级变质作用事件。峰期麻粒岩相变质导致柳毛地区 (502± 10)Ma (2σ )深熔花岗岩的形成。正、副片麻岩变质年龄的一致性表明它们已在变质前发生了构造叠置。西麻山副片麻岩中含有在后期麻粒岩相变质过程中未重结晶的碎屑锆石，由此形成从协和一致的 550 Ma到弱不一致 1 900 Ma的较大 207Pb/206Pb年龄变化范围，表明其原岩具有从新元古代到中元古代-古元古代的年龄。柳毛地区变质的片麻状闪长岩中所含的古老锆石的 207Pb/206Pb年龄为 546～ 1 460 Ma表明，该闪长岩大约在 1 400 Ma就位，并受到 500 Ma变质事件的影响，从而说明柳毛地区存在中元古代基底。然而，与以前的认识相反，麻山杂岩不存在具有太古宙基底的同位素证据。晚泛非期变质事件年龄的确定对重塑晚前寒武纪-显生宙早期麻山杂岩和佳木斯地块的古地理位置具有重要意义。根据目前获得的有关证据，认为佳木斯地块可能曾经位于冈瓦纳大陆北缘的华北克拉通附近。     

Characterisation of igneous terranes by zircon dating: implications for UHP occurrences and suture identification in the Central Rhodope, northern Greece

The occurrence of ultra high pressure (UHP) and high pressure (HP) relicts associated with oceanic material suggests the presence of a suture zone within the Rhodope Massif. Characterisation of the accreted igneous terranes and their relationship with the UHP/eclogite occurrences provide new constraints on the location of this suture. Single-zircon evaporation and sensitive high-resolution ion microprobe dating of orthogneiss protoliths define two groups of intrusion ages: Permo-Carboniferous and Late Jurassic–Early Cretaceous. Structurally, the Late Jurassic gneissic complex overthrust a unit with Permo-Carboniferous orthogneisses. A “melange zone” marked by mylonites, eclogites, amphibolites, and UHP micaschists separates these two units. We interpret these observations in terms of two distinct igneous terranes, the Thracia (Permo-Carboniferous) and Rhodope (Late Jurassic) terranes, separated by the Nestos suture, and assembled during the closure of an oceanic basin of the Tethys. Geochemically, the Late-Jurassic rocks are akin to subduction magmatism, possibly the same subduction that caused the UHP metamorphism of metasediments within the “melange zone”. Observed UHP–HP relicts are restricted to the tectonic contact zone, suggesting that a single subduction/collison event can explain the occurrences of UHP relicts and eclogites in the Central Rhodope, and that subducted rocks are exhumed only within the Nestos suture.     

Detrital zircon ages from a Lower Ordovician quartzite of the İstanbul exotic terrane (NW Turkey): evidence for Amazonian affinity

NW Turkey is a mosaic of several continental and oceanic units, that were amalgamated by collisional and strike–slip tectonics. One of the continental units is the İstanbul terrane, the Palaeozoic strata of which comprise an Early Ordovician to Early Carboniferous transgressive sedimentary sequence. The basement to the Palaeozoic succession is exposed in the Bolu Massif and is represented by a thick meta-volcanic unit intruded by Late Neoproterozoic (ca. 575 Ma) felsic magma. This meta-igneous basement is considered to be a sliver of the Cadomian magmatic arc. Unconformably overlying Early Ordovician strata start with red fluvial clastics (ca. 3,000 m), overlain by shallow marine quartzites (<250 m). Detrital zircons were separated from an Ordovician quartzite sample located 5 m above the contact with the underlying red fluvial clastics. We have dated a total of 99 zircon grains by U–Pb method using laser ablation-sector field-inductively coupled plasma mass spectrometer (LA-SF-ICP-MS). The ages (97% are >90% concordant) range from 530 to 2550 Ma. The dominance (44 grains) of Neoproterozoic zircon ages (around 540, 570, 600–640 and 700–800 Ma) rules out any link of the İstanbul terrane with Baltica. The large amount (40 grains) of Grenvillian and Meso- to Palaeo-proterozoic ages (around 1.0–1.2, 1.3–1.6, 1.7–1.8, and 1.9–2.1 Ga) in the zircon population further rules out a link to Cadomian and Minoan terranes, but is indicative for Amazonian source areas. The new age data suggest the İstanbul terrane to be a peri-Gondwanan terrane that was located at the northwestern margin of Gondwana close to Amazonia. This terrane should have travelled a large distance in the Phanerozoic times.     

拉萨地体东南缘始新世早期变质作用及其构造意义

本文对位于青藏高原拉萨地体东南缘林芝杂岩中的片麻岩进行了岩石学和锆石U-Pb年代学研究.所研究的样品包括正片麻岩和副片麻岩,它们经历了中压角闪岩相变质作用.岩石地球化学分析结果表明,所研究的正片麻岩的原岩具有钙碱性岛弧岩浆岩的特征.锆石U-Pb年代学分析结果表明,副片麻岩中的碎屑锆石核部为岩浆成因,它们给出的206Pb/238U年龄范围为3012～ 522Ma,其锆石的增生边给出了～51Ma的变质年龄.在正片麻岩中,黑云母片麻岩给出了～67Ma的原岩结晶年龄和～ 55 Ma的变质年龄;石榴石角闪黑云斜长片麻岩给出了～58Ma的原岩结晶年龄和～54Ma的变质年龄.因此,所研究的林芝杂岩并不能代表拉萨地体中的前寒武纪变质基底,而是古生代的沉积岩和晚白垩纪至早新生代的岩浆岩在始新世早期变质而成.这一时期,表壳岩和侵入岩一起经历的中压角闪岩相变质作用很可能跟新特提斯洋俯冲导致的地壳增生、加厚有关.     

秦岭造山带东段秦岭岩群的年代学和地球化学研究

对东秦岭地区的陕西省洛南县、宁陕县、长安县和河南省淅川县出露的四个秦岭岩群变质岩进行的岩石学和地球化学研究表明,样品主要由变质火山岩和变质沉积岩组成.详细的锆石U-Pb定年结果显示三个正变质岩均形成于新元古代早期(971～843Ma),而副变质岩中富集大量新元古代碎屑锆石,根据最年轻的谐和年龄(859Ma)和早古生代的变质年龄,推测其沉积时代为新元古代中晚期.因此,北秦岭南部的秦岭岩群的变质岩主要由新元古代早期的火成岩和新元古代中晚期的沉积岩组成.变质作用主要发生在加里东期,局部有燕山期的变质作用叠加.指示北秦岭的造山作用主要发生在早古生代.岩石地球化学研究还显示秦岭岩群的新元古代火山岩均形成于火山弧构造环境,沉积岩沉积于大陆弧-活动大陆边缘环境,指示秦岭造山带在新元古代早期是一个火山弧.秦岭岩群的火山岩和沉积岩在形成时代和构造环境方面与扬子克拉通西缘的特征非常相似,表明位于北秦岭造山带的秦岭岩群应归属于扬子克拉通陆块,是扬子北缘的一个大陆边缘弧.