U/Pb zircon ages of basement gneisses and discordant felsic dykes from Vestranden,westernmost Baltic Shield and central Norwegian Caledonides

Four new U/Pb-zircon ages are reported from northern Vestranden, the northernmost part of the Western Gneiss Region of Norway, Scandinavian Caledonides. The age of a basement tonalite (1819±6 Ma), together with earlier datings on granites, indicates that the dominantly granitoid basement in Vestranden began to form more than 1800 Ma years ago. The minimum age of a subsequent Precambrian migmatization event is given by a U/Pb age of 1630±80 Ma found a granite dyke which cuts migmatitic veining in the basement gneisses. While age and lithology suggest general similarity and thereby favour a correlation between the basement granites of the northern part of Vestranden and Svecofennian granites farther to the east, no detailed chronological information is at present available that support such a correlation.No Sveconorwegian-Grenvillian ages have previously been reported from northern Vestranden. Also in the presently studied cases events between c. 1630 Ma and 430 Ma ago did not affect the U-Pb isotope system of zircons. About 430 Ma ago, substantial parts of Vestranden experienced Caledonian high-grade metamorphism and partial anatectic melting. Migmatite neosomes in northern Vestranden, yielded an U/Pb zircon age of 434±22 Ma. Relatively high temperatures persisted throughout much of the Silurian and probably into the Devonian. Large amounts of pegmatite intruded at the end of this period. One of these dykes yielded an U/Pb-zircon age of 404±2 Ma.

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Zusammenfassung Vier neue U-Pb-Zirkonalter liegen aus dem nördlichen Vestranden-Bereich des westnorwegischen Gneissgebiets vor. Das Alter einer Tonalitintrusion (1819±6 Ma) bestätigt zusammen mit anderen Altersbestimmungen, daß das präkambrische Fundament der zentralskandinavischen Kaledoniden sich im Vestrandengebiet bereits vor mehr als 1800 Ma zu bilden begann. Das Minimalalter einer darauf folgenden präkambrischen Migmatisierung wird durch ein 1630±20 Ma Alter durchsetzender Granitgänge begrenzt. Zwar deuten die bisher vorliegenden Ergebnisse eine Gleichaltrigkeit der Granitgesteine im nördlichen Vestranden und den Svekofenniden weiter östlich an, ermöglichen aber derzeit noch keine präzise Korrelation des geologischen Geschehens.Sveconorwegische (Grenville) Isotopenalter sind bisher nicht aus dem nördlichen Vestranden bekannt. Auch die neuen Daten weisen keine Störung der U-Pb-Isotopensysteme zwischen etwa 1630 und 430 Ma auf.Beträchtliche Teile des Vestrandenbereiches wurden vor ca. 430 Ma einer kaledonischen, höhergradigen Metamorphose und anatektischer Aufschmelzung unterworfen. Die Neosome eines Migmatites ergaben ein Alter von 434±22 Ma, was erneut die Existenz zweier verschiedener Migmatisierungsetappen im Präkambrium bzw. in Silur bestätigt. Höhere Temperaturen bestanden danach bis in das Devon. Größere Pegmatitmengen wurden am Ende dieser Zeitspanne gebildet. Einer dieser Pegmatitkörper ergab ein U-Pb-Zirkonalter von 404±2 Ma.

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Résumé Cette note fait état de 4 nouvelles déterminations d'âge (méthode U/Pb sur zircon) dans le Vestranden septentrional, qui forme la partie nord de la région gneissique occidentale de la Norvège (Calédonides). L'âge d'une tonalite (1819±6 Ma), joint à des datations antérieures effectuées sur des granites, indique que la formation du socle granitoîde dans le Vestranden a commencé il y a plus de 1800 Ma. L'âge maximal d'une migmatitisation précambrienne ultérieure est donné par la mesure de 1630±80 Ma effectuée sur un dyke de granite qui recoupe le rubanement migmatitique. L'âge et la lithologie indiquent une similitude d'ensemble et, partant, suggèrent une corrélation entre les granites du nord du Vestranden et ceux des Svécofennides situés plus à l'est; toutefois, on ne dispose aujourd'hui d'aucune donnée chronologique à l'appui d'une telle corrélation.Jusqu'à présent, aucun âge svéconorvégien-grenvillien n'a été enregisré dans le Vestranden septentrional. De même, dans les cas étudiés ici, aucun événement n'a affecté le système isotopique U-Pb entre 1630 et 430 Ma. A environ 430 Ma, des parties importantes du Vestranden ont enregistré un métamorphisme calédonien de degré élevé et une fusion anatectique partielle. Les néosomes d'une migmatite ont fourni un âge U/Pb sur zircon de 434±22 Ma. Des températures relativement élevées se sont maintenues pendant la plus grande partie du Silurien et jusqu'au cours du Dévonien. De grandes quantités de pegmatite ont été intrudées à la fin de cette période; un de ces dykes a donné un âge U/Pb sur zircon de 404±2 Ma.

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- - . 1819±6 . , , - 1800 . . , , 1630±80 . , . , .- - 1630-430 . . . 434=22 . . , , , ; , , -, 404=2 . .

     

U/Pb and Pb/Pb zircon ages for arc-related intrusions of the Bolu Massif (W Pontides, NW Turkey): evidence for Late Precambrian (Cadomian) age

We present new U/Pb and Pb/Pb radiometric age data from two tectono-stratigraphic units of the regionally extensive Bolu Massif, in the W Pontides (İstanbul Fragment), N Turkey. A structurally lower unit (Sünnice Group) is cut by small meta-granitic intrusions, whereas the structurally higher unit comprises meta-volcanic rocks (Çaşurtepe Fm) cut by meta-granitic plutons (Tüllükiriş and Kapıkaya plutons). U/Pb single-crystal dating of zircons from the Kapıkaya Pluton yielded a concordant cluster, with a mean ²³⁸U/²⁰⁶Pb age of 565.3 ± 1.9 Ma. Zircons from the Tüllükiriş Pluton (affected by Pb loss) gave a ²⁰⁷Pb/²⁰⁶Pb age of 576 ± 6 Ma age (Late Precambrian). Small meta-granitic intrusions cutting the Sünnice Group yielded a less precise ²⁰⁷Pb/²⁰⁶Pb age of 262 ± 19 Ma (Early Permian). The older ages from the Bolu Massif confirm the existence of latest Precambrian arc magmatism related to subduction of a Cadomian ocean. We infer that the Bolu Massif represents a fragment of a Cadomian active margin. Cadomian orogenic units were dispersed as exotic terranes throughout the Variscan and Tethyan orogens, and the Bolu Massif probably reached its present position prior to latest Palaeozoic time. Our dating results also confirm that NW Turkey was affected by Hercynian magmatism related to subduction of Palaeotethys, as inferred for other areas of the Pontides.     

Magma development of the Leka Ophiolite Complex, central Norwegian Caledonides

The magmatic products of the Leka Ophiolite Complex of Lower Ordovician age, indicate formation in different tectonic settings and generation from different mantle sources. Harzburgites of the mantle tectonite, clinopyroxenes from wehrlites of the ultramafic cumulates, the metabasalts of the dyke complex and earliest pillow lavas (IAT/MORB, boninites) all show characteristics compatible with formation above a subduction zone in an intra-oceanic setting. Nd-isotopes indicate that some of the IAT and boninites may have been derived from a source contaminated by continental material, and a CAB source differ significantly from that of the IAT and boninites. The later pillow lavas of MORB composition show only minor influence of subduction-related processes (minor or no negative Ta-anomalies), and the supposed latest volcanic sequence of an alkaline OIB-type, none at all. The MORB- and OIB-type magmas are thought developed by spreading in a back-arc setting, in which the latter magma type developed in a remote position from the subduction zone.     

Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China. To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MS U/Pb analysis of zircon was conducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim of gaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granite samples: Early–Middle Triassic ( 245 to 229 Ma), Late Triassic ( 219 to 216 Ma) and Cretaceous ( 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable ε_Hf values and Mesoproterozoic ( 1.6 Ga) depleted-mantle model ages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous granite shows higher and less variable ε_Hf values and slightly younger model ages ( 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of  1.5 Ga is calculated from the pooled Triassic and Cretaceous samples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton, which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.     

U–Pb ages of the Dalsfjord Complex, SW Norway, and their bearing on the correlation of allochthonous crystalline segments of the Scandinavian Caledonides

The Dalsfjord Complex is an allochthonous crystalline unit in the hanging wall of a major extensional detachment of the Caledonian orogen on the western coast of Norway. U-Pb geochronology has been carried out to test a previously proposed correlation between this unit and the Jotun Nappe Complex, a member of the Middle Allochthon now occupying a foreland position to the east. A monzonite sample of the Dalsfjord suite yields a zircon age of 1,634Dž Ma, whereas a crosscutting gabbro was formed at 1,464Lj Ma. Both rocks were strongly overprinted during the Sveconorwegian orogeny, especially the monzonite whose zircons U-Pb data were strongly pulled towards a lower intercept age of 882ᆱ Ma, and whose titanite indicates a two-stage growth history at >960 Ma and <920 Ma. These relationships support the correlation of these units with the Jotun Complex, and to some degree also with the Lindås Nappe in the Bergen Arcs.     

Early Ordovician U–Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Eclogite-grade metamorphism of the Seve Nappe Complex (SNC) in Norrbotten, Sweden, records the attempted subduction of the Baltic continental margin during the early Palaeozoic evolution of the Iapetus Ocean. Metamorphic titanite sampled from several calcsilicate gneisses of the SNC in Norrbotten occurs as part of a prograde, eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant U/Pb ages of 500–475  Ma. Later structural disruption of these rocks occurred during the Siluro-Devonian Scandian phase of the Caledonide orogeny, but the U/Pb systematics show no evidence of a second generation (metamorphic or recrystallized) of titanite, or of post-Early Ordovician disturbance through Pb loss. Hence the U/Pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the SNC.
These results support earlier Sm/Nd and ⁴⁰Ar/³⁹Ar studies indicating an Early Ordovician metamorphic age for the eclogitic Norrbotten SNC, and confirm the Early Ordovician destruction of at least this segment of the Palaeozoic passive margin of Baltica. These results indicate that the SNC in the northern Scandinavian Caledonides was subducted and metamorphosed to high grade some 50–70  Myr prior to the high-grade metamorphism of the SNC in the central Scandinavian Caledonides. This result requires significantly different early Palaeozoic tectonic histories for rocks mapped as SNC in the northern Caledonides and those in the central Caledonides, despite a seemingly similar tectonostratigraphic position and broadly similar high-grade metamorphism.     

中祁连西段巴嘎德尔基岩体LA-ICP-MS锆石U-Pb年龄及地质意义

巴嘎德尔基岩体位于中祁连山西段的野马南山地区,主要由钾长花岗岩和二长花岗岩组成。LA-ICP-MS锆石U-Pb测年表明,巴嘎德尔基岩体侵位年龄为462.9±1.4Ma。该岩体地球化学方面表现出强过铝、高K、K/Na,低Mg、Fe、Ca的主量元素特征,并富集K、Rb、Ba、Th,亏损Zr、Hf、Y、Yb等微量元素,轻稀土富集,弱的Eu负异常。在微量元素判别图解上,所有岩石投影点均落入同碰撞区域。结合区域地质背景,认为巴嘎德尔基岩体是加里东造山作用晚期陆陆碰撞的产物。     

U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area: constraints on magmatic and tectonic evolution of Precambrian rocks of western Ethiopia

Summary The Precambrian rocks in western Ethiopia consist of high- and low-grade terranes intruded by granitoids with a wide compositional spectrum. The formation ages of these granitoid rocks are, so far, poorly understood. Single-grain zircon Pb/Pb evaporation and conventional U/Pb dating conducted on four granitoids places time constraints on their emplacement and tectonothermal events. Three granitoid magmatic events were identified at 815 Ma, 700–730 Ma, and 620–625 Ma, which were marked by emplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatectic Suqii-Wagga two-mica granite and the Guttin K-feldspar megacrystic granite, and the anorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark a major magmatic episode in this part of Africa. A tectonothermal event at ∼ 630 Ma preceded the emplacement of the within-plate granitoids at 620–625 Ma. The decrease of ages from the calc-alkaline to anorogenic granitoids suggests a shift of magmatic styles and tectonic setting of the granitoids over a period of 200 million years. The Suqii-Wagga and Guttin granites, representing the granitoid population in the migmatitic terrane, formed as part of the successive evolution of the granitoid magmatism in the region. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoid populations emplaced into the low-grade volcanosedimentary sequence and the high-grade, often migmatitic, gneisses suggest contribution of pre-Pan-African crust to the origin and evolution of the granitoids. Conventional U/Pb studies of zircons from the Guttin K-feldspar megacrystic granite and the Ganjii monzogranite yielded upper intercept ages of ∼ 3 Ga and ∼ 2 Ga, respectively, possibly indicating the presence of reworked Archean-Proterozoic crustal material.

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Zusammenfassung U/Pb und Pb/Pb Zirkonalter granitoider Gesteine aus dem Gebiet von Wallagga: Hinweise zur magmatischen und tektonischen Entwicklung pr?kambrischer Gesteine in A¨thiopien Das Pr?kambrium im westlichen ?thiopien besteht aus hoch- und niedrigmetamorphen Basement Serien, die von Granitoiden unterschiedlichster Zusammensetzung intrudiert werden. Die Bildungsalter dieser Magmatite sind bisher nur ungenügend bekannt gewesen. Neue Pb/Pb-Evaporations- und konventionelle U/Pb-Datierungen an Einzelzirkonen von vier verschiedenen Plutoniten erlauben nun Rückschlüsse auf deren Intrusionsalter und die damit verbundene tektonische Entwicklung der Region. Drei zeitlich getrennte magmatische Ereignisse lassen sich unterscheiden: Intrusion der kalk-alkalischen Ujjukka Granite um 815 Ma; Bildung der anatektischen Zweiglimmer Granite der Suqii-Wagga Suite um 700–730 Ma; Intrusion der grob porphyrischen K-Feldspat Granite von Guttin und der anorogenen Ganjii Monzogranite um 620–625 Ma. Das 815 Ma Ereignis wird als wichtige magmatische Phase in diesem Teil von Afrika interpretiert. Ein thermisches Ereignis um 630 Ma geht der Platzname von “within-plate” Granitoiden um 620–625 Ma voraus. Die beobachtete Altersabnahme von den kalk-alkalischen zu den anorogenen Granitoiden spricht für eine pr?gnante ?nderung des tektonischen Regimes über einen Zeitraum von ca. 200 Ma. Die Suquii-Wagga und Guttin Granite sind in das hochgradige, migmatische Basement intrudiert. Dies mag für eine sukzessive tektonische Entwicklung dieser Abfolgen sprechen. Ererbte, mesoproterozoische Zirkone deuten auf die Aufarbeitung pr?-panafrikanischer Gesteine hin. Obere Einstichpunkte von den U/Pb Analysen im Altersbereich von ca. 3 Ga in den Guttin Graniten und von ca. 2 Ga in den Ganjii Monzograniten sprechen ebenfalls für die Inkorporation von proterozoischen bis archaischen Krustenkomponenten.

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Received June 7, 2000; accepted October 29, 2000     

Formation and emplacement ages of the SSZ‐type Neotethyan ophiolites in Central Anatolia,Turkey: palaeotectonic implications

Isolated outcrops of ophiolitic rocks, termed the Central Anatolian Ophiolites, are found as allochthonous bodies in the Central Anatolian Crystalline Complex, that represent the metamorphosed passive northern edge of the Tauride–Anatolide Platform, central Turkey. In terms of pseudostratigraphic relationships of the magmatic units and their chemical designation, the Central Anatolian Ophiolites exhibit a supra‐subduction zone (fore‐arc) setting within the Vardar–İzmir–Ankara–Erzincan segment of the Neotethys. The epi‐ophiolitic sedimentary cover of the Central Anatolian Ophiolites is generally characterized by epiclastic volcanogenic deep‐sea sediments and debris flows intercalated with pelagic units. The richest and most significant planktonic foraminiferal association recorded from the lowest pelagic members infer a formation age of early–middle Turonian to early Santonian. K/Ar ages of post‐collisional granitoids (81–65 Ma) intruding the basement rocks as well as the Central Anatolian Ophiolites suggest a post‐early Santonian to pre‐middle Campanian emplacement age. The marked high volume of epiclastic volcanogenic sediments intercalated with the pelagics of the Central Anatolian Ophiolite is suggestive of rifting in a marginal sea adjacent to a volcanic arc. Penecontemporaneous tectonism is reflected in repetitions in the stratigraphy and in debris flows, which result from major slides and mass‐gravity reworking of pre‐existing units and of arc‐derived volcanics and sediments. Correlating the rock units and formation/obduction ages of the Central Anatolian Ophiolites with further supra‐subduction zone type ophiolites in the eastern (Turkey) and western (Greece) parts of the Vardar–İzmir–Ankara–Erzincan segment of Neotethys we conclude that the intraoceanic subduction in the east is definitely younger and the closure history of this segment is more complex than previously suggested. Copyright © 2000 John Wiley & Sons, Ltd.     

Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

The Western Gneiss Region (WGR) marks the outcrop of a composite terrane consisting of variably re-worked Proterozoic basement and parautochthonous or autochthonous cover units. The WGR exhibits a gross structural, petrographic and thermobarometric zonation from southeast to northwest, reflecting an increasing intensity of Scandian (late Palaeozoic) metamorphic and structural imprint. Scandian-aged eclogites have been widely (though for kinetic reasons not invariably) stabilised in metabasic rocks but have suffered varying degrees of retrogression during exhumation. In the region between the Jostedal mountains and Nordfjord, eclogites commonly have distinctively prograde-zoned garnets with amphibolite or epidote–amphibolite facies solid inclusion suites and lack any evidence for stability of coesite (high pressure [HP] eclogites). In the south of this area, in Sunnfjord, eclogites locally contain glaucophane as an inclusion or matrix phase. North of Nordfjord, eclogites mostly lack prograde zoning and evidence for coesite, either as relics or replacive polycrystalline quartz, is present in both eclogites (ultrahigh pressure [UHP] eclogites) and, rarely, gneisses. Coesite or polycrystalline quartz after coesite has now been found in eight new localities, including one close to a microdiamond-bearing gneiss. These new discoveries suggest that, by a conservative estimate, the UHP terrane in the WGR covers a coastal strip of about 5000 km² between outer Nordfjord and Moldefjord. A “mixed HP/UHP zone” containing both HP and UHP eclogites is confirmed by our observations, and is extended a further 40 km east along Nordfjord. Thermobarometry on phengite-bearing eclogites has been used to quantify the regional distribution of pressure (P) and temperature (T) across the WGR. Overall, a scenario emerges where P and T progressively increase from 500°C and 16 kbar in Sunnfjord to >800°C and 32 kbar in outer Moldefjord, respectively, in line with the distribution of eclogite petrographic features. Results are usually consistent with the silica polymorph present or inferred. The P–T conditions define a linear array in the P–T plane with a slope of roughly 5°C/km, with averages for petrographic groups lying along the trend according to their geographic distribution from SE to NW, hence defining a clear field gradient. This P–T gradient might be used to support the frequently postulated model for northwesterly subduction of the WGC as a coherent body. However, the WGC is clearly a composite edifice built from several tectonic units. Furthermore, the mixed HP/UHP zone seems to mark a step in the regional P gradient, indicating a possible tectonic break and tectonic juxtaposition of the HP and UHP units. Lack of other clear evidence for a tectonic break in the mixed zone dictates caution in this interpretation, and we cannot discount the possibility that the mixed zone is, at least, partly a result of kinetic factors operating near the HP–UHP transition. Overall, if the WGC has been subducted during the Scandian orogeny, it has retained its general down-slab pattern of P and T in spite of any disruption during exhumation. Garnetiferous peridotites derived from subcontinental lithospheric mantle may be restricted to the UHP terrane and appear to decorate basement-cover contacts in many cases. P–T conditions calculated from previously published data for both relict (Proterozoic lithospheric mantle?) porphyroclast assemblages and Scandian (subduction-related?) neoblastic assemblages do not define such a clear field gradient, but probably record a combination of their pre-orogenic P–T record with Scandian re-working during and after subduction entrainment. A crude linear array in the P–T plane defined by peridotite samples may be, in part, an artifact of errors in the geobarometric methods. A spatial association of mantle-derived peridotites with the UHP terrane and with basement-cover contacts is consistent with a hypothesis for entrainment of at least some of them as “foreign” fragments into a crustal UHP terrane during subduction of the Baltic continental margin to depths of >100 km, and encourages a more mobilistic view of the assembly of the WGC from its component lithotectonic elements.     

Early Cretaceous U–Pb zircon ages for the Copiapó plutonic complex and implications for the IOCG mineralization at Candelaria, Atacama Region, Chile

Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike–sill system exposed in the Candelaria open pit have been dated with the U–Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz–monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu–Au mineralization (Re–Os molybdenite ages at ∼115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria–Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz–monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu–Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified ⁴⁰Ar/³⁹Ar and K/Ar systematic in some areas.     

U/Pb radiochronology of two granitic plutons from the eastern Cordillera (Peru) — Extent of Permian magmatic activity and consequences

Two granitic bodies of the eastern Cordillera (Peru) have been dated by U/Pb method on zircons: 257±3 my and 238±11 my ages are respectively obtained for P 438 Quillabamba and P 381 Coasa granites. P 381 zircon fractions show a degree of discordancy which can be related to their quite high uranium contents (continuous lead loss model, with variable diffusion coefficient Wasserburg, 1963). Recent data and these new radiochronological results allow to define in the eastern Cordillera an extensive magmatic province of nearly 1000 km in length. This activity should be related to a distension phase. Consequently kinematic Hercynian and Andean intrusives are only present in minor proportion in Peruvian eastern Cordillera.

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Zusammenfassung Zwei Granitkörper wurden in der östlichen Kordillere von Peru mit Hilfe der U/Pb-Methode an Zirkonen datiert. 257±3 mill. Jahre wurden für den P 438 Quillamba Granit und 238±11 mill. Jahre für den P 381 Coasa Granit ermittelt. Die P 381 Zirkon Werte zeigen eine Diskordanz, die auf einen erhöhten Urangehalt zurückgeführt werden kann (nach dem Modell für kontinuierlichen Bleiverlust mit variablem Diffusionskoeffizienten, Wasserburg 1963). Zusammen mit weiteren Datierungen kann man in der östlichen Kordillere eine magmatische Provinz von einer Länge bis zu 1000 km definieren. Diese Aktivität sollte im Zusammenhang mit einer Dehnungsphase stehen. Daraus folgt, daß herzynische und andin Intrusionen in der östlichen Kordillere von Peru nur von untergeordneter Bedeutung sind.

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Resume Deux massifs de granite appartenant à un vaste cortège magmatique de la Cordillère orientale du Pérou ont été daté par la méthode U/Pb sur zircons; des âges de 257 ± 3 m.A. et de 238 ± 11 mA ont été ainsi déterminés respectivement pour les granites de Quillabamba (P 438) et de Coasa (P 381). Le notable degré de discordance des échantillons P 381 parait être dû à leurs relativement hautes teneurs en uranium (modèle de perte continue, à coefficient de diffusion variable de Wasserburg, 1963). Des travaux récents et ces nouvelles données radiochronologiques permettent de définir dans la Cordillère orientale du Pérou une province magmatique permienne d'environ 1000 km de long, dont la mise en place serait dûe à une phase de distension. Les instrusions hercyniennes et andines ne représentent qu'une proportion mineure des massifs granitoïdes de la Cordillère orientale.

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- . Quilamba 438 257 ±3 , Coasa 381 — 238±11 . 381 , ( : , Wasserburg, 1963). 1000 . . , .

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Contribution C.G.G. n^o 222.     

Dating the exhumation of UHP rocks and associated crustal melting in the Norwegian Caledonides

U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.     

In-situ U–Pb zircon ages for Early Ordovician magmatism in the eastern Pyrenees, France: the Canigou orthogneisses

U-Pb age determinations with an ion probe on zircons within orthogneisses from the Canigou Massif, eastern Pyrenees, give an emplacement age for their granitic protolith of 475ᆞ Ma (Arenigian). This invalidates the previous interpretation of basement-cover relationships between the orthogneisses and the overlying Lower Cambrian Canaveilles metasedimentary series, and corroborates the intrusive character of the orthogneisses, which is further supported by field evidence. The data also define two groups of inherited zircons (Paleo/Meso-Proterozoic and Cadomian) in addition to partial lead loss during the Hercynian Late Carboniferous high temperature-low pressure metamorphic event. These results are in agreement with ages reported from other orthogneissic bodies across the entire Variscan chain of western Europe, and indicate the existence of a widespread Cambro-Ordovician magmatic event.     

Three varieties of alpine-type ultramafic rocks in the Norwegian Caledonides and basal gnesis complex

Three varieties of alpine-type ultramafic rocks are distinguish in the Norwegian Caledonides associated Basal Gneiss Complex. Type one rocks have primary (magmatic) olivine, clinopyroxene, orthopyroxene and chromite, and are partly or completely serpentinised. They are found exclusively in rocks of Cambro-Silurian age. Type two are polymetamorphic metaperidotites or sagvandites consisting of olivine, enstatite and carbonate minerals, with talc and amphibole commonly being present. They are found in medium- to high-grade metamorphic rocks. Type three also show a metamorphic mineral association of olivine, orthopyroxene and minor chromite, while clinopyroxene, amphibole and chrome-bearing chlorite may also be present in some samples. Garnet may or may not occur and, where present, is often surrounded by reaction rims of spinel and amphibole. The type three ultramafic bodies are serpentinised to varying degrees and occur in high-grade metamorphic gneisses which may also contain eclogites and anorthosites. Distinction of these three varieties of ultramafic body may be useful for correlation purposes and for more detailed studies on the nature of their metamorphism.     

阿尔泰造山带南缘富蕴后造山线形花岗岩体锆石U-Pb年龄及其地质意义

阿尔泰造山带南缘发育一系列后造山花岗岩体。富蕴南额尔齐斯断裂带中发育具有片麻理的黑云母花岗岩体,其中还侵入有未变形的黑云母花岗岩体。本文对这两个岩体进行了 TIMS 法锆石 U-Pb 测年,结果分别为 281±5 Ma(变形岩体)和275±2 Ma(不变形岩体),显示其为后造山花岗岩体。该研究表明,阿尔泰造山带在281± 5 Ma 时仍存在一定规模的后造山剪切作用,275±2 Ma 之后变形作用开始变弱。     

Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

A Paleozoic subduction complex in Korea: SHRIMP zircon U–Pb ages and tectonic implications

The Wolhyeonri complex in the southwestern margin of the Korean Peninsula is divided into three lithotectonic units: Late Paleozoic Zone I to the west, Middle Paleozoic Zone II in the middle and Early Paleozoic Zone III to the east. Zones II and III display characteristics of continental arc magmatic sequence. Zone II is dominated by mafic metavolcanics, whereas zone III is characterized by the presence of dismembered serpentinite bodies including chaotic mélange. These zones are proposed to have been formed in a convergent margin setting associated with subduction. Here we present zircon SHRIMP U–Pb ages from the various units within the Wolhyeonri complex which reveal the Paleozoic tectonic history of the region. The Late Carboniferous ages obtained from the main shear zone between the Wolhyeonri complex and the Paleoproterozoic Gyeonggi massif are thought to mark the timing of continental arc magmatism associated with the subduction process. In contrast, Zone I with Neoproterozoic arc magmatic remnants might indicate deposition in a forearc basin. The Wolhyeonri complex also preserves strong imprints of the Triassic collisional event, including the presence of Middle Triassic high-pressure metabasites and eclogites near the eastern boundary of the Zone III. These range of radiogenic ages derived from the Wolhyeonri complex correlate well with subduction and accretion history between the North and South China cratons. Similar geochronological features have also been indentified from the Qinling, Tongbai–Xinxian, and northern Dabie areas in east-central China. The existence of Paleozoic coeval subduction in East Asia prior to the Triassic collision is broadly consistent with a regional tectonic linkage to Gondwana.     

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and ²⁰⁷Pb/²⁰⁶Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The ⁸⁷Rb/⁸⁶Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. ⁸⁷Rb/⁸⁶Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The ⁸⁷Rb/⁸⁶Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.     

Zircon U/Pb and Pb/Pb geochronology of the Rastenberg granodiorite,South Bohemian Massif,Austria

Summary A combined zircon typology, zircon Pb-Pb evaporation, and conventional U-Pb study of the late- to post-tectonic Rastenberg granodiorite yields the following results: Typological investigations show two distinguishable zircon populations. Type l: subtype S24 ofPupin, colourless to slightly pink, clear to turbid, often with cores, few to abundant inclusions, long prismatic; type 2: subtype S4 ofPupin, colourless to reddish or slightly pink, clear to slightly turbid, no visible cores, abundant inclusions, tabular habit, short prismatic.At least 4 different zircon-forming events can be distinguished: Inherited cores with ages around 623±22Ma and single ages > 1206Ma from type 1 zircons imply the reworking of rocks derived from Cadomian and Proterozoic to Archean crust. Ages around 353±9Ma from type 1 zircons are interpreted as timing a first magma formation or the onset of a long-lasting magma-generating event during the Variscan plutonism in the South Bohemian pluton. The actual intrusion of the granodioritic magma into the middle crust took place around 338±2Ma (type 2 and rims of type 1 zircons).Only type 1 zircons are found as inclusions in large K-feldspar phenocrysts providing evidence that these phenocrysts have grown before the 338Ma event and may be as old as 353 Ma.[]

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Zirkon U/Pb und Pb/Pb Geochronologie des Rastenberger Granodiorits, südliche Böhmische Masse, Österreich

Zusammenfassung Zirkon-typologische Untersuchungen, Einzelzirkon Pb-Pb-Evaporations- und konventionelle U-Pb-Altersbestimmungen an Gesteinen des spät- bis postkinematischen Rastenberger Granodiorits geben folgende Resultate: Typologisch lassen sich zwei Zirkonpopulationen unterscheiden: Typ 1: S24 Subtyp vonPupin, farblos bis leicht rosa, klar bis getrübt, häufig mit Kernen, wenige bis viele Einschlüsse, lang prismatisch; Typ 2: S4 Subtyp vonPupin, farblos bis rötlich oder leicht rosa, klar bis leicht getrübt, keine sichtbaren Kerne, viele Einschlüsse, flachtafeliger Habitus, kurzprismatisch.Mindestens 4 unterschiedliche Altersgruppen lassen sich unterscheiden: Ererbte Kerne von Typ 1 Zirkonen mit Altern um 623±22Ma und einzelnen Altern > 1206Ma sprechen für die Aufarbeitung von Gesteinen, die von cadomischen und proterozoischen bis archaischen Gesteinen im Krustenbereich des Südböhmischen Plutons abstammen könnten. Alter um 353±9Ma von Typ 1 Zirkonen werden einer ersten magmatischen Phase oder dem Beginn der Krustenaufschmelzung im Zuge der Bildung der variszischen Plutonite zugeordnet. Die eigentliche Intrusion des Granodiorits um 338±2Ma wird mit Typ 2 und mit Randpartien von Typ 1 Zirkonen erfaßt.In den großen K-Feldspat-Phänokristallen finden sich nur Zirkone vom Typ 1. Dies deutet darauf hin, daß die Phänokristalle vor dem 338Ma Ereignis gebildet wurden, eventuell also bis 353Ma alt sein können.

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With 6 Figures