U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus

U/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmøy ophiolite is dated at 493+7/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+/–2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9/–5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903–6677 ppm) zircon. Lower intercepts of 247+/–68 and 191+/–70 Ma defined by the plagiogranite and clinopyroxene-phyric gabbro best-fit lines may reflect a real low-T alteration/rift-related event.A plagiogranite differentiate of the Gullfjellet ophiolite complex is dated at 489+/–3 Ma and a crosscutting arc-related tonalite is 482+6/–4 Ma. Both of these ages overlap with those of the correlative rocks at Karmøy suggesting that they are parts of one ophiolitic terrane with a common history.Trondhjemite associated with the Leka ophiolite is dated at 497+/–2 Ma, indicating that supra-subduction zone magmatism there may be coeval with spreading which formed the Karmøy axis sequence.The U/Pb zircon ages of Norwegian ophiolites reported here, combined with ages of other Appalachian-Caledonian ophiolite complexes in Britain and Canada, indicate a narrow age range for the generation of at least two marginal basins in the Tremadoc-Arenig. Two spreading episodes documented at Karmøy are separated in time by intrusion of arc-related trondhjemite magmas at 485+/–2 Ma and may correlate with two separate spreading events documented in other ophiolites.The Solund/Stavfjorden ophiolite, at 443+/–3 Ma, is the only late Ordovician ophiolite yet documented in the entire Appalachian-Caledonian Orogen and it probably represents a small, short-lived marginal basin late in the history of the Iapetus Ocean. It is correlative with Caradocian ensialic marginal basin magmatism in Wales and the Trondheim region, and with tholeiitic gabbro-diorite plutons that intruded Newfoundland ophiolites in a tensional regime after emplacement of the ophiolites over the continental margin.     

右江盆地酸性脉岩继承锆石成因及地质意义

卡林型金矿与岩浆作用的关系一直以来是矿床学家关注的热点。右江盆地发育众多卡林型金矿床,但由于区内岩浆岩出露十分有限,岩浆活动对金成矿有无贡献目前还无定论。火成岩中的继承锆石常常可以用来指示早期的、隐伏的岩浆活动。桂西北地区晚白垩世巴马、料屯和下巴哈石英斑岩脉中含有大量的继承锆石。其中,料屯和下巴哈两处脉岩分别与料屯和明山金矿在空间上紧密相关。脉岩中锆石的LA-ICP-MS和SIMS原位U-Pb定年表明,其主要集中于130~140Ma、242Ma左右、400~450Ma、700~1000Ma和1700~1800Ma等五个年龄段。关于较年轻的两个时间段,来自料屯和下巴哈石英斑岩脉的继承锆石分别获得两组谐和年龄,即136.3±3.9Ma(2σ)和242.3±1.7Ma(2σ)、128.2±2.3Ma(2σ)和243.1±3.6Ma(2σ)。这些脉岩侵入的最年轻地层为中三叠统百逢组,因而前三叠纪继承锆石可能捕获自地层,而晚于三叠纪的130~140Ma锆石则应来自于深部隐伏岩体。关于242Ma左右锆石,尽管其与围岩地层百逢组1~2段近于同期,但其晶形较差且多呈不规则碎片状,暗示其更可能源自百逢组中的沉积碎屑锆石。结合区域地质背景,认为该期锆石所代表的岩浆事件可能与古特提斯洋闭合有关。以上130~140Ma和约242Ma左右的两组年龄,与初步确定的右江盆地卡林型金矿的两次成矿的时代基本一致,据此推断金矿化与岩浆作用可能有一定成因联系。     

广西北部新元古代花岗岩锆石U-Pb年代学及其构造意义

广西北部（扬子块体南缘）３个代表性的新元古代花岗岩体（本洞、三防和元宝山岩体）的矿物组合、岩石化学、元素和同位素地球化学特征表明,它们均为过铝质Ｓ型花岗岩。高精度的ＳＨＲＩＭＰ和颗粒级锆石ＵＰｂ定年结果表明,本洞、三防和元宝山岩体的形成时代分别为（８２０±７）Ｍａ、（８２５±６）Ｍａ和（８２４±４）Ｍａ,在误差范围内基本相同,排除了本区存在中元古代末／新元古代初形成的花岗岩。结合野外地质关系和扬子块体东南缘蛇绿岩的定年结果,扬子块体南缘新元古代晋宁期造山运动的时限应为１．０～０．８０Ｇａ,很可能以０．８２Ｇａ的晚造山花岗岩和约０．８Ｇａ的陆相双峰式火山岩的形成而告终,同时,震旦系底界年龄也应小于８２０Ｍａ。     

Detrital zircon ages from southern Norway – implications for the Proterozoic evolution of the southwestern Baltic Shield

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have ²⁰⁷Pb/²⁰⁶Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core ²⁰⁷Pb/²⁰⁶Pb age of 2383 Ma. Two discrete, unzoned zircons have ²⁰⁷Pb/²⁰⁶Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U ratios and maximum ²⁰⁷Pb/²⁰⁶Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997     

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     

北山地区古元古界北山岩群中长英质脉内锆石U- Pb年龄及其地质意义

深熔作用是大陆地壳分异、元素迁移富集的重要地质过程。位于中亚造山带南缘的北山地区,笔者等对前寒武系北山岩群进行了研究,结果表明该岩群普遍经历了角闪岩相变质及深熔作用,长英质浅色脉体分布广泛,却鲜有文献报道。绵山地区出露的古元古代北山岩群斜长角闪岩及相关的长英质浅色脉体的野外地质特征、相互关系共同指示了北山岩群变质地层经历了部分熔融,熔体在原地分凝聚积。锆石LA-ICP-MS U-Pb测定结果表明,顺北山岩群变质地层片理产出的长英质浅色脉体锆石n(²⁰⁶Pb)/n(²³⁸U)加权平均年龄约为370 Ma,表明深熔作用发生时间为晚泥盆世;一测点的n(²⁰⁷Pb)/n(²⁰⁶Pb)年龄为1623±12 Ma,与前人报道的北山岩群测年结果高度相近,虽此年龄值不足为证,但结合产出特征,可能指示长英质浅色脉体的源岩为北山岩群。此外,北山岩群中深熔作用普遍发育,露头可见大量的石榴子石、电气石、云母类矿物,指示区内部分稀有元素成矿潜力巨大。     

北山地区古元古界北山岩群中长英质脉内锆石U- Pb年龄及其地质意义

深熔作用是大陆地壳分异、元素迁移富集的重要地质过程。位于中亚造山带南缘的北山地区,笔者等对前寒武系北山岩群进行了研究,结果表明该岩群普遍经历了角闪岩相变质及深熔作用,长英质浅色脉体分布广泛,却鲜有文献报道。绵山地区出露的古元古代北山岩群斜长角闪岩及相关的长英质浅色脉体的野外地质特征、相互关系共同指示了北山岩群变质地层经历了部分熔融,熔体在原地分凝聚积。锆石LA- ICP- MS U- Pb 测定结果表明,顺北山岩群变质地层片理产出的长英质浅色脉体锆石n(206Pb)/n(238U)加权平均年龄约为370 Ma,表明深熔作用发生时间为晚泥盆世; 一测点的n(207Pb)/n(206Pb)年龄为1623±12 Ma,与前人报道的北山岩群测年结果高度相近,虽此年龄值不足为证,但结合产出特征,可能指示长英质浅色脉体的源岩为北山岩群。此外,北山岩群中深熔作用普遍发育,露头可见大量的石榴子石、电气石、云母类矿物,指示区内部分稀有元素成矿潜力巨大。     

Mid-Miocene (post 12 Ma) displacement along the central Karakoram fault zone in the Nubra Valley,Ladakh, India from spot LA-ICPMS U/Pb zircon ages of granites

The Karakoram fault zone is a prominent right lateral fault that connects the frontal thrust of the North Pamir with the Indus suture zone near Mount Kailas. Its nature and age of initiation is controversial. In the Nubra valley, Ladakh, India, a Karakoram range granite is thrust over Cretaceous magmatic arc rocks and this thrust is cut by a western strand of the Karakoram fault zone. Three different lithologies from this granite gave weighted mean zircon U/Pb ages of 12.92±0.77 Ma, 12.41±0.43 Ma, and 11.72±0.31 Ma. The ages indicate a relatively short intrusive history of about 1 Ma for the phases: the geochemistry is practically identical to the Pangong leucogranites in the same tectonic block. The Karakoram fault zone in this area is thus less than ~12 Ma old which supports a post middle Miocene (Serravallian) age of Karakoram fault initiation in this area.     

Structural evolution and U–Pb SHRIMP zircon ages of the Neoproterozoic Maria da Fé shear zone,central Ribeira Belt – SE Brazil

The Maria da Fé Shear Zone (MFSZ) is a sinistral strike-slip kilometric-scale structure developed in the late Neoproterozoic during the assembly of Gondwana. The MFSZ development is related to the NW–SE collision between the São Francisco Paleocontinent and the Rio Negro Magmatic Arc, which formed the Ribeira Belt. This paper describes the shear zone in detail, concluding that the orientation and age are consistent with NW–SE shortening during the afore mentioned collision. A U–Pb SHRIMP Concordia age of 586.9 ± 8.7 Ma is reported from zircon grains of a granitic dyke that crystallised synkinematically to the main tectonic activity of the shear zone. Another group of zircon grains from the same sample generated an upper intercept age of 2083 ± 43 Ma anchored in the younger Concordia age. These zircon grains are interpreted as relict grains of the basement from which the granite dyke was generated by partial melting. The temperature during mylonitization in the MFSZ was estimated in the range from 450 to 600 °C, based on microstructures in quartz and feldspar. An earlier collision in the same region, between 640 and 610 Ma, led to an extensive nappe-stack with tectonic transport to ENE, integrating the southern Brasilia Belt. One of the thrust zones between these nappes in the studied area is the Cristina Shear Zone with mylonites that were generated under upper amphibolite to granulite facies conditions. Brittle-ductile E–W metric-scale shear zones are superimposed on the MFSZ, which were active in similar, but probably slightly cooler, metamorphic conditions (≈500 °C).     

U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Garze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement

The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U–Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have magmatic crystallization ages of 221 ± 3.8 Ma and 216 ± 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb–Sr–Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan-Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.     

塔里木盆地中部辉绿岩脉LA-ICP-MS锆石U-Pb年龄及其地质意义

塔里木盆地塔中地区地震资料显示,寒武系、奥陶系、二叠系中发育大量辉绿岩岩脉,在塔中隆起断裂带附近采集了侵入到志留系与奥陶系中的辉绿岩脉样品(中1井、中16井岩心)。用LA-ICP-MS测定技术,测定了其中锆石的U-Pb年龄和微量元素。锆石U-Pb年龄显示,这些辉绿岩岩脉形成于早二叠世(273~296Ma);测得的700~930Ma(新元古代)、396~544Ma(古生代)年龄显示,盆地早期有构造岩浆活动;121.0±8.5Ma(早白垩世)锆石存在,显示其形成与盆地内部同期热液活动有关。     

Cathodoluminescence studies and U/Pb dating of zircons in pre-Mesozoic gneisses of the Tauern Window: Implications for the Penninic basement evolution

The pre-Mesozoic basement in the eastern and central part of the Tauern Window (Eastern Alps) consists mainly of anatectic plagioclase gneisses and amphibolites intruded and migmatized by Carboniferous granitoids. The gneisses are geochemically similar to calcalkaline dacites of modern primitive island arcs. Cathodoluminescence investigations and U/Pb-data on zircons indicate an uppermost Proterozoic or Cambrian extrusion age of the dacitic protoliths and a Carboniferous syngranitoid anatexis around 320 Ma. High-K calcalkaline meta-rhyolites in the north central Tauern Window form a separate part of the basement not affected by Carboniferous anatexis and plutonism. Their protoliths extruded in the Lower Carboniferous. In the Permian, high-K granitoids intruded the anatectic basement.Results are consistent with an island-arc system evolving from a Late Proterozoic and Early Paleozoic primitive stage to a Lower Carboniferous mature stage represented by K-rich rhyolite volcanism. An Upper Carboniferous collisional event caused anatexis, affecting mainly the K-rich calcalkaline rocks. The last calcalkaline magmas intruded in a Permian post-collisional setting.

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Zusammenfassung Das prä-mesozoische Grundgebirge des östlichen und zentralen Tauernfensters (Ostalpen) besteht größtenteils aus anatektischen Plagioklasgneisen und Amphiboliten, welche von karbonischen Granitoiden intrudiert und migmatisiert werden. Die Plagioklasgneise entsprechen geochemisch kalkalkalischen Daziten primitiver Inselbögen. Die anhand von Kathodolumineszenzaufnahmen ausgewerteten U/Pb-Daten der Zirkone weisen auf ein jüngst-proterozoisches oder kambrisches Extrusionsalter der dazitischen Protolithe und auf eine karbonische syn-granitoide Anatexis um 320 Ma. High-K kalkalkalische Metarhyolithe im nördlichen Teil des zentralen Tauernfensters bilden einen separaten Teil des Grundgebirges, der von karbonischer Anatexis und Plutonismus nicht berührt wurde. Ihre Protolithe extrudierten im unteren Karbon. Das anatektisch überprägte Grundgebirge aus Plagioklasgneisen und Amphiboliten wurde im Perm von high-K Granitoiden intrudiert.Die Ergebnisse sind konsistent mit der Entwicklung von einem jung-proterozoischen und alt-paläozoischen primitiven Inselbogen zu einem unter-karbonischen reifen Inselbogen mit K-reichen Rhyolith-Vulkanismus. Ein ober-karbonisches Kollisions-Ereignis führte zur Anatexis insbesondere der K-reichen kalkalkalischen Gesteine. Die letzten kalkalkalischen Magmen intrudierten postkollisional im Perm.

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Résumé Le socle pré-Mésozoïque du centre et de l'est de la Fenêtre des Tauern (Alpes orientales), est principalement composé de gneiss plagioclasiques anatectiques et d'amphibolites. Des granitoïdes carbonifères sont intrusifs dans ce socle et le migmatitisent. Les gneiss sont géochimiquement comparables à des dacites calco-alcalines d'arc insulaire immature. Des études sur zircons par cathodoluminescence et datation U-Pb indiquent, pour les dacites, un âge d'extrusion protérozoïque supérieur ou cambrien. L'anatexie contemporaine de la mise en place des granitoïdes est datée aux alentours de 320 Ma. Dans le nord de la partie centrale de la fenêtre, des méta-rhyolites calco-alcalines riches en potassium forment une zone distincte du socle et ne sont pas affectées par l'anatexie et le plutonisme carbonifère. Leur extrusion date du Carbonifère inférieur. Au Permien, des granitoïdes riches en potassium ont recoupé le socle anatectique.Les résultats sont en accord avec une évolution d'arc insulaire depuis un stade primitif d'âge protérozoïque supérieur ou paléozoïque inférieur jusqu'à un stade de maturité d'âge carbonifère inférieur représenté par les rhyolites riches en potassium. Au Carbonifère supérieur, un événement de collision a provoqué l'anatexie qui affecte principalement les roches calco-alcalines riches en potassium. Les derniers magmas calco-alcalins se sont intrudés dans le contexte post-collisionnel Permien.

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Latest Precambrian to Early Cambrian U–Pb zircon ages of augen gneisses from Calabria (Italy), with inference to the Alboran microplate in the evolution of the peri-Gondwana terranes

In situ U–Pb dating of zircons from five samples of Calabrian augen gneisses shows that their protoliths are Latest Precambrian to Early Cambrian in age (562 ± 15, 547 ± 7, 540 ± 4, 539 ± 16 and 526 ± 10 Ma), and contain Archaean (3.1 Ga), Palaeoproterozoic (1.7–2.4 Ga) and Neoproterozoic (0.6–0.9 Ga) inheritance. Geochemical signature of augen gneisses is typical of high-K calc-alkaline post-collisional magmatism. Their Sr–Nd isotopic compositions [0.7093 < (⁸⁷Sr/⁸⁶Sr)_i < 0.7139; −3.2 < ɛ_Nd(t) < −5.4; 1.5 < T _DM < 1.7 Ga] indicate the involvement of a crustal component in significant proportions. The Calabrian augen gneisses have, therefore, to be distinguished from the orthogneisses of Sardinia and northern Algeria, and from the porphyroids of Sicily, which are Middle Ordovician. By contrast, the Calabrian augen gneisses show a close similarity to the Pan-African post-collisional granitoids of the northern edge of the West African craton (e.g. the Moroccan Anti-Atlas). This suggests a peri-Gondwana origin and corroborates previous palaeogeodynamic reconstructions attributing the Alboran microplate to the northern margin of the West African craton.     

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.     

The basement of the Mount Athos peninsula, northern Greece: insights from geochemistry and zircon ages

The Mount Athos Peninsula is situated in the south-easternmost part of the Chalkidiki Peninsula in northern Greece. It belongs to the Serbo-Macedonian Massif (SMM), a large basement massif within the Internal Hellenides. The south-eastern part of the Mount Athos peninsula is built by fine-grained banded biotite gneisses and migmatites forming a domal structure. The southern tip of the peninsula, which also comprises Mount Athos itself, is built by limestone, marble and low-grade metamorphic rocks of the Chortiatis Unit. The northern part and the majority of the western shore of the Mount Athos peninsula are composed of highly deformed rocks belonging to a tectonic mélange termed the Athos-Volvi-Suture Zone (AVZ), which separates two major basement units: the Vertiskos Terrane in the west and the Kerdillion Unit in the east. The rock-types in this mélange range from metasediments, marbles and gneisses to amphibolites, eclogites and peridotites. The gneisses are tectonic slivers of the adjacent basement complexes. The mélange zone and the gneisses were intruded by granites (Ierissos, Ouranoupolis and Gregoriou). The Ouranoupolis intrusion obscures the contact between the mélange and the gneisses. The granites are only slightly deformed and therefore postdate the accretionary event that assembled the units and created the mélange. Pb–Pb- and U–Pb-SHRIMP-dating of igneous zircons of the gneisses and granites of the eastern Athos peninsula in conjunction with geochemical and isotopic analyses are used to put Athos into the context of a regional tectonic model. The ages form three clusters: The basement age is indicated by two samples that yielded Permo-Carboniferous U–Pb-ages of 292.6?±?2.9?Ma and 299.4?±?3.5?Ma. The main magmatic event of the granitoids now forming the gneiss dome is dated by Pb–Pb-ages between 140.0?±?2.6?Ma and 155.7?±?5.1?Ma with a mean of 144.7?±?2.4?Ma. A within-error identical age of 146.6?±?2.3?Ma was obtained by the U–Pb-SHRIMP method. This Late Jurassic age is also known from the Kerdillion Unit and the Rhodope Terrane. The rather undeformed granites are interpreted as piercing plutons. The small granite stocks sampled have Late Cretaceous to Early Tertiary ages of 66.8?±?0.8?Ma and 68.0?±?1.0?Ma (U–Pb-SHRIMP)/62.8?±?3.9?Ma (Pb–Pb). The main accretionary event was according to these data in the Late Jurassic since all younger rocks show little or no deformation. The age distribution together with the geochemical and isotopic signature and the lithology indicates that the eastern part of the Mount Athos peninsula is part of a large-scale gneiss dome also building the Kerdillion Unit of the eastern SMM and the Rhodope Massif. This finding extends the area of this dome significantly to the south and indicates that the tectonic boundary between the SMM and the Rhodope Massif lies within the AVZ.     

Geochemistry and volcanic setting of the Ordovician Forbordfjell and Jonsvatn greenstones,Trondheim Region,central Norwegian Caledonides

Geochemical patterns in two, correlative, Ordovician, basaltic greenstone formations from the central Norwegian Caledonides corroborate field indications of a tripartite stratigraphic subdivision of the units. Lower and upper members show similar chemical traits and are transitional to LREE-enriched, high-magnesia to low-magnesia basalts. The thicker, more massive, middle member carries the geochemical signature of LREE-depleted, normal mid-oceanic ridge tholeiites. Stratigraphic-upward petrochemical variation discerned within the individual members points to processes of either closed or open-system low-P fractional crystallization. The geochemical trends can be ascribed to a mechanism of spreading accompanied by lithospheric thinning in a marginal ocean basin situation, followed by renewed crustal thickening. Geochemical and lithovolcanic polarity indicators denote that the basin was located to the east of an Arenig-Llanvirn magmatic arc sited within the Caledonian Iapetus Ocean.     

桐庐、相山火山-侵入杂岩单颗粒锆石U-Pb年龄

利用单颗粒锆石Ｕ－Ｐｂ法分别测定了桐庐和相山火山－侵入杂岩中早期及晚期形成的岩石单元的结晶年龄,桐庐杂岩体中早期火山岩的喷发年龄为１３４．９Ｍａ,晚期浅成岩的结晶年龄为１３４．４Ｍａ,杂岩体基本上属于同时形成,为早白垩世岩浆活动的产物;相山杂岩体中第二阶段岩石的结晶年龄为１４０．３Ｍａ,最晚阶段超浅成岩的结晶年龄为１３５．４Ｍａ,杂岩体各单元的形成时间相近,为晚侏罗世末期岩浆活动的产物。可见利用单颗粒锆石Ｕ－Ｐｂ年龄能非常有效地讨论一套岩石组合是否是同时间形成的,而这正是火山－侵入杂岩研究的重要内容之一。