UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland,Swedish Caledonides

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP metamorphic conditions of ~ 3 GPa and 800 °C, within the stability field of coesite, are constrained from geothermobarometry and calculated phase equilibria for the peak-pressure assemblage garnet + omphacite + kyanite + phengite. A prograde metamorphic evolution from a lower P–T (1.5–1.7 GPa and 700–750 °C) stage during subduction is inferred from inclusions of pargasitic amphibole, zoisite and kyanite in garnet cores. The post-UHP evolution is constrained from breakdown textures, such as exsolutions of kyanite and silica from the Ca-Eskola clinopyroxene. Near isothermal decompression of eclogite to lower crustal levels (~ 0.8–1.0 GPa ) led to formation of sapphirine, spinel, orthopyroxene and diopside at granulite facies conditions. Published age data suggest a Late Ordovician (460–445 Ma) age of the UHP metamorphism, interpreted to be related to subduction of Baltoscandian continental margin underneath an outboard terrane, possibly outermost Laurentia, during the final stages of closure of the Iapetus Ocean. The UHP rocks were emplaced from the hinterland collision zone during Scandian thrusting of the nappes onto the Baltoscandian foreland basin and platform. The record of P–T conditions and geochonological data from UHP rocks occurring within the allochthonous units of the Scandinavian Caledonides indicate that Ordovician UHP events may have affected much wider parts of the orogen than previously thought, involving deep subduction of the continental crust prior to final Scandian collision between Baltica and Laurentia.     

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Ts?kkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in J?mtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and J?mtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.     

40Ar/39Ar data on the age and metamorphism of the Ottfjället dolerites,Särv Nappe,Swedish Caledonides

The age of the Ottfjället dolerites, an extensive dyke swarm within the Särv Nappe of the central Scandinavian Caledonides, has earlier been suggested from K/Ar and Rb/Sr data to be either at least 1800 Ma or about 700 Ma. New K/Ar data on mineral separates confirm that the rock contains excess Ar while ⁴⁰Ar/³⁹Ar analyses of plagioclase indicate that intrusion and alteration of the dykes took place 600–700 Ma ago. Reassessment of previous K/Ar data confirms this and provides a more precise age of 665±10 Ma. The K/Ar data indicate that, despite Caledonian orogeny and tectonic transport, much of the Särv Nappe has not been above 300°C since dyke emplacement. Using neutron produced Ar and Kr the excess Ar in plagioclase is shown to be associated with anion sites containing Br and Cl.     

The Arcuate Nappe Structure on the Northern Margin of the Wuliang Moutnains in Western Yunnan

The arcuate nappe structure on the north edge of the Wuliang Mountains in westernYunnan Province is a complex nappe structural system with multiple superimposed structures.The autochthonous system is a WNW-trending arcuate fold belt consisting of the Jurassic andCretaceous and the allochthonous system is mainly composed of Upper Triassic rocks. Generally,the nappe structure moved from south to north, with the hanging wall thrusting in a WNW direc-tion for a distance of over 10km. The deep nappe structural system was formed at depths ofabout 5-10km in an environment not exceeding the greenschist facies. It occurred in theOligocene (about 40-20 Ma).     

Zircon geochronology constraints on the age and nature of ‘Precambrian metamorphic rocks’ in the Xing’an block of Northeast China

The Great Xing’an Range in Northeast China is located in the eastern part of the Central Asian Orogenic Belt. From north to south, the Great Xing’an Range is divided into the Erguna, Xing’an, and Songliao blocks. Previous U–Pb zircon geochronology results have revealed that some ‘Precambrian metamorphic rocks’ in the Xing’an block have Phanerozoic protolith ages, questioning whether Precambrian basement exists in the Xing’an block. We present laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb dating results for zircons from suspected Precambrian metamorphic rocks in the Xing’an block. Meta-rhyolites of the Xinkailing Group in Nenjiang yield magmatic ages of 355.8 Ma. Detrital zircons from phyllites of the Xinkailing Group in Duobaoshan yield populations of ca. 1505, ca. 810, and ca. 485 Ma, with the youngest peak constraining its depositional age to be <485 Ma. Zircons from amphibolitic gneisses of the Xinkailing Group in Nenjiang have magmatic ages of 308.6 Ma. Mylonitic granites of the Xinkailing Group in Nenjiang have zircon magmatic ages of 164 Ma. Detrital zircons from two-mica quartz schists of the Luomahu Group in the Galashan Forest yield ca. 2419, ca. 1789, ca. 801, ca. 536, ca. 480, and ca. 420 Ma, with the youngest peak indicating its depositional age as <420 Ma. Detrital zircons from mylonitized sericite–chlorite schist of the Ergunhe Formation in Taerqi yield populations of 982–948, ca. 519, and ca. 410 Ma, with the youngest peak demonstrating that its depositional age is <410 Ma. These zircon ages for a range of lithologies show that the Great Xing’an Range metamorphic rocks formed during the Phanerozoic (164–485 Ma) and that this crust is mostly Palaeozoic. Based on these results and published data, we conclude that there is no evidence of Precambrian metamorphic basement in the Xing’an block. In summary, the age data indicate that Precambrian metamorphic basement may not exist in the Xing’an region.     

Chronology of eclogite retrogression within the Seve Nappe Complex,Råvvejaure,Sweden: Evidence from40Ar/39Ar mineral ages

Amphibole concentrates from four samples of variably retrogressed eclogite from the Tsäkkok Lens of the Seve Nappe Complex display discordant⁴⁰Ar/³⁹Ar age spectra suggestive of extraneous argon contamination.³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar isotope correlations are well-defined for two samples and yield ages of 464.7 ± 1.3 Ma and 463.2 + 6.3 Ma. Inverse ordinate intercepts are very much larger than 295 and confirm significant extraneous argon components. Two white mica concentrates from metasedimentary units of the Tsäkkok Lens display internally concordant⁴⁰Ar/³⁹Ar age spectra which define plateau ages of 448.2 ± 1.6 Ma and 468.4 ± 0.9 Ma. An amphibole concentrate from a metamorphosed basic dike within the Sarek Lens of the Seve Nappe Complex yields an isotope correlation age of 463.8 ± 12.3 Ma, suggesting a post-metamorphic thermal evolution similar to that of the Tsäkkok Lens. The amphibolite and white mica ages are interpreted to date post-metamorphic cooling through appropriate temperatures for intracrystalline retention of argon following amphibolite facies retrogression of the eclogite facies assemblages. The Seve Nappe Complex appears to have originated within distal facies of the late Proterozoic-early Paleozoic miogeocline of Baltica. The present⁴⁰Ar/³⁹Ar results provide a record of early Paleozoic (pre-Middle Ordovician) tectonothermal activity and therefore are compatible with previous suggestions of significant early Caledonian orogenesis within the Scandinavian Caledonides.

---

Zusammenfassung Die Amphibolitkonzentrate aus vier Proben unterschiedlich regressiver Eklogite aus der Tsäkkok-Linse des Seve-Deckenkomplexes spiegeln diskordante⁴⁰Ar/³⁹Ar Altersspektren wieder. Diese deuten an, daß eine Argonkontamination von außen stattgefunden hat.³⁶Ar/⁴⁰Ar gegen³⁹Ar/⁴⁰Ar Isotopen sind für zwei Proben genau bestimmt und liefern Alter von 464,7 ± 1,3 Mio. und 463,2 ± 6,3 Mio. Jahre. Inverse Ordinatenabschnitte sind sehr viel größer als 295 und bestätigen erhebliche Argon-Fremdkomponenten. Zwei Hellglimmerkonzentrate aus metasedimentären Einheiten der Tsäkkok-Linse zeigen intern konkordante⁴⁰Ar/³⁹Ar Altersspektren, welche Plateaualter von 448,2 ± 1,6 Mio. und 468,4 ± 0,9 Mio. Jahre besitzen. Ein Amphibolkonzentrat aus einem metamorphen basischen Gang innerhalb der Sarek-Linse des Seve-Deckenkomplexes liefert ein Isotopenkorrelationsalter von 463,8 ±12,3 Mio. Jahre, welches eine postmetamorphe thermale Entwicklung andeutet, die ähnlich der Tsäkkok-Linse ist. Die Amphibol- und Hellglimmeralter werden dahingehend interpretiert, daß sie die postmetamorphe thermale Abkühlung durch entsprechende Temperaturen für das intrakristalline Zurückhalten des Argons datieren, die im Anschluß an die Amphibolitfaziesregression der Eklogitfaziesvergesellschaftung folgte. Der Seve-Deckenkomplex scheint aus einer distalen Fazies spätprotero- bis frühpalaeozoischen Miogeosynklinale von Baltika hervorgegangen zu sein. Die jetzigen⁴⁰Ar/³⁹Ar Ergebnisse liefern eine Aufzeichnung von frühpaleozoischen (Vor-Mittelordovizium) tektono-thermalen Aktivitäten, und sie sind daher vergleichbar mit früheren Vermutungen von einer bedeutenden frühkaledonischen Orogenese innerhalb der skandinavischen Kaledoniden.

---

Résumé Les concentrés d'amphiboles de quatre échantillons d'éclogites, rétromorphosés à des degrés divers, provenant de la lentille de Träkkok («Seve Nappe Complex») montre un spectre discordant d'âges⁴⁰Ar/³⁹Ar, ce qui suggère une contamination par un apport d'argon. Les corrélations entre³⁶Ar/⁴⁰Ar et³⁹Ar/⁴⁰Ar sont bien définies pour deux échantillons et fournissent des âges de 464,7 ± 1,3 Ma et 463,2 ±6,3 Ma. Les rapports⁴⁰Ar/³⁶Ar sont beaucoup plus élevés que 295 et confirment un apport extérieur d'argon significatif. Deux concentrés de mica blanc provenant des unités métasédimentaires de la lentille de Tsäkkok présentent des spectres d'âges⁴⁰Ar/³⁹Ar concordants, qui définissent des âges-plateau de 448,2 ± 1,6 Ma et 468,4 ± 0,9 Ma. Un concentré d'amphibole provenant d'un dyke basique métamorphisé dans la lentille des Sarek du «Seve Nappe Complex» fournit par corrélation isotopique un âge de 463,8 ± 12,9 Ma, ce qui suggère une évolution thermique post-métamorphique similaire à celle de la lentille de Tsäkkok. Les âges fournis par les amphiboles et les micas blancs sont interprétés comme correspondant au refroidissement post-métamorphique, à travers des températures permettant la rétention intracristalline de l'argon, à la suite de la rétromorphose des éclogites dans le faciès des amphiboles. Le «Seve Nappe Complex» a dû être engendré dans les faciès distaux du miogéosynclinal tardi-protérozoïque à éo-paléozoïque de la Baltique. Les valeurs actuelles de⁴⁰Ar/³⁹Ar enregistrent l'activité tectonothermique du Paléozoïque inférieur (pré-Ordovicien moyen); elles sont donc en accord avec l'hypothèse antérieurement émise d'une orogenèse éo-calédonienne au sein des Calédonides Scandinaves.

---

Tsäkkok Seve, , ⁴⁰Ar/³⁹Ar. , -. ³⁶Ar/⁴⁰Ar ³⁹Ar/⁴⁰Ar 464,7±1,3 , 463,2±6,3 . 295 , . Tsäkkok , 448,2±1,6 468,4±0,9 . Sarek Seve 463,8±12,3 ., , Tsäkkok. , , , coxpa . Seve, , - . ⁴⁰Ar/³⁹Ar - , .. . .

     

Complex basin evolution in the Gökova Gulf region: implications on the Late Cenozoic tectonics of southwest Turkey

Southwestern Turkey experienced a transition from crustal shortening to extension during Late Cenozoic, and evidence of this was recorded in four distinct basin types in the Mu?la–Gökova Gulf region. During the Oligocene–Early Miocene, the upper slices of the southerly moving Lycian Nappes turned into north-dipping normal faults due to the acceleration of gravity. The Kale–Tavas Basin developed as a piggyback basin along the fault plane on hanging wall blocks of these normal faults. During Middle Miocene, a shift had occurred from local extension to N–S compression/transpression, during which sediments in the Eskihisar–T?naz Basins were deposited in pull-apart regions of the Menderes Massif cover units, where nappe slices were already eroded. During the Late Miocene–Pliocene, a hiatus occurred from previous compressional/transpressional tectonism along intermountain basins and Yata?an Basin fills were deposited on Menderes Massif, Lycian Nappes, and on top of Oligo–Miocene sediments. Plio-Quaternary marked the activation of N–S extension and the development of the E–W-trending Mu?la–Gökova Grabens, co-genetic equivalents of which are common throughout western Anatolia. Thus, the tectonic evolution of the western Anotolia during late Cenozoic was shifting from compressional to extensional with a relaxation period, suggesting a non-uniform evolution.     

Stratigraphy and evolution of the Galve sub-basin (Spain) in the middle Tithonian–early Barremian: Implications for the setting and age of some dinosaur fossil sites

A review of the stratigraphy of the Galve sub-basin (western Maestrazgo Basin, eastern Spain) around the Jurassic–Cretaceous transition is presented here, based on new data acquired after extensive geological mapping and logging complemented with facies analysis, new biostratigraphic data and a revision of the published information available. The results obtained are relevant for a more detailed understanding of the tecto-sedimentary evolution of the studied basin during the transition between two stages of rift evolution (i.e., syn-rift sequences 1 and 2). In addition, new information on the age and setting of numerous dinosaur fossil- and track-sites found across the Galve sub-basin and in the northern part of the nearby Penyagolosa sub-basin is provided here. Two new lithostratigraphic units are defined and characterized, the Aguilar del Alfambra and the Galve formations. The previous stratigraphic framework considered only two lithostratigraphic units (the Villar del Arzobispo and El Castellar formations) bounded by a single regional unconformity, and this resulted in significant misinterpretations. The whitish limestones, red lutites and cross-bedded sandstones of the Aguilar del Alfambra Formation were deposited in transitional environments, ranging from coastal lutitic plains to restricted lagoons. Of particular interest are the laminated micritic-peloidal limestones with abundant fenestral porosity (supratidal ponds to intertidal flats), which preserve common dinosaur footprints. This unit is bounded by widespread unconformities and is of very variable thickness (0–450 m), controlled by extensional tectonics operating at the climax of syn-rift sequence 1 during the latest Tithonian–middle Berriasian. The overlying Galve Formation is of variable thickness (from 0 to 100 m) and is also bounded by regional unconformities described in detail here. It consists of red lutites with cross-bedded and tabular-burrowed sandstones representing channel and overflow deposits in an alluvial floodplain. The sauropod dinosaur Aragosaurus ischiaticus found in this unit has a controversial age assignment. The age of the Galve Formation is poorly constrained from late Berriasian to Hauterivian, but new biostratigraphic data presented here, combined with the correlation with the nearby Penyagolosa and Salzedella sub-basins, suggest a possible equivalence to the upper Berriasian–lower Valanginian sequence deposited during the initial stage of syn-rift sequence 2.     

Zircon geochemistry and U–Pb age at rare metal deposits of syenite in the Ukrainian Shield

The distribution of rare and rare earth elements in zircon at the Yastrebets, Azov (Zr–REE–Y), and Perzhan (Be) rare metal deposits of the Ukrainian Shield was studied. Additional evidence for magmatic genesis of these deposits is obtained: unaltered zircon is characterized by a magmatic REE distribution spectrum with a somewhat higher δ¹⁸O value than that of the mantle (6.6‰ on average). The final formation stage of the deposit was marked by predominance of fluids enriched in Y, REE, Nb, and heavy oxygen, resulting in anomalous geochemical characteristics of zircon rims and alteration zones (up to 81500 Y ppm, over 10300 ppm Nb, and 13.9‰ δ¹⁸O). The age of zircon formed in ore-bearing Yastrebets and Azov nonnepheline syenite deposits was estimated at ~1770 Ma (U–Pb, SHRIMP-II).     

The metamorphic evolution of migmatites from the Ötztal Complex (Tyrol,Austria) and constraints on the timing of the pre-Variscan high-T event in the Eastern Alps

Within the Ötztal Complex (ÖC), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the ÖC, the Winnebach migmatite in the central part and the Verpeil- and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central ÖC. The P-T data range from 670–750 °C and < 2.8 kbar for the pre-Variscan event, 550–650 °C and 4–7 kbar for the Variscan event and 430–490 °C and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-Th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-Silurian metamorphic event in the ÖC.     

Origin of atoll garnet in schists from the Alpuj��rride Complex (Central zone of the Betic Cordillera, Spain): Implications on the P-T evolution

Atoll-shaped and normal garnets from schists of the Betic Cordillera (Spain) were studied by electron microprobe, scanning electron microscopy and microstructural electron backscattered scanning diffraction (EBSD). Medium-grade schists contain a textural variety of atoll garnets, characterized by the presence of muscovite, annite, and quartz as main ??core?? phases. Zoning patterns, EBSD analyses and themobarometric data indicate that the micaceous intergrowths formed in most cases through breakdown of a first garnet generation, with orientation of micas being controlled by garnet (c*_mica//[111]*_Grt and c*_mica//[110]*_Grt as main crystallographic relationships). Rings formed from multiple nucleation and coalescence, with orientation being controlled, in some cases, by the relics of the initial garnet and more generally by mica orientation. P-T estimates indicate that the first stage of garnet growth occurred at relatively high P (9?C12?kbar/500?C550°C) whereas the second metamorphic stage occurred at lower P and slightly higher T conditions (5?C7?kbar/500?C600°C).     

Zircon U–Pb age constraints on the Paleoproterozoic sedimentary basement of the Ediacaran Porongos Group,Sul-Riograndense Shield,southern Brazil

Thick quartzites record significant information on cratonic environments during long geological periods. The capacity to resist weathering and deformation turn the quartzite covers especially useful in the provenance studies of Precambrian basins. Provenance of 194 detrital zircon grains from two samples of thick quartzite cover on the Paleoproterozoic Encantadas Complex displays mostly Paleoproterozoic (95%) and minor Archean (5%) sources. The results indicate that sediments were derived from the La Plata Craton with the maximum depositional age at 2.03 Ga possibly up to 1.7 Ga. In comparison, the adjacent Porongos Group has provenance data of 61 detrital zircon grains indicating mostly Mesoproterozoic (69%), subordinately Paleoproterozoic (26%) and minor Archean ages (5%). Considering previous published data, the Porongos Group is Ediacaran in age and probably chronocorrelated with sedimentary basins from the Tandilia Belt (Argentina). Therefore, the quartzite cover and the Porongos Group require distinct evolution in time and in tectonic environment.     

U-Pb zircon age from the base of the Ediacaran Doushantuo Formation in the Yangtze Gorges,South China： constraint on the age of Marinoan glaciation

The reported new U-Pb age by sensitive high-resolution ion microprobe (SHRIMP Ⅱ) on zircon was obtained from a tuff sample at the basal Doushantuo Formation in the Jiuqunao section, which situated at the western limb of the Huangling anticline in the Yangtze Gorges in Zigui, Hubei, South China. Eighteen spots of zircons were analyzed and they form two clusters: one includes three spots, with an inherited age of 784+ 15 Ma (MSWD=0.05); the other consists of 15 spots and gives a weighted mean ^206pb/^238U age of 628.3&#177;5.8 Ma (MSWD=0.86). It is the first SHRIMP U-Pb zircon age obtained nearly the base of the Doushantuo Formation of Ediacaran and represents a maximum age of the Doushantuo Formation It also forms an age constraint on the upper limit age of the Nantuo (Marinoantype) glaciation.     

U–Pb TIMS age constraints on the evolution of the Neoproterozoic Meatiq Gneiss Dome,Eastern Desert,Egypt

Ages are used to constrain the temporal evolution of the Meatiq Gneiss Dome, Eastern Desert, Egypt, by dating (ID-TIMS) pre-, syn-, and post-tectonic igneous rocks in and around the dome. The Um Ba’anib Orthogneiss, comprising the deepest exposed structural levels of the dome, has a crystallization age of 630.8 ± 2 Ma. The overlying mylonites are interpreted to be a thrust sheet/complex (Abu Fannani Thrust Sheet) of highly mylonitized metasediments (?), migmatitic amphibolites, and orthogneisses with large and small tectonic lenses of less-deformed intrusives. Two syn-tectonic diorite lenses in this complex have crystallization ages of 609.0 ± 1.0 and 605.8 ± 0.9 Ma, respectively. The syn-tectonic Abu Ziran diorite, cutting across the tectonic contact between mylonite gneisses of the Abu Fannani Thrust Sheet and a structurally overlying thrust sheet of eugeoclinal rocks (“Pan-African nappe”), has a magmatic emplacement age of 606.4 ± 1.0 Ma. Zircons from a gabbro (Fawakhir ophiolite) within the eugeoclinal thrust sheet yielded a crystallization age of 736.5 ± 1.2 Ma. The post-tectonic Fawakhir monzodiorite intrudes the ophiolitic rocks and has an emplacement age of 597.8 ± 2.9 Ma. Two other post-tectonic granites, the Arieki granite that intrudes the foliated Um Ba’anib Orthogneiss, and the Um Had granite that cuts the deformed Hammamat sediments, have emplacement ages of 590 ± 3.1 and 596.3 ± 1.7 Ma, respectively. We consider formation of the Meatiq Gneiss Dome to be a young structural feature (<631 Ma), and our preferred tectonic interpretation is that it formed as a result of NE–SW shortening contemporaneous with folding of the nearby Hammamat sediments around 605–600 Ma, during oblique collision of East and West Gondwana.     

Geochronologic Constraints on the Magmatic Underplating of the Gangdise^ Belt in the India-Eurasia Collision： Evidence of SHRIMP II Zircon U-Pb Dating

Abstract  Abundant small mafic intrusions occur associated with granitoids along the Gangdisê magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro‐pyroxenite zone occurs along the southern margin of the Gangdisê belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends ～1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdisê gabbro‐pyroxenite assemblage is most likely a result of underplating of mantle‐derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa‐Xigazê segment of the mafic intrusive zone, and was followed by zircon SHRIMP II U‐Pb dating. In addition to the ages of two samples previously published (47.0±1 Ma and 48.9±1.1 Ma), the isotopic ages of the remaining four gabbro samples are 51.6±1.3 Ma, 52.5±3.0 Ma, 50.2±4.2 Ma and 49.9±1.1 Ma. The range of these ages (47–52.5 Ma) provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdisê belt at ca. 50 Ma. This underplating event post‐dated the initiation of the India‐Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP II U‐Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479–526 Ma (weighted average age 503±10 Ma), thus yielding information about the pre‐existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian‐Eurasia collision, possibly corresponding in time to the formation of the 14–16 km‐thick “crust‐mantle transitional zone” characterized by Vp = 6.85–6.9 km/s.     

Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Rämepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz-tourmaline-sulphide veins located mainly within an intensively altered and sheared zone about 20-30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H₂O-CH₄ (Й equiv. wt% NaCl), and (2) H₂O-CO₂ (⢬ equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310-350 and 220-250 °C respectively. Both types of fluid inclusions contain 1-3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH₄) to a more oxidising (CO₂) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.     

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit,China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.     

Isotope-geochemical constraints on the formation of the early Earth’ crust

Analysis of isotope-geochemical data obtained for the early crustal complexes of the Earth provided constraints on the formation time, scales of development, and geochemical features of protocrust. Most informative were isotope-geochemical and geochemical data on the oldest zircons with ages up to 4.4 Ga, short-lived ¹⁴⁶Sm/¹⁴²Nd isotope system, and lead isotope composition of the oldest rocks of Greenland. The presence of positive ¹⁴²Nd anomaly in the rocks of West Greenland and negative anomaly in the amphibolites of the oldest Nuvvuagittuq greenstone belt of the Superior province (O’Neil et al., 2008) indicates the early differentiation of the Earth material into depleted mantle and enriched (basaltic) crust (Caro et al., 2006; Benett et al., 2007a, b; O’Neil et al., 2008). Pb-Pb isotopic systematics of the oldest crustal rocks from West Greenland and Labrador testifies that high μ enriched crust (²³⁸U/²⁰⁴Pb = 10.9) of basaltic composition already existed 3.9 Ga ago (Kamber et al., 2003). Based on isotope-geochemical and geochemical features of the oldest zircons in the Late Archean greenstone belts of the Yilgarn block (Western Australia), the crust of intermediate-felsic composition and water on the Earth’s surface already existed 4.4 Ga ago (Wilde et al., 2001).     

Timing of juvenile arc crust formation and evolution in the Sapat Complex (Kohistan–Pakistan)

The combination of age determination and geochemical tracers allows understanding the source evolution during magmatism. We studied the Sapat Complex, in the exhumed Cretaceous Kohistan Paleo-Island Arc, to reconstruct the formation of the juvenile lower arc crust and the evolution of the mantle source during arc magmatism. High precision ID-TIMS U/Pb dating on zircon, shows that a protracted period of magmatic accretion formed the Sapat Complex between 105 and 99 Ma. Since continued melt percolation processes that formed the lower crust obscured the original bulk rock Nd–Pb–Sr isotopic composition, we rely on the Hf isotopic composition of zircons of different ages to unravel the source evolution. Nd and Pb bulk isotopic compositions coupled with Hf isotopic composition on zircons allow reconstructing a geodynamical scenario for the Sapat Complex, and the Cretaceous history of the Arc. We suggest that trenchward migration of the hot mantle source at 105 Ma explains the small heterogeneous εHf signal between + 14 and + 16. This heterogeneity vanished within ca. 2 million years, and the εHf of the source evolved from + 16 to + 14 at 99 Ma. Integrated to the Kohistan Cretaceous history, which has a baseline of εHf ≈ 14, these data pinpoint two geodynamical events, with slab retreat and the formation of the Sapat Complex followed by splitting of the Kohistan island arc at 85 Ma.