S. A. Sergeev

A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain)

The hot emplacement of the Ronda peridotites (Betic Cordilleras) developed a dynamothermal aureole and partial melts that led to the intrusion of granite dykes in the peridotites. Previous geochronological data place rather broad limits for this event between 22 and 19 Ma. Analyses of neocrystalline zircon rims from large zircon populations yield a U–Pb SHRIMP age of 22.3±0.7 Ma for the dynamothermal aureole formation, and intrusion ages of granite dykes between 22.6±1.8 and 21.5±3.8 support that conclusion. Therefore, these new ages provide a more robust constraint on the hot emplacement of the Ronda peridotites at middle crustal levels.

Single zircon U-Pb ages and geochemistry of granitoid gneisses from SW Poland: evidence for an Avalonian affinity of the Brunian microcontinent

Seven granitoid gneisses from the contact zone between the eastern margin of the Variscan belt and the Brunian microcontinent in SW Poland have been dated by ion-microprobe and 207 Pb/ 206 Pb single zircon evaporation methods. The zircons define two age groups for the gneiss protoliths: (1) late Neoproterozoic c. 576–560 Ma and (2) early Palaeozoic c. 488–503 Ma granites. The granitoid gneisses belonging to the basement of the Brunian microcontinent contain abundant Mesoproterozoic to latest Palaeoproterozoic inherited material in the range of 1200–1750 Ma. The gneisses of the Variscan crustal domain lack Mesoproterozoic inherited zircon cores. Trace element geochemistry of Proterozoic gneisses reveals features resembling either volcanic arc or post-collisional granites. The studied rocks are geochemically similar to other Proterozoic orthogneisses derived from the basement of the Brunian microcontinent. Gneisses with early Palaeozoic protolith ages are geochemically comparable to granitoid gneisses widespread in the adjacent Sudetic part of the Bohemian Massif and are considered characteristic of peri-Gondwanan crust. Our data prove the dissimilarity between the Brunia plate and the westerly terranes of the Variscan belt. The occurrence of granitic gneisses with late Neoproterozoic protolith ages and widespread Mesoproterozoic inheritance in our dated samples support an East Avalonian affinity for the Brunian microcontinent. In contrast, the abundance of gneisses derived from an early Palaeozoic granitic protolith and devoid of Mesoproterozoic zircon cores supports the Armorican affinity of the Variscan domain bordering on the Brunia plate from the west. Structural evidence shows that the eastern segment of the Variscan belt is juxtaposed against the Brunian microcontinent along a N–S-trending tectonic contact, possibly equivalent to the Rheic suture.

SHRIMP U-Th-Pb zircon dating of the granitoid massifs in the Malé Karpaty Mountains (Western Carpathians): evidence of Meso-Hercynian successive S- to I-type granitic magmatism

SHRIMP U-Th-Pb zircon dating of the granitoid massifs in the Malé Karpaty Mountains (Western Carpathians): evidence of Meso-Hercynian successive S- to I-type granitic magmatism Representative granitic rock samples from the Malé Karpaty Mountains of the Western Carpathians (Slovakia) were dated by the SHRIMP U-Th-Pb isotope method on zircons. Oscillatory zoned zircons revealed concordant Mississippian magmatic ages: 355±5 Ma in Bratislava granodiorite, and 347±4 Ma in Modra tonalite. The results document nearly synchronous, successive Meso-Hercynian plutonic events from S-type to I-type granites. The Neo-Proterozoic inherited zircon cores (590±13 Ma) were identified in the Bratislava S-type granitic rocks whereas scarce Paleo-Proterozoic inherited zircons (1984±36 Ma) were detected within the Modra I-type tonalites.

Age and geochemistry of zircons from the ancient granitoids of the Vyg River, Southeastern Karelia

The structure and composition of accessory zircons from the tonalites of the Vyg River, southeastern Karelia, were investigated. Their local U-Pb SHRIMP dating yielded ages between 3127±15 and 3146±25 Ma. It was shown that the zircons consist of three zones, a central part containing solid and melt inclusions and zoned magmatic and metasomatic shells. The obtained ages correspond to the magmatic and metasomatic stages of zircon crystallization. In general, the zircons have elevated contents of LREE (up to 867 ppm La), which were mainly accumulated in the outer metasomatic shell. Apatite and CO2 inclusions are widespread. Orthoclase, orthopyroxene, ilmenite, galena, quartz, and bastnaesite were identified in a solid inclusion in one zircon core using a CAMSCAN MX 2500 electron microscope. The presence of bastnaesite accentuates the relation of LREE with a CO2-rich fluid. It was shown that REE content is not correlated with U, Th, and U/Th ratio.

Paleoarchaean tonalites in the Orekhovo-Pavlogradskaya palaeoproterozoic collsional zone (Ukrainian Shield)

The novel detailed geological and geochronological data for the tonalitie rocks and products of its structural and metamorphic overprint in the Orekhovo-Pavlogradskaya collision zone (Ukrainian Shield) are discussed. Magmatic crystallization of tonalites (3500 ± 13 Ma) followed by amphibolite (2863 ± 22 Ma) and granulite (2105 ± 40 Ma) facies metamorphic events, resulting in biotite-garnet gneisses formation. The last date also indicating the time of collision event. The earliest age of 3.64 Ga was found in the zonal zircon fragment within 3.5-Ga old magmatic zircon from tonalite.

Extremely Radiogenic Hafnium in the Zircons from Precambrian Calciphyres

In order to identify the possible presence of a mantle component, we studied the hafnium isotopic composition of zircons from the calciphyres. Zircon crystals are heterogeneous with a blocky structure. Besides zircon generations with ages corresponding to the crystallization age of calciphyres, there are ancient cores. Geochemical studies of zircons, in particular the study of local REE distribution in them using SIMS SHRIMP-II, showed that these minerals are characterized by specific REE distribution patterns. A distinctive feature of zircons is their prominent geochemical heterogeneity in terms of REE distribution patterns and variations of the Th/U ratio (from 0.01 to 0.4) even within individual grains. These features make it possible to suggest that zircons grew from fluid whose parameters varied widely owing to geochemical fractionation in the carbonate‐carbonic fluid system [2]. Taking into account the coherent behavior of hafnium and neodymium in the mantle-derived rocks, the e Hf ( T ) elevated with respect to the “terrestrial array” in the diagram e Hf – e Nd [3, 4] (i.e., e Hf ( T ) > 1.4 e Nd ( T ) , in the given case e Hf ( T ) > ‐23) can be considered as a criterion for the presence of mantle components in the crustally contaminated rocks. The Hf isotopic composition in zircons was measured by the local method using the LA-MC-ICPMS complex consisting of a ThermoFinnigan Neptune multicollector ICP mass spectrometer and the New Wave DUV-193 laser ablation system. The configuration of the collectors of the mass spectrometer allowed simultaneous measurement of the isotopes 172 Yb, 174 Yb, 175 Lu, 176 Hf, 177 Hf, 178 Hf, and 179 Hf . All ratios were corrected for mass discrimination by normalizing to the 178 Hf/ 177 Hf ratio. The correct 176 Hf value was obtained by subtraction of 176 Yb and 176 Lu ( 172 Yb and 175 Lu free from interferences were measured). The measurement error was no more than ± 1.5 e Hf units. The studied zircons are close to standard 91 500 in terms of Hf concentrations (about 6000 ppm) and lower in the 176 Lu/ 177 Hf ratio (<0.002). This indicates that the correction for radiogenic hafnium at the expense of in situ radiogenic decay of 176 Lu is lower than the measurement errors of the hafnium isotopic composition.

Is there a time lag between the metamorphism and emplacement of plutons in the Axial Zone of the Pyrenees

Detailed petrographic and geochemical studies conducted on zircons from the Lys-Caillaouas pluton reveal their igneous and metamorphic affinities. The igneous zircons constrain the emplacement of the pluton to 300±2 Ma. By contrast, the metamorphic zircons yield an older age of 307±3 Ma, which probably dates the thermal peak of the HT/LP Variscan metamorphism. Therefore, a short time lag of c . 7 Ma emerges between the metamorphic climax and emplacement of the pluton in the Axial Zone (Pyrenees).

New age data on the Archean rocks of the Vodlozero domain, Baltic shield, and their significance for geodynamic reconstructions

1 The Vodlozero domain composes the eastern part of the Fenno Karelian province of the Baltic shield. A significant part of its area is occupied by rocks of the tonalite–trondhjemite–granodiorite (TTG) series. The available U–Pb zircon datings on some of these rocks are ~3.24 Ga [1–3], which makes it possible to consider the domain as the “ancient core” of the Bal tic shield [4, 5]. The marginal parts of the domain are made up of greenstone belts consisting mainly of meta morphosed komatiite–basaltic volcanics. The Sm–Nd age of the volcanics is 3.0–2.9 Ga [5, Table 4.1].

Composition and age of metaorthopyroxenite xenoliths and host enderbite gneisses in the Pobuzhie granulite complex, Ukrainian Shield

The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite, diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to 6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence, the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the Novopavlovsk complex in the Azov Region.

Geochronological and isotope geochemical characteristics of mafic intrusions of the Norilsk region

Layered intrusions of the Norilsk group contain unique deposits of copper-nickel, cobalt and plati� noid ores, and platinoids. The origin of silicate matrix was traditionally considered to be mantle. However, the first study of the isotopic composition of fluid components (He and Ar (1) and sulfur (2)) revealed largescale mantle-crust interaction. The complex isotopic investigations of rocks and ores of mafic intrusions of the Norilsk region performed at the Karpinskii Geological Research Institute in 2003- 2008 demonstrated the quantitative genetic indicators allowing us to refine the models of the formation of unique Cu-Ni-PGE deposits. We performed several hundred isotopic analyses: isotopic composition of helium and argon from fluid microinclusions in rocks and ores; sulfur, copper, and nickel from sulfides. The absolute (isotopic) age of sul� fide ores and zircons from rocks in intrusions with var� ious ore potential was determined. To obtain an inte� grated pattern, we also used the latest data of Russian and foreign researchers in this region (3-5 and refer� ences therein). The helium isotopic composition ( 3 Не/ 4 Не ratio) is considered to be the only reliable and strong criterion of the link between mineralforming fluids and the mantle. It is revealed that the ratio of helium isotopes in the Earths upper mantle (1.2 × 10 -5 ) is ~1000 times higher than in helium formed in rocks of the Earths crust (~ 2 × 10 -8 ). This allows us to reveal and calculate the portion of mantle helium, if it exceeds 1% of the total helium in the sample. Measurements demon� strated that crustal helium dominated in paleofluids from the Norilsk intrusions and the contribution of mantle helium ranged from 1 to 22%. The contribu� tion of helium of such origin is especially low (1-4%) in rich and intermediate intrusions. Poor intrusions significantly differ by this parameter (4-22%). The argon isotopic composition ( 40 Ar/ 36 Ar) pro�