TIME CONSTRAINTS ON THE FORMATION OF THE KANDALAKSHA AND KERETSK GRABENS OF THE WHITE SEA PALEO-RIFT SYSTEM FROM NEW ISOTOPIC GEOCHRONOLOGICAL DATA

Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations.

Multi-chronometer dating of the Souter Head complex: rapid exhumation terminates the Grampian Event of the Caledonian Orogeny

ABSTRACTThe Souter Head sub-volcanic complex (Aberdeenshire, Scotland) intruded the high-grade metamorphic core of the Grampian Orogen at 469.1 ± 0.6 Ma (uranium-238–lead-206 (238U–206Pb) zircon). It follows closely peak metamorphism and deformation in the Grampian Terrane and tightly constrains the end of the Grampian Event of the Caledonian Orogeny. Temporally coincident U–Pb and argon/argon (40Ar/39Ar) data show the complex cooled quickly with temperatures decreasing from ca.800 °C to less than 200 °C within 1 Ma. Younger rhenium–osmium (Re–Os) ages are due to post-emplacement alteration of molybdenite to powellite. The U–Pb and Ar/Ar data combined with existing geochronological data show that D2/D3 deformation, peak metamorphism (Barrovian and Buchan style) and basic magmatism in NE Scotland were synchronous at ca.470 Ma and are associated with rapid uplift (5–10 km Ma−1) of the orogen, which, by ca.469 Ma, had removed the cover to the metamorphic pile. Rapid uplift resulted in decompressional melting and the generation of mafic and felsic magmatism. Shallow slab break-off (50–100 km) is invoked to explain the synchroneity of these events. This interpretation implies that peak metamorphism and D2/D3 ductile deformation were associated with extension. Similarities in the nature and timing of orogenic events in Connemara, western Ireland, with NE Scotland suggest that shallow slab break-off occurred in both localities.

Permian post-collisional basic magmatism from Corsica

<p>Post-Variscan early Permian magmatism is widespread in Corsica with mafic dykes emplaced during the extensional tectonic phase which followed the Variscan orogeny. This study focuses on a mafic dyke swarm intruded in the region of Ajaccio (Corsica, France). New U-Pb zircon geochronological data show that these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes have a calc-alkaline affinity, while a few dykes show tholeiitic affinity resembling N-MORB basalts. Calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. We interpret these data as evidence that an enriched mantle source, which was likely formed during Variscan subduction, sourced the calc-alkaline suite, while a depleted mantle component dominates the source of the tholeiitic suite. Notably, coeval Permian mafic intrusive bodies from throughout Corsica and from the Southern, Central and Western Alps display similar ages and geochemical features to the Ajaccio dyke swarm. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt</p>

Geochemistry of basic magmatism of Western Antarctic Rift: implications for volatiles storage and recycling in the mantle

<p>The petrologic study of olivine-hosted melt inclusions (MIs) from alkaline primary Cenozoic basalts of Northern Victoria Land (Antarctica) provide new insights on the role of volatiles in the onset of rift-related magmatism. The concentration of volatile species (H<sub>2</sub>O, CO<sub>2</sub>, F, Cl) have been determined by Secondary Ion Mass Spectrometry (SIMS) on a selection of MIs which have been previously re-homogenized at high pressure and temperature conditions in order to avoid any heterogeneity and reducing the H diffusion. The least differentiated MIs vary in composition from basanitic to alkaline basalts, analogously to what is found in McMurdo volcanics, while their volatile concentrations reach up to 2.64 wt% H<sub>2</sub>O, 3900 ppm CO<sub>2</sub>, 1377 ppm F and 1336 Cl. Taking into account the most undegassed MIs a H<sub>2</sub>O/(H<sub>2</sub>O+CO<sub>2</sub>) ratio equal to 0.88 was determined, which in turn brings the CO<sub>2</sub> content in the basanitic melt with the highest water content up to 8800 ppm.</p><p>Major and trace element melting modelling indicate that basanite and alkali basalt composition can be reproduced by 3 and 7% of partial melting of an amphibole-bearing spinel lherzolite respectively. Assuming a perfect incompatible behavior for H<sub>2</sub>O and CO<sub>2</sub> these melting proportions allow to constrain the water and CO<sub>2</sub> contents in the mantle source in the range 780-840 and 264-273 ppm respectively. The resulting CO<sub>2</sub>/Nb, CO<sub>2</sub>/Ba and H<sub>2</sub>O/Ce ratio are lower than those estimated for Depleted MORB Mantle (DMM), suggesting that the NVL Cenozoic alkaline magmatism could be originated by an enriched mantle source composed by a range from 70% to 60% of Enriched Mantle (EM) and from 30% to 40% of Depleted Morb Mantle (DMM).</p><p>A global comparison of fluid-related, highly incompatible and immobile/low incompatible elements such as Li, K, Cl, Ba, Nb, Dy and Yb allow to put forward that the prolonged (~500 to 100 Ma) Ross subduction event played a fundamental role in  providing the volatile budget into the lithospheric mantle before the onset of the Cenozoic continental rifting.</p>

Paleoarchean U–Pb (SIMS SHRIMP-II) age of mafic granulites of the bug complex (Ukrainian shield)

Mafic granulites (metamorphosed tholeite-komatiite volcanic rocks) represent the inclusions in gneissenderbite of the Bug complex (southwest of the Ukrainian shield). The studied mafic inclusion has two zircon generations of magmatic origin that yield, respectively, SHRIMP zircon U–Pb ages of 3628±58 Ma and 2845±65 Ma. The first age is the oldest date the earliest stage of basic magmatism on the Ukrainian shield. The age of the second zircon is interpreted as a result of partial melting, synchronous with the metamorphism in the area.