Zircons of the Permian age (280–290 Ma) from intrusive magmatic rocks in Riphean structure of the Southern Urals

Research subject. Magmatic complexes that are developed in the lower (volcanogenic-sedimentary) part of the Ai Formation of the Lower Riphean of the Taratash anticline in the Southern Urals. Results. For the frst time, the Lower Permian SHRIMP dating (288.6 ± 3.1 Ma by U-Pb method on zircon from monzogabbro) was obtained for a dike cutting the basalts of the Lower Paleozoic (420–450 Ma) age. Conclusions. On the eastern slope of the Urals there is a chain of massifs which are close in the age. The chain belongs to the Lower Permian Stepninsky monzogabbro-granosyenite-granite complex, represented by the Uiski, Vandyshevski, Biryukovski and Stepninski intrusions with the age of 281 ± 2, 280 ± 2 and 286 ± 2 Ma, correspondingly (U-Pb method after zircons, SHRIMP-II, VSEGEI) and earlier obtained dates 281 ± 4 Ma (Rb-Sr isochrone) and 283 ± 2 Ma (isotope Pb-Pb method after zircons). The Stepninsky complex was described earlier as plume-dependent. The monzogabbro dike, described in this paper, although being at a considerable distance from the Sepninsky complex, is situated at a strike of the chain of the stepninsky intrusions, is close to them by the composition and age and can be ascribed to the same plume episode. The idea of the plume character of the complex was ехрressed by us relatively long ago based on a superimposed character of the chain of the intrusions over the earlier, collisional Uralian structures. As for the geochemical character (monzogabbro) the dike conforms with one of two standard trends of the Stepninsky complex – monzonite (monzogabbro, monzodiorites, syenites).

THE FIRST U-PB SHRIMP DATING OF ZIRCONS FROM CAPITANIAN (MIDDLE PERMIAN) DEPOSITS OF THE OMOLON MASSIF (NORTHEAST RUSSIA)

U-Pb dating of zircons from two samples of stratotype sections of the Gizhiga Formation of the Middle Permian of the Omolon Massif was first performed. Weighted average ages are 266 ± 2 and 265 ± 3 Ma taking into account the error, are consistent with Capitanian age previously established by paleontological determination. The presence of a detrital zircon population made it possible to identify several sources in the Omolon Basin. Pre–permian zircons are associated with erosion of basement deposits of the Omolon Massif, and the Middle Paleozoic volcanic rocks of the Kedon series and Permian population of zircons linked with volcanic activity of the Okhotsk-Taigonos volcanic arc.

Visean overprint of the Devonian-Early Carboniferous granites: result of Variscan collisional stage revealed by zircon SHRIMP dating (Tribeč Mts., Western Carpathians)

<p>The granites with I- and S-type affinity in the Variscan segments of the Alpine West-Carpathian edifice belong to the oldest intrusions within the European Variscides. Granites and granodiorites of the West-Carpathian crystalline basement are mostly classified as S-type, whereas tonalities and granodiorites belong to the I-type suite. Both suites probably originated in the volcanic arc setting as product of subduction-related regime in the Galatian superterrane (Broska et al. 2013). The I- and S-type granite bodies were firstly identified in the West-Carpathian Tribeč Core Mountains and the new SHRIMP and CHIME datings recognised their Visean geotectonic overprint. The subduction-related I-type granites show the age span 364-358 Ma followed by the intrusion of the S-type granites dated by SHRIMP on 358 Ma. The bimodal SHRIMP data of a dyke placed within S-type granites show ages 351 Ma and 330 Ma, or primary vs. alteration age. The CHIME age from monazite dating shows 347 Ma because monazite indicate probably early stage of massive granite alteration perhaps during collisional process, younger zircons represents later phase of the event.  CHIME dating of newly formed monazite in greisenised S-type granite gives the age 344 Ma. The granite showing strong greisenization (total degradation of feldspars and formation of quartz - white mica assemblages) is dated by SHRIMP on 355 Ma. The greisenised granite contains abundant tourmaline with high dravitic molecule, Sr-rich apatite and common monazite. Abundant tiny stoichiometrically pure apatite grains in this granite indicate their exsolution from feldspars enriched in phosphorus. The S-type granite dyke from the ridge of the Tribeč Mts gives zircon SHRIMP age 355 Ma and CHIME monazite age 342 Ma. The dating results of the Tribeč granites identified: (<strong>1</strong>) older Upper Devonian/Lower Mississippian subduction-related I-type tonalites (ca. 364-351 Ma), and (<strong>2</strong>) S-type granites Middle/Upper Mississippian (Visean) intruding in time span 342-330 Ma reflecting probably of the collisional event in the Variscan orogeny. Dual evolution of the Tribeč Mts. Variscan granitic rocks is partly corroborated by Hf isotopes from the dated zircons with εHf<sub>(t)</sub> = +3.5 ~ –2.4 for the older granites, and εHf<sub>(t)</sub> = –0.3 ~ –4.9 for the younger ones. The evolution of the I- and S-type granites seems to be rather different from the granite evolution known in the Bohemian Massif and therefore the origin of Variscan hybrid granites from the Western Carpathians we placed on the SW side of Galatian volcanic arc as result of Paleo-Tethys subduction (see Stampfli and Borel, 2002, Stampfli et al. 2013).</p><p>Acknowledgments: Support from Slovak Research and Development Agency: APVV SK-KR-18-0008, APVV-14-0278/, APVV-18-0107, and VEGA 2/0075/20 are greatly appreciated.</p>

Timing of HP/HT alpine metamorphism: new data from Cima di Gagnone (Central Alps)

<p>Keywords: HP-HT metamorphism, microstructures, U-Pb-Th dating, P-T-t-d path.</p><p>The occurrence of (ultra)high pressure and high temperature mineralogical assemblages developed during the Alpine phases makes the Cima di Gagnone area (Cima Lunga unit) one of the most studied area in the Central Alps. It consists of continental basement rocks (orthogneisses, paragneisses and metapelites) enveloping (ultra-) mafic bodies of oceanic crust (eclogite, amphibolites and peridotites) which record pressure and temperature up to 3 GPa and 800 °C, respectively (e.g. Nimis and Trommsdorff, 2001; Scambelluri et al., 2015). This high-grade metamorphism is constrained between 40 and 35 Ma by U-Pb dating from the ultra-mafic and mafic rocks (e.g. Gebauer, 1999). The metamorphism peak of the surrounding gneiss complex is instead constrained at considerably lower conditions (up to 0.8 GPa and 660 °C; Grond et al., 1995). The temperature peak in the felsic rocks is dated at ca. 32 Ma (Gebauer, 1996), coeval with the Bergell emplacement. Several models have been proposed to explain the coupling between ultrahigh- and middle- pressure rock pairs resulting in a large uncertainty in the adopted subduction-exhumation models.</p><p>We performed new petrological, micro-structural and geochronological data from the gneissic rocks, with the aim to investigate how the pressure and temperature conditions experienced by the felsic and mafic rocks are truly different. We explored the spatial variation of the metamorphic record through sample collection the structural control of the inclusion-matrix couples. Petrological and microstructural (SEM-EBSD) analyses are performed to define the deformation and metamorphic patterns of samples collected. Our results indicate that some portions of the gneissic matrix preserve relicts of higher pressure and temperature than previously suggested. The high-T conditions are temporally constrained by U-(Th)-Pb dating of monazite and zircon, which provides peak age estimations similar to the mafic rocks. The new data shed a light on heterogeneous metamorphism recorded by different rocks, providing new elements for the discussion on the most fitting geodynamic models.</p><p>REFERENCES</p><p>- Gebauer, 1996. A P-T-t Path for an (Ultra?-) High-Pressure Ultramafic/Mafic Rock-Association and its Felsic Country-Rocks Based on SHRIMP-Dating of Magmatic and Metamorphic Zircon Domains. Example: Alpe Arami (Central Swiss Alps). Earth Processes Reading the Isotopic Code, Geophysical Monograph 95, 307-329, AGU.</p><p>- Gebauer, 1999. Alpine geochronology of the Central Alps and Western Alps: new constraints for a complex geodynamic evolution. Schweiz. Mineral. Petrogr. Mitt., 79, 191-208.</p><p>- Grond, R., Wahl, F. and Pfiffner, M., 1995. Mehrphasige alpine Deformation und Metamorpshe in der nordlichen Cima Lunga-Einheit, Zentralalpen (Scweiz). Schweiz. Mineral. Petrogr. Mitt., 75, 371-386.</p><p>- Nimis, P. & Trommsdorff, V., 2001. Revised thermobarometry of Alpe Arami and other garnet peridotites from the central Alps. J. of Petrology, 42, 103-115.</p><p>- Scambelluri, M., Pettke, T., & Cannaò, E. (2015). Fluid-related inclusions in Alpine high-pressure peridotite reveal trace element recycling during subduction-zone dehydration of serpentinized mantle (Cima di Gagnone, Swiss Alps). Earth and Planetary Science Letters, 429, 45-59.</p>