Lithogenetic systematization and formation conditions of the Wenlockian sequences belonging to the Subpolar Urals and the Chernyshev Ridge

Research subject. Wenlock deposits in the Subpolar Urals and southern part of the Chernyshev Ridge. Materials and methods. Carbonate and terrigenous-carbonate rock samples from sections of the Subpolar Urals (outcrops 212, 217) and the southern part of the Chernyshev Ridge (outcrop 479) were studied by isotope analysis of carbonate carbon and oxygen.Results. The isotopic composition of the studied sections varies across a wide range of δ13С (–6.4…–0.05‰) and δ18O (20.0–26.9‰). Therefore, three time intervals characterized by distinctive isotopic characteristics can be distinguished, roughly corresponding to early Sheinwoodian (I), late Sheinwoodian (II) and Homerian (III). Interval I is characterized exclusively by the rocks of outcrop 479 with δ13С (–3.6…–3.0‰) and δ18O (22.4–23.6‰). In Interval II, the average values of isotopic characteristics of outcrop 479 indicate a tendency to weighting carbon (–5.5…–3.5‰) and somewhat heavier oxygen (23.2–25.2‰) isotopes. In outcrop 212, the average isotopic values for carbon and oxygen vary from –2.9 to –1.3‰ and from 21.9 to 24.3‰, respectively. In outcrop 217, the average values are δ13C (–1.8…–0.8‰) and δ18O (22.4–25.4‰). In Interval III, the average values of carbon isotopic composition in outcrop 479 are becoming heavier from –2.5 to –0.7‰. In outcrop 212, the isotopic values of oxygen (21.9–23.1‰) and carbon (–4.9…–2.5‰) tend to become weighting; however, in outcrop 217, the average isotopic values of δ13C (–1.9…–0.5‰) and δ18O (22.3–24.5‰) remain constant. The conducted litho-facial studies showed that the weighting carbon isotopic composition ranging from –6.4…to –3.0‰ is associated with an increase in microbial activity in sediments, the manifestation of vadose-phreatic conditions, and the intake of light carbon dioxide with a flow of terrigenous material from the earth. In the latter case, oxygen isotopic values are also the most lightweight (20.0–23.0‰).Conclusions. The obtained isotopic characteristics of the Wenlock rocks under study indicate the expedience of identifying three time intervals and their correlation with paleogeographic reconstructions of Wenlockian sedimentation in the Timan-northern Ural region.

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A UNIQUE FINDING OF ZIRCON INTO BADDELEYITE TRANSFORMATION (THE ICHETJU ORE OCCURRENCE, THE MIDDLE TIMAN)

Proceedings of higher educational establishments Geology and Exploration ◽

10.32454/0016-7762-2018-1-27-35 ◽

2018 ◽

pp. 27-35

Author(s):

S. G. Skublov ◽

A. O. Krasotkina ◽

A. B. Makeyev ◽

O. L. Galankina ◽

A. E. Melnik

Keyword(s):

High Temperature ◽

Experimental Studies ◽

Middle Timan ◽

Natural Analogues ◽

Opposite Situation ◽

Formation Conditions

Findings of the growth relationships between baddeleyite and zircon are rare, due to significant differences in the formation conditions of the minerals. A reaction replacement (partial to complete) of baddeleyite by zircon is possible during metamorphism accompanied by the interaction with high-Si fluids. The opposite situation, when zircon is replaced by baddeleyite, is extremely rare in the nature. Transformation of zircon from polymineral (compound) ore occurrence Ichetju (the Middle Timan) with the formation of microaggregates of baddeleyite, ratile and florencite has been found out. The size of the largest segregations of baddeleyite does not exceed 10 microns in diameter. Microaggregates are unevenly related to the rim of zircon with a thickness of 10 to 50 rfn, voids and cracks across the grain. Altered zircon rim (a mixture of newly formed minerals) is characterized by sharply increased composition of REE (especially LREE), Y, Nb, Ca, Ti. The composition of Th and U also increases. An overview of the experimental studies on the reaction between zircon and baddeleyite and single natural analogues allows to make a conclusion that the most likely mechanism of the transformation of zircon from ore occurrence Ichetju to baddeleyite (intergrowth with ratile and florencite) is due to the effect of interaction of primary zircon with high-temperature (higher than 500—600°C) alkaline fluids transporting HFSE (REE, Y, Nb, Ti). This is indirectly confirmed by the findings of zircon with anomalous high composition of Y and REE up to 100000 and 70000 ppm respectively.

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