U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Pb-Sr-O-C isotope compositions of metacarbonate rocks of the Derbina Formation (East Sayan): Chemostratigraphic and geochronological significance

The Pb-Sr-O-C isotope compositions of calcite marbles of the Derbina Formation, exposed in the northwestern part of the Derbina block of the East Sayan, were studied. Rocks of the Derbina Formation were metamorphosed under high-temperature amphibolite facies conditions. The carbonate constituent of marbles contains (ppm) 15–130 Mn, 130–160 Fe, 0.008–0.039 Rb, 645–2190 Sr, 0.565–0.894 U, and 0.288–1.42 Pb. These concentrations are similar to those in modern carbonate sediments. The values of δ¹³C in marbles of the Derbina Formation range from–0.6 to +1.4‰ PDB; the values of δ¹⁸O range from 21.5 to 28.6‰ SMOW. The ⁸⁷Sr/⁸⁶Sr ratio values in the two least altered rocks, which meet geochemical criteria of the Rb-Sr system preservation in high-grade carbonate rocks, are 0.70804 and 0.70829. The protolith ages of marbles determined using Sr and C chemostratigraphy lie within the interval of 560–530 Ma, which is regarded as the period of carbonate sedimentation. The slope of the straight line on the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram (n = 9, MSWD = 19) constructed on the basis of the data points of bulk carbonate constituents of all samples studied and those representing leachate steps of one of them in 0.5N HBr yields Late Vendian age (556 ± 31 (2σ) Ma. Taking into account the data on Sr and C isotope systematics of Derbina marbles, this age is regarded as the age of early diagenesis of carbonate sediments close to the age of sedimentation. Thus, metacarbonate rocks of the Derbina Formation preserved the pre-metamorphic chemostratigraphic and isotope-geochronological information. The age obtained testifies that formation of the carbonate cover of the Derbina block occurred in the Late Vendian. At the end of the Cambrian, carbonate deposits were metamorphosed during the Early Caledonian tectonic event in the southeastern part of the Central Asian Fold Belt.     

The Sr isotopic characterization and Pb-Pb age of carbonate rocks from the Satka formation,the Lower Riphean Burzyan Group of the southern Urals

The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formations corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the ⁸⁷Sr/⁸⁶Sr ratio of sedimentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leaching are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial ⁸⁷Sr/⁸⁶Sr ratio in the least altered limestones of this formation is within the range of 0.70460–0.70480. Generalization of the Sr isotopic data published for the Riphean carbonates from different continents showed that 1650–1350 Ma ago the ⁸⁷Sr/⁸⁶Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux.     

U-Pb age and Sr-chemostratigraphy of limestone from the Sorna Formation,Azyr-Tal Range,Kuznetsk Alatau

The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation out- cropped on the Azyr-Tal Range, Kuznetsk Alatau. The weighted average value from three calculated values is 523 ± 5 Ma (MSWD = 0.02, 2σ). This age coincides with the Early Cambrian age interval determined with Sr isotopic chemostratigraphy (⁸⁷Sr/⁸⁶Sr ratio is 0.70850–0.70852). The low value of μ₂ = ²³⁸U/²⁰⁴Pb for limestone is due to the evolution of diagenetic fluid containing lead from the mantle reservoir into the Sorna sediments.     

Age of Carbonates and Phosphorites in the Sedimentary Cover of the Tuva–Mongolian Microcontinent

The ages of the two main stages of carbonate cover formation within the Tuva–Mongolian microcontinent have been determined. The Pb–Pb age and Sr–chemostratigraphic characteristics of the carbonate rocks of the Irkutnaya, Agaringol, and Zabita formations, as well as of the Zabita Formation phosphorite, demonstrate that the first stage began in the Middle Riphean (Late Ectasian) at about 1290 Ma and the second stage started in the Early Vendian (Early Ediacaran) at 630–600 Ma. The accumulation of phosphorite deposits started in the Ediacaran at about 580 Ma.     

Pb—Pb Age of the Bakal Ore Field Riphean Magnesite

For the first time, the age of magnesite in the Lower Riphean Bakal Formation of the Southern Urals is determined by the U—Pb (Pb—Pb) method: it is equal to 1366 ± 47 Ma (MSWD = 18). The stage of magnesite formation of the Bakal ore field was associated with the Mashak rifting pulse and took place prior to the formation of industrial deposits of the Bakal siderite.     

Diagenesis of Carbonate and Siderite Deposits of the Lower Riphean Bakal Formation,the Southern Urals: Sr Isotopic Characteristics and Pb-Pb Age

The Rb-Sr and U-Pb systematics were studied for carbonate rocks of the Lower Riphean Bakal Formation of the southern Urals and related siderite ores of the Bakal iron deposit. The least-altered limestones taken at a significant distance from the Bakal ore field satisfy the strict geochemical criteria of retentivity: Mn/Sr < 0.2, Fe/Sr < 0.5, and ⁸⁷Sr/⁸⁶Sr (difference between the measured ⁸⁷Sr/⁸⁶Sr values in secondary and primary carbonate phases) < 0.001. The least-altered carbonate phases were extracted by the stepwise dissolution in 0.5 N HBr. The Pb-Pb date of limestones (1430 ± 30 Ma) defines the age of early diagenesis of carbonate sediments of the Bakal Formation. The ⁸⁷Sr/⁸⁶Sr ratio in the sedimentary environment of the Bakal carbonates (0.70457–0.70481) yields isotopic signature for the Early Riphean seawater. The Pb-Pb age of metasomatic siderites (1010 ± 100 Ma), which formed at the end of the main ore formation stage and did not undergo late epigenesis, corresponds to the final phases of the Grenville tectonogenesis. Siderites of the main ore formation stage are confined to central parts of the thickest carbonate units and have high ratios of ⁸⁷Sr/⁸⁶Sr (0.73482–0.73876) and ²⁰⁸Pb/²⁰⁴Pb (41.4–42.9). Iron-bearing solutions formed during the diagenesis of mainly Lower Riphean clayey rocks and migrated along low-density zones and faults. The solutions discharged at the interformational unconformity between the Bakal and Zigalga formations. At the contact with shales, carbonate rocks and siderites experienced the later epigenetic dolomitization (partial desideritization) caused by the circulation of solutions enriched in radiogenic ⁸⁷Sr and low-radiogenic ²⁰⁶Pb. This dolomitization occurred simultaneously with the Cadomian tectonothermal activation of the region.__________Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2005, pp. 227–249.Original Russian Text Copyright © 2005 by Kuznetsov, Krupenin, Ovchinnikova, Gorokhov, Maslov, Kaurova, Ellmies.     

Diversity of Archaeocyaths and Sr Chemostratigraphy of the Lower Cambrian of West Transbaikalia,Uda–Vitim and Birami–Yanguda Zones

Doklady Earth Sciences - The morphology of archaeocyaths from the Oldynda Formation of the Uda–Vitim lithofacial zone and the Aikta Formation of the Birami–Yanguda Zone of West...