Origin and composition of fluid responsible for metasomatic processes in shear zones of the Bolshie Keivy tectonic nappe,baltic shield: Carbon isotope composition of graphite

Doklady Earth Sciences -     

First Evidence of the Archean Age of Orogenic Gold of the Russian Part of the Karelian Craton (Fennoscandian Shield): Sm–Nd Mineral Isochron for Gold-Bearing Metasomatites of the Novye Peski Deposit

One unsolved problem of Archean metallogeny of the Russian part of the Fennoscandian Shield is related to the period of formation of orogenic gold deposits, the age of which was previously considered Early Proterozoic. Comprehensive studies identified the complex evolution of ores from the Novye Peski deposit and the first Neoarchean Sm–Nd isochron isotopic age (2583 ± 16 Ma) of the formation of orogenic gold of the Karelian Craton. A significant positive ε_Nd(t) value of +4.7 of gold-bearing metasomatites probably points to the link between the ore-forming fluid and the deep mantle source.

     

Scheme of mineral facies of metamorphic rocks

The proposed PT grid of mineral facies of metamorphic rocks, which retains the commonly adopted nomenclature (greenschist, epidote-amphibolite, amphibolite, granulite, glaucophane-schist, and eclogite facies), is based on original calculations and the published calculated and experimental data on mineral equilibria. To validate the facies and subfacies PT boundaries, the mineral equilibria in metapelitic and metabasic rocks have been used.     

The age of high-pressure metasomatism in shear zones during collision-related metamorphism in the Lapland granulite belt: The Sm-Nd method of dating the paragenesises from sillimanite-orthopyroxene rocks of Por’ya Guba nappe

The Sm-Nd isotope-chronological method is used for the first time to investigate the whole rock and rock-forming minerals from metasomatic sillimanite orthopyroxene rocks in the Por’ya Guba nappe of the Lapland granulite belt. As a result isochrone values of the age are obtained, which allows estimate the minimum time intervals that have passed from the moment of formation of the investigated rocks. They indicate that peak conditions of collision metamorphism in fluid-permeable shear zones of the LGB occurred in the Paleoproterozoic Svekofennian age when high-temperature high-pressure sillimanite-orthopyroxene metasomatic rocks were formed. The obtained data make it possible to assume that, with regard to the measurement errors, high-pressure metasomatism did not last over 10 Ma, and this process did not coincide in time with older medium pressure metamorphism.     

An Abnormal Isotopic Composition of Oxygen in Zircon from Corundum-Bearing Metasomatites of the Dyadina Gora Ore Occurrence,Belomorian Mobile Belt

Doklady Earth Sciences - For the first time, the isotopic composition of oxygen, the content of trace elements, and the U–Pb zircon age were determined for kyanite–gedrite metasomatites...