Amphibolites of the Central zone of metamorphism of the Rai-Iz massif (Polar Urals)

   Research subject. The material composition, formation conditions, and age of amphibolites of the Central zone of metamorphism (CZM) of the Rai-Iz massif.

   Materials and methods. Microprobe studies of minerals were conducted
using a Cameca-SX100 microanalyzer; the content of petrogenic elements was determined by an X-ray multichannel spectrometer CPM-35; the REE content was determined by a mass spectrometer with inductively coupled plasma NexION 300S in the Collective Use Center “Geoanalitik”, Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences. ⁴⁰Ar/³⁹Ar dating was carried out at the Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences according to A.V. Travin’s method. Results. The petrography and geochemistry of the amphibolites of the CZM Rai-Iz massif were studied; the parameters of their metamorphism were established, and the age of ⁴⁰Ar/³⁹Ar was determined. Two types of amphibolites were identified: garnet amphibolites, epidote-garnet amphibolites, and clinopyroxene-amphibole and amphibole-phlogopite-garnet rocks. The main minerals were found to be amphibole and
garnet. Amphibole corresponds to edenite, pargasite, and ferropargasite. In some amphibole samples, chemical zonation was established, manifested in the depletion of the marginal parts of Al₂O₃ and FeO grains relative to the central ones, while the MgO content increased from the center to the edge. Garnets from garnet and epidote-garnet amphibolites exhibited an almandine-grossular composition with pyrope (3–16 %) and spessartine (3–13 %) components; in amphibole-phlogopite-garnet rocks, garnet demonstrated an almandine–pyrope composition. In the pomegranate, the following chemical zonation was established: the FeO content increases from the center to the edge of the grains, while the MnO content decreases. The age of amphibolites determined by the 40Ar/39Ar method (405.2 ± 5.4 Ma) corresponds to the Lower Devonian. The averaged formation parameters of amphibole-bearing rocks correspond to P = 6–13 kbar, T = 430–860° C, and to the boundary of amphibolite and greenschist facies and the boundary of amphibolite and granulite facies. REE spectra in the studied rocks, normalized relative to chondrite, showed the predominance of heavy lanthanides over light ones. The character of REE distribution is close to N-MORB basalts.

   Conclusion. The studied rocks formed at the boundary of the early and middle Devonian. Their formation was associated with regional metamorphism in the conditions of the onset of collision.

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