Установлен возраст щелочных рудных (редкометалльно-редкоземельных) метасоматитов Новобобровского рудного поля. Калиевый полевой шпат является одним из ведущих минералов фенитов и фенитизированных пород. Согласно рентгеноструктурным данным, калиевый полевой шпат является максимальным (низким) микроклином, температура кристаллизации оценивается на уровне 450 С. В химическом составе микроклина содержание Na2O – 0,3–0,4 мас. %. Полученный возраст (40Ar/39Ar) составил 564,0 ± 7,8 млн лет. We determined the age (Ar-Ar by microcline) of alkaline ore (rare-metal-rare-earth) metasomatites of the Novobobrovskoe ore field. The Novobobrovskoe ore field is a part of the Kosyu group of rare-metal-rare-earth occurrences developed on the Chetlassky Kamen of the Middle Timan. Quartz veins and fenites (alkaline metasomatites - albite, microcline, aegirine) are developed on the Late Riphean quartzite sandstones of the Svetlinskaya and Novobobrovskaya formations of the Chetlassky series. Riphean alkaline picrites of the Chetlassky complex also take part in the structure of the ore field. The latter is presumably associated with complex (Nb-LREE) mineralization. The main ore minerals are columbite and monazite. Potassium feldspar is one of the fenite and fenitized rock minerals. Its composition and structure were studied at the CCU “Geonauka” (Syktyvkar). The structural control of the mineral affiliation was carried out by X-ray diffraction analysis (diffractometer Shimadzu XRD-6000 Cu-anode, current – 30 mA, voltage – 30 kV, Ni filter, scanning angles 2θ from 2 to 60 degrees, scanning step 2θ – 0.05, shooting speed – 1 g/min). The unit cell parameters of the minerals were calculated by the Unit Cell software. Microprobe studies were carried out by Tescan Vega 3 LMH spectral electron microscope with X-MAX 50mm Oxford Instruments energy dispersive detector. 40Аr/39Ar dating was carried out by the method of stepwise heating of potassium feldspar monofraction in the Institute of geology and mineralogy SB RAS. According to the X-ray diffraction studies, potassium feldspar is the maximum microcline, the crystallization temperature is estimated at 450 °C. The chemical composition of the microcline presents the sodium oxide content not more than 0.5 wt %. According to the Ar-Ar data, the age of the microcline of the ore vein rocks (fenitized quartzite sandstones) of the Novobobrovskoe ore field is 564.0 ± 7.8 Ma. Geochronological data correlate with previously obtained isotope-geochronological methods (U-Th-Pb, Sm-Nd) data on ore minerals monazites, thorites, and columbites – 552 ± 31 and 581 ± 47 Ma. Also, the obtained result is close to the age of ultrabasic rocks (Ar-Ar method based on phlogopites from alkaline picrites) 598.1 ± 6.2 Ma.