| Summary: | The paper provides details of local geology and mineralogy of the Tazheran Massif, which was the sampling site of perovskite used as an external standard in perovskite U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively-coupled plasma (LA−ICP−MS) mass spectrometry. The Tazheran Massif is a complex of igneous (mafic dikes, syenite, nepheline syenite), metamorphic (marble), and metasomatic (skarn, calc−silicate veins) rocks. Metasomatites are thin and restricted to the complex interior being absent from the margins. Perovskite has been studied at four sites of metasomatic rocks of three different types: forsterite−spinel calc−silicate veins in brucite marble (1); skarn at contacts between nepheline syenite and brucite marble (2), and skarn-related forsterite−spinel (Fo-Spl) calc−silicate veins (3). Pervoskite from Fo-Spl calc−silicate veins (type 1) is almost free from impurities (<1 wt.% in total: 0.06%−0.4% REE<sub>2</sub>O<sub>3</sub>, 0.10%−0.22% Nb<sub>2</sub>O<sub>5</sub>, ≤0.1% ThO<sub>2</sub>). The U contents are from 0.1 to 1.9 wt.% UO<sub>2</sub> and are relatively uniform in perovskites from the same vein but differ from vein to vein of this type. Perovskite from the contact skarn (type 2) contains 1.5 to 4.5 wt.% REE<sub>2</sub>O<sub>3</sub> but is poor in other impurities. Perovskite grains from skarn-related Fo-Spl calc−silicate rocks (type 3) belong to two generations that differ in REE, Nb, Th, Fe, and Na concentrations. Early-generation perovskites occurs as compositionally homogeneous octahedral or cubic-octahedral crystals with contents of impurities higher than in other varieties (3.6 wt.% REE<sub>2</sub>O<sub>3</sub>, 1.6 wt.% Fe<sub>2</sub>O<sub>3</sub>, 1.3 wt.% Nb<sub>2</sub>O<sub>5</sub>, 0.7 wt.% ThO<sub>2</sub>, 0.6 wt.% UO<sub>2</sub>, and 0.6 wt.% Na<sub>2</sub>O) but the lowest is at the respective site. Late-generation varieties show highly variable impurity concentrations of 1.5 to 22.7 wt.% REE<sub>2</sub>O<sub>3</sub>, 0.4 to 8.4 wt.% Nb<sub>2</sub>O<sub>5</sub>, and 0.8 to 4.5% ThO<sub>2</sub>, while the perovskite component may be as low as 65%. In addition to the lueshite and loparite, components, they contain REEFeO<sub>3</sub> and Th<sub>0.5</sub>TiO<sub>3</sub> endmembers which have no natural analogs.
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