Chevkinite-(Се) and Perrierite-(Се) from the Island Arc Quartz Gabbronorite–Dolerite of the Ayu-Dag Intrusion, Crimean Mountains

Abstract

Chevkinite-(Се) and perrierite-(Се), which are characteristic minerals of alkaline igneous rocks and high-temperature alkaline alterations, are typical accessory minerals in the Mesozoic island-arc gabbros of the Crimean Mountains. The early magmatic mineral assemblages from these low-alkali gabbros of the Ayu-Dag intrusion contain accessory Ti–Zr–LREE minerals depleted in SiO2: chevkinite-(Се), perrierite-(Се), zirconolite, and baddeleyite. Chevkinite and perrierite occur as isolated short prismatic crystals of up to 0.1 mm and strong corroded inclusions in the cores of the late-magmatic allanite-(Ce) crystals. Both minerals are one of the major concentrators of lanthanides in the Aju-Dag gabbros. Lanthanides are distributed in chevkinite and perrierite as follows: Ce $$ \gg $$ La > Nd $$ \gg $$ Pr > Gd, Sm, Tb, and Dy. The lanthanide ratios in chevkinite and perrierite are similar, % REE: 54–58 Ce, 25–31 La, and 12–20 Nd. The Zr, Th, Y, and Sc concentrations in both polymorphic modifications from the Ayu-Dag gabbros are identical. Chevkinite-(Ce) that occurs as isolated crystals is Mg-dominant; its composition is [Cе1.5La0.8–0.9Nd0.3–0.4Pr0.1(Gd + Sm + Tb + Dy)0.1Y0.2Sc0.1Ca0.7–0.8Na0.1]4(Mg0.5 $${\text{Fe}}_{{{\text{0}}{\text{.5}}}}^{{{\text{3 + }}}}$$ )1( $${\text{Fe}}_{{{\text{1}}{\text{.1}} - 1.2}}^{{{\text{3 + }}}}$$ Ti0.4–0.7Zr0.1–0.2Al0.1–0.2)2Ti2[(Si3.7–4Al0.3–0)4O22]. Chevkinite-(Ce) enclosed in allanite is depleted in Mg; its composition is [Cе1.5–1.7La0.6–0.9Nd0.5–0.6Pr0.1(Gd + Sm)0.1Y0.1–0.2Sc0.1Ca0.6–0.8Na0.1]4( $${\text{Fe}}_{{{\text{0}}{\text{.6}} - {\text{1}}}}^{{{\text{2 + }}}}$$ Mg0–0.1 $${\text{Fe}}_{{{\text{0}} - 0.4}}^{{{\text{3 + }}}}$$ )1(Ti0.8–0.9 $${\text{Fe}}_{{{\text{0}}{\text{.3}} - 0.7}}^{{{\text{3 + }}}}$$ Al0.2–0.4Zr0–0.2)2Ti2[(Si3.9–4Al0.1–0)4O22]. Perrierite-(Ce) that forms euhedral crystals is Mg-dominant; its composition is [Cе1.2–1.6La0.6–0.8Nd0.3–0.4 Pr0.1(Gd + Sm + Tb + Dy)0.1Y0.1Sc0.1Ca1.0–1.1Sr0–0.1Na0.1]4(Mg0.9–1 $${\text{Fe}}_{{{\text{0}}{\text{.1}} - 0}}^{{{\text{3 + }}}}$$ )1( $${\text{Fe}}_{{{\text{1}}{\text{.0}} - 1.2}}^{{{\text{3 + }}}}$$ Al0.2–0.7Ti0.3–0.5Zr0–0.2)2 Ti2[(Si3.9–4Al0.1–0)4(O21.3–21.5OH0.7–0.5)22]. Perrierite-(Cе) enclosed in allanite is depleted in Mg; its composition is [Cе1.3–1.5La0.7–0.8Nd0.3–0.5Pr0.1(Gd + Sm)0.1Y0.1Sc0.1Ca1.1–1.3Na0.1]4( $${\text{Fe}}_{{{\text{0}}{\text{.4}} - 0.5}}^{{{\text{2 + }}}}{\text{Fe}}_{{{\text{0}}{\text{.2}} - 0.4}}^{{{\text{3 + }}}}$$ Mg0.2)1(Al0.5–0.7 Ti0.3–0.8 $${\text{Fe}}_{{{\text{0}}{\text{.3}} - 0.9}}^{{{\text{3 + }}}}$$ Zr0.1–0.3)2Ti2[(Si3.9–4Al0.1–0)4(O21.4–22OH0.6–0)22]. The presence of chevkinite, perrierite, and zirconolite in an unusual geochemical environment is possibly caused by weak late-magmatic processes in the shallow gabbro intrusions, which are depleted in fluids, of the Crimea Mesozoides.