Чевкинит-(Се) и перрьерит-(Се) из островодужных кварцевых габбро-норит-долеритов интрузива Аю-Даг, Горный Крым

Abstract

Сhevkinite-(Се) and perrierite-(Се) are minerals characteristic for alkaline magmatic rocks and high-temperature alkaline metasomatites, and yet they are typical accessory minerals in Mesozoic island-arc gabbroids of the Rocky Crimea. Early magmatic mineral associations in these low-alkaline rocks of Ayu-Dag intrusion include accessory Ti—Zr—LREE minerals under-saturated in silica: chevkinite-(Се), perrierite-(Се), zirconolite, baddeleyite. Сhevkinite and perrierite occur as isolated shortly prismatic crystals up to 0.1 mm in size and strongly corroded inclusions in center of late-magmatic allanite-(Ce) crystals. Both minerals are the main concentrators of lanthanides in Aju-Dag gabbroids. Distribution of lanthanides in them is as following: Ce ˃> La > > Nd >> Pr > Gd, Sm, Tb, Dy. Contents of lanthanides in chevkinite and perrierite are identical: Ce (54—58 % REE) — La (25—31 %) — Nd (12—20 %). Contents of Zr, Th, Y, Sc in both polymorphs are identical also. Сhevkinite-(Ce) of isolated crystals is magnesium-dominant; its composition is: [Cе 1.5 La 0.8—0.9 Nd 0.3—0.4 Pr 0.1 (Gd+Sm+Tb+Dy) 0.1 Y 0.2 Sc 0.1 Ca 0.7—0.8 Na 0.1 ] 4 (Mg 0.5 Fe 3+ 0.5 ) 1 × (Fe 3+ 1.1—1.2 Ti 0.4—0.7 Zr 0.1—0.2 Al 0.1—0.2 ) 2 Ti 2 [(Si 3.7—4 Al 0.3—0 ) 4 O 22 ]. Сhevkinite-(Ce) from inclusions in allanite is poor in Mg, its formula is: [Cе 1.5—1.7 La 0.6—0.9 Nd 0.5—0.6 Pr 0.1 (Gd+Sm) 0.1 Y 0.1—0.2 Sc 0.1 Ca 0.6—0.8 Na 0.1 ] 4 × (Fe 2+ 0.6—1 Mg 0—0.1 Fe 3+ 0—0.4 ) 1 (Ti 0.8—0.9 Fe 3+ 0.3—0.7 Al 0.2—0.4 Zr 0—0.2 ) 2 Ti 2 [(Si 3.9—4 Al 0.1—0 ) 4 O 22 ]. Perrierite-(Ce) composing idiomorphic crystals is magnesium-dominant, its composition is: [Cе 1.2—1.6 La 0.6—0.8 Nd 0.3—0.4 Pr 0.1 × (Gd+Sm+Tb+Dy) 0.1 Y 0.1 Sc 0.1 Ca 1.0—1.1 Sr 0—0.1 Na 0.1 ] 4 (Mg 0.9—1 Fe 3+ 0.1—0 ) 1 (Fe 3+ 1.0—1.2 Al 0.2—0.7 Ti 0.3—0.5 Zr 0—0.2 ) 2 Ti 2 × [(Si 3.9—4 Al 0.1—0 ) 4 (O 21.3—21.5 OH 0.7—0.5 ) 22 ]. Perrierite-(Ce) occurring inside allanite is poor in Mg, with the formula: [Cе 1.3—1.5 La 0.7—0.8 Nd 0.3—0.5 Pr 0.1 (Gd+Sm) 0.1 Y 0.1 Sc 0.1 Ca 1.1—1.3 Na 0.1 ] 4 (Fe 2+ 0.4—0.5 Fe 3+ 0.2—0.4 Mg 0.2 ) 1 × (Al 0.5—0.7 Ti 0.3—0.8 Fe 3+ 0.3—0.9 Zr 0.1—0.3 ) 2 Ti 2 [(Si 3.9—4 Al 0.1—0 ) 4 (O 21.4—22 OH 0.6—0 ) 22 ]. Presence of chevkinite, perrierite and zirconolite in unusual geochemical settings may be explained perhaps by poor development of late-magmatic processes in near-surface intrusions of Crimeaʼs Mesozoids. Perhaps, the presence of chevkinite and perrierite, as well as zirconolite in unusual geochemical settings can be explained by poor development of late-magmatic processes in near-surface intrusives of the Mountain Crimea mesozoids.