Zirconolite, Baddeleyite, Zircon, and Thorite of Island-Arc Quartz Gabbronorite-Dolerites of the Ayu-Dag Intrusive, Crimean Mountains

Abstract

The Early Bajocian island-arc quartz gabbronorite-dolerites of the Ayu-Dag intrusive contain syngenetic zirconolite and baddeleyite. Zirconolite includes Ca-dominant and atypical Y-dominant (Y, Ca, Th, REE)₂FeZr₂Ti₃O₁₄ varieties. Two genetic types of zircon and thorite occur. The dominant xenogenic zircon-I with thorite inclusions and numerous melting traces (lacunes) is enriched in Hf, Th, Y, and P (up to 6 wt % HfO₂, 5 wt % ThO₂, 6 wt % Y₂O₃, 3 wt % P₂O₅). Thorite contains 7–9 wt % UO₂. Zircon-I and thorite-I of similar compositions are typical minerals of highly radioactive granites. The isotope age of xenogenic zircon is more than 2 Ga. Rare syngenetic zircon-II is extremely depleted in Th, U, Y, and Hf. Thus, the Ayu-Dag basic magma was contaminated by Early Precambrian highly radioactive granites, whose bodies probably occur in the basement of Crimean Mountains.