Armalcolite-bearing island arc plagiolherzolites and olivine gabbro-norite-dolerites of the area of Balaklava, Crimea

Abstract

Armalcolite (Mg,Fe)Ti2O5 is characteristic mineral of titanian basalts of the Moon, as well as lamproites, other alkaline magmatic Earth rocks, impactites and other formations. It is widely represented in island-arc low-alkaline plagiolherzolites and olivine gabbro-norite-dolerites of the Early-Bajocian Pervomaysk–Ayu-Dag complex of mesozoids in Rocky Crimea, specifically, in the littoral Balaklava cliffs near Sevastopol. Crystals of cumulative Mg-olivine contain the inclusions of alumomagnesiochromite and Mg alumochromite in the center and the inclusions of alumochromite partly replaced by chromespinels of the 2nd generation (Ti-Al ferrychromite and chrometitanomagnetite) in the external zones. Chromespinels of the 1st generation are moderately chromic and ferrian (f = 4555), contain 8–14 mass.% Fe2O3 and up to 1,2% TiO2; that sharply distinguishes them from chromespinels of alpinotypes ultrabasic rocks. Chromespinels of the 2nd generation are more ferrian (f = 5875) and contain up to 26 mass.% Fe2O3, 2,5–9% TiO2 and up to 1,8% V2O3. The typical basic trend from ferrian alumomagnesiochromite to chrometitanomagnetite is characteristic for Balaklava chromespinels. Tabular armalcolite crystals are up to 0,60,2 mm included in intercumulus magnesian bronzite and augite, anorthite and bytownite. Balaklava armalcolite contains 63–66 mass.% TiO2, 6,9–8,4% MgO, 9,9–11,7% FeO, 13–18% Fe2O3, up to 1,5% Al2O3 and V2O3, up to 0,7% Cr2O3. The armalcolite formulae is (Mg0,38–0,45Fe2+0,30–0,34Fe3+0,36–0,49V0,04–0,05Al0–0,06Cr0–0,02Ti1,73–1,77)3O5; in minals, molar %: 38–45 MgTi2O5, 31–36 Fe2+Ti2O5, 23–27Fe3+2TiO5. Rare baddeleyite with 1,6–3,1% HfO2(ZrO2:HfO2 = 57–30, av. 44) and zirconolite associate with armalcolite. Zirconolite contains 4–6,5 mass.% Y2O3, 4–8% REE2O3, from traces up to 4,5% ThO2, <0,5% UO2, 0,7–1,4% HfO2 (ZrO2:HfO2 = 47–25, av. 38). The distribution of lanthanides in Balaklava zirconolite is Ce = Nd > Dy > Gd > Sm, Yb, Er, Lu, Tb > Pr. The zirconolite formulae is (Ca0,7Y0,2REE0,1)Zr(Ti1,6Fe2+0,3Fe3+0,1)2O7. The recent formations are ilmenite-I with 5–8 mass.% MgO and 0,3–0,6% MnO, titanomagnetite, hydroxyl-fluorapatite, ilmenite-II, poor in Mg and enriched in Mn. Armalcolite is partly replaced by ilmenite-II. Perhaps, an existence and preservation of armalcolite and zirconolite within the near surface basic intrusions of the Crimean Mountains is caused by poor development of late magmatic processes.