Armalcolite-Bearing Island Arc Plagiolherzolites and Olivine Gabbro–Norite–Dolerites of the Balaklava Area (Crimea)

Abstract

Armalcolite (Mg,Fe)Ti2O5 is a characteristic mineral of lunar titaniferous basalts that occurs widely in island-arc low-alkaline plagiolherzolites and olivine gabbro–norite–dolerites of the Early Bajocian Pervomaiskoe–Ayu-Dag complex of mesozoids in Crimea, especially in Balaklava sea cliffs near Sevastopol. Cumulose Mg-olivine contains inclusions of chromium spinels. Chromium spinels I are moderately chromic and ferruginous (f = 45–55) containing 8–14 and up to 1.2 wt % of Fe2O3 and TiO2, respectively; they are distinguished clearly in this way from chromium spinels of alpinotype ultrabasites. Chromium spinels II are more ferruginous (f = 58–75) and contain up to 26, 2.5–9, and up to 1.8 wt % of Fe2O3, TiO2, and V2O3, respectively. The typical basic trend from ferruginous aluminomagnesiochromite to chromium titanomagnetite is characteristic for Balaklava chromium spinels. Tabular armalcolite crystals of approximately 0.6–0.2 mm are included in intercumulus magnesium bronzite and augite, as well as in anorthite and bytownite. The treated armalcolite contains (wt %) TiO2, 63–66; MgO, 6.9–8.4; FeO, 9.9–11.7; Fe2O3, 13–18; Al2O3 and V2O3, up to 1.5 each, and Cr2O3, up to 0.7. The armalcolite formula is (Mg0.38–0.45$${\text{Fe}}_{{0.30 - 0.34}}^{{2 + }}$$$${\text{Fe}}_{{0.36 - 0.49}}^{{3 + }}$$V0.04–0.05Al0–0.06Cr0–0.02Ti1.73–1.77)3O5; minals contain (mol %): MgTi2O5, 38–45; Fe2+Ti2O5, 31–36; and Fe3+2TiO5, 23–27. Baddeleyite and zirconolite are associated with armalcolite. The composition of zirconolite is (Ca0.7Y0.2REE0.1)Zr(Ti1.6$${\text{Fe}}_{{0.3}}^{{2 + }}{\text{Fe}}_{{0.1}}^{{3 + }}$$)2O7. The more recent minerals are ilmenite I, which contains 5–8 wt % MgO and 0.3–0.6 wt % MnO, as well as Mg-deficient and Mn-enriched ilmenite II replacing armalcolite. The occurrence and preservation of armalcolite and zirconolite within the near-surface basic intrusions of the Crimean Mountains is probably caused by poor development of late magmatic processes.