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

摘要

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, 9.911.7; 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 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)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.