Geochemistry of Ti and P in alkaline and basic rocks from the Ukrainian Shield
Kryvdik S.G., Guravsky T.V., Dubyna A.V.
Semenenko Institute of geochemistry, mineralogy and ore formation NAS of
Within the Ukrainian shield (USd) magmatic rocks from different areas of this region and from various rock complexes are often of different geochemical characteristics. Alkaline and subalkaline ultrabasic and basic rocks are usually enriched in Ti and P initially. In some dikes and volcanites of these rocks the contents of TiO2 amounts to 9% and P2O5 to 1-2%. But within USd, there are rocks with a high concentration of titanium and low of phosphorus and vice versa.
High contents of TiO2 (to 5.4%) and P2O5 (to 2.3%) occur in kimberlites from Azov area and central part of US.
In different gabbroids Ti and P concentrate together during Áprocess of magmatic differentiation. It is typical or anorthosite-rapakivi granite plutons which contain complex deposits of ilmenite, apatite and titanomagnetite. But apatite-ilmenite (Stremygorod, Fedorivka, Davydky) and essential ilmenite (Nosachiv) deposits sometimes occur in these plutons separately. It is established that increased contents of apatite is characteristic for subalkaline gabbroids (olivine gabbro, gabbro-troctolite, troctolite) in which clinopyroxene is presented by titanian augite. Wheares essential ilmenite gabbroids are of norite composition, i.e. they belong to rocks of normal alkalinity. It is considered that initial contents of TiO2 was not higher than 4-6% and P2O5 about 0.7-1.2% in parental magmas (type of jotunites or enriched in Ti and P alkaline basalts) of such ore gabbroids.
In alkaline-ultrabasic (carbonatic) complexes titanium and phosphorus are distinctly separated. Titanium concentrates and can form deposits in early rocks ã pyroxenites, jacupirangites, olivinites. The main titanian mineral of these rocks is ilmenite (in Ukrainian complexes), and in another worldôs region perovskite and titanomagnetite may be form large deposits too. Whereas phosphorus concentrates in later carbonatitic and phoscoritic differentiates.
Titanium distribution between pyroxene and other minerals depends on two factors ã alkalinity of rocks and depth (pressure) of their crystallization. In ultrabasic and basic alkaline and subalkaline rocks from hypabyssal or slightly eroded massifs a considerable part of titanium (2-6% TiO2) consist in clinopyroxenes (titanian augite, titanaugite). Whereas in abyssal massifs of these rocks the titanium contents in clinopyroxene strongly decreased (to 0.1-1.0 TiO2) even in cases of high amount ofÁ TiO2 (to 10%) in containing rocks (e.g. alkaline magnetite-ilmenite pyroxenites in Cherigovka carbonatite complex, Azov area).
In rocks of normal alkalinity clinopyroxenes are always characterizedÁ by low contents of titatium (0.1-0.3 TiO2) independently on its amount in rocks (including their ore types, e.g. Nosachiv ilmenite deposit).
Within the USd some regional features of magmatic rocks in reference to concentration of Ti and P are discovered. In the Eastern part of USd (Azov area) alkaline and subalkaline basic and ultrabasic rocks are rich in titanium, while in the Western part of this region they are depleted by this element (even in cases of their significant enrichment of phosphorus). This is explained by different geodynamic situation of melting of deep-seated magmas, which formedÁ the titanium and phosphorus deposits during the process of differentiation.