Geochemistry
of Ti and P in alkaline and basic rocks from the Ukrainian Shield
Kryvdik S.G., Guravsky T.V., Dubyna A.V.
N.P.
Semenenko Institute of geochemistry, mineralogy and ore formation NAS of
Ukraine, Kyiv,
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.