Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Eu/Eu* in apatites of ore gabbroids (P-Ti) as indicator of their fO₂ formation conditions

Kryvdik S.G., Dubyna A.V.

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

kryvdik@ukr.net

Anorthosite-rapakivi granite plutons of the Ukrainian Shield (Korosten and Korsun-Novomirgorod), as well as the similar but substantially South Kalchik syenitic massif (Azov area) are characterized by the presence of phosphate-titanium ore gabbroids. There are two main types of gabbroids: 1) olivine gabbroids and troctolite with apatite-titanomagnetite-ilmenite mineralization (Stremygorod, Feodorivka, Davydky, Volodarsk, etc.) are more widespread; 2) ilmenite rich and the poor in apatite (without magnetite) ore norites (Nosachiv, Penyzevychy). Ilmenite gabbroids of the second type are very similar to those from Telnes deposit in Norway. Geochemical studies of apatites from the mentioned types of ore gabbros of the Ukrainian Shield have shown essential differences in degree of negative Eu-anomalies of chondrite-normalized patterns of REE and but slopes of their lines are in general similar (Fig.). In the olivine gabbros (with titanomagnetite) the REE patterns are characterized by moderate negative Eu-anomalies (0,35-0,56), while in the ilmenite-rich norites (without magnetite) this anomaly is very deep - 0,06-0,10 (Fig.).

In our opinion, such differences between the values ​​of Eu/Eu* are due to diverse oxygen fugacity (fO₂), as well as to intensive feldspar fractionation (the formation of anorthosite) in the process of ore gabbro's formation.

Megnetitefree ilmenite norites were formed under reducing conditions when almost all Eu was in divalent state and rapidly extracted by plagioclase (anorthosite). The presence of graphite in these rocks as well as almost completely absence of hematite minal in ilmenite confirms that.

At the same time the titanomagnetite ore gabbros were formed in conditions of fO₂ close to QFM buffer when a significant part of Eu had the trivalent state and entered largely into the apatite. In addition, olivine gabbroids have often a subalkaline composition (clinopyroxene - Ti-augite). A rocks of such composition are usually formed under increased fO₂. Perhaps an increased alkalinity of these melts due to the enrichment in apatite (10-15%).

Based on the geological data and mineralogical parageneses anorthosite rapakivigranite plutons of the Ukrainian Shield have the deeper erosion cut as compared similar plutons from other regions of the world. Thereby it can explain the reducing fO₂ in the formation of ore gabbroids.