CHEMICAL-COMPOSITION AND PETROGENETIC IMPLICATIONS OF APATITES FROM SOME ULTRA-ALKALINE ITALIAN ROCKS

The compositional characteristics of apatite group minerals from some Italian late Pleistocene ultra-alkaline rocks were studied and compared with apatites from other ultra-alkaline suites. The new data, together with data from the literature, were investigated using Principal Component Analysis (PCA). Three genetic groups of apatites were identified in terms of their compositional differences by means of the PC diagram and a Tetrahedra Substitution Index (TSI = 100*(Si + S + C)/P atom/apfu). Group I is made up of apatite samples from the ijolite-sovite association, with high contents of P, Sr, REE and considerable F (TSI = 1.6-6.8); Group II samples are apatites from fine-grained extrusive calcite carbonatites with high C, Si, OH, low S and a dominant substitution of C and Si for P (TSI = 7.9-48.5); Group III corresponds to the apatites from alkaline rocks with high S, F, and low C and OH contents and a dominant substitution of S and Si for P (TSI = 2.9-14.0). The apatite group geochemistry suggests that an ijolite-sovite magma existed as a single parent system over a relatively large range of P-X-T conditions. Major geochemical differences between the apatites from extrusive calcite carbonatites and alkaline rocks could well reflect separation of the ijolite-sovite magma into silicate and carbonate liquids under sub-volcanic or extrusive conditions.