ZIRCONOLITE AND Zr-Th-U MINERALS IN CHROMITITES OF THE FINERO COMPLEX, WESTERN ALPS, ITALY: EVIDENCE FOR CARBONATITE-TYPE METASOMATISM IN A SUBCONTINENTAL MANTLE PLUME

摘要

Zirconolite (CaZrTi_2O_7) associated with baddeleyite (ZrO_2), thorianite (ThO_2), uraninite (U02), thorite or huttonite (ThSiO_4) annd zircon (ZrSiO_4) has been discovered in chromitites of the Finero mantle-derived massif, in the Western Alps of Italy. The "exotic" minerals do not occur in late veins or fractures; they are part of the accessory assemblage (phlogopite, amphibole, apatite, ilmenite, geikielite, rutile, molybdenite, and Mg-Ca carbonates) formed as a result of metasomatism of the Finero mantle. Textural relations of zirconolite and the Zr-Th-U minerals indicate crystallization with the chromite-olivine-orthopyroxene assemblage between 800 deg and ~600 deg C, at a given pressure of 1.0 GPa. Zirconolite is an indicator of silica-undersaturated conditions. Its composition, characterized by the prevalence of Th and U over Nb and Ta, is similar to that of zirconolite from some carbonatite complexes. The close association with Zr-Th-U minerals, and with the carbonatite-compatible elements Y, Hf, Pb, and LREE, is symptomatic of the carbonatitic character of metasomatism at Finero. The formation of a carbonatite-type liquid and related hydrous fluid is commonly related to the emplacement of mantle plumes in continental rift systems worldwide. This supports the inference that the metasomatism of the Finero mantle was a result of mantle diapirism at the base of the continental crust induced by extensional tectonics in pre-Hercynian times.