NIOBIAN AND ZINCIAN ILMENITES IN SYENITES FROM CAPE ASHIZURI, SOUTHWEST JAPAN

Ilmenite in alkali feldspar quartz syenite from Cape Ashizuri contains up to 4.4 wt.% Nb2O5 Niobium substitutes for Ti in the octahedral sit e of the ilmenite structure. Substitution of Nb for Ti may involve a c oupled exchange to maintain charge balance, and an exchange of 2Nb + 2 Fe(3+)=3Ti + 2Fe(2+) is advocated. An Fe-Ti oxide geothermometer obtai ned from mineral pairs of granular and lamellar intergrowths indicates a subsolidus re-equilibration temperature of 510-640 degrees C and fO (2) between the FMQ and MW-buffers, implying that it is very undersatu rated with respect to water. On the other hand, Zn-ilmenite, containin g up to 5.4 wt.% ZnO, occurs in miarolitic cavities in peralkaline rhy olite which cuts the quartz syenite. The Zn-ilmenite is one of the las t crystalline phases of the Ashizuri magmatic activities under volatil e-rich conditions. Nb-oxides, such as fergusonite, samarskite, columbi te and a pyrochlore-like mineral, are Ta- and Mn-poor, which correspon ds to those of less-fractionated rocks of anorogenic alkali granite an d pegmatitic granite in the continental situation. Ta- and Mn-poor Nb- oxides in F- and Li-rich alkaline felsic magmas such as the Ashizuri s yenites are unusual; this may be related to a rapid emplacement and co oling of mantle-derived small-volume magma in the island are situation .