ZIRCON SYENITE PEGMATITES IN THE FINERO PERIDOTIE (IVREA ZONE) - EVIDENCE FOR A SYENITE FROM A MANTLE SOURCE

Zircon syenite pegmatite dikes in the phlogopite peridotite of Finero are rich in volatiles, incompatible elements and in rare earths with (La/Yb)n ratios in the range of 14-15. The main mineral constituents in the alkaline dikes (Na > K) are albite and magnesium-rich biotites whereas nepheline, cancrinite, sodalite, zircon and apatite are minor or accessory. With their initial Sr-87/Sr-86 ratio of 0.70371 the noncontaminated syenite pegmatites originate from partial melts of the upper mantle. The magma is derived from a depleted mantle source with isotopic signatures (epsilon-Nd(T) = 5.7 and 5.1) of ocean island basalts (OIB). The ascent of the syenite pegmatites and the marginal uplift of the granulitic crust in the northern Ivrea zone may be correlated with the intrusion of alkaline melts at depth. The Triassic zircon age of 225 +/- 13 Ma is interpreted as the time of the dikes' emplacement. From geochemistry and setting it is concluded, that the syenites are related to a rift magmatic stage at the early Mesozoic in the southern Alps.