Sources of matter for the Il’meno-Vishnevogorsky alkaline complex: Evidence from Lu—Hf isotopic data for zircons

The composition of hafnium isotopes in zircons iscurrently an important instrument for reconstructionof the source of magmatic rocks. Zircon is stable inmany hypogene and hypergene processes. Metamictization and later processes of alteration practically donot affect the composition of Hf isotopes in zircon[12]. The relative stability of Hf isotopes in zirconsmakes them a powerful instrument for the study of thesource age, as well as the origin and isotopic evolutionof various rocks [6, 11]. The combination of dating ofan individual zircon grain by the U–Pb method withthe study of Hf isotopes of the same grains (with application of local isotopic methods, such as SHRIMP,SIMS, and laser ablation) is the most effective. Such acombination provides important information on therock source even in the case when other isotopic systems are significantly disturbed.We obtained the first Lu–Hf isotopic data (in combination with the U–Pb isotopic data) by the methodof laser ablation for zircons from carbonatite and alkaline rocks of the Il’menoVishnevogorsky Complex inorder to study the age of rocks and the source of theirmaterial. Since the Sm–Nd and Rb–Sr isotopic systems were disturbed to various degrees during the Hercynian collision [5, 9], we considered application ofthe Lu–Hf isotopic system as the most stable andinformative to be especially important.The Il’menoVishnevogorsky Complex occurs atthe boundary between the Middle and Southern Uralsand is located within the Sysert–Ilmeny Precambrianblock, which has a twolevel structure and a long history of the formation. The core of the block is composed of granulitic gneiss and migmatite of the Selyankino Series (the U–Pb age of its zircons is 1820 ±70 Ma), as well as plagiogneiss and amphibolite of theVishnevye Gory Series (PR