Introduction

Eight zircon grains from six rock samples of dunite, metadunite and clinopyroxenite of Kytlym and Galmoenan zonal Ural-type massifs were extracted using "ppm-mineralogy" technique (Knauf V.V., at al., 2007) for U-Pb dating and four of them from Galmoenan massif were analyzed by Lu-Hf isotope method (SIMS SHRIMP-II, VSEGEI, Russia, ICP-LA MS GEMOC, Australia). U-Pb isotope data of dunite-clinopyroxenite cores of Kytlym and Galmoenan massifs has revealed wide range of ages for both massifs: 2800 - 310 Ма. Lu-Hf method has indicated that zircons from dunites and clinopyroxenites of Galmoenan massif have mantle isotope footprints.

Results

The 207Pb/206Pb ages of dunite-clinopyroxenite cores estimated on zircon grains range from 2772 to 790 Ма (Table 1), but three groups of the ages are present: 2600-2800, 1800-1900 and 790 Ма. The values of the 176Hf/177Hf ratio of zircon grains from dunite-clinopyroxenite cores of Galmoenan massif vary from 0.281110 to 0.281941 that indicate the mantle origin of zircon grains (Fig. 1).

Table 1. Aages of dunite-clinopyroxenite cores

Fig. 1. 176Hf/177Hf ratio		Fig. 2. Groups of ages

Combining our data with the data received earlier (Bea F., at al., 2001), ages of dunite-clinopyroxenite cores form practically continuous spectrum of values 2800 - 310 Ма, but three groups of ages (2600-2800, 1800-1900, 310-440 Ма) are stand apart clearly enough (Fig. 2). Suggesting that the most ancient ages reflect the age of dunite formation (2800 Ma), that imply the younger ages indicate the stages of alteration of primary rocks which coincide with global cycles of mantle activation. Accent on age of dunite formation not casual: as it was reported earlier (Knauf O.V., 2005), the formation of clinopyroxenite rims around dunites resulting alteration of dunite core partly. So, zircon grains as a resistant and stable mineral may be found in clinopyroxenite matrix as a relic phase.

Concluding remaks

Presence of a wide interval of U-Pb ages obtained on zircon grains, and the fact of mantle genesis of zircons (Lu-Hf data) indicate the absence of an isotope homogenization during the all long history of formation and alteration of dunite cores of Kytlym and Galmoenan zonal massifs from 2.8 Ga to the last thermal event - 310 Ма.

The hypothetical model of dunite-clinopyroxenite massifs forming may by following:

I. Dunites were formed to ~2.8Ga, as a one of the inner layer (or chain of lenses) of ancient Earth.

II. Dunites were incorporated at different time to different structures of Earth Crust .

III. Dunites-clinopyroxenite zones resulting of overheated dunite incorporation to fluidic Earth Crust. The gabbrio rocks were formed (if they exist) as the interaction of melting products of dunite+clinopyroxenites cores with surrounded fluidic rocks of Earth Crust.

Acknowledgements

Special thanks go to K.N.Malitch and GEMOC colleagues for Lu-Hf analysis.

References

1. Bea F., Fershtater G.B., Montero P., Whitehouse M., Levin V.Ya., Scarrow J.H., Austrheim H., Pushkariev E.V. Recycling of continental crust into the mantle as revealed by Kytlym dunite zircons, Ural Mts, Russia. Terra Nova, vol 13, No 6, pp 407-412, 2001.
2. Knauf V.V., Guseva N.S., Knauf O.V. Application of the "ppm-mineralogy" technique for isotope dating and mineralogical study of ore deposits in mafic-ultramafic complexes based on minute accessory minerals. The proceedings of the 9th biannual SGA meeting, extended abstracts, pp 777-779, 2007.
3. Knauf O.V. The sparse platinum group minerals (PGM) in dunite and clinopyroxenite rocks of zonal complexes Ural type (Kytlym and Galmoenan massifs, Russia). Extended abstracts, Xth international platinum symposium, pp 400-403, 2005.