| Popis: |
The lower continental crust and subcontinental lithospheric mantle (SCLM) are very interesting and important part of the Earth planet that has influenced significantly processes, which are continually altering surface of the Earth. Knowledge of the nature, structure and evolution of these layers may lead to understanding of the deep Earth’s processes. The Siberian craton is one of a few regions where samples of lower crust and underlying subcontinental lithospheric mantle can be found. Many geological and geophysical studies were carried out in this region but the role of crust and SCLM in the Earth’s evolution is not completely understood yet. Modern analytical techniques allow solving problems that were outstanding up to now. For example, isotopic and trace-element studies on zircon can specify not only the age of rock but also the source of melts and thus identify different stages in the craton evolution. The Siberian craton has experienced widespread Phanerozoic kimberlite magmatism. Besides diamonds, kimberlites also carry up a unique set of xenogenic material (xenoliths and xenocrysts) derived from depths of the lithosphere-asthenosphere transition zone (230-250 km) to the upper crust. Thus, kimberlitic pipes are natural drill holes, supplying with upper-mantle and crustal xenoliths and allowing us to investigate and reconstruct the structure of the deeper sections of the lithosphere. More than 1000 zircon grains from five terranes (Magan, Markha, Daldyn, Khapchan and Birekta) of the Siberian craton were analysed for U-Pb ages, trace-element characteristics, hafnium and oxygen isotopes composition. These data were supplemented by Re-Os isotope information collected on sulphide inclusion in silicate minerals from the mantle xenoliths. This combined isotopic information show that there were several events that involved both reworking of older crust and some addition of mantle-derived material. Moreover, integration of U-Pb age and Hf-isotopic data for zircons from different terranes indicates that the main magmatic-tectonic events are coeval for all studied terranes. For kimberlitic fields located within the Magan and Markha terranes the results of U-Pb and model ages show a good agreement with previous data. However, the ages from the eastern part of the craton (Khapchanand Birekta terranes) are much older than those reported previously. This implies that the amalgamation of the Siberian craton happened much earlier than it was previously suggested,which might have a significant impact on the diamond-bearing potential of the eastern part of the craton. |
