THE GEOCHEMICAL EVOLUTION OF GRANITOID MAGMATISM IN THE LATHERAL CROSSECTION OF THE MIDDLE URALS (KYSHTYM AREA)

Shardakova G.Yu.*, Saveliev V.P.**, Khuttor-Kukkonin K.N.**

*Institute of Geology and Geochemistry UrB RAS, Ekaterinburg, Russia, shardakova@igg.uran.ru

**Open Society "Chelyabgeosiemka", Chelyabinsk, Russia, chelgeo@yandex.ru

Granitoids located within the structures with different type of crust strongly vary on the structure and age. It reflects the source composition, i.e. the ratio between crustal and mantle (continental and oceanic) material. It is shown on example of the Uralian granitoid series, that there is lowing change in petrogeochemical features from the west to the east, from the Main Uralian Fault zone (MUFZ). It means an increase in the part of the sialic material in the source in this direction (Orogenic…, 1994).

During the geological mapping (1:200 000, Kyshtym area) we investigated the granitoid composition in the wide area, where it is possible to choose the approximately lateral crossection (from the west to the east, near Nizhny Ufaley) through some structural zones and track back the evolution of granitoid magmatism from the western border of the Urals and the Russian plate (from the MUFZ to the paleocontinental sector).

A swarm of submeridional granitic dykes (extent of 0,5-3 km) is observed in the most western part of described area, among the metasedimentary rocks of Taganay suite (R₁tg). By petrogeochemical characteristics, these granites are very similar to ones of the rocks of Jurma complex (Shardakova et al, 2007) located 10-15 km to the south-west, and according to our data, it has the age about 510-540 Ma (U-Pb, SHRIMP, zircons). The rocks are formally close to continental-riftogenic series (such as granites of the Berdyaush pluton and Kuvash-Mashak paleorift (about 1380 Ma) in Bashkirian meganticlinorium). Gneisso-granites of the Jurma complex contain plenty of zircon grains from the source corresponding to «mashak riftogenic event», that means the granite genesis due to ancient continental material (substance of Russian plate). Complex of dykes described above has apparently the similar age and genesis.

The next structure where granitoids are widespread, is the Ufaley block (UB), called «the alien terrane» by some uralian scientists (Necheukhin et al, 2000). In the central part of the structure, the ancient granitic gneisses (1100-900 million years) are present, and they are very similar to continental-riftogenic granites of Kuvash-Mashak paleorift by the petrogeochemical characteristics. Therefore, they have similar origin. Gneisses of UB are cut by younger granites of Nizhny Ufaley massif (317 m. a.) (Shardakova et al, 2006); the latter ones, by geochemical characteristics, correspond to Carboniferous early orogenic ("above-subduction") series such as verkhisetsk, syrostan ones, etc. The analogue of this massif (called Sukhovyaz) with the same age and geochemistry is on the eastern border of UB with MUFZ Based on the Sr and Nd isotopic data, we concluded that the oceanic (island arcs) material, that is «really Uralian material», in addition to the continental one, played a significant role in substrate for melting such granites. We mapped earlier unknown granite body named Nikolsky to the east from Nizhny Ufaley massif. Its rocks are cut by Nizhny Ufaley massif granites, and they cut the gneisses of the UB central part. By geochemistry, Nikolsky granites are close to ancient gneisses. Ar-Ar age of Nikolsky granites is about 579 Ma (2008); it is close to the age of Jurma complex (see above). We should note that the Vendian orogenic granites that are rare for the Middle and Southern Urals, but widespread in the Polar Urals (Kuznetsov et al. 2005), have the similar geochemical features.

Thus, the Ufaley block is the complicate heterogeneous structure. Granite formation here was multistage. And both continental and oceanic («Uralian») material played various role in the substrate of the granite melting.

The next sector is Sysert-Ilmenogorsk zone (ancient plate area), where there are large bodies of gneissic granites, being concordant with kyshtym suite (PR₁). These gneisses are very close to the described above ancient (Riphean-Vendian) within-plate series by the geochemical parameters. Also, kyshtym suite is cut by young granitic dykes characterized by lower contents of Al₂O₃, K₂O, total REE and La/Yb. Probably, particularly it is particularly the result of the secondary alterations. The higher values of the mentioned parameters are specific for the typical Permian granites of the Urals eastern slope.

The most eastern structure is Shilovo-Konyevskaya zone within the paleocontinental sector of the Urals. Here is Allakyozyorsk massif (C₁?), represented by high-K granites with the geochemical characteristics that are similar (as well as Nizhny Ufaley massif) to the above-subduction series of MUFZ and the western margin of the Kazakhstan plate. Allakyozyorsk massif is the most eastern member of investigated row. Granites have the maximal contents of K, Rb, Th, U, total REE and La/Yb, this fact reflecting an increase in thickness of the continental crust in this direction.

Based on the analysis of the petrogeochemical and age peculiarities of granitoids in the lateral crossection described above, we can conclude the following. 1) The most western members of the row (swarms of granite dykes cutting the Taganay suite) correspond, by the composition, to continental-riftogenic series of the Russian plate eastern periphery of Russian plate and, probably, fix (as well as Jurma complex) the kadomsky activity stage in northern part in the Bashkirian meganticlinorium. 2) The heterogeneity of Ufaley block is reflected in some stages of magmatic events and in variety of granite types – from ancient continental-riftogenic (granitic gneisses of the central part of structure and Nikolsky massif) to subduction-collisional series (Nizhny Ufaley and Sukhovyaz massifs). The formation of the latter one fixes the end of activity of the Main Uralian Fault zone in this part of the Middle Urals. In the South Urals, the Syrostan-Turgoyak group is a marker of this process. 3) Granites of the extreme eastern member of row (Allakyozyorsk massif) have the higher contents of sialic components. It reflects the general geochemical zonation of the Uralian Carboniferous granitic series (an increase in the thickness of continental crust to the east). 4) Within the Sysertsko-Ilmenogorsk microcontinent (terrane?), there are granitoids that characterize both the ancient and young stages of geological history of the Uralian orogen.

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