Chelyabinsky granitoid pluton: the formation phases and the source of magmageneration (South Urals, Russia)

Kallistov G.A.

A.N.Zavarizky Institute of Geology and Geochemistry UB RAS, Ekaterinburg, Russia

kallistov@igg.uran.ru

The Chelyabinsky pluton is the most «long-leaving» (Late Devonian–Early Carboniferous – Middle Triassic) and one of the largest granitoid massiffs of the Urals Main Granite Axis. It is the utmost northern intrusive body in the south part of the East-Uralian Rise. The host rocks are PZ_1-2 metavolcanic and metasedimentery green-sheets.

Granites, which constituent Chelyabinsky massif, form four petrochemical series: high-K granitoids (quartz-diorites, granodiorites and high-K granites); medium-K granites; subalcaline and peraluminous leucogranites (garnet-bearing leucogranites). The rocks of each series have a specific petrographic and geochemical features.

According to U-Pb (zircon), Rb-Sr and Ar-Ar data the granitic rock of this 4 intrusive unites have intruded during 4 main stages. I – Late Devonian–Early Carboniferous: quartz-diorite and cross-cutting it granodiorite contain zircons, which have uniform U-Pb (Shrimp II) ages of 358 ± 5 Ma and 361 ± 5 Ma respectively; and zircons from high-K granite has U-Pb (Shrimp II) age of 344 ± 5 Ma. II – Carboniferous: zircons of medium-K granite yielded U-Pb (ID-TIMS, lower concordia-discordia crossing) age of 317 ± 12 Ma. III – Permian: fluorine-bearing leucogranite yielded Rb-Sr (whole rocks and minerals isochrone) 274.7 ± 2.6 Ma and contains zircons with 271 ± 5 Ma U-Pb (Shrimp II) age. IV – Middle Triassic: Ar-Ar age of muscovite and plagioclase from garnet-bearing leucogranite is 236 ± 2 Ma and 225 ± 3 Ma respectively. It is very important, that the K-Ar age of biotite from quartz-diorite, granodiorite, fluorine-bearing leucogranite and host metamorphic rocks vary from 290 to 250 Ma according Grabezhev et al., 1998 [5] and Osipova et al., 2001 [8].

The generation of massif corresponds to general stages of paleozoic intrusion magmatism of the Urals [3, 4]. Granites with more mafic composition are compared with rocks of margin continental type massifs.

The formation of the high-K series of granites (360-345 Ma) of Chelyabinsky pluton corresponds to the first episode of supersubduction magmatism [1]. The generation of the medium-K series of granites (317 Ma) corresponds to stage of the finishing of subduction , which characteristics with break of slab [9]. The significance content of mantle component in the source of the medium-K granites is related with realization of such mechanism. The age of the formation of subalcaline granites and leucogranites (275-260 Ma) of the Chelyabinsky massif corresponds to final stage of collision [6, 9]. The age of generation of the peraluminous (Gt-Mu) leucogranites (235 Ma) coincided with platform stage and postcollision dilatation [9]. According to Puchkov [9] a numerous cases of acid magmatism in this age in the Urals corresponds to heat introduce, the agent of which was basalt magma. This magma was a result of active trappean magmatism in the East Siberia - the giant superplum [2].

The D₃-C₁ and P chelyabinsky granitiods Sr and Nd isotopic signature (⁸⁷Sr/⁸⁶Sr_i = 0.7051-0.7067, eNd_i = 0 – -5.2) vary from typical to significantly less primitive than the most granitic rocks of the south part of the East-Uralian Rise. The possible source of that very granitiods may be the mixture of both Precambrian and ‘young Uralian’ crustal material. The presence of PR₃ (665 Ma) and S₂ (416 Ma) zircon grains at the same time with the ‘own’ magmatic zircons in the C₁ and P granites respectively supports this contemplation.

The Carboniferous medium-K granite is one of the most primitive in Sr (⁸⁷Sr/⁸⁶Sr_i = 0.7051) and Nd (eNd_i = +3) isotopic composition in South Urals, perhaps, due to the predominance of the young ‘properly Uralian’ crust material in the source in spite of the presence of PR₁ (1905 Ma, upper concordia-discordia crossing) zircons, which is close to detrital zircon age (1930 Ma) reported by Krasnobaev et al. [7] for metasedimentary gneisses from the large (2 x 0.5 km) xenolite in granodiorite.

The T₂ garnet-bearing leucogranite has the typical ‘crust’ Sr and Nd isotopic composition (⁸⁷Sr/⁸⁶Sr_i = 0.711, eNd_i = -1) and contains Early Carboniferous zircon (345 Ma, U-Pb, Shrimp II). So, as T₂ leucogranite doesn’t contain Precambrian zircons– the ‘direct’ evidence of old crust participation in magma source, it can be considered as the derivate of composite heterogenic crust of the south part of the East-Uralian Rise.

This study has been supported by the Ural Branch of RAS (grant 09-T-5-1023).

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