THE MAGMA SOURCES OF THE COLLISION- AND SUBDUCTION-RELATED GRANITE

BATHOLITHES IN THE SOUTH URAL

Osipova T.A.

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

The most of the granite rocks of the South Ural are situated in the East-Uralian Rise. The new isotopic-geochemistry data for the granite rocks (The Orogen…, 1994; Shatagin et al., 2000; Popov et al., 2002, 2003; Bea et al., 2002; Paleozoidy…, 2006; etc.) can be used for the reconstruction of the magma sources and composition crust of the East-Uralian Rise south part.

The most important results of the last ten years isotopic-geochemistry South Ural granitoid study are: most of them have primitive isotopic composition (⁸⁷Sr/⁸⁶Sr_i = 0.704 – 0.706 and ¹⁴³Nd/¹⁴⁴Nd_i = 0.5121 – 0.5124) independent of its age or subduction- or collision- related origin; almost absence of the typical continental-crust-derived granites; the absence of correlation between Sr and Nd isotopic composition; the geochemistry different rocks are isotopically very similar; the isotopically similarity of the Carboniferous and Permian granitic and Permian basic rocks.

There are two main hypothesis for the explanation of the primitive isotopic signature: the granite magma source is young island-ark crust or, alternatively, the crust-mantle interaction.

The Rb-sr and Sm-Nd isotope data is in contradiction with the model of the isochemic remelting of the Early- Middle Paleozoic granitoids: that very mode origin rocks must show notable more radiogenic Sr (⁸⁷Sr/⁸⁶Sr_i ~ 0,707). Today it is known only rare examples of such isotope Sr composition medium-K Early-Permian granite in the South Ural. The acid-basic magma mixture model based on the Sr and Nd isotope-geochemistry data about specific South Ural East-zone Upper-Paleozoic basic rocks (Osipova et. al, 2008) is useful only for only private, rather rare examples. Moreover, in common there is a lot of high-silica peraluminos high-K granites without any geological acid-basic magma evidence. That very granites are extremely abundant in volume comparative with the monzonitoid granite rocks which are often exhibit the magma-mingling geological evidence. But both they are isotopically very similar.

The Middle-Paleozoic subduction-related granitic rocks are of close Sr-isotope composition and slightly poor with radiogenic Nd comparative with the Upper-Paleozoic collision-related granitoids. Most of them exhibit the geological and isotopic-geochemistry evidence of the heterogenic source derived.

The Upper-crust sources role in the granite origine in the East zone of South Ural is very restricted: today it is known only one example (Popov et al., 2003) of Upper-crust derived Early-Carboniferous leucogranite (⁸⁷Sr/⁸⁶Sr_i = 0,717, ¹⁴³Nd/¹⁴⁴Nd_i = 0,5122). The Early-Proterozoic ~1930 Ma metaterrigeniс rocks which had been amphibole-facie metamorphosed at the Cambrian time ~ 530 Ma (Krasnobayev et. al, 1998) can be the source of that very leucogranite. And only the presence of rare residual zircon in the granite rocks is the other evidence of the pre-Paleozoic material participation in the granite-magma source. But the other type of metamorphic rocks is known in the East-Uralian Rise south part: it is the basic magmatic rocks being amphibole-facie metamorphosed at the Ordovician (460 Ma) time (Vinogradov et. al, 2000). It’s isotopic-geochemistry features are similar to the most of the Middle- and Upper-Paleozoic granite rocks of the the East-Uralian Rise south part and can be considered as a protolite for granite-melting and the typical representative of the Early-Paleozoic East-Uralian Rise south part crust. So, most all of the geochemicaly different granite types of that very part of the Ural mobile belt can be the vary evolved melts derived from this source but under different melting conditions. The existence of the Upper-Paleozoic basic rocks which are accompany the granitoids is the reason for the assumption about the notable basic rocks role in the heat-production for the melting process, but all in all, they are not of great important for the granite magma and source composition.

This study has been supported by the Russian Fond for Basic Research (RFFI) grant 07-05-01023.

Reference:

Bea F., Fershtater G.B., Montero P. Granitoids of the Uralides: Implications for the Evolution of the Orogen // Mountain Building in the Uralides: Pangea to the Present Geophysical Monograph. 132. Copyright by the American Geophysical Union. 2002. P.211-232.

Krasnobaev A.A., Kuznetsov G.P., Davydov V.A. The age and origin of the chelyabinsk complex gneisses // ДАН, 1998. V. 360. № 3. P.386-389.

Orogenic granitoid magmatism of the Ural. Fershtater G.B., Borodina N.S., Rapoport M.S. et al. Miass, 1994. 247p.

Osipova T.A., Tevelev Al.V., Popov V.S., Belyatsky B.V. Sm–Nd and Rb–Sr Age of Gabbroic Rocks in the Dzhabyk Batholith, the Southern Urals Doklady Earth Sciences, 2008, Vol. 419, No. 2, P.275–280.

Paleozoidy of the East-Uralian and Back-Uralian joint zone. Tevelev Al.V., Kosheleva I.A., Popov V.S. et al. M.: MSU, 2006. V. 4. 293p.

Popov V.S., Bogatov V.I., Zhuravlev D.Z.. Sources of granite magmas and formation of an Earth's crust of Middle and South Ural: Sm-Nd and Rb-Sr the isotope data // Petrology, 2002. V. 10. № 4. P.389–410.

Popov V.S., Tevelev Al.V., Belyatsky B.V., Bogatov V.I., Petrova A.Yu., Zhuravlev D.Z., Osipova T.A. Isotopic composition of Nd and Sr in the Urals granites as an evidence for the mantle-crust interaction // Proceedings of the Russian Mineralogical Society, 2003. Pt. CXXXII. №3. P.17-38.

Shatagin K.N., Astrakhantsev O.V., Degt’arev K.E., Luchitskaya M.V. The continental crust heterogeneity of the East Ural: the evidence from the isotopic-geochemistry study of the Paleozoic granitoides // Geotectonics. 2000. №5. P.44-60.

Vinogradov V.V., Sherbakov S.A., Gorozhanin V.M., Goltsman Ju.V., Buyakajte M.I. The age of the metamorphic rocks of the East-Uralian Rise: Sm-Nd and Rb-Sr isotopic dating // DAS, 200. V. 371. № 6. P.784-787.