ABOUT THE AGE AND NATURE OF KREMENKUL RARE-METAL LEUCOGRANITE

(SOUTH URAL) – EVIDENCE FROM ISOTOPIC AND GEOCHRONOLOGY DATA

Kallistov G.A., Osipova T.A.

Institute of Geology and Geochemistry UrD RAS, Ekaterinburg, Russia

Kallistov@igg.uran.ru, osipova@igg.uran.ru

The Kremenkul rare-metal (W-Mo) massif is located not far from the Chelyabinsk city (South Ural, East-Ural Raise). According to the geophysical data (Kuznetsov, 1999) the Kremenkulsky massif is a steeply-dipping stock 3 km size and 6 km deep. It is hosted by the subduction-related granite rocks of the Chelyabinsky batholite with the zircon Pb-Pb evaporation age 360-330 Ma (Bea et al., 2002). The Kremenkulsky granite is a pinkish leucocratic fluorite-bearing two-feldspar granite with ~ 1 wt.% (Grabezhev et. al., 1987) magnetite content. The Kremenkul leucogranite is quite close to the Central-Kazakhstan rare-metal granites on their geochemistry signature (Grabezhev, etc., 1987; Kallistov, 2007). The modern isotopic and geochronology data allow to approach the of South Ural rare-metal leucogranite magma sources problem.

The Kremenkul leucogranite isotopic Sr and Nd composition (⁸⁷Sr/⁸⁶Sr_i = 0.7057; ¹⁴³Nd/¹⁴⁴Nd_i = 0,51209) is rather different from the majority of the South Ural Early-Permian granites which are notably primitive (⁸⁷Sr/⁸⁶Sr_i is less than 0.706 and ¹⁴³Nd/¹⁴⁴Nd_i varies 0.51225-0.51235 - Shatagin et. al., 2000; Popov et. al., 2002).

According to the recent data the Kremenkul granite is the youngest in Chelyabinsk batolite. Up today there are K-Ar biotites and muskovites age 260-280 Ma (Grabezhev et. al., 1998) which is in good agreement with Rb-Sr whole-rocks plus both microcline and biotite isochrone age 274.52.5 Ma (author's data).

Now we have new U-Pb-SHRIMP zircon data. 2 different zircon types due to their morphology and chemistry composition were determined in the Kremenkul leucogranite: zircon-I have the typical middle-level granites morphology (Krasnobaev, 1986), and rather high U (200-600 ppm) and Th (120-650 ppm) content, Early Permian age 2715 Ma; zircon-II have the typical abissal-level granites morphology (Krasnobaev, 1986) and lower U (20-60 ppm) and Th (10-50 ppm) content, Late-Silurian – Early Devonian age 4167 Ma. It is of great important that host rocks have the Early Carboniferous age 360-330 Ma. So, the zircon-II can’t be the host rock xenocrysts, but can be the relic zircon from the protolite – Middle Paleozoic intrusive granite rock.

The reliable isotopic-geochronology data about Late-Silurian – Early Devonian granites in East zone of South Ural are very limited. There are only three granite massifs can be discussed. They are the tonalite-plagiogranite Rassypn’ansky, Plastovsky and Nizhnesanarsky; and independently of its different geological and tectonic settings, all of them are considered to be formed due to the ensimatic island arc evolution (Fershtater et al., 2007; Yazeva, Botchkarev, 1995; Shatagin et al., 2000). According to (Yazeva, Botchkarev, 1995) the Nizhnesanarsky granitoids are the typical examples of “young” Paleozoic “continental” crust of the South Ural. So, it can be considered as the Kremenkul leucogranite source.

But the isotopic Sr and Nd data are in contradiction with the both hypothesis of Kremenkul leucogranite origin: as due to isochemic remelting of the Middle Paleozoic granite rock, as due to the mixture of this melt with the basic magmas.

There is one more important fact: the Early Proterozoic metaterrigene gneisses are identificated in the contact zone of Chelyabinsk batolite (Krasnobaev et al., 1998). Due to the absence of the isotopic Sr and Nd data for this very gneisses it is impossible get the correct conclusion on the significance of this source for the granite origin. But it is clear that the Kremenkul leucogranites isotopic signature can’t be explained by the Middle Paleozoic crust remelting model. So, we suggest that the Kremenkul leucogranite were generated by remelting of Proterozoic material as (Popov et al., 2002) did, but we consider the two-episode remelting of the metaterrigene Proterozoic material and involving the mantle-derivated magma or solution.

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

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