2011

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About the age of melanocratic carbonatites of chagatay complex

(Western Uzbekistan)

Divaev F.K., Golovko A.V., Golovko A.R.

SE «Central geologic-geophysical expedition», Samarkand, Uzbekistan

divaev2749@mail.ru

Melanocratic carbonatites of chagatay complex form a swarm of subparallel dikes capacity from 0,5 to 6,0 metres and a length of 20 to 1000 metres, abrupt (from about 60 dykes) and a few tubes of explosion in diameter from 100 to 200 metres. They are located in the central part of the ridge South Nuratau among Cambrian carbonate-terrigenous and Silurian volcanic-terrigenous rocks and are blocked Quaternary alluvial-deluvial deposits. Questions of mineralogical and petrographic and chemical composition, and genesis of diamond in these rocks have been considered previously in several publications (Divaev, 1996; Golovko et al., 2007, Kogarko et al., 2010). The question of the age of these formations is just not clear. Direct geological relationships provide a wide age interval from Silurian to Quaternary. The undertaken attempts of definition of absolute age (rubidium-strontic and uranium-lead method on zircons) have not crowned success, so a conditional Triassic age was given to these rocks on the basis of absence in them of any traces of regional and contact metamorphism (the age of occurring in the immediate vicinity of granitoids is C₂-P₁ ), and by analogy with the age of alkaline basaltic dikes regionally distributed South Tien Shan complex, for which it received a lot of definitions of absolute age K-Ar and Rb-Sr methods, most of which has the Triassic age.

Back again to the question about the age of carbonatites has prompted the authors the finding of well-preserved fossils of gastropods, in appearance very similar to the genus Cerithium, having an age range from K2 to Q (Mikhailova et al., 1996).

This finding makes it possible, with caution, to raise the lower limit of age Chagatay carbonatites to the level of K2-P? (Upper Cretaceous - Paleogene). This younger age of Chagatay carbonatites looks quit realistic, if to consider the published data about the discovery of the basalt dike in the Upper Cretaceous sediments of south-western spurs of the Hissar range (Tuaev, 1964), as well as data of absolute age (97 Ma, Rb-Sr method) of subalkaline basalt dikes of dzhigaristan complex, which lies in the Jurassic sediments in the basin of Angren river (Ganiev, 1996).

This finding is interesting not only from geology, but also with the petrological point of view.

Firstly, it is interesting, the mechanism of entering the xenoliths with fossils in carbonatite dikes at the present level. Depth of cut denudation Chagatay carbonatites according to various estimates range from 1000 to 1500 metres. Consequently, it is at such a depth and had to "fail" carbonate xenolith with fossils. Previously, such cases of "failures" described xenoliths in kimberlite pipes of Zolotitsky and Chernoozersky fields of Arkhangelsk diamond province, as well as in kimberlite pipe on Olenek raising in Yakutia (Malkov, 2008).

In the latter case xenoliths from the Jurassic belemnites were installed in the tube with a slice of denudation 1500-2000 meters. According to K.M. Alekseevsky and A.I. Botkunov (Alekseevsky et al., 2007) the movement of the debris toward the main mass movement occurs only in the kimberlites, having extremely low viscosity. Now we can say that similar phenomena are observed not only in kimberlites, but in carbonatites, which underlines the genetic affinity of these formations.

No less interesting is the fact that a surprising preservation of carbonate xenoliths in the pipes and dikes of carbonatite composition. The samples clearly shows that even very small carbonate xenoliths (3-5 mm) retain their sharp-angled shapes and are not subject to corrosion and assimilation. This indicates an extremely low activity of carbonatite magma (fluid-melt?) and extremely high speed of crystallization. The scientific explanation of these phenomena requires further study.

References:

Alekseevsky K.M., Botkunov A.I. Thermodynamic aspects of studying kimberlites // Ores and metals. № 5. 2007. P. 78-81. (in Russian)

Ganiev I.N. The problem of the upper age limit of magmatism of Middle Tien Shan. Int. Conf. "The evolution of the geological processes of the Tien Shan." Tashkent: Fan. 1996, P. 70-72 (in Russian)

Golovko A.V., Divaev F.K. Diamond-bearing rocks of Uzbekistan. Proceedings of the VII International Workshop "Alkaline magmatism, its sources and plumes. Irkutsk, Naples. 2007. P. 124-146.

Divaev F.K. Chagatay carbonatite complex - a new type of igneous rocks in Uzbekistan, "Uzbek. Geol. Journal, 1996. № 6. P. 32-41. (in Russian)

Kogarko L.N., Ryabchikov I.D., Divaev F.K., Wall F. Mode of carbon compounds diamondiferous carbonatites Uzbekistan (data isotopes of carbon and Thermodynamic Calculations) / / Geokhimiya. № 11. 2010. P. 1123-1131.

Malkov B.A. Belemnites and eclogite in kimberlite pipe Bared on Olenek raising (Yakutia), Vestnik. № 6. 2008. P. 12-14. (in Russian)

Mikhailova I.A., Bondarenko, O.B., Obrucheva O.P. General paleontology. Moscow: Moscow State University. 1989. P. 219. (in Russian)

Tuaev N.P. Depression of the Top Amudarya as an example of an activated platforms in dial with questions about oil and gas presence. In the book. "Tectonics of the Pamir and Tien Shan." Moscow: Science. 1964. P. 95-109. (in Russian)