Source areas for neoproterozoic sediments of oseloc formation, Sayanian Birusa: geochemical and isotopic evidences

Proshenkin A.I.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

During the geodynamical researches of Siberian platform and its folded framing the particular attention was given to igneous and metamorphic complexes. Geochemical and isotopic researches, including Zr dating of neoproterozoic-paleozoic terrigenous formations on the border of Siberian platform make it possible to see more correctly the alteration of igneous and metamorphic events in the range of this structure. Main objective of this research is reconstruction of sedimentation environments, composition and, in some cases, age of source areas for sediments from oceloc formation, Sayanian Birusa. Age determinations for studied sediments are arguable, older than 740 mln. ages and younger than Cambrian. Oselok formation lie with erosion contact on karagass formation and in the range of presayanian deflection has no contacts with Siberian platform basement rock complexes.

Petrography researches show that oseloc formation rocks are similar in mineral composition but differ in separate grain size from coarse-grained to siltstones. Some clayey samples were marked also. Fragmental part of terrigenous oseloc formation rocks is represented mostly by quartz, which is cemented by gray-green mass of clayey material (montmorillonite, chlorite, illite). The amount of plagioclase in these rocks is pretty low – no more than 10%. Among the accessory minerals the most remarkable are tourmaline and zircon.

Oseloc formation terrigenous deposits in their geochemical characteristics are similar to average composition of upper continental crust and close to post archean average scale (PAAS). The exceptions are green-violet siltstones and mudstones, mostly with chlorite composition, in the upper part of the section. Petrochemical and isotopic characteristics of these rocks show that there is a great admixture of tuffagenous material with andesite composition. They are characterized by high contents of Fe, Mg, elements from Fe group and younger model age for source areas (1750 Ma.). For the typical terrigenous rocks from oseloc formation source areas model age is 1900-2100 Ma. (Table).

Probe number	Sm, ppm	Nd, ppm	^147Sm/^144Nd	^143Nd/^144Nd	T, mln. age	T(DM), mln. age	eNd (T)
М15/07	6.259	31.86	0.11877	0.512065			-4.2
C18/07	5.174	25.98	0.12037	0.511846			-8.6
C20/07	7.416	37.53	0.11946	0.511936			-6.8
C23/07	5.963	31.94	0.11285	0.511856			-7.8
C65/07	12.32	46.02	0.16185	0.512050			-8.4
C71/07	6.365	32.29	0.12294	0.511942			-7.0

Table. Sm-Nd data for oseloc formation terrigenous deposits of Sayanian Birusa

Note. Т – stratigraphy subdivision age, accepted for ε_Nd(Т) и T(DM)-2st calculations. During the Sm-Nd isotopic composition analysis of terrigenous rocks Nd model age values in response to depleted mantle T_Nd(DM) were determined on the basis of known values [Goldstein, 1988]: ¹⁴⁷Sm/¹⁴⁴Nd = 0.21365 and ¹⁴³Nd/¹⁴⁴Nd = 0.513151. The value ε_Nd(T) is counted with a glance of stratigraphic age of sedimentary rocks and modern values ¹⁴³Nd/¹⁴⁴Nd = 0.512638 и ¹⁴⁷Sm/¹⁴⁴Nd = 0.1967 [Jacobsen, 1984] for homogenous chondrite reservoir (CHUR).

On the basis of petrochemical, geochemical and isotopic characteristics source areas are specified as rocks with granite composition. Younger model ages for source areas of these deposits with respect to the Siberian platform basement rocks, and also the presence of tuffa rocks show that there was a significant juvenile source in the range of border platform parts, such as, for example, late Cambrian island arc on the active continental border.

Evidences of this fact are also Zr measuring data (Fig.) for several sandstone samples from oseloc formation (right shore of river Birusa, settlement Nersa area). Zircon Isotopic composition researches were made in the University of Hong Kong, China on VG PQ ExCall ISP-MS equipment. Among the main zircon bearing provinces it is possible to separate neoarchean (2600-2700 Ma) and paleoproterozoic (1900-2100 Ma) igneous rocks, undertaken metamorphic thermal event on, approximately, 1860 Ma (Fig.). It is important to note that paleoproterozoic igneous source area is the most significant among the other ones and, perhaps, is connected with the formation of green-scale belts, like the akitkansk formation, Western Baikal. Less revealed is the igneous zircons source area on the border between mezo- and neoproterozoic (1000-1100 Ma), which is most likely connected with the development of the island arcs on the border of Siberian platform at that time. In neoproterozoic time in the range of 800 Ma there was a formation of igneous zircons – this group is largely represented among our samples. Two grains also have determined age of 620-660 Ma. They represent the processes of preVendian collision on the border of Siberian platform.

Fig. Probability density plot for oseloc formation zircons. Data wes received using ICPMSDataCal7.2 and isoplot software for 44 zircons dated on VG PQ ExCall ISP-MS equipment in Hong Kong.

Minimal determined age of 620 Ma for all our zircons make it possible to limit the age of their sedimentation no older than this level. Thus, the age of oseloc formation terrigenous rocks is Vendian, because in the ust-tagulsk formation overlapping it is the presence of Cambrian fauna.

Reconstructions of geodynamical sedimentation environments for oseloc formation rocks show that the sedimentation took place in the active continental border environment, where the main sources of clastic material were both from Siberian platform and from ensialic volcanic island arc in the range of it’s active continental border.

The work was supported by the Russian Foundation of Basic Research (projects no12-05-00569) and the Presidium of the Siberian Branch of Russian Academy of Sciences (project no. 68).

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