ON THE ABSOLUTE AGE OF SYENITES AND FELDSPAR VEINS IN THE NORTHERN PART OF THE YENISEI RIDGE

A. D. NOZHKIN, R. M. GOLD, R. D. MELNIKOVA, N. K. GRIGOR’EV

(Presented by Prof. Dr. A. M. Kuz’min)

Among the igneous rocks developed in the northern part of the Yenisei Ridge, syenites—as well as intrusive formations of generally increased alkalinity—are very limited in distribution. Moreover, under conditions of poor exposure of the territory and the small size of the bodies, they are identified only with great difficulty. In this connection it is no accident that all occurrences of subalkaline rocks have been established only in the last 3–4 years, when large-scale geological investigations began to be carried out.

Outcrops of syenites and quartz syenites are known in the basin of the upper reaches of the Isakovka, Vorogovka, Tyrada, and Zakhrebetnaya rivers, and also in the lower reaches of the Vorogovka River. Brief information about them is given in the reports of the geologists L. A. Rumyantsev, V. P. Bordonosov, Yu. A. Kudryavtsev, G. E. Massov, F. P. Krendelev, and others. It is noteworthy that almost all the named investigators, with the exception of F. P. Krendelev, consider these formations to be Precambrian, although convincing arguments on this question are usually not provided.

In 1961, one of the authors discovered new outcrops of quartz syenites, syenites, and also alkaline syenite-porphyries in the upper reaches of the Noiba River. Syenites and quartz syenites usually occur together and are connected with one another by gradual transitions. They probably occur as small central-type bodies, the size of which does not exceed 400–500 m in diameter. One such body is mapped at the headwaters of the first upper right tributary of the Srednyaya Noiba River; the second is located in the upper reaches of the second upper left tributary of the Levaya Noiba River. Syenite-porphyries have been established on the watershed of the Levaya and Srednyaya Noiba rivers, where they occur as dikes of northwestern (310°) strike. The thickness of the dikes is 10–20 m.

All intrusive bodies of syenites are emplaced among crystalline schists of the Penchenga Formation of the Upper Precambrian. The quartz syenites of the left bank of the Pravaya Noiba have an intrusive contact with the youngest of the Precambrian fine-grained granites.

Among the petrographic features of the rocks, the following should be noted: quartz syenites and syenites are pink or pinkish-gray, evenly fine-grained. The former consist of K-feldspar (65–70%), albite (15–20%), quartz (5–10%), and biotite (0–10%); the latter are composed almost entirely of K-feldspar (90–95%), with biotite or amphibole of subordinate importance (5–7%), and sometimes albite (up to 5%). Accessory minerals are sphene, zircon, apatite, magnetite, rutile, corundum, fluorite, and especially characteristic phosphorus-bearing thorite.

Syenite-porphyries have a light-brown color and a porphyritic texture. In the porphyritic segregations there are plates of potassium feldspar. The groundmass is fine-crystalline; its microstructure is trachytoid, close to bostonitic. They consist of potassium feldspar (70–75%), albite (10–15%), and barkevikite (2–5%). Magnetite, sphene, apatite, fluorite, and also phosphorus-bearing thorite are present as accessory minerals.

Contact changes in the country rocks near the syenite outcrops are very slight. The processes of autometasomatism and especially of alkaline metasomatism were manifested much more widely; they were expressed in the potassic feldspathization, muscovitization, and albitization both of the intrusive and of the enclosing rocks, as well as in the formation of vein-like and vein bodies of essentially feldspar composition. Metasomatic feldspar formations are distributed considerably more widely than the syenites themselves and occur not only in the upper reaches of the Noiba River, but also far to the north and south of it. Together with the above-mentioned outcrops of alkaline rocks they form a rather extended zone of northwestern strike, which can be traced from the valley of the Uvolga River to the headwaters of the Noiba River and farther to the valleys of the Chapa and Vorogovka rivers. Bodies of feldspathized rocks occur in rocks of different composition and age, beginning with the most ancient and deeply metamorphosed formations of the Penchenga Formation and ending with sedimentary deposits of the Vendian complex of the Upper Precambrian [3].

On the basis of a study of the geological-petrographic features and mineralogical composition of the feldspar formations, several varieties are distinguished among them: quartz-feldspar, quartz-feldspar of granitic aspect, feldspar, apatite-feldspar, and others. The most widespread of these are the quartz-feldspar rocks, a detailed characterization of which is given in the work of A. D. Nozhkin and F. N. Krendelev [1]. Individual large bodies of quartz-feldspar composition very much resemble granites. They have a coarse-grained structure and consist of potassium feldspar (orthoclase, microcline-perthite), albite, quartz, and biotite. The feldspar rocks proper are composed almost entirely of potassium feldspar, albitized to varying degrees. In rocks of apatite-feldspar composition, along with potassium feldspar and albite, apatite is present in increased amounts. Fluorite, cyrtolite, orthite, monazite, orangite, and phosphorus-bearing thorite occur as accessory minerals in the named varieties of feldspar rocks.

Correct resolution of the question of the age of the syenites and of the metasomatic formations of feldspar composition is of great importance. In this connection the authors carried out geochronological studies to determine the absolute age of thorite, monazite, and galena separated from these rocks. The absolute-age investigations were carried out in the problem laboratory of the Geological Prospecting Faculty of the Tomsk Polytechnic Institute. The age was determined by the lead-isotope method. The following constants were used in the calculation:

\[ \lambda U^{238} = 1.54 \cdot 10^{-10}\ \text{yr}^{-1}, \]

\[ \lambda U^{235} = 9.72 \cdot 10^{-10}\ \text{yr}^{-1}, \]

\[ \lambda Th^{232} = 0.499 \cdot 10^{-10}\ \text{yr}^{-1}. \]

The U and Th contents were determined by the emanation method on an SG-1M instrument. The lead content was determined colorimetrically on an SF-5 spectrophotometer and by the isotope dilution method. In po-

in the latter case each sample was decomposed with two standards of lead enriched in isotopes with mass 204 and mass 207, which made it possible to check the reliability of the analytical determinations.

The relative errors of repeated (6–8 times) measurements of the U and Th contents are 2–6%, and the relative errors in determining the lead contents are 0.5–2%.

The isotopic composition was determined from lead iodide on an MI 1305 mass spectrometer with a furnace source. The age was calculated according to the tables of Stieff and Stern [7]. The results of the determinations are given in Table 1.

Table 1

Results of age determination for accessory minerals from rocks of the northern part of the Yenisei Ridge

In the same laboratory the isotopic composition of lead from galena was studied; the galena was found as fine disseminations in the quartz–feldspar veins. The isotopic composition is as follows:

\[ \mathrm{Pb}^{204}=1;\quad \mathrm{Pb}^{206}=17.09;\quad \mathrm{Pb}^{207}=15.29;\quad \mathrm{Pb}^{208}=36.55. \]

Using Cumming’s equations [2], the age of the galena can be calculated. It proved to be, respectively:

\[ t\,\frac{\mathrm{Pb}^{206}}{\mathrm{Pb}^{204}}=523\ \text{Ma}, \]

\[ t\,\frac{\mathrm{Pb}^{208}}{\mathrm{Pb}^{204}}=465\ \text{Ma}. \]

Considering the results of the absolute-age determination, it is not difficult to note that the figures obtained show fairly satisfactory agreement both for different minerals and for different isotopic ratios.

The somewhat different age value for sample 170 is apparently explained by removal of lead from the mineral. The possibility of such a fact is indicated both by the mineralogical study of the sample and by the features of the isotopic composition of the lead. Here thorite is represented by an altered variety—orangite, in which the greater part of the ferrous iron has been converted into the ferric form. Such changes in a mineral usually occur under the influence of later hydrothermal processes or in the zone of hypergenesis [4]. Sample 170 was taken from a surface outcro-

of the carbonatized quartz–feldspar body. Under these conditions, removal of lead from thorite is quite probable. Furthermore, despite the similar conditions of formation of thorite from sample 138 and orangeite from sample 170, the latter contains considerably less admixed lead. This is well illustrated by the isotopic composition of lead given in Table 1. Evidently, the missing part of the admixed lead was removed, since it is precisely this lead which, in the opinion of I. E. Starik [5], migrates first. Hence the age value obtained from sample 170 should probably be regarded as understated. Taking this circumstance into account, the geochronological data make it possible to consider the age of the rocks studied to be \(470 \pm 30\) million years, which, according to the scale adopted by the IX Session of the Commission for Determining the Absolute Age of Geological Formations [6], corresponds to the boundary between the Ordovician and the Cambrian.

The closeness of the absolute-age values for syenites and metasomatic formations of essentially feldspar composition, together with facts of a general geological nature, indicates their probable genetic connection.

REFERENCES

1. F. P. Krendelev, A. D. Nozhkin. On post-Cambrian magmatism in the Yenisei Ridge. Geologiya i geofizika, No. 9, 1961.

2. R. D. Russell, R. M. Farquhar. Lead Isotopes in Geology. Foreign Literature Publishing House, 1962.

3. Resolution of the conference on the stratigraphy of Upper Precambrian deposits of Siberia and the Far East. Novosibirsk, 1962.

4. M. V. Soboleva, A. L. Pudovkina. Uranium Minerals. Gostoptekhizdat, 1957.

5. I. E. Starik. Nuclear Geochronology. Publishing House of the Academy of Sciences of the USSR, 1961.

6. Proceedings of the IX Session of the Commission for Determining the Absolute Age of Geological Formations. Publishing House of the Academy of Sciences of the USSR, 1961.

7. J. R. Stiff. T. W. Stern. Oshiro Seiki. F. E. Senelle. Tables for the Calculation of Lead Isotope Ages, Washington, 1959.