VOLCANIC ROCKS OF THE BOGRAD BRIDGE AREA

(BATENEVSKY RIDGE)

D. I. TSAREV

(Presented by the scientific seminar of the Department of General Geology)

Small, scattered areas of volcanic rocks located around the Yuliya ore field (Batenevsky Ridge) have long attracted geologists, but because of the complexity of their structure they have so far been little studied. Their stratigraphic position, and consequently their age, have been interpreted differently by various investigators.

On the basis of detailed studies of the volcanic rocks of the Yuliya mine area, the author sets forth in the present article his view on the structure, stratigraphic position, and material composition of one of the areas of volcanic rocks situated 5 km southwest of the Yuliya mine, in the Bograd Bridge area, and extending in a latitudinal direction from the left to the right side of the valley of the Sukhaya Erba River for a distance of 6 km. The maximum width of the belt of volcanic-rock outcrops is 1.8 km.

In the structure of the Batenevsky anticlinorium, the volcanic rocks of Bograd Bridge compose a small syncline of superimposed character, the basement of which consists of Upper Proterozoic, Lower and Middle Cambrian rocks. The eastern part of the northern limb of the syncline is cut off by a disjunctive fault that was established even before the formation of the volcanic rocks and was subsequently renewed. The rocks of the southern limb and of the western part of the northern limb have stratigraphic contacts with the basement rocks. On the right side of the valley of the Sukhaya Erba River, the rocks described rest with azimuthal unconformity on a terrigenous-carbonate sequence of faunistically characterized Middle Cambrian age (Karasuk Formation). On the left side, likewise with azimuthal unconformity, they rest on Upper Proterozoic limestones with lenses and interbeds of siliceous rocks (Martyukhino Formation); nowhere are they overlain by other, younger deposits. Taking as a basis the data on the geological structure of the Batenevsky Ridge (4, 5), the author compared the petrographic and petrochemical compositions of the volcanic rocks of Bograd Bridge with unquestionably Devonian rocks of the environs of the village of Bograd and other areas bordering the Minusinsk Depression. As a result, the conclusion was drawn that the rocks described belong to the Lower Devonian age.

The material composition of the volcanic sequence tends toward an intermediate composition, with subordinate acid varieties. Rocks of intermediate composition are represented by andesite porphyrites, while the acid rocks are represented by felsites and felsite—

porphyries; transitional varieties—trachyandesite porphyries and plagioporphyries. The lower part of the sequence is composed predominantly of flows of andesitic porphyrites. In the upper part, trachyandesite and plagioclase porphyries, felsites, and felsite porphyries predominate. Petrographic and petrochemical analyses of the rocks show a continuous series of magma differentiation from andesitic to rhyolitic compositions. Below is a description of the most complete section in the eastern part of the volcanic sequence, from bottom to top. Because of a disjunctive disturbance of the northern limb of the eastern part of the syncline, at the place where the section was compiled the volcanogenic sequence is represented only by one southern limb, complicated by additional folds and small disjunctive disturbances. The general dip of the rocks here is northeastward, with angles from 10 to 70°.

1. Andesitic porphyrites with brown, lilac, and greenish shades, with thin interbeds of lithoclastic tuffs of the same composition and trachyandesite porphyries . . . . . 200 m.
2. Tuffs of felsites and felsite porphyries, pink in color . . . 15 m.
3. Pink banded felsites . . . . . 65 m.
4. Tuffs of felsites, pink in color . . . . . 10 m.
5. Pink banded felsites . . . . . 50 m.
6. Trachyandesite porphyries, brown in color, with thin lenses of tuffs of the same composition . . . . . 20 m.
7. Andesitic porphyrites with brown and lilac shades . . . 65 m.
8. Trachyandesite porphyries with brown and cherry-colored shades . . . . . 7 m.
9. Tuffs of pink felsites and felsite porphyries . . . 5 m.
10. Andesitic porphyrites with lilac and brownish shades . . . . . 7 m.
11. Trachyandesite porphyries of brown and pink tones . . . 10 m.
12. Banded felsites, pink in color . . . . . 7 m.
13. Trachyandesite porphyries . . . . . 8 m.
14. Felsites and felsite porphyries with pink and lilac shades . . . 6 m.
15. Trachyandesite porphyries and plagioporphyries . . . 10 m.
16. Tuffs of pink felsites and felsite porphyries . . . 4 m.
17. Andesitic porphyrites . . . . . 8 m.
18. Trachyandesite porphyries . . . . . 12 m.
19. Pink felsites . . . . . 7 m.
20. Trachyandesite porphyries . . . . . 6 m.
21. Pink felsites . . . . . 6 m.
22. Trachyandesite porphyries . . . . . 9 m.
23. Tuffs of pink felsites and felsite porphyries . . . 10 m.
24. Tuffs of andesitic porphyrites . . . . . 15 m.
25. Andesitic porphyrites . . . . . 20 m.
26. Felsites and felsite porphyrites, pink and lilac in color . . . . . 22 m.
27. Trachyandesite porphyries . . . . . 12 m.
28. Andesitic porphyrites . . . . . 17 m.
29. Trachyandesite porphyries . . . . . 15 m.
30. Andesitic porphyrites . . . . . 10 m.
31. Trachyandesite porphyries and plagioporphyries . . . 31 m.

Total thickness 689 m.

It should be noted that the banded felsites are represented mainly by extrusive and transitional facies from extrusive to effusive. At times it is difficult to determine to which of these facies the rocks belong. In places the banded felsites form dikes and irregular bodies cutting the porphyrites.

Below are tables characterizing the petrographic properties of the effusive rocks of the Bograd Bridge area. The mean values of the principal characteristics of the rocks were determined from thirty-member samples at a 95% confidence interval. The distribution in the samples corresponds to a lognormal law.

Table 1

Chemical composition of the rocks

1, 2, 3, 4 — andesite porphyrites,
5 — plagioclase porphyry,
6, 7 — trachyandesite porphyries,
8, 9 — felsites,
10 — dike of quartz microsyenites,
11 — quartz syenite of Yuli.

Of the physical properties of the volcanic rocks, radioactivity (γ-activity) was determined, and for the andesite porphyrites, density and magnetic properties (1,3). The gamma activity of the andesite porphyrites averages 11.75, of the trachyandesite porphyries and plagioporphyries—14.7, and of the felsites and felsite porphyries—21.2 microroentgens per hour. The mean density of the andesite porphyrites is 2.62 within the limits of the 95% confidence interval, equal to 2.602—2.636, and with a standard deviation \(S = 0.037\) (6). The mean residual magnetization is \(J_n = 22350 \cdot 10^{-6}\) CGS. The magnetic susceptibility \(\chi_{\mathrm{av}} = 1860 \cdot 10^{-6}\) CGS.

Results of spectral analyses

Table 2

Average element contents, in %

Indicators of the primary composition and morphology of the intratelluric phases

Table 3

The intrusive facies of the volcanism is represented by dikes of porphyrites, felsites, felsite-porphyries, quartz microsyenites, and pyroclastic dikes. Some dikes and irregularly shaped subvolcanic bodies of acidic composition are surrounded by halos of explosive

Table 4

Indicators of late crystallization of lavas, propylitic alteration, and early postvolcanic mineralization

breccias. An indium ore occurrence is associated with one such body, representing an ancient vent volcanic apparatus. Dikes of acidic rocks are close in chemical composition to effusive felsites and quartz syenites of the Yulinskaya intrusion. This circumstance to some

Table 5

which in a certain way indicates the genetic connection between the intrusive and effusive magmatism of the area.

REFERENCES

1. A. F. Belousov, A. I. Dudarev. An attempt at analyzing rock densities in ancient effusive-sedimentary complexes of the Gorny Altai. Geology and Geophysics, No. 10, Publishing House “Nauka,” Siberian Branch, Academy of Sciences of the USSR, 1965.

2. A. F. Belousov, V. V. Velinskii, Yu. N. Kochkin. Plagioclases in basaltic effusives of the Upper Proterozoic and Cambrian in Altai and Salair. Geology and Geophysics, No. 3, Publishing House “Nauka,” Siberian Branch, Academy of Sciences of the USSR, 1965.

3. Yu. B. Evdokimov, A. G. Komarov. An experiment in determining the age of volcanic rocks by measuring natural remanent magnetization. Proceedings of the All-Union Scientific-Research Geological Institute, vol. 67, 1961.

4. I. V. Luchitskii. Volcanism and tectonics of the Devonian depressions of the Minusinsk intermontane trough. Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.

5. A. A. Mossakovskii. The tectonic development of the Minusinsk depressions and their mountainous framing in the Precambrian and Paleozoic. Gosgeoltekhizdat, Moscow, 1963.

6. I. P. Sharapov. The application of mathematical statistics in geology. Publishing House “Nedra,” Moscow, 1965.