Petrochemistry of the Tilek Intrusive Complex on the Eastern Slope of the Kuznetsk Alatau
Abstract Generated abstract
This paper describes the Devonian Tilekovsky intrusive complex identified in the Nenya-Bazinsk district on the eastern slope of the Kuznetsk Alatau. Using petrographic observations and chemical analyses recalculated by A. N. Zavaritskii’s method, the study characterizes a sequence from wehrlites through transitional noritic and gabbroic rocks to leucocratic gabbro-essexites and gabbro-diabases. The author argues that the observed geological, mineralogical, and chemical gradations reflect local differentiation within a single intrusive body. The compositional data indicate an alkaline trend in magma evolution, extending toward monzonitic and syenitic varieties.
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PETROCHEMISTRY OF THE TILEKOVSKY INTRUSIVE COMPLEX ON THE EASTERN SLOPE OF THE KUZNETSK ALATAU
V. I. Bogdanov
(Presented by the scientific seminar of the Department of Petrography)
As a result of our investigations of the geological structure of the Nenya-Bazinsk district on the eastern slope of the Kuznetsk Alatau, an original complex of magmatic rocks of Devonian age was identified. We have named this complex the Tilekovsky intrusion. Since Devonian intrusions have not been sufficiently well studied, the compositional features of the complex that we have distinguished are of definite interest. Outcrops of the rocks of this complex are observed on the watershed of the Beika-Kug rivers, 5 km northwest of the settlement of Tilekov, and form a linear body elongated in a latitudinal direction. The length of the body is about 2.5 km; the width of the outcrop at the surface is about 1.0 km, narrowing in the southwestern direction to 0.25 km. Several small stock-like and dike-like outcrops of rocks of this complex were noted on the right side of the valley of the Nenya River, southeast of the village of Sinyavino, 3.5 km southeast of Lake Bulan-Kul.
The Tilekovsky intrusion cuts through and hornfelses the red-colored sandstones of the Kazanovskaya Suite and the lower part of the overlying Bolshesyrskaya Suite of effusive-pyroclastic composition. The age of these sedimentary-effusive deposits is determined by most geologists as Lower Devonian (A. G. Sivov [3]; I. V. Luchitskii [2], and others).
In the intrusive body of the Tilekovsky intrusive complex, leucocratic gabbro-essexites and gabbro-diabases predominate. Approximately at the center of the massif there is a small field of rocks with transitions from troctolites to feldspathic wehrlites. The most widespread are leucocratic gabbro-essexites and gabbro-diabases. From the former there is a gradual transition to wehrlites.
The most basic rocks of this complex are the wehrlites (Table 1, Fig. 1). From the average composition of wehrlites according to Daly they differ by a somewhat elevated value of $c$, higher $m'$ and lower $c'$, and have a sharply elevated value of $\varphi$ (sample 1). The chemical composition of the wehrlites is not constant. A rather clearly expressed regularity is observed (see samples 2, 3, 4, 5) in the form of an increase in the value of $Q$ (from 24.8 to 18.23); $a$ (from 2.72 to 3.4—5.7); $c$ (from 2.35 to 4.65); $f$ (from 22.81 to 24.9—27.7), while the value of $b$ decreases (from 50.3 to 41.9). The value of $m'$ fluctuates, but is everywhere higher than in the average wehrlite; the values of $n$ and $\varphi$ are variable, but everywhere lower than in the average wehrlite.
Table 1
| Sample No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SiO₂ | 42,80 | 42,82 | 42,78 | 42,98 | 44,02 | 46,04 | 47,30 | 47,04 | 51,72 | 51,99 | 50,96 | 51,14 | 49,28 | 50,36 | 49,72 | 51,10 |
| TiO₂ | 0,74 | 0,81 | 0,55 | 0,50 | 0,60 | 1,20 | 1,17 | 0,74 | 1,32 | 0,84 | 0,75 | 1,38 | 1,41 | 1,31 | 1,42 | 1,23 |
| Al₂O₃ | 9,60 | 7,13 | 8,61 | 10,01 | 12,85 | 14,30 | 16,02 | 18,76 | 17,56 | 21,88 | 19,69 | 19,56 | 19,21 | 19,68 | 19,59 | 19,22 |
| Fe₂O₃ | 2,99 | 2,33 | 3,06 | 2,57 | 3,95 | 6,73 | 3,42 | 2,12 | 5,30 | 1,46 | 3,64 | 5,11 | 5,12 | 4,89 | 4,89 | 4,92 |
| FeO | 10,58 | 13,97 | 11,96 | 10,93 | 10,20 | 5,54 | 5,95 | 5,71 | 3,56 | 3,07 | 3,80 | 3,6 | 3,18 | 3,74 | 4,06 | 3,72 |
| MnO | 0,07 | 0,11 | 0,19 | 0,13 | 0,12 | 0,07 | 0,09 | 0,08 | 0,07 | 0,04 | 0,03 | 0,07 | 0,10 | 0,10 | 0,07 | 0,09 |
| MgO | 23,46 | 21,70 | 23,80 | 22,50 | 18,46 | 12,38 | 8,01 | 7,7 | 3,43 | 3,85 | 3,46 | 3,87 | 1,93 | 3,83 | 4,28 | 3,32 |
| CaO | 4,83 | 3,88 | 3,97 | 4,15 | 6,4 | 6,28 | 9,02 | 9,18 | 7,43 | 9,06 | 5,60 | 6,72 | 9,11 | 6,28 | 7,08 | 6,33 |
| Na₂O | 1,01 | 2,22 | 1,18 | 1,05 | 1,43 | 2,56 | 2,75 | 2,62 | 3,95 | 3,37 | 4,95 | 3,54 | 3,48 | 3,94 | 3,54 | 3,64 |
| K₂O | 0,57 | 1,00 | 0,82 | 0,65 | 0,87 | 1,50 | 0,87 | 1,30 | 1,50 | 2,50 | 1,61 | 2,18 | 1,55 | 1,99 | 2,13 | 2,69 |
| P₂O₅ | 0,55 | 0,35 | 0,21 | 0,23 | 0,51 | 0,54 | 1,16 | 0,55 | 0,68 | 0,41 | 1,58 | 0,54 | 0,85 | 0,61 | 0,49 | 0,42 |
| S | 0,05 | 0,07 | 0,05 | 0,06 | 0,09 | 0,04 | 0,03 | 0,04 | 0,02 | 0,02 | 0,02 | 0,03 | 0,01 | 0,04 | 0,04 | 0,02 |
| ignition loss | 1,74 | 1,67 | 2,63 | 2,85 | 1,73 | 2,51 | 3,14 | 3,06 | 2,67 | 1,90 | 3,69 | 1,75 | 4,05 | 2,53 | 2,13 | 2,45 |
| total | 100,10 | 99,50 | 100,30 | 99,84 | 99,60 | 100,41 | 99,55 | 99,53 | 99,52 | 99,51 | 100,12 | 99,15 | 99,61 | 99,63 | 99,82 | 99,52 |
Numerical characteristics according to A. N. Zavaritsky
| Characteristic | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| a | 2,72 | 5,70 | 3,4 | 3,2 | 4,27 | 7,7 | 7,87 | 8,33 | 11,6 | 12,10 | 14,56 | 11,8 | 11,16 | 12,65 | 11,68 | 13,1 |
| c | 2,35 | 7,4 | 3,4 | 4,65 | 5,77 | 5,6 | 7,3 | 9,27 | 16,6 | 9,75 | 7,11 | 8,26 | 8,80 | 8,15 | 8,34 | 7,4 |
| b | 50,3 | 48,9 | 49,1 | 46,6 | 41,9 | 33,7 | 27,4 | 24,4 | 17,70 | 13,9 | 14,13 | 18,84 | 15,74 | 15,7 | 17,55 | 15,5 |
| s | 44,63 | 44,0 | 44,1 | 45,35 | 48,04 | 53 | 57,43 | 58,0 | 64,0 | 64,25 | 64,2 | 61,1 | 64,3 | 63,5 | 62,43 | 64,0 |
| a′ | — | — | — | 0,27 | — | — | — | — | — | — | — | — | — | — | — | — |
| f′ | 22,81 | 27,7 | 24,6 | 24,9 | 26,0 | 32,6 | 32,8 | 31,7 | 48,2 | 32,7 | 52,38 | 51,5 | 54 | 53,3 | 49,60 | 54,1 |
| m′ | 71,31 | 66,6 | 73,5 | 71,83 | 70,6 | 61,6 | 51,8 | 57,5 | 34,9 | 51,1 | 44,99 | 44,8 | 23,3 | 44,8 | 44,52 | 39,7 |
| c′ | 5,08 | 5,7 | 1,86 | — | 3,4 | 5,8 | 15,4 | 10,8 | 16,9 | 16,2 | 2,63 | 3,7 | 22,7 | 1,9 | 5,88 | 6,2 |
| n | 72,7 | 76,6 | 67,9 | 70,8 | 69,6 | 71,9 | 81,6 | 75,5 | 80 | 66,7 | 82,5 | 71,4 | 76,7 | 75,4 | 72,2 | 67,2 |
| φ | 4,66 | 3,47 | 4,73 | 4,27 | 3,72 | — | 10,93 | 7,76 | 27,3 | 9,68 | 24,3 | 29,8 | 32, | 28,3 | 25,2 | 29,7 |
| l | 1,24 | 1,38 | 0,97 | 0,83 | 1,1 | 1,9 | 1,87 | 1,14 | 1,93 | 1,7 | 1,7 | 2,07 | 2,14 | 1,87 | 2,13 | 1,73 |
| Q | 18,53 | 24,8 | 22,0 | 20,35 | 18,23 | 15,0 | 8,18 | 5,93 | 1,9 | 4,45 | 7,83 | 5,66 | 2,54 | 6,5 | 7,32 | 5,6 |
| α/c | 1,15 | 4,07 | 1,0 | 0,7 | 0,77 | 1,4 | 1,1 | 0,89 | 1,7 | 1,3 | 2,1 | 1,42 | 1,4 | 1,5 | 1,4 | 1,77 |
Figure 1. Diagram of the chemical composition of rocks of the Tilekovsky intrusive complex (after A. N. Zavaritsky).
1, 2, 3, 4, 5 — verlites; 6, 7, 8, 9, 10, 11 — transitional varieties from verlites to leucocratic gabbro, gabbro-diabases; 12, 13, 14, 15 — leucocratic gabbro, gabbro-diabases; 16 — leucocratic gabbro with an alkaline tendency.
...of Deli. The value of \(t\) is at first greater than the \(t\) of average wehrlite and then steadily decreases from 1.24 to 0.83.
The transition from wehrlites to the most widespread leucocratic gabbro-essexites and gabbro-diabases takes place gradually through rocks that, in chemical composition, correspond to norites and olivine-free gabbro (see specimens 6, 7, 8, 9, 10, 11).
The value of \(a\) increases steadily (from 7.7 to 14.56), \(c\) increases (from 5.6 to 7.11), \(b\) decreases (from 33.7 to 14.13), and the value of \(s\) increases to 64.2. The characteristic \(f'\) also increases, \(m'\) decreases, and \(n\) is variable. The values of \(\varphi\), \(t\), and \(Q\) increase sharply.
The most widespread varieties of the Tilekovsky intrusive complex are represented by leucocratic gabbro-essexites and gabbro-diabases; in chemical composition they vary within fairly broad limits, from leucocratic gabbroids through shonkinites and monzonites to alkaline-earth syenites. The value of \(a\) increases from 11.8 to 13.1; the amount of silica and the value
\[ \frac{a}{c} \]
increase appreciably (from 1.42 to 1.77). The characteristic \(n\) is variable, decreasing with the transition to more alkaline rock varieties.
Considering the petrographic and chemical features of the rocks of the intrusive complex as a whole, it should be noted that all the recorded rock varieties are products of local differentiation within the body of the massif, as is demonstrated by the gradual transitions of the rocks both with respect to their geological position and mineralogical composition and with respect to their chemistry.
The rock varieties plotted on the diagram by the method of A. N. Zavaritskii show compositional points from which the alkaline trend of magma differentiation is clearly visible, up to the formation of monzonites and syenites.
In conclusion, the author considers it his duty to express gratitude to his supervisor S. S. Il’enok for consultations in the present work.
REFERENCES
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A. N. Zavaritskii. Recalculation of chemical analyses of igneous rocks. Publishing House of the Academy of Sciences of the USSR, 1941.
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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, 1960.
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A. G. Sivov. On the principles of constructing a regional stratigraphic scale and its subdivisions. Proceedings of the Conference on the Study of Geological Formations. Publishing House of the West Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1953.