SUBDIVISION OF VOLCANOGENIC FORMATIONS OF THE TELBES SERIES IN THE TALANOVSKY GRABEN AREA (NORTHWESTERN SLOPE OF THE KUZNETSK ALATAU)

V. G. Kryukov and V. Z. Mustafin

(Presented by Professor A. M. Kuzmin)

Serious attention is being given to the problems of subdividing Devonian deposits and studying their stratigraphy within the Kuznetsk Alatau. In the territory of the Talanovsky graben we carried out geological mapping of the undivided effusives of the Telbes series of Lower–Middle Devonian age, $D_1—2tl$. In the process, material was obtained on the petrography, stratigraphy, and tectonics of the sequence, making it possible to distinguish in it the following principal groups of rocks (from bottom to top): 1) effusives of basic composition, 2) effusives of intermediate composition, 3) effusives of alkaline composition, 4) effusives of the feldspathoid group, 5) effusives of basic composition, 6) effusives of basic–intermediate composition.

1. Effusives of Basic Composition

The sequence of effusives of basic composition forms the base of the Telbes series. In the graben area it is predominant, being traceable throughout its entire length along the eastern boundary. A north–northeastern strike of the sequence and a monoclinal easterly dip at an angle of $30—60^\circ$ are observed. About six flows are counted in it (Fig. 1), separated from one another by a slaggy crust or by a sill-like analogue of these rocks. Among the basic effusives there predominate hypocrystalline fine- to coarse-porphyritic and aphyric porphyrites with amygdules and without them. Labrador-pyroxene, labrador-pyroxene with olivine, and labrador porphyrites and aphyrithes, as well as their lavas, clastolavas, and tuffs, are widely distributed. In addition to textural-structural features, the rock varieties listed have certain differences in mineralogical composition.

Labrador-pyroxene porphyrites and aphyrithes form flows constituting a significant part of the sequence of effusives of basic composition. Labrador-pyroxene porphyrites and aphyrithes are characterized by: a dark-gray to black color with a lilac tint; a massive or amygdaloidal-porous texture; an aphanitic or finely recrystallized structure of the groundmass. In most cases they are distinguished by a fine-porphyritic structure, the amount of phenocrysts varying from individual grains to $30—40\%$.

Plagioclase predominates in the porphyritic segregations, together with which pyroxene is present. In flows of the upper part of the section

in the phenocrysts of the labradorite-pyroxene porphyries of the unit being described, isolated grains of olivine are sometimes recorded. The plagioclase of the phenocrysts has a tabular habit. Its angle of symmetrical extinction is 32–38°; refractive index, by \(Ng \approx 1.570\); \(2V_{Np} = 100^\circ\); \(PcNg = 33^\circ\); \(PcNm = 62^\circ\); \(PcNp = 70.3^\circ\); \(BcNg = 30^\circ\); \(BcNm = 30^\circ\); \(BcNp = 62^\circ\), which corresponds to labradorite no. 60–65. The pyroxenes of the porphyritic segregations are represented mainly by a monoclinic variety, corresponding in composition predominantly to augite. The latter is characterized by a light-brownish color in transmitted light, an angle \(c \wedge Ng\) of about 43–48°, \(2V_{Ng} \approx 54^\circ\)—68°, and birefringence equal to 0.022. In rare cases diopside is identified, distinguished by a positive sign of the optical axes, birefringence of about 0.032, an angle \(c \wedge Ng\) equal to 42–48°, and \(2V_{Ng} = 56\)—60°. The olivine is almost completely opacitized—replaced by serpentine, chlorite, and magnetite. As a rule, it has high relief, a specific composition of replacement products, and well-developed faces (021) and (010).

The groundmass of the rock, with an intersertal or hyalopilitic texture, consists chiefly of plagioclase, which makes up from 30–40% to 60–80%. Its refractive indices are \(Ng \approx 1.567\) and \(Np \approx 1.560\), which corresponds to labradorite no. 52–56. The dark-colored minerals are represented by augite, for which faces (100), (110), and (001) are characteristic. In its principal properties it is analogous to the augite of the phenocrysts. The ore mineral, amounting quantitatively to 7–12% of the groundmass, when observed in reflected light proves to be magnetite, more rarely magnetite and pyrrhotite. Products of devitrification of glass and its relics are noted almost everywhere in amounts of 3–6%, increasing sharply to 20–30% in lavas.

Labradorite porphyries occur in the form of flows in the middle and upper parts of the sequence of basic effusives. In addition, their nearly meridional dikes are also noted.

The rocks described are distinctive among the effusives because of the abundance of porphyritic phenocrysts of plagioclase, the amount of which varies from 3–5% to 15–20%, and their sizes, respectively, from \(1 \times 5\) mm to \(3 \times 25\) mm. Not infrequently 3–6 plagioclase phenocrysts form stellate glomeroporphyritic aggregates. They are represented by labradorite, characterized by: an angle of symmetrical extinction of about 32–34°, \(2V_{Np} = 104^\circ\); \(BcNg = 77^\circ\); \(BcNp = 46^\circ\); \(BcNm = 46^\circ\); albite and Carlsbad twinning laws. By these properties the plagioclase corresponds to labradorite no. 50–60.

The degree of crystallization of the groundmass of the labradorite porphyries varies, and therefore products of its devitrification are either completely absent (intersertal texture), or their amount reaches 30–40% (hyalopilitic texture). The composition of the groundmass is comparatively uniform: labradorite no. 50–55 about 50–70%, monoclinic pyroxene in an amount of 5–20%, magnetite about 10–20%, devitrification products (albite, calcite, chalcedony), and isolated grains of apatite.

Lavas and clastolavas of basic rocks have a chemical composition analogous to that of the rocks described above, differing from the latter only in textural and structural features. The transitions from porphyries and aphyrites to lavas, which apparently represent the roofs of large flows, are gradual.

Tuffs of effusives of basic composition are relatively widespread, bordering the sheets of basic effusives from the east as a belt of variable, 10–400 m, width. Among them are noted psephitic, pelitic, and psammitic varieties, interbedded with one another and having a similar composition. Tuffaceous formations

Scale: −100 0 100 200 300 400 m

Visible locality and stratigraphic labels in the diagram: Perevozninskaya — Tashtyp, Ust-Tashtyp; Voroshilovo; north of Batrada; Kopevo; Cheremushno; Lake Shira; Talanovsky graben. Kharadzhul suite; Tarbagan suite; Karagaimek suite; Tolochkov suite; Tashtyp–Ulus suite; Bolshe-Syr suite; Belbei suite.

Fig. 1. Correlation scheme of stratigraphic columns of Devonian volcanogenic formations of the Kuznetsk Alatau.

1. Recent deposits. Devonian formations. 2. Andesite porphyrites. 3. Pyroxene–olivine porphyrites. 4. Sandstones, limestones, aleurolites, conglomerates. 5. Trachytoid phonolites. 6. Trachyte porphyrites. Orthophyres. 7. Plagioclase–pyroxene porphyrites. 8. Labradorite porphyrites. 9. Labradorite–pyroxene porphyrites and aphyrites. 10. Lavas. 11. Pelitic, psammitic, psephitic tuffs. 12. Unsaturated effusives of basic composition. 13. Volcanic conglomerates and breccias. 14. Basement rocks.

are characterized by a red-brown to brown color with a cherry tint, caused by an admixture of pelitic material, amounting from 40–50% to 70–80%. Fragments of lapilli form, 1–10 mm in size, are observed; they are represented by labradorite-pyroxene and labradorite porphyrites and aphyrites.

2. Effusives of intermediate composition

The effusives of intermediate composition apparently constitute a subvolcanic facies of the basic effusives (labradorite-pyroxene and labradorite porphyrites). This is confirmed by the close chemical composition (Table 1) of the intermediate and basic effusives, by the better recrystallization of the groundmass, by the broad development of hydroxyl-bearing minerals, and by their intrusive contacts with the enclosing labradorite porphyrites. Rocks of intermediate composition form a large dike-like body traceable along the western boundary of the graben and in its central part. Plagioclase–hornblende porphyrites predominate in the body of effusives of intermediate composition; among them, lens-shaped bodies of tuff-conglomerates are noted.

Table 1

Average oxide contents according to chemical analyses of rocks of the Talanovsky graben

Plagioclase-hornblende porphyrites have a dark-gray coloration with greenish and lilac spots. The quantity of porphyritic phenocrysts is relatively constant and fluctuates within the range of 30–35%. Plagioclase predominates among them, accounting for 50–80% of their quantity. The remaining part of the phenocrysts is represented by hornblende.

On the basis of the angle of symmetrical extinction ($27—33^\circ$), the angle $2V_{Np}\approx104^\circ$, $B(001)$, $II(010)$, and the development of Carlsbad twins, the composition of the plagioclase is determined as andesine-labradorite, No. 45—55.

Hornblende occurs as long-prismatic crystals. Its extinction relative to the elongation of the grains is $14—16^\circ$. Pleochroism ranges from brown to light brown or from blue-green to light greenish-yellow. Absorption scheme: $Ng > Nm > Np$; angle $2V_{Na}\approx80^\circ$.

The groundmass of the rock is usually well crystallized (intersertal structure). About 50—60%, more rarely 70%, of it consists of andesine No. 40 (refractive indices $Ng\approx1.560$ and $Np\approx1.552$). In addition, hornblende is observed in amounts of 20—40%, magnetite 5—10%, and, sporadically, pyrrhotite.

Tuff-conglomerates are characterized by a coarse-blocky texture and gray color. The size of the fragments varies widely, from 0.1—1 to 30—50 cm. They make up 70—80%. The main part of the fragments consists of plagioclase-hornblende porphyrites. They are cemented by finer clastic material.

3. Effusives of Alkaline Composition

Effusives of alkaline composition are developed in the middle parts of the section of the Telbes Series. They are of relatively limited distribution, forming lens-shaped bodies 700—1200 m long, more rarely 2500—3000 m, with a width of 100—400 m, oriented in a north-northeasterly direction.

Lithologically, among the rocks under consideration there are distinguished trachytic porphyries, their lavas and clastolavas, which form extrusive bodies, as well as orthophyres, alkaline syenites, and xenoclastolavas, representing a vent facies. The latter are cut by labrador-pyroxene and labrador porphyrites. In addition, the rocks of basic composition of the first unit are overlain by trachytic porphyries.

Trachytic porphyries predominate among the listed alkaline varieties. They are characterized by light-brownish, light-pink, and brownish coloration. The amount of porphyritic phenocrysts in them is variable: from 3—5% to 20—25%, and their dimensions vary from 2—3 mm to 10—15 mm in length. About 40—50% of the phenocrysts consist of orthoclase, for which Manebach twins and the angle $2V_{Ng}=60^\circ$ are characteristic; microperthite accounts for 5—10%. In addition, up to 40% of grains of albite-oligoclase No. 8—15 are observed: $BcNg=80^\circ$, $BcNp=83^\circ$, $BcNm=15^\circ$; $PcNg=13^\circ$; $PcNp=88^\circ$; $PcNm=77^\circ$. In the central parts of the more massive dome-shaped bodies, in the trachytic porphyries, monoclinic pyroxene is established in amounts of 3—5%. It is characterized by: angle $c \wedge Ng = 42—46^\circ$, weak pleochroism from greenish-yellow along $Ng$ to light with a violet tint along $Nm$; refractive indices $Ng\approx1.720$ and $Np=1.700$. This makes it possible to assign it to titanium-bearing augite with 40% $(Ca, Mg)$ and 60% $(Ca, Fe)$ [4].

The groundmass of trachytic structure is represented chiefly, 70—90%, by feldspar with approximately equal quantities of orthoclase ($Ng\approx1.525$) and albite ($Ng\approx1.535$ and $Nm\approx1.527$). In addition to them, pyrite is recorded in amounts of 1—3% and about 3—10% of a weakly polarizing mineral corresponding in composition to albite, apparently formed during devitrification.

Lavas and clastolavas of trachytic porphyries make up the roof of the extrusive bodies. These rocks have fluidal, vesicular, and indistinctly fragmental textures. The size of the fragments is the first centim—

rocks. Their composition and the structural features of both them and the cementing mass are identical. Gradual transitions are observed between clastolavas and trachyte porphyries.

Orthophyres, petrographically and in chemical composition, are close to trachyte porphyries, representing either a subvolcanic facies or a facies transitional to it. The contacts of these rock types are gradual and are characterized by varieties of transitional aspect within a belt 15–30 m wide. Orthophyres differ from their effusive analogues (trachyte rocks) by better recrystallization, the predominance of microperthite among the phenocrysts, an insignificant amount of albite and amphibole in them, and an orthophyric structure of the groundmass.

Amphibole is characterized by a short-prismatic grain shape, cleavage at an angle of 62–65°, pleochroism from colorless along Ng to light pink along Nm, absorption scheme \(Nm \gg Ng > Np\), angle \(c \wedge Ng \approx 50^\circ\), and birefringence about 0.014. All the noted features make it possible to assign the mineral described to the alkaline amphiboles of the kataphorite series.

Alkaline syenites are a facies variety of orthophyres. They are characterized by textures from fine- to coarse- and even gigantic-grained. The latter appear with increasing depth (at 100–300 m from the present-day surface). The block structure of the graben and the differing erosion of the blocks account for the appearance of alkaline syenites at the surface in its southern and northern ends, in the form of broad, 150–250 m, dike-like bodies. The syenites differ from the alkaline rocks described above by a holocrystalline texture, the practically complete absence of mafic minerals, the comparatively broad development of potassium feldspars—microcline and microcline-perthite—and also by comparatively rare grains of nepheline.

Xenoclastolavas develop at the contacts of orthophyres and effusives of basic composition. The number of fragments, represented by labrador-pyroxene, labrador porphyrites and their lavas, reaches 10–30%. The cementing mass has a trachytoid aspect, alkaline character, close to orthophyres.

4. Effusives of the feldspathoids group

This group of effusive rocks includes only trachytoid phonolites. Their form of occurrence is sills and dikes. The latter cut sheets of trachyte porphyries; more rarely they are observed outside the fields of development of alkaline rocks, where they intrude labrador-pyroxene porphyrites.

Trachytoid phonolites are characterized by a light brownish, brownish-violet, or lilac coloration. Their texture is porphyritic. The amount of phenocrysts is variable: both along strike and down the dip of the bodies it ranges from 7–10% to 20–25%, with average sizes of 5–7 mm.

In the porphyritic segregations, thick-tabular isometric crystals of nepheline play a substantial role, from 30–40 to 80%. Their form is square or subsquare, with faces (1010). Nepheline is commonly replaced by cancrinite, whereas the other components of the rock remain fresh. Only rarely are unreplaced grains of nepheline observed. The remaining part of the phenocrysts is accounted for by potassium feldspar. In the latter, sanidine predominates; it is characterized by a fresh appearance of the grains, the presence of simple Carlsbad twins, angle \(2V_{Nb} = 40^\circ\), \(P(010)\), \(B(100)\); \(PcNg = 0^\circ\), \(PcNp\) and \(Nm = 90^\circ\); \(BcNg = 88^\circ\), \(BcNp\) and \(Nm = 20^\circ\), which corresponds to 65% Ort and 35% Ab [4]. In addition, orthoclase and microperthite occur among the phenocrysts.

The trachytoid groundmass of the phonolites is represented mainly by K-feldspar \((N_g \approx 1.525)\) and albite \((N_g \approx 1.535\) and \(N_m \approx 1.527)\), with a small, up to 1–3%, admixture of pyrite.

In individual blocks of the graben (northern and southern), an unconformable overlap on the alkaline and basic rocks of the effusive-sedimentary sequence is established. At the base of the section of the latter, a member of conglomerates of variable thickness, 10–30 m, is recorded. The clastic material, which constitutes 60–80% of them, is represented by labrador, labrador-pyroxene porphyrites, trachyte porphyries, and their lavas. Upsection, the conglomerates are replaced by a variegated sequence with a thickness of not less than 80–120 m. It is characterized by interbeds that are either sheet-like or sill bodies of pyroxene-olivine porphyrites. The lower parts of the sequence are composed of coarse-grained cherry-colored, brownish-reddish polymictic sandstones with an admixture of tuffaceous material. Upward they are replaced by fine-grained brownish polymictic sandstones with interbeds, 10–20 m thick, of grayish-greenish siltstones with poorly preserved plant remains. In the upper part of the section of the sequence, interbeds of brownish-greenish argillites appear. The sequence being characterized is crowned by a comparatively thin, about 20 m, horizon of dark-gray clayey limestones.

The section of the Devonian formations is completed by a second sedimentary-volcanogenic sequence of small thickness, 30–60 m (Fig. 1). At its base a bed of conglomerates is noted; in its pebbles, besides effusive rocks, isolated fragments of sedimentary formations are established. In this sequence, as in the preceding one, horizons of lavas of pyroxene-olivine and andesite porphyrites are recorded. It is composed mainly of brownish-gray polymictic sandstones with interbeds of light grayish-greenish siltstones, brownish argillites, and dark-gray clayey limestones with a brownish tint.

In the last two sequences, pyroxene-olivine and andesite porphyrites and their lavas and clastolavas are quite specific.

Pyroxene-olivine porphyrites, in chemical composition and textural features, are close to labrador-pyroxene porphyrites. Their higher stratigraphic position, confinement to the eastern boundary of the graben, the multifacial character of their occurrence (dikes, sills, sheets, vents), and a somewhat different mineralogical composition make it possible to resolve unambiguously the question of their position in the section.

In the porphyritic segregations of these rocks, along with widely developed augite with \(2V_{N_g} = 64^\circ\) and with \(\angle N_g\) about \(42–48^\circ\), grains of olivine are present. The latter differs from the olivine of the labrador-pyroxene porphyrites by its wide distribution and comparative freshness. For it, \(c \parallel N_m\) and the angle \(2V_{N_m} \approx 72^\circ\) have been established. Diopside is less developed.

The groundmass has a doleritic (in sills), microdoleritic (in dikes cutting the rock complex from alkaline effusives to sedimentary sequences), or hyalopilitic structure (in sheet formations).

In its composition, labrador No. 50–60 predominates, 50–80%. In a smaller amount, 20–40%, monoclinic pyroxene is established. There is an insignificant, 5–10%, admixture of magnetite and rare apatite.

Andesite porphyrites, judging by their composition, may be included in the group of basic–intermediate rocks. Their mineralogical composition is close to that of pyroxene-olivine porphyrites. The difference from the latter consists in the lower basicity of the plagioclase, corresponding to andesine-labrador No. 45–55, and in textural features. In the sheets of andesite porphyrites, horizons of coarse-porphyritic-

dark-brown porphyrites having the appearance of labradorite porphyrites from the lower unit. The presence of a gradual transition from them to andesitic fine-phenocryst porphyrites, and then to pyroxene–olivine porphyrites, leaves no room to doubt their younger age.

Lavas and clastolavas represent the cover of large flows of pyroxene–olivine and andesitic porphyrites. They are characterized by a brown color and by porous, indistinctly fragmental, weakly fluidal textures.

The studies carried out in the section of the volcanogenic formations establish several breaks. The most significant of them, of regional scale, is established at the boundary between the formation of trachytoid phonolites and the sedimentary unit that includes horizons of pyroxene–olivine and andesitic porphyrites. This makes it possible to divide the process of volcanic activity in the area described into two megacycles. Breaks of a lower order correspond to the period of time between the formation of effusives of intermediate and alkaline composition, and also between the intervals of formation of two sedimentary units.

According to A. G. Sivov [9], the principal criterion in distinguishing suites is a break in the sequence of formation of units corresponding to the rhythm of a geotectonic process of lower order than a megacycle. In accordance with this, three suites can be distinguished in the area of the Talanovsky graben. The first of them includes basic and intermediate effusives. The second suite, in volume, corresponds to alkaline effusives, including trachytoid phonolites. The volcanogenic-sedimentary unit is assigned to the third suite.

The absence of plant remains of good preservation does not make it possible to date precisely the time of formation of the individual suites. However, most investigators [3, 5, 6, 7, 9] consider it possible to correlate sections of “mute” Devonian deposits from different parts of the eastern slope of the Kuznetsk Alatau, obtaining quite good results. The basis of such correlation is the fact that the manifestation of volcanism within a single structure, despite local variations, is characterized by a definite general trend in magma development—for the northeastern slopes of the Kuznetsk Alatau—toward the alkaline side [3, 7], and also by the presence of major stages of the geotectonic process [2].

Nevertheless, even with this principle of correlation, comparison of sections of Devonian volcanogenic units of individual regions encounters serious difficulties: in the works of some investigators there are no data not only on the chemical composition, but also on the petrographic characteristics of the rock types distinguished. The absence of marker horizons substantiated faunistically or phytostratigraphically makes correlation of effusives under such conditions impossible [8]. Therefore we compare the Talanovsky section with the comparatively reliably subdivided sections of neighboring regions [5, 7]. Variations in their composition require the use of several stratigraphic columns of substantially effusive character and their comparison with a typical effusive-sedimentary section.

The effusive types of sections (Talanovsky, Lake Shira, and others, Fig. 1) are clearly comparable with the substantially sedimentary Devonian units developed in the area of the villages of Tashtyp, Ust-Tashtyp, and Perevozinskaya. At the same time it must be pointed out that some confusion in comparing the sections arises because of the differing application of the terminology of effusive rocks, defined sometimes by petrographic-genetic features and sometimes by their chemical composition [7, 8]. Thus, diabases, diabase porphyrites—

the basalts of the Kharadzhul formation [7], in chemical and mineralogical composition, as well as in their form of occurrence (covers, flows), are close to the effusives of the Bolshesyr formation [5, 10] and, apparently, should be defined as labrador-pyroxene and andesitic porphyrites. The first (lower) suite of effusives of the graben must be correlated with them. The middle suite is comparable with the Karagai suite [5], while the upper one is parallelized with the Shunet and Charkov suites [5], or with the Imek and Tolochkov suites [7].

Thus, the time of formation of the Devonian volcanogenic rocks of the Talanovsky graben lies within the interval from the Zhedin—Koblenzian stages to the Givetian, and the boundary between the Lower and Middle Devonian, drawn with regard to the change in stages of the geotectonic process, is established clearly.

REFERENCES

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2. A. R. Anan’ev. On breaks in the deposits of the continental Devonian in the Sayan-Altai mountain region according to paleontological data. In: Reports of the Paleobotanical Conference. TGU Publishing House, Tomsk, 1962.

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