Trachydolerite Intrusions of the Western Part of the Siberian Platform
Abstract Generated abstract
This paper describes the limited occurrences of trachydolerite intrusions in the western Siberian Platform and interprets them as an early, independent phase of trap magmatism. On the basis of field observations, petrographic descriptions, and chemical and spectral analyses, the study characterizes these bodies as mostly sill-like intrusions in Lower Permian coal-bearing deposits, with weak internal differentiation, silica undersaturation, elevated alkalis, titanium, phosphorus, and distinctive trace-element features. It also examines contact effects produced both by younger dolerite intrusions on trachydolerites and by trachydolerite sills on enclosing sedimentary rocks, concluding that the latter mainly underwent hornfelsing with only limited local migration of petrogenic components. The enrichment of these rocks in ilmenite is noted as a possible source for ancient and modern placer accumulations.
Full Text
Trachydolerite Intrusions of the Western Part of the Siberian Platform
B. V. Oleinikov
(Presented by Prof. A. M. Kuzmin)
Trachydolerite intrusions within the western part of the Siberian Platform have a limited distribution. Brief descriptions of this group of trap bodies are given in the works of M. N. Godlevsky [1] and M. L. Lur’e, V. L. Masaitis, and L. A. Polunina [2]. Following the above-named researchers, the author [4] regards the intrusions of trachydolerite magma as an independent, earliest phase of trap magmatism. The trachydolerite bodies were studied by us in the middle course of the Gorbiachin River, on the right bank of the Kureika River, near Lake Anomal’noe, and from the core of the Bolsheporozhskaya reference borehole.
Trachydolerites usually form sills, gently discordant or almost concordant with the enclosing coal-bearing Permian deposits. The thickness of the bodies studied ranged from 30 to 80 m. The described group of rocks is characterized by blocky and thick-platy forms of jointing.
In the marginal parts of the sills there are thin zones of black, fine-grained tholeiitic rocks possessing a porphyritic texture. They consist of microlites of basic plagioclase immersed in an altered glassy base. The porphyritic segregations are represented by labradorite No. 60–64.
The main part of the section of the deposits is composed of mottled fine- and medium-grained trachydolerites. Their modal composition may be expressed by the following figures (in volume %): plagioclase No. 45–60—30–55, anorthoclase—5–20, titaniferous augite—15–30, hortonolite—0–12, biotite—5–10, ilmenite, titanomagnetite—5–8, analcime—0–4, apatite—3–5; secondary minerals: iddingsite, bowlingite, talc, serpentine—5. In thin sections the trachydolerites show a poikilo-ophitic texture with a taxitic arrangement of femic and ore components and a broad development of feldspathic mesostasis.
In the upper part of the trachydolerite bodies, small lenticular and schlieren-like segregations of leucocratic coarse-grained rocks are recorded, which, by analogy with similar segregations in dolerites, may be called trachydolerite pegmatites. They differ from trachydolerites in their increased content of anorthoclase and apatite, a lower number of plagioclase; enstatite and quartz are noted in them.
In the intrusions, weak differentiation is established. The upper horizons of the bodies become enriched in alkaline feldspar, apatite, and analcime, while in the bottom parts of the deposits an accumulation of olivine is observed.
The results of chemical analysis of the rocks are given in Table 1. Trachydolerites differ from the average chemical composition of normal dolerites [3] in silica undersaturation, a reduced magnesium content, an almost twofold increase in alkalis and titanium, and an even more considerable increase in phosphorus pentoxide.
By semiquantitative spectral determination, the following elements were found in the trachydolerites: cerium—0.1%; zinc, barium, yttrium—hundredths; copper, gallium, scandium, germanium, molybdenum, tin—thousandths of a percent; beryllium—0.0003%. By quantitative spectral analysis, the following were determined in the trachydolerites (average of 19 analyses, calculated by the method of N. K. Razumovsky [5]): strontium—0.0064, manganese—
Fig. 1. Metasomatic veinlets of serpentine and diopside (1) in biotitized trachydolerite (2).
Fig. 2. Structure of metasomatic veinlets: biotitized trachydolerite (1); zone of diopside with pseudomorphs of serpentine after plagioclase (2); band of serpentine with large grains of diopside and sphene (3); chain of sphene grains (4).
0.095, vanadium—0.0086, cobalt—0.0015, and nickel—0.0056%.
In comparison with rocks of other trap intrusions [4], trachydolerites contain the smallest amount of nickel and the maximum share of strontium, barium, zirconium, and beryllium. Zinc is very typomorphic for trachydolerites; it is present in 15 samples and occurs only sporadically in rocks of other phases of intrusion. It is interesting that chromium was recorded in 9 samples and only in traces, whereas in traps of other phases it is constantly present.
The Bolshenorozhskaya borehole established that the trachydolerite deposit is overlain by a younger dolerite body 165 m thick. Under the influence of this intrusion, the trachydolerites undergo biotitization, and in them there appears a dense network of veinlets and lenticular bodies of symmetrical-zonal structure (Figs. 1 and 2), composed of serpentine, diopside, and sphene. The thickness of the zone of contact influence is about 10 m. Taking into account the scale of the phenomenon described, it may be assumed that diopsidization and later serpentinization were accompanied by the introduction of magnesium into the metamorphosed rocks and the removal of alkalis from them.
On the northern side of Lake Anomalnoye, the contact of the trachydolerite body with a sill of dolerites about 50 m thick was also observed. Here
in the near-contact band, 10–15 cm wide, the trachydolerites undergo slight lightening, discernible against the general dark-gray background and caused by partial replacement of the primary femic minerals by pale-green amphibole and biotite. A comparison of chemical analyses of the lightened trachydolerite and of the unaltered variety, taken at a distance of 1.5 m from the contact surface, shows their very close similarity (Table 1).
In the exocontact aureoles of the trachydolerite bodies, the formation of metamorphic rocks belonging to the formation of contact hornfelses and marbles has been recorded; they are rarely cut by veinlets of zeolites. Metasomatic transformations in the enclosing rocks of the near-contact zone are practically absent. All trachydolerite occurrences known to the author are situated in continental coal-bearing deposits of Lower Permian age, as a result of which they usually are in contact with siltstone rocks. Under the influence of thermal metamorphism, the latter are transformed into sphene, andalusite, and biotite hornfelses, in which a small amount of metasomatic albite sometimes appears, arranged in the form of microveinlets and small lenses. The outer zones of the metamorphosed rocks are composed of nodular spotted hornfelses. The thickness of the zone of contact hornfelses ranges from 5 to 10 m. As an example, let us consider the character of metamorphism of the enclosing rocks of the footwall and hanging wall of a trachydolerite intrusive from the area of the middle course of the Gorbiachin River.
At the boundary with the trap body there are sphene hornfelses, formed at the expense of siltstones and argillites. This variety has a light-gray or gray color, a fine-grained texture, and a conchoidal fracture. Titanite is evenly distributed throughout the entire mass of the rock; its content ranges from 3 to 7%. The quartz–feldspar groundmass is recrystallized to a greater or lesser degree. This process has a spotted character. With increasing distance from the contact, among the new formations biotite becomes dominant; its content is in direct dependence on the amount of clayey material in the original rock and on its position relative to the contact. The maximum presence of biotite (15–20%) was observed in apoargillite hornfelses. Away from the boundary with the trap body, brown biotite is replaced by greenish biotite; at the same time chlorite appears in the hornfelses. The thickness of the zone of sphene and biotite hornfelses is 2–4 m.
The outer aureoles of the exocontact border are composed of nodular and finely spotted apoargillite and aposiltstone hornfelses. The nodular hornfelses have a dark-gray color and preserve the bedding of the primary rock. Under the microscope it is seen that the nodules are more transparent than the groundmass surrounding them. In the nodules, weak regeneration of rare quartz grains is established and flakes of mica appear. In the spotted hornfelses, reliable new formations cannot be detected. The thickness of the zone of nodular and spotted hornfelses does not exceed 2–3 meters.
In one case, in contact with a large protrusion in the roof of the trachydolerite body, calcareous argillites are in contact. In contact with the trap they are transformed into a chlorite–calcite rock. In it, areas of coarse-crystalline calcite alternate with patches of chlorite, represented by diabantite. In addition to the minerals mentioned, in thin sections there have been identified: oligoclase, quartz, sphene, orthite, epidote, and apatite; pyrite is present. The thickness of the recrystallization zone is about 1 m. Table 2 gives the results of chemical analysis of the described variety.
(Laboratory of SNIGGIMS)
Results of chemical analyses of trachydolerites (wt. %)
Analysts: M. I. Matveev and V. V. Taskina
Table 1
| Components | Trachydolerite from the lower half of the body, Nizhnyaya Tunguska River | Trachydolerite from contact with dolerite, Lake Anomalnoye | Light-colored trachydolerite in contact with dolerite, Lake Anomalnoye | Trachydolerite 1.5 m from contact with dolerite, Lake Anomalnoye | Trachydolerite from the upper half of the body, Gorblyachin River | Average of 167 analyses of normal dolerites [3] |
|---|---|---|---|---|---|---|
| SiO₂ | 44.15 | 44.53 | 44.95 | 45.53 | 45.42 | 46.57 |
| TiO₂ | 3.55 | 3.45 | 3.10 | 2.85 | 2.90 | 1.48 |
| Al₂O₃ | 15.48 | 15.54 | 15.58 | 15.64 | 13.32 | 14.81 |
| Fe₂O₃ | 2.36 | 4.24 | 3.68 | 2.02 | 4.13 | 4.32 |
| FeO | 12.79 | 11.28 | 11.06 | 12.20 | 13.04 | 8.91 |
| MnO | 0.28 | 0.20 | 0.22 | 0.20 | 0.24 | 0.20 |
| MgO | 4.36 | 4.34 | 4.20 | 4.59 | 4.74 | 6.21 |
| CaO | 8.75 | 8.33 | 8.69 | 9.18 | 7.56 | 8.86 |
| Na₂O | 3.64 | 3.90 | 3.67 | 3.45 | 3.90 | 1.88 |
| K₂O | 2.38 | 2.25 | 2.11 | 1.78 | 2.16 | 0.81 |
| P₂O₅ | 1.10 | 0.69 | 0.64 | 0.75 | 0.78 | 0.08 |
| H₂O | 0.11 | 0.19 | 0.08 | 0.04 | 0.05 | — |
| L. o. i. | 1.51 | 1.49 | 1.66 | 1.54 | 1.95 | 4.52 |
| Sum | 100.46 | 100.43 | 99.70 | 99.71 | 100.19 | 100.00¹ |
| Bulk density | — | — | 2.93 | 2.94 | — | — |
¹ Including BaO — 0.12, Cr₂O₃ — 0.05, V₂O₅ — 0.05.
Table 2
Results of chemical analyses of metamorphosed rocks from the exocontact of trachydolerites
(SNIIGGiMS Laboratory)
Analyst M. I. Matveeva
| Components | Chlorite–carbonate rock: wt. % | Chlorite–carbonate rock: wt. %, reduced to 100% | Chlorite–carbonate rock: amount of substance, g/100 cm³ | Apodoleritic spotted hornfels 0.2 m from contact with trapp (1): wt. % | Apodoleritic spotted hornfels 0.2 m from contact with trapp (1): wt. %, reduced to 100% | Apodoleritic spotted hornfels 0.2 m from contact with trapp (1): amount of substance, g/100 cm³ | Biotite hornfels in contact with trapp (2): wt. % | Biotite hornfels in contact with trapp (2): wt. %, reduced to 100% | Biotite hornfels in contact with trapp (2): amount of substance, g/100 cm³ | Gain (+), loss (−), g/100 cm³ (2–1) |
|---|---|---|---|---|---|---|---|---|---|---|
| SiO₂ | 33.39 | 33.14 | 84.00 | 54.28 | 54.16 | 135.00 | 58.52 | 58.30 | 150.70 | +15.70 |
| TiO₂ | 0.73 | 0.73 | 1.85 | 1.00 | 1.00 | 2.50 | 0.90 | 0.90 | 2.30 | −0.20 |
| Al₂O₃ | 9.98 | 9.90 | 25.30 | 15.70 | 15.63 | 39.00 | 16.08 | 16.00 | 41.00 | +2.00 |
| Fe₂O₃ | 1.68 | 1.67 | 4.30 | 1.42 | 1.42 | 3.60 | 1.37 | 1.37 | 3.50 | −0.10 |
| FeO | 6.22 | 6.15 | 15.60 | 6.18 | 6.18 | 15.50 | 6.00 | 6.00 | 15.40 | −0.10 |
| MnO | 0.28 | 0.28 | 0.70 | 0.15 | 0.15 | 0.40 | 0.14 | 0.14 | 0.40 | — |
| MgO | 9.18 | 9.10 | 21.80 | 4.29 | 4.29 | 10.70 | 2.83 | 2.83 | 7.30 | −3.40 |
| CaO | 20.44 | 20.30 | 51.00 | 10.08 | 10.05 | 25.00 | 5.95 | 5.93 | 15.20 | −2.80 |
| Na₂O | 0.31 | 0.31 | 0.80 | 1.88 | 1.88 | 4.70 | 4.04 | 4.04 | 10.40 | +5.70 |
| K₂O | 2.32 | 2.30 | 5.80 | 1.68 | 1.68 | 4.20 | 1.74 | 1.74 | 4.50 | −0.30 |
| P₂O₅ | 0.22 | 0.22 | 0.55 | 0.07 | 0.07 | 0.20 | 0.07 | 0.07 | 0.20 | — |
| H₂O | 0.11 | 0.11 | 0.30 | 0.48 | 0.48 | 1.20 | 0.08 | 0.08 | 0.20 | −1.00 |
| Ign. loss | 15.84 | 15.62 | 40.50 | 3.04 | 3.04 | 7.50 | 2.61 | 2.60 | 6.30 | −1.20 |
| Total S | 0.06 | 0.06 | 0.10 | not determined | — | — | not determined | — | — | — |
| Sum | 100.76 | 100.00 | 254.00 | 100.25 | 100.00 | 250.00 | 100.33 | 100.00 | 257.00 | +2.5; −8.80 |
| Volumetric weight | 2.54 | 2.50 | 2.57 |
In the sole of the intrusion in the middle course of the Gorbyachin River, in contact with trachydolerite, argillites have been transformed into biotite hornfelses possessing a light-gray color, a platy-lenticular texture, and a fine-grained composition. In addition to biotite, veinlet-lenticular segregations of albite are observed in them. With distance from the contact, the biotite variety is rapidly replaced by a light-gray spotted apoargillite hornfels. The thickness of the rocks described is about 2 m. Comparison of the chemical compositions of the biotite variety and the apoargillite spotted hornfels (Table 2) shows their great similarity. An introduction of sodium from the trap body into the narrow zone of direct contact is established. All the other recorded differences are probably connected with local migration of petrogenic components within the exocontact zone.
Andalusite hornfelses were observed in the sole of a trachydolerite body exposed by a reference borehole at Bolshoy Porog (Lower Tunguska River). They possess a blastopelitic structure and a spotted texture. Among the recrystallized quartz-feldspar spots, small prisms of andalusite form small accumulations. Within the spots, carbonaceous pigment is completely absent. The andalusite varieties are replaced by biotite hornfelses with rare mica flakes. The thickness of the exocontact aureole zone here is about 10 m.
Thus, the sedimentary rocks enclosing the trachydolerite sills undergo hornfelsing. The nature of the metamorphic processes is in direct dependence on the thickness of the trap body and the material composition of the original rocks. In the presence of lithologically homogeneous members, a definite zonality is observed in the development of the metamorphosed rocks. The inner zones of lithologically homogeneous belts of hornfelsing are composed of sphene, biotite, and andalusite varieties, which are replaced by nodular and spotted hornfelses that have not undergone significant mineral changes. The process of hornfelsing was not accompanied by the introduction of large quantities of petrogenic elements, but was reduced to their insignificant, narrowly local migration within small spots, lenses, and bands.
Taking into account the enrichment of the rocks of the group of bodies under consideration with ilmenite, it should be noted in conclusion that, during the destruction of trachydolerites, both ancient and modern placers of ilmenite could have formed.
REFERENCES
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V. L. Masaitis. Intrusions of the fissures of the Vilyuy-Markha flexure. Materials of VSEGEI, issue 24, 1959.
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B. V. Oleinikov. Trap intrusions and questions of metamorphism of the enclosing rocks. In: Geological Structure and Prospects of Oil-and-Gas Potential of the Northwest of the Siberian Platform, Proceedings of SNIIGGiMS, issue 28, Gostoptekhizdat, 1963.
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N. K. Razumovsky. The arithmetic mean sample and its relation to logarithmic moments. In: Problems of Exploration Geophysics, issue 1, Gostoptekhizdat, 1962.