ON THE STUDY OF CENTRAL-TYPE PALEOVOLCANOES: THE EXAMPLE OF DEVONIAN FORMATIONS IN THE MOUNT STOG AREA (NORTHEASTERN MARGIN OF THE MINUSINSK DEPRESSION)

V. G. Kryukov, Yu. A. Fomin

(Presented by Professor A. M. Kuzmin)

In accordance with existing concepts \[1, 3, 5, 4\], and in addition to them, the term “paleovolcano” is used by the authors to denote an ancient volcanic edifice whose composition includes vent and near-vent formations, while its internal structure is determined by a combination of radial, mantle, ring, conical, and peripheral structures. The boundaries of such edifices are their outer ring faults, often emphasized by dikes and veins of subvolcanites and small intrusive bodies.

Despite the increased interest of researchers in ancient volcanic apparatuses, prompted by the occurrence of ore formations within them, these edifices very often prove to be insufficiently studied. The reason for their inadequate study apparently lies above all in the difficulty of detecting such edifices. In particular, indications of possible finds of paleovolcanoes are found in the work of Ya. S. Edelshtein \[7\]. Since then many geologists have dealt with problems of volcanism in this region. The most detailed and complete investigations in this direction were carried out by I. V. Luchitskii, A. I. Anatol’eva, and others \[2, 6\]. However, they devoted insufficient attention to questions of reconstructing paleovolcanoes for the indicated region.

We attempt here to set forth the principal features that make it possible to identify central-type paleovolcanoes, using as an example a comparatively small area—namely, the northeastern part of the mountainous framing of the Minusinsk basin. These features were developed as a result of a three-year study of the products of Devonian volcanism in this territory and are as follows:

1. All volcanic apparatuses of the region under consideration are confined to faults of northwestern and sublatitudinal trends, or to areas of their intersection. These faults, which were initiated as early as the Precambrian, are expressed quite clearly in the Devonian effusives as powerful weakened zones with a downthrow type of displacement along individual displacers.

Along with the indicated tectonic control, a tendency is observed for such edifices to be confined to the closure parts of anticlinal folds.

1. The next important feature is the morphology of the paleovolcanoes. All of them are characterized by more or less isometric—oval or circular in plan—forms, emphasized by ring faults. The sizes of the apparatuses are highly varied: from the first tens or hundreds of meters to [[unclear: continuation cut off on this page]].

tens of meters across for monogenic volcanoes, to the first kilometers for polygenic volcanoes.

1. The complexity of the facies composition of volcanogenic formations and, first of all, the presence of vent facies proper (necks). Necks are distinguished by an oval or rounded planform and by steeply dipping, cutting contacts with respect to the enclosing rocks. In addition to necks, in such areas a significant number of mutually intersecting subvolcanic veins, dikes, and sills are observed, often having a very complex form. For cover facies near paleovolcanoes, a sharp increase in the thickness of sections is characteristic.

2. Lithologic-petrographic features of vent and near-vent formations.

In particular, the rocks filling the vents are often represented by terminal magmatic differentiates—quartz porphyries and orthophyres—or by fragmental varieties of rocks: agglomerate tuffs, eruptive breccias, and lava breccias. Lava breccias are characterized by a banded texture with fancifully curving, steeply dipping fluidality. The predominant orientation of the latter reflects the morphology of the body; therefore, the compilation of a structural scheme showing elements of rock occurrence will facilitate the identification of eruption centers. Finds of fragments of basement rocks within eruptive formations are of interest.

Highly indicative for paleovolcanoes are facts of the presence within them of veins composed of tuffaceous material. Cover facies near eruption centers abound in rocks with a significant explosivity index—tuff and lava breccias, tuff lavas, ashes, etc.

1. A characteristic feature is the presence of ring, conical, and radial rupture disturbances, emphasized by dikes or confined to the boundaries of vent bodies. In a number of cases, the indicated structures testify to processes of caldera formation on areas composed of paleovolcanoes. In addition, mantle structures are noted, along which sills were intruded.

2. Geomorphological features and the decipherability of paleovolcanoes. The vents themselves are especially well deciphered, appearing as isometric bowl-shaped depressions, and the ring rims expressed on the ground by ridges. Also noteworthy in this respect is the radial type of river network.

In addition to the main features indicated above, a number of indirect ones are established, and among them, above all, the wide development within paleovolcanoes of secondary quartzites, which are products of the activity of ancient solfataras and fumaroles. Postvolcanic small intrusions, predominantly of granosyenitic composition, are also associated with these structures.

In conclusion it should be noted that, despite a certain similarity in the structure of ancient eruption centers in this region, they are to a significant degree characterized by their own distinctive features. Therefore, when identifying and subsequently studying paleovolcanoes, along with using the general features listed above, it is very important to seek out the individual traits inherent to each specific apparatus separately.

REFERENCES

1. V. N. Kotlyar, On ore-bearing volcanic veins and their spatial distribution. Notes of the All-Union Mineralogical Society, part 91, issue 4, 1962.

2. I. V. Luchitskii, Volcanism and Tectonics of the Devonian Depressions of the Minusinsk Intermontane Trough. Publishing House of the USSR Academy of Sciences, 1960.

1. E. F. Maleev. Neogene Volcanism of Transcarpathia. “Nauka,” 1966.

2. Paleovolcanological Reconstructions, Lavas, and Ores of Ancient Volcanoes. Proceedings of the Laboratory of Paleovolcanology, issue 3, Alma-Ata, 1964.

3. A. Rittmann. Volcanoes and Their Activity. “Mir” Publishing House, 1964.

4. Comparative Paleovolcanology of the Middle and Upper Paleozoic of Southern Siberia and Eastern Kazakhstan. “Nauka” Publishing House, Siberian Branch of the USSR Academy of Sciences, 1966.

5. Ya. S. Edelstein. Brief Explanatory Text to Sheet II-22 (Lake Shira Sheet) of the Geological Map of Khakassia (Former Minusinsk Territory). ONGI NKTP USSR, 1936.