bles the form of a bagel, but now not a structural one, but a petrogeochemical one. It is smaller than the first one that is tectonic, and only partially coincides with.
ISSN 1028334X, Doklady Earth Sciences, 2009, Vol. 429, No. 8, pp. 1359–1363. © Pleiades Publishing, Ltd., 2009. Original Russian Text © Yu.P. Masurenkov, A.L. Sobisevich, N.I. Laverova, 2009, published in Doklady Akademii Nauk, 2009, Vol. 428, No. 6, pp. 810–814.
GEOPHYSICS
Relation between Structural and Material Circular Motions in a Volcanic Center Yu. P. Masurenkov, A. L. Sobisevich, and N. I. Laverova Presented by Academician N.P. Lavrov January 21, 2009 Received March 4, 2009
Abstract—Comparison between some petrogeochemical features of magmatic types of the volcanic center with an area of Mohorovichich structure is made for the first time. On the basis of correlation signs, the fact that they belong to a single system is established. DOI: 10.1134/S1028334X09080261
A volcanic center is a set of interconnected old vol canoes, intrusions, and hydrothermal displays united by an endogenous stream of substance and energy localized in space and steady in time that generates magma and has structural embodiment in the form of telescoping circular dome caldera associations. For the first time, comparison of the petrogeochemical features of magmatic types from Pyatigorsk volcanic center with Mohorovichich area structure has been performed, and on the basis of the analysis, correla tion signs of their correspondence to a uniform system are established. A volcanic center and the isometric structure cor responding to it represent the projection area of a dynamic vertically stretched system on the Earth, which drains Earth’s depths and carries out one of the major functions of planetogenesis, that is, localized encrustation [3, 6, 7]. The reason for the occurrence of systems of vertically stretched crust and sinks localized in space is the periodically emerging mantle asteno liths, that is dropshaped diapirs, which carry material and mantle heat [2]. Pyatigorsk volcanic center, the magmatic display of which is represented by subvolcanic bodies of lacco liths, was ranked among typical representatives of vol canic structures with concentric ash value of the petro geochemical signs of rhyolite, trachyte, and trachyli parite [6]. Detailed heliometric studies of the territory of the North Caucasus revealed, among other things, the Mineralovodskaya deep circular structure [1], and
Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Bolshaya Gruzinskaya 10, Moscow, 123995 Russia
the independently confirmed confinement of Pyatig orsk laccoliths to circular structure. In work [4] data about the deep structure of the area of Caucasian Min eral Waters are cited on the basis of geophysical research. In aggregate with data about petrogeochem ical properties of the type of laccoliths [8], the last author’s research allows analysis of the interrelations of structural and material signs in Pyatigorsk volcanic center, the very problem to which this work is devoted. The mantle area in the volcanic center according to materials [4] contains about 80 points (points are cal culated by authors of the stated work, according to the data of Method of Metabolic Earthquake Waves) pre sented in Fig. 1. This is only one of the variants of pos sible interpretation of the data. We constructed the mantle area on the basis of the author’s quoted work about three geophysical profiles made in terms of interpretation of records of exchange waves from earthquakes and waves from industrial explosions, made by scientists from the Neftegeofizika Scientific Development and Production Center, and a variant that represents a smoothed picture of the mantle area (sliding quadrate method on an area 400 × 400 km2 at 10km intervals). Despite the essential differences in details, the specified variants are uniform in one point: the mantle area at the basis of the volcanic center rep resents a dome with vertex fan pressure. Figure 1 can be interpreted in the following way. The depression is immersed in relation to the circular elevation sur rounding it (by 4–6 km). It represents an oval with dimensions of 27.5 × 37.5 km (in other variants 22 × 27 and 40 × 50 km with immersion of 2–4 and l–2 km, respectively). It is necessary to pay attention to the ring fault interpreted in arc directions of river valleys and their dissymmetric structure. The position of these partially hypothesized faults will be coordinated with
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Fig. 1. Mohorovichich area and circular faults in Pyatigorsk volcanic center. (1) Horizontal and depth to the area M in km; (2) circular faults on the Earth’s surface; (3) axis of circular reclamation, which delimits the topmost dome fan pressure.
the Mohorovichich pattern of the horizontal area, which acknowledges the sufficient reliability of the geophysical and geomorphological data. In turn, these data testify to the single nature of these structural ele ments that are spread vertically over space, that is, cir cular faults on Earth’s surface and mantle area. A series of timetables concerning the distribution of some petrogeochemical characteristics in the area of Pyatigorsk center are given in [6]. In Fig. 2 the distri bution of porphyritic phenocrysts of the second gener ation as the most typical is shown. In the very middle of the volcanic center, the quantity of crystals of the second generation in magma is quite large—27%. Toward the periphery, at first it gradually decreases to 13% in all directions, and then gradually increases to values close to those in the middle: 24–30%. Owing to this fact, a concentrically zone field with maximums
both in the middle and at the edges with a circular zone of minimum between them is created. The zone of minimum configuration is an ellipse extended in the north–south direction. Its axes have the following parameters, 27 and 13 km. The type distribution in phenocryst laccoliths of the first, most likely deeper and certainly earlier, generation shows an obvious similarity to the distribution of phenocrysts of other generations: in the middle of the center 25%, in the elliptic minimum zone 15%, on periphery 24–29%. It is natural to ask what factors define such a natu rally organized distribution of crystals in separate injected bodies of laccoliths, first of all, making them part of a uniform system that defines them and, sec ond, testifying to the influence of one mechanism for the whole system determining melting crystallization. Naturally, role of temperature is considered. It would DOKLADY EARTH SCIENCES
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Fig. 2. Concentric zonation in distribution of secondgen eration phenocrysts in types of laccoliths (average values in %). (1) Laccoliths and maintenance of phenocrysts in them; (2) isolines of phenocryst concentration; (3) axis of circular zone of lowered phenocryst concentration; (4) zone of lowered phenocryst concentration, containing less than 15 vol %.
be rather simple if the quantity of crystals naturally increased from the center to the system periphery as though the magmatic body cooled down consistently from the edges to the middle. But we have a twice confirmed case concerning not only cold system peripheries, but also its middle, with an almost ideal presence of warmedover circular between them. Hence, it is necessary to involve one more factor influ encing crystallization of silicate melting for explana tion of this phenomenon. Shifting constituents may be such a factor and, primarily, water: an increase in its quantity in the melt reduces the quantity of crystals. It is possible to surmise that the greater the quantity of dissolved water that was in the melt, the greater the amount of it that would remain in residual form. This could serve as a certain orientator, specifying what quantity of water there was in transformation of melt ing rock. Actually, the pattern of residuary water distri bution shows a similarity to the distribution of phe nocrysts but with the opposite sign: it is fixed concen trically to the zone structure of the system and of particular importance is the presence of a minimum in the center (10% and less) and on the periphery (3– 24%) of the structure and the maximum (30–79%) between them in circular form that surrounds the cen DOKLADY EARTH SCIENCES
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Fig. 3. Correlation of circular area of reclamation area M and circular petrogeochemical abnormal zones. (1) Axes of circular reclamations of M area: (1') according to three profiles from Method of Metabolic Earthquake Waves, (2') according to the points calculated for the whole cen tral area, (3') according to smoothing of points settlement; (2) area of uncertainty in which the circular reclamation is located, it surrounds the topmost fan pressures; (3) axes of circular anomalies in distribution of petrogeochemical characteristics: (A, B) axes of zones with lowered concen tration of phenocrysts of the first and second generation, (C) axis of subcircular zone of increased concentrations of residual water in laccolith types; (4) axes of zone location of petrogeochemical anomalies.
ter. The size of elliptic axis of this positive anomaly is 33 × 18 km. Thus, a comparison of the distribution of residual water and primary crystallization and second ary crystallization of inversely proportional depen dence types [6] is planned. In Fig. 3, for the time of construction of the mantle area according to geophysical data, we dealt with a uniform system of circular reclamation, the position of which varied in the timetable depending on the way the data was interpreted, which is why we do not know its real location. In this timetable three reclamation axes are placed, each of them correlates to one of the applied ways: according to data that are contained only in Method of Metabolic Earthquake Waves cuts; according to data of calculations of depth subject to area (Fig. 1); according to smoothed data of calcula tions subject to area. Axes constructed on smoothed data delimit the outside area of uncertainty, which arises because of the
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contradictoriness of calculation data according to areas and cuts. It is also possible to assume that the area located in the zone that is occupied with axes sketched in other ways is deprived of circular reclama tions. Then a circular zone, which includes all axes of reclamations, forms like a figure of uncertainty in the form of a bagel. Most likely, there is an axis of true cir cular reclamation round the middle fan pressure of the volcanic center in this zone. This zone is shown in Fig. 3. In this figure, the zone of axes position of petro geochemical anomalies is emphasized, in particular, the zone of residual water and melting crystallinity that generated the Pyatigorsk laccoliths. It also resem bles the form of a bagel, but now not a structural one, but a petrogeochemical one. It is smaller than the first one that is tectonic, and only partially coincides with it, but some connection between them definitely exists. As all their centers practically coincide, and at the time of their superposition, an almost ideal scheme of the model “a bagel in a bagel” comes out. The distribution of fluorine content in types of lac coliths is like the distribution of the water content: minimum in the structure center, a circle of maximum around it, and concentration decrease toward the periphery. This is understandable as H2O and HF are dissolved in silicate melting [5]. However, their behav ior, which coincides in the main, is characterized with rather considerable independence, forming in the timetable ellipses that are quite independent and strongly differing in size, form, and orientation. CO2 as an antipode of water, and fluoric acid in the solubil ity in silicate melting behaves accordingly: the maxi mum in the center and the minimum on the periphery. However, unlike water and fluorine, it does not form a circular anomaly between the center and the periph ery. The Q characteristic, which is reflected in A.N. Zavaritskii’s system of recalculations of the quantity of free earth silicon, is distributed like fluo rine. Characteristic n also behaves in a similar way, showing a share of sodium oxide in the sum of alkali. All this was noted earlier [6], but without the tie to concrete deep structures. The cited data specify the high probability of the following geological objects and factors belonging to a uniform interconnected system: ring faults with a dif ficult pattern of intermittent concentric reclamations and deflections on the Earth’s surface; subvolcanos in the circular structure with concentrically zoned distri bution of petrogeochemical signs; the dome structure on the area of the geophysical stratum at a depth of 7– 16 km; the circular dome fan pressure structure on the Mohorovichich area at a depth of 40–46 km. All these elements of unified system, as a whole coinciding in plan, differ in sizes and are characterized by some displacement relative to each other. Subvol canic intrusions, projected on a horizontal plain of the daylight surface chemical and temperature structure of the zone of magma formation and crystallization, pos sibly also contain information about the vertical heter
ogeneity of this zone. The differences in the concen tration of water and carbonic gas in intrusion types of sites of different structures reach such high values that it is necessary to admit the difference in pressure for these sites, which is correlated to a difference of depths of 12 km [5]. Such an overfall at a distance of only 5– 6 km can be presented as the apical prominence of the magma chamber roof, which forms a resemblance to a large ring dike on its arch. Its relative enrichment with water, fluorine, sulfur, alkalis (mainly sodium), and earth silicon reduced the crystallization temperature. Therefore, the quantity of crystal phase is significantly lower at the same or even at lower temperature than in the chamber center. Certainly, this is one of the possi ble variants for explaining the phenomenon described above. It is possible to imagine another model but not in the form of a single magma chamber, as the area that is under a premelting condition and, therefore, in a position for disjunctive dislocation. Then the circular faults, which go through the entire crust and mantle thickness and are conductors of mantle gas and fluid streams, provide for the occurrence of local magma chambers, the properties of which submit to thecon centric zonal structure of the general intratelluric sub stance and energy stream. Detailed information about the geological aspects of the crust in the interval between the mantle and the daylight surface within a volcanic center are necessary to solve this problem. These data could clarify the role of structural hetero geneity of the mantle area in the mechanisms and ways of transformation of crust material. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project no. 090500066a), Program no. 16 of the Presidium of the Russian Acad emy of Sciences (projects nos. 1.7, 2.7, 3.6, and 7.5), and the Foundation for Support of Russian Science (www.sciencesupport.ru) REFERENCES 1. V. N. Bashorin, N. I. Laverova, A. P. Pronin, et al., Seis mically Active FluidMagmatic Systems of Northern Cau casus (IFZ Ross. Akad. Nauk, Moscow, 2005) [in Rus sian]. 2. V. V. Belousov, Earth’s Crust and the Upper Mantle of Continents (Nauka, Moscow, 1966) [in Russian]. 3. O. A. Braitseva, I. A. Egorova, L. D. Sulerzhitskii, and I. A. Nesmachnyi, Volcanic Center: Structure, Dynam ics, Composition (the Karym Structure) (Nauka, Mos cow, 1980) [in Russian]. DOKLADY EARTH SCIENCES
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RELATION BETWEEN STRUCTURAL AND MATERIAL CIRCULAR MOTIONS 4. I. V. Garetovskaya, G. V. Krasnopevtseva, F. V. Sizov, et al., in The Main Problems of Seismotectonics (Nauka, Moscow, 1975) [in Russian]. 5. A. A. Kadik, in Physicochemical Problems of Hydrother mal and Magmatic Processes (Nauka, Moscow, 1975), pp. 5–32 [in Russian]. 6. Yu. P. Masurenkov, Volcanoes above Intrusions (Nauka, Moscow, 1979) [in Russian].
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7. Yu. P. Masurenkov and L. A. Komkova, Geodynamics and OreForming Processes in the Dome–Ring Structure of a Volcanic Belt (Nauka, Moscow, 1978) [in Russian]. 8. N. D. Sobolev, A. A. LebedevZinov’ev, A. S. Naz arova, et al., Neogene Intrusive Rocks and Premesozoic Base of the Caucasus Mineral Waters Region (Gos geoltekhizdat, Moscow, 1959) [in Russian].
