ON THE ROLE OF ASSIMILATION AND METASOMATISM IN THE FORMATION OF ROCKS OF THE IEDYGEM INTRUSIVE MASSIF (GORNY ALTAI)

A. I. BAZHENOV

(Presented by Professor A. M. Kuzmin)

In the basin of the lower course of the Karagem River, the right tributary of the Argut River, erosion has exposed three large granitoid massifs—the Iedygem, Karasu, and Yungur massifs. In addition, several small intrusive bodies are known in the left-bank part of the Argut River [23].

The present work describes the petrography of the southeastern part of the Iedygem intrusion, studied by the author, which is confined to the sharply dissected region of the western termination of the North Chuya Range.

In its southeastern part the Iedygem intrusive massif cuts through and metamorphoses rocks of the Ordovician–Silurian sequence. Dikes of aplites and certain vein formations genetically related to the Iedygem intrusion cut through the Devonian sequence, which includes sediments of the upper part of the Givetian and lower Frasnian stages; this permits the age of the intrusion to be regarded as Upper Paleozoic.

The intrusive body lies discordantly with respect to the bedding of the enclosing rocks; in numerous outcrops it truncates the bedding of the latter at various angles. The contact line is weakly wavy, but in the area of the southeastern closure of the massif it becomes more complicated and becomes complexly serrated because the magma penetrated into the rocks of the continent along bedding planes. In these sectors apophyses up to 1.5–2.0 m thick extend from the intrusion.

The petrographic composition of the rocks of the Iedygem massif is complex. This is due to the fact that erosion exposes its apical part with diverse hybrid formations. Over a considerable area the intrusion is composed of granodiorite. The rock consists of plagioclase, microcline, quartz, and biotite. Among accessory minerals present are apatite, sphene, zircon, pyrite, magnetite, hematite, monazite, epidote, and tourmaline. The quantitative-mineralogical composition of the granodiorite is given in Table 1. The texture of the rock is granitic.

Plagioclase—andesine No. 31–33¹—is observed as equidimensional individuals, idiomorphic with respect to biotite, giving rectangular sections. A zonal structure of the crystals is quite common, with, for example, the following sequence of zones (from center to periphery): andesine No. 40–No. 37—

¹ All compositions of plagioclases and alkali feldspars cited in the article were determined on the Fedorov stage.

No. 34—No. 37—No. 34—No. 32. The boundaries between adjacent zones are rather sharp. Plagioclase is replaced by sericite and zoisite, especially in the central parts of zoned crystals, where zoisite is more common. Toward the periphery the process develops more weakly; here there is sometimes a tendency for sericitization to increase, with zoisite playing a more modest role. Secondary alteration of plagioclase intensifies in the endocontact part of the intrusive massif; moreover, near the contact, plagioclase is often found to be completely filled with flakes of sericite or grains of zoisite.

Microcline, in contrast to plagioclase, is very fresh. Usually it is equidimensional with quartz, and sometimes contains idiomorphic individuals of the latter. The microcline lattice is well expressed, but grains lacking a lattice are sometimes encountered, or else the latter is confined only to their central part. The coordinates of the cleavage pole (001) are \(N_g = 81^\circ\), \(N_m = 12^\circ\), \(N_p = 82^\circ\); \(2V = (-)83^\circ\). Rarely, microcline surrounds grains of plagioclase.

Table 1

Quantitative mineralogical composition of the rocks of the Iedygem massif

Biotite is present in the form of flakes with pleochroism typical of it, from dark brown to light brownish-yellow.

In its optical properties (\(N_g \simeq 1.633\), \(N_p = 1.577\), \(N_g - N_p = 0.056\), \(2V \simeq (-)0^\circ\)) the mineral belongs [3] to a moderately ferruginous variety containing 53% of the lepidomelane molecule and 47% phlogopite. Biotite contains a sagenitic lattice of rutile. Sometimes grains of monazite and zircon are confined to the central parts of the mineral flakes.

Quartz is present as grains of irregular shape, and more rarely as idiomorphic individuals, and fills intergranular spaces—

previously separated minerals. It usually has weak cloudy extinction.

In the endocontact part of the intrusive body, an admixture of diopside appears in the composition of the granodiorite \((N_g - N_p = 0.030,\ cN_g = 38^\circ)\), as a result of which a tendency is outlined toward a transition of the rock into banatite.

The chemical composition of the granodiorite is given in Table 2, which also contains analyses of the most closely related rocks from other Upper Paleozoic massifs of Altai.

According to the numerical characterization of A. N. Zavaritskii, the rock described is closest (Fig. 1) to the average composition of granodiorite according to R. Daly [6]. It is interesting that the composition of the Iedygem granodiorite is close (Table 2) to the average composition of intrusive rocks of Western Siberia [17].

Fig. 1. Diagram of the chemical compositions of rocks of the Iedygem intrusive massif.
I—Rocks of the Iedygem intrusive body. 1. Granodiorite. 2. Quartz biotite meladiorite. 3. Quartz biotite diorite. 4. Quartz leucodiorite. II—Average compositions of igneous rocks according to R. Daly [6]. 9. Granodiorite. 10. Quartz diorite. III—8. Biotite hornfels from the exocontact part of the aureole of the Iedygem massif. IV—Rocks from other intrusive massifs of Altai. 5. Granodiorite of the Zmeinogorsk complex of Rudny Altai [4]. 6. Granodiorite of the Talitskii massif (after A. I. Aleksandrov). 7. Average composition of intrusive rocks of Western Siberia [17].

In the marginal part of the Iedygem massif, dioritoid varieties of rocks are locally widespread, developing exclusively in contact with biotite hornfels. In rocky outcrops one can very clearly observe the contact of biotite hornfels with biotite diorite. Along the contact there develops a narrow band up to 2.0 m thick, composed of quartz biotite meladiorite intensely enriched in biotite. With increasing distance from the contact, the mica content rapidly decr—

Table 2

Chemical compositions of rocks of the Iedygem intrusive massif

Numerical characteristics (according to A. N. Zavaritsky)

Rock formulas according to Barth [24, 22]

1. K₃₃ Na₅₃ Ca₂₇ Mg₁₇ Fe²⁺₂₆ Fe³⁺₆ Al₁₄₃ Ti₅ Si₅₃₃,₁ P₁ O₁₅₃₃ OH₄₀.
2. K₄₀ Na₁₂ Ca₆₃ Mg₇₁ Fe²⁺₆₆ Fe³⁺₂₁ Al₁₅₄ Ti₁₅ Si₄₆₆ O₁₄₀₀ OH₁₀₀.
3. K₂₅ Na₇₈ Ca₄₂ Mg₄₇ Fe²⁺₂₃ Fe³⁺₁₉ Al₂₀₄ Ti₃ Si₅₁₃ O₁₅₃₀ OH₅₀.

4. K₃₅ Na₄₉ Ca₄₄ Mg₁₁ Fe²⁺₁₅ Fe³⁺₄ Al₁₃₂ Ti₃ Si₅₁₇ O₁₅₃₀ OH₅₀.

5. Granodiorite of the Iedygem massif. Chemical laboratory of the Tomsk Complex Expedition of the West Siberian Geological Administration (TKE).

6. Quartz biotite meladiorite. Chemical laboratory of the Krasnoyarsk Geological Administration.
7. Quartz-biotite diorite. Chemical composition obtained by recalculation of the quantitative-mineralogical composition of the rock.
8. Quartz leucodiorite. Chemical laboratory of the Tomsk Polytechnic Institute.
9. Granodiorite of the Zmeinogorsk complex of Rudny Altai [4].
10. Granodiorite of the Talitsa massif (according to A. I. Aleksandrov).
11. Average composition of intrusive rocks of Western Siberia [17].
12. Biotite hornfels from the contact aureole of the Iedygem massif. Chemical laboratory of TKE.

…is expressed through an increase in the amount of leucocratic rock-forming minerals, and the rock passes into a normal quartz biotite diorite. In the latter, schlieren enriched in biotite are quite common; their characteristics are given below. Quartz biotite diorite gradually passes into granodiorite through a further decrease in the biotite content of the rock and a parallel increase in the amount of feldspars.

Macroscopically, quartz biotite meladiorite and diorite differ from granodiorite in their darker color. Microscopic study makes it possible to trace the nature of the change in the material composition of the rock with increasing distance from the contact with the intrusive body. Such a dependence may be illustrated by data from counts of the contents of rock-forming minerals (Table 1) in meladiorite at 0.3 m and in diorite at 10 m from the contact.

Under the microscope a granitic structure of the rock is revealed. Plagioclase is analogous to plagioclase from granodiorite. Biotite in the described rock varieties is represented by two generations, of which the earlier one, in its physico-optical properties, is analogous to the mineral from granodiorite. Biotite of the second generation belongs to later formations; it composes streaky and veinlet-like aggregates confined to the boundaries of individuals of plagioclase, biotite, and quartz, and replaces them. Microcline is especially readily replaced by it, relics of which can occasionally be observed. In biotite of the second generation, inclusions of rutile are entirely absent, whereby in thin sections it is very easily distinguished from the mineral of the first generation. In comparison with the latter, flakes of late biotite usually have smaller dimensions. With increasing distance from the contact of the intrusive body, the amount of second-generation biotite in the rock rather rapidly decreases to complete disappearance.

The chemical composition of the mineral of the second generation depends on proximity to the contact with the biotite hornfels. According to optical constants \((N_g \simeq N_m = 1.630,\ N_p = 1.580,\ N_g - N_p = 0.050,\ 2V \simeq (-)0^\circ)\), biotite from meladiorite contains [3] 50% of the lepidomelane molecule and 50% phlogopite; in the mineral from diorite \((N_g \simeq N_m \simeq 1.631,\ N_p = 1.574,\ N_g - N_p = 0.057,\ 2V \simeq (-)0^\circ)\) an increase in the amount of the lepidomelane molecule to 53% is noted, with a corresponding decrease in the content of the magnesian component to 47%.

Microcline is usually absent from the composition of meladiorite, but in some thin sections single grains of the mineral are noted; in diorite its content does not exceed 5%. Quartz in the rock is analogous to quartz of granodiorite. Diopside occurs as an impurity in meladiorite and in diorite \((cN_g = 38^\circ,\ N_g - N_p = 0.030)\).

The accessory minerals are represented by sphene, monazite, apatite, and zircon, whose physico-optical properties do not differ from those of the minerals from granodiorite. It should only be noted that there is a marked increase, in the biotite-enriched rock, in the content of sphene, which gives rhomboid sections of individuals in thin sections. Chemical analysis (Table 2) indicates a significant concentration of Ti in the endocontact part of the intrusive body in comparison with granodiorite; moreover, as noted earlier, the content of rutile in the rocks of the endocontact sharply decreases. This phenomenon is explained by an increase in the amount of sphene in the rock, apparently formed owing to the relatively high concentration in this part of the massif not only of \(\mathrm{TiO_2}\), but also of \(\mathrm{SiO_2}\) and \(\mathrm{CaO}\).

Among secondary processes, vermiculitization of biotite should be noted, with the process developing selectively in separate areas. As a result, in some thin sections the biotite is fairly fresh, while in others—

selected in other areas, only relics of biotite can be observed in the central parts of the flakes. Vermiculite in thin section is colorless; \(N_g \simeq N_m = 1.588\), \(N_p = 1.562\); \(N_g - N_p = 0.026\), \(2V \simeq (-)0^\circ\). Vermiculite, in turn, is replaced by clinochlore; in thin sections one can observe all transitional varieties from vermiculite to pseudomorphs of chlorite after vermiculite.

The chemical compositions of meladiorite and quartz biotite diorite are given in Table 2.

Granite belongs to the less widespread formations. It forms small areas (up to 10 m in their greatest dimension), confined to the apical part of the massif. The genesis of this rock should apparently be associated with the process of crystallization differentiation of the magma, i.e., it forms hysterogenic schlieren [6].

The quantitative-mineralogical composition of the granite is given in Table 1, and the chemical composition in Table 2.

Microscopically, a granitic structure of the aggregate is revealed. Microcline is the most important component of the rock. Its grains are usually sharply xenomorphic with respect to plagioclase and biotite. In composition, plagioclase corresponds to albite–oligoclase No. 9–11 and is intensely sericitized. Biotite, in its optical properties, differs from the biotite from granodiorite by an increased content [3] of the lepidomelane molecule, up to 57% (\(N_g \simeq N_m = 1.637\), \(N_p = 1.582\), \(N_g - N_p = 0.055\), \(2V \simeq (-)0^\circ\)). Among the accessory minerals in the rock, rare prisms of apatite are noted, confined to flakes of biotite. Along the periphery of the mica flakes epidote develops; its appearance apparently should be associated with a postmagmatic process of mineral formation.

Distinctive formations in the Iedygem intrusive are the leucocratic varieties of rocks, which clearly fall into three groups. 1. Leucogranite, confined to the complex festoon-like closure of the massif and forming apophyses extending from the main intrusive body. 2. Quartz leucodiorite, forming a border of the exposed apical part of the massif. 3. Aplite-like granites, appearing in the near-contact zone of the intrusive at hypsometrically lower levels than the first two rock varieties.

Leucogranite is composed of microcline, plagioclase, and quartz; as an admixture, sporadically scattered flakes of biotite appear (Table 1). The structure of the rock is hypidiomorphic-granular. The alkali feldspar is represented by microcline with a clearly expressed grating; however, it should be noted that some grains lack such a grating, while in other cases it is only incipient. Nevertheless, according to measurements on the Fedorov stage, such individuals lacking a grating belong to triclinic feldspar: \(N_g = 80^\circ\); \(N_m = 13^\circ\), \(N_p = 82^\circ\), \(2V = (-)84^\circ\). Microcline is usually equidimensional with quartz and fills the interstices between prismatic individuals of plagioclase. The mineral is slightly pelitized. Plagioclase—oligoclase No. 18—forms idiomorphic prismatic individuals, as a rule intensely sericitized. The quartz present in the rock and—accessorily—biotite are analogous to the quartz and biotite of granodiorite.

Quartz leucodiorite forms a continuous border of the apical part of the intrusive massif along the contact with the country rocks. The thickness of the border does not exceed 3 m. Quartz leucodiorite is connected with granodiorite by a gradual transition, which occurs through the disappearance of biotite.

Under the microscope, the rock shows a hypidiomorphic-granular structure. In its material composition (Table 1), the rock differs from typical leucodiorite by a somewhat increased content of micro-

clay: according to A. N. Zavaritskii [6], it should not be more than 12.5%, and according to E. A. Kuznetsov [11]—not more than 13.3%.

The principal rock-forming mineral is plagioclase, corresponding in composition to andesine No. 32. As in the other rock varieties, the mineral is intensely replaced by sericite and zoisite. Microcline is distinguished in the rock by a remarkably fresh appearance. Sometimes, in the central parts of microcline grains, there are areas with slightly noticeable pelitization, which possibly indicates the presence of two generations of microcline. Potassic feldspar intensely replaces plagioclase, relicts of which are observed quite often in thin sections. Biotite is also readily replaced; microcline penetrates it along cleavage cracks. Quartz grains have weak cloudy extinction. Among the accessory minerals, apatite is present.

The chemical composition of the leucodiorite is given in Table 2.

Aplite-like granites compose a small body confined to the endocontact part of the intrusion and situated at a hypsometrically lower level than the two other leucocratic rock varieties described above. The size of the body is about \(20 \times 1\) m. Its boundaries with the other rocks of the endocontact are indistinct. The transition takes place by a change in the rock structure, grain size, and composition of the rock-forming minerals.

Under the microscope an allotriomorphic-granular structure of the rock is revealed.

The aplite-like granite is composed of alkali feldspar, quartz, plagioclase, and biotite (Table 1).

Orthoclase is the predominant constituent of the rock. Its slightly pelitized grains have very irregular outlines. The coordinates of the cleavage plane \((001)\) are \(N_g = 90^\circ\), \(N_m = 5^\circ\), \(N_p = 85^\circ\); \(2V = (-)66^\circ\). The content of plagioclase, represented by albite No. 2, ranges from 0 to 20%. It is concentrated chiefly in the marginal part of the bodies, whereas in the remaining part this mineral is completely absent.

The amount of biotite in the rock varies from 0 to 3%. In its optical properties it is analogous to that of the granodiorite. Muscovite flakes occur as an admixture in the granite, confined to the boundaries of grains of quartz and microcline.

Among the secondary minerals, calcite should be noted; its content noticeably increases toward the contact with the country rocks. Selective replacement of feldspars by calcite is observed.

In connection with the presentation of material on the leucocratic varieties of rocks of the massif, it is necessary to dwell on the petrography of a small intrusive body lying in marbled limestone conformably with its bedding. The size of the body is about \(20 \times 6\) m. It is probably an apophysis of the intrusive massif. The body is composed of hornblende diorite, which along the periphery of the occurrence passes into labradorite. Changes in composition are manifested by the disappearance of amphibole and quartz and their replacement by plagioclase. The contacts with the enclosing rocks are sharp.

The hornblende diorite is composed of plagioclase, green hornblende, and quartz. Among the accessory minerals, wedge-shaped individuals of sphene are noted (Table 1). Microscopically, the rock shows a granitic structure.

Plagioclase is represented by andesine No. 35; its individuals are equidimensional with amphibole. Plagioclase is replaced by an aggregate consisting of sericite and zoisite. Hornblende in thin section has a weak green color and pleochroism: \(N_g\)—light greenish, \(N_m\) and \(N_p\)—colorless; \(cN_g = 22^\circ\), \(2V = (-)72^\circ\), \(N_g = 1.656\), \(N_m = 1.648\), \(N_p = 1.633\),

$N_g - N_p = 0.023$. In its optical properties [19], the mineral belongs to the low-iron variety. Quartz fills the spaces between grains of plagioclase and hornblende.

The chemical composition of the rock is given in Table 2. The superimposed process of alkaline metasomatism led to the appearance of microcline, developing chiefly along cleavage cracks in the plagioclase. The microcline has a beautifully expressed lattice and is distinguished by its freshness.

Labradorite composes the periphery of the body, forming a continuous rim up to 2 m thick. Under the microscope the structure of the rock is panidiomorphic-granular. The sole rock-forming mineral is plagioclase—labrador No. 50, with a rather fresh appearance. Deformation of mineral individuals is sometimes observed, expressed in the bending of twin lamellae, which acquire undulose extinction.

In a small number of grains in the rock, rhombic pyroxene—hypersthene—is encountered ($N_g = 1.685$, $N_p = 1.675$, $N_g - N_p = 0.010$). In its optical properties it belongs to the low-iron variety [3]. Among other accessory minerals, sphene is present in the labradorite, analogous to the mineral from the diorite.

The dike suite, genetically related to the Iedygem intrusion, is represented by aplites and pegmatites. Depending on mineral composition, the aplites may be divided into three groups: 1. Aplites. 2. Plagioaplites. 3. Sphenic aplites. The quantitative-mineralogical composition of the aplites is given in Table 1.

As for the facies character of the intrusive body, according to the classification of Yu. A. Kuznetsov [12], by the nature of the contact metasomatism and the presence in the rocks of zoned plagioclase and intensive metasomatism of the dike rocks, it belongs to massifs corresponding, on the one hand, to hypabyssal formations and, on the other, to the mesoabyssal group of facies—more precisely, to the batholith-laccolith facies of this group. The features of the latter include the appearance of aplite and pegmatite dikes, the absence of perthites and antiperthites in the intrusive rocks, and the skarnification of the enclosing rocks.

The material presented on the petrography of the Iedygem intrusive massif attests to a great diversity of rocks, which cannot be explained by a multiphase process of intrusion formation, since all rock varieties are connected with one another by gradual transitions. The latter circumstance apparently gives grounds for believing that the genesis of the rocks of the intrusive body is due to other petrogenetic processes.

In the petrographic literature, numerous works have been devoted to the problem of hybridism, including those of Kh. M. Abdullaev [1], V. S. Koptev-Dvornikov [8], V. V. Lyakhovich [14], M. A. Favorskaya [21], Kh. M. Abdullaev, I. M. Isamukhametov, and I. Kh. Khamrabaev [2], and others. It is not our task to examine this complex and important problem of petrogenesis; therefore, only conclusions following from the material set forth above will be presented here.

In the spatial distribution of rocks in the massif, a connection is clearly established between the composition of the rocks of the endocontact part of the intrusive body and the composition of the enclosing rocks.

The significance of the assimilation process in the formation of hybrid rocks is especially emphasized in the study of the body described above, composed of hornblende diorite and labradorite. The change in the chemical composition of its rocks can be explained only by assimilation of limestone and the associated increase in CaO content. It should be emphasized here that the transition from diorite to labradorite is gradual, which

is also manifested in the disappearance of amphibole and in an increase in the basicity of the plagioclase.

Dioritic varieties of rocks with biotite always develop in contact with biotite hornfels. The latter is the earliest contact formation, genetically related to the metamorphism of calcareous sandstone. Under these conditions the limestones recrystallize and are transformed into marble. The subsequent stage of the contact process is associated with the appearance of calcareous–magnesian–silicate rocks (with grossular, diopside, clinozoisite, actinolite, vesuvianite, albite, and other minerals), which intensively replace both the marbles and the biotite hornfels. The formation of these mineral concentrations was preceded by a deformational break, accompanied by the establishment of a system of fractures that controlled the circulation of metasomatic solutions.

In the rocks of the endocontact part of the intrusive massif, schlieren and xenoliths are quite common, the latter always being observed only in apophyses extending from the main intrusive body. As a rule, xenoliths preserve their position in space and therefore it is not difficult to restore their place in the contact. As microscopic study shows, no changes in the material composition of the xenoliths, compared with the composition of biotite hornfels, are observed. The association of xenoliths with apophyses is most probably due to the low chemical activity of the magmatic melt in such small bodies.

Schlieren are confined to the endocontact part of the intrusive body. They have a rounded form, with a maximum dimension of no more than 0.1 m. The boundaries of the schlieren are sharp, which is clearly revealed both in macroscopic and microscopic study. The schlieren consist of rock enriched in biotite; the sizes of the mica flakes in different schlieren are not uniform, varying from 0.1 to 3 mm. The microscope shows that the schlieren aggregates have a lepidogranoblastic structure. They consist of biotite, oligoclase No. 28, microcline, quartz, apatite, and sphene. The secondary minerals are chlorite (clinochlore) and epidote.

Biotite, in contrast to biotite from granodiorite, lacks a sagenitic rutile network. The mineral composition, \(N_g \simeq N_m = 1.627\), \(N_p = 1.577\), \(N_g - N_p = 0.050\), \(2V = (-)0^\circ\), includes [3] about 57% phlogopite molecule and 43% lepidomelane; i.e., this variety has an intermediate composition between the biotites from meladiorite and hornfels—in the latter \(N_g \simeq N_m = 1.621\), \(N_p = 1.569\), \(N_g - N_p = 0.052\), \(2V \simeq (-)0^\circ\).

It is very interesting that quartz grains sometimes preserve their semirounded shape, representing relicts of the original clastic nature of the material. Microcline is usually filled with secondary minerals, especially epidote. Plagioclase is affected by secondary processes to a considerably lesser extent. The appearance of clinochlore is connected with the decomposition of biotite.

The study of schlieren gives grounds to believe that assimilation of a specific substrate took place in the formation of the dioritoid rocks. Similar relationships of hybrid dioritoid rocks with biotite hornfelses have repeatedly been described in the literature [21, 8]. However, the mechanism of formation of rocks of the diorite family (quartz biotite meladiorite and diorite) in the Iedygem intrusion cannot be explained solely by the process of assimilation of specific continental rocks. The petrographic composition of contact-metasomatic formations developing at the expense of carbonate country rocks is rather complex. But of greatest interest is the fact that such magnesium-bearing minerals as diopside, vesuvianite, actinolite, and others are widespread in them. Data from chemical analyses (in this article they are not pre-

...show that the MgO content in the various zones of the contact aureole is variable and ranges from 1.73 to 4.34%, but is higher than in the granodiorite (1.29%) and in the unaltered country rocks (1.63%). Nevertheless, in contact with them, dioritoid rocks do not develop; rather, granodiorite is preserved, bearing more clearly expressed traces of autometamorphic reworking.

The MgO content in the rocks of the intrusion and in the contact-metasomatic formations increases regularly with distance from the central part of the massif. Such a distribution of this element sheds some light on the question of the origin of the diorites. An important role here is played by the multistage character of the contact process, as a result of which the biotite hornfels is an earlier formation in comparison with the lime–magnesian–silicate metasomatites that compose a broad belt around the intrusion. The formation of dioritoid rocks is most probably synchronous with the formation of lime–magnesian–silicate metasomatites developing at the expense of carbonate and silicate rocks. The source of the solutions is the magmatic melt, during the differentiation of which, at a certain stage, specific highly magnesian solutions separate. The migration of the latter as a whole has a centrifugal character, but the composition of the rocks of the continent, or more precisely their physical properties, also plays a significant role.

In explaining the mechanism of formation of the dioritoid rocks of the massif, the influence of the filtration effect is most probable; this effect is expressed in the lagging of dissolved matter behind the solvent during filtration. According to the data of Ershler and Gakker, the amount of dissolved matter \(dm\) that has passed through a filter is determined by the following relation: \(dm = \varepsilon \cdot c \cdot dv\), where \(c\) is the concentration of the solution, \(dv\) is the volume of solution that has passed through the filter, and \(\varepsilon\) is the coefficient of the filtration effect [10], representing the ratio of the mean flow velocities of the dissolved matter and of the solution. The theoretical investigation of this effect belongs to D. S. Korzhinskii [9]. Experimentally the phenomenon was studied by L. N. Ovchinnikov and V. G. Maksenkov [15], who showed that the filtration effect is not the same for different substances. Owing to this, even when a solution passes through a homogeneous filter, a noticeable increase in the concentration of the dissolved matter above the latter is observed. The values of the coefficient \(\varepsilon\) vary within wide limits, depending on the concentration of the solution, the size of the pores, and other causes.

In the case of contact between two rocks (in the present case, the rocks of the intrusion and the rocks of the continent), porosity changes; moreover, this is manifested especially sharply at the contact with fine-grained hornfels and, to a lesser degree, with marmorized limestone. As a result, at the boundary of two anisotropic media, some components whose coefficient \(\varepsilon\) is small are “blocked,” whereas other components may continue to move. Such a conclusion about the possibility of separating mixtures follows from the inconstancy of this coefficient for solutions of different substances. In this respect, interesting material is provided by analysis of the paragenetic associations of lime–magnesian–silicate metasomatites containing albite in their composition. Probably sodium (like other alkalis) under these conditions has an insignificant filtration effect and thereby possesses high mobility. Unfortunately, this can be spoken of only as a first approximation, since the question of the change in the coefficient \(\varepsilon\) for compounds of magnesium and sodium has not been investigated experimentally.

An increase in the concentration of MgO (its “affinity” at the contact of the intrusion with hornfels) leads, under favorable thermodynamic conditions, to the onset of deposition of biotite replacing the minerals of the intrusive rock. Since the concentration of components near the “filter” naturally increases, a larger part of the mica is precipitated here; its amount gradually decreases with distance from the contact. Similar solutions probably passed readily through the contact of the intrusion with limestone, replacing the latter. As a result, only an admixture of the magnesian silicate—diopside—appears in the granodiorite from these endocontact parts.

Thus, as a consequence, on the one hand, of assimilation of the enclosing rocks and, on the other, of high-temperature magnesian metasomatism, an equalization takes place in the bulk chemical compositions of the rocks of the continent (biotite hornfels) and of the intrusive formations in contact with them; this is also confirmed by the diagram of chemical compositions of the intrusive rocks (Fig. 1). However, the significance of assimilation should not be overestimated: in all cases, when schlieren whose dimensions are small are studied, complete reworking and migration of matter are not observed, owing to which the outlines of the schlieren remain sharp. Therefore, large-scale assimilation is unlikely to have occurred in the contact zone of the intrusion with the enclosing rocks.

Recalculation of the chemical analyses according to Barth [24, 22], the results of which are given in Table 2, shows that the material balance during the formation of biotite meladiorite at the expense of granodiorite has the following form (in ions).

Addition: K — 7, Ca — 36, Mg — 56, Fe² — 40, Fe³ — 15, Al — 11,

Ti — 12, OH — 80.

Removal: Na — 41, Si — 127, O — 80.

The formation, around the apical part of the Iedygem massif, of a leucocratic rim composed of leucodiorite is due to another petrogenetic process. It should be noted that rims composed of leucocratic varieties of rocks rather often develop in the apical parts of granitoid massifs. Such rocks were distinguished by A. I. Aleksandrov in the Talitsa massif. Alaskite rims in granitoid intrusive bodies of the Caucasus have been described by S. K. Savich-Zablotskii [16], M. A. Favorskaya [20], and V. V. Lyakhovich [13, 14]. V. V. Lyakhovich [13] explains the formation of alaskite rims in the Cenozoic neointrusions of the Caucasus by reworking, by alkaline (essentially potassium) solutions, of the apical part of the massif during the final stages of emplacement of the intrusion. A. N. Zavaritskii [5] also pointed to the possibility of concentration of such solutions. In another of his works [14], V. V. Lyakhovich also noted the high mobility of sodium in some neointrusions of the Caucasus. According to studies by S. S. Il’enko [17] and N. G. Sumina [18] on the petrography of Paleozoic intrusions of southern Siberia, sodium also possesses a high capacity for migration.

In its chemistry, the leucodiorite of the Iedygem massif is very close to granodiorite (Fig. 1), with which it is connected by a gradual transition. The formation of leucodiorite was accompanied by a certain change in the chemical composition of the rock and by the removal of a number of components. As shown by the data obtained by recalculating the chemical compositions of granodiorite and leucodiorite according to Barth [22, 24], the material balance during the formation

the formation of leucodiorite by metasomatism of granodiorite is characterized by the following data (in ions):

Input: K — 2, Al — 1, Si — 24, O — 1.

Output: Na — 4, Ca — 13, Mg — 6, Fe² — 11, Fe³ — 2, OH — 1.

Attention is drawn to the removal of sodium from the rock, but on the whole the scale of this phenomenon is small. On the other hand, the introduced components are probably partly bound in the microcline molecule. Therefore, evidently, the potassium feldspar in the leucodiorite is distinguished by its fresh appearance and replaces plagioclase and biotite. Consequently, the formation of the leucodiorite selvage around the apical part of the Iedygem intrusive is probably genetically connected with the activity of metasomatic alkaline solutions released in the process of differentiation of the magmatic melt. V. V. Lyakhovich’s conclusions [13] on the genesis of leucocratic selvages in non-granitic intrusions of the Caucasus are fully applicable to explaining the origin of the leucodiorite of the intrusive massif described. The components removed in this process, such as sodium, magnesium, and iron, migrated into the exocontact part of the aureole, where they were fixed in the composition of various minerals—albite, diopside, grossular, vesuvianite, actinolite, and others.

Thus, the material presented above indicates that the diversity of the material composition of the rocks of the Iedygem intrusive massif is due to a number of petrogenetic processes—crystallization differentiation of the magmatic melt, assimilation, and especially high-temperature magnesian and alkaline metasomatism of the magmatic stage.

LITERATURE

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2. Abdullaev Kh. M., Isamukhametov I. M., Khamrabaev I. Kh. The role of assimilation processes in the formation of intrusive complexes of Western Uzbekistan. In: Problems of Petrography and Mineralogy, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1953.

3. Winchell A. N., Winchell H. Optical Mineralogy. Foreign Literature Publishing House, 1953.

4. Eliseev A. N. Petrography of Rudny Altai and Kalba. Publishing House of the Academy of Sciences of the USSR, 1938.

5. Zavaritsky A. N. Granites and aplites. Transactions of the All-Union Mineralogical Society, issue 2, 1950.

6. Zavaritsky A. N. Igneous Rocks. Publishing House of the Academy of Sciences of the USSR, 1955.

7. Il’enok S. S. The Belsk intrusive complex. Proceedings of the Tomsk Polytechnic Institute, vol. 65, issue 2, 1950.

8. Koptev-Dvornikov V. S. Hybridization phenomena on examples of certain granite intrusions of the Paleozoic of Central Kazakhstan. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, petrographic series, issue 148, 1953.

9. Korzhinsky D. S. The filtration effect in solutions and its significance for geology. Izv. AN SSSR, geol. ser., No. 2, 1947.

10. Korzhinsky D. S. An outline of metasomatic processes. In: Basic Problems in the Study of Magmatogenic Ore Deposits, Publishing House of the Academy of Sciences of the USSR, 1953.

11. Kuznetsov E. A. Petrography of Magmatic and Metamorphic Rocks. Moscow State University Publishing House, 1956.

12. Kuznetsov Yu. A. Facies of magmatic rocks. In: Problems of the Geology of Asia, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1955.

13. Lyakhovich V. V. On one feature of the contact of granite with hornfelses (Northern Caucasus). In: Problems of Petrography and Mineralogy, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1953.

14. Lyakhovich V. V. Endomorphic alteration of young granites of Baksan in contact with limestones. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, petrographic series, issue 148, 1953.

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1. Savnich-Zablotskii S. K. Petrographic study of rocks of granodiorite magma of Mountain Ossetia. Transactions of the All-Union Mineralogical Society, issue 1, 1927.

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3. Sumin N. G. On sodium metasomatism in skarn iron-ore deposits. In: Problems of Petrography and Mineralogy, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1953.

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5. Favorskaya M. A. The role of processes of contamination and metasomatism in the formation of tertiary intrusions of the monzonite type in certain regions of the Far East. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, petrographic series, issue 148, 1953.

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