Features of Contact Phenomena Associated with Dike Rocks of Basic, Alkaline, Intermediate, and Acidic Compositions

(A Case Study of the Elekmonar Intrusive Massif in the Altai Mountains)

A. I. Bazhenov

(Presented by Prof. A. M. Kuzmin)

The Elekmonar intrusive massif occupies an extensive area in the right-bank part of the middle course of the Katun River. In the south it is exposed by the valleys of the Kuyum, Elekmonar, and Kuba rivers. In the structure of the pluton, rocks of four intrusive phases are clearly distinguished; their formation was separated in time by a deformational hiatus. Petrographically, these phases have the following composition: the first (early) phase—gabbro and pyroxenites; the second—diorites; the third—granodiorites; the fourth—granites.

The dike suite, genetically related to the Elekmonar intrusion, is widely developed and is represented by the following groups: 1. Aplites. 2. Pegmatites. 3. Gabbroid rocks. 4. Krinanites. 5. Microdiorites. 6. Diorite-porphyrites. 7. Granite-porphyries.

The few dikes of aplites and pegmatites are confined to areas occupied by rocks of the first, second, and fourth phases. Composite dikes of aplites were encountered, the central parts of which are composed of pegmatite. Alteration of the intrusive rocks enclosing them is expressed in weak epidotization.

Gabbroid dike rocks (norite and gabbro) are spatially associated with outcrops of rocks of the first intrusive phase. Norites compose small veinlike bodies in intrusive gabbro. True, in the valley of the Kuyum River, within the schist sequence, a small norite stock is known, measuring \(1.5 \times 0.05\) km. The rock described consists of rhombic pyroxene—bronzite (60–45%)¹, biotite (0–15%), and plagioclase No. 50–52. Microcline, quartz, magnetite, ilmenite, and apatite are present as accessory minerals. Secondary minerals are serpentine (antigorite) and hornblende. Contact phenomena associated with the injection of norite have not been studied.

Gabbro dikes, in the form of a series of closely spaced bodies, are exposed in the valley of the Kuyum River. Their thicknesses range from 0.5 to 5 m. The gabbro is a medium-grained mesocratic rock possessing a gabbroic struc-

¹ Quantitative mineralogical composition of the rocks in volume %.

composed of titanaugite and plagioclase (60–55%) and monoclinic pyroxene (40–45%). The accessory minerals are sphene, apatite, magnetite, and biotite. The non-zonal individuals of plagioclase have a variable composition from No. 51 to No. 59. In zonal plagioclases the central parts of the crystals have a composition of No. 57–59, whereas their periphery is No. 50–52; in dikes of small thickness zoning is usually absent. The monoclinic pyroxene composes brownish grains in thin section, often forming simple and polysynthetic twins. In optical properties—\(cN_g = 42^\circ\), \(N_g = 1.726\), \(N_p = 1.698\), \(N_g - N_p = 0.028\), \(2V = (+)52^\circ—(+)50^\circ\)—it belongs to ferroaugite [13]. Weak serpentinization of the mineral is sometimes observed. The chemical composition of the rock is given in Table 1. In its numerical characteristics it is closest to the composition of olivine-free gabbro [4].

The marbled limestones enclosing the dikes bear traces of recrystallization. In the limestones (and in the marbles from the near-contact part) there is an admixture of hydrogen sulfide, released when struck with a hammer. Microscopic study reveals a granoblastic (marble-like) structure of the rock. During recrystallization the pelitic material was squeezed into the intergranular spaces of the calcite individuals. The chemical composition of this variety of the enclosing rocks is given in Table 1, analysis 4.

Along with contaminated limestones there are also those containing no impurities and corresponding to the theoretical composition of calcite (Table 1, analysis 5).

The contact phenomena associated with the emplacement of the gabbro dikes were manifested especially intensely and affected both the enclosing rocks and the endocontact part of the dike. The scale of metasomatism depends on the thickness of the dikes: the contact zone of bodies more than 5 m thick has a width of about 2 m, whereas at thicknesses of less than 1 m it decreases to 0.4–0.5 m. The width of the band of recrystallized limestone forming the outer rim around the metasomatites correspondingly decreases from 3 to 1 m.

The contact-metasomatic formations clearly fall into two genetic groups separated in time by a deformational hiatus. The mineral concentrations of the first group form a continuous selvage along the contacts of the dikes, whereas the minerals of the second group occur as small cross-cutting lenticular and vein-like bodies. In the material composition of the first group of metasomatites, zoning is well expressed; in its most complete form it is represented by the following zones (from the dike contact toward the periphery): 1. Scapolite. 2. Grossular. 3. Wollastonite. 4. Wollastonite–albite–diopside. 5. Quartz. These associations are distributed unevenly: in the aureoles of some dikes the fifth zone is absent; more rarely the fourth is also indistinctly expressed.

The scapolite zone always develops at the expense of the dike rock; its width usually does not exceed 0.1 m. Its sole metasomatic mineral—scapolite—develops selectively by replacement of the plagioclase of the gabbro. Microscopic study makes it possible to reveal details of the mechanism of replacement of the dike rock. The front of scapolitization is sharply expressed. At the initial stage, scapolite appears as a scattering of the finest plates, which then merge into larger individuals. Plagioclase, filled with a scattering of scapolite particles, still preserves its twinned structure. As the size of the scapolite particles increases, the twinning of the replaced plagioclase is quickly extinguished; the size of the scapolite grains reaches 3–4 mm. Macroscopically the mineral is white in color. In thin sections it has a slightly brownish tint. The mineral is uniaxial, optically positive;

$N_o = 1.618$, $N_e = 1.600$, $N_o - N_e = 0.018$. Such high birefringence is possibly due to the high potassium content, as follows from the chemical-analysis data (Table 1). Elevated refractive indices are characteristic of potassium scapolites [12].

The monoclinic pyroxene of the gabbro under these conditions possesses considerable stability. However, it undergoes coalescent recrystallization, owing to which large individuals arise, imparting a pegmatoid appearance to the rock. The newly formed individual has a single system of cleavage cracks, but sometimes blocky extinction is clearly visible in it, apparently representing a relic of recrystallization that did not proceed to completion. Simultaneously with recrystallization, the chemical composition of the mineral also changes: its refractive indices decrease to $N_g = 1.612$, $N_p = 1.598$, which corresponds to a decrease in the ferruginous component in it [2]. In the more completely altered zones of the metasomatic column, pyroxene is absent.

The apatite of the gabbro in the scapolite zone undergoes no changes. Sphene is also fairly stable, but it undergoes granulation, owing to which its crystals break down into small rounded grains; however, in the distribution of the latter the original wedge-shaped form of the sphene crystals can be discerned.

Apparently, the plagioclase No. 32–35, which is scarcely affected by scapolitization and is distinguished by its fresh appearance, belongs to later formations.

The chemical composition of the scapolite rock, containing an insignificant admixture of unreplaced monoclinic pyroxene, is given in Table 1, analysis 6.

The grossular zone is composed, in a considerable part of its mass, of garnet replacing the scapolite rock and the formations of the more external wollastonite zone. In details of structure the zone described is somewhat heterogeneous, which is reflected in the content of the andradite molecule in the garnet and in the appearance, near the wollastonite zone, of an admixture of vesuvianite and hydrogarnet. The garnet zone is sharply separated from the scapolite and wollastonite zones. However, ahead of the continuous replacement front, small rounded garnet grains, measuring thousandths and hundredths of a mm, appear within the mineral associations being replaced; their number rapidly increases (over a distance of 0.1–0.3 mm), and, merging, they form a continuous front. Relics of the most stable mineral—ferroaugite—are preserved at a distance of no more than 1–5 cm from the zone boundary. At some distance (up to 1–1.5 cm), in addition to augite, relics of apatite are also observed. Scapolite, plagioclase, and sphene do not pass through the replacement front at all. Zoned garnet crystals appear rarely (up to 8 zones are distinguished). The color of the darker zones is brown. Within each zone its intensity gradually weakens toward the inner part of the crystal, while the contact surface of two zones is covered by a brown coating. Garnet aggregates have a brown color, the intensity of which noticeably increases in the direction toward the dike. In thin sections the mineral is optically isotropic, but grains are sometimes observed that possess clearly expressed anisotropy, manifested in the appearance of four sector zones extinguishing in pairs (for example, the first and third sectors). The chemical composition of the garnet rock is given in Table 1 (analysis 7), and it corresponds to the average composition of the metasomatite (an average sample was taken).

Measurement of the optical constants of garnet makes it possible to elucidate details of the change in the chemical composition of the mineral along the normal thickness of the zone. The content of the andradite molecule in it gradually decreases—

decreases from 28% \((N = 1.780)\) near the contact with the scapolite zone to 15% \((N = 1.767)\) near the contact with the wollastonite zone.

Hydrogarnet forms rims around some idioblasts of garnet. In thin sections the mineral is colorless, optically isotropic, and has a lower refractive index than garnet. Hydrogarnet regularly overgrows individuals of grossular, and its external crystallographic boundaries are parallel to the faces of the idioblasts of the latter.

Vesuvianite was found as several colorless grains in thin sections, situated within the garnet aggregate.

The wollastonite zone consists predominantly of a monomineralic aggregate. The sizes of crystals and grains of this mineral vary from \(7 \times 25\) mm in the contact aureoles of large dike bodies to \(1 \times 10\) mm in the contacts of small dikes. Microscopically the mineral is colorless, slightly brownish; \(cN_p = 31^\circ\), \(2V = -38^\circ\), \(N_g = 1.634\), \(N_p = 1.620\), \(N_g - N_p = 0.014\). Calcite is noted as an impurity in the composition of the zone. The chemical composition of the rock described is given in Table 1, analysis 8. The boundary of the wollastonite zone with the grossular zone is sharp: the microscope shows that practically complete replacement of wollastonite by garnet takes place over an interval of about 0.5 mm. The thickness of the zone described is variable and does not exceed 1.5 m.

The wollastonite–albite–diopside zone is not always clearly expressed. It is composed of a fine-grained aggregate consisting mainly of wollastonite; albite and diopside are present in subordinate amounts. The thickness of the zone usually does not exceed 0.1 m; around small dike bodies it is sometimes altogether absent. In its physico-optical properties wollastonite is analogous to the mineral of the preceding zone. Diopside forms a more or less uniform dissemination in the wollastonite aggregate. In thin sections it is colorless, slightly brownish. In its optical properties \((cN_g = 40^\circ, N_g = 1.698, N_p = 1.668, N_g - N_p = 0.030, 2V = +60^\circ)\) it corresponds to diopside [2]. Albite is present in amounts up to 5–8%, in colorless grains that, as a rule, lack polysynthetic twinning. The chemical composition of the metasomatite described is given in Table 1, analysis 9.

The outer zone of the metasomatic column is composed of quartz; it appears only where the dike is of relatively small thickness. The thickness of the zone is usually 0.1 m. The quartz metasomatite is a fine-grained monomineralic rock composed of quartz, whose grains do not exceed 0.01–0.03 mm. The boundaries with the adjacent zones are very sharp and linear. As microscopic study shows, during replacement of the quartz metasomatite by wollastonite–albite–diopside formations the latter develop as a dense network of veinlets which, merging, pass into a rock containing no relics of quartz.

The contact-metasomatic formations described are cut by veinlets of second-generation garnet, and in most cases the latter affect the mineral concentrations of the wollastonite zone. This is evidently due to the favorable mechanical properties of wollastonite, which readily reacts to tectonic stresses by developing systems of fractures. Under these conditions the garnet metasomatite is much less affected by fracturing, since it is composed of a mechanically stronger mineral. Consequently, veinlets of late mineral associations in it are, as a rule, absent. Macroscopically the garnet has a brownish coloration, darker than that of the variety described earlier. The mineral is optically isotropic, \(N = 1.774\). The chemical composition of the rock is given in Table 1, analysis 11. According to these data, the mineral belongs to a moderately ferruginous variety of the isomorphous grossular–andradite series.

¹ Loss on ignition, including CO₂.

Numerical characteristics according to A. N. Zavaritsky

1. Gabbro from dike. 2. Krinait. 3. Granite-porphyry. 4. Marmorized limestone altered by a pelitic-metasomatic process, deprived of the admixture of clastic material. 6. Skarnite metasomatite. 7. Garnet metasomatite. 8. Wollastonite metasomatite. 9. Wollastonite–albite–diopside metasomatite. 10. Quartz metasomatite. 11. Garnet rock from veins, crosscutting calcite–wollastonite–diopside formations.

Analyses were carried out in the laboratory of Tomsk State University; analyst A. V. Krylov.

Table 2

Balance of substance (in grams) during the formation of a metasomatic column in gabbro contacts
(calculated per 100 cm³ of rock)

It is interesting to note that garnet veinlets are not traceable into the enclosing limestones, which is probably explained by their fairly high plasticity owing to heating in the near-contact part.

To clarify the mechanism of formation of the rocks of the metasomatic column, valuable material is provided by analysis of the paragenetic associations of each zone, as well as by their chemical compositions. The chemical analyses given in the work were recalculated by Barth’s oxygen method [14] and by the method taking porosity into account of N. I. Nakovnik [11]; the resulting graphs proved to be analogous, with discrepancies concerning only details. The material balance during formation of the metasomatic column is given in Table 2, compiled on the basis of recalculation of chemical analyses by the method taking porosity into account. Table 2 clearly shows the change in the weight quantities of components in the process of metasomatism; it becomes apparent that the gross amount of introduced and removed material changes quite regularly and has a not entirely usual character. It is assumed [8] that, with increasing distance from the contact with the intrusive body, the composition of metasomatic neogenic formations approaches ever more closely the chemical composition of the enclosing rock and, consequently, the amount of displaced material should gradually decrease. In the present case, other relations are found. Figure 1 shows the change in the gross compositions of metasomatites of different zones. The graph distinctly shows two minima—one each in the endocontact and exocontact belts. Attention is drawn to the fact that formation of the outer members of the metasomatic column is accompanied by an increase in the displacement of matter; it is especially sharply expressed for the outer exocontact zone (483 g are displaced for every 100 cm³ of rock). The insignificant amount of displaced material during formation of the wollastonite zone at the expense of the wollastonite–albite–diopside zone is most likely due to small differences in their chemical compositions.

Fig. 1. Amount of material displaced during formation of zones of the metasomatic column in the contacts of gabbro dikes (in grams per 100 cm³ of rock). I — introduction; II — removal; III — total amount of displaced material: 1. Replacement of gabbro by scapolite rock. 2. Replacement of scapolite metasomatite by garnet metasomatite. 3. Replacement of wollastonite metasomatite by garnet metasomatite. 4. Replacement of wollastonite–albite–diopside metasomatite by wollastonite metasomatite. 5. Replacement of quartz metasomatite by wollastonite–albite–diopside metasomatite. 6. Replacement of limestone by quartz metasomatite.

Changes in the chemistry of metasomatic neogenic formations and the direction of the replacement process are illustrated by the composition–paragenesis diagram (Fig. 2). From this diagram a regular change in the composition of metasomatites is clearly visible. The curve successively connecting the average compositions of the metasomatic rocks clearly splits into two branches: the branch corresponding to endocontact formations, associated with alteration of the dike rock, and the branch uniting exocontact formations, which owe their origin to replacement of limestone. The direction of change in the chemistry of the newly formed mineral concentrations in these parts of the contact aureole is diametrically opposite. In the endocontact formations, as the

as the host limestones are approached, the CaO content increases regularly, and therefore the resultant curve has a descending character, approaching on the diagram the CaO apex. Its slight deviation toward the Al₂O₃ apex is due to metasomatic desilication of the dike rock in the scapolite zone and, to a considerably lesser degree, in the grossular zone. Thus, for the metasomatites described, the above-noted equalization of the chemical compositions of these rocks and the host rocks does indeed occur. The change in the chemistry of the exocontact zones, with movement from the dike toward the periphery of the aureole, changes regularly in the direction of increasing silica content, with a simultaneous decrease in the metasomatic associations of lime and alumina. Therefore the resultant curve has an ascending appearance.

Fig. 2. Composition–paragenesis diagram for metasomatic formations in contacts of gabbro dikes: 1. Gabbro. 2. Scapolite rock. 3. Garnet rock. 4. Wollastonite rock. 5. Wollastonite–albite–diopside rock. 6. Quartz rock. 7. Limestone of the host sequence. Ab—albite. An—andradite. Gr—grossular. Wol—wollastonite. Qz—quartz. Pir—diopside. Ma—marialite. Me—meionite. Dots connect the chemical compositions of endocontact formations; the solid line, the compositions of exocontact formations.

form. In contrast to the zonality of the endocontact part, here there is no equalization of the chemical compositions of the metasomatic newly formed rocks with the host rocks. On the contrary, an antagonism is revealed in the behavior of CaO and SiO₂. As the limestones are approached, the CaO content in the metasomatic zones gradually decreases. The behavior of silica is opposite: with increasing distance from the silicate rock (in the exocontact area), its content rises sharply; moreover, in the outer zones it considerably exceeds the amount of SiO₂ in the gabbro.

One should dwell on the question of the mobility of other petrogenic elements in the process of formation of the metasomatic column. From Fig. 3 it follows that, in contrast to silica, alumina almost does not leave

beyond the limits of the endocontact. The ratios \(Al_2O_3 : SiO_2\) in rocks of the different zones are indicative: gabbro—0.376; scapolitic metasomatite—0.424; grossularite—0.386; wollastonitic—0.032; wollastonite–albite–diopsidic—0.024; quartz—0.000. In the endocontact metasomatites the ratio \(Al_2O_3 : SiO_2\) is more than 10 times greater than this ratio in the new formations that arose at the expense of the enclosing limestones. Gradually, as the outer part of the aureole is approached, it decreases to zero. Consequently, in the endocontact formations there is desilicification of the rocks, caused by the higher mobility of silica diffusing toward the country rocks. \(Fe_2O_3\) has relatively high mobility in the grossularite zone. The migration of the remaining elements has much in common: their content gradually decreases with increasing distance from the dike.

Fig. 3. Variation diagram of the chemical compositions of the zones of a metasomatic column in contacts of gabbro dikes (intersections according to the method taking porosity into account): 1. Gabbro. 2. Scapolitic rock. 3. Garnet rock. 4. Wollastonitic rock. 5. Wollastonite–albite–diopsidic rock. 6. Quartz rock. 7. Limestone of the enclosing sequence.

An interesting feature is noted on the composition–paragenesis diagram (Fig. 2) in the part concerning the chemistry of the wollastonitic and wollastonite–albite–diopsidic zones. The figurative points of the latter on the diagram are located close together, which indicates the similarity of their compositions and, apparently, is the reason for the insignificant displacement of material during replacement of the wollastonite–albite–diopsidic aggregate by a wollastonitic aggregate (Fig. 1).

Contact phenomena genetically associated with basic rocks are discussed in the geological literature mainly using examples of bodies of the Siberian traps. A direct parallel with the latter cannot be drawn, since their geological positions and mineral parageneses differ from the formations described. Nevertheless, in con-

in the contacts of Siberian trap bodies, in individual cases a zonal distribution of metasomatism products is observed [3, 9, 10, etc.]. Zoning is more common for the contact zones of granitoid massifs. Doscarn metasomatites similar in mineral composition from the contacts of a granodiorite massif were recently described by the author [1].

Dikes of alkaline rocks, represented by crinanite, are spatially associated with the areas of distribution of gabbroid dikes and rocks of the first intrusive phase. The thickness of such dikes reaches 5 m.

Microscopically, an ophitic texture is observed. Plagioclase (50%) forms idiomorphic individuals; the larger crystals correspond in composition to No. 68–70. The periphery of zoned crystals has composition No. 38. It is interesting that its small crystals have the same composition (No. 38), which is probably due to the high rate of crystallization of the magmatic melt. Monoclinic pyroxene (35%), by its optical properties \((N_g = 1.736, N_p = 1.708, N_g - N_p = 0.028; 2V = +54^\circ; cN_g = 45^\circ)\), belongs to a more ferruginous variety of ferroaugite [13] than the pyroxene of the gabbro. Quartz and pyrrhotite are present as impurities. The rock contains about 15% of a serpentine-like, almost isotropic substance. The chemical composition of the rock is given in Table 1. In terms of numerical characteristics it is closest to crinanite [4].

The contact effect of the dikes of alkaline rocks was weak, despite the fact that the enclosing rocks are the same carbonate sequence as in the case of the gabbroid bodies, and spatially the dike formations occur 30–40 m from one another. The limestones underwent weak recrystallization. The thickness of the contact aureole is about 2–2.5 m, and that of the metasomatic neoformations is 0.1 m. The contacts of this metasomatic zone are sharp. The metasomatite is a dense aggregate; its color depends on the admixture of pelitic material in the replaced substrate: during metasomatism of contaminated varieties the color of the newly formed aggregate is dark gray, whereas pure varieties of limestones give rise to a light color. Microscopic study reveals a microgranoblastic texture of the rock, composed of a fine-grained aggregate of quartz, among which clotted segregations of pelitic material are distributed.

Dikes of granite-porphyries, diorite-porphyrites, and microdiorites belong to the latest formations. Their mutual relations remain unclear, since spatially they are usually separated. The thicknesses of the dike bodies vary from 1 to 10 m.

The microdiorite is composed of andesine and chloritized and epidotized hornblende. Quartz probably belongs to the later minerals, and its appearance is accompanied by the deposition of clinozoisite.

Diorite-porphyrite usually composes large dikes, the marginal parts of which are represented by microdiorite. In addition, rounded xenoliths (?) of the latter are often observed among the diorite-porphyrite. The rock has a porphyritic texture with a microcrystalline-granular groundmass. Porphyritic segregations are represented by tabular individuals of non-zoned andesine No. 36–38 and rounded corroded grains of quartz. Brown biotite is replaced by pennine; judging by the shape of the aggregates, biotite is a pseudomorph after hornblende. The groundmass is composed of microlites of andesine No. 32–34. Accessory minerals are apatite and magnetite.

Granite-porphyry dikes are especially numerous and diverse in their structural-textural features; their thicknesses vary from 0.3 to 5–8 m. Their bodies cut and metamorphose the gabbro dikes. Along with porphyritic varieties in thin dikes and in the endocontact parts of thick bodies, weakly recrystallized rocks appear.

of felsitic appearance, containing no porphyritic segregations. The rock structure is micrographic, porphyritic with a microcrystalline granular microaplitic, micrographic, spherulitic structure of the groundmass. The porphyritic segregations are composed of oligoclase No. 20–22 (in other dike bodies it sometimes has the composition No. 27–29), quartz, in some varieties of the rocks surrounded by a spherulitic rim, and sparse flakes of brown biotite. Accessory minerals are magnetite, zircon, and apatite. Secondary minerals are pennine, replacing biotite, and green hornblende, composing lath-shaped aggregates. The sagenitic network of rutile contained in the biotite is well preserved also in the aggregate of chlorite. The chemical composition of the granite porphyry (which has a porphyritic structure with a microaplitic structure of the groundmass) is given in Table 1.

The contact effect of dikes of diorite porphyrites, microdiorites, and granite porphyries, differing in material composition and chemistry, is entirely analogous. The differences are manifested only in its intensity, which increases in the chilled margins of thick bodies. The greatest width of metamorphosed carbonate rocks does not exceed 2–3 m, and in the overwhelming majority of cases it decreases to 0.5–1 m. A characteristic feature distinguishing the aureoles of late dike bodies is the absence of even traces of metasomatic newly formed minerals. In the immediate contact the enclosing rocks are transformed into marbles with grain sizes up to 20–30 mm, but with distance from it the size of the latter decreases. In such recrystallized rocks the pelitic material contained in them, as in the contact zones described earlier, is squeezed out into intergranular spaces, as a result of which calcite individuals grow completely pure.

The factual material set forth above indicates the following features of the contact phenomena associated with the formation of dikes of different composition.

The dike bodies genetically connected with the Elekmonar intrusion occur in an area composed of carbonate rocks of one and the same suite and are often spatially close together, which makes it possible to compare the character of the contact effects of different generations of magmatic injections and to trace the development over time of metasomatic phenomena. The intensity of metasomatism weakens in the chilled margins of later formations: if the emplacement of the earliest gabbro dikes is accompanied by a complex assemblage of metasomatites, then in the contacts of crinanite bodies a narrow band of chalcedony-like quartz develops, while the dikes of microdiorites, diorite porphyrites, and granite porphyries are not accompanied by metasomatic redistribution of matter.

In the spatial distribution of the products of the earliest stage of the metasomatic process, associated with the emplacement of gabbro dikes, zoning is clearly revealed, expressed in the regular replacement of some paragenetic associations by others, with sharp boundaries between individual parageneses. According to the change in the chemistry of the newly formed parageneses, as shown by analysis of the composition–paragenesis diagram, the variation curve splits into two branches depending on the features of the material being replaced. In endocontact formations a tendency is revealed toward equalization of the contents of CaO, SiO₂, and Al₂O₃ in the outer zones with the content of these components in the adjoining marble. In the composition of the exocontact zones, antagonism in the behavior of SiO₂ and CaO is clearly manifested: as the metasomatizing solutions move and the outer zones of the column form, the CaO content decreases sharply with a corresponding increase in the amount of SiO₂; the outer part of the contact aureole is composed of quartzite, almost devoid of CaO impurities.

The study of minerals of variable composition (this is especially clearly manifested in garnet) indicates that, within the zone, the content of elements in them does not remain constant. Therefore, the development of a continuous rim around the parent magmatic body, and the absence of stability in the chemistry of minerals of variable composition [7, 8], make it possible to assign the metasomatic column to the diffusion type.

The study of the contact aureoles of the dike bodies of the intrusive massif described here, it seems, makes it possible to clarify the question of the causes of a weakly expressed, or entirely unmanifested, process of metasomatism. The features of the material composition of the selvage parts of gabbroid dikes—in particular, the absence of a quartz zone in a number of bodies of great thickness—testify to the specificity of the physicochemical conditions. The intensity of alteration of the metasomatic column in most cases is also directly related to the mass of injected material. Apparently, the possibility that metasomatizing solutions could be released into the enclosing rocks was affected by heating of the contact region, which can to some extent be judged from the thickness of the recrystallized limestones. For dikes of the same thickness (about 2 m), the width of such a band decreases approximately in the following order: gabbro—3 m, crinanite—1.5 m, microdiorite and diorite-porphyrite—0.5 m, granite-porphyry—0.5–0.3 m. Directly related to this is also the amount of petrogenic elements released by the injected magmatic melt. Apparently, weak heating of the contact region substantially hindered the separation of postmagmatic solutions and the concentration in the latter of silica, the content of which in granite-porphyry exceeds that in gabbro by more than 1.5 times, and in the former rock it occurs in a free state. Other elements behave in an analogous way. From these standpoints, the appearance of free quartz in the metasomatic column in the presence of excess $\mathrm{CaCO_3}$, and the development of chalcedony-like quartz in the contacts of crinanite dikes, become understandable, although it would seem that under these conditions the reaction $\mathrm{CaCO_3 + SiO_2 = CaSiO_3 + CO_2}$ should proceed, with the formation of calcium silicate. Wollastonite belongs to the high-temperature minerals that develop under conditions of deep zones [5]. To be sure, the available materials are rather contradictory. Thus, Harker [15] believes that even slight heating of impure varieties of limestone containing $\mathrm{SiO_2}$ leads to the formation of calcium silicate. The thermodynamic analysis of the parageneses of wollastonite with other minerals of the zones belongs to D. S. Korzhinskii [5], who showed the dependence between the appearance of calcium silicate and the partial pressure of $\mathrm{CO_2}$. Recently, a corrected theoretical equilibrium curve for calcite–wollastonite was presented [16], and the possibility was noted of the coexistence of $\mathrm{CaCO_3}$ and $\mathrm{SiO_2}$ even in deep zones of metamorphism.

Under the conditions of the contacts of the gabbroid dikes described here, the heating of the enclosing limestones was relatively low (though sufficient for pyroxene in the endocontact parts of the dikes, owing to collective crystallization, to form rather large individuals, and for the gabbros not to bear traces of “quenching”). Therefore, in those cases where the amount of metasomatizing solutions released during crystallization of the injected magmatic melt and diffusing into the country rocks was relatively small, while heating of the latter was sufficiently high, the outer zone in the metasomatic column is composed of wollastonite, or of an association of wollastonite, albite, and diopside. In dikes of small thickness, especially if the separating solutions were sufficiently abundant and migrated beyond the limits of the heated region, the reaction between calcite and silica did not develop; instead, replacement of the country rocks (calcite) by fine-grained ...

... by a pure aggregate of quartz, which in this case constitutes the outer zone of the metasomatic column.

An analogous phenomenon also develops in the contacts of crinanite dikes, whose thermal energy was considerably lower, as is indicated by the weaker recrystallization of the near-contact parts. Therefore, here the process of influence by the magmatic melt resembles, in its general features, the character of metasomatism in the quartz zone of gabbro contacts, differing, apparently, only in the lower temperature of the metasomatizing solution, to which the chalcedony-like appearance of the quartz precipitated from the solution corresponds.

The magmatic melt from which the dikes of microdiorite, diorite-porphyrite and especially granite-porphyry formed apparently possessed insignificant reserves of heat and, judging from the fluctuation structural features (the regular arrangement of plagioclase laths around porphyritic quartz segregations), the eutectoid chemical composition (abundance of micropegmatitic and spherulitic formations), and the clearly expressed fine-grained character (“chilling”) of the dike rocks in the near-contact area, crystallization proceeded under conditions of relatively low temperatures.

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