PETROLOGY OF THE LYUBAVINSKY GRANITOID STOCKS

(Central Transbaikalia)

G. V. SHUBIN, V. M. VOLKOV

(Presented by Professor A. M. Kuzmin)

The presentation of the materials in this work is prompted by the circumstance that, in genetic terms, the granitoid stocks of the Lyubavinsky ore field are regarded by some investigators (Bilibin, 1961; Leshkevich, 1965, et al.) as an element of an independent post-batholithic (up to the intrusions of the Kyrinsky complex) Khangaruk intrusive complex, with which they also associate the manifestation of gold-ore mineralization. Other investigators (Valitskaya, 1933; Kozhevina, 1954; Busygina, 1957; Bindeman, 1966, et al.) believe that the granitoid stocks are derivatives of the Kyrinsky intrusive complex, forming its various phases or apophyses of the large Khamaro-Tyrinsky granitoid batholith. Finally, there are opinions that the granitoid stocks are an independent part of a post-batholithic complex of small intrusions (Tulokhonov, 1959, et al.).

Such divergent views on the genesis of the granitoid stocks are connected, as it seems to us, with the incompleteness of geochemical and petrological studies and their comparisons for the magmatic formations under consideration, and with the absence of a marker geological object relative to which it would be possible to speak of the time of origin of the intrusive formations. The absolute-age data have a very strong scatter (180—190—200—250 million years) and cannot be reliably applied for subdividing the magmatic rocks.

Naturally, in order to evaluate the above-mentioned views, it is first of all necessary to have a more or less clear idea of the petrological-geochemical features of the granitoids of the Lyubavinsky ore field. These features, to which the present work is devoted, may subsequently be used as the basis for the questions considered, taking into account the available geological data on the position of the magmatic formations of this region.

General remarks on the geology of the granitoid stocks

Granitoid rocks are very widely distributed within the Lyubavinsky ore field, which is part of the Verkhne-Ononsky gold-ore district, located in the south of the Daurian zone of Central Transbaikalia.

Magmatic formations of granitoid composition are represented by separate, small-area, stock-like bodies arranged in a chain (the Evgrafovsky, Lyubavinsky, and Bolshe-Fedorov—

stocks), which at the present erosional level have an elongated shape in the latitudinal direction and irregular outlines (Fig. 1). The spatial magmatic bodies are confined to a sublatitudinal zone of increased fracturing (Radkevich, 1956; Thomson, 1964), cutting Permian sandy-shaly rocks that make up the northern limb of a large folded structure of sublatitudinal strike. In relation to the latter, the stocks as a whole are discordant, and only locally conform to the strike of the enclosing rocks, tending toward concordant occurrence; this is especially true of their small apophyses.

In terms of the nature of the interaction of the magmatic bodies with the enclosing rocks, clearly expressed intrusive contacts of complex configuration with an overall steep southerly dip are observed. The magnitude

Fig. 1. Schematic layout of granitoid bodies.
1 — elements of bedding of the sandy-shaly sequence; 2 — granitoids; 3 — aureole of contact-altered rocks; 4 — Lyubavinsky ore zone; 5 — disjunctive disturbances;
I — Khamaro-Tyrinsky massif; II — Evgrafovsky stock; III — Lyubavinsky stock; IV — Bolshe-Fedorovsky stock.

of the aureole of contact metamorphism associated with the emplacement of the stocks described is measured in the first tens of centimeters; at the same time, the thickness of the metamorphic zone depends on the size of the stocks and the composition of the enclosing rocks. Metamorphism is most conspicuous in sandstones and shales with interbeds of siltstones. An increase in newly formed quartz and biotite is noted, with the disappearance of pelitic material, and the appearance of small amounts of andalusite, cordierite, and staurolite. The newly formed minerals, with their spatial distribution, often lead to the formation of “spotted” or “nodular shales,” which in the immediate vicinity of the granite bodies pass into dense, non-foliated quartz-biotite hornfelses. In some cases, injections of quartz-feldspar material are observed in the zone of contact metamorphism, most often oriented in the direction of the primary layering of the enclosing rocks. Such phenomena were encountered rather clearly in the apical part of the Evgrafovsky stock.

Aplites and quartz-feldspar veins, occurring mainly—

mainly in the bodies of the stocks and only rarely (quartz–feldspar veins) in their exocontacts. The modes of occurrence of the described formations are varied; their thickness and extent are extremely insignificant. Outwardly, the aplites have the usual appearance of fine-grained rocks, often with a granophyric texture. Cases are observed in which quartz–feldspar veins gradually pass along strike into formations resembling fine-grained pegmatites. The feldspars (anorthoclase) in the veins have well-expressed crystals, sometimes forming Carlsbad twins. It is characteristic that the bulk of these crystals is located in the endocontacts of the veins; in individual cases the vein consists entirely of feldspar with a small participation of quartz. In the quartz–feldspar formations muscovite may also be encountered, concentrated in the form of rosettes in the central parts of the veins. The development of aplites and quartz–feldspar veins is also associated with the phenomenon of potassium feldspathization of the granitoids (Fig. 2), and, as will be shown below, potassium feldspar in the granite stocks is one of the latest minerals in time of crystallization and, in its development, evidently has a close genetic connection with the phenomena that led to the formation of the vein formations of the granitoids.

Fig. 2. Diagram of the interaction of a quartz–feldspar vein and granitoids (Evgrafovsky stock).
1 — quartz–feldspar vein; 2 — potassium-feldspar leucites; 3 — zone of lightening of granitoids; 4 — granodiorites.

As for the metallogeny of the granitoid stocks under consideration in the Lyubavinsky deposit, no connection with any deposits or ore occurrences is established for them. It is true that some researchers are inclined to believe that tungsten mineralization may be connected with granitoids of the type of the stocks described. The idea that gold mineralization may have a genetic connection with the stocks proceeds from an erroneous interpretation of the dobatolithic age of the magmatic bodies under consideration. As studies of recent years have shown (Shubin, Matsyushevsky, 1968), the Lyubavinsky-type gold mineralization is genetically connected with an independent Middle–Upper Jurassic dike complex or, as is now believed, with the volcano-plutonic formation of Central Transbaikalia. This question requires independent investigation and is not considered by us in the present work.

Petrographic and petrochemical characteristics of the stocks

In external appearance, the granitoids composing the stocks of the Lyubavinsky ore field are dark- and light-gray rocks, sometimes with a greenish tint, having a medium- and uneven-grained, often porphyritic structure. Porphyritic texture is most clearly manifested in the exocontacts and apical parts of the magmatic bodies, with a gradual transition to rocks of uneven-grained structure composing the central and deeper parts of the stocks. In a number of cases, in the marginal parts of the magmatic bodies under consideration, xenoliths of rocks of the sandy-shale sequence may be observed; these are often strongly reworked. Under the microscope one can see in them relict bedding-

...osity and schistosity, along which newly formed minerals are strictly oriented, especially light-colored fine-scaled biotite and other rock-forming minerals characteristic of granitoids.

The principal minerals of the granitoid rocks are: plagioclase, quartz, orthoclase, biotite, and hornblende. Accessory minerals: sphene, apatite, zircon, garnet, monazite, corundum, arsenopyrite, pyrite, chalcopyrite, sphalerite, ilmenite. Secondary minerals: sericite, chlorite, muscovite, epidote, leucoxene, rutile, calcite. The quantitative mineralogical composition is given in Table 1.

On the basis of structural features, among the rocks described there are distinguished granitoids with a granitic texture and porphyritic varieties with an allotriomorphic-granular groundmass, against which are distinguished

Table 1

Quantitative mineralogical composition of the granitoids of the Lyubavinsky ore zone

large or small (2–7 mm) grains of the principal rock-forming minerals. Depending on the quantity of porphyritic segregations and the groundmass, transitions are observed from porphyritic granitoids through a cryptic texture to rocks with a granitic texture. The diversity of structural features is due to different conditions of rock formation at different hypsometric levels. In the apical parts, endocontacts, and apophyses of the stocks described, the rocks have a distinct porphyritic texture; rocks of the central and deep parts of the stocks possess a texture close to granitic. In direct contact with the enclosing rocks, the granitoids distinctly change their...

their structure to granoblastic serrate. There occurs an almost complete disappearance of mafic minerals; plagioclases lose their zoning; porphyritic segregations and grains of the groundmass acquire sinuous, serrate outlines, undulose extinction, and, locally, a blocky structure.

Below is a description of the principal rock-forming minerals in the order of their crystallization.

Plagioclase, belonging to oligoclase (Nos. 18–25) and oligoclase-andesine (Nos. 30–32), is represented by regular short- and long-prismatic crystals 2 to 5, more rarely up to 8 mm in size, and is always idiomorphic with respect to all the other rock-forming minerals. In the endocontact and apical parts of the stocks, the plagioclase has a distinct zonal structure of recurrent character, expressed in repeated periodic changes in the composition of the zones from the core to the periphery of the grains according to the following schemes: 1. Nos. 28—28—22—25—31—24; 2. Nos. 26—33—29; 3. Nos. 28—33—27. The boundaries between zones are usually sharp, sometimes with faint traces of resorption. Individual large plagioclase grains have a distinct blocky structure: each block is essentially an independent plagioclase grain with its own zoning. These individual “plagioclase blocks” are clearly overgrown by later zonal plagioclase, and as a result a “single” large grain is formed, which may be regarded as a metacryst (Fig. 3—1).

A number of grains are characterized by polysynthetic twinning, and the “twin lamellae” for the most part have uneven, wedge-shaped outlines, pinching out within the grain. It should be noted that one very interesting phenomenon is observed when, in two adjoining, differently oriented plagioclase grains, a twinned structure is clearly traceable, being common to these two grains (Fig. 3—2). This suggests a deformational character for the formation of this type of twins.

The porphyritic plagioclase segregations are quite readily, mainly from the periphery of the grains, affected by secondary alteration processes—sericitization and saussuritization. In the case of zonal plagioclases, selective alteration of individual zones can be observed. Altered plagioclases, as a rule, acquire lower refractive indices ($Np = 1.539—1.542$) and correspond to numbers 18—22, whereas unaltered plagioclases have a refractive index of $Np = 1.543—1.546$, corresponding to numbers 25—32.

Quartz is characterized by a rounded or irregular grain shape, 4—5 mm in size, with sinuous, corroded margins. It has undulose extinction, a blocky structure, and is not infrequently fractured; the fractures are filled by later minerals of the groundmass, often accompanied by biotite and sphene (Fig. 3—3, 3—7). Large quartz grains sometimes contain inclusions of fragments of plagioclase grains and, in some cases, are overgrown by a rim of radially fibrous quartz of the groundmass, with the formation of a micrococarde structure.

Hornblende in the rocks described forms rare grains both of regular shape and idiomorphic elongate-prismatic crystals 0.2 to 2 mm in size, often replaced by chlorite, epidote, and biotite, in places to complete pseudomorphs. Skeletal forms of hornblende occur, formed during its corrosion by potassium feldspar. The pleochroism of hornblende is: along $Ng$—dark green, along $Np$—light yellowish-green, along $Nm$—yellowish-brown; $cNg = 17—20^\circ$; $2V = 75—80^\circ$; refractive index along $Ng = 1.660—1.668$. According to the optical constants given, hornblende, following V. E. Tröger (1968), is substantially magnesian ($Mg = 70—80\%$).

Fig. 3. Sketches of the interrelationships of rock-forming minerals (explanations in the text).
O. M. — fine-grained groundmass

The groundmass consists of quartz (25–30%), orthoclase (35–40%), and plagioclase (30–35%). Among secondary minerals, sericite is present, and more rarely muscovite and epidote. Among accessory minerals, sphene is relatively abundant; zircon, apatite, and ore minerals are also noted. The principal minerals of the groundmass, determining its initial composition, are plagioclase (No. 25–30) and quartz, whose grain sizes range from 0.01 to 0.2 mm. As for potassium feldspar, biotite, and accessory minerals, in terms of their time of formation they are later, superimposed on the minerals of the groundmass. Quantitatively, biotite together with the earlier hornblende described above makes up 5–10% of the groundmass.

During crystallization, the minerals of the groundmass form aggregates that distinctly corrode the large porphyritic segregations of the early plagioclase and quartz grains, penetrating into the latter along cracks with the formation of “bay-like” and “embayment-like” forms of corrosion (Fig. 3—4, 3—7).

Orthoclase, with refractive index \(N_p = 1.519\), in the described

in the rocks it occurs in the form of small, irregularly shaped grains among the minerals of the groundmass, in a number of cases forming large segregations (0.6–5 mm), in which inclusions of early minerals (plagioclase and quartz) are observed. These large segregations of orthoclase essentially represent porphyroblasts that arose in the process of replacement of early rock-forming minerals. In all cases orthoclase distinctly penetrates the rock along intergranular spaces and cracks in the early minerals (Figs. 3–5). All these phenomena clearly indicate its later separation. This pattern is most clearly revealed by artificial staining of orthoclase in polished specimens and thin sections.

Biotite crystallized after orthoclase, and its development was metasomatic in character. This is indicated by the facts of penetration of biotite in the form of xenomorphic segregations into the intergranular spaces and cracks of all the minerals described above (Fig. 3–6). At the same time, biotite, owing to its high crystallization capacity, often forms more or less regular platy segregations up to 2 mm in size. According to the optical constants \(N_g = 1.639\), \(N_p = 1.584\), and \(N_g - N_p = 0.055\), the mineral described, according to V. E. Treger, corresponds to a composition of 45% annite and 55% phlogopite. Along with this, biotites with a higher annite content, reaching 55%, are noted. The pleochroism of biotite is yellow-green along \(N_m\), and brownish-brown along \(N_g\). The mineral described is replaced by muscovite, epidote, and most energetically by chlorite, especially in the case of strong deformation of biotite grains. During replacement by muscovite, magnetite is released and sagenitic lattices are formed. In biotite grains, inclusions are often observed of well-formed apatite crystals, zircon with pleochroic halos, and accumulations of small irregular grains of sphene, as well as poikilitic intergrowths of hornblende.

The results of spectral analyses of biotite showed the presence of copper (0.001–0.003%), nickel (trace–0.001%), cobalt (trace–0.001%), titanium—0.01%, chromium (0.001–0.006%), vanadium (0.01–0.03%), manganese (0.03–0.06%), and barium (trace–0.03%).

Accessory minerals. Most often in the rocks there are observed sphene, apatite, and zircon (Table 2). They occur as separate, well-formed crystals and their aggregates. Spatially, the accessory minerals listed above occur mainly as inclusions in biotite or in the immediate vicinity of accumulations of the latter. In some cases, apatite inclusions can be observed in grains of plagioclase and quartz. Accessory minerals are often concentrated in the form of a “chain” in the intergranular space of the rock-forming minerals (Fig. 3–8). The foregoing leads us to the conclusion that the accessory minerals arose during the final stages of crystallization of the melt, mainly after the formation of orthoclase and biotite or close to the time of crystallization of the latter.

Spectral analyses of monomineralic fractions showed the presence in sphene of lead (trace–0.001%), copper (trace), arsenic (0.006–0.06%), zirconium (trace–0.3%), chromium (0–0.003%), manganese (0.01–0.1%), and germanium (0–0.001%); in zircon—traces of lead, copper, and nickel, titanium (0.001–0.03%), manganese (up to 0.003%), niobium (up to 0.03%), and hafnium (up to 0.1%).

Comparing the quantitative mineralogical composition of the rocks on diagrams (I. V. Ginzburg et al., 1962; V. E. Gendler, 1956), among the granitoids described one may distinguish leucocratic normal granites and granodiorites. In some cases rocks of plagiogranite and tonalite composition appear (Figs. 4 and 5). Leucocratic granites are characteristic of the Bol’she-Fedorovskii stock, located in the western

Table 2 (continued)

Content of accessory minerals in granitoids of the Lyubavinsky ore zone, g/t

part of the ore field, and the granodiorites for the Evgrafovsky stock on the eastern flank of the deposit (Fig. 1).

The chemical composition of the above-mentioned varieties of granitoids is shown in Table 3 and in the diagram (Fig. 6). In comparison with Daly’s granites, the leucocratic granites are characterized, above all, by a smaller amount of femic oxides (parameter “b,” on average 3.9 versus 5.4 in Daly’s granites), increased alkalinity (ratio \(a:c = 4.1—9.6\), versus 5.4 in Daly’s granites), and also by a larger amount

Fig. 4. Diagram of the quantitative-mineral composition of rocks of the Lyubavinsky ore zone according to I. V. Ginsburg et al. (1962).
1 — Khamaro-Tyrinsky massif; 2 — Evgrafovsky stock; 3 — Lyubavinsky stock; 4 — Bolshe-Fedorovsky stock. Fields on the diagram: A — normal granites; B — granodiorites; C — plagiogranites and tonalites. The numbers of the points correspond to the serial numbers in Table 1.

of free silica (\(Q = 32.9\) versus 28.4). The granodioritic varieties are close to Daly’s granodiorites, differing in somewhat increased alkalinity (\(a:c = 3.2\) versus 2.8).

The differences between the identified groups of granitoids in trace elements are insignificant; a certain increase in the content of lead, gallium, strontium, and arsenic in the leucocratic varieties can be explained by the fact that the rocks of the Bolshe-Fedorovsky stock underwent noticeable hydrothermal alteration (Table 4).

Comparing the petrographic and petrochemical characteristics of the Lyubavinsky granitoid stocks with data on the batholithic intrusion of the area, represented by the Khamaro-Tyrinsky massif in the eastern part of the ore field, we come to the conclusion that there is a close genetic connection between these two intrusive manifestations and that the stocks should be regarded as apophyses or acmoliths of the granodioritic Khamaro-Tyrinsky massif lying at depth in this area.

Without giving here a detailed petrographic description of the rocks of this massif, which may be found in the work of Tikhomirov (1964), we shall note that the mode of occurrence, the crystallization sequence, and the quantitative-mineralogical composition of the magmatic formations under consideration are entirely analogous. The same may be said of the chemical composition and of the quantitative ratios of trace elements (Table 4). From the latter table it is clearly seen that the content of trace elements in the granitoids under consideration generally corresponds to the average

Fig. 5. Diagram of the quantitative-mineralogical composition of rocks of the Lyubavinsky ore zone according to V. E. Gendler (1956).

1 — Khamaro-Tyrinsky massif; 2 — Evgrafovsky stock; 3 — Lyubavinsky stock; 4 — Bolshe-Fedorovsky stock. Fields in the diagram: I — leucocratic rocks; II — normal rocks; III — melanocratic rocks; IV — field of basic rocks. The numbers of the points correspond to the serial numbers in Table 1.

contents of the corresponding elements in acidic rocks of the granite series (A. P. Vinogradov, 1962). The presence in the rocks of the Lyubavinsky stocks of molybdenum, zinc, arsenic, and an elevated content of copper is evidently connected with the occurrence, within the magmatic bodies under consideration, of quartz veins with gold-sulfide mineralization.

In addition, recalculating the chemical analyses of the rocks by the Niggli method, we obtained the same tonalitic type of magma for all the magmatic bodies described above (Table 5).

Finally, it should be noted that many features similar to those of the rocks described above can also be found in the granitoids of the Ilinsky deposit (Kuzmin, Shubin, 1966).

All the magmatic formations discussed above belong to the well-known Kyrinsky complex, widely distributed in Central Transbaikalia, and are attributed to the “post-ore” or “post-inversion” type of granitoid magmatism (Kuznetsov, Yanshin, 1967).

Table 3

Chemical composition of granitoids of the Lyubavinsky ore zone

Numerical characteristics according to A. N. Zavaritsky

Numerical characteristics according to A. N. Zavaritsky

Continuation of Table 3

Fig. 6. Diagram of the chemical composition of granitoid rocks of the Lyubavinsky ore belt (according to A. N. Zavaritsky):

1 — Khamaro-Tyrinsky massif; 2 — Evgrafovsky stock; 3 — Lyubavinsky stock; 4 — Bolshie-Fedorovsky stock. The vector numbers in the diagram correspond to the serial numbers of the analyses in Table 3.

Table 4

Content of trace elements in granitoids. (1 = 1·10⁻²)

Origin of the Granitoid Rocks

In this section, considerations are set forth concerning the sequence of formation of the rock-forming minerals and of individual types of rocks genetically related to one another, as is evident from the material presented above.

In the study of the granitoids, attention was directed to the sequence of crystallization of the rock-forming minerals and to those phenomena of autometasomatism that took part in the formation of the magmatic rocks. For this purpose, a large number of thin sections and polished sections with preliminary staining were examined, which helped to clarify the interrelations of the rock-forming minerals.

Table 5

Numerical characteristics according to P. Niggli

I — Khamaro-Tyrinsky massif
II — Lyubavinsky stocks

Phenomena associated with the evolution of the magmatic melt that gave rise to the rocks described may conventionally be divided into two stages: the first is crystallization of the “principal” rock-forming components of the magmatic rocks; the second stage is potassium–magnesium autometasomatism. The first stage is closely connected with the phenomenon of crystallization differentiation (Kuzmin, 1963—1968), during which crystallization of plagioclase, quartz, and, in insignificant amounts, hornblende took place in the magmatic melt. At this stage, the minerals that had formed were in suspension in the mobile part of the melt, still largely uncrystallized. This uncrystallized part of the magma, as a result of internal dynamics and tectonic processes, had the capacity for movement, drawing plagioclase and quartz crystals into this movement. The dynamics of movements,

Obviously, it was governed by two factors: the phenomenon of concentration flows of Wulff (Kuz’min, 1964) and filter-pressing (F. Gordon Smith, 1968). As a result of the movement of such magma, there are cases of crushing and cracking of previously separated minerals, with simultaneous replacement and cementation of them by a fine-grained aggregate of newly arising minerals as a result of recrystallization of the remaining part of the melt, with formation of the so-called “groundmass.” During this period, quartz and plagioclase were separated, composing the fine-grained groundmass. As a result, rocks with a porphyritic texture arose, which is most characteristic of the apical parts of massifs and of small bodies. The porphyritic segregations here are usually represented by relatively large segregations of early quartz and plagioclase minerals, bearing distinct traces of dissolution, crushing, replacement, and cementation by their aggregate of groundmass minerals. Recrystallization of the melt that yielded the groundmass, judging from the character of mineral development, took place under conditions of a sharp decrease in temperature, an increase in supersaturation, and the mass appearance of crystallization centers. In the deeper parts of magmatic bodies, recrystallization proceeded in a calmer setting, with the formation of comparatively uniformly grained rocks. Ultimately, a magmatic rock was formed that in composition corresponds to plagiogranites (trondhjemites).

At the second stage, in a rock that had not yet had time to cool sufficiently and consisted of minerals that had arisen at the first stage, potassium–magnesian autometasomatism begins to develop, with the formation of orthoclase, biotite, and accessory minerals. Aggregates of these minerals develop in the intergranular space, being superimposed on the groundmass and replacing all earlier minerals, of which relicts not infrequently remain.

The manifestation of autometasomatism was associated with the separation, in the course of development of the intrusive chamber and the formation of plagiogranites, of a residual melt. The early influence of derivatives of the residual melt was manifested in the development of orthoclase, whose distribution in the magmatic rock was uneven, in the form of streams of various scale with replacement of earlier-formed minerals and, in a number of cases, with the appearance of large porphyroblasts bearing poikilitic inclusions of plagioclase and quartz. That accumulation of the feldspar component occurs toward the end of crystallization of the melt, and that migration of the potassium-feldspar component takes place in the intergranular spaces of the rock with obvious elements of metasomatic replacement of early minerals, was also noted by Tuttle (O. F. Tuttle, 1953) and Mehnert (K. Mehnert, 1963). Depending on the facies conditions of orthoclase recrystallization, it was manifested as xenomorphic grains or their aggregates, which is characteristic of apical parts or small bodies; or, together with xenomorphic grains, it forms large segregations with inclusions of fragments of early rock-forming minerals.

Not infrequently, in granitoid massifs one can observe a spatial transition from areal or zonal development of potassium feldspar into vein-like bodies represented by aplite–pegmatites or quartz–feldspar and feldspar veinlets. It is not excluded that the formation of a number of dikes or dike-like bodies of acidic composition will, in time, correspond to the period of alkaline or silica-alkaline autometasomatism occurring during the establishment of parent magmatic bodies.

Following the recrystallization of the groundmass, orthoclase begins to separate out biotite, aggregates of which develop in the intergranular space of early minerals, being superimposed on the groundmass with obvious

features of metasomatic replacement. In places, large porphyroblasts arise, containing poikilitic inclusions of plagioclase, quartz, and orthoclase. An uneven influx of solutions causing biotitization also leads to uneven growth of biotite grains, which most often have sharply xenomorphic outlines and penetrate with peculiar “tongues” into the main mass of the rock and the minerals composing it.

Statistical observations show that accessory minerals are spatially closely associated with biotite. Accumulations or individual grains of zircon, sphene, ilmenite, and apatite, as a rule, occur in biotite or near its grains. Thus, determining the place of biotite in the general process of crystallization of the magmatic body resolves the important question of the time of formation of the accessory minerals.

The development of autometasomatic processes with the formation of a certain amount of orthoclase, biotite, and other minerals ultimately leads to the formation of rocks corresponding to granites, biotite granites, or granodiorites.

Taking into account, in general, the uneven autometasomatic development of biotite and orthoclase within the total volume of the original plagiogranitic composition of the massif (in individual hand specimens the amount of orthoclase ranges from 10 to 55%), we may expect, at one and the same hypsometric level of the exposed area of the magmatic body, plagiogranites, granites, and granodiorites. These circumstances should be taken into account when mapping magmatic bodies.

For granitoid rocks, the main classificational and genetic parameters are the quantitative ratios of the salic minerals; their use for physicochemical interpretation is expanded by applying the “albite–orthoclase–quartz–water” system (Tuttle, Bowen, 1958). This system can be widely used for petrological conclusions in the study of natural rocks. On

Fig. 7. Diagram of the normative compositions of granitoids of the Lyubavinskaya ore belt.
1 — Khamaro-Tyrinsky massif; 2 — Evtrafovsky stock; 3 — Lyubavinsky stock; 4 — Bolshoi-Fedorovsky stock; A — field of normative compositions (statistical maximum) of “standard” granites and rhyolites with albite + orthoclase + quartz totaling more than 80%; B — field of quartz crystallization; V — field of plagioclase crystallization; G — field of potassium feldspar crystallization. I — cotectic line quartz–potassium–sodium feldspar at water-vapor pressure of 500 bar; II — cotectic line quartz–potassium–sodium feldspar at water-vapor pressure of 4000 bar; \(M_1, M_2, M_3, M_4, M_5\) — positions of the triple-minimum points at water pressures of 500, 1000, 2000, 3000, and 4000 bar, respectively (Tuttle, Bowen, 1958). \(\alpha\) — line of change in melt composition during the separation of rock-forming minerals.

this diagram (Fig. 7) also reflects fairly clearly the sequence of separation of rock-forming minerals discussed above. Thus, the points for rocks of the Khamaro-Tyrin massif and the Evgradovka and Lyubavinsky stocks lie in the field of crystallization of plagioclase and quartz. The moment at which either of these begins to separate will depend on the pressure of water vapor. As quartz and plagioclase separate, the composition of the melt will approach the ternary minimum, which is represented by the point for rocks of the Bol’she-Fedorovsky stock. This circumstance indicates the possibility of simultaneous separation of quartz, plagioclase, and potassium feldspar during the final stages of development of the melt, which ends with potassium–magnesium autometasomatism. The latter, manifested unevenly in different granitoid bodies, leads to the formation of the diverse magmatic rocks characteristic of the Lyubavinsky ore field.

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