Paragenetic Relationships of Minerals Depending on the Chemical Potentials of Alkalis for the Batanayul Alkaline Rock Massif

A. A. Mityakin

(Presented by the scientific seminar of the Department of Petrography)

The Batanayul massif of alkaline rocks is located in the northern part of the Kuznetsk Alatau along the Batanayul spring, a left tributary of the middle course of the Uryup River. The total area of the massif exposed at the surface is about 5.5 km², with maximum dimensions of 1.2 km across and 5 km along strike.

The geological position of the region that includes this area is determined by its location within the marginal Devonian stage of the Kuznetsk Alatau, bordering in the north on the Chulym–Yenisei depression. The region is composed of Upper Proterozoic and Lower Paleozoic deposits, overlain by a thick sedimentary-volcanogenic sequence of the Lower and Middle Devonian. All these sequences are intruded by a large number of intrusive massifs of alkaline nepheline-bearing rocks.

The nepheline rocks of the Batanayul massif were first discovered in the form of bereshites by M. M. Grunina and K. V. Ivanov during geological mapping work in 1940. Subsequently it was visited by M. V. Voroshilov (1957) and Yu. D. Skobelev (1959). However, until recently the massif had been poorly studied. The bereshites were considered a constituent part of the Devonian sedimentary-effusive sequence. In petrographic descriptions, only bereshites and nephelinites were noted.

As a result of field investigations carried out by the author in 1965, facts were established confirming the intrusive character of the bereshite body, which occurs in the form of a sill:

1. At the boundary with the enclosing rocks, a chilled zone with a glassy groundmass of the rock is observed in the bereshite body.

2. The internal structure of the bereshite sill is characterized by the presence of three zones in the direction from the floor to the roof, distinguished by the character of the included minerals and by bulk chemical composition. These differences are caused by an uneven rate of cooling, clearly manifested under near-surface conditions, as well as by gravitational differentiation that occurred in place.

3. In the enclosing rocks there is a system of fractures conformable with the contact planes of the bereshite sill with the enclosing rocks.

The massif is characterized by a varied assemblage of rocks, forming the following series, beginning with the first products of differentiation: labradorite bereshite, andesine bereshite, teschenite-porphyrite, nepheline monzonite, nepheline syenite, lime-alkaline hornblende ...

and alkaline leucocratic syenites. All the rocks are characterized by a clearly expressed porphyritic structure—the size of the grains of the phenocryst minerals exceeds the grain size of the groundmass by tens of times, which is caused by the rapidly changing thermal regime under near-surface conditions. These same conditions did not favor any appreciable interaction of the magma with the enclosing rocks at the site of final recrystallization, except for the loss of a certain amount of volatiles. Nor are there facts indicating any noticeable effect of later injections of magma on the early products of differentiation. The only exception is the process of formation of the alkaline potassium syenites, with which the feldspathization both of the enclosing rocks and of the earlier-formed rocks of the massif is connected. The foregoing makes it possible to conclude that the composition of the rocks of the massif, formed in successive stages, closely approximates the composition of the magma that arrived in separate portions from deeper horizons of the Earth’s crust, and to trace the course of differentiation of the magmatic melt at depth. A study of the structures of the rocks, taking the above facts into account, shows that nepheline-bearing rocks and, in particular, nepheline arose from a magmatic melt as a result of the normal course of crystallization. The following features of the mineralogical composition are characteristic of the massif as a whole:

1. A large role of calcium is observed in the formation of femic and salic minerals. In bereshites—early products of differentiation—despite considerable undersaturation with silica, olivine is absent. Instead of it, monoclinic pyroxene of the diopside group is formed. In these same rocks, despite their high alkalinity, plagioclase of high basicity (No. 70) is present. The fact of the complete replacement of nepheline by cancrinite and zeolites, and the presence of free carbonate, also indicates the large role of CaO.

2. Alkaline aluminosilicates played the chief role in the formation of the rocks of the massif. At the same time, a decrease in the role of sodium alkalis from the early differentiates toward the late ones, and an increase in the activity of potassium alkalis in the same direction, can be clearly traced. Thus, in the labrador and andesine bereshites, as well as in the tinguaite-porphyries, potassium does not form independent minerals. Its content reaches 2%, whereas Na₂O amounts to 6–7%. Beginning with nepheline monzonite, the role of potassium alkalis becomes dominant. In the monzonite, plagioclase is present in subordinate quantity relative to feldspar. In the nepheline syenite, the latter is the only mineral among the feldspars. In the nepheline-free alkaline syenites, feldspar likewise plays the principal role.

3. In the direction from the early products of differentiation to the late ones, the CaO content in the rocks of the massif decreases and SiO₂ increases.

Proceeding from the features of the mineralogical composition, it may be concluded that the course of the mineral-forming process in the massif was determined to a significant degree by the chemical activity of the alkalis. Using the methods of constructing diagrams for multicomponent systems developed by D. S. Korzhinskii and V. S. Sobolev, it is possible to depict graphically how the chemical activity of individual components and the parageneses of minerals changed.

The principal mineral-forming components of the rocks of the massif are Na₂O, K₂O, CaO, SiO₂, Al₂O₃, MgO, FeO, P₂O₅.

Silicate analyses of the rocks also show a significant content of CO₂ and H₂O (the content of each of these components reaches 4%). However, these components are included in the composition of cancrinite and zeolites—minerals formed in the postmagmatic stage; therefore they will not be included in the calculation.

Taking into account the presence of highly potassic minerals, as well as a large amount of free carbonate, CaO is taken to be an excess component. As determinations of the optical constants of pyroxenes show, their composition varies rather strongly; however, this is caused mainly by the behavior of CaO and MgO, and also of Na$_2$O when aegirine-augite appears. Iron oxide enters into the composition of pyroxenes in limited amounts, and the greater part of the iron is separated in the form of magnetite. Therefore FeO is taken as an isomorphous component. Assuming, on the basis of the foregoing, that Na$_2$O and K$_2$O are quite mobile component-parameters determining the process of mineral formation, and taking P$_2$O$_5$ as an isolated component, since this oxide enters only into the composition of apatite and does not affect the paragenetic relationships of the minerals, we obtain three inert virtual components—SiO$_2$, Al$_2$O$_3$, Mg(Fe)O.

Thus, all oxide components of the system may be arranged in order of decreasing chemical activity in the following series: Na$_2$O, K$_2$O, SiO$_2$, Al$_2$O$_3$, Mg(Fe)O, CaO, P$_2$O$_5$.

In view of the impossibility of determining the real composition of the rock-forming minerals because of their replacement by secondary minerals, and the difficulty of obtaining monomineral fractions for chemical analyses, for further calculations of the mineral-forming reactions the compositions of all minerals—diopside (Dp), aegirine-augite (Eg), hornblende (Rog), plagioclase (Pl), potassium feldspar (Ksh), nepheline (Nf)—are taken as standard.

Let us now consider the behavior of the system as a function of the chemical potentials of the alkalis. With the adopted distribution of oxide components, one can calculate a series of reactions for constructing monovariant bundles tied into a definite multisystem. Denoting the diopside-free, nepheline-free, and other nonvariant nodes as [Dp], [Nf]..., we write all possible mineral-forming reactions:

\[ [\mathrm{Dp}] \]

\[ \begin{aligned} 1)\quad &(\mathrm{Eg})\; 8\mathrm{Pl} + 2\mathrm{K}_2\mathrm{O} + 2\mathrm{Na}_2\mathrm{O} \rightarrow 4\mathrm{Ksh} + 8\mathrm{Nf},\\ 2)\quad &(\mathrm{Ksh})\; 9.25\mathrm{Pl} + 0.25\mathrm{Rog} + 8.65\mathrm{Na}_2\mathrm{O} \rightarrow 4\mathrm{Eg} + 14.38\mathrm{Nf},\\ 3)\quad &(\mathrm{Pl})\; 8.5\mathrm{Ksh} + 0.5\mathrm{Rog} + 12.6\mathrm{Na}_2\mathrm{O} \rightarrow 8\mathrm{Eg} + 10.25\mathrm{Nf} + 4.6\mathrm{K}_2\mathrm{O},\\ 4)\quad &(\mathrm{Nf})\; 14.38\mathrm{Ksh} + 0.5\mathrm{Rog} + 9.9\mathrm{Na}_2\mathrm{O} \rightarrow 8\mathrm{Eg} + 10.25\mathrm{Pl} + 7.2\mathrm{K}_2\mathrm{O},\\ 5)\quad &(\mathrm{Rog})\; \mathrm{Ksh} + 2\mathrm{Nf} \rightarrow 2\mathrm{Pl} + 0.5\mathrm{K}_2\mathrm{O} + 0.5\mathrm{Na}_2\mathrm{O}. \end{aligned} \]

\[ [\mathrm{Eg}] \]

\[ \begin{aligned} 1)\quad &(\mathrm{Ksh})\; 2.5\mathrm{Pl} + 0.5\mathrm{Rog} + 1.25\mathrm{Na}_2\mathrm{O} \rightarrow \mathrm{Dp} + 4.75\mathrm{Nf},\\ 2)\quad &(\mathrm{Pl})\; 2.5\mathrm{Ksh} + \mathrm{Rog} + 1.25\mathrm{Na}_2\mathrm{O} \rightarrow 8\mathrm{Dp} + 4.5\mathrm{Nf} + 1.25\mathrm{K}_2\mathrm{O},\\ 3)\quad &(\mathrm{Nf})\; 4.75\mathrm{Ksh} + \mathrm{Rog} + 0.125\mathrm{Na}_2\mathrm{O} \rightarrow 8\mathrm{Dp} + 4.5\mathrm{Pl} + 2.38\mathrm{K}_2\mathrm{O}. \end{aligned} \]

\[ [\mathrm{Nf}] \]

\[ \begin{aligned} 1)\quad &(\mathrm{Ksh})\; 4.75\mathrm{Eg} + 1.5\mathrm{Rog} \rightarrow 10.75\mathrm{Dp} + 2\mathrm{Pl} + 5.75\mathrm{Na}_2\mathrm{O},\\ 2)\quad &(\mathrm{Pl})\; 4.55\mathrm{Eg} + \mathrm{Rog} + \mathrm{K}_2\mathrm{O} \rightarrow 10.25\mathrm{Dp} + 2\mathrm{Ksh} + 5.5\mathrm{Na}_2\mathrm{O},\\ 3)\quad &(\mathrm{Rog})\; \mathrm{Eg} + \mathrm{Pl} + 0.75\mathrm{K}_2\mathrm{O} \rightarrow 0.5\mathrm{Dp} + 1.5\mathrm{Ksh} + 0.62\mathrm{Na}_2\mathrm{O}. \end{aligned} \]

\[ [\mathrm{Rog}] \]

\[ \begin{aligned} 1)\quad &(\mathrm{Ksh})\; 0.5\mathrm{Eg} + 1.5\mathrm{Nf} \rightarrow 0.25\mathrm{Dp} + \mathrm{Pl} + \mathrm{Na}_2\mathrm{O},\\ 2)\quad &(\mathrm{Pl})\; \mathrm{Eg} + \mathrm{Nf} + 0.25\mathrm{K}_2\mathrm{O} \rightarrow 0.5\mathrm{Dp} + \mathrm{Ksh} + 1.5\mathrm{Na}_2\mathrm{O},\\ 3)\quad &(\mathrm{Nf})\; \mathrm{Eg} + \mathrm{Pl} + 0.75\mathrm{K}_2\mathrm{O} \rightarrow 0.5\mathrm{Dp} + 1.5\mathrm{Ksh} + 1.25\mathrm{Na}_2\mathrm{O}. \end{aligned} \]

Theoretically, for this six-mineral system one can construct six five-mineral nonvariant nodes. But under the given physicochemical conditions of mineral formation only four nodes retain stability, forming a stable multisystem. The nepheline-free and plagioclase-free nodes form between themselves a metastable multisystem not connected with the first.

Of all sixteen fields of parageneses obtained as a result of constructing the multibeam diagram (Fig. 1), for the six-mineral system under consideration the fields No. 16, No. 5, and the group of fields 6—7—8—9—10—11 are of interest. The three-mineral paragenesis diopside–plagioclase–nepheline is characteristic of the early products of differentiation—bereshites and teralite-porphyries. As is evident from the diagram, this paragenesis is characterized by the highest sodium potential and the lowest potassium potential. The peculiarity of this paragenesis consists in the fact that here, despite the high activity of sodium, high-calcium minerals are present—diopside and plagioclase No. 70. This circumstance emphasizes the great role of calcium in the process of mineral formation.

In the group of fields 6—7—8—9—10—11 there is present the three-mineral paragenesis plagioclase of intermediate basicity—38-hornblende–potassium feldspar, characteristic of nepheline-free lime-alkaline syenites. The diagram shows that this paragenesis with hornblende is characterized by medium and approximately equal values of the sodium and potassium potentials. With an increase in the sodium potential, hornblende is replaced by diopside in field No. 16, and with an increase in the potassium potential, instead of hornblende there appears aegirine-augite and diopside of field No. 5. The association of aegirine-augite with nepheline and kali-feldspar without plagioclase (field No. 5) is characteristic of nepheline syenites. This field lies in the region of the greatest value of the potassium potential and the smallest sodium potential. It is interesting that aegirine-augite appears in this region with the greatest potassium potential. This circumstance is evidently explained by the fact that here, at high potassium activity, the formation of plagioclase becomes impossible; therefore sodium goes into the construction of the molecule of aegirine-augite and nepheline. Of the feldspars, only the potassium feldspar is present here. For the diagram as a whole, a division of all fields by the lines (Hbl, Di, Aeg) into two groups is characteristic—the group without plagioclase (right-hand part of the diagram) and without kali-feldspar (left-hand part of the diagram). The joint occurrence of kali-feldspar and plagioclase is therefore possible only in a narrow zone along the line (Di, Aeg, Hbl). This indicates that, between the potassium and sodium alkalis, there was competition in the process of magmatic differentiation and mineral formation. An increase in the activity of one of the alkalis caused a decrease in the other, which was immediately reflected in the character of the parageneses (presence of plagioclase or kali-feldspar). What has been said fully corresponds to the character of the actual parageneses—plagioclase and kali-feldspar occur together only in nepheline monzonite and lime-alkaline syenite, where the alkali potentials were approximately equal. In all the other rocks, only one of the named minerals is present.

Thus, the general character of the mineral parageneses in the rocks of the massif is determined by a gradual decrease in the sodium potential from the first products of differentiation to the last, and by an increase in the potassium potential in the same direction.

The question arises as to the causes of the mobilization of sodium and the formation of alkaline, nepheline-bearing rocks in the early stages of differentiation. Chemical analyses of the rocks show a rather high content of volatile components in the rocks of the massif. Thus, in bereshites the average con—

the content of CO₂ reaches 1.30%, and H₂O, 4.79%. If one takes into account that the massif formed under near-surface conditions, as a result of which most of the volatiles were lost, then one may imagine what role volatiles played in magmatic differentiation. In this connection it may be assumed that a reaction took place, as indicated by P. O. Daly (1936):

\[ \mathrm{NaAlSi_3O_8 + CaCO_3 = Na_2CO_3 + Ca_3Al_2Si_3O_{12} + CaSiO_3 + CO_2,} \]

i.e., when magma is charged with carbonate, formation is possible of an easily volatile mineral—Na₂CO₃—which tends to move toward the roof of the magmatic chamber. This effect may be expressed still more sharply when a large quantity of water is present. At the roof of the chamber the following reactions occur:

\[ \mathrm{NaAlSi_3O_8 + Na_2CO_3 = NaAlSiO_4 + Na_2SiO_3 + CO_2,} \]

\[ \mathrm{CaAl_2Si_2O_8 + Na_2SiO_3 = NaAlSiO_4 + CaSiO_3,} \]

and even when the temperature falls sufficiently low:

\[ \mathrm{CaAl_2Si_2O_8 + Na_2CO_3 = NaAlSiO_4 + CaCO_3} \]

The enrichment of the rocks of the Batanayul massif in CO₂ and CaCO₃, and the presence of high-calcium minerals, provide grounds for assuming an interaction of the original magma with carbonate rocks.