ON ALKALI FELDSPARS IN SYENITOIDS OF THE KONDOM COMPLEX AND THE TEMPERATURE OF THEIR CRYSTALLIZATION

G. M. IVANOVA

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

Alkali potassic feldspars are the principal rock-forming minerals of all rocks of the syenite series of the Kondom magmatic complex. Therefore, determination of their composition is of great importance in considering the genesis of alkaline rocks, all the more so because they reflect the cooling conditions and the geological features of the formation of the intrusion. In this connection we give a brief summary characterization of the alkali feldspars based on the investigations we have carried out.

The optical constants presented in Table 1 indicate a fairly persistent constancy and similarity of the optical properties of the alkali feldspars of different syenites, irrespective of their composition and locality within the district. Thus, the angle \(2V\) lies within the limits from \(-72^\circ\) to \(-(80—82^\circ)\). The content of the albite molecule in potassic feldspar, according to optical data, as a rule varies from 23 to 26–27%. Determinations of the minerals by the Fedorov method showed that in the syenitoids microclines are most often present and, more rarely, orthoclases.

For more precise diagnosis of the minerals, three complete silicate analyses were made of potassic feldspars (selected as carefully as possible) from the alkaline syenites of the Tashtagol, Kochurin, and Taenzin massifs (Table 2). The obtained contents of components in weight percent were recalculated into molecular amounts, and then into the composition of potassic feldspar. The resulting data indicate contents in them of 15; 23.3; and 26% of the albite molecule.

The remaining chemical analyses fell to perthites and to potassic feldspars albitized to varying degrees, as a result of which analyses of samples 410, 064a, 065, and 1 showed in their composition a mixture of the minerals orthoclase and albite (Table 3).

In addition, the elevated content of the sodium and calcium components in the chemical analyses of potassic feldspars given in Table 3 is apparently connected with the presence in a number of cases of plagioclase Nos. 14–30, which cannot always be eliminated when selecting a monomineralic fraction. The increased content here of the sodium component is apparently connected primarily with intense albitization that occurred in the syenitoids, which made sample selection extremely difficult.

The miscibility of \(KAlSi_3O_8\) and \(NaAlSi_3O_8\), forming solid solu-

Optical properties of nepheline feldspars in syenitoids of the Kondom gabbro–syenite complex

Table 1

Table 2

Chemical composition of alkali feldspars in the syenitoids
of the Kondom gabbro-syenite complex

Table 3

Chemical composition of microperthites in the syenitoids
of the Kondom gabbro-syenite complex

Note: sample 410—from the Tashtagol massif,
sample 1—from the Shalym massif.
samples 065, 064ᵃ—from the Kochura massif.

Table 4

Crystallization temperature of some potassic feldspars
(after K. Fogg and E. Spencer)

Note: the table is taken from the work of O. A. Bogatikov (1966).

… and representing the composition of potassium feldspars, depends, as a rule, on temperature and certain other conditions. The probable crystallization temperatures of some alkali feldspars according to K. Fogg and E. Spencer are given by us in Table 4.

If one proceeds from the chemical composition of the potassium feldspars, then our samples, containing from 15 to 26% of the albite molecule, and, by optical properties, 25—27% of the albite molecule, belong, according to this table, to rather high-temperature varieties formed at temperatures of 750—950°.

The crystallization temperatures of the analyzed alkali feldspars can be determined somewhat more precisely by using T. F. Barth’s two-feldspar “thermometer” (1962), later refined by I. D. Ryabchikov

Fig. 1. Relationship between temperature and the degree of distribution of albite between alkali feldspar and plagioclase (after Barth, 1962). Points—distribution coefficients ($K$) in syenites of the Kondom gabbro-syenite complex.

Table 5

Alkali content and molecular compositions of some alkali feldspars (without taking account of the anorthite component) and the corresponding compositions of the granitoids from the syenitoids of the Kondoma gabbro-syenite complex

(1964). This method is based on the assertion that, in equilibrium mineral associations, the mole fractions of albite in alkali feldspar and in coexisting plagioclase, as well as the pressure and temperature of mineral formation, are related by a single-valued dependence. T. F. Barth constructed a graph of the dependence of the albite distribution coefficient \((K)\) on temperature. The coefficient \((K)\) is equal to the ratio of the mole fraction of albite in alkali feldspar to the mole fraction of albite in calc-alkali feldspar. Below, in Table 5, we present the molecular compositions of the alkali feldspars and the compositions of the plagioclases coexisting with them, and calculate the albite distribution coefficient \((K)\). The values of \(K\) were then plotted on T. F. Barth’s diagram (Fig. 1). The points obtained fall in the region of rather high temperatures—from 860 to 700°, and in only one case near a temperature of 540°.

Because of the absence of the necessary experimental data, T. F. Barth, in constructing the diagram, neglected a number of factors and, in particular, the influence of pressure, assuming that the ratio of the concentrations of the albite component in the two feldspar phases, being a function of temperature, depends little on the chemistry of the medium.

Later I. D. Ryabchikov (1963) processed a series of experimental data and, taking into account the results obtained, constructed the corresponding dependence graph (Fig. 2). When plotted on this graph, our data are located mainly in the region between the two isotherms corresponding to temperatures of 700–600°, and only one point falls near a temperature of 530–540° (Fig. 2).

As T. F. Barth and I. D. Ryabchikov noted, the temperature obtained from both graphs is the minimum possible temperature at which equilibrium was still established between the coexisting feldspars. Depending on a number of factors—in particular, the rate of cooling of the magma, the size of the mineral grains, and the presence and composition of the interstitial fluid—the temperature may deviate toward lower values from the crystallization temperature of the rock.

Thus, from the foregoing the following conclusions may be drawn:

Fig. 2. Isotherms of distribution of the albite component in coexisting feldspars (after I. D. Ryabchikov, 1964), \(N_1\)—mole fraction of albite in alkali feldspar; \(N_2\)—mole fraction of albite in calc-sodic feldspar; points—feldspars from syenites of the Kondom gabbro-syenite complex.

1. The potassic feldspars from the alkaline and lime-alkaline syenites of the studied massifs of the Kondom gabbro-syenite complex are characterized by considerable similarity in their optical and chemical properties.

2. Determination of the crystallization temperature of the alkaline feldspars by the two-feldspar thermometry method indicates rather high crystallization temperatures of the syenites of the Kondom gabbro-syenite complex, namely temperatures on the order of 860–700°. The relatively lower temperatures (700–600° and, in one case, 540°) are apparently associated with the superposition of residual postmagmatic solutions on the minerals.