DENSITY OF IGNEOUS ROCKS AND ITS SIGNIFICANCE FOR PETROLOGY

A. M. Kuz’min

If we turn to works on the physicochemical foundations of petrography, we do not find in them attempts to connect the composition and density of rocks into a single interrelated whole. True, P. P. Chirvinskii, using a large number of examples of granites he had studied, showed that the density of a rock is most closely connected with the quantitative composition of the rock-forming minerals, and that the specific gravity of igneous rocks is, to a certain extent, their characteristic \[2\].

The same investigator, dwelling on cosmogonic and geophysical considerations, says that, in order to clarify the geological role of granites in the structure of the terrestrial globe, it is necessary to know the significance of specific gravity in the differentiation of magma; and this problem, as he believed, can be solved only on the condition of broad use of average figures, so as to avoid erroneous conclusions \[2\].

Our founders of petrography—F. Yu. Levinson-Lessing, A. K. Zavaritskii, D. S. Belyankin, and others—also did not touch upon the question under consideration. H. Rosenbusch in Descriptive Petrography and R. A. Daly in Igneous Rocks and the Depths of the Earth give figures for specific gravities and analyses for many intrusive rocks and rock-forming minerals, but neither author draws any ensuing conclusions concerning the role of rock density.

Among the diagrams characterizing the variation of a series of igneous rocks both chemically and mineralogically, we do not find diagrams of the same order constructed with allowance for their density.

In order to form an idea of the role of rock density in relation to their chemical—and therefore also their mineralogical—composition, a group of igneous intrusive rocks was taken that is most widely distributed and for which, on the basis of a large number of analyses, data are known on the average density, D g/cm³, of the rocks and on their average chemical composition. Here we shall consider both in relation to granites, granodiorites, diorites, gabbros, and peridotites (Table 1).

With reference to the figures just given, let us construct a diagram (Fig. 1) in which on the ordinate we plot average data on rock density from 2 to 3.5 g/cm³, and, parallel to the abscissa, at levels of density corresponding to 2.6, 2.64, 2.75, 2.85, 2.42, and 3.32 g/cm³, the average density values characterizing the average composition of alkaline gra-

Table 1

syenites, alkaline-earth granites, granodiorites, diorites, gabbros, and peridotites.

We shall divide the abscissa into 100 equal parts, corresponding in total to 100%. At the density level corresponding to one or another rock, we shall lay off segments equal to the number of percent of the content of the components \( \mathrm{SiO_2} \), \( \mathrm{TiO_2} \), \( \mathrm{Al_2O_3} \), \( \mathrm{Fe_2O_3} \), \( \mathrm{FeO} \), \( \mathrm{MgO} \), \( \mathrm{CaO} \), \( \mathrm{Na_2O} \), \( \mathrm{K_2O} \), \( \mathrm{H_2O} \), and \( \mathrm{P_2O_5} \).

The ends of the segments of one and the same component at adjacent density levels are connected, as they say, by curves. As a result of such a construction we obtain a diagram (Fig. 1) in which the fields of the corresponding components of igneous rocks are delineated by curves.

Since density in the terrestrial sphere increases from its surface toward the center, we shall arrange the constructed diagram so that the density value increases from top to bottom, as shown in the figure.

Fig. 1. Diagram of the regularity of the relationship between the density of igneous rocks and the percentage content in them of the principal compositional components.

From an analysis of the diagram, what first attracts attention is the regularity and smoothness of the change in the fields of the corresponding components in passing from one rock to another, from gabbro–peridotites to granites. This feature of the regular change in chemical composition with depth is evidently closely connected with the Earth’s force field and is strictly determined by the law of gravity and its particular expression—the hypsometric law. As for diagrams illustrating the regu-

regularity of the qualitative and quantitative changes in the mineralogical composition of the same series of magmatic rocks, they can be found, for example, in G. W. Tyrrell’s textbook of petrography [1], but the diagrams he gives should be interpreted in accordance with Fig. 1.

Diagrams similar to Fig. 1 can be constructed for a number of well-studied basic rocks, for example for the dolerites of the island of Tasmania, occurring as thick sills of 45–285 m. Their chemical composition was studied at various levels from the floor to the roof. For example, from the diagram (Fig. 2) for the dolerite sill of Mt. Nelson [3] it follows that the chemical composition of the rock changes markedly and regularly from bottom to top, as in the case of Fig. 1; consequently, its density must also change in the same direction.

Fig. 2. Change in the bulk chemical composition of the Mt. Nelson dolerite sill within 45–285 m of its thickness.

Finally, I shall allow myself to present one more diagram and section of the Lugar sill [1], studied by G. W. Tyrrell (Fig. 3), and in this case it is clearly seen that the inner part of the sill, 26 m thick, underwent gravitational differentiation, as if it had stratified. The density and mineralogical composition within each differentiate increase from top to bottom; in the same direction, the total density of the entire differentiated part of the sill increases.

From the factual material presented, it follows quite convincingly that, along with the study of the quantitative chemical minera-

logical analysis of rocks one should provide for them data on density, which, as follows from Fig. 1, Table 1, and the data of P. P. Chirvinsky [2], is to a certain extent their characteristic, their constant.

Fig. 3. Regularities in the change of the mineralogical composition and density of the rocks of the Lugar intrusive body in a vertical section (4).

The regularity, indicated by the diagrams, in the change of the density of intrusive rocks with depth is a function of gravity, and it should be manifested in every deep-seated magmatic body, i.e., change according to the hypsometric law expressed by the formula:

\[ h=\frac{RT\ln p_0/p_h}{Mg} =\frac{RT\ln c_0/c_h}{m_hNg} =\frac{RT\ln n_0/n_h}{m_hNg}. \]

There is every reason to expect that, in individual magmatic chambers, the composition of rocks with depth \((h)\) will change from granites in the apical part of the massif to gabbro-peridotites at the base.

REFERENCES

1. Tyrrell G. V. Principles of Petrology, OATI, 1932.

2. Chirvinsky P. P. Quantitative, mineralogical, and chemical composition of granites and greisens. Moscow, 1910.

3. Edwards A. B. Differentiation in the dolerites of Tasmania. In: Geology and Petrography of Trap Formations. Foreign Literature Publishing House, 1950.