PETROCHEMICAL CHARACTERISTICS OF ROCKS OF THE LEBED PLUTON

(GORNY ALTAI)

V. I. Yarygin

(Presented by Prof. A. M. Kuzmin)

The geology and petrography of the pluton under consideration have been described by the author in earlier works, which are in press.

The main features of the geological structure of the pluton are as follows (in brief).

The Lebed pluton is confined to the southern part of the Kondoma–Lebed mobile zone, which is a transitional belt between the southwestern plunge of the Shor anticlinorium and the northeastern limb of the Uimeno–Lebed synclinorium.

The Kondoma–Lebed mobile zone is composed predominantly of metamorphosed effusive and effusive-sedimentary Cambrian deposits, which substantially overlap the carbonate deposits of the Shor anticlinorium. Middle Paleozoic, chiefly Devonian effusive-terrigenous deposits make up the Uimeno–Lebed synclinorium. The Lebed pluton is structurally fitted within the limits of the Kondoma–Lebed mobile zone. In plan it is a linearly elongated batholithic body trending in a meridional direction. Its host-rock boundaries are established within narrow limits—from the Middle Cambrian to the Lower Ordovician.

Various rocks participate in the structure of the Lebed pluton, among which two separate groups are distinguished. Rocks of basic composition form a series of independent bodies at the mouth and headwaters of the Yaman-Sadra River, in the lower course of the Sadra River, and in the area of the settlement of Suronash. A rather broad range of rocks takes part in their composition, the principal ones being pyroxenites, gabbro, hornblende gabbro, and gabbro-diorites. The other varieties (hornblendites, monzonites, gabbro-norites) have a sharply subordinate significance. All varieties are connected with one another by gradual transitions.

Granitoids, which in their areal distribution greatly exceed the gabbroids, are single-phase formations and are likewise connected with one another by gradual transitions. In the southern part of the pluton, plagiogranites, trondhjemites, and tonalites are distinguished, while in the north there are granodiorites and quartz monzonites. Quartz diorites and diorites are rare rock types and represent products of related hybridism of basic rocks and acid magma. The dike complex is very poor. According to the degree of genetic relationship, two groups of dikes are distinguished. The first group includes plagioaplites, aplites, and granite porphyries. Dikes of the second group predominate over dikes of the first group and are characterized by a certain diversity: quartz

diabases, microgabbro, spessartites, odinites; diabases and pyroxene-plagioclase porphyrites occur more rarely.

The petrographic varieties cited clearly indicate the complex composition of this natural association of rocks, with a sharp “polarity” from ultrabasic to acidic types. Petrochemically, they are closely related to one another and form a single genetic series. The petrochemical investigations are based on 37 complete silicate analyses carried out on our samples in the chemical laboratory of the complex thematic expedition of ZSGU.

The petrochemical regularities of the rocks are well illustrated by the vector diagram (Fig. 1), constructed according to the method of A. N. Zavaritskii [1]. It shows the compositions of the pluton rocks and the average types of igneous rocks according to Daly, as well as variation lines connecting the points of the rocks of the Lebed pluton and the points of average rock types according to Daly. The diagram is more conveniently analyzed by the behavior of the variation lines, which smooth out particular deviations in rock compositions caused by processes of contamination and assimilation during formation of the pluton.

A noteworthy fact is the closeness in position of the variation lines of the rocks of the Lebed pluton and of the average types of igneous rocks according to Daly. This indicates that the rocks of the pluton belong to the group of rocks of the calc-alkaline series. However, local mutual deviations of the variation lines and the character of the arrangement of the vectors indicate substantial differences in the chemical composition of the pluton rocks and Daly’s average types, which determine the petrochemical features of the rocks of the Lebed pluton.

The chemical compositions of the plagiogranites of the pluton (analyses 1–6) are characterized by an elevated content of silica and feldspathic lime and by a reduced content of alumina and alkalies; moreover, sodium predominates among the alkalies in comparison with Daly’s average granites. These features are reflected in the diagram by the fact that, on the ASB projection, the origins of the vectors are situated closer to the vertex S, their inclination is rightward and steep, and on the CSB projection it is almost vertical. The quartz monzonites and granodiorites of the pluton are located on the diagram closer to the SB axis and farther from the vertex S on the ASB projection than the corresponding average rock types according to Daly, which indicates a lower content of SiO₂ and alkalies and enrichment in iron and magnesium. In addition, they are poorer than the corresponding average rocks according to Daly in alumina (the presence of the parameter “C”) and richer in feldspathic lime (in the left part of the diagram, the points of the granodiorites and quartz monzonites are located to the left of the points of the average compositions of granodiorites according to Daly).

Quartz diorites and tonalites are, in almost all parameters, analogous to the corresponding rocks of the average types according to Daly. The parameter m¹ has a somewhat lowered-anomalous value, indicating a smaller iron content in comparison with the average rocks according to Daly.

The diorites of the Lebed pluton have an elevated content of femic components (a large parameter “v”) and, correspondingly, a low content of SiO₂ (analyses 20, 22). Gabbro is characterized by an extremely inconsistent composition. Most of the rocks (analyses 23, 24, 26, 27), having a high content of the oxides of iron, magnesium, and calcium (a large value of “v” and “c”) and a low content of alkalies, display an eucritic character. Another part (analysis 25) of the gabbro is enriched in silica. In the pyroxenites and ferrogabbro there is an almost complete absence of alkalies and a higher SiO₂ content than in Daly’s diallagites and wehrlites. The dike rocks, in chemical composition, generally correspond to their intrusive analogues.

Fig. 1. Diagram of the chemical compositions of rocks of the Lebed pluton (after A. N. Zavaritskii): 1–6—leucocratic plagiogranites and trondhjemites, southern part of the pluton, samples 1487, 1494, 1581, 1542, 125/65, 1525; 7–13—medium-grained quartz monzonites, granodiorites, samples 323, 52, 329, 46, 1630, 41/65, 140/65; 14–17—medium-grained aplite-granites, aplites, plagioclase aplites, granite-porphyries, samples 1092, 1075, 1540б, 1644; 18–22—tonalites, quartz diorites, diorites, gabbro-diorites, samples 1498, 1559, 65, 1472б, 54/65; 23–26—gabbro, gabbro-norites; 27–29—pyroxenites, ferro-gabbro, kosvites, samples 104/65, 1080, 1612б; 30–31—lamprophyres (odinites, spessartites), samples 123, 312; 32–37—dikes of intermediate and basic composition (porphyrites, diabases), samples 187а, /65, 122/65, 1495, 1517а, 1472а, 68. Average types of igneous rocks after Daly: g—granites, Qm—quartz monzonites, gd—granodiorites, Qd—quartz diorites, d—diorites, go—gabbro, og—olivine gabbro, Pr—pyroxenites. I—variation line of rocks of the Lebed pluton; II—variation line of the average rock types after Daly.

From the foregoing, the following conclusions may be drawn:

1. Almost all rocks of the Lebed pluton are characterized by an elevated silica content, in comparison with Daly’s average rock types.

2. All the rocks have a sodic specialization of the alkalies, with the exception of granodiorites and quartz monzonites.

3. In its chemistry the pluton should be assigned to the calc-alkaline series of rocks.

4. Almost all rocks of the pluton contain less alkali than Daly’s average rocks.

5. The ratio FeO : MgO in the rocks of the pluton increases with increasing acidity of the rocks.

6. The group of hybrid rocks (quartz diorites, diorites) occupies an intermediate position on the diagram between gabbroids and acid rocks (vectors 20, 21, 22).

LITERATURE

1. A. N. Zavaritskii. Recalculation of chemical analyses of igneous rocks. Gosgeoltekhizdat, 1960.