Some Features of the Chemistry of the Chezima Gabbro-Plagiogranite Complex of Gornaya Shoria

V. I. Salomatin

(Presented by the seminar of the Department of Petrography, GRF)

The Chezima gabbro-plagiogranite complex consists of several massifs, small in area, located in northeastern Gornaya Shoria in the basin of the Fedorovka River. The massifs are composed of rocks of identical composition, among which granites and plagiogranites predominate. The most representative of the massifs, the Chezima massif, has an area of about 6 sq. km. The main body of the massif is composed of granites and plagiogranites. On the western side it is bordered by a band of gabbroic rocks up to 300 m wide. At the contact of the gabbroic rocks with the granitoids, hybrid rocks—diorites and granodiorites—are developed. Dike rocks are widely distributed within the massif; these are divided into a group of diaschistic rocks and a group of aschistic rocks. The massif is surrounded by a band of contact-metamorphic rocks, among which skarns of varied composition develop in separate areas.

By age, we assign the Chezima complex to the Upper Cambrian—Ordovician.

To clarify the features of the chemical composition of the rocks of the complex, 41 chemical analyses were collected, of which 35 were made from our samples in the central laboratory of ZSGU and 6 were taken from the production report of A. N. Krivorotenko. All analyses were recalculated by the method of A. N. Zavaritsky.

According to Peacock’s classification, the rocks of the complex belong to the lime-alkaline series of rocks. The Peacock alkali-lime index for them is 55.2.

In chemical composition the rocks are very diverse. Among them, basic rocks are distinctly separated from granitoid rocks, which confirms the existing viewpoint regarding the two-phase formation of the massifs.

The silica content in the rocks varies from 43.83% to 57.75%, with alkalies ranging from 1.8% in the basic rocks to 9.8% in the acidic ones. In the first phase of intrusion, in the basic rocks there is an almost linear increase in alkali content at the expense of sodium. This increase in sodium caused the appearance of monzonites and gabbro-monzonites, in which sodium enters into the composition of microcline. Along with sodium, the role of ...

potassium. The alkali content fluctuates especially in the hybrid rocks, which results from the very uneven distribution of potassium feldspar in them. In granites and plagiogranites the role of potassium increases, but sodium always predominates over it, which is associated with a fairly high content of the albite component in the plagioclases.

The sum of alumino-alkalis varies chiefly from 20% to 25%. With increasing acidity the sum of alumino-alkalis increases, but at a silica content above 64% it begins to decrease. As alkalis and silica separate in the magma, the amount of calcium and magnesium increases; accumulation, and then removal, of total iron and magnesium takes place. This is manifested in magnesian-ferruginous metasomatism.

These are the most general features of the evolution of the chemistry of the rocks.

Granitoid rocks are characterized by supersaturation with silica. They are moderately rich in, or poor in, alkalis. The content of feldspar lime varies within small limits. In all rocks sodium predominates over potassium. Supersaturation with alumina is observed very rarely. Ferrous iron and magnesium occur in approximately equal amounts.

Diorites and granodiorites have an increased content of alkalis and feldspar lime. In some samples an elevated magnesium value is observed, owing to the presence of magnesian hornblende, the content of which varies sharply in these rocks. The silica content also varies strongly.

Basic rocks are characterized by an increased content of feldspar lime. The amount of iron and magnesium is equal in some rocks; in most cases, however, magnesium predominates over iron. Sodium in all rocks sharply predominates over potassium.

The group of diabases and dike rocks is characterized by a significant role of potassium in the composition, although sodium still predominates. The amount of alkalis sharply exceeds the content of feldspar lime.

Aschist rocks possess the same chemical features as the corresponding rocks of the massif that enclose them.

The group of metasomatic rocks is completely distinct. In them magnesium sharply predominates; alkalis are absent. The iron content increases considerably, with ferric iron greatly exceeding ferrous iron. The small calcium content in the rocks is connected with the presence of carbonate in the form of dolomite and calcite, and partly with apatite.

Thus, having considered certain features of the chemical compositions of the rocks of the gabbro-plagiogranite complex, the following conclusions may be drawn:

1. From the entire diversity of rocks by chemical composition, two groups are distinguished: a group of gabbroids and a group of granitoids. Between them are situated hybrid rocks. As products of metasomatic reworking, metasomatic formations occupy a separate position.

2. For all rocks of the complex, in comparison with average rock types, an increased content of alkalis is characteristic; these alkalis have a sodium specialization.

3. As the acidity of the rocks increases, the value of the alkali parameter gradually rises and the value of feldspar lime decreases, which is characteristic of the normal course of magma differentiation.

4. The rocks of the gabbro-plagiogranite complex are united into a lime-alkaline series of rocks of the Lassen Peak volcano type.