Intrusive Complexes in the Northeastern Part of Mountain Shoria

V. I. Salomatin

(Presented by Professor S. S. Ilyenok)

In the northeast of Mountain Shoria, within the mobile zone that borders the Mrass median massif from the north, magmatic formations are widely developed. Among them we distinguish gabbro-diorite and gabbro-plagiogranite complexes, forming an almost continuous arc-shaped chain of massifs of small area. On the basis of geological data and absolute age, a Late Cambrian–Ordovician age is accepted for the gabbro-plagiogranite complex, and a Middle–Late Cambrian age for the gabbro-diorite complex.

The complexes are characterized by their distinctive features in the petrographic composition of the rocks composing them, and by the peculiarities of their chemistry and origin.

Gabbro-diorite Complex

It consists of two massifs—the Bazas and the Ortotag massifs. The Bazas massif is situated in the lower course of the Bazas River and occupies an area of 15 sq. km. The entire area of distribution of the massif is composed of rocks of the diorite series: diorites and quartz-bearing diorites, quartz diorites, and melanocratic diorites. Among these rocks, in the form of irregular patches, basic rocks are distributed: pyroxene-hornblende gabbros, norites, and gabbro-norites. Closer to the marginal and apical parts of the massif, acidic rocks develop in the form of lens-shaped and dike-like bodies: plagiogranites, aplites, and pegmatites. In the northeastern endocontact zone, a large number of thin quartz veins are observed. In the very center of the massif, in the form of narrow bands and thin dikes, amphibolites, hornblendites, quartz porphyries, and albitophyres occur.

The Ortotag massif is located 1 km southwest of the Bazas massif. Its area is 7 sq. km. The massif is composed of various diorites, among which melanocratic varieties predominate. In the southwestern part of the massif gabbros are distributed, which in the west are in contact with pyroxenites and hornblendites. Acidic rocks form veinlets in the apical parts of the massif. In composition they are analogous to the rocks of the Bazas massif.

At the contact of the massifs with the enclosing rocks, metamorphic rocks are developed, among which skarns occur in limited amounts.

The intermediate rocks consist of plagioclase, up to 50%; hornblende, up to 40%; and quartz, up to 10%. In quartz diorites the quartz content reaches 20%. The secondary minerals are epidote, chlorite, and sericite. The accessory minerals are represented by isolated grains of apatite, sphene, and magnetite. In melanocratic diorites the hornblende content is 65–70%.

A dioritic texture predominates in the rocks. Alongside it, elements of diabase and gabbro textures are encountered.

Plagioclase varies in composition from acid oligoclase No. 13 to labradorite No. 65. Hornblende is represented by the ordinary green variety. Quartz, in xenomorphic grains, fills the interstices between other minerals. In many samples it forms large accumulations of grains.

Gabbro rocks are characterized by a gabbro texture with elements of ophitic texture.

They include plagioclase No. 49–52, up to 50%; augite or, in norites, hypersthene, up to 40%; and ordinary hornblende, up to 20%. As a secondary mineral, green chlorite develops on hornblende and pyroxenes. The accessory minerals are represented by small, isolated grains of apatite and magnetite.

To study the chemical characteristics of the massifs, 55 chemical analyses were carried out, recalculated by the method of A. N. Zavaritsky. These analyses show how the chemical properties gradually change from gabbro to diorites. The rocks are characterized by sharp fluctuations in silica content, a smooth increase in alkali content from gabbro to diorites, and, on average, a uniform content of feldspar lime.

A distinct position is occupied by acid rocks supersaturated with alumina.

The entire series of rocks is characterized by an elevated value of the alkali parameter, a lowered content of feldspar lime, and a predominance of sodium over potassium.

In their origin, the diorites of this complex belong to metasomatic rocks formed through recrystallization and metasomatic replacement of earlier intruded basic rocks. The following facts support this point of view: the presence among the diorites of irregular patches of gabbro and norites; relics of gabbro and ophitic textures in diorites; the variable composition of plagioclases; the presence, at contacts with gabbro, of melanocratic diorites and amphibolites; the widespread development of low-temperature minerals—chlorite, epidote, and sericite; and the uneven distribution of minerals in the rocks.

Toward the northeast, the gabbro-diorite complex is replaced by the gabbro-plagiogranite complex.

Gabbro-plagiogranite Complex

It consists of a series of small massifs, 4–6 sq. km in area, located in the basin of the Fedorovka River. The most representative of them are the Chezim and Pistek massifs.

The main field of the Chezim massif is composed of granitoid rocks, which in the western, southwestern, and northeastern parts are bordered by a belt of gabbroid rocks.

In the Pistek massif, basic rocks make up a significant part of the area. The group of basic rocks includes pyroxene gabbro—

hornblende and leucocratic gabbros, norites and gabbro-norites, quartz monzonites, essexites, anorthosites, gabbro-anorthosites.

Despite such a diversity of rocks, they have a number of common features. They are characterized by a gabbroic texture, and the monzonites by idiomorphism of plagioclase relative to microcline. The composition of the plagioclase varies within narrow limits and corresponds to labradorite No. 60–65. Among the mafic minerals, pyroxene predominates; in the monzonites and norites it is represented by pinkish hypersthene, and in the other rocks by greenish augite. In smaller quantity, with the exception of the pyroxene-hornblende gabbros, hornblende is present, represented in most rocks by the common variety, and in the essexites and leucocratic gabbros by the alkaline variety. In individual patches and in intergrowths with hornblende, biotite is present.

Accessory minerals: apatite, magnetite, sphene.

Secondary minerals: epidote, chlorite, sericite, calcite, which develop unevenly after plagioclase and hornblende.

The group of acid rocks includes granites, plagiogranites, and granodiorites. The rocks are characterized by a hypidiomorphic-granular texture and acid zoned plagioclase (No. 30–33 in the core and No. 11–18 at the margins).

Among the mafic minerals, common green or subalkaline hornblende predominates; ferruginous biotite is present in subordinate quantity.

At the contact between acid and basic rocks, hybrid rocks of dioritic composition develop.

Dike rocks of various composition are widely developed in the massifs: granite-porphyries, grano-syenites, syenites, diabases, dolerites, spessartites, odinites, and others.

The massifs are surrounded by a belt of contact-metamorphic rocks, among which skarns of various composition develop in individual areas.

To clarify the features of the chemical composition of the rocks, 41 chemical analyses were carried out. By chemical composition, two groups are distinguished: the gabbro group and the granite-plagiogranite group. Hybrid rocks are situated between them. All rocks of the complex are characterized by an increased content of alkalies, which have a sodium specialization. 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.

In our observations we adhere to the view of previous investigators concerning the two-phase formation of the massifs. In the first phase, gabbroids formed; in the second phase, the emplacement of granites and plagiogranites took place.

Such are the most general features of the gabbro-diorite and gabbro-plagiogranite complexes distinguished within a single structural-tectonic zone.