On Distinctive Plicative-Disjunctive Structures of Gabbroids in Hyperbasites

A. G. Bakirov

(Presented by the Department of Mineralogy and Crystallography)

On the surface of the erosional section of the Kempirsai hyperbasite massif, elongated bodies of gabbroids are observed. What do they represent—dikes or remnants of a gabbro roof? With what tectonic structures are they associated—with plicative or disjunctive ones? This question is the subject of our article.

The Kempirsai massif of ultrabasic rocks is located in the Southern Urals within the Aktobe Region. It is composed of peridotite and dunite serpentinites. In their exocontact zone gabbroids are observed in the form of an interrupted border (Fig. 1)¹. In the hyperbasites numerous vein formations occur, of which gabbro-diabases are the most developed.

In most cases the gabbroids are represented by hornblende and actinolitized gabbros, gabbro-amphibolites. Among them occur gabbros, gabbro-norites, olivine gabbros, and gabbro-diorites. Both melanocratic and leucocratic varieties are encountered. The textural and structural features of the gabbroids vary. Massive, banded, gneiss-like, and schistose gabbros occur, with the latter three developed predominantly in the contact zones with the hyperbasites. Gradual transitions are observed between many varieties of gabbroids, especially between hornblende gabbros and gabbro-amphibolites.

The gabbroids, on the one hand, and the hyperbasites, on the other, are in complex and contradictory relationships with one another. A taxitic structure is noted in their contact zone, where separate xenoliths of gabbro occur in ultrabasic rocks, as well as hyperbasites in gabbroids. In outcrops one can see veins of banded and gneiss-like gabbros in hyperbasites and, conversely, veinlets of ultrabasic rocks in gabbro. At the same time, gradual transitions are often observed from serpentinites to gabbroids through olivine gabbro, forellensteins, and plagioclase serpentinized peridotites. In the zone of mutual contact between gabbroids and hyperbasites, both types of rock are altered: an increased degree of serpentini-

¹ The geological scheme of the massif shown in the figure was compiled by the author of the article from the data of personal surveys and materials of the Kempirsai geological party.

zation of ultrabasic rocks; in addition, chlorite and amphibole appear in them; in the gabbroids we see newly formed actinolite, epidote, garnet, spinel, more rarely chlorite, sphene, and apatite.

Fig. 1. Geological sketch map of the Kemptirsai hyperbasite massif:
1 — hyperbasites, 2 — gabbroids, 3 — rocks enclosing and overlying the hyperbasites and gabbroids, 4 — disjunctives. Gabbroid belts:
AA — Buranovskaya, BB — Shcherbakovskaya, VV — Dzharlybutakskaya,
GG — Kyzylkainskaya.

We take into account the close spatial connection between hyperbasites and gabbroids, the presence of gradual transitions between them, and also of taxites in their contact zone. These facts testify to a genetic connection between the two mentioned types of rocks and give grounds for assuming the simultaneous intrusion of ultrabasic and basic magmas in the form of a single differentiated magmatic complex. The gabbroic magma, occupying a peripheral position in the intrusion, began to solidify

first. A consequence of this circumstance was the earlier formation of the gabbroids as rocks relative to the peridotites and dunites. At the same time, completion of the crystallization of the gabbro magma apparently occurred later than the beginning of solidification of the ultrabasic melt, which caused the mutually injectional relations of the gabbroids and hyperbasites in their contact zone, as noted by us above.

The massif is situated in the axial part of the southern end of the Ural-Tau anticlinorium, composed mainly of rocks of the Proterozoic, Lower Paleozoic, Silurian, and Devonian. The axis of the anticlinorium passes through the middle of the area. The anticlinorium is complicated by second-order structures. Overturning of folds to the east is observed. Latitudinal and meridional faults of the anticlinorium are noted. Thus, the eastern limb of the Ural-Tau anticlinorium is cut off by the meridional Borodinovka normal fault, passing through the eastern part of the area. The northern end of the massif adjoins a meridionally elongated fault-type dislocation in the metamorphic sequence, extending beyond the limits of the area. Both disjunctives are of pre–Lower Devonian age. The Borodinovka normal fault was repeatedly renewed later as well, in particular in post-Paleogene time.

Disjunctives of normal-fault and normal-fault–strike-slip type are developed in the hyperbasites. They split the massif into a number of tectonic blocks along cracks of latitudinal, or nearly latitudinal, strike. The fractures are emphasized by numerous gabbro-diabase dikes. Reverse faults and small thrusts of NW and NE strike occur, imparting a scaly structure to individual parts of the massif.

The magnitude of the dip angle of the planar banding of the hyperbasites and gabbroids in the western parts of the massif reaches 50–60°; in the eastern, central, and northern parts of the massif it is 25–30°. In both cases the dip is westward, with an almost meridional strike. The gabbroids occur on both the footwall and hanging-wall sides of the hyperbasites. Large folded structures are observed in the ultrabasic rocks. They are marked by planar banding and by chromite bodies, which in most cases have a western and eastern dip in different parts of the massif with an almost meridional strike. In the cores of the synclines of these structures, gabbroids are localized, forming elongated bands in the erosional section of the hyperbasites. Thus, in the northern, narrowest part of the massif there is one fold, and in the southern, widest part there are three (Fig. 1). Smaller folded forms can also be observed in outcrops of hyperbasites. Their amplitudes are measured in centimeters, meters, and tens of meters. The dip angles of the limbs of folds in the hyperbasites are from 25 to 60°.

On the basis of the foregoing, we regard the hyperbasites and gabbroids as an intrusive body folded into folds. In our opinion, it is confined to a tectonically complicated contact surface of Precambrian and Cambro-Ordovician sequences on one side and Silurian–Lower Devonian on the other. In this connection we cannot agree with A. A. Nepomnyashchikh [3], who interprets the Kempirsai massif as an almost horizontally lying intrusive body with approximately parallel lower and upper boundaries.

On the territory of the massif there are four gabbroid bands: the Buranovka, Shcherbakovka, Dzharlybutak, and Kyzylkain bands, elongated mainly in the meridional direction. The length of the Kyzylkain band is 4 km, of the Buranovka band 14 km, and of the Shcherbakovka band 15 km. The width of the first is from 50 to 200 m, of the second from 150 to 350 m, and of the third from 100 to 200 m. The Dzharlybutak band consists of two meridionally elongated groups of gabbro bodies with a total extent of

about 3 km, situated at a distance of 4 km from one another; its width is from 50 to 200 m. Each belt includes from three to six large gabbroid bodies, elongated in the form of a chain and sometimes accompanied by smaller bodies (satellites), most often located next to the large ones. The small gabbro bodies have a lenticular form. Their dimensions in erosional section are measured in tens of meters across. As drilling has shown, they gradually pinch out and in places even at depths on the order of 15–20 m. Locally, especially in the Buranovskaya belt, the gabbroid bodies are broken into separate tectonic blocks, somewhat displaced relative to one another along disjunctives, chiefly of latitudinal strike. Within the belts the gabbroids form bodies dipping either westward at angles from 40 to 65° (the Shcherbakovskaya and Buranovskaya belts), or eastward at angles of 60–80° (the Kyzylkainskaya belt). Tectonic contacts of the gabbroid bodies with the hyperbasites enclosing them are widespread.

We believe that the gabbroids located in the exocontact zone of the hyperbasite massif, as well as those composing dike-like bodies in the form of belts among serpentinites, are essentially one and the same formations. This is indicated by the commonality of their composition and texture. The Shcherbakovskaya belt is an obvious tongue-like projection of gabbroids located in the roof of the massif. The Kyzylkainskaya belt passes along strike into the gabbro-amphibolites of Mamyt.

At the same time, the gabbroids of the belts as a whole are characterized by a great diversity of composition and of textural and structural features. These traits are most sharply manifested in the Buranovskaya belt. In it, among banded and gneiss-like gabbroids, varieties of massive texture, medium-grained, fine-grained, and in places also porphyroid structure, are encountered. They are represented by gabbro, gabbro-norites, and norites, which sometimes form veinlets in the banded and gneiss-like gabbroids. Gabbro-pegmatites are also noted. Within the gabbroid bodies in the area of the Buranovskaya gabbro belt, veins of hyperbasites are locally encountered, cutting the banded gabbros.

We suppose that in the initial stage of the volcanic cycle a zone of intermediate structure was established in the tectonically weakened zone of the Ural-Tau anticlinorium, in the area of the territory of the Kempirsai district. Subsequently, during Middle Devonian tectogenesis, when the ultrabasic and gabbroic magmas had for the most part recrystallized, crumpling occurred of the Precambrian and Paleozoic rocks, which had also undergone earlier folding. Together with them, the gently lying intrusive deposit of ultrabasic rocks was folded, along with the accompanying gabbroids situated on the footwall and hanging-wall sides of the hyperbasites. In the process of crumpling, owing to the considerable plasticity of the serpentinites, synclinal downwarps of the roof of the gabbroic rocks were formed. The roots of these downwarps, during the process of fold formation, played the role of tectonic wedges. In a number of places they split the hyperbasites and led to the formation of fractures. Along these, later, in the concluding stage of the Middle Devonian volcanic cycle, fluids penetrated from depth. They caused metasomatic alteration of the gabbroids, and possibly also of the hyperbasites. This led to the formation of massive and porphyroid gabbros, gabbro-norites, and norites, as well as gabbro-pegmatites. Hence the diversity of composition and of textural and structural features of the rocks of the aforementioned gabbro belts, especially the Buranovskaya.

Subsequently, in the Variscan cycle of tectogenesis, the disjunctive dislocations established in the hyperbasites were emphasized still more sharply,

in the Middle Devonian, new ruptures of the massif also arose. As a result of a complex and prolonged tectonic history, the roots of synclinal downwarps of gabbro in the hyperbasites acquired dike-like forms with steep angles of dip and tectonic contacts with serpentinites, as we see them at the present time. At the same time, the banding and gneiss-like character of the gabbroids were emphasized. Consequently, in the present case, the gabbro bands in the hyperbasites constitute distinctive plicative structures of synclinal downwarps of the gabbro roof, which pass into disjunctive ones under the conditions of folding of the hyperbasite–gabbro intrusive body. As a formulation of the question, we propose to call these structures plicative-disjunctive.

On the northern continuation of the Buranovka band of gabbroids there is a zone of tectonic disruption that extends beyond the limits of the district. Farther north, along the strike of the Kyzylkain band, runs the Borodino fault. These facts testify to the connection of plicative-disjunctive structures in the hyperbasites with disjunctive structures manifested in the rocks enclosing the hyperbasites, and to the influence of the latter upon the former; that is, in the case described we observe the adaptation of synclinal downwarps of the gabbro roof to the disjunctive structures of the surrounding medium and the transition of the synclinal structures of the gabbroids, in the course of their historical development, into disjunctive ones. This circumstance should be taken into account when identifying the structures mentioned in geological conditions similar to those of the Kempirsai district.

As is evident from the foregoing, the elongated gabbro bodies within the Kempirsai hyperbasite massif represent more complex formations than gabbro xenoliths [1] or ordinary remnants of an ancient roof [2], as they are often regarded.

Many silicate-nickel ore occurrences of the district are confined to areas of the weathering crust of hyperbasites that lie in the contact zone with gabbroids forming bands in the erosional section of ultrabasic rocks. Herein lies the practical importance of identifying gabbro plicative-disjunctive structures in hyperbasites. They are potentially ore-bearing for silicate nickel where preserved areas of an ancient weathering crust are present, even if relatively small, within the area of development of ultrabasic-rock massifs situated in geological conditions close to those of the Kempirsai district.

LITERATURE

1. Aleshkov A. N. On the relationships between ultrabasites and gabbroids. In: Collection dedicated to the 75th anniversary of the birth and the 45th anniversary of the scientific activity of Academician D. S. Belyankin. Publishing House of the Academy of Sciences of the USSR, 1943.

2. Loginov V. P., Pavlov N. V., Sokolov G. A. Chromite-bearing character of the Kempirsai ultrabasic massif in the Southern Urals. Chromites of the USSR, vol. II, Publishing House of the Academy of Sciences of the USSR, 1940.

3. Nepomnyashchikh A. A. Study of the form and dimensions of the Kempirsai ultrabasic massif. Sov. Geol., No. 9, 1959.