On the Geology of the Kempirsai Hyperbasite Massif

A. G. Bakirov

Introduction

The Kempirsai hyperbasite massif is located on the territory of the Novorossiisk and Stepnoy districts of Aktyubinsk Oblast, Kazakh SSR, 70 km south of the city of Orsk, Chkalov Oblast.

The massif is spatially confined to the Or–Ilek watershed, which is the southern continuation of the Guberlinsk Mountains of the Southern Urals. The watershed itself is a hilly projection of the ancient metamorphic series of the Urals, the so-called Ural-Tau zone. The massif is elongated in a north–northwestern direction. Its length is 70 km. Its width varies from 3 to 30 km. The area of the massif is 1100 km². A satellite of the Kempirsai pluton is the small Mamyt massif in the northeastern part of the study area.

The hyperbasite pluton is composed of apoperidotite and apodunite serpentinites, bordered along the periphery by gabbroids. The enclosing sequences are metamorphic rocks of the Precambrian and Cambrian, and schists and effusives of the Silurian and Devonian. The cover sequence consists of Upper Cretaceous and Paleogene rocks of marine facies, and sandy-clayey deposits of the continental Paleogene and Neogene.

Until recent years this massif was little known in the literature. Attention was first drawn to it in 1930–33.

A number of researchers have carried out geological investigations of individual parts of the massif and neighboring areas; however, there are almost no published materials on the geology of the district.

Among those who carried out mapping on the territory of the massif, mention should be made of A. A. Petrenko, A. K. Konev, Yu. P. Kurazov, and N. P. Kheraskov.

A number of works belong to A. N. Aleskov, who considers the gabbroids of the Kempirsai intrusive complex to be earlier than the peridotites.

V. P. Loginov and N. V. Pavlov [6] investigated the structure and chromite-bearing character of the massif. Certain questions of the tectonics of the district were elucidated by A. D. Zinovkin [5].

Works by I. I. Ginzburg [3; 4] and his collaborators, A. N. Aleskov [1], A. A. Filimonova and D. N. Kofanov [7], as well as other researchers, are devoted to the characterization of the processes of ancient weathering of ultrabasic rocks.

Physical-Geographical Sketch

The work area is situated on the Or–Ilek watershed. This watershed is a dissected hilly upland with an average absolute elevation of 420 m. The elevation of individual parts of the study area varies from a maximum of 507 m to a minimum of 340 m.

Within the area of the Kempirsai massif it is possible to distinguish two main categories of relief, represented by a gently rolling plain and by low hills.

The plain relief is expressed in areas where Quaternary, Tertiary and, in part, Cretaceous deposits are developed. Low hills occur where igneous and metamorphic rocks crop out at the surface. Within the low-hill terrain there are remnants of an ancient pre-Cretaceous plateau. These remnants are elevated, dissected areas of the terrain with a preserved weathering crust.

The area is treeless. Shrub vegetation (karagainik) and small thickets of alder, birch and poplar are confined exclusively to riverbanks and ravines. In the settlements there are artificial plantings of poplars.

From the Or–Ilek watershed originate the tributaries of the Or and Ilek rivers. The rivers of the western part of the area—the Kok-pekty and Ku-agach—flow into the Ilek. The rivers of the eastern and southern parts of the area—the Tas-Sai, Dzharla-Butak, Kyzyl-Kain, Mamyt, and Kairakty—flow into the Or.

The rivers of the study area in most cases flow across the strike of the structures, more rarely along the strike. A confinement of certain river valleys to tectonic faults of latitudinal strike (the Kyzyl-Kain, Mamyt, Shandasha rivers) and meridional strike (the Tas-Sai and Dzharla-Butak rivers) is indicated. The type of drainage network is pinnate, and in places rectangular.

In rivers flowing in a latitudinal direction, an asymmetrical structure of their valleys is clearly expressed. The southern slopes are more exposed and steeper than the northern ones. The slopes of the river valleys are convex throughout their entire length. In the mouth sections the river valleys have the form of canyons and narrow gorges, whence the geomorphological contrasts between the mountainous landscape in the mouth sections of the river valleys and the plain over which the rivers flow.

Here and there along the Tas-Sai and Kyzyl-Kain rivers there occur “hanging” dry valleys of former tributaries. Alluvial deposits in the river valleys are poorly developed.

The sharply continental climate of the study area, strong winds, the small amount of precipitation, and the low absolute humidity of the air almost throughout the entire year are extremely unfavorable factors for the formation of groundwater. Under these conditions its recharge is difficult and occurs mainly in June and July through infiltration of water along fractures in the rocks during the period of maximum precipitation and the greatest values of absolute humidity, and, in part, in spring after the melting of snow.

Geological-Stratigraphic Outline

In the present outline the principal stratigraphic, petrographic, and tectonic features of the area of our investigations are briefly considered.

Stratigraphy

In the study area we distinguish four main complexes of rocks. The first of them is represented by various metamorphic formations of the Precambrian and Cambrian; the second by basic effusives and shales of the Silurian and Devonian; the third by rocks of the gabbro-hyperbasite intrusive complex; the fourth by unconsolidated marine and continental sediments of the Mesozoic and Cenozoic.

Ancient Metamorphic Complex of the Precambrian and Cambrian

This complex includes three sequences of rocks composed of siliceous, sericite, actinolite-chlorite, and graphite schists. These are: (a) the Kairakty sequence, (b) the Dzhusaly sequence, and (c) the Shelekty sequence.

The Kairakty sequence is widespread in the eastern part of the area. It is represented by a thick complex of siliceous schists with subordinate micaceous, hornblende, and chlorite schists. The thickness of the sequence is about 5000 m.

In the upper parts of the Kairakty sequence, siliceous schists alternate with thin-bedded chlorite, zoisite-, and sericite-chlorite schists. A horizon of dark-gray fine-grained marbles about 50 m thick has been encountered.

The Dzhusaly sequence is represented by a variegated complex of rocks, among which green schists and strongly altered basic effusive rocks predominate. The thickness of the sequence is about 4500 m.

Characteristic of the entire Dzhusaly sequence is the presence of fine folding and corrugation of the schists, as well as the inconstancy of the composition of the rocks along their strike.

The complex of rocks of the Shelekty sequence has been distinguished in the western part of the surveyed area. This sequence lies in the roof of the Kempirsai pluton and is represented mainly by a siliceous-schist complex of rocks. The thickness of the latter is approximately 1200 m.

Characteristic of the Shelekty sequence is the presence of garnet in certain varieties of schists and the occurrence in its upper horizons of a member of graphite schists.

In the rocks of the Kairakty and Dzhusaly sequences there are many quartz veins, which conform to the schistosity of the rocks and often have a lenticular structure.

We did not possess data that would make it possible to determine the age of the described metamorphic sequences. By analogy with northern and southern areas, we assign the Kairakty and Dzhusaly sequences, with some reservation, to Precambrian formations, and the Shelekty sequence to Cambrian formations.

Effusive-Schist Complex of the Silurian and Devonian

This includes basic effusive rocks of the Upper Silurian—Lower Devonian, as well as Upper Devonian conglomerates, schists, and limestones.

The spilite-diabase sequence of the Upper Silurian—Lower Devonian. The rocks of this sequence are distributed in the western and southern parts of the area studied. This sequence is composed of a greenstone effusive complex of rocks with subordinate jaspers. The thickness of this suite is 1800 m.

Among the effusive rocks, spilites, diabases, diabase porphyrites, and albitophyres have been identified.

Among the greenstone complex of rocks, predominantly in the middle part of its section, occur jaspers. They are characterized by massive structure, fine grain, and variegated coloration (gray, dark gray, pink, flesh-red, and dark green). Under the microscope, in crossed nicols, rounded spots are noticeable in thin sections of the jaspers—traces of radiolarian shells. The thickness of the greenstone complex of rocks is 1200 m, and that of the jaspers is 600 m.

Spilites and diabases lie stratigraphically above the rocks of the Shelekty sequence. In turn, these basic effusive rocks are overlain by faunistically characterized deposits of the Upper Devonian. We

we consider it possible to correlate the spilite-diabase sequence with the lithologically similar Irendyk Formation of the Upper Silurian—Lower Devonian, distinguished farther north in the area studied by N. P. Kheraskov.

Conglomerate-sandstone-shale sequence. Rocks of this sequence crop out in the southwestern part of the mapped area.

Within the rock complex of the conglomerate-sandstone-shale sequence it is possible to distinguish:

1) conglomerates,
2) sandstones and finely crystalline effusives,
3) a series of clay shales interbedded with limestones,
4) a series of clay and thin-platy siliceous shales.

The conglomerates consist of rounded pebbles and poorly rounded fragments of various rocks. The sizes of the pebbles and fragments vary from 8 cm to 0.5 cm in cross section. The conglomerates contain pebbles of quartz, diabases, diabase porphyrites, spilites, quartz albitophyres, quartz porphyries, and gabbro-diabases.

The gabbro-diabases encountered among the pebbles are, macroscopically and microscopically, indistinguishable from analogous dike rocks cutting the serpentinized peridotites.

Clay and siliceous shales are represented by ordinary varieties, and the limestones by light-gray, fine-grained rocks with abundant Upper Devonian goniatite fauna (as determined by A. K. Nalivkina).

The thickness of the conglomerates is about 200 m, that of the sandstones 350 m, and that of the siliceous and clay shales with limestones about 400 m.

On the basis of the fauna, the age of this sequence is determined as Upper Devonian.

Devonian gabbro-hyperbasite intrusive complex

In our area of investigation, rocks of the gabbro-hyperbasite complex are widely distributed. Hyperbasites predominate, composing the central part of the intrusion. On their periphery are gabbroids in the form of an interrupted and bordering rim.

Hyperbasites. The ultrabasic rocks¹ are represented by apoperidotite and apodunite serpentinites. Among the peridotite varieties, harzburgite types are predominantly developed, whereas lherzolite types occur rarely.

In the contact zone with the gabbroids, amphibole peridotites are recorded.

Apoharzburgite serpentinites are dark-green rocks of gray and brown shades. They consist of serpentinized olivine and bronzite. Along with massive varieties, banded ones also occur, most often in the contact zone with the gabbroids; in these, bands enriched in pyroxene or amphibole alternate with bands depleted in it.

Apodunite serpentinites are rocks of dark-green and black color. They consist mainly of serpentinized olivine. Pyroxene and chromospinelides are present as accessory minerals.

In serpentinized dunites and peridotites, the olivine has almost entirely been transformed into serpentine. If grains of olivine do occur, they are present in amounts not exceeding 5–20%. The pyroxenes are bastitized. In the field, in most cases it is possible without particular difficulty macroscopically to estab—

establish the primary peridotitic or dunitic nature of the serpentinites, since bastite is quite clearly visible in them.

Gabbroids. The gabbroids include a rather diverse complex of rocks, represented by olivine gabbros, ordinary gabbros, hornblende gabbros, gabbro-diorites, and gabbro-amphibolites.

The most widespread are the hornblende gabbros. These varieties, like ordinary gabbros, are gray, unevenly grained rocks, rather diverse in their textural and structural features. Their coloration is also varied: from gray in the melanocratic varieties to light gray in the leucocratic ones.

In the contact zone with the hyperbasites, banded gabbros may also be encountered, in which bands enriched in plagioclase alternate with bands enriched in amphibolitized pyroxene. Massive, gneiss-like, and schistose varieties occur.

Through the hornblende gabbroids there are gradual transitions to gabbro-amphibolites, containing, in places, garnet and sphene, and to rather distinctive schistose rocks of greenish color, consisting of plagioclase and amphibole-edenite.

Olivine gabbros are confined to the contact zone of the gabbroids with the hyperbasites and are connected by gradual transitions both with them and with other rocks.

In the rocks of the gabbro-hyperbasite complex there occurs a whole series of vein formations, of which the most widespread are pyroxenites, hornblendites, gabbros, gabbro-diabases, and plagioclasites.

On the history of the formation of the intrusive complex. According to the prevailing point of view, the gabbroids¹ of the Kempirsai massif are older formations relative to the hyperbasites. These views, as far as I know, are shared by all investigators of the massif, including A. N. Aleshkov, N. P. Kheraskov, V. N. Razumova, Yu. P. Kurazova, V. P. Loginov, N. V. Pavlov, M. A. Tsibulchik, and other geologists.

We regard the gabbroids and hyperbasites as formations of the same age. Their contemporaneity is indicated, first, by the commonality of their structure. The elements of the occurrence of the planar banding and fracturing of both types of rocks, as a rule, coincide. Second, in the mutual contact of the gabbroids and hyperbasites there is not only no change in grain size in either one or the other rocks, but even transitional formations occur. Among these are the olivine gabbros, which are connected by gradual transitions both with ordinary gabbroids and with hyperbasites. Third, in the endocontact zones both the gabbroids and the hyperbasites are banded.

The presence of fine-grained varieties of gabbroids in the peripheral parts of the intrusion gives grounds to suppose that the massif began to solidify from the periphery. The variegation of textural and structural features and of the composition of the gabbroids testifies to the non-simultaneous solidification of their different parts, which took place under conditions of tectonic stresses that were not relieved uniformly within the pluton.

The subsequent recrystallization of the gabbroids that had solidified on the periphery of the intrusive complex under conditions of stress led to the appearance of crystalloblastic structures in the gabbro, to a gneiss-like character that accentuated the banding, and to schistosity; hence also the greater degree of metamorphism of the gabbroids relative to the later-solidified hyperbasites.

The peripheral position of the gabbroids in the intrusive complex accounted for the entire specificity of their textural and structural features, as well as the nature of their relationships with the hyperbasites.

The contact zone of the gabbroids with the hyperbasites, being tectonically weakened, favored the localization within it of processes of autometamorphism associated with the thermal activity of the pluton. From the depths of the intrusion came solutions containing aluminum, silica, calcium, alkalis, and titanium, which altered both these and other rocks at the contact. Hence the increased degree of alteration of the gabbroids and hyperbasites at their contact, and the appearance of secondary minerals such as actinolite, epidote, chlorite, garnet, spinel, and sphene.

Stratigraphic position of the intrusive complex. It is rather difficult to determine the age of the Kempirsai hyperbasite massif.

There are as yet no reliable data indicating the presence of intrusive contacts between the serpentinites of the Kempirsai massif and rocks of the Upper Devonian and Carboniferous. In the neighboring southern district, according to G. I. Vodorezov, serpentinites cut through faunistically characterized deposits of the Lower Devonian. Somewhat north of our area of investigation, small satellites of the Kempirsai pluton have been found to cut through faunistically characterized deposits of the Middle Devonian (oral communication of N. P. Kheraskov).

In neighboring southern districts, G. I. Vodorezov notes the presence of pebbles of serpentinites in Upper Devonian deposits. In the pebbles of faunistically characterized Upper Devonian conglomerates exposed in the area of our investigations, gabbro-diabases occur that are indistinguishable from the veins of the same type that cut the serpentinites of the Kempirsai massif.

The factual material cited above compels us to conclude that the gabbro-hyperbasite intrusive complex is connected with the Middle Devonian (Telbes) volcanic cycle. The time of emplacement was between the Middle and Upper Devonian.

Our views on the Telbes age of the Ural serpentinites, and in particular of the Kempirsai serpentinites, were set forth by us as early as 1941 in the article “On the Cycles of Tectogenesis of the Urals” [2]. We adhere to these views at the present time as well.

In recent years N. P. Kheraskov has also arrived at the opinion that the Kempirsai hyperbasite massif is of Middle Devonian age, as he stated in a personal conversation.

Most investigators of the district, including A. N. Aleshkov, N. V. Pavlov, V. P. Loginov, M. A. Tsibulchik, Yu. P. Kurazova, and others, regard the Kempirsai pluton as a Carboniferous intrusion; G. I. Vodorezov regards it as Caledonian, emplaced at the boundary between the Lower and Middle Devonian.

Complex of Unconsolidated Meso-Cenozoic Deposits

In the area of investigation, unconsolidated deposits of marine and continental facies are rather widely distributed. The former are represented by various glauconite-bearing sands, sandstones, opokas, clays, and phosphorite conglomerates of the Upper Cretaceous and Paleocene. The continental facies are represented by unconsolidated formations of the pre-Jurassic weathering crust, by variegated deposits of the Jurassic—Lower Cretaceous, Eocene, and Quaternary, as well as by Oligocene, Neogene, and Quaternary sandy-clayey deposits.

Tectonics

In accordance with the stratigraphic subdivision of the rocks, in this outline we shall consider successively the tectonics of the metamorphic and effusive-schist complex, then the structural features of the hyperbasite-gabbro pluton, and the tectonics of the Meso-Cenozoic cover deposits.

Tectonics of the metamorphic rocks

The suites of Precambrian and Paleozoic rocks make up a monoclinal sequence dipping to the west-southwest at an angle of 50–70°. This monocline is part of the western limb of the anticlinal uplift of the Ural-Tau—Or-Ilek watershed zone. The eastern limb of the anticlinorium is only partly represented in the northeastern part of the area studied, since for the most part it is cut off by the meridional Borodinovka fault. The western limb is complicated by a small syncline.

In the rocks of the Kairakty, Dzhusaly, and Shelekty formations, small subordinate folds are developed. In the upper horizons of the Dzhusaly suite, fine folding passes into cleavage. In these sequences there are also intraformational ruptures of subordinate folds, having the character of upthrow dislocations.

In the spilite-diabase sequence, in contrast to those mentioned above, subordinate folds occur more rarely, and cleavage is absent. Upper Devonian rocks are gathered into small cover-type folds, well expressed in the section along the Eissyl-Kara River. Plicative structures of the Upper Silurian and Devonian are locally complicated by small ruptures with well-expressed slickenside surfaces.

In the rocks of the Kairakty, Dzhusaly, Shelekty, and spilite-diabase sequences there occur small sill-like bodies of serpentinites and gabbroids, confined to the cores of small anticlinal folds.

Structural features of the gabbro-hyperbasite intrusive complex

The gabbro-hyperbasite intrusion is confined to the western limb of the anticlinal uplift of the Ural-Tau zone. The schists, lying in the footwall and hanging wall of the serpentinites and gabbroids, dip to the southwest at an angle of 50–70°. The planar banding of the gabbro and peridotites likewise in most cases has a southwestern dip, the angles of which vary from 60 to 90°. The attitude elements of the ore bodies of chromite iron ores coincide with the strike and dip of the primary banding of the hyperbasites.

The coincidence of the attitude elements of the planar banding of the intrusive complex and of its enclosing rocks testifies to a conformable type of occurrence of the hyperbasites and gabbro in relation to the structure of the monocline of the western limb of the anticlinal uplift of the Ural-Tau zone. This gives grounds for regarding the Kempirsai pluton as a body dipping steeply to the southwest and approaching, in its conditions of occurrence, a sheetlike interformational intrusion.

Within the massif, the zone of predominant development of apodunite serpentinites is situated in its eastern part, and the zone of apoperidotite serpentinites in its western part. There are banded areas in which the alternation of bands of dunites and peridotites is distinctly expressed. Evidently, in the process of formation of the intrusive body, the peridotitic varieties of the ultrabasic magma were differentiated toward the hanging wall of the intrusion, and the dunitic varieties, with the richest chromite deposits of the Donskoy group, toward the footwall.

Between peridotites and dunites, in most cases, there are gradual transitions. The most common are schlieren segregations of dunites in the zone of predominant development of peridotites and, conversely.

The average thickness of the hyperbasite pluton is approximately 9–10 kilometers. The greater width of the southern part of the massif relative to the northern part is apparently due to the structure of the folded enclosing strata in the area of the anticlinal uplift. It should be noted that in the southeastern part of the massif there are also some deviations in the dip of the planar banding from southwestern to steep southeastern.

In the hyperbasites, four systems of fractures have been established: the first has a northwestern strike of 340°, the second—a northeastern strike of 30°, the third—meridional and close to it, and the fourth—latitudinal and close to it. The oldest fractures are those of northwestern and northeastern strike; the youngest are the latitudinal ones. Dikes of pyroxenites, hornblendites, and plagioclasites are predominantly confined to the first two fracture systems, which represent diagonal fractures of the massif; gabbro and gabbro-diabases—to latitudinal extension fractures. Meridional fractures, unlike the above-mentioned ones, only in very rare cases serve as host structures for vein rocks.

For almost each of the fracture systems of northwestern, northeastern, and meridional strike, there are paired fractures intersecting at a right angle or close to it, with western and eastern dips. Within the chromite deposits, these paired fractures of northeastern and northwestern strike with dips to the east and west are quite clearly expressed; they cut the chromite bodies into tectonic blocks displaced relative to one another (the deposits “Gigant,” “Spornoe,” and named after XX Years of the Kazakh SSR).

In the hyperbasites, zones of crumpling are often encountered, as well as reverse faults and thrusts of northwestern and northeastern strike. The latter give a scaly structure to certain parts of the massif (the upper reaches of Kyzyl-Kain, Kishkine-sai, Dzhangiz-agach).

In their structural features, the gabbroids in many respects resemble the hyperbasites. The banding of the gabbro and the planar banding of the ultrabasic rocks have identical elements of occurrence. In the gabbroic rocks the same four fracture systems occur as in the ultrabasic rocks (in the gabbro the predominant fractures are those of northwestern strike with southwestern and northeastern dips).

At the same time, however, the gabbroids also have their own distinctive feature, determined by their position in the peripheral zone of the intrusive complex. Among these are the banding and gneiss-like structure of the gabbroic rocks, which are most distinctly expressed in the gabbroids in the zone of contact with the hyperbasites.

Also noteworthy is the sharp variability of the texture and structure of the gabbroids. Within small areas of 1–4 m² one can observe transitions from massive coarse-grained varieties through weakly gneiss-like medium-grained ones to gneiss-like fine-grained ones (the Mamut, Shelekta, and Shandasha rivers). Apparently, such textural-structural variability of the rocks is due to the nonsimultaneous solidification of different parts of the gabbro and their unequal reaction to tectonic stresses.

In the gabbroic rocks there occur zones of foliation 0.6–6 m thick.

Tectonics of the Meso-Cenozoic

The Meso-Cenozoic rocks lie horizontally on the eroded folded basement of the Ural–Tai anticlinorium.

Disjunctive disturbances occur in the Cretaceous and Tertiary deposits. One of these disjunctives is the large meridional Borodinovka fault, passing through the eastern part of the area studied. It accounts for the tectonic contacts of the Paleogene with serpentinites, rocks of the Kairakta suite, and Upper Cretaceous deposits. The amplitude of the fault is measured at 60–90 m.

The meridional and latitudinal faults to which the river valleys of the Dzharly-Butak and Kyzyl-Kain rivers are confined are of young age. Here, together with the hyperbasites, the Cretaceous rocks overlying them are also disturbed.

Small tectonic disturbances with an amplitude of 6–10 m, affecting Upper Cretaceous and Paleogene deposits, have been established in the area of the Don deposits and in the headwaters of the Eissyl-Kara and Kyzyl-Kain rivers.

The young disjunctive disturbances are of post-Paleogene age. Most likely, they are connected with the Miopliocene phase of Alpine tectogenesis, which was clearly manifested in the Southern Urals.

Conclusion

In the present work, certain questions of the geology of the Kempirsai hyperbasite massif have been briefly set forth. Attention has been directed to the age of the ultrabasic rocks, to their relationships with the gabbroids, and to the structural features of the gabbro-hyperbasite pluton. The resolution of these questions should be regarded as one of the attempts to interpret the geology of the Kempirsai ultrabasic complex.

Literature

1. Aneshkov A. N.—On the question of the age of the “weathering crust” in the Southern Urals. Priroda, No. 1, 1941.

2. Bakirov A. G.—On the cycles of tectogenesis of the Urals. Izv. Tomsk Industrial Institute, vol. 62, issue 1, 1941.

3. Ginzburg I. I. and Dvug.—Ancient weathering crust on ultrabasic rocks of the Urals. Part I. Types and morphology of the ancient weathering crust. Tr. Inst. Geol. Nauk, Academy of Sciences of the USSR, issue 80(1), 1946.

4. Ginzburg I. I.—Ancient weathering crust on ultrabasic rocks of the Urals. Part II. Geochemistry and geology of the weathering crust in the Urals. Tr. Inst. Geol. Nauk, Academy of Sciences of the USSR, issue 81(2), 1947.

5. Zinovkin A. D.—On the tectonics of the Kempirsai pluton. Sovetskaya Geologiya, No. 12, 1940.

6. Loginov V. P., Pavlov N. V. and Sokolov G. A.—Chromite-bearing capacity of the Kempirsai ultrabasic massif in the Southern Urals. Chromites of the USSR, vol. II, 1940.

7. Filimonova A. A. and Kofanov D. N.—Ancient weathering of the South Kempirsai chromite deposits “Gigant” and “Spornoye.” Tr. Inst. Geol. Nauk, Academy of Sciences of the USSR, issue 41(5), 1941.

1. We have in mind the gabbroids of the intrusive complex situated on the periphery of the ultrabasic rocks. ↩↩