RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES5003, doi:10.2205/2005ES000188, 2005
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Figure 1 |
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Figure 2 |
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Figure 3 |
[5] The gabbro amphibolite and quartz diorite are composed of elongated prismatic crystals of bluish green amphibole and of tabular plagioclase crystals wholly replaced by saussurite or by an aggregate of epidote, zoisite, sericite, and quartz grains. The quartz diorite contains large granulated quartz grains with amphibole accounting for 30% of the rock volume. These rocks have a relict subhedral structure. The schistosity planes are marked by the aggregates of secondary chlorite and quartz grains. The main minerals of the plagiogranites are quartz and plagioclase, the latter being slightly altered or wholly replaced by epidote, chlorite, and sericite. The accessory minerals are sphene, zircon, and apatite.
[6] The zircon of Sample 2114 is represented by the perfect crystals showing predominantly hyacinth habitus. The crystals are translucent and slightly brownish. Many grains are highly fractured. The morphology of the zircons suggests their magmatic origin. The U-Pb isotope study of the accessory zircons from the rocks of the Polar Ural region was carried out using the Krogh method [Krogh, 1973] and the zircon microspecimens. The uranium and lead concentrations were determined using a mixed 235U+208Pb tracer. The isotope composition was measured using a TSN-206A solid-phase mass spectrometer of the Cameca production. The resulting ages were corrected for the common lead admixture using the model reported by Stacey and Kramers [1975]. The ages were calculated using the PBDAT program [Ludwig, 1991] and the commonly used uranium decay constants. The results are presented here in Table 1.
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Figure 4 |
[8] The mapping of the area including serpentinite melange, carried out by [Dushin et al., 1997] proved that the melange blocks resided in the field of some sedimentary rock sequence consisting of highly schistose sandstone and siltstone with gradation layering, including numerous blocks of serpentinite, tectonized volcanomictic sandstone, and conglomerates. This creates the impression that we deal with a relatively deep-sea rock sequence including olistostrome horizons, as well as the melanged rock blocks of some disintegrated ophiolite rock complex, occurring as olistoliths and tectonic lenses.
[9] In the west of this region the sedimentary rock sequence borders, along a steep tectonic contact, the undifferentiated sequence of tholeiite basalts (Figure 2). A tectonic sheet, composed of volcanogenic sedimentary rocks, is thrust over the sedimentary rocks. Mapped at the base of this sheet are aphyric basalts, altered to green schists, and overlain by a layer of pillow lava injected by diabase dikes. The width of the exposed volcanic rocks is about 800 m. Above follows a cherty volcanic rock unit (200 m thick) composed of the interbedded layers of aphyric and occasional porphyritic basalts and dark grey cherts and siltstones. Associated with the cherts are aphyric rocks with ovoidal parting and variolite layers. Boninites were reported from the volcanic rocks of this sheet [Dushin, 1989].
[10] To sum up, the Enganepe Ridge includes a series of tectonic sheets which were superposed in pre-Late Cambrian time. These sheets include the fragments derived from volcanic arcs and their slopes, as well as the fragments of the crust of the adjacent basins. In some places the floors of these basins were composed of ultramafic rocks which were subjected to underwater weathering. The isotopic geochronological data obtained in this study suggest that all of these structural features might have existed at the end of the late Riphean (Neoproterozoic).
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Figure 5 |
[12] The Khanmenkhoi Formation is represented there as a monotonous sequence of alternating albite amphibolite, chlorite-micaceous, biotite, and amphibole-micaceous schists, and paragneiss. Three tectonic sheets were mapped in the rocks of the Nyarovei and Nemyuryugan formations. The lower sheet is composed of volcanic and terrigenous, often poorly sorted, rocks. The volcanic rocks were identified by their petrochemical characteristics as metamorphosed high-Ti basalts [Dushin, 1987]. The upper tectonic sheet is composed mostly of metamorphic flyschoid terrigenous rocks.
[13] Of particular interest are the rocks of the intermediate sheet including the largest number of ultramafic rock and gabbroid bodies. The basis of this sheet is composed of metamorphic carbonaceous-siliceous schists sequence, saturated with tholeiitic metabasalt dikes and sills. The upper part of this sheet is composed of a metamorphosed volcanogenic sedimentary rock sequence represented by chlorite schists, epidote-chlorite schists, chlorite-magnetite schists, and amphibole schists, alternating with metasiltstones and metapelites. This rock sequence seems to be composed of metamorphosed volcanogenic sedimentary rock complex with lavas, tephraturbidites, volcanomictic sandstones, and shales. Our study revealed that the bodies of the ultramafic rocks and gabbroids were restricted to the area composed by the sedimentary rocks of this rock sequence, or occupy a position as a chains of outcrops at the contacts of the rock members with different types of the rock sequences.
[14] In the area of the upper reaches of the Mollybdenite Creek, the left tributary of the Bolshoi Kharbei River, a fragment of an ophiolite allochthon composed of dunite at the base, which is followed by a layred dunite-wehrlite-clinopyroxenite complex with taxite gabbroids at the top was mapped by [Dushin, 1987]. The base of the allochthon is granitized. Its contact shows metasomatically altered ultramafic rocks. The average composition of the antigoritized ultramafic rocks agrees with that of harzburgite. The structural position of this allochthone is not quite clear, except for the fact that it is thrust over the schistsose metamorphic sandstone and siltstone.
[15] In the area of the Kuz-Shor Creek, which is the right tributary of the Bolshoi Kharbei River, the structural position of the ultramafic and gabbroid rocks was found to be more obvious. The ultramafic rocks and gabbroids occur as olistoliths in the olistostrome units inside the sedimentary rocks of the volcanic-sedimentary sheet. They also occur as tectonic lenses at the contacts of the sedimentary and volcanic rock members, see Figure 5. An outcrop at a distance of 3 km from the Kuz-Shor Creek shows an olistostrome sequence with a schistose and metamorphic sedimentary and volcanic matrix and elongated rounded olistoliths, completely enclosed in the matrix. The talc-bearing serpentinized ultramafic rocks, including antigorite serpentinite, compose olistoliths, 0.5 m to 5 m in size, the maximum thickness of the gabbroids being 3 m. The lenticular serpentinite bodies, often composed of smaller block accumulations with foliated serpentinite at the contacts, extend over hundreds of meters.
[16] For the purpose of dating the volcanogenic sedimentary rocks and their metamorphism large samples were collected from the paragneisses of the highest-grade metamorphic rocks of the Khanmenkhoi Formation and also from the metamorphic and, hence, gneissic quartz diorite localized in the intermediate sheet composed of volcanic and sedimentary rocks. These diorites were expected to mark the youngest age of the volcanic and sedimentary rock sequence.
[17] The zircons from Sample 2127 of the Khanmenkhoi Formation rocks were represented by prismatic to isometric crystals with a corroded surface. Some of the grains were found to be perfectly shaped, translucent, and brownish in color. The translucent and well-shaped grains, supposed to be of magmatic origin, were selected for the analysis.
[18] The zircons from Sample 2128 had a small size (below 60 m m), a prismatic form with the well developed prism faces, and almost colorless, possibly being of magmatic origin.
[19] The ages obtained for the zircons of both samples are highly discordant
(Table 1). It appears that the gneiss of Sample 2127 represents some metasedimentary rock. Proceeding
from the metamorphic origin of the zircons from Sample 2127, all zircons contain an admixture of some
older radiogenic lead, this precluding the exact dating of the metamorphism. The diagram with a concordia
(Figure 4)
shows that data points of zircons from sample
2128 (black ellipses) define a regression line
with concordia intercepts about 640 and 435 Ma
with large error, which may corresponds with ages
of zircon crystallisation and their metamorphic
transformation. The regression line for data
points of zircons from from sample 2128 and for
two less discordant data points of zircons from
sample 2127 (grey ellipses) define a regression
line (MSWD=1.6) with concordia intercepts at
643
16 Ma and 434
12 Ma.
These data suggest that the gneiss of Sample 2128 represents some orthorock with
an age of about 640 Ma, which experienced metamorphism in Late Ordovician-Early Silurian.
[20] To conclude, the metamorphism of these two rock samples can be dated 434
12 million years.
[21] The isotopic-geochronologic data obtained in this study suggest that the metamorphic volcanogenic sedimentary rocks of the Kharbei Complex originated in pre-Vendian time. The fragments of the rocks of ophiolite origin occur as olistoliths inside the olistostrome units and seem to be even older. The rocks of the Kharbei Complex were transformed to metamorphic rocks in Late Ordovician-Early Silurian.
[23] The rock massifs of the mafic-ultramafic belt were characterized by Sm-Nd and Ar-Ar isotopic
geochronological data, which placed the rocks into a large Ordovician to Devonian age range.
Sm-Nd study of the whole-rock samples of the Voikar-Synya Massif
yielded an age of 387
34 million years
[Sharma et al., 1995].
In terms of the dating method used, this value seems to
be doubtful, because it was made using the samples collected from different parts of the ophiolite
complex. The study of the gabbroids and diabase of the Syumkeu Massif using the Ar-Ar method yielded
the data corresponding to a large time interval, namely from 491 Ma to 419 Ma. In the case of the gabbroids
and dikes of the Voikar-Synya Massif the resulting ages varied from 497 Ma to 426 Ma
[Kurenkov et al., 2002].
In our study we dated the rocks of the dikes from the Voikar-Synya Massif, using their geological
and isotopic-geochemical studies for the purpose of dating the ophiolite rock association of the
Voikar-Synya Massif.
[24] Developed in the study area are three voluminous rock complexes. In the NW-SE direction these rocks are (1) the dunite-harzburgite complex of mantle tectonites, (2) dunite-pyroxenite-gabbro, and (3) the intrusive gabbro complex. Found in subordinate volumes was the diabase of the parallel dikes, which had been undoubtedly associated with the origin of the ophiolite gabbroids.
[25] The rocks of the dunite-harzburgite complex (1) composing the water-shed area of the Polar Ural segment were studied in detail and described in the literature [Bogdanov, 1978; Savelieva, 1987; Sobolev and Dobretsov, 1977, to name by a few]. As a result of our study we obtained new information about the structure of the second and third rock complexes mentioned above and their relationships. Proceeding from their relationships, we identified a complex of intrusive gabbroids, differing from the complex that had been identified earlier in terms of its volume and rock types.
[26] The ophiolite complex of mafic-ultramafic rocks (2) includes dunite, pyroxene-bearing dunite, the layered sequence of rocks ranging from verlite to leucocratic gabbroid, as well as the isotropic gabbro and gabbro-diabase complex of parallel dikes. The dunite was found to be associated with massive coarse-grained pyroxenite via gradual transitions. Another characteristic feature of these rocks is the banded alternation and the reticular and spotty relationships between the dunite and massive pyroxenite bodies. These rocks also compose bodies as large as a few kilometers, located mostly in the watershed area of the Lagortayu and Trubayu rivers. The rock bodies of this type are believed to be characteristic of mantle-crust transition zones or are interpreted as the upper parts of the mantle rock complexes in the well-known ophiolite rock sequences. The layered rock complex includes the rhythmically alternating sequence of wehrlite, pyroxenite, and gabbroid bands, ranging from their melanocratic to leucocratic varieties. These bands, ranging from a few millimeters to some decimeters, usually measuring 1-1.5 cm in width, seem to be of the primary, magmatic origin, yet, the orientation of the mineral grains suggests that these rocks experienced some metamorphic transformation which was responsible for the folds deforming their primary banding. The layered wehrlite-pyroxenite-gabbro sequence, and also the isotropic gabbro associated with the diabase, are known to be typical of the ophiolite association, which is confirmed by geochemical data [Bogdanov, 1978; Kurenkov et al., 2002].
[27] Exposed in the Right Payera River basin is a zone of massive gabbro to diabase transition, the structure of which proves that these rocks belong to the same rock complex. This zone is composed of numerous gabbroid screens localized between diabase body packets and individual diabase bodies, some of them showing a stepwise configuration. A specific feature of this zone is the presence of small gabbro-pegmatite bodies, devoid of any chill zones, which cut across both the older gabbro and diabase bodies. The diabase and gabbroids of the transition zone are cut by a series of leucocratic rock veins, composed of felsite, plagioclase rocks, amphibole-plagioclase pegmatoid rocks, and plagiogranite, which include igneous rock breccias with gabbro and diabase fragments. The rocks developed in the direction toward the contact with the ultramafic rocks along the Right Payera River are gabbroids and the less developed rocks of the wehrlite-pyroxenite-gabbro association, which show distinct doubling marked by a serpentinite zone. The rocks of a parallel double complex are also developed in the area of the Lagortayu R. Canyon. The specific feature of this rock sequence is the wide development of serpentinized ultramafic rocks in the form of screens in the dike complex.
[28] The rocks of the reconstructed dunite-pyroxenite-gabbro association (Complex 2) occur as a large geologic body of complex structure and configuration. Its largest outcrop, not less than four kilometers wide, was found at the latitude of the Right Payera River.
[29] The rocks of this association (2) contact mantle tectonites (1) in the west and large intrusive gabbroid bodies (3) in the east, some small gabbroid bodies being also restricted to the planes of tectonic displacement in the area underlain by the rocks of Complex 2. Their relations with the rocks of Complex 3 control the complex configuration of the geological body composed of the rocks of the dunite-pyroxenite-gabbro complex.
[30] The intrusive bodies of the gabbro norite and olivine gabbroids (3) occupy the significant volume of the Voikar-Synya mafic-ultramafic rock complex. The large bodies of these rocks, being tens of kilometers long, are expressed very well in the topography of the eastern slope of the Polar Ural Ridge and compose an isolated mountain ridge between the water-divide area of the Polar Ural Mountain Range and its eastern piedmont. These bodies extend in the northeastern direction along the strikes of most of the contacts, along the boundaries of the mantle ultramafic rock bodies, and along the contacts inside the dunite-pyroxenite-gabbro complex. We studied one of the largest gabbro-norite and olivine gabbroid bodies which occupies the significant part of the watershed area of the Lagortayu and Trubayu rivers, and also some smaller bodies in this area. We discovered some relatively smaller intrusions of the same complex in the upper reaches of the Right Payera River.
[31] The body of the gabbro-norite and olivine gabbroid, located in the water-shed area of the
Lagortayu and Trubayu rivers was found to be about 4 km wide and more than 10 km long. In the
northwest this body borders banded gabbroid and pyroxene-bearing dunite. In terms of their grain size the
rocks were found to vary gradually from their coarse-grained varieties (Norite Creek area) to medium-grained
varieties in the left bank of the Lagortayu River between the Norite Creek and the next large
tributary of this river in its upper course. Discovered in the area west of this tributary was a chill zone in the
gabbro-norite massif. Associated with the exocontact zone of the gabbro-norite massif are pegmatoid rock
bodies. The pegmatite located in the vicinity of the exocontact zone of the gabbro-norite massif was found
to contain amphibole crystals as large as 0.5 in size. At a distance of a few hundred meters from the contact
zone the banded pyroxenite and gabbroid were found to include a body of pegmatoid gabbro norite and
pyroxenite with crystals 1-2 cm in size, some of them being as large as 10-15 cm. At a distance of about 5 km
along the strike of the exocontact zone, the left side of the Trubayu River valley includes another
outcropping body of characteristic gabbro norite. The southeastern contact of this body is exposed in the right
side of the Trubayu River valley, where the gabbroids contact a large lenticular dunite body, 0.5
5 km in size,
elongated along the general strike of the contacts. Here, the endocontact zone of the massif is composed of
gabbro and pegmatite with oriented pyroxene and plagioclase crystals amounting to a few centimeters in length.
[32] The area of the Right Payera River does not expose any intrusive gabbro-norite bodies of Complex 3. This seems to have been responsible for the good preservation of the rock sequence enclosing the mafic rocks the ophiolite complex. This explains its classification as the Payera Sheet [Saveliev and Savelieva, 1977]. However, in the course of studying the banded rocks of the ophiolite association in the vicinity of the surface of their tectonic doubling we discovered a series of gabbro-norite and websterite veins (3) with distinct chill zones, bordering the layered gabbroids (2) and the rocks of a dunite-harzburgite complex (1). The rocks enclosing them contain large pyroxene and plagioclase crystals which seen to have been impregnated during the intrusions of new mafic magma portions into the ophiolites along some weak zones.
[33] To sum up, the new data available for the geologic structure of the Voikar-Synya rock complex confirm the previously offered view about the presence of two ophiolite and intrusive rock associations in it. However, its intrusive rock association is reported here to have a new volume and a new rock composition. Proceeding from our new data, we conclude that the wide development of intrusive gabbro bodies seems to explain some specific features of the Voikar-Synya ophiolite association, such the absence of an effusive-sedimentary rock sequence and the subordinate development of diabase complexes. It can be assumed that the rocks of the upper part of the ophiolite sequence were intruded by new magma portions from some intermediate-depth magma chambers, which were responsible for the formation of gabbro-norite and olivine gabbroids massifs existing in the present-day structure. It is possible that these rocks can be found as skialiths or xenoliths in these rock massifs in the course of their more detailed study.
[34] At the present time a special study is carried out to examine the compositions of the rocks and minerals, as well as of the distribution of trace and rare earth elements in them to verify the above conclusions which were base thus far mainly on the geological data available.
[35] In terms of our geochronological study a particular attention was given to studying the series of the parallel dyke complex (2) in the vicinity of its contact with the gabbroids of the same complex, where the plagiogranite bodies of the ophiolite association could be expected. In the rocks exposed by the Lagortayu River, plagiogranites were found at two sites: in the gabbroids residing at a distance of several tens of meters west of the contact with the dike complex and also among the parallel dikes at a distance of 1.5 km this contact (Site 2570).
[36] A plagiogranite sample was collected at this site from a vein 20 cm wide, which intersected the early generations of diabase and plagioclase porphyries of the parallel dike series, and was intruded by mafic dikes of the later generation (Figure 2). Thus, the plagiogranite had been sealed up inside the dike complex, this allowing one to date the upper age boundary of the ophiolite formation.
[37] The accessory zircon derived from the plagiogranite (Sample 2570) was found to be represented by subidiomorphic nontransparent colorless semimetamictic crystals with intensively corroded surfaces. Some grains showed metamictic cores of irregular form. Accounting for not more than 20% of the total sample volume were idiomorphic and subidiomorphic, translucent colorless or light yellow crystals of prismatic or short-prismatic form of zircon habit, which were used for dating the rocks. Some grains showed slightly corroded surfaces. Characteristic of the internal structure of the zircon crystals was the presence of "fine" magmatic zones and sectors, also of solid-phase mineral inclusions. The crystal size varies from 50 m m to 250 m m, the elongation of the crystals ranging from 1.6 to 2.0.
[38] The U-Pb isotope studies were carried out using three zircon samples collected from the size
fractions ranging from
- 100+60
m m to
>
60
m m. The zircons larger than 60
m m were
subject to preliminary air abrasion
[Krogh, 1982].
As follows from Table 1 and Figure 4, the zircon used in this study showed
insignificant discordance, its data points producing a discordia, whose intersection with the concordia
corresponded to the age of 489
20 million years, the lower intersection being almost zero (MSWD=0.4).
The average age value calculated using the
207 Pb/
206 Pb ratio for the three fractions of the
examined zircon was found to be 490
7 million years (MSWD=0.21) and coincided with
the age obtained using the upper intersection of the discordia. Taking into account the fact that the
morphologic features of the studied zircons suggested their magmatic origin, the age value of 490
7
million years can be taken as the most exact dating of the plagiogranite crystallization in the Voikar-Synya Massif
(E. V. Khain et al., in press, 2006).
[39] The resulting age of the plagiogranite from the ophiolites of the Voikar-Synya Massif (490
7 Ma)
corresponds roughly to the Cambrian-Ordovician boundary of the modern scale. Consequently, the
ophiolite of this massif can be dated late Cambrian or somewhat older. The problem of dating its gabbro-norite
complex remains to be solved.
Citation: 2005), The Neoproterozoic and Early Paleozoic geological history of the Ural-Kazakhstan margin of the Paleoasian Ocean using new isotopic and geochronological data obtained for the Polar Ural region, Russ. J. Earth Sci., 7, ES5003, doi:10.2205/2005ES000188.
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