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Geochronology of the Aldan Shield, southeastern Siberia'
A. I. TOUGARINOV F'ernadsky Institute of Gec~chemistry, Academy of Sciences of the U.S. S. R., Mosco ws U. S. S. R.
Received November 16, 1964 Accepted for publication January 31, 1968
Crystalline Precambrian rocks exposed in the basins of the Olekma and Aldan Rivers represent the southeastern part of the Siberian Platform. To the north and east the basement is covered by Paleozoic and Upper Proterozoic formations. To the south and west lie younger deformed belts.
The Aldan Shield can be divided into four regions. The Tcharskaja massif, in the west, is the most ancient block. Pb-U dates on allanite and zircon show a good concordance at 2950 m.y. The prevalent rock type is gneiss and migmatite.
In the Aldan River basin three thick series of Lower Proterozoic rocks were deposited during the time interval 2000 to 2400 m.y. Granulite fxies metamorphism of these rocks occurred 1800 to 2100 m.y. ago. The time of deposition of the lowest formations (Iengra Series) is given approxinlately by measurement of the isotopic composition of lead in different syngenetic mineral phases. Using mus- covite and magnetite, the 207Pb/204Pb:206Pb/204Pb isochron yields a value of 2550 9 300 m.y.
In the east the crystalline basement of the Aldan consplex is covered by thick Proterozoic sedimentary formations. These formations are intruded by alkaline roclts, dated at 1100 n3.y. and small ring struc- tures with associated carbonatites, dated at 600 to 650 m.y. The magmatic cycles, combined with glauconite age determinations, indicate a nearly complete, Middk and Upper Proterozoic stratigraphic section. The base of the Middle Proterozoic is established at 1900 9_ 50 m.y.
The Aldan complex, and the Stanovoi conlplex to the south are separated by a wide belt of tectonic fault zones. The prevailing inetamorphic grade of the Stanovoi coinplex is ainphibolite fasies, with only local areas of granulite facies rocks. Pb-U ages from monazites, zircons, and allanites from peg- islatites cutting the complex date the amphibolite facies metamorphism at 1980 m y . Available data suggest an Archean age for the granulite facies rocks. The Stanovoi complex has been affected by at least two subsequent magmatic periods; the first were syenitic intrusions at 1100 m y . , and the second Jurassic granites at 150 m y . The last magmatic cycle, found in many parts of the Aldan Shield, is an excellent demonstration of the recurrent rejuvenation of Precambrian terrains.
htraduetisn The Aldan Shield forms the southeastern part
of the Siberian Platform. It comprises a block of crystalline rocks in the basin of Olekma and Aldan Rivers. The crystalline basement of the shield descends to the north under a thick cover of Paleozoic sediments; to the east it is covered by Upper Proterozoic formations; to the south crystalline rocks of the Aldan Shield are in tectonic coiltact with metamorphic rocks of the Stansvoi complex; to the west it is surrounded with fold mountains of Baicalides (Fig. 1).
Tn our report we will examine the details of the geochronology of four Aldan regions and their nearest provinces.
BBekma-Tchara Region The most ancient nucleus of the Aldan Shield
consists of the Tcharskaja massif exposed in the western part of the shield, in the basin of Tchara and Tokko Rivers. It comprises gneisses and migmatites with various pegmatites. In the south --
lPrcsented at the International Conference "Geo- chronology of Precambrian stratified rocks," held in Edmonton, Alberta, June 12-14, 1964.
this nucleus is in intrusive contact with granite dated by different methods at 2000 1n.y. To the east the crystalline basement is covered by the schists and gneisses of the Blekminskaja Series, which is exposed in the Olekma Valley (Table I). Because rocks of this series are only weakly metamorphosed, and because their tectonic con- tact with metamorphic rocks of the Aldan com- plex is obscure, they were classed as Proterozoic by Dsevanovsky.
However, six years ago, at the General Assembly of the International Union of Geodesy and Geophysics, in Helsinki, Prof. A. P. Vino- gradov and the author published the first tenta- tive age measurement of Rhara migmatites as approximately 2658 m.y. Because an altered allanite was used and because of a remarkable disconcordance of the age values given by dif- ferent isotopic ratios, we decided to continue our investigations in this region.
In the basin of Tchara and Tokko, A. F. Petrov and E. P. Miromuk discovered metamor- phic rocks similar to the Olekma Series. They noted the resemblance of the stratigraphic sec- tions and magnetic anomalies of both regions
Canadian Journal of Earth Sciences, 5, 649 (196s
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650 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 5. 1968
FIG. 1. Geochronologic map of the Aldan Shield. (I) Proterozoic 2 and 3; (2) Stanovoi (Proterozoic I ) ; (3) Jal- tula; (4) Timpton; 45) Iengra (3, 4, and 5 are all Proterozoic 1, and part of the Aldan Shield); (6) Olekma-Chara (Archean); (7) 2950 m.y.; (8) 2600 my. ; (9) 2550 m.y.; (10) 1800 to 2200 m y . Insert shows general location of the n a p area.
and suggested the structure and stratigraphy were uniform for the whole area.
Now as is shown in Figs. 1 and 2, and Table 11, different age measurements for this area by Pb-U and K-Ar methods coi~firm an Archean age of the Olekminskaja and the Kurultinskaja Series. In Table 11, four allanites show, with reasonable agreement 2950 m.y. for different migmatites developed in the rocks of both lower
series. The same data were calculated by the isochron method (Fig. 2). It is remarkable that some samples of biotite and muscovite from the schists and gneisses of the Olekininskaja Series have approximrttely the same ages indicated by the argon method, approximately 3000 m.y. This means that a big block of Archean rocks in that part of the Aldan Shield has experienced later, thermal events during its long geological history.
TABLE I
Precambrian stratigraphy of the Tchara-Olekn~a region
Series Suite Lithology Thickness (m)
Barsalinskaja - Biotite gneisses 1400
Olekminskaja Mordginskaja Biotite-amphibole gneisses Krestiachskaja Biotite-amphibole and pyroxene gneisses 7800 Bldongsinskaja Biotite gneisses partly granitized
Kurultinskaja Avinskaja Biotite-amphibole, amphibole-pyroxene gneisses
Kurbalikitskaja Garnet-pyroxene gneisses Zverevskaja Biotite and sillimanite-garnet gneisses 10000 Imangrakanskia Pyroxene-garnet gneisses Karurakskaja Pyroxene gneisses
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TOUGARINOV: D A N SHIELD, SOUTHEASTERN SIBERIA
TABLE I1 The age of minerals of the Aldan Shield
- --
Ages in may. Isotopic composition of lead
207 206 207 208 - - - No. Locality h.Zineral Pb U Th 204Pb 206Pb "'Pb 20sPb 2Q6 238 235 232
1. Chara basin, granit ogieiss Allanite
2. S&OO basin, granitogneiss Allanite
3. Sakoo basin, granitogneiss Allanite
4. Beresovy, Chara basin granitogneiss Allanite
1. Illimah basin, v. Arnaljak, mica schist Muscovite
2. .I.I Magnetite
3. , , Magnetite
1 . O 18.98 15.56 43 .87 1 . O 27 .70 87.00 65 .OO 2550+ 300 (by 1 . 0 3 0 . 5 87.5 7 0 . 0 isochron method)
Aldan River Region East of the Olekma Series, in the basin of
Aldan River and its tributaries, are the meta- morphic rocks of the Aldan complex itself, metamorphosed to the granulite facies. This complex is divided into three series, which strike northward and dip to the east. The stratigraphy of the Aldan complex is shown in Table 111.
It is remarkable that the lithological sequence of the tvhole Aldan stratigraphic section, from the lotvest to the uppermost, repeats perfectly amazingly the common sequence of a typical section of Lotver Proterozoic rocks. In fact, the sedimentary history of the Aldan complex re- flected all global changes of geochemical con- ditions typical for Lower Proterozoic rocks, i.e.
FIG. 2. The age of minerals from the Aldan Shield.
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CANADIAN JOURNAL OF EARFH SCIENCES. VOL. 5, 1968
TABLE 111 Precambrian stratigraphy of the Aldan region
Series Suite Lithology Thickness (m)
Jaltula Lurikanskaja Diopside-garnet-biotite gneisses Chaikanskaja Marble, scapolite-diopside rocks Sutalnskaja Marble, calciphyres, diopside and garnet-biotite gneisses
Tirnptsn Kurikanskaja Hypersthens and biotite-garnet gneisses Sunnaginskaja Hypersthene and biotite gneisses Uluntchinskaja Hypersthene and biotite gneisses
Hengra Idgakskaja Biotite and hypersthene-biotite schists Fedorovskaja Pyroxene-amphibole and diopside schists and gneisses with phlogopite
inetasomatites 6500 Upper Aldanian Cordierite, garnet-sillimanite gneisses, quartzites
the time interval between 2700-2060 m.y. In spite of this, the curious idea that high-rank metamorphism to the granulite facies is typi- cally restricted to the ancient Precambrian in- clined many geologists to the view that the Aldan complex is 01 Archean age.
In attempting to decide this problem with straight geochroiaological ineasurcn~ents onIy one difficulty was encountered; a great amount of data provided by K-Ar, Pb-U, and Rb-Sr measurements for different intrusions, inclu- ding peginatites and gneisses in this area, always gave the same time dates-1800-2108 m.y. But the fact that synorogeilic intrusions in
Zircon 0"
0 100 k ? o 0 ~ ~ ~ ~ ( 1 4 ~ ~ %a
FIG. 3. Tlae agc of Iengra migrnatites.
the Aldan complex had everywhere the same age, and that no intrusions earlier than 2000 my . were found, was very suspicious. It implied that there was no great time interval between sedi- mentation and magmatic events in this Pre- cambrian geosyncline. If this were ss, the Archean age of the Aldan coinplex is certainly questionable. This cornplicatioii suggested that different methods of age determination sho~ild be used in attempts to determine the real age of the Aldan complex. We therefore separated different accessory minerals from the earliest migmatites cut by granites, and measured their age. As is shown in Table IV and Fig. 3, the age of these migmatites Is approximately the same, 1950 m.y. A few zircon grains asrere obtained from a sample of upper Aldanian quartzites. We hoped these would provide information concern- ing the age of the pre-Aldanian basement, because the zircons were of two distinct types, but the measured ages are 2308 m.y.
Finally we used our Pb-Pb method of age measurements s f sediinentary rocks, a method which proved successful in the Ukraine. This method is based on the determination ol' the isotopic composition of leads in different, syn-
TABLE IV The age of Tengra rniginatites
- . . - - -- - - - " - - -- - - -- -- - - - - - -- -- -- -- -
Age values in m.y. % Isotopic compogition of Bead
-- - --
Samples -- A- 206 207 207 208 - -- . Pb U Th "O"Pb 'OVb 2"7P& '08Pb -238 235 206 sf
Allanite 0.091 0.009 0.65 0.056 3.32 1.146 95.48 1680 1890 1900 2500 Spkene 0.038 0.084 0.17 0.14 20.14 4.24 75.48 3540 2630 1980 4080 Zircon 0.044 0.057 0.105 0.12 46.74 7.07 46.07 2230 2050 1920 3650
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TOUGARINOV: ALBAN SHIELD, SOUTHEASTERN SIBERIA 653
genetic, mineral phases of sediments. Tf these phases were originally characterized by different Pb-U ratios, and during their geological lives represented a closed system for uranium and its decay products, the measured lead isotope ratios will give, on a graph with coordinates 2 0 7 ~ b - 204~b:2"~b-204~b, a reliable isochron age of sedimentation. Our experience shows that even a loss of uranium or lead up to 30% has a limited influence on the ages so determined. By this method we have determined the age of sedimentation of the Krivoi Rog basin as Early Proterozoic, 2600 m.y. and latully and Iotny in central Karelia as Middle Proterozoic, 1800 m.y., and so on. With this method in mind we selected, in the northern part of the AIdanian Shield, an outcrop of mica schist from the Iengra Series. The series was weakly metamorphosed and the outcrop is sufficiently distant from granite intrusions. TW b different fractions, one composed sf micas and the other of magnetite were ob- tained from the scl~ist. The lead isotope com- positions in both samples show a remarkable enrichment of these iron oxide compounds by radiogenic lead. It seems that this may be due to a high absorption effect of iron hydroxide. Therefore one might suppose that the difference in isotopic composition of lead is a result of a primary, disproportionate distribution of uran- ium in the sediments. The isochroil calculated on the basis of these results has given 2550 -t- 300 m.y. These data cannot be considered precise, but in spite of this it does give additional evidence of an Early Proterozoic age for the Aldan complex, and the absence of Archean rocks (Fig. 4).
o I b io 30
206~$ /204~b I = Muscovite 2=Mognetite
FIG. 4. The age of sedimentation of the Iengra Series.
Timpton River Region
In the east the crystalline basement of the Aldan Shield is covered by thick Upper Pre-
cambrian sediments. A schematic section is given in Fig. 5. There, as in the Ural Utchuro-Maisky region, there is one of the most nearly complete stratigraphic sections of Middle and Upper Proterozoic rocks in the world.
The combination of glauconite age measure- ments on glauconite from different sedimentary layers in this section, and radiological dating of intra-Proterozoic intrusions has provided a fortunate opportunity to subdivide these Upper Precambrian rocks. A definite boundary be- tween the Lower and Middle Proterozoic rocks is based on age determinations of the Ulkan granites. These granites intruded a crystalline basement, were intensively eroded and then overlapped by alkaline porphyries, which are the basal rocks of the Middle Precambrian. Accessory zircons from both types of rocks have given very similar ages 1900 Ifr 50 m.y. These ages are supported also by K-Ar age deter- minations.
The next definite interruption in sedimentation is observed between the hfaiskaja and Utchurs- kaja Series. In some suites of the Utchurskara Series glaucoi~ites occur with a K-Ar age of 1400-1500 m.y., and in the hfaiskaja Series similar glaucornites were dated at 1100 m.y. In the basal conglon~erates of the Maiskaja Series pebbles of alkaline ii~trusions comparable with outcrops of Ulkan iiltrusions were observed. It is interesting to note that zircons from the Ulkan intrusions were dated by the Pb-U method and gave dates of 1100 m.y.
At least in the upper part of the Precambrian section in many places of the Aldan Shield itself" and among Upper Precambrian formations in the Maia basin sniall intrusions of so-called 'central type9 ring structures were discovered. They cut the youngest Precambrian sediments, sometimes developing a fold belt around themselves. Their denuded surface was overlapped by the paleonto- logically well-defined Judom Series of Early Cambrian age. Glauconites obtained from Judom sandstones are dated at 578 may.
The age of carbonatites, which are genetically related to these intrusions, measured on gatche- tolite and other accessory minerals by the Pb-U method is 600450 m.y.
In summary, in the eastern part of the Aldan Shield it is possible to distinguish at least two Late Precambrian magmatic cycles, 1100 and 650 m.y. old.
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654 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 5, 1968
Utchurskaja Sn;!
Anorthosite of
FIG. 5. GeochronoBogic column of the eastern part of the AIdan SBnieId.
Stansvsi Region One of the most complicated and until now
insoluble problems in the geochronology of the Aldan Shield was the geochronological relation between the Aldan Shield and the Stanovoi region. The Stansvoi complex is represented by metan~orphic rocks. The boundary between the Aldan Slaield and the Stanovoi region is a wide
belt of tectonic fault zones thousands of kilo- meters in length.
D. S. Konlgii~sky was thc first to suggest a similar age for the Aldan and Stanovoi corn- plexes. He paid attention to the different meta- morphism of both complexes. Stanovoi rocks were metamorphosed inostly to the aar~phibslite facies and Korg'inskj~ held that the Stanovoi
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TOUGARINOV: ALBAN SHIELD, SOUTHEASTERN SIBERIA 655
region had been subjected to retrograde meta- morphisim. This idea explained the appearance in some parts of the Stanovoi ranges of pyroxene- garnet rocks typical of the granulite facies, which were considered remnants of the primary for- mations. Recent geological mapping of this area has led many geologists to conclude that such remnants are derived from a more ancient for- mation than tbe Stanovoi complex itself. For instance, A. V. Koodriavzev mapped a block of pyroxenite gneisses in the upper basin of Timpton River of the Stanovoi complex as part of the Kurultino-Gonamsky complex of Archean age.
Contradictions in geological ideas are reflected in varied iilterpretatioi~s of a great amount of age data determined by different methods for the rocks from this area.
The upper geochronological limit for the Stanovoi complex is now magnificently lineated by 40 age detern~inations of monazite, zircons, and allanites from pegmatites disseminated along fault zones on the boundary between the Aldan and Stanovoi regions. Their age, 1900 may., is the age of intensive magmatism, which completed the development of the Aldan Shield at the end of Early Proterozoic times. It means that the same events were typical for the Stanovoi region as well.
There is a prii~cipal difference between the regions. E. K. Gerling has discovered in the Stanovoi range some amphibolite-pyroxene For- illations with age values of more than 3000 m.y. This means that the suggestion of Koodriav- zev is not so far from the truth. However, in the Aldan complex where granulite formations are more typical, such great age values are absent.
The Pb-U method used for age determinations of apatites from anorthosites of the Jug Jur region give high age values, i.e. 2300 my.
Many geologists now hold the idea that the Olekminskaja Series and the Stanovoi complex are two Proterozoic formations rejuvenated from the most ancient formations of the Aldan Shield.
The Stanovoi region is also interesting from another point of view. This region gives a typical example of the influence of young meta- morphisna on ancient rocks.
First sf all, in tbe upper basin of Blekma and
Jukga Rivers and among Udokan formations to the west of the Stanovoi range, there were dis- covered granosyenite intrusions dated at 1000- 1100 m.y. This means that Late Precambrian magmatic events of this time, which we detected as well in the eastern part of the shield, were typical of tbe whole of the surrounding area and reflected an intense tectono-magmatic cycle, which developed over Transbaikalia.
But, as it follows from Fig. 1, in the southern part of the Stanovoi range there is typically a broad distribution of very young age values, 150-200 m.y., as determined by the K--Ar method. Sometimes the K-Ar and Pb-U data are contradictory, but there are also some young veinlets with accessory uranium-bearing min- erals, which repeat the young ages derived by the K-Ar dating of biotites.
This phenomena has given rise to the follow- ing speculation. Tt is suggested that at great depths in zones of high geothermal gradient, argon easily migrates from K-minerals and there is no accumulation of radiogenic argon in such minerals. Accumulation of argon becomes possible only after the floating of such deeply buried rocks to the surface to a level of low geothermal gradient. Some of our geologists suggest that these young argon dates indicate the time of rapid uplifting of the Stanovoi com- plex. But such speculation ignores the data received by the Pb-U method, and the develop- ment in the same area of younger Jurassic granites as well as the remnant of rocks with pri~llary high age values received by the same K-Ar method.
The influence of Mesozoic magmatism on the structure and isotopic ratios ill Precambrian metamorphic rocks of the Aldan Shield is a special peculiarity of this area.
Even in the central part of the Aldan Shield we find alkaline magmatic rocks of Jurassic age (150 m.y.). This last magmatic cycle occurred in many parts of the Aldan Shield and is iin- postant evidence for the intensive rejuvenation of broad Precambrian areas.
[NOTE: AS the galley proofs had not been received from the author in time to be incorporated in this symposium issue, this paper has bcen published ~vithout benefit of the author's corrections. Ed. Dept. 5 June, 19681
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