N. V. Popov

Collisional metamorphism as a result of thrusting in the Transangara region of the Yenisei Ridge

The Yenisei Ridge is one of the most interesting regions in the southwestern folded framework of the Siberian Craton with respect to geodynamics. The regional structure of the Transangara sector of the Yenisei Ridge is traditionally displayed as a NW-trending system of tectonic sheets divided by faults characterized by the collision of blocks and thrusting. Therefore, this region was subject to pressure-variable regional metamorphism expressed in the juxtaposition of lowand moderate-pressure metamorphic facies. Collisionrelated moderate-pressure metamorphism is locally superimposed on the low-pressure (presumably, younger) metamorphic rocks. As a result, andalusite is replaced with kyanite with the formation of new mineral assemblages and deformational structures. The prograde replacement of andalusite with kyanite in the Yenisei Tange is a rare phenomenon, because the stationary continental geotherm commonly does not intersect the andalusite‐kyanite equilibrium line. Such replacements are usually referred to the retrograde stage of metamorphism, but this interpretation comes into conflict with the regional geological situation. Only a few examples are known in the literature (Northwest Cordillera in the United States and Canada, Dalradian in Scotland, central and northwestern Appalachians in the United States, and the Kola Peninsula and Yenisei Ridge in Russia), where prograde transformation of andalusite into kyanite is assigned either to the metastable state of andalusite in the PT stability field of kyanite or to an increase in pressure as a result of thrusting or magmatic loading characterized by different PT trends. While studying collisional metamorphism in the Transangara sector of the Yenisei Ridge, we selected three (Chapa, Mayakon, and Angara) areas composed of Paleoproterozoic, Middle Riphean, and Upper Riphean rocks (Fig. 1). The Chapa and Mayakon areas are located in the Central uplift between the Ishimbino and Tatarka deep faults. The Angara area covers the junction of the Transangara structural units and the Angara‐ Kan block. The Angara area is situated at the interfluve of the Angara, Belokopytovka, and Malaya Sploshnaya rivers. The reference sections are exposed in the Tatarka shear zone along the right bank of the Angara River between the mouths of the Babkin and Polovinkin creeks. The area is composed of the Upper Riphean low-pressure metasedimentary rocks (rhythmic intercalation of quartzites and phyllites of the Sukhoi Ridge Formation). In the study area, this sequence is largely made up of phyllites of the greenschist facies represented by quartz (Qtz), muscovite (Ms), chlorite (Chl), and ilmenite (Ilm). These rocks underwent high-pressure collisional metamorphism with the formation of new (kyanite-bearing) mineral assemblages. Metamorphism occurred simultaneously with the development of steep (80 ° ‐85 ° NW and SE) near-meridional cleavage. The increase in the metamorphic grade in the near-latitudinal direction is marked by the successive formation of chloritoid (apparent thickness 0.5‐0.8 km) and kyanite (~1.5‐1.7 km) zones. These minerals correspond to the conditions of kyanite schist facies. The eastern boundary of these rocks is hidden beneath unmetamorphosed Paleozoic rocks of the Pogromnino basin. In the Kulakovo uplift on the left bank of the Angara River, collisional metamorphism is expressed in the crystallization of kyanite (Ky), chloritoid (Cld), and ilmenite in metapelites consisting of staurolite (St), plagioclase (Pl), Ms, biotite (Bt), Qtz, and garnet (Grt) [2].

Mesoproterozoic granitoid magmatism in the Trans-Angara segment of the Yenisei Range: U-Pb evidence

Marginal parts of cratons yield valuable informa� tion on the evolution of the lithosphere, which is con� trolled by different geodynamic processes. This explains the interest in problems of reconstruction and development of accretionary-collisional continental structures at craton margins (1). The Yenisei Range representing a foldthrust belt in the southwestern margin of the North Asian Craton is geodynamically one of the most interesting regions of Siberia. Its TransAngara segment hosts widespread Proterozoic substantially metasedimentary rocks of the Tei Group. The basal part of its section corresponds to the Malaya Garevka and Nemtikha metamorphic complexes, pre� sumably Early Precambrian in age (2). The strati� graphic position and age of these rocks are debatable. Recent studies of presumably the oldest granitoids in the TransAngara part of the Yenisei Range using pre� cise geochronological methods revealed their Neopro� terozoic age (865-880 Ma) (3). Based on these data, many researchers have arrived at the conclusion that no Grenvillian collisional events related to the forma� tion of the supercontinent Rodinia took place in this region and that its stratified and intrusive complexes are Neoproterozoic in age, not older. The original geochronological data indicate devel� opment of Mesoproterozoic granitoid magmatism in this region. These results are important for periodiza� tion of the Precambrian and understanding the geody� namic evolution of the western North American Cra� ton as well as for solving the widely discussed problem concerning the probable position of this continental block in the ancient supercontinents Pangea I and Rodinia (4, 5). Periodization of the Mesoproterozoic is one of the fundamental problems dealing with the long geologi� cal period (1.6-1.0 Ga) spanning from termination of the intense growth of the continental crust to the for� mation of Rodinia. Inasmuch as Mesoproterozoic rocks are characterized by a limited distribution, the formation mode of the crust at this stage remains unclear, which prompts the question: did the super� continent exist continuously as a single block during this stage or was it first broken similarly to Pangea I (its predecessor) and then later amalgamated at the end of the Mesoproterozoic after the Grenvllian Orogeny to form epiGrenvilian Rodinia (1).

Collision Metamorphism of Precambrian Complexes in the Transangarian Yenisei Range

Three complexes in the zones of the Ishimbinskii and Tatarka deep faults in the Transangarian part of the Yenisei Range were studied to reproduce their metamorphic evolution and elucidate distinctive features of regional geodynamic processes. The results of our geological and petrological studies with the application of geothermobarometry and P-T metamorphic paths indicate that the Neoproterozoic kyanite-sillimanite intermediate-pressure metamorphism overprinted regionally metamorphosed rocks of low pressure of Middle Riphean age. The kyanite-sillimanite metamorphism was characterized by (1) the development of deformational structures and textures and kyanite-bearing blastomylonites with sillimanite, garnet, and staurolite after andalusite-bearing regional-metamorphic mineral assemblages; (2) insignificant apparent thickness of the zone of intermediate-pressure zonal metamorphism (from 2.5 to 7 km), which was localized near overthrusts; (3) a low geothermal gradient during metamorphism (from 1–7 to 12°C/km); and (4) a gradual increase in the total metamorphic pressure from southwest to northeast with approaching the overthrusts. These features are typical of collisional metamorphism during the thrusting of continental blocks and testify that the rocks subsided nearly isothermally. The process is justified within the scope of a model for the tectonic thickening of the crust via rapid thrusting and subsequent rapid exhumation and erosion. The analysis of our results with regard for the northeastern dips of the thrusts allowed us to consider the intermediate-pressure metapelites as products of collision metamorphism, which were formed in the process of a single thrusting of ancient rock blocks from the Siberian Platform onto the Yenisei Range.

Age and tectonic position of the Chiney Layered Massif, Aldan shield

The Chiney Complex of basic rocks includes several layered gabbroid massifs. The largest and the best stud- ied massif is the Chiney pluton, which is located among the terrigenous sequence of the Udokan Group, in the southeastern part of the Chara-Olekma geoblock of the Aldan shield. The Udokan sequences are also intruded by the rapakivilike granites of the Kodar Complex. The Chiney Massif is widely known owing to its associated economic mineralization represented by the unusual association of Ti-magnetite V-bearing and PGE-bearing sulfide copper ores. The age of this massif is presently estimated on the basis of 20-year old data: Thus, the age of the considered massif is bracketed between 2180 ± 50 and 1830 ± 50 Ma. This range is too wide to determine the position of the massif in the mod- ern integrated geodynamic models of the evolution of the Precambrian complexes of the Aldan shield (3, 4) and to estimate the promise of this region for Fe-Ti-V and Cu-PGE mineralization. In addition, more data need to be obtained to constrain the upper and lower age limits of the different units of the Udokan Group, which is the Lower Proterozoic stratotype of the Siberia and Far East and serves as an age marker in the regional stratigraphic scale. In order to solve this problem, we conducted U-Pb geochronological studies of the Chiney Massif, which are presented in this paper. The Chiney Massif is situated in the southeastern part of the Kodar-Udokan trough, which is filled mainly with metasedimentary rocks of the Udokan Group. It penetrates terrigenous-carbonate rocks of the upper Chiney (Aleksandrovskaya and Butunskaya For- mations) and lower Kemen (Sakukan Formation) sub- groups of the Udokan group. The massif is exposed as a W-E-trending elongated body with the maximal size of 9 × 16 km and an area of 150 km 2 . The study results are reported in detail in the generalized monographs (5-8).

The first find of rapakivi granite in the Yenisei ridge: Age, PT conditions, and tectonic settings

365 Rapakivi granites discovered within most ancient cratons worldwide are products of the Precambrian evolution of the Earth. According to the tectonic set ting, they are related to intraplate formations, the ori gin of which was controlled by the processes of conti nental rifting and activity of mantle plumes. The abundance of these rocks in space and time is very uneven. The formation of most rapakivi granite plu tons occurred in Proterozoic foldbelts and covered the range from the end of the Paleoproterozoic to the Neoproterozoic. The peak of this magmatism is usu ally related to Grenville orogeny, the first significant epoch of folding in the Neogean, during which the Rodinia supercontinent was formed [1]. In recent years special interest in these rocks was caused by their ore generating role in the formation of a number of tin and Cu–U–Au–Ag–REE deposits.

The Oldest Granitoids in the Transangarian Part of the Yenisey Ridge: U-Pb and Sm-Nd Data and Geodynamic Settings

The existence of several alternative geodynamic models of the Yenisey Ridge leads to competing inter pretations of a number of the key geological problems of the region. Current geodynamic scenarios for the evo lution of the Yenisey Ridge imply that this region showed little endogenic activity for approximately 1 Gyr, from the time of emplacement of the Tarak gra nitic plutons (1900–1840 Ma) into the Angara–Kan granulite–amphibolite facies terrane to the Middle Neoproterozoic (~750 Ma) [1]. In view of this implica tion, previous geochronological data have been revised in several recent studies suggesting the absence of Mesoproterozoic and Grenvillian events within the Yenisey Ridge [2]. However, the character of crustal evolution during this stage is still speculative because of the lack of reliable isotope data. Our recent SHRIMP II U–Pb and 40Ar/39Ar ages of metapelites provide a link between regional metamor phism in the Yenisey Ridge and the Grenville orogeny [3]. Interpretation of these results in the context of a supercontinent cycle [4] may both contribute to the understanding of the crustal evolution in this region and help find posthumous traces of collisional orogens, which have been identified in most Mesoproterozoic cratons. Based on recent geochronological and geochemical data, we attempted to evaluate the geodynamic nature and age of apogranitic rocks of the Nemtikha Complex and their protolith. The tectonic setting of this crystal line complex and its age, which has been estimated from the Archean [5] to Neoproterozoic [6], are issues of ongoing interest and debate.

New data on Late Riphean intraplate granitoid magmatism in the Transangarian Yenisei Ridge

1100 Problems in estimation of the geochronological boundaries in the history of the formation and collapse of the ancient supercontinent of Rodinia are largely far from receiving final solution. This is explained by deficiency of precise age dating and ambiguity of pale oreconstructions of the location of individual cratons in the composition of the supercontinent, which sig nificantly limits the possibilities of age correlations of geological processes on the global scale. Because of this, the problem of distinguishing and dating of Neoproterozoic complexes indicating the processes of continental riftogenesis resulting from breakdown of Late Proterozoic Rodinia is still relevant [1].

Late Vendian postcollisional leucogranites of Yenisei Ridge

The Late Vendian (540–550 Ma) U–Pb zircon age of postcollisional granitoids in the Osinovka Massif was obtained for the first time. The Osinovka Massif is located in rocks of the island-arc complex of the Isakovka Terrane, in the northwestern part of the Sayany–Yenisei accretion belt. These events stand for the final stage of the Neoproterozoic history of the Yenisei Ridge, related to the completing accretion of the oceanic crust fragments and the beginning of the Caledonian orogenesis. The petrogeochemical composition and the Sm–Nd isotopic characteristics support the fact that the granitoid melt originated from a highly differentiated continental crust of the southwestern margin of the Siberian Craton. Hence, the granite-bearing Late Riphean island-arc complexes were thrust over the craton margin at a distance considerably exceeding the dimensions of the Osinovka Massif.

Paleoproterozoic granitoids from the basement of the central Siberian Platform (Borehole Mogdinskaya-6): U-Pb age and composition

334 The Siberian Platform is the largest structure of Northeast Asia, which records practically its entire geological history since the Early Archean. The base ment of the platform crops out in the Aldan Shield in the southeast, Anabar Shield, Olenek Uplift in the north, and as small inliers along its southwestern periphery (Kan, Sharyzhalgai, Irkutnyi, and Angara– Kan) (Fig. 1). The remaining part of the platform is overlain by a thick cover of Phanerozoic sedimentary and igneous rocks. The age and composition of the buried basement of the Siberian Platform is deter mined by analogy with the well known metamorphic and igneous rocks cropping out in the above men tioned shields and inliers. The data on the occurrence, depth, thickness, and lateral distribution of regional rock complexes of the basement under the cover are usually derived from geophysical data whole rock composition of blocks (terranes) and specific features of boundaries between these blocks (Fig. 1).

Kuranakh complex diabases in the western part of the Aldan-Stanovoi Shield: Age and tectonic setting

The age of Kuranakh Complex diabases in the western part of the Aldan-Stanovoi Shield determined by the zircon U-Pb method is virtually identical to that of basic rocks in the Chinei stratified pluton and granites in the Kodar Complex. Thus, it is possible to suggest that they form a unified bimodal magmatic association and belong to the South Siberian postcollision magmatic belt, which extends along the southwestern framing of the Siberian Craton for more than 2500 km from the Yenisei mountain range to the Aldan-Stanovoi Shield. The occurrence of the diabase dike swarms in magmatic associations of this belt testifies to formation under lithospheric extension conditions.