P. S. Kozlov

Contact metamorphism of Fe- and Al-rich graphitic metapelites in the Transangarian region of the Yenisei Ridge, eastern Siberia, Russia

Abstract Prograde evolution of minerals in Fe- and Al-rich graphitic metapelites in the Ayakhtinsk aureole, Siberia, produced the unusual contact metamorphic mineral assemblages: chloritoid+biotite, chloritoid+biotite+andalusite and cordierite+garnet+muscovite. Field-petrologic observations show that: (1) the grade of contact metamorphism ranges from chloritoid to sillimanite–alkali feldspar zone; (2) chloritoid+biotite assemblages have a restricted temperature interval and give way up-grade to garnet+chlorite assemblages; and (3) garnet+chlorite assemblages have a wide temperature interval and give way to cordierite+biotite parageneses with increasing grade. Geothermobarometry and thermodynamic analysis of mineral equilibria give estimates of the P – T – X H 2 O conditions of the contact metamorphism: the temperatures increase toward the intrusive contact from 430 to 640 °C at P =3.2±0.3 kbar, X H 2 O for the metamorphic fluid decreases toward the intrusive contact from 0.89–0.85 at T =450 °C to 0.49–0.36 at T =640 °C, assuming ideal and non-ideal mixing, respectively, of H 2 O–CO 2 in the fluid phase. The stability of the rare mineral assemblages chloritoid+biotite and chloritoid+biotite+andalusite within the contact aureole can be explained by the unusual combination of pressure (>3 kbar) and Fe- and Al-rich bulk-rock compositions. The local occurrence of the cordierite+garnet+muscovite mineral assemblage is controlled by Mn in the garnet composition and Fe- and Al-rich bulk-rock composition than P – T conditions. Our data are compatible with the KFMASH grid of Spear and Cheney [Contrib. Mineral. Petrol. 101 (1989) 149.].

Grenville tectonic events and evolution of the Yenisei Ridge at the western margin of the Siberian Craton

Geological, petrologic, geochemical, and isotopic geochronological evidence for Grenville events at the western margin of the Siberian Craton are considered. These events were related to assembly of the Rodinia supercontinent. Multiple manifestations of riftogenic and within-plate magmatism at the final stage of orogenic evolution gave rise to breakdown of Rodinia and the formation of the Paleoasian ocean. The results allowed us to develop a new concept on the Precambrian geological evolution of the Yenisei Ridge and the processes that created its tectonic structure. The chronological sequence of events in the history of the Transangarian Yenisei Ridge is based on geological evidence and isotopic dating of Precambrian complexes variable in geodynamic nature. Four tectonic stages dated at 1.4−1.1, 1.1−0.9, 0.90−0.85, and 0.8−0.6 Ga were controlled by collision and extension recognized from large regional linear crustal structural elements. The evolution of the Transangarian Yenisei Ridge, which lasted for ∼650 Ma, corresponds in duration to supercontinental cycles that begin from rifting and breakdown of the predated supercontinent and was completed by orogeny and the formation of a new supercontinent. The regional geodynamic history correlates with the synchronous sequence and similar style of tectonothermal events at the periphery of the large Precambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which confirm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400−600 Ma ago and indicate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.

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].

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.

Three Metamorphic Events in the Precambrian P-T-t History of the Transangarian Yenisey Ridge Recorded in Garnet Grains in Metapelites

A study of gneisses and schists from the Yenisey regional shear zone (Garevka complex) at the western margin of the Siberian Craton has provided important constraints on the tectonothermal events and geodynamic processes in the Yenisey Ridge during the Riphean. In situ U-Th-Pb geochronology of monazite and xenotime from different garnet growth zones and the calculation of P-T path derived from chemical zoning pattern in garnet were used to distinguish three metamorphic events with different ages, thermodynamic regimes and metamorphic field gradients. The first stage occurred as a result of the Grenville orogeny during late Meso-early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism at ∼4.8–5.0 kbar and 565–580°C and a metamorphic field gradient with dT/dH = 20–30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) collision-related medium-pressure metamorphism at ∼7.7–7.9 kbar and 630°C with dT/dH ≤ 10°C/km. The final stage evolved as a syn-exhumation retrograde metamorphism (785–776 Ma) at ∼4.8–5.4 kbar and 500°C with dT/dH ≤ 12°C/km and recorded a relatively fast uplift of the rocks to upper crustal levels in shear zones. The range of exhumation rates at the post-collisional stage (500–700 m/Ma) correlates with the duration of exhumation and the results of thermophysical numerical modeling of metamorphic rocks within orogenic belts. The final stages of collisional orogeny are marked by the development of rift-related bimodal dyke swarms associated with Neoproterozoic extension (797 ± 11 and 7.91 ± 6 Ma; U-Pb SHRIMP II zircon data) along the western margin of the Siberian craton and the beginning of the breakup of Rodinia. Post-Grenville metamorphic episodes of regional evolution are correlated with the synchronous succession and similar style of the later tectono-metamorphic events within the Valhalla orogen along the Arctic margin of Rodinia and support the spatial proximity of Siberia and North Atlantic cratons at about 800 Ma, as indicated by the latest paleomagnetic reconstructions.

The first data on mesoproterozoic tectonic events in the geological history of the South Yenisei ridge

1274 Special attention in scientific publications of recent years has been paid to shear zones of continen tal margins, which are the areas of bulk fragile and plastic flows of rock masses localized along the narrow linear structures of the Earth’s crust. These publica tions demonstrate some characteristic effects accom panying intense shear deformations during tectogene sis. Despite increasing interest in these problems, one of the least studied is the problem of the role of syn shear metamorphic processes in the formation of the foldbelt structure. The significance of these processes for reconstruction of the evolution of orogens is strongly underestimated. In this paper we consider new geochronological and petrological data for metapelites of the South Yenisei ridge with different ages, T–P parameters of metamorphism, and intensi ties of deformations. These data allowed us to distin guish the Mesoproterozoic stage in the tectonic his tory of the region for the first time. In addition to the solution of regional problems, these results are impor tant for correlation of regional peculiarities with global geological processes, in particular, entrance of the Siberian Craton into the composition of the Nuna and Rodinia supercontinents [1].

The Teya polymetamorphic complex in the Transangarian Yenisei Ridge: An example of metamorphic superimposed zoning of low- and medium-pressure facies series

Two successive phases of metamorphism can be recognized based on mineralogical and petrological observations coupled with geothermobarometric estimates for chemical zoning in Fe- and Al-rich metapelites from the Teya crystalline rocks of the Transangarian Yenisei Ridge. The first phase is marked by the formation of low-pressure regional metamorphic complexes of the andalusite-sillimanite type (P = 3.9–5.1 kbar; T = 510–640°C), which were most likely related to the Middle Riphean Grenville events. In the second phase, metapelitic rocks underwent Late Riphean medium-pressure collisional metamorphism of the kyanite-sillimanite type (P = 5.7–7.2 kbar, T = 660–700°C), which resulted locally in an increase in pressure in the vicinity of thrusts. These results suggest that medium-pressure kyanite-bearing metapelitic rocks were formed as a result of collision-related metamorphism caused by thrusting of the Siberian cratonal blocks onto the Yenisei Ridge in the vicinity of the Tatarka deep fault.

U-Pb and 40Ar/39Ar evidence for Grenvillian activity in the Yenisey Ridge during formation of the Teya metamorphic complex

Reconstruction of the geologic history of the Yenisey Ridge, which developed as an accretionary collision orogen on the western margin of the Siberian craton is essential to understanding the evolution of mobile belts surrounding older cratons, as well as to resolving the recently much debated problem of whether Siberia was part of the supercontinent Rodinia [1]. Available paleo tectonic models suggest that this supercontinent was assembled at the Middle–Late Riphean boundary (1100–900 Ma) as a result of the Grenville orogeny, the first long lived mountain building event which occurred in geosynclinal areas during the Neogaea [2]. However, the character of crustal evolution at that stage is still speculative due to the lack of reliable and conclu sive isotope data. In many current geodynamic models, a common underlying assumption is that the Yenisey Ridge showed very little endogenic activity for 1 Gyr, from the time of Tarak granite emplacement (1900– 1840 Ma) to the Middle Neoproterozoic (~750 Ma). On the basis of this assumption, several recent studies suggested the absence of Grenvillian collisional events within the Yenisey Ridge [e.g., 3]. The results of the SHRIMP II U–Pb analysis of rift related plagiogranites of the Nemtikha Complex, Yeni sey Ridge (1380–1360 Ma) [4] suggest an increase in magmatic activity in the Mesoproterozoic. Interpreta tion of these results in terms of a supercontinent cycle [5] may help find evidence for possible occurrence of the Grenville orogeny on the western margin of the Siberian craton. With this in mind, we attempted to reconstruct using recent geochronological constraints the evolution of metapelitic rocks from the Teya poly metamorphic complex (TPMC), which is a good example of superimposed zoning of low and medium pressure facies series [6]. High precision age determi nations from rock complexes formed in different geody namic settings under different thermodynamic condi tions and geothermal gradients were used to distinguish several major metamorphic events and unravel their time relations with tectonic and magmatic activity in the region.

Neoproterozoic metamorphic evolution in the Transangarian Yenisei Ridge: Evidence from monazite and xenotime geochronology

Chemical mapping and in situ dating of U-Th-rich minerals in zoned garnets from gneisses of the Garevka metamorphic complex were used to constrain multiple metamorphic events in the Transangarian Yenisei Ridge. The data provide supporting evidence for three distinct metamorphic stages. The first episode occurred as a result of the Grenville orogeny during the Late Mesozoic and Early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism and a metamorphic field gradient with dT/dH = 20−30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) syn-collisional medium-pressure metamorphism with a low metamorphic field gradient (dT/dH ≤ 10°C/km). The final stage evolved as a synexhumation dynamic metamorphism (785–776 Ma) with dT/dH ≤ 12°C/km and reflected rapid exhumation of rocks in shear zones. The sequence of collisional events within the western margin of the Siberian craton affected by the Valhalla orogen suggests that Siberia and cratons of the North Atlantic region were in close proximity to one another at about 800 Ma, which is supported by recent paleomagnetic reconstructions.

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.