I. I. Likhanov

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].

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).

Neoproterozoic collisional metamorphism in overthrust terranes of the Trans-Angarian Yenisey Ridge, Siberia

Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).

Lower proterozoic metapelites in the northern Yenisei Range: Nature and age of the protolith and the behavior of material during collisional metamorphism

The variable P-T metamorphic conditions studied in the Fe-Al metapelites of the Karpinskii Range Formation are regarded as typical of collision-related metamorphism in the trans-Angara part of the Yenisei Range. Recently obtained geochronologic (SHRIMP-II U-Pb zircon dating) and geochemical data on the distribution of major and trace elements are used to reproduce the composition of the protolith, the facies conditions under which it was formed, the tectonic setting, and the age of the eroded rocks. The metapelites are determined to be redeposited and metamorphosed material of Precambrian kaolinite-type weathering crusts of predominantly kaolinite-illite-montmorillonite-quartz composition. The protolith of the rocks was formed via the erosion of Lower Proterozoic granite-gneiss complexes of the Siberian craton (dated mainly within the range of 1962–2043 Ma) and the subsequent accumulation of this material in a continent-marginal shallow-water basin in a humid climate and tectonically calm environment. These results are consistent with data of lithologic-facies analysis and geodynamic reconstructions of the Precambrian evolution of geological complexes in the Yenisei Range. Mass-transfer analysis with the use of the evaluated rock compositions and calculated chemical reactions indicates that the differences in the REE patterns of metapelites from distinct zones can be explained mostly by the chemical heterogeneity of the protolithic material and, to a lesser extent, by metamorphic reactions at a pressure increase.

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.

Geochemistry, age, and petrogenesis of rocks from the Garevka metamorphic complex, Yenisey Ridge

The mineralogical, petrological, geochemical and geochronological data on the Garevka metamorphic complex (GMC) of the Yenisey Ridge were used to evaluate the age, nature, and provenance of their protoliths. The evolution of the GMC occurred in two stages with different ages, thermodynamic regimes, and metamorphic field gradients. The final emplacement of granitoids was marked by high-pressure (HP) amphibolite facies regional metamorphism (970 Ma). At the second stage, these rocks experienced Late Riphean (900–870 Ma) retrograde epidote-amphibolite facies metamorphism accompanied by the formation of blastomylonitic complexes within narrow zones of brittle-ductile deformation. The metamorphism of migmatites (850 Ma) is coeval with the collisional medium-pressure metamorphism of the kyanite-sillimanite type. The GMC is different from the other rock complexes of the Yenisey Ridge in the presence of rapakivi-type granites. The geochemistry of these rocks, which is characterized by stronger enrichment in K2O, FeO, Y, Th, U, Zr, Hf, Nb, Ta, and REE relative to the other mineral assemblages of the GMC, is typical of anorogenic (A-type) within-plate granites. Among other distinctive features of these rocks are the strong iron enrichment of the melanocratic minerals, the presence of ilmenite as the sole Fe-Ti oxide, and crystallization from higher temperature (T = 825°C vs. T = 750°C) water-poor magmas under reducing conditions below the FMQ buffer. Significant variations in the geochemical and petrological characteristics of the GMC rocks suggest that they could not be derived from a single source. The main volume of the high-K rocks varying in composition from A-type to S-type granites was generated by melting of mixed mantlecrustal sources. The products of melting of the Late Archean-Early Proterozoic infracrustal gneisses of the Siberian Craton could be a possible source for the least oxidized rocks.

Precambrian Fe- and Al-Rich Pelites from the Yenisey Ridge, Siberia: Geochemical Signatures for Protolith Origin and Evolution during Metamorphism

In the Trans-Angarian region of the Yenisey Ridge in eastern Siberia (Russia), Fe- and Al-rich low-pressure, andalusite-bearing pelitic schists of the Teya Sequence were subjected to prograde pressure increase in the vicinity of the thrust and were overprinted by medium-pressure, kyaniteand sillimanite-bearing assemblages and microtextures of collisional metamorphism. Major, trace, and rare-earth element contents of rock samples from the different metamorphic zones were determined in order to examine the composition, nature, and evolution of their protolith during metamorphism. Results indicate that these rocks are the redeposited and metamorphosed products of Precambrian kaolinitic weathering crusts. The protolith of the Teya Sequence metapelites was produced by erosion of post-Archean granitoid rocks, which accumulated under humid conditions in shallow-water basins along the continental margin. These results agree with lithofacies data and with the geodynamic reconstruction of the Precambrian evolution of the Yenisey Ridge. Geochemical and petrological data coupled with a mass balance analysis show that the coherent mobility of REEs during collisional metamorphism may be attributed both to mineral reactions responsible for modal changes and to local chemical heterogeneity inherited from the initial protolith, with the prevalence of one or another process at different stages of rock evolution.

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.