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.     

Kyanite–sillimanite metamorphism of the Precambrian complexes,Transangarian region of the Yenisei Ridge

Four Precambrian metamorphic complexes in the vicinity of regional faults in the Transangarian region of the Yenisei Ridge were examined. Based on geothermobarometry and P-T path calculations, our geological and petrological studies showed that the Neoproterozoic medium-pressure metamorphism of the kyanite–sillimanite type overprinted regionally metamorphosed low-pressure andalusite-bearing rocks at about 850 Ma. A positive correlation between rock ages and P-T estimates for the kyanite-sillimanite metamorphism provide evidence of the regional structural and tectonic heterogeneity. The medium-pressure metamorphism was characterized by (1) the development of deformational structures and textures, and kyanite-bearing blastocataclasites (blastomylonites) with sillimanite, garnet, and staurolite after andalusite-bearing regional metamorphic rocks; (2) insignificant apparent thickness of the zone of medium-pressure zonal metamorphism (from 2.5 to 7 km), which was localized in the vicinity of the overthrusts; (3) a low metamorphic field gradient during metamorphism (from 1–7 to 12 °C/km); and (4) a gradual increase in lithostatic pressure towards the thrust faults. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence for near-isothermal loading. This event was justified within the framework of the crustal tectonic thickening model via rapid overthrusting and subsequent rapid uplifting and erosion. The results obtained allowed us to consider medium-pressure kyanite-bearing metapelites as a product of collision metamorphism, formed either by unidirectional thrusting of rock blocks from Siberian craton onto the Yenisei Ridge in the zones of regional faults (Angara, Mayakon, and Chapa areas) or by opposite movements in the zone of splay faults of higher orders (Garevka area).     

Fe- and Al-Rich metapelites of the Teiskaya Group, Yenisei Range: Geochemistry, protoliths, and the behavior of their material during metamorphism

The Fe- and Al-rich metapelite of the Teiskaya Group in the trans-Angara part of the Yenisei Range are characterized by variable P-T parameters of their metamorphism. Geochemical data on these rocks were used to reproduce the nature and composition of their protolith. The metapelites were determined to be redeposited and metamorphosed Precambrian weathering crusts of the kaolinite type, which were produced by the erosion of Archean rocks of predominantly acid (granitoid) composition in shallow-water continental-margin basins in a humid climate. These results are consistent with the results of the lithological-facies analysis and geodynamic reconstruction of the Precambrian geological evolution of complexes in the Yenisei range. Differences were revealed between the REE patterns in metapelites in various metamorphic zones caused by both the compositional heterogeneity of the protoliths and the prograde (in terms of pressure) mineral reactions of collision metamorphism with the predominant effects of various processes during different evolutionary stages.     

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.     

Inverse modeling approach for obtaining kinetic parameters of diffusion-controlled metamorphic reactions in the Kharlovo Contact Aureole (South Siberia, Russia)

Summary A new approach for deriving kinetic parameters of diffusion-controlled metamorphic reactions is described. It is based on comparison of models of temperature field evolution near a cylindrical intrusion and observable chemical changes in biotite at the contact aureole of the Kharlovo massif. Unlike other methods used to construct quantitative models of reactions which produce distinctly expressed zoned microtextures, the proposed approach allows better understanding of the processes occurring in texture-homogeneous rocks, where complete consumption of some reactant mineral phases prevent accurate reconstruction of the metamorphic process. The calculations yielded estimates of the effective rates of metamorphic reactions (10⁻¹² s⁻¹) and the effective diffusion coefficient (5.29 × 10⁻¹⁶ cm² s⁻¹). These values are consistent with experimentally and theoretically derived estimates of solid-state diffusion, including grain-boundary diffusion.

---

Zusammenfassung Inverse Modellierung zur Bestimmung kinetischer Parameter diffusionskontrollierter metamorpher Reaktionen in der Kontaktaureole von Kharlovo (Südsibirien, Ru?land) Ein neuer Ansatz zur Herleitung kinetischer Parameter diffusionskontrollierter metamorpher Reaktionen wird beschrieben. Er basiert auf einem Vergleich der Modelle für die Temperaturentwicklung um eine zylindrische Intrusion mit beobachtbaren ?nderungen im Chemismus von Biotit in der Kontaktaureole des Kharlovo Massivs. Im Unterschied zu anderen Methoden, die quantitative Modelle von Reaktionen mit deutlich zonierten Mikrotexturen verwenden, erlaubt dieser Ansatz ein besseres Verst?ndnis der Prozesse in texturell homogenen Gesteinen, in denen die vollst?ndige Reaktion einiger Reaktanden (Mineralphasen) sonst die Rekonstruktion des Metamorphoseprozesses verhindert. Die Berechnungen erlauben eine Absch?tzung der metamorphen Reaktionsraten und des effektiven Diffusionskoeffizienten . Diese Werte sind mit experimentell und theoretisch ermittelten Werten der Diffusion in Festk?rpern, einschlie?lich der Diffusion entlang von Korngrenzen, konsistent.

---

---

Received July 14, 1998; revised version accepted June 28, 2000     

Nature and Age of Detrital Zircons from Rocks of the Shear Zone: The Problem of Occurrence of the Archean Basement in the Transangarian Yenisei Ridge

Petrogeochemical and geochronological study of boudined quartzite sandstones and tonalites, as well as host amphibolites, in the shear zone showed that potential sources of detrital zircons were Neoarchean–Paleoproterozoic rocks of the Angara–Kan block and products of their metamorphism. Interpretation of the available data does not confirm the inferred presence of the Early Precambrian basement of the Siberian Craton in the Transangara region.