Home – Home – Jornals – Russian Geology and Geophysics 2019 number 9

Due to its size and high altitude, the growth of the Tibetan Plateau remains an enigma. Based on a synthesis of anterior collisions, paleoaltimetric data, geochemistry of ultrapotassic lava and their rare mantle enclaves, combined with a reinterpretation of tomographic data, we suppose that Tibet’s growth took take place in two main stages. Initially, the accretion of Gondwana terranes to the margin of South Asia, especially during the Early Triassic-Cretaceous period, resulted in the first episode of plateau growth, which affected an area of about 2/3 of the current plateau. We suppose that during the Late Cretaceous, the Tibetan crust reached a thickness of about 50-55 km, which is equivalent to an altitude of about 2500 to 3000 m, with local landforms that could have exceeded 4000 m. Another important consequence of these successive accretions was a strong metasomatism and a softening of the upper part of the Tibetan cover. The P wave low-velocity anomaly currently observed under the central part of Tibet would correspond not to a temperature anomaly but to a composition anomaly. From 50 Ma onwards, the convergence between India and Asia, estimated at about 1000 km on the Tibetan side, led to a shortening of the plateau by about 40%. We suppose that this additional shortening, which has led to the current thickness of the Earth’s crust of about 70 km and an average altitude of 4800 m, has been compensated by the reactivation of the continental slabs along the previous sutures and by the homogeneous shortening of the crust.

A quantitative assessment of the fractionation of elements during sedimentation is made based on long-term comprehensive studies with the participation of geochemists, hydrobiologists, soil scientists, and chemists. Analytical studies of the chemical composition of water, soil, bottom sediments, and biota were carried out at the Center for Collective Use of Scientific Equipment for Multielement and Isotope Studies and at the Institute of Catalysis, Novosibirsk. Based on a mineralogical and geochemical approach, we chose lakes with different types of biogenetic formation and different classes of organomineral sediments and performed detailed studies of the relationship between the organic and mineral parts of the sediments. It has been established that the organomineral sediments of different classes and biogenetic types of formation differ not only in the contents of major elements (Si, Ca, C, and O) but also in the group composition of organic matter. The direct effect of the transformation of organic matter on the mineral composition of bottom sediments has been revealed. The contents of other elements vary in a narrow range of values. At the same time, the difference in the contents of elements between organomineral sediments of different types and classes is comparable with their difference within a class. The leading role in the formation of the geochemical and mineral compositions of the organomineral sediments of small lakes belongs to intricate biological, biochemical, and physicochemical processes depending mainly on azonal factors and occurring under prolonged freezing-up (anaerobic conditions).

The paper presents a detailed review of chemostratigraphic data on the Bonarelli oceanic anoxic event (OAE2) at the Cenomanian-Turonian boundary from different regions worldwide, with a focus on the respective redox conditions in the basins, metal anomalies, and δ¹³C excursions. Marine sediments with OAE2 signatures in Eastern Kamchatka are compared with their counterparts from submarine volcanic rises in the northwestern Pacific. The available isotopic ages of lavas from some large igneous provinces are correlated with the age of OAE2. The Bonarelli event was triggered and governed by several interrelated factors: volcanism, climate, and ocean circulation. The supply of nutrients into oceans as a result of enhanced magmatism, hydrothermal activity, and weathering on continents played an important role in maintaining anoxia. The spatial patterns of black-shale deposition were controlled by ocean circulation. Geochemical and biochemical processes associated with high productivity ultimately led to its reduction and to the recovery of oxic conditions in oceans.

Studies of helium isotope ratios in groundwaters discharging as springs within the Tunka-Oka-Sayan rift in the East Sayan area reveal an anomaly extending for 350 km along the East Sayan fault on the extension of the Tunka anomaly in the southwestern flank of the Baikal rift system. The calculated heat flow values ( qR ) agree with the measured ones ( qT ), and the heat flow anomalies correlate with those in the ³He/⁴He ratios throughout the Oka-Sayan and Bilin-Busiyn-Gol rifts. The correlation of the geophysical and geochemical parameters confirms that both are controlled by heat and mass transfer. Both mantle helium and heat apparently originate from a mantle plume beneath the South Baikal volcanic province (SVVP). The concordant ³He/⁴He and heat flow patterns in the Oka-Sayan fault record ongoing rifting and magmatism along the East Sayan Fault zone. The helium isotope and heat flow anomalies are consistent with the presence of low-velocity zones in the upper 200 km beneath the southwestern Baikal rift zone as far as the southern edge of the Siberian craton, which are detectable in seismic tomography images. Magmatism in the extreme southwest of the Baikal rift zone was maintained by active tectonic movements in the Pliocene and by the activity of the SBVP mantle plume. The obtained helium isotope data and high heat flow values indicate that rifting and magmatism propagate northwestward along the East Sayan fault zone. This pattern fits the geothermal model for continental rifts implying that magmatic activity in the western end of BRS has been controlled by lithospheric deformation. The geothermal model for the Baikal-Mongolia region covers the area northwest of the system of three rift basins along the East Sayan faults.

This paper considers the use of radioactive elements for the solution of geochemical problems related to granitoid magmatism. The metallogenic types of granites are recognized on the basis of the contents of radioactive (Th-bearing, rare-metal, U-bearing, gold ore, etc.) elements in them. Special attention is focused on the Th/U ratio, which is higher than 2.5 in primary igneous rocks, such as Li-F granites of the Ongon-Hayrhan deposit (Mongolia). Granitoids with Th/U < 2 (and, the more so, Th/U < 1) cannot be considered igneous. They are either metasomatized or of metasomatic genesis. Petrochemical and geochemical data on these types of rocks should be used for petrological implications with great care. Radiogeochemical indices can and must be used as additional indicators of the genesis of rocks, when it is controversial because of the widely manifested convergence and metasomatism processes.

The paper presents results of a comprehensive study of ferromanganese crusts from a submarine volcano on the Sonne Ridge, which was discovered during the 178th cruise of the RV Sonne (2004) investigating the northern slope of the Kuril basin in the framework of the KOMEX program. Results of study of the structures, textures, and mineral and elemental compositions of the crusts suggest their hydrogenous origin. The contents of major, trace, and rare-earth elements and Y (REY) in four mineral phases of ferromanganese crusts of the Sea of Okhotsk have been determined by phase analysis for the first time. The REY patterns of the manganese and ferrous in-situ phases confirms the hydrogenous origin and indicates no hydrothermal influence.

New thermobarogeochemical and isotope-geochemical data are presented, which show the intricate and long history of the formation of the unique Olimpiadinskoe gold deposit with predicted gold reserves of >1000 tons on the Yenisei Ridge. Metal-bearing oxidized water-carbon dioxide and reduced carbon dioxide-hydrocarbon fluids participated (at the same time or successively) in the formation of the deposit at 220-470 ºC and 0.6-2.5 kbar. Fluids of gold-bearing mineral assemblages include CO₂, hydrocarbons, and S-, N-, and halogen-containing compounds capable of transporting ore elements, including gold. Highly mobile carbon dioxide-hydrocarbon fluids were responsible for the appearance of disseminated gold mineralization in large bodies of quartz-carbonate-mica schists serving as geochemical barriers in the Olimpiadinskoe deposit. The deposit formed in the period from 817 to 660 Ma, which fits the time interval from crystallization to cooling (868-721 Ma) of the most proximal multiphase Chirimba granitoid pluton. The hydrothermal activity of the fluids that formed the Olimpiadinskoe deposit lasted at least 100-150 Myr.

The Aksug porphyry Cu-Mo deposit is located in a region of long-lasting magmatic activity. Gabbroids of the Khoito-Oka complex are the earliest intrusive rocks, in which the Aksug granitoid pluton hosting ore-bearing small porphyry intrusions is localized. The intrusive activity was terminated with emplacement of late leucogranite dikes. There are different viewpoints on the age of magmatism and mineralization of the Aksug deposit, with the concept of their Devonian age prevailing. To solve the debatable issue, we performed isotope geochronological studies and analyzed new results of U-Pb (SHRIMP-II) zircon dating and previously published Re-Os molybdenite dates (518 ± 2, 516 ± 2, and 511 ± 2 Ma). The concordant U-Pb zircon ages for igneous rocks are younger than the Re-Os age for mineralization. New U-Pb dating of Khoito-Oka gabbro-diorites has yielded an age of 503 ± 2 Ma. The U-Pb SHRIMP zircon age of tonalites from the Aksug pluton has been estimated at 504 ± 5 Ma. The U-Pb zircon ages for ore-related tonalite porphyry I and tonalite porphyry II are 500 ± 6 and 499 ± 6 Ma, respectively. The obtained SHRIMP age for leucogranite dike is 509 ± 4 Ma. Two groups of U-Pb dates have been obtained for each of the analyzed zircon samples: close to the Re-Os dates (518-511 ± 2 Ma) and younger (507-486 Ma). The weighted average zircon ages calculated for early and late populations from post-ore leucogranites are 515 ± 4 and 500 ± 4 Ma, respectively. We suggest that zircons with an age close to the Re-Os dates found in post-ore leucogranites were assimilated from the underlying substrate and zircons with an age of 500 ± 4 Ma crystallized from melt. The oldest U-Pb dates (509-515 Ma) of individual zircon grains from ore-bearing tonalite porphyry are consistent with the Re-Os molybdenite ages. Zircons from tonalite, tonalite porphyry, and Khoito-Oka gabbroids sometimes show internal textures indicating secondary alteration. The younger U-Pb concordia zircon ages relative to the Re-Os dates might be due to the influence of late thermal processes on the U-Pb isotopic system. The younger dates (486-507 Ma) of individual zircon grains probably reflect the time of the impact of a thermal fluid process. The weighted average of these younger dates (502 ± 2 Ma) falls within the weighted average age of post-ore leucogranites (500 ± 4 Ma). According to the Re-Os dates, the Aksug deposit formed at the end of the early Cambrian. Ore occurrences similar in magmatism and mineralization to the Aksug ore deposit are widespread in Tuva and in the Lake Zone in Mongolia. Therefore, it is necessary to reassess the role of the Cambrian and Devonian magmatism in the development of porphyry Cu-Mo mineralization both in Tuva and in the Altai-Sayan orogenic area.