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New data are presented on the geologic structure, age, petrogeochemical composition, and conditions of formation of the Late Proterozoic Meteshikha ultramafic-mafic pluton of the Ikat complex. Mafic rocks are the main rocks of the massif, whereas ultramafic rocks are secondary; both of them correspond to two intrusive phases. The first phase includes a layered rock series enriched in intercumulus amphibole, which varies in composition from olivine gabbro to leucocratic gabbro-anorthosite; the second is composed of wehrlite, plagiowehrlite, and olivine clinopyroxenite. Mineralogical, petrographic, geochemical, and isotope studies show that the rocks of both phases crystallized from the same mantle melt; note that the PT -conditions of their formation were considerably different. We suppose that they were separated in the intermediate chamber during fractional crystallization and the accumulation of early minerals (olivine and, probably, clinopyroxene) in the lower part of the chamber. Using the COMAGMAT software, we have found the composition of the parental melt for the rocks of the first phase - normal tholeiitic basalt with 0.2-0.5 wt.% water, which might have crystallized at 3.0-3.5 kbar and the oxygen activity controlled by the QFM buffer. The differentiated series is characterized by gradual depletion with Cr and Ni and enrichment with Sr, Ti, Cu, and REE during the evolution of melt. The REE patterns for the massif rocks have a similar low-fractionation trend with domination of light lanthanides over heavy ones and (La/Yb) _N = 1.25-2.75. Multielement spectra are characterized by negative anomalies of K, Th, Nb, and Zr and positive anomalies of Ba, U, Sm, and Sr. The geochemical characteristics of the rocks are similar to those of the tholeiitic basalts of present-day island arcs. Studies show that the Meteshikha massif formed in the subduction setting of the active margin of the Siberian continent in the Late Riphean (809 Ma).

We summarize results of geological, geochronological, petrogeochemical, and isotope-geochemical (Sr-Nd) studies of Late Vendian-Early Paleozoic granitoid batholiths in Eastern Tuva (Kaa-Khem, East Tannu-Ola, Khamsara, etc.). Analysis of geochronological (U-Pb, Ar-Ar) data has shown that the Late Vendian-Early Paleozoic granitoids in Eastern Tuva formed in several stages in the time interval 562-450 Ma and at different geodynamic stages of the regional evolution: island-arc (562-518 Ma) and accretion-collision (500-450 Ma), with the latter stage characterized by more intense granitoid magmatism. Diorite-tonalite-plagiogranite associations with different petrogeochemical parameters are the most widespread in the region. Petrogeochemical studies of the Late Vendian-Early Paleozoic plagiogranitoid associations have revealed high- and low-alumina varieties reflecting different conditions of formation of parental melts. At the island-arc stage of the regional evolution, only low-alumina plagiogranites of tholeiitic ( M -type) and calc-alkalic ( I -type) series formed. Their parental melts were generated at 3-8 kbar through the partial melting of N-MORB-type metabasalts in equilibrium with amphibole restite. Isotope-geochemical studies have shown positive e _Nd values (6.9-6.3) and low Sr isotope ratios (( ⁸⁷Sr/ ⁸⁶Sr) ₀ = 0.7034-0.7046). The lower (as compared with the depleted mantle) e _Nd values and specific petrogeochemical composition (positive Nb-Ta and negative Ti anomalies) of the plagiogranites reflect the subduction nature of metabasic substratum and the subordinate role of ancient crustal material. At the accretion-collision stage of the regional evolution, high- and low-alumina plagiogranitoids of calc-alkalic series (I-type) formed. The high-alumina plagiogranitoids are products of melting of N-MORB-type metabasalts in equilibrium with garnet restite at ³15 kbar in the lower part of the collisional structures, and the low-alumina ones formed through the melting of metabasites in equilibrium with amphibole restite at ≤8 kbar in the upper part of the same structures. The Sr-Nd isotope data for the high- and low-alumina plagiogranitoids generated at the accretion-collision stage show that the rejuvenation of rocks is accompanied by the decrease in e _Nd (from 6.2 to 3.4) and the increase in their model Nd age T _Nd(DM) (from 0.73 to 0.92 Ga) and ( ⁸⁷Sr/ ⁸⁶Sr)0 (0.7036-0.7048). This points to the essentially metabasic composition of the parental substratum, as in the case of the island-arc plagiogranitoids, and the progressive supply of ancient crustal material to the magma chamber.

Palaeoseismological studies were performed within the Yaloman graben (Gorny Altai). Five Quaternary sections with coseismic deformation structures (seismites) have been recognized in the lower coarse of the Malyi Yaloman River. Traces of ancient earthquakes are localized at two levels (Late Pleistocene-Holocene). The most likely mechanisms of the seismite formation are brittle failure, liquefaction, and fluidization. The types of coseismic deformations and their sizes suggest that the Yaloman graben was the locus of prehistoric earthquakes with M ≥ 5-7, although modern-day seismic activity consists of smaller-magnitude earthquakes. This should be taken into account in assessing the seismic hazards during construction of gas pipeline to China and tourism infrastructure facilities.

Experimental data on diamond crystallization in model systems at 5.2-7.5 GPa and 1150-1800

To simulate a natural carbonate-silicate medium parental for both diamondites and their syngenetic mineral inclusions, melanocratic carbonatites of the Chagatai complex, Uzbekistan, were used. The carbonatites are characterized by a high percentage of silicate components and high-pressure eclogitic silicate minerals. The experiments carried out at 7.0-8.5 GPa and 1800

Experimental data on synthesis of potassium-bearing clinopyroxene in systems of different compositions are reviewed. The partitioning of K between clinopyroxene and melt is governed mainly by such factors as pressure, concentrations of K, Si, and Al in coexisting aluminosilicate melt, and jadeite and Ca-Tschermak contents in clinopyroxene. Temperature has a minor effect on partition coefficient, especially for natural clinopyroxenes of complex composition. Based on these observations, a thermodynamic equation for the KAlSi₂O₆ (Cpx) = KAlSi₂O₆ (melt) equilibrium at 1-11 GPa and 1100-1900

The goal of this work was to study phase relations on melting of the system CaCO₃-Mg₃Al₂Si₃O₁₂ (pyrope)-FeS (pyrrhotite) modeling the compositions of mineral phases associated with natural diamonds. The experiments were carried out at 7 GPa and 1200-1600

Hot cathode cathodoluminescence (CL) microscopy is introduced as a powerful tool to distinguish between different silica phases and between different generations of the same breakdown product of coesite. The rock type investigated is a fine grained pyrope-quartzite from Pangi, Dora Maira Massif, Italy. While coesite inclusions within pyrope display bright bluish-green luminescence colours, their palisade-like breakdown products of quartz are characterized by heterogeneously brownish-violet colours. Cracks within coesite are filled by quartz which shows brown luminescence colours indicating a different generation. Quartz of the matrix exhibits a homogeneously dark blue luminescence. Fine-grained, partly also fibrous chalcedony which is difficult to distinguish from quartz breakdown products of coesite under the polarizing microscope shows bright brownish-yellow luminescence colours. Consequently, even tiny crystals of coesite can easily be separated from different quartz generations or chalcedony due to their characteristic CL emission. Thus, CL-microscopy is an elegant and inexpensive method to discover chronological breakdown sequences of silica and to reveal specific features and to obtain information of minerals which may have become otherwise overlooked.
Twinning of metamorphic coesite resembles polysynthetic plagioclase exhibiting albite- and pericline-law individuals. Two respective types of lamellae were identified which intersect each other at an angle of ca. 90

Eclogitization of gabbronorites in the temperature range 600-700

Low-K medium-Ti tholeiite basic rocks represented by weakly altered pillow basalts as well as by their metamorphosed counterparts (amphibolites and blueschists) are exposed on tectonic slices in the southeast of Bol'shoi Lyakhovsky Island. The rocks are depleted in light rare-earth elements and were melted out of a depleted mantle source. A different mantle source enriched in incompatible high-field strength elements, such as Th, Nb, Zr, also contributed to the rock formation. The magma sources were not affected by subduction-related fluids or melts. The basalts represent the upper portion of oceanic crust of the Jurassic South Anyui ocean. The blueschists belong to the same oceanic crust subducted beneath the more southern Anyui-Svyatoi Nos arc to a depth of 25-30 km. Pressure and temperature of metamorphism indicate a