Petrologiya

Tectonic sheets of various size along the southern slope of the Mongolian and Chinese Altai ranges and in eastern Kazakhstan include high-grade metamorphic rocks, which are collectively referred to as the Southern Altai Metamorphic Belt. Rocks of the sheets show traces of amphibolite-facies elevated-pressure metamorphism of the kyanite–sillimanite type M₂. Some of the tectonic sheets display evidence of polymetamorphism: the rocks preserve textures and mineral assemblages of an earlier metamorphic episode (of elevated temperature and relatively low pressure) of the andalusite–sillimanite facies series M₁. The earlier metamorphic episode occurred at 390–385 Ma, and the later one, at ~370–356 Ma. The protoliths of the high-grade metamorphic rocks were mostly Early Paleozoic terrigenous rocks and subordinate amounts of volcanic rocks analogous to the weakly metamorphosed or unmetamorphosed rocks in their northern surroundings. Typical rocks of the tectonic sheets are mafic dikes and massifs of the Gashun Nuur Complex, which were emplaced between metamorphic episodes M₁ and M₂. According to their geochemistry and Nd isotopic parameters, most of the metabasites are similar to enriched basalts of mid-oceanic ridges and oceans plateaus. The quantitatively subordinate group of the layered mafic bodies displays geochemical characteristics of subduction-related rocks. Correlations between the metamorphic events and magmatism in the continental (Mongolian and Chinese Altai) and paleoceanic (Trans-Altai Gobi and eastern Junggar) regions led us to suggest a geodynamic model for the development of the Southern Altai Metamorphic Belt. The volcano-terrigenous rocks, which were later metamorphosed, were accumulated mostly in the Early Paleozoic as an accretion wedge on an active continental margin. The earlier episode of high-temperature metamorphic M₁ and coeval large-scale calc–alkaline magmatism occurred at the same active continental margin after the magmatic front shifted southward (in modern coordinates). The emplacement of the swarms of mafic bodies of the Gashun Nuur Complex and simultaneous rifting in the southern Chinese Altai were triggered by the subduction of an spreading ridge of an oceanic or backarc basin beneath the active margin. The second metamorphic episode (elevated-pressure metamorphism) M₂ and overthrusting in the structures of the Altai are correlated with deformations at low angles and the transition from oceanic to continental volcanism in the Trans-Altai Gobi and Junggar. These tectonic processes were induced by the accretion of a system of mid-Paleozoic ensimatic island arcs of the Trans-Altai Gobi and Junggar to the Altai margin of the Siberian paleocontinent.

The paper presents newly acquired data on concentrations of major and trace elements and on Sr, Nd, and Pb isotope composition in Pliocene and Late Pleistocene–Holocene mafic volcanic rocks of the Uksichan volcanic center, one of the largest in the Sredinnyi Range of Kamchatka. Based on these data, the mafic Pliocene volcanics are thought to be produced by the melting of heterogenized mantle material, which had been hybridized by subduction and asthenospheric processes. The behavior of HFSE and Pb isotopic systematics provide evidence of the melting of subducted sediment and origin of pyroxenite segregations in the peridotite matrix. The low ∆8/4Pb values of the Pliocene lavas of Uksichan shield volcano and in modern large volcanic centers in the Central Kamchatka Depression are correlated with the magmatic productivity, which indicates, when considered together with HFSE and HREE behavior, that the Pacific asthenosphere was involved in the magma-generating processes. The Late Pleistocene–Holocene basalt volcanism, which was spatially constrained to the peripheries of the Pliocene shield edifice, developed in an extensional environment as a result of the melting of an enriched mantle source. The attenuation and then complete termination of volcanic activity in the Sredinnyi Range in the Late Pleistocene–Holocene was associated with an increase in the ∆8/4Pb of the mafic lavas, which indicates that the center of the activity related to the oceanic asthenosphere shifted eastward toward the Central Kamchatka Depression. The influence of the oceanic asthenosphere on subduction-related magmatism is not unique to convergence zones alone and should be taken into consideration when models are constructed for the origin of juvenile continental crust.

The paper reports the isotope-geochronological and petrological-geochemical studies of the Pliocene moderately-acid volcanism of the Akhalkalaki Plateau in the central part of the Lesser Caucasus (Javakheti highland, Georgia). K-Ar dating showed that young dacitic lavas and pyroclastic rocks were formed in the Mid-Pliocene (3.28 ± 0.10 Ma) in relation with the explosive–effusive eruptions of small composite volcanic cones and formation of minor extrusive domes confined mainly to the eastern margin of the region. Isotope-geochronological data in the combination with results of structural drilling indicate that the considered short-term pulse of the volcanic activity occurred during a short gap between two phases of the Pliocene–Early Pleistocene mafic magmatism widespread within the Akhalkalaki plateau. The studied Pliocene dacites were erupted at the post-collisional stage of the evolution of the Lesser Caucasus, but bear petrological-geochemical affinity of adakitic series. They are characterized by the steady presence of amphibole phenocrysts, the elevated contents of Sr, Ba, LILE and the lowered contents of Y, Nb, Ta, and HREE, and have depleted Sr isotopic composition (⁸⁷Sr/⁸⁶Sr < 0.7045). Analysis of petrogenetic models earlier proposed to explain the generation of adakite-like magmas in the modern collision zones showed that the origin of the Pliocene dacitic lavas of the Akhlkalaki plateau is best described by the crystallization differentiation of watersaturated calc-alkaline basaltic melts with removal of common mafic rock-forming minerals (first of all, amphibole and pyroxene) and accessory phases (apatite, titanite, Ti-magnetite) as cumulus minerals. Crustal assimilation of evolved magmas only insignificantly contributed to the petrogenesis of the dacites.