Geodynamic setting,structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO₂ contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO₂ (< 1.7 wt.%) and Fe₂O_3tot and higher contents of Al₂O₃ in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.     

Composition and Sources of Silurian Terrigenous Rocks at the Periphery of the Tekturmas Ophiolite Zone (Central Kazakhstan)

Doklady Earth Sciences - The paper presents new petrographic and geochemical data from terrigenous rocks and first U–Pb ages of detrital zircons from sandstones of the Tekturmas zone of...     

Plume magmatism in the northeastern part of the Altai–Sayan region: Stages,source compositions,and geodynamics (exemplified by the Minusinsk Depression)

The results of geochronological (U–Pb, Ar–Ar), geochemical, and isotopic (Sr, Nd) studies of the Ordovician and Devonian mafic volcanic–subvolcanic rock associations of the Minusinsk Depression are presented. The obtained ages of magmatic associations and the basite composition, considering previous studies, witness to the impact of two mantle plumes different in age (Late Cambrian–Ordovician and Devonian) on suprasubduction rock complexes in active continental margin settings.     

Orogenic Volcanism in Eastern Kazakhstan: Composition,Age, and Geodynamic Position

Studies of volcanic rocks in orogenic troughs of Eastern Kazakhstan were carried out. The troughs were formed at late-orogenic stages of evolution of Hercynian Altai collision system. Volcanic rocks are represented by basalts, andesites, dacites and rhyolites. Based on geochemical and isotopic data, the basalts and andesites derived from mafic magmas that formed as a result of partial melting of garnet peridotites in the upper mantle under the orogen. U–Pb zircon data prove two volcanic stages: more-scaled Middle Carboniferous (~311 Ma) and less-scaled Early Permian (297–290 Ma). Basalts and andesites in lower parts of the orogenic troughs and independent dacite-rhyolite structures were formed at the Middle Carboniferous stage. Parental mafic magmas were formed as a result of partial melting of mantle substrates in local transtensional zones along large shear faults. The formation of dacites and rhyolites could have been caused by partial melting of crustal substrates under effect of mafic magmas. Transtensional movements in the lithosphere of orogenic belts may indicate the beginning of collapse of orogens. A smaller volume of basalts and andesites formed at the Early Permian stage. Geochemical data prove the independent episode of partial melting in upper mantle. Synchronous basalts and andesites also appeared at wide territory in Tian Shan, Central Kazakhstan, and Central and Southern Mongolia. Early Permian volcanism indicates general extension of the lithosphere at the postorogenic stages. Large-scaled Early Permian mafic and granitoid magmatism in Central Asia has been interpreted in recent years as the Tarim Large Igneous Province caused by Tarim mantle plume activity. Thus, the extension of the lithosphere and associated volcanism in the Early Permian can be an indicator of the onset of the plume–lithosphere interaction process.