Orogenic Volcanism in Eastern Kazakhstan: Composition, Age, and Geodynamic Position
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enic Volcanism in Eastern Kazakhstan: Composition, Age, and Geodynamic Position S. V. Khromykha ,b, *, D. V. Semenovaa, P. D. Kotlera, b, A. V. Gurovab, E. I. Mikheeva, and A. A. Perfilovaa, b aSobolev
Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia b Novosibirsk State University, Novosibirsk, 630060 Russia *e-mail: [email protected] Received February 3, 2020; revised March 18, 2020; accepted March 24, 2020
Abstract—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. Keywords: volcanism, basalts and andesites, partial melting in the mantle, U–Pb age, orogeny, fold belts, Eastern Kazakhstan, Central Asia, Tarim Large Igneous Province DOI: 10.1134/S0016852120040044
INTRODUCTION Volcanic deposits are an important indicator of endogenic activity, while the shapes, scales, duration of manifestation, and compositional features of volcanic rocks depend on geodynamic settings. To understand the evolution of fold belts, orogenic volcanism is of great importance: it reflects the different stages of accretion–collision processes. One of the largest intracontinental accretion–collision belts is the Central Asian Orogenic belt (CAOB), which formed during the Phanerozoic during closing
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