GEOCHEMISTRY, PETROGENESIS AND GEODYNAMIC ORIGIN OF BASALTS FROM THE KATUN' ACCRETIONARY COMPLEX OF GORNY ALTAI ( southwestern Siberia )
I.Yu. Safonovaa,b, M.M. Buslova, V.A. Simonova, A.E. Izokha, T. Komiyac, E.V. Kurganskayaa, and T. Ohnoc
a V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
b Korean Institute of Geoscience and Mineral Resources (KIGAM), 92 Gwahang-no, Daejeon, 305-350, Korea
c University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
Keywords: Paleoasian Ocean, Late Neoproterozoic-Middle Cambrian, oceanic plate stratigraphy, volcanism, subduction, accretion, basalts, mantle sources
Pages: 421-442
Abstract
The paper presents new data on geochemistry (major and trace elements, isotopes) and petrology (composition of phenocrysts and mineral thermometry) of basalts from the Early Cambrian Katun' accretionary complex of the Kuznetsk-Altai island arc (Gorny Altai). We also discuss the geodynamic settings of the formation of basalts based on petrological, geochemical, and isotope data taking into account their relationships with associated sedimentary rocks of oceanic origin and with terrigenous sedimentary rocks of the accretionary complex. The Late Neoproterozoic basalts associated with siliceous sediments are characterized by medium TiO2 and Zr/Nb, flat REE patterns and Nb/LaPM < 1 (MORB). The oceanic floor basalts are represented by high- and low-magnesium varieties. The Early Cambrian basalts (Manzherok Formation) formed in an oceanic island setting (OIB). They occur in sections consisting of paleo-oceanic island basalts alternating with siliceous-terrigenous-carbonate sediments of slope facies. The Manzherok basalts are characterized by high crystallization temperatures (1175? C) and are enriched in incompatible elements (LREE, Ti, Nb), whose variations suggest a mantle plume source and variable degrees of partial melting in the spinel and garnet stability fields. The low- to high-Mg Middle Cambrian basalts of the Ust'-Sema Formation occur as subparallel dikes and lava flows, which cut and overlap the accretionary complex. The basalts are characterized by low TiO2, Nb, LREE, high Zr/Nb and, compared to the OIB, lower temperatures of crystallization, 1047? C and 1138? C for the low- and high-Mg varieties, respectively. Geochemically, the low-Mg basalts of the Ust'-Sema Formation are close to MORB, and the high-Mg basalts, to island-arc tholeiites. Such a combination of geochemical and geological data suggest that the low-Mg basalts of the Ust'-Sema Formation formed during ridge subduction, and the high-Mg basalts formed in a suprasubduction setting, at the crust-mantle boundary.
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