HR: 0830h
AN: V11E-0542    [PDF]
TI: Oceanic Basalts of the Paleo-Asian Ocean: Geochemistry and Structural Setting in Folded Zones of Altai and East Kazakhstan (Central Asia)
AU: * Safonova, I Y
EM: inna@uiggm.nsc.ru
AF: Institute of Geology SB RAS, 3 Kopyuga ave., Novosibirsk, 630090 Russian Federation
AU: Buslov, M M
EM: misha@uiggm.nsc.ru
AF: Institute of Geology SB RAS, 3 Kopyuga ave., Novosibirsk, 630090 Russian Federation
AU: Kokh, D A
EM: misha@uiggm.nsc.ru
AF: Institute of Geology SB RAS, 3 Kopyuga ave., Novosibirsk, 630090 Russian Federation
AB: Four periods of basaltic volcanism of the Paleo-Asian Ocean have been studied: 1) Vendian-Early Cambrian (Kurai zone); 2) Early-Middle Cambrian (Katun zone); 3) Late Cambrian-Early Ordovician (Zasurin Formation) and 4) Late Devonian-Early Carboniferous (Chara belt) ages. The studied entities are oceanic basalts incorporated in four Caledonian accretion-collision zones.The Early Cambrian collision of the Siberian continent and Gondwana-derived terranes formed the Kurai and Katun accretionary zones in Gorny Altai. The Late Cambrian-Early Ordovician collision of the Siberian continent and Gondwana-derived terranes formed shear zones comprising oceanic sediments and basalts (Zasurin Formation). The Late Devonian-Early Carboniferous collision of the Siberian and Kazakhstan continents formed the Chara shear belt including basaltic terranes (Buslov et al., 2001). Geochemical features of tholeiites, subalkaline and alkaline basalts incorporated in accretionary terranes illustrate the history of oceanic volcanism with OIB, OPB and MORB affinities. According to REE there are LREE-enriched, transitional and MORB/OPB-like samples. LREE vary and are considerably enriched (LaN (ppm) 25-65 for Kurai, 35-55 for Katun, 100-160 for Zasurin, and 35-105 for Chara OIB) relative to chondrites. HREE have much smaller ranges: 6 to 13 times that of chondrites for Kurai and Katun and 10 to 20 times for Zasurin and Chara. There is also a positive correlation between La/Yb and SiO2. In the multi-element diagrams Kurai samples display Sr enrichment relative to K, whereas Zasurin and Chara samples show negative Sr peaks. Except for the Katun wedge, most display moderate to strong Nb-Ta depletion suggesting fractionation of ilmenite and magnetite at the base of the crust or a previous melting episode in the source. For all groups of basalts, transitions between end members of basalts: from N-MORB, through E-MORB and T-MORB (OPB?) to OIB were found. Zr/Y and Sm/Nb ratios show two distinct linear trends suggesting two independent magma sources regardless of ages: near-constant Zr/Y=2-3 and Sm/Nb=1-1.5 probably represent MORB or OPB while Zr/Y=6 and Sm/Nb=5-6 represent OIB-type rocks. The oceanic metabasalts from those four areas display many common geochemical features. On one hand, they could have been melted at different degrees of partial melting and at different depths. On the other hand they could have melted from one mantle source, but then differently influenced and/or contaminated by the upper mantle, for example, a weakly depleted oceanic mantle could interact with upwelling plume melts. Thus, the fragments of oceanic crust from Gorny Altai, North-West Altai and East Kazakhstan comprise not only ophiolitic rock units, but also oceanic plateau and oceanic island units as important markers in reconstruction of the structure of foldbelts. It is concluded that intra-plate volcanism was active in the Vendian to Early Carboniferous period of the evolution of the Paleo-Asian Ocean. The fragments of weakly to strongly differentiated oceanic basalts have been preserved in accretion-collision zones and their geochemical data indicate that the Altai and East Kazakhstan metabasalts could have been formed at mid-oceanic ridges, oceanic islands or oceanic plateaux of the Paleo-Asian Ocean in Vendian to Early Carboniferous times. This study was supported by RFBR grants no. 01-05-65228, 02-05-64627 and 03-05-64668.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting