Geochemistry of volcanic rocks from transform margins: Evidence from the Alchan basin,northwestern Primorie |
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Authors: | V P Simanenko V V Golozubov V G Sakhno |
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Institution: | (1) Far East Geological Institute, Far East Division, Russian Academy of Sciences, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022, Russia |
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Abstract: | Geological, petrochemical, and geochemical data are reported for volcanic rocks of a Cretaceous pull-apart basin in the Tan
Lu strike-slip system, Asian continental margin. A comparison of these volcanic rocks with magmatic rocks from typical Cenozoic
transform margins in western North America and rift zones of Korea made it possible to distinguish some indicator features
of transform-margin volcanic rocks. Magmatic rocks from strike-slip extension zones bear island-arc, within-plate, and, occasionally,
depleted MORB geochemical signatures. In addition to calc-alkaline rocks, there are bimodal volcanic series. The rocks are
characterized by high K2O, MgO, and TiO2 contents. They show variable enrichment in LILE relative to HFSE, which is typical of island-arc magmas. At the same time,
they are rich in compatible transition elements, which is a characteristic of within-plate magmas. The trace-element distribution
patterns normalized to MORB or primitive mantle usually display a negative Ta-Nb anomaly typical of suprasubduction settings.
Their Ta/Nb ratio is lower, whereas Ba/Nb, Ba/La, and La/Yb are higher than those of some MORB and OIB. In terms of trace-element
systematics, for example, Ta-Th-Hf, Ba/La-(Ba/La)n, (La/Sm)n-La/Hf, and others, they fall within the area of mixing of magmas from several sources (island arc, within plate, and depleted
reservoirs). The magmatic rocks of transform settings display a sigmoidal chondrite-normalized REE distribution pattern, with
a negative slope of LREE, depletion in MREE, and an enriched or flat HREE pattern. The magmas with mixed geochemical characteristics
presumably originated in a transform margin setting in local extension zones under the influence of mantle diapirs, which
caused metasomatism and melting of the lithosphere at different levels, and mixing of melts from different sources in variable
proportions.
Original Russian Text ? V.P. Simanenko, V.V. Golozubov, V.G. Sakhno, 2006, published in Geokhimiya, 2006, No. 12, pp. 1251–1265. |
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