GEOCHEMISTRY OF PERMO-TRIASSIC VOLCANIC ROCKS OF WEST SIBERIA
A.Ya. Medvedev, A.I. Al'mukhamedov, and N.P. Kirda*
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
* Institute of Oil and Gas Geology, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Basalts, shoshonites, superplume, West Siberian Plate
Pages: 82-98
Abstract
We consider the lithology of Permo-Triassic buried volcanic rocks of the West Siberian Plate. By now, three types of volcanic rocks have been recognized: basalts, rocks of shoshonite-latite series, and acid effusions (rhyolites and rhyodacites). Basalts, amounting to >80% of all effusions, are referred to subalkaline and tholeiitic series and are enriched in mobile trace elements relative to N-MORB. The rocks of shoshonite-latite series are subdivided into moderate- and high-potassium. Their common feature is enrichment in easily mobile elements (Rb, K, Ba, Sr, Nb, etc.). Acid effusions are referred to typical rhyolites and rhyodacites of calc-alkalic series.
We have shown that all the studied rocks are the products of rifting magmatism caused by the action of a superplume beneath the Siberian craton. Though the rock series differ significantly in mineral and chemical compositions, they show a similar distribution of trace elements, which confirms the influence of plume smoothing the compositions of volcanic rocks.
The available geochemical, including isotope, data evidence that the parental magmas for the three series had different sources. Basalts and moderate-potassium shoshonites formed from undepleted mantle; high-potassium shoshonites, from enriched mantle; and acid effusions, from crustal substratum.
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