SILURIAN BONINITE SERIES IN THE URALS

Abstract

Among volcanic rock associations, the indicators of geodynamic environments, special attention has recently been paid to the boninite association: a series of high-magnesian, low alumina rocks, whose mafic members are close to pyroxenite in chemical composition. The boninites described in this article are confined to the eastern part of the Tagil-Magnitogorsk megazone in the central Urals. Products of Silurian-Early Devonian igneous activity here constitute a typically eugeosynclinal series of volcanic solid basalt-rhyolite, andesite-basalt and potassic basalt-trachyte associations. The boninite series is sometimes considered to be an ophiolite component. In the Urals it is closely associated in the section with andesitic and dacitic, volcaniclastic and subvolcanic rocks of the typical calcalkalic series and was formed after the ophiolites. Thus, as in the Bonin Islands, the boninite series in the Urals seems to mark the initial, submarine stage of island-arc evolution.