ULTRAMAFIC VOLCANIC ROCKS OF THE SHOSHONITE-LATITE SERIES

Abstract

The lithology of the sediments (siliceous, clayey-siliceous, graywacke, and tuffogenic deposits) of the lower Drozdovskaya Formation suggests that the lavas were erupted in a deep-sea basin (tholeiite) and on its flanks (calc-alkalic basalt). The volcanics of the three upper formations were produced under shallow-water and subaerial conditions. The sequence of ultramafic volcanics, which is relatively thin (100 to 300 m) is dominated by coarse-fragment tuff of agglomerate and block size. The chemical composition of the ultramafic extrusives varies within narrow limits, being equivalent to that of potassium-rich picrite or wehrlite. Most of the analyses show 27 to 33 percent magnesium oxide and 5 to 3 percent alumina and lime. The titanium and sodium contents are low, while that of potassium oxide ranges from tenths of a percent to a few percent. These mineralogic and chemical properties of the volcanics lead their assignment to the shoshonice-latice series, putting them in the same association with absarokite, shoshinite and latice, which are dominant in the Poputnovo Formation and occur, along with subalkalic potassisodic basalt, in the Temnorechenskaya Formation [8]. The above ultramafic extrusives, previously unknown in the shoshonite-latite association and thus constituting a special property of the Valaginskiy association should be identified as a new petrographic species, called valaginite.