THE VARIABLE CONTENT OF POTASSIUM IN SUBALKALINE ULTRABASIC LAVAS OF THE NORTHERN SIBERIAN PLATFORM AND THEIR GENESIS

Abstract

We have studied the geologotectonic and petrochemical peculiarities of the occurrences of subalkaline ultrabasic lavas of the northern Siberian Platform. The constructed petrochemical diagrams evidence the existence of compact rock series from meymechites through porphyritic peridotites, picrite porphyrites, picrites, and picrite basalts to plateau basalts. The TiO2-MgO and K2O/(K2O + Na2O)-MgO diagrams show a broad scatter of composition points. The K2O-MgO diagram supports the existence of low-, medium-, high-, and superhigh-potassium rock series. We suggest that K serves as an indicator of the genetic relationships between the rocks from meymechites to plateau basalts (traps). Formation of K-rich magmas begins with metasomatism of the mantle substratum under its interaction with deep-seated K-enriched fluids, which then passes into potassic magma metasomatism of ultrabasic and, then, daughter basic melts. Potassic magma metasomatism is indicated by the increase in K2O/(K2O + Na2O) of the melt with increase in its Mg content.