THE PROBLEM OF PHANEROZOIC KOMATIITES ON THE SIBERIAN PLATFORM

Abstract

Picrite and picritoid volcanogenic rocks in Permo-Triassic traps in the northwest of the Siberian Platform include komatiite-like varieties with CaO/Al2O3 ≥1, low concentrations of TiO2, and contents of MgO corresponding to real komatiites (in the range 9-25 wt.%). Similar rocks also occur among picritic differentiates of ultrabasic-basic intrusions, including ore-bearing ones, in the north and west of the Siberian Platform. These komatiite-like rocks might be compositionally similar not only to famous Phanerozoic komatiites of Gorgona Island and Vietnam but also to komatiites of some Precambrian greenstone belts. However, there is no complete similarity between the compared objects on the classification diagrams (Fe2O3+FeO)-Al2O3/TiO2, Al2O3/TiO2-CaO/Al2O3, Al2O3/TiO2-Cf, and CaO/Al2O3-Cf (Cf is a fractionation coefficient). Therefore, we refer them to as komatiite-like rocks. The trends on the above-mentioned diagrams show a sequence of hypogene evolution of the magnesian compositions of parental melts generated in the mantle plume beneath the Siberian Platform in the Permo-Triassic: kimberlites, meimechites, picritic porphyrites-komatiite-like rocks-picritic basalts of the Gudchikha Formation-picritic basalts of the Tuklon Formation-ultrabasic-basic intrusions-normal basalts of various regions of the Permo-Triassic trap formation of the Siberian Platform. This sequence determines the general pattern and corresponding model of development of platform (within-plate continental) magmatism on decompression melting of the deep-seated plume and further fractionation, though the occurrence of the above rocks near or on the Earth's surface may not correspond to this sequence because of the specific regional tectonics.