LOCAL AND REGIONAL VARIABILITY IN COMPOSITION OF PERMO-TRIASSIC EFFUSIVE TRAPS OF THE SIBERIAN PLATFORM

Abstract

The sections existing in the Permo-Triassic effusive traps of the Siberian Platform (SP) are of three types: (1) monotonous, with slightly altered composition of tholeiites within several succesive formations in the same section; (2) with lava composition regularly changed from trachybasalts to picritoids, with a certain petrochemical trend in particular "magmatic cycles" as well as in the entire local section of effusions; (3) with alkaline or other rocks "sandwiched" or "enclosed" into a monotonous section of low-K tholeiites or with the inverse arrangement of the rocks. The available data are not sufficient to outline the local development of these types, and only general tendencies of their possible predominance in separate provinces of the SP have been recognized. The first type seems to be the most widespread in the central part of the platform, though the third type locally occurs there too; the second type is typical of the northwestern margin of the SP, and the third one, for the northeastern region of lava occurrence. Statistical comparison of petrochemical information on the composition of Permo-Triassic effusions of the SP shows that there is a regional zoning in their composition. A "tholeiitic shield" with monotonous sections of low-K basalts exists at the center. In the west, north, and northeast it is enveloped by more alkaline, Ti- and Mg-enriched lavas, among which magnesian and alkaline rocks alternatively play a leading role. On the area where the effusions develop, the spatial petrochemical trends of effusive and intrusive rocks demonstrate some similarity. The maximum extents of intrusive magmatism in the cover of the SP are recorded west and south of the effusive "tholeiitic shield". Within analogous time sections of development of effusions on the SP, spatial petrochemical and geochemical trends are well expressed. They suggest that there were differences in compositions of basic melts in different provinces of the SP.