Intrusive traps in the western Siberian Platform: Petrochemistry and regional zoning of composition

Three petrochemical groups of basic rocks are statistically recognized amon g the intrusive traps of the Siberian Platform (SP) and Taimyr Peninsula. T hese are: (1) normal tholeiitic (predominant), (2) Ti- and Fe-enriched, wit h increased contents of alkalies (ferrodolerites), (3) low-Ti picritoid (of Noril'sk type), which are least abundant. The platform cover is most satur ated with intrusions in the west, where in places up to 17 separate magmati c bodies occur and their total thickness may be more than 2000-2500 meters. In some sections two or three petrochemical groups of intrusions are prese nt, whereas others are denominated by bodies of one type. There is distinct regional zoning in distribution of compositions of intrus ive traps. In the southern and southwestern SP, the Noril'sk-type massifs a re rare to absent. Only one deposit contained about 15 wt.% MgO, whereas th e maximum contents of MgO at the bottom of differentiated sills are, as a r ule, less than 11 wt.%. Striking differences are observed in occurrence of ferrobasites. In the western SP they make up no more than 11%, and in the s outhern SP, about one quarter of all rock groups. The rock enriched in TiO2 and FeO* (ferrodolerites) amount to about 70% in the south of the platform . The most basic, magnesian, and low-Ti melts occur within the Noril'sk-Kha raelakh district. South and north of it, the intrusive traps are, on averag e, more silicic, high-Ti, and alkaline. The molten intrusive basites in the north and south are richer in iron than those in the central part of the p latform. In the east of the platform the melts are close to "normal tholeii tic". They contain less iron but are richer in Ca and Mg than the melts in the southwestern SP. The compositional zoning of intrusive traps is correla ted with structure-formational zones and the structure of the Earth's crust : (1) magnesian and trachybasaltic differentiated intrusions are conjugate chiefly with the Yenisei-Khatanga depression, (2) Fe- and Ti-enriched basit es are predominant in the Angara block, which is characterized by the thinn est consolidated Earth's crust in its central part, (3) normal tholeiitic b asites seem to dominate in localities with the thick consolidated Earth' cr ust.