Spinels and Mg ilmenites from the Noril'sk 1 and Talnakh intrusions and other mafic rocks of the Siberian flood basalt province

Spinels from the mineralized Noril'sk 1 and Talnakh intrusions show a broad and distinctive compositional spectrum, from normal chromite to Ti-enrichc d aluminous magnetite. In the Talnakh intrusion, in particular, the range o f spinel compositions extends across a compositional space where spinels ar e rare in other layered intrusions. Spinel compositions vary widely on a sc ale of individual thin sections and are influenced by the nature of the enc losing silicate phase. Spinels enclosed in olivines are less Ti enriched th an those outside olivine grains. Compositional variance in the spinels is attributed to variable but general ly advanced reaction between cumulus chromite grains and solidifying trappe d intercumulus liquid. The advanced nature of the process is attributed to the open nature of the intrusions. Continuous, prolonged flow-through of ho t magma enabled cumulates within the intrusions to remain above their solid us temperature for longer than would be normal for sills of comparable thic kness. The barren Chibacheka River intrusion shows some of the same features but d iffers from Talnatkh in that spinel-olivine pairs record equilibration temp eratures higher than those at Talnakh. Temperatures recorded by spinel-oliv ine pairs at Talnakh are anomalously low for such a small intrusion, again indicating prolonged slow cooling consistent with a flow-through model for the sill. Spinel compositions support a hypothesized comagmatic relationship between the Chibacheka River intrusion and the Ti-, Cr-rich Gudchichinsky picritic basalts. Spinels in the Noril'sk and Talnatkh intrusions imply a parent mag ma with a lower Cr/Al ratio than the Gudchichinsky picritic basalts. Spinels from the Siberian intrusions show a similar range in compositions t o the feeder intrusions of the Karoo flood basalt province. Jinchuan spinel s share the features of high Ti contents and anomalously low equilibration temperatures with the Noril'sk-Talnakh spinels. Mg ilmenites are a feature of the Siberian and Jinchuan intrusions. They ar e attributed to crystallization of ilmenite from evolved trapped intercumul us liquid buffered by surrounding Mg-rich cumulus phases. Their existence i mplies moderately but not unusually Ti-rich parent magmas, consistent with compositions of the Siberian Traps lavas.