A RECONNAISSANCE AR-40 AR-39 GEOCHRONOLOGICAL STUDY OF ORE-BEARING AND RELATED ROCKS, SIBERIAN RUSSIA/

Ar-40/Ar-39 age spectra of biotite from a mineralized vein in the ore- bearing, Noril'sk I intrusion and from picritic-like gabbrodolerite fr om the weakly mineralized, Lower Talnakh intrusion show that these bod ies were emplaced at 249 +/- 2 Ma, which is not significantly differen t from the age of the Permian Triassic boundary. The ore-bearing intru sions postdate the lower third of the flood-basalt sequence in the Nor il'sk area and, on the basis of geochemistry, can best be correlated w ith lavas slightly younger than those which they cut. Thus, flood basa lt was erupted at the time of the Permian-Triassic mass extinction eve nt, although its role in this event is, as yet, ill defined. Additiona l new Ar-40/Ar-39 age data for a group of intrusive and extrusive rock s on the western margin of the Siberian craton indicate that mafic mag matism extended over a period of several tens of million years, wherea s paleomagnetic data suggest that the bulk of the Siberian flood-basal t sequence near Noril'sk has been erupted in only a million years or s o. Ar-40/Ar-39 ages of plagioclase from early flood-basalt flows are a bout 2% younger than those obtained for biotite from the crosscutting, Noril'sk I intrusion, probably because of slight alteration and Argon loss from the plagioclase.