Geochemical,Isotopic, and SHRIMP Age Data for Precambrian Basement Rocks,Permian Volcanic Rocks,and Sedimentary Host Rocks to the Ore-bearing Intrusions,Noril'sk-Talnakh District,Siberian Russia

Petrographic, geochemical, and isotopic data have been obtained for 33 samples selected to provide constraints on contamination models for the volcanic and intrusive components of the Late Permian to Early Triassic, Siberian flood-volcanic province. Twenty-one of these samples were carried from great depth in an explosive diatreme of Triassic age, whereas 12 were collected from drill core from depths of tens to 2000 m. The studied diatreme xenoliths are: (1) fragments of the crystalline basement; and (2) fragments of a basaltic-to-rhyolitic volcanic suite.

Prompted by an unexpected, Late Paleozoic, Rb-Sr isochron age for this compositionally diverse volcanic suite, a SHRIMP U-Pb zircon age of ～270 Ma was obtained for a rhyodacite xenolith. Previously, a SHRIMP zircon U-Pb age of ～910 Ma had been determined for a leucogranite xenolith from the crystalline basement; this sample also contains substantial amounts of inherited, Early Proterozoic and Archean zircon.

The presence of this volcanic suite, only ～20 m.y. older than the 251 Ma, flood-volcanic sequence, is an extremely provocative result, inasmuch as hundreds of exploration drill holes in the Noril'sk area, and throughout the Siberian platform, have encountered only Tungusskaya Series coal-bearing sedimentary rocks in this stratigraphic/time interval. These data support arguments that subduction/underthrusting from the West Siberian Lowland under the northwest margin of the Siberian craton took place in Late Permian time.

The isotopic data obtained for the xenolith suite indicate that the upper part of the crystalline basement under the northwest margin of the Siberian craton is composed of Late Proterozoic (Riphean) rocks-alkaline granites, trondhjemites, crystalline schists, gneisses, and amphibolites-with much in common with rocks of the Central zone of the Taymyr folded area, which has been interpreted as an accretionary block formed and joined to Siberia in Late Riphean to Vendian time.

Measured isotopic characteristics for the Precambrian crystalline basement, and the Paleozoic sedimentary rocks that host the ore-bearing intrusions in the Noril'sk region, provide parameters for quantitative modeling of crustal contamination during evolution of the Siberian flood-volcanic rocks and related intrusions, both while en route to the surface and at the site of intrusion emplacement.