Shalkar Ophiolite Complex, northern Kazakhstan: structural setting, age, geochemistry and genesis

The Shalkar ophiolite complex consists of a series of tectonic slabs including lavas, a complex of parallel dikes, and a layered complex of gabbros and gabbro-pyroxenites intruded by dikes of plagiogranites. The tectonic slabs are subsided northward and are confined to the base of a large nappe made up of rocks of the Kokchetav microcontinent. The ophiolitcs and fragment of the microcontinent are jointly thrust over the Kokchetav megamelange complex with HP-UHP rocks. It is inferred from the chemical composition and structural position that the Shalkar ophiolites formed under the conditions of suprasubduction zone of extension and occur in the setting of the initial stage of formation of the Stepnyak island arc. The upper age of the ophiolites is 485 ± 6 Ma, as determined for zircons from plagiogranites using the U-Th-Pb SHRIMP method. The age of the Shalkar ophiolites is within 495-485 Ma. This interval is marked by a considerable rearrangement in the subduction zone, which might have been due to the jump of the subduction zone to the hinterland of the Kokchetav microcontinent and to the origin of a zone of extension within its limits with the formation of ophiolites

The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis.

We present new high-precision 40Ar/39Ar ages on feldspar and biotite separates to establish the age, duration and extent of the larger Siberian Traps volcanic province. Samples include basalts and gabbros from Noril'sk, the Lower Tunguska area on the Siberian craton, the Taimyr Peninsula, the Kuznetsk Basin, Vorkuta in the Polar Urals, and from Chelyabinsk in the southern Urals. Most of the ages, except for those from Chelyabinsk, are indistinguishable from those found at Noril'sk. Cessation of activity at Noril'sk is constrained by a 40Ar/39Ar age of 250.3 ± 1.1 Ma for the uppermost Kumginsky Suite.\ud \ud The new 40Ar/39Ar data confirm that the bulk of Siberian volcanism occurred at 250 Ma during a period of less than 2 Ma, extending over an area of up to 5 million km2. The resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates, or is synchronous with, the end-Permian mass extinction, further strengthening an association between volcanism and the end-Permian crisis. A sanidine age of 249.25 ± 0.14 Ma from Bed 28 tuff at the global section and stratotype at Meishan, China, allows us to bracket the P–Tr boundary to 0.58 ± 0.21 myr, and enables a direct comparison between the 40Ar/39Ar age of the Traps and the Permo–Triassic boundary section.\ud \ud Younger ages (243 Ma) obtained for basalts from Chelyabinsk indicate that volcanism in at least the southern part of the province continued into the Triassic