Permo-Triassic igneous rocks of Siberia, Russia

Widespread basaltic volcanism occurred in the region of the West Siberian Basin (WSB) and the Taimyr Peninsula in central Russia, and voluminous A-type magmatism within the Mongolian-Transbaikalian belt in southeast Siberia, during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the WSB reveal that the basalts were erupted at ~250 million years ago. This is synchronous with the main period of the Siberian Traps volcanism, which was located farther east. The age and geochemical data presented confirm that the WSB basalts are part of the Siberian Traps, and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction. Furthermore, it is argued that the WSB and Taimyr basalts are genetically related to the Siberian Traps basalts, especially the Nadezhdinsky Suite found at Noril’sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth’s largest continental flood basalt province. The Mongolian-Transbaikalian granitoid belt covers over 600,000 km2 with over 350 single A-type plutons. New U-Pb geochronological data presented here demonstrate that no plutonic complex dated is 250 Ma old. Although mantle-derived material played a prominent role in the granitoid generation, these melts may have been generated by processes other than decompressional melting within the head of a mantle plume. The new U-Pb ages and other observations contradict the idea of a relation between the Siberian plume and magmatic activity in the territory of Transbaikalia. An alternative preferred model inducing up rise of asthenospheric material includes slab break-off after a long period of subduction

Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis

In a recent paper, Xie et al. (2010) elegantly demonstrate that cyanobacterial blooms recorded in Permian–Triassic (P–Tr) rocks are closely associated with local volcanic activity in South China, and with the development of large negative global carbon isotope excursions. Using a compiled data set of published U/Pb and 40Ar/39Ar ages obtained on volcanic ash layers from South China, and volcanic rocks of the Siberian Traps (ST), respectively, Xie et al. argue that the volcanism associated with the ST is predominantly younger than the P–Tr boundary age. Xie et al. note that the majority of ST 40Ar/39Ar ages (e.g., Reichow et al., 2009) are similar to U/Pb zircon ages for two Triassic boundaries, and consequently that ST volcanism was likely responsible for the prolonged stress in the Early Triassic ecosystems. However, the suggested age correlation is flawed, and the purpose of this Comment is to challenge the comparison based on ages obtained by different methodologies, and demonstrate that one of the conclusions drawn by Xie et al. is invalid

Distinguishing and Correlating Deposits from Large Ignimbrite Eruptions Using Paleomagnetism: the Cougar Point Tuffs (Mid-Miocene), Southern Snake River Plain, Idaho, USA

In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid-Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

Mid-Miocene record of large-scale Snake River–type explosive volcanism and associated subsidence on the Yellowstone hotspot track: the Cassia Formation of Idaho, USA

The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River–type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km³). It covers 14,000 km² and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau