TEMPO OF INTRUSIVE MAGMATISM IN THE SIBERIAN TRAPS

The Siberian Traps large igneous province (LIP) constitutes the most voluminous Continental Flood Basalt (CFB) preserved on Earth and is associated with the most severe ecological disaster in the Phanerozoic—the end-Permian mass extinction. Targeted geochronology elucidates the timing, duration, and tempo of magmatism in order to reconstruct the eruptive and intrusive history of the Siberian Traps and test potential causal relationship with extinction. High-precision zircon U-Pb dates of volcanic ash beds from the Permian-Triassic boundary exposed in Meishan, China (Burgess at al., 2014) indicate that the extinction interval coincided with extrusive magmatism in the Siberian Traps (Burgess et al., 2015). Shallow intrusive bodies represent ~50% of the total LIP volume (Reichow et al., 2009) yet few of these bodies have been precisely (< 0.1% 2σ) dated since the advent of EARTHTIME U-Pb tracer solutions. This study exploits subsurface borehole samples of dolerite intrusions to enhance the spatial coverage and resolution of high-precision dates and further assess the role of intrusive magmatism as a possible trigger mechanism.

Dolerite sills emplaced into Paleozoic sedimentary host rocks occur at multiple depth intervals and range in thickness from 10 to 150 m, forming lenticular bodies. Zircon and/or titanite occur as accessory minerals in varying abundances within dominantly plagioclase and clinopyroxene assemblages. Titanite grains are highly radiogenic (Pb*/Pb_c > 50) and yield dates with uncertainties comparable to zircon (± 0.2 Ma on single grains). Zircon and titanite from the same rock yield indistinguishable U-Pb dates, indicating fast cooling rates and limited diffusion of Pb. Furthermore, the lack of discernable age differences between samples from different depths within the same intrusion indicates that sills were emplaced as monogenetic bodies with no evidence for inheritance, protracted crystallization, pulsed emplacement, or mixing of distinct magma batches. In general, most intrusive bodies yield weighted mean ages that postdate the extinction interval, however, evidence for pre-extinction intrusive magmatism suggests that intrusive magmatism cannot be ruled out as a potential trigger mechanism.