A petrologic, geochemical and Sr–Nd isotopic study on contact metamorphism and degassing of Devonian evaporites in the Norilsk aureoles, Siberia

Abstract

Devonian evaporites and associated sedimen- tary rocks in the Norilsk region were contact metamor- phosed during emplacement of mafic sills that form part of the end-Permian (*252 Ma) Siberian Traps. We present mineralogical, geochemical and Sr-Nd isotopic data on sedimentary rocks unaffected by metamorphism, and meta- sedimentary rocks from selected contact aureoles at Norilsk, to examine the mechanisms responsible for magma-evaporite interaction and its relation to the end- Permian environmental crisis. The sedimentary rocks include massive anhydrite, rock salt, dolostone, calcareous siltstones and shale, and the meta-sedimentary rocks comprise calcareous hornfels, siliceous hornfels and minor meta-anhydrite and meta-sandstone. Contact metamor- phism took place at low pressure and at maximum temperatures corresponding to the phlogopite-diopside stability field. Calcareous hornfels have high CaO, MgO, CO2 ,S O3, low SiO2 and initial Sr isotopic ratios of 0.7079-0.7092, features indicative of calcareous siltstone protoliths. Siliceous hornfels, in contrast, have high SiO2, Al2O3 ,N a 2O, low in other major element oxides and initial Sr isotopic ratios of 0.7083-0.7152, consistent with pelitic or shaley protoliths. Loss of CO2 in a subset of calcareous hornfels can be explained by decarbonation reactions dur- ing metamorphism, but release of SO2 from evaporites cannot be accounted for by a similar mechanism. Occur- rences of wollastonite and a variety of hydrous minerals in the calcareous hornfels are consistent with equilibration with hydrous fluid, which was capable of leaching large quantities of anhydrite in the presence of dissolved NaCl. In this way, substantial sediment-derived sulfur could have been mobilized, incorporated into the magmatic system and released to the atmosphere. The release of CO2 and SO2 from Siberian evaporites added to the variety of toxic gases generated during metamorphism of organic matter, coal and rock salt, contributing to the end-Permian envi- ronmental crisis.