The Silfari Volcano: Recently Discovered Ladinian-Carnian Triassic Volcanism Offshore Mid-Norway Characterized by the 6307/1-1S Borehole

Abstract

Borehole 6307/1-1S was drilled in the Froan Basin offshore mid-Norway in 2018. The bottom of the well penetrated the upper c. 650 m of a seismically imaged mound revealing a previously unknown volcano-sedimentary sequence, here termed the Silfari Volcano. Borehole analyses reveal four volcanic packages (V1-V4), dominated by deeply altered extrusive lava flows and subsidiary pyroclastic units interlayered with (S1-S3) and then capped by (S4) four prominent terrestrial volcaniclastic sedimentary intervals. Petrological analyses of core and sidewall core samples reveal a strongly alkaline series of basic-intermediate to evolved igneous rocks ranging from foidites, through tephri-phonolites to phonolites. Sheet intrusions and cored hydrothermal baryte-calcite-fluorite veins with very low δ³⁴S_VCDT (9.9‰–11.8‰) subsequently cut the sequence. The vein formation is provisionally attributed to mixing of shallow Ba-enriched waters with saline and sulphate-rich brines ultimately derived from sub-volcanic magma-evaporite interactions with an interpreted potential Permian Zechstein sequence below the well penetration. Calcite δ¹³C and δ¹⁸O (mean −2.10‰ δ¹³C_VPDB and −14.79‰ δ¹⁸O_VPDB, respectively) support a modified seawater component in the vein-forming fluids. High precision U–Pb TIMS age dating on zircons extracted from the lowermost V1 (238.02 ± 0.13 Ma) and uppermost V4 (234.96 ± 0.47 Ma) volcanic packages bracket the activity of the Silfari Volcano across c. 3 Myr spanning the Ladinian-Carnian boundary of the Middle-Upper Triassic conformable with biostratigraphic results from overlying sediments. Magmatic zircons from within the V3 package reveal a date of 183.11 ± 0.12 Ma, indicating that a previously undocumented Early Jurassic intrusive magmatic event influenced the area. The Silfari Volcano shares some chemical affinities with the Skagerrak Centred Large Igneous Province (SCLIP) and the Oslo Rift; however, it is located several 100 km north of the nearest SCLIP deposits and post-dates the youngest robustly dated volcanism by several million years. The identification of Triassic volcanism has significant implications for understanding the tectono-magmatic history of the NE Atlantic margins.