Ordovician shoshonitic to ultrapotassic volcanism in the central Norwegian Caledonides: The result of sediment subduction, mantle metasomatism and mantle partial melting

Abstract

Shoshonitic to ultrapotassic, mantle-derived volcanic rocks found within certain accretionary and collisional settings have trace element patterns comparable to those of common arc-related rocks, but with extreme enrichments in highly incompatible elements. Such rocks, previously unknown in the Scandinavian Caledonides, have been discovered in the Oppdal area in the Trondheim Nappe Complex, central Norway. The volcanic rocks are part of the Skarvatnet unit, which consists of (1) the Kinna volcanic succession, (2) the Storgruvpiken rhyolite, and (3) the Skaret conglomeratic succession. The Kinna volcanic succession is interpreted as consisting mainly of submarine pyroclastic flows. A trachytic bed from within this succession is dated to 474 ± 1 Ma. The Storgruvpiken rhyolite is interpreted as a shallow, subvolcanic intrusion or volcanic dome, and is dated to 470 ± 1 Ma. The Skaret conglomerates were deposited in a shallow-marine, tectonically active setting post-dating the Storgruvpiken rhyolite. The Kinna volcanic succession is highly enriched in Th, U, Pb and LREE, with trace element signatures remarkably similar to high-K to ultrapotassic rocks of the Alpine-Himalayan and other orogenic belts. By analogy with these recent examples, the Kinna volcanic succession is interpreted as the result of a two-stage process: (1) subduction and partial melting of continent-derived material caused extensive metasomatism of the overlying mantle wedge, (2) partial melting of this heterogeneous mantle source produced the highly enriched mantle melts. The Storgruvpiken rhyolite, particularly enriched in beryllium, is interpreted as the result of partial melting of Kinna-type rocks at depth. The Skarvatnet unit is interpreted as the result of a complex tectonic evolution at the margin of the Iapetus Ocean, involving the following stages: (1) intra-oceanic subduction, producing the Løkken-Vassfjellet-Bymarka ophiolite at 487–480 Ma, followed by (2) the arrival of the Laurentian margin or an associated micro-continent at ca. 480 Ma, leading to the subduction of large amounts of continent-derived material and subsequent metasomatism of the overlying mantle wedge, (3) arc-continent-collision and slab retreat/slab break-off, causing an intermittent phase of mantle-derived, highly enriched magmatism preserved as the Kinna volcanic succession and the Storgruvpiken rhyolite at 475–470 Ma.