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.
Highlights
Magmatism above subduction zones is important for the growth of the continental crust and for refining its chemistry
Based on the interpretation of these Mediterranean examples of calc-alkaline to ultrapotassic rock suites, we propose that the Kinna and Hølonda volcanic rocks are related, and that they resulted from the same process of partial melting of variably enriched mantle after arc-continent collision (Fig. 12c)
The Kinna volcanic succession is interpreted as a series of submarine pyroclastic flows extruding at ca. 474 Ma, which were intruded by the subvolcanic Storgruvpiken rhyolite at ca. 470 Ma and were subsequently overlain by the shallow-marine Skaret succession in a tectonically active setting
Summary
Magmatism above subduction zones is important for the growth of the continental crust and for refining its chemistry. Arc-related andesites are relatively enriched in large ion lithophile elements (LILE, e.g. K, Rb, Ba), light rare earth elements (LREE) and certain high field strength elements (HFSE) such as Th and U, with abundances commonly on the order of 10–100 x primitive mantle. Some basaltic to intermediate volcanic rocks can be far more enriched in these incompatible elements (on the order of 100–1000 x primitive mantle), indicating a similar, but more extreme process of formation compared to the more common arc-related rocks They are the product of potassic to ultrapotassic mantle-derived magmatism found in accretionary, collisional and post-collisional magmatic settings (e.g. Conticelli et al, 2009; Ersoy and Palmer, 2013; Peccerillo, 1999; Peccerillo and Martinotti, 2006)
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