Subduction‐flip during Iapetus Ocean closure and Baltica–Laurentia collision, Scandinavian Caledonides

Abstract

Evidence is presented here from the northern Scandinavian Caledonides for development of an extensional basin of Ashgill to Mid Llandovery age along the Baltoscandian margin immediately prior to Baltica–Laurentia collision. U/Pb multigrain and ion microprobe zircon dating of plagiogranites in the Halti Igneous Complex complement previous baddeleyite and zircon dating of a dolerite dyke, and zircon dating of anatectic granite; they demonstrate that this dunite, troctolite, gabbro, sheeted‐dyke complex ranges in age from c. 445 to 435 Ma. The dolerite dykes intruded and melted arkoses of inferred Neoproterozoic age. This evidence, taken together with previous documentation of ophiolites (Solund–Stavfjord), ophiolite‐like associations (Sulitjelma Igneous Complex) and several other mafic suites (e.g. Råna, Artfjället) of Ashgill to Llandovery age further south in the northern Scandinavian Caledonides, implies that Scandian collisional orogeny along this nearly 2000‐km‐long mountain belt was immediately preceeded by development of short‐lived marginal basins. The latter developed during the final closure of the Iapetus Ocean and are inferred to be of back‐arc origin, some (perhaps all) related to E‐dipping subduction. Collision of the continents at c. 435 Ma is inferred to have induced a flip in subduction polarity, leading to underthrusting of Laurentia by Baltica.

Early Silurian mafic–ultramafic and granitic plutonism in contemporaneous flysch, Magerøy, northern Norway: U–Pb ages and regional significance

“…The fact that these dates were older than the fossiliferous host-rocks was problematic and generated a heated debate (Andersen 1989;Krill & Rodgers 1989) that was eventually terminated by additional finds of Early Silurian graptolites inside the hornfelsed part of the contact metamorphic aureole near the base of the Honningsvåg Igneous Complex (Krill et al 1993), thus confirming the original interpretation. The reason for the too-old Sm-Nd and Rb-Sr ages remains unexplained but analogous cases have been observed for the Halti Complex (Andréasson et al 2003), and in plutons of the Seiland Igneous Province (Krogh & Elvevold 1990;Daly et al 1991;Roberts et al 2005).…”

“…Time scale is based on the most recent compilation (Gradstein et al 2004). Sources: (1) Krill et al 1993;(2) this study; (3) Corfu et al 2004;(4) Vaasjoki & Sipilä 2001;Andréasson et al 2003;(5) Tucker et al 1990; (6) Northrup 1997; (7) Pedersen et al 1991;(8) Nordgulen et al 2002;(9) Tucker et al 2004;(10) Dunning & Grenne 2000;(11) Nilsen et al 2003;(12) Wilson et al 1983;(13) Hansen et al 2002;(14) R. B. Pedersen, in Hansen et al 2002;(15) Dunning & Pedersen 1988;(16) Hartz et al 2002;F. Corfu, unpubl.…”

“…In most cases there are strong geological as well as faunal and/or isotopic arguments supporting an outboard Baltica and possibly Laurentian affinity of these marginal basins (Stephens & Gee 1989;Pedersen et al 1992;Andersen & Andresen 1994). An apparent exception is the Halti Complex, which was placed by Andréasson et al (2003) into a Baltic setting. The critical link for this interpretation is the presence of mafic dykes in the underlying nappe, a correlative of the Kalak Nappe system.…”

“…Thereafter, predominantly crustal processes accompanied the main collisional phase. In the Scandinavian Caledonides the events at the transition into the Silurian are recorded all along the orogen and formed such prominent rock suites as the Solund-Stavfjord ophiolite, the Røros volcanogenic massive sulphide deposit, the equally mineralized Sulitjelma and Råna layered intrusions and the Halti Igneous Complex, as well as felsic plutons such as the 447 AE 7 to 430 AE 7 Ma Bindal Batholith, the Hitra-Smøla and related intrusive rocks, the Bremanger Granitoid Complex, and the granites of Sunnhordland (Andersen & Jansen 1987;Dunning & Pedersen 1988;Fossen & Austrheim 1988;Nordgulen et al 1993;Hansen et al 2002;Andréasson et al 2003;Tucker et al 2004). With few potential exceptions (e.g.…”

“…Pedersen et al (1992) pointed to the Laurentian affinity of faunas and detrital zircon populations, which imply a development of the Late Ordovician-Early Silurian spreading complexes on the Laurentian margin of the narrowing Iapetus ocean. Roberts (2003), however, suggested an origin related to Baltica, and Andréasson et al (2003) interpreted the Halti Complex to have formed on Baltic crust above a temporarily east-dipping subduction zone. Grenne et al (1999) suggested that such complexes may have been formed on both margins during a transcurrent event accompanying the process of suturing of the two continental masses.…”