Thrusting and Extension in the Scandian Hinterland, Norway: New U-Pb Ages and Tectonostratigraphic Evidence

Abstract

A new regional compilation map and U-Pb ages on a suite of variably deformed, Ordovician, calc-alkaline intrusive igneous rocks requires a reinterpretation of the nature of continental collision and extensional exhumation of deep-seated rocks of the Western Gneiss Region west and northwest of Trondheim. A suite of calc-alkaline plutonic rocks, in the age range 482 to 438 Ma, previously known from the region of Smøla-Hitra-Ørlandet-Froan above the Høybakken extensional detachment fault associated with the Devonian 'Old Red Sandstone' basins, is shown to extend over wide areas below the fault, commonly as strongly foliated and lineated gneisses that had been previously mistaken for parts of the Proterozoic Western Gneiss Region. At Follafoss, a member of this intrusive suite is unconformably overlain by weakly metamorphosed conglomerate and volcanogenic sedimentary rocks of probable Late Ordovician age, suggesting that both the sedimentary rocks and the underlying intrusions correlate with the Støren Nappe in the upper part of the local sequence of Caledonide nappes. U-Pb evidence of metamorphic ages from deep-seated rocks of the Western Gneiss Region include: 1) zircon reaction rims on Proterozoic igneous baddeleyite in the Selnes Gabbro at 401 ؎ 2 Ma; 2) widespread development of zircon overgrowth and metamorphic zircon with omphacite and coesite inclusions from the Hareidlandet eclogite at 402 ؎ 2 Ma; and 3) extreme Devonian thermal resetting and neocrystallization of titanite over a wide area of the Proterozoic basement gneisses ending at 395 ؎ 3 Ma, here fully documented for the first time. At Kjørsvika, west of Trondheimsfjord, a ductilely deformed gneiss of the Ordovician intrusive suite contains igneous titanite dated at 455 Ma that shows little evidence for Devonian thermal resetting. This gneiss lies only 1 to 2 km northwest from a large area of Proterozoic gneisses with 100 percent Devonian reset titanite across a ductilely deformed contact that must represent a phase of extensional detachment (Agdenes detachment) much older than the more brittle detachments (compare Høybakken detachment) associated with some of the present outcrops of the Devonian clastic basins. These and other relationships suggest the following broad sequence of Siluro-Devonian events in the region: A) Early Scandian (430 -410 Ma) thrusting with emplacement of the composite Støren Nappe onto the relatively cool Baltoscandian margin of Baltica during contemporaneous subduction of its distal part, locally

Role of crustal contribution in the early stage of the Damara Orogen, Namibia: New constraints from combined U–Pb and Lu–Hf isotopes from the Goas Magmatic Complex

“…estimated based on various studies to be around 600-700°C (Cherniak, 1993;Scott and St-Onge, 1995;Verts et al, 1996), and, according to Pidgeon et al (1996) and Zhang and Schärer (1996), a closure temperature exceeding 710°C can be estimated for very short-lived magmatic events. However, in slowly cooled metamorphic terranes titanite is often affected by diffusive lead loss (Tucker et al, 1987;Tucker and Krogh, 1988;Tucker et al, 1990Tucker et al, , 2004, and magmatic titanites whose U-Pb dates record cooling below ca. 600-700°C with late subsolidus titanite growth are well documented (Schoene et al, 2012;Warren et al, 2012).…”

Metamorphic rocks seek meaningful cooling rate: Interpreting 40Ar/39Ar ages in an exhumed ultra-high pressure terrane

“…The pervasive amphibolite-facies structural and mineralogical overprint suggests that this happened isothermally, with extensional faulting subsequently facilitating the final exhumation to the surface. At the PT conditions estimated for the overprint, Ar diffusion out of muscovite should be efficient (Harrison et al, 2009;Warren et al, 2012b), and hence 40 Ar/ 39 Ar ages should be expected to constrain the timing of cooling following the amphibolite facies overprint (estimated at 395 Ma in the northern WGC, Tucker et al, 1987Tucker et al, , 1990Tucker et al, , 2004 Ma in the central WGC Kylander-Clark et al, 2008). Since the published 40 Ar/ 39 Ar ages for the Nordfjord region currently lie within the 399-395 Ma timeframe, further investigation of whether these ages represent cooling under open system conditions is warranted.…”

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

“…Because of their links to subduction and/or collisional processes, the distribution-and age-of eclogites have important implications for the geodynamic reconstructions of the orogen. The most important, and most scrutinized, set of eclogites, yielding ages from 400 to 420 Ma from various locales (e.g., Carswell et al 2003a, b;Root et al 2004;Tucker et al 2004;Kylander-Clark et al 2009;Peterman et al 2009;Krogh et al 2011), is hosted by the deeply subducted Western Gneiss Complex (WGC) consisting of reworked Baltica crust ( Fig. 1; Roberts and Gee 1985).…”