Timing of sedimentation, metamorphism, and plutonism in the Helgeland Nappe Complex, north-central Norwegian Caledonides

Abstract

The Helgeland Nappe Complex consists of a sequence of imbricated east-dipping nappes that record a history of Neoproterozoic-Ordovician, sedimentary, metamorphic, and magmatic events. A combination of U-Pb dating of zircon and titanite by laser-ablation-inductively coupled plasma-mass spectrometry plus chemostratigraphic data on marbles places tight constraints on the sedimentary, tectonic, and thermal events of the complex. Strontium and carbon isotope data have identifi ed Neoproterozoic marbles in the Lower Nappe, the Horta nappe, and Scandian-aged infolds in the Vikna region. The environment of deposition of these rocks was a continental shelf, presumably of Laurentia. Detrital zircon ages from the Lower Nappe are nearly identical to those of Dalradian sedimentary rocks in Scotland. Cambrian rifting caused development of one or more ophiolitefl oored basins, into which thick sequences of Early Ordovician clastic and carbonate sedi-ments were deposited. On the basis of ages of the youngest zircons, deposition ended after ca. 481 Ma. These basin units are now seen as the Skei Group, Sauren-Torghatten Nappe, and Middle Nappe, as well as the stratigraphically highest part of the Horta nappe and possibly of the Upper Nappe. The provenance of these sediments was partly from the Lower Nappe, on the basis of detrital zircon age populations in metasandstones and cobbles from proximal conglomerates. However, the source of Cambrian-Ordovician zircons in all of the Early Ordovician basins is enigmatic. Crustal anatexis of the Lower and Upper Nappes occurred ca. 480 Ma, followed by imbrication of the entire nappe sequence. By ca. 478 Ma, the Horta nappe was overturned and was at the structural base of the nappe sequence, where it underwent migmatization and was the source of S-type magmas. Diverse magmatic activity followed ca. 465 Ma, 450-445 Ma, and 439-424 Ma. Several plutons in the youngest age range contain inherited 460-450 Ma zircons. These zircons are interpreted to refl ect a deep crustal zone in which mafi c magmas caused melting, mixing, and hybridization from 460 to 450 Ma. Magmatic reheating of this zone, possibly associated with crustal thickening, resulted in voluminous, predominantly tonalitic magmatism from 439 to 424 Ma.

Crustal properties of the northern Scandinavian mountains and Fennoscandian shield from analysis of teleseismic receiver functions

“…The collision generated a stack of four major allochthonous nappes (Roberts & Gee 1985) with a vergence toward the west. These nappes are composed of late Proterozoic and lower Palaeozoic continental margin rocks and basement metamorphosed to granulite and amphibolite grade (Stephens & Gee 1985, 1989Grenne et al 1999;Barnes et al 2007;Roberts et al 2007;Hollocher et al 2012). In the regions discussed in this paper, the nappes were thrust onto the Norrbotten craton in northern Sweden.…”

Orogenic degassing, scapolitization and K- metasomatism during Caledonian exhumation, Helgeland, Norway

“…A tourmaline-rich, S-type granitic pegmatite body, similar to other pegmatites that formed from decompression melting during gneiss dome uplift (Barnes et al, 2007), intruded at the stratigraphic level of the hornblende gneiss and the W-Mo zone. It consists of subconformable sheets 5-20 m thick, with subvertical dike offshoots intersecting the overlying lithologies.…”

“…Corre lations of the BNC to either the HNC or the RNC ( Gustavson & Gjelle, 1991;Gustavson, 1996) have been proposed, but it may very well be seen as a separate complex (Augland et al, 2012). Zircon ages in the HNC resemble those in sedimentary zircons from the Dalradian Supergroup of Scotland (Barnes et al, 2007), while ε Hf(t) -values in zircons from magmatic intrusions in the Beiarn Nappe correspond to those in the Hurry Inlet plutonic terrane in Liverpool Land, NE Greenland (Augland et al, 2012). Furthermore, Taconian structures in the HNC (Yoshinobu et al, 2002;Barnes et al, 2007) and the RNC underline the exotic origin of the Uppermost Allochthon.…”

U–Pb zircon ages of felsic rocks in the Forbordfjell ophiolite fragment, Støren Nappe, Mid-Norwegian Caledonides

“…The Upper Allochthon embraces the diverse ocean-floor and marginal basin assemblages and volcanic arc complexes, considered to have originated within the exotic offshore realm of the Iapetus Ocean, and traditionally referred to in Scandinavia as the 'Köli nappes' . Above this is the Uppermost Allochthon which consists largely of magmatic and metasedimentary rocks considered to have originally formed parts of the continental margin of Laurentia (Stephens & Gee, 1989;Roberts et al, 2002aRoberts et al, , 2007Barnes et al, 2007). Some modifications have been made to the original four-fold subdivision (e.g., Andréasson & Gee, 2008) and questions have been raised about the scheme in general (Corfu et al, 2014), but it continues to provide a very convenient and useful framework in which to cate gorise the rocks in the many individual nappes and thrust sheets.…”