Ordovician–early Silurian intrusive rocks in the northwest part of the Upper Allochthon, mid-Norway: Plutons of an Iapetan volcanic arc complex

Abstract

The Scandinavian Caledonides are a late Silurian–early Devonian orogen that is an amalgam of thrust sheets derived from the Baltican margin, Iapetan arcs, and the Laurentian margin. Here we discuss intrusive rocks in two Iapetan arc complexes of Ordovician–early Silurian age that are now part of the Upper Allochthon in the central-western part of Norway.

Samples from the older of the two arcs were collected near Lensvik and Rissa, on both sides of Trondheimsfjord. These include mafic to felsic, mostly tholeiitic metamorphosed intrusive rocks, with one previously reported age of 482 Ma. These plutons intrude volcanic rocks of the Støren Group, a suite of 495 to 482 Ma oceanic arc rocks that include ophiolites. We assign these plutonic rocks to the Støren Group, based on their similar ages and strong geochemical similarities with Støren Group plutonic and volcanic rocks in the inner Trondheimsfjord region. Part of the Støren Group is unconformably overlain by earliest Middle Ordovician sedimentary sequences that contain warm-water Laurentian faunal assemblages.

Plutonic rocks of the younger arc have previously reported ages of 477 to 440 Ma, are calc-alkaline, span a composition range from ultramafic to granitic, and include a large fraction of adakites. Rocks in the Upper Allochthon with similar ages and lithologic and geochemical characteristics also occur farther north, across the Grong-Olden culmination, and in southwestern Norway, indicating an extensive arc complex. Throughout that extent, the plutonic rocks are spatially associated with older ophiolite-bearing oceanic arc assemblages, thought to be equivalent to the Støren Group.

The Helgeland Nappe of the Uppermost Allochthon contains an extensive suite of igneous rocks that have almost identical characteristics to the younger Upper Allochthon arc we describe. They are also spatially associated with 500 to 480 MA ophiolites, and in addition cut metamorphosed sedimentary rocks that contain evidence for development near or on the Laurentian margin. This evidence suggests that the calc-alkaline rocks of the Upper and Uppermost Allochthons may belong to the same arc complex. The fact that similar-age calc-alkaline arc rocks occur in the Taconian arc system of western New England, the Notré Dame arc of Newfoundland, and in the Midland Valley Terrane and related units in Scotland and Ireland, suggest extensive calc-alkaline Iapetan arc complexes that developed before and during late Ordovician closing of an arm of Iapetus.