Geochemistry of amphibolite-facies volcanics and gabbros of the Storen Nappe in extensions west and southwest of Trondheim, western gneiss region, Norway: A key to correlations and paleotectonic settings

Abstract

The Upper Allochthon of the Scandinavian Caledonides is composed of thrust slices of a variety of metamorphosed volcanic, plutonic, and sedimentary rocks. The rocks are largely interpreted to have been formed in Cambrian to Ordovician oceanic to peri-continental arc environments that were thrust onto Baltica during the Silurian-Devonian Scandian Orogeny. We review the literature on Upper Allochthon igneous rock geochemistry broadly. We also present 87 new analyses of metamorphosed igneous rocks taken from the Surnadal and Moldefjord synforms and the Rissa area in the northern part of the Western Gneiss Region, where highly deformed Upper Allochthon rocks are preserved. Here the Upper Allochthon is dominated by basaltic volcanics and gabbros with sparse intermediate and felsic rocks. Most of the mafic rocks are transitional between MORB- and arc-type compositions in tectonic discriminant diagrams and for REE and multi-element patterns. We present two new discriminant diagrams to illustrate this subtle transitional characteristic. We interpret these volcanics as having been erupted in a mature back-arc basin environment where mantle sources for the magmas were only slightly influenced by the subduction zone component from an adjacent arc. Another set of mafic volcanic rocks, coming only from the Storås and Rissa areas, were extruded in an oceanic arc-type environment. These two geochemical types, back-arc and oceanic-arc, are closely matched by Støren Group ophiolites in the Trondheim Region which are dominated by basaltic volcanics and gabbros and generally lack felsic igneous and sedimentary rocks. Felsic igneous rocks, sedimentary rocks, and calc-alkaline and alkaline volcanics are more abundant in stratigraphically younger units of the Upper Allochthon, including the Lower and Upper Hovin Groups. We correlate the Upper Allochthon igneous rocks in the Moldefjord and Surnadal synforms and the Rissa area with ophiolites of the Støren Group. Early oceanic-arc and back-arc volcanic rocks, followed by more calc-alkaline to alkaline volcanics, are the dominant pattern for ophiolite sequences in the rest of the Upper Allochthon of the Scandinavian Caledonides. We envisage a paleotectonic scenario in which a Late Cambrian to Tremadocian, oceanic-arc system developed above a subduction zone dipping oceanward from a microcontinent that had earlier rifted away from Baltica or possibly Ganderia. The ophiolitic and primitive arc rocks are inferred to have been obducted in Late Tremadocian–Early Floian time upon epicontinental rocks flanking the microcontinent, which was then drifting rapidly across the Iapetus Ocean approaching Laurentia. A new arc and marginal basin developed, following a subduction polarity reversal, with the Dapingian-Darriwilian sedimentary infill being replete with Laurentian faunas, and locally punctuated by calc-alkaline volcanics and dikes, with fringing reefal limestones. The entire volcano-sedimentary assemblage was later affected by Scandian (Silurian–Early Devonian) orogenesis during emplacement of the major nappes onto the Baltoscandian margin.