Southernmost nappes in the Scandinavian Caledonides: correlations, evidence for a Tonian marine volcanic-sedimentary terrane and paleogeographic implications

Abstract

<p>Nappes of the Scandinavian Caledonides are the repository of information on both Caledonian orogenic evolution and pre-Caledonian geologic evolution of the Baltica and Laurentia margins and the Iapetus ocean. We report geological mapping, zircon U–Pb geochronological data on 33 samples, and mica 40Ar/39Ar data on 4 samples, along five profiles in the southernmost Caledonides in the Stavanger-Ryfylke region (Stavanger, Suldal, Nedstrand, Randøy, Røldal). <br>In Stavanger, the lowermost phyllite nappe –Buadalen nappe– is overlain by the Madla and Sola nappes (former Jæren Nappe). The Madla nappe comprises c. 1510–1495 Ma orthogneiss with Sveconorwegian metamorphism (c. 1025 Ma). The overlying Sola nappe comprises a sequence of mica schist, metasandstone, marble, amphibolite and felsic metavolcanic rocks. The metavolcanic rocks – Snøda metadacite-rhyolite – are fine-grained, frequently porphyritic, mica gneisses, with calc-alkaline, peraluminous, composition and negative Nb-Ta anomaly. Their extrusion ages of c. 941 and 934 Ma date deposition of the whole sequence. Detrital zircons in a metasandstone sample (n=138) yield main age modes at c. 1050 and 1150 Ma, significant Proterozoic and Archaean modes, and a maximum deposition age of c.990 Ma. The Sola nappe was affected by Taconian metamorphism peaking in eclogite-facies conditions at c.470 Ma (Smit et al., 2010), followed by regional cooling between c.446 Ma (white-mica) and 438 Ma (biotite). Trondhjemite dykes intruded at c.429 Ma, cutting the pre-Scandian fabric. <br>At regional scale, the lower nappes correlate over long distances. The lowest phyllite nappes –Buadalen, Holmasjø, Lower Finse and Synnfjell– represent the Cambro-Ordovician sediment cover of the Baltic margin, containing thin tectonic slivers of the underlying c. 1521 to 1225 Ma orthogneiss. The overlying nappes –Madla, Storheia, Dyrskard, Hallingskarvet, Espedalen– consist of felsic metavolcanic or metaplutonic rocks with a consistent age between c. 1525 and 1493 Ma with c. 1040 Ma intrusive, corresponding to the Telemarkian crystalline basement in S Norway. The Kvitenut nappe hosts metaplutonic rocks ranging from c. 1625 to 1039 Ma and metasedimentary rocks. It requires additional characterization. The overlying far-travelled nappes do not correlate well. The metasedimentary Revseggi nappe in Røldal is affected by a Taconian metamorphism (470–450 Ma) and hosts c. 434–428 Ma felsic intrusives (Roffeis & Corfu, 2014). Detrital zircons (n=33) in a kyanite-mica-gneiss sample constrain deposition of the sequence after c. 890 Ma. The Revseggi nappe may correlate with the Sola nappe. In Nedstrand, a c. 932 Ma augen gneiss is overlain by amphibolite and mica schist, tentatively attributed to the Boknafjord nappe. Detrital zircon data (n=11) imply an Ordovician (<459 Ma) deposition, therefore refuting a correlation of this transect with the Sola nappe.<br>The Sola nappe exposes a far-travelled Tonian marine volcanic-sedimentary sequence. The Taconian metamorphism suggests an evolution in the Iapetus ocenic realm. The Sola sequence may represent the microcontinent onto which the Karmøy ophiolite complex (c. 493–470 Ma) was obducted. By analogy to several other Tonian sequences preserved in far-travelled allochthons in the Scandinavian and Greenland Caledonides, the Sola sequence may originate from the active Neoproterozoic Renlandian margin of Laurentia and Rodinia before opening of Iapetus.</p>