Southernmost nappes in the Scandinavian Caledonides: correlations, evidence for a Tonian marine volcanic-sedimentary terrane and paleogeographic implications
Authors: Bingen Bernard , Torgersen Espen , Ganerød Morgan , Roberts Nick M W 
Published: March 2020

&lt;p&gt;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&amp;#8211;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, Rand&amp;#248;y, R&amp;#248;ldal).&amp;#160;&lt;br&gt;In Stavanger, the lowermost phyllite nappe &amp;#8211;Buadalen nappe&amp;#8211; is overlain by the Madla and Sola nappes (former J&amp;#230;ren Nappe). The Madla nappe comprises c. 1510&amp;#8211;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 &amp;#8211; Sn&amp;#248;da metadacite-rhyolite &amp;#8211; 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.&amp;#160;&lt;br&gt;At regional scale, the lower nappes correlate over long distances. The lowest phyllite nappes &amp;#8211;Buadalen, Holmasj&amp;#248;, Lower Finse and Synnfjell&amp;#8211; 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 &amp;#8211;Madla, Storheia, Dyrskard, Hallingskarvet, Espedalen&amp;#8211; 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 R&amp;#248;ldal is affected by a Taconian metamorphism (470&amp;#8211;450 Ma) and hosts c. 434&amp;#8211;428 Ma felsic intrusives (Roffeis &amp; 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 (&lt;459 Ma) deposition, therefore refuting a correlation of this transect with the Sola nappe.&lt;br&gt;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 Karm&amp;#248;y ophiolite complex (c. 493&amp;#8211;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.&lt;/p&gt;