Bernard Bingen

Geological mapping, zircon U–Pb dating of 28 samples, and mica 40Ar–39Ar dating of 7 samples in the Stavanger–Ryfylke region (Stavanger, Suldal, Nedstrand, Randøy) characterizes the tectonostratigraphy of the southernmost nappes in the Scandinavian Caledonides. Four main tectonostratigraphic levels are described. (1) The lowest phyllite/mica schist nappes –Buadalen, Holmasjø, Lower Finse, Synnfjell– represent the Cambro–Ordovician sediment cover of the Baltic margin. (2) The overlying nappes –Madla, Storheia, Dyrskard, Hallingskarvet– consist of felsic metaigneous rocks with a consistent age between c.1525 and 1493 Ma. They host c.1040 Ma intrusives and c.1025 Ma Sveconorwegian metamorphism. They likely represent transported Baltican (Sveconorwegian) basement, widely exposed in S Norway. (3) The overlying nappes –Sola, Boknafjord, Kvitenut, Revseggi– are more diverse and lack counterparts in the exposed Baltican crust. The Sola nappe, near Stavanger, comprises a marine succession –Kolnes succession– of mica schist, metasandstone, marble, amphibolite and felsic metavolcanic rocks. The metavolcanic rocks –Snøda metadacite–rhyolite– are fine-grained mica gneisses, with calc-alkaline composition. Their extrusion age of c.941–934 Ma date deposition of the sequence. Detrital zircons in a metasandstone sample (n=138) yield main age modes at c.1040, 1150 and 1395 Ma, as well as significant Paleoproterozoic and Archaean modes. The Kolnes succession was affected by Taconian/Grampian metamorphism peaking in eclogite-facies conditions between c.471 and 458 Ma (Smit et al., 2010), followed by regional cooling around 445–435 Ma. Leucogranite bodies (c.429 Ma) cut the Grampian fabric. Several 40Ar–39Ar white mica and biotite plateau ages constrain the timing of Scandian top-to-the SE nappe stacking at c.420 Ma. The Boknafjord nappe in Nedstrand comprises a c.932 Ma augen gneiss, overlain successively by amphibolite and mica schist units. Preliminary detrital zircon data (n=11) imply an Ordovician (<459 Ma) deposition for the mica schist. (4) The highest nappes –Karmsund and Hardangerfjord– host the Karmøy and Bømlo ophiolite complexes. These complexes comprise a c.493 Ma supra subduction zone ophiolite, intruded by c.485–466 Ma volcanic arc plutonic rocks, and unconformably overlain by fossiliferous upper Ordovician (<c.445 Ma) clastic sediments (Pedersen and Dunning, 1997).We propose that the Iapetan Karmøy–Bømlo ophiolite complexes were accreted onto the Kolnes succession on the Laurentian side of the Iapetus realm, during the Grampian orogeny, before integration of both in the Scandian nappe pile. The age of HP metamorphism in the Kolnes succession (471–458 Ma) matches the inferred timing for obduction of the Karmøy–Bømlo complexes (485–448 Ma). The evidence for a Laurentian margin obduction stems from a conspicuous similarity with Shetland. On Shetland, the c.492 Ma Unst–Fetlar ophiolite complex was obducted during the Grampian orogeny onto Neoproterozoic Laurentian marine sequences (psammite-marble-mica gneiss) of the Westing, Yell Sound and East Mainland successions. The Westing and Yell Sound successions are characterized by a c. 944–925 Ma, Renlandian, high-grade metamorphism, a dominant detrital zircon mode at 1030 Ma, and common Archean detrital zircons. They correlate well with the Kolnes succession and suggest an ancestry along the Neoproterozoic Renlandian active margin of Laurentia and Rodinia, before opening of Iapetus.

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). 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. 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.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.

In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.