Earth-Science Reviews 203 (2020) 103142

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Earth-Science Reviews

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The -Venn Massif (Ardenne, ), a rift shoulder basin ripped off the West African craton: Cartography, stratigraphy, sedimentology, new T U-Pb on zircon ages, geochemistry and Nd isotopes evidence ⁎ Alain Herboscha, Jean-Paul Liégeoisb, , Andreas Gärtnerc, Mandy Hofmannc, Ulf Linnemannc a Département Géosciences, Environnement et Société de l’Université Libre de Bruxelles, B-1050 Brussel, Belgium b Geodynamics and Mineral Resources, Royal Museum for Central Africa, B-3080 Tervuren, Belgium c Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Sektion Geochronologie, GeoPlasma Lab, D-01109 Dresden, Germany

ARTICLE INFO ABSTRACT

Keywords: The Stavelot-Venn Massif (SVM) is the major Cambrian-Ordovician inlier of the Ardenne Allochthon (Southern Rheno-Hercynian basement Belgium). The SVM belongs to the Avalonian microcontinent and was affected by the Caledonian and Variscan detrital zircon ages orogenies. The Ardenne Allochthon constitutes the northwestern part of the Rheno-Hercynian domain, just south Cambrian-Ordovician sedimentology of the Variscan northern front. By contrast, the comparable Avalonian Cambro-Silurian Brabant Massif (northern crustal magma geochemistry Belgium) is located just to the north of this front and is not affected by the Variscan orogeny. Rheic rift shoulder In this study, we compiled the SVM available but dispersed data concerning the the Cambrian-Ordovician West African craton margin series (field, geochemistry, isotopes). In addition, we acquired new data especially LA-ICP-MS detrital and magmatic zircon ages and Nd isotopes. The comparison with the Brabant Massif, which also belongs to Avalonia and whose geological history is well known, is particurlarly enlightening. SVM Cambrian and lowermost Ordovician depositional environments are similar to the Brabant Massif but sediment thickness is significantly lower (2000 m vs 9000 m). During the remaining Ordovician and Silurian, after the opening of the Rheic Ocean, Brabant and SVM behaved differently, pointing to the existence of two different basements, whereby the basement underneath the SVM acted more rigidly (metacratonic). The SVM environment matches a rift shoulder while the Brabant was located in the rift itself. Our comprehensive study of detrital zircon ages from Cambrian-Ordovician sediments establishes the relative contributions over time of three composite sources: the West African craton, the Western Amazonian craton and the Pan-African orogen. This allowed us to determine a fine record of the tectonic events, distant or local, at the origin of the supply variations from these major sources. The detrital zircon age pattern of the Pridoli con- glomerate (Ardennian unconformity) deposited after a sedimentation hiatus of 45 Myrs, is very distinctive. It shows a continuous record between 467 Ma and 420 Ma (97% zircons) that we relate to the activity of a large igneous province (LIP). The latter would be located along the south-eastern boundary of Avalonia, denying the existence of a Silurian island arc as previously proposed. The geochemical evidence indicates that most mag- matic rocks have a crustal origin in relation to melting in an intracontinental setting due to stress applied at the Avalonia plate margin. Finally, we propose a geodynamic model in which the rigid basement of the Rheno-Hercynian domain ori- ginated from the tearing of a metacratonic fragment of the West African craton, which left a large scar on its western margin (Mauritania/Senegal). This metacratonic fragment includes only the Silurian Brabant western foreland and extends to England, including the Midlands microcraton.

1. Introduction closure of the Iapetus Ocean; or by the Variscan orogeny during the closure of the Rheic Ocean. During the latter, the peri-Gondwanan Western Europe is largely made of far-traveled peri-Gondwanan terranes were sandwiched between Laurussia and Gondwana in a terranes that were later rejoined to Gondwana or stayed in its vicinity. complex assemblage of smaller intervening oceans and terranes. This These terranes were affected by the Caledonian orogeny during the intricate story has been enlightened particularly by a combination of

⁎ Corresponding author. E-mail address: [email protected] (J.-P. Liégeois). https://doi.org/10.1016/j.earscirev.2020.103142 Received 26 October 2019; Received in revised form 18 February 2020; Accepted 19 February 2020 Available online 26 February 2020 0012-8252/ © 2020 Elsevier B.V. All rights reserved. A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 paleomagnetic and paleontological studies (e.g. Cocks and Torsvik, 1998). In the northern domain, the Caledonian metamorphic conditions 2002, 2005; Franke et al., 2017; Franke et al., 2019), and by the study (180-270°C) are lower but also comparable to the Variscan conditions of detrital zircon in Paleozoic sediments (e.g. Pereira et al., 2012; (c. 200°C) as indicated by the crystallinity of illite (Spaeth et al., 1985). Linnemann et al., 2014; Orejana et al., 2015; Siegesmund et al., 2018 The latter suggests that the Caledonian metamorphism is the result of among many others). These approaches have made significant progress burial and/or tectonic loading of an earlier generation of thrust sheets but have arrived at rather unclear constraints concerning the paleo- (Frank and Spaeth, 1991). This means that the SVM is severely tecto- position of the terranes, due to a lack of knowledge of longitude for nized, having been affected by both the Caledonian and the Variscan paleomagnetic studies and the size and complexity of the sources con- orogenies but always under low-grade conditions (Fig. 2; Fielitz and sidered for detrital zircon studies. It is thus now time for using addi- Mansy, 1999). tional arguments such as the consideration of the geological hetero- The SVM shows a continuous sedimentation from the upper part of geneity inside each peri-Gondwanan terrane and of their large supply the lower Cambrian to the Middle Ordovician (Lamens, 1985a; sources. This implies to incorporate other geological techniques, such as Geukens, 1986, 1999; Vanguestaine, 1992; Verniers et al., 2001). This sedimentology, geochemistry of magmatic rocks or isotopic signatures. sedimentation is mainly terrigenous with minor volcanic episodes We consequently compiled and acquired data about the Stavelot- (Corin, 1965; Geukens, 1976; Lamens and Geukens, 1984) and even Venn Massif (SVM) which is the largest Cambro-Ordovician inlier of the rarer small magmatic intrusions (Corin, 1965; Kramm and Buhl, 1985; Ardenne Allochthon, itself part of the western Rheno-Hercynian do- Dejonghe, 2003). The estimated thickness of the different units varies main. The Ardenne inliers belong to the Avalonia microcontinent as according to the authors (von Hoegen et al., 1985; Lamens, 1985b; well as the Brabant Massif with which the SVM will be compared. Also, Geukens, 1999, 2008; Verniers et al., 2001), but the total thickness of we present an update on the stratigraphy, sedimentology, basin evo- the Lower Paleozoic of the SVM is about 3000 m (range > 2700 to 3500 lution, detrital zircon constraints, dating of magmatic rocks, geo- m). chemistry including Nd isotopes of magmatic and sedimentary rocks of Two main structural domains, separated by a N50°E oriented graben the SVM. This allows a systematic correlation with the well-known structure filled with red conglomerates of presumably Permian age, are Brabant Massif. Based on their similar stratigraphy, we conclude that recognized (Geukens, 1957, 1975, 1986, 1999)(Fig. 2). The northern the Brabant and the SVM massifs belong to the same basin but they rest domain is mainly made of rocks of the Revin Group (upper Cambrian) on different basements. This conclusion has significant repercussions showing a succession of closed synclinal structures with a N50°E Var- allowing to propose a specific paleoposition for the Ardenne basement, iscan trend. The presence of the Brabant Massif close to the north and so for Avalonia, before the opening of the Rheic Ocean. Finally, we during the NW-oriented Variscan deformation can explain this trend propose a global geodynamic evolution from Cambrian to the Lower (Fig. 1, 2; Hance et al., 1999; Mansy et al., 1999). These structures Devonian for the Brabant and the Ardenne massifs (SE Avalonia). overprint earlier Caledonian E-W oriented structures. The meta- morphism is weak and limited to anchizonal conditions. In the north- 2. Geology of the Stavelot-Venn Massif (SVM): overview and western Chevron Syncline, the carpholite paragenesis shows P-T con- general features ditions of about 300° C and 1-2 kbar (Theye et al., 1996). The southern domain is marked by Caledonian E-W oriented Among the four Ardenne Cambrian-Ordovician inliers (Stavelot, structures with upright isoclinal folds and flat overthrusts reactivated Rocroi, Serpont and Givonne; Fig. 1), the SVM is the largest and the best during the Variscan deformation (Geukens, 1986; Piessens and studied. Together with the Brabant Massif basement, these Ardenne Sintubin, 1997). A steeply dipping slaty cleavage coincides with the inliers belong to the Avalonia microcontinent (Cocks and Torsvik, 2002; axial plane of the folds but most often the Variscan strain partitioning Verniers et al., 2002; Winchester and the PACE TMR Network, 2002). has generated a multiple crenulation cleavage (Piessens and Sintubin, The SVM belongs to the northeastern part of the Ardenne Allochthon 1997). Two intricate folded and faulted anticlinal structures (Grand- close to the Variscan front complex (inset Fig. 1). The SVM is the Halleux and Ligneuville anticlines; Fig. 2) typify the eastern part of the northernmost Variscan thrust nappe of the Rheno-Hercynian foreland southern domain. In the southernmost metamorphic area (Recht- fold-and-thrust belt (Franke, 2000; Oncken et al., 1999, 2000). It was Vielsam-; Fig. 2), the P-T conditions are 360-420°C and 2-3 transported at least 20 to 30 km to the northwest towards the Brabant kbar (andalusite-chloritoid-spessartine-white K-mica paragenesis; Massif (Adams and Vandenberghe, 1999; Hance et al., 1999; Oncken Kramm, 1982; Fransolet and Kramm, 1983). Slightly lower P-T condi- et al., 2000). As a consequence, it has undergone Variscan folding and tions are recorded along the northeast rim of the SVM towards Germany very low (anchizone) to low-grade (epizone) metamorphism during (Fig. 2). Upper Carboniferous times (e.g. Kramm, 1982; Theye et al., 1996; Fielitz and Mansy, 1999; Nierhoff et al., 2011). 3. Stratigraphy, sedimentology and basin evolution of the Many authors have described an earlier Caledonian tectono-meta- Stavelot-Venn Massif morphic event (Theunissen, 1971, 1977; Geukens, 1986; Piessens and Sintubin, 1997; Musschoot, 2000; Debacker et al., 2014) but was denied 3.1. Stratigraphy and sedimentology by some other ones (Van and Sintubin, 2008; Sintubin et al., 2009). It is indeed difficult to distinguish the respective imprint of the We use the most recent estimations of sediment thickness (Geukens, Caledonian and the Variscan metamorphic events. However, an earlier 2008). The old stratigraphic nomenclature, Dv1-2 (Devillian), Rv1-5 Caledonian folding accompanied by a metamorphism is highlighted by (Revinian), Sm1-3 (Salmian) (Beugnies et al., 1976) is given within an angular unconformity between the SVM and the overlying Pridoli- brackets. Lochkovian basal conglomerate (Ardennian unconformity) that sur- rounds the SVM (Fig. 2). The latter is affected by the Variscan meta- 3.1.1. The Deville Group morphism but contains pebbles also affected by the Caledonian meta- The Deville Group (upper part of lower Cambrian; Figs. 2, 3) morphism. Indeed, a geothermometric study of two generations of comprises two formations: (1) the Hourt Formation (Dv1), which is the quartz veins in this metaconglomerate, observed both within pebbles oldest formation of the SVM, is > 200 m thick and consists of grey (Caledonian veins) and cross-cutting the pebbles and the matrix (Var- sandstones in thick beds with some greenish slate intercalations. This iscan veins) (Ferket et al., 1998), shows the existence of two superposed formation is devoid of fossils or microfossils. (2) The Bellevaux Forma- metamorphic events. In the southern part of the SVM, they are of tion (Dv2), c. 250 m thick, is formed by green to purplish slates evolving roughly similar intensity (280-380°C for the Caledonian metamorphism in rhythmically alternating sandstone-siltstone-slate (von Hoegen et al., and minimum 300-415°C for the Variscan metamorphism; Ferket et al., 1985; Geukens, 2008). Magnetite slates are described in the upper part

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Brussels Netherlands +

+ Maastricht + B R A B A N T M A S S I F St. Truiden + Aachen Wavre + BRABANT PARAUTOCHTHON 1 Aachen Thrust Fault Tournai Liège + . + T. S s e nlier e u France - M enn I r e window Namur b Germany + + a m Midi Thrust Fault + H a i n e - S Charleroi Stavelot-V Condroz Inlier Meso- and Cenozoic 2 4 3 Upper Carboniferous A R D E N N E A+ L L O C H T H O N Fig. 2 Middle Devonian to Dinant Lower Carboniferous + Lower Devonian Givet Cambro-Silurian (Brabant Massif & Condroz inlier) Bastogne Cambro-Ordovician + Rocroi (Ardenne inliers) + Serpont Inlier Rocroi Inlie 5 Stavelot-Venn Massif r N

BRA Rhen KÖLN BANT MA Fig. 1 + ish Massif Bouillon S AACHEN+ + SIF LIEGE + + GIESSEN A N Midi Fault NAMUR + Y N I Givonne Inlier + FRANKFURT R H E N O - H E RC + Arlon A r d Doleritic magmatism e n n e + France MAINZ Fault, thrust

+TRIER 30 km 50 km Brabant Massif southern limit

Fig. 1. Simplified geological map of the southern part of Belgium and adjacent countries showing the major structural units of the Paleozoic, including the four Cambrian-Ordovician inliers (Stavelot-Venn, Rocroi, Serpont, Givonne) of the Ardenne Allochthon. The white stars indicate the location of the doleritic magmatism (described in the text). Inset: position of the Fig. 1 in the Rheno-Hercynian Belt; the red star shows the location of the Stavelot-Venn Massif. The black rectangle shows the position of the Fig. 2. of the formation (“phyllade du Pont” Anthoine, 1940; Pirlet, 1976; unpublished observation), always deposited in a deep anoxic basin. On Raynaud, 1977). The ichnofossils Oldhamia radiata and Oldhamia an- the base of acritarchs (Vanguestaine, 1977, 1986, 1992) the Revin tiqua were discovered at the end of the 19th century (Malaise, 1874, Group could be attributed to the Miaolingian and the Furongian series 1876, 1878) in the middle part of the formation (Oldhamia in Fig. 3). (Fig. 3; former “middle” and “upper Cambrian”) within the 509-485 Ma The uppermost part was dated by acritarchs indicating a mid- or late age interval. The top of the La Gleize Formation indeed coincides with Early Cambrian to early Mid Cambrian age (informal Biozone 0, the end of the Cambrian (Wang and Servais, 2015). Vanguestaine, 1974, 1992). In the global chronostratigraphy (Peng et al., 2012), these two formations could be assigned to the interval from the upper part of the Stage 2 (?) to the upper part of the Stage 4 3.1.3. The Salm Group (former “lower Cambrian”)(Vanguestaine, 1992; Herbosch and The Lower to Middle Ordovician Salm Group, c. 1200 m thick, Verniers, 2011; Herbosch and Servais, in preparation), i.e. latest Ter- comprises the , Ottré and Bihain formations subdivided into 8 reneuvian and Series 2 (525-509 Ma). The succession is interpreted as a members (Fig. 3; Geukens, 1999, 2008; Verniers et al., 2001). The shallow marine deposit evolving rapidly to deep marine turbidites and Jalhay Formation (Sm1) includes: (1) the Solwaster Member (Sm1a), hemipelagites (von Hoegen et al., 1985; Herbosch, unpublished ob- 200-250 m thick, made of sandstone, siltstone and slate displaying servations). frequent Bouma turbidite sequences (Bouma, 1962); (2) the Spa Member (Sm1b), c. 200 m thick, formed by wavy bedded siltstone in- terpreted as low density silt turbidites (sensu Stow and Piper, 1984); (3) 3.1.2. The Revin Group the Lierneux Member (Sm1c), about 100 m thick, that shows greenish The Revin Group comprises three formations: the Wanne (Rv1-2), slates and siltstones. A detailed sedimentological study (Lamens, 1985a, La Venne (Rv3-4) and La Gleize (Rv5) formations, more than 1100 m 1985b, 1986) has shown that the Jalhay succession corresponds to a thick altogether. The two first formations, c. 300 m and > 500 m thick, regressive sequence with successively the basin plain (Solwaster Mbr), are formed by sandstone, siltstone and black or green slates that form the slope (Spa Mbr) and finally the upper part of the slope or even the thick turbiditic sequences and hemipelagic deposits (Geukens, 1963b; shelf (Lierneux Mbr). Biostratigraphically, the two lower members of von Hoegen et al., 1985). They are interpreted as deposits from a deep the Jalhay Formation are dated from the early Tremadocian by grap- anoxic marine basin. The La Gleize Formation, > 300 m thick, is formed tolites (Rhabdinopora sps.; Malaise, 1874; Geukens, 1950, 1954; Bulman by wavy bedded dark siltstones that evolve rapidly to black slates. and Geukens, 1970; Wang and Servais, 2015; Graptolite in the Fig. 3) These black slates are enriched in uranium and contain pyrrhotite, and by acritarchs (informal Zone 7 from Vanguestaine, 1974, 1992). properties that permitted remote cartography (Geukens, 1981; Based on the range of the early Tremadocian graptolites, Wang and Vanderschueren et al., 2007). They are interpreted as fine-grained Servais (2015 fig. 2) show that the Solwaster Member comprises half of turbidites (Stow model) evolving to pelagic sediments (Herbosch, the Tr1 Stage-Slice, from its extreme base (R. praeparabola) to near its

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Permian? 6° 0' E Fm. N Lower Devonian SCHEVENHUTTE Marteau/Fépin fms. NEDERLANDS Unknown age, post-Sm3 Petites Tailles Fm. GERMANY VICHT Ordovician Bihain Fm. (Sm3) Ottre Fm. (Sm2)

enn Fault Jalhay Fm. (Sm1) BELGIUM V Cambrian La Gleize Fm. (Rv5) Lammersdorf

La Venne Fm. (Rv3-4) EupenFault Vesdre Wanne Fm. (Rv1-2)

Hourt & Bellevaux Fm. Vesdre (Dv1-2) fault

thrust fault D Ardennian unconformity Helle St11

Gileppe MONSCHAU magmatic intrusion Helle Northern limit of epizone Hogne Eupen Fault GERMANY THEUX Theux window A 50° 30' N 50° 30' N

SPA Fault BELGIUM Theux BELGIUM

Sl13 St4 Sl4 E MALMEDY Amble ve Sl5 arche ult W Fa Stn: zircon sample ris Xho STAVELOT Ligneuv Sln: slate sample ille Chevron B * Antic Ambl line Ambleve ev Challes sill e TROIS-PONTS St2,3 * Fault Sl3 Grand-Halleux sill Xhoris Salm * in borehole

RECHT Grand-Halleux Sl2 BRA

St1Sl1 Anticline Lienne St6 D E St5 * LIERNEUX Jalhay E Pridoli Lochk. upen F. Syncl. Rv3-4 Rv5 Sm1 Sm2 Sm1 Rv5 Sm1 Rv5 St7,9 B Sm1 C C A Rv1-2 Dv1-2 Sm3 DOCHAMPS St10 Dv1-2 Lochk. Pridoli St8 Sl6,8,9 X Sl7 Th hor Sm1 Rv5 Sm1 Rv5 eux FRv3-4 is Rv3-4 Rv1-2 Rv5 Rv3-4 F. Sm1 . 5 km 6° 0' E

Fig. 2. Geological map and N-S cross sections (ABC and DE) of the Stavelot-Venn Massif modified after Geukens (1986, 1999). Location of the samples for zircon datation (black St numbers) and of slates for Nd isotopic analysis (blue Sl numbers) is reported on the map. Lochkov. = Lochkovian. The old stratigraphic no- menclature is given within brackets (Dv1-2, etc.; Beugnies et al., 1976)

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Age Series Global Acritarch Stavelot-Venn Inlier Groups Samples Ma Stages zonation formations Zircon Isotope Geoch- (Vanguestaine) datation Sm/Nd emistry LOWER LOCHKOVIAN Fm. DEVONIAN 419 St10 420 Ardennian Unconformity St10 St10

NAIRULIS PRIDOLI 423 LUDLOW 427

430 hiatus 450 KATIAN LATE 453 ORDOVICIAN SANDBIAN

458 N 460 I C I V O D R R D O V I C I DARRIWILIAN MIDDLE ORDOVICIAN 467 ? Sl7 Salm-Château DAPINGIAN BIHAIN Fm. Mbr.

470 Zone 9 (Sm3) Ruisseau d'Oneux Mbr. FFLOIAN Colanhan Mbr. OTTRE Fm. LOWER Les Plattes Mbr. Sl6,9 Sl6,9 478 (Sm2) (St8) (St8) ORDOVICIAN Meuville Mbr. Lierneux Mbr. St7

480 Z.8 OA TREMADOCIAN JALHAY Fm. Spa Mbr. SALM GROUP

Z.7 (Sm1) Solwaster Mbr. St6

485 Graptolite Sl5 Stage 10 LA GLEIZE Fm. (St5) (St5) (St5) 490 Z.6 (Rv5) 490 FURONGIAN JIANGSHANIAN (Rv4) 494 Z.5 PAIBIAN LA VENNE Fm. St4 Sl4 497 (St11) (St11) (St11) "upper Camb." (Rv3)

GUZHANGIAN Z.4b 500 501 Z.4a

MIAOLIN- DRUMIAN REVIN GROUP GIAN 505 Z.3 (Rv2) St3

I R B M B R I WANNE Fm. Z.2 WULIUAN (Rv1) Sl3 "middle Camb." 509 Z.1 510 Phyllades du Pont

Z.0 St2 Stage 4 BELLE- Quartzite de Fourire 514 Oldhamia VAUX Fm. Sl2

AN SERIE 2 (Dv2) ?

Stage 3 (Dv1) CA HOURT Fm. 520 521 St1 Sl1 ? TERRENEU- Stage 2 VIAN DEVILLE GROUP 529

Fig. 3. New chronostratigraphy of the Stavelot-Venn Massif (Herbosch and Servais, in preparation). Ages, series and global stages after Cohen et al. (2013, updated). Avalonian trilobite biozones (Rushton and Molyneux, 2011) and graptolite zones (Wang and Servais, 2015) allow to link up the informal Vanguestaine acritarch zones with the global stratigraphy (Vanguestaine, 1974, 1986, 1992; Ribecai and Vanguestaine, 1993; Vanguestaine and Servais, 2002; Vanguestaine et al., 2004). Dotted line: limit between formations or members of uncertain age. Old nomenclature in brackets (Beugnies et al., 1976). The Petites Tailles Formation, of unknown age, is not reported. St and Sl numbers: stratigraphic position of the sample used for zircon ages, Nd isotopes and geochemistry; sample numbers within brackets indicate magmatic rocks. middle part (R. f. flabelliformis and R. f. socialis; 485.4-482 Ma). Then trifidium acritarch assemblage” (Servais and Mette, 2000; Vanguestaine the Spa Member belongs to the upper part of the Tr1 Stage-Slice (R. f. and Servais, 2002; Breuer and Vanguestaine, 2004). So, the Jalhay socialis and R. f. anglica; 482-480 Ma). The upper part of the Lierneux Formation coincides with most of the Tremadocian (485-478 Ma). Member can be assigned to the late Tremadocian “messaoudensis- The middle Ottré Formation (Sm2) is mainly composed of red to

5 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 purple pelitic rocks very rich in Fe and Mn where the famous yellow The conglomerate was sampled on the south-eastern rim of the SVM coticule layers occur (Renard, 1878; Kramm, 1976; Lamens et al., 1986; (St10, Fig. 2, Table 1) in the former Gami quarry, which shows an al- Krosse and Schreyer, 1993; Herbosch et al., 2016). (1) The lower ternation of white granule conglomerate, white sandstone and greenish Meuville Member (Sm2a), c. 150 m thick, is formed by red silty slate slate. This succession belongs to the Waimes Formation and occurs a with some green siltstone. (2) The middle Les Plattes Member (Sm2b), few meters above the basal cobble conglomerate (“poudingue de 0-50 m thick, is formed by red to violet slate interstratified with mm to Quareux”; Graulich, 1951; Prévinaire, 1970; Fiermans, 1982), which cm yellowish coticule layers. These are fine-grained highly mangani- lies in angular unconformity on the Salm Group at the sampling site. ferous metasedimentary rocks mainly composed of spessartine, K-mica Rare volcanic elements are present in this conglomerate (Geukens, and quartz. (3) The upper Colanhan Member (Sm2c), c. 100 m thick, 1976). St10 sample is a metaconglomerate belonging to the epizonal shows red and green slates. These three members are completely devoid metamorphic zone of the SVM (Fig. 2, Chap. 2). of acritarchs due to their highly oxidizing depositional environment. Microscopically, the St10 metaconglomerate is essentially com- However, the discovery of conodonts of the Parastodus proteus Zone at posed of quartz, muscovite and minor chlorite. Detrital quartz of the base of the Les Plattes Member in the Lienne Syncline (Vanguestaine varying sizes is embedded in a very fine matrix of muscovite, quartz and et al., 2004) gives a likely lower Floian age (Early Ordovician, Fig. 3) chlorite. Muscovite has an oriented structure defining a Variscan fo- for this member. This allows attributing a short duration of about 2 to 4 liation. This metamorphic matrix forms about 20 to 30% of the total Myr to the Les Plattes Member (Herbosch et al., 2016). volume of the rock. Detrital quartz is most often observed in sub-an- The uppermost Bihain Formation (Sm3), c. 300 m thick, is formed by gular monocrystalline grains with uniform or undulating extinction. (1) the Ruisseau d’Oneux Member (Sm3a), 30 m thick, and (2) the Their size ranges from 0.05 to 6-7 mm, the most frequent size is be- Salmchâteau Member (Sm3b). The formation consists of dark laminated tween 0.5 and 1 mm. Aggregates of polycrystalline quartz grains with slate and dark greenish bioturbated siltstone. Some thicker siltstone interlocking boundaries are observed in lower abundance. beds show turbiditic sedimentary structures that indicate a SE pa- No K-feldspar is observed in sample St10. However, K-feldspar is leoslope and a not very deep anoxic depositional environment (Lamens, reported in small amounts in the Waimes Formation on a regional scale, 1985a, 1986). The upper member has been dated on relatively long- sometimes well-preserved but most often highly saussuritized range acritarchs from the late Darriwilian to early Sandbian (informal (Prévinaire, 1970; Fiermans, 1982; Vandenven, 1990; Boulvain, pers.

Biozone 9, Vanguestaine, 1986, 1992; Verniers et al., 2001). Recently, comm., 2018). The high K2O contents between 1.50 and 4.75% tough, a slightly older age has been proposed based on an integrated (Fiermans, 1982; Van den Bleeken and Corteel, 2007; St10 analysis acritarch-chitinozoan study (upper Dapingian to early Darriwilian age, Table 2) can be attributed to K-feldspar but also to muscovite whose c. 467 Ma; Servais, comm. pers.; Fig. 3). K2O content is about 10% (Van den Bleeken and Corteel, 2007). K- The Petites Tailles Formation lies in unconformity over this succes- feldspar, even episodic or in low amount, can explain the formation of sion in the southern end of the SVM (Fig. 2). Its age is unknown but it is muscovite by metamorphism and indicates that the protolith of the older than the Ardennian unconformity (Pridoli-Lochkovian) that caps metaconglomerate was K-feldspar-bearing. this formation. 3.2. Comparison of subsidence and depositional environments with the 3.1.4. Above the Ardennian unconformity, the Waimes Formation Brabant Massif The SVM Cambro-Ordovician succession is interrupted by an an- gular unconformity, the “Ardennian unconformity” (Fig. 2; after the A better estimate of sediment thicknesses (Geukens, 2008) and the “Ardennian phase” of Michot, 1980; Verniers et al., 2002). This un- recent progress chronostratigraphy (see above; Fig. 3), allow to build a conformity is highlighted by a conglomerate, from uppermost Pridoli cumulative thickness curve for the whole Lower Paleozoic sedimentary (Silurian) to Lochkovian (Lower Devonian) in age. This conglomerate pile of the SVM (Fig. 4A). This curve is not a true subsidence curve, belongs to the Marteau Formation to the W and N (Dejonghe et al., because it has not been corrected for the compaction and water column. 1994), to the Fépin Formation to the SW (Meilliez and Blieck, 1994) Nevertheless, this curve reveals major changes in the rate of sedi- and to the Waimes Formation to the SE (Graulich, 1951; Vandenven, mentation: the curve shows a progressive increase of the rate of sedi- 1990; Van den Bleeken and Corteel, 2007). The diachronic onset of the mentation (convex curve) from Hourt to La Gleize formations followed transgression (historically the "Gedinnian transgression") upon the SVM by a sharp rate increase in the Jalhay Formation, going back to a lower was directed to the NW (Steemans, 1989; Bless et al., 1990). In con- rate in the Ottré and Bihain formations. This rate change in the Jalhay sequence the conglomerate along the SE flank is older (uppermost Formation can be correlated with its strong regressive character (§ Pridoli to lower Lochkovian) than that from the NW flank (lower to 3.1.3). This slope discontinuity corresponds to a major event in the upper Lochkovian), well shown by the brachiopod assemblages basin development (Rheic Ocean opening, see below). (Godefroid and Cravatte, 1999 fig. 6; Jansen, 2016). The nearest Avalonian sedimentary basin is the Brabant-Condroz

Table 1 Description and location of the sample used for geochemical and U-Pb zircon geochronology.

Sample Lithology Lithostratigraphy Acritarchs Zone Chronostratigraphy Location Coordinates

Sedimentary rocks St1 White quartzite Middle Hourt Formation no acritarch Stage 3 Rocher de Hourt, road km 106 N 50°18’36.2” E 5°54’25.6” St2 Grey-green sandstone Upper Bellevaux Fm. Biozone 0 Upper Stage 4 Lôdômé, Laid Trou gully N 50°23’13.6” E 5°58’22.9” St3 Grey pyritic quartzite Middle Wanne Fm. Biozone 1 Upper Wuliuan Lôdômé, Laid Trou gully N 50°23’4.1” E5°58’26.4” St4 Black quartzite Middle La Venne Fm. Biozone 4b Upper Paibian Old railways km 29.6, Stavelot N 50°26’12.9” E 5°57’50.7” St6 Grey turbiditic sandstone Solwaster Member Biozone 7a Lower Tremadocian Road Trou de Bra km 14 N 50°19’6.5” E 5°46’4.7” St7 Green argillaceous sandstone Uppermost Lierneux Mbr. Biozone 8 Uppermost Tremadocian Vielsalm, station trench N 50°20’4.1” E 5°44’3.0” St9 Black siltstone Salmchâteau Member Biozone 9 Lower Darriwilian E side Salm cluse, Salmchâteau N 50°60’26.6” E 5°54’24.4” St10 Metaconglomerate Waimes Formation (Brachiopod) Uppermost Pridoli Old quarry, Salmchâteau N 50°15’54.4” E 5°54’32.7”

Intrusive and volcanic rocks St5 Interstratified volcanic rocks Upper La Gleize Fm. Biozone 6 Upper Stage 10 Road to Lierneux km 17.2 N 50°19’6.5” E 5°46’4.7” St8 Interstratified volcanic rocks Les Plattes Member (Conodont) Lower Floian NW Thier del Preu hill N 50°16’45.9 E 5°50’13.8” St11 Helle quartz diorite sill Borehole He4 5.2-5.6 m N 50°32’55.9” E 6°08’23.8”

6 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Table 2 Major, trace elements and Nd isotopes of sample St10 Pridoli metaconglomerate.

% SiO2 Al2O3 Fe2O3t MnO MgO CaO Na2OK2O TiO2 P2O5 LOI Total

84.36 9.22 0.59 0.03 0.21 0.14 0.16 2.91 0.17 0.02 1.85 99.67 ppm V Rb Sr Y Zr Nb Ba Ga Cs Th U 11.6 141 66 28 90 9.6 249 9.6 4.0 10.0 1.22 ppm La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu 11.0 25 3.2 11.0 2.5 0.34 3.1 4.0 0.92 2.7 3.0 0.45 ppm Hf Ta W Pb Co Cr Zn Cu Ge Ni Sc 2.5 1.92 7.3 15.4 4.1 4.2 11.7 177 1.41 4.5 10.0 147 144 143 144 Sm Nd Sm/ Nd Nd/ Nd 2 s εNd, 420Ma TDM 2.54 11.03 0.1390 0.512201 0.00001 -5.44 1713

2001; Debacker et al., 2005; Sintubin et al., 2009; Linnemann et al., 0 + Hourt + Bellevaux A 2012 fig. 6) and concave for the SVM (Fig. 4). This can be linked to the + Wanne Stavelot-Venn Massif Blanmont + La Venne development of a Cambrian rift for the Brabant Massif (huge deposition + La Gleize of 9 km; Verniers et al., 2002; Linnemann et al., 2012). The thinner + Jalhay sedimentary pile in the SVM (c. 3 km) indicates another tectonic en- 2000 + Ottré vironment. Tubize + Bihain + Hiatus However, the SVM and Brabant basins display important common features. First, except for the subsidence rate, the depositional en- vironments in the two basins and their evolution during the ~467 Ma Megasequence 1 are very similar (Fig. 5): the sedimentation began with 4000 Megasequence 1 Oisquercq sandstone deposited in a shallow shelf (Blanmont vs Hourt formations) that evolved rapidly to a deep suboxic basin with turbidites and pelagic shales (Tubize vs Bellevaux formations). In both basins, the sedi- 6000 mentation continued for a long time (30-35 Ma) in a deep basin with Jodoigne turbiditic and pelagic sediments (Oisquercq / Jodoigne / Mousty / Chevlipont formations vs Wanne / La Venne / La Gleize / lower Jalhay (Solwaster and Spa members) formations). This long period was char- acterized by a deep anoxic basin where the turbiditic sequences were 8000 more frequent in the SVM than in the Brabant Massif. Secondly, and this ~480 Ma Megasequence 2 Mousty is a major point, the slope discontinuity at the top of the Jalhay For- Chevlipont Abbaye de Villers mation is contemporaneous (Fig. 4, 5) with the onset of the hiatus that Cumulative thickness (meters) Hiatus Tribotte has been recognized between the Megasequence 1 and the Mega- 10000 Rigenée Ittre B Bornival sequence 2 in the Brabant Basin (Vanguestaine, 1992; Verniers et al., Brabant Massif C.Grand-Manil Huet+Fau. 2002; Vanmeirhaeghe, 2006; Linnemann et al., 2012 fig. 6; Herbosch and Verniers, 2014). This hinge period (c. 480 to 467 Ma) is thus Cambrian Meg. 3 Ordovician marked by a stratigraphic hiatus with unconformity in the Brabant Basin and, in the SVM, by a sudden change in the deposition depth 525 500 475 450 between the upper part of the Jalhay Formation (upper slope to shelf) Time (millions years) and the Ottré Formation (deep basin; Lamens, 1985a, 1986; Herbosch et al., 2016)(Fig. 4, 5). Fig. 4. A. Cumulative thickness curve for the Lower Paleozoic sediments by By contrast, after Megasequence 1, the evolution in the two basins is formations of the Stavelot-Venn Massif plotted against the stratigraphic age the opposite. The stratigraphic hiatus in the Brabant Massif corresponds (time-scale from Cohen et al., 2013, updated). Estimated thickness from Geukens (2008) and estimated stratigraphic ages from fig. 3. B. Same graph for in the SVM to the Ottré and Bihain formations (dash lines in Fig. 4). the Brabant Massif towards Megasequence 3 (modified after Linnemann et al., This means that the slope discontinuity (bathimetric break) observed at 2012). The dashed lines correspond the Brabant Massif hiatus period. the top of the Jalhay Formation marks the end of Megasequence 1 and that the Ottré and Bihain formations form the beginning of Mega- sequence 2. Thus, the 480-467 Ma Brabant sedimentary hiatus is absent Basin (Verniers et al., 2002; Vanmeirhaeghe, 2006; Herbosch and in the SVM, where the sedimentary deposition became deeper and Verniers, 2014). The south-eastern margin of the Brabant Massif is slower. In the SVM, a hiatus is observed later when the sedimentation currently hidden under the Midi-Aachen thrust fault that marks the resumed in the Brabant Massif with the shelf deposits of the Abbaye de front of the Ardenne Allochthon (Fig. 1). The distance between the SVM Villers and Tribotte formations (Herbosch and Verniers, 2014; Fig. 4, and the Midi-Aachen Fault is currently of c. 40 km. Before the Cale- 5). This inverse behaviour can be related to the opening of the Rheic donian and Variscan folding and thrusting, this distance can be esti- Ocean that occurred at c. 480 Ma, i.e. the rift-drift transition of Ava- mated at a minimum of 100 km taking into account a regional short- lonia leaving Gondwana (Cocks and Torsvik, 2002, 2005; Torsvik et al., ening of c. 50% (Adams and Vandenberghe, 1999; Oncken et al., 2000; 2012; Linnemann et al., 2012; Herbosch et al., 2016). The premises of Nierhoff et al., 2011). This implies that the Brabant-Condroz and the the drifting are a higher sedimentation rate leading to a regression in SVM basins have been close during their development. This hypothesis the SVM (Jalhay Formation; Fig. 4) and a lower sedimentation rate has been suggested before, but rather evasively (Martin, 1969; Owen coupled with an uplift in the Brabant Massif (uppermost Mousty and and Servais, 2007; Linnemann et al., 2012; Debacker et al., 2014; Chevlipont formations) (Fig. 4, 5). The Rheic drifting (opening) itself Herbosch and Verniers, 2014) except recently, based on the genesis of generated an abrupt deepening of the basin in the SVM (Ottré and Bi- the coticule (Herbosch et al., 2016). hain formations) and the emergence of the Brabant Massif (hiatus). On The "subsidence" curves of the Brabant and SVM massifs are mark- the other hand, the rift-drift initiation is marked in both regions by an edly different (Fig. 4A, 4B), it is convex for the Brabant (Debacker,

7 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Brabant Massif Stavelot-Venn Massif Age Series Global Stages Formations Depositional Depositional Formations Ma environment environment 460 RIGENEE Fm. Siltstone.Transition N from shelf to slope DARRIWILIAN Emersion hiatus MIDDLE TRIBOTTE Fm. Argillaceous sand- I C I V O D R R D O V I C I ORDOVICIAN stone to siltstone 467 ABBAYE de VILLERS Shallow shelf Megaseq. 2 Dark bioturbated DAPINGIAN BIHAIN Salm-Chât. Mbr. 470 siltstone, rare turbidi. Fm.(Sm3) Deep anoxic basin R. d'Oneux Mbr.

FFLOIAN hiatus Emersion Colanhan Mbr. Red Fe-Mn slate, OTTRE coticule layers Les Plattes Mbr. LOWER 478 Fm.(Sm2) Megaseq. 2 ORDOVICIAN Deep oxic basin Meuville Mbr. Lierneux Mbr. 480 Turbidites & slate Fine-grained turbidites JALHAY Spa Mbr. TREMADOCIAN Regression from a CHEVLIPONT Fm. Regression to slope Fm.(Sm1) Solwaster Mbr.

OA deep basin to shelf 485 Black slate & low Stage 10 LA GLEIZE Fm. Black pelagic slate, density turbidites (Rv5) 490 Mn enriched levels Deep anoxic basin 490 FURONGIAN MOUSTY Fm. JIANGSHANIAN Deep anoxic basin (Rv4) 494 Turbidites & black PAIBIAN slate LA VENNE 497 Fm. GUZHANGIAN Deep anoxic basin 500 Turbidites & black (Rv3) pelagic slate MIAOLIN- DRUMIAN JODOIGNE Fm. GIAN 505 Deep anoxic basin Turbidites & black (Rv2) I R B M B R I slate WANNE WULIUAN ? Deep anoxic basin Fm. (Rv1) 509 Violet & green 510 OISQUERCQ Fm. pelagic slate Sandstone,siltstone, Stage 4 Deep oxic basin slate, also turbidites BELLEVAUX Fm. 514 Rapid transition to a (Dv2) ? M e g a s q u n c 1 M e g a s q u n c 1 AN SERIE 2 Turbidites & pelagic deep suboxic basin TUBIZE Fm. green slate ? Stage 3 Deep suboxic basin Quartzite, rare slate CA ? HOURT Fm. 520 521 Quartzite, rare slate Shelf environment BLANMONT Fm. Shallow environ. in a (Dv1) rapidly subsiding rift Stage 2 ? ? TERRENEU- VIAN 529 530 FORTUNIAN

Fig. 5. Comparison of the lithology and the depositional environment between the Brabant Massif (from Linnemann et al., 2012; Herbosch and Verniers, 2013, 2014) and the Stavelot-Venn Massif (from Lamens and Geukens, 1984; von Hoegen et al., 1985; Verniers et al., 2001; Geukens, 2008; Herbosch, unpublished). Chron- ostratigraphy of the Brabant Massif from Herbosch and Verniers (2013, 2014) and of the Stavelot-Venn Massif from the Fig. 3 (Herbosch and Servais, in preparation). Time-scale from Cohen et al. (2013, updated). episode of volcanism, in the upper part of La Gleize Formation in the La Venne Formation (Rv4, Jiangshanian) from c. 505 to 490 Ma. Also, SVM (§ 4.1) and in the Chevlipont Formation in the Brabant Massif some volcanism is present in the Salm Group, in the lower part of the (Linnemann et al., 2012). These features highlight the contrasting Ottré Formation (Sm2ab, Meuville and Les Plattes members, Floian, rheological behavior of the two regions within the same basinal en- 478-473 Ma; Lamens and Geukens, 1984). All these volcanic rocks are vironment, suggesting one basin on two distinct basements. interstratified in the sediments with a maximum thickness of some meters but can be observed along several kilometres. This indicates a 4. Magmatism in the Stavelot-Venn Massif and related areas progressive increase of the volcanism from 505 Ma to a climax at 490- 485 Ma with a sharp decrease afterwards. All these volcanic rocks are 4.1. Felsic magmatic rocks locally affected by both a slaty and a crenulation cleavage attributed to the Caledonian orogeny (Geukens, 1976). A series of conformable acid volcanic rocks ("eurite" and "kerato- The Helle and Lammersdorf sills are the only significant felsic ig- phyre") associated with the SVM Cambrian and Ordovician sediments neous occurrences. They are situated in the north-western part of the and also scarce intrusive igneous rocks have been described since the SVM, respectively in Belgium and Germany (Fig. 2). They are hosted in 19th century (Dumont, 1848; Poussin and Renard, 1876; von Lasaulx, the La Venne Formation (Rv3-4), giving them a maximum age of c. 490 1884). Due to their very limited volume, none of the volcanic rocks can Ma. The Helle intrusion was recognized by four drillings down to a be mapped at the scale of fig. 2. An extensive review of all occurrences, depth of about 100 m (Dejonghe and Melchior, 1996; Dejonghe, 2003). their petrography and some chemistry, was published by Corin (1965). This quartz diorite to granodiorite intrusion has the shape of a sill The extrusive rocks, presumably tuffs, were deeply weathered during dipping 30° towards the SE, stretches along about 500 m in a NE-SW the deposition and the following metamorphisms (Daneels and Vogel, trend and has a maximum thickness < 100 m. The Helle sill has been 1978). dated by the U-Pb ID-TIMS on bulk zircons method that gave an age of Conformable volcanic rocks are absent in the Deville Group and are 381 ± 16 Ma (Middle-Late Devonian; Kramm and Buhl, 1985). This is mostly observed in the Revin Group (Fig. 3), especially in its upper part actually a minimum age as it resulted from a lower intercept de- (La Gleize Formation, Rv5, Stage 10, 490-485 Ma) where they are termined by four discordant zircon fractions affected by Pb loss. In particularly abundant (Geukens, 1976, 1999; Lamens and Geukens, consequence, the age of the Helle intrusion can only be constrained 1984). They are also observed, but more scarcely, from the upper part between 490 and 380 Ma. Finally, there are also some meter-thick felsic of the Wanne Formation (Rv2, Drumian) towards the upper part of the dykes in the Spa region that crosscut the strata of the Revin Group.

8 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Fig. 6. A. Concordia plot of U-Pb zircon data of the volcanic rocks St5 A: St5, La Gleize volcanic rock: 485 ±6 Ma interbedded in the upper part of the La Gleize Formation (upper Stage 0.082 206 500 data-point error 10, Fig. 3; Biozone 6 of Vanguestaine, 1992). To the left: all the zircon Pb ellipses are 2s age gives a Discordia with the lower intercept at 0 ± 0 Ma and the upper intercept at 487 ± 15 Ma. To the right: the two most concordant 0.078 2 most concordant grains 238 Concordia Age = 484.8 ± 6.3 M a zircons give a Concordia age of 484.8 ± 6.3 Ma which is interpreted to U (2, decay-const. errs. included), MSWD (of concordance) = 0.0048, be the age of the extrusion. B. Concordia plot of U-Pb zircon data of Probability (of concordance) = 0.94 the quartz diorite of the Helle sill that intruded the Furongian sedi- 0.074 460 0.0805 mentary rocks (sample St11). In the centre: all zircon age. To the n = 22/30, 78-100% conc. upper left: Discordia with the lower intercept at 0 ± 0 Ma and the 0.0795 upper intercept at 446 ± 15 Ma. To the lower right: the 13 most 0.070 490 concordant zircons give a Concordia age of 446.2 ± 2.7 Ma which is 0.0785 420 interpreted to be the age of the intrusion.

0.066 0.0775 480

0.0765 380 0.63 0.062 470 0.0755

0.0745 0.058 0.59 0.61 3 most concordant grains 207 Intercepts at Concordia Age = 483.0 ± 5.2 M a Pb 0 ± 0 Ma & 487 ± 15 [± 16] Ma (2, decay-const. errs. included), 0.054 MSWD = 0.0033 MSWD (of concordance) = 0.019, 235 Probability (of concordance) = 0.89 U

0.44 0.48 0.52 0.56 0.60 0.64 0.68 B: St11, Helle sill: 446 ±3 Ma ellipses are 2 d a

206 Intercepts at ta-p 0.076 470 0 ± 0 Ma & 446 ± 15 [± 16] Ma

0.26 Pb o i

MSWD = 0.0065 n t 238 erro 450 U 0.072  r 1300 1357 ±21 Ma 0.22 430 0.62 0.068 n = 31/60, 90-110% conc. 1072 0.075 410 0.58 ±24 Ma 0.18 0.064 0.073 390 450

0.54 0.060 900 0.071 0.14 0.42 0.46 0.50 430 0.069 0.60 700 650 ±16 Ma 0.58 0.067 13 most concordant grains 0.10 (99-101% conc.) 561 ±13 Ma 410 500 0.56 486 ±6 Ma 0.065 0.48 0.50 0.52 0.54 Concordia Age = 445.9 ± 2.7 M a 207 0.06  Pb 300 (2 , decay-const. errs. included), MSWD (of concordance) = 0.013, 235 Probability (of concordance) = 0.97 U 0.02 0123

They are made of porphyritic rhyolite with phenocrysts of quartz, albite Goffette, 1991), they display an albitic plagioclase skeletal frame and and chloritized biotite (Daneels and Vogel, 1978). xenomorphic crystals of retrogressed clinopyroxene (augite) sur- rounded by actinolitic hornblende and actinolite. The secondary para- 4.2. Mafic magmatic rocks genesis comprises epidote, chlorite, actinolite, albite, quartz, calcite and ilmenite partly transformed into titanite. These metadolerites are In addition to the above felsic rocks, there are several mafic rocks tholeiitic in nature (Schreyer and Abraham, 1978; André, 1983; André relevant for SVM evolution (stars 1 to 5 in Fig. 1): (1) two intrusions in et al., 1986; Goffette et al., 1990). the SVM itself (Grand Halleux and Challes), (2) a bimodal dyke swarm At Grand-Halleux, a drill core presents two levels of dolerite among in the Rocroi Massif (Grande-Commune-Mairupt), another Ardenne the quartzites and slates of the Deville Group. The second level (depth: inlier (Fig. 1) and (3) two hypabyssal bodies (Voroux-Goreux, Mozet) 1892-1900 m), better preserved, is an 8 m thick sill (Corin, 1965). At near the Ardenne - Brabant thrust contact (Variscan Front). All these Challes, the intrusion is a 5 m thick sill in the slates of the upper part of mafic rocks are dolerites (diabase or microgabbro) affected by a low- the Wanne Formation (Rv2; Geukens, 1963a; Corin, 1965; Schreyer and grade greenschist facies metamorphism, being actually metadolerites. Abraham, 1978). Their age is unknown except that they are younger They are all heavily altered, showing a variably preserved subophitic than the Deville Group and the Wanne Formation, respectively, and are texture. Typically (Denayer and Mortelmans, 1954; Corin, 1965; thus younger than c. 500 Ma. Schreyer and Abraham, 1978; André, 1983; Goffette et al., 1990; In the southern Rocroi Massif, the bimodal dyke swarm has been

9 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 recently dated at 421 ± 3 Ma (Pridoli-Lochkovian) on the microgranitic adopt the age of 485 ± 6 Ma for the La Gleize volcanic event. This age dyke of Mairupt (LA-ICP-MS U-Pb zircon; Cobert et al., 2018). This is a agrees with a syn-sedimentary emplacement of this level within the dense swarm although volumetrically unimportant in which the do- upper part of Stage 10 (latest Furongian) whose upper limit is dated at lerites are predominant (80%). The dykes are WSW/ENE oriented and 485.4 ± 1.9 Ma (Cohen et al., 2013, updated). No inherited zircons crosscut the folds affecting the Cambrian formations (Meilliez, 1981; have been identified in this sample; the eight unused spots are too Goffette et al., 1991). discordant to give meaningful ages and their 206Pb/238U ages are in the Close to the Variscan front, the Voroux-Goreux and Mozet doleritic range 346-498 Ma (Table S1). This magmatic age of 485 ± 6 Ma places bodies, sometimes associated with felsic rocks, are related to the Haine- this volcanic rock within the climax (490-485 Ma) of the interstratified Sambre- (HSM) overturned thrust sheets structurally located just volcanism described above (§ 4.1). below the Midi thrust Fault classically marking the base of the Ardenne Allochthon (Fig. 1). This HSM tectonic unit comprises sedimentary 5.1.2. The Helle quartz dioritic sill (sample St11) rocks from both the Ordovician-Silurian basement and the Middle De- The published 381 ± 16 Ma zircon age being only a minimum age vonian to Carboniferous cover of the Brabant Massif (Bélanger et al., (Kramm and Buhl, 1985), a new age was needed for this key intrusion; 2012). The Voroux-Goreux volcanic complex (including the Hozémont the sample was chosen in a borehole (Table 1). Sixty zircon grains of sill), comprises basaltic pillow lavas associated with gabbroic hypa- sample St11 were analysed, from which 8 are inherited and, among the byssal intrusions, acidic pyroclastic rocks and rhyolites (Denayer and magmatic zircons, 31 were concordant in the range of 90 to 110% Mortelmans, 1954; Corin, 1965; André, 1983) and constitutes a bi- (Table S1). These 31 spots form a Discordia with an upper intercept at modal association. The lava lies discordantly over lower Silurian (upper 446 ± 15 Ma (MSWD= 0.007, imposed lower intercept at 0 Ma; Llandovery, c. 435 Ma) sediments (Michot, 1930; Corin, 1965) that Fig. 6B). The 13 most concordant zircons (99-101% concordance) constitute a maximum age for this volcanic activity. The Mozet dolerite better define this age at 445.9 ± 2.7 Ma (Concordia age, probability of is intrusive in the HSM along the southern side of the Midi Fault and concordance = 0.97; Fig. 6B); the intrusion age of the Helle sill is thus crosscut the Upper Ordovician and lowermost Silurian formations. considered to be 446 ± 3 Ma. This age corresponds to the Katian-Hir- Except for the doleritic dykes in Rocroi indirectly dated at 421 ± 3 nantian transition (445.2 ± 1.4 Ma), near the end of the Ordovician Ma (uppermost Silurian; Cobert et al., 2018), these tholeiitic intrusions (443.8 ± 1.8 Ma; Cohen et al., 2013 updated). Kramm and Buhl (1985) are not dated. Considering the geological constraints, they can belong were right to consider their 381 ± 16 Ma lower intercept age as a to the Ordovician and/or Silurian. They cannot be younger as none of minimum age, their analysed zircons having suffered Pb loss probably them have been found crosscutting lower Devonian or younger series. It during the Variscan orogeny. The Helle sill contains also inherited is not known if this magmatism lasted a long time, e.g. during the in- zircons, with 3 concordant spots at 486 ± 6 Ma, corresponding to the dependent life of Avalonia, from the beginning of the Ordovician (after La Gleize volcanic episode (485 ± 6 Ma). Older ages are based on single the Rheic Ocean opening, c. 480 Ma) to the end of the Silurian concordant spots: 561 ± 13 Ma, 650 ± 16 Ma, 1072 ± 24 Ma and (amalgamation with Laurentia, c. 420 Ma) or if it was brief and limited 1357 ± 21 Ma (Fig. 6B); the remaining spots indicate older ages to the end of the Silurian. (207Pb/206Pb ages between 1.7 and 1.9 Ga).

5. New U-Pb LA-ICP-MS on SVM magmatic and detrital zircons: 5.2. U-Pb ages on detrital zircons data Sample location of rocks, from which zircons grains were analyzed The location of the investigated samples is shown in Table 1 and by LA-ICP-MS are shown in Table 1 and Figs. 2, 3. Seven sandstone, Fig. 2, 3. Analytical techniques of the analysis of U-Pb isotopes of quartzite and siltstone samples that regularly cover the whole SVM magmatic and detrital zircons by Laser Ablation Inductively Coupled from Stage 2 (lower Cambrian) to lower Darriwilian (Middle Ordovi- Plasma Mass spectrometry (SF-ICP-MS) are described in Annex 1. cian) and one additional uppermost Pridoli (end of Silurian) meta- Concordia diagrams (with 2 σ error ellipse) and Concordia or Discordia conglomerate from the base of the Ardennian unconformity, were ages (95% confidence level) were produced using isoplot/Ex 2.49 sampled for zircon dating (Fig. 3). The sample St9, a siltstone from the (Ludwig, 2001), and frequency and relative probability plots using upper part of the sedimentary succession (Bihain Formation), has not AgeDisplay software (Sircombe, 2004). The 207Pb/206Pb age was taken given suitable zircon for dating. for interpretation for all zircons > 1.0 Ga and the 206Pb/238U ages for The studied samples display time intervals rich in detrital zircons the younger grains. Analytical results are given in Table S1 (Data re- and others without any zircons (gap), which are often similar from one pository). sample to another. Taking advantage of this feature, several time per- iods have been setup and shown in each detrital zircon figures (Figs. 7 5.1. U-Pb zircon ages of magmatic rocks to 9). In Table 3, the zircon percentages are available for each sample and periods (for facility, the period names are used even if the age Two interstratified volcanic rocks, in the La Gleize and Ottré for- bracket considered does not exactly match the official period length). mations were sampled but zircons have been recovered only in the first They are: 419-480 Ma (Ordovician-Silurian), 480-541 Ma (Cambrian), one. This sample (St5) is interbedded in the upper part of the La Gleize 541-635 Ma (Ediacaran), 635-800 Ma (Cryogenian), 800-950 (Tonian), Formation (Fig. 2, 3; upper Stage 10; Biozone 6 of Vanguestaine, 1992). 950-1700 Ma (Mesoproterozoic), 1700-1950 Ma (upper Paleoproter- In addition, a sample from the Helle sill, intruded into the La Venne ozoic), 1950-2250 Ma (middle Paleoproterozoic), 2250-2400 Ma Formation, has been dated (sample St11; Fig. 2, 3). (Siderian), 2400-3600 Ma (Archean). In sample St1 (middle Hourt Formation, Stage 3; Table 1 and Fig. 2, 5.1.1. La Gleize volcanic rock (sample St5) 3), 120 detrital zircon grains were analysed. Of these, 92 grains pro- Thirty zircon grains were analysed (Table S1), from which 22 spots vided U-Pb ages that can be considered as concordant (specifically in define a Discordia age of 487 ± 15 Ma with a lower intercept imposed the 90-110% range of concordance; Fig. 7A; Table 3). Only one zircon is at 0 Ma (Fig. 6A). This discordia indicates a small regular Pb loss, Cambrian (534 ± 8 Ma, 1%). Most zircons are old, with 33% Meso- meaning that a more precise age can be obtained using only the most proterozoic, 26% Paleoproterozoic (18% middle and 8% upper) and concordant grains (99-100% concordance): the two most concordant 22% Archean. In addition, there are 9% Ediacarian and 9% Cryogenian grains give an age of 484.8 ± 6.3 Ma (probability of concordance = (18% Neoproterozoic). Two gaps are prominent as they are no Tonian 0.94). Adjoining a third one gives a concordia age of 483.0 ± 5.2 Ma and no Siderian zircons. (probability of concordance = 0.89) (Fig. 6A). Conservatively, we In sample St2 (upper part of the Bellevaux Formation, upper Stage

10 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

St1, n=92/120, 90 -110% conc., middle Hourt Fm., Stage 3 0.003 6 2650 Ma 2105 1573 Ma Ma A 5 604 Ma 520 Ma 558 Ma 1207 Ma 0.002 2083 Ma Shelf 4 Frequency

3 760 Ma Probability 1045 Ma 3067 Ma 0.001 1877 Ma 2

1 Tonian gapTonian gap gapTonian Tonian Siderian gap 0.000 0 400 500 600 700 800 900 1000 1 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3600 3500 100 Age 1 (Ma) 93389 0018 22% 480 635 800 1700 2250 2400 541 950 1950 3600 419

St2, n=89/120, 90-110 conc., upper Bellevaux Fm., upper Stage 4 0.004 8 2058 0.003 7

572 Ma B

2027 Ma 6 0.003 510 Ma

603 Ma Suboxic Frequency

647 Ma deep basin 5 0.002 2608 Ma

2651 Ma 4 0.002 523 Ma 2727 Ma 1665 Ma

Probability 3 0.001 1251 Ma 2

0.001 3163 Ma 1 Tonian gap Tonian Siderian gap 0.000 0 400 500 600 800 1000 1100 1300 1800 2100 2500 2700 3100 3300 3500 700 900 1200 1400 1500 1600 1700 1900 2000 2200 2300 2400 2600 2800 2900 3000 3200 3400 3600 Age 2 (Ma) 157 0 201 310 24 %

St3, n=93/120, 90-110 conc., middle Wanne Fm., upper Wuliuan 0.004 8 596 Ma 0.004 C 7

0.003 505 Ma 6 Deep 529 Ma Frequency 0.003 1788 Ma anoxic basin 5 620 Ma 554 Ma 0.002 1340 Ma 4 666 Ma 1737 Ma 2032 Ma 1989 Ma 1253 Ma 1472 Ma

Probability 0.002 3

0.001 2 Tonian gap Tonian 3581 Ma 2650 Ma

0.001 Siderian gap 1

0.000 0 2500 2700 3100 3300 400 500 600 700 800 900 1000 1 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2600 2800 2900 3000 3200 3400 3500 3600 100 Age (Ma) 6 224 0 3714 12 0 5 %

Fig. 7. Combined binned frequency and probability density distribution plots of U–Pb ages of detrital zircon grains from: A. Sample St1 (middle Hourt Formation, Cambrian Stage 3). B. Sample St2 (Upper Bellevaux Formation, Cambrian upper Stage 4). C. Sample St3 (middle Wanne Formation, Cambrian, upper Wuliuan Stage). The scale bar below the graphs represents the percentage of detrital zircon age within time intervals reported in Table 3.

4; Table 1 and Figs. 2, 3), 120 detrital zircon grains were analysed. Of Mesoproterozoic (20% vs 33%) and less upper Paleoproterozoic (1% vs these, 89 grains provided U-Pb ages that can be considered as con- 8%) and much more middle Paleoproterozoic (31% vs 18%). Most cordant (Fig. 7B; Table 3). The youngest concordant grain has a zircons are however old, with 20% Mesoproterozoic, 31% Paleopro- 521 ± 6 Ma 206Pb/238U age (2% Cambrian zircons). The detrital sig- terozoic and 24% Archean with the oldest zircon having a 207Pb/206Pb nature of St2 share similarities with that of St1 but with less age of 3163 ± 28 Ma. There are slightly more Neoproterozoic zircons

11 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

St4, n=91/120, 90–110% conc., middle La Venne Fm., Paibian 0.006 10 611 Ma 9 0.005 A 8 495 Ma 7

0.004 Frequen

628 Ma Deep anoxic basin 6 0.003 5 558 Ma c 673 Ma

4 y Probability 1207 Ma 0.002 2101 Ma 3 988 Ma 1464 Ma 2729 Ma 2428 Ma Siderian gap 1835 Ma 2 0.001 gap Tonian 1 0.000 0 2800 30 3100 33 340 3500 3600 1 21 24 25 500 600 800 1 1300 1400 15 22 2 260 2700 2900 3200 1600 1800 19 20 400 7 900 1000 1200 700 0 100 3 Age 0 00 00 00 00 00 0 00 0 00 0 0 0 0 0 0 (Ma) 1 823 0128 1116 0 3% St6, n=39/120, 90-110% conc. Solwaster Mbr.,lower Tremadocian 0.009 7 649 Ma 0.008 6 0.007 B 5 0.006 483 Ma 619 Ma Frequency

702 Ma Deep 0.005 anoxic basin 4 (evolving to shelf) 0.004 [uplift] 3 Probability 0.003 2116 Ma 2116 2 0.002 Satherian gap 495 Ma 1 0.001 Siderian gap Mesoproterozoic gap

0.000 0 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2700 2800 2900 3000 3200 3300 3400 3600 2600 3100 3500 Age (Ma) 2.5

44 323 2.5 2 18 0 5 %

St7, n=73/120, 90-110% conc., Lierneux Mbr., uppermost Tremadocian 0.008 10 657 Ma 0.007 9 8 0.006 C 596 Ma 7 479 Ma Frequency 0.005 Shelf 6 0.004 5 2151 Ma 766 Ma 4

Probability 0.003 793 Ma

2089 Ma 3

0.002 505 Ma 2 0.001 Siderian gap

Satherian gap 1 0.000 0 400 600 700 900 1000 1300 1400 1600 1700 1800 2000 2100 2200 2400 2500 2700 2900 3100 3200 3500 3600 500 800 1 1200 1500 1900 2300 2600 2800 3000 3300 3400 100 Age 3 (Ma) 38 319 8 7 18 0 4 %

Fig. 8. Combined binned frequency and probability density distribution plots of U–Pb ages of detrital zircon grains from: A. Sample St4 (middle La Venne Formation, Cambrian, Paibian Stage). B. Sample St6 (Solwaster Member, Jalhay Formation (Sm1a), Ordovician, lower Tremadocian). C. Sample St7 (Lierneux Member, Jalhay Formation (Sm1c), Ordovician, uppermost Tremadocian). The scale bar below the graphs represents the percentage of detrital zircon age within time intervals reported in Table 3. than in St1 (22% vs 18%) with 15% Ediacaran and 7% Cryogenian. The 508 ± 9 Ma 206Pb/238U age (6% Cambrian zircons). Compared to St1 two Tonian and Siderian gaps are also observed (0% zircon). and St2, St3 bears more Ediacaran zircons (22%) but less Cryogenian In sample St3 (middle part of the Wanne Formation, uppermost zircons (4%) and much less Archean zircons (5%), but with the oldest Wuliuan Stage; Table 1 and Fig. 2, 3), 120 detrital zircon grains were dated SVM zircon (207Pb/206Pb age of 3581 ± 7 Ma). Mesoproterozoic analysed. Of these, 93 grains provided U-Pb ages that can be considered (37%) and upper Paleoproterozoic (14%) become again abundant while as concordant (Fig. 7C; Table 3). The youngest concordant grain has a middle Paleoproterozoic zircons fall to 12%. The two Tonian and

12 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

St10, n=62/120, 90-110 conc. Waimes Fm. uppermost Pridoli 0.030 40 439 Ma Metaconglomerate above the Ardennian unconformity 35 0.025 A 420 Ma 30 0.020 Shelf 25 Frequency

0.015 20

Probability 15 0.010 10 0.005 5 1331 Ma 1992 Ma 0.000 0 3200 3300 400 500 600 700 800 900 1000 1 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3400 3500 3600 100 Age (Ma)

97 1.5 1.5 %

14 La Gleize Helle sill volcanism B 12 Rheic Ocean

10 opening Relativ

6 zircons

8 e prob 421 ±3 Ma (PF: 0.997) ability

Number 6

4

2

0 400 420 440 460 480 500 Ma Devonian Silurian Ordovician Cambrian 206 Pb/ 238 U age

Fig. 9. Combined binned frequency and probability density distribution plots of U–Pb ages of detrital zircon grains for Sample St10 (Waimes Formation, Silurian, uppermost Pridoli Series). A. All zircons; the scale bar below the graphs represent the percentage of detrital zircon age within time intervals reported in Table 3.B. Zoom on the 400-520 Ma age span comprising 97% of the detrital zircon ages. La Gleize Formation volcanic rock (upper Stage 10), Rheic Ocean opening (c. 480 Ma), corresponding to the base of the hiatus in the Brabant Massif (Fig. 3, 5) and Helle sill (446 ± 3 Ma; Fig. 6b) are added for comparison.

Siderian gaps are once more present (0% zircon). Paleoproterozoic (16% vs 12%). The Neoproterozoic zircon distribution In sample St4 (middle part of the La Venne Formation, upper is similar with 23% Ediacaran and 8% Cryogenian, while there is a new Paibian; Table 1 and Fig. 2, 3), 120 detrital zircon grains were analysed. outbreak of Archean zircons (13%). The Tonian and Siderian gaps are Of these, 91 grains provided U-Pb ages that can be considered as con- still well marked. cordant (Fig. 8A; Table 3). The youngest concordant grain has a In sample St6 (lower Solwaster Member of the Jalhay Formation, 530 ± 6 Ma 206Pb/238U age (1% Cambrian zircon). Compared to St3, lower Tremadocian; Table 1 and Figs. 2, 3), 120 detrital zircon grains St4 detrital zircon signature shows less Mesoproterozoic (28% vs 37%) were analysed. Of these, only 39 grains provided U-Pb ages that can be and less upper Paleoproterozoic (11% vs 14%) but more middle considered as concordant (Fig. 8B, Table 3). The youngest concordant

Table 3 Zircon proportions in the SVM sediments following ages.

Ord.-Sil. Cambrian Ediacarian Cryogenian Tonian Mesoprot. up. Paleoprot. mid. Paleoprot. Siderian Archean

Formation Age (Ma) 419-480 480-541 541-635 635-800 800-950 950-1700 1700-1950 1950-2250 2250-2400 > 2400 St1-Hourt 520 0 1 9 9 0 33 8 18 0 22 St2- Bellevaux 510 0 2 15 7 0 20 1 31 0 24 St3-Wanne 505 0 6 22 4 0 37 14 12 0 5 St4-La Venne 495 0 1 23 8 0 28 11 16 0 13 St6-Solwaster 483 0 2.5 23 44 3 2.5 2 18 0 5 St7-Lierneux 479 0 3 19 38 3 8 7 18 0 4 St10-Waimes 420 97 0 0 0 0 1.5 0 1.5 0 0

13 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

50 A. West African craton be considered as concordant (Fig. 8C, Table 3). The youngest con- cordant grain has a 502 ± 6 Ma 206Pb/238U age (3% Cambrian zircon). 45 3600-1950 Ma The St7 detrital zircon signature is close to that of St6: a large pro- 40 portion of Cryogenian zircons (38%) in addition to 19% Ediacaran 35 zircons. The Mesoproterozoic (8%), upper Paleoproterozoic (7%) and Bellevaux Archean (4%) remain low while the middle Paleoproterozoic zircons 30 stays high (18%). The Siderian gap persists and similarly as in St6, 3% % Zircon abundance 25 Hourt zircons somewhat fill the former Tonian gap. The large Mesoproter- Lierneux 20 Solwaster ozoic-Statherian gap of St6 is here narrowed to a Statherian gap only. La Venne

Wanne In sample St10, a metaconglomerate from the Ardennian un- 15 middle conformity (lower part of the Waimes formation, uppermost Pridoli; 10 Paleoproterozoic Table 1, Fig. 2, 3, § 3.1.4), 120 detrital zircon grains were analysed. Of 5 Archean these, 62 grains provided U-Pb ages that can be considered as con- cordant. The detrital zircon signature (Fig. 9A; Table 3) is here totally 0 ff 470 480 490 500 510 520 530 di erent from the previous samples with 97% of the zircons being < Sediment age (Ma) 475 Ma, with only two older zircons at 1331 ± 26 Ma and 1992 ± 28 50 Ma (concordance at 95% and 99%, respectively). The Paleozoic zircons 45 B. Western Amazonian craton are regularly distributed between 472 ± 10 Ma and 419 ± 8 Ma 1950-950 Ma (206Pb/238U age) covering most of the Ordovician and the Silurian 40 (Fig. 9B) with a maximum at c. 439 Ma (Llandovery). The six younger 35 zircons define an age of 421 ± 3 Ma (probability of fit = 0.997; 30 Fig. 9B) corresponding to a maximum age of deposition close to the % Zircon abundance uppermost Pridoli stratigraphical age of the St10 sample (Godefroid 25 opening Uplift prior to Rheic Ocean and Cravatte, 1999; Jansen, 2016), i.e. an age of c. 420 Ma. 20 Mesoproterozoic 15 upper 6. U-Pb zircon ages: interpretation Rheic Ocean opening Paleoproterozoic 10 6.1. U-Pb ages of magmatic rocks and comparison with the Brabant Massif 5 The La Gleize volcanic age (485 ± 6 Ma) corresponds to the climax 0 470 480 490 500 510 520 530 of the syn-sedimentary volcanic episode (490-485 Ma) that char- Sediment age (Ma) acterizes the Wanne-La Venne-La Gleize formations (505-485 Ma) in 50 C. Pan-African orogen s.l. the SVM (§ 4.1). This climax can be correlated with the earliest known 45 950-541 Ma volcanic activity in the Brabant Massif dated at 490-480 Ma marking 40 & Cambrian (Meseta?) 541-480 Ma the uplift that preceded the emersion there linked to the separation of Avalonia from Gondwana (Linnemann et al., 2012; Fig. 4). This context 35 of ocean opening at c. 480 Ma is in agreement with that of the following 30 Cambrian Ottré Formation volcanism (478-473 Ma; § 4.1) that occurred in the

% Zircon abundance Ediacaran SVM during the initial stages of the Rheic ocean opening (coticule 25 Cryogenian event; Herbosch et al., 2016). 20 Tonian The 446 ± 3 Ma age of the Helle intrusion corresponds to an upper 15 Katian to Hirnantian age (Upper Ordovician; Cohen et al., 2013, up- dated). This age corresponds to a period of tectonic instability of the 10 Avalonia microplate that is related to its soft docking with the Baltica 5 plate (c. 448 Ma; Cocks and Torsvik, 2005; Cocks and Fortey, 2009; 0 Linnemann et al., 2012). This period of instability is also recorded in 470 480 490 500 510 520 530 the Brabant Massif by a major magmatic episode that lasted between Sediment age (Ma) 460 and 430 Ma with a climax at 450-440 Ma (Linnemann et al., 2012). The Helle sill is contemporaneous with the Madot dacite (444 ± 6 Ma) Fig. 10. Evolution during the SVM sedimentation of the proportion of zircons ff following the three main regional sources (A: West African craton; B: and volcanic tu s (445 ± 2 Ma) but is older than the Quenast pluton Amazonian craton; C. Pan-African orogen) and their constituting different or- (430 ± 3 Ma) from the Brabant Massif (Linnemann et al., 2012). In ogenies based on age brackets. consequence, the Helle sill can be considered as a Caledonian intrusion, even if the sedimentary record in the SVM stopped during the Middle Ordovician (Fig. 3). grain has a 495 ± 8 Ma 206Pb/238U age (2.5% Cambrian zircon). The St6 detrital zircon signature marks a break in the detrital supply with a 6.2. U-Pb detrital zircon ages large arrival of Cryogenian zircons (44%) in addition to the constant 23% Ediacaran zircons. There is almost disappearance of Mesoproter- 6.2.1. Attribution of the zircon populations to regional sources ozoic (2.5%), upper Paleoproterozoic (2%) and Archean (5%) zircons It appears that a general statistical approach for determining the while the middle Paleoproterozoic maintain at 18%. The Siderian gap is sources of the Brabant Massif was not efficient. Indeed, such a study still there while 3% Tonian zircons partly fill the Tonian gap. By con- dedicated to peri-Gondwanan terranes (compilation of 58 601 detrital trast, with the exception of one single zircon, there is a large gap cov- zircon U–Pb ages from 770 Precambrian to Lower Paleozoic shelf se- ering the Mesoproterozoic and the Statherian. dimentary rocks; Stephan et al., 2019), showed that, although a suffi- In sample St7 (upper Lierneux Member of the Jalhay Formation, ciently large number of zircon grains were dated, the Brabant zircon uppermost Tremadocian; Table 1 and Figs. 2, 3), 120 detrital zircon fingerprint cannot be attributed to a particular province of Gondwana grains were analysed. Of these, 73 grains provided U-Pb ages that can in contrast with a more focused study (Linnemann et al., 2012). This

14 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 indicates that, as for Brabant, a dedicated approach must be adopted for the Ediacaran vs Cryogenian-Tonian zircons can be related to the determining the source variability of the SVM. contrasted terranes present in the Pan-African belts around the For the six older samples (St1 to St5 and St7), the previous de- WAC, i.e. the Trans-Saharan belt (Liégeois et al., 1998, 2003; Fezaa scription of the relative abundance of the detrital zircons (Figs. 7, 8) et al., 2010), the Anti-Atlas belt (Ennih and Liégeois, 2008; shows that some age groups behave in a correlative manner all over the Toummite et al., 2013; Belkacim et al., 2017), the Souttouf massif time period or for a part of it. We consider that when two age groups (Gärtner et al., 2016), the Bassarides and the Rockelides show a similar evolution trough time, this means that they are provided (Villeneuve et al., 2010, 2015). In these terranes, the juvenile from a unique composite source (Fig. 10). Cryogenian and in a much lesser amount Tonian island arc terranes have been generated before the ocean closures (> 630 Ma), while (1) The Archean and the middle Paleoproterozoic zircon populations the 630-545 Ma period corresponds to collision and post-collision behave similarly except after 490 Ma, when the uplift prior to the events with large amounts of high-K calc-alkaline magmatism Rheic Ocean opening occurred (Fig. 10A). This uplift is marked by (Liégeois et al., 1998). Most of the preserved Cryogenian-Tonian the regression of the Jalhay Formation (Fig. 4; Lamens, 1985b). terranes are thrust on rigid cratonic or metacratonic boundaries This suggests a distant source made of Archean and lower Paleo- (Liégeois et al., 2003; Ennih and Liégeois, 2008; Brahimi et al., proterozoic (3.6 – 1.95 Ga) lithologies, corresponding to the West 2018; Liégeois, 2019). This can induce different behaviours when African craton (WAC). Indeed, the latter is made of two Archean an external stress is applied. This suggests that the pre-Rheic uplift nucleus (Man and Amsaga; e.g. Potrel et al., 1998; Kouamelan et al., brought to the surface and then to erosion Cryogenian and Tonian 2015; Rollinson, 2016) that collided at 2.1 Ga with the lower Pa- terranes. The relatively small areas occupied by these terranes in leoproterozoic Birimian juvenile domain (Wane et al., 2018) that the Pan-African belts around the WAC and the high amount of has a maximum age of 2.25 Ga (Tshibubudze et al., 2013). Post- Cryogenian zircon arrival (Fig. 10C) can only be reconciled if these collisional magmatism occurred until 2.02 Ga (Parra-Avila et al., terranes were closed to the SVM basin. We thus consider that the 2017; Wane et al., 2018) with late events until 1.95 Ga (Liégeois Cryogenian-Tonian and by consequence Ediacaran zircon popula- et al., 1991). By contrast, the WAC comprises very few Mesopro- tions were delivered by close Pan-African belts, i.e. those to the terozoic rocks (Ennih and Liégeois, 2008) not prone to deliver zir- west of the WAC, from the Rockelides to the Anti-Atlas, and not cons such as rare dolerite dykes (El Bahat et al., 2013). We thus from the Trans-Saharan Belt, located much further east and more- consider that these 3.6-1.95 Ga zircon populations were delivered over covered by the Tassilis sandstones from the late Cambrian by the WAC. (Beuf et al., 1971). We group hereafter Tonian, Cryogenian and (2) In the same way, the upper Paleoproterozoic and Mesoproterozoic Ediacaran zircons as the Pan-African orogen. Indeed, even if het- zircon populations behaves similarly (Fig. 10B) but in a reverse way erogeneous, it can be considered as one geographical source. when compared to the evolution of the Archean / middle Paleo- (4) The Cambrian zircons are very rare (a few %; Fig. 10C) with a small proterozoic population. There is only a greater loss of Mesopro- peak at 505 Ma, confirming that something occurred at this time terozoic zircons than that of upper Paleoproterozoic zircons during (Fig. 10A, B, C). As there are big changes in distant West African the uplift prior to the Rheic ocean opening. This indicates another and Western Amazonian craton sources, this indicates that these large distant composite source with ages between 1.95 and 0.95 Ga. scarce Cambrian zircons could come from a remote region. An at- This corresponds to the western portion of the Amazonian craton to tractive source would be the Cambrian volcanic activity recently the west of the Purus Arch, with the Rio Negro (1.86-1.52 Ga), dated at 507 ± 5 Ma in the Moroccan Meseta (El Attari et al., Rondonia-Juruena (1.81-1.52 Ga) and Sunsas (1.45-0.99 Ga) pro- 2019). However, additional constraints are needed to sustain that vinces (e.g. Bahlburg et al., 2009; Ibanez-Mejia et al., 2011; hypothesis that will remain hereafter with a question mark. Albuquerque et al., 2017). This is also the case for smaller cratons in its vicinity such as the Rio de la Plata craton (Gaucher et al., 6.2.2. The particular case of the Pridoli metaconglomerate (sample St10) 2011) or the San Luca range (Cuadros et al., 2014). The eastern part Nearly all the detrital zircons from this sample (60 on 62 concordant of the Amazonia craton, to the east of the Purus Arch, has ages spots) crystallized between 472 and 419 Ma, a time interval which between 3.1 and 1.7 Ga (Transamazonas, Carajas provinces, Ama- corresponds to the SVM hiatus (Fig. 3, 9). The age of the youngest zonia central, Tapajos-Parima provinces; Albuquerque et al., 2017). zircon (419 ± 8 Ma) corresponds to the stratigraphic age of the meta- Considering the opposite behaviour of the 3.6-1.95 Ga and 1.95- conglomerate itself (Pridoli-Lochkovian boundary at 419 ± 3 Ma; 0.95 Ga zircon populations in the SVM (Fig. 10A and 10B), we Cohen et al., 2013, updated). The two older zircons are concordant and consider that the western Amazonian craton west of the Purus Arch give ages at 1331 ± 26 Ma and 1992 ± 28 Ma but their very small represents a suitable second source for the SVM. Avalonia, which number does not allow any conclusions to be drawn. As a whole, this comprises both the SVM and the Brabant Massif, was located to the Ordovician-Silurian signature indicates that the source of this sediment NE of Amazonia in Cambrian times (Cocks and Torsvik, 2002; Cocks cannot correspond to the SVM or to the Brabant Massif or, a fortiori, to and Fortey, 2009). This implies that, at this time, the Amazonia any of their sources, close or remote, which did not provide nearly any craton should have its eastern basement portion buried and its zircon. The almost Gaussian age repartition of the St10 zircons suggests western basement portion at the erosion. This appears to be in that they originated from one single long-lived magmatic source. agreement with the fact that the deep Paleozoic basins of South Constraints for its identification will be given in the next chapter and America are located to the east (Eastern Amazon, Parnaíba, and discussed in the geodynamical interpretation chapter. Parana cratonic basins; Daly et al., 2014). We thus consider that these 1.95-0.95 Ga zircon populations were derived from the wes- 7. Geochemistry and Nd isotopes tern Amazonian craton. (3) The different Neoproterozoic zircon populations do not behave in 7.1. Geochemistry the same way: the Ediacaran zircons, after an initial increase from 520 to 505 Ma, remain relatively constant afterwards (Fig. 10C). By 7.1.1. Origin of data contrast, the Cryogenian zircon abundances, constantly low from The aim of this chapter is to characterize geochemically the SVM 520 to 495 Ma, strongly increase during the pre-Rheic uplift (45% magmatic rocks and to compare them with those of the Rocroi Massif of the Neoproterozoic detrital zircons). Tonian zircons, absent be- (also belonging to the Ardenne allochthon; Fig. 1) and of the Brabant fore 490 Ma, appear during the uplift and can be considered as Massif based on published analyses for setting up a global geodyna- correlated to the Cryogenian zircons. The different behaviours of mical interpretation. Available geochemical analyses are dispersed in a

15 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 series of papers sometimes quite old and have never been interpreted agreement with Cobert et al. (2018) conclusion for the Rocroi Mairupt together. Several of the analyzed rocks are no longer accessible (former felsic dykes. In the tholeiitic rocks, the alteration is marked by a great water pipes, backfilled quarry…) and cannot be resampled. This implies variability of the peralkaline index values, the peraluminous index that the number of elements analyzed in each sample set is variable. being buffered by much higher values of less mobile CaO. In consequence, we have compiled the geochemical composition of The observations made in Fig. 11A and 11B imply that diagrams the SVM magmatic rocks from André (1983; 1 Challes sill, 3 Grand- devoted to deciphering the magmatic trend of the magmatic rocks Halleux sill), Daneels and Vogel (1978; 1 Helle sill, 1 Lammersdorf sill, based on alkalic elements are not appropriate here. This is shown in 3 Spa dykes, 5 conformable volcanic rocks), Herbosch et al. (2016; 1 Fig. 11C (MALI diagram; Frost and Frost, 2008) where the SVM felsic Helle sill, 2 conformable volcanic rocks), Lamens (1985a; 3 conform- magmatic rocks form a trend towards the composition of the Pridoli able volcanic rocks), Ronchesne (1934a; 1 Challes sill), Schreyer and conglomerate while the tholeiitic rocks determine a trend perpendi- Abraham (1978; 1 Helle sill, 1 Challes sill) and Van Wanbeke (1955; 1 cular to the MALI magmatic series. Only the Rocroi Mairupt dykes Helle sill). The Rocroi Massif analysis are from André (1983; 1 Grande follow a magmatic trend (calc-alkaline). The exact nature of the SVM Commune dyke), Beugnies (1964, 1 Grande Commune dyke), Cobert magmatic rocks can only be approached by using immobile elements et al. (2018; 6 Mairupt dykes), Goffette et al. (1990; 2 Grande such as those used in Fig. 11D (Th/Yb vs Ta/Yb; Pearce, 1982a). In this Commune and 2 Laifour dykes). The Brabant Massif (including the diagram, all the rocks are grouped in the high-K calc-alkaline ("shosh- HSM) analyses are from André (1983; 4 Hozémont sill), Linnemann onitic") field and in the calc-alkaline field close to the latter. The Les et al. (2012; 5 conformable volcanic rocks, 1 Bierghe sill, 4 Quenast Plattes volcanic rocks (Ottré Fm, Sm2b) cluster with the Brabant pluton), Ronchesne (1934b, 1 Mozet sill; 1934c, 1 Hozémont sill, magmatic rocks on the boundary zone between the two above fields. 1934d, 3 basalt from Voroux-Goreux volcanic complex). The La Gleize (Rv5) and Meuville (Ottré Fm, Sm2a) volcanic rocks, For the SVM sedimentary rocks, we used, mostly as reference, the together with the Helle sill are by contrast well inside the "shoshonitic" geochemical rock composition from and compiled by Herbosch et al. field. The Rocroi felsic dykes also follow the boundary zone with two (2016; 115 sediments from Cambrian to Ordovician but mostly from the more Ta enriched samples close to the Pridoli conglomerate. The ele- Floian Ottré Formation and 62 Floian coticules) and for the Brabant ments considered here are all incompatible, meaning that their ratios Massif the geochemical rock composition from Linnemann et al. (2012; are only slightly modified during the magmatic differentiation, and in 16 sediments from Cambrian to Silurian). For the Pridoli metaconglo- turn, are close to the magmatic rock source (e.g. Hofmann and Jochum, merate, in addition to four analyses from Van den Bleeken and Corteel 1996). These incompatible element ratios point to a rather enriched (2007), one new analysis is provided here, including Nd isotopes potassic sources for the SVM magmatic rocks, similar to the Brabant (Table 2). All the above geochemical data are given in Table S2 (data magmatic rocks (Linnemann et al., 2012). Let us remark that in all these repository). diagrams (Fig. 11A-D), the tholeiitic rocks from the SVM, the Rocroi Massif and the south-eastern Brabant Massif, have very similar com- 7.1.2. Alteration and nature of the rocks positions, in agreement with a common origin. The SVM magmatic rocks are known to be highly altered (e.g. The Les Plattes volcanic rocks (Sm2b) present a strong enrichment

Daneels and Vogel, 1978). This can be seen in Fig. 11A that presents in MgO (7.3-8.2%) for an intermediate andesitic composition (SiO2: two ratios with a potentially mobile element (Na2O and K2O) and a 58.8 – 61.2%), actually richer in MgO than all the basic rocks in the usually immobile element (Al2O3). Low Na2O/Al2O3 are non-magmatic SVM, Brabant and Rocroi massifs (Fig. 11E). Such a high magnesian/ and typically sedimentary (Gallet et al., 1998). In the latter field, a silica composition implies that the source of Les Plattes rocks must be of typical shale trend, to which the NASC (North American Shale Com- andesitic composition as demonstrated by the experimental liquid lines posite) belongs, extends from the illite composition towards a mean of of descents determined for basaltic and primitive andesitic melts (Wang Phanerozoic juvenile crust (Condie, 1993), the upper continental crust et al., 2018; Fig. 11E). Indeed, melts deriving from a basaltic source,

(UCC) being slightly more potassic (Fig. 11A). All SVM volcanic rocks even high-Mg (i.e. with Mg# > 50 at 55% SiO2; He et al., 2015) such as (Wanne-La Venne, La Gleize, Ottré) lie within a low-K part of the se- experimental melts of Wang et al. (2018), are always lower in MgO at c. dimentary field, being superposed on the SVM slates and coticule. This 60% SiO2. The Meuville volcanic rock (Sm2a) and Helle sill composi- agrees with their volcano-sedimentary nature, an environment very tions are compatible with a similar primitive andesitic source, MgO favourable for an underwater intense alteration process. The Pridoli being strongly decreasing from 58 to 66% SiO2 but leaving however conglomerate, also with very low Na2O/Al2O3 ratios, lies in a higher-K quite magnesian rocks at 65-70 % SiO2 (Fig. 11E). part of the sedimentary field, closer to the shale trend. The SVM sills The relatively high TiO2 content of Les Plattes volcanic rocks (Sm2b; and dykes are grouped close to the magmatic/sedimentary boundary, 1.01-1.08% TiO2 at c. 60% SiO2) is in line with the TiO2 increase in the on the magmatic side. This means that they are less altered than the 58-65 % SiO2 range shown by the experimental LLD of primitive an- volcano-sedimentary rocks and have compositions closer to their desitic melts (Fig. 11F; Wang et al., 2018). Volcanic rocks deriving from magmatic composition. The Brabant magmatic rocks show a more a basaltic melt such as the Karoo series have strongly decreasing TiO2 in variable alteration from rather pristine to strongly altered. The Rocroi this silica range (Fig. 11F). The Meuville volcanic rock and Helle sill felsic dykes display variable composition but always in the magmatic have adequate TiO2 contents for being derived from primitive andesitic field. The tholeiitic rocks have generally low Na2O/Al2O3 ratios, espe- melts (Fig. 11F). This indicates a crustal source. cially those from SVM, indicating high alteration, in agreement with their petrography (§ 4.2). 7.1.3. Magmatic characterization and equivalents elsewhere The peralkalinity (NK/A, molar) vs peraluminosity (A/CNK, molar) Rare earth elements (REE) of the SVM intermediate magmatic rocks diagram (Fig. 11B) confirms the high alteration of the SVM (and Bra- (59-61% SiO2), represented by the Les Plattes volcanic rocks (Ottré Fm, bant) volcanic rocks that results in very high peraluminosity values Sm2b), display enriched LREE (LaN:56–73) and HREE (YbN:7–12) with similar to the SVM slates. This is due to a high loss in alkalis. The Pridoli a moderate LREE/HREE fractionation (LaN/LuN: 4.5–8.8) and Eu ne- conglomerate lies on the less peraluminous side of the SVM sediments. gative anomalies (Eu/Eu*: 0.54) (Fig. 12A). These are typical values for The SVM sills and dykes, the Rocroi felsic dykes and the Brabant plu- high-K calc-alkaline granitoids (Liégeois et al., 1998). The larger var- tonic rocks form a trend in the peraluminous field with peralkaline iation in HREE than in LREE suggests the presence of garnet in the index/peraluminous index from 0.9/1.05 to 0.5/1.8. This trend, in line crustal source. The Devonian Altai high-Mg dacites display similar REE with that of the sedimentary and volcano-sedimentary rocks, suggests patterns with some samples even more enriched in HREE (He et al., that the plutonic rocks are also altered and have lost alkali elements, 2015). even if to a lesser extent than the volcano-sedimentary rocks, in The more felsic SVM volcanic rocks (La Gleize Fm and Meuville

16 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

1.2 A Juvenile crust from Peralkalinity Na2O/Al2O3 R Stavelot-Venn Massif Rocroi Massif 0.50 Archean (A) to Recent (R) A Index B Helle-Lammersdorf sills Mairupt felsic dykes 1 Wanne-La Venne volc. Tholeiite La Gleize volcanics 0.40 Ottré volcanics Brabant Massif 0.8 Tholeiite Intrusive Magmatic field Spa dyke Volcanics 0.30 Slate Tholeiite 0.6 Coticule Sediment UCC Pridoli conglomerate 0.20 Non-magmatic field 0.4 (sedimentary) 0.10 NASC 0.2 K 2O/Al2O3 A/CNK 0.00 Illite 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 01234567 12 10 Oceanic Active continental margin MALI (%K2O+%Na2O-%CaO) Th/Yb arc + post-collisional- La Gleize 10 C Helle Meuville D SHO 8 CA Les Plattes E-MORB, 6 OIB 1 4 A 2 CA A-C Helle sill 0 TH La Gleize volcanics Stavelot-Venn Massif Rocroi Massif SVM Ottré volcanics -2 C-A Helle-Lammersdorf sills Mairupt felsic dykes Wanne- La Venne volc. Tholeiite 0.1 Primordial mantle Pridoli conglomerate -4 Ottré volcanics Mairupt felsic dykes (Rocroi) C Brabant Massif Tholeiite Intrusives (Brabant) -6 Intrusive Spa dyke Volcanics (Brabant) -8 La Gleize volcanics Volcanics N-MORB Pridoli conglomerate Tholeiite %SiO2 Ta/Yb -10 0.01 45 55 65 75 85 95 0.01 0.1 1 10 80 1.7

Mg# SVM slate %TiO2 Experimental LLD SVM coticule E of basaltic melts F 70 Les Plattes Brabant sediment

60 Meuville Experimental 1.2 LLD of primitive 50 andesitic melts Les 40 Plattes

30 Meuville 0.7 high- Altaï 20 Mg dacites Experimental LLD of primitive andesitic melts Wudaoliang 10 Experimental LLD of basaltic melts high-Mg diorites Helle Karoo %SiO %SiO2 2 0 0.2 45 55 65 75 85 95 45 5055 60 65 70

Fig. 11. Major and trace element bivariate diagrams for the SVM rocks together with rocks from the Rocroi and Brabant massifs (André, 1983; Cobert et al., 2018; Corin, 1965; Daneels and Vogel, 1978; Goffette et al., 1990; Herbosch et al., 2016; Lamens, 1985a; Linnemann et al., 2012; Ronchesne, 1934a, 1934b, 1934c;

Schreyer and Abraham, 1978, Van den Bleeken and Corteel, 2007; Van Wambeke, 1955; Table S2). A. K2O/Al2O3 vs Na2O/Al2O3 allowing distinguishing magmatic from non-magmatic fields (sedimentary) with the position of the upper continental crust (UCC), illite and shale trend (Gallet et al., 1998). The juvenile crust evolution is from Condie (1993) and the NASC (North America Shale Composite) is from Gromet et al. (1984); see B for symbol caption. B. A/CNK (Al2O3/CaO

+Na2O+K2O, molar values) vs Peralkalinity index ((Na2O+K2O)/ Al2O3, molar values). C. SiO2 vs MALI (K2O+Na2O-CaO) (Frost and Frost, 2008). D. Ta/Yb vs Th/

Yb (Pearce, 1982b). E. SiO2 vs Mg# (MgO/(MgO+Fe2O3t), molar value. Experimental liquid lines of descent (LLD) from Wang et al. (2018) See C for symbol caption. F SiO2 vs TiO2 LLDs from Wang et al. (2018). Altai high-Mg dacites from He et al. (2015), Wudaoliang from Wang et al. (2018) and Karoo dolerites from Jourdan et al. (2007).

Mbr/Ottré Fm, Sm2a) and the Helle sill (66-77% SiO2), already distinct crustal source and a lower degree of partial melting for these more felsic and clustered in Fig. 11D, display similar REE patterns. They are rocks. characterized by more enriched LREE (LaN: 120-132) and lower HREE Only one REE pattern is available for the SVM tholeiitic rocks (YbN:5–8) implying a higher LREE/HREE fractionation (LaN/LuN: 15- (Grand-Halleux dyke). It is nearly identical to that of the Hozémont sill 24). They display also less negative Eu anomalies (Eu/Eu*: 0.82-0.86) (Voroux-Goreux complex) located on the south-eastern margin of the (Fig. 12B). This reveals a slightly higher amount of garnet left in the Brabant Massif (Fig. 12C). They are rather flat and moderately enriched

17 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

1000 1000 A Ottré 2 B La Gleize St5 Ottré 3 Ottré St8 Ottré 1 (Meuville Mbr) - (59 61 % SiO2 ) Helle St11 100 (Les Plattes Mbr) Devonian Altaï 100 high-Mg dacites (66-77 % SiO 2)

10 10 Normalized to chondrite Normalized to chondrite 1 1

1000 1000 Grand-Halleux (SVM) Mairupt (Rocroi) C D (67-78% SiO2 ) Hozémont (S. Brabant) St10-Pridoli conglomerate (SVM) (47-51% SiO2 ) South Taunus metarhyolites 100 100 Rhyolites in mafic LIPs

10 10

Karoo low-Ti Giessen-South Harz Rhyolites in SLIPs Normalized to chondrite Normalized to chondrite 1 1

1000 16 Ce /Yb E Brabant Massif F N N Helle 14 Stavelot-Venn Massif (57-67 % SiO 2) Helle sill Ottré volcanics SVM felsic Grand-Haleux tholeiite 12 Meuville La Gleize volcanics 100 Pridoli conglomerate La Gleize Rocroi Massif 10 Mairupt felsic dykes Brabant Massif Intrusive 8 Volcanics Hozémont Tholeiite SVM 6 10 interm. Brabant 4 interm. SVM-BM Pridoli mafic South conglo. 2 sic Taunus Rocroi fel Ce ppm Normalized to chondrite 1 0 020406080100

Fig. 12. Rare Earth element (REE) diagrams normalized to chondrite (Taylor and Mc Lennan, 1985) for: A. SVM Les Plattes Member volcanic rocks (Sm2b; Ottré Formation; Lamens and Geukens, 1984; Herbosch et al., 2016), Devonian Altai high-Mg dacites from He et al. (2015). B. SVM La Gleize Formation (Rv5) and Meuville Member (Sm2a; Ottré Formation; Lamens and Geukens, 1984; Herbosch et al., 2016) volcanic rocks and Helle sill (Herbosch et al., 2016). The grey area corresponds to the Les Plattes volcanic rocks. C. SVM (Grand-Halleux) and southern Brabant (Hozémont) tholeiitic basalts (André, 1983); the Karoo low-Ti basalts are from Jourdan et al. (2007). D. SVM Pridoli conglomerate (this study; Table 4) and Rocroi Mairupt felsic dykes (Cobert et al., 2018). The rhyolites from mafic LIPs and from SLIPs are from Pankhurst et al. (2011). E. Brabant Massif intermediate volcanic rocks (Linnemann et al., 2012) compared with the Les Plattes volcanic rocks in grey.

F. Ce ppm vs CeN/YbN (normalized to chondrite) for the Belgian rocks present in Fig. 12 AtoE.

18 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

in LREE (LaN:33–34), rather enriched in HREE (YbN:15–18) with a low Karoo low-Ti tholeiites (Fig. 12C), suggesting a LIP linkage rather than LREE/HREE fractionation (LaN/LuN: 1.9–2.1) and no Eu anomalies (Eu/ a T-MORB signature (Floyd, 1995). They are neither similar to island Eu* ~1). The Lower Devonian (Emsian) tholeiitic basaltic lavas from arc basalts even if taking enriched calc-alkaline basalts as a reference the Giessen-South Harz region (eastern Rheno-Hercynian region; Floyd, (Mariana arc, Pearce, 1982a, 1982b; Fig. 12C). Indeed, in island arc 1995) display very similar REE patterns (Fig. 12C). As a whole, if these setting, LILE are markedly enriched over HFSE and most often Nb-Ta patterns are MORB-like (Floyd, 1995), they are also close to the Karoo display negative anomalies due to subduction fluid effects (e.g. Pearce, mafic low-Ti sills and flows (Fig. 12C; mean of 8 rocks; Jourdan et al., 1982a and references therein). The Pridoli conglomerate, again except 2007). Their slightly higher REE abundance can be linked to their for mobile LILE, shows a remarkable similarity with the Pridoli-Loch- slightly higher TiO2 contents (1.3-1.9 % TiO2 vs 0.9-1.7 % TiO2), the kovian Mairupt felsic rocks, especially with sample Ms2, nearly a du- high-Ti Karoo mafic flows and sills being enriched in REE (Jourdan plicate except for P, which can be attributed to more apatite fractio- et al., 2007). These patterns are strongly different from the Mg-rich Les nation as also shown by some classical LIP provinces (e.g. Taupo) or A- Plattes (Sm2a) andesitic rocks. type granites and not to volcanic arc granite such as those present in The REE pattern of the SVM Pridoli metaconglomerate is very si- Jamaica (Pearce (1982a, 1982b); Fig. 13D). Compared to the Les Plattes milar to that of the Mairupt felsic dykes from Rocroi (Fig. 12D; LaN: rocks, the Mairupt felsic dykes show enrichment in Th, Ta and Y-Yb. 35.5 vs 18-85, YbN: 14.1 vs 15-24, LaN/LuN: 2.5 vs 1.2-4.1, Eu/Eu*: The other HFS (high-field strength) elements confirm that the Brabant 0.37 vs 0.3-0.6, respectively). This suggests that the incompatible ele- Massif intermediate magmatic rocks have close patterns to that of the ments of this conglomerate can be considered as a proxy for its mag- SVM intermediate rocks although slightly more enriched (Fig. 13E). matic source. The Middle Ordovician/Lowermost Devonian South Among global similarities, the more striking differences, if excepting Taunus metarhyolites (465-410 Ma when including errors on zircon U- mobile elements (LILE) and elements often removed by fractionation Pb ages; Sommermann et al., 1992) have close patterns although more (Ti, P), are the variations observed for Y and Yb. This can be sum- enriched and slightly more fractionated (Meisl, 1995). Such patterns marized in a diagram Y vs Zr, the former representing the HREE and the can be found in the felsic magmatic rocks present in large igneous latter the other HFSE (Fig. 13F). In this diagram, the different magmatic province, although not only (see spidergrams below for more con- rocks are indeed well discriminated. The SVM intermediate and felsic straints). In these provinces, the felsic rocks can be subordinate (Mafic rocks have distinctly lower Y values for Zr values similar to the other LIPs, Parana, Etendeka …) or predominant (Silicic LIPs or SLIPs; Chon reported rocks. On the other hand, the Pridoli conglomerate and the Aike,) even if the latter provinces are less abundant. The REE patterns Rocroi felsic rocks determine a Zr-Y positive trend to which belong the of the two groups share major similarities but the SLIP felsic rocks have most Y enriched samples among the studied rocks. The three mafic lower REE abundances and more negative Eu anomalies (Pankhurst groups have similar Y values and increasing Zr values from SVM, Bra- et al., 2011; Fig. 12D). The Pridoli conglomerate displays a REE pattern bant to Rocroi massifs. The SVM mafic rocks have values close to the close to that of the SLIP rhyolites (low rather flat REE and large Eu Pridoli conglomerate. Finally, the Brabant intermediate rocks have anomaly), although with slightly higher HREE contents. The Mairupt values close to the latter mafic rocks, distinct from the SVM inter- felsic rocks have REE patterns similar to LIP felsic rocks, with the less mediate and felsic rocks as far as Y is concerned. REE enriched samples being similar to the Pridoli conglomerate and the SLIP rhyolites and the more enriched samples being similar to the mafic 7.2. Nd isotopes of the magmatic and sedimentary rocks LIP rhyolites (Fig. 12D). The Brabant intermediate (57-67 % SiO2) magmatic rocks have REE We compiled here the available Nd isotopic data on the Belgian patterns close to that of the Les Plattes (Sm2b) volcanic rocks, being lower Paleozoic magmatic and sedimentary rocks. Data on the SVM only slightly more enriched (Fig. 12E). LREE abundance (Ce ppm) and slates, coticule and magmatic rocks are from Herbosch et al. (2016),

REE fractionation (CeN/YbN) permits to summarize the differences be- that on the tholeiites from SVM, south-eastern Brabant and Rocroi tween the Belgian lower Paleozoic magmatic rocks (Fig. 12F). The SVM massifs from André et al. (1986), on the Rocroi Mairupt felsic dykes felsic rocks (Helle, Meuville, La Gleize) are distinctly enriched and from Cobert et al. (2018), on the Brabant Massif magmatic and sedi- fractionated. The SVM intermediates rocks (Les Plattes, Sm2b) are mentary rocks from Linnemann et al. (2012). Data on the SVM Pridoli much less enriched and less fractionated. Their fractionation values are conglomerate is from this study (Table 2). Data from the north-eastern similar to the Brabant intermediate rocks although the latter are gen- part of the Rheno-Hercynian belt (Ordovician slates from the Ebbe erally more enriched in LREE. The Rocroi felsic rocks determine a po- Anticline, Germany) are from Samuelsson et al. (2002). sitive trend, i.e. the more the rock is fractionated, the more it is en- In Fig. 14A (age of magmatic emplacement or sedimentary deposi- riched in Ce. The South Taunus lavas are located at the enriched tip of tion vs ɛNd), four different groups can be distinguished. The tholeiites this trend while the Pridoli conglomerate and the SVM and south- from the three Belgian regions (SVM, BM, RM) show similar signatures eastern Brabant tholeiites are located towards the less enriched tip. marked by strongly positive ɛNd (+3.1 to +6.3), indicating a mainly Spidergrams normalized to MORB (Fig. 13) allow us to test the juvenile mantle origin. The SVM slates and coticules together with the conclusions extracted from the REE. The SVM intermediate rocks (Les Ebbe anticline slates display the more negative ɛNd (-8.1 to -10.2) in- Plattes, Sm2b) are again very similar to the Altai high-Mg dacites when dicating an important old crustal component in their source region. considering additional incompatible elements (Fig. 13A). The more They are slightly more negative than the Brabant Massif sediments felsic SVM rocks (La Gleize, Meuville, Helle) display very comparable having ɛNd from -5.5 to -7.7, with two additional samples at -9.0 to -9.3 spidergrams (Fig. 13B), generally more enriched than the intermediate lying inside the SVM field. All the intermediate and felsic magmatic rocks, in agreement with the incompatibility character of these ele- rocks have ɛNd between the tholeiites and the sediments (from +0.6 ments. They are only depleted in P and Ti, which can be attributed to and -6.5). The Mairupt felsic dykes (Rocroi Massif) are on the more the fractionation of usual minerals such as apatite and titanite. The negative side with a range in ɛNd from -4.0 to -6.5, in which the sub- tholeiitic rocks from SVM (Grand-Halleux), Brabant Massif (Hozémont) contemporaneous SVM Pridoli conglomerate falls (ɛNd: -5.4). The SVM and Rocroi Massif (Grande Commune) share the same pattern magmatic rocks have a rather homogeneous slightly negative Nd sig-

(Fig. 12C) except for the LILE (large ion lithophile elements, i.e. Sr, K, nature (ɛNd: -1.7 to -2.4), despite their age range varying from 487 to Rb, Ba), which can be attributed to the metamorphic alteration. Even if 446 Ma. incomplete, the Grande Commune pattern is so close to that of Grand- The Nd TDM model ages, opposed to ɛNd (Fig. 14B), allow to un- Halleux that they can be associated. These patterns are, except for the derstand the different isotopic signatures without considering the em- mobile LILE, very close to the Emsian Giessen-South Harz MORB-like placement or deposition time and to determine the mean age of the tholeiitic basalts (Floyd, 1995). Together they are comparable to the sources at the origin of the studied magmas and sediments. TDM model

19 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

1000 1000 Normalized Normalized La Gleize St5 to MORB Ottré 2 to MORB Ottré 1 (Meuville Mbr) Ottré 3 100 100 Helle St11 Ottré St8 (66-77 % SiO )

(Les Plattes Mbr) 2 (59-61 % SiO ) 10 2 10

1 1

ABAltaï Devonian high-Mg dacites 0.1 0.1 Sr K Rb Ba Th Nb Ta Ce Nd P Hf Zr Sm Ti Y Yb Sr K Rb Ba Th Nb Ta Ce Nd P Hf Zr Sm Ti Y Yb

1000 1000 Mairupt (Rocroi; 67-78% SiO2) Normalized Normalized St10 Pridoli conglo. (SVM) to MORB Grand-Halleux (SVM) to MORB Karoo Hozémont (S. Brabant) Etendeka–Parana (Low-Ti) 100 100 Taupo Volcanic zone (47-51% SiO2 ) Grande Commune Typical A-type granite

10 10

Ms2 1 1 Mariana Jamaica calc-alkaline volcanic arc island arc Karoo low-Ti granite C basalt Giessen-South Harz D 0.1 0.1 Sr K Rb Ba Th Nb Ta Ce Nd P Hf Zr Sm Ti Y Yb Sr K Rb Ba Th Nb Ta Ce Nd P Hf Zr Sm Ti Y Yb 1000 60 Normalized Y ppm to MORB Brabant Massif 50 (57-67 % SiO 2) 100 40 Brabant interm.

SVM mafic 10 30 Rocroi felsic + Pridoli conglo.

Rocroi mafic 20 Karoo Brabant mafic 1 10 SVM felsic EFSVM interm. Zr ppm 0.1 0 Sr K Rb Ba Th Nb Ta Ce Nd P Hf Zr Sm Ti Y Yb 50 100 150 200 250 300

Fig. 13. Spidergrams normalized to MORB (Sun, 1980; Pearce, 1982b). A. SVM Les Plattes Member volcanic rocks (Sm2b; Ottré Formation; Lamens and Geukens, 1984; Lamens, 1985a; Herbosch et al., 2016), Devonian Altai high-Mg dacites from He et al. (2015). B. SVM La Gleize Formation (Rv5) and Meuville Member (Sm2a; Ottré Formation; Lamens and Geukens, 1984; Lamens, 1985a; Herbosch et al., 2016) volcanic rocks and Helle sill (Herbosch et al., 2016). The grey area corresponds to the Les Plattes volcanic rocks. C. SVM (Grand-Halleux) and southern Brabant (Hozémont) tholeiitic basalts (André, 1983); the Karoo low-Ti basalts are from Jourdan et al. (2007); Mariana arc from Pearce (1982a, 1982b). D. SVM Pridoli conglomerate (this study; Table 4) and Rocroi Mairupt felsic dykes (MS2; Cobert et al., 2018); the Karoo, Etendeka-Parana, Taupo and A-type granite are from Pankhurst et al. (2011), Jamaica volcanic arc granite from Pearce (1982a, 1982b).E. Brabant Massif intermediate volcanic rocks (Linnemann et al., 2012) compared with the Les Plattes volcanic rocks in grey. F. Zr ppm vs Y ppm for the Belgian rocks present in Fig. 13 A to E. Symbols as in Fig. 12F.

147 144 ages have been calculated for Sm/ Nd < 0.15 for getting con- considered with care but the fact that their TDM model ages are rather strained model ages (Liégeois and Stern, 2010) except for tholeiites, all close and coherent suggest that we can however rely on them. That of them having > 0.15 ratios. Calculated model ages for the latter rocks being said, we can observe that the tholeiites have the younger TDM must then be considered as having large uncertainties and must be model ages varying from 560 to 990 Ma pointing to a preponderant

20 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

+8 source during that period of 60 million years.  Nd, t SVM slates A Coupled with their Mesoproterozoic TDM model ages (Fig. 14B), this +6 SVM coticule SVM-RM-BM SVM Pridoli conglomerate indicates a preponderant crustal component in the SVM magmatic +4 tholeiites SVM magmatic rocks source. This is in line with the existence of Mg-rich intermediate rocks SVM-RM-BM tholeiites +2 Rocroi Mairupt microgranite (especially Les Plattes, Sm2b; Fig. 11E) that point to a high-Mg garnet- Brabant slates Brabant magmatic rocks bearing andesitic source, much more easily found in the lower crust 0 Ebbe Anticline slates than in the mantle. The slight geochemical variability encountered in

-2 SVM the SVM intermediate and felsic magmatic rocks can be explained by RM BM SVM-RM-BM small variations in the source composition, either geochemically (SiO2 -4 intermediate & felsic and MgO contents) or mineralogically (garnet), coupled with variation Pridoli magmatic rocks -6 conglo. Brabant sedimentary province of the degree of partial melting. SVM, Brabant/HSM, Rocroi and Giessen-South Harz tholeiitic ba- -8 salts display alike geochemical signatures. Similarly, felsic Spa dykes in -10 SVM and Mairupt dyke in Rocroi Massif display similar major element SVM-Ebbe sedimentary province Age (Ma) signatures and their REE and spidergrams are similar to the SVM Pridoli -12 410 430 450 470 490 510 530 metaconglomerate (Fig. 12D, 13D). In turn they are comparable to that +8 of the South Taunus rhyolites (Meisl, 1995), the latter being only more  Nd, t +6 SVM-RM-BM B enriched in REE (Fig. 12D). tholeiites The Nd isotopic signature of the tholeiitic rocks from the three +4 massifs is similar and highly positive (ɛNd,420Ma from + 3 to +6; +2 Fig. 14A) and NdTDM model ages between 560 and 990 Ma (Fig. 14B) SVM-RM-BM indicate a mostly asthenospheric mantle origin, which is typical for LIP 0 intermediate & felsic tholeiites where an old lithospheric mantle is feebly participating (e.g. magmatic rocks SVM -2 Deckart et al., 2005). By contrast, the Rocroi felsic dykes and the Pridoli ɛ BM conglomerate have negative Nd,420Ma from -4.0 to -6.5 with NdTDM at -4 1.6 – 1.7 Ga indicating an important crustal contribution. In LIPs, as- -6 RM Pridoli sociated felsic rocks can be generated through the differentiation of the Brabant sedimentary conglo. mafic rocks or through the melting of the lower crust induced by these -8 province mafic rocks. Both types can be encountered in the same province such SVM-Ebbe -10 sedimentary as in the Emeishan Province (Zhong et al., 2011). However, this crustal province T (Ma) melt does not correspond to that of the SVM intermediate-felsic mag- -12 DM 400 600 800 1000 1200 1400 1600 1800 2000 matism as their geochemical characteristics are different (Fig. 12, 13). This can be related to a heterogeneous crust (already pointed by the ɛ Fig. 14. Nd isotopes. A. Age in Ma vs Nd calculated at the time (t) of empla- difference observed between the high-Mg andesites vs the other SVM cement (magmatic rocks) or of deposition (sedimentary rocks). B. Nd TDM ɛ magmatic rocks; Fig. 12 A, B) probably accompanied by a lower partial model age vs Nd. TDM model ages following Nelson and DePaolo (1985). Ex- – cept for tholeiites, samples with 147Sm/144Nd < 0.15 only have not been melting rate in line with the more alkaline character of the Mairupt considered (Liégeois and Stern, 2010). Data from Herbosch et al. (2016; SVM South Taunus felsic magmatism (e.g. Fig. 12D). slates, coticule and magmatic rocks), André et al. (1986; tholeiites), Cobert et al. (2018; Rocroi Mairupt microgranite), Linnemann et al. (2012, Brabant 8. Geodynamical interpretation slates and magmatic rocks), Samuelsson et al. (2002; Ebbe anticline), this study (Pridoli conglomerate, Table 2). 8.1. Close but contrasted basements for the Ardenne and Brabant massifs mantle component in these rocks. All the SVM and Ebbe sediments Although separated by a main thrust (Midi-Aachen thrust fault; display model ages between 1500 and 1900 Ma, similarly as the Bra- Fig. 1), the Lower Paleozoic sediments of the Brabant Massif (BM) and bant sediments, indicating a major input of Precambrian sources, in the Ardenne Allochthon, here represented by its main inlier, the Sta- agreement with the detrital zircon signatures (Figs. 7 and 8). The SVM velot-Venn Massif (SVM) can be considered to have been close during magmatic rocks, whatever their age, have Nd model ages in the 1000- their deposition (Cambrian to Silurian). Indeed, the Variscan northerly 1200 Ma age range, i.e. on the young side of the whole magmatic field movement along this thrust is estimated between 40 and 120 km based together with some Brabant magmatic rocks. By contrast the Rocroi on ECORS data in western Ardenne allochthon (Raoult and Melliez, Mairupt felsic dykes, as well as the Pridoli conglomerate, have older Nd 1987; Lacquement et al., 1999) but less to the east (20-30 km; Adams model ages in the 1600-1700 Ma age range, similar to the SVM sedi- and Vandenberghe, 1999) and even probably 10-15 km at the extreme ments. east, close to SVM (Hance et al., 1999). As shown above (§3.2), during the Megasequence 1 (525-480 Ma), the depositional environment is very similar in the SVM and BM basins 7.3. Geochemistry and Nd isotopes: interpretation although their subsidence curves are much contrasted (> 9000 m in BM, > 2100 m in SVM; Fig. 4). From lower Ordovician to lower De- The above geochemical study has shown that the SVM magmatic vonian, after the Rheic Ocean opened, there is a remarkable opposite rocks are all altered and that mobile elements, mostly LILE (large ion development of the SVM and BM (§3.2): (1) at c. 480 Ma, the sedi- lithophile elements, which include alkalis) cannot be used for de- mentation stopped in BM and resumed in SVM; (2) at c. 467 Ma, the termining the nature of these rocks (Fig. 11 A, B). Immobile elements sedimentation stopped in SVM and restarted in BM and (3) at c. 419 Ma, indicate that the SVM intermediate and felsic rocks correspond to a K- the sedimentation ceased in BM and started again in SVM. (Fig. 15). rich calc-alkaline series eventually shoshonitic-type rocks (Fig. 11D). Alike, magmatism shows similar periods of activities but dissimilar This signature is similar to that of the magmatic rocks of the Brabant intensities: during the Rheic opening (c. 480 Ma), magmatism is im- Massif (Linnemann et al., 2012). We showed above that the SVM portant in SVM, low in BM; during the docking with Baltica (c. 445 Ma), magmatic rocks emplaced sporadically from c. 505 Ma to 446 ± 3 Ma the magmatism is very limited in SVM and substantial in BM (§ 6.1, Fig. 15). This implies a continuous availability of the K-rich (Linnemann et al., 2012 and this study).

21 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Fig. 15. Comparison between the Brabant Massif and the Stavelot-Venn Massif from the base of the Cambrian to the Upper Devonian with: (1) System, Series and Stage name from the International Stratigraphic Chart (Cohen et al., 2013 updated); (2) detailed lithostratigraphy of the Brabant Massif (from Herbosch and Verniers, 2013, 2014; Verniers et al., 2001) and SVM (Chap. 3 and references therein); (3) depth deposition; (4) regional events (5) Megasequence number and total thickness; (6) magmatic events with their ages (for Brabant from Linnemann et al., 2012 and unpublished; for SVM; Chap. 4); (7) large scale events. Abbreviation: s.b.: shallow basin; shal.: shallow; s.: shelf; m.: marine

These observations can be reconciled if a more rigid basement is situation of the Silurian foreland basin to the W and S of the Brabant present below SVM, implying two different basements under SVM and Cambrian-Ordovician core (Fig. 2 in Linnemann et al., 2012). BM. This corresponds to the model of the Midlands microcraton being present below the Ardenne on geological (e.g. Sintubin et al., 2009) and 8.2. Variation through time in zircon supply from regional sources in geophysical (Smit et al., 2018) basis. The BM is located in a much more Ardenne and Brabant massifs subsiding region belonging to a failed arm of the rift that led to the Rheic Ocean opening (Verniers et al., 2002; Sintubin and Everaerts, The change in relative abundance of detrital zircons coming from 2002; Linnemann et al., 2012). In that context, we can deem that the the three determined regional sources (West African craton, Western SVM was located on one of its shoulder. Rheological contrasted beha- Amazonian craton, Pan-African orogen), with in addition the Cambrian viors between rift and its shoulder (graben/horst structure) are well zircon group (Meseta?) is shown for the SVM in fig. 16A. For compar- known and continue to display contrasted evolution after the ocean ison, the same set of sources is displayed for the Brabant Massif in fig. opened (e.g. Faleide et al., 2008). 16B and Table 4 (data reprocessed from Linnemann et al., 2012). ff The BM being located in the rift itself will be a ected by much more Grouping the detrital zircons in these three physical sources allows us deepening under extension that the SVM with its more rigid basement. to relate the variation in their relative abundance to actual geodynamic While under compressional stress, the BM will be deformed and uplifted processes. The proportion of these three sources can be affected by the more easily. This corresponds to a rift inversion situation such as the abundance of zircons within source lithology (fertility bias; Spencer current High Atlas range in Morocco, a former Mesozoic rift that has et al., 2018 and references therein) but this is minimized when looking been inverted during the Cenozoic (e.g. Ellouz et al., 2006; Missenard at their relative abundance through time. The erosional rate may also et al., 2008; Frizon de Lamotte et al., 2009). This inversion induced a induce a fertility bias but it can be incorporated in the notion of source fl Cenozoic basin on the High Atlas southern ank (Ouarzazate basin; El availability. fi Har et al., 2001), which is located on the more rigid margin of the Within the rift itself, the variations in the zircon supply are buffered West African craton (Ennih and Liégeois, 2008). This corresponds to the by the width and the opened situation of the basin. By contrast, the off-

22 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

St5 (La Gleize) rift position of the SVM renders it more prone to have more limited 485 ±6 Ma A. Stavelot-Venn Massif Volcanism Zircon age supply sources implying more rapid and important variations in these Strat. Sm2ab Rv5 Rv4 Rv3 Rv2 supplies. These variations can be used to foresee events occurring in the % zircon source regions. 70 In SVM, from 520 Ma (Hourt Fm) to 495 Ma (La Venne Fm), the Distant tectonic Amazonian and West African craton sources show an opposite behavior, period 60 one being increasing when the other is decreasing (Fig. 16A). This confirms the merits of considering these two sources as distinct. The 50 amount of Pan-African orogen zircons is continuously slightly in-

uplif creasing. During the same period, in the Brabant Massif, the WAC zir- 40 HIATUS Pre-drifting cons are stable while the Amazonian craton zircons have an opposite behavior to the Pan-African orogen zircons. For both massifs, the var- 30 opening ocean Rheic iations observed in the 512-505 Ma period suggest that tectonic pro- lwaster

So cesses were at work. In the SVM, this is the WAC and W Amazonian Coticule event, narrow RheicCoticule event,narrow Ocean

20 Venne craton zircons that are highly variable and not the Pan-African orogen

La zircons, suggesting that these tectonic processes were remote 10 (Fig. 16A). This conclusion is in agreement with the absence of any Hourt Wanne Bellevaux particular events of this age in the SVM sedimentary record (Fig. 5). The Lierneux 0 SVM Pan-African orogen zircon increase is not recorded in the Brabant 460 470 480 490 500 510 520 Ma Massif (Fig. 16B), where they are very abundant since 520 Ma, showing West African craton (3.6-1.95 Ga) only slight variation mostly above 50% of the total amount. Especially, W. Amazonian craton (1.95-0.95 Ga) % the Cryogenian zircon arrival marking the SVM at c. 485 Ma is not Pan-African orogen (0.95-0.541 Ga) 70 Cambrian (Meseta?) (0.541-0.480 Ga) recorded in the Brabant Massif (Linnemann et al., 2012). The SVM signature can be attributed to higher vertical mobility of the local Pan- 60 African terranes and Pan-African metacratons, the Cryogenian terranes being geographically small all over the Pan-African Belt around the 50 WAC (Liégeois et al., 2003; Gärtner et al., 2016; Triantafyllou et al., B. Brabant 2016; Inglis et al., 2017; Liégeois, 2019). This indicates that the SVM, in uplif 40 HIATUS Massif contrast to the Brabant Massif, had poor communication with the main Pre-drifting rift where local source inputs were drowned within the zircon popu- lipont 30 Jodoigne-Orbais ff v lations coming from everywhere. This o -rift situation of the SVM is Rheic ocean opening ocean Rheic Tribotte

Che also shown by the much lower cumulative thickness of the Cambrian 20 sediments than that of the in-rift Brabant Massif (Fig. 4).

ont In SVM, from 495 Ma (La Venne Fm) to 484 Ma (Solwaster Fm), the

10 Tubize Pan-African orogen zircon abundance strongly increased from 30% to Blanm

Jodoigne-Maka 70% (Fig. 16A). Let us recall that the strong increase between La Venne 0 and Solwaster formations is actually due to a major arrival of Cryo- 460 470 480 490 500 510 520 Ma genian zircons (Fig. 10C). The importance and the rapidity of this Fig. 16. Evolution during the SVM sedimentation of the proportion of zircons variation suggest that it had a close tectonic origin related to the pre- following the three main regional sources in (A) the Stavelot-Venn Massif drift, rift-related subvertical movements, which is marked by the re- (SVM) and (B) the Brabant Massif (BM; Linnemann et al., 2012). In each dia- gression observed in the Jalhay Formation (Fig. 4, 5; § 3.1.3 and 3.2). gram, the zircons have been grouped following the three main regional sources At the same time, the Amazonia source strongly decreased (from 40% to (West African craton; Amazonian craton; Pan-African orogen) with in addition 5%) while the West African source decreased only slightly. This sug- the Paleozoic zircons. gests that the local uplift at the origin of the high detrital arrival of the Pan-African orogen zircon formed a barrier for the arrival of the remote

Table 4 Zircon proportions in the SVM and Brabant Massif following sources.

Ord.-Sil. % Cambrian % Pan-African orogen % W. Amazonian craton % West African craton %

Sample Formation Sample age Zircon nb. 419-480 Ma 480-541 Ma 541-950 Ma 950-1950 Ma 1950-3600 Ma

Stavelot Venn Massif St1 Hourt 520 92 0 1.0 18 41.0 40 St2 Bellevaux 510 89 0 2.0 22 21.0 55 St3 Wanne 505 93 0 6.0 26 51.0 17 St4 La Venne 495 91 0 1.0 31 39.0 29 St6 Solwaster 484 39 0 2.5 70 4.5 23 St7 Lierneux 479 73 0 3.0 60 15.0 22 St10 Waimes 420 62 97 0.0 0 1.5 1.5

Brabant Massif BRM 01 Blanmont 522 92 0 0.0 61 19.6 20 BRM 12 Tubize 515 88 0 0.0 54 25.8 20 BRM 02 Jod., Maka 507 103 0 0.0 45 35.3 20 BRM 03 Jod., Orbais 505 83 0 3.6 54 22.9 19 BRM 09 Chevlipont 482 120 0 19.8 55 12.4 12 BRM 10 Tribotte 466 108 0 13.0 67 5.6 14.8

23 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Amazonia zircons. This can be related to a more distal position of the to (1) a heterogeneous lower crust, several sources have been identified, Amazonian craton relative to the SVM, which can be considered to have basaltic, high-Mg andesitic and more felsic ones and (2) varying been located off West Africa during Cambrian times. This agrees with melting rate linked to fluctuating available heat (Fig. 11E, 11F). Heat the fact that the WAC is surrounded by Pan-African belts or Pan-African flow variations would be linked to the epoch of magma generation: metacratonic zones, which have been regularly affected by vertical higher partial melting degree could explain the lower enrichment in movements during the Phanerozoic (Ennih and Liégeois, 2008). In the LREE in the Les Plattes volcanic rocks (Sm2b) compared to older (La Brabant Massif, being located in the main rift (Verniers et al., 2002; Gleize, Rv5 and Meuville, Sm2a; Fig. 15, 16) and younger (Helle) Linnemann et al., 2012), which played a buffer role, Amazonia and magmatic events (Fig. 12B). This suggests a short higher heat flow in WAC zircons abundances are nearly steady from 505 to 482 Ma the SVM during the Les Plattes volcanism emplacement. By contrast, (Fig. 16B). This shows that the zircon delivery in the SVM is more af- the Brabant magmatic rocks as a whole have a geochemical signature fected by the Rheic ocean opening, through associated vertical tectonic close to the Les Plattes magmatism (Fig. 12E), indicating a constant movements than in the Brabant Massif higher heat flow in Brabant Massif compared to the SVM. This suggests In SVM, from 484 (Solwaster Fm) to 479 Ma (Lierneux Fm), the a thinner lithosphere for the Brabant Massif in agreement with its lo- zircons from the Pan-African orogen diminished (70 to 60%), in cation within the rift itself. In the Ardenne, located on the rift shoulder, agreement with the disappearance of the uplift relief at the dawn of the such magmatism would occur only through ephemeral higher heat flux Rheic Ocean (Fig. 16A). This lower relief situation allowed an im- maybe linked to shear zone functioning in relatively rigid areas (me- mediate greater abundance of the W Amazonia zircons after 484 Ma, tacratonic areas; Liégeois et al., 2013). In turn, the mostly crustal source the narrow ocean allowing the arrival of sediments from more internal of the SVM intermediate and felsic rocks implies that the "subduction- parts of Gondwana. During this period, no change is seen for the WAC related" signature marked in the SVM magmatic rocks must be attrib- source, in accordance with its proximal situation. uted to the source and not to the SVM magmatic rocks themselves (Liégeois, 1998). Such a remelting of previous active margin rocks is frequent in post-collisional setting or in synorogenic intracontinental 8.3. Four episodes of magmatism, witnessing three geodynamic settings (Liégeois et al., 1998, 2013; Fezaa et al., 2010). This origin is environments also suitable for the Brabant Massif magmas. The SVM displays three local episodes of magmatism (Figs. 15, 17): (1) an intermediate to felsic volcanism before and just after the Rheic 8.3.2. The bimodal magmatism Ocean opening (505-474 Ma); even if it is modest, this is the most Hypabyssal tholeiitic rocks are present in the Stavelot-Venn, Rocroi important magmatic episode, including the here dated La Gleize vol- and Brabant massifs but are always located in particularly faulted area. canic rock (485 ± 6 Ma); (2) a punctual low-volume late Ordovician In SVM, the two known sills (Challes and Grand-Halleux) are located in felsic magmatic event, including the here dated Helle sill (446 ± 3 Ma) the southern more fractured part of the inlier (stars 3, 4 in Fig. 1; linked to the tectonic instability that affected the Avalonia microplate Fig. 2). In the south-eastern Brabant/HSM area, Mozet and Voroux- during its soft docking with the Baltica plate (c. 448 Ma; Cocks and Goreux magmatic bodies are located at the front of the Ardenne Al- Torsvik, 2005; Cocks and Fortey, 2009; Linnemann et al., 2012) and (3) lochthon (§ 4.1; stars 1, 2 in Fig. 1). In the Rocroi Massif (Grande a few tholeiitic mafic dykes (Grand-Halleux, Challes) whose ages are Commune), they belong to a bimodal dyke swarm parallel to thrust only known to be Ordovician to Silurian (Fig. 17). (4) In addition, the faults well developed in the southern part of the inlier (fig. 1 of Cobert Pridoli metaconglomerate reveals the existence of a remote fourth et al., 2018; star 5 in Fig. 1). None of these tholeiitic rocks are dated. major magmatic activity from 472 Ma to 419 Ma (Fig. 9) that has to be Only a c. 420 Ma age can be ascribed to the Rocroi tholeiitic dykes that located in the erosion area of the Devonian transgression that came are subcontemporaneous with the Mairupt felsic dykes dated at from the SE (Fig. 9, 17; § 3.1.4). The two first episodes are intermediate 421 ± 3 Ma (Cobert et al., 2018). For the other occurrences, only to felsic in composition while the two last are bimodal. maximum ages can be determined: c. 435 Ma for Voroux-Goreux, c. 440 Ma for Mozet and of c. 500 Ma for SVM sills and dykes (§ 4.2). This 8.3.1. The intermediate to felsic magmatism means that these tholeiitic rocks could represent (Fig. 17) (1) a punc- The geochemical study has shown that all the intermediate to felsic tual event at c. 420 Ma similarly as other tholeiitic large igneous pro- magmatic groups are potassic and have a predominantly lower crustal vinces dominated by dykes (such as the Central Atlantic magmatic source generated much earlier (mean Nd model age of 1.2 Ga) within a province at c. 201 Ma; e.g. Marzoli et al., 2011). In that case, it would subduction environment. Differences in composition can be attributed represent a magmatic echo of the Brabantian tectonic inversion

Magmatic occurrences SE Rheno-Hercynian zone (LIP)

SVM, RM, SE BM ? ?

rocks ty ili

SVM, intermediate instab

to felsic rocks aur Rheic ocean opening with L with Docking with a BM, intermediate Rheic Ocean Narrow ion to felsic rocks , tectonic, Collis prior to dprior to ing

420 430 440 450 460 470 480 490 500

Devonian Silurian Ordovician Cambrian Age (Ma) Abundant Scarse Very scarse Supposed, unconstrained

Fig. 17. Summary of the available ages of the magmatic rocks of the Stavelot-Venn Massif (SVM; this study) and the Brabant Massif (BM; Linnemann et al., 2012) and the ages of the proposed LIP (large igneous province) based on detrital zircons in the SVM Pridoli conglomerate (this study).

24 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

CALEDONIDESPOLISH Elbe-Odra fault zone

宑宲宵宷宫季宖宨室

Rhine Magdeburg

Lower Rhine Graben HARZ MTS.

BRABANT MASSIF

E G R I B E Brussels G Köln R ALLINE Z. E F 宅宵室宥室宱宷季害室宵室宸宷孱 IE H Aachen thrust C S RUHLCRYSTA 宱宯宬宨宵 ES GIESSEN - HARZ NAPPES Midi thrust CH 宆宲宱宧宵宲宽季完 NIS Dill - Syncl. HEI Giessen SVM R Lahn- Syncl. THERN PHYLLITE ZONE RHENO - HERCYNIAN ZONE AN ZONE Rocroi M. Serpont M. NOR GERMAN TAUNUS

ÜCK Givonne M. NSR SPESSART HU ODEN- Rhine graben extension Bray fault zone WALD Thrust fault 宬宷 SAXO - THURINGI 宵宱季宯宬宰 MID - Variscan boundary zone 宄容室宯宲宱宬室季家宲宸宷宫宨 Brabant Massif PALATINATE Rheno-Hercynian Zone Cambro-Ordovician inliers Northern Phillite Zone Upper Rhine Graben Strasbourg Mid-German Crystalline Zone BLACK VOSGES Saxo-Thuringian Zone FOREST MOLDANUBIAN孳 ZONE學孳 孴孳孳季宮宰 Moldanubian Zone

Fig. 18. Sketch map of the Rheno-Hercynian Zone and surrounding regions. To the North-West, the delineated Brabant Massif has not been affected by the Variscan orogeny. The Northern Phyllite Zone and the Mid-German Crystalline Zone marks the Rheic and Rheno-Hercynian sutures with mixed Avalonian and Armorican characteristics (Stephan et al., 2016; Franke et al., 2017). The Saxo-Thuringian Zone and the Moldanubian Zone belong to Armorica microcontinent (active margin). This map has been drawn based on Dallmeyer et al., 1995; Oncken and Weber, 1995; Sintubin et al., 2009; Dörr et al., 2017; Vanneste et al., 2013).

(Fig. 15), taking advantage of favourable tectonic conditions for (Oncken, 1997). Such Variscan oblique movement could imply their reaching the surface (magma transfer zone; Vernon et al., 2012; dela- existence before the oceanic closure provoked by an oblique Rheic ridge mination along shear zones; Liégeois et al., 2003, 2013) or (2) a long- push disposition. The SE rigid boundary of Avalonia could have con- lived and sporadic magmatic sequence extending from some time after centrated the strain induced. This situation maybe lasted during lower- the Rheic Ocean opening to the collision climax with Laurentia (from c. middle Devonian considering the 390-384 Ma trachytic volcanism in 472 Ma to c. 419 Ma). In that case, they would record pulses during the the Lahn-Dill area (SE Rheno-Hercynian; Schulz-Isenbeck et al., 2019), continuous LIP life recorded by the 472-419 Ma zircons present in the which display a continuous inherited zircon age record from 460 to 384 Pridoli conglomerate. Indeed, it is a general feature of LIPs to generate, Ma (Schulz-Isenbeck et al., 2019), reminiscent of the Pridoli conglom- besides long-lived but scattered and low-volume phases of magmatism, erate zircon signature (Fig. 10). short-lived voluminous phases (Ivanov et al., 2005). Robust geochro- The erosion of such a large magmatic province could explain that nological data are needed for definitely solve that point. 97% of the detrital zircons of the Pridoli conglomerate are coming from In the Rheno-Hercynian domain, during the same age period, oc- a unique magmatic source: it is large enough for covering the whole curred the early Devonian basaltic lavas from the Giessen-South Harz basement consequently unable to provide detrital zircons. Such a si- region (eastern Rheno-Hercynian region; Floyd, 1995), and the Middle tuation could be imaged by the late Ediacaran Ouarzazate volcanic Ordovician- early Devonian South Taunus metarhyolites (southern province in the Moroccan Anti-Atlas, which is very large (80 x 600 km), Rheno-Hercynian region; Meisl, 1995; Sommermann et al., 1992). They mostly felsic and potassic in composition, that lasted 60 Myr and which have been interpreted as T-MORB (Floyd, 1995) and island arc rocks emplaced along the northern boundary of the West African craton (e.g. (Meisl, 1995), respectively. These interpretations can be questioned Ennih and Liégeois, 2008; Gasquet et al., 2008; Belkacim et al., 2017). especially in South Taunus where the nature of the volcanic rocks Another example is the Early Cretaceous bimodal (basalts and rhyo- (Serizitgneis, Felsokeratophyr, Keratophyr; Meisl, 1995) are mostly lites) Parana Province associated with the opening of the South Atlantic trachy-andesite, dacite, rhyo-dacite and rhyolite that are more in line Ocean, which is very large with a preserved surface of > 1.2 million with a SLIP than with an island arc. Moreover, even if more work and km2 and an average thickness of 0.7 km (Peate, 1997 and references data are needed, we showed that, geochemically, these rocks could therein). The characteristic of such a volcanism, emplaced at the constitute a bimodal event linked to a LIP. In our model, the generation margin of large rigid bodies, is the abundance of dyke swarms that are of this LIP is linked to reactivation of existing structures by a distant actually the way followed by the magmas for reaching the surface. This stress and not by a hypothetical plume as more and more shown (e.g. means that, when erosion has done its work enough, which is rapid Liégeois et al., 2005; Hastie et al., 2014; Peace et al., 2020 and refer- when taking into account the friability of these volcanic deposits, there ences therein). Middle Ordovician to early Devonian tectonic move- is almost nothing left as a witness to the huge volcanic province, the ments at the southern boundary of the Rheno-Hercynian segment are vast majority of its volume being present in the superstructure only. not documented due to the sedimentary cover. However, strike-slip This can explain that this formerly large province is now only re- movements are documented there in the Middle Devonian (Franke, presented mostly by dyke swarms or sills (SVM, BM, RM) or rare lavas 2007) and in the early Carboniferous during the Variscan orogeny (Harz, Taunus). The detrital zircon signature of the Pridoli

25 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142 conglomerate indicates that this large volcanic province lasted from (Fig. 19B). In this 486-473 Ma drifting period magmatism occurred in 472 to at least 419 Ma, i.e. during about 50 Myr, which is the same both BM and SVM, but just before the opening in BM and just after the order of magnitude than the Ouarzazate Supergroup (610-545 Ma, i.e. opening in SVM (Fig. 17), a difference that again has to be related to 65 Myr), which emplaced in transtensive conditions with some pure their contrasted basement. The basement of Ardenne appears to be a strike-slip periods (Belkacim et al., 2017 and references therein). fractured rigid body invaded by magmas. These are typical features of a metacraton (Liégeois et al., 2013). This is in line with the structure 8.4. The particular situation of the Rheno-Hercynian domain in Avalonia proposed by several studies indicating that the Ardenne basement is a microcontinent distinct Precambrian block (Chacksfield et al., 1993) that can be con- sidered as a prolongation of the Midlands microcraton (Sintubin, 1999; Geophysically, the Rheno-Hercynian domain (SE Avalonia) is lim- Sintubin and Everaerts, 2002). More recently, Smit et al. (2018) pro- ited to the N by the Midi-Aachen thrust Fault zone, by major faults to posed that the entire Rheno-Hercynian domain is located above an the W (Bray Fault; Lefort, 2010; Smit et al., 2018) and to the E (Elbe- extension of the Midlands microcraton. Here, we propose further that Odra Fault; Guterch and Grad, 2006; Smit et al., 2016) and to the S by this south-eastern portion of Eastern Avalonia is made of a metacratonic the Northern Phyllite Zone (Smit et al., 2018; Fig. 18). The extensions Gondwana basement (Fig. 19B). This metacratonic margin, on which is towards the NE beyond the Thor suture until the Trans-European suture located the Ardenne Allochthon, is bordered to the north by several zone (e.g. Franke et al., 2017) and to the E beyond the Rheic suture (e.g. basins, the Brabant, East Anglia and Welsh basins (Fig. 19C). Oczlon et al., 2007) sometimes proposed for Avalonia are not validated by geophysical data (Smit et al., 2016). Correlations mostly based on 8.5.2. The SVM and BM intermediate and felsic magmatism paleontology and sedimentology can point to Avalonian affinities but The first set of magmatism is linked to the vertical movements and cannot demonstrate their bearing to a given tectonic terrane (Domeier, tectonic instabilities that occurred before the end of the main rifting 2016). period, largely before the Rheic Ocean opening approached (505-486 Currently, the Northern Phyllite Zone, a major tectonic structure, is Ma; Fig. 19A). It is restricted to the SVM where it constitutes the main composed of both Avalonian and Armorican lithologies as a result of the magmatic episode (Fig. 15, 17). It is mostly of crustal origin but with an Variscan orogeny (Franke, 2000; Stephan et al., 2016; Franke et al., important mantle contribution (Fig. 14). The second magmatic set oc- 2017). Before the Variscan orogeny, this zone was a cold passive margin curred around the Rheic Ocean opening (486-473 Ma; Fig. 19B), just (Henk, 1995) of lithospheric scale (Smit et al., 2018) and existed earlier before in the Brabant Massif (486-477 Ma), just after in the SVM (478- at least from Early Devonian (Anderle, 1987) and most probably since 473 Ma; Fig. 15, 17). The third magmatic set occurred after a magmatic the early Ordovician (Anderle, 1998). We can thus propose that the quietness (449-443 Ma in the SVM, 460-427 Ma in the BM; Fig. 17). A Rheno-Hercynian domain constituted a south-eastern spur protruding major distant role of the Avalonia/Baltica docking followed by the from the southern margin of the Avalonia microcontinent (Fig. 18), collision with Laurentia has been proposed for the generation of the giving it a particular sensibility to a stress applied from the south on Brabant felsic-intermediate magmatism (460-427 Ma; Linnemann et al., Avalonia. During the Cambrian, the rigid Rheno-Hercynian (RH) 2012) rather than to invoking a SSE-dipping subduction plane (e.g. basement was separated from the Avalonia mainland by the Brabant Verniers et al., 2002; Pharaoh, 2018) that does not fit with the passive rift. While the latter failed to evolve to an ocean, the RH basement margin sedimentary sequences, the very low volume and the shosho- detached and moved away with Avalonia. This means that the RH could nitic nature of the magmatism. In addition to distant collisions, the be either a part of Avalonia or a part of the continent from which Rheic Ocean push should be also considered, allowing including the Avalonia detached. Considering its rigid behavior, the development of limited amounts of magmatism emplaced before docking in the model the Brabant rift on its northern margin and the protuberance it forms to (Fig. 19). In the SVM, located on a more rigid basement, magmatism the SE of Avalonia, we consider that there is a greater probability that it occurred only when the stress was high, i.e. during the docking with is a piece of lithosphere torn from the other continent. Detrital zircons Baltica (449-443 Ma; Helle pluton; Figs. 15, 17). from this study have shown that the closer continent was West Africa, especially the West African craton with some Neoproterozoic super- 8.5.3. The large igneous province (LIP) structures. As a result, we propose that the RH basement is a piece of The 473-420 Ma period (Fig. 19C) corresponds to the drifting of lithosphere ripped off from the West African craton during the opening Avalonia away from Gondwana, its soft docking with Baltica followed of the Rheic Ocean. by the collision with Laurentia. This induced continuous lower stress (Rheic ridge push) and episodic higher stress (docking and collision) to 8.5. The geodynamical model which the Brabant Massif and the Stavelot-Venn Massif reacted in a contrasted way as described above. During this period, the southern 8.5.1. The regional structural model boundary of Avalonia was facing the Rheic Ocean all the time (Cocks The proposed geodynamical model is represented in Fig. 19. The and Torsvik, 2005; Golonka and Gawęda, 2012; Domeier, 2016; Torsvik structure of the Albuquerque basin within the Rio Grande rift (Russell and Cocks, 2017; Franke et al., 2017). The existence and evolution of and Snelson, 1994) can be adapted to the Brabant-Ardenne situation the Rheno-Hercynian Ocean, which is a short-lived Middle Devonian (Fig. 19A). The 525-486 Ma period corresponds to the main rifting small oceanic segment related to the Rheic Ocean (Eckelmann et al., period that preceded the Rheic Ocean opening (at c. 480 Ma) and the 2014; von Raumer et al., 2017; Franke et al., 2017) is outside the period uplift marked by regression (from 486 Ma). The Ardenne (including of time considered in this paper. The major point is that high-pressure SVM) is located on the shoulder of the rift made of Gondwana litho- metamorphism demonstrates a southerly dipping subduction below the sphere (Fig. 19A). The Brabant Massif is located in the main rift on a Saxothuringian active margin during at least from early Upper Devo- thin Avalonia lithosphere. The Condroz, marked by no Cambrian and nian to Lower Carboniferous times (Ganssloser et al., 1996). thin Ordovician series is located on a shoal between the latter two. This Geochemically, we show arguments for a LIP parentage for the situation can explain the similarities and di fferences in the sedi- mafic/felsic Ardenne-Brabant rocks to which we relate the late mentology of SVM and BM described above (§ 8.1, also § 3.2, Fig. 5). Silurian/early Devonian mafic and felsic rocks from South Taunus and Magmatism occurred in the 505-486 Ma only in the Ardenne segment Harz areas, dismissing the MORB and island arc setting proposed by (SVM), close to the main eastern discontinuities. Besides, a main tec- Floyd (1995) and Meisl (1995), respectively. We are aware that this tonic discontinuity is postulated to the east of the Ardenne (Fig. 19A). considerably alters the regional geological evolution paradigm that When the Rheic Ocean opened, we suggest that a part of Gondwana relies a lot on the existence of a Silurian island arc (e.g. Franke et al., is detached with Avalonia and drifted away from Gondwana mainland 2017). Clearly, more work is needed to verify this new proposition. But

26 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

SVM Fig. 19. The proposed geodynamic model for the south-eastern 525-486 Ma, main rifting period magmatism Avalonia that contains the Ardenne region to which the SVM belongs. A 505-486 Ma A. Cambrian rifting period (upper Terreneuvian to upper Furongian, km Brabant Condroz Ardenne 525-486 Ma). This cross-section is based on the current Albuquerque 0 basin structure (Russel and Snelson, 1994) with the Ardenne located

5 on the rift shoulder, the Brabant in the most subsiding area, separated Avalonia Gondwana by a shoal (Condroz zone). During this period, magmatism was present 10 only in Ardenne. B. During lower Ordovician (Tremadocian-Floian; Albuquerque basin cross-section, adapted 486-473 Ma), the Rheic Ocean opened to the east of the Brabant- BM SVM Ardenne rift zone, within the Gondwana fractured basement. The magmatism magmatism 478-473 Ma southern margin of Avalonia is thus made of a Gondwana metacra- 486-477 Ma tonic margin, which meets the lithospheric zones proposed by Smit Avalonia et al. (2018). Magmatism occurred both in Brabant and in Ardenne. C. (main) B The Middle Ordovician–upper Silurian (Dapingian-Pridoli; 473-420 486-473 Ma Ma) corresponds to the main life of the Rheic Ocean whose ridge is End of main rifting, able to apply a stress on the southern margin of Avalonia, especially pre-drift uplift during the docking of Avalonia with Baltica (around 448 Ma) and later (regression) with Laurentia (from 430 Ma). This induced strike-slip movements

and drifting away Rheic Ocean Rheic Ocean along the south-western boundary (Bray Fault) and along the south- of Avalonia with a eastern margin of Avalonia (SE Rheno-Hercynian margin). This gen- portion of the erated a large igneous province (LIP) along the latter and limited Gondwana margin Gondwana continent magmatism in Brabant and Ardenne regions, mostly of crustal origin. (metacratonic) It can be seen that the Gondwanan Avalonia metacratonic margin is C bordered to the north by several basins (Brabant, East Anglia, Welsh). 473-420 Ma, Rheic Ocean period

Avalonia Elbe-Odra fault

SVM, BM, RM: tholeiitic magmatism at c.420 Ma (and earlier?) East Anglia BM limited SVM rare Midlands basin magmatism magmatism microcraton 460-427 Ma c. 446 Ma llochthon Welsh basin spur A Brabant basin N-A FZ heno-Hercynian R

ont M. rgin at depth Serp a Bray fault gin 420 Ma Avalonia m Avalonatia d marepth 472 - Rheic ocean

Oceanic ridge

we must note that this is the same situation for the existence of the episodic (docking with Baltica and collision with Laurentia) but rather Silurian island arc only based on a few samples present in a paper intense and produced additional tholeiitic magmatism in the LIP without any data tables (Meisl, 1995). We show here that this inter- (Laurentian episode climax, c. 420 Ma), very rare (rigid shoulder, SVM) pretation can be questioned. Additionally, we can note that this sub- and more abundant felsic-intermediate magmatism (aborted rift, BM) duction plane to the north-west below Avalonia (Franke et al., 2017)is during the Baltica (c. 445 Ma) and initial Laurentia episodes (430-420 superfluous, a much major subduction zone existing on the other side of Ma). the Rheic ocean, e.g. in the Odenwald area (Armorica; Dörr and Stein, 2019 and references therein). 8.6. The pre-drift Cambrian position of Avalonia

8.5.4. A global model for the magmatism of the Brabant and Ardenne We have shown (§8.4) that a part of the Gondwana basement has massifs been ripped off and left with Avalonia at the Cambrian-Ordovician As a whole, the c. 460 Ma to c. 420 Ma magmatism (or from 472 Ma boundary. The paleoposition of Avalonia during the Cambrian before if the felsic LIP is included) in the SVM and in the BM (Fig. 19C) can be drifting is classically off West Africa (e.g. Domeier, 2016) or even off considered as intraplate magmatism linked to stress applied at plate South America (e.g. Nance et al., 2008). Based on detailed interpreta- boundaries: (1) to the south, the stress is continuous but rather weak tion of the detrital zircon age spectra and on rheological considerations, and produced abundant magmatism on the continental boundary (felsic we propose a more constrained paleogeographic position for Avalonia LIP), no magmatism in the rigid shoulder of the rift (SVM) and scarce before its drifting from Gondwana at the dawn of the Ordovician magmatism in the aborted rift (BM); (2) to the north, the stress is (Fig. 20). Indeed, the West African craton (WAC) is made of two main

27 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

Mesozoic to Recent sedimentary cover 1000 km Paleozoic sedimentary cover Rabat Neoproterozoic (+ Cambrian) sedimentary cover West Africa Pan-African belts (+ Variscan in Mauritanides) B e c h a r WAC Paleoproterozoic (2.3-1.95 Ga) Saoura Ougarta WAC Archaean Tindouf South American cratons Reguibat Geophysical WAC eastern boundary Shield Chegga Major discontinuities and terrane boudaries Zouerate Rheno-Hercynian realm (RHr) Amsaga Taoudeni (with Ardenne inliers) Tuareg Metacratonic Avalonia Taoudeni Main Eastern Nouakchott Basin shield Avalonia WACTimbouctou Dakar AvaloniaEast Anglia RHr G a o basin Kayes Brabant Gourma Midlands spur basin Kenieba (’microcraton’) B a m a k o Niamey Ouagadougou Harlech basin

Man Volta ? Shield Western Avalonia d Sao Luis craton

South America Paranaiba Andean Block Orogen Amazonian craton Sao Francisco craton

Fig. 20. The proposed location of Avalonia before the Rheic Ocean opening (Cambrian times). This location is based on several arguments: (1) the proximal source of the SVM sediments corresponds to West Africa (zircon detrital ages, this study); (2) the Ardenne needs a fractured rigid basement (metacratonic) and a metacratonic margin is known to surround the West African craton (Ennih and Liégeois, 2008); (3) the eastern boundary of Avalonia fits perfectly the western embayment of the West African craton existing to the west of the Taoudeni basin and the Kayes and Kenieba inliers, i.e. at the contact zone between the Man Shield to the south and the Reguibat Shield to the north. This leaves an off-shore location for the more subsiding Brabant basin and, more to the west for western Avalonia, more juvenile in nature. The white area marked by the question mark is classically attributed to Florida (e.g. Nance et al., 2008; Domeier, 2016). West African geology from Ennih and Liégeois (2008), South American cratons and Pan-African belts delineations from de Castro et al. (2014). shields, the Man Shield to the south and the Reguibat Shield to the (Fig. 20). This paleoposition helps to explain several key features of north, both with an Archean domain to the west and a Paleoproterozoic Avalonia: (1) The Gondwana metacratonic margin below the Rheno- domain to the east; in the middle a mostly unknown basement is cov- Hercynian domain join the western Reguibat and Man and thus com- ered by the Meso- to Neoproterozoic Taoudeni basin (Fig. 20). The pletes the WAC western margin while the Brabant basin is off the Kayes and Kenieba small inliers indicate however that this basement is Gondwana coast, in agreement with its position linked to the main rift. likely to be Paleoproterozoic in age. To the west, in front of the This WAC lithosphere would have been ripped off the WAC similarly as Taoudeni basin, the WAC displays an embayment that corresponds the Icartian basement in Brittany (Cadomia; Auvray et al., 1980; Calvez exactly in size to the Rheno-Hercynian eastern spur of Avalonia and Vidal, 1978; d'Lemos and Brown, 1993; Samson and D’Lemos,

28 A. Herbosch, et al. Earth-Science Reviews 203 (2020) 103142

1998) although the latter seems to be much smaller. (2) Most of Ava- these sources are more important than those observed in the Brabant lonia is off the coast of Gondwana, which allows all the Neoproterozoic Massif. This is related to the position of the Brabant in the main rift arc systems to develop (e.g. Murphy et al., 2018). (3) Western Avalonia, where these variations are buffered by opposition to the SVM located on with that disposition, would be located in a more oceanic position (at the more rigid rift shoulder. Especially, a distant tectonic event is re- the place of the current Andean orogen). This is in agreement with its corded between 512 and 505 Ma that strongly modified the proportion more juvenile signature than that of Eastern Avalonia, that show much of the West African and West Amazonian cratons inputs (Fig. 16). By more involvement of ancient crustal material, similarly to Cadomia contrast, during the uplift just before the Rheic Ocean opening, an (Murphy et al., 2018 and references therein). important arrival of Cryogenian zircons points to a local tectonic event The extension of the Avalonian Gondwanan metacratonic margin to that generated a relief forming a barrier that induced a strong drop in the west is not known but the juvenile signature of W Avalonia and the the number of Amazonian zircons. fracture between W and E Avalonia during the Atlantic opening sug- The geochemical study, looking beyond the problems of secondary gests that it did not extend much further west than its representation in alteration, has demonstrated that the SVM intermediate and felsic fig. 20. The nature of the terrane that would have been located to the rocks, which emplaced sporadically from c. 505 Ma to 446 ± 3 Ma south of Avalonia (indicated by a question mark in Fig. 20) is un- (Fig. 17), correspond to a K-rich calc-alkaline series eventually shosh- constrained but this is usually the location of Florida (Suwannee ter- onitic-type rocks, similarly to the Brabant Massif magmatic rocks. This rane) that display Amazonia connections (Nance et al., 2008 and re- implies a continuous availability of a K-rich source during that period of ferences therein). The reason why a part of the WAC was ripped off and 60 million years, which, taking into account the Mesoproterozoic TDM accompanied Avalonia in its drifting must be in relation with the de- model ages of these magmatic rocks, has to be located mostly in an old velopment of the Taoudeni basin between the Man and Reguibat Shield, lower continental crust. More specifically, this source comprises high- which is still a poorly known process. This could have to do with the Mg garnet-bearing andesitic component (especially Les Plattes Sm2b structure of the Sarmatia craton that was attached to the WAC before 1 magmatism). Slight variations in the source composition (mainly SiO2, Ga (Wane et al., 2018). MgO and garnet content) and of the degree of partial melting can ac- count for the slight geochemical variability observed in the SVM in- 9. Conclusion termediate and felsic magmatic rocks. Ultimately, this can be related to the heat flow value that had to be higher in Brabant (rift centre) and The Ardenne and Brabant basements belong both to the Avalonia lower and more variable in SVM (rift shoulder submitted to intermittent microcontinent but only the Ardenne basement has been affected by the higher heat flow due to shear zone). As a whole, this means that the Variscan orogeny, leading to the concept of Ardenne Allochthon. This "subduction-related" signature marked in the SVM magmatic rocks must implies that the correlations between the two Cambro-Ordovician be attributed to the crustal source and not to the SVM magmatic rocks basements are not straightforward and have been much discussed but themselves, within a synorogenic intracontinental setting (Liégeois not updated. This study, focused on the Stavelot-Venn Massif (SVM), et al., 1998, 2013; Fezaa et al., 2010). the largest Ardenne basement inlier, demonstrates that while the The geochemical characteristics of the SVM, Brabant/HSM, Rocroi Cambrian and lowermost Ordovician (extensional Megasequence 1) and Giessen-South Harz tholeiitic basalts (sills, dykes and flow) are si- thickness is very different (> 9000 m in Brabant Massif, c. 2000 m in milar to low-Ti basaltic sills and flows of large igneous provinces (LIP) SVM), their depositional environments are strikingly similar (Fig. 5). and associated felsic rocks in Ardenne and in South Taunus to LIP felsic After a short initial shelf stage (c. 525-515 Ma), a main deep anoxic rocks. This lead us to question the existence of a Silurian island arc on basin developed ending with a regression (lower Tremadocian) corre- the south-eastern boundary of Avalonia and to rather propose the sponding to the uplift announcing the opening of the Rheic ocean former presence of a LIP, now largely eroded, leaving only subvolcanic (Fig. 4, 5, 15). After the opening of the Rheic Ocean, Brabant and SVM remnants. This erosion event is recorded in the Pridoli-Lochkovian behaved differently: in Brabant the regression led to emersion (hiatus conglomerate covering the SVM (Ardennian unconformity), which from c. 480 to c. 467 Ma) while a deep sedimentation resumed rapidly contain 97% of 472 to 420 Ma old detrital zircons, without any con- in SVM. When this sedimentation stopped in SVM at c. 467 Ma (Da- tribution of the Rheno-Hercynian or Brabant basement, in agreement pingian-Darriwilian boundary), it restarted in Brabant (Fig. 15). Simi- with a large and thick volcanic cover. The generation of this large larly, at c. 419 Ma (Pridoli-Lochkovian boundary), the sedimentation volcanism is attributed to the stress imposed by the Rheic oceanic ridge, stopped in Brabant and started again in SVM (Ardennian unconformity; especially in late Silurian times when Avalonia collided with Laurentia. Fig. 15). All these observations are in agreement with two different The Silurian arc model was initially loosely constrained (Meisl, 1995) basements below SVM (and Ardenne as a whole) and below Brabant, and was never re-examined even if often used in the literature (Franke the basement under Ardenne being more rigid. et al., 2017 and references therein). The proposed LIP model needs also Magmatism (volcanic and subvolcanic) is very scarce in SVM to be strengthened. Obviously, more data are needed for elucidating (Fig. 17). There are (1) volumetrically limited flows and tuffs in the that key period of Avalonia life. period around the Rheic Ocean opening just before (La Gleize flow, The Rheno-Hercynian basement constitutes a bulge located to the 485 ± 6 Ma; Fig. 6A) and just after (478-473 Ma), associated with the SE of Avalonia (Fig. 18, 19), which corresponds exactly in size to the coticule event (Herbosch et al., 2016); (2) the Helle (446 ± 3 Ma; western embayment of the West African craton observed in southern Fig. 6B) and Lammersdorff sills related to a distant effect of the Ava- Mauritania and Senegal to the W of the Taoudeni Basin and to the W lonia-Baltica docking; (3) the Grand Halleux and Challes mafic dykes, and NW of the Kayes and Kenieba inliers (Fig. 20). We thus propose that undated but probably Pridoli/Lochkovian in age. Geochemistry points this bulge was ripped off the West African metacratonic margin during to post-collisional or intracontinental orogenic potassic magmatism the drifting of Avalonia from Gondwana (Fig. 19), which explains the with a mostly crustal source for the two first groups and a mostly more rigid behavior of the Ardenne basement. asthenospheric mantle for the late mafic tholeiitic magmatism. Supplementary data to this article can be found online at https:// Our comprehensive detrital zircon study has strongly constrained doi.org/10.1016/j.earscirev.2020.103142. the sources of SVM sediments and their availability variations through time. Homogeneous zircon age responses have suggested the existence Acknowledgements of three sources: the West African craton (3.6–1.95 Ga zircons), the Western Amazonia craton (1.95-0.95 Ga zircons) and the Pan-African AH thanks his colleague N. Mattielli for the Nd isotopic analysis of orogen (0.95-0.54 Ga zircons) with, in addition, rare Cambrian the sample St10. He also thanks the Département de Géosciences, (Meseta?) zircons (Fig. 10). Relative variations in the contribution of Environnement et Société from the Université Libre de Bruxelles for the

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