Petrogenesis of the Mairupt microgranite: A witness of an Uppermost Silurian magmatism in the Rocroi Inlier, Ardenne Allochton

Abstract

Magmatism in the Rocroi inlier (Ardenne Allochton, southeastern Avalonia during eo-Hercynian times) consists of a swarm of bimodal dykes (diabase and/or microgranite) emplaced in Middle to Upper Cambrian siliciclastics (Revin Group). Felsic volcanites interbedded within the Upper Silurian/Lower Devonian transgressive strata on the eastern edge of the inlier were interpreted as belonging to the same magmatic event. This was subsequently invalidated by zircon U-Pb dating of the Mairupt and Grande Commune magmatic rocks, which yielded an Upper Devonian age. Here we report a reevaluation of the age of the Mairupt microgranite based on LA-ICP-MS in situ U-Pb zircon geochronology, which yields a concordant age of 420.5 AE 2.9 Ma (Late Silurian/Early Devonian). This new dating restores the consistency between the different magmatic occurrences in the Rocroi inlier. The geochemical and petrographical data furthermore indicate a major crustal contribution, which fits well within the context of crust thinning of the Ardenne margin (southeastern Avalonia) in the transtensional Rheno-Hercynian back-arc basin.

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“…The SVM sills and dykes, the Rocroi felsic dykes and the Brabant plutonic rocks form a trend in the peraluminous field with peralkaline index/peraluminous index from 0.9/1.05 to 0.5/1.8. This trend, in line with that of the sedimentary and volcano-sedimentary rocks, suggests that the plutonic rocks are also altered and have lost alkali elements, even if to a lesser extent than the volcano-sedimentary rocks, in agreement with Cobert et al (2018) conclusion for the Rocroi Mairupt felsic dykes. In the tholeiitic rocks, the alteration is marked by a great variability of the peralkaline index values, the peraluminous index being buffered by much higher values of less mobile CaO.…”

“…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).…”

“…recently dated at 421 ± 3 Ma (Pridoli-Lochkovian) on the microgranitic dyke of Mairupt (LA-ICP-MS U-Pb zircon; Cobert et al, 2018). This is a dense swarm although volumetrically unimportant in which the dolerites are predominant (80%).…”

“…Except for the doleritic dykes in Rocroi indirectly dated at 421 ± 3 Ma (uppermost Silurian; Cobert et al, 2018), these tholeiitic intrusions are not dated. Considering the geological constraints, they can belong to the Ordovician and/or Silurian.…”

“…In consequence, we have compiled the geochemical composition of the SVM magmatic rocks from André (1983; 1 Challes sill, 3 Grand-Halleux sill), Daneels and Vogel (1978; 1 Helle sill, 1 Lammersdorf sill, 3 Spa dykes, 5 conformable volcanic rocks), Herbosch et al (2016; 1 Helle sill, 2 conformable volcanic rocks), Lamens (1985a; 3 conformable volcanic rocks), Ronchesne (1934a; 1 Challes sill), Schreyer and Abraham (1978; 1 Helle sill, 1 Challes sill) and Van Wanbeke (1955; 1 Helle sill). The Rocroi Massif analysis are from André (1983; 1 Grande Commune dyke), Beugnies (1964, 1 Grande Commune dyke), Cobert et al (2018;6 Mairupt dykes), Goffette et al (1990; 2 Grande Commune and 2 Laifour dykes). The Brabant Massif (including the HSM) analyses are from André (1983;4 Hozémont sill), Linnemann et al (2012; 5 conformable volcanic rocks, 1 Bierghe sill, 4 Quenast pluton), Ronchesne (1934b, 1 Mozet sill;1934c, 1 Hozémont sill, 1934d, 3 basalt from Voroux-Goreux volcanic complex).…”