40Ar/39Ar phlogopite geochronology of lamprophyre dykes in Cornwall, UK: new age constraints on Early Permian post-collisional magmatism in the Rhenohercynian Zone, SW England

Journal of the Geological Society (2015), http://jgs.lyellcollection.org/content/early/2015/06/03/jgs2014-151. Copyright © Geological Society of London 2015The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation of Pangaea. The SW England peninsula is composed largely of Early Devonian to Carboniferous volcano-sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low-grade regional metamorphism during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows, largely on the basis of geochronological data from lamprophyric lavas in Devon. Although published geochronological data for Cornish lamprophyre dykes are consistent with this interpretation, these data are limited largely to imprecise K–Ar whole-rock and biotite analyses, hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional Variscan tectonics. 40Ar/39Ar geochronological data for four previously undated lamprophyre dykes from Cornwall, combined with published data, suggest that lamprophyre magmatism occurred between c. 293.6 and c. 285.4 Ma, supporting previous inferences that their emplacement was coeval with the Cornubian Batholith. These data provide insights into (1) the relative timing between the lamprophyres and basalts, the Cornubian batholith and post-collisional magmatism elsewhere in the European Variscides, and (2) the post-collisional processes responsible for the generation and emplacement of lamprophyres, basalts and granitoids.NSERC (Canada) Discovery grant

Does the Meguma Terrane Extend into SW England?

The peri-Gondwanan Meguma terrane of southern Nova Scotia, Canada, is the only major lithotectonic element of the northern Appalachian orogen that has no clear correlatives elsewhere in the Appalachians and lacks firm linkages to the Caledonide and Variscan orogens of western and southern Europe. This characteristic is in contrast with its immediate peri-Gondwanan neighbor, Avalonia, which has features in common with portions of Carolinia in the southern Appalachians and has been traced from the Rhenohercynian Zone of southern Britain eastward around the Bohemian Massif to the Carpathians and western Pontides. At issue is the tendency in Europe to assign all peri-Gondwanan terranes lying outboard of the Rheic suture to Avalonia, characterized by relatively juvenile basement and detrital zircon ages that include Mesoproterozoic populations, and those inboard of the suture to Cadomia, characterized by a more evolved basement and detrital zircon ages that match Paleoproterozoic and older sources in the West African craton.    Although the unexposed basements of Avalonia and Meguma are thought to be isotopically very similar, the Meguma sedimentary cover contains scarce Mesoproterozoic zircon and is dominated instead by Neoproterozoic and Paleoproterozoic populations like those of Cadomia. Hence, felsic magma produced by crustal melting in the Meguma terrane (e.g. the ca. 370 Ma South Mountain Batholith) is isotopically more juvenile (eNd = –5 to –1, TDM = 1.3 Ga) than the rocks it intruded (eNd= –12 to –7, TDM = 1.7 Ga). By contrast, felsic magma produced by crustal melting in Avalonia (eNd = –1 to +6, TDM = 0.7–1.2 Ga) is isotopically similar to its host rocks (eNd = –3 to +4, TDM = 0.9–1.4).    The isotopic relationship shown by the Meguma terrane has also been recognized in the South Portuguese Zone of southern Spain, which is traditionally assigned to Avalonia. However, the Sierra Norte Batholith of the South Portuguese Zone (ca. 330 Ma; eNd = +1 to –3, TDM = 0.9–1.2 Ga) is on average more juvenile than the Late Devonian host rocks (eNd = –5 to –11) it intruded, suggesting instead an extension of the Meguma terrane into Europe. Available data for the Cornubian Batholith of SW England (ca. 275–295 Ma; eNd = –4 to –7, TDM = 1.3–1.8 Ga) and the Devonian–Carboniferous metasedimentary rocks it intruded (eNd = –8 to –11) suggests this may also be true of that part of the southern Britain (Rhenohercynian Zone) with which the South Portuguese Zone is traditionally correlated.SOMMAIRELe terrane péri-gondwanien de Meguma en Nouvelle-Écosse au Canada, est le seul grand élément lithotectonique de l’orogène des Appalaches du Nord qui n’ait pas de correspondant avéré ailleurs dans les Appalaches et qui ne montre aucun lien sûr avec les orogènes calédonienne et varisque de l’ouest et du sud de l’Europe.  Cette situation contraste avec celle de son voisin péri-gondwanien immédiat, l’Avalonie, qui partage certaines caractéristiques avec des portions de Carolinia des Appalaches du sud et qui a été suivi à partir de la zone rhénohercynienne dans le sud de la Grande-Bretagne vers l’est autour du massif bohémien jusqu’aux Carpates et l’ouest de la chaîne pontique.  Ce qui est en question ici c’est la tendance en Europe à assigner l’Avalonie à tous les terranes péri-gondwaniens situés à l’extérieur de la suture rhéïque lesquels sont caractérisés par un socle relativement juvénile et des âges de zircons détritiques qui comportent des populations mésoprotérozoïques, et ceux situés à l’intérieur de la suture à Cadomia, lesquels sont caractérisés par un socle plus évolué et des âges de zircons détritiques qui correspondent à des sources du craton ouest africain paléoprotérozoïques et plus anciennes.     Bien que l’on estime que les socles non-exposés des terranes d’Avalonie et de Meguma soient très similaires isotopiquement, le couvert sédimentaire de Meguma ne renferme que de rares zircons mésoprotérozoïques, et ce sont plutôt les populations de zircons néoprotérozoïques et paléoprotérozoïques qui dominent, comme c’est le cas pour Cadomia.  Il en ressort que le magma felsique produit par la fusion de croûte dans le terrane de Meguma (par ex. le batholite de South Mountain de 370 Ma env.) est isotopiquement plus jeune (eNd = –5 à –1, TDM = 1.3 Ga) que les roches qu’il recoupe (eNd= –12 à –7, TDM = 1.7 Ga).  Par opposition, le magma felsique produit par la fusion de la croûte dans le terrane d’Avalonie (eNd = –1 à +6, TDM = 0.7–1.2 Ga) est isotopiquement similaire aux roches de son encaissant (eNd = –3 à +4, TDM = 0.9–1.4).     Le profil isotopique du terrane de Meguma, traditionnellement assignée à l’Avalonie,  a aussi été détecté dans la Zone sud-portugaise du sud de l’Espagne.  Cependant, le batholite de Sierra Norte de la Zone sud-portugaise (ca. 330 Ma; eNd = +1 à –3, TDM = 0.9–1.2 Ga) est en moyenne plus jeune que l’encaissant du Dévonien moyen (eNd = –5 à –11) qu’il recoupe, ce qui permet de penser à une extension du terrane de Meguma en Europe.  Les données disponibles du batholite de Cornubian dans le S-O de l’Angleterre (ca. 275–295 Ma; eNd = –4 à –7, TDM = 1.3–1.8 Ga) et des roches métasédimentaires dévono-carbonifères qu’il recoupe (eNd = –8 to –11) permet de penser qu’il pourrait en être de même de cette portion du sud de la Grande-Bretagne (Zone rhénohercynienne) avec laquelle la Zone sud-portugaise est traditionnellement corrélée

⁴⁰Ar/³⁹Ar phlogopite geochronology of lamprophyre dykes in Cornwall, UK: new age constraints on Early Permian post-collisional magmatism in the Rhenohercynian Zone, SW England

<p>The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation of Pangaea. The SW England peninsula is composed largely of Early Devonian to Carboniferous volcano-sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low-grade regional metamorphism during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows, largely on the basis of geochronological data from lamprophyric lavas in Devon. Although published geochronological data for Cornish lamprophyre dykes are consistent with this interpretation, these data are limited largely to imprecise K–Ar whole-rock and biotite analyses, hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional Variscan tectonics. ⁴⁰Ar/³⁹Ar geochronological data for four previously undated lamprophyre dykes from Cornwall, combined with published data, suggest that lamprophyre magmatism occurred between <em>c</em>. 293.6 and <em>c</em>. 285.4 Ma, supporting previous inferences that their emplacement was coeval with the Cornubian Batholith. These data provide insights into (1) the relative timing between the lamprophyres and basalts, the Cornubian batholith and post-collisional magmatism elsewhere in the European Variscides, and (2) the post-collisional processes responsible for the generation and emplacement of lamprophyres, basalts and granitoids. </p

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Journal of the Geological Society (2015), http://jgs.lyellcollection.org/content/early/2015/06/03/jgs2014-151. Copyright © Geological Society of London 2015The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation of Pangaea. The SW England peninsula is composed largely of Early Devonian to Carboniferous volcano-sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low-grade regional metamorphism during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows, largely on the basis of geochronological data from lamprophyric lavas in Devon. Although published geochronological data for Cornish lamprophyre dykes are consistent with this interpretation, these data are limited largely to imprecise K–Ar whole-rock and biotite analyses, hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional Variscan tectonics. 40Ar/39Ar geochronological data for four previously undated lamprophyre dykes from Cornwall, combined with published data, suggest that lamprophyre magmatism occurred between c. 293.6 and c. 285.4 Ma, supporting previous inferences that their emplacement was coeval with the Cornubian Batholith. These data provide insights into (1) the relative timing between the lamprophyres and basalts, the Cornubian batholith and post-collisional magmatism elsewhere in the European Variscides, and (2) the post-collisional processes responsible for the generation and emplacement of lamprophyres, basalts and granitoids.NSERC (Canada) Discovery grant