Neoproterozoic recycling of the Sveconorwegian orogenic belt: Detrital-zircon data from the Sparagmite basins in the Scandinavian Caledonides

Abstract

Arkose-dominated sequences exposed in the Lower and Middle Allochthons of the Scandinavian Caledonides, known as the Sparagmites, are the product of sedimentation along the Baltoscandian (northwestern) margin of Baltica during Cryogenian and Ediacaran times; they notably include the glaciogenic Moelv Formation. These sequences were thrusted southeastward during the Caledonian orogeny. U-Pb and Lu-Hf LA-ICP-MS data from detrital zircons are reported from five samples of arkose or greywacke distributed over the Hedmark Group, to represent the Lower Allochthon Osen-Røa Nappes in southeast Norway. Three samples of arkose from the Särv, Kvitvola and Valdres Nappes represent the Middle Allochthon. All samples share a multimodal detrital-zircon age distribution ranging from 1988 to 862 Ma. Zircons older than 2000 Ma are very rare. Zircons with ages between 677 and 620 Ma are present in only one sample. Mesoproterozoic detrital zircons generally have positive ɛHf values, and a general trend of decreasing ɛHf with decreasing age reflects crustal recycling. Most detrital-zircon populations in the samples can be attributed to primary magmatic sources exposed in the Sveconorwegian orogenic belt sensu lato (including the Paleo- to Mesoproterozoic crust in western Baltica affected by Sveconorwegian reworking). A 1580–1380 Ma detrital population with negative ɛHf, found in several samples of the Hedmark Group, is nevertheless unknown from this belt, and may reflect sourcing from granitoids located in Fennoscandia east of the Sveconorwegian belt. The correspondence between known magmatic events in the Sveconorwegian belt and the ages of detrital zircons is strongest in the samples of the Middle Allochthon. The pooled distribution of new and published detrital-zircon data in arkose from the Middle Allochthon has age modes at 1645, 1600, 1490, 1280, 1155, 965 and 930 Ma. Thick Tonian to Cryogenian siliciclastic sediments exposed in the Caledonides of Scandinavia, East Greenland, Scotland and Svalbard all have detrital-zircon populations rich in Palaeo- to Mesoproterozoic ages, which are very similar to those of the Cryogenian–Ediacaran Sparagmite basins. They attest to pan-continental transport of sediments, derived from the combined Grenvillian and Sveconorwegian belts of Laurentia and Baltica, to the margin of the Rodinia supercontinent. Younger immature Cryogenian to Ediacaran sediments exposed in the Lower and Middle Allochthons are interpreted to reflect resumed erosion of the Sveconorwegian belt at the end of the Neoproterozoic, when rifting led to the opening of the Iapetus Ocean. They do not display the distinct enrichment in Archaean detrital zircons recorded in the coeval Dalradian Supergroup of Scotland. This is consistent with an interpretation in which the Sparagmite and Dalradian sequences were deposited on the conjugate margins of Baltica and Laurentia, respectively, during the opening of the Iapetus Ocean.