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The nappe pile of the Scandinavian Caledonides contains thrust sheets of crustal fragments derived from the western margin of Baltica, or from microcontinents within the Iapetus Ocean or possibly continents being exotic relative to Baltica. The Kvitvola Nappe Complex of the Middle Allochthon of the Caledonides in South Norway is thrust some hundreds of kilometers from west to east and is dominated by several thousand metres thick Neoproterozoic sandstones. One thrust sheet in the nappe complex, far west in the outcrop area of the nappe complex in South Norway, consists of granitic gneiss, overlain by the Rosten Formation.
The Rosten Formation consists of a strongly tectonized sandstone and matrix-supported conglomerate with scattered boulders of granite, foliated granite and gneiss, deposited on a mylonitic gneiss basement. The textural properties of the clasts reveal that they were derived from a granitic lithosome that had been deformed before erosion, deposition and Caledonian thrusting and deformation of the Rosten Formation. Our hypothesis is that the Rosten Formation represents an alluvial or marine conglomerate formed along a fault escarpment at the western rifted margin of Neoproterozoic Baltica. The purpose of this study is to map out the chronological variety and petrogenetic history of basement and conglomerate deposit of this particular thrust sheet in order to uncover the identity and history of rocks at the outer reaches of continent Baltica before the final breakup of Rodinia and formation of the Iapetus Ocean at the end of Neoproterozoic time.
Zircons analysed for U-Pb and Lu-Hf by laser-ablation multi-collector mass spectrometry gave the depositional basement an early Gothian concordia age of 1620±9Ma and early Sveconorwegian concordia ages (1177±6Ma, 1197±12Ma and 1205±12Ma) for three granitic clasts. The Hf-isotopic signature of the basement can be explained by remelting of crust similar in Hf-composition to the ca. 1650-1710 Ma Transscandinavian Igneous Belt (TIB2+3) with addition of juvenile input from the mantle. Renewed melting of the basement with similar juvenile input can explain the Hf-signature in clasts of early Sveconorwegian deformed granite. The results give new data points on the pre-Caledonian western extent of Mid- to early Neoproterozoic crustal blocks of Neoproterozoic Baltica.
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