A U-Pb zircon age for the Råna intrusion, N. Norway : new evidence of basic magmatism in the Scandinavian Caledonides in Early Silurian time

Four morphologically distinctive zircon types from quartz-bearing gabbronorite in the Rana intrusion in the Narvik Nappe Complex yield coincident and concordant U-Pb analyses indicating a crystallization age of 437+1-2 Ma. The age of the Rana intrusion, together with other zircon ages from similar layered mafic intrusions and ophiolites in Norway, provide evidence for a distinct and possible short-lived period of back-arc spreading and mafic plutonism in the upper part of the Upper Allochthon and in the Uppermost Allochthon of the Scandinavian Caledonides in Early Silurian time. The intrusion was emplaced during a period of regional crust extension that post-dates the local D1 deformational phase and pre-dates phases D2-D6. The main period of regional metamorphism (and D2-6) in the Narvik Nappe Complex post-dates emplacement of the Rana intrusion, and it is concluded that these latest deformational phases are probably related to the medial Silurian Scandian orogeny and not the Cambrian-Early Ordovician (?) "Finnmarkian' orogeny, as previously supposed