A U-Pb zircon age for the Råna intrusion, N. Norway: New evidence of basic magmatism in the Scandinavian Caledonides in Early Silurian time
pp. 229-239

Four morphologically distinctive zircon types from quartz-bearing gabbronorite in the Råna intrusion in the Narvik Nappe Complex yield coincident and concordant U-Pb analyses indicating a crystallization age of 437:':1 Ma. The intrusion, which consists of ultramafic and mafic cumulates derived from a highMgO tholeiitic parental magma, was emplaced into argillaceous sediments within a probable marginal basin or back-are setting prior to its tectonic emplacement onto the Baltoscandian Shield. The age of the Råna intrusion, together with other zircon ages from similar layered mafic intrusions and ophiolites in Norway, provide evidence for a distinct and possibly short-lived period of back-are 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 crustal extension that post-dates the local D1 deformational phase and pre-dates phases DrD6• The main period of regional metamorphism (and DH) in the Narvik Nappe Complex post-dates emplacement of the Råna intrusion, and it is concluded that these latest deformational phases are probably related to the media! Silurian Scandian orogeny and not the Cambrian-Early Ordovician (?) 'Finnmarkian' orogeny, as previously supposed.