An evaluation of the age of high-grade metamorphism in the Caledonides of Biskayerhalv⊘ya, NW Svalbard

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Abstract
New geochronological data on eclogite-bearing gneisses of the Richarddalen Complex from Biskayerhalv⊘ya in the Caledonides of NW Svalbard indicate these rocks were strongly recrystallized during a major Ordovician (c. 455 Ma) metamorphic and deformational event of high amphibolite or higher grade. This event was not recognized in previous studies, which identified igneous protoliths of the Richarddalen Complex as Middle and latest Proterozoic in age and provided evidence suggesting that high-pressure eclogitic metamorphism was latest Proterozoic in age. New zircon 207Pb/206Pb ages indicate that the c. 650 Ma igneous crystallization ages previously identified in felsic dikes (agmatite) associated with the eclogites are widespread in mafic and felsic gneisses of the Richarddalen Complex. The results of this study do not rule out the possibility of a latest Proterozoic to Cambrian high P and T metamorphism, previously inferred from a zircon age from an eclogite. However, the more widespread occurrence of c. 650 Ma prismatic zircons of igneous appearance in amphibolitic (many retroeclogite) and felsic gneisses associated with the eclogites suggests that the zircon ages reflect a contribution from latest Proterozoic magmas rather than zircon growth during eclogite metamorphism. The igneous appearance of the zircons and the field occurrence of now-eclogitized mafic and felsic rocks as dikes favor the interpretation that the mafic and felsic magmas were emplaced during a late Proterozoic extensional (rift-related) event. Latest Proterozoic rift magmatism and a superimposed c. 455 Ma (early Caledonian) tectonothermal event are within the range of ages recognized elsewhere in western Svalbard and in Laurentia for Iapetus rifting and early convergent phases leading to Iapetus closure.

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