Ion microprobe U-Pb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland-Kola Orogen, northern Fennoscandian Shield

The Lapland-Kola Orogen (LKO; former Kola craton) in the northern Fennoscan dian Shield comprises a collage of partially reworked late Archaean terrane s with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5 -2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic sepa ration was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, Sm-Nd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these t erranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, w ith negative a,, signatures, contributed relatively little detritus, sugges ting a basin of considerable extent. Subduction of the resulting Lapland-Ko la ocean led to are magmatism dated by the NORDSIM ion probe at c 1.96 Ga i n the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk are took place before c 1.91 Ga as shown by ion probe U-Pb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk are rocks, juvenile metasedim ents as well as Archaean gneisses in the footwall of the orogen. Deep buria l during collision under high-pressure granulite-facies conditions was foll owed by exhumation and cooling between 1.90 and 1.87 Ga based on Sm-Nd, U-P b and Ar-Ar data. Lateral variations in deep crustal velocity and Vp/Vs rat io, together with reflections traversing the entire crust observed in repro cessed seismic data from the Polar Profile, may be interpreted to image a t rans-crustal structure - possibly a fossilised subduction zone - supporting an are origin for the protoliths of the Lapland Granulite, UGT and Tersk t erranes and the location of a major lithospheric suture - the Lapland-Kola suture. (C) 2001 Elsevier Science B.V. All rights reserved.