Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0–1.8 Ga accretionary orogen, Sweden

Abstract

Absolute ages of migmatization and protolith formation, and constraints on the timing of ductile deformation in two major lithotectonic units in the south-western part of a 2.0–1.8Ga orogenic belt in the Fennoscandian Shield, south-eastern Sweden, have been determined using U–Pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite. Detrital and xenocrystic zircon in paragneiss and garnet-bearing leucogranite, respectively, in the Bergslagen lithotectonic unit show ages of 2.1–2.0Ga and 1.9Ga. Deposition of the sedimentary material occurred during or after a subduction-related magmatic event at 1.91–1.87Ga. Two orthogneiss protoliths formed during this magmatic event around 1.88Ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1.85Ga igneous protolith. A polyphase tectonothermal evolution with anatexis under low-P metamorphic conditions around 1.86Ga (M1) and 1.84–1.81Ga (M2) is inferred for the migmatitic gneisses in the Bergslagen unit; garnet-bearing leucogranite crystallised around 1.84–1.83Ga, close in time to major folding of the M1 gneissic fabric. A previously unrecognised 1.86–1.85Ga ductile deformational event under medium-grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south (Småland lithotectonic unit), close in time to the M1 event in the Bergslagen unit to the north.By constraining the timing of anatexis and comparing with information bearing on crustal thickness, excess mass at depth and the character and age of magmatic activity, it is inferred that anatexis in the Bergslagen lithotectonic unit is related to pulses of mafic underplating, during the early stages of two separate, subduction-related magmatic episodes after the 1.91–1.87Ga magmatic event. It is suggested that each pulse was related to intra- or back-arc spreading above a subduction boundary, which had entered a retreating mode with separate, long periods (20–50Ma) of extension or transtension. This study challenges the need to invoke crustal thickening related to plate collision at 1.9–1.8Ga as a mechanism to explain high-grade metamorphism in the southern part of the 2.0–1.8Ga orogen. Instead, a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred.