Isabel Sophie Masek Carter: Petrochronology of the Köli Nappe Complex: Investigating the tectonic evolution of Iapetus oceanic terranes in the Scandinavian Caledonides

Abstract

The Köli Nappe Complex (KNC) in the Scandinavian Caledonides originated as terranes within the Iapetus Ocean, derived from subduction-related magmatic and basin systems. These terranes have characteristics of magmatic arcs and associated forearc or back-arc basins and underwent several periods of rifting and magmatism prior to their accretion to the Baltican margin. This thesis aimed to investigate the tectonometamorphic history of these terranes, using a range of modern petrochronological and geochronological techniques to reconstruct their magmatic and metamorphic evolution through time.  The results of the study shed new light on the early development of the Caledonian Orogenic Belt in Northern Scandinavia during late Proterozoic to early Palaeozoic times, and thus provide insights into the evolution of oceanic terranes within orogenic belts globally. 

In the Lower Kӧli Nappes in Västerbotten, Sweden, U–Pb zircon ages of 512 ± 3.5, 497 ± 2, 491 ± 1 and 488 ± 4 Ma were determined from dacitic to andesitic metavolcanites. The three younger ages fit with previous magmatic ages for Lower Köli volcanic rocks, but the 512 Ma age is older than any previous age for this unit. These dates constrain the age of magmatism in an island arc system within Iapetus. Magmatic ages within the KNC overlap with ages for an early episode of ultrahigh-pressure (UHP) metamorphism within the underlying Seve Nappe Complex (SNC), supporting the hypothesis that attributes UHP metamorphism within the SNC to subduction beneath the island arc now preserved within the Lower Köli Nappes. At least part of the arc system must have been close to Baltica at an early stage, which is supported by similarity of the Ordovician rock sequences of both. 

In the Tännfors Synform, Jämtland, Sweden, Lu-Hf dating of garnet from sequences of calc-pelitic schists (equivalent to the Lower and Middle Kӧli Nappes) resulted in garnet-whole rock isochrons defining ages of  433.5 ± 2.7 Ma and 427.8 ± 4.7 Ma. Garnet chemical zoning profiles are smooth and display normal zoning, so it is likely that garnet growth occurred in a single generation. Garnet growth within individual samples occurred in a period of less than 10 Ma. The ages are similar to those found by previous studies for the Lower Kӧli Nappes and Seve-Kӧli boundary, relating to the early stages of assembly of the Caledonian orogenic wedge. Metamorphism in the Tännfors Synform is therefore likely to related to shearing and nappe stacking, associated with assembly of the orogenic wedge in the early Scandian collision prior to its final translation and imbrication.

The Krutfjellet Nappe in Västerbotten, Sweden and the Gasak Nappe in Nordland, Norway are both part of the Upper KNC. In both of these units, siliciclastic, carbonate and volcanic protoliths underwent amphibolite facies metamorphism (involving in places extensive migmatisation) which was of a distinctly higher grade than the Lower and Middle KNC. In particular, the migmatites had received little previous study, and their age and origins were uncertain. Monazites in pelitic schists from the Sulitjelma area of the Gasak Nappe were dated in-situ using LA-ICP-MS, yielding concordia ages of 427-428 Ma.  Monazites in migmatites from the Norra Storfjallet Lens, Krutfjellet Nappe produced Th-U-Pb concordia ages of between 428-424 Ma. The monazites yielded a large number of discordant analyses which fall on discordia lines at high angles to the concordia curve, interpreted to be due to the presence of an initial Pb component in the monazites. The monazites from the Krutfjellet Nappe often have complex zoning patterns in Y and REEs, however this zoning appears to be decoupled from the Th-U-Pb isotopic systems. This suggests that the monazites experienced resetting via dissolution-reprecipitation reactions, leading to selective redistribution of elements. 

Migmatites from the Krutfjellet Nappe have leucosomes with an unusual trondhjemitic composition, and were likely formed primarily via water-fluxed melting reactions. Garnet cores from migmatites gave peak P-T conditions of c. 680-730°C and 6-8 kb. Garnet rims gave conditions of c. 560-630°C and 7-10 kb, with approximately isobaric cooling from peak P-T conditions. These results produce a segment of an apparently anticlockwise P-T path. The formation of this P-T path may be caused due to increased heat flux from emplacement of nearby gabbroic intrusions leading to partial melting, followed by cooling occurring simultaneously with nappe stacking in the Scandian. Similar anticlockwise P-T path segments were also produced for two samples from the Gasak Nappe, in this case the isobaric cooling path is thought to be a local effect related to a nearby granitic intrusion. The Upper Kӧli Nappe appears to have experienced fluid fluxing during metamorphism, postdating but likely connected to emplacement of large igneous intrusions within the region around 445-435 Ma. Migmatisation may be associated with enhanced heat flow from rift- related magmatism, but the age record has been severely overprinted during subsequent Scandian orogenic wedge assembly and nappe emplacement.