A key fold structure within a Sveconorwegian eclogite-bearing deformation zone in Halland, south-western Sweden: geometry and tectonic implications Brendan Dyck Masters thesis in Geology at Lund University, no. 279 (45 hskp/ECTS) Department of Earth- and Ecosystem Sciences Division of Geology Lund University 2011 A key fold structure within a Sveconorwegian eclogite-bearing deformation zone in Halland, south-western Sweden: geometry and tectonic implications Master Thesis Brendan Dyck Department of Earth and Ecosystem Sciences Division of Geology Lund University 2011 Contents 1 Introduction ............................................................................................................................................................... 1 2 Geological setting ........................................................................................................................................................ 2 3 Methods ....................................................................................................................................................................... 3 3.1 Mapping and structural analysis 5 3.2 Petrography, Scanning Electron Microscope-Energy Dispersion Spectroscopy (EDS) and P-T-calculations 5 3.3 U-Pb zircon analysis 6 4 Results.......................................................................................................................................................................... 6 4.1 Rock types/lithologies in the Ätran area 6 4.1.1 Grey gneiss (Gällared type) 6 4.1.2 Leucocratic gneiss 6 4.1.3 Porphyritic granite-augen gneiss (Tjärnesjö type) 7 4.1.3.1 Sample (26-0) porphyritic granite 7 4.1.3.2 Sample (26-1) augen gneiss 7 4.1.3.3 Sample (26-4) migmatized gneiss 7 4.1.4 Retro-eclogite 8 4.1.5 Garnet-rich gneiss 16 4.1.6 Migmatization 16 4.2 Metamorphic assemblages and P-T conditions 17 4.2.1 Eclogite facies 17 4.2.2 Medium-pressure granulite facies 17 4.2.3 Greenschist facies 17 4.3 P-T estimation using TWQ 19 4.4 Structure 22 4.4.1 Structural map and the Ätran closure 22 4.4.2 Folding 25 4.4.3 Fabrics 25 4.4.4 Shearing spatially associated with folding 26 4.4.4 Sense of shear in the bulk rock mass 26 4.4.5 U-Pb zircon analysis 27 4.5 Zircon geochronology 27 4.5.1 Sample descriptions and isotopic results 27 4.5.1.1 Undeformed granite 27 4.5.1.2 Deformed granite 27 4.5.1.3 Migmatized granite 27 4.5.1.4 Grey gneiss 28 4.5.1.5 Pegmatite 28 5 Interpretations ......................................................................................................................................................... 33 5.1 Metamorphism 33 5.1.1 1.39 Ga granite 33 5.1.2 Stabilities of mineral assemblages 35 5.2 Structure and strain 35 5.2.1 Heterogeneous strain distribution 35 5.2.2 Shearing and folding relations 35 5.2.3 Strain model 35 5.2.4 Tectonostratigraphic marker 36 5.3 Geochronology 37 6 Tectonic interpretation and discussion ................................................................................................................... 37 6.1 Suggestions for future studies 39 7 Conclusions ............................................................................................................................................................... 39 8 Acknowledgments ..................................................................................................................................................... 40 9 References ................................................................................................................................................................. 40 2 A key fold structure within a Sveconorwegian eclogite-bearing deformation zone in Halland, south-western Sweden: geometry and tectonic implications BRENDAN DYCK Dyck, B., 2011: A key fold structure within a Sveconorwegian eclogite-bearing deformation zone in Halland, south -western Sweden: geometry and tectonic implications. M.Sc. Thesis in geology at Lund University, Nr. xxx, 42 pp. 45 hskp/ECTS. Abstract: Retro-eclogites and associated high-pressure rocks are found in the Ullared area, southwest Sweden. These rocks are evidence of the high-pressure deformation which occurred during the formation of the c. 500 km long Sveconorwegian orogenic belt around one billion years ago. Their presence is restricted to a domain north of the towns Ätran, Gällared and Ullared. Detailed structural, petrographic and geochronologic studies of the Ätran area were made with the aim of further understanding the regional structure and metamorphic history, with particular focus on a fold closure sug- gested by airborne magnetic anomaly maps. A cylindrical inclined south-vergent isoclinal fold with fold axis paral- lel stretching lineations forms a c. 4 km wide fold closure around the town of Ätran. The southern demarcation of the Ätran closure is a lithotectonic boundary where eclogites are restricted to the area north of the boundary. U-Pb zircon SIMS geochronology of a granitic meta-intrusion (Ätran granite) found just south of the Ätran closure yields an igneous intrusive age of 1388±7 Ma and a Sveconorwegian migmatization & amphibolitization age of 955±15 Ma. The intrusive age of the Ätran granite is coeval to those of the nearby Torpa and Tjärnesjö granitic meta- intrusions, which are now recognized as tectonostratigraphic markers to the aforementioned lithotectonic boundary. P-T estimates for microdomains with biotite+hornblende+garnet indicate post-eclogite re-equilibrium at conditions of 8.5±1.1 kbars and 690±50°C. Thermodynamic modelling using Domino-Theriak software, combined with petrological data from samples within the Ätran area suggest decompression from a peak metamorphic pressu- re of c. 17 kbars. Pseudosection calculations describe a c. 2 kbar (6 km Δ depth) zone where eclogite assemblages are stable in mafic and not felsic rocks. The recognition of this eclogite facies variable zone serves as a plausible explanation for the observation of eclogite mineral relicts in the mafic but not the felsic rocks found north of the lithotectonic boundary. The parallelism of the fold axes and stretching lineations of the cylindrical folding near Ätran prompt a wrench shear component in the regional strain model. Following the regional strain model, the Ätran fold is inter- preted as the lateral tip of a proposed ≥50 km wide fold nappe that propagated at least 75 km into the Eastern Seg- ment. A probable tectonic scenario for the formation of this fold nappe and related regional deformation is a Sveco- norwegian late-orogenic deformation with a present day E-W shortening. Keywords: Sveconorwegian, fold nappe, eclogite, Ätran area, U-Pb geochronology, Domino-Theriak Brendan Dyck, Division of Geology, Department of Earth and Ecosystem Sciences, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden. E-mail: [email protected] 3 1. Introduction al, 2001). This orogen has tectonic counterparts around the world, including the Grenville Orogen in Canada The Sveconorwegian Orogen in southwestern and USA. The Sveconorwegian Orogen is composed Scandinavia exposes deep sections of a mountain cha- of metamorphosed and deformed, Proterozoic intrusi- in that formed by continent-continent collision one ve and supracrustal rocks and resulted from a collision billion years ago. It is the tectonic counterpart to the between Baltica and at least one other major continent. Grenville Orogen in North America and was built du- One tectonic interpretation suggests Amazonia is the ring the formation of Supercontinent Rodinia. The other colliding continent, but subsequent evidence is southern parts of the Eastern Segment (counties still lacking to confirm this theory (Hoffman, 1991). Halland and Västergötland), host remnants of very The Sveconorwegian Orogen has been divided into high-pressure rocks – eclogites - that formed by deep five distinct lithotectonic units (Bingen et al, 2005). crustal metamorphism around 0.97 Ga. These units are all N-S-trending and are separated by Field data (Möller and Andersson, unpublished) deeply rooted deformation zones (Park et al, 1991). suggest that the eclogites are bound to a specific struc- The primary lithotectonic units from west to east are; tural domain within the Eastern Segment; one example the Telemarkia terrane, the Kongsberg terrane, the of this is that eclogites are lacking east of Ätran, cen- Bamble terrane, the Idefjorden terrane and the Eastern tral Halland. The ―disapperance‖ of eclogites east of Segment (figure 1). The foreland of the Sveconorwegi- Ätran coincides with an apparent fold structure shown an orogenic belt to the east, in Sweden and Finland, is by the airborne magnetic anomaly map. The present made up of Paleoproterozoic rocks predominantly af- study was initiated in order to investigate the field re- fected by orogenic activity between 1.9 and 1.8 Ga; lations and structural geology of the Ätran area, with more or less undeformed rocks that formed around and particular focus on the suggested fold closure. after 1.7 Ga are also present. Geochronological data Objectives: from a multitude of samples taken in the past forty years confines the Sveconorwegian orogeny to 1140- To document the structure and deformation 900 Ma (Bingen et al, 2008). The ages interpreted as fabrics in a presumed fold hinge in a part of the peak metamorphic ages youngs from c. 1140 Ma in the eclogite-bearing Ullared Deformation Zone western units to c. 970 Ma in the Eastern Segment To document the deformation fabrics and kine- (Johansson et al, 2001; Bingen et