Timing and characterization of recurrent pre-Sveconorwegian metamorphism and deformation in the Varberg-Halmstad region of SW Sweden

The Atran terrane of southwestern Sweden is a consequence of westward crustal growth away from an Archean core and records both Gothian (c. 1.75-1.55 Ga) and c. 1.0 Ga Sveconorwegian (Grenvillian) tectonism. In spite of widespread consensus on the first-order aspects of this westward growth model, there is a current debate as to whether the bulk of the deformation and migmatization in the Atran terrane should be attributed to Gothian or Sveconorwegian events. U-Pb zircon, titanite, monazite and rutile ages from gneiss samples and three tectonostratigraphic groups of dykes in the Varberg-Halmstad region constrain the timing of major tectonothermal events. A felsic orthogneiss yields a protolith age of 1664 +/- 7 Ma. Mafic dykes of a first dyke group (Steninge dykes) intruded after the first recognized deformation (DI) and were subsequently metamorphosed at 1654 +/- 9 Ma. The Steninge dykes constrain DI to the 1664-1654 Ma interval and D2 at 1654 +/- 9 Ma, requiring the D1/D2 gneissosity to have formed during Gothian orogenesis. Steninge dykes and migmatitic D1/D2 structures in the host gneisses are cross-cut by less deformed dykes of a second dyke group, one regional granitic suite dated at 1426 + 9/-4 Ma and a pegmatite swarm at 1399 + 7/-6 Ma. The second group of dykes therefore confirms a pre-1.43 Ga age for the main D1/D2 gneiss-forming event(s) consistent with previous models of regional Gothian deformation in this terrane. Group 2 magmatism also includes the Varberg Charnockite-Granite Association, from which a sample of charnockite is now dated at 1399 + 12/-10 Ma. In addition, metamorphism at 1438 + 12/-8 Ma resulted in new zircon growth in a mafic gneiss. Together, the Group 2 magmatic rocks and metamorphic zircons represent an inter-orogenic, 1.44-1.38 Ga thermo-magmatic episode in the Varberg-Halmstad region that has c. 1.4 Ga equivalents elsewhere in southern Sweden. Previous models of high heat flow and comprehensive metamorphism during a thermomagmatic Hallandian event at c, 1.4 Ga is thus supported. Sveconorwegian deformation and recrystallization (D3-D4) occurred before 946 + 6/-4 Ma, as constrained by a post-kinematic third group of dykes. U-Pb ages for monazite (948 +/- 9 Ma), titanite (935 +/- 7 Ma and 932 +/- 5 Ma) and rutile (878+/-9 Ma) reflect Sveconorwegian cooling after peak metamorphism. During this stage, U-Pb ages and closure temperatures suggest slow cooling rates of 5-11 degrees C/m.y. from 948 to 932 Ma and 2.5-5 degrees C/m.y. from 932 to 878 Ma, attributed to late-stage erosion and isostatic uplift. The lack of regional penetrative migmitization during the Sveconorwegian granulite conditions is consistent with metamorphism of previously dehydrated rocks (during Gothian and Hallandian times) during eastward Sveconorwegian thrusting of western segments over the Atran terrane. (C) 1999 Elsevier Science B.V. All rights reserved.