The Baltica‐Laurentia Connection: Sveconorwegian (Grenvillian) Metamorphism, Cooling, and Unroofing in the Bamble Sector, Norway

Abstract

The Sveconorwegian Bamble Sector of south Norway is a segment of the high‐grade metamorphic basement of the Baltic shield generally considered an integral piece of the global Grenvillian orogenic belt. In particular, the Bamble Sector is generally directly linked to the Grenville Province of North America in middle Proterozoic tectonic reconstructions. However, few high‐precision data exist on the metamorphic chronology of the Bamble Sector to test such models. In this paper high precision U‐Pb data for monazite, allanite, and titanite indicate that rocks from the Bamble Sector near Arendal underwent granulite facies metamorphism at ∼1140 Ma. A second phase of metamorphism may be indicated by U‐Pb titanite ages of ∼1105 Ma, or alternatively these may simply record delayed cooling or growth of titanite below temperatures for significant lead diffusion. Hornblende 40Ar/39Ar ages indicate cooling of the same rocks below ∼550°C between 1090 and 1080 Ma. Metamorphic cooling rates of ∼3°–8°C/Ma between ∼725° and ∼550°C, and ∼2°–4°C between ∼550°C and ∼300°C are consistent with published models for the retrograde P‐T path. Calculated unroofing rates of 0.5 to 1.0 mm/a from depths of ∼30 km to ∼15 km may indicate relatively fast initial unroofing of the Bamble metamorphic basement. While slow relative to active orogens like the Alps or Himalaya, such cooling and unroofing rates represent the fastest yet determined for regional metamorphic basement of Grenvillian age. The timing of metamorphism and cooling in the Bamble Sector, when compared to the limited data available for elsewhere in the Baltic shield, indicate that Sveconorwegian metamorphism may not have been synchronous throughout the Baltic shield. However, these data compare favorably with the timing of metamorphism and metamorphic cooling in the Grenville Province of North America, providing strong evidence for tectonic models directly linking Baltica to Laurentia during the middle Proterozoic Sveconorwegian/Grenvillian orogeny.