1.86–1.85 Ga intrusive ages of K-feldspar megacryst-bearing granites in the type area of the Revsund granites in Jämtland County, central Sweden

Abstract

The type area of the coarse-grained, K-feldspar megacrystbearing Revsund granites is located in Jämtland County. These granites cover an area of c. 6.000 km² in the western part of northern Sweden, and their emplacement ages have been established to c. 1.80 Ga from granites located outside and at the margin of the type area. Some of the rocks originally classified as Revsund granite in the southeastern part of Jämtland have later been interpreted to belong to the early Svecofennian suite. U-Pb SIMS analyses of zircon from one such rock and two Revsund granites from the type area, yield indistinguishable ages: 1854±8, 1858±9, and 1859±11 Ma, respectively. These ages are considerably older than the established age, suggesting that there either are two magmatic events forming the Revsund granites or that further K-feldspar megacryst-bearing granites in the type area must be re-classified. The ages achieved partly overlap with the ages of the 1.84–1.85 Ga, Kfeldspar megacryst-bearing Ljusdal granitoids located to the southeast. However, these rocks are metamorphosed and penetratively deformed, and have thus experienced a different tectonometamorphic evolution than have the Revsund granites in Jämtland, which are mainly isotropic outside shear zones.

Key Words:

• Palaeoproterozoic
• Svecofennian
• U-Pb
• SIMS
• zircon
• Revsund granite
• Jämtland
• Sweden