A 1,220�60 M.Y. Rb-Sr isochron age representing a Taylor-convection caused recrystallization event in a granitic rock suite

Abstract

In the Tisselskog area (N. Dalsland), in the eastern, external part of the Grenvillian belt in N. Europe, two formations of granitic rocks form the basement for the basal sediments of the Grenvillian, Dalslandian Group. Basement and cover went through a low grade regional metamorphism at 1,030±40 m.y. (2σ) ago. The granitic rocks are strongly recrystallized, but have preserved their granitic texture. Most magmatic crystals are pseudomorphed by a suite of metamorphic crystals of quartz, albite, chlorite, white mica, epidote, titanite, hematite, pyrite and carbonate. It is remarkable that these rocks with a perfect magmatic appearance have lost so much of their magmatic mineralogy. The older of the two intrusions, the Ballsjon Granodiorite Formation, yields a 7 point, 1,220±60 m. y. Rb-Sr isochron (initial Sr isotope ratio=0.7048±0.0004, λ87Rb=l.42×10−11·y−1). This age is interpreted as representing the pervasive recrystallization in the rocks, which was induced by a hydrothermal convective circulation system — a Taylor convection — set up by the intrusion itself or by the younger Tisselskog Leucogranite Formation which intrudes it. In dating a phase of pervasive hydrothermal resetting by whole rock Rb-Sr dating one should try to strike a balance between maximal Rb/Sr variation and maximal similarity in the starting mineralogy in the rock samples, the latter aiming at geochemical coherence during the hydrothermal alteration.