Implications of the emplacement age of the Kläppsjö gabbro; a mafic layered intrusion in the Bothnian basin, north central Sweden

Abstract

The 6X3.5 km large Kläppsjö gabbro in the county of Västernorrland is a tholeiitic mafic layered intrusion that was emplaced into metaturbidites of the Bothnian basin, and is partly bordered by early- and late-orogenic Svecofennian granitoids. The intrusion consists of ultramafic, gabbroic, leucogabbroic and ferrogabbroic mega-units, tens to hundreds of metres thick forming a c. 3 km thick layered sequence. The gabbro is exposed in cross section, with a sub-vertical igneous layering that is sub-parallel to the bedding in the surrounding metasediments. U-Pb SIMS zircon dating yields an emplacement age of 1877±5 Ma, and the gabbro is thus contemporaneous with 1.88–1.86 Ga early-orogenic Svecofennian magmatism in the Bothnian basin. This includes a tonalite bordering on the gabbro here dated to 1870±10 Ma. Extension related magmatism of this age characterises major parts of the Svecofennian domain, and the emplacement of the Kläppsjö gabbro was probably related to extension in the Bothnian basin.

Key Words:

• ion-probe dating
• zircon
• layered intrusion
• Svecokarelian
• Svecofennian
• Bothnian basin
• Fennoscandian shield