Geochronology of the Rymmen gabbro, southern Sweden; implications for primary versus inherited zircon in mafic rocks and rheomorphic dykes

Abstract

The age relation between gabbroic rocks and granitoids of the Transscandinavian Igneous Belt (TIB) is based on field observation. Since granitoids often intrude gabbros as dykes, gabbroic rocks have been interpreted as older. However, the existence of rheomorphic granitoid dykes is demonstrated in this paper. Two samples from the Rym-men gabbro; a pegmatitic gabbro located in the central part and a hybrid-gabbro, close to an unexposed contact with surrounding granitoids, yielded zircon. Zircon from the contact zone formed due to contamination of the gabbro with partial melts from the surrounding older granitoid and there is no evidence in the data to support inheritance of zircon from the granitoid. Zircon from both samples yield well-defined Pb-Pb evaporation plateaus, which are identical within error. The Pb-Pb age of 1692±7 Ma (2[sgrave]) is interpreted as the crystallisation age of the Rymmen gabbro. This age and ages of foliated TIB-granitoids located some tens of kilometres away from the Rymmen gabbro, prove that gabbroic and granitoid magmas were coeval. Ion probe data on zircon from a melted granitoid, close to an unexposed contact with the Rymmen gabbro, is reported. These zircon grains consist of two generations, core and rim. The core gave a minimum age of c. 1.77 Ga, coeval with the oldest phase of TIB rocks, and the rim is suggested to have formed at the gabbro emplacement.