Pyroxene (re-)equilibration in the Precambrian terrain of SW Norway between 1030--990 Ma and reinterpretation of events during regional cooling (M3 stage)

Pyroxenes in the Precambrian terrain of SW Norway include metamorphic ortho- and Ca-rich clinopyroxenes and reequilibrated igenous pyroxenes. The compositions indicate a thermal regime of c. 750° to c. 590°C. They belong to a period of regional cooling (M3) in the metamorphic history of the area around
1030-990 Ma. Events during the M3 stage are reinterpreted and indicate that Rb-Sr whole rock and zircon U-Pb isotope systems may have become reset. This may be coeval with the formation of halogenbearing amphiboles. Intrusion of the (Quartz) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion took place between c. 1050-1035 Ma and c. 1030-990 Ma.