A Strontium Isotopic Investigation of the Bjerkreim—Sokndal Layered Instrusion, Southwest Norway

Abstract

The 230 km2 Proterozoic Bjerkreim—Sokndal layered instrusion has a monzonoritic bulk composition and comprises a 6 km thick, broadly leuconoritic Layered Series (LS) overlain by unlayered mangerite and quartz mangerite. In the Bjerkreim lobe the LS comprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV) in a syncline. This lobe is surrounded by migmatitic gneisses; the roof to the instrusion is missing. The mg-number opx, An % and whole-rock initial 87Sr/86Sr isotope ratios (Sr0) display broadly parallel trends through MCUs II and III, and into MCU IV, with decreasing mg-number (75–58) and An % (50–40) accompanying increasing Sr0 (0.7050−0.7085). This correlation breaks down in the upper part of MCU IV and mangerite; the quartz mangerite has Sr0 values of ∼0.7085. Abrupt reversals occur across the MCU boundaries. The LS crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses. Strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense, primitive ferrobasaltic magma (Sr0 = 0.7049) along the floor and the development of extensive buoyant roof melts. Assimilation took place on a massive scale (up to ∼50%) in the upper parts of the chamber.