A Sr-, Nd- and Pb-isotopic investigation of the transition between two megacyclic units of the Bjerkreim–Sokndal layered intrusion, south Norway

Abstract

The Bjerkreim–Sokndal layered intrusion forms part of the Proterozoic South Rogaland Igneous Complex of southwest Norway. The intrusion has been divided into two parts; an anorthositic to gabbro-noritic Layered Series, and a generally more massive mangeritic, quartz-mangeritic to charnockitic upper part. On the basis of repeated lithological sequences, the Layered Series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment. Initial (940 Ma) Sr, Nd and Pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit III and megacyclic unit IV range from 87 Sr / 86 Sr i =0.7048, 143 Nd / 144 Nd i =0.51156 ( ε Nd=+2.6) and 206 Pb / 204 Pb i =17.50; to 87 Sr / 86 Sr i =0.7061, 143 Nd / 144 Nd i =0.51143 ( ε Nd=+0.1) and 206 Pb / 204 Pb i =17.73. The isotopic compositions vary systematically with stratigraphic height, and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts: replenishment and post-replenishment. Across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma. The replenishing magma was characterized by 87 Sr / 86 Sr i =0.7048, 143 Nd / 144 Nd i =0.51156 ( ε Nd=+2.6) and 206 Pb / 204 Pb i =17.50. The positive ε Nd of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle. In contrast the resident magma had 87 Sr / 86 Sr i =0.7061; 143 Nd / 144 Nd i =0.51143 ( ε Nd=+0.1) and 206 Pb / 204 Pb i =17.73 indicating that it had assimilated crustal material. In addition, the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of Nd and Pb in the resident magma had been strongly reduced, probably by fractionation of apatite. Above the replenishment interval, Nd and Pb isotopic variations are apparently decoupled from Sr isotopic variations. Two mechanisms are proposed for achieving this. The first is heterogeneous mixing of up to 20% of a third magmatic component into an already isotopically stratified magma chamber. The second is the presence of 10%–20% of post-cumulus plagioclase that crystallized from a migrating intercumulus melt.