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

The Bjerkreim-Sokndal layered intrusion forms part of the Proterozoic South Rogaland Igneous Complex of southwest Norway. The intrusion has been divid ed 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 h as been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment. Initial (940 Ma) Sr, Nd and Pb isot ope ratios for plagioclases separated from samples crossing the transition between megacyclic unit III and megacyclic unit TV range from Sr-87/Sr-86(i ) = 0.7048, Nd-143/Nd-144(i) = 0.51156 (epsilon(Nd) = +2.6) and Pb-206/Pb-2 04(i) = 17.50; to Sr-87/Sr-86(i) = 0.7061, Nd-143/Nd-144(i) = 0.51143 (epsi lon(Nd) = +0.1) and Pb-206/Pb-204(i) =17.73. The isotopic compositions vary systematically with stratigraphic height, and the interplay between the th ree isotopic systems is used to divide the stratigraphic profile into two p arts: replenishment and post-replenishment, Across the replenishment interv al isotopic variations are consistent with mixing between a resident and a replenishing magma. The replenishing magma was characterized by Sr-87/Sr-86 (i) = 0.7048, Nd-143/Nd-144(i) = 0.51156 (epsilon(Nd) = +2.6) and Pb-206/Pb -204(i) = 17.50, The positive epsilon(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 Sr-87/(86)Sri = 0.7061; Nd-1 43/Nd-144(i) = 0.51143 (epsilon(Nd) = +0.1) and Pb-206/Pb-204(i) = 17.73 in dicating that it had assimilated crustal material. In addition, the isotopi c correlations across the replenishment interval indicate that prior to mix ing the concentrations of Nd and Pb in the resident magma had been strongly reduced, probably by fractionation of apatite. Above the replenishment int erval, Nd and Pb isotopic variations are apparently decoupled from Sr isoto pic variations. Two mechanisms are proposed for achieving this. The first i s heterogeneous mixing of up to 20% of a third magmatic component into an a lready isotopically stratified magma chamber. The second is the presence of 10%-20% of post-cumulus plagioclase that crystallized from a migrating int ercumulus melt. (C) 2000 Elsevier Science B.V. All rights reserved.