STRONTIUM ISOTOPE STUDIES RELATED TO PETROGENESIS IN CALEDONIAN BASIC IGNEOUS PROVINCE OF NE SCOTLAND

Rubidium-strontium isotope analyses are reported for 90 whole-rocks from the Newer Gabbros and associated rocks of NE. Scotland. The Sr87/Sr86 ratio of the regional gabbro magma at Insch and Belhelvie underwent a progressive increase, in the former case from 0·703 to 0·712 during differentiation to syenogabbros and syenites. This is ascribed to a process of isotopic equilibration with country rocks of the Dalradian Series. The wider implications of a process by which the Sr87/Sr86 ratio of a basic magma can be increased in the crust without any other signs of petrological contamination are considered. Cordierite-norites and related rocks of the Haddo House and Arnage masses have initial Sr87sol;Sr86 ratios of 0·720-0·730, essentially the same range as for the country rocks at the time of intrusion. This is interpreted as support for the hypothesis that such rocks were formed by the partial fusion of pelitic sediments adjacent to the basic igneous sheet. © 1969 Oxford University Press.