The relation of discordant Rb-Sr mineral and whole rock ages in an igneous rock to its time of crystallization and to the time of subsequent Sr 87/Sr 86 metamorphism
Interpretation of discordant Rb - Sr ages of coexisting biotite and K-feldspar in igneous rocks, mostly from Sudbury, Ontario, has been attempted using supplementary whole-rock ages. Following a model proposed by COMPSTON and JEFFERY, it is postulated that, if the igneous body is a closed system, and a post-crystallization thermal event interrupts the accumulation of Sr 87 in biotite and K-feldspar, the whole-rock analysis will give the true age and, owing to diffusion of radiogenic Sr out of biotite and K-feldspar, the apparent ages of these two minerals would be less than the whole-rock age. The common intersection of the three radiogenic growth lines (Sr 87/Sr 86 plotted against age) gives the time of metamorphism. For the majority of the twelve examples the model offers an apparently valid explanation of the discrepant ages in terms of known field relations and two orogenic events at 1·2 billion years and 1·6 billion years.