The effect of mineral age discordancies on total-rock Rb-Sr isochrons of the Heemskirk Granite, western Tasmania

Abstract

The red and white series A divisions of the Heemskirk granite, western Tasmania, have a high internal scatter of total-rock data points about their respective isochrons. Consideration of the mineral phases of these rocks shows that the scatter is due predominantly to open-system behavior of some of the samples, plus a small range in ‘true’ initial Sr87/Sr86 in at least one granite type. The open-system behavior of the mineral phases has a consistent pattern of loss of radiogenic Sr from K-feldspar and gain by plagioclase. Biotite either retains radiogenic Sr or leaks a fractional amount (up to 4%), which has led to a 14-m.y. range in indicated age. The upper limit of the range (367±3 m.y.) is significantly older than the total-rock isochron estimates (red granite 346±8 m.y., white series A granite 352±4 m.y., white series B granite 357±6 m.y.), which suggest that the latter have been biased toward young values by the inclusion of open total-rock systems. The K-feldspar age discordancy relates to the degree of secondary alteration (and triclinicity), which was apparently caused by dynamic stresses associated with faulting 80 m.y. after granite formation. The youngest chlorite age of 296 m.y. supports the existence of this event, which has led by means of the isotopic redistribution response in the mineral phases, to open-system total rocks.