Rubidium‐strontium age determinations on micas from a geologically controlled, composite batholith

Thirty biotite and two muscovite ages are reported from thirteen plutons in the Berridale Batholith, a thoroughly mapped composite batholith in southeastern Australia. Middle Ordovician country rocks place an older limit on the age of the batholith. The mica ages of samples from any one pluton agree within experimental error except in a few cases where later plutons caused metamorphic resetting. Significant age differences have been found between plutons. The mica ages are consistent with the mapped intrusive sequence and range between 420 m.y. and 429 m.y., which indicates cooling of the intrusives to the biotite blocking temperature of about 300°C during the Late Silurian and Early Devonian. The agreement between the muscovite and biotite ages and also one age from a total rock with high Rb/Sr suggests that this cooling interval was short.