The radiometric age of the Mount Keith Granodiorite, a maximum age estimate for an Archaen greenstone sequence in the Yilgarn block, Western Australia

The 'granite' pluton previously shown by Durney (1972) to be unconformably overlain in the Jones Creek area of Western Australia by a boulder conglomerate and a younger greenstone sequence has a Rb-Sr total rock age of 2689 ± 17 Ma. Two boulders analysed from the conglomerate are not distinguished isotopically from the underlying 'granite'. Biotite ages from the 'granite' reflect the final effects of a regional upper greenschist facies metamorphism at approximately 2622 ± 15 Ma. Potassium feldspars in the 'granite' indicate ages less than 2600 Ma and are presumably related to late stage tectonic adjustments. Thus the isotopic dating shows that the 'granite', now formally named the Mount Keith Granodiorite, belongs to the early part of the 2750-2600 Ma Yilgarn age group (Compston and Arriens, 1968) and places an older limit on the age of the upper greenstone sequence. This greenstone was laid down within a period of 70 Ma or less and was followed by metamorphism which had ended by 2622 Ma, the time of closure of the biotite Rb-Sr system. Details of the petrography, particularly the metamorphic textures and paragenesis, indicate that despite their externally massive appearance, all the samples except the pegmatite have been recrystallized to varying degrees. Major-element analyses are given also, to aid in the chemical classification of the Granodiorite and in understanding its petrogenesis.