Lead isotopic systematics and ages of Archaean acid intrusives in the Kalgoorlie-Norseman area, Western Australia
Lead-lead mineral isochrons consisting of K-feldspar + plagioclase + total rock have been determined for eight intrusive bodies in the Kalgoorlie-Norseman area of Western Australia. The ages found range from 2630 to 2760 m.y. Variation of initial lead isotopic composition within plutons has been identified in two cases. For the remaining plutons, whole rock lead-lead isochrons give the same age as the mineral isochrons. Attempts to determine uranium-lead ages reveal large-scale loss of uranium from surface rocks in recent times. The initial lead isotopic composition of the intrusions, as estimated from age corrected K-feldspar data, shows that the source regions for the plutonic magmas must have experienced multistage histories prior to magma generation. The time integrated μ value for the source region in all cases exceeds the mean μ value for the Earth. Multistage model calculations show that continental type rocks must have been present in the Norseman area as early as 3300 m.y. ago. Original formation of continental crust near Kalgoorlie either occurred much later than at Norseman, or consisted of more basic rock types.