A detailed analysis has been made of the Rb-Sr geochronology of an area of regional metamorpnism at Cooma, N.S.W. The area contains a central granite mass surrounded by meta-morphic zones decreasing radially in metamorphic grade. Rb–Sr measurements on total-rock and mineral samples define a high uniform Sr87/Sr86 ratio, 0·7179±0·0005, in the granite at 415±12 m.yrs. This indicates either the inheritance of radiogenic Sr, if the 415 m.yr age represents the time of formation of the granite, or else a re-arrangement of radiogenic Sr within the granite after its formation. The values for the indicated age and initial Sr87/Sr86 of the high-grade metasediments are very close to those of the granite, suggesting a genetic relationship between the two. In contrast to this, the more distant greenschist facies metasediments have a significantly greater age, 460±11 m.yrs, and a lower initial Sr87/Sr86, 0·710±0·002. By considering the movement of strontium isotopes necessary to produce the linear correlations between Sr87/Sr86 and Rb87/Sr86 observed in the various samples, and the restrictions to such movement apparently set by geological evidence, it is concluded that no extensive rearrangement of isotopes occurred at Cooma later than the high-grade metamorphism. The high initial Sr87/Sr86 in the granite is therefore regarded as an original feature. The age and Sr/Rb ratios of the adjacent metasedimentary rocks yield a range in the Sr87/ Sr86 values at the times of granite formation whose mean value would approximate to the primary value observed in the granite. Production of the granite by anatexis of these sediments is, therefore, quite compatible with its particular Sr87 abundance.
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Pidgeon et al. (1965) studied this question.