Abstract

Two contrasting types of Palaeozoic granitoids are of widespread occurrence in SE Australia and can generally be distinguished by their chemistry, mineralogy, field relations and initial strontium isotope ratios. Chappell & White (1974) have proposed that the granitoids are derived by partial melting of two different types of source materials: (1) igneous or ‘I-type,’ and (2) sedimentary or ‘S-type.’

Oxygen and hydrogen isotope compositions were measured on 63 whole-rock samples of both fresh and altered granitoids and xenoliths. Analyses were also made of representative mineral separates from these rocks. For ‘S-type’ plutons, average δ18O values range from 9.9 to10.5, whereas for the ‘I-types’ the range is 7.9 to 9.4 Xenoliths are about 1 per mil depleted in 18O and generally enriched in D relative to the hose rocks. The average δD values (and water contents) are –62 ± 4 (1.10%) and –77 ± 12 (0.73%) for ‘S’ and ‘I’ granitoids, respectively. Individual δD values range from –50 to –102 and correlate well with water contents: the more water-rich the granitoid the greater the deuterium content. Per mil 18O fractionations between quartz and biotite are rather large ranging from 5.0 to 6.9 (independent of granitoid type), with a mean of 6.0. Thus typical isotope temperatures of about 520°C are inferred and retrograde effects are indicated.

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