Oxygen and hydrogen isotope relations in the Berridale batholith

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 δ ¹⁸O values range from 9.9 to 10.5, 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 host 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 rail ¹⁸O fractionafions 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.